Epitaxial Growth of κ-(AlxGa1−x)2O3 Layers and Superlattice Heterostructures up to x = 0.48 on Highly Conductive Al-Doped ZnO Thin-Film Templates by Pulsed Laser Deposition

Abstract

(AlxGa1−x)2O3 thin-film layers in the metastable orthorhombic κ-modification are deposited heteroepitaxially on unintentionally doped ZnO buffer layers on heavily Al-doped and highly conductive ZnO back contact layers by pulsed laser deposition. The back contact layers potentially serve as electrodes for ferroelectric hysteresis measurements and improve the performance of device applications such as Schottky barrier diodes or quantum-well infrared photodetectors (QWIPs). The alloy layers cover a broad composition range of 0 ≤ x ≤ 0.48 for which X-ray diffraction (XRD) patterns confirm the same high crystal quality as for layers grown on single-crystalline substrates. Epitaxial relations of the κ-phase layers to the ZnO thin-film templates are determined and both in- and out-of-plane lattice constants follow a linear evolution with Al content in agreement with Vegard's law. Smooth surface morphologies are verified in atomic force microscopy images. Finally, a 15-layer-pair κ-(Al0.27Ga0.73)2O3/κ-Ga2)O3 quantum-well superlattice (SL) structure is similarly deposited on the ZnO:Al back contact layer and shows sharp SL fringes up to the fifth order in XRD measurements confirming excellent crystal quality and abrupt interfaces in the heterostructure. The results render ZnO:Al as a promising back contact layer for QWIP applications and the determination of electrical polarizations of κ-phase alloy layers.