Disorder effects on the density of states of the II-VI semiconductor alloys Hg0.5Cd0.5Te, Cd0.5Zn0.5Te, and Hg0.5Zn0.5Te

Great reduction of the hole effective mass in wide bandgap semiconductors by highly mismatched alloying

Many wide bandgap semiconductors suffer from a large hole effective mass, and inherent defects severely limit their performance. LiGaSe2, as a direct wide bandgap selenide semiconductor, also faces such challenges. In this work, we present a strategy for valence band engineering of LiGaSe2 through high mismatch O-alloying. Hybrid functional calculations show that LiGa(Se1-xOx)2 alloys can greatly reduce their hole effective mass, drastically improving the hole mobility. Specifically, at x = 6.25%, the hole effective mass of the alloy along the Γ-Y direction is only 0.295m0, indicating an approximate 80% reduction compared to LiGaSe2. This physically counterintuitive reduction can be attributed to the introduction of a small amount of O-2p orbitals into the valence band, which strongly overlaps with Ga-3d orbitals, forming strong p-d hybridization. Furthermore, the band anticrossing interaction between the O-2p orbitals and the original orbitals pulls down the conduction band, reducing the band gap of the LiGa(Se0.9375O0.0625)2 alloy to 3.037 eV, which is sufficient to maintain excellent visible light transparency. These findings highlight the potential of semiconductor LiGa(Se1-xOx)2 alloys as transparent conducting materials and offer a novel solution for other similar wide bandgap semiconductors.

Atomic-Level Sn Doping Effect in Ga2O3 Films Using Plasma-Enhanced Atomic Layer Deposition

In this work, the atomic level doping of Sn into Ga₂O₃ films was successfully deposited by using a plasma-enhanced atomic layer deposition method. Here, we systematically studied the changes in the chemical state, microstructure evolution, optical properties, energy band alignment, and electrical properties for various configurations of the Sn-doped Ga₂O₃ films. The results indicated that all the films have high transparency with an average transmittance of above 90% over ultraviolet and visible light wavelengths. X-ray reflectivity and spectroscopic ellipsometry measurement indicated that the Sn doping level affects the density, refractive index, and extinction coefficient. In particular, the chemical microstructure and energy band structure for the Sn-doped Ga₂O₃ films were analyzed and discussed in detail. With an increase in the Sn content, the ratio of Sn-O bonding increases, but by contrast, the proportion of the oxygen vacancies decreases. The reduction in the oxygen vacancy content leads to an increase in the valence band maximum, but the energy bandgap decreases from 4.73 to 4.31 eV. Moreover, with the increase in Sn content, the breakdown mode transformed the hard breakdown into the soft breakdown. The C-V characteristics proved that the Sn-doped Ga₂O₃ films have large permittivity. These studies offer a foundation and a systematical analysis for assisting the design and application of Ga₂O₃ film-based transparent devices.

Antagonism between Spin-Orbit Coupling and Steric Effects Causes Anomalous Band Gap Evolution in the Perovskite Photovoltaic Materials CH3NH3Sn1-xPbxI3

Halide perovskite solar cells are a recent ground-breaking development achieving power conversion efficiencies exceeding 18%. This has become possible owing to the remarkable properties of the AMX3 perovskites, which exhibit unique semiconducting properties. The most efficient solar cells utilize the CH3NH3PbI3 perovskite whose band gap, Eg, is 1.55 eV. Even higher efficiencies are anticipated, however, if the band gap of the perovskite can be pushed deeper in the near-infrared region, as in the case of CH3NH3SnI3 (Eg = 1.3 eV). A remarkable way to improve further comes from the CH3NH3Sn1-xPbxI3 solid solution, which displays an anomalous trend in the evolution of the band gap with the compositions approaching x = 0.5 displaying lower band gaps (Eg ≈ 1.1 eV) than that of the lowest of the end member, CH3NH3SnI3. Here we use first-principles calculations to show that the competition between the spin-orbit coupling (SOC) and the lattice distortion is responsible for the anomalous behavior of the band gap in CH3NH3Sn1-xPbxI3. SOC causes a linear reduction as x increases, while the lattice distortion causes a nonlinear increase due to a composition-induced phase transition near x = 0.5. Our results suggest that electronic structure engineering can have a crucial role in optimizing the photovoltaic performance.