Shuai-Wei Fan

The functionality of electronic devices critically depends on high-quality p-n junctions, which fabrication in turn relies on bipolar semiconductors. Till now, bipolar wide gap semiconductors remain severely scarce due to the inherent limitations in achieving balanced n-type and p-type electrical conductivity. In this work, we utilize the band engineering scheme to screen the bipolar wide gap semiconductors in ternary chalcogenides LiAX2 (A = Al, Ga and In; X = S and Se). Band alignment and ionization energy analyzes identify wide gap semiconductor LiInSe2 as the optimal candidate among compounds LiAX2, as its band edges strictly satisfy the empirical doping thresholds, with exceptionally low acceptor and donor ionization energies. Defect analysis further confirms LiInSe2 has good bipolar doping. The primary n (p)-type defects are Br (Cd) substituting Se (In). And the electron (hole) mobility reaches 202.3 (14.1) cm2/V·s. As carriers' density reaches 1019 cm-3, the n (p)-type electrical conductivity exceeds 104 (103) S/m. In addition, LiInSe2 also exhibits high visible light transparency and superior thermal conductivity. This study clarifies the fundamental properties of LiInSe2, and opens up new avenues for exploration of bipolar wide gap semiconductors.

LiYS2, as a wide gap semiconductor, suffers from the flat valence band maximum, which is characterized by the large hole effective mass that limits hole mobility and p-type electrical conductivity. To address this issue and effectively modulate its electronic properties, band engineering strategy is proposed for LiYS2 by forming the highly mismatched LiY(S1-xOx)2 alloys with LiYO2. Systematic computational results confirm oxygen significantly reconstructs the electronic structures of LiY(S1-xOx)2 alloys. Especially, when the oxygen composition reaches 25%, this reconstruction dramatically reduces the hole effective mass, and relocates the conduction band maximum. Notably, the LiY(S0.75O0.25)2 alloy achieves a direct wide band gap (3.15 eV) with dispersive band edges and exhibits promising p-type electrical conductivity. These favorable properties make LiY(S0.75O0.25)2 be a potential candidate for transparent conducting materials.

Wide gap semiconductors (WGS) are crucial for optoelectronics. Till now, high-performance p-type WGSs remain scarce compared to the maturity of n-type counterparts. Herein, we investigate the electronic structure, doping, and hole transport properties of Ba2GeSe4 by using the optimized Heyd-Scuseria-Ernzerhof hybrid functional and transport theory. Obtained results demonstrate p-type doping is achievable in Ba2GeSe4 due to its high valence band maximum. Cs substituting Ba is identified as an effective p-type defect, yielding maximum hole mobility of 17.0 cm2/V·s and room-temperature conductivity of 5.2×103 S/m at hole density of 3.0×1019 cm-3. These findings highlight the potential of Ba2GeSe4 for transparent electronics.

Panel (a): atomic orbital energies of group VI _A elements. Panel (b): VB composition of M–Ch mixed state. Panel (c): VBM position relative to vacuum level. Panel (d): bandgap for AgMCh ₂ . Panel (e): formation energies for main native defects in AgAlS ₂ .

Commercially available transparent conducting materials (TCMs) are typically n-type, and high-performance p-type TCMs are rare, which would impede the development of optoelectronics. Previous studies have shown that Ba₂SiSe₄ possesses a wide band-gap, low hole effective mass and high transmittance in the visible light region [R. Kormath Madam Raghupathy et al., Chem. Mater., 2018, 30, 6794–6800]. These characteristics imply that Ba₂SiSe₄ is a potential p-type TCM. However, research studies on its electrical conductivity are limited. In this work, p-type defects are screened based on HSE hybrid functional calculations. We find that Cs substituting Ba(Cs_Ba) is an ideal p-type defect with a transition energy (ε(0/−)) of 0.081 eV above the valence band maximum. Under the thermodynamic equilibrium fabrication scheme, Cs₂Se is the ideal dopant source for Cs dopants, and Se-rich, Ba (Si)-poor conditions are necessary to fabricate Cs_Ba defects. When doped samples are quenched from the preparation temperature to room temperature, their hole density reaches 4.04 × 10¹⁷ cm⁻³, and their p-type electrical conductivity reaches 32.3 S m⁻¹. When a non-equilibrium fabrication scheme is considered, as the hole density reaches 10²⁰ cm⁻³, the corresponding p-type electrical conductivity exceeds 10⁴ S m⁻¹. These results indicate that Ba₂SiSe₄ is a promising p-type TCM, which is valuable in developing high-performance transparent electronic devices.

Transparent conducting materials (TCMs) are vital in transparent electronics. Previous investigations show that ternary telluride α-LiAlTe₂ possesses a wide band gap of 3.13 eV, a smaller hole-effective mass (0.69m₀) and high transmittance in the visible light region [S. Kang et al., Phys. Scr., 2024, 99, 035923]. These characteristic features imply that α-LiAlTe₂ is a potential TCM. However, the research on its electrical conductivity is still lacking. In this work, we utilize a hybrid functional method (Heyd–Scuseria–Ernzerhof (HSE06)) to screen the possible binary impurity phase. Under the thermodynamic equilibrium conditions, the regions for the stable phase of α-LiAlTe₂ are demarcated. The effects of the intrinsic defects and the possible p-type extrinsic defects are evaluated. We find that the intrinsic defects Li_int and Al_int can induce n-type conductivity. Substitution defects Mg_Al, Ca_Al, N_Te, P_Te and As_Te are p-type defects. Both Mg_Al and N_Te are shallow acceptor defects with the transition energy levels ε(0/−) at 0.24 eV and 0.21 eV above the valence band maximum (VBM). Under Te-rich and Li-relatively poor conditions, the acceptor defect N_Te possesses a lower formation energy of 0.35 eV, implying that defect N_Te could be fabricated by using the thermodynamic equilibrium fabrication method. At room temperature, the p-type electrical conductivity exceeds 60 800 S m⁻¹ when the hole density reaches 10²⁰ cm⁻³. These results indicate that α-LiAlTe₂ is a promising candidate for optoelectronic applications.