ingaas∕inp quantum well intermixing studied by high-resolution x-ray diffraction and grazing...

$: InGaAs∕InP quantum well intermixing studied by high-resolution x-ray diffraction and grazing incidence x-ray analysis$
InGaAs ∕ InP quantum well intermixing studied by high-resolution x-ray diffraction andgrazing incidence x-ray analysisP. G. Piva, I. V. Mitchell, Huajie Chen, R. M. Feenstra, G. C. Aers, P. J. Poole, and S. Charbonneau Citation: Journal of Applied Physics 97, 093519 (2005); doi: 10.1063/1.1870114 View online: http://dx.doi.org/10.1063/1.1870114 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/97/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characterization of interdiffusion around miscibility gap of lattice matched In Ga As ∕ In P quantum wells by highresolution x-ray diffraction J. Appl. Phys. 101, 013502 (2007); 10.1063/1.2404784 Microbeam high-resolution x-ray diffraction in strained InGaAlAs-based multiple quantum well laser structuresgrown selectively on masked InP substrates J. Appl. Phys. 97, 063512 (2005); 10.1063/1.1862769 Concentration dependent interdiffusion in In Ga As ∕ Ga As as evidenced by high resolution x-ray diffraction andphotoluminescence spectroscopy J. Appl. Phys. 97, 013536 (2005); 10.1063/1.1825613 Photoluminescence and x-ray diffraction studies of the diffusion behavior of lattice matched InGaAs/InPheterostructures J. Appl. Phys. 94, 988 (2003); 10.1063/1.1586975 A comparison of spectroscopic and microscopic observations of ion-induced intermixing in InGaAs/InP quantumwells Appl. Phys. Lett. 72, 1599 (1998); 10.1063/1.121185

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InGaAs/ InP quantum well intermixing studied by high-resolution x-raydiffraction and grazing incidence x-ray analysis

P. G. Pivaa! and I. V. MitchellDepartment of Physics, University of Western Ontario, London, Ontario, N6A 3K7, Canada

Huajie Chenb! and R. M. FeenstraDepartment of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

G. C. Aers, P. J. Poole, and S. CharbonneauInstitute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6,Canada

sReceived 20 August 2004; accepted 14 January 2005; published online 21 April 2005d

Following a study of implantation enhanced interdiffusion of InGaAs/ InP multiple quantum wellsMQWd structures by cross-sectional scanning tunneling microscopysXSTMd, the techniques oflow temperature photoluminescence spectroscopy, high-resolution x-ray diffractionsHRXRDd, andgrazing incidence x-ray analysissGIXA d are used to independently investigate the suitability of asquare well model for the interdiffused MQW profiles, and the observed dependence of straindevelopment as a function of the implanted ion range relative to the MQW stacks. In agreement withprevious XSTM findings, when ions are implanted through the MQWs, HRXRD measurementsindicate equivalent extents of interdiffusion occurring on both sublattices, while GIXAmeasurements further indicate the compositional profiles to be non-Fickian and compatible withuniformly broadened square well distributions. Following shallow ion implantsswhere ions aredeposited between the MQWs and the sample surfaced, s004d HRXRD measurements indicatepreferential group V interdiffusion. Dynamical simulations of the superlattice envelope in thes004dHRXRD rocking curves show the compositional profiles to be non-Fickian and compatible with asquare well model for the broadened compositional profiles. Additional analysis of thes001d bilayerspacing from previously published XSTM linescan data for this structure is also consistent with thisfinding. Results of a preliminary photoluminescence and HRXRD investigation of disorder-ing induced by indium implants and the effects of extended annealing on a series of MQWsamplesswith and without implantationd are presented. Implications for the implantation enhance-ment of interdiffusion in the InGaAs/ InP material system are discussed. The interpretation ofquantum well interdiffusion experiments in this material system in terms of Fickian diffusionmodels warrants revision in light of the present findings. ©2005 American Institute of Physics.fDOI: 10.1063/1.1870114g

I. INTRODUCTION

Quantum well interdiffusionsQWId has been pursuedover the last two decades as a spatially selective means ofmodifying the band gap of epitaxially grown semiconductorquantum wellsQWd heterostructures. In regions where modi-fication of the QW band gap is desired, QWI methods relyupon supplying a QW heterostructure with a local source ofpoint defects which decrease the energy barrier against inter-diffusion of QW slow band gapd and barriershigh band gapdmaterial during subsequent thermal annealing. The resultingexchange of material between the QW and barrier modifiesthe confinement potential for the electrons and holes in theQW region leading to a modification of the local QW bandgap. While QWI techniques have been refined with the pri-mary goal of fabricating and integrating planar optoelec-tronic devices such as QW lasers, modulators, and

waveguides, where specific variations in the QW band gapacross the structure are desired, QWI has also provided aunique opportunity to study the diffusion of semiconductorheterointerfaces over nanometer length scales.

Given the strong bias which has existed toward deviceapplications, reports of intermixing have made dominant useof low temperature photoluminescencesPLd measurementsto characterize the extent of the QW band gap modificationas a function of the various process parameters involvedse.g., anneal temperature and time, implantation fluence, di-electric capping layer thickness, etc.d. Such studies, however,do not allow the compositional profiles of the intermixedQW structures to be determined as a range of broadenedcompositional profiles for a given structure can reproducethe observed changes in the fundamental band gap energy.These difficulties are compounded in QW structures wherechemical gradients exist on two separate sublatticesfas forinstance in the InGaAs/ InP multiple quantum wellsMQWdstructures studied hereg.

Studies seeking to determine the changes in the compo-

adPresently a Visiting Fellow with the National Institute for Nanotechnology,Edmonton, Canada; electronic mail: [email protected]

bdPresent address. IBM Fishkill, NY.

JOURNAL OF APPLIED PHYSICS97, 093519s2005d

0021-8979/2005/97~9!/093519/16/$22.50 © 2005 American Institute of Physics97, 093519-1


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sitional profiles of QW structures following QWI have there-fore enlisted measurement techniques which can in principleprovide direct spacefe.g., cross-sectional transmission elec-tron microscopysXTEMd methodsg, or reciprocal lattice im-aging of the crystal structurese.g., x-ray diffraction methodsdwith monolayersML d scale resolution. Early interdiffusionstudies1 made use of XTEM to detect the onset of completealloying in intermixed regions, subsequent work used XTEMto obtain information on interfacial abruptness for interme-diately disordered QWs.2 Schwarzet al.3 made use of XTEMto study interdiffused InGaAs/ InP MQWs and were able tomake complementary use of electron diffraction measure-ments to obtain values for the strain induced variation in thes001d lattice parameter across the interdiffused structure. Sig-nal averaging across the foil thicknesss,20 MLd, and inter-facial roughness generated during sample thinning limitsmeasurement sensitivity at the ML scale.

Reciprocal space mapping using high-resolution x-raydiffraction sHRXRDd has also been used in conjunction withsecondary ion mass spectroscopy,4,5 PL,6 and XTEM.7 Giventhe interpretation of a rocking curve as a Fourier transform ofthe spatial distribution of the scattering power in the kine-matic approximation, this technique has been used to inferchanges to the interfacial abruptness of MQW structures fol-lowing interdiffusion by following changes to the superlat-tice sSLd satellite intensities.8 While the modeling of diffrac-tion in both the dynamical and the more approximatekinematical theories is capable of yielding information onthe compositional profiles of uniform SL structures with MLscale resolution,9 analysis has generally been constrained toinferring qualitative changes to the QW structure based onoverall changes to the SL satellite intensity levels or changesto the first order SL satellite intensity levels. This limitationhas existed, however, as this technique was used to charac-terize samples where the extent of QWI across the SL stackwas known to vary appreciably, or because no complemen-tary means of investigating the ML scale uniformity of theQW response across the stack had been applied, or becausethe structures studied possessed an insufficient number ofobservable SL satellites.

In this article, low temperature PL, HRXRD, and graz-ing incidence x-ray analysissGIXA d are used to study thecompositional profiles of lattice-matched InGaAs/ InP MQWsamples prior to and following interdiffusion by ion implan-tation enhanced intermixing.10 With the uniform spatial peri-odicity of our MQW structures established in prior XTEM11

and cross-sectional scanning tunneling microscopysXSTMd11,12 investigations, full dynamical simulations in-volving fits including contributions of up to 12th order SLpeaks ins004d HRXRD and GIXA rocking curves are per-formed. This allows the earlier conclusions concerning theobserved models for strain development as a function of theimplanted ion range relative to the MQW stacks to be bothverified and extended. In agreement with previous publishedXSTM measurements, when ions are implanted through theMQWs, HRXRD measurements indicate equivalent extentsof interdiffusion occurring on both sublattices, while GIXAmeasurements further indicate the compositional profiles tobe non-Fickian and compatible with uniformly broadened

square well distributions. Following shallow ion implantQWI swhere implanted ions are deposited between the QWsand the sample surfaced, s004d HRXRD measurements indi-cate preferential group V interdiffusion. Dynamical simula-tions of the SL envelope in thes004d HRXRD rockingcurves, show the compositional profiles to be non-Fickianand compatible with a square well model for the broadenedcompositional profiles. Additional analysis of the bilayerspacing in the XSTM linescan data for this structure is alsofound to be consistent with this finding. Results of a prelimi-nary HRXRD investigation of disordering induced by indiumimplants and the effects of extended annealing on a series ofMQW samplesswith and without implantationd are also pre-sented. Implications for the mechanism of implantation en-hanced intermixing in the InGaAs/ InP material system arediscussed.

II. EXPERIMENT

The lattice-matched InGaAs/ InP MQW samples used inthe present study were grown by chemical beam epitaxysCBEd as described in the previously published XSTM por-tion of this study.13 For ease of reference, sample structuresslabeled as in Ref. 13d are depicted schematically for wafersA, B, C, and D in Fig. 1 along with nominal growth param-eters and implant conditions in Table I. Samples were grownon s100d orientedsS-dopedd InP and dopedn-type with Sissee Table I for concentration valuesd to increase the sampleconductivity for the earlier XSTM experiments. Wafer A wasgrown with 20 InGaAs MQWs in the near surface region forthe through the wellimplant QWI experimentsswith implantion energies selected to place the ions beyond the MQWstackd. Wafer B was grown with a similar 20 periodInGaAs/ InP MQW stack overgrown with 1.56µm of InP forthe shallow implant QWI experimentssions implanted be-tween the sample surface and the MQW stackd. Wafer C was

FIG. 1. Schematic of lattice-matched InGaAs/ InP MQW structures used inthe PL, XSTM, and XRD/GIXA study of QWI. Left: wafer A used in thethrough the well QWI experiments in Sec. III A, middle: wafer B used in theshallow and through the well QWI experiments in Secs. III B and III C,respectively, and right: wafer CsDd used in the P and In QWI experimentsin Sec. III D sSec. III Ad. Grey regions depict relative locations of MQWstacks in these samples. Arrows show the relative positions of the implantedspeciessindicated at the top of each sample structured ranges relative to theMQW stacks. Compositional details appear in Table I.

093519-2 Piva et al. J. Appl. Phys. 97, 093519 ~2005!


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grown with two 10 period InGaAs/ InP MQW stacks in thenear surface and near substrate regions, respectivelyfwithdifferent barrier thicknesses being used to allow signals fromeach stack to be separately resolved in thes004d HRXRDrocking curvesg. The 1.34-µm-thick spacer layer grown be-tween both the uppersnear surfaced and lower snear sub-strated MQW stacks allowed both thethrough the wellandshallow implant QWI modes to be investigated simulta-neously in a given sample by implanting ions into the spacerlayer sand short of the lower stackd. Wafer D was used toinvestigate through the well QWI of the shallow MQW stacksonlyd by GIXA.

Ion implantation was performed using the 1.7 MV Tan-dem accelerator facility at the University of Western Ontario.Specific implant conditions are listed in Table I. Indium andphosphorus ion implants were performed at 7° off-normal tominimize ion channeling. Ion flux and fluence were26 nA/cm2 and 131014 ions/cm2, respectively, unless oth-erwise indicated. Following implantation, rapid thermal an-nealssRTAd were performed using a nitrogen purged Heat-pulse 610 RTA unit to mobilize the exchange of QW andbarrier materialsi.e., to initiate the intermixing response inthe implanted areas leading to a modification of the QWband gapd.14 Specific annealing parameters are listed in TableI. In all instances, a ramp rate of 20°C/s wasused to reach

the temperature set-points. As-grown control and implantedsamples were annealed simultaneously in a graphite suscep-tor and proximity capped with fresh pieces of cleaved InPsubstrate material to protect sample surfaces against In and Pdesorption. Temperatures were monitored using a chromel-alumel cantilever thermocouple. For wafers A, B, and D,anneal temperatures and times ranging from 675 to 725 °C,and 90 to 180 s were selected to saturate or maximise the PLblueshift, while preserving the as-grown QW band gap of theunimplanted material. For wafer C, additional annealing tem-peratures and times of 750 °C and 270 s were investigated toexamine the response of the implanted and as-grown mate-rial to extended annealing conditions.

Low temperatures4.2 Kd continuous wave PL measure-ments were performed in a liquid helium cooled cryostat tomonitor changes to the QW band gap by the interdiffusionprocess. Unless otherwise indicated, samples were excitedusing an Ar+ pumped Ti:sapphire laser tuned below the bulkInP band gap, and PL spectra were acquired using a BomemFourier-transform infrared spectrometer fitted with a thermo-electrically cooled InGaAs detector. System resolution was8 cm−1.

While the PL emission energy cannot determine on itsown the suitability of a given model for the QW profile, itplays an indispensable role here in restricting the locus of

TABLE I. Growth parameters for wafers A, B, C, and D along with sample implantation and annealing conditions. Parameters for the second near substrateMQW stack in wafers C and D appear between parentheses.

Waferand

sample ID

Silicondoping

sAtoms/cm3d

InP capsbufferd

thicknessfnmg

ThicknessLQW/LInP

fnmg

In1−xGaxAscomposition

sxdNumber ofQWs/stack

Implantparameters

RTAconditions

A 831018 19 8.0/19.0 0.47 201014 P/cm2

25 nA/cm2

1.0 MeV675 °C, 90 s

BIa 231017 1560 6.0/18.0 0.47 201014 P/cm2

26 nA/cm2

500 keV

675 °C, 90 s1725 °C, 90 s

BIb 231017 1560 6.0/18.0 0.47 20 1014 P/cm2

26 nA/cm2

500 keV740 °C, 90 s

BIY 231017 1560 6.0/18.0 0.47 20 1014 P/cm2

26 nA/cm2

4.7 MeV740 °C, 90 s

CIP 431018

s431018d88

s1340d6.0/16.0

s6.0/28.0d0.47

s0.47d10

s10d1014 P/cm2

6 nA/cm2

625 keV725 °C, 180 s

CIln 431018

s431018d88

s1340d6.0/16.0

s6.0/28.0d0.47

s0.47d10

s10d231013 In/cm2

6 nA/cm2

2.1 MeV725 °C, 180 s

DIP 331016

s331016d19.0

s1480d8.5/19.0

s8.5/19.0d0.45

s0.45d10

s10d1014 P/cm2

23 nA/cm2

850 keV725 °C, 270 s

DIIn331016

s331016d19.0

s1480d8.5/19.0

s8.5/19.0d0.45

s0.45d10

s10d

1014 In/cm2

7 nA/cm2

2.4 MeV725 °C, 270 s



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allowed QW profile changes by requiring that the fundamen-tal transition energy calculated for an assumed compositionalprofile for the intermixed QW structure reproduce the experi-mentally observed PL blueshift in emission energy relative tothe as-grown structure. Thus, for a specified interdiffusionmodel, and knowledge of the starting QW width and compo-sition, an experimentally determined PL blueshift can beconverted into a characteristic interdiffusion length whichwill describe the extent of interdiffusion required in thesample. Details on this procedure can be found in Refs.11–13.

To extract information on the compositional profile ofInGaAs/ InP MQW samples both prior to and followingQWI, samples with minimum dimensions of 5 mm35 mmand 2 cm32 cm were analyzed in the HRXRD and GIXAbeam geometries, respectively. The GIXA experiments wereperformed using a commercial Philips 1820U-2U verticalgoniometer, using a 2.5 kW generator, and CuKa s0.1542nmd radiation. The incident x-ray beam was conditioned us-ing a flat Si crystal monochromator, coupled to a flat platecollimator. Instrumental 2U resolution was,0.02° with abackground signal corresponding to a reflectivity of,5310−7. Specular reflectivity curves were analyzed using arecursive formalism to account for dynamical effects.15 Op-timization of fits was achieved using commercial PhillipsGIXA simulation software.

HRXRD experiments were performed in as004d con-figuration using a Philips MRD instrument. While this setuppossessed a similar x-ray source to the GIXA setup describedearlier, additional beam conditioning using a four crystalsGe220d Bartel-type monochromator produced an incident beamwith a ,12 arc s angular divergence.s004d rocking curveswere simulated using Philips HRS dynamical calculationsoftware which includes corrections for the nonlinear varia-tion of lattice parameter of In1−xGaxAsyP1−y alloys with vary-ing x andy fractions.

III. RESULTS

A. HRXRD and GIXA results of through the wellimplanted samples

Figure 2sad shows the experimentals004d HRXRD rock-ing curves obtained for an as-grown sample from wafer A.The central dominant feature at,31.7° corresponds to thes004d reflection of the InP substrate material. Visible on ei-ther side of this main substrate peak, and reduced in intensityby an order of magnitude or more, are the SL satellite peaks.These have been labeled according to convention. The zerothorder SL peak is unresolved from the central InP peak, andindicates that the mean QW composition is unstrained rela-tive to the substrate lattice constant. Figure 2sbd shows thesimulateds004d rocking curve for the as-grown structure as-suming the well widths for the as-grown material of 8.3 nmdetermined previously by XTEM and XSTM, and a barrierthickness of 19.0 nmsrequired to reproduce the periodicityof the SL peaks in the experimental rocking curvesd. Assum-ing an average InAsyP1−y barrier composition withy=0.013to account for arsenic tailing forces an In1−xGaxAs composi-tion in the QWs withx=0.487. The corresponding 4.2 K PL

band gap obtained by solving for then=1 bound state levelsfor electrons and holes in the conduction and valence bands,respectively, is 859 meV, in favorable comparison with theactually measured value of 865 meV. While effective in re-producing the +/−1, + /−2, and +3 SL peak intensity levels,this model for the as-grown structure fails to reproduce theobserved x-ray intensity levels in the higher order SL satel-lites. Better fits to these higher order satellites can be ob-tained by including ML strain dipole layers resulting fromproposed As/P and In/Ga exchange reactions occurring inthe interfacial regions without adversely affecting the corre-spondence between the experimental and simulated PL tran-sition energies. A simple square well model for the startingwell compositional profile will be preferred heresand in sub-sequent determinations of the as-grown structuresd as we areprincipally interested in obtaining a good estimate for theeffective QW width and composition prior to interdiffusion.

Justification for this approach is based on the applicabil-ity of Vegard’s law, and the assumption that interdiffusion inthese structuressindependent of the model assumedd will actto average out growth related ML scale compositional fluc-tuations in the QW regions. Empirical justification for thislatter assumption is supported by the relatively smoothlyvarying topographical XSTM linescan profiles obtained inthe QW centers for the intermixed structuressfollowing ei-ther through the well, or shallow implant QWId in compari-son to the corresponding as-grown data.

Figure 2, panelscd shows the experimentals004d rockingcurves for as-grown and intermixed samples from wafer A.

FIG. 2. Experimental and simulateds004d XRD rocking curves forInGaAs/ InP MQW structuresswafer Ad: sad experimental as-grown,sbdsimulated as-grownsaverage compositiond, scd experimental as-grownsleftdand penetrating P QWIsoffset rightd curves,sdd experimental penetrating PQWI, simulated square well QWI, and simulated Fickians004d QWI rock-ing curvessoffset in order from left to rightd.



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These rocking curves were acquired from samples previouslyimaged by XSTM in Ref. 11. Significantly, interdiffusion inthis sample acts to roughly preservesin the case of the +/−1 order satellitesd, or degrade the SL satellite intensities. Adecrease in intensity across all orders in the SL satellite en-velope is to be expected for the case of equal interdiffusionon the group III and V sublattices, as in this case, both thechemical compositions and lattice constants in the barrierand well regions will monotonically tend toward commonvalues leading to gradual extinction of the SL structure inreal and in reciprocal space.

Figure 2, panelsdd shows the experimentals004d rockingcurve datasleftmost curved followed by simulated curves as-suming square wellsmiddle curved and Fickian modelssrightmost curved for interdiffusion, respectively. In bothsimulated curves, length scales selected for the composi-tional broadening were chosen to reproduce the experimen-tally determined PL blueshift for this sample of 82 meV. Themiddle curve was generated assuming equal broadening ofthe square well distributions on both the group III and Vsublattices by 2.0 nmsi.e., DIII =DV =2.0 nmd. The rightmostrocking curve was generated assuming Fickian interdiffusionlengths16 of DIII =DV =2.7 nm on the group III and V sublat-tices, respectively.

While both simulated curves in Fig. 2sdd reasonably ap-proximate the intensity levels of the +/− 1, and +2 SL peaksto within a factor of 2, neither model distinguishes itself as amore consistent description for the interdiffused profile ofwafer A. To allow for better experimental discrimination ofthe high order SL components, a series of complementaryexperiments were conducted in the GIXA geometry. Al-though the strain sensitivity in rocking curves acquired in thegrazing incidence geometry approaches a minimum, the con-tribution of atomic scattering factors to the overall structurefactor reaches a maximum, leading to increased chemicalsversus lattice constantd sensitivity.17

Figure 3sad shows the experimental GIXA rockingcurves obtained from large area samples cleaved from waferD. Curvessid, sii d, and siii d, correspond to rocking curvescollected from as grown, phosphorus implant + RTA, and Inimplant + RTA samples, respectively. While the 850 keV Pimplants, and 2.4 MeV In implantsswith ion ranges of 0.78and 0.81µm, respectivelyd place the implanted ions midwaythrough the InP spacer layer separating both the upper andlower stacks, the strong attenuation of grazing incidence xrays as a function of depth from the sample surface elimi-nates any detectable signal from the deep MQW stack, al-lowing the response of the upper stack to be probed in iso-lation.

Figure 3sbd shows the simulation results using the Phil-lips GIXA simulation software for the as-grown and im-planted samples from wafer D. Curvesid shows the result forthe as-grown sample assuming the nominal growth param-eters in Table I for wafer D. Curvessii d andsiii d correspondto the simulation results assuming a square well model forthe interdiffusion. The square well distributions were broad-ened by DIII =DV =3.65 nm, and DIII =DV =4.36 nm consis-tent with the observed low temperature PL blueshiftss125and 143 meV for samples DIP and DIIn, respectivelyd.

Curve svd compares the simulation output assuming a Fick-ian model for the interdiffused profile withDIII =DV

=2.54 nm consistent with the 125 meV PL blueshift ob-served for sample DIP.

Curves sid, sii d, and siii d in panel sbd may be seen assuccessfully reproducing the overall shape of the SL enve-lopes, with the changes observed in the SL beat frequenciesbeing reproduced in the simulated curves. While the attenu-ation in SL intensity at rocking angles in excess of 5 deg incurvesii d and 3 deg in curvesiii d panelsad suggest that somelevel of interfacial roughening has occurred, the QWs remainabrupt in comparison to the profiles obtained in the case of aFickian model for interdiffusion. In curvesvd panel sbd, noevidence of the MQW structure remains in the simulatedrocking curve data beyond a rocking angle of 1.5 deg. Toprobe the sensitivity of this conclusion to the accuracy of theassessment of the interdiffusion length, curvesivd, shows thesimulated result assuming Fickian QWI andDIII =DV

=1.27 nm. While the PL blueshift predicted for such a struc-ture would be 41 meVsconsiderably less than the 125 meVblueshift actually observed for sample DIPd, this structurefails to produce any evidence of the SL structure at rockingcurve angles in excess of 3 deg. In terms of an interpretationof the HRXRD/GIXA experiments as a Fourier transform ofthe spatial distribution of the scattering power in thesesamples, the double error-function description for the com-

FIG. 3. Experimental and simulated GIXA rocking curves for as-grown, andpenetrating ion QWI wafer D samples. Panelsad: curve sid experimentalas-grown wafer D, curvesii d experimental DIP, and curvesiii d experimentalDIIn. See Table I for sample processing parameters. Panelsbd: curve sidsimulated as-grown wafer D, curvesii d simulated DIP ssquare well model,k=1d, curvesiii d simulated DIIn ssquare well model,k=1d, curvesivd simu-lated curve for intermediate QWI between as-grown and DIP sFickianmodel, k=1,Di=1.27 nmd, curve svd simulated DIP sassuming FickianQWI, k=1, Di=2.54 nmd consistent with the observed PL shift for thisstructure.



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positional profiles is seen to possess insufficient high fre-quency components to be considered compatible with experi-ment. sSimilar conclusions were also found in upper stackGIXA rocking curves obtained from the through the well Pand In implanted samples from wafer C—not shownd. On thebasis of these results, a Fickian model for the though the wellQWI data is rejected in favor of a square well description.

B. HRXRD results for shallow implant QWI

Figure 4 shows the experimental and simulateds004drocking curves for wafer B. Panelsad shows the experimentalrocking curve for the as-grown wafer B sample. Substrateand SL satellite features are as discussed in the last section.The appearance of a peak on the small angle side of thecentral InP peak corresponds to the zeroth order SL peak andindicates that the mean lattice constant of the epitaxial layersexceeds that of the InP substrate. While the observed peakseparation of 0.0300° translates to a positive strain distortionof 426 ppmsrelative to InPd along thes001d axis, the sym-metric SL intensity about the zeroth order SL peak of thefirst, second, and third SL orders indicate the QWs to beunstrained relative to the SL barrier material. Figure 4sbdshows a “best fit” simulated rocking curve for the as-grownstructure yielding a MQW structure comprising 5.3 nmIn0.538Ga0.462As QWs, and 19.9 nm InAs0.013P0.987 barriers.Again being primarily interested in obtaining a good esti-mate for the effective QW width and composition, no at-tempt was made to include strain dipole layers into the com-positional profile for the as-grown structure as discussed insection III A.

Figure 4scd shows the experimentals004d rocking curvesfor the as-grownsleft curved and shallow P implanted QWIsamplessright curved. These rocking curves were acquiredfrom samples previously imaged by XSTM in Ref. 12.

Whereas both the position and intensity of the zeroth orderSL peak remain unaffected by the QWI, the intensity of theSL satellite peaks is seen to increase by a factor of 10 ormore sorder dependentd. The SL envelope also develops anasymmetric line shape relative to the zeroth order SL peak.Whereas the negative order SL envelope preserves a 5:1 QWto SL periodicity ratio, the positive order SL peaks take on anincreased value of 6:1. While the overall increase in SL in-tensity is consistent with the formation of strain dipoleswithin the periodic MQW structuresimplying unequal ex-tents of diffusion on the cation and anion sublatticesd, theparticular direction of the asymmetry of the SL envelope ischaracteristic of the preferential group V interdiffusion oc-curring in this sample.sThe assumption of QWI with prefer-ential group III motion leads to simulated rocking curveswith a similarly shaped SL envelope reflected about the ze-roth order SL peak.d Hence, the explicit strain dependence ofIn1−xGaxAsyP1−y on thex andy fractions intrinsically estab-lishes the sensitivity of thes004d HRXRD experiment to theextent of interdiffusion occurring separately on each of thesublattices.

To obtain a more quantitative determination of the com-positional profile of the shallow P implant sample, and to testthe relative suitability of a square well versus a Fickianmodel for the interdiffused profile, rocking curves for a num-ber of trial QWI structures were calculated consistent withthe experimentally observed 110 meV blueshift observed forthis sample. To allow for ready comparison between bothmodels we define a parameterk, such that

k =DV

DIII, for Fickian diffusion

or

k =DV

DIII, for a square well model,

whereDi denotes the broadening of the square well distribu-tion on theith sublattice, andDi denotes the interdiffusioncoefficient18 on theith sublattice assuming Fickian diffusion.

Figure 5sad shows the peak labeling convention used togenerate the curves in panelssbd and scd later. To facilitatepresentation of the resultssgiven the larger number of SLpeaks involved in determining a good fit between the simu-lated and experimental rocking curvesd, the SL peak intensi-ties were separated into four bins as depicted in Fig. 5sad,and the value of the average SL peak intensity in a given binwas expressed as a fraction of the zeroth order SL peak in-tensity. As the full width at half maximum of the simulatedSL peaks are constant throughout the idealized structures,algebraic manipulation of the peak intensity values was suf-ficient. The results of this calculationsas a function of kvalue defined earlierd are shown for simulated Fickian QWIstructures in Fig. 5sbd, and square well diffusion QWI struc-tures in Fig. 5scd. In the case of the experimental data ob-tained for sample BIa, the integrated peak intensities wereused in a similar fashion to correct for the effect of slightvariations in SL periodicity across the MQW structure. Theresults of this calculation for the experimental rocking curve

FIG. 4. Simulated and experimentals004d XRD rocking curves forInGaAs/ InP MQW structuresswafer Bd: sad wafer B as-grownsexperimen-tald, sbd wafer B as-grownssimulatedd, scd experimental rocking curves forwafer B as-grownsleftd, and P implant + RTA, BIa soffset rightd.



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appear as horizontal reference lines across Figs. 5sbd and5scd. Intersection points between the simulated and experi-mental bin data are highlighted by hollow circles. Ideally,any simulated structure generated using either of the twomodels which at any fixedk value intersects the horizontalreference lines for the experimental structure at all fourpoints simultaneously would represent a strong candidate forthe compositional profile of the experimental structure.

Figure 5sbd shows the results of the bin-averaging pro-cedure outlined earlier for a series of QWI simulations as-suming a range ofk values between unitysequal extents ofFickian diffusion on both sublatticesd and infinity si.e., noQWI on the group III sublatticed. The actualx axis limits arelimited to k values between 1 and 20 for presentation pur-poses. Fork values between 1 and 3, a rapid extinction in themean SL peak intensities in each of the four bins is apparent.This observation is consistent with the strong analogy of anHRXRD rocking curve as a spatial Fourier transform of asample. As the double error-function profilesswhich describewell rounded and smoothly varying compositional gradientson both sublattices fork values near unityd are well repre-sented by a small number of sinusoidal terms, the simulatedHRXRD rocking curves indicate extremely low levels of in-tensity in the high order binssa and bd. As k increases, how-ever, the relative displacement of atoms on the group IIIsublattice tends toward zero, and the abrupt interfaces whichremain on the group III sublattice produce a sharp straingradient along thes001d axis sin spite of the slow varyinggroup V compositiond and significant increases in HRXRDintensity levels are observed across all bins.

As in HRXRD experiments, measuredsintegratedd inten-sity levels can be expected to fall below simulated levels dueto structural imperfectionssspatial frequencies present in theactual sample, but absent from the idealized structures usedin the simulationsd, simulated structures which predict lowerintensity levels than those observed experimentally can berejected. In terms of the results for the Fickian model, apply-ing this criteria would require that all Fickian models forsample BIa be rejected fork,3.65 sintersection of simu-lated “a” curve with experimental referenced.

Turning to the parallel series of results for the squarewell model in Fig. 5scd, the mean SL peak intensities in eachof the four bins ask varies from 3 to 1 do not fall as rapidlynor to as great an extent as in the Fickian diffusion caseshown in Fig. 5sbd. In this case, the square well model pre-serves sharp interfacessand, hence, abrupt discontinuities interms of strain gradients in the interfacial layers between thewell and barrier materiald on both sublattices. Correspond-ingly, the greater number of Fourier components required inthe expansion to suitably reproduce these abrupt interfacesresults in increased levels of x-ray diffraction into high ordersatellite peaks. Again applying the requirement that the simu-lated intensity levels be greater than or equal to the experi-mentally detected levels would require that structures withk,4.26 sestablished by the intersection of simulated “c”curve with the experimental referenced to be rejected.

As the bin averaging procedure ignores the more de-tailed information on the compositional profile contained inthe envelope of the HRXRD rocking curve, Figs. 5sbd and5scd are best used as a guide for what may be deemed allow-able candidates for BIa. Correspondingly, simulated rockingcurves from a range of candidate structures will be examinedin the next two figures. For the Fickian model, these willconsist in structures withk=1.6, 4, and 6soverlapping withthe range ofk values 3.7, 2.2, 1.9, and 1.6, corresponding tointersections between simulated data in bins a, b, c, and d,respectively, with the experimental data for BIad. For thesquare well model, these will consist of structures withk=1.8, 3, and 4soverlapping with intersections between ex-perimental data and simulated bins a, b, c, and d, correspond-ing to k values of 1.8, 2.8, 4.3, and 2.6, respectivelyd.

Figures 6sad–6scd show the simulateds004d rockingcurves for BIa assuming Fickian interdiffusion, andk valuesof 1.6, 4, and 6, respectively. In each of these panels, theexperimentals004d rocking curves for BIa sdotted curved isoffset to the right of the simulated curves by 0.07° for pre-sentation purposes. Consistent with the trends depicted inFig. 5sbd, the simulated and experimental intensity levels areseen to be in best agreement in Fig. 6sad for SL peaks locatedin bins c and d, in Fig. 6sbd with SL peaks in bins a, c, and d,and in Fig. 6scd with bin a and to a somewhat lesser extentwith bins c and d. The explicit shape of the simulated enve-lopes more precisely reveal the inadequacy of the error-function description for the compositional profile to repro-duce the intensity levels observed experimentally in thehigher order SL satellitesspeaks −12 to −7d for k valuessituated in the vicinity of 2. While the intensity levels inthese higher order SL peaks do increase withk as evidencedin panelssbd and scd, the overall fit of the SL envelope re-

FIG. 5. Experimentals004d XRD rocking curve and simulation results forBIa. Panelsad shows the bin partitioning scheme used in panelssbd andscd.Panelssbd and scd show simulation results for the average SL intensity ineach bin ratioed to the zeroth order SL intensity peak as a function ofkvalue for Fickian and square well models, respectively. Horizontal lines insbd andscd show experimental bin values for BIa. Hollow circles insbd andscd indicate intersection points between simulated and experimental bin in-tensity values.



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mains poor. The HRXRD simulations also incorrectly predictintensity nodes in the SL envelope at SL orders −4fpanelsadg, −8,−4, +6fpanelsbdg, and −8,−4, +6fpanelscdg. Simi-lar difficulties exist with the predicted positions of the inten-sity antinodes.

Figures 7sad–7scd show a series of simulateds004d rock-

ing curves for BIa generated assuming a square well model,andk values of 1.8, 3, and 4, respectively. In each of thesepanels, the experimentals004d rocking curves for BIa sdot-ted curved is offset to the right of the simulated curves by0.07° for comparison. Strong agreement is seen in the overallshape of the simulated SL envelope in all three panels. Con-sistent with trends exhibited in Fig. 5scd, the simulated andexperimental intensity levels are seen to be in best agreementin Fig. 7sad for SL peaks located in bins a and d, in Fig. 7sbdwith SL peaks in bins a, b, and d, and in Fig. 7scd with binsb, c, and d. While the eye may be tempted to attribute thislevel of agreement to encompass all bins in each panel, thesimulated curve in Fig. 7scd correctly compensates for theincreased widthsand intensityd exhibited in the experimentalSL satellite peaks.

The square well model in all three panels accurately pre-dicts the locations of the SL minimasand extinctionsd andoverall, is more compatible with experiment than the Fickianmodel. The greatest discrepancy is in the periodicity of thepositive order SL peaks in panelscd, and in particular, thepredicted intensity node at the location of the sixth order SLsatellite. fAdditional simulationssnot shownd suggest thismay be attributed to inwards motion of the Ga distributionon the group III sublattice on the scale of a ML or less. Thispossibility will not be pursued here given the larger uncer-tainties involved in determining the as-grown structure forB.g While the k=3 data yield a best fit in this regard, thepredicted intensity levels in bin c remain less than thoseobserved experimentallysby a factor of 1.3d. Given thegoodness of fit of thek=3 envelope, and the modest magni-tude of the disagreement in bin c,k may be reasonably as-sumed to lie within the range of 3–4 for a square well modelfit to the HRXRD data.

On the basis of these simulation results, a Fickian modelfor the interdiffused profile is discarded in favor of a squarewell model with ak value of ,3.5 for sample BIa. Thisconclusion is consistent with previous XSTM findings of-fered in conjunction with finite element simulations of the

elastically relaxeds11̄0d cleave surface for this sample whichsuggested the suitability of a square well modelsover a Fick-ian descriptiond with k,2.4 for the QWI profile.13

Figure 8 compares thes001d bilayer spacings predictedby the best fit square wellsk=3.5d and Fickiansk=4d modelsto the HRXRD data with the experimentally determined val-ues derived from the XSTM linescan data presented Fig. 6sfdof Ref. 13. s001d lattice parameters in this plotssolid line,solid circlesd were obtained by subtracting the slowly vary-ing topographic envelope from XSTM linescan data and byfitting Gaussian distributions to the atomic corrugationmaxima. After extracting the bilayer spacings, the data werescaled to set the average value of the rightmost eight bilayerspacing pointsscorresponding to the last 8 ML of pristineInP deposited before the MQW stackd equal to the latticeconstant of InPs0.587 nmd. The single error bar shown wasderived from the maximum variance in the experimental datain the InP barrier regions and approximates the uncertainty inthe other experimental data points on this graph. In the bi-layer spacing data one can clearly distinguish the InP barrierregions on the left and right-hand portions of the curve, the

FIG. 6. Simulatedssolid linesd and experimentalsdotted linesd s004d XRDrocking curves for BIa. Simulated curves insad, sbd, andscd were generatedassuming Fickian QWI withk values of 1.6, 4, and 6, respectively. Experi-mental curves are offset to the right for presentation purposes.

FIG. 7. Simulatedssolid linesd and experimentalsdotted linesd s004d XRDrocking curves for BIa. Simulated curves insad, sbd, andscd were generatedassuming a square well model for QWI withk values of 1.8, 3, and 4,respectively. Experimental curves are offset to the right for presentationpurposes.



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compressive interfacial regionsssituated at approximately+/−3 nmd, and the tensile nature of the QW center—observations all consistent with preferential QWI on thegroup V sublattice. In the case of the InP barrier regions, theslightly increased lattice constant on the left-hand side, andthe increased width of the compressive interfacial peak arealso consistent with presence of arsenic tailing identified inXSTM for the as-grown material.

The dashed and dark solid curves show the simulatedlattice spacing data generated and used by the HRS x-raysimulation software for thek=3.5 square well, andk=4Fickian models for sample BIa, respectively. While bothmodels fail to account for the magnitude of the observedlattice constants at the QW interfaces, the square well modelmore accurately reproduces the highly localised variations inthe s001d lattice parameter in the interfacial regionssi.e., fullwidths at half maximum of 0.60, 0.62, and 1.65 nm for theexperimental, square well, and Fickian distributions, respec-tively, relative to a baseline of 0.587 nmd.

C. Through the well implants revisited—sample B

Based on the results presented up to this point, twoqualitatively different models have emerged for describingthe compositional profiles of intermixed InGaAs/ InP MQWstructures contingent upon the relative range of implantedions. In the case of through the well phosphorus ion irradia-tion sSec. III Ad, the compositional profiles of the intermixedMQW structures are consistent with equivalent extents ofinterdiffusion on both the group III and V sublattices,whereas QWI by shallow phosphorus implantssSec. III Bdindicates the compositional profiles of these intermixed QWsto be consistent with a square well model for QWI accom-panied by preferential interdiffusion on the group V sublat-tice. Although the implant ion range relative to the MQWstack suggests itself as a reasonable explanation for the di-vergent behaviors observed in these samples, a number of yetto be addressed variations in intrinsic sample characteristicsand experimental conditions could equally be responsible. In

particular, the factor of 40 variation in Si doping levels be-tween samples taken from waters A and B could modulatedefect formation and mobilization dynamics though a positedFermi-level effect.19 Equally significant is the difference inannealing conditions used: 675 °C, 90 s for wafer A, versus675 °C, 90 s plus 725 °C, 90 s for sample BIa.

In order to ascertain what role these additional factorsmay have contributed to producing the qualitatively differentresponses observed between the shallow and deep QWIMQW structures, BIb was irradiated withsshallowd 500 keVphosphorus ions, and BIg was irradiated with 4.7 MeV phos-phorus ionssthrough the welld. Both samples were annealedat a temperature of 740 °C for 90 s. Low temperature 4.2 KPL measurements were carried out on these samples as de-scribed in the preceding section. PL blueshifts were102 meV+ /−5 meV, and 156 meV+ /−5 meV for samplesBIb and BIg, respectively.

Figure 9 showss004d XRD scans taken from BIb andBIg. Figure 9sad shows the rocking curves for both wafer Bsas-grownd and BIb offset from one another by 0.07°. Thispanel is to be compared with Fig. 4scd for BIa. As in Fig.4scd, an increase in the SL envelope intensity relative to theas-grown levels is apparent. While thek.1 nature of QWIin this structure can be inferred from the nature of the asym-metry observed in the envelope, the suitability of a squarewell model for BIb is also established given the one to onecorrespondence existing between the observed intensity lev-els across all observed SL orders in BIa and BIb. Given thesimilarity in the measured blueshifts for both sampless110and 102 meV for BIa and BIb, respectivelyd the overallenvelopes would be expected follow one another closely.sAdecrease by a factor of 1.05 in the overall SL envelope forBIb relative to BIa is also consistent with the trends exhib-ited by the magnitudes of the PL band gap shifts.d Hence, theelevated temperaturesand extended timed RTA used to pro-

FIG. 8. Experimental and simulated profiles of thes001d bilayer spacingacross a single intermixed well in sample BIa. Solid line, solid circles—bilayer spacing determined from XSTM linescans for sample BIa sextractedfrom data presented in Ref. 13d. Solid sdashedd line—bilayer spacing pre-dicted from x-ray rocking curve simulations assuming a Fickianssquarewelld model withk=4 sk=3.5d.

FIG. 9. sad s004d rocking curves for wafer B as-grownsleft curved andsample BIb soffset rightd, sbd rocking curves for BIg as-grownsleft curved,and BIg postimplant + RTAsoffset rightd.



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cess this sample has not altered the nature of the composi-tional profile from that observed previously for BIa.

Figure 9sbd shows a comparison between as-grownsBIgd, and implant + RTAsBIgd. In this figure, each of thesecurves are offset from the previous one by 0.07° for ease ofcomparison. The effect of the implant + RTA is to redistrib-ute the intensity among orders −4 to +4, and significantlyreduce the intensity diffracted into orders −8 to −6, and +7.The tell tale signature of preferential group III or V migra-tion in this structuresi.e., increased SL intensity levels acrossall ordersd is absent, and is consistent with uniform broaden-ing of the QW width identified in Sec. III A. The slight in-tensity increases observed in some of the low order SL peakss−1 and −4d, however, are consistent with structure resolvedin both constant current imaging and XSTM linescan data20

indicating that some level of strain modulation has devel-oped across the QWs. These results hint that a slightk.1regimesresolved more clearly in the next sectiond may occurin deep phosphorus implanted structures annealed at thesehigher temperatures.

D. PL and HRXRD of the double stack samples

To allow simultaneous investigation ofk=1, andk.1regimes initiated by penetrating and shallow phosphorus ionQWI, a sample with two 10 period MQW stacks separatedby a 1.34 µm InP buffer layer was grown by CBEsseesample parameters for wafer C in Table Id. In these experi-ments, the energy of the implanted ions was adjusted to placethe mean ion ranges within the InP barrier layer separatingboth MQW stacks. A parallel series of experiments involvingindium implants was also undertaken to determine the sensi-tivity of the interdiffusion response in these structures to thechemical identity of the implanted species. Implant param-eters listed in Table I for the phosphorus and indium ionsplaced the mean range of these ionsspredicted byTRIMd21 at0.63 and 0.64µm from the sample surfaces, respectively. Theindium fluence was scaled back from the P fluence by afactor of 5 si.e., from 1014 ions/cm2 to 2.031013 ions/cm2dto produce comparable levels of displacement damage inthese samples using vacancy production rates predicted byTRIM s3500 vacancies/ion versus 17 500 vacancies/ion forphosphorus and indium, respectivelyd.

To allow thes004d HRXRD measurements to discrimi-nate between diffracted SL satellite orders originating fromeither the upper or lower stacks in wafer C, each MQW stackwas grown with a different periodicity. To minimize anyasymmetry in energy deposition across the upper MQWstack, a smaller period of 22.0 nm was selected, while thebottom MQW was grown with a periodicity of 34.0 nmsseeTable Id. An intentionally selected ratio for the periodicity ofthe bottom to upper MQW stacks of 3:2 allowed two out ofevery three SL peaks to emerge from one or the other of thetwo MQW stacks without overlapping. QWs in both of theMQW stacks were grown with a nominal width of 6.0 nm.Results from an XSTM study of indium implanted samplesfrom wafer C are presented in Ref. 13.

PL measurements were performed following successive90 s anneals at the indicated temperatures up to a cumulative

anneal time of 270 s. Residual levels of implant damagepresent in the upper MQW stacks of each sample allowed thePL emission from each stack to be unambiguously identified.Laser diode emission at 980 nm was used to photoexcite theMQWs below the barriers allowing signal from the undam-aged lower MQW stack to dominate in intensity. HeNe ex-citation s632.8 nmd was used to photoexcite carriers pre-dominantly at the sample surface. In this case, both theopacity of the InP at this wavelength, as well as the distribu-tion of nonradiative centers in the InP buffer layer betweenboth MQWs would impede photocarrier transfer/excitationof the bottom MQW stack, allowing PL emission from theupper stack to dominate. In all instances, room temperaturemeasurementssnot shownd were repeated, allowing the ori-gin of the PL emission from the bottom MQW to be con-firmed. In this case, PL emission from the upper stack in allsamplessexcept those samples annealed in excess of 725 °Cfor 180 s or more, where some emission could be observedfrom the upper MQW stackd would be entirely quenched.Given the inherent difficulty in precisely determining thefundamental MQW transition energies from continuous wavePL measurements for highly doped MQW samples, the ob-served shifts in the PL emission energies are offered prima-rily as a guide to monitor the relative extents of interdiffu-sion in the upper and lower MQW stacks.

Figure 10 shows PL spectra acquired from a subset ofthe samples studied from wafer C. Given the similarity ingrowth parameters between wafers A and C, the reader isreferred to the discussion of the PL spectra for wafer A inRef. 11 for details concerning the interpretation of the PLline shape. Fine structure between 873 and 918 meV super-

FIG. 10. Low temperatures4.2 Kd PL spectra for a subset of wafer Csamples annealed at 675 °C. Sample processing parameters, and excitationsources980 nm laser diode or 632.8 nm HeNe laserd for each curve areindicated in the right hand margin. Upward pointing arrows indicate theapproximate location of the fundamental PL transition energy. Curvessid–siii d show the negligible effect of RTA on the unimplanted MQW samples.Curvessivd and svd show PL blueshifts for the In implant + RTA samples.Curvessvid and svii d show PL blueshifts for the P implant + RTA samples.Alternate use of HeNe and laser diode excitation in curvessii d–svii d allowthe PL response of the upper and lower MQW stacks in each of the samplesto be differentially probed.



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imposed on all spectra results from atmospheric water ab-sorption. Curvesid shows the PL spectrum for an as-grownsample from wafer C. The observed low energy peaksindi-cated by the upward pointing arrowd at 895 meV is in agree-ment with the calculated fundamental QW transition energyfor the MQWs s892 meV based on Table I parametersd.The two high energy shoulders resolved at 1000 and1025 meV are consistent with Fermi-level broadening ofthe emission envelope by doping levels of 331018/cm3 and431018/cm3, respectively. As the 980 nm excitation stimu-lates luminescence from the upper and lower MQW stacks inequal portions, the presence of the two shoulders suggests avariation in doping concentration between both MQWstacks.

Curvessii d and siii d show the negligible effect of a cu-mulative 270 s anneal at 675 °C on the unimplanted material.The PL spectrum in curvesii d was collected under 980 nmexcitation and shows no difference relative to curvesid.Curve siii d was obtained using 632.8 nm excitation to pref-erentially generate photocarriers in the upper MQW stack.The low energy shoulder remains unchanged in energy rela-tive to curvessid and sii d indicating an absence of any sig-nificant interdiffusion. The appearance of a single Fermi-level broadened shoulder at 1000 meV indicates the dopingconcentration in the upper MQW stack is,331018/cm3.

Curvessivd and svd show the PL spectra for the indiumimplanted material annealed for 90 s at 675 °C. Curvesivdwas acquired using 980 nm excitation to preferentially excitethe lower MQW stack. Under these annealing conditions, asmall PL blueshift of 15 meV is observed in the lower stackindicating that few of the QWI enhancing point defects havereached these wells. Curvesvd shows the PL spectrum ob-tained under HeNe excitation to preferentially probe lumi-nescence from the upper MQW stack. The much larger PLblueshift s96 meVd is consistent with the proximity of theradiation point defects to the upper stack. Also present in theHeNe excitation spectrafcurves svd and svii dg is a defectband centred at 1.2 eV. This feature is visible only in theimplanted samples under HeNe excitation and is attributed tolattice disorder generated in the InP by the ionimplantation.22 Curvessvid andsvii d show PL spectra for thephosphorus implanted samples annealed for a cumulativetime of 270 s at 675 °C. Shifts of 17 and 115 meV are ob-served for the lower and upper MQW stacks, respectively.

Figure 11sad summarizes the results of the low tempera-ture PL measurements taken from the phosphorus QWIsamples from wafer C. The hollow-symbol-solid-line andsolid-symbol-solid-line curves depict the dynamic responseof the band gap shifts induced in the upper and lower MQWstacks as a function of anneal time and temperaturessee fig-ure caption for legendd. While the blueshift induced in theRTA-only material remains less than 20 meV, a PL blueshiftof up to 166 meV is exhibited by the shallow stack followinga cumulative 270 s RTA at 750 °C. The overall time andtemperature responses exhibited by both the upper and lowerMQW stacks are quite different. Looking at the shallowMQW data, a PL blueshift of 92 meV results immediatelyfollowing the first 90 s RTA at 675 °C, whereas for the bot-tom MQW stack, a 90 s anneal at 750 °C, or a cumulative

180 s anneal at 725 °C is required before a comparable PLshift can be observed. Hence, the behavior of this 431018 Si/cm3 doped structure is consistent with the differ-ent annealing requirements exhibited between the penetrat-ing implants employed in wafer A samples, and the shallowimplants employed in the BIa and BIb samples.sSimilarresults were obtained in the case of the wafer D sampleswhich possessed a silicon concentration of 331016 atoms/cm3d. Hence, the different annealing require-ments for samples reported in Secs. III A and III B resultprimarily from the proximity of the implant damage from theMQWs and not from differences in the doping levels.

Similar qualitative conclusions may also be drawn fromthe set of indium implanted samples in Fig. 11sbd and willnot be reiterated. Of significance, however, is the fact that thePL blueshifts in the shallow MQW stack samples saturate toa lower level and smaller rangesi.e., PL emission rangingbetween 991 and 1026 meV vs 991 and 1065 meV for the Pimplantsd than the P implants. While chemical effectssrelat-ing to the excess In or P introduced by the implants intothese samplesd on point defect annealing dynamics cannot beruled out, another plausible mechanism may involve differ-ences in defect trapping. Given that the indium implants cre-ate on average five times as many defects per ion track, thegreater defect densities produced by these ions may promotea greater number of stable defect centres in the shallowMQW region. As HRXRD data and analysis presented laterwill eliminate differences ink values as an explanation forthe different PL blueshifts observed in bottom stacks of theindium and phosphorus implanted samples, the smaller blue-shifts observed in the deep MQW stack following In implan-tation suggest a smaller number of QWI enhancing defectsare being released from the implant damage regionsconsis-tent with defect trappingd. Without a more detailed investi-gation of the QWI response using other implant species, dif-

FIG. 11. Plot of 4.2 K cw PL emission energy as a function of RTA tem-perature forsad phosphorus implanted, andsbd indium implanted samplesfrom wafer C. Dashed and solid lines refer to PL emission from as-grown +RTA only, and implant + RTA samples. Circles, triangles, and squares referto samples annealed for 13 ,23, and 3390 s, respectively. Hollow sym-bols denote PL emission from the near surface MQW stacks in all samples.



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ferences resulting from possible interactions between theexcess phosphorus or indium atoms implanted into thesesamples and QWI enhancing point defects cannot be ruledout.

s004d HRXRD rocking curves were measured for allsamples shown in Fig. 11. To verify that sample nonunifor-mity across the CBE grown wafer would not impede com-parison of HRXRD data from samples originating from dif-ferent locations, a HRXRD investigation of the radialuniformity was carried outsnot shownd. A series of rockingcurves collected from the wafer center out to the edge of the2 in. wafer indicated a radial growth uniformity of betterthan 5% in terms of layer thickness across the entire wafersand consistent with the degree of PL emission uniformityexhibited in PL mapping experimentsd. Uniformity in SLpeak intensity across all orders variedssystemically andmonotonicallyd outward from the wafer center over a radialdistance of 24 mm by 20%. Samples used in the presentHRXRD study in this section were restricted to a contiguousregion lying within 10 mm of the wafer center limiting theSL intensity variations to,4%d.

Figures 12sad and 12sbd show thes004d rocking curvesmeasured for the phosphorus implanted samples annealed attemperatures of 675 and 725 °C, respectively. Curves are off-

set from left to right and are shown for the as-grown, andimplant + RTA samples following cumulative anneal timesof 90, 180, and 270 s. For reference purposes peaks originat-ing from the upperslowerd SL have been labeled withswith-outd primes. In panelsad sconcentrating on the primed peakswhich do not overlap with nonprimed peaks originating fromthe bottom MQW stackd the effect of the first 90 s, 675 °Canneal is to decrease the intensity of the SL satellite peaksattributed solely to the upper MQW stack. This behavior isconsistent with the qualitative changes induced in theHRXRD measurements from wafer A, and reflects the ex-pected behavior of the SL envelope for interdiffusion withk=1. Some of these SL peaks do, however, begin to overtakethe intensity of as-grown SL peaks following a cumulativeanneal time of 270 s. Looking at the nonprimed SL peaksoriginating from the bottom MQW, a slight monotonic in-crease in all satellite peak intensities is observed with in-creased anneal time. The relatively small changes in intensityobserved for these peaks, however, are consistent with theslight PL shifts exhibited by the bottom MQW structures13.4 meV following 270 s at 675 °Cd and interdiffusion oc-curring with ak value greater than unityfsee Fig. 11sadg.

In Fig. 12sbd, the 725 °C data amplify the trends ob-served in panelsad. The overall envelope formed by the non-primed peaks from the bottom MQW stack displays the fa-miliar enhancement attributed to preferential group V QWIfsee also positive order SL peaks in Fig. 14sadg. The in-creased amplitude of the nonprimed SL envelope relative toFig. 12sad is consistent with the larger PL shifts observedfollowing these higher temperature annealss41, 120, and 135meV following each of the three cumulative 90 s annealsd.Interestingly, the primed SL peaks from the upper stack be-gin to display increased x-ray intensities at these higher tem-peratures as well. This indicates that after a rapid initial onsetof interdiffusion with k=1 in the shallow stack followingpenetrating phosphorus QWIfrefer to panelsadg, a second,more gradual phase sets in during extended time or elevatedtemperature anneals where QWI on the group V sublatticebegins to outpace that occurring on the group III sublatticei.e., k.1.

Figure 13 shows thes004d rocking curves measured forthe indium implanted samples annealed at temperatures of675 and 725 °Cfsad and sbd, respectivelyg. Following the675 °C anneals, the x-ray intensities of the primed SL peakssupper stackd show a rapid decrease across all orders follow-ing the first 90 s RTA with intensities slowly recovering tothe initial signal levels in the lower order SL peaks over thecourse of the two subsequent 90 s annealssand again sug-gestive of a slight tendency toward QWI withk.1 for pro-longed RTA times as observed in the phosphorus implantedsamplesd. The nonprimed peaks originating from the lowerMQW stack, however, again display monotonically increas-ing intensity levels. These behaviors are again consistentwith initial k=1 QWI in the shallow stack following pen-etrating QWI, and QWI withk.1 in the case of the deepMQW stack.

In Fig. 13sbd, these trends are amplified following RTAat 725 °C. The significantly larger increases in thesnon-primedd SL intensities from the lower MQW stack are both

FIG. 12. s004d rocking curves measured for phosphorus implanted wafer Csamples annealed atsad 675 °C andsbd 725 °C. Arrow indicates leftmostas-grownsno RTAd reference curve. Curves in both panels corresponding to13 ,23, and 3390 s anneals at a given temperature are offset 0.02, 0.04,and 0.06°, relative to the as-grown reference curve, respectively. Primedsnonprimedd SL orders denote XRD signal originating from topsbottomdMQW stack.



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consistent with thek.1 QWI, and with the larger overall PLblueshiftss35, 96, and 115 meV following each of the threecumulative 90 s annealsd observed for the deep MQW stackfsee Fig. 11sbdg. Significantly, variations in intensity levels ofSL peaks associated with the shallow MQW stacksprimedpeaksd are relatively subdued following the three anneals. Inspite of the fact that the PL blueshifts in these indium im-planted shallow MQW stackss105, 110, 115 meVd are com-parable to the blueshifts exhibited by the shallow MQWstack implanted with phosphoruss123, 138, 147 meVd, theincreases in SL intensity in the indium implant shallowMQW remain significantly smaller than the equivalently an-nealed phosphorus implanted samples. Implications of thisbehavior for QWI processes will be discussed in Sec. IV.

The rightmost rocking curve in Fig. 13sbd allows a for-tuitous comparison to be made. Following the 270 s anneal at725 °C, the PL shifts exhibited by the upper and lower MQWstacks are 115 and 114 meV, respectivelyfsee Fig. 11sbdg.Given the similarity of the band gap shifts exhibited by thetwo MQW stacks in this particular sample, the differencesobserved between the primed and nonprimed SL satellite en-velopes corespond to explicit differences in the composi-tional profiles in each of the interdiffusion MQW stacks.Hence, of all the data presented for wafer C, this sample best

indicates the differential QWI response of the QWs to therelative position of the implanted ion range.sXSTM mea-surements performed on CIIn following a 180 s RTA at725 °C also allowed simultaneous observation ofk=1, andk.1 QWI in the upper and lower MQW stacks, respec-tively.d

To investigate whether the compositional profiles of thebottom MQW stacks are comparable following QWI by shal-low indium or phosphorus implants,s004d rocking curvestaken from samples exhibiting similar PL blueshiftssand an-nealed at identical temperaturesd are compared in Fig. 14sad.Rocking curves are shown for an indium implanted sampleannealed at 725 °C for 270 ssblueshift =115 meVd, and aphosphorus implanted sample annealed at 725 °C for 180 ssblueshift =124 meVd. The nonprimed SL peaks originatingfrom the bottom MQW stacks in the two samples are seen tofollow very similar envelopes. The SL envelope for the in-dium QWI bottom stack is reduced in intensity relative to theP implant sample by a factor of 1.4. This observation, how-ever, is consistent with the smaller PL blueshift observed inthis sample’s deep MQW stack. Given the interpretation ofthe HRXRD rocking curve as a Fourier transform of thestructure factor distribution throughout the MQW SL, onemay conclude both these intermixed MQW stacks shareclosely matched compositional profiles.

FIG. 13. s004d rocking curves measured for indium implanted wafer Csamples annealed atsad 675 °C, andsbd 725 °C. Arrow indicates leftmostas-grownsno RTAd reference curve. Curves in both panels corresponding to13 ,23, and 3390 s anneals at a given temperature are offset 0.02, 0.04,and 0.06°, relative to the as-grown reference curve, respectively. Primedsnonprimedd SL orders denote XRD signal originating from topsbottomdMQW stack.

FIG. 14. sad s004d rocking curves measured for phosphorussleft curved andindium sright curved implanted wafer C samples annealed at 725 °C for 23 and 3390 s, respectively. Only odd SL orders from the bottom MQWstack are labeled.sbd Effect of RTA only on wafer C. Curves from left toright are for as-grown, 13 ,23, and 3390 s RTAs at 750 °C, respectively.



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X-ray rocking curves collected from the annealed-onlycontrol samples allow a limited investigation of thermal-onlyQWI. Figure 14sbd shows the effect of cumulative 90 s, 750°C anneals on as-grown samples from wafer C. Apparent inthe 725 °C datasnot shownd, and to a greater extent in the750 °C data, the intensities of the nonprimed SL peaks origi-nating from the bottom MQW stack indicate a significantincrease in intensity. The relative increase by a factor of 3 ofthe nonprimed SL envelopesfollowing 3x90 s, at 750 °Cdrelative to the as-grown rocking curve is consistent with themagnitude of the PL shift observed for this samples19 meVd.These observations are consistent withk.1 interdiffusion.That no accompanying increase in SL intensity has yet oc-curred in the case of the uppersprimedd SL peaks indicatesthat QWI enhancing defects are not being supplied in uni-form fashion throughout the epitaxially grown layers.

Excluding gradients of some unidentified impuritywithin the epitaxial layers, two explanations present them-selves:s1d defects existing in the epitaxy/substrate interfacialregion sor in the substrate itselfd beneath the structure aremobilized during the anneal serving to promote QWI en-hancement in the bottom MQW stack,s2d chemical reactionsduring RTA between the sample surface and ambient gasesare introducing defects from the surface which suppress theaction of QWI enhancing defects in the upper MQW stackotherwise present throughout the epitaxial layers. Comple-mentary experimental work reported by Haysomet al.23 in-volving similarly grown samplessusing the same CBE reac-tor as in the present studyd demonstrated that intentionalinclusion of low temperature grown InP layers in these struc-tures were responsible for enhanced thermal-only QWIshifts. Thermal QWI in these samples was found to proceedwith k.1 sinferred from photoabsorption measurementsdover a similar regime of anneal times and temperatures.Hence, the QWI enhancing defects in the present contextsi.e., RTA-only responsed appear to be correctly identified asbeing intrinsic to the epitaxially grown layers.

IV. DISCUSSION AND CONCLUSIONS

Results presented over the course of this study allow anumber of conclusions to be drawn relating to the nature ofthe interdiffused compositional profiles of lattice-matchedInGaAs/ InP MQW structures following QWI under varyingexperimental conditions. Penetrating implant damage gener-ated by either indium or phosphorus ions results in QWI withk=1. Suggested first by a combination of PL and XSTMmeasurementsswith the caveat that displacement damage didnot impede the interpretation of strain-free QWI in theXSTM measurementsd, subsequent HRXRD measurementspresented here were able to confirm strain-free QWI with akvalue of unity. GIXA measurements performed on similarlyprocessed structures revealed the resulting profiles to be in-compatible with a Fickian description, and consistent with asquare well model. HRXRD measurements further indicatedthat while QWI withk=1 resulted following RTA,675 °C,90 s, annealing for extended time and/or at elevated tempera-ture resulted in additional QWI withk.1.

Following shallow P implant QWI, interdiffusion was

first determined by XSTM to proceed withk.1. Subsequentintroduction of either the Fickian or square well models forthe QWI profilesconsistent with the magnitude of the experi-mental PL blueshiftd into finite-element simulations for theelastic surface relaxationsobserved in XSTMd also indicateda better fit to the data in the case of the square well model.HRXRD measurements on these samples presented here al-lowed independent verification of these findings. While in-creased levels of x-ray intensity into the SL satellite peakswere indicative of preferential interdiffusion on one of thetwo sublattices, the asymmetric modification of the SL enve-lope has been shown to be consistent with interdiffusion withk.1 in this material system. Simulations presented here ofthe HRXRD rocking curves assuming Fickian and squarewell models for the interdiffused profile also indicated amore consistent fit to the SL satellites resulted in the case ofa square well model for QWI. As a final comparison betweenthe XSTM and HRXRD data set for this sample, the bilayerspacing was extracted from previously published XSTMdata, and was compared with thes001d bilayer spacings gen-erated by the Philips HRS software used to simulate theHRXRD data. While both the Fickian and square well mod-els failed to reproduce the magnitudes of the observeds001dlattice parameters in the interfacial regions, the square wellmodel offered a better description of the spatial distributionof the distortions to the lattice constant in the interfacialregions.

While other groups have reported that interdiffusion inthe InGaAsP/ InP material system can proceed differently onthe group III and V sublattices,3–6 the present results requirethat the assumption of a Fickian model for interpreting inter-diffusion in this material system be revised. Whether thesefindings impact the entire range of conceivableIns1−xdGaxAsyPs1−yd / InP multilayers and interdiffusion condi-tions si.e., all fractional compositions ofx and y, varyingdoping species and concentrations, well and barrier thick-ness, implant species, annealing conditions, etc.d, or the lim-ited range of experimental parameters investigated here, isnot clear. It seems a similar application of the GIXA andHRXRD methods could be employed to map out the atomicscale dependence of interdiffusion on these variables. It re-mains, however, to interpret the findings of non-Fickian dif-fusion in the samples studied here in terms of relevant ther-modynamic quantities and their relationship to themiscibility gap known to exist in this material system.24

Samples were also studied possessing two MQW stacksseparated by an InP buffer layerssee Table Id. By implantingions into the InP buffer layer between both MQW stacks, theQWI response following penetratingsin the upper MQWstackd and shallowsin the bottom stackd ion implantationcould be investigated simultaneously. While anneals per-formed between 675 and 725 °C for 180 s or less allowedk=1 andk.1 regimes to be observed in the upper and lowerMQW stacks, respectively, anneals performed at higher tem-peratures or for longer times resulted in some additionalQWI in the upper stack withk.1.

Separate indium and phosphorus implants were per-formed on double stack samples to determine whether therelative stochiometric imbalance of phosphorus or indium



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atoms introduced by the implants might modulate the ob-served QWI response through the intermediary of phos-phorus or indium-rich point defects. As both MQW stacks inthese samples were grown with different SL periods, it waspossible to study the differential response of the upper andlower MQWs in HRXRD.k=1 andk.1 trends in the upperand lower stacks, respectively, were reproduced followingirradiation by either indium or phosphorus ions. SL enve-lopes for shallow phosphorus and indium implant QWIMQWs exhibiting similar PL blueshifts were found to beessentially indistinguishable. Correspondingly, the QWI re-sponse following shallow indium and phosphorus ion QWIwas concluded to be dominated by the release of point de-fects from the lattice disorder region, and not by the chemi-cal identity of the implanted ions.

Although comparable band gap shifts following bothpenetrating In and P QWI at the lowest anneal times andtemperatures were observedsconsistent with the equivalentdamage levels predicted byTRIM

21 for the implant energiesand fluences investigatedd, differences were noted in thesaturation response of the shallow MQW stacks. Indium im-planted QWs exhibited a more rapid saturationsaccompaniedby a smaller overall PL shift following RTA at elevatedtemperature/extended timed than similarly annealed phos-phorus implanted QWs. QWI occurring in these phosphorusimplanted MQWs following elevated RTA times and tem-peratures also showed a greater propensity toward QWI withk.1 than their indium implanted counterparts.

While interactions between the implant species and theQWI enhancing point defects during RTA cannot be ruledout from this data setsas for instance inferred from QWIsaturation effects following shallow aluminum QWI in zincdoped InGaAsP/ InP laser structuresd,25 a simple model con-sistent with these results can be formulated. The existence oftwo classes of QWI enhancing defects is proposed:type 1point defects which induce QWI on the group III sublatticeonly, andtype 2point defects which promote group V QWIonly. Furthermore, type 2 defects should posses a lesser pro-pensity to form defect aggregates in radiation damaged InP.

In the case of penetrating implant QWI, type 1 and 2defects are assumed to be produced in comparable levelsduring implantation, allowing QWI to proceed equivalentlyon both sublattices. Saturation effects then result from thecoalescence of both defect types onto stable defect structuresseeded by primary lattice disorder generated by the implants.At sufficiently high defect concentrations, extinction ratesfor both defect types are comparable and QWI proceedsequivalently on both sublattices. Saturation of QWI also oc-curs at an increased rate, withk values remaining close tounity following RTA for prolonged times and/or at elevatedtemperatures. At lower defect density levels, type 1 defectswould tend to precipitate to a larger extent than type 2 de-fects, allowing type 2 defects to promote preferential QWIon the group V sublattice following an initialk=1 QWI re-gime. Also, these proposed differences in the propensity toform defect aggregates would result in preferential release oftype 2 defects following either indium or phosphorus im-plantation from the implant damage region during RTA, and

would be consistent with observations ofk.1 QWI inMQW stacks located beyond the implant ion range.

Finally, the thermal response of the as-grown materialwas investigated for the double stack structures over therange of interest to QWI applications. While XSTM analysiswas not undertaken, HRXRD and PL allowed the identifica-tion of a regime of thermal only QWI, mediated most likelyby defect release from the sample substrate/lower epitaxiallayers. The differing periodicities of the upper and lowerMQW stacks allowed combined PL and HRXRD analysis toidentify QWI occurring primarily in the bottom MQW stackwith k.1. Recent work on structures grown from the sameCBE reactor by Haysomet al.23 indicated, however, that thethermal stability against QWI in these temperature regimescan be modulated by slight variations in growthsandcrackerd temperaturessand consistent with growth conditionsused in this studyd. The source of these defects is thereforemost likely intrinsic to the bulk epitaxial InP itself. That theQWI enhancing defects in low temperature grown InP alsocouple preferentially to the group V sublattice inInGaAs/ InP MQW structures suggests they may constitutethe same mobile point defects as those responsible for thepreferential group V migration observed here following shal-low implant QWI.

ACKNOWLEDGMENTS

This work was partially supported by OGS, NSERC, andthe NSF. One of the authorssP.G.P.d wishes to thank Dr.Jean-Marc Baribeau for helpful discussions, and assistancewith the GIXA portion of the measurements.

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