Compositionally Graded III-Nitride Alloys: Building Blocks for Efficient Ultraviolet Optoelectronics and Power Electronics.

Wide bandgap semiconductor aluminum gallium nitride (AlGaN) alloys have established themselves as the key materials for building ultraviolet (UV) optoelectronic and power electronic devices. However, further improvements of device performance are lagging behind largely due to the difficulties in precisely controlling the carrier behavior, both carrier generation and carrier transport within the AlGaN-based devices. Fortunately, it has been discovered that instead of using AlGaN layers with fixed Al content, by grading the Al composition along the growth direction, we are capable of (1) generating high-density electrons and holes via polarization-induced doping; (2) manipulating carrier transport behavior via energy band modulation, or so-called "band engineering". Consequently, such compositionally graded AlGaN alloys have attracted extensive interest as the promising building blocks for efficient AlGaN-based UV light emitters and power electronic devices. In this review, we focus on the unique physical properties of graded AlGaN alloys and highlight the key roles that such graded structures play in device exploration. Firstly, we elaborate on the underlying mechanisms of efficient carrier generation and transport manipulation enabled by the graded AlGaN alloys. Thereafter, recent progress of UV light emitters and power electronic devices incorporating the graded AlGaN structures are comprehensively summarized and discussed. Finally, we outline the prospects associated with the implementation of graded AlGaN alloys in the pursuit of high-performance optoelectronic and power electronic devices.