AlGaN multiple quantum wells by PA-MBE for deep UV emission: Effect of growth interruptions

Abstract

Growth of AlGaN multiple quantum wells by Plasma Assisted-Molecular Beam Epitaxy has been optimized for use in deep ultraviolet light emitting diodes. A series of samples were grown on c-plane sapphire using an AlN buffer layer, an ~1 μm AlGaN layer, followed by 40-pairs of wells and barriers. The III/V flux ratio was varied from near stoichiometric to excess group III and a plasma exposure step was introduced for some samples after the deposition of wells and barriers. The quality of the well-barrier interface was estimated by the number and intensity of X-Ray Diffraction (XRD) superlattice peaks. Our results indicate that interface abruptness is greatly enhanced by the addition of this process step. While the room temperature photoluminescence (PL) intensity increases significantly upon introduction of the interruption after the barrier layer, this effect is reversed with additional interruption after the well layers. Temperature dependent PL measurements show an anomalous variation with increasing temperature with a maxima around 110°-150°C. This is due to photogeneration of e-h pairs in the underlying AlGaN layer and its subsequent recombination in the quantum wells by diffusion through percolative barrier layers with in-plane potential fluctuations. This is arrested at lowest temperatures causing a reduction in PL intensity. The PL intensity ratio I300K/I4K is highest for samples grown under excess group III, but reduces with the addition of the growth interruption step after the barrier layer. Growth under stoichiometric conditions together with dual growth interruptions was found to be optimal, with improved PL properties while maintaining high interface quality.