Vadim Sh. Yalishev

Electrical and Optical Properties of ZnO Films Grown on GaAs Substrates

Undoped ZnO films were deposited on GaAs substrates by conventional rf magnetron sputtering technique. After thermal annealing at temperatures of 500°C and higher for 20 min, the Hall coefficient of ZnO films on GaAs substrate becomes positive. The long-time annealing of 550 min at a temperature of 400°C also converts the sign of the Hall coefficient to positive. X-ray microanalysis shows that the diffusion of Zn atoms into the GaAs substrate and Ga atoms from the GaAs substrate into the ZnO film during thermal annealing occurs. The results of Hall measurements were analyzed by using the two-layer model of conductivity. It was shown that the positive sign of the Hall coefficient for the annealed ZnO film on the GaAs substrate is due to p-type conductivity of the GaAs substrate as a result of the diffusion of the Zn atoms from ZnO film into the GaAs substrate. With increasing annealing temperature or annealing time the ZnO films become more n-type due to the diffusion of Ga atoms from the GaAs substrate into the ZnO film.

Magnetoresistance of Ga1-xMnxAs Epitaxial Layers Doped by Be

Magnetoresistance (MR) measurements were performed on Ga1-xMnxAs (x=0.03) additionally doped with Be. At low temperatures and in low magnetic fields, a positive MR signal was observed for measurements in the transverse (B⊥I) geometry. However, at high temperatures, the MR becomes negative for all fields and all field orientations. The value of the negative MR has a maximum near the Curie temperature, and its magnitude depends strongly on the concentration of free holes. The observed MR behavior can be described by the magnetoimpurity scattering model in both the paramagnetic and the ferromagnetic temperature regions. Quantitative analysis of the Ga1-xMnxAs MR data yields the value of the p–d exchange energy as |N0β|≈1.6 eV.

Blue luminescence and Schottky diode applications of monoclinic HfO2 nanostructures

Schottky diodes based on metal–semiconductor (MS) and metal–insulator–semiconductor (MIS) configurations are nowadays widely regarded as key components for the realization of a number of improved electronic and optoelectronic functions. In this regard, hafnium dioxide (HfO2) nanostructures were processed through a facile chemical route for application in MIS Schottky diodes. Their monoclinic phase and micro-structural characteristics were studied in detail using the X-ray diffraction, Raman and electron microscopic measurements. The nanostructures were studied to evolve in form of particulate structures at an average scale of 8–10 nm. The low-temperature photoluminescence measurements revealed the optical activity of HfO2 to spread across the blue region of electromagnetic spectrum. And their origin has been related to the transitions taking place across the intermediary energy levels established by the oxygen related vacancies. The power of incident laser irradiation was also noted to have a significant influence on the surface-state related defect emissions. The electrical properties of HfO2 were studied using the Bode, Nyquist and Mott–Schottky type plots extracted from the impedance spectroscopic measurements. MIS Schottky diode architectures were finally fabricated using the HfO2 thin films that were spin cast on n-Si. A significant improvement in the diode characteristics were noted for the heat treated devices, suggesting the improved tunnelling and limiting of charge leakages across the integrated heterojunctions.

Nonpolar Resistance Switching in Anodic Oxide Alumina Films

At an applied voltage, anodic oxide alumina (AOA) films exhibit reproducible resistance switching between two states: a high-resistance state with insulating properties and a metallic low-resistance state. This resistance switching does not depend on the polarity of the applied voltage. A reduction in photoluminescence signal intensity of points switched to the low-resistance state was observed. The filamentary electronic model was employed to explain the resistance switching in the AOA films, which assumed that rupture and recovery of filaments are caused by the migration of electrons from and to filaments, respectively.

Dependence of Resistance Switching Voltage on the Potential Barrier in ZnO Thin Films

We have studied the electric properties of Au/ZnO/Al structures that exhibit a reproducible resistance switching under the applied voltage. Rupture and recovery of the conductive filaments at the ZnO/Au interface were suggested as responsible for this switching. Trapped charge electrons in the interface states can change width and height of a Shottky‐like barrier that exists on the Au/ZnO interface. However, the changing of the interface barrier characteristics leads to change of the resistance switching properties, decreasing or increasing their values.