Abstract
AlGaN has attracted considerable interest for ultraviolet (UV) applications. With the development of UV optoelectronic devices, abnormal carrier confinement behaviour has been observed for c-plane-oriented AlGaN quantum wells (QWs) with high Al content. Because of the dispersive crystal field split-off hole band (CH band) composed of pz orbitals, the abnormal confinement becomes the limiting factor for efficient UV light emission. This observation differs from the widely accepted concept that confinement of carriers at the lowest quantum level is more pronounced than that at higher quantum levels, which has been an established conclusion for conventional continuous potential wells. In particular, orientational pz orbitals are sensitive to the confinement direction in line with the conducting direction, which affects the orbital intercoupling. In this work, models of Al0.75Ga0.25N/AlN QWs constructed with variable lattice orientations were used to investigate the orbital intercoupling among atoms between the well and barrier regions. Orbital engineering of QWs was implemented by changing the orbital state confinement, with the well plane inclined from 0° to 90° at a step of 30° (referred to the c plane). The barrier potential and transition rate at the band edge were enhanced through this orbital engineering. The concept of orbital engineering was also demonstrated through the construction of inclined QW planes on semi- and nonpolar planes implemented in microrods with pyramid-shaped tops. The higher emission intensity from the QWs on the nonpolar plane compared with those on the polar plane was confirmed via localized cathodoluminescence (CL) maps.
Highlights
AlGaN can be used in light-emitting diodes that can emit light ranging from ultraviolet (UV)-A to UV-C; in addition, it has excellent thermal conductivity, high physical hardness, high chemical resistance, and a high melting point
Since the valence band maximum (VBM) in an AlxGa1−xN well with x > 0.5 is dominated by pz orbitals and the heavy-hole (HH) and light-hole (LH) bands are composed of px and py states, the abnormal radiative interband transition in a quantum wells (QWs) is associated with the transition to the pz state
It would be of interest if the underlying mechanism of orbital intercoupling based on the quantum confinement direction in a QW could be determined because this would provide a reliable starting point for orbital engineering that could be useful in the fabrication of other novel materials and optoelectronic devices
Summary
AlGaN can be used in light-emitting diodes that can emit light ranging from ultraviolet (UV)-A to UV-C; in addition, it has excellent thermal conductivity, high physical hardness, high chemical resistance, and a high melting point. A comparable energy variation with its sign depending on the orbital configuration together with the confinement direction should be considered in addition to the band offset.
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