First-principles Defect Calculations Research Articles

Perspective on electrically active defects in β-Ga2O3 from deep-level transient spectroscopy and first-principles calculations

The ultra-wide bandgap of gallium oxide provides a rich plethora of electrically active defects. Understanding and controlling such defects is of crucial importance in mature device processing. Deep-level transient spectroscopy is one of the most sensitive techniques for measuring electrically active defects in semiconductors and, hence, a key technique for progress toward gallium oxide-based components, including Schottky barrier diodes and field-effect transistors. However, deep-level transient spectroscopy does not provide chemical or configurational information about the defect signature and must, therefore, be combined with other experimental techniques or theoretical modeling to gain a deeper understanding of the defect physics. Here, we discuss the current status regarding the identification of electrically active defects in beta-phase gallium oxide, as observed by deep-level transient spectroscopy and supported by first-principles defect calculations based on the density functional theory. We also discuss the coordinated use of the experiment and theory as a powerful approach for studying electrically active defects and highlight some of the interesting but challenging issues related to the characterization and control of defects in this fascinating material.

NiGa2O4 interfacial layers in NiO/Ga2O3 heterojunction diodes at high temperature

NiO/Ga2O3 heterojunction diodes have attracted attention for high-power applications, but their high temperature performance and reliability remain underexplored. Here, we report the time evolution of the electrical properties in the widely studied p-NiO/n-Ga2O3 heterojunction diodes and formation of NiGa2O4 interfacial layers at high temperatures. Results of our thermal cycling experiment show an initial leakage current increase which stabilizes after sustained thermal load, due to reactions at the NiO–Ga2O3 interface. High-resolution TEM microstructure analysis of the devices after thermal cycling indicates that the NiO–Ga2O3 interface forms a ternary compound at high temperatures, and thermodynamic calculations suggest the formation of the spinel NiGa2O4 layer between NiO and Ga2O3. First-principles defect calculations find that NiGa2O4 shows low p-type intrinsic doping and hence can serve to limit electric field crowding at the interface. Vertical NiO/Ga2O3 diodes with intentionally grown ∼5 nm thin spinel-type NiGa2O4 interfacial layers show an excellent device ON/OFF ratio of >1010 (± 3 V), VON of ∼1.9 V, and increased breakdown voltage of ∼1.2 kV for an initial unoptimized 300 μm diameter device. These p–n heterojunction diodes are promising for high-voltage, high temperature applications.

Regulation of Hole Concentration and Mobility and First-Principle Analysis of Mg-Doping in InGaN Grown by MOCVD.

This work studied the regulation of hole concentration and mobility in p-InGaN layers grown by metalorganic chemical vapor deposition (MOCVD) under an N-rich environment. By adjusting the growth temperature, the hole concentration can be controlled between 6 × 1017/cm3 and 3 × 1019/cm3 with adjustable hole mobility from 3 to 16 cm2/V.s. These p-InGaN layers can meet different requirements of devices for hole concentration and mobility. First-principles defect calculations indicate that the p-type doping of InGaN at the N-rich limiting condition mainly originated from Mg substituting In (MgIn). In contrast with the compensation of nitrogen vacancy in p-type InGaN grown in a Ga-rich environment, the holes in p-type InGaN grown in an N-rich environment were mainly compensated by interstitial Mg (Mgi), which has very low formation energy.

Self-consistently determining structures of charged defects and defect ionization energies in low-dimensional semiconductors

Determination of defect ionization energy in low-dimensional semiconductors has been a long-standing unsolved problem in first-principles defect calculations because the commonly used methods based on jellium model introduce an unphysical charge density uniformly distributed in the material and vacuum regions, causing the well-known divergence issue of charged defect formation energies. Here in this work, by considering the physical process of defect ionization, we propose a charge correction method based on jellium model to replace the unphysical jellium background charge density with the band edge charge density to deal with charged defects. We demonstrate that, our method is physically meaningful, quantitatively accurate and technically simple to determine the defect ionization energies, thus solving the long-standing problem in defect calculations. Our proposed method can be applied to any dimensional semiconductors.

Experimental and theoretical study on device-processing-incorporated fluorine in AlGaN/GaN heterostructures

This paper reports on an in-depth research on the etching-process induced current degradation in AlGaN/GaN heterostructures by experimental research and theoretical calculations. The channel current degrades by 60% after the etching process and recovers to 90% after thermal annealing at 400 °C. It is found that fluorine is incorporated into the heterostructures of both AlGaN and GaN layers during the etching process. The degradation and recovery of the current is accompanied by the change in the concentration of fluorine in both AlGaN and GaN layers during the experiment. First-principles defect calculations of fluorine in GaN indicate that the dominant defect configuration of F is negatively charged interstitial (Fi−) with the lowest formation energy. The accompanied technology computer-aided design simulation of the energy bands reveals that Fi− raises the conduction band minimum of GaN and, thus, decreases the density of the two-dimensional electron gas, causing degradation of the channel current. This result leads to the post-etching recovery treatment without a high temperature process and ohmic contact degradation.

Discovery of n-Type Zintl Phases RbAlSb4, RbGaSb4, CsAlSb4, and CsGaSb4

Zintl phases are excellent candidates for high-efficiency thermoelectrics (TEs) due to their extremely low lattice thermal conductivity. The manufacturing of an all-Zintl module is particularly attractive for practical applications, as it alleviates concerns regarding the electronic, thermal, and mechanical compatibility of the p- and n-type legs. To date, a large majority of Zintl phases have been realized as p-type TE materials. Our recent discovery of n-type transport in Ba-doped KAlSb4 and KGaSb4 has helped demonstrate the potential of n-type Zintl thermoelectrics. In this paper, we report the experimental discovery of 4 ABX4 Zintl phases: RbAlSb4, RbGaSb4, CsAlSb4, and CsGaSb4. Transport measurements on Ba-doped RbGaSb4 and CsGaSb4 demonstrate near glassy lattice thermal conductivity (<0.5 W m–1 K–1, 350 °C) and lightly doped n-type transport. However, the doping efficiency of Ba in RbGaSb4 and CsGaSb4 is significantly impeded when compared to our prior work on KGaSb4. To investigate the underlying mechanism, we performed first-principles defect calculations and found that the effect of compensating alkali metal vacancies increases in the Rb- and Cs-based analogues. Considering the TE potential of the known ABX4 n-type materials, we have also performed a computational survey over 27 plausible compositions where A = (K, Rb, Cs), B = (Al, Ga, In), and X = (As, Sb, Bi) to investigate the effect of chemistry on potential TE performance.

Effective n-type doping of Mg3Sb2 with group-3 elements

The recent discovery of high thermoelectric performance in Mg3Sb2 has been critically enabled by the success in n-type doping of this material, which is achieved under Mg-rich growth conditions, typically with chalcogens (Se and Te) as extrinsic dopants. Using first-principles defect calculations, we previously predicted that higher electron concentrations (∼1020cm−3) can be achieved in Mg3Sb2 by doping with La instead of Se or Te [P. Gorai et al., J. Mater. Chem. A 6, 13806 (2018)]. Subsequent experiments [K. Imasato et al., J. Mater. Chem. A 6, 19941 (2018)] showed that free electron concentration in La-doped Mg3Sb2−xBix indeed exceeds those in the Te-doped material. Herein, we further investigate n-type doping of Mg3Sb2 and predict that, in addition to La, other group-3 elements (Sc and Y) are also effective as n-type dopants; Y is as good as La while Sc slightly less. Overall, we find that doping with any group-3 elements should lead to higher free electron concentrations than doping with chalcogens.