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Materials Chemistry and Physics 100 (2006) 457–459

Effect of nitridation on crystallinity of GaN grown on GaAs by MBE

O. Maksimov a,∗, P. Fisher b, M. Skowronski b, V.D. Heydemann a

a Electro-Optics Center, Pennsylvania State University, 559A Freeport Road, Freeport, PA 16229, United Statesb Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, United States

Received 6 October 2005; received in revised form 29 December 2005; accepted 23 January 2006

bstract

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrateitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and theependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperatureo achieve c-oriented �-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and �-GaN inclusions

2006 Elsevier B.V. All rights reserved.

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eywords: Molecular beam epitaxy; GaN; GaAs

GaN materials are technologically important for a variety ofevice application [1,2]. They are ideal candidates for fabri-ation of high power microwave devices, high frequency fieldffect transistors, high electron mobility transistors, light emit-ers and detectors operating in the visible to UV spectral range.igh quality hexagonal �-GaN films and heterostructures aresually grown either by metal organic chemical vapor depositionMOCVD) or by molecular beam epitaxy (MBE) on sapphirend 6H-SiC substrates [3,4]. Growth on [0 0 1] GaAs is muchess studied, although these substrates provide several advan-ages, such as, low cost, easy cleavage along [0 1 1] direction,loser thermal expansion coefficient matching, and possibilityo stabilize cubic �-GaN.

We have reported that direct deposition on a thermally des-rbed GaAs results in the growth of a polycrystalline poorly ori-nted �-GaN containing mis-oriented domains and large cubicnclusions. However, a significant improvement of the crys-allinity is achieved by adopting the growth procedure that con-ists of a substrate nitridation, deposition of a low-temperatureuffer layer, and epitaxial overgrowth at elevated temperature5]. The nitridation conditions are extremely critical for this pro-ess and have to be carefully controlled to achieve high-quality

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Here we investigate the influence of the substrate tempera-ure during nitridation on the structural properties of GaN film.

∗ Corresponding author. Tel.: +1 724 295 6624; fax: +1 724 295 6617.E-mail address: Maksimov@netzero.net (O. Maksimov).

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254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2006.01.024

e observe that low-temperature (400 ◦C) nitridation promotesrowth of c-oriented �-GaN. When nitridation is performedt higher temperature, crystalline quality degrades and filmecomes polycrystalline.

The samples are fabricated in a custom-built MBE systemquipped with a Ga effusion cell, a radio frequency (RF) excitedlasma source (SVT Associates, Inc.), a retractable ion gauge forux calibration, and a reflection high-energy electron diffractionRHEED) system. GaN is grown on semi-insulating epi-ready0 0 1] GaAs substrates indium-mounted to molybdenum hold-rs. The substrate temperature is measured by a thermocouplen contact with the backside of the mounting block. To prevents incorporation in the GaN alloy, the oxide layer is desorbed at00 ◦C in the absence of As flux. The GaAs wafer is exposed toub-monolayer Ga pulses to facilitate oxide desorption throughhe conversion of Ga2O3 to a more volatile Ga2O [6]. This pro-ess results in a slightly distorted GaAs surface. Kikuchi linesre clearly visible in a RHEED pattern, indicating that GaAsurface is free of oxide layer, Fig. 1A.

After oxide desorption wafer temperature is adjusted to theesired setting and nitridation is performed by exposing sub-trate to nitrogen plasma. The nitridation rate is controlled withmass flow controller through which a high purity (6N) N2

as (gas flow is ∼2.5 sccm) is introduced into the RF-plasmaource (input power is ∼400 W). The nitridation is performed

or 15 min with the substrate temperature kept constant.

When the wafer is exposed to nitrogen plasma, surfaceeconstruction disappears during the first few minutes sug-esting formation of an amorphous GaAsN layer. We observe

458 O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459

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fintemperature (400 C) to achieve c-oriented �-GaN. Higher sub-strate temperature promotes formation of mis-oriented domainsand �-GaN inclusions.

ig. 1. RHEED patterns for: (A) GaAs substrate after oxide desorption at 500eposited at 600 ◦C, (D) GaN film grown at 750 ◦C.

n arc pattern after approximately 5 min indicating develop-ent of a preferred out-of-plane orientation in a disordered

ayer, Fig. 1B. Since it does not change when the wafer isotated around the surface normal, layer is not oriented in-lane. Spot-like features with hexagonal symmetry developfter, approximately, 10 min. This reconstruction does not sig-ificantly change when the wafer is exposed to nitrogen plasmaor a longer period of time. Therefore, we limit nitridationo 15 min.

In the next step we close nitrogen plasma source shutternd increase wafer temperature to 600 ◦C. Annealing, ∼30 min,harpens diffraction spots demonstrating recrystallization of �-aN phase, Fig. 1C. The diffraction spots are broad signifying

hat very defective GaN layer forms at the beginning. How-ver, they become significantly sharper and elongated duringhe growth of a relatively thin (50-nm) buffer layer, indicatinghat GaN buffer has a better crystalline quality and a smootherurface.

Finally, wafer temperature is raised to 750 ◦C for GaNrowth. A slightly diffused (1 × 1) reconstruction is observeduring the film growth, Fig. 1D.

Crystalline quality of the GaN films is studied by X-rayiffraction (XRD). All the films are deposited in one growthun under identical conditions and differ only in the nitridationemperature (A 400 ◦C, B 500 ◦C, C 550 ◦C, D 600 ◦C). A XRD–2θ scan demonstrates that low temperature nitridation pro-otes growth of c-oriented �-GaN, Fig. 2A. Mis-oriented grains

〈1 0 1 1〉, 〈1 0 1 2〉, 〈1 1 2 0〉, 〈1 0 1 3〉) and cubic �-GaN inclu-ions (〈0 0 2〉) develop when nitridation is performed at 500 ◦C,ig. 2B. The intensity of 〈0 0 0 2〉 diffraction decreases while

ther peaks become more pronounced with the further increasef nitridation temperature indicating degradation of crystallineuality of the film, Fig. 2C and D. This trend is, most prob-bly, due to the surface etching that is activated by substrate

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(B) GaAs substrate after 5 min of nitridation at 400 ◦C, (C) GaN buffer layer

emperature during nitridation [7]. It results in a rough defec-ive epilayer/substrate interface and can promote polycrystallinerowth.

In conclusion, we demonstrate that crystalline quality of GaNlms grown on [0 0 1] GaAs substrates is extremely sensitive toitridation conditions. Nitridation has to be performed at low-

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ig. 2. XRD θ–2θ scans of ∼2 �m thick GaN films grown on a GaAs substrate.ubstrate nitridation is performed at (A) 400 ◦C, (B) 500 ◦C, (C) 550 ◦C, (D)00 ◦C.

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[American MBE Conference, Santa Barbara, CA, 2005.

[6] Z.R. Wasilewski, J.M. Baribeau, M. Beaulieu, X. Wu, G.I. Sproule, J. Vac.

O. Maksimov et al. / Materials Che

cknowledgement

This material is based upon work supported by Dr. Colinood, ONR under Contract No. N00014-05-1-0238.

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