Nitrogen pair-induced temperature insensitivity of the band gap of GaNSb alloys

The temperature dependence of the band gap of GaN x Sb1−x films with x ≤ 1.3% has been studied in the 1.1–3.3 m (0.35–1.1 eV) range using infrared absorption spectroscopy between 4.2 and 300 K. As with other dilute nitride semiconductors, the temperature dependence of the band gap is reduced by alloying with nitrogen when compared to the host binary compound. However, for GaNSb, the smallest variation of the band gap with temperature is observed for samples with the lowest N content for which the band gap is almost totally insensitive to temperature changes. This contrasts with the more widely studied GaN x As1−x alloys in which the band gap variation with temperature decreases with increasing N content. The temperature-dependent absorption spectra are simulated within the so-called band anticrossing model of the interaction between the extended conduction band states of the GaSb and the localized states associated with the N atoms. The N next-nearest neighbor pair states are found to be responsible for the temperature insensitivity of the band gap of the GaNSb alloys as a result of their proximity to the conduction band edge giving them a more pronounced role than in GaNAs alloys

Growth and Characterization of Sb2Se3 Single Crystals for Fundamental Studies

Three methods of growing bulk crystalline samples of Sb2 Se3 to provide material for basic studies have been investigated and preliminary racterization is reported. These growth methods were: A) melt-growth, similar to vertical Bridgman, b) dynamic vapor transport over a temperature gradient (Piper-Polich method) and c) a static vapor method in which the source material is transported in nearly iso-thermal conditions. The melt-growth method produced the largest single crystals (up to 4 mm diameter), while the vapor methods both yielded polycrystalline boules with mm-sized grains. Powder XRD confirmed the boules to comprise orthorhombic Sb2 Se3, having lattice parameters a = 11.7808 Å b = 3.9767 Å and c = 11.6311 Å. Cleavage facets were parallel to (100). Raman peaks at 191 (A g and 211 cm -1 were excited anisotropically. FTIR reflectance features showed some sensitivity to s- and ppolarization

Band gap temperature-dependence and exciton-like state in copper antimony sulphide, CuSbS2

The temperature-dependence of the band gap of the proposed photovoltaic absorber copper antimony sulphide (CuSbS2) has been studied by Fourier-transform infrared spectroscopy. The direct gap rises from 1.608 to 1.694 eV between 300 and 4.2 K. Below 200 K an exciton-like feature develops above the absorption edge at 1.82 eV. First-principles calculations evaluate band structure, band symmetries, and dipole selection rules, suggesting distinctly enhanced absorption for certain excitonic optical transitions. Striking consistency is seen between predicted dielectric and absorption spectra and those determined by ellipsometry, which reveal rapidly strengthening absorption passing 105 cm−1 at 2.2 eV. These results suggest beneficial photovoltaic performance due to strong optical absorption arising from unusually strong electron–hole interactions in polycrystalline CuSbS2 material

Vacancy-Ordered Double Perovskite Cs2TeI6 Thin Films for Optoelectronics

Alternatives to lead- and tin-based perovskites for photovoltaics and optoelectronics are sought that do not suffer from the disadvantages of toxicity and low device efficiency of present-day materials. Here we report a study of the double perovskite Cs2TeI6, which we have synthesized in thin film form for the first time. Exhaustive trials concluded that spin coating CsI and TeI4 using an anti-solvent method produced uniform films, confirmed as Cs2TeI6 by XRD with Rietveld analysis. They were stable up to 250°C, had an optical band gap of ~1.5 eV, absorption coefficients of ~6 x 104 cm-1, carrier lifetimes of ~2.6 ns (unpassivated 200 nm film), a work function of 4.95 eV and had p-type surface conductivity. Vibrational modes probed by Raman and FTIR spectroscopy showed resonances qualitatively consistent with DFT Phonopy-calculated spectra, offering another route for phase confirmation. It was concluded that the material is a candidate for further study as a potential optoelectronic or photovoltaic material

A hard x-ray photoemission study of transparent conducting fluorine-doped tin dioxide

Fluorine-doped tin oxide (FTO) is a commercially successful transparent conducting oxide with very good electrical (resistivities < 1×103 Ω·cm) and optical properties (transmittance > 85%). These properties coupled with cheap and large-scale deposition on float-glass lines means FTO has found commercial use in, for example, low emissivity windows and solar cells. However, despite its widespread application, a detailed understanding is lacking of the doping and defects in FTO. Recent work [1] has suggested that the fluorine interstitial plays a major role in limiting the conductivity of FTO. Here we present synchrotron radiation high energy x-ray photoemission spectroscopy (XPS) of the fluorine 1s core level of FTO films without in situ surface preparation. This probes deeper than standard XPS and shows that the fluorine interstitial is present not just at the surface of the films and is not an artefact of argon ion sputtering for surface preparation

Resonant doping for high mobility transparent conductors: the case of Mo-doped In2O3

Transparent conductors are a vital component of smartphones, touch-enabled displays, low emissivity windows and thin film photovoltaics. Tin-doped In2O3 (ITO) dominates the transparent conductive films market, accounting for the majority of the current multi-billion dollar annual global sales. Due to the high cost of indium, however, alternatives to ITO have been sought but have inferior properties. Here we demonstrate that molybdenum-doped In2O3 (IMO) has higher mobility and therefore higher conductivity than ITO with the same carrier density. This also results in IMO having increased infrared transparency compared to ITO of the same conductivity. These properties enable current performance to be achieved using thinner films, reducing the amount of indium required and raw material costs by half. The enhanced doping behavior arises from Mo 4d donor states being resonant high in the conduction band and negligibly perturbing the host conduction band minimum, in contrast to the adverse perturbation caused by Sn 5s dopant states. This new understanding will enable better and cheaper TCOs based on both In2O3 and other metal oxides

Self-Compensation in Transparent Conducting F-Doped SnO2

The factors limiting the conductivity of fluorine-doped tin dioxide (FTO) produced via atmospheric pressure chemical vapor deposition are investigated. Modeling of the transport properties indicates that the measured Hall effect mobilities are far below the theoretical ionized impurity scattering limit. Significant compensation of donors by acceptors is present with a compensation ratio of 0.5, indicating that for every two donors there is approximately one acceptor. Hybrid density functional theory calculations of defect and impurity formation energies indicate the most probable acceptor-type defects. The fluorine interstitial defect has the lowest formation energy in the degenerate regime of FTO. Fluorine interstitials act as singly charged acceptors at the high Fermi levels corresponding to degenerately n-type films. X-ray photoemission spectroscopy of the fluorine impurities is consistent with the presence of substitutional F O donors and interstitial F i in a roughly 2:1 ratio in agreement with the compensation ratio indicated by the transport modeling. Quantitative analysis through Hall effect, X-ray photoemission spectroscopy, and calibrated secondary ion mass spectrometry further supports the presence of compensating fluorine-related defects