We report on the synthesis and processing, and structure - property correlations in gallium doped ZnO films grown on (0001) sapphire and glass substrates by pulsed laser deposition. Films with varying microstructure were grown on amorphous glass by changing the pulsed laser deposition parameters, namely temperature and oxygen partial pressure. The results corresponding to these films were compared with those from epitaxial single crystal films grown on (0001) sapphire. It is shown that resistivities and transmittance comparable to epitaxial Zn0.95Ga0.05O films (&rgr;=1.4x10-4&OHgr;-cm, %T>80) can be achieved in the nanocrystalline films (&rgr;=1.8x10-4&OHgr;-cm, %T>80) deposited on glass by carefully controlling the deposition parameters. We have investigated and modeled the conduction mechanisms (carrier generation and carrier transport) in the novel Ga:ZnO films through detailed structural characterization, chemical analysis, and electrical and optical property measurements. The device applications based on these highly conducting and transparent films as electrodes will also be discussed. Our preliminary results have demonstrated that power conversion efficiencies comparable to indium tin oxide (ITO) based organic photovoltaic devices can be achieved using ZnGaO films as the anode.
Initial stages of misfit relaxation process in Ge epitaxial films grown by pulsed laser deposition on (001) Si substrates have been investigated by high-resolution transmission electron microscopy. Special emphasis is placed on conditions leading to a 2D (layer-by-layer) growth mode. The evolution of the dislocation network as a function of film thickness and thermal annealing is controlled by surface undulations and interactions between dislocations. The dislocation interactions leading to rearrangements in a nonequilibrium dislocation network driven by elastic interaction between parallel 60 degree(s) dislocation segments are discussed in detail. Based upon our experimental observations, we propose a model for the formation of stacking faults in heterostructures.
This paper reviews our recent work on laser processing and characterization of epitaxial TiN/Si heterostructures. Pulsed laser deposition (PLD) technique has been employed to grow TiN films on H- terminated Si(100) substrates at various temperatures in the range of 25 to 600 degree(s)C. A pulsed KrF excimer laser ((lambda) equals 248 nm, (tau) equals 25 X 10-9 sec) was used with the deposition chamber maintained at a base pressure of 10-7 Torr/. The films were characterized by x-ray diffraction technique, Auger electron spectroscopy, Raman spectroscopy, scanning and high resolution electron microscopy, Rutherford backscattering spectroscopy and four probe electrical resistivity. Auger and Raman spectroscopy revealed that the films were purely TiN and free from oxygen impurities. The x-ray diffraction and TEM results showed that the TiN films deposited at 600 degree(s)C were single crystal in nature with epitaxial relationship <100>TiN<100>Si. The RBS channeling yield for these films was found to be in the range of 10-13%. Four-point-probe electrical resistivity measurements showed characteristic metallic behavior of these films as a function of temperature with the lowest value of resistivity of about 15(mu) (Omega) -cm at room temperature. The epitaxial growth of TiN on Si(100) is rationalized in terms of domain matching epitaxy, where four unit cells of TiN match with three unit cells of Si with less than 4% misfit. This paper also describes the fundamental issues related to thin film growth, defect formation, atomic structure of defects and interfaces in semiconductor heterostructures.
Atomic structure of defects and interfaces in semiconductor heterostructures determine properties and operation of device structures of these materials. In order to minimize their adverse effects, we need to understand the atomic structure of these defects and correlations with physical properties and device operation. A three-step iterative procedure has been developed to determine atomic structure of dislocations, twins, stacking faults and grain boundaries. These steps consist of : (a) calculation of atomic structure of defects using appropriate interatomic potentials, (b) simulation of high-resolution TEM images, and (c) comparison with experimental images. By manipulating atomic structure of defects, electrically active defects can be converted into inactive defects, thus reducing their effectiveness as trap or recombination centers. We also discuss various methods which can be employed to reduce the number density or preferably eliminate the process-induced defects.
The superconducting properties of YBa2Cu3O7 thin films subjected to controlled pulsed nanosecond laser irradiation were investigated. Irradiated films on (100) LaAlO3 substrates showed excellent thermal stability with the temperatures for zero resistance of approximately 90 K even after irradiation with energy densities greater than 250 mH/sq cm. The critical current density Jc of the films showed an enhancement at low energy densities, followed by a drastic decrease in Jc above a certain energy threshold. This decrease has been correlated with the melting threshold. Similar results are observed for YBa2Cu3O7 films on (100) yttria-stabilized zirconia substrates; however, the energy density was found to be much smaller.
Metal organic chemical vapor deposition (MOCVD) has the potential of emerging as
a viable technique to fabricate ribbons, tapes, coated wires, and the deposition of films of
high temperature superconductors, and related materials. As a reduced thermal budget
processing technique, rapid isothermal processing (RIP) based on incoherent radiation as
the source of energy can be usefully coupled to conventional MOCVD. In this paper we
report on the deposition and characterization of high quality superconducting thin films
of Y-Ba-Cu-O (YBCO) on MgO, SrTiO3, and YSZ substrates by RIP assisted MOCVD.
Some preliminary results are also presented for the deposition of BaF2, Y203 and MgO
on silicon substrates. It is envisaged that high energy photons from the incoherent light
source and the use of a mixture of N2O and 02 as the oxygen source, assist chemical
reactions and lower the overall thermal budget for processing of YBCO films.
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