New twists in LEDs and HFETs based on nitride semiconductors

摘要

Despite notable progress in light-emitting and charge transport devices based on GaN heterostructures containing In, there is still controversy as to the light emission characteristics at high injection levels in InGaN-based light-emitting diodes (LEDs) and hot-carrier scattering in GaN-based field effect transistors (FETs) with AlGaN or AlInN barriers. For LEDs to be inserted into conventional lighting systems, reasonably high efficiencies would have to be retained at high injection levels to meet intensity requirements which is not yet borne out by many experiments but might be mitigated by use of nonpolar m-plane varieties according to the most recent data. The efficiency degradation at high injection levels, beyond that which is expected due to heating and current crowding, has been attributed by what is shaping up to be two camps to mainly Auger recombination and carrier spillover. The latter has been attributed to or helped by polarization-induced fields which is contrary to again recent experiments on LEDs. In terms of the FETs the conventional wisdom of increased carrier concentrations leading to better devices does not seem to holdrnbeyond a certain point. This is due to strong electron LO phonon coupling in this highly ionic material and the resultant hot phonon population. Hot phonon lifetime decreases with increasing carrier concentration up to a point owing to plasmon-phonon interaction. But beyond the concentration at which the plasma frequency and phonon frequency match, the phonon lifetime begins to increase again. Increased phonon lifetimes lead to reduced carrier velocity and inefficient heat transfer, and thus performance degradation ensues. Another intriguing feature is that the aforementioned phenomenon is electric field dependent at least because increased field in FETs means widening of the channel and thus for the same volume density the resonance occurs at higher sheet densities. In this paper the details of carrier recombination in the context of InGaN LEDs both on polar c-plane and nonpolar m-plane GaN at high injection levels, and hot-carrier-scattering-related physics in the context of HFETs based on the GaN material family containing In will be elucidated.