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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 Ω 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.

A fast piezoelectric detection system was utilized to record time-resolved photoacoustic signals reflecting the thermal expansion and the rate of material ablation in infrared matrix-assisted laser desorption/ionization (IR-MALDI). Glycerol was employed as a liquid and 2,5-dihydroxybenzoic acid (2,5-DHB) as a crystalline matrix. An Er:YAG laser (λ ) 2.94 µm; τ L ∼ 100 ns) and a wavelength-tunable optical parametric oscillator (OPO) laser system (λ ) 1.4-4.0 µm; τ L ) 6 ns) were used for desorption/ionization. Material ejection and MALDI ion signals were recorded as a function of laser fluence, pulse duration, and wavelength. For glycerol as matrix, material ejection was found to be temporally confined to about the laser pulse duration when the OPO was employed. For excitation with the Er:YAG laser, a prolonged material ejection with an approximately exponential decay was observed with a characteristic time constant of ∼1 µs. For both lasers, material ejection was observed already at fluences substantially below the MALDI ion threshold. For excitation with the OPO, a correlation between a change in the desorption/ablation mechanism and the onset of ion generation was found. Crystalline 2,5-DHB preparations were investigated with the OPO only. Material ejection from this matrix was found to exhibit a significantly different dependence of the overall ejected material on laser fluence. The article also provides an introduction to the underlying photoacoustic theory adapted to the MALDI process.

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Leaky mode analysis of luminescent thin films: The case of ZnO on sapphire J. Appl. Phys. 112, 063112 (2012) ZnO/ZnSxSe1−x core/shell nanowire arrays as photoelectrodes with efficient visible light absorption Appl. Phys. Lett. 101, 073105 (2012) Study of the photoluminescence emission line at 3.33eV in ZnO films J. Appl. Phys. 112, 013528 (2012) Optical analysis of doped ZnO thin films using nonparabolic conduction-band parametersWe investigated the effects of post-growth annealing in the temperature range of 873 to 1273 K on the spectral features of photoluminescence (PL) vis-à-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 made a quantum leap at 1073 K. In contrast to that, intensity of PL due to the near band-edge emission (NBE) in UV region of $3.15 to 3.45 eV increased very steeply up to the annealing temperature of 973 K, which is found to be due to improvement in the crystalline and compositional qualities of the films and beyond that it dropped drastically due to deteriorations of these qualities. The high resolution PL spectra at 10 K in the NBE region mainly consisted of peaks due to the recombinations of neutral donor bound excitons' complexes (D 0 X) at $3.36 eV, free excitons (FX A ) at $3.38 eV with their conspicuous LO phonon replicas and some other features such as exciton complexes bound to surface states or transitions of conduction band electrons to acceptor levels located in stacking faults and recombination of neutral acceptor bound excitons. The relative intensities of these individual features were strongly dependent on the annealing temperature of the films and the ensuing crystalline and compositional qualities. The 10 K PL spectra from the interfacial region of the annealed ZnO films and the sapphire substrates observed from the backside of the samples showed that the annealing temperature affected the crystalline and compositional qualities at the interface in a complex manner. Particularly, the features corresponding to the interface deteriorations resulting from the diffusion of Al into the ZnO films and the crystalline defects at the interface caused by the sputtering due to the PLD plume were prominently present in these PL spectra. These studies provide deeper insight...