Surface and Interface Bands of the CdTe–HgTe–CdTe Heterostructure: Evidence of Metallicity

Performed full-relativistic DFT calculations have demonstrated that thin HgTe layers are metallic and, with increasing thickness, do not become insulators – either ordinary band insulators or topological insulators. The variations of the potential at the CdTe–HgTe interfaces are found to be negligible in comparison with those at the terminating surfaces of the CdTe–HgTe–CdTe films, so that the interfaces in fact do not form any potential well. It is shown that the interface-related bands of the CdTe–HgTe–CdTe films are situated well below EF, so that a dominant input into the density of states at EF and, therefore, to the conductivity is provided not by the interface states, but by the surface bands of the net layered system. It is reasonable therefore to consider an alternative interpretation of the reported thickness dependence of the conductivity of the system, such as the possible surface segregation of components or unavoidable contaminations, which seems much more realistic than the interpretation based on involving topological insulators and topologically protected surface states.

Microstrucral, Optical and Electrical Characteristics of Cu-doped CdTe Nanocrystalline Films for Desinging of Absorber Layer in Solar Cell Applications

This paper reports the microstrcure, optical and electrical characteristics of undoped and Cu doped CdTe nanostructured thin films prepared on glass substrates by electron beam evaporation technique. The Crystallographic study of X-ray diffraction shows that CdTe and Cu doped CdTe films crystallize in the form of a cubic zinc blende structure. Microstructure analysis reveals that as the Cu doping level increases, the average crystallite size increases, while the microstrian decreases due to the improvement of the crystallinty, thereby reducing defects. XRD and AFM investigations confirmed the nanostructure characteristic of undoped and Cu doped films. It was found that the optical band gap energy increases from 1.485 eV to 1.683 eV as the Cu concentration increases from 0 wt. % to 10 wt. %, which may be related to the Burstein-Moss effect. The refractive index is calculated from the Swanepoel envelope method and found to decrease with the increase of the Cu doping due to the decrease in the prolizability. Similarly, the extinction coefficient decreases with the increase of Cu in CdTe matrix. The dc electrical conductivity is found to increase with increasing Cu doping, which is attributed to the increase in the grain size, thereby reducing the scattering of the grain boundary. Furthermore, two conduction mechanisms of the carrier transport in nanostrcutured undoped and Cu doped CdTe films were observed. The low temperature dependence of the conductivity of undoped and Cu doped CdTe nanostructured films is explained based on Mott’s variable range hopping conduction mechanism model (VRH). Interestingly, the calculated values of hopping distance R, the hopping energy W and the the density of states at the Fermi level N(EF) are consistent with the Mott's VRH. Finally, Hall effect measurements show that all the films have p- type conduction behavior. Besides, the results show that as Cu doping level increases, the carrier concentration and the Hall mobility increase due to the decrease in grain boundary scattering with the increase in grain size. Accordingly, it can be concluded that by increasing the Cu doping level in the CdTe film, the conductivity is increased, thereby improving the performance of the CdTe absorber layer in the solar cell structure.

Preparation and Characterization of CdS/CdTe Device for Radiation Sensing

Several Techniques had been applied to measure Ionizing Radiation. Majority of thistechniques are costly and very complicated. We focus on this research to chemically deposition of CdS to formwith CdTe junction x-ray sensor. CdTe has been electrodeposited onto CdS/FTO glass substrate to formwith previously fabricated CdS layer 4 µm thickness. The optimum potential for CdTe deposition hasbeen studied by potentiostat measurement, it shows that -1.3 is the optimum working potential. The XRDanalysis showed that the CdTe films have highly oriented crystallites with the cubic phase zinc blend withpreferred orientation (111). The band gap Eg extrapolated to be 1.4 eV. Four stacked sensors wereconnected in series to measure the device performance. It was observed that amplitude of the pulseformed due to exposed FTO/CdS/CdTe/Mo detector to X-ray of 33 keV and 1mA intensity is 1.03 V.