Band offset

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

Modelling, simulation, optimization of Si/ZnO and Si/ZnMgO heterojunction solar cells

Abstract: Currently, crystalline and polycrystalline silicon wafer based solar cells dominate the photovoltaic market, which employs expensive manufacturing processes. Zinc oxide is one among the inexpensive alternatives for acting as emitter/anti-reflection coating layer which can be used in combination with widely available p-type silicon. Hence it becomes essential to gain in-depth knowledge about the charge transport mechanisms involved in the n-type ZnO/p-type silicon (Si) heterojunction devices. Therefore, Si/ZnO solar cell is modeled and the carrier transport mechanisms are carefully examined by analyzing the band edge discontinuities, electric field distributions at the Si/ZnO interface, carrier generation-recombination profile for varying ZnO thickness, carrier density and affinity values. The photo- generation rate degrades with the increase in ZnO emitter thickness owing to increased absorption in the blue region and is independent of ZnO affinity and donor concentration. The cliff like configuration with high conduction band discontinuity at the Si/ZnO interface for higher values of ZnO affinity has resulted in the formation of poor electric field and the lattice mismatch resulting in high defects formation (recombination centers) at Si/ZnO interface are the predominant causes for deficit in open circuit voltage and fill factor. Hence the conduction band offset alignment is further engineered by designing Si/MgZnO solar cell model and Mg doped ZnO emitter has significantly improved the band offset alignment at the p-n heterojunction when compared to ZnO emitter. Moreover, the simulation results revealed that the stronger electric field strength developed at the heterojunction is suggested to favor the carrier separation process by drift motion thereby improving the open circuit voltage and fill factor of the device. As a next step, both the models are optimized for varying emitter parameters like thickness, carrier density etc using Silvaco ATLAS simulator. Based on this investigation, the optimized Si/ZnO and Si/MgZnO designs anticipate improved conversion efficiencies of 11.57% and 14.46% respectively.

Band Discontinuity and Band Gap of MBE Grown HgTe/CdTe(001) Heterointerfaces Studied by k‐Resolved Photoemission and Inverse Photoemission

Abstract: We have investigated the valence band offset (ΔE VBO ) of the molecular beam epitaxially grown heterostructure HgTe/CdTe(001) by k-resolved ultraviolet (UPS) and X-ray (XPS) photoemission. Our angle-dependent UPS measurements at two different photon energies demonstrate that the dispersion of the valence band (VB) must be checked carefully in order to find the true position of the VB maximum in the Brillouin zone. With this information the valence band discontinuity was determined as (0.53 ± 0.03) eV, which is different from previously found photoemission values but now agrees well with magneto-optical investigations. In addition, we have determined the energy gap of CdTe(001) and HgTe(001) by a combination of UV and inverse photoemission. The energy gap of CdTe is determined to (1.57 ± 0.06) eV and that for HgTe as (0.0 ± 0.06) eV. We therefore conclude that HgTe is a semi-metal with inverted band structure in agreement with other results.

Effect of collector‐base valence‐band discontinuity on Kirk effect in double‐heterojunction bipolar transistors

Abstract: The effect of valence‐band discontinuity at the collector base heterojunction on the current gain and base charge storage is modeled. It is shown that the onset of the Kirk effect is accompanied by a sharp drop in the current gain and ft due to the formation of a potential barrier. The variation of barrier height with collector current density is determined and its effect on current gain and base transit time described. The results discussed here are applicable to Si/SiGe double‐heterojunction bipolar transistors.

Growth conditions, stoichiometry, and electronic structure of lattice-matched SrO BaO mixtures on Si(100)

Abstract: The low temperature growth conditions of mixed Ba0.7Sr0.3O layers on Si100 were investigated using the combination of low energy electron diffraction, x-ray photoemission XPS, and electron energy loss spectroscopy. With these methods crystallinity, stoichiometry, and electronic structure of both occupied and unoccupied levels were studied as a function of layer thickness. Oxide layers were generated by evaporating the metals in oxygen ambient pressure with the sample at room temperature after determination of the minimum oxygen pressure necessary for full oxidation. Good crystallinity with perfect lattice matching was only obtained starting with preadsorbed metallic layers at a concentration close to one monolayer ML, with best results starting from the Sr5 1. The XPS analysis shows that a sharp interface is formed during oxidation. The chemical species present at the interface are up to 1 ML of mono-oxidized Si and 1 ML of a mixed Sr –O–S i species. No silicide and silicate species or SiO2 formation at the interface after oxidation were found. Both interface and mixed oxide layers turned out to be stable to temperatures up to at least 600 ° C. Starting already at 1M L of Ba 0.7Sr0.3O, the band gap was found to be 4.3 eV, independent of layer thickness. We determined the valence band offset with respect to n-Si to be �2.2 eV, resulting in a conduction band offset of +1.0 eV.