ion implanted dopants in gan and aln: lattice sites, annealing behavior, and defect recovery

JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 5 1 MARCH 2000

Ion implanted dopants in GaN and AlN: Lattice sites, annealing behavior,and defect recovery

C. Ronning,a) M. Dalmer, M. Uhrmacher, M. Restle, U. Vetter, L. Ziegeler,and H. HofsassUniversitat Gottingen, II. Physikalisches Institut, Bunsenstr. 5-7, D-37073 Go¨ttingen, Germany

T. Gehrke, K. Jarrendahl,b) and R. F. DavisNorth Carolina State University, Department of Materials Science and Engineering, Box 7907,Raleigh, North Carolina 27695

ISOLDE CollaborationCERN, CH-1211 Geneva 23, Switzerland

~Received 1 September 1999; accepted for publication 17 November 1999!

The recovery of structural defects in gallium nitride~GaN! and aluminum nitride~AlN ! afterimplantation of111In1 and89Sr1 in the dose range~0.1–3! 1013 cm22 and ion energies of 60–400keV has been investigated as a function of annealing temperature with emission channeling~EC!and perturbedgg angular correlation spectroscopy. The implanted In and Sr atoms occupiedsubstitutional sites in heavily perturbed surroundings of point defects after room temperatureimplantation. No amorphization of the lattice structure was observed. The point defects could bepartly removed after annealing to 1473 K for 10–30 min. Lattice site occupation of implanted lightalkalis, 24Na1 in GaN and AlN as well as8Li1 in AlN, were also determined by EC as a functionof implantation and annealing temperature. These atoms occupied mainly interstitial sites at roomtemperature. Lithium diffusion and the occupation of substitutional sites was observed in GaN andAlN at implantation temperatures above 700 K. A lattice site change was also observed for sodiumin AlN, but not in GaN after annealing to 1073 K for 10 min. ©2000 American Institute ofPhysics.@S0021-8979~00!01005-7#

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I. INTRODUCTION

The III–V nitrides have generated considerable interbecause of their realized and future potential for blue lisources,1–6 UV light sources,3,7 UV detectors,8–10microwavesources,11,12 and high power and high frequencdevices.6,13–16 Reviews regarding the preparation, functioand possibilities of these devices are given in Refs. 3, 1517–19. For such applications it is necessary to dope thsemiconductors, and, commonly, Si or O are useddonors,20,21 and Mg, Be, Zn, or Ca as acceptors.17,22–27

The incorporation of dopants into gallium nitride~GaN!and aluminum nitride~AlN ! by diffusion is very difficult dueto their high thermal stability.28–30 Moreover, the realizationof lateral doping structures during film growth is impossibIon implantation is an attractive alternative method of dopincorporation for device fabrication because~i! the concen-tration as well as the lateral and depth distributions ofdopants are precisely controllable and~ii ! almost all ele-ments can be implanted with sufficiently high purity. However, this process is compromised by the radiation damwhich has to be removed via annealing. Amorphization moccur for high implantation doses.31,32 The determination ofannealing sequences that heal lattice damage, reduce la

a!Author to whom correspondence should be addressed; [email protected]

b!Present address: Linko¨ping University, Department of Physics and Mesurement Technology, SE-581 83 Linkoping, Sweden.

2140021-8979/2000/87(5)/2149/9/$17.00

Downloaded 13 Nov 2007 to 134.76.86.189. Redistribution subject to AIP

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strain, and drive the dopants to electrically active sitesessential for a successful doping by ion implantation. Itimportant to know how the dopants can be incorporatedthe favorable lattice sites, how they can be activated,how much of the remaining damage compensates the elecal activation.

For ion implantation into GaN the high thermastability20,28,33 and the limited success in electricactivation1,20,22,24,34,35of the implanted species has beemainly reported, but there are also several studies aboucreated radiation damage.31–36 Tan et al. found that GaN isextremely resistant to amorphization and occurs only at vhigh doses of approximately 431016cm22 at liquid nitrogentemperature.31 Below this dose the created disorder consimainly of clusters, small dislocation loops, and planar dfects. The concentration of the various defects can beduced by implantation at high temperatures or by subseqannealing procedures, but a complete recovery has not bachieved with different annealing techniques32,36 because ofthe decomposition of the GaN surface at high temperature37

Several approaches to protect GaN during annealing, e.ghigh pressure annealing, annealing under a flux of NH3 or thedeposition of protective AlN cap layer have beereported.32,36,38,39 However, maximum annealing tempertures of 1573 K have been employed at this writing formaximum time of 30 min, which is still too low for a complete recovery of the implanted GaN.

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9 © 2000 American Institute of Physics

license or copyright, see http://jap.aip.org/jap/copyright.jsp

2150 J. Appl. Phys., Vol. 87, No. 5, 1 March 2000 Ronning et al.

TABLE I. Implantation parameters of radioactive implanted atoms: ion energyEimpl , ion dose, implantation depth, and straggles calculated withTRIM ~seeRef. 51!. Implanted ions were used for EC and PAC spectroscopy.

Target IonEimpl

~keV!Dose

~cm22!Depth ~s!

~Å!Implantation

facilityParticle

energy~keV!Decay

particles Half timeTechnique

applied

GaN 111In 100–400

331013 264 ~103!–952 ~655!

Gottingen 1711245 g1Con.electrons

2.83 d EC1PAC

GaN 85Sr 60 231013 205 ~85! ISOLDE ,1460 b2 electrons 50.5 d ECGaN 8Li 60 231013 a 2367 ~874! ISOLDE 1500–5000 a 0.8 s ECGaN 24Na 30 231013 322 ~160! ISOLDE ,1390 b2 electrons 15.01 h ECAlN 111In 285 331013 895 ~228! Gottingen 1711245 g1Con.

electrons2.83 d EC1PAC

AlN 8Li 60 231013 a 3129 ~786! ISOLDE 1500–5000 a 0.8 s ECAlN 24Na 30 231013 422 ~170! ISOLDE ,1390 b2 electrons 15.01 h EC

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Few reports regarding the ion implantation into AlN aavailable. Borowskiet al. implanted Ti into AlN and pro-duced the formation of an additional phase after annealin40

Wilson et al. implanted Er into AlN to investigate the 1.5mm luminescence of optically excited Er31 atoms.41 Nothingis known regarding the implantation-induced defects, thetice site occupation of the implanted impurities and thenealing behavior of the implanted single crystalline AlN.

Lithium is a well-known shallow interstitial donor in elemental semiconductors and shows donor or acceptorbehavior in most compound semiconductors, dependingthe Li lattice site occupation. Lithium is also a fast diffusin most semiconductors and, similar to hydrogen, it mayteract with other donors or acceptors resulting in electricainactive complexes. Early optical investigations of Li-dopGaN show that lithium doping by implantation42 andgrowth43 produced deep levels in the band gap. Howevthese deep levels are most likely due to the low qualitythese early GaN samples or due to implantation defects.42,43

We have determined that ion implanted Li mainly occupthe interstitial lattice sites in the center of thec-axis hexa-gons at low temperature.44 The onset of Li diffusion above700 K and the strong coulomb interaction with vacancyfects lead to the formation of substitutional Li.44 The activa-tion energy for Li diffusion was determined asEact

'1.7 eV. Nothing is known about the behavior of Li in AlNHowever, one can expect a similar behavior to GaN. Nothhas been published so far about the behavior of sodiumboth GaN and AlN. In both cases the alkali atoms will actshallow dopants for eithern- or p-type doping, depending onthe occupied lattice sites.

In this study we investigated the lattice site occupatof 111In and 89Sr in GaN and AlN with the emission channeling ~EC! technique and the recovery of defects with pturbedgg angular correlation~PAC! spectroscopy as a function of annealing temperature. In the second part of tarticle we present the results regarding the occupied lasites ~measured by EC! and the behavior of Li and Na inGaN and AlN after ion implantation and annealing. We adetermined the temperature of the onset of Li diffusion,interaction of Li and Na with vacancies and the thermal sbility of the alkalis on different lattice sites.


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II. EXPERIMENT

Undoped, 1.0–2.0-mm-thick GaN films were grown onon-axisn-type, Si-face 6H-SiC~0001! substrates at 1273 Kand 63103 Pa, using a vertical, cold-wall, rf inductivelyheated metalorganic vapor phase epitaxy deposisystem.45,46 Prior to the GaN growth a 0.1mm high-temperature~1373 K! AlN buffer layer was deposited on thsubstrate. Deposition of the AlN and GaN layers were pformed using triethylaluminum and triethylgallium, respetively, in combination with 1.5 spatial light modulato~SLM! of ammonia (NH3) and 3 SLM of H2 diluent.Magnesium-doped GaN films of 0.5mm thickness weregrown on top of the undoped GaN films. Thesep-type GaNfilms had a dopant concentration of 4.431019cm23 andcapacitance–voltage measurements revealed about 10electrically active dopant atoms after rapid thermal anneafor 30 s at 1073 K.

Aluminum nitride films of 0.35–0.5mm thickness weregrown on ~4H!-SiC substrates at 1373 K in a gas-sourmolecular beam epitaxy system with an ultimate base psure of 1028 Pa (10210Torr).47,48 The Al source was a 30cm3 effusion cell heated to 1500 K and filled with pellets99.9999% pure material. High purity NH3 ~Solkatronix‘‘blue grade’’! introduced at a flow rate of 7.5 sccm was usas the nitrogen source. Studies by reflection high eneelectron diffraction, atomic force microscopy, and transmsion electron microscopy showed that the AlN surfaces wvery smooth and x-ray diffraction also showed the filmsbe highlyc-axis oriented and single phase.47,48

Ion implantation of different radioactive atoms for thEC technique and PAC spectroscopy were conducted aton-line isotope separator PSB–ISOLDE at CERN49 or at theion implantation facility at the University of Go¨ttingen.50

Implanted ion species, ion energies, and doses as well acalculated depth profiles byTRIM51 are summarized in TableI. The choice of the implanted species was also dictatedthe measurement techniques used, which are describedHowever, a variety of isotopes are suitable for EC measuments; whereas, only a few species can be used for Pmeasurements.


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2151J. Appl. Phys., Vol. 87, No. 5, 1 March 2000 Ronning et al.

The lattice location of the implanted impurities has bedetermined using the EC technique. For these measuremthe samples were pre-oriented by Laue x-ray photograand mounted on a six-axis goniometer in a vacuum chamConversion electrons,b2 electrons, or alpha particles emited in the decay of the implanted nuclei were detected wisilicon surface barrier detector. The EC spectra werecorded typically for an angular range of63° around themajor crystal axes by measuring the emitted intensity afunction of the emission angle. The angular resolution varbetween 0.12–0.44°. The data acquisition was accomplisusing a setup described elsewhere.52,53 For the alpha par-ticles, the emission from a position within an atomic rowplane results in a minimum emission yield along the resptive axis or plane~blocking minimum!; whereas, the emission from interstitial sites will result in a peak in the emision yield ~channeling maximum!. EC with positivelycharged particles is therefore analogous to the channeand blocking effects of ions. For either conversion electroor electrons fromb2 decay, the screened Coulomb potentof the lattice atoms is attractive, and the electrons canbound to atomic axes and planes and can result in anhanced emission yield along an axis or plane, if the emiatoms are located within such an axis or plane. Electremitted from interstitial sites do not get bound in axialplanar potentials and will produce an almost isotropic emsion yield. Details of the EC technique are describelsewhere.52

Most of the radioactive isotopes of interest for EC eperiments emit electrons with energies ranging from 1keV to 5 MeV ~see Table I!. A quantitative theoretical description of electron emission channeling is therefore theto quantitative data analysis. Electrons with these enerbehave quantum mechanically, and EC patterns for electcan be calculated with the many beam formalism basedthe dynamical theory of electron diffraction.52,54 For a con-tinuous electron energy spectrum as in the case ofb2 elec-trons, a special fitting procedure is necessary, as describRef. 55. The effects of dechanneling and thermal vibratiof the probe nuclei must also be taken into account ascussed in Ref. 52. EC of alpha particles or positrons cantreated classically, in complete analogy to ion channeling

The immediate neighborhood of the implanted111In andits possible interactions with other defects was monitowith PAC spectroscopy using the emittedg radiation. Theelectric field gradient tensor~EFG! at the site of the radioactive probe atom is measured. The EFG interacts withquadrupole moment and causes a hyperfine splitting oexcited state of the111Cd daughter nuclei, which is measureby PAC. The EFG is described by the quadrupole couplconstant,nQ , and the asymmetry parameter,h. These valuesare characteristic for specific defects nearby the probe aand for the intrinsic lattice EFG in the case of hexagolattices. Usually the time-dependent anisotropyR(t) of theemittedg radiation is plotted. A unique EFG gives riseundamped periodic oscillations in theR(t) spectra. An EFGdistribution that reflects different defect that surroundprobe atoms, gives a decreasing anisotropy with increa


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coincidence time. Details of the PAC technique are descrielsewhere.56

The annealing of the EC samples was carried outvacuum (,13105 mbar) for 10–30 min. The samples usein the PAC measurements were sealed in evacuated (,13104 mbar) quartz ampoules and subsequently annewithout breaking the ampoule. For annealing the Gsamples at high temperatures~PAC studies! a thin, 300-Å-thick AlN layer was previously grown on the top of the Gasamples to inhibit surface decomposition.

III. RESULTS AND DISCUSSION

A. Heavy ion implantation of In and Sr: Lattice sitesand defect recovery

The EC spectrum measured along thec axis immediatelyafter room temperature implantation of111In into GaN isshown in Fig. 1~open symbols!. The normalized emissionyield was obtained after correcting for the backgroucaused by Compton electrons and normalization to theaxis random yield. Clearly visible is a higher conversielectron intensity along thec axis. The maximum yield in thec-axis direction and the sixfold symmetry of the twodimensional EC spectrum~not shown, see Ref. 36! show thatthe In atoms occupy mainly substitutional lattice sites afroom temperature implantation. We cannot distinguishtween Ga and N sites, but it is reasonable that the In atooccupy Ga sites, as these two species are isoelectronicmeasurements were also conducted after each 100 K incr

FIG. 1. Normalized emission yield of the conversion electrons emitted

the c-axis direction along the (3210) plane after implantation of111In inGaN as a function of the tilt angle. Open circles represent data from msurements directly after implantation and closed circles represent datameasurements after subsequent annealing to 1073 K. The calculatedneling effect ~solid line! assumed fractions off s590(10)% and f r

510(10)%.


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in temperature to 1073 K and annealing for 10 min at estep. No increase of the normalized yield along thec axiswas observed as can be seen in Figs. 1 and 2~closed sym-bols!. The calculated emission yield was fitted to the expemental data in Fig. 1. The calculated spectra~solid line! fitwell both in the region of the maximum emission yield ain the width and shape of the spectrum. From the fit a sstitutional fraction (f s) of 90% and a remaining fraction (f r)of 10%, including randomly distributed emitter atoms wedetermined~all EC results are summarized in Table II!.

Similar EC measurements were conducted on111In im-planted AlN for annealing temperatures to 1273 K. T

FIG. 2. Normalized emission yield of the conversion electrons in thec-axisdirection after implantation of111In in GaN ~closed circles! and AlN ~opencircles! as a function of annealing temperature.

TABLE II. Lattice location distribution of the implanted emitter in GaN anAlN at room temperature and as a function of annealing.

Sample Probe Ta ~K! f sa ~%! f i

a ~%! f ra ~%!

GaN 111In 300 90 ¯ 10GaN 111In 1073 90 ¯ 10GaN 89Sr 1073 70 ¯ 30GaN 8Li 300b ,40 .60 ¯

GaN 8Li 770b .60 .40 ¯

GaN 24Na 300 44 56 ¯

GaN 24Na 1073 44 56 ¯

AlN 111In 300 90 ¯ 10AlN 111In 1073 90 ¯ 10AlN 8Li 300 ,40 .60 ¯

AlN 8Li 800 .60 ,40 ¯

AlN 24Na 300 40 60 ¯

AlN 24Na 1073 65 35 ¯

aThe error in all measurements is around 10%.bIn case of the Li measurements, the annealing temperature (Ta) is also themeasuring temperature (Tm) due to the short half life of the nuclei.f s

5substitutional fraction,f i5 interstitial fraction, andf r5random fraction.


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maximum emission yield in thec-axis direction is alsoshown in Fig. 2~open symbols! as a function of annealingtemperature. As in the case of GaN, no change in the mmum yield was observed within the experimental accurafor higher annealing temperatures. The lower channelingfect compared to GaN is mainly due to the higher implantion energy used. The emitted particles have to pass a lodistance from the deeper implanted nuclei which results ihigher dechanneling yield. However, such effects were tainto account in the calculations of the theoretical emissyields.52,54 A fit of the experimental data also resulted insubstitutional fraction of 90% and a remaining fraction10%, including randomly distributed emitter atoms~TableII !. The implanted In occupies most probably Al sites.

These results clearly show that the radiation damageated during the implantation process does not influencechanneling effect; thus, no substantial damage to the latis generated in GaN and AlN even after the implantationheavy indium ions for the doses used. This is in agreemwith Rutherford backscattering measurements on GaNshowed that the GaN lattice is very resistant to damage fion radiation,31,32and can also be confirmed by the followinEC study on ion implanted Sr in GaN. Figure 3 shows tEC effects ofb2 particles from the decay of89Sr measuredfor the ^0001&, ^0223&, and ^1101& axial directions afterroom temperature implantation and annealing at 1073 K10 min. The solid line represents the calculated emissyield, and the fit of this data indicated a high substitutionfraction of 63~10!%. This is slightly lower compared to thIn results. However, we cannot exclude that the groupelement Sr may also occupy interstitial sites with a smfraction, which would not contribute to the channeling effe

Figure 4 shows PAC time spectra for111In in AlN ~a!directly after implantation and~b!–~e! after subsequent annealing to 1473 K for 30 min. The fast drop in anisotropseen in theR(t) spectrum measured directly after implanttion indicates that the probe atoms are exposed to a distrtion of different EFGs. This environment is characterizedan average valuenQ578(2) MHz and an associated distrbution width of aboutDnQ530(5) MHz. Accordingly, the

FIG. 3. EC spectra ofb2 particles emitted from89Sr and measured for threeaxial directions after implantation at room temperature and annealin1073 K for 10 min. The solid line in the left graph shows the calculaemission yield. A substitutional fraction (f s) of 63~10!% was obtained fromthe fit of the latter curve.


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majority of the indium atoms is incorporated in highly ditorted nonuniform environments after implantation. After anealing to 873 K theR(t) spectrum shows only one EFGindicating that a significant fraction of the In probe atomoccupy a unique lattice site and have the identical surrouings. From the amplitude of the modulation it was deducthat 40% of the In atoms are at this site. A fit to the expemental data gives a coupling constant ofnQ539(1) MHzand a nonaxial symmetry of the EFG tensor (h50.3). Fromthese and the earlier EC results, it is concluded that this wEFG is seen by In atoms at substitutional sites with undturbed surroundings in the noncubic lattice, in agreemwith results on CdS.57 The remaining probe atoms are stexposed to a distribution of different EFGs. However, frothe weaker EFG distribution with an average value ofnQ

553(2) MHz@DnQ520(5) MHz# we claim that these are Inatoms on substitutional sites with point defects in the secor further nearest neighborhood. Annealing at 1023 K for

FIG. 4. PAC spectra of AlN implanted with111In at 400 keV at~a! roomtemperature and~b! subsequent annealing in vacuum to 873, 1023~c!, and1473 K (d1e) for 30 min. Spectra~d! and ~e! were taken using a differengeometry, but were fitted with the identical parameters.


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min resulted in a further increase of the indium atom fracton undisturbed lattice sites, as shown in Fig. 4~c! and Fig. 5~top!. However, a decrease of this fraction was observedannealing at higher temperatures due to the reaction ofAlN with evaporated oxygen of the quartz ampoule formipolycrystalline or amorphous AlOx . Furthermore, our fittingroutine was checked by PAC measurements in differententations@Figs. 4~d! and 4~e!# resulting in satisfactory fitswith identical fit parameters.

We found similar PAC results for111In implanted intoGaN.36 For isochronal annealing treatments in vacuumgradual recovery of the implantation damage in the sroundings of the indium atoms also occurred, but at higannealing temperatures compared to AlN@see Fig. 5~bot-tom!#. After the 1173 K annealing step, 53~5!% of the probeatoms still had point defects in their surrounding. The mamum temperature in that experiment was limited to 1173due to the decomposition of GaN in the vacuum37 environ-ment. For this work, GaN was used with a thin AlN surfalayer ~300 Å!. The111In implantation was conducted througthis AlN layer into the GaN with an ion energy of 400 keV,5% of the In atoms were stopped in the AlN~compare withTable I!. The implanted samples were annealed to 1473 Kvacuum. The associated PAC measurements showed ather increase in the fraction of In atoms occupying deffree lattice sites, as shown in Fig. 5. However, a complrecovery~i.e., 100% on undisturbed sites! was not achieved.

Summarizing this part, the EC studies showed thatand Sr atoms primarily occupy substitutional lattice sitesrectly after room-temperature implantation and thatchange in their locations was observed after annealing trments. Furthermore, these experiments demonstrate thacrystalline structure of GaN and AlN is not significantly dstroyed by heavy ion implantation, as observed with EC, athat the created damage can be annealed out to a large eas observed with PAC. However, point defects remainedthe materials even after the highest temperature anne

FIG. 5. PAC results measured for111In in ~a! AlN and ~b! GaN as a functionof annealing temperature.


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These conclusions are in agreement with photoluminesceand x-ray diffraction ~XRD! measurements on implanteGaN.58 The luminescence, which is sensitive to point dfects, was not recovered even after high temperature anning. By contrast the XRD peaks, which are sensitive to strtural damage, reached the strength and shapenonimplanted GaN after annealing at 1173 K for 15 min.

B. Lithium and sodium: Lattice sites and annealingbehavior

EC spectra were taken of8Li-implanted AlN, GaN, andp-GaN. The normalized yield measured in thec-axis direc-tion is plotted in Fig. 6 as a function of temperature. Tchanneling effect in this direction proves that Li mainly ocupies interstitial lattice sites in AlN at low temperatures~seeFig. 6, right side!. We determined the occupation of the iterstitial site in the center of the hexagonal at positionsc/4and 3c/4 from channeling measurements in different othaxial directions. The onset of Li diffusion around 700 K athe coulomb interaction between the Lii

1 and the vacancies(Vn2) resulted in substitutional Li in AlN and blocking effects above 700 K. Vacancy diffusion can be excluded in treaction due to the absence of a recovery stage at theseperatures in AlN, as shown in the previous PAC studies.predict that the Li occupy the Al sites due the low stabilitythis atom ~and H! on N sites.59 In a previous study,44 wefound a similar behavior for Li in GaN~see Fig. 6, left, solidsymbols!. The onset of Li diffusion with an activation energof '1.7 eV was also observed at 700 K. Additionally, tpossible formation of Mgs-Li i complexes was investigateon Mg-doped GaN samples with a Mg concentration of31019cm3. Such complexes should have a higher thermstability compared to interstitial Li; however, we observthe identical behavior and a transition temperature of 700as shown in Fig. 6~left, open symbols!. This demonstratesthat the interaction with implantation-induced vacancies

FIG. 6. Left side: normalizeda-emission yield in thec-axis direction as afunction of implantation temperature for undoped GaN~solid symbols! andMg-doped GaN~open symbols!. The change from channeling (yield.1) toblocking (yield,1) effects around 700 K shows the lattice site change osignificant fraction of Li from interstitial to substitutional sites. The slighthigher normalized yield for the Mg-doped sample is due to a reduced bground achieved with a different experimental arrangement. Right sideidentical measurements conducted using wurzite AlN.


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the dominating process in our experiments, primarily bcause of the high vacancy concentration in the vicinity ofimplanted Li atom. Nevertheless, we also expect thatdiffusing Li will strongly interact with the Mg acceptors tform Li–Mg complexes. However, since the Mg concenttion was much lower compared to the local vacancy conctration, this interaction was not observed, i.e., the Li atowere trapped in vacancies before they could form Mg–complexes.

In a further experiment, we implanted the next largalkali atom sodium to investigate its behavior in AlN anGaN. 24Na was used to determine the lattice site locationa function of annealing temperature. Figure 7~top! shows themeasured emission distribution around thec axis directlyafter implantation at room temperature in GaN. The channing effects in the direction of thec axis and the three plane@(2110), (1010), and (1100)# indicate the occupation of afraction of Na emitters on substitutional sites. However,missing channeling effects in the direction of the three otplanes@(3120), (3210), and (0110)# exclude the exclusiveoccupation of substitutional sites.36 The channeling effects

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FIG. 7. Top: channeling spectrum ofb2 particles from the decay of24Naalong thec axis in GaN directly after implantation at room temperatuBottom: theoretical channeling spectrum assuming that 44% of the emioccupy substitutional sites and 56% interstitial sites.


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must be compensated by emitters on interstitial sites caublocking effects. To obtain a quantitative fraction of Na aoms occupying different lattice sites, EC patterns forelectrons were calculated for substitutional and interstisites. For sodium at the threefold interstitial sites in the cter of the hexagons, the calculations gave weak channeeffects along three planes as well as blocking effects althe c axis and the other three planes. For sodium on subtutional lattice sites the calculations indicate electron chneling effects along thec axis and all six planes. The calculated patterns were fitted as a superposition to the measspectra, the best fit is displayed in Fig. 7~bottom! whichseparates into fractions off s50.44 andf i50.56. The resultsindicate that sodium occupies mainly interstitial sites direcafter room temperature implantation, but a considerable ftion of the Na atoms are also located on substitutional siAdditional evidence for substitutional sodium is the channing effect in the 0110& direction of GaN, which is 47° tiltedto the c axis along the (2110) plane, as shown in Fig. 8Substitutional Na is related to implantation-assisted pcesses including kinetic replacement collisions with lattatoms in the final stage of the stopping process. This effenegligible for large differences in mass of the implanted ipurity and host atoms, as in the case of Li on Ga sites in G~Li on N sites should be unstable!, but not for the moreheavier sodium. Annealing of the implanted sample to 10K did not change the lattice location distribution of NaGaN.

For sodium implanted AlN, we found a similar behavidirectly after room temperature implantation. Figure 9~top!shows the measured spectrum around thec axis and the re-spective calculated pattern~bottom!. The simulation resultedin fractions off s50.40 andf i50.60, which are almost identical to the situation for Na in GaN directly after room temperature implantation. However, we found a different sitution after annealing in AlN. Figure 10~top! shows themeasured EC spectrum around thec axis after annealing to

FIG. 8. Channeling spectra ofb2 particles from the decay of24Na along the

^0111& axis, 47° along (2110) plane, in GaN directly after implantation aroom temperature.


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1073 K for 10 min. For this spectrum we had to assume t65% of the emitter occupies substitutional sites and 35%the emitter occupies interstitial sites to achieve the besbetween experiment and simulation@Fig. 10 ~bottom!#. Thislattice site change of sodium in AlN is related to the diffsion of vacancies and the recombination with Na as indicaby the earlier described PAC studies. In these studies reery of the implantation damage was observed in this teperature range in AlN, but not in GaN~see again Fig. 5!.However, the diffusion of Na cannot completely excludedthese experiments.

IV. CONCLUSIONS

The results of EC and PAC spectroscopy experimentsion implanted radioactive111In, 89Sr, 8Li, and 24Na in GaNand AlN have been presented. The EC studies showed thand Sr atoms mainly occupy substitutional lattice sitesrectly after room-temperature implantation and no chanwas observed after annealing treatments. The crystastructure of GaN and AlN is not significantly destroyed

FIG. 9. Top: channeling spectrum ofb2 particles from24Na around thecaxis in AlN directly after implantation at room temperature. Bottom: theretical channeling spectrum ofb2 particles from the24Na decay in AlNassuming that 40% of the emitters occupy substitutional sites and 60%terstitial sites.


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heavy ion implantation, as observed with EC and the creadamage can be essentially annealed out, as observedPAC. However, point defects remained in the materials eafter high temperature annealing. Li and Na atoms maoccupy interstitial lattice sites in GaN and AlN directly aftroom-temperature implantation. Recombination of the intstitial alkalis with vacancies and the formation of substitional alkalis was observed at higher implantation andnealing temperatures due to the diffusion of Li in GaN adue to the diffusion of vacancies or Li in AlN. A highesubstitutional fraction of Na in GaN is also expected, buttemperatures above 1073 K.

ACKNOWLEDGMENTS

This work was financially supported by the GermBundesminister fu¨r Bildung, Wissenschaft, Forschung unTechnologie. The work at NCSU was supported by thefice of Naval Research via Contract No. N00014-96-1-07monitored by Max Yoder.

FIG. 10. Above: channeling spectra ofb2 particles from24Na along thecaxis in AlN after subsequent annealing to 1073 K. Below: theoretical chneling spectrum ofb2 particles from the24Na decay in AlN supposing tha65% of the emitters occupy substitutional sites and 35% interstitial site


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ion implanted dopants in gan and aln: lattice sites, annealing behavior, and defect recovery

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