effect of nitridation on crystallinity of gan grown on gaas by mbe
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Materials Chemistry and Physics 100 (2006) 457–459TRANSCRIPT
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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
lm.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: [email protected] (O. Maksimov).
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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
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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 substrateFS6
(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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