Zincblende GaN Promises High Hole Concentrations
University of Illinois team reveal a bright future for zb- III- nitride research, particularly for bipolar devices
Achieving high n- and p-type doping in GaN materials is essential to increase efficiency and power in solid-state lighting and RF/power electronics. The dopability of GaN is determined by the formation, activation, and self-compensation of dopants, which depend on the bonding nature between dopants and GaN.
GaN forms in wurtzite (wz-) and zincblende (zb-) phases. Yet, the knowledge of n- and p-type doping has been mostly developed on wz-GaN due to its stability and current industrial adaption.
Today, p-type doping of wz-GaN and their alloys remain challenging, particularly for reaching high (>1019 cm-3) hole concentrations. The main factor in the p-doping limitation has been attributed to the large activation energies of common acceptors (e.g. Mg with an activation energy of ~ 250 meV). Compared to the low (< 30 meV) activation energies of donors, this asymmetry between electron and hole concentrations limits performance of GaN photonics (e.g.LEDs, laser diodes) and electronics (e.g. RF/power transistors).
In Computational Materials Science 190 (2021) 110283, University of Illinois researchers led by Cam Bayram, associate professor of Electrical and Computer Engineering, Innovative COmpound semiconductoR LABoratory (ICORLAB), use first-principles calculations to study formation, activation, and self-compensation of Si, Ge, C, Be, and Mg for wz- and zb- GaN.
The results reveal impacts of symmetricity not only on the activation energies but also on the formation energies and self-compensation effects. Specifically, Mg activation energy is reduced to ~ 153 meV and vibrational analysis suggests that Mgi compensating donor is less favourable to form in zb- than wz-GaN, since the higher symmetricity of the interstitial site in zb- GaN renders a much smaller vibrational entropy. The authors estimate fourfold higher hole concentration is achievable in zincblende (w.r.t. wurtzite) GaN. These results promise a bright future for the zb- III- nitride research, particularly for bipolar devices.
The figure above shows formation energies of MgGa (acceptor) and Mgi (double donor) in wz-GaN (left) and zb-GaN (right). The reference lines highlight the activation energies (EA).
'Mitigate Self-Compensation with High Crystal Symmetry: A First-Principles Study of Formation and Activation of Impurities in GaN' by Y.-C. Tsai and C. Bayram; Computational Materials Science 190 (2021) 110283.