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

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

Sn 5 s 2 lone pairs and the electronic structure of tin sulphides: A photoreflectance, high-energy photoemission, and theoretical investigation

The effects of Sn 5 s lone pairs in the different phases of Sn sulphides are investigated with photoreflectance, hard x-ray photoemission spectroscopy (HAXPES), and density functional theory. Due to the photon energy-dependence of the photoionization cross sections, at high photon energy, the Sn 5 s orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn 5 s state contribution at the top of the valence band in the different Sn-sulphides, SnS, Sn 2 S 3 , and SnS 2 , to be clearly identified. SnS and Sn 2 S 3 contain Sn(II) cations and the corresponding Sn 5 s lone pairs are at the valence band maximum (VBM), leading to ∼ 1.0 –1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast, SnS 2 only contains Sn(IV) cations, no filled lone pairs, and therefore has a ∼ 2.3 eV room-temperature band gap and much lower VBM compared with SnS and Sn 2 S 3 . The direct band gaps of these materials at 20 K are found using photoreflectance to be 1.36, 1.08, and 2.47 eV for SnS, Sn 2 S 3 , and SnS 2 , respectively, which further highlights the effect of having the lone-pair states at the VBM. As well as elucidating the role of the Sn 5 s lone pairs in determining the band gaps and band alignments of the family of Sn-sulphide compounds, this also highlights how HAXPES is an ideal method for probing the lone-pair contribution to the density of states of the emerging class of materials with n s 2 configuration

A high-performance “fueled” photodetector based on few-layered 2D ternary chalcogenide NiGa2S4

Few-layered binary 2D transition metal chalcogenides have been comprehensively employed in photodetector systems thanks to their intrinsic band gap structure and a high in-plane charge carrier mobility. Their rich chemistry is further broadened when considering ternary 2D chalcogenide materials, giving the possibility to prepare isomorphic materials characterized by various metal distributions within the framework of a crystal structure. Whereas the “inverse” A(τ)B(Ω)(τ)2X4 hexagonal structures (e.g. ZnIn2S4) have been widely studied in photocatalysis, “normal” AII(Ω) BIII(τ)2XVI4 phases have not been considered so far. In this study, a NiGa2S4 photodetector has been prepared by drop-casting the exfoliated crystals onto ITO glass. Their photoresponse has been analyzed either in the absence or in the presence of an electron donor species (EtOH) in the electrolyte solution. Ethanol acts as a fuel in the PEC photodetector system boosting their performance by a factor of ∼x49 in terms of the responsivity at a given wavelength. Moreover, the spectral response is expanded from blue to far IR wavelengths, reaching responsivity values from 48 to 7 mA W−1. The dynamics of photogenerated electrons and holes has been studied using laser flash photolysis. The stability of the “fueled” PEC photodetector has been evaluated through long-term tests which have shown a stable response over extended periods. It is anticipated that the present work can provide fundamental insight into the field of PEC semiconductor-based photodetectors, offering an extendable strategy to engineer high-performance PEC devices.11 página

Low- and high-energy photoluminescence from GaSb1−xBixwith 0 <x≤ 0.042

Two photoluminescence (PL) peaks were observed in temperature-dependent PL spectra of GaSb1−xBix layers with 0 < x ≤ 0.042. The high-energy (HE) peak was found to be associated with the bandgap-related emission in GaSb1−xBix, since its energy corresponds to the bandgap determined from photoreflectance measurements. The low-energy (LE) peak was attributed to the optical transition between the conduction band and native acceptor states, and was observed at low temperatures where acceptor states are not occupied by electrons. With increasing temperature, the intensity of the LE peak is quenched with the activation energy corresponding to the energy difference between HE and LE peaks

Optical properties and dynamics of excitons in Ga(Sb,Bi)/GaSb quantum wells: evidence for a regular alloy behavior

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Direct Measurements of Fermi Level Pinning at the Surface of Intrinsically n-Type InGaAs Nanowires

Surface effects strongly dominate the intrinsic properties of semiconductor nanowires (NWs), an observation that is commonly attributed to the presence of surface states and their modification of the electronic band structure. Although the effects of the exposed, bare NW surface have been widely studied with respect to charge carrier transport and optical properties, the underlying electronic band structure, Fermi level pinning, and surface band bending profiles are not well explored. Here, we directly and quantitatively assess the Fermi level pinning at the surfaces of composition-tunable, intrinsically n-type InGaAs NWs, as one of the prominent, technologically most relevant NW systems, by using correlated photoluminescence (PL) and X-ray photoemission spectroscopy (XPS). From the PL spectral response, we reveal two dominant radiative recombination pathways, that is, direct near-band edge transitions and red-shifted, spatially indirect transitions induced by surface band bending. The separation of their relative transition energies changes with alloy composition by up to more than ∼40 meV and represent a direct measure for the amount of surface band bending. We further extract quantitatively the Fermi level to surface valence band maximum separation using XPS, and directly verify a composition-dependent transition from downward to upward band bending (surface electron accumulation to depletion) with increasing Ga-content x(Ga) at a crossover near x(Ga) ∼ 0.2. Core level spectra further demonstrate the nature of extrinsic surface states being caused by In-rich suboxides arising from the native oxide layer at the InGaAs NW surface