L. M. Kukreja

Effect of Si doping on electrical and optical properties of ZnO thin films grown by sequential pulsed laser deposition

The sequential pulsed laser deposition technique was used to grow highly transparent and c-axis oriented thin films of Si doped ZnO on sapphire substrates. On doping with Si, the resistivity of the virgin ZnO thin films was found to decrease from ~3.0 ? 10?2 to 6.2 ? 10?4???cm and its bandgap increased from about 3.28 to 3.44?eV at different doping concentrations. XPES measurements revealed that Si predominantly occupies the Zn lattice sites in the Si+3 state. The increase in the bandgap of the ZnO films with increasing Si concentration was found to be due to the collective effects of high carrier concentration induced Burstein?Moss blue shift and bandgap narrowing. Efficient photoluminescence (PL) was observed at room temperature from these Si doped ZnO films. The bandgaps obtained from the PL measurements were found to be Stokes shifted as compared with those obtained from the transmission spectra. Si doping of ZnO offers the possibility of developing superior transparent conducting electrodes for applications such as in display panels, solar cells and transparent resistive non-volatile memories.

Field emission studies on well adhered pulsed laser deposited LaB6 on W tip

Lanthanum hexaboride films have been deposited on a tungsten tip by pulsed laser deposition technique. The field emission studies have been performed in the conventional field emission geometry. The Fowler-Nordheim plot obtained from the current-voltage characteristic is found to be linear in accordance with the quantum mechanical tunneling phenomenon. A current density of 1.2×104A∕cm2 is drawn from the deposited tip. The field enhancement factor is calculated to be 5537cm−1, indicating that the field emission is from nanoscale protrusions present on emitter surface. The emission current-time plots show the very good stability of the emitter.

Room temperature photoluminescence from ZnO quantum wells grown on (0001) sapphire using buffer assisted pulsed laser deposition

Efficient room temperature (RT) photoluminescence (PL) is achieved on ZnO multiple quantum wells (MQWs) grown on sapphire by pulsed laser deposition using a buffer assisted growth scheme. Absorption spectra of these MQWs at RT showed the excitonic features entwined with the band edges, which pointed to the excitonic nature of the PL transitions. At RT the band edge of these MQWs shifted from ∼3.36to3.78eV on decreasing the well layer thickness from ∼4to1nm. In the range from 10K to RT, the PL spectral linewidth increased and the peak shifted monotonically towards red with increasing temperature.

Correlation of spectral features of photoluminescence with residual native defects of ZnO thin films annealed at different temperatures

We investigated the effects of post-growth annealing in the temperature range of 873 to 1273 K on the spectral features of photoluminescence (PL) vis-a-vis the crystalline and compositional native defects of ZnO thin films grown at 773 K by pulsed laser deposition (PLD) on sapphire substrates. It is found in the PL spectra at 10 K that the deep level emission (DLE) shifted from red-orange spectral region of ∼1.8–2.4 eV to yellow–green region of ∼2.4–2.9 eV with the increasing temperature of annealing. We propose that the PL in red-orange region originating from the singly ionized oxygen vacancies diminished due to increased replenishment of oxygen with increasing annealing temperature and that in the yellow–green region originating from the oxygen interstitials and/or zinc vacancies increased due to enhanced concentration of these point defects. As the annealing temperature was increased, the overall intensity of PL in the DLE region increased slightly up to 973 K but beyond that it increased steeply and ...

Band alignment and interfacial structure of ZnO/Ge heterojunction investigated by photoelectron spectroscopy

Studies on band-offset and band-alignment of heterojunction of highly c-axis oriented ZnO thin films grown on n-Ge (1 1 1) by pulsed laser deposition show a type-II band alignment with the valence band offset (ΔEV) of 3.1 ± 0.2 eV. The valence band spectra of this heterojunction show band onsets corresponding to Ge, interfacial GeOx, and ZnO layers. This observation also enabled us to determine ΔEV of ZnO/GeOx heterojunction to be 1.4 ± 0.2 eV. These studies provide further insight into the band alignment of ZnO/GeOx/Ge system wherein the observed large value of ΔEV of ZnO/Ge can be used for heterojunction based optoelectronic devices.

Effect of disorder on carrier transport in ZnO thin films grown by atomic layer deposition at different temperatures

We have grown ∼200 nm thick ZnO films on (0001) sapphire substrates using atomic layer deposition at different substrate temperatures ranging from ∼150 to 350 °C. X-ray diffraction and photoluminescence spectra of these films showed that crystalline and compositional native defects were strongly dependent on the substrate temperature. Room temperature Hall measurement showed that all the films were degenerate with carrier concentration exceeding the Motts critical density nc required for metallic conduction. The lowest value of room temperature resistivity ∼3.6 × 10−3 Ω cm was achieved for the film deposited at ∼200 °C, which had an estimated carrier concentration ∼5.7 × 1019 cm−3 and mobility ∼30 cm2/V s. The films deposited both below and above ∼200 °C showed increased resistivity and decreased mobility presumably due to the intensified defects and deteriorated crystalline quality of these films. To investigate the effect of disorder on the underlying charge transport mechanisms in these films, the ele...

Studies on temperature dependent semiconductor to metal transitions in ZnO thin films sparsely doped with Al

For a detailed study on the semiconductor to metal transition (SMT) in ZnO thin films doped with Al in the concentration range from 0.02% to 2%, we grew these films on (0001) sapphire substrates using sequential pulsed laser deposition. It was found that the Al concentration in the films increased monotonically with the ratio of ablation durations of the alumina and ZnO targets used during the deposition. Using x-ray photo electron spectroscopy, it was found that while most of the Al atoms occupy the Zn sites in the ZnO lattice, a small fraction of the Al also gets into the grain boundaries present in the films. The observed SMT temperature decreased from ∼270 to ∼50 K with increase in the Al concentration from 0.02% to 0.25%. In the Al concentration range of ∼0.5% to 2%, these doped ZnO films showed metallic behavior at all the temperatures without undergoing any SMT. A theoretical model based on thermal activation of electrons and electron scatterings due to the grain boundaries, ionic impurities, and p...

Defect-free ZnO nanorods for low temperature hydrogen sensor applications

Uniformly distributed and defect-free vertically aligned ZnO nanorods (NRs) with high aspect ratio are deposited on Si by sputtering technique. X-ray diffraction along with transmission electron microscopy studies confirmed the single crystalline wurtzite structure of ZnO. Absence of wide band emission in photoluminescence spectra showed defect-free growth of ZnO NRs which was further conformed by diamagnetic behavior of the NRs. H2 sensing mechanism based on the change in physical dimension of channel is proposed to explain the fast response (∼21.6 s) and recovery times (∼27 s) of ZnO NRs/Si/ZnO NRs sensors. Proposed H2 sensor operates at low temperature (∼70 °C) unlike the existing high temperature (>150 °C) sensors.

Phase-coherent electron transport in (Zn, Al)Ox thin films grown by atomic layer deposition

A clear signature of disorder induced quantum-interference phenomena leading to phase-coherent electron transport was observed in (Zn, Al)Ox thin films grown by atomic layer deposition. The degree of static-disorder was tuned by varying the Al concentration through periodic incorporation of Al2O3 sub-monolayer in ZnO. All the films showed small negative magnetoresistance due to magnetic field suppressed weak-localization effect. The temperature dependence of phase-coherence length ( lφ∝T−3/4), as extracted from the magnetoresistance measurements, indicated electron-electron scattering as the dominant dephasing mechanism. The persistence of quantum-interference at relatively higher temperatures up to 200 K is promising for the realization of ZnO based phase-coherent electron transport devices.

Studies on laser peening using different sacrificial coatings

Abstract Laser shock peening usually requires the presence of a sacrificial coating to protect the substrate from undesirable thermal effects generated by laser irradiation. This paper describes an experimental study to identify an adherent sacrificial coating for laser peening of smooth metallic surfaces where black paint does not adhere well. Normal black polyvinyl chloride electrical insulation tape has been found to be a good choice as a sacrificial coating. Laser peening with black insulation tape generated effective laser peening in three different substrates, namely, Ti6Al4V alloy, SAE 9260 spring steel and DIN X20Cr13 martensitic stainless steel. The depth of peening was comparable or better than that generated with black paint. With respect to widely used black paint, black insulation tape not only provides clean, simple and uniform coating but also literally eliminates the time involved in its application (black paint requires multilayer coating with long intermediate curing periods) and removal.