*Corresponding author. Tel.: #81 43 290 3331; fax: #81 43290 3360; e-mail: taniyasu@semi.te.chiba-u.ac.jp.

Journal of Crystal Growth 189/190 (1998) 305—309

Spectroscopic ellipsometry study on initial growth stages ofGaN films on GaAs(0 0 1) in low-pressure MOVPE

Yoshitaka Taniyasu!,*, Ryouichi Ito!, Norio Shimoyama", Megumi Kurihara", Anwei Jia!,Yoshinori Kato!, Masakazu Kobayashi!, Akihiko Yoshikawa!, Kiyoshi Takahashi#

! Department of Electrical and Electronics Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan" TRI Chemical Laboratory Inc., 8154-217 Uenohara, Kitatsurugun, Yamanashi 409-01, Japan

# Department of Electronics and Information Science, Teikyo University of Science and Technology, 2525 Yatsuzawa, Uenohara, Kitatsurugun,Yamanashi 409-01, Japan

Abstract

Initial growth stages of cubic GaN (c-GaN) films on GaAs(0 0 1) in low-pressure MOVPE were studied by spectro-scopic ellipsometry. The GaN buffer layer was deposited at 500°C and was then annealed at 700°C with H

2/monomethyl-

hydrazine (MMHy) ambient. Spectroscopic ellipsometry revealed that the buffer layer thickness was increased and thesurface was roughened during the annealing process. The change of surface morphology was also confirmed by theatomic force microscopy measurement. These observations would be associated with the nitridation of the GaAssubstrate by the ambient MMHy and re-crystallization of the GaN buffer layer. Spectroscopic ellipsometry is a helpfultechnique to study the initial growth stages of GaN films on GaAs substrates. ( 1998 Elsevier Science B.V. All rightsreserved.

PACS: 68.55.Jk; 78.20.Ci; 81.05.Ea; 81.15.Gh

Keywords: Spectroscopic ellipsometry; In-situ monitoring; Cubic GaN; MOVPE; Monomethylhydrazine

1. Introduction

GaN films with the hexagonal structure (h-GaN)have been extensively grown on sapphire substratesand applied to practical device structures [1,2]. The

growth of c-GaN films on GaAs(0 0 1) substrateshave been also studied because such structures caneasily have cleaved edges and the electrode can beformed on the substrate [3—6]. One of the mainproblems for the realization of high-quality c-GaNfilm is the inclusion of the h-GaN phase during thegrowth. The formation of the GaN buffer layer andthe annealing of the buffer layer are importanttechniques to eliminate the h-GaN phase [4,5]. The

0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 7 1 - 1

Fig. 1. Schematic illustration of the experimental apparatus.Spectroscopic phase modulated ellipsometry system was at-tached to the MOVPE reactor.

detailed understanding of the initial growth stagesof the GaN film is crucial to achieve high-qualityc-GaN layers. It is well known that the ellipsometrymeasurement is very sensitive to the sample struc-ture (layer thickness, surface roughness, etc.) as wellas the optical constants of the material [7]; hencespectroscopic ellipsometry was used in this study toevaluate the structure of the initial growth stages ofthe GaN film.

2. Experimental procedure

Fig. 1 shows a schematic illustration of the ex-perimental apparatus used in this study. Sampleswere grown by low-pressure (50 Torr) MOVPE.Trimethylgallium (TMG) and MMHy were used asa Ga source and a nitrogen source, respectively. H2was used as a carrier gas. The surface of the GaAs(001) substrate was etched in a solution ofH

2SO

4: H

2O

2: H

2O"5 : 1 : 1 and was then ther-

mally cleaned in H2/tertiarybutylarsine (TBAs) am-

bient at 600°C. The atomically flat GaAs substratesurface with well-defined mono-layer steps was ob-tained after this process. The GaN buffer layer wasformed at 500°C and was then annealed at 700°Cwith H

2/MMHy ambient. The GaN epilayer was

grown at 700°C. A UVISEL (JOBIN YVON) spec-troscopic phase-modulated ellipsometry systemwas mounted on the MOVPE reactor. All ellip-sometry data W, D were converted to the pseudo-dielectric function SeT"Se

1T!iSe

2T using the

equation [7]

SeT"Se1T!iSe

2T

"sin2 h#sin2 h tan2 h[(1!o)/(1#o)]2. (1)

Here o"r1/r

4"tan(W) exp (iD), where r

1and

r4are the complex reflectivities of the surface for p-

and s-polarized light, and h is the angle of inci-dence. For the ellipsometry measurement in thekinetic mode (KM), the pseudo-dielectric functionwas measured as a function of the time with a fixedprobing photon energy throughout the growth pro-cesses of the GaN film on the GaAs(0 0 1) substrate.The signal was monitored at 3.25 eV; the dielectricfunction of GaAs at this photon energy shows theminimal variation within the temperature rangeused for this study [8]. The spectroscopic mode(SM) analysis was carried out in situ after the filmgrowth at the room temperature; the probingphoton energy was varied in the range of1.5—5.0 eV. The curve fitting of the data was per-formed to understand the variation of the bufferlayer structure during the annealing process. Themorphology of the GaN buffer was characterizedby the atomic force microscopy (AFM) measure-ment.

3. Results and discussion

Fig. 2 shows the KM signal during the growthprocesses of the GaN film. The buffer layer growthwas started at t"0 s and the signal change wasobserved simultaneously. The signal change duringthe buffer layer growth would be mainly originatedfrom the variation of the buffer layer thickness. Thesignal variation was also observed during the ram-ping process of the substrate temperature from500°C to 700°C. The signal change was furtherconfirmed during the annealing where the temper-ature was maintained at 700°C. The signal vari-ations during latter two steps would include theinformation related to the structural change of thebuffer layer as well as the change of the dielectricconstant of the buffer layer during the ramping andannealing. On the growth stage of the GaN epi-layer, the oscillation of the signal due to the optical

306 Y. Taniyasu et al. / Journal of Crystal Growth 189/190 (1998) 305–309

Fig. 2. Pseudo-dielectric function of the GaN buffer layer on the GaAs(0 0 1) substrate during the growth processes of the GaN film.The growth sequence of the GaN layer is also shown.

Fig. 3. The measured (solid circle) pseudo-dielectric functiontrajectories (Se

1T,Se

2T) at the initial growth stages of the GaN

film. The calculated trajectory (solid line) was based on a uni-formly growth model. Markers denote 1 nm thickness in-crements.

interference was gradually damped because pro-bing photon energy is more than the energy of thefundamental absorption edge of c-GaN.

Fig. 3 shows the pseudo-dielectric function tra-jectories at the initial growth stages of the GaNfilm; the calculated trajectory is shown along withthe measured values. The calculated trajectory wasobtained under the assumption that the GaN filmwas homogeneously grown. The calculated datapoints of the pseudo-dielectric function agree fairlywell with the experimental data. As a result offitting procedure, the thickness of the as-grownbuffer layer was obtained to be 4.5 nm. With com-

paring these trajectories, it was confirmed that thelayer thickness was increased from 4.5 to 5.5 nmduring the ramping process of the substrate tem-perature from 500°C to 700°C. It was further con-firmed that the layer thickness was increased from5.5 to 7 nm during the annealing process.

The structural variation of the buffer layer dur-ing the annealing process was analyzed by SMalong with the theoretical estimation. Fig. 4a andFig. 4b show the Se

1T and the Se

2T, respectively, of

samples without and with the annealing process.The dielectric function of the GaAs are also in-dicated in the same figures. It is shown that the SeTis drastically affected by both the deposition of thebuffer layer and its annealing process. The differ-ence of the Se

1T at around 2.9 eV and the Se

2T at

around 4.8 eV are especially apparent. It is knownthat the optical response of Se

2T for the GaAs

substrate at around 4.8 eV is sensitive to the pres-ence of surface overlayer and any surface defects[7], and it would be the contributing factor toobtain the distinct signal variation. The differenceof the SeT of samples without and with the anneal-ing process was also clearly distinguished. Thesesignal variation are mainly related to the structuralchange of the buffer layer.

These data were curve-fitted using the four-phasemodel where the sample would be ambient/a roughoverlayer of GaN/a homogeneous GaN layer/aGaAs substrate in order to estimate the variation ofthe buffer layer structure quantitatively. The roughoverlayer can be represented by the layer consisted

Y. Taniyasu et al. / Journal of Crystal Growth 189/190 (1998) 305–309 307

Fig. 4. The pseudo-dielectric function of the GaN buffer layer on the GaAs(0 0 1) substrate as a function of the photon energy. Solidlines indicate the experimental data and the best fitting results were shown by the open circles.

Fig. 5. AFM images of the buffer layer surface (a) without the annealing, and (b) with annealing.

of the bulk and void (density deficit) [7]. The Brug-geman effective medium approximation (EMA) [9]was employed to calculate the effective dielectricfunction of the rough overlayer. The effective di-electric function e of the rough overlayer is given bythe solution of

0"f"6-,

(e"6-,

!e)/(e"6-,

#2e)

#f70*$

(e70*$

!e)/(e70*$

#2e), (2)

where e"6-,

(e70*$

) is the dielectric function of thebulk (void) and f

"6-,( f

70*$) is the relative volume

fraction of bulk (void). A fitting procedure wasperformed in the photon energy range from 1.5 to3.1 eV, because referred dielectric function of c-GaN was reported only in this photon energy range[10]. A good fitting result was obtained when the

sample without the annealing was estimated tohave a 4.5 nm homogeneous layer and a 0.4 nmrough overlayer with 85% of the void and 15% ofthe bulk. The annealed sample was further esti-mated to have a 6.4 nm homogeneous layer anda 2.0 nm rough overlayer with 77% of the void and23% of the bulk. The SM analyses indicate that thelayer thickness was increased and the surface wasroughened by the annealing. These phenomenawould be related to the nitridation of the GaAssubstrate by the ambient MMHy and re-crystalli-zation of the GaN buffer layer.

In order to confirm the results of the SM ana-lyses, the morphology of the GaN buffer layerswere characterized by the AFM measurements.Fig. 5a and Fig. 5b show AFM images of the GaN

308 Y. Taniyasu et al. / Journal of Crystal Growth 189/190 (1998) 305–309

buffer layer surfaces without and with the anneal-ing process, respectively. The as-grown buffer layersurface was fairly flat and the rms roughness was assmall as 0.12 nm. The surface was roughened by theannealing and large grains (100 nm]100 nm) wereobserved after the annealing; the rms roughnesswas increased to 0.51 nm. The formation of grainsimplies that the re-crystallization of the GaN bufferlayer took place during the annealing process. TheSM analyses were confirmed by the AFM measure-ments.

As shown above, it was found that the spectro-scopic ellipsometry was effective to monitor andstudy the initial growth stages of GaN films onGaAs(0 0 1) in low-pressure MOVPE. It is reportedthat the deep understanding of the initial growthstages of the GaN film on GaAs substrate is veryimportant to control the crystalline structure andto improve the crystal quality of the c-GaN epi-layer. Spectroscopic ellipsometry would be a help-ful in-situ optical monitoring method to control thegrowth process of the GaN film in MOVPE.

4. Summary

The structural variation of the GaN buffer layerson the GaAs(0 0 1) substrates in LP-MOVPE sys-tem were studied by the spectroscopic ellipsometrymeasurement. The variation of the KM signal wasobserved during the annealing process, whichwould indicate the structural variation of the bufferlayer. SM was used to analyze the change of thepseudo-dielectric function and correspondingstructural change of the GaN buffer layer. Thetheoretical calculation indicated that the thicknessof the buffer layer was increased and surface was

roughened by the annealing process. The nitrida-tion of the GaAs substrate by the ambient MMHyand re-crystallization of the GaN buffer layerwould occur during the annealing process. Spectro-scopic ellipsometry is a versatile technique to studythe initial growth stages of GaN films on GaAs andwould be a helpful technique to realize high qualityc-GaN layers.

Acknowledgements

This work was partly supported by the “Re-search for the Future” Program, Japan Society forthe Promotion of Science (JSPS-RFTF 96R16201).

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