Growth Kinetics and Structural Quality in GaN Epitaxy by Low Pressure MOVPE
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BY LOW PRESSURE MOVPE
O.BRIOT, J.P. ALEXIS, B.GIL and R.L.AULOMBARD GES-CNRS CC074, Universit6 Montpellier II, Place E.Bataillon, 34095 MONTPELLIER Cedex 5, FRANCE
ABSTRACT GaN epilayers have been grown using low pressure MOVPE (76 Torr) onto (0001) sapphire substrates. Triethylgallium (TEGa) and Ammonia (NH 3) were used as precursors. The growth rate has been determined versus growth parameters. The growth rate versus NH3 flow displays a rather unusual behavior, in particular at low growth temperature (buffer layer), where the growth rate decreases strongly when the NH3 flow increases. A growth mechanism involving a competitive adsorption step onto the surface is proposed, and results in a strong dependence of the growth rate on the V/Ill ratio. The structural quality of the layers has been assessed by X-ray diffraction versus the growth parameters, and will be reported here. A substrate nitridation step, prior to buffer layer deposition has been introduced in our growth process. The effect of substrate nitridation on the structural and optical properties of the epilayers is studied and we propose an optimized pre-treatment for the MOVPE growth of GaN onto (0001) sapphire substrates. INTRODUCTION Gallium nitride (GaN) is a promising material for optoelectronic applications, as it has been brilliantly demonstrated by the realization of candela-class blue, blue-green and green light emitting diodes [1]. Moreover, in addition to optoelectronic devices, field effect transistors and photodetectors have been successfully demonstrated by several groups. Although nice devices have been achieved, many fundamental aspects of the growth of these materials are still unclear and request further investigations. For example, the MOCVD growth of GaN, based on the socalled « introduced by Amano et al.[2], leads to high quality material, but the process parameters must range in very narrow «>. The understanding of the growth mechanisms is a major concern in order to be able to predict the very influence of each growth parameter and the relationship between them. In this paper, we have studied the influence of process parameters on the growth kinetics. This gives insights on the growth mechanism, which is found to be quite different from more classical MOCVD processes.
EXPERIMENT The GaN layers studied here have been grown by low pressure MOVPE (76 Torrs) in an ASM OMR 12 MOCVD equipment. The precursors were triethylgallium (TEGa) and ammonia (NH3 ). The substrates were (0001) sapphire, with both sides polished to enable transmission experiments. Different growth temperatures, in the range 950 - 1040 0 C have been used. The influence of the V/Ill molar ratio was studied by keeping the molar flow rate of TEGa constant (65 sccm @ 200 C) and changing the NH 3 flow rate in the range 300 - 4000 sccm. The sapphire substrate was cleaned with organic solvents and etched using a H3PO4 : 3 H2 SO4 hot solution. A substrate nitridation step 207 Mat. Res. Soc. Symp. Proc. Vol. 395 © 1996 Materials Research Society
was realized by heating th
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