(Invited) Polarization-Induced Doping in Graded AlGaN Epilayers Grown on AlN Single Crystal Substrates
Authors: Dalmau Rafael , Moody Baxter 
Published: February 2020

Wide-bandgap nitride semiconductor-based lateral and vertical power switching devices are reaching a mature state and gaining increasing commercial acceptance. This is due to the nitrides’ outstanding properties, including high critical electric fields, mobilities, and saturation velocities, and to progress made in the understanding of key issues, such as controllable doping and polarization engineering. Since many of the figures of merit for power device performance scale with the critical electric field, itself a strong function of the bandgap, the ultrawide-bandgap (UWBG) materials, with bandgaps beyond the 3.4 eV of GaN, have the potential for improved power device performance [1]. However, research and development into these emerging materials is still at a relatively immature stage. Significant challenges remain in the growth of high Al composition AlGaN, including efficient n-type doping at very high Al compositions, and, even more so, p-type doping in general. In spite of these challenges, high-quality, controllably-doped AlGaN epilayers grown on AlN single crystal substrates have been demonstrated in devices, including UV emitters [2] and Schottky diodes [3]. Improved approaches to doping the high Al composition AlGaN layers required for operation of these devices will lead to increased device performance. AlGaN epilayers are commonly doped with substitutional impurities, typically Si for n-type and Mg for p-type, which require thermal activation to generate free carriers. As the Al composition, x, in AlxGa1-xN increases, the Mg impurity activation energy increases, while the Si impurity undergoes a DX transition beyond x~ 0.84, leading to rapidly decreasing free carrier concentrations in high Al content layers. In contrast to this approach, doping in compositionally graded AlGaN epilayers by taking advantage of the distributed polarization charge has been demonstrated for both n-type [4] and p-type [5] doping. The polarization-doped layers thus formed exhibit high sheet carrier densities and are resistant to carrier freeze-out at low temperatures, since they do not depend on thermal activation. We have investigated graded AlGaN epilayers grown on Al-polar AlN single crystal substrates by metal-organic chemical vapor deposition (MOCVD), and measured n-type or p-type doping in increasing or decreasing Al composition layers, respectively. The structural quality and compositional grading were assessed by high-resolution x-ray diffraction (HRXRD) and secondary ion mass spectrometry (SIMS). The space charge profile in an n-type layer graded from x= 0.66 to x= 1.0 was calculated from capacitance-voltage (CV) measurements of a Schottky diode structure. Furthermore, the temperature-dependent resistivity, carrier concentration, and mobility in p-type layers was determined by Hall effect measurements using the Van der Pauw method, and a room-temperature resistivity of 1 Ωcm was obtained for a p-type layer graded from x= 1.0 to x= 0.36. This talk will review MOCVD growth and characterization of both n-type and p-type doped, compositionally graded AlGaN epilayers by HRXRD, SIMS, CV, and Hall effect measurements. Results of integrating these layers into UWBG device structures will also be discussed.
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