Polarity Inversion by Supplying Group-III Source Firstin MOMBE of GaN/AlN or GaNon GaAs (111)B (As Surface)

F. Hasegawa (a), O. Takahashi (a), T. Nakayama (a), and R. Souda (b)

(a) University of Tsukuba, Institute of Applied Physics, Tsukuba, Ibaraki 305-8573, Japan

(b) Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba,Japan

(Received June 19, 2001; accepted August 4, 2001)

Subject classification: 68.35.Bs; 81.15.Hi; S7.14

In order to investigate the influence of the Ga or Al and N supply sequence on the polarity, GaN/AlNor GaN was grown on a GaAs (111)B (As surface) from Ga/Al source or N source in MOMBE.300 nm GaN/10 nm AlN/GaAs (111)B grown with N source supply first in N rich condition ex-hibited N polarity dominance, but it became Ga polarity dominant when about five monolayersAl were deposited prior to N source supply. The same phenomenon was observed whenGaN/GaAs (111)B was grown in N rich condition by supplying Ga source first. These results indi-cate that polarity of GaN is controlled not only by substrate polarity but also by the surface condi-tion of the substrate.

Introduction It is well known that the polarity of hexagonal GaN influences greatlythe crystal quality of the grown layer, but it is not understood well how the polarity ofthe grown layer is controlled. GaN grown by MOVPE on a sapphire substrate is knownto have usually Ga polarity [1]. Polarity of GaN grown by GSMBE is much more com-plicated than the MOVPE case and depends on the reactive gas and the buffer layer.GaN grown by rf plasma-assisted MBE is usually a mixture of Ga and N polarities, andgenerally the N polarity is dominant [2, 3]. Layers grown with NH3 as the N source arereported to have Ga polarity [4]. Ga polarity layers can be obtained by an AlN bufferlayer or AlN/GaN superlattice [5, 6]. The quality of GaN grown by rf plasma-assistedMBE is improved very much by controlling the polarity to be the Ga one [5, 6].When GaN is grown on a substrate having its own polarity, it should depend on the

substrate polarity. However, it is not made clear yet how the polarity of the grownlayer is governed by the substrate, surface and growth conditions. Influence of an AlNintermediate layer is not known well.The purpose of this work is to clarify how the polarity of GaN is governed by the

surface condition, an AlN intermediate layer or the substrate polarity itself, by growingGaN or GaN/AlN on GaAs (111)A and B surfaces.

Experimental GaN and AlN were grown by a conventional MOMBE apparatus, usingmetal Ga as the Ga source and Di-methyl-aluminum-hydride (DMAH) as the Alsource. Di-methyl-hydrazine (DMHy) was used as the nitrogen source for both growths[7]. The beam equivalent pressure (BEP) of Ga was kept at 2 � 10––8 Torr (Ga celltemperature; 866 �C) so that the growth condition was N rich. The growth rate was200 nm/h compared to the normal growth rate of 400 nm/h in stoichiometry condition,and the grown GaN normally exhibits N polarity dominant. The BEPs of DMAH and

phys. stat. sol. (b) 228, No. 2, 549–552 (2001)

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DMHy were fixed at 1 � 10––6 Torr and 2 � 10––4 Torr, respectively. The growth tem-perature was 700 �C for both AlN and GaN layers.(111) GaAs substrates were etched in an etchant of NH4OH:H2O2 :H2O = 1 :1 :5 at

0 �C for 2 min and dipped into HF solution for 15 min. A thermal cleaning was per-formed at 600 �C for 10 min in vacuum prior to the growth to remove the surfaceoxide. GaAs (111)A and B substrates were set together in the same Mo holder, andGaN/AlN was grown on the both wafers in the same run with the same growth condi-tions.Crystal quality was measured by RHEED and XRD, and the polarity was character-

ized by CAICISS (Coaxial Impact Collision Ion Scattering Spectroscopy) [8].

Results and Discussion GaN grown on GaAs (111)A (Ga surface) always exhibits Gapolarity independently of V/III ratio, but polarity of GaN grown on GaAs (111)B (Assurface) depends on the V/III ratio and it becomes N polar for large V/III ratio, i.e., Nrich growth condition. By insertion of 20 nm AlN layer, it became Ga polar even onGaAs (111)B with N rich growth condition. However, when the AlN thickness wasreduced to 10 nm, GaN grown on it was N polarity dominant for usual growth proce-dure with DMHy supply first.In order to see the effect of DMAH pre-flow, DMAH corresponding to 2 ML, 5 ML

and 10 ML of Al was supplied on GaAs (111)B before supply of DMHy. Although theRHEED pattern was spotty ring for GaN grown with DMHy first, a spotty streakyRHEED pattern was obtained for GaN grown on AlN with the 5 ML Al pre-flow asshown in Fig. 1. The RHEED pattern became spotty ring again with the 10 MLAl pre-flow.As to the polarity characterized by CAICISS, N polarity was dominant when DMHy

was supplied first as shown by the top curve of Fig. 2. Simulated CAICISS spectra ofGa polar and N polar GaN are also shown in Fig. 2 for reference; the best Ga polarity

550 F. Hasegawa et al.: Polarity Inversion by Supplying Group-III Source First

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Fig. 1. Dependence of RHEEDpattern on supply sequence andamount of Al for GaN/10 nm AlN/GaAs (111)A&B. Left: on A-face,right: on B-face

spectrum was obtained for 5 ML Al as can be seen in Fig. 2. The CAICISS spectrumdeteriorated slightly for 10 ML Al pre-flow, but it was still Ga polarity dominant. GaNgrown on GaAs (111)A (Ga surface) showed always Ga polarity independently of thegrowth condition and of the sequence of the source material supply as shown in Fig. 3.An almost similar result was obtained for Ga metal pre-deposition as shown in Fig. 4.

When GaN was directly grown on GaAs (111)B in a usual way with DMHy first in Gadeficient condition, a CAICISS spectrum corresponding to a layer including a lot of N

phys. stat. sol. (b) 228, No. 2 (2001) 551

Fig. 2. Dependence of the polarityon supply sequence and amount ofAl for GaN/10 nm AlN/GaAs (111)B.Ga polarity became dominant by2 ML Al pre-flow

Fig. 3. Dependence of CAICISSspectra on supply sequence andamount of Al for GaN/10 nmAlN/GaAs (111)A. The grownlayers have always Ga-polarity in-dependently of the supply se-quence on GaAs (111)A surface

polarity columns was observed as shown in the top curve of Fig. 4. Ga polarity dominantGaN layer was obtained with 1.5 ML Ga pre-deposition. These results indicate thatgroup-III materials supplied first are very effective to obtain Ga polarity dominant GaNlayers.

Summary In order to investigate influence of the supply sequence of source materialson the polarity, GaN/AlN or GaN was grown on GaAs (111)B (As surface) from Ga/DMAH or DMHy in MOMBE. It was found that the polarity greatly depended on whichsource was supplied first; when a group-III source of five monolayers was supplied first, aGa polarity layer was always obtained independently of the growth condition.

Acknowledgements The authors would like to express their sincere thanks to Prof.M. Sasaki for his kind help on the maintenance of the MOMBE system, and to Dr.T. Suemasu, Messrs. M. Namerikawa and T. Sato for their support to the experimentsand for discussions on the experimental results.

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552 F. Hasegawa et al.: Polarity Inversion by Supplying Group-III Source First

Fig. 4. Dependence of CAICISS spectraon supply sequence and pre-depositionof Ga for GaN/GaAs (111)B