Y. Q. Li

TRIBOLOGY OF DIAMOND-LIKE CARBON SLIDING AGAINST ITSELF, SILICON NITRIDE, AND STEEL

Diamond-like carbon (DLC) films were deposited on (100) silicon wafers and silicon nitride balls by RF plasma-assisted chemical vapor deposition at a pressure of 700 mTorr and a substrate temperature of 360 K. The friction coefficient and the wear rates were measured using a pin-on-disk tribometer in 40% humid and dry air. Friction coefficients are near 0.05 in all cases measured. In dry air, the wear of silicon nitride and steel against DLC is below measurement capability because of a protecting DLC transfer layer, and wear of DLC is 2.5 ± 10 −8 mm 3 /Nm against silicon nitride and 6.5 ± 10 −9 mm 3 /Nm against steel. In humid air, the DLC transfer layer does not adhere to the solids, and wear of both bodies is larger. Unmeasurable wear is obtained when DLC slides against itself in humid air; the wear rate is 5 ± 10 −9 mm 3 /Nm in dry air. These results are interpreted in terms of the properties of a friction-induced transformation of the surface layer of DLC.

Epitaxial growth of BaTiO3 thin films by plasma‐enhanced metalorganic chemical vapor deposition

High‐quality BaTiO3 thin films have been epitaxially grown on (001) LaAlO3 and (001) NdGaO3 substrates by plasma‐enhanced metalorganic chemical vapor deposition at a substrate temperature of 680 °C. X‐ray diffraction θ–2θ, ω, and φ scan results all indicate that single‐crystalline BaTiO3 thin films were epitaxially grown on the substrates with 〈100〉 orientation perpendicular to the substrates. The high degree of epitaxial crystallinity is further confirmed by Rutherford backscattering spectrometry which gives a minimum yield of 7.5% and 11% for films deposited on LaAlO3 and NdGaO3, respectively. Cross‐section high‐resolution electron microscopy images also showed that the layer epitaxy of BaTiO3 was characterized by an atomically abrupt film/substrate interface. Scanning electron micrographs showed that these films had very smooth surface morphologies.

HIGH CRITICAL CURRENT DENSITIES IN YBA2CU3O7-X THIN FILMS FORMED BY METALORGANIC CHEMICAL VAPOR DEPOSITION AT 730 C

YBa2Cu3O7−x superconducting thin films with a critical current density of 2.3×106 A/cm2 at 77.7 K and 0 T were prepared by a metalorganic chemical vapor deposition process. The films were formed in situ on LaAlO3 at a substrate temperature of 730 °C in 2 Torr partial pressure of N2O. Resistivity and magnetic susceptibility measurements of the as‐deposited films show a sharp superconducting transition temperature of 89 K with a narrow width of less than 1 K. Critical current densities were measured by the dc transport method with a patterned bridge of 120 μm×40 μm. Both x‐ray diffraction and high‐resolution electron microscopy measurements indicate that films grew epitaxially with the c axis perpendicular to the surface of the substrate.

Low‐temperature in situ formation of Y‐Ba‐Cu‐O high Tc superconducting thin films by plasma‐enhanced metalorganic chemical vapor deposition

Highly textured, highly dense, superconducting YBa2Cu3O7−x thin films with mirror‐like surfaces have been prepared, in situ, at a reduced substrate temperature as low as 570 °C by a remote microwave plasma‐enhanced metalorganic chemical vapor deposition process (PE‐MOCVD). Nitrous oxide was used as the oxidizer gas. The as‐deposited films grown by PE‐MOCVD show attainment of zero resistance at 72 K. PE‐MOCVD was carried out in a commercial scale MOCVD reactor.

Effects of composition on microstructure and superconducting properties of YBa2Cu3O7−x thin films prepared by plasma enhanced metalorganic chemical vapor deposition

Abstract The microstructure and the superconducting properties of YBa 2 Cu 3 O 7− x thin films prepared by plasma-enhanced metalorganic chemical vapor deposition have been investigated systematically as a function of metal composition. Yttria precipitates are not apparent on the surface of yttrium-rich films. They are densely distributed within the films, their average size is of the order of 5–10 nm and their density can be as high as 10 24 /m 3 . Excess copper leads to the precipitation of copper oxide (CuO) particles on the surface of the films, but they are not found in the bulk. High transition temperatures and high critical current densities have been obtained over a wide range of compositions. Transition temperatures higher than 86 K are always obtained when the Cu/Ba ratio is larger than the stoichiometric ratio of 1.5. Films with Cu/Ba ratio larger than 1.5 and a Ba/Y ratio less than 1.7 usually have a critical current density larger than 10 6 A/cm 2 at 77 K and 0 T. The dependence of the critical current density on temperature follows a power law, J c  A (1− T / T c) n . The value of n is 2 for stoichiometric and barium-rich films and 1 for yttrium-rich films. The best films with transition temperatures of 90K, critical current densities in excess of 10 6 A/cm 2 at 77.5 K, and smooth surfaces are observed when the Ba/Y ration is around 1.6 and the Cu/Ba ratio is around 1.8.

Compositional effects on plasma-enhanced metalorganic chemical vapor deposition of YBa2Cu3O7-x thin films

Epitaxial YBa2Cu3O7−x superconducting thin films with a zero resistance transition temperatures of about 90 K have been prepared, in situ, on LaAlO3 by a plasma‐enhanced metalorganic chemical vapor deposition process at a substrate temperature of 670 °C in 1 Torr partial pressure of N2O. The composition of the films was varied systematically to investigate the effect of changes in the Ba/Y and Cu/Y ratio on the film properties. The results indicated that superconducting current densities exceeding 106 A/cm2, measured at 77 K by a transport method, could be obtained on films with an anomalously wide range of film compositions. Excess Cu (up to 60%) and deficiency in Ba (down to 30%) from their stoichiometric values did not significantly degrade the superconducting properties of the films. As the composition approached the Y‐Ba‐Cu ratio of 1‐2‐3, an improvement in surface morphology and a decrease in superconducting transition temperature were found.

Epitaxial thin films of YBa2Cu3O7−x on LaAlO3 substrates deposited by plasma‐enhanced metalorganic chemical vapor deposition

High quality epitaxial YBa2Cu3O7−x (YBCO) superconducting thin films (0.3 μm thick) were grown on the closely lattice and thermal expansion matched substrate, LaAlO3, which has low dielectric loss. The YBCO layers were prepared, in situ, by a microwave plasma‐enhanced metalorganic chemical vapor deposition process. The films, which had mirror‐like smooth surfaces, were deposited at a substrate temperature of 730 °C with a partial pressure of 2 Torr of N2O. The electrical resistance and magnetic susceptibility versus temperature of the as‐deposited films show metallic behavior in the normal state and sharp superconducting transitions with Tc (R=0) of 88 K. Critical current densities measured on patterned bridges were 5×105 A/cm2 at 78 K for the films deposited on LaAlO3. X‐ray diffraction measurements indicate that films grow epitaxially in the plane of the substrate with axis perpendicular to the substrate surface.

Superconducting YBa2Cu3O7−x thin films on silver substrates by in situ plasma‐enhanced metalorganic chemical vapor deposition

An in situ microwave plasma‐enhanced metalorganic chemical vapor deposition process was used to fabricate highly c‐axis oriented YBa2Cu3O7−x superconducting thin films on metallic Ag substrates. The films were deposited at a reduced substrate temperature of 740 °C in about 270 Pa of N2O ambient. Magnetic susceptibilities versus temperature of the as‐deposited films show attainment of zero resistance of 85 K and composition of single (high Tc) phase. X‐ray diffraction measurements reveal that the films deposited at 740 °C have highly preferential orientation of the crystallite c axes perpendicular to the substrate surface.

High‐quality YBa2Cu3O7−x thin films by plasma‐enhanced metalorganic chemical vapor deposition at low temperature

Single‐crystalline epitaxial YBa2Cu3O7‐x thin films with a sharp superconducting transition temperature of 90 K and a critical current density of 3.3×106 A/cm2 at 77 K were prepared by a plasma‐enhanced metalorganic chemical vapor deposition (PE‐MOCVD) process. The films were formed in situ on (100) LaAlO3 substrates at a temperature of 670 °C in 2 Torr partial pressure of N2O. X‐ray analysis indicated that films grew epitaxially with the c‐axis perpendicular to the substrate and the a and b axes uniformly aligned along the LaAlO3 [100] directions. High‐resolution transmission electron microscopy along with electron diffraction revealed that the films grew epitaxially with atomically abrupt film‐substrate interfaces. The high degree of epitaxial crystallinity of the films was also confirmed by Rutherford backscattering spectroscopy which gave a minimum channeling yield of 9%.

Effects of Y2O3 precipitates on critical current anisotropy in YBa2Cu3O7−x thin films prepared by plasma‐enhanced metalorganic vapor deposition

Yttrium‐rich YBa2Cu3O7−x thin films containing yttria precipitates with an average size of less than 10 nm and with densities of up to 1024/m3 have been prepared by metalorganic chemical vapor deposition. The field dependence of the critical current density of the yttrium‐rich films with the c‐axis normal to the surface indicates that the precipitates act as effective pinning centers when the magnetic field is perpendicular to the CuO2 planes while their presence has no effect on the critical current density when the magnetic field is parallel to the CuO2 planes. In this case, intrinsic pinning of the weakly coupled CuO2 planes may be the dominant pinning mechanism in c‐axis oriented YBa2Cu3O7−x thin films.