Characterization of a-plane GaN templates grown by HVPE and high efficiency deep UV emitting AlGaN/AlN MQWs grown by MBE on such templates

Thick GaN templates (>50 μm) were grown by hydride-vapor-phase epitaxy (HVPE) on the r-plane of sapphire. While X-ray diffraction indicates that the GaN grows with its a-plane parallel to the r-plane of sapphire, the surface has a triangular morphology of 120^o, which is the result of the faster growth rate of the a-plane compared to the m-plane. Electron microscopy studies indicate that these GaN templates have a high concentration of threading defects (>10¹⁰ cm^-2) and basal plane stacking faults (7x10⁵ cm^-1). Several identical deep-UV emitting Al_0.7Ga_0.3N/AlN multiple quantum wells (MQWs) were grown on such GaN templates by molecular-beam epitaxy (MBE). The wells were grown under more gallium than required by stoichiometry, which changes the growth mode from physical vapor deposition to liquid phase epitaxy. This growth mode leads to deep band structure potential fluctuations. The structure and microstructure of these MQWs were evaluated by transmission electron microscopy (TEM) and found to have even higher density of threading defects than the GaN template. In spite of this, the internal quantum efficiency (IQE) of these MQWs was estimated from the temperature dependence of the cathodoluminescence spectra to be 87%. This remarkable result was attributed to the deep band structure potential fluctuations introduced by the liquid-phase epitaxy growth mode as well as the high concentration of basal plane stacking faults.