Wide-Bandgap III-Nitride Tunnel Junctions and Novel Approaches towards Improving Optoelectronic Devices
Clinton, Evan A.
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A combination of novel techniques, materials, and devices are explored to enhance III-nitride optoelectronics from the infrared to the deep ultraviolet wavelengths. Low-bandgap, high indium content III-nitride materials are investigated for longer wavelength applications. High indium incorporation into the crystal is achieved via plasma-assisted molecular beam epitaxy (PAMBE) at low growth substrate temperatures < 400 °C, but damage to the crystal from the plasma is observed in the form of elevated unintentional background electron concentrations and a textured surface morphology. The plasma is extensively characterized and an optimal condition is determined which reduces the unintentional background electron concentration by 74% in InN films, providing insight towards future high-indium content III-nitride devices. Wide-bandgap GaN homojunction tunnel junctions are investigated to enhance shorter wavelength optoelectronics. Extreme n- and p-type doping is established with carrier concentrations above 1x1020 cm-3, which is essential to form tunnel junctions in such a wide bandgap material. The extreme doping is then applied to form GaN standalone tunnel diodes at various n- and p-type doping levels and even with delta doping. Negative differential resistance is measured at a low peak voltage of 1.3 V, and is measurable down to cryogenic temperatures as low as 77 K. Tunnel junctions are then demonstrated as tunnel contacts to p-i-n diodes exhibiting a state of the art low voltage penalty of ~0.1 V. Additionally, tunnel-contacted InGaN multi-quantum well (MQW) solar cells and LEDs are fabricated, which demonstrate the tunnel junctions are compatible in both forward and reverse bias applications. Finally, ultra-wide-bandgap AlGaN material is synthesized to enable ultraviolet optoelectronics. Techniques are established to limit phase separation in the ternary material, while still maintaining smooth crystal morphology, and allow for the extreme doping necessary for tunnel junctions. Standalone homojunction AlGaN tunnel diodes are formed and exhibit state of the art negative differential resistance for Al contents up to 19%. Reverse-bias tunneling is observed up to 60% Al, promising for deep-UV optoelectronics. A 276 nm tunnel-contacted LED is formed and displays >2x light output power when compared to a control.