Influence of Precursor Concentration on Crystalline Quality of GaN Thin Films Grown on a Sapphire Wafer

Abstract

Chemical vapor deposition (CVD), which involves chemical reactions in gases for deposition on a heated surface, is an extensively used manufacturing technique for obtaining thin films of materials like silicon, graphene, silicon carbide, aluminum nitride, and gallium nitride (GaN). The process is driven by heat and mass transfer, fluid flow, and chemical reactions in the gases and at the surface. GaN is one of the most promising materials for manufacturing optical and electronic devices. However, the reliability and durability of the GaN-based devices depend on the crystalline quality of the thin films used. In this study, the epitaxial growth of GaN thin films on sapphire (Al2O3) wafers is carried out in a vertical rotating disk metalorganic chemical vapor deposition (MOCVD) system. Epitaxial growth refers to the process of growing a crystal of a particular orientation on the top of another crystal, with the orientation being determined by the underlying crystal. MOCVD reactors are CVD systems that use metalorganic compounds that consist of metal and organic ligands, leading to materials like GaAs, AlN, InN, and GaN. The quality of the thin films is largely determined by the choice of operating conditions such as the flowrate, surface temperature, and concentration of the metalorganic precursors that decompose due to heat in the reactor, react, and deposit the desired material on the surface of a wafer or a heated susceptor. In this experimental study, the crystalline quality and surface morphology of GaN thin films are evaluated using atomic force microscopy (AFM), X-ray diffraction (XRD), and Raman spectroscopy. The correlation between the crystalline quality of GaN thin films and the flowrate of the precursors is examined in detail on the basis of an evaluation of the dislocation density. The results indicate that a low concentration (V/III) ratio, where V and III refer to elements in the fifth and third groups of the periodic table, is beneficial for obtaining a high deposition rate since a low value of this ratio implies a high precursor concentration. However, it negatively affects the crystalline quality of the thin film. Similarly, high V/III ratios lead to low deposition rates and better crystalline quality, indicating the need to optimize the process.