Gallium Nitride Growth Using Diethylgallium Chloride as an Alternative Gallium Source
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(C2 H 5 ) 2 GaCI-- GaCl + 2 C2 H4 +H2
This source was successfully used in GaAs growth that resulted in high uniformity, low carbon content GaAs film 4' 5. GaAs selective area growth, with a complete selectivity of GaAs growth with respect to Si0 2 , Si3N4 , and A120 3 masking materials, has been demonstrated over a wide range of process window 6' 7' 8. The temperature range for GaAs using this source was 400-800'C and the higher temperature range of 1000-1 150'C, required for conventional GaN growth, has not been investigated. We report here on the use of diethyl gallium chloride (DEGaCl) for the large area growth of GaN within a MOVPE system. The present work demonstrates the utility of the DEGaCI source in GaN epitaxial growth. EXPERIMENTAL PROCEDURE: The GaN growth was carried out in a horizontal MOCVD reactor operated at a pressure of 76 Torr. RF induction was used to heat the graphite susceptor. Trimethyl gallium (TMGa), diethyl gallium chloride (DEGaCI) and NH 3 were used as precursors in a hydrogen carrier gas. The DEGaCl bubbler was kept at 60 °C which requires a heated source line to the reactor. Sapphire c-plane substrates were initially etched in H3P0 4 :H2S0 4 =l:3 solution at 700C for 15 minutes, loaded into the reactor and then heated in a flowing H2 ambient to I100"C for 10 minutes prior to the growth. Three comparison sample structures were used in this study. Samples A and C consists of an all DEGaCl or all TMGa-based growth respectively, allowing comparison of the growth chemistry under identical growth process conditions. Sample B is a structure comprised of an initial GaN layer that is identical to Sample C with an additional homoepitaxial GaN layer grown using DEGaCl. Samples A and B have a similar final thickness allowing a comparison of the impact of the initial nucleation of the GaN buffer layer on materials properties. These structures are summarized in Table 1. All buffer layers were deposited at a temperature of 525°C and heated at a rate of 25"C per minute to 1050'C, then annealed at 1050°C for 10 minutes. The subsequent GaN epilayers in all samples were grown at a temperature of 1000 to 11 50'C for 1 hour. The additional GaN layer grown in Sample B using the DEGaCl source was initiated by heating the sample structure (Sample C) to the growth temperature under a high NH 3 partial pressure and directly starting the subsequent growth. The typical gas flows employed were 65[tmol/min for Ga, 4.9 slm for NH 3 and an additional 4 slm for H2 carrier gas. Crystalline quality was measured by double crystal x-ray rocking curve at (002) and (104) Bragg peaks. Surface morphology was determined using an atomic force microscope (AFM). Room temperature Hall and C-V measurements were also obtained. RESULT AND DISCUSSION: Figure 1 shows the growth efficiency, which is related to the growth rate, as a function of growth temperature using DEGaCI and TMGa. The growth efficiency (GE) is defined as the thickness of epitaxial film GaN (iim/min) deposited per Ga source flux (ýtmole/min) in the reactor feed. Th
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