Band offset

About: Band offset is a research topic. Over the lifetime, 2446 publications have been published within this topic receiving 53450 citations.

Copper phthalocyanine on InSb(111)A: interface bonding, growth mode and energy band alignment : Organic-inorganic semiconductor interfaces

Abstract: The growth of the organic semiconductor CuPc on the InSb(111)A surface at 300 K has been studied using photoelectron spectroscopy. Core level emission data obtained using low energy synchrotron radiation reveal that the interface is abrupt with very weak bonding between the InSb surface atoms and the adsorbed molecules. The coverage dependence of the substrate and overlayer core level peak intensities follows the prediction of a uniform growth mode at high growth rates, but the organic film follows a Stranski-Krastanov growth mode at lower growth rates. C 1s and N Is photoelectron emission data obtained with Mg Kα radiation confirm that the CuPc molecules are intact within the layer, and shake-up satellites associated with benzene and pyrrole C and N peaks provide an insight into the energy and spatial distribution of the highest occupied and lowest unoccupied molecular orbitals. Photoelectron emission from the occupied bonding states of the CuPc and the valence band states of InSb provides the band offset for the filled states and the overall energy band profile for this organic-inorganic heterojunction. The presence of an interface dipole at the interface disproves a simple band alignment based on the vacuum level; the energy bands have a nested arrangement where both band edges in the InSb lie within the HOMO-LUMO gap of the CuPc.

Impact of the Al/Hf ratio on the electrical properties and band alignments of atomic-layer-deposited HfO2/Al2O3 on S-passivated GaAs substrates

Abstract: HfO2, Al2O3 and HfO2/Al2O3 (AHO) nanolaminates with various Al/Hf ratios (including 1:3.0, 1:2.1 and 1:1.3) were fabricated on S-passivated GaAs substrates by atomic layer deposition (ALD). The interface structure and band alignments of various dielectric/GaAs structures have been investigated systematically. The AHO films with the Al/Hf ratio of 1:1.3 suppress the formation of As oxides and elemental As overlayers around AHO/GaAs interfaces more effectively than other samples, showing higher accumulation capacitance, less hysteresis width (ΔVFB = 415 mV) and lower leakage current density. The band alignments of interfaces of HfO2/GaAs, Al2O3/GaAs and AHO/GaAs were established. The results indicate that ALD HfO2/Al2O3 nanolaminate structures could effectively tune the interface quality and band offset of gate dielectric films on n-GaAs.

Band offset in semiconductor heterojunctions.

Abstract: Semiconductor heterojunctions are widely applied in solid-state device applications, including semiconductor lasers, solar cells, and transistors. In photocatalysis they are of interest due to their capability to hinder charge carriers' recombination. A key role in the performance of heterojunctions is that of the alignment of the band edges of the two units composing the junction. In this work, we compare the performances of three widely applied approaches for the simulation of semiconductors heterostructures, based on density functional theory calculations with hybrid functionals. We benchmark the band offsets of ten semiconductors heterostructures for which experimental values are available: AlP/GaP, AlP/Si, AlAs/GaAs, AlAs/Ge, GaAs/Ge, GaP/Si, ZnSe/Ge, ZnSe/AlAs, ZnSe/GaAs, and TiO2/SrTiO3. The methods considered are (i) the alternating slabs junction (ASJ), (ii) the surface terminated junction (STJ), and (iii) the independent units (IU) approach. Moreover, two different ways to determine a common reference have been considered, (i) the plane averaged electrostatic potential, and (ii) the energy of the core levels. Advantages, drawbacks and overall performances of each method are discussed. The results suggest that the accuracy in the estimation of the band offsets is ∼0.2 eV when the ASJ method is applied. The STJ approach provides a similar accuracy, while the neglection of any interface effect, as in the IU method, provides only a qualitative estimate of the band offset and can result in significant deviations from the experiment.

High-pressure photoluminescence study of GaAs/GaAs1-xPx strained multiple quantum wells.

Abstract: We report low-temperature photoluminescence studies of GaAs/GaAs 1-x P x strained quantum-well samples grown by gas-source molecular-beam epitaxy as a function of pressure. We have found that the transitions between the lowest Γ-confined electron and hole states shift toward higher energy with increasing pressure, and also that the pressure coefficients of the transitions depend on the alloy concentrations and the quantum-well structures. From the observation of the pressure-induced crossover of the lowest Γ-confined electron state in the wells against the conduction-band (001) X minima in the barriers, we are able to determine the valence-band offset for the GaAs/GaAs 0.68 P 0.32 heterostructure

First-principles insights into the electronic structure, optical and band alignment properties of earth-abundant Cu2SrSnS4 solar absorber.

Abstract: Cu2SrSnS4 (CSTS) is a promising alternative candidate to Cu2ZnSnS4 (CZTS) for single- or multi-junction photovoltaics (PVs) owing to its efficient light-absorbing capability, earth-abundant, nontoxic constituents, and suitable defect properties. However, as a novel absorber material, several fundamental properties need to be characterized before further progress can be made in CSTS photovoltaics. In this letter, hybrid density functional theory (DFT) calculations have been used to comprehensively characterize for the first time, the electronic structure, band alignment, and optical properties of CSTS. It is demonstrated that CSTS possesses the ideal electronic structure (direct band gap of 1.98 eV and small photocarrier effective masses) and optical properties (high extinction coefficient and wide absorption) suitable for photovoltaic applications. Simulated X-ray photoelectron spectroscopy (XPS) valence band spectra using variable excitation energies show that Cu-3d electronic state dominates the valence band maximum of CSTS. Furthermore, the vacuum-aligned band diagram between CSTS and other common absorbers (CZTS, CIGS, CdTe) and the common n-type partner materials (CdS, ZnO) was constructed, which indicate staggered type-II band alignment at the CSTS/CdS and CSTS/ZnO interfaces. Based on these results, interface band offset engineering and alternative device architectures are suggested to improve charge carrier separation and power conversion efficiencies of CSTS.