UC Santa Barbara

Gallium nitride (GaN) has proven to be a semiconductor material system that is ideal for the development of lasers and light-emitting diodes (LEDs) with emission wavelengths spanning the visible spectrum. In particular, the prevalence of white LEDs has been made possible by pairing GaN-based optoelectronic devices emitting at 450 nm (blue) with phosphors for down-conversion. The efficiency of these devices, although better than alternatives, is limited at high current densities (efficiency droop), elevated temperatures (thermal droop) and is markedly worse at longer wavelengths. Understanding the physical origins of these efficiency problems can provide a pathway for engineering solutions to these long-standing issues. Despite extensive efforts to improve and understand these limitations, unambiguous identification of these efficiency loss mechanisms has remained an open topic of research.

Most studies of the efficiency loss mechanisms in GaN LEDs rely on an indirect measurement of carrier dynamics by analysis of the emitted light. The inadequacy of these indirect measurements has motivated the development of an electron spectroscopy technique, capable of directly probing several electron transport and recombination mechanisms in GaN-based optoelectronic devices. In 2013, electron emission spectroscopy (EES) was used for the first time to measure high energy electrons generated via Auger recombination in the active region of an LED. This result brought an end to the long-standing controversy over the causes for efficiency droop at high carrier densities.

Our analysis of the kinetic energy of vacuum emitted electrons from GaN-based LEDs has unambiguously identified carrier overshoot as the dominant mechanism responsible for thermal droop. Control experiments from p-n diodes show evidence for a previously unknown hot carrier generating process, likely trap-assisted Auger recombination (TAAR) occurring in AlGaN/GaN barrier heterostrucutres. We also discuss the possibility of TAAR occurring in InGaN quantum wells through examination of low-efficiency LEDs grown by molecular beam epitaxy. Finally, we discuss preliminary EES results regarding long wavelength GaN LEDs as well as the first EES measurements from semipolar (202 ̅1 ̅) GaN LEDs and p-n diodes.