SURFACE AND INTERFACE ANALYSIS, VOL. 26, 220È223 (1998)

Depth Profiling of an In0.53

Ga0.47

As/InPMultilayer Sample using Grazing-incidenceSputtered Neutral Mass Spectrometry with LaserPost-ionization

Yasuhiro Higashi,* Tetsuya Maruo¤ and Yoshikazu HommaNTT Science and Core Technology Laboratory Group, 3-9-11 Midori-cho, Musashino-shi, Tokyo 180, Japan

Depth proÐling of thin layers in InP using sputtered neutral mass spectrometry with grazing-In0.53

Ga0.47

Asincidence ion beam sputtering and laser post-ionization was performed and compared with SIMS and AES depthproÐling. The depth resolution was improved by using grazing incidence (at an incident angle of 77Ä) of a 10 keVAr‘ primary beam and was better than that in the AES measurement with 1 keV Ar‘ bombardment at 70.3Ä andsample rotation. The distortion of the indium proÐle at the interface that was observed in theIn

0.53Ga

0.47As/InP

SIMS measurement with 2 keV bombardment at 81Ä was not observed in the grazing-incidence sputteredO2‘

neutral mass spectrometry measurement. 1998 John Wiley & Sons, Ltd.(

Surf. Interface Anal. 26, 220È223 (1998)

KEYWORDS: sputtered neutral mass spectrometry ; depth resolution ; grazing incidence ; SIMS; AES

INTRODUCTION

Ion beam bombardment at a grazing angle of incidencewith respect to the sample surface can improve thedepth resolution in SIMS,1 basically because the ionmomentum component that is perpendicular to thesample surface is reduced. In beam bombardment,O2`the use of a higher incident angle also reduces thebombardment-induced segregation that causes theproÐle distortion.2 Unlike the method using a speciallow-energy ion source,3,4 grazing-incidence ion beamsputtering can easily be done in a conventional SIMSsystem with a high ion beam current, which produces ahigh sputtering rate and a short enough measuring timefor practical depth proÐling. However, the matrix e†ectat grazing incidence is often more serious than that atnormal incidence and hinders quantitative understand-ing of the measured proÐles. Furthermore, the second-ary ion yield greatly decreases and the sensitivitybecomes lower.

Sputtered neutral mass spectrometry with laser post-ionization (laser-ionization SNMS) considerablyreduces matrix e†ects.5 Depth proÐling analyses usinglaser-ionization SNMS have mostly been studied withthe combination of multiphoton resonance ionizationand magnetic-sector mass analysis,6,7 whereas staticSNMS studies have been done with the combination ofnon-resonant multiphoton ionization (NRMPI) andtime-of-Ñight mass analysis.8,9 If we could apply grazingincidence to dynamic SNMS analyses, we couldperform high-resolution depth proÐling with little

* Correspondence to : Y. Higashi, NTT Science and TechnologyLaboratory Group, 3-9-11 Midori-cho, Musashino-shi, Tokyo 180,Japan.

¤ Present address : NTT Multimedia Business Department, 2-2-2Otemachi, Chiyoda-ku, Tokyo 100, Japan.

matrix e†ect. However, it is not easy to change the inci-dent angle in the magnetic-sector-type SNMS systembecause the ion-extraction electrode set-up and condi-tions are Ðxed to detect photoionized neutrals. At agrazing incidence near 90¡, the quality of the ion beamshape must also be considered because the ion beammay not be in focus over the whole analysed area of thesample at that angle, so the sputtered crater may not beÑat. The crater Ñatness depends on the quality of theion beam shape and the focusing conditions of the laser-ionization SNMS apparatus.

We have developed a laser-ionization SNMS systemthat is suitable for depth proÐling by combiningNRMPI with quadrupole mass analysis,10,11 and havepresented almost matrix-e†ect-free depth proÐling bythis system.12,13 The use of NRMPI enables the quasi-simultaneous analysis of multiple elements and the useof a quadrupole mass spectrometer makes it easy itdetect photoionized neutrals in the pulse countingmode.11 Our system can also be operated as a conven-tional quadrupole-type dynamic SIMS system. Its ionextraction electrode set-up and conditions are notstrictly Ðxed and the working distance around itssample holder is big enough to tilt the holder at anyangle. The crater had a Ñat bottom even when the inci-dent angle was 77¡.

In this paper, we present depth proÐling ofthin layers in InP by grazing-incidenceIn0.53Ga0.47As

SNMS using our system and compare the measuredproÐles with the SIMS and AES results.

EXPERIMENTAL

The multilayer sample was pre-In0.53Ga0.47As/InPpared by metal-organic vapour-phase epitaxy in NTT

CCC 0142È2421/98/030220È04 $17.50 Received 27 May 1997( 1998 John Wiley & Sons, Ltd. Accepted 28 October 1997

DEPTH PROFILING OF In0.53Ga0.47As/InP WITH LASER-IONIZATION SNMS 221

opto-electronics laboratories. The InP andlayers were alternately deposited on aIn0.53Ga0.47As

sulphur-doped 2-inch InP wafer. The in-depth structureis shown schematically in Fig. 1. The four

layers were each 12 nm thick. The devi-In0.53Ga0.47Asation of the Ðlm thickness was 1% in the central 30 mmdiameter region of the sample wafer. The sample waferwas cut into several pieces to be measured by laser-ionization SNMS, SIMS and AES.

In the laser-ionization SNMS measurements, a 10keV Ar` continuous ion beam was used for sputtering.The tilt angle of the sample holder was set to 0¡, 45¡ or75¡ with respect to the horizontal plane. As the primaryion beam was already tilted by 2¡, the angle of incidencewas calculated as 2¡, 47¡ or 77¡ with respect to thesample surface. A Lambda Physik LPX 240iF excimerlaser system was newly added to our SNMS system.The KrF excimer laser beam (248 nm) was focusedabove the sample surface by a spherical lens with 30 cmfocal length. Sputtered arsenic and indium atoms whoseÐrst ionization potentials are 9.8 eV and 5.8 eV, respec-tively, should be ionized by non-resonant two-photonprocesses, and phosphorus atoms whose Ðrst ionizationpotential is 10.5 eV by non-resonant three-photon pro-cesses. During the measurements, the laser pulse energywas held at 100 mJ in the constant energy mode of thelaser system. The repetition rate of the laser pulses was333 Hz.

The SIMS measurements with an Ar` beam (Ar`-SIMS) were done in the SIMS mode of the same SNMSsystem. The sputtering conditions were the same asthose in the SNMS measurements. The SIMS measure-ment with an beam was made with aO2` (O2`-SIMS)quadrupole-type dynamic SIMS system (Atomika SIMS4000). The beam energy was 2 keV and the angleO2`of incidence was 81¡. Depth proÐling measurements byAES were made with a PHI SAM 660. The samplesurface was intermittently bombarded by a 1 keV Ar`beam at the incident angle of 70.3¡, with or withoutrotating the sample. The MNN spectra were measured

Figure 1. Structure of the multilayer sample.In0.53

Ga0.47

As/InPThe layers are each 12 nm thick. All of the depos-In

0.53Ga

0.47As

ited layers were doped with selenium.

for indium and the LMM spectra for the other ele-ments.

Although the sputtering rates in InP andare, of course, di†erent from each other,In0.53Ga0.47As

the depth scales in all of the following proÐle data werecalculated directly from the sputtering time because the

layers were thin enough to assume thatIn0.53Ga0.47Asthe total average sputtering rate was almost equal tothe sputtering rate in InP.

RESULTS AND DISCUSSION

Figure 2 shows the indium, phosphorus and arsenicproÐles measured by The main feature inO2`-SIMS.the indium proÐle is that the indium secondary ionintensity seems to increase at the interface from InP to

and decrease at the interface fromIn0.53Ga0.47Asto InP, and does not reÑect the actualIn0.53Ga0.47As

indium in-depth concentration proÐle. This interfacee†ect resulted from the combination of the change inthe surface indium concentration itself and the changein the indium secondary ion yield during sputtering.The former was caused by the preferential sputteringunder beam bombardment. The latter was causedO2`by the changes in composition, the surface oxygen con-centration and the surface morphology during sputter-ing. The interface e†ects on the indium intensity inSIMS will be discussed elsewhere.14 Of course, not onlythe indium proÐle but also the P and As proÐles will besubjected to these e†ects to some extent.

Figures 3 and 4 show the proÐles measured by Ar`-SIMS and laser-ionization SNMS at an incident angleof 77¡. The indium proÐle by Ar`-SIMS shows small“peaksÏ at the layers, whereas the indiumIn0.53Ga0.47AsproÐle by SNMS shows, “dipsÏ that correspond to thedecrease in the indium concentration at the layers. Thegeneration of “peaksÏ and “dipsÏ cannot be explained bypreferential sputtering because the sputtering conditions

Figure 2. Depth profiles of indium, arsenic and phosphorus mea-sured by when the angle of incidence was 81¡.O

2½-SIMS

( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 220È223 (1998)

222 Y. HIGASHI ET AL .

Figure 3. Depth profiles of indium, arsenic and phosphorus mea-sured by Ar½-SIMS when the angle of incidence was 77¡.

are exactly the same in both the Ar`-SIMS and SNMSmeasurements, or by the change in the surface concen-tration of the primary ion species because the surfaceargon concentration in Ar`-SIMS does not inÑuencethe secondary ion yield as much as the surface oxygenconcentration in Therefore, the “peaksÏO2`-SIMS.might reÑect the indium secondary ion yield changeinduced by the in-depth changes in the sample composi-tion and/or the surface morphology during sputtering.It was reported that sputtering caused the growth ofcone-shaped spikes with indium-rich balls on their topson the InP surface.15 The size and shape of the spikesmay change greatly at the inter-In0.53Ga0.47As/InPfaces, but their e†ect on the indium secondary ion yieldhas not yet been clariÐed. The spike growth duringsputtering may also cause the Ñuctuation in the indium

Figure 4. Depth profiles of indium, arsenic and phosphorus mea-sured by laser-ionization SNMS when the angle of incidence was77¡.

intensity in the InP layers because the sputtering rate ofthe InP is di†erent from that of the indium-rich balls.

Judging from the di†erence in the phosphorus pro-Ðles in Figs 3 and 4, one might feel that the depthresolution in the Ar`-SIMS measurement is better thanthat in the SNMS measurement. However, their depthresolutions must be almost the same because the sput-tering conditions were exactly the same. This proÐle dif-ference is caused by the di†erence in the phosphorussecondary ion yield between InP and inIn0.53Ga0.47Asthe Ar`-SIMS measurement. One must pay carefulattention to this when determining the depth resolutionfrom SIMS proÐles.

Figure 5 shows the proÐles measured by AES withsample rotation. Without sample rotation, only the Ðrst

layer was observed due to the consider-In0.53Ga0.47Asable degradation in depth resolution. This might becaused by the growth of the above-mentioned “spikesÏ.As the spikes grow in the direction parallel to the ionbeam incidence, the sample rotation could e†ectivelysuppress the spike growth in the AES measurement. InFig. 5, the AES proÐles show no distortion caused bypreferential sputtering and are very similar to theSNMS proÐles except that the detection limit of As inAES was much higher than that in SNMS. In order tocompare their resolutions in detail, we evaluated thedecay lengths at the trailing edges of the As proÐles. InFig. 6, the decay lengths in the AES and SNMS mea-surements at the di†erent incident angles are plottedwith respect to the in-depth number of the

layers. Obviously, the decay lengthIn0.53Ga0.47Asseries in the SNMS measurements was shortest at 77¡incidence and was almost constant up to the fourth

layer, i.e. the depth resolution wasIn0.53Ga0.47Asimproved by the grazing incidence and was notdegraded to the depth of D 400 nm in the SNMS mea-surement. In the AES measurement, the decay lengthwas longer in the deeper region because of the degrada-tion in depth resolution. The di†erence in the depthresolution between the SNMS and AES measurements

Figure 5. Depth profiles of indium, arsenic and phosphorus mea-sured by AES with sample rotation. The angle of incidence was70.3¡.

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 220È223 (1998) ( 1998 John Wiley & Sons, Ltd.

DEPTH PROFILING OF In0.53Ga0.47As/InP WITH LASER-IONIZATION SNMS 223

Figure 6. Decay lengths of the arsenic profiles measured bylaser-ionization SNMS (open plots) and AES (closed plots) at dif-ferent depths. The angles of incidence in the SNMS measurementsare shown in the figure.

might be caused by the di†erence in the sputtering ioncurrent density.16 Because the ion current density in theAES measurement was lower than that in the SNMS

measurement, the “spikesÏ might grow larger and theInP surface might get rougher in the AES measure-ment.16

CONCLUSIONS

Depth proÐling using laser-ionization SNMS withgrazing-incidence ion beam sputtering was performed.Grazing-incidence SNMS provides depth proÐles withmuch smaller matrix e†ect that SIMS, with muchhigher sensitivity than AES and with much higherresolution than normal-incidence SNMS. These featuresare highly attractive for analysing layered sample struc-tures and shallow dopant proÐles, and will be helpfulfor investigating phenomena in the interfacial and near-surface regions. Furthermore, laser-ionization SNMSmeasurements at di†erent incident angles can supportquantitative studies on ion beam sputtering processeson solid surfaces.

Acknowledgements

We are grateful to Dr C. Amano of NTT Opto-electronics Labor-atories for o†ering the samples, and to Dr M. Suzuki of NTT, Ms A.Masamoto and Mr H. Ando of NTT Advanced Technology Corpora-tion for the AES and SIMS measurements.

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( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 220È223 (1998)