Supersonic Beam Epitaxy of Wide Bandgap Semiconductors

The advent of wide band gap semiconductors is ushering in an era of high frequency semiconductor devices: blue LED’s, high temperature/high power transistors, “visible blind” ultraviolet photodetectors, and blue to ultraviolet solid state lasers [1]. III-N nitrides are the materials of choice for such applications, particularly GaN with its direct band gap of 3.4 eV. Unlike conventional semiconductors, III-N semiconductors cannot be grown from the melt but must be deposited expitaxially onto a suitable substrate under carefully controlled conditions. The established techniques for this are Metal-Organic Chemical Vapor Deposition (MOCVD) and, to a lesser extent, Molecular Beam Epitaxy (MBE) [2]. Alternative techniques based on supersonic beams are now attracting attention, both as a deposition tool and as a probe of the still poorly understood fundamentals of the III-N deposition. Of specific utility are the energy selectivity, state selectivity, enthalpy storage, and high flux attainable in a supersonic free-jet expansion. We discuss two supersonic jets of particular relevance to III-N deposition, seeded supersonic free-jets [3] and discharge supersonic free-jets [4].