High Efficiency Mid and Deep Ultraviolet Optoelectronic Devices

Abstract

Ultraviolet (UV) light is a critical component of future technological products, having applications in curing polymers, sensors, medical diagnostics, as well as in the sterilization of pathogens – a need which is of prime importance to curtail the spread of diseases and possibly a future pandemic. Solid state UV devices can replace existing sources, such as mercury lamps and xenon lamps, by providing non-hazardous, scalable, easy to use, durable, compact and more efficient performance. The III-nitride material system has established itself as the basis for optoelectronic devices operating in the visible and ultraviolet (UV) wavelength range. While InGaN-based devices have already been commercialized for visible light applications, demonstrating a high external quantum efficiency (EQE) and wall-plug efficiency (WPE), the adoption of AlGaN-based UV devices has been hindered due to their correspondingly lower efficiencies. The primary reasons for the low efficiency of AlGaN LEDs include the low internal quantum efficiency because of defects and dislocations in the device active region, inadequate light extraction due to the primarily transverse-magnetic (TM) polarized light emission, and inefficient carrier injection efficiency from the poor p-type doping of the wide band-gap materials. In this work, we have investigated the design, epitaxy, fabrication and characterization of high efficiency AlGaN devices operating in the mid and deep UV wavelength regime.

We used molecular beam epitaxy (MBE) to grow high-quality Mg-doped AlGaN layers under slightly Ga-rich conditions. The unique growth conditions pinned the Fermi level away from the valence band during epitaxy, which improved Mg incorporation by over an order of magnitude as compared to conventional epitaxy. We demonstrated Mg-doped AlGaN layers having Al compositions up to 90% with resistivities several orders of magnitude lower than previous reports, which is further supported by the dramatically improved EQE of LEDs with emission at 280 nm grown using this technique.

Despite significantly improving the p-type doping, the disparity in the electron and hole concentrations and mobilities is large for Si-doped and Mg-doped AlGaN, respectively. The imbalance of the electron and hole injection to the active region can cause reduced injection efficiency. To address this issue, we investigated different electron-blocking layers (EBLs) and their positioning. We demonstrated that by placing the EBL before the active region as an n-type EBL, instead of a conventional p-type EBL, the flow of electrons can be impeded without hindering hole transport.

We have also utilized polarization-engineered tunnel junctions to increase the hole injection from the p-contacts, which is a critical challenge for wide-bandgap AlGaN. The thickness of the critical tunnel junction layer was optimized for an LED at 265 nm, and we demonstrated a maximum EQE of 11%, the highest value ever reported for devices operating at this wavelength. We also extended this heterostructure design towards shorter wavelengths. Extensive temperature-dependent optical and electrical measurements of 245 and 255 nm LEDs indicate the pivotal role of carrier overflow on device performance and efficiency.

This work provides a unique path for achieving high efficiency mid and deep UV LEDs that were not previously possible. The techniques developed here can be extended to even shorter wavelengths to maximize the efficiency of UV-C AlGaN light sources. Future work includes the development of AlGaN mid and deep UV laser diodes and UV-C and far UV-C LEDs with efficiency comparable to commercial blue LEDs.