S. K. Jha

Enhanced performance by incorporation of zinc oxide nanowire array for organic-inorganic hybrid solar cells

We study the solar power conversion efficiency in hybrid solar cells based on zinc oxide (ZnO)/antimony trisulfide (Sb2S3)/poly-3(hexylthiophene) heterojunctions. The incorporation of ZnO nanowire arrays (NAs) structure results in power conversion efficiency of 2.9%, or 20% higher than the control device. Absorption spectra and numerical simulation analysis provide strong evidence revealing that the enhanced performance is mainly induced by (1) enhanced optical absorption from light-trapping effect of NAs and (2) reduced bulk recombination rate in Sb2S3 from shortened electron injection pathway into ZnO. Significantly, numerical simulations show that the expected interface recombination increase from larger interfacial area effects is negligible.

Plasma-assisted growth and nitrogen doping of graphene films

Microwave plasmas were employed to synthesize single- or double-layer graphene sheets on copper foils using a solid carbon source, polymethylmetacrylate. The utilization of reactive plasmas enables the graphene growth at reduced temperatures as compared to conventional thermal chemical vapor deposition processes. The effects of substrate temperature on graphene quality were studied based on Raman analysis, and a reduction of defects at elevated temperature was observed. Moreover, a facile approach to incorporate nitrogen into graphene by plasma treatment in a nitrogen/hydrogen gas mixture was demonstrated, and most of the nitrogen atoms were verified to be pyridinelike in carbon network.

ZnO-nanorod-array/p-GaN high-performance ultra-violet light emitting devices prepared by simple solution synthesis

Pure ultra-violet (UV) (378 nm) electroluminescence (EL) from zinc oxide (ZnO)-nanorod-array/p-gallium nitride (GaN) light emitting devices (LEDs) is demonstrated at low bias-voltages (∼4.3 V). Devices were prepared merely by solution-synthesis, without any involvement of sophisticated material growth techniques or preparation methods. Three different luminescence characterization techniques, i.e., photo-luminescence, cathodo-luminescence, and EL, provided insight into the nature of the UV emission mechanism in solution-synthesized LEDs. Bias dependent EL behaviour revealed blue-shift of EL peaks and increased peak sharpness, with increasing the operating voltage. Accelerated bias stress tests showed very stable and repeatable electrical and EL performance of the solution-synthesized nanorod LEDs.

Electronic structure and electrical transport in ternary Al-Mg-B films prepared by magnetron sputtering

Nanostructured ternary Al-Mg-B films possess high hardness and corrosion resistance. In the present work, we study their electronic structure and electrical transport. The films exhibit semiconducting characteristics with an indirect optical-bandgap of 0.50 eV, as deduced from the Tauc plots, and a semiconductor behavior with a Fermi level of ∼0.24 eV below the conduction band. Four-probe and Hall measurements indicated a high electrical conductivity and p-type carrier mobility, suggesting that the electrical transport is mainly due to hole conduction. Their electrical properties are explained in terms of the film nanocomposite microstructure consisting of an amorphous B-rich matrix containing AlMgB14 nanoparticles.

Study of temperature distribution in the channels of AlGaN/GaN HEMT devices by μ- Raman characterization techniques

AlGaN/GaN and InAlN/GaN high electron mobility transistors (HEMTs) were fabricated and their electrical and thermal properties were examined. The influence of irradiation to direct current (DC) and low-frequency noise properties of these devices have already been investigated and published. However, the thermal processes in the active layers of these devices can also induce significant changes in the performance of devices. Since thermal processes are directly associated with temperature distribution, temperature mapping in the devices has been suggested using a μ-Raman technique. Considering that the phonon frequencies are sensitive to the sample temperature, the shift of first-order Raman scattering can conveniently be used to directly measure the device temperature with a high spatial resolution. In this context, we report time resolved and sample mapping Raman spectra inside the HEMTs channel as measures of temperatures.