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.

Six decades of research on ZnO has recently sprouted a new branch in the domain of resistive random access memories. Highly resistive and c-axis oriented ZnO thin films were grown by us using d.c. discharge assisted pulsed laser deposition on Pt/Ti/SiO 2 /Si substrates at room temperature. The resistive switching characteristics of these films were studied in the top-bottom configuration using current-voltage measurements at room temperature. Reliable and repeated switching of the resistance of ZnO thin films was obtained between two well defined states of high and low resistance with a narrow dispersion and small switching voltages. Resistance ratios of the high resistance state to low resistance state were found to be in the range of 2-5 orders of magnitude up to 20 test cycles. The conduction mechanism was found to be dominated by the Ohmic behaviour in low resistance states, while Poole-Frenkel emission was found to dominate in high resistance state. The achieved characteristics of the resistive switching in ZnO thin films seem to be promising for nonvolatile memory applications.

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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 Mott's critical density n c required for metallic conduction. The lowest value of room temperature resistivity $3.6 Â 10 À3 X cm was achieved for the film deposited at $200 C, which had an estimated carrier concentration $5.7 Â 10 19 cm À3 and mobility $30 cm 2 /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 electrical resistivity was measured in the temperature range of $4.2 to 300 K. The films grown at $150, 300, and 350 C were found to be semiconducting in the entire range of the measurement temperature due to the intensified disorder which impeded the metallic transport in these films. However, the films grown at $200 and 250 C showed a transition from metallic to semiconducting transport behaviour at lower temperatures due to the reduced defects and improved crystalline quality of these films. The observed semiconducting behaviour below the transition temperature for these films could be well explained by considering quantum corrections to the Boltzmann conductivity which includes the effect of disorder induced weak localization and coulomb electron-electron interactions. V C 2013 AIP Publishing LLC. [http://dx.

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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 2 scatterings due to the grain boundaries, ionic impurities and phonons has been developed to explain the observed concentration and temperature dependent SMT.Keywords: Al doped ZnO, sequential PLD, metal to semiconductor transitions I. IntroductionStudies on the ZnO thin films doped with different elements is an area of contemporary research for the development of new functional materials mainly for photonic and photovoltaic applications 1 . One of the ramifications of this area of research is to obtain and study highly conductive and transparent films of ZnO using n-type dopants such as Al, In, B, Ga and Si 2-7 . Al has been extensively used for this purpose because of its easy availability, low cost, ease of doping and superior properties of the ensuing films. Studies on temperature dependent transport properties of such Al doped ZnO (AZO) films 8, 9 are essential to understand the conduction mechanisms under different conditions of the doping. An important effect that has been observed in the temperature dependent resistivity measurements of AZO thin films is the semiconductor to metal transition (SMT) 10-12 . SMT is an important phenomenon because it can be used to elicit the basic processes involved in the transport properties of AZO films. It is well known that as grown pure ZnO thin films are semiconducting in nature showing decrease of resistivity (ρ) with increase of temperature (i.e. it has negative temperature coefficient of resistivity) 2 . When doped with small amounts of (~ 2%) n-type dopant, the ZnO thin films become metallic showing increase of resistivity with increasing temperature (i.e. positive temperature coefficient of resistivity) 6, 10 . Hence it is expected that the 3 3 temperature dependent resistivity of sparsely doped AZO films with Al concentration < ~ 2% would show SMT. Although SMT in AZO thin films has been reported previously [10][11][12] , a systematic and detailed study of SMT in sparsely doped AZO thin films with controlled doping concentration is lacking. We studied the SMT in sparsely doped AZO thin films as a function of Al doping concen...

Resistance switching characteristics, observed in metal oxide thin films, has recently attracted a great deal of attention to develop next generation low power, low cost, high speed, rugged and nonvolatile resistive random access memory (RRAM) devices. The memory effect in these materials is realized through the switching of the resistance of their thin films between two states of high and low resistances. Amongst the known metal oxides currently being explored for the development of RRAM, ZnO has been demonstrated to be a potential candidate. ZnO is an n‐type wide bandgap semiconductor and is highly transparent in the visible spectral region. Moreover its conductivity can be tailored in a broad range from metal like to insulator like by suitable impurity doping. Therefore it is possible to develop a fully transparent RRAM entirely based on ZnO. In this paper we report growth of a novel transparent RRAM devices based on ZnO and its variants. We studied the resistive switching characteristics of these devices and associated conduction mechanisms responsible for the switching. The details of this study will be presented in the paper (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)