Anisotropy and thermal properties in GeTe semiconductor by Raman analysis.

Low-symmetric GeTe semiconductors have attracted wide-ranging attention due to their excellent optical and thermal properties, but only a few research studies are available on their in-plane optical anisotropic nature that is crucial for their applications in optoelectronic and thermoelectric devices. Here, we investigate the optical interactions of anisotropy in GeTe using polarization-resolved Raman spectroscopy and first-principles calculations. After determining both armchair and zigzag directions in GeTe crystals by transmission electron microscopy, we found that the Raman intensity of the two main vibrational modes had a strong in-plane anisotropic nature; the one at ∼88.1 cm-1 can be used to determine the crystal orientation, and the other at ∼124.6 cm-1 can reveal a series of temperature-dependent phase transitions. These results provide a general approach for the investigation of the anisotropy of light-matter interactions in low-symmetric layered materials, benefiting the design and application of optoelectronic, anisotropic thermoelectric, and phase-transition memory devices based on bulk GeTe.

Deep defects limiting the conversion efficiency of Sb2Se3 thin-film solar cells.

Quasi-one-dimensional (Q1D) semiconductor antimony selenide (Sb2Se3) shows great potential in the photovoltaic field, but the photoelectric conversion efficiency (PCE) of Sb2Se3-based solar cells has shown no obvious breakthrough during the past several years, of which the intrinsic reasons are pending experimentally. Here, we prepare high-quality Q1D Sb2Se3 thin films via the vapor transport deposition technique. By investigating the bandedge electronic level structure and carrier relaxation/recombination dynamics, we find that (i) the optimized Se-rich growth conditions can highly improve the crystal quality of the Q1D Sb2Se3 thin films, the carrier lifetime of which is substantially increased up to ∼8.3 µs; (ii) the Se-rich growth conditions have advantages to annihilate the deep selenium vacancies VSei (i = 1 and 3 for non-equivalent Se atomic sites) but is not effective for the deep donor VSe2, which locates at ∼0.3 eV (300 K) below the conduction band and intrinsically limits the PCE value of devices below ∼7.63%. This work suggests that further optimizing the Se-rich conditions to technically eliminate this kind of deep defect is still essential for preparing high-performance Sb2Se3 film solar cells.

Abnormal vibrational anisotropy and thermal properties of a two-dimensional GeAs semiconductor.

Anisotropy in a crystal structure plays a striking role in determining the optical, electrical and thermal properties of the condensed matter. Here, we investigated in-plane vibrational anisotropy in a two-dimensional (2D) van der Waals (vdW)-layered GeAs narrow-gap semiconductor by combining microstructural characterization and polarization Raman spectroscopy. Interestingly, not only the intensities but also the Raman shifts in all modes evolved periodically with different symmetries as the polarization angle changed continuously, which could be well-analyzed using the Raman tensors and further interpreted from the phonon dispersion relations. More importantly, the temperature-dependent Raman intensities of the Raman modes in the range from 83 K to 823 K gave a thermal-related uniform constant, based on which key parameters, including the thermal expansion coefficient, Grüneisen constant and quasi-particle lifetime, could be directly derived, which were in line with the calculated predictions. This investigation provides a comprehensive understanding of structure-dependent optical anisotropy in 2D vdW-layered GeAs and suggests a new idea for exploring the thermal properties of related materials using temperature-dependent Raman spectroscopy.

Modulation of Ferroelectric and Optical Properties of La/Co-Doped KNbO3 Ceramics.

The phase transition, microscopic morphology and optical and ferroelectric properties are studied in a series of La- and Co-doped KNbO3-based ceramics. The results show that the doping induces the transformation from the orthorhombic to the cubic phase of KNbO3, significantly reduces the optical bandgap and simultaneously evidently improves the leakage, with a slight weakening of ferroelectric polarization. Further analysis reveals that (i) the Co doping is responsible for the obvious reduction of the bandgap, whereas it is reversed for the La doping; (ii) the slight deterioration of ferroelectricity is due to the doping-induced remarkable extrinsic defect levels and intrinsic oxygen vacancies; and (iii) the La doping can optimize the defect levels and inhibit the leakage. This investigation should both provide novel insight for exploring the bandgap engineering and ferroelectric properties of KNbO3, and suggest its potential applications, e.g., photovoltaic and multifunctional materials.