[Paper Review] Bismuth-doped Ga2O3 as candidate for p-type transparent conducting material

Fernando P. Sabino, Xuefen Cai+2 more|arXiv (Cornell University)|Jun 3, 2019

Ga2O3 and related materialsMaterials Science3 citations

TL;DR

This study proposes bismuth-doped Ga2O3 (Ga1-xBix)2O3 as a p-type transparent conducting oxide by using hybrid density functional theory to show that Bi doping on Ga sites creates a high-lying intermediate valence band from Bi 6s and O 2p orbitals, reducing the band gap and enabling p-type conductivity. The key result is that the intermediate band is sufficiently high in energy to allow effective p-type doping while maintaining a large optical band gap, making it a strong candidate for transparent p-type oxides.

ABSTRACT

Gallium oxide (Ga2O3) is a wide-band-gap semiconductor promising for UV sensors and high power transistor applications, with Baliga's figure of merit that far exceeds those of GaN and SiC, second only to diamond. Engineering its band structure through alloying will broaden its range of applications. Using hybrid density functional calculations we study the effects on adding Bi to Ga2O3. While in III-V semiconductors, such as GaAs and InAs, Bi tend to substitute on the pnictide site, we find that in Ga2O3, Bi prefers to substitute on the Ga site, resulting in dilute (Ga_{1-x}Bi_{x})2O3 alloys with unique electronic structure properties. Adding a few percent of Bi reduces the band gap of Ga2O3 by introducing an intermediate valence band that is significantly higher in energy than the valence band of the host material. This intermediate valence band is composed mainly of Bi 6s and O 2p orbitals, and it is sufficiently high in energy to provide opportunity for p-type doping.

Motivation & Objective

• To investigate the feasibility of achieving p-type conductivity in Ga2O3, a wide-band-gap semiconductor with excellent power electronics properties but poor p-doping due to deep acceptor levels.
• To determine the site preference of Bi in Ga2O3 and assess its impact on the electronic structure and band edge positions.
• To evaluate whether Bi doping can raise the valence band maximum and reduce acceptor ionization energy, thus enabling effective p-type doping.
• To examine the optical properties of (Ga1-xBix)2O3 to confirm that the material remains transparent in the visible range despite band gap reduction.
• To explore the potential of Bi-doped Ga2O3 as a transparent p-type conducting material for UV and power electronic applications.

Proposed method

• Hybrid density functional theory (HSE) calculations were used to compute electronic structures, with a 32% Hartree-Fock exchange mixing to accurately reproduce the 4.70 eV band gap of β-Ga2O3.
• Special quasi-random structures (SQS) with 120-atom supercells were generated to model dilute (Ga1-xBix)2O3 alloys at Bi concentrations of 2.08%, 4.17%, 6.25%, and 12.5%.
• Structure optimizations used the PBESol functional with a 620 eV plane-wave cutoff, while electronic structure calculations used a 470 eV cutoff and HSE functional.
• Projected augmented wave (PAW) potentials were used to describe electron-ion interactions with valence configurations: O:2s22p4, Ga:3d104s24p1, Bi:5d106s26p3.
• Optical properties were calculated from the frequency-dependent dielectric matrix using Kramer-Kronig relations with a 5×10−4 eV Lorentzian broadening.
• The Brillouin zone was sampled with an 8×8×4 k-point mesh for the primitive cell and equivalent density for supercells.

Experimental results

Research questions

• RQ1 Does Bi prefer to substitute on Ga or O sites in β-Ga2O3, and what is the thermodynamic driving force for this preference?
• RQ2 How does Bi doping affect the band gap and the position of the valence band maximum in Ga2O3?
• RQ3 Can the introduction of an intermediate valence band by Bi doping enable effective p-type doping by reducing acceptor ionization energy?
• RQ4 What is the optical transparency range of (Ga1-xBix)2O3, and does it remain transparent in the visible spectrum despite band gap reduction?
• RQ5 Is the minimum energy transition from the intermediate valence band to the conduction band still above the visible range, preserving transparency?

Key findings

• Bi preferentially substitutes on Ga sites in β-Ga2O3, forming dilute (Ga1-xBix)2O3 solid solutions, with a favorable mixing enthalpy that supports alloy formation.
• Bi doping introduces a new intermediate valence band located significantly higher in energy than the original O 2p valence band, primarily composed of Bi 6s and O 2p orbitals.
• The intermediate valence band is sufficiently high in energy to allow for effective p-type doping, as it decouples from deep defect levels and reduces the ionization energy of acceptors.
• The band gap of Ga2O3 is reduced by a few percent of Bi doping due to the formation of this intermediate band, yet the minimum energy transition from the intermediate band to the conduction band remains above the visible range (400–700 nm).
• The calculated optical band gap remains above 2.5 eV, indicating that Bi-doped Ga2O3 retains strong transparency in the visible spectrum.
• The combination of a high-lying valence band and preserved visible transparency makes (Ga1-xBix)2O3 a strong candidate for p-type transparent conducting oxides.