Donghwan Kim

Pulsed laser deposition of VO2 thin films

High quality vanadium dioxide (VO2) thin films have been successfully deposited by pulsed laser deposition without postannealing on (0001) and (1010) sapphire substrates. X‐ray diffraction reveals that the films are highly oriented with (010) planes parallel to the surface of the substrate. VO2 thin films on (0001) and (1010) substrates show semiconductor to metal transistions with electrical resistance changes as large as 4×104, 105, respectively. Thin films on (1010) substrate have a transition at as low as 55u2009°C with a hysteresis less than 1u2009°C. These transition properties are comparable with single crystal VO2.

Pulsed laser deposition of BaTiO3 thin films and their optical properties

Highly oriented a‐axis BaTiO3 thin films were grown by pulsed laser deposition on (001) MgO substrates. The full width at half‐maximum of the (200) BaTiO3 rocking curve was as narrow as 0.6°. A large hysteretic quadratic electro‐optic effect was observed in a transverse geometry at 6328 A. A birefringence shift up to 3×10−3 was measured at an applied dc electric field of 1 kV/mm. It was found that laser repetition rate plays an important role on the surface morphology of the deposited films.

Correlation between plasma dynamics and thin film properties in pulsed laser deposition

The pressure-distance scaling law for pulsed laser deposition is examined for several thin film systems. This scaling law is due to the plasma dynamics occurring within the laser plasma plume near the location of the substrate. Time-of-flight studies of both ions and neutrals confirm the existence of an optimal velocity distribution for optimal film deposition. Fast ions play a major role in determining the quality of the films deposited. They may provide surface activation of the film or induce damage to the film depending on their kinetic energies.

Pulsed laser deposited crystalline ultrathin indium tin oxide films and their conduction mechanisms

Abstract Crystalline ultrathin indium tin oxide (ITO) films were deposited on yttrium stabilized zirconia (YSZ) substrates by pulsed laser deposition. An X-ray rocking curve as narrow as 0.08° was obtained. The conductivity mechanism in these films was studied as a function of temperature down to 10 K. It was found that the temperature dependence of the conductivity was substantially different between thin (>10 nm) and ultrathin (

Study of photoresponse of high‐Tc Y‐Ba‐Cu‐O superconducting ultrathin films using a picosecond laser pulse train

We have systematically studied the photoresponse of Y‐Ba‐Cu‐O (YBCO) ultrathin films to a train of picosecond laser pulses. The onset and decay of nonequilibrium superconductivity due to different levels of optical excitation can be observed within the single pulse train. Thermal and nonthermal responses can be clearly identified. In addition, the superconducting film resistance was found to increase progressively with laser power above the onset threshold. The YBCO thin films were possibly excited into a nonequilibrium intermediate (resistive) state from the superconducting state. The laser intensity dependence indicated a nonthermal origin for these voltage pulses.

In and out‐diffusion of oxygen during YBa2Cu3O7−δ thin film deposition by in situ resistance measurement

The instantaneous oxygen content of YBa2Cu3O7−δ thin films was measured during pulsed laser deposition by in situ resistance measurements. The oxygen deficiency (δ) of the surface layer was found to be ∼0.835 during the laser plume and increase to 0.90 when the plume was turned off. Using a theoretical fit it was found that the thickness of the oxygen‐rich layer at the surface increased with increasing film thickness and saturated at ∼300 A for thick films. The out‐diffusion of oxygen in the absence of the laser plume can be well characterized by a simple diffusion equation, with a diffusivity of D=1.5×10−13 cm2/s. On the other hand, the in‐diffusion of oxygen during the laser plume occurs with a diffusivity of D≳10−8 cm2/s. The vastly different diffusivity is due to the presence of activated atomic oxygen in the laser plume.

Ultra Thin Indium Tin Oxide Films On Various Substrates By Pulsed Laser Deposition

Indium Tin Oxide (ITO) thin films with low resistivities of 0.1~0.2 mΩ-cm were deposited on various substrates such as YSZ, glass, and ZnO buffered glass by pulsed laser deposition (PLD). The X-ray rocking curve of crystalline (200) ITO films grown on (100) YSZ had a FWHM as narrow as 0.08. ITO films grown on ZnO (0001) buffered glass had an single (222) orientation and the X-ray rocking curve had a FWHM of 2.1. Ultrathin ITO films of 3.6nm were fabricated on YSZ and their electrical properties were measured from 10K-300K. ITO films fabricated on ZnO buffered glass and bare glass were characterized by Hall effect measurements as a function of temperature. The results indicate that the resistivity of ITO films grown by PLD does not depend on the orientation or the structure of the thin film. The resistivity is dominated by impurity scattering in the range of 10K-300K. We show that ZnO/glass is a good alternative to bare glass for producing commercial ITO films.

Critical current densities in BI‐textured polycrystalline superconducting thin films

The Jc of a polycrystalline film was studied experimentally and modeled by a percolation calculation. The superconducting thin film was modeled as a single layer of grains so that current limitation was considered as a linear optimization problem. Since the degree of in plane a-axis texturing of the grains in in situ Y-Ba-Cu-0 thin films was known, the critical current densities in polycrystalline superconducting thin films could be calculated. Comparison of transport and SQUID critical current densities from the experimental data and the calculated values in terms of intragrain critical current density, gave the estimation of Jc reduction across the 45” grain boundary.

In‐plane textured YBa2Cu3O7−δ thin films and their critical current characteristics

Bi‐textured YBa2Cu3O7−δ (YBCO) thin films with different ratios of [100] to [110] grains can be grown controllably on (100) yttria‐stabilized ZrO2 (YSZ) substrates using pulsed laser deposition. It was found that the ratio of [100] to [110] grains was determined predominantly by the laser fluence. By comparing the transport and intragranular critical currents, it is shown that the 45° grain boundaries (GBs) are only correspondent to about 18x reduction in Jc across polycrystalline bi‐texture samples. A percolation simulation was carried out to find out the relationship between Jc reduction across a single GB and in a polycrystalline thin film sample with many GBs. The calculated results were compared with the experimental data. The transport critical current for different films were also studied in the presence of applied magnetic field.