*Corresponding author. Fax: #81 886 56 9060; e-mail:sakai@ee.tokushima-u.ac.jp.

Journal of Crystal Growth 189/190 (1998) 471—475

Growth of InNAs on GaAs(1 0 0) substrates bymolecular-beam epitaxy

Shiro Sakai!,*, Tin S. Cheng", Thomas C. Foxon", Tomoya Sugahara!, Yoshiki Naoi!,Hiroyuki Naoi!

! Department of Electrical and Electronic Engineering, The University of Tokushima, Minami-josanjima, Tokushima 770, Japan" Department of Physics, University of Nottingham, University Park, Nottingham NG7 2RD, UK

Abstract

An InNAs ternary alloy which was predicted to have a very narrow or even a negative band-gap energy was grown onGaAs(1 0 0) substrate by molecular-beam epitaxy (MBE). A ternary alloy with a nitrogen content of about 38%, whichwas almost lattice-matched to GaAs(1 0 0) substrate, was successfully grown. ( 1998 Elsevier Science B.V. All rightsreserved.

Keywords: InNAs; MBE; III-nitride; III—V nitride

1. Introduction

Group III—V-nitride alloys such as InNAs,GaNAs or GaNP were found to have a very largeband-gap bowing due to the large valence electronenergy of the nitrogen atom when compared tothose of the other group V atoms [1]. It was experi-mentally demonstrated that the addition of nitro-gen into GaAs or GaP made their band-gap energysmall [2,3]. Although it is still unclear if the bowingis large enough to produce zero or even negativeband-gap energy in these alloys, the probability ofproducing such a band-gap energy must be high inthose alloys, such as InNAs and InNSb, because

the band-gap energy of InAs or InSb is already verysmall. In this work, we have studied the growth ofInNAs by the molecular-beam epitaxy (MBE).

InNAs was first synthesized by Naoi et al.[4] by metalorganic chemical vapor deposition(MOCVD). Materials with nitrogen content up to6% were obtained, and the band-gap energy wasshown to decrease with increasing nitrogen con-tent. However, a material with higher nitrogen con-tent was not obtained due to insufficient NH

3decomposition at relatively low growth temper-ature.

MBE, on the other hand, is a useful technique tosupply a high density of active nitrogen species atlower growth temperature. Kao et al. [5] havereported the MBE growth of InNAs with nitrogencontent of 38%. Although a preliminary MBEgrowth of InNAs was reported [5,6], the optimum

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 3 - 9

Fig. 1. RHEED intensity as a function of time.

Fig. 2. R1/R

0versus OED.

growth conditions are not reported so far. Thispaper reports the growth of InNAs on (1 0 0)GaAssubstrate by MBE.

2. Growth conditions

MBE was performed at 500°C using a solid Ga,an As cracker to produce As

2and plasma-cracked

nitrogen whose flux was monitored by the opticalemission detector (OED) (in arbitrary units). Theflux of Ga and In was adjusted such that the growthrates of GaAs and InAs were 0.5 and 0.15 lm/h,respectively. A GaAs buffer layer was grown ona semi-insulating GaAs(1 0 0) substrate at 500°C,and then the Ga shutter was closed and the In andN shutters were opened at the same time.

The streaked reflection high-energy electron dif-fraction (RHEED) pattern which was observedduring GaAs growth, disappeared at about 10—20 safter the commencement of the InNAs growth, anda new spotty pattern was observed. The intensityof the RHEED specular beam was monitored asa function of time as shown in Fig. 1, in which anInNAs was grown for 5 s in between the two GaAslayers. The RHEED intensity suddenly decreasedas soon as the InNAs growth was started, and thenrecovered almost to the original intensity observedduring the growth of GaAs. The intensity ratioR

1/R

0was measured as a function of the N/As flux

ratio as shown in Fig. 2, where relative N/As fluxratio was defined as OED (arb. units)/As flux(Torr)]10~4. In the experimental results shownin Fig. 2, the As flux was kept constant at4]10~6 Torr and OED was changed. The inten-sity ratio R

1/R

0has a clear dependence on N/As

flux ratio and has a maximum at OED"0.6 (N/Asratio"15). The decay in RHEED intensity iscaused by the lattice mismatch between the InNAsand the GaAs. The time constant for the decayincreases as the lattice-matching condition is ap-proached and is the longest for a N/As ratio of 15.

3. Layer characterization

InNAs layers were grown on GaAs at differentN/As flux ratios. The X-ray diffraction patterns of

the resultant layers near GaAs(2 0 0) and (4 0 0)diffraction peaks are shown in Fig. 3a and b, re-spectively. When N/As ratio is too low, onlya broad peak of an InAs and a sharp GaAs peak arevisible, while a broad InN(2 0 0) peak also appearsat N/As"80. In addition to these peaks, a peakclose to the GaAs(4 0 0) is clearly visible when N/Asratio is 15. This ratio is the same as that which givesthe longest RHEED decay time.

The surface of MG515, however, is milky withtriangular-shaped islands as shown in Fig. 4. Theenergy-dispersive X-ray (EDX) mapping indicates

472 S. Sakai et al. / Journal of Crystal Growth 189/190 (1998) 471–475

Fig. 3. XRD of the grown samples. (a) near (2 0 0) peak,(b) near (4 0 0) peak.

Fig. 4. Surface microphotographs of the grown layers.

that In, As, and N exist within the triangular islandand the outside of the island is GaAs. White pitsshown on the surface of MG509 is found to be ofInN by EDX analysis. On the contrary, no nitrogensignal was detected from MG510.

A secondary ion mass spectroscopy (SIMS) in-depth measurement were performed as shown inFig. 5. Although the SIMS sensitivity to nitrogenatom was not strong, a clear nitrogen signal wasdetected in the ternary layer. Ga signal was alsodetected, since the ternary layer do not cover theentire GaAs surface. A SIMS nitrogen intensitydivided by that of the In is shown as a function of

S. Sakai et al. / Journal of Crystal Growth 189/190 (1998) 471–475 473

Fig. 5. In-depth SIMS profile.

the relative nitrogen supply N/As in Fig. 6. Almostno nitrogen is detected below N/As of 10, andnitrogen intensity increases with increasing nitro-gen supply.

Summarizing all these results, it can be con-cluded that InNAs can be grown only when itslattice constant is close to that of the GaAs substra-te. Assuming the Vegard’s law, the nitrogen contentof MG515 is estimated to be 38%. An u-scan X-raydiffraction was also measured for InNAs(4 0 0)peak, and the full-width at half-maximum was350 arcsec. In order to investigate the crystal struc-ture of an InNAs, X-ray u—2H scan was measuredwhile changing a tilt angle of the sample. Onlypeaks from cubic GaAs, InNAs and InAs were seen,

and no peak was visible at the points correspond-ing to the hexagonal phase.

4. Summary

It was demonstrated that an InNAs layer can begrown by MBE by adjusting the flux ratio of N/Asso that the grown layer is nearly lattice-matched tothe GaAs substrate. The ternary layer grown in thisstudy had a rough surface and included a phase-separated InAs. The details of this phase separationis reported separately [7]. Multi-layer structures of(GaAs/InNAs) were also grown of n periods wheren"7. In this case the diffraction peak from InAs

474 S. Sakai et al. / Journal of Crystal Growth 189/190 (1998) 471–475

Fig. 6. Relative nitrogen intensity as a function of nitrogensupply.

was totally suppressed. The results shown in thispaper indicate that further improvement is possibleby the optimization of the MBE growth conditions.

For the 38%-nitrogen InNAs obtained in thisstudy, tight binding calculation predict a negativeband-gap energy [8]. The electronic characteriza-

tion is now under investigation and will be reportedin future.

Acknowledgements

A part of this work was supported by the Satel-lite Venture Business Laboratory of the Universityof Tokushima.

References

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[4] H. Naoi, Y. Naoi, S. Sakai, Solid State Electron. 41 (1997)319.

[5] Y.C. Kao, T.P.E. Broekaert, H.Y. Liu, S. Tang, Abstracts ofthe Material Research Society Spring Meeting, 1996, E12.4,p. 100.

[6] W.G. Bi, C.W. Tu, Abstracts of the Material ResearchSociety 1996 Fall Meeting, 1996, N3.41, p. 318.

[7] M.S. Hao, S. Sakai, T. Cheng, C.T. Foxon, J. CrystalGrowth 189/190 (1998) 481.

[8] T. Yang, S. Nakajima, S. Sakai, Jpn. J. Appl. Phys. 36 Part2 (1997) L320.

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