R. T. Holm

Formation of high Tc superconducting films by organometallic chemical vapor deposition

The first growth of superconducting YBaCuO films by organometallic chemical vapor deposition is described. Metal β‐diketonates were decomposed thermally on MgO substrates in an oxygen‐rich atmosphere to produce amorphous brown films. Subsequent annealing in oxygen yielded dull gray films whose thickness corresponded to deposition rates of approximately 8 nm min−1. These films showed semiconductor‐like behavior at higher temperatures, followed by a broad resistive transition from 80 to 36 K with the resistance becoming zero at ∼20 K. Analysis of x‐ray data indicated the presence of the orthorhombic superconducting phase and various other metal oxides. Profilometer measurements yielded film thicknesses up to 950 nm, and scanning electron microscopy revealed faceted grains from 0.5 to 1.0 μm in size.

Infrared reflectance studies of bulk and epitaxial‐film n‐type GaAs

The reflectance spectra of n‐type GaAs samples with carrier densities from 3×1016 to 3×1018 cm−3 have been measured in the spectral range 55–1000 cm−1. Except for slight discrepancies in the vicinity of the reflectance minima, the classical Drude model provides an excellent description of the free‐carrier effects in the reflectance. The discrepancies may be accounted for by a depletion layer at the surface or by a quantum‐mechanical scattering theory. Inhomogeneities in the carrier density are measured by scanning the surface of the sample with a small aperture at a fixed frequency. The effect of mechanical polishing on the reflectance is analyzed in terms of a damaged layer. Reflectance measurements are also used to characterize samples consisting of epitaxial films of GaAs on GaAs substrates.

Infrared reflectance evaluation of chemically vapor deposited β‐SiC films grown on Si substrates

We use infrared reflectance from 400 to 4000 cm−1 to evaluate cubic SiC films grown by chemical vapor deposition on Si substrates. From different regions of the spectra we determine precise film thicknesses, estimate carrier concentrations in highly doped specimens, observe roughness at both film surfaces, and detect conducting regions at the interface. We show how the roughness and the interfacial conducting region introduce nonideality into the spectra. The method is nondestructive and the information can be obtained in less than 1 h after film growth.

Guided resonances in asymmetrical GaN photonic crystal slabs observed in the visible spectrum

We demonstrate that guided resonant modes can be readily observed in asymmetrical photonic crystal slabs on high-index substrates. In spite of the high radiative loss associated with all optical modes in these cases, the guided resonant modes are found to give rise to strong high-Q features in the transmission spectra. Since these photonic crystal structures are far more robust and easier to fabricate than the free-standing photonic crystal membranes used in previous studies of guided resonant modes, detailed studies of relevant optical phenomena and the implementation of proposed applications are greatly simplified.

Self-Assembled Monolayers of Alkylphosphonic Acid on GaN Substrates

In this paper we describe the formation and characterization of self-assembled monolayers of octadecylphosphonic acid (ODPA) on epitaxial (0001) GaN films on sapphire. By immersing the substrate in its toluene solution, ODPA strongly adsorbed onto UV/O 3-treated GaN to give a hydrophobic surface. Spectroscopic ellipsometry verified the formation of a well-packed monolayer of ODPA on the GaN substrate. In contrast, adsorption of other primarily substituted hydrocarbons (C n H 2 n+1 X; n = 16-18; X = -COOH, -NH 2, -SH, and -OH) offered less hydrophobic surfaces, reflecting their weaker interaction with the GaN substrate surfaces. A UV/O 3-treated N-polar GaN had a high affinity to the -COOH group in addition to ODPA, possibly reflecting the basic properties of the surface. These observations suggested that the molecular adsorption was primarily based on hydrogen bond interactions between the surface oxide layer on the GaN substrate and the polar functional groups of the molecules. The as-prepared ODPA monolayers were desorbed from the GaN substrates by soaking in an aqueous solution, particularly in a basic solution. However, ODPA monolayers heated at 160 degrees C exhibited suppressed desorption in acidic and neutral aqueous solution maybe due to covalent bond formation between ODPA and the surface. X-ray photoelectron spectroscopy provided insight into the effect of the UV/O 3 treatment on the surface composition of the GaN substrate and also the ODPA monolayer formation. These results demonstrate that the surface of a GaN substrate can be tailored with organic molecules having an alkylphosphonic acid moiety for future sensor and device applications.

Infrared dielectric constant of gallium arsenide

The real dielectric constant of gallium arsenide has been determined at 300 and at 5 K from fits to observed interference in transmission of thin samples with parallel surfaces. Measurement was carried out over all or part of the range 30–4000 cm−1. An accuracy of ±0.5% is estimated based on the quality of the analytic fits and the sample thickness measurement technique. We find at 300 K, e0=12.90 and e∞=10.86. At 5 K, e0=12.46 and e∞=10.58. An analytic expression for the dielectric function is given which allows accurate values of the real dielectric constant to be determined throughout most of the 0–4000 cm−1 spectral range. The observed ratio e0/e∞ agrees with the Lyddane–Sachs–Teller relation calculated with Raman values of transverse and longitudinal optical phonon frequencies to better than 0.1% at 300 and at 5 K.

OMCVD of thin films from metal diketonates and triphenylbismuth

Films containing the metals copper, yttrium, calcium, strontium, barium, and bismuth were grown by organometallic chemical vapor deposition (OMCVD). Depositions were carried out at atmospheric pressure in an oxygen-rich environment using metal beta-diketonates and triphenylbismuth. The films were characterized by Auger electron spectroscopy, Nomarski and scanning electron microscopy, and x-ray diffraction. The results show that films containing yttrium consisted of Y 2 O 3 with a small amount of carbidic carbon, those with copper and bismuth were mixtures of oxides with no detectable carbon, and those with calcium, strontium, and barium contained carbonates. Use of a partially fluorinated barium beta-diketonate gave films of BaF 2 with small amounts of BaCO 3 .

Nondestructive analysis of threading dislocations in GaN by electron channeling contrast imaging

Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107to109cm−2. A comparison between the ultraviolet electroluminescent output measured at 380nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence.

Growth of superconducting thin films of bismuth-strontium-calcium-copper oxide by organometallic chemical vapor deposition

Abstract The first growth of superconducting Bi-Sr-Ca-Cu-O films by organometallic chemical vapor deposition is described. Triphenyl bismuth and beta-diketonates of strontium, calcium, and copper were decomposed thermally at 500°C on MgO substrates in an oxygen-rich atmosphere to produce fine grained or amorphous, dark brown to black films. Subsequent annealing in oxygen yielded gray-black films with thicknesses of 0.95 to 1.25 μm. X-ray analysis indicated the presence of a tetragonal superconducting phase along with other metal oxides. The sheet resistance versus temperature for the sample with the highest T c had a resistive transition with a midpoint at 78.8 K, a width of 7.7 K, and R = 0 at 73.2 K.

Lowered dislocation densities in uniform GaN layers grown on step-free (0001) 4H-SiC mesa surfaces

We report that very low threading dislocation densities (8×107∕cm2) were achieved in uniform GaN layers grown by metalorganic chemical vapor deposition on (0001) 4H-SiC mesa surfaces 50μm×50μm in area that were completely free of steps. Transmission electron microscopy (TEM) indicated that all observable GaN film threading dislocations were of edge type. TEM analysis of the defect structure of the nucleation layer (aluminum nitride, AlN) revealed a lack of c-component dislocations, and the clean annihilation of lateral, a-type dislocations within the first 200 nm of growth, with no lateral dislocations developing threading arms. These results indicate that the elimination of steps on the initial (0001) 4H-SiC growth surface may play an important role in the removal of mixed and c-type dislocations in subsequently grown AlN and GaN heteroepitaxial layers.