Wide bandgap (WBG) semiconductors play a crucial role in the current solid-state lighting technology. The AlGaN compound semiconductor is widely used for ultraviolet (UV) light-emitting diodes (LEDs), however, the efficiency of these LEDs is largely in a single-digit percentage range due to several factors. Until recently, AlInN alloy has been relatively unexplored, though it holds potential for light-emitters operating in the visible and UV regions. In this dissertation, the first axial AlInN core-shell nanowire UV LEDs operating in the UV-A and UV-B regions with an internal quantum efficiency (IQE) of 52% are demonstrated. Moreover, the light extraction efficiency of this UV LED can be further improved by 63% by utilizing appropriate hexagonal photonic crystal structures.
The carrier transport characteristics of the LEDs have been carefully engineered to enhance the carrier distributions and reduce the current leakage, leading to a significantly improved IQE of the LEDs. In this regard, the p-type AlGaN electron blocking layer (EBL) has been utilized to suppress electron leakage. Although the EBL can suppress the electron leakage to an extent, it also affects the hole injection due to the generation of positive polarization sheet charges at the hetero interface of EBL and the last quantum barrier (QB). Moreover, the Mg acceptor activation energy of the Al-rich AlGaN EBL layer is elevated, affecting the Mg doping efficiency. To mitigate this problem, in this dissertation, EBL-free UV LED designs are proposed where the epilayers are carefully band-engineered to notably improve the device performance by lowering the electron overflows. The proposed EBL-free strip-in-a-barrier UV LED records the maximum IQE of -61.5% which is -72% higher, and IQE droop is -12.4%, which is -333% less compared to the conventional AlGaN EBL LED structure at 284.5 nm wavelength. Moreover, it is shown that the EBL-free AlGaN deep UV LED structure with linearly graded polarization-controlled QBs instead of conventional QBs in the active region could drastically reduce the electrostatic field in the quantum well (QW) region due to the decreased lattice mismatch between the QW and the QB. The carrier transport in the EBL-free deep UV LEDs is significantly improved, attributed to the increased radiative recombination, quantum efficiency, and output power compared to the conventional EBL LEDs. Overall, the study of EBL-free UV LEDs offers important insights into designing novel, high-performance deep UV LEDs for practical applications.
Further, it is demonstrated that novel WBG materials could be perfectly employed for emerging non-volatile memory (resistive random access memory, RRAM) applications. The resistive switching (RS) capability has been observed in Ga2O3 at low power operation. Importantly, for the first time, the multi-bit storage capability of this types of RRAM devices with a reasonably high Roff/Ron ratio is experimentally demonstrated. In addition, integrating a thin SiNx layer in the conventional SiO2 RRAM device could effectively facilitate the formation of a conducting filament. It is reported that the proposed RRAM device exhibits excellent RS characteristics, such as highly uniform current-voltage characteristics with concentrated SET and RESET voltages, excellent stability, and high Roff/Ron (> 103) even at ultra-low current (10 nA) operation. The multi-bit RS behavior has been observed in these RRAM devices, which pave the way for low-power and high-density data storage applications.
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