A Numerical Study of the V/III Ratio in the MOCVD Process with Pulsed Injection Method

Abstract

AlN layer grabs lots of attention as a great super lattice layer between Si or sapphire substrate and GaN-based material. V/III ratio in the AlN MOCVD process is regard as an important parameter which causes different AlN growth modes on the substrate surface. The growth mode changes from 3D nucleation to 2D growth while the V/III ratio decreases. On the other hand, the pulsed injection method is applied in the MOCVD process to depress parasitic reaction and improve the quality of the AlN film. However, the growth conditions at the wafer surface are strongly affected by the mass transport in the reactor chamber which depends on the flow pattern and gas phase reaction. The momentum, heat and mass transfers are complicated by the pulse action. Different types of pulse sequences and pulse times are reported for finding a better process condition. Until now, it still depends on the empirical data very much and induces enormous time consumption and financial cost in the chamber design process. A suitable numerical simulation is needed to be developed for overcoming this difficulty. In this study, a time-dependent numerical model is developed to analysis the flow motion and the heat and mass transfer caused by different pulse conditions. For the case with the TMAl gas mixture maintained a constant and a pulsed NH3 gas mixture, the V/III ratio at the surface of substrate increases with the process time after the pulse of NH3 mixture ejected into the chamber. It approaches to a constant when the time is greater than a certain value. The V/III ratio decreases to zero after the pulse of NH3 mixture ends. The V/III changes quickly in a little time when the pulse of NH3 mixture is turned on or off. When the NH3 and TMAl are pulsed into the chamber alternatively, the V/III ratio at the surface of the substrate keeps increasing after the pulse of NH3 mixture turned on. The V/III ratio decreases to zero after the pulse of NH3 mixture turned off. The V/III ratio does not stay at a certain value like the cases which has the constant supply of TMAl mixture. Based on the variation of V/III ratio at the surface of the substrate, we can predict the history of the growth mode at the surface of the substrate during the process.