Shavkat U. Yuldashev

Photovoltaic device on a single ZnO nanowire p?n homojunction

A photovoltaic device was successfully grown solely based on the single ZnO p-n homojunction nanowire. The ZnO nanowire p-n diode consists of an as-grown n-type segment and an in situ arsenic-doped p-type segment. This p-n homojunction acts as a good photovoltaic cell, producing a photocurrent almost 45 times larger than the dark current under reverse-biased conditions. Our results demonstrate that the present ZnO p-n homojunction nanowire can be used as a self-powered ultraviolet photodetector as well as a photovoltaic cell, which can also be used as an ultralow electrical power source for nanoscale electronic, optoelectronic and medical devices.

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.

Correlation of magnetic property with electrical transport property for ferromagnetic (Zn1−xMnx)O thin films

The correlation of electrical properties with magnetic properties for As+-implanted p-(Zn0.93Mn0.07)O thin films was investigated. For electrical transport measurements, it was clearly observed that the electrical mobility at the cryogenic temperature region is increased with decreasing temperatures. In Arrhenius plots of the carrier mobility, the critical point was observed at 65K. For Arrhenius plots of Mn2+-related emissions in photoluminescence measurements, the similar behavior was also observed at the same temperature region. The values of critical points are very closed to the value of the Curie temperature, thus it is expected that the increase of carrier mobility might be related to the activation of Mn2+ ions. Below the Curie temperature region, the activated Mn2+ ions will provide higher magnetic moments, and then the exchange interaction may increase in the material system. As a result, it will lead to enhance the spin-ordering effect and to reduce the probability of spin-disorder scattering. ...

Green Photoluminescence Suppression in ZnO Embedded in Porous Opal

The photoluminescence (PL) and transmittance characteristics of zinc oxide (ZnO) embedded in voids of silicon dioxide synthetic opal by the technologically simple sol–gel method are reported. The uniform formation of ZnO nanoparticles inside of the opal matrix can be obtained after its infiltration in an aqueous solution containing a zinc nitrite hexahydride precursor followed by thermal annealing. The green-PL suppression is observed due to the inhibition of spontaneous emission through oxygen vacancies in ZnO. The strong redshift of the transmittance characteristics signifies the essential filling of voids in the fcc packed structure. The infiltration of nanocrystals into synthetic opal may be used as an inexpensive method for the fabrication of polycrystalline ZnO with dominant ultraviolet-blue PL. This technology may also be promising for the fabrication of color light sources, such as RGB pixels in secondary-electron-emission or field-emission displays.

Thermal Conductivity of ZnO Nanowires Embedded in Poly(methyl methacrylate) Matrix

The thermal conductivity of ZnO nanowires (NWs) was determined from the thermal conductivity measurement of the ZnO NW/poly(methyl methacrylate) (PMMA) composite in the temperature range of 30–300 K. The thermal conductivity of ZnO NWs at room temperature is approximately two times lower than that of bulk ZnO. The results of this study show that the thermal conductivity of ZnO NWs is mainly determined by the scattering of phonons on the defects, as well as by the increased phonon-surface boundary scattering. These results could be useful for the design of ZnO nanowire-based devices.

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.

The role of zinc vacancies in bipolar resistance switching of Ag/ZnO/Pt memory structures

We have presented a study of the bipolar resistance switching characteristics in the Ag/ZnO/Pt cell. This switching is accompanied by a change in intensity of the photoluminescence emission at 3.33 eV which is attributed to zinc vacancy related transitions in ZnO film. Besides voltage-driven resistance switching phenomena, a transition from a high-resistance state to a lower one is observed under laser illumination at low temperature. These results demonstrate that the bipolar resistance switching can originate due to an electron trapping/de-trapping process at zinc-vacancy defects localized in the interface layer. The Mott metal-insulator transition is proposed as a possible mechanism of the memory effect.

White Light Emission from ZnO/Zn0.9Mg0.1O Heterostructures Grown on Si Substrates

Light-emitting n-Zn0.9Mg0.1O/ZnO/p-Zn0.9Mg0.1O heterojunction structures were grown on single-crystal p-type Si(100) substrates using a simple process of ultrasonic spray pyrolysis. Aqueous solutions of zinc acetate, magnesium acetate, and ammonium acetate were used as the sources of Zn, Mg, and N, respectively. p-Type conductivity in the nitrogen-doped ZnO and Zn0.9Mg0.1O films has been observed. A distinct visible electroluminescence was observed at room temperature from the heterojunction structures under forward bias conditions.

Highly Sensitive Flexible Photodetectors Based on Self-Assembled Tin Monosulfide Nanoflakes with Graphene Electrodes

Tin monosulfide (SnS) nanostructures have attracted huge attention recently because of their high absorption coefficient, high photoconversion efficiencies, low energy cost, ease of deposition, and so on. Here, in this paper, we report on the low-cost hydrothermal synthesis of the self-assembled SnS nanoflake-like structures in terms of performance for the photodetectors. High-performance photodetectors were fabricated using SnS nanoflakes as active layers and graphene as the lateral electrodes. The SnS photodetectors exhibited excellent photoresponse properties with a high responsivity of 1.7 × 104 A/W and have fast response and recovery times. In addition, the photodetectors exhibited long-term stability and strong dependence of photocurrent on light intensity. These excellent characteristics were attributed to the larger surface-to-volume ratio of the self-assembled SnS nanoflakes and the effective separation of the photogenerated carriers at graphene/SnS interfaces. Additionally, a flexible photodetector based on SnS nanoflakes was also fabricated on a flexible substrate that demonstrated similar photosensitive properties. Furthermore, this study also demonstrates the potential of hydrothermal-processed SnS nanoflakes for high-performance photodetectors and their application in flexible low-cost optoelectronic devices.

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.