Session Index

Polarization fields due to spontaneous and piezoelectric polarization are a distinguishing feature of nitride semiconductors. They are exploited in transistors to create high-density carrier gases, while in light emitters they need to be controlled to avoid efficiency loss. The impact of polarization on growth of nitrides has received less attention, however. We have recently developed a first-principles methodology that enables us to rigorously address polarization fields in surface calculations. The results show that spontaneous polarization affects the stability and nature of surface reconstructions. Compensation of the polarization fields requires the presence of significant amounts of charge, which manifests in the form of fixed charge associated with surface reconstructions or the presence of heterovalent impurities. Consequences for defect incorporation will be discussed.
Work performed in collaboration with C. E. Dreyer, J. Neugebauer, M. Todorova, S. H. Yoo, and D. Vanderbilt.
[1] S. H. Yoo, M. Todorova, J. Neugebauer, and C. G. Van de Walle, Phys. Rev. Appl. 19, 064037 (2023).

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Annealing behaviors of vacancy-type defects in low-dose Si-implanted GaN were studied by positron annihilation spectroscopy. Si ions were implanted into GaN with a concentration of 1E12 cm‒3. From measurements of Doppler broadening spectra of the annihilation radiations, the major defect species for these samples were determined to be Ga-vacancy-related defects, such as divacancies. After annealing above 900C, the net donor concentration was higher than the concentration of implanted Si. The presence of vacancy-type defects detected by positron annihilation suggests that vacancies were the origin of donor-like defects introduced by post-implantation annealing.

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This work analyzes the effects of the GaN substrate quality and p-layer doping profile on the performance of p-i-n homojunction avalanche photodiodes (APDs). Single-step p-type doped GaN APDs were grown by MOCVD on three different (0001) n-type free-standing and bulk GaN substrates. Besides, APDs with a two-step p-type doped layer were evaluated. All APDs were fabricated into shallow-beveled mesas to evaluate their optical gain and spectral responsivity. Substrate comparison revealed improved electrical characteristics for devices grown on substrates with better crystal quality, surface morphology and lower threading dislocations, while implementation of a two-step p-layer improved responsivity without degrading electrical performance.

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We demonstrated the high-temperature annealing (HTA) of MBE-grown AlN films on sapphire annealed from 1200 °C to 1625 °C. Post-HTA, AlN films exhibited smooth surface morphology with RMS roughness below 0.3 nm over 4 µm² areas, and improved crystal quality, indicated by the reduced FWHM and threading dislocation density (TDD) values. The lowest TDD, 2.5 × 10⁹ cm⁻² was achieved for 300 nm film at 1625 °C, highlighting the role of thickness in minimizing TDD. Additionally, MOCVD regrowth on MBE-grown AlN templates was assessed, resulting in a smooth surface with RMS roughness reduced to 0.1 nm over 4 µm² areas, showing the epi-ready quality of the templates.

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First-principles Density Functional Theory (DFT) and phonon calculations were performed for the AlGaN(0001) surface under Metalorganic Vapor Phase Epitaxy (MOVPE) growth conditions. Thermodynamically stable surface reconstructions were identified as a function of temperature and the ammonia-to-hydrogen ratio. Potential Energy Surface (PES) maps for Al and Ga adatoms revealed energetically favorable adsorption sites and diffusion pathways. Nudged Elastic Band (NEB) calculations determined migration barriers, with Al atoms exhibiting stronger binding than Ga. Phonon-based free-energy corrections confirmed stability trends at high growth temperatures.

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We study the formation of acceptor-type charge carrier traps in AlN and high Al-content AlGaN alloys. With positron annihilation experiments we observe formation of high concentrations of native vacancies and their complexes with oxygen impurities likely contributing to the overall electrical compensation in AlN. Moreover, silicon-doped AlGaN samples with 90% Al exhibit transition of Si donors to a deep acceptor Si DX state. Finally, first principles calculations on the interactions between silicon and native defects and impurities are undergo.

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We investigate the impact of native oxide removal, via argon ion milling, on the electrical properties of ohmic contacts to p-type N-polar GaN. Our findings indicate that N-polar surface is terminated with a native oxide layer, and it can be effectively removed through in situ ion milling. The voltage threshold observed in the linear I-V curve was reduced by 2V for any studied [Mg], while the specific contact resistivity ρc slightly increased. The decrease in voltage threshold highlights the native oxide removal, whereas the increase of ρc is due to compensating defects introduced by ion milling.

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