Tailoring the Electronic Properties of Monoclinic (InxAl1-x)2O3 Alloys via Substitutional Donors and Acceptors

TL;DR

This work uses first-principles density functional theory with the HSE06 hybrid functional to map defect energetics, ionization levels, and defect concentrations for a range of donors and acceptors in $(In_xAl_1-x)_2O_3$ alloys. The results show that transition-metal donors such as $Hf$ and $Zr$ offer favorable n-type doping, especially under oxygen-poor conditions and low Al content, while Si, Ge, Sn, and C exhibit varying behavior and potential deep-level formation as Al content increases. Acceptors Mg, Zn, and Cu can compensate unintentionally doped material but do not enable practical p-type conduction, highlighting the persistent challenge of p-doping in ultra-wide-bandgap oxides and motivating heterojunction strategies (e.g., with NiO) for bipolar devices. The findings provide a roadmap for engineering $(In_xAl_1-x)_2O_3$-based power electronics, balancing dopant chemistry, alloy composition, and growth conditions to optimize n-type performance and interface rectification.

Abstract

Ultra-wide bandgap semiconductors such as \b{eta}-Ga2O3 are ideal materials for next-generation power electronic devices. Electronic and mechanical properties of \b{eta}-Ga2O3 can be tuned by alloying with other sesquioxides, notably Al2O3 and In2O3. Moreover, by tuning the In content of a (InxAl1-x)2O3 alloy, its lattice constants can be matched to those of Ga2O3, while preserving a large conduction-band offset. In view of potential applications to \b{eta}-Ga2O3-based heterostructure, we performed atomistic modelling of (InxAl1-x)2O3 alloys using density functional theory to investigate thermodynamic and electrical properties of conventional group IV dopants (Si, Sn, C, Ge), alternative metal donors (Ta, Zr, Hf), and acceptors (Mg, Zn, Cu). The hybrid Heyd-Scuseria-Ernzerhof functional (HSE06) is used to accurately quantify the defect formation energies, ionization levels, and concentrations over a wide range of experimentally relevant conditions for the oxygen chemical potential and temperature. In our atomistic models, Hf and Zr show favourable properties as alternative donors to Si and other group IV impurities, especially under oxygen-poor conditions. Our findings also suggest that acceptors Mg, Zn, and Cu, while they cannot promote p-doping, can be still beneficial for the compensation of unintentionally n-doped materials, e.g., to generate semi-insulating layers and improve rectification.

Figures (12)

• Figure 1: (a) Conventional monoclinic unit cell showing non-equivalent tetrahedral (I) and octahedral (II) configurations. (b) Supercell representation of a $1 \times 3 \times 2$ expanded conventional unit cell containing one Hf substitutional impurity at an octahedral site (II).
• Figure 2: Formation energies of substitutional defects C, Ge, Si, and Sn (in blue, orange, green, and red, respectively) in Al$_2$O$_3$ (a, d), (In_0.5Al_0.5)_2O3 (b, e), and In$_2$O$_3$ (c, f) as a function of the Fermi level. Panels (a, c, d) correspond to oxygen-rich conditions, while panels (b, e, f) correspond to oxygen-poor conditions. The faded lines indicate regions where the defect charge states are unstable. Chemical potentials are reported at the top of the panels. For the sake of simplicity, the values are given with respect to their standard state, e.g., $\mu_O=0.00$ means extreme oxygen rich condition, see SI.
• Figure 3: Temperature sensitivity of the defect concentration of substitutional dopants C, Si, Ge, and Sn in (a, c) $(\text{In}_x\text{Al}_{1-x})_2\text{O}_3$ for $x = 0.5$, and (b, d) $x = 1$ over an indicative range of temperature. Panels (a and b) correspond to oxygen-rich conditions, while (c and d) correspond to oxygen-poor conditions.
• Figure 4: First charge-state transition levels (+/0) or (+/−) for the C, Ge, Si, and Sn dopants in (In$_x$Al$_{1-x}$)$_2$O$_3$ alloy. Explicit calculations for $x=0$, $0.5$, and $1$ are indicated with points, which are connected by linear interpolation. Black crosses represent the calculated indirect bandgap values, while the dashed black line shows a quadratic fit of the bandgap. The subscripts 'I' and 'II' denote the tetrahedral and octahedral positions, respectively.
• Figure 5: The calculated charge-state transition levels for group IV substitutional dopants in (In$_x$Al$_{1-x}$)$_2$O$_3$ alloys, for $x=0$ (left), $x=0.5$ (center), and $x=1$ (right). Numerical values are provided in Supplementary Table S3.