Eiji Fujimoto

Hetero-Epitaxial Growth of ZnO Film by Temperature-Modulated Metalorganic Chemical Vapor Deposition

ZnO films were fabricated by metalorganic chemical vapor deposition while a laser substrate heating system was used to rapidly cycle the sample temperature. When a hydrogen gas atmosphere was used together with the temperature modulation, ZnO films deposited from diethyl-zinc and oxygen exhibited oxygen-face polarity and a smooth surface. The quality of ZnO films was good enough to observe phonon replicas corresponding to the transition from donor-bound to free exciton in the temperature dependence of photoluminescence. The growth of ZnO film is discussed in relation to the combined effect of growth temperature modulation and the hydrogen atmosphere.

Reduction of nonradiative recombination center for ZnO films grown under Zn-rich conditions by metal organic chemical vapor deposition

ZnO films were grown by metalorganic chemical vapor deposition using the repeated temperature modulation in an H2 ambient. The crystalline quality, as defined by the full-width at half-maximum of the ω (101¯1) reflection, was found to be correlated with the photoluminescence (PL) lifetime at 300 K. The fine structure of the PL spectra for the samples grown under Zn-rich condition at 8 K indicated the higher order (n=2) structure of free exciton A, two electron satellites and phonon-replicas. The internal quantum efficiency and the PL lifetime at 300 K were 5.5% and 2.6 ns, respectively, indicating a reduction in Zn vacancies in the ZnO films.

Development of a new laser heating system for thin film growth by chemical vapor deposition.

We have developed a new laser heating system for thin film growth by chemical vapor deposition (CVD). A collimated beam from a high-power continuous-wave 808 nm semiconductor laser was directly introduced into a CVD growth chamber without an optical fiber. The light path of the heating laser inside the chamber was isolated mechanically from the growth area by bellows to protect the optics from film coating. Three types of heat absorbers, (10 × 10 × 2 mm(3)) consisting of SiC, Ni/NiO(x), or pyrolytic graphite covered with pyrolytic BN (PG/PBN), located at the backside of the substrate, were tested for heating performance. It was confirmed that the substrate temperature could reach higher than 1500 °C in vacuum when a PG/PBN absorber was used. A wide-range temperature response between 400 °C and 1000 °C was achieved at high heating and cooling rates. Although the thermal energy loss increased in a H(2) gas ambient due to the higher thermal conductivity, temperatures up to 1000 °C were achieved even in 200 Torr H(2). We have demonstrated the capabilities of this laser heating system by growing ZnO films by metalorganic chemical vapor deposition. The growth mode of ZnO films was changed from columnar to lateral growth by repeated temperature modulation in this laser heating system, and consequently atomically smooth epitaxial ZnO films were successfully grown on an a-plane sapphire substrate.

Fabrication of all YBaCuO trilayer Josephson junctions with YBaCuO wiring layer

We report a fabrication process of c-axis oriented all YBaCuO trilayer Josephson junctions with YBaCuO wiring layers. The trilayer junctions consisted of YBaCuO (170nm)/PrBaCuO (28 nm)/YBaCuO (170 nm) structures. CeO/sub 2/ films (520nm) and YBaCuO films (1.2 /spl mu/m) were used as insulating layers and wiring layers, respectively. A junction with a dimension of 7 /spl mu/m/spl times/7 /spl mu/m showed a critical current I/sub c/ of 1.6 mA and a junction resistance R/sub n/ of 1.2 /spl Omega/ at 4.2 K. The values of the critical current density and the I/sub c/R/sub n/ product were estimated to be 3.3kA/cm/sup 2/ and 1.9 mV, respectively. Note that the junction exhibited I/sub c/R/sub n/ product of 1.0 mV even at 30 K. From these results, the junction is promising candidate for satisfying the requirements for HTS integrated circuits.

In-situ characterization of engineered surfaces of c-YBCO films for sandwich type junctions

In-situ characterization of the surface nature and re-crystallization of c-axis YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films engineered by Ar ion etching and subsequent annealing has been carried out. Effects of kinetic energy of the Ar ions and annealing conditions have been examined. RHEED and XPS measurements showed that etching with the ion energy above a few hundred eV for longer than 60 sec yielded insulating and amorphous surface layers. Analysis of XPS core signals revealed a deviation of surface composition toward a Y-rich one with an increase of the product of [ion flux] /spl times/ [ion energy] /spl times/ [etching time], whereas thickness and degree of reduction of the amorphous layer were dominated by the kinetic energy of the Ar ions: the higher ion energy resulted in a thicker and less reduced layer. For the amorphous layer created by the 500 eV-beam etching, oxidation-annealing at 630/spl deg/C for 1h is sufficient to convert it into a metallic 123 structure. For the surfaces treated by the 1 keV etching, an insulating feature was conserved even after the annealing of 710/spl deg/C, 1 h or 660/spl deg/C, 3 h. These results mean that the higher energy etching and the shorter subsequent process are desired to properly fabricate barrier layers for the interface-engineered sandwich type of junctions.