Abstract. Alloys of silicon (Si), germanium (Ge) and tin (Sn) are continuously attracting research attention as possible direct band gap semiconductors with prospective applications in optoelectronics. The direct gap property may be brought about by the alloy composition alone or combined with the influence of strain, when an alloy layer is grown on a virtual substrate of different composition. In search for direct gap materials, the electronic structure of relaxed or strained Ge

Using empirical pseudopotential theory, the direct (Γ) and indirect bandgaps (L and X) of unstrained crystalline SixGe1−x−ySny have been calculated over the entire xy composition range. The results are presented as energy-contour maps on ternary diagrams along with a ternary plot of the predicted lattice parameters. A group of 0.2 to 0.6 eV direct-gap SiGeSn materials is found for a variety of mid-infrared photonic applications. A set of “slightly indirect” SiGeSn alloys having a direct gap at 0.8 eV (but with a smaller L-Γ separation than in Ge) have been identified. These materials will function like Ge in various telecom photonic devices. Hetero-layered SiGeSn structures are described for infrared light emitters, amplifiers, photodetectors, and modulators (free carrier or Franz-Keldysh). We have examined in detail the optimized design space for mid-infrared SiGeSn-based multiple-quantum-well laser diodes, amplifiers, photodetectors, and quantum-confined Stark effect modulators.

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Efficient chemical gas detection is of great importance for various functionalities (such as leakage detection of hazardous and explosive gases in industrial safety systems). The recent discovery of 2-dimensional black phosphorene (BlackP) has created intensive interests towards nano-sensors because of its maximized surface-to-Meanwhile, tri and quad vacancies induce the dissociative adsorption, not suitable for the reversible adsorption-desorption cycles. Substitutional doping by Fe atoms is found to be a feasible approach to enhance the sensing resolution of SO 2 detection because of the remarkable adsorption energy incorporated with the substantial variation in DOS after gas exposure. This modification in electronic properties is facilitated by the charge transfer mechanism from Fe-3d to P-3p which can generate the measurable electrical signal detect by the external circuit of the sensor.

Advancement in doping other elements, such as Ce, Dy, Ni, Sb, In and Ga in ZnO[1], have stimulated great interest for high-temperature thermoelectric application. In this work, the effects of Al-doping in a ZnO system on the electronic structure and thermoelectric properties are presented, by experiment and calculation. Nanosized powders of Zn1−xAlxO (x = 0,0.01, 0.02, 0.03 and 0.06) were synthesized by hydrothermal method. From XRD results, all samples contain ZnO as the main phase and ZnAl2O4 (spinel phase) peaks were visible when Al additive concentrations were just 6 at%. The shape of the samples changed and the particle size decreased with increasing Al concentration. Seebeck coefficients, on the other hand, did not vary significantly. They were negative and the absolute values increased with temperature. However, the electrical resistivity decreased significantly for higher Al content. The electronic structure calculations were carried out using the open-source software package ABINIT[2], which is based on DFT. The energy band gap, density of states of Al-doped ZnO were investigated using PAW pseudopotential method within the LDA + U. The calculated density of states was then used in combination with the Boltzmann transport equation[3] to calculate the thermoelectric parameters of Al-doped ZnO. The electronic band structures showed that the position of the Fermi level of the doped sample was shifted upwards in comparison to the undoped one. After doping Al in ZnO, the energy band gap was decreased, Seebeck coefficient and electrical conductivity were increased. Finally, the calculated results were compared with the experimental results. The good agreement of thermoelectric properties between the calculation and the experimental results were obtained.

ZnO is widely studied for several applications such as a photocatalyst, a working electrode for dye-sensitized solar cells and for thermoelectric devices. This work studies the effect of an increase in the number of carriers by doping ZnO with Al and Ga. The 6.25 mol% of Al-doped ZnO, 6.25 mol% of Ga-doped ZnO, and 12.5 mol% of (Al, Ga) co-doped ZnO nanoparticles were prepared using combustion method. The prepared samples were then characterized by X-ray diffraction, transmission electron microscope, energy-dispersive X-ray spectroscopy and UV-visible spectroscopy techniques. Moreover, density functional theory (DFT) was also employed for computational studies of Al and Ga doped ZnO. Optimized structures, density of states (DOS) and band structures of these materials were calculated using Vienna Ab initio Simulation Package code. From this study, Al and Ga are found to play an important role in morphology and optical properties of the ZnO, changing the band gap and Fermi level of ZnO. Then, the prepared samples were characterised for their thermoelectric properties, and modelling of thermoelectric properties of ZnO, Al-doped ZnO, Ga-doped ZnO and (Al, Ga)-co doped ZnO was performed using BoltzTraP code. Furthermore, the Seebeck coefficient, electrical conductivity, relaxation time, electronic thermal conductivity and power factor were studied. The experimental and computational results are pointing in the same direction, that the thermoelectric properties of the ZnO are changed by doping: the semiconducting ZnO transforms into metallic ZnO when doped with Al and Ga. This leads to ZnO showing new thermoelectric properties, in particular the Ga-doped ZnO and (Al, Ga)-co doped ZnO: they provide high electrical conductivity and power factor. Therefore, it is expected that these good properties might promote the ZnO to be a potential candidate for applications, especially in high efficiency thermoelectric devices.

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