The (Al,Ga,In)N materials system has impacted energy efficiency on the world-wide scale through its application to blue light-emitting diodes (LEDs), which were invented and developed in the 1990s. Since then, cost reductions and performance improvements have brought GaN-based LEDs into the mainstream, supplanting outdated lighting technology and improving energy efficiency.

One of the main challenges that still limits commercial LEDs, however, is “efficiency droop,” which refers to the reduction in efficiency as the input current density (and with it, the carrier density) increases. This phenomenon especially plagues high power LEDs, which operate in the current density range of 100-1000 A/cm2.

Few practical options exist to directly eliminate efficiency droop, however we investigated two complementary approaches to circumvent the phenomenon. The first “high power solution” would employ blue laser diodes as the engine of solid state white lighting in lieu of LEDs. When laser diodes reach the threshold current density for stimulated emission, the carrier density in the active region clamps, simultaneously clamping droop. The wall plug efficiency of the laser diodes can then continue to rise as input current density increases until another effect (usually thermal) overrides it. The second “low power solution” maintains the blue LED as the solid state lighting engine, but shifts the operation point to low current density (and low carrier density) where efficiency droop effects are negligible and other thermal and electrical constraints in the device design are alleviated, enabling designs for high wall-plug efficiency. Both approaches to circumventing efficiency droop are likely to find a home in diverse future technologies and applications for lighting and displays.

The challenge to produce high performance blue laser diodes was approached from an m-plane epitaxy platform. m-Plane is a non-polar orientation of the wurtzite (Al,Ga,In)N, which is free from deleterious polarization-related electric fields in the growth direction. m Plane is a naturally occurring crystal plane with high material gain due to its non-degenerate valence band structure, and thus should be well-suited for laser diode applications. However, m plane blue emission suffers from low indium uptake and broad spontaneous emission linewidth. The use of surface “double miscut” was investigated to improve the local step structure and morphology, resulting in higher indium uptake, narrower linewidth and higher peak power in the blue spectrum.

The complementary challenge to improve the wall-plug efficiency for LEDs at low power operation focuses primarily on improved light extraction efficiency and low voltage operation. The main sources of extraction efficiency losses in typical c-plane blue LEDs on patterned sapphire substrates are absorption on the metal contacts, in the current spreading layer and on the metallic reflector, which also doubles as the heat sink. With the relaxed constraints at low power operation, new designs become possible. High light extraction designs were vetted with ray tracing software prior to experimental implementation. The highest demonstrated wall-plug efficiency resulting from these designs was 78.2%, and was accompanied by a greater than unity electrical efficiency (1.03) resulting from thermoelectric pumping, suggesting a pathway for 100% or greater wall-plug efficiency.

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.

As the average luminous efficacy of light emitting diodes (LEDs) has increased over the years, the energy performance of LEDs has surpassed preceding lighting technologies such as incandescent and fluorescent lighting. One way to reduce the cost per lumen.hour would be to maximize the wall-plug efficiency (WPE), where the internal quantum efficiency (IQE), η_rad, plays a major role. η_rad represents the number of photons created per injected electron-hole (e-h) pair and is known to peak at a low current density ~1-10 A cm-2. This reduction in efficiency at higher current densities has been referred to as efficiency droop, or simply droop, posing a roadblock to full penetration of the lighting market and for applications in display technologies.An effective technique to study efficiency droop is electron emission spectroscopy (EES). EES is capable of measuring and detecting hot electrons generated in the active region of an LED, thus allowing direct investigation of the recombination, and scattering processes in the device. Electrons were detected to be emitted from a side valley at ~0.9 eV above the Γ-valley of GaN, which can only be generated by Auger recombination. The intensity of these hot electrons was showed to increase with increasing droop, thereby providing direct evidence of Auger recombination as the cause of droop. The work presented here concerns taking the experimental technique towards a full quantitative approach. Using the light output power of an LED as a proxy for active region carrier density n, we were able to directly obtain the power law dependence of the various valley peaks on n, distinguishing between 2-body trap-assisted Auger recombination (TAAR) and 3-body band-to-band Auger combination. Efficiency and thermal droop studies in c-plane blue and green III-nitride LEDs were made to quantify the prevalence of TAAR and investigate the sources of the defects. We report on detection of a new high energy upper valley at ~1.7 eV above Γ-valley, which was predominantly generated by TAAR. Its detection was contingent on a low number of pre-well InGaN/GaN superlattice (SL) period, higher [In] quantum wells, and presence of AlGaN in the active region – indicating the defect reduction capabilities of SLs and presence of a deep trap at the AlGaN/(In)GaN interfaces. Through systematic thinning of the p-region by growth or by ex-situ etching, we present work leading towards obtaining the absolute TAAR and 3-body Auger electron currents for full LED recombination physics quantification.