High-quality Crystalline Films Research Articles

Epitaxial ferromagnetic metal/GaAs(100) heterostructures

Ferromagnetic bcc-FexCo1−x(100) films have been successfully grown on GaAs(100) and ScyEr1−yAs(100) by molecular beam epitaxy. X-ray diffraction combined with reflection high energy electron diffraction and low energy electron diffraction patterns revealed the epitaxial orientation of bcc-FexCo1−x(100)〈010〉‖GaAs(100)〈010〉 and bcc-FexCo1−x(100) 〈010〉‖ScyEr1−yAs(100)〈010〉. Rutherford backscattering channeling minimum yields, χmin∼3%, suggest epitaxial films of high crystalline quality. Vibrating sample magnetometry measurements show in-plane uniaxial anisotropy and fourfold in-plane anisotropy for FexCo1−x grown on GaAs(100) and ScyEr1−yAs(100), respectively. The difference in magnetic anisotropy is interpreted as arising from the ScyEr1−yAs interlayer altering the surface symmetry from twofold symmetry for GaAs(100) to fourfold symmetry. Misoriented substrates were also used to increase the step density in the [011] direction, which induced an additional uniaxial anisotropy with a [011] easy axis and a [011̄] hard axis. This step structure symmetry-induced magnetic anisotropy generated a split field ∼50 Oe in the hard axis for bcc-FexCo1−x(100) grown on ScyEr1−yAs(100) surfaces.

Comparison of Ta2O5 Thin Films Deposited by “Off-axis” and “On-axis” Pulsed Laser Deposition Technique

The crystalline properties of Ta2O5 thin films deposited by an off-axis aperture-installation-type pulsed laser deposition (PLD) technique are investigated and the results are compared to those of films deposited by the conventional on-axis technique. When the repetition frequency is lowered, the X-ray diffraction intensity of (001) peak increases in both films. This tendency, however, is more pronounced in the off-axis films. The dielectric constant of off-axis films fabricated at 20 Hz indicates a high value of about 40, and is higher than the measured value of bulk Ta2O5. Therefore, it is concluded that the off-axis aperture-installation-type pulsed laser deposition technique is effective not only for decreasing the density of droplets, but also for obtaining high-quality crystalline films.

Transfer of Ultrathin Silicon Layers to Polycrystalline SiC Substrates for the Growth of 3C‐SiC Epitaxial Films

A novel approach for the production of large area 3C‐SiC substrates is described. Ultrathin (<20 nm) Si seed layers were transferred to high purity polycrystalline 3C‐SiC substrates through a unique wafer bonding process. The ultrathin Si seed layer was subsequently carbonized and used as the nucleation layer for high temperature (>1500°C) growth of epitaxial 3C‐SiC. The use of more optimal growth temperatures, not limited by the melting point of Si, led to 3C‐SiC films of high crystalline quality. Double crystal X‐ray diffraction measurements of the 3C‐SiC(200) reflection gave peak widths of 660 arcsec. © 1999 The Electrochemical Society. All rights reserved.

Realization of epitaxial barium ferrite films of high crystalline quality with small resonance losses

We report the results of systematic studies of the effect of thin film deposition conditions, such as deposition temperature, oxygen pressure during deposition, etc., on the microstructural, magnetic, and microwave properties of pulsed laser deposited epitaxial thin films of barium ferrite on single crystal sapphire substrates. Ferromagnetic resonance (FMR) linewidths are very sensitive to the presence of defects and inhomogeneities and therefore change markedly with the variation of deposition parameters. After careful optimization of the deposition conditions, relatively narrow resonance lines were realized in these films. For further improvement in the film quality, these films were annealed at elevated temperatures in flowing oxygen. As a result of the high degree of epitaxy, good stoichiometry, and reduced concentration of defects, FMR linewidths as small as 37 Oe were obtained in films deposited at 920 °C and subsequently annealed at 1000 °C in an oxygen atmosphere.

Crystallization of In2Se3 semiconductor thin films by post-deposition heat treatment. Thickness and substrate effects

Polycrystalline thin films of -In2Se3 were grown on various substrates by sequential thermal evaporation of In and Se. The crystallization was achieved by annealing the as-deposited films in flowing nitrogen. The depositions were carried out for different atomic ratios (1.5 R = [Se]/[In]5) and the annealings performed at 400 °C for 0.5 h. X-ray diffraction, energy dispersive x-ray analysis, scanning electron microscopy, x-ray photoelectron spectroscopy and Raman scattering have shown that thin films of high crystalline quality were obtained. The influence of the substrate nature as well as the film thickness on the crystallite preferential orientation and size is studied.

Study of Low-Energy Ion Assisted Epitaxy of Gan Films: Influence of the Initial Growth Rate

AbstractThe deposition of thin epitaxial hexagonal gallium nitride films on c-plane sapphire by low-energy nitrogen ion assisted deposition is shown to result in films of high crystalline quality. The quality can be further heightened by using the concept of an isothermal growth rate ramp. Characterization of film structure, defect density distribution and surface topography by XRD, RBS/C, and AFM, respectively, reveals the importance of the nitrogen ion energy and the ion to atom ratio on the film properties.

Low-Temperature Si (111) Homoepitaxy and Doping Mediated by a Monolayer of Pb

ABSTRACTThe codeposition of Pb during Si (111) molecular beam homoepitaxy leads to high-quality crystalline films at temperatures for which films deposited on bare Si (111) are amorphous. Like other growth mediating elements-- commonly called surfactants-- Pb segregates to the film surface. Ion channeling and transmission electron microscopy reveal nearly defect-free epitaxy for a Pb coverage of one monolayer and temperatures as low as 310 °C. We have deposited films up to 1000 Å in thickness with no indication that this is an upper limit for high-quality epitaxy. However, a decrease in the Pb coverage during growth by only one tenth of a monolayer leads to highly defective films at these temperatures. The codeposition of both As and Pb results in a striking enhancement of the film quality as well. In this case, while the Pb again segregates to the film surface, the As is incorporated into the film with no apparent segregation. Lead-mediated Si epitaxy on As-terminated Si (111) produces high-quality films in which the As remains buried at the substrate-film interface. These results show Pb-mediated Si (111) homoepitaxy to be a promising strategy for the synthesis of layered structures having abrupt nanoscale dopant profiles

Growth of GaN Layer from the Single-Source Precursor (Et2GaNH2)3

In recent years, there has been a great interest in new routes for depositing GaN films in the application of III−V semiconductors. We report herein on the deposition of highly crystalline GaN films by low-pressure MOCVD (in the low-temperature range of 500−700 °C and the pressure range of 77−177 mbar) using the single-source precursor (Et2GaNH2)3. This process was investigated for a variety of substrates (Si(100) and polycrystalline Al2O3) using a cold wall chemical vapor deposition reactor. The thickness of films grown under these conditions ranged from 6 to 8 μm, and the growth rates varied from 7 to 8 μm/h. Films deposited at lower temperatures (500−550 °C) had a pale yellowish color and were amorphous. At 600 °C slightly gray colored films were obtained, while above 650 °C high-quality crystalline films were formed, which show diffraction patterns characteristic of the hexagonal wurtzite structure. The films are consistent with the 1:1 stoichiometry of GaN and have carbon and oxygen as impurities; ho...

Reduced optical losses in MOCVD grown lithium niobate thin films on sapphire by controlling nucleation density

Lithium niobate (LiNbO 3) epitaxial thin films of high crystalline quality have been grown on c-plane sapphire substrates by the solid-source MOCVD method. For waveguiding applications, low optical propagation losses (less than 2 dB/cm) are required, and this has been difficult to achieve in films grown by various vapor growth techniques. Since scattering from rough film surfaces is a major source of optical attenuation, a knowledge of the mechanisms involved in film roughening and their relationship with the initial stage of film formation is essential. In this study, it was observed that roughening and optical losses in lithium niobate films grown on sapphire substrates increased with grain size and the depth of grain grooves formed between them. Increasing the nucleation density during the initial stage of growth can inhibit grain growth, and thereby reduce surface roughness even at high growth temperatures. Based on these results, a two-step growth process was developed in which the nucleation and growth stages were controlled separately. This process involves (1) creating a high nucleation density by using either a low deposition temperature or high source partial pressure during the early stages of film growth, and (2) enhancing crystallinity by growing the balance of the film at a higher temperature or lower source partial pressure. Using this refined growth process, rms roughness was decreased to less than 2 nm, while the crystalline quality remained equal to the best films grown by the conventional one step growth method. Optical losses were reduced to below 1.8 dB/cm for the TE o mode at a wavelength of 632.8 nm.

Growth of epitaxial CoSi 2 films on strained [formula omitted] heterostructures

Thin epitaxial films of CoSi 2 have been grown on strained Si 1 − x Ge x Si(001) heterostructures with Ge contents between 20 and 25 at% by molecular beam epitaxy (MBE). Single-oriented CoSi 2(001) films of high crystalline quality ( x min = 4.6%) and smooth surface morphology with low resistivities (17 μΩ · cm) could be obtained using a special template method and an additional thin silicon sacrificial cap layer on top of the Si 1 − x Ge x Si(001) heterostructure. The films were characterized using RHEED, LEED, in-situ AES, SIMS, RBS, SEM, TEM and sheet resistance measurements. The results are compared with films grown on mere Si(001) substrates and on Si 1 − x Ge x Si(001) heterostructures grown without the use of a silicon cap layer. It is demonstrated that a silicon sacrificial cap layer is essential for the growth of high-quality single-domain CoSi 2(001) films on Si 1 − x Ge x Si(100) heterostructures.

Ion-energy effects in silicon ion-beam epitaxy.

Direct ion-beam deposition of $^{28}\mathrm{Si}^{+}$ ions for homoepitaxial film growth on Si{100} has been studied over the ion-energy range of 8\char21{}80 eV in the low-temperature range of 40\char21{}500 \ifmmode^\circ\else\textdegree\fi{}C. Deposition was performed by means of a mass-selected, low-energy, ultrahigh-vacuum ion-beam system with a well-defined ion energy (E) for which the energy spread is \ensuremath{\Delta}E=\ifmmode\pm\else\textpm\fi{}3 eV. The films were analyzed in situ at growth intervals by reflection high-energy electron diffraction and Auger-electron spectroscopy, and ex situ by cross-section high-resolution transmission electron microscopy, Rutherford backscattering spectrometry, and secondary-ion-mass spectrometry (SIMS) depth profiling. The growth mode, crystalline quality, and number of defects in the films are found to be extremely sensitive to both substrate temperature (at low temperature) and ion energy (at low energy). Layer-by-layer epitaxial growth is observed down to \ensuremath{\sim}160 \ifmmode^\circ\else\textdegree\fi{}C with appropriate ion energies; below this temperature, island growth with a transition to an amorphous phase occurs. An optimum ion-energy window for achieving layer-by-layer epitaxial growth and high crystalline quality films which are relatively defect free is observed. This energy window, which illustrates ion beam enhanced epitaxy, is extremely narrow at low temperature, i.e., \ensuremath{\sim}20\ifmmode\pm\else\textpm\fi{}10 eV at 160 \ifmmode^\circ\else\textdegree\fi{}C, and broadens out on the low-energy side at higher temperatures, e.g., at 290 \ifmmode^\circ\else\textdegree\fi{}C. Within this energy window, the films have the same level of crystallinity as the single-crystal silicon substrate. This behavior is discussed in terms of the changes in the phenomena which dominate the growth process as a function of ion energy and temperature. For the conditions 290 \ifmmode^\circ\else\textdegree\fi{}C and 20 eV, epitaxial high crystalline quality films up to 352 nm thick have been grown, and there is no indication of a limiting epitaxial layer thickness. SIMS analysis shows that the isotropic enhancement ratio is $^{28}\mathrm{Si}$/${(}^{29}$Si ${+}^{30}$Si)\ensuremath{\gtrsim}${10}^{4}$. \textcopyright{} 1996 The American Physical Society.

High T c step-edge Josephson junctions on silicon substrates

The epitaxial buffer system CeO2/YSZ was used to prepare YBCO films of high crystalline quality on silicon substrates by laser deposition. An advanced technique of step preparation in the YSZ buffer layer preserved the critical current densities of jC (77 K)∼106 A/cm2 for 30 nm thick YBCO films also in the etched parts of the substrate. Further, a homogeneous growth over the 35 to 45 nm deep steps was observed. So we were able to realize step-edge Josephson junctions with critical temperatures up to 77 K. Our dc SQUIDs on silicon substrates showed voltage modulation up to 74 K.

Amorphous PbCl2 Films and Their Crystallization Studied by Optical Absorption and Reflection Measurements

Amorphous PbCl2 films with high transmittance are obtained by quench-deposition onto 77 K. substrates. On heating, they show sharp crystallization in a narrow temperature range of 1.5 K near 282 K. For films with thickness less than 50 nm, the high transmittance is little impaired by the crystallization, leading to a high quality crystalline film never achieved by deposition onto hot substrates. The absorption spectrum in the amorphous phase is characterized by a non-excitonic, prominent first band due to intra-atomic transitions within Pb2+ ions affected by inhomogeneous electric fields in the disordered network.

Growth of MoSe 2 thin films with Van der Waals epitaxy

The concept of Van der Waals epitaxy that has been recently introduced removes severe lattice matching requirement by using materials which only have strong bonding in two dimensions. We demonstrate that an epilayer of MoSe 2 deposited on various substrates can produce films of high crystalline quality despite of large mismatch. RHEED oscillation, observed in-situ for growing MoSe 2 epilayers, shows a layer-by-layer growth with evidence for bilayer type growth, from which the 2H b polytype is determined. STM provides real space images of the morphology of the epilayer, and shows novel structures resulting from the large lattice mismatch where the epilayer atoms are commensurated.

Heteroepitaxial growth of Ge films on (100) GaAs by pyrolysis of digermane

Pyrolysis of high-purity digermane (Ge2 H6 ) has been used to grow epitaxial Ge films of high crystalline quality on (100) GaAs substrates in a low-pressure environment. X-ray double-crystal diffractometry shows that fully commensurate, coherently strained epitaxial Ge films can be grown on (100) GaAs at digermane partial pressures of 0.05–40 mTorr for substrate temperatures of 380–600 °C. Amorphous films also were deposited. Information about the crystalline films surface morphology, growth mode, and microstructure was obtained from scanning electron microscopy, cross-section transmission electron microscopy, and in situ reflectivity measurements. The amorphous-to-crystalline transition temperature and the morphology of the crystalline films were both found to depend on deposition conditions (primarily the incidence rate of Ge-bearing species and the substrate temperature). Epitaxial growth rates using digermane were found to be about two orders of magnitude higher than rates using germane (GeH4 ) under similar experimental conditions.

Epitaxial growth of LaF3 on GaAs(111)

We report the first epitaxial growth of lanthanum trifluoride on the (111) surface of gallium arsenide. Smooth, crack-free, and high-crystalline quality films of thickness up to 200 nm were grown at 500 °C on a GaAs(111) surface that was cleaned by ion heat treatment and post-anneal. The film surfaces were examined in situ by low-energy electron diffraction (LEED) and ex situ by reflection high-energy electron diffraction (RHEED) and Nomarski optical microscopy. Capacitance-voltage (C-V) and current-voltage (I-V) measurements showed a breakdown strength of 2×106 V/cm and no significant flat-band shift due to insulator charges.

Photoluminescence, Raman Scattering and Rbs/Channeling of Epitaxial Fluorides

AbstractGrowth conditions are described for producing high quality crystalline films of CaF2 on (100) and (111) GaAs and LaF3, CeF3 and NdF3 on (111) Si. Rutherford backscattering/channeling, low temperature (&lt;2K) photolumin-escence and Raman scattering are used as diagnostic techniques to probe the crystalline quality of the films and the stress/disorder induced at the film-substrate interface due to lattice mismatch.