A new mechanism in the growth process of GaN by HVPE
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A new mechanism in the growth process of GaN by HVPE A. Trassoudaine*, E. Aujol*, R. Cadoret*, T. Paskova** and B. Monemar** *Lasmea, Université Blaise Pascal Clermont II, F-63177 Aubière Cedex, FRANCE **Department of Physics and Measurements Technology, Linköping University, S-581 83 Linköping, SWEDEN ABSTRACT Experimental results obtained in two different HVPE reactors are analyzed and compared to the theoretical curves, taking into account the surface kinetics, the mass transfer, and parasitic deposition on the glass walls before the substrate. Unexpected high growth rate values, up to 50 µm/h, were measured in conditions of expected substrate etching by HCl. A systematic experimental study of this new phenomenon is presented together with a theoretical analysis. This analysis suggests a new mixed general mechanism of growth. INTRODUCTION The modeling of GaN HVPE growth in the [00.1] direction [1] performed by using the experimental curves by Seifert et al [2] made it possible to analyze and understand the physics of the system. The model growth processes involve the following steps: adsorption of NH3 molecules, of N atoms by surface NH3 decomposition, of GaCl molecules over the N atoms and finally the chlorine desorption. Two desorption mechanisms of chlorine are considered: desorption of HCl gaseous molecules following a surface reaction with H2, leading to an intermediate HCl adsorption state, and desorption of GaCl3 gaseous molecules following an adsorption of GaCl on two GaCl underlying molecules. The two mechanisms are labelled as the H2 and GaCl3 mechanisms. The spiral growth and step flow mechanisms are applied to the surface diffusion of both NGa and NGaCl molecules. The solution of the diffusion equations are approached in a simple way by considering separate balance equations. The mass transfer effect is approximated by considering a diffusion layer thickness equal to the velocity gradient thickness of an established Poiseuille regime, providing that the substrate is parallel to the flow and that the total flow is not too small. When the substrate makes an angle with respect to the horizontal plane, the mass transfer effect can be approached by varying the wall-substrate distance, equal to the diffusion layer thickness in the model. The extraneous deposit on the quartz walls of the reactor upstream the substrate is taken into account as a mass of GaN deposited in units of g/h. A software taking into account all these phenomena has been produced. All the experimental parameters are entered in a data file, such as: the Ga source HCl, added HCl, H2, N2, He, Ar and total flows, substrate and source temperatures, reactor diameter for the mass transfer, NH3 decomposition rate, GaCl production rate and parasitic deposition rate before the substrate. One of five loops in substrate temperature, Ga source HCl, added HCl , H2 and NH3 flows is selected in each run. Several results have already been published, demonstrating a good agreement between the model and the experimental results [3,4,5]. In the first part o
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