Photo-assisted MOCVD Research Articles

Photo-assisted MOCVD growth of YBCO thick films for wire applications

Abstract Photo-assisted MOCVD has been applied to the fabrication of thick film YBa 2 Cu 3 O 7− x for wires and tapes. The application of photo-irradiation as the sole source of energy for the MOCVD reaction has resulted in extremely rapid growth rates for YBCO films with high superconducting performance. Typical YBCO films of thickness >1 μ m exhibit J c ∽1×10 6 A cm −2 and T c >90 K on single crystal oxide substrates. Such films have now been integrated to metal substrates including both single crystal Ni(100) and roll-textured nickel foil. YBCO films grown on these substrates with CeO 2 and YSZ buffer layers show J c >3×10 5 A cm −2 for films of >1 μ m thickness grown in 2–4 min time. Such performance for thick films made in such a short time is promising for YBCO wire fabrication with optimization of the photo-assisted MOCVD process expected to yield J c >1×10 6 A cm −2 for 1–5 μ m thick YBCO films on flexible nickel substrates.

Optimization of Jc of YBCO thin films prepared by photo-assisted MOCVD through statistical robust design

Intrinsically, the thin-film preparation technique of metalorganic chemical vapor deposition involves a large number of control variables which are especially critical in work with quaternary or higher high- T c materials. Thus, effective methods are needed to optimize variable settingsin the preparation of high- T c films. A matrix experimental plan and robust design methodology has been employed for this purpose. The first design phase was based on existing knowledge regarding growth temperature, pressure and annealing temperature for photo-assisted preparation of YBCO thin films on LaAlO 3 (100) substrates. A battery of only nine experiments plus three confirmation depositions were then used to optimize the process control parameters of precursor oven temperature, carrier gas (Ar) flow rate, and the flow rates of two oxidizing gases, O 2 and N 2O, with respect to critical current density ( J c) of the films. J c (77 K, zero field) values for deposited YBCO films (with thickness in the 0.4–0.6 μm range) were found to be improved from about 1 to 3 × 10 6 A/cm 2. J c for a thinner sample (0.15 μm) reached 5.0 × 10 6 A/cm 2. It was found that under these processing conditions the most significant process control parameter affecting J c is the flow rate of O 2, or more precisely, the oxidizing agent flow-rate ratio of O 2 and N 2O. Analysis of the variance of the J c data indicated that there likely exist interactions among some of the control variables, most obviously between the flow rates of O 2 and N 2O.

High-rate growth of purely a-axis oriented YBCO high- Tc thin films by photo-assisted MOCVD

The growth of purely a-axis oriented YBCO thin films on LaAlO 3 (100) substrates with no template layer addition has been accomplished by photo-assisted MOCVD. In addition to the requirement of lattice-matched substrates, the difficulty of growth of purely a-axis oriented YBCO films with this processing technique has been overcome by utilizing three appropriate processing/growth factors: substrate temperature ( T s), total partial pressure of oxidizing gases O 2 and N 2O ( P(O 2 + N 2O)), as well as high film growth rate. A high growth rate (up to 7000 a/min) has been accomplished through MOCVD growth assisted by photo-irradiation with tungsten-halogen lamps. Variance of the three factors leads to tailored growth of either a-axis, c-axis or [110] oriented growth for YBCO. The growth of a-axis YBCO thin films under different precursor flow rates also affects the film morphology.

Optimization of Jc for Photo-Assisted MOCVD Prepared YBCO Thin Films by Robust Design

AbstractMetalorganic chemical vapor deposition (MOCVD) is emerging as a practical high Tc superconducting thin film preparation technique for industrial application. Intrinsically this technique involves a large number of variable parameters. This is especially critical for the quarternary or higher high Tc materials. Thus, effective methods are required to optimize the parameters for the preparation of high Tc films. A matrix experimental design named Robust Design has been employed for this purpose. The first-phase design was based on a starting knowledge of growth temperature and pressure, and annealing temperature for MOCVD preparation of YBCO thin films. A minimum lab effort of only nine deposition experiments was then used to optimize the process control parameters of precursor oven temperature, carrier gas (Ar) flow rate, O2 flow rate and N2O flow rate. The results were then followed by three confirmation depositions. The Robust Design resulted in the growth of YBCO film with Tc consistently in the range of 87.0 K to 90.2 K and Jc improved from about 1.0 × 106 A/cm2 to 3–5 × 106 A/cm2.

Reaction measurements and growth of HgTe/CdTe by photo-assisted MOCVD using an excimer laser

Abstract The photoabsorption and photodecomposition properties of source gases for Cd x Hg 1− x Te including new Te sources have been measured and in-situ measurements of MOCVD have been carried out by photoabsorption and laser spectroscopy. The results of concentration and decomposition measurements of source gases show that these in-situ measurements proved to be useful for MOCVD. Based on these techniques, the epitaxial growth of HgTe/CdTe has been demonstrated by photo-assisted MOCVD using an excimer laser at growth temperatures of 100 and 150°C.

The effects of photo-assisted MOCVD on the doping of Na acceptors into ZnS epitaxial layers

Photo-assisted (PA) MOCVD has been used under low-pressure conditions of 3 Torr to grow ZnS films on (100) GaAs substrates. This method has been shown to enhance the Na acceptor-bound exciton line at 3.783 eV at 4.2 K in intentionally Na-doped ZnS films. The growth rate was not enhanced by the PAMOCVD, but the crystal quality was found to be significantly improved as evidenced by the appearance of free-exciton emission around 3.80 eV. It is tentatively proposed that the observed effects are due to Zn atom desorption which allows Na ions to be incorporated as acceptors.

Photo-Assisted Metal Organic Chemical Vapor Deposition Preparation of Polycrystalline ZnS:Mn Films for Thin Film Electroluminescent Devices

Polycrystalline ZnS:Mn films with good crystallinity are grown by photo-assisted MOCVD using dimethylzinc and diethylsulphide. The growth temperature is decreased from 550 to 400°C by photo-irradiation (high-pressure mercury lamp) and no dead layer results. Patterned ZnS films can also be grown by this method. Thin film electroluminescent devices fabricated using these films have lower threshold voltages.