High-quality Epitaxial Thin Films Research Articles

Preferred growth of epitaxial TiN thin film on silicon substrate by pulsed laser deposition

Epitaxial TiN thin films on silicon substrates were prepared by pulsed excimer laser (KrF, 34 ns) ablation of a hot-pressed TiN target in nitrogen gas atmosphere. X-ray diffraction (XRD) showed that the preferred orientations of TiN thin films did not change with substrate temperatures, nitrogen gas pressure and film thickness; however, they did change with the orientations of substrates. The epitaxial orientation relationships between high-quality epitaxial TiN thin films and silicon substrates [2 4 2] TiN∥ [2 4 2] Si, (1 1 1) TiN∥(1 1 1) Si and [3 1 1] TiN∥ [3 1 1] Si, (1 0 0) TiN∥(1 0 0) Si. The full-width at half-maximum (FWHM) of the rocking curve of XRD and the minimum channelling yield of Rutherford backscattered spectroscopy (RBS) of the epitaxial TiN thin film were estimated to be 0.3 ° and 7.3%, respectively, indicating excellent crystalline quality of the grown film. X-ray photoelectron spectroscopy confirmed that the binding energies of Ti 2p 3/2 and N 1s core levels in epitaxial thin film were 455.2 and 397.1 eV, respectively, corresponding to those of TiN bulk. By calibrating the RBS spectra, the chemical composition of TiN thin films was found to be titanium-rich. The typical surface roughness of TiN thin film observed by scanning probe microscopy was about 1.5 nm. © 1998 Chapman & Hall

Effect of sputtering parameters on superconducting properties of GdBaCuO epitaxial thin film deposited by ICP method

Thin films of GdBaCuO (GBCO) have been deposited in situ onto LaAlO3 single crystal substrates by inverted cylindrical sputtering pattern (ICP). The superconductive properties of the thin films' dependence on the substrate temperature and sputtering pressure have been systematically investigated. By optimization of the deposition parameter, high-quality c-axis epitaxial GBCO thin films of Tc0>92 K were reproducibly grown. The Tc of the best sample is as high as 93.2 K. Upon changing the target composition to GdBa2Cu4O y (Gd124), it was observed that the samples always show some a-axis oriented films, implying that excess copper would favor a-axis growth in thin films. The superconductivity of the thin films under higher substrate temperature (Ts>800°C) was clearly improved by the procedure of special post-oxygenization at 400°C with an ozone atmosphere. This is very useful for preparing large-area thin films of GBCO.

Tunneling junctions of the heavy-fermion superconductor UPd 2Al 3

Tunneling spectroscopy on planar Giaever-type junctions is a powerful tool for the investigation of the superconducting state of metals. Since it is possible to prepare high-quality epitaxial thin films of the heavy-fermion compound UPd 2Al 3, this method can be used to examine the energy gap of this presumably unconventional superconductor. We prepared cross-junctions consisting of a UPd 2Al 3 base electrode and a metal counter electrode (Au, Al or Ag). These small area contacts without artificial barriers have only low junction resistances and suffer from irreproducibility. On the other hand, on some of those junctions we observed BCS-like tunneling conductivity. In order to increase the junction resistances AlO x or UO x were used as a artificial barrier with Al as a counter-electrode. As an alternative to quasi-particle tunneling, the preparation of Josephson-junctions could be an easier approach. A supercurrent between a small In-dot and an UPd 2Al 3 film was observed.

A structural investigation of high-quality epitaxial thin films

The (PZT 53/47) and (PZT 20/80) films deposited on substrates by RF magnetron sputtering are single-phase epitaxial films. In order to prevent large thermal stresses being induced in the films during the preparation process, the PZT films were cooled slowly through the Curie temperature. Scanning electron microscopy (SEM) showed that the PZT 53/47 film had a pyramid structure on its surface, whereas the PZT 20/80 film had a smooth surface without any observable features. Transmission electron microscopy (TEM) observations showed similar features: the PZT 53/47 film had a coarse columnar structure, whereas the PZT 20/80 film had a fine crystalline structure; this is due to the better lattice matching in the latter.

Control of Crystallinity in Sol-Gel Derived Epitaxial LiNbO3 Thin Films on Sapphire

Control of crystallinity in solid phase epitaxial growth of LiNbO3 thin films derived from methoxyethoxide solution on sapphire substrates has been attempted by investigating growth variables. Crystallization at 700° C for longer than 30 min was necessary for full crystallization of LiNbO3 thin films, although the orientations of the LiNbO3 crystal planes parallel to the substrates were almost independent of crystallization temperature and crystallization time. Layer-by-layer crystallization and preparation of an ultrathin initial layer have been found to be important in the growth of high-quality epitaxial LiNbO3 thin films. X-ray diffraction analysis revealed that perfectly single-plane-oriented epitaxial LiNbO3 thin films, with rocking curve full width at half-maximum of less than 0.07°, were produced on sapphire (001) substrates. Refractive index of 2.24 and optical propagation loss of 3.0 dB/cm were achieved.

Superconducting properties of epitaxial laser ablated thin films

We present experimental results obtained for high quality epitaxial thin films (film thicknesses around 5000 Å; rocking curve FWHM down to 0.1°). These films are obtained by laser ablation of REBa 2Cu 3O 7−δ (RE = Y, Er) deposited on SrTio 3 and YSZ substrates. The superconducting properties have been studied by complex susceptibility in an extended AC field range ( h ac ≤ 300 Oe) and show sharp magnetic transition width and high critical currents (10 7A cm −2 < j c (77K) < 10 8A cm −2). The loss peak is only weakly depressed by an increasing AC field and the observed shifts are up to an order of magnitude lower than those observed for intragranular contributions in bulk samples. The agreement with the behaviour expected from a critical state model is also discussed.

Study of high-quality epitaxial YBCO thin films grown directly on Y-Cut LiNbO3

Pulsed laser deposition was used to deposit high-quality YBa2Cu3O7-δ (YBCO) thin films directly on y-cut LiNbO3 substrates. The as-deposited YBCO films had a high degree of in-plane orientation and showed superconducting transition temperature (Tco) at 91K with a transition width of less than IK. Transport critical current densities were found to be ∼106 A/cm2 at 77K and zero field. An ion beam minimum channeling yield of 16% was obtained for YBCO films, indicating high crystallinity. High-resolution transmission electron microscopy studies showed that the interface between the film and the substrate was quite smooth and free from interfacial interdiffusion. The defects in thin films are also identified. The work showed that high-quality high Tc thin films can be deposited directly on LiNbO3. Novel devices based on the properties of both YBCO and LiNbO3 could be realized based on these results.

Growth of La–Ca–Mn–O films by ozone-assisted molecular beam epitaxy

We have grown very high quality epitaxial thin films of La–Ca–Mn–O on SrTiO3 and LaAlO3 using ozone-assisted molecular beam epitaxy and the block-by-block deposition technique. Films of thicknesses of at least 600 Å are fully strained and are tetragonal on both SrTiO3 (100) and LaAlO3 (100) substrates, and are single phase with extremely low rocking curve widths (about 0.067°), suggesting a high degree of structural perfection. Magnetoresistance values of 900% at an applied field of 5.5 T have been obtained for as-deposited La0.58Ca0.33MnO3 films.

Structural and electronic properties of defects in semiconductors

The development of growth techniques such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy during the last fifteen years has resulted in the growth of high quality epitaxial semiconductor thin films for the semiconductor device industry. The III-V and II-VI semiconductors exhibit a wide range of fundamental band gap energies, enabling the fabrication of sophisticated optoelectronic devices such as lasers and electroluminescent displays. However, the radiative efficiency of such devices is strongly affected by the presence of optically and electrically active defects within the epitaxial layer; thus an understanding of factors influencing the defect densities is required.Extended defects such as dislocations, twins, stacking faults and grain boundaries can occur during epitaxial growth to relieve the misfit strain that builds up. Such defects can nucleate either at surfaces or thin film/substrate interfaces and the growth and nucleation events can be determined by in situ transmission electron microscopy (TEM).

High critical current density in epitaxial YBa 2Cu 3O 7 thin films

Very large critical current densities are obtained reproducibly in epitaxial YBa 2Cu 3O 7−δ thin films prepared on (1 0 0) SrTiO 3 and MgO substrates by inverted cylindrical magnetron sputtering. The structure properties studied by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, and Rutherford backscattering show a very high-quality epitaxial thin film growth. Critical current densities of the order of 10 7 A/cm 2 at 77 K were measured directly and are in agreement with indirect AC-susceptibility measurements. A flux creep model gives a good agreement with the experiment data.

Atomically regulated layer-by-layer growth of SrTiO 3 on a-axis oriented YBa 2Cu 3O 7−δ thin films

The intensity oscillation of reflection high energy electron diffraction (RHEED) was observed during the pulsed laser deposition of ultra-thin SrTiO 3 films on a-axis oriented YBa 2Cu 3O 7−δ (YBCO) thin films. This enabled us to control digitally the overlayer thickness on an atomic scale. In the same deposition chamber, we could fabricate high quality epitaxial YBCO thin films on SrTiO 3 with Tc exceeding 80K and 90K for a-and c-axis oriented films, respectively. Reducing the particle density below 10 5cm −2 on the bottom YBCO films should be promising to construct Josephson tunnel junctions.

Systematic studies on the growth process of superconducting YBa2Cu3O7−δ and Bi2Sr2CuOy thin films by scanning tunneling microscopy

We have systematically studied the growth mechanism and surface topography of YBa2Cu3O7−δ (YBCO) and Bi2Sr2Ca0Cu1Oy-2201 (BSCCO) epitaxial grown superconducting films by scanning tunneling microscopy. Here we report two growth mechanisms of screw dislocation growth and layered growth; the surface characterization; and surface modification of thin films under a controlled manner. Excimer laser ablation technique was applied to synthesize high-quality epitaxial thin films. YBCO thin films were epitaxially grown with c axis perpendicular to the SrTiO3 (100) substrate surface. If the YBCO films were grown on the substrate which normal is slightly off the ideal [001] orientation, the films show layered growth. Whereas on the flat substrate, the films were nucleated and grown with the screw dislocation manner. By using the effect of field-induced evaporation, we were able to analyze the initial stage of growth. Epitaxial grown BSCCO-2201 films on ZrO2 substrates were synthesized and studied by scanning tunneling microscopy (STM). Layered growth and larger atomic flat areas were observed, which are more stable under STM than YBCO films. No screw dislocations were identified in BSCCO-2201 films. The reasons of the different behaviors of the two kinds of thin films are discussed.

Fundamental Materials-Issues involved in the Growth of GaN by Molecular Beam Epitaxy

AbstractGallium nitride is one of the most promising materials for ultraviolet and blue light-emitting diodes and lasers. Both Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) have recently made strong progress in fabricating high-quality epitaxial GaN thin films. In this paper, we review materials-related issues involved in MBE growth. We show that a strong understanding of the unique meta-stable growth process allows us to correctly predict the optimum conditions for epitaxial GaN growth. The resulting structural, electronic and optical properties of the GaN films are described in detail.

The growth mechanism and topography of superconducting YBa 2Cu 3O 7−δ and BiSrCaCuO-2201 films studied by scanning tunneling microscopy

The growth mechanism and surface topography of YBa 2Cu 3O 7 − δ (YBCO) and BiSrCaCuO-2201 (BSCCO) epitaxially grown superconducting films have been studdied by scanning tunneling microscopy. We report here two growth mechanisms: screw dislocation growth and layered growth, and also the surface characterization and surface modification of thin films under a controlled manner. High-quality epitaxial thin films were grown in situ by excimer laser ablation. YBCO thin films were epitaxially grown with the c-axis perpendicular to the SrTiO 3 (100) substrate surface. On the flat substrate, the YBCO films were nucleated and grown in the screw dislocation manner. On the titled substrate (α<1 deg.), the films show layered growth. The etching caused by field-induced evaporation has been utilized to observe the initial stage of growth. Epitaxially grown BSCCO-2201 films were firstly synthesized in our laboratory with the c-axis perpendicular to the ZrO 2 substrates. Layered growth has been observed by STM. No screw dislocations were identified in our samples. Larger atomic flat areas were present, which are more stable than YBCO films. As a parent phase for the BSCCO 2212, 2223 series, the study of the growth mechanism of 2201 films is very important to the synthesis of superconducting 2212, 2223 or other artificial layered thin films.

Angular dependence of the zero resistance superconducting glass state in YBCO thin films

The authors present current versus voltage and resistivity data as a function of angle, magnetic field, and temperature. The critical scaling exponents extracted from the data are consistent with the new theories of the vortex and Bose glass. However, the angle dependence of the glass transition temperature is inconsistent with the vortex glass model and is explainable in a model utilizing both the vortex and Bose glass theories. The samples, high quality epitaxial thin films of YBCO, were also studied using the atomic force microscope and X-ray diffraction techniques. The effects of the microstructure on angle-dependent current versus voltage measurements within the vortex glass framework are discussed. >

Superconducting properties and structural perfection of epitaxial YBa 2 Cu 3 O 7- x thin films

High quality epitaxial YBa2Cu3O7-x thin films have been successfully prepared by dc magnetron sputtering deposition, on (100) and (110) aligned SrTiO3, LaAlO3 and yttria-stabilized zirconia (YSZ) substrates. The films showed zero resistance around 90 K and had a Jc (at 77 K, H = 0) over 106 A/cm2. It was found that superconducting properties and structures of the films were strongly dependent on oxygen pressure and substrate temperature. The epitaxial structure of the films have been studied by x-ray diffraction, Rutherford backscattering and channeling spectroscopy, x-ray double-crystal diffraction and transmission electron microscopy. The experimental results demonstrated that the epitaxial YBa2Cu3O7-x films had excellent superconducting properties and quite perfect structure.

Epitaxial growth of monoclinic and cubic ZrO 2 on Si(100) without prior removal of the native SiO 2

High-quality epitaxial thin films of pure and yttria-stabilized ZrO 2 (YSZ) were deposited onto Si(100) by electron beam evaporation at substrate temperatures around 880°C. No specific wafer cleaning was employed to remove the native SiO 2. X-ray diffraction revealed a monoclinic structure for the pure ZrO 2, whereas a cubic structure was observed for the YSZ. X-ray pole figure analysis showed that in both cases the in-plane axes of the films were essentially parallel to the Si[010] and [001] directions. Rutherford backscattering and channelling analysis resulted in minimum yield values of 7% and 6% for the pure ZrO 2 and the YSZ films respectively, thus indicating a high degree of crystalline perfection. A 20 Å thick amorphous SiO 2 layer was observed at the interface using high resolution transmission electron microscopy. It was probably regrown during film deposition, after the original surface oxide of the wafer had been removed in situ.

Radiation damages and flux pinning in YBa2Cu3O7 thin films

We were able to increase the critical current density (Jc) and implicitly, enhance the flux pinning in high quality epitaxial YBCO thin films by 200 keV proton irradiation. The critical temperature (Tc) of the film does not change until radiation-induced localized defects begin to overlap over distances to the extent of a coherence length. The point defect induced by radiation damage or the radiation induced weak center (RWC) is believed to be responsible for the observed flux pinning enhancement. The self-field of the transport current transforms the RWCs into pinning centers. Compared to the bulk materials, the much lower Jc enhancement factor observed after irradiation of the thin films is due to the already very strong pinning force of the pinning defects in thin films. The extra pinning force of radiation defects is just a small perturbation on top of a large background.

Magnetic and electrical characterization of YBa 2Cu 3O 7 thin films made by laser ablation

High quality epitaxial thin films of YBa 2Cu 3O 7 have been deposited on single crystal MgO (100), SrTiO 3 (100), and LaAlO 3 (100) by laser ablation. The transport critical current density at 77 K is in the range of 10 6 – 10 7 A/cm 2 depending on the type of substrate. An alternative way to measure the critical current density based on high probe field ac-susceptibility measurements, avoiding the needs of patterning, is described, and the results are compared to transport measurement.

Deposition, characterization, and laser ablation patterning of YBCO thin films

High quality epitaxial thin films of YBa 2Cu 3O 7 have been deposited on single-crystal MgO(001) substrates by 355 nm Nd:YAG laser ablation. Through a systematic optimization of the deposition parameters, it was found that for a target-substrate distance of 30 mm, the optimal laser intensity, substrate temperature, and deposition oxygen pressure were 300 MW/cm 2, 750 ° C, and 0.5–1.0 mbar, respectively. Microstrips with dimensions down to 10 μm across were fabricated using both a photoresist technique and laser ablation through a metal mask. The superconducting transition takes place over 1 K, and the critical temperature is reproducible within ±1.5 K, the best result being T c,0 = 90 K. The highest critical current density measured on a 10 X 0.15 μm 2 strips was 4 X 10 6 A/cm 2 at 77 K . Film patterning using laser ablation through a metal mask was studied in detail to investigate the applicability of this method. Etch rates as a function of laser intensity were measured, and the process was followed in situ by on-line monitoring of the film resistivity.