Design of gas inlets for the growth of gallium nitride by metalorganic vapor phase epitaxy

The problem of gas inlet design for metalorganic Vapor phase epitaxy (MOVPE ) of group III nitrides from metal alkyls and ammonia is addressed. The foc us is on GaN growth from trimethyl-gallium and ammonia. In traditional reac tors with well-mixed inlet streams, parasitic gas-phase reactions between t he two precursors may lead to the formation of stable adducts that can cond ense on cool inlet walls, thus reducing the him growth efficiency. Such rea ctions may also lead to the formation of particulates through gas-phase con densation reactions (e.g. during AlN growth). A fundamentally-based model w as developed to describe the MOVPE of GaN and was used to study the effect of inlet design and reactor operating conditions on film thickness uniformi ty in vertical stagnation-flow and rotating-disk reactors. The model includ es a description of gas-phase kinetics and a simple gas-surface reaction me chanism. The kinetic model was coupled to a two-dimensional transport model describing flow, heat and mass transfer in a vertical MOVPE reactor. Predi ctions of growth rate compare well to experimental observations from a vert ical rotating-disk reactor, without any adjustable parameters. The model wa s also used to study the distribution of gaseous species in the reactor and their role in film growth. Finite element simulations using a massively pa rallel computer code (MPSalsa) indicate that the species responsible for fi lm growth are Ga-alkyls and not their adducts with ammonia. Sensitivity ana lysis was also performed to assess the relative importance of each reaction in determining the growth rate. The model was subsequently employed in the design of axisymmetric, multi-aperture gas inlets feeding precursors into the reactor in an alternating (not well-mixed) fashion. Simulations were pe rformed to study the effect of key design parameters, such as inlet velocit ies, susceptor rotating speed, inlet to susceptor distance as well as the n umber and distribution of inlets, on GaN film growth rate and uniformity in industrial scale reactors. Optimal cases are presented that lead to unifor m films over large-area substrates. An alternating precursor feed scheme ba sed on concentric rings was found to lead to more uniform films as the numb er of inlets increased. The other important reactor parameters were inlet v elocity, relative size of inlet "rings" and susceptor distance from the inl et. (C) 2000 Published by Elsevier Science B.V. All rights reserved.