*Corresponding author. Fax: #81 3 3481 4509; e-mail:biwa@photonics.rcast.u-tokyo.ac.jp.

Journal of Crystal Growth 189/190 (1998) 485—489

Metalorganic vapor-phase epitaxy of GaP1~x~y

AsyN

xquaternary alloys on GaP

Goshi Biwa!,*, Hiroyuki Yaguchi!, Kentaro Onabe!, Yasuhiro Shiraki"! Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

" Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan

Abstract

We report on a successful growth of GaP1~x~y

AsyN

xquaternary alloys on GaP by metalorganic vapor-phase epitaxy

(MOVPE). The arsenic solid composition increases with increasing AsH3

supply during the MOVPE growth, while thenitrogen composition is unchanged. The alloys with compositions (x&2.3%, 0(y(19%) are obtained, and thecontrol of the lattice constant in GaP

1~x~yAs

yN

xquaternary alloys is demonstrated. From the dependence of

photoluminescence (PL) intensities on lattice mismatch to GaP substrates, we found that lattice-matching to substrates isnecessary to obtain GaP

1~x~yAs

yN

xalloys with high quality and intense PL. ( 1998 Elsevier Science B.V. All rights

reserved.

PACS: 81.05.E; 81.15.K; 78.55.C

Keywords: GaPAsN; GaPN; Metalorganic vapor-phase epitaxy (MOVPE); Nitride alloy; Metastable alloy; Latticematch

1. Introduction

In the GaP1~x

Nx

ternary semiconductor alloys,there is an extremely large miscibility gap due tolarge differences in lattice constant (&20%) andstructure (GaP: zincblende, GaN: wurtzite) be-tween GaP and GaN. Therefore, it has been verydifficult to grow GaP

1~xN

xalloys with high nitro-

gen contents. Recently, however, growth of the

GaP1~x

Nx

alloys on GaP with nitrogen contentsup to several percent has been successfully achievedby molecular beam epitaxy (MBE) (x(7.6%) [1]and metalorganic vapor-phase epitaxy (MOVPE)(x(4%) [2]. A latest study reports the growth ofthe GaP

1~xN

xalloy with nitrogen contents as high

as 16% [3]. At high nitrogen contents (x'2%),nevertheless, misfit dislocations are produced ow-ing to the lattice mismatch between GaP substratesand GaP

1~xN

xepitaxial layers and behave as

non-radiative recombination centers, which de-crease the photoluminescence (PL) intensitiesdrastically. Thus, the lattice mismatch between 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 3 3 6 - 4

substrate and the epilayer as well as the intrinsicmiscibility in the GaP

1~xN

xalloy system limits the

material quality and nitrogen contents.In this work, we demonstrated the MOVPE

growth of GaP1~x~y

AsyN

xquaternary alloys to

obtain the epitaxial layers lattice-matched to theGaP substrates. Adding N to GaP decreases thelattice constant, while adding As to GaP increasesthe lattice constant. Therefore, GaP

1~x~yAs

yN

xquaternary alloys can be lattice-matched to GaP byadjusting the alloy compositions, and the epitaxiallayer with few misfit dislocations and high nitrogencontents may be obtained. We have grown theGaP

1~x~yAs

yN

xalloys under the identical growth

condition with the GaP1~x

Nx

ternary alloys(x"2.3%) except that the AsH

3flow was adopted

during the MOVPE growth. The growth conditiondependence of solid alloy compositions and PLintensities in GaP

1~x~yAs

yN

xquaternary alloys

were investigated to clarify the effects of adding Asto GaP

1~xN

xternary alloys.

2. Experimental procedure

GaP1~x~y

AsyN

xquaternary alloys were grown

on GaP(1 0 0) substrates by low-pressure (60 Torr)MOVPE. Trimethylgallium (TMG), PH

3, AsH

3and 1—1 dimethylhydrazine (DMHy) were used asGa, P, As and N sources, respectively. Prior to thegrowth of the GaP

1~x~yAs

yN

xalloy, a 0.3 lm thick

GaP buffer layer was grown at 750°C with TMGand PH

3. Then the temperature of the substrate

was lowered to 630°C for the growth ofGaP

1~x~yAs

yN

xalloy, and TMG, PH

3, AsH

3and

DMHy were supplied simultaneously. The V/IIIratio was&80. The growth rate and epilayer thick-ness were 1.3 lm/h and 0.4 lm, respectively. Theseconditions were based on the MOVPE growth forGaP

1~xN

xternary alloys [2]. Adopting DMHy as

a N source made it possible to grow alloys withhigh nitrogen contents, since DMHy decomposesmore efficiently than ammonia. To investigate theinfluence of the addition of the AsH

3flow during

the MOVPE growth to the solid alloy composition,we grew the samples with various AsH

3flow rates

from 0 (GaP1~x

Nx

ternary alloy) to 3.9 lmol/min,while flow rates of the other sources were kept

constant; TMG: 4.5 lmol/min, PH3: 140 lmol/min

and DMHy: 220 lmol/min.We determined the solid compositions (x, y) in

the GaP1~x~y

AsyN

xquaternary alloy from its lat-

tice constant and PL peak energy. We assumedVegard’s law between the solid compositions (x, y)and lattice constant a. We also assumed that the PLpeak energies in GaP

1~x~yAs

yN

xquaternary

alloys are a sum of the contributions from bothincorporated As and N into GaP; PL peak energyin the GaP

1~xN

xalloy would shift by the band-gap

shift from GaP to GaP1~y

Asy. The PL peak ener-

gies in GaP1~x

Nx(0.028%(x(3.2%) alloys and

the band-gap energies in GaP1~y

Asy

alloys werederived from Refs. [2,4], respectively. The latticeconstants of GaP

1~x~yAs

yN

xalloys were measured

by double crystal X-ray diffractometry usingGaP(4 0 0) as the first crystal. Low-temperature PLmeasurements were performed using a He—Cd laseroperating at 325 nm as the excitation source.

3. Results and discussion

The surface morphologies and X-ray (4 0 0) rock-ing curves of three samples are shown in Figs. 1and 2, respectively. These three samples differ onlyin AsH

3flow rates during the MOVPE growth

((a) 0 lmol/min, (b) 1.8 lmol/min, (c) 3.9 lmol/min),and other growth conditions, such as flow rates ofthe other sources and growth temperature, arecommon to all the samples. When no AsH

3was

supplied, many crosshatches due to misfit dislo-cations are clearly seen on the surface as shownin Fig. 1a. In the X-ray rocking curve of thisGaP

1~xN

xalloy, the peak associated with the alloy

layer is observed to be about 1000 s distant fromthe GaP(4 0 0) peak, indicating the large differencein the lattice constant between the GaP substrateand the epitaxial layer (*a/a&!0.4%). On theother hand, the surface of sample (b) is mirror-likeand, moreover, smooth just like a GaP homo-epi-taxial layer. In the X-ray rocking curve, the peak isseen near the GaP(4 0 0) peak and its full-width athalf-maximum (FWHM) is much narrower thanGaP

1~xN

xternary. These indicate that adding

AsH3

flow during the growth of the alloy increasesthe lattice constant and that this GaP

1~x~yAs

yN

x

486 G. Biwa et al. / Journal of Crystal Growth 189/190 (1998) 485—489

Fig. 1. Nomarski interference contrast micrographs of(a) GaP

1~xN

xalloy (x"2.3%), (b) GaP

1~x~yAs

yN

xalloy

(x"2.3%, y"13%) and (c) GaP1~x~y

AsyN

xalloy (x"1.9%,

y"19%).

Fig. 2. Double crystal X-ray(4 0 0) rocking curves of(a) GaP

1~xN

xalloy (x"2.3%), (b) GaP

1~x~yAs

yN

xalloy

(x"2.3%, y"13%) and (c) GaP1~x~y

AsyN

xalloy (x"1.9%,

y"19%). The peaks pointed by arrows are diffractions fromGaP

1~xN

xor GaP

1~x~yAs

yN

xalloy layers.

quaternary alloy is almost lattice-matched to GaPand improved in lattice uniformity. In the sample(c), however, crosshatches appear on the surfaceagain and the peak in the X-ray rocking curve is farfrom that of GaP, showing largely increased latticeconstant due to the excess AsH

3supply. Such

crosshatches do not appear at all on the surfacewhen the lattice mismatch is less than 0.3%. Fig. 3shows the low-temperature PL spectra of thesethree samples. The broad PL peak in (a) is typical inGaP

1~xN

xalloys, and such luminescence occurs at

the density of states tails due to the weak localiza-tion of excitons [5]. PL peak of GaP

1~x~yAs

yN

xquaternary alloys shifts to lower energies with in-creasing AsH

3supply, indicating the band-gap de-

crease due to As and N incorporation into GaP. Itis notable that the integrated PL intensity insample (b) is larger than those in other samples andabout twice as much as that in (a), suggesting thedecrease of nonradiative recombination centers asa result of lattice-matching to the GaP substrate.

The solid compositions (x, y) in theGaP

1~x~yAs

yN

xalloys grown under various AsH

3

G. Biwa et al. / Journal of Crystal Growth 189/190 (1998) 485—489 487

Fig. 3. Low-temperature PL spectra of (a) GaP1~x

Nx

alloy(x"2.3%), (b) GaP

1~x~yAs

yN

xalloy (x"2.3%, y"13%) and

(c) GaP1~x~y

AsyN

xalloy (x"1.9%, y"19%).

Fig. 4. Solid compositions (x, y) in GaP1~x~y

AsyN

xalloys with

various AsH3

flow rates. Numbers located beside the markersare the AsH

3flow rates during the MOVPE growth of

GaP1~x~y

AsyN

xalloys.

Fig. 5. The dependence of integrated PL intensities inGaP

1~x~yAs

yN

xalloys (x&2.3%, 0(y(19%) on the relative

lattice mismatch to GaP substrates.

flow rates from 0 to 3.9 lmol/min are shown inFig. 4. Numbers located beside the markers are theAsH

3supply during the MOVPE growth. Solid

compositions of As simply increase with increasingAsH

3flow rates. On the other hand, N composi-

tions of all samples fall within the range 1.9%(

x(2.4% and are almost unchanged from that ofthe GaP

1~xN

xternary alloy. From this, it is clear

that N and As compositions can be controlledeasily by adjusting the DMHy and AsH

3flow rates,

respectively. The lattice constant is variable withthe alloy composition; GaP

1~x~yAs

yN

xalloys can

be lattice-matched to GaP when y"4.7x (dashedline in Fig. 4), and their lattice constant would bedecreased (y(4.7x) or increased (y'4.7x) fromthat of GaP. Fig. 5 shows the dependence of integ-rated PL intensities in GaP

1~x~yAs

yN

xalloys on

the relative lattice mismatch to GaP. Integrated PLintensities around *a/a&0 are more than twice asmuch as those in GaP

1~xN

xternary (Da/a&

!0.4%), and PL intensities suddenly decreasewhen Da/a exceeds 0. This suggests that non-radiative recombination centers are clearlydecreased when GaP

1~x~yAs

yN

xalloys are lat-

tice-matched to GaP and that lattice matching to

488 G. Biwa et al. / Journal of Crystal Growth 189/190 (1998) 485—489

substrates is important to obtain high-quality alloys with intense PL. Moreover, the lat-tice-matched GaP

1~x~yAs

yN

xalloys with no

strain may be more favorable for the N incorpora-tion from the point of view of lattice-matching tosubstrates [6]. Therefore, adopting GaP

1~x~yAs

yN

xquaternary alloys lattice-matched to GaP

substrates can be useful for obtaining alloys withmuch higher N content as well as high qualityepilayers.

4. Conclusions

The GaP1~x~y

AsyN

xquaternary alloys have

been successfully grown on GaP(1 0 0) substratesby MOVPE. Solid compositions of As inGaP

1~x~yAs

yN

xalloys can be controlled by ad-

justing the flow rates of AsH3

during the MOVPEgrowth, while N compositions are independent ofAsH

3flow rates. The GaP

1~x~yAs

yN

xquaternary

alloys with compositions (x&2.3%, 0(y(19%)were obtained, and the integrated PL intensities

reached the maximum when the alloy was lattice-matched to GaP.

Acknowledgements

The authors would like to express their appreci-ation to J. Wu, S. Akiyama, C. Setiagung, T.Tsujikawa, M. Ishikawa, M. Sato, M. Kudo, andD. Aoki for assistance in the MOVPE growth andS. Ohtake and N. Usami for their technical support.

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G. Biwa et al. / Journal of Crystal Growth 189/190 (1998) 485—489 489