POSTER SESSION 1PO4: Thin Film and Device Processing

Monday, Feb. 21, 2:30 p.m. – 4:30 p.m., Hall D (GRB)

1PO4-1 New Methods to Obtain Large Area Thin Films

Armen S. Kuzanyan 1, Georgi R. Badalyan 1, Volodya E. Karapetyan 1, and Armen M. Gulian 2. 1 Institute for Physical Research, National Academy of Sciences, Ashtarak-2, 378410, Armenia. 2 NRL, Washington, DC, USA.

Presenting Author: A.S. Kuzanyan

New relatively simple methods of pulsed laser deposition of large area thin films are proposed. The peculiarity of the first method is the laser deposition of the compound upon a rotating substrate through a mask. The possibilities for different configurations of the split in the mask are considered. This consideration involves a preliminary determination of the required split in the mask at different distances from the rotation center by performing deposition of the compound on the static substrate, and fabrication of the corresponding corrected mask by taking into accounts the mass transfer of the deposited material at different parts of the substrate.

The second method employs controlled tilting of the target around the axis parallel to the substrate plane, while the respective positions of the laser beam, the focal spot, and the substrate are kept constant. The thickness uniformity of films obtained with these methods was preserved within the limits of ±3.5% on 100 mm-diameter substrates.

1PO4-2 Deposition Rate as the Key Parameter in Pulsed Laser Deposition of Oxide Films: A Practical Model and Experiment

Mikhail Strikovski 1, John H. Miller Jr. 1,2, and Jaroslaw Wosik 1,3. 1 Texas Center for Superconductivity, Houston, TX. 2 Department of Physics, University of Houston, Houston, TX. 3 Department of Electrical Engineering, University of Houston, 4800 Calhoun Rd., Houston, TX 77204, USA.

Presenting Author: M. Strikovski

Pulsed laser deposition (PLD) in ambient oxygen utilizes a jet of laser-produced plasma, the parameters of which depend strongly on distance from the target. We present a simple, practical model that describes the space - time evolution of the velocity and temperature of the propagating jet. The model defines and calculates the plume range (optimum target - substrate distance in PLD processes) and deposition rate as functions of oxygen pressure and jet intensity. One prediction is that, when the distance is optimized, the deposition rate will always be about 0.1 nm/pulse.

Experiments with PLD of superconducting (YBCO) and magnetoresistive (NSMO) films confirm the model predictions. We show that the oxygen pressure, P, is not an independent optimization parameter, but that the deposition rate, R, rather than P, often controls the as-deposited film quality. Practical PLD systems should allow for in-situ adjustment of the target - substrate distance if different materials are to be deposited. Higher quality films can be grown at reduced pressure, since R ~ P ^2/3 scaling is predicted when the distance is optimal. Scaling of the deposition parameters with laser pulse energy will be discussed.

1PO4-3 Thickness Limitation for Superconductivity of YBa₂Cu₃O_y Ultrathin Films

W.H. Tang, C.Y. Yau, C.Y. Ng, and J. Gao, Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong

Presenting Author: W.H. Tang

Superconducting YBa₂Cu₃O_y ultrathin films with thickness from 2nm to 25nm were deposited on (100) SrTiO₃ substrates. The thickness dependence of superconductivity of YBa₂Cu₃O_y thin films was investigated. It was found that T_c almost did not decrease for films with thickness greater than 15nm, but dropped significantly when less than 10nm. The variation of T_c with the film thickness fits an empirical relation of T_c0= T_cB(1-dm/d). Based on this relation, a critical thickness dm=1.56nm was deduced, suggesting that superconductivity can be expected for one unit-cell thick YBa₂Cu₃O_y films.

1PO4-4 YBCO Nanopowder: Novel Material for PLD Preparation of Thin Films

Reino Laiho, Hannu Huhtinen, Erkki Lahderanta, Petriina Paturi, and Jussi Raittila. Wihuri Physical Laboratory, University of Turku, FIN-20014, Turku, Finland

Presenting Author: R. Laiho

The main requirements for high-T_c superconductor films in device technology are high critical temperature, high critical current density, J_c, and very smooth surface (£ 2 nm roughness) with minimum amount of macroscopic defects. We discuss the influence of the microstructure of the deposition target on the quality of YBCO films prepared by pulsed laser deposition (PLD).

Our target is sintered from YBCO nanopowder prepared with a sol-gel technique to achieve high homogeneity of the material and small grain size without mechanical grinding. The powder is calcinated at a low temperature (@ 800 ° C) to limit thermal agglomeration of flake shaped particles with a width typically £ 50 nm. The main superconducting component of this material has T_c @ 60 K but 92 K can be achieved by annealing in air at T ³ 870 ° C, preserving the width of the particles still less than 120 n. Using a XeCl laser and the target prepared from the powder calcinated at 800 ° C, films with T_c @ 90 K and J_c @ 8x10⁶ A/cm² at 77 K are obtained.

Investigations by atomic force microscopy show that by controlling the laser energy a growth mode is found where only particles with thickness from 1 to 3 nm and width @ 50 nm are deposited from the target, forming a very smooth film. This type of growth obviously leads to a high density of internal defects, providing an efficient mechanism for flux pinning and high J_c.

1PO4-5 Thin-Film Surface Temperature Variations during Pulsed Laser Deposition

Cristina Buzea 1, Kensuke Nakajima 1,2, and Tsutomu Yamashita 1,2,3. 1 Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan. 2 CREST Japan Science and Technology Corporation (JST). 3 New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan.

Presenting Author: C. Buzea

Among the successful techniques used to date, the pulsed laser deposition has proven to be the most suitable for preparation of films with desired properties. Many studies have been devoted to the optimization of the laser deposition process. However, the variation of film temperature during deposition was neglected. It has been thought that if the substrate temperature, which is an essential factor for obtaining good quality thin films, is kept fixed, the film temperature will be also fixed during deposition.

This paper presents new results on the variation of the film surface temperature (T) during pulsed laser deposition as a function of process parameters (oxygen pressure in the chamber and target-substrate distance). Thin films of La-Sr-Cu-O were deposited on La-Sr-Ga-O(100) substrates by using a KrF excimer laser (5 Hz repetition rate). During the deposition, the temperature of the film surface was measured with an infrared pyrometer. For high O₂ pressure, T decreases with the deposition time (number of laser pulses), and follows the exponential law of radiation absorption in a material. The extent of T decrease depends strongly on process parameters. After 6000 laser pulses, it can attain nearly 40C. An important deviation from the theoretical law, of up to 10C, occurs at low O₂ pressures. The onset of deviation is depending on the target-substrate distance. Possible explanations of the film surface temperature variations are related to changing velocity of the ablated species. This change is explained by different target removal mechanisms - evaporation versus explosive removal.

1PO4-6 Interaction of YBCO Thin Film and Substrates and Its Effects on the Quality of the Film

H. Zhang, L.P. Yu, L.L. Cheng, Y.Z. Wang, and Z.Z. Gan, Materials Physics Laboratory, State Key Laboratory for Artificial Microstructure and Mesoscope Physics, Department of Physics, Peking University, Beijing 100871, China

Presenting Author: H. Zhang

YBCO thin films grown on SrTiO₃, yttria-stabilized zirconia (YSZ), and YSZ with CeO₂ buffer layer, by pulsed laser deposition, were studied by high resolution x-ray diffraction, including Rocking curve, and Phi-scan. It is shown that the distortion of the thin films is different in the different substrates. The mismatch between them can partially explain it. The larger mismatch corresponds to the larger distortion of the films, which can be determined by the full width at half maximum (FWHW) in the rocking curve and Phi-scan curve. As the mismatch between the film and substrate increases, the FWHW of both rocking curve and Phi-scan curve increases simultaneously. But the YBCO on the YSZ with CeO₂ buffer layer does not obey this regulation. The careful calculation of the cohesive energy of the thin films and substrates, especially, considering the effect of the interaction of the YBCO and substrates, indicate there is a close relationship between the cohesive energy and the distortion of the films. Using it to explain the distortion is better than using mismatch.

1PO4-7 YBa₂Cu₃O₇ on Sputter Deposited YSZ and ZrO₂ Buffered (100) Si

Philip D. Brown 1, Tasharuff Khan 1, Peter Batista 1, Felipe Leon 1, Dianne Dixon 1, Yuri A. Vlasov 1,2, Grover L. Larkins 1, patricia Stampe 3, and Robin Kennedy 3. 1 Future Aerospace Science and Technology Center for Space Cryoelectronics, Florida International University, Miami, FL, 33199. 2 On leave from the Ioffe Physico-Technical Institute, St. Petersburg, Russia. 3 Department of Physics, Florida Agriculture and Mechanical University, Tallahassee, FL, 32307.

Presenting Author: P.D. Brown

The focus of this work is to evaluate and compare Yttria Stabilized Zirconia (YSZ) and Zirconium Oxide (ZrO₂) thin films deposited on (100) silicon substrates, suitable for use as buffer layers for YBa₂Cu₃O₇ (YBCO). The YSZ and ZrO₂ layers were grown using on-axis pulsed D.C. (PDC) and R.F. magnetron sputtering. After the buffer layer was grown, few samples were annealed at T³ 900°C for 1 hour in the presence of oxygen, and cooled down. YBCO films, 200 nm thick, were grown on the buffered Si in the pulsed-laser deposition system in an atmosphere of 0.5 Torr O₂ at 750°C. The critical temperature (T_c(R=0)) of the films produced is currently 65 K - 75 K with an onset in the range of 85 K - 90 K.

1PO4-8 Development of Epitaxial YBCO films on Epitaxial layer of LaBa₂NbO₆ by pulsed laser ablation

J. Koshy 1, J. Kurian 1, J. James 1, S.P. Pai 2, and R. Pinto 2. 1 Regional Research Laboratory (CSIR), Trivandrum -695 019, India. 2 Tata Institute of Fundamental Research, Mumbai - 400 005, India.

Presenting Author: J. Koshy

LaBa₂NbO₆ has been synthesised and developed, for its use as substrate for YBCO and Bi(2223) superconductors. LaBa₂NbO₆ is now grown as an epitaxial layer on (100) MgO and (100) LaAlO₃ from sintered LaBa₂NbO₆ pellet by pulsed laser ablation. The conditions for the epitaxial growth of LaBa₂NbO₆ film on MgO and LaAlO₃ by pulsed laser ablation have been optimised. The epitaxial nature of LaBa₂NbO₆ film was confirmed by X-ray diffraction and AFM studies. Superconducting YBCO films are then in situ grown on the epitaxial thin layer of LaBa₂NbO₆. The YBCO thin films thus produced gave a T_c(0) = 90K with a transition width of 0.5K. The YBCO films exhibited (00l) orientation and showed almost perfect metallicbehavior in the normal state with resistance ratio (R300/R100) = 2.98. A transport current density, J_c = 6x10⁶ A/cm² was obtained for YBCO film developed on epitaxial LaBa₂NbO₆ film consistently.

1PO4-9 Step-flow Growth of High T_c YBa₂Cu₃O₇ Thin Films With Atomically Smooth Surface

Lin Li 1, Wen-fei Hu 1,2, Ying-fei Chen 1,2, and Tian-sheng Wang 1,2. 1 National Laboratory for Superconductivity, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China. 2 Department of Material Science, Yanshan University, Qinhuangdao 066004, China.

Presenting Author: Y.F. Chen

Most of the semiconductor devices, which require films grow epitaxially are fabricated using step-flow growth by molecular beam epitaxy due to better crystal quality than obtained by layer-by-layer growth. Recently, step-flow growth of SrTiO₃ (STO) thin films were produced by pulsed laser ablation, the high crystallinity of the films resulted in a marked increase of the dielectric properties, approaching to that of the bulk sample. Growing single crystalline and smooth continuous thin films of YBa₂Cu₃O_7-d (YBCO) is essential for the fabrication of devices, in particular those devices with heterostructures. However, there are still no reports of step-flow growth of the YBCO thin films. In our study, the normal and vicinal (STO) substrates were etched in a buffered NHF4-HF solution, and annealed subsequently at 800-1000° C in pure flowing O₂ for 2-6h. These STO substrates were examined by ex-situ atomic force microscopy (AFM), which displayed step-and smooth-terrace morphology. Then the YBCO thin films were deposited on these substrates with thickness varying from 5-100 unit cells of c-axis YBCO by pulsed laser ablation. Systematic study by AFM and transmission high-resolution electron microscopy showed that the YBCO films initiates with 2D nucleation on the terraces and form islands, (the 2D nucleation on the terraces changes with substrate temperatures following an Arrhenius-type behavior [A· exp(-E/kT]), when the islands coalesced and the terrace surfaces are completely overgrown, and many of the defects of the films were mended concurrently by insertion of stacking faults, step-flow takes place. As a result, c-axis oriented YBCO thin films with smooth surfaces can be produced on the STO substrate with a moderate miscut.