(Invited) Deep Level Defects in AlN Studied By UV-Visible Spectroscopy
Authors: Dalmau Rafael , Kirby Samuel , Britt Jeffrey , Schlesser Raoul 
Published: November 2022

The ultrawide-bandgap (UWBG) AlGaN alloy system is emerging as a promising material for next generation power semiconductor devices. The increase in bandgap as the alloy composition is varied from the binary endpoints GaN to AlN leads to an increase in the critical electric field, which is a key parameter determining the performance of power semiconductor devices. As development of GaN and SiC materials for high-frequency and high-power devices reaches a state of maturity, pursuit of improved device performance is generating interest in UWBG materials with larger bandgaps. High Al composition AlGaN alloys, in addition to offering increased critical electric fields, offer prospects for improved device performance in high temperature operation. Fundamental challenges to the development of AlGaN-based devices, such as controllable doping, electrical contacts, and effective passivation, remain, but a variety of power devices have already been demonstrated (1-3).
Point defects strongly affect the material properties of semiconductors, with important consequences for device performance. In traditional semiconductors, such as Si, point defects have been studied extensively, and point defect engineering methods have been developed to enhance device performance. However, the properties of point defects in UWBG materials are not as well understood. Point defects may be introduced during device growth, fabrication, and operation. On the one hand, intentionally introduced impurities (dopants) are critical to the control of electrical transport, while on the other hand, unintentionally introduced impurities may act deleteriously as carrier traps or recombination centers. In AlGaN, compensation by deep level, native point defects or unintentionally incorporated impurities limits achievable free carrier concentrations in electronic devices, while defects and their complexes affect the optical properties in optoelectronic devices. Since point defects exert such a strong influence on material properties, and are often responsible for device degradation, knowledge of their formation mechanisms and material effects are essential to develop strategies for the required control of point defect concentrations (4).
In this work, we studied the optical signatures of deep level defects in single crystal AlN substrates. AlN possesses a high thermal conductivity and a close lattice match to high Al composition alloys, which make it an excellent substrate choice for growth of AlGaN-based power electronic and optoelectronic devices. High-quality, 2-inch diameter AlN substrates with average threading dislocation densities below 103 cm-2 were recently demonstrated (5). However, high optical absorption in the ultraviolet-C (UV-C) region was observed in AlN substrates grown by physical vapor transport (PVT), due to a deep level absorption band related to the carbon impurity. This absorption band negatively impacts the efficiency of UV-C optoelectronic devices that require light propagation through the substrate. In order to reduce the unwanted UV-C absorption, we studied the optical properties of double-side polished, 2-inch, c-plane AlN substrates by ultraviolet-visible (UV-Vis) spectroscopy and developed strategies for point defect control. Spatially uniform absorption coefficients below 30 cm-1 at 265 nm were demonstrated across 2-inch substrates (6). In this talk, UV-Vis absorption, photoluminescence (PL) emission, and PL excitation spectra will be presented and correlated with measured impurity concentrations from secondary ion mass spectrometry (SIMS) data, in order to identify deep level defects in AlN. Finally, the mechanisms for reduction of UV-C absorption in PVT AlN will be discussed.
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