Chemical Reaction Pathways for MOVPE Growth of Aluminum Nitride

Abstract

The chemical reaction pathways for metal-organic vapor phase epitaxial growth of aluminum nitride (AlN) were investigated using elementary reaction simulations and density functional theory calculations. We found that alkyl-aluminum amide (DMA-NH 2 ), which initiates parasitic chemical reactions such as oligomerization, is one of the major reactive species involved in AlN growth. The simulation results indicated that DMA-NH 2 is adsorbed on the surface, followed by the elimination of methane with no energy barrier. The results clarify the role of parasitic reactions and their products in the growth of AlN. Aluminum nitride (AlN) is an attractive semiconductor material due to its wide bandgap of 6.1 eV and high thermal conductivity. Moreover, AIN is commonly used as a buffer layer for the heteroepitaxial growth of gallium nitride (GaN) 2 because it has both a low lattice mismatch with and a similar thermal expansion coefficient to GaN. In AlN growth, metal-organic vapor phase epitaxy (MOVPE) is the most widely used technique because it is capable of large-area growth and because both the layer thickness and composition can be precisely controlled. However, the growth rate is generally limited by parasitic chemical reactions initiated by trimethyl-aluminum (TMA) and ammonia (NH 3 ) precursors.3-8 Creighton and Wang 6 investigated the early stages of the reaction between TMA and NH 3 using IR spectroscopy and density functional theory (DFT) calculations. They reported a parasitic reaction mechanism in which a TMA-NH 3 adduct was first formed and subsequently decomposed into amide and methane, finally producing amide oligomers. Thus, the chemical pathway through the TMA-NH 3 adduct is believed to cause the parasitic reactions that negatively impact AlN growth. However, the contribution of molecules derived from the parasitic reactions of AlN growth remains unclear, although several kinetic models that consider an AlN growth pathway through adduct-related molecules have been developed. [7][8][9] Therefore, in the present study, we investigated the possibility of a chemical reaction pathway through the TMA-NH 3 adduct using elementary reaction simulations and DFT calculations.Elementary reaction simulations were carried out to determine the reactive species using a kinetic model for AlN growth reported by Mihopoulos et al. 9 The basic concept of this growth mechanism is similar to that described by Creighton and Wang. 6 The kinetic model has 10 gas-phase and 7 surface reactions. A perfectly stirred reactor (PSR) was selected for the simulation. In the calculations assuming a PSR, the gas conditions were computationally-optimized by adjusting parameters such as residence time, pressure, and temperature. Additionally, it is assumed that the gas is homogeneous at the moment of gas supply. The residence time of the gas was set to 1 s, the pressure was set to 100 Torr, and the V/III ratio was set to 115. These values were determined by referencing previous reports by Amano 10,11 describing AlN growth with reduced parasit...

Quantum Chemistry Study on Gas Reaction Mechanism in AlN MOVPE Growth

“…Quantum chemistry calculations based on the density functional theory (DFT) have been widely used to study the gas reaction mechanism of AlN MOVPE. − Nakamura et al , studied the elimination reactions of TMX adducts (X = Al, Ga, and In) assisted by excess NH 3 , TMX, and the TMX adduct, respectively. They proved that the energy barrier for CH 4 elimination is reduced considerably in the excess NH 3 and the Al–N bond interaction is most strong in TMAl/NH 3 system.…”

“…Although it has been confirmed that the strong parasitic reactions between TMAl and NH 3 in TMAl/NH 3 /H 2 system are responsible for both nanoparticle generation and thin film growth, − ,− the detailed reaction paths that lead to nanoparticles or stable gas precursors for AlN epitaxy are still unclear, and the existence of [AlN] 1–6 clusters is not justified. Besides this, previous researchers determined the reaction paths mainly by examining the changes of potential energy or Gibbs free energy along the reaction paths. − However, the occurrence of a reaction along certain reaction path requires not only the Gibbs energy difference between the products and the reactants, Δ G , decreasing but also the Gibbs energy of activation, Δ G * , at the transition state being small among competing reactions . Thus, the previous sole potential energy or Gibbs energy criterion can not give correct predictions of reaction pathways in AlN MOVPE growth.…”

Chromium Carbide Growth by Direct Liquid Injection Chemical Vapor Deposition in Long and Narrow Tubes, Experiments, Modeling and Simulation

“…The rate and efficiency of the process are strongly dependent on reaction kinetics, fluid flow, heat transfer and mass transport in the reaction zone. An in-depth understanding of such reaction-transport phenomena requires computational fluid dynamics (CFD) models that are able to predict growth rate and its uniformity over complex geometries [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46].…”

High-quality AlN grown with a single substrate temperature below 1200 °C

“…2(b) , the pit issue is solved on the SPF sample attained by interrupting the NH 3 supply during the HT-AlN growth. Unlike the case in the CF sample, where the synthesis of AlN immediately takes place once the TMAl:NH 3 adduct or the monomethyl-Al (MMAl, decomposed from TMAl) reach the reactive surface 21 , 22 , the Al precursors decomposed during the NH 3 pulse interval are more likely to diffuse to the vacant atomic sites (created either by inherent in-plane strain or by H 2 partial etching) 23 , and thereby flatten the pitted surface. The other benefit of pulsed NH 3 flow can be explained using Fig.…”