Optimization of Reactor Geometry and Growth Conditions for GaN Halide Vapor Phase Epitaxy
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absorption spectroscopy. By changing the solid composition, a tunable direct bandgap ranging from that of pure GaN to that of pure AIN was obtained. The heteroepitaxial growth of thin GaN films on sapphire leads to defects arising from lattice mismatch and difference in thermal expansion coefficient. The development of GaN substrates is likely to be a key advance in nitride epitaxial technology, making it feasible to grow homoepitaxial thin GaN films [2]. A promising route for the development of GaN substrates is the heteroepitaxial growth of GaN films by rapid growth techniques, such as halide vapor phase epitaxy (HVPE), followed by the in situ etch removal of the initial substrate to leave a freestanding GaN film. The HVPE technique has been used previously to grow thick layers of GaAs [3], GaN [2, 4, 5], and InP [6]. Mochizuki et. al. [7] have studied the direct reaction between AsH 3 and surface adsorbed GaCl in order to understand the growth chemistry involved in HVPE of GaAs. However, no comparable GaN based studies have been reported. The development of predictive models of the HVPE process can substantially reduce the time and cost associated with reactor optimization and scale-up by minimizing the required experimental trial and error. It would also aid development of an improved understanding of the HVPE process. In this study we will describe a two-zone hot wall reactor used to grow thick HVPE GaN films. The emphasis will be on studying the effect of process and geometric parameter variation on film thickness, uniformity and material properties. Experimental results will be compared to 227
Mat. Res. Soc. Symp. Proc. Vol. 423 ©1996 Materials Research Society
computational predictions. The effect of local gas phase concentrations on film properties will be discussed. REACTOR MODEL & GROWTH STUDIES The experiments were carried out in an atmospheric pressure quartz reactor which has been presented earlier[8]. The reactor has three separate concentric inlets for the reaction gases. A mixture of N 2 and HCI is introduced through the central tube, N 2 through the middle annular region, and NH 3 through the outer annular region. The reactor is divided into two separate temperature zones of 850 and 1050 °C. In the first reaction zone, operated at 850 °C, Ga metal is reacted with HCI gas (typical flow - 30 sccm) to yield GaCl and H 2 reaction products. The extent of reaction, obtained from decrease in the mass of Ga
Computational Domain
-I
Z
metal, was in the range of 5070%. These reaction products
are transported to the second
Sa.ple. Sample ..............
N2 + HCI
Holder zone through the central tube. N2C ,_________________ NH,-I--In the second zone, typically maintained at 1050 °C, a high Substrate Isothermal Zone flowrate N 2 buffer and NH 3 were introduced from the Figure 1: Schematic of the horizontal HVPE reactor. The shaded middle and outer annular region defines the computational domain. regions, respectively. The I NH 3/HCI ratio was typically 30:1. Two inch diameter (0001) sapphire substrates
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