Polycrystalline MgO Films Research Articles

Synthesis and Characterization of Physical Properties of MgO Thin Films by Various Concentrations

Abstract In this work, magnesium oxide was elaborated on a glass substrate at 450°C by a pneumatic spray technique. The structural, optical, and electrical properties were studied at different MgO concentrations (.05, .10, .15, and .2 mol L−1). Poly-crystalline MgO films with a cubic structure with a strong (002) preferred orientation were observed at all sprayed films, with a maximum crystallite size of 21.4 nm attained by the sprayed film at .2 mol L−1. Good transmission was found in the deposited MgO thin films with lowest molarity. The transmission of MgO thin films decreases rapidly as the wavelength increases in the range of 300–400 nm and then increases slowly at higher wavelengths. The bandgap of MgO thin films decreases as the molarity increases, and the band gap values range between 4.8 and 4.3 eV. The Urbach energy values range between 375 and 519 meV. The electrical resistance of our films is on the order of 2 × 107Ω. The prepared MgO thin films were suitable for electronic packaging; they are capable to provide very stable and high secondary electron emission combined with low bandgap energy and low electrical resistance.

Understanding of Preferred Orientation Formation in Rock-Salt Materials: The Case of MgO

We developed a kinetic model to predict the preferred orientation for the case of rock-salt MgO crystal by assuming random walk diffusion of the adatom between (111) and (100) planes during growth. This model suggests that the tendency of the (111) preferred orientation increases under high deposition rate and low activation energy for diffusion of the adatom between (111) and (100) grains because the adatoms on the (111) surface stick to the surface more strongly compared to those on the (100) plane. The theoretical model was confirmed experimentally by growing several polycrystalline MgO films using the electron beam evaporation method. We could easily control the flux and energy of MgO adatoms by modulating the emission current of the electron gun, substrate temperature, and extra energy source such as ion beam assisted deposition. Our experimental results show good qualitative agreement with the model for the various polycrystalline MgO films deposited at several growth conditions.

Surface water reactivity of polycrystalline MgO and CaO films investigated using x-ray photoelectron spectroscopy

The authors report a study comparing the surface reactivities of the alkaline earth oxides MgO and CaO with respect to water vapor under ultrahigh and high vacuum conditions. Using x-ray photoelectron spectroscopy and a series of in vacuo exposures spanning ∼10−10Torr vacuum, to 10−6Torr flowing oxygen, the extent of surface hydroxylation was investigated by monitoring the O 1s photoelectron line. After the most aggressive exposures, the MgO surface reacted to form a maximum hydroxyl coverage of ∼1.3 ML (monolayer), while the CaO surface reached a maximum coverage of ∼3.0 ML. Both surface hydroxides could be removed by in vacuo thermal anneals; exposure to 250 and 500°C were required for MgO and CaO, respectively. These results are framed in terms of the suitability of these oxides in complex oxide-compound semiconductor heteroepitaxial multifunctional devices and with respect to understanding the development of surface morphology in epitaxial 111-oriented CaO and MgO thin films.

Spray pyrolysis deposition and characterization of highly (100) oriented magnesium oxide thin films

AbstractTransparent dielectric thin films of MgO has been deposited on quartz substrates at different temperatures between 400 and 600°C by a pneumatic spray pyrolysis technique using Mg(CH3COO)2·4H2O as a single molecular precursor. The thermal behavior of the precursor magnesium acetate is described in the results of thermogravimetry analysis (TGA) and differential thermal analysis (DTA). The prepared films are reproducible, adherent to the substrate, pinhole free and uniform. Amongst the different spray process parameters, the substrate temperature effect has been optimized for obtaining single crystalline and transparent MgO thin films. The films crystallize in a cubic structure and X‐ray diffraction measurements have shown that the polycrystalline MgO films prepared at 500°C with (100) and (110) orientations are changed to (100) preferred orientation at 600°C. The MgO phase formation was also confirmed with the recorded Fourier Transform Infrared (FTIR) results. The films deposited at 600°C exhibited highest optical transmittivity (>80%) and the direct band gap energy was found to vary from 4.50 to 5.25 eV with a rise in substrate temperature from 500 to 600°C. The measured sheet resistance and the resistivity of the film prepared at 600°C were respectively 1013Ω/□ and 2.06x107Ω cm. The surface morphology of the prepared MgO thin films was examined by atomic force microscopy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reflection high-energy electron diffraction experimental analysis of polycrystalline MgO films with grain size and orientation distributions

Analysis of biaxial texture of MgO films grown by ion-beam-assisted deposition (IBAD) has been performed using a quantitative reflection high-energy electron diffraction (RHEED) based method. MgO biaxial texture is determined by analysis of diffraction spot shapes from single RHEED images, and by measuring the width of RHEED in-plane rocking curves for MgO films grown on amorphous Si3N4 by IBAD using 750 eV Ar+ ions, at 45° incidence angle, and MgO e-beam evaporation. RHEED-based biaxial texture measurement accuracy is verified by comparison with in-plane and out-of-plane orientation distribution measurements made using transmission electron microscopy and x-ray rocking curves. In situ RHEED measurements also enable the analysis of the evolution of the biaxial texture which narrows with increasing film thickness. RHEED-based measurements of IBAD MgO biaxial texture show that the minimum in-plane orientation distribution depends on the out-of-plane orientation distribution, and indicates that the minimum obtainable in-plane orientation on distribution is 2°.

Rheed in-plane rocking curve analysis of biaxially-textured polycrystalline MgO films on amorphous substrates grown by ion beam-assisted deposition

We have developed a method for biaxial texture determination in polycrystalline films using reflection high energy electron diffraction (RHEED) in-plane rocking curves. Experimental RHEED in-plane rocking curves were taken at 25keV and 2.7° incidence angle from 11nm thick, nominally [100]-textured MgO films grown on amorphous Si3N4 by ion beam-assisted deposition (IBAD). The experimental RHEED in-plane rocking curves were analyzed by comparing them with RHEED in-plane rocking curves calculated using a kinematical simulation. The model enables a quantitative correlation between biaxial texture and RHEED in-plane rocking curve measurements. RHEED results are compared to X-ray rocking curve film analysis.

Electron ejection from MgO thin films by low energy noble gas ions: Energy dependence and initial instability of the secondary electron emission coefficient

Low energy ion-induced secondary electron emission from the surface of thin (500–5000 Å) polycrystalline MgO films has been investigated with various noble gas ions at energies ranging from 45 to 300 eV. The dependence of secondary electron emission coefficient γi on the type and energy of ions is reported and interpreted in terms of electron ejection mechanisms. As-deposited MgO films showed an initial fluctuation in the secondary emission current, which upon annealing or after a certain ion bombardment time irreversibly disappeared.

Growth of magnesium oxide thin films using single molecular precursors by metal–organic chemical vapor deposition

Thin films of MgO have been deposited on Si(100) and c-plane sapphire substrates by the metal–organic chemical vapor deposition (MOCVD) method using Mg(tmhd) 2 and Mg(acac) 2 as single molecular precursors and oxygen as carrier gas. We have synthesized the metal–organic precursors used in this study. Strongly [111] oriented polycrystalline MgO films were obtained on both Si(100) and c-plane sapphire substrates using these precursors in the temperature range 500–600°C. The MgO thin films grown on Si(100) at 600°C and on c-plane sapphire at 500°C with Mg(tmhd) 2 are highly oriented in the [111] direction, whereas the MgO film grown on Si(100) at 350°C and then annealed at 520°C has no preferred orientation. In the case of using Mg(acac) 2 as precursor, however, the MgO film deposited on c-plane sapphire surface at a deposition temperature above 500°C was grown with more [110] and [100] dominant orientation relatively rather than [111]. Furthermore, we have also shown that the synthesized precursors Mg(tmhd) 2 and Mg(acac) 2 are suitable precursors for obtaining MgO thin films by MOCVD and the substrate or precursor type and the growth temperature will be important factors influencing either the crystal growth direction or the crystallinity of the films.

Quantitative Rheed Analysis of Biaxially-Textured Polycrystalline MgO Films on Amorphous Substrates Grown by Ion Beam-Assisted Deposition

AbstractWe have developed a computer simulation based on analytic calculation of reflection high energy electron diffraction (RHEED) patterns in the kinematic approximation for mosaic polycrystalline films for given values of electron beam incidence angle, polycrystalline texture, in-plane orientation distribution, and grain size. Although RHEED is most appropriately modeled using dynamical scattering theory, the computational efficiency of the kinematic approximation has enabled the development of a model suitable for real time measurement of crystallographic texture and in-plane orientation distributions for biaxially-textured films grown by ion beam-assisted deposition (IBAD). Using the simulation, we can quantitatively determine how RHEED spot shapes and relative intensities depend on the crystallographic texture and in-plane orientation distribution of polycrystalline films. RHEED patterns taken at 25 keV with incidence angle in the range 1–5 degrees from 10 nm thick, nominally [100]-textured MgO films grown on amorphous Si3N4 by IBAD were analyzed by comparing experimental RHEED phi rocking curves with those predicted by the simulation. For some films, an additional 200 nm thermally-grown MgO homoepitaxial layer was grown on top of the IBAD MgO layer. The model enables a quantitative correlation between biaxial texture and RHEED measurements. RHEED results are compared to X-ray rocking curve film analysis.

Preparation of MgO films on GaAs by metalorganic chemical vapor deposition

MgO films have been prepared on GaAs(100) substrates by low-pressure metalorganic chemical vapor deposition using Mg(tmhd) 2 as a precursor and oxygen as a carrier gas, where tmhd is 2,2,6,6-tetramethyl-3,5-heptanedionate. Polycrystalline MgO films oriented mainly in the [100] direction were obtained at the substrate temperature of 300 °C. However, [110] oriented growth began to appear as the deposition temperature was increased. Deposition at 500 °C was found to induce breaking of the GaAs surfaces.

Growth and characterization of (111) and (001) oriented MgO films on (001) GaAs

The effects of substrate preparation on the structure and orientation of MgO films grown on (001) GaAs using pulsed laser deposition has been investigated. Textured MgO films displaying a (111)MgO∥(001)GaAs orientation relation with x-ray rocking curve full width at half maximum (FWHM) values as low as 1.8° were obtained in cases where the native GaAs surface oxide was only partially desorbed prior to growth. Reflection high-energy electron diffraction, transmission electron microscopy (TEM), and x-ray pole figure analysis of these films reveals a preferential orientation within the plane of the substrate: [11̄0]MgO∥[11̄0]GaAs and [112̄]MgO∥[110]GaAs. An interfacial layer (∼5 nm thick) was observed in high resolution TEM analysis, and was attributed to a remnant native GaAs oxide layer. Complete desorption of the native GaAs oxide at ∼600 °C in vacuum prior to MgO growth led to significant surface roughening due to Langmuir evaporation, and resulted in randomly oriented polycrystalline MgO films. Growth of MgO on Sb-passivated GaAs substrates, which provided smooth, reconstructed surfaces when heated to 350 °C in vacuum, resulted in cube-on-cube oriented films [i.e., (001)MgO∥(001)GaAs,[100]MgO∥[100]GaAs] with x-ray rocking curve FWHM values as low as 0.47°. TEM analysis of the cube-on-cube oriented films revealed evidence of localized strain fields at the MgO/GaAs interface, indicating the presence of misfit dislocations in the MgO layer.

Preparation and characterization of thin films of MgO, Al2O3 and MgAl2O4 by atomic layer deposition

MgO, Al2O3 and MgAl2O4 thin films were deposited on silicon substrates at various temperatures by the atomic layer deposition (ALD) method using bis(cyclopentadienyl)magnesium, triethylaluminum, and H2O and were characterized systematically. High-quality polycrystalline MgO films were deposited for a substrate temperature above 500°C, and amorphous thin films were deposited around 400°C. The deposited Al2O3 and MgAl2O4 thin films were characterized as amorphous in structure. Applicability of ALD to complex oxides is discussed.

Microstructure and Superconductivity in Epitaxial MgO/NbN Multilayers

The aim of this investigation is to achieve high transition temperature, low resistivity films under conditions suitable for use in all-NbN Josephson junction fabrication. For this purpose, the heteroepitaxy of NbN on MgO films is investigated. For substrate temperature of 200°C or thereabouts, polycrystalline MgO films of (200) orientation were formed on (100) silicon, (11̄02) α-Al2O3, and fused silica. Based on XTEM, HEED and X-ray data, heteroepitaxial growth ((100)NbN//(100)MgO) was obtained in NbN films and NbN/MgO/NbN trilayers deposited on (200) oriented MgO films. The epitaxial NbN films of 10 nm thickness had high transition temperatures of about 14 K, and relatively low residual resistivities of less than 200 µΩ·cm. Using the GL relationships, we calculated the Ginzburg-Landau parameters, λ(0) and ξ, of our NbN films in the dirty limit. The calculated ξ(4.2 K) and λ(0) are ∼4.5 nm and ∼300 nm, respectively.

Secondary electron emission and conductivity mechanisms of epitaxial MgO films

Secondary electron emission and electron bombardment induced conductivity measurements have been made on epitaxial MgO films (50–200 Å thick) deposited on a (100)Mo substrate. The films were deposited and measurements made under UHV conditions. Changes in the sample surface potential ( V s) were determined from changes in the threshold energy of the secondary emission energy distribution, thus allowing current ( I) versus V s curves to be made. It was found that the conductivity data could be fitted best by a Poole-Frenkel conductivity mechanism. Mechanisms for electron injection from the substrate into the film were not consistent with the data, implying that holes are the dominant charge carriers. This is in contrast to most results on polycrystalline MgO films which conclude that electron tunneling is probably dominant in such films. The possible ways that these differences in conduction mechanisms can affect processes that degrade the secondary emission properties of these films are discussed.