Phillip L. Jones

On the thermoelastic properties of hydrogenated amorphous silicon

Abstract Direct mechanical tests were used to estimate both the coefficient of thermal expansion and the elastic modulus of hydrogenated amorphous silicon (a-Si:H) thin films. Films varying in thickness from approximately 2 to 10 μm were deposited on thin foils of aluminum, Invar and copper by conventional capacitive glow discharge decomposition of silane. The coefficient of thermal expansion was subsequently calculated from the temperature dependence of the radius of curvature of the coated foils, and the elastic modulus was then estimated from the thermal expansion coefficient and the film-to-substrate thickness ratio. The mean values obtained for the expansion coefficient and the elastic modulus of a-Si:H are 4.4 × 10−6 °C−1 and 44 000 MPa respectively. These values differ significantly from those reported for crystalline silicon but are in qualitative agreement with observations reported for the thermoelastic properties of other amorphous materials when compared with their crystalline counterparts. This knowledge of the thermal expansion coefficient of a-Si:H can be expected to be of aid in the preparation of thick films by helping to improve the match between film and substrate expansion coefficients.

The optical band gap of hydrogenated amorphous‐boron thin films: The effect of thermal treatment

Hydrogenated amorphous‐boron thin films produced by glow‐discharge decomposition of a gas mixture of 3.1 vol. % diborane and 96.9 vol. % hydrogen have been heat treated at 360 and 400 °C. The resulting absorption coefficients and optical band gaps have been measured as a function of heat‐treatment time and temperature. Both the absorption coefficients and the optical band gaps of these films can be drastically changed by such treatments. Thermal treatment at 400 °C for 200 h results in a systematic decrease in the optical band gap from 2.19 to 0.90 eV. From quantitative hydrogen analyses it is shown that hydrogen evolution during heating plays a strong role in reducing the apparent optical band gap. These results indicate that it might be possible to optimize theoretical amorphous‐boron thin films through the tailoring of the optical band gap by such thermal treatment.

The deconvolution of Doppler-broadened positron annihilation measurements using fast Fourier transforms and power spectral analysis

Abstract A deconvolution procedure which corrects Doppler-broadened positron annihilation spectra for instrument resolution is described. The method employs fast Fourier transforms, is model independent, and does not require iteration. The mathematical difficulties associated with the incorrectly posed first order Fredholm integral equation are overcome by using power spectral analysis to select a limited number of low frequency Fourier coefficients. The FFT/power spectrum method is then demonstrated for an irradiated high purity single crystal sapphire sample.

Hydrogenated amorphous‐silicon thin films produced by ion plating

Ion plating techniques have been used to produce amorphous‐silicon thin films which contain up to 25 atomic percent hydrogen. The Si–H bond stretching mode observed for hydrogenated amorphous‐silicon thin films produced by glow‐discharge decomposition methods is also observed for these ion‐plated films. Optical absorptivity measurements for ion‐plated films give band‐gap values between 1.58 and 1.90 eV.

The dependence of the texture of tellurium thin films on vacuum deposition angle

Abstract Vacuum-deposited tellurium thin films can show substantially different surface morphologies depending on the angle with which the vapor stream impinges on the substrate surface. These tellurium thin films have a tendency to grow as acicular crystallites but as the deposition angle is increased so that the vapor stream becomes tangetial to the substrate surface the spacing between crystallites increase and approaches, at stream angles of approximately 80° from the normal, dimensions roughly once or twice the average wavelength of visible light. Such films may have application in solar energy collector systems because of the high absorptivity of sunlight shown by such films. Mechanisms which describe the tendency for crystallite spacing to increase with increasing angle are discussed.

The optical band gap of hydrogenated amorphous boron thin films: The effect of substrate temperature

Hydrogenated amorphous boron thin films have been produced on quartz substrates heated to 150, 250, and 350°C by the glow discharge decomposition of a gas mixture of 5 v/o BoHg and 95 v/o H2. Doped films have also been prepared by the addition of 1 a/o C2H4 or 0.5 a/o SiH4. The absorp-tion coefficients as a function of wavelength across the visible spectrum and the concomitant optical band gaps of all films were measured. The optical band gaps of the films produced at 150 and 250°C were approximately 2.1 eV while those films produced at 350°C showed band gaps between 1.6 and 1.4 eV. These results indicate that it is possible to optimize theroretical photovoltaic conversion efficiencies of solar cells based on hydrogenated amorphous boron thin films produced by glow discharge methods through the tailor-ing of the optical band gap by control of substrate temper-ature.

A positron annihilation study of the evolution of amorphization in Nb3Sn by mechanical milling

Abstract In an attempt to probe the effect of vacancy-related defects on the crystalline-to-amorphous phase transformation, positron annihilation lifetime measurements were performed on Nb 3 Sn with the A15 structure as a function of mechanical milling. The lifetime spectra were deconvoluted into two components. Both components increased during the initial stage of milling, consistent with an increase in the concentration of the vacancy-type defects and void-type defects formed as a result of milling. The formation of these types of defects plays an important role in the amorphization reaction of A15 Nb 3 Sn.

On the characterization of glassy polymers by positron annihilation lifetime spectroscopy

Abstract The present paper reviews some of our recent results exemplifying the use of positron annihilation lifetime spectroscopy in evaluating the effects of thermal history on the physical properties of glassy polymers. Specifically, studies on thermal cycling, physical aging and structural relaxation in polycarbonate are discussed. It is shown that the ortho-positronium pickoff component intensity I 3 is sensitive to structural transitions below the glass transition and the thermal response of I 3 is a sensitive structural parameter that can be used in the study of the thermodynamics and kinetics of physical aging and structural relaxation.

A comparison of the doppler-broadened positron annihilation spectra of neutron irradiated Al2O3 and MgAl2O3

Radiation damage studies of oxides and ceramics have become of increasing importance due to the projected use of these materials in thermonuclear fusion reactors as electronic insulators and first wall materials. In addition these materials are important in RAD waste disposal. As part of a study of the defect structure in radiation damaged ceramics Doppler-broadened positron annihilation spectra have been obtained for a series of single crystal sapphire (α-Al2O3) and polycrystal (1:1) and (1:2) magnesium aluminate spinel (MgO·Al2O3 and MgO-2Al2O3) samples. These samples were irradiated in EBR-II to a fluence of 3 × 1025 n/m2 (E > 0.1 MeV) at 740°C, and 2 × 1026 n/m2 (E > 0.1 MeV) at ~ 550°C respectively. Positron annihilation spectra lineshapes for the irradiated, annealed, and as-received samples of both materials were compared using S parameter analysis. These calculations were made on deconvoluted gamma ray spectra that were free of any instrumental broadening effects. In this way, absolute S parameter changes could be calculated. The observed changes in the S parameter are consistent with independent volume swelling measurements for both the α-A12O3 and the (1:2) MgAl2O4 samples. However, the change in S parameter measured for the (1:1) spinel is contrary to the measured volume change. This apparent anomaly indicates a predominence of interstitial as opposed to vacancy type defects in this material.

Positron evaluation of hydrogen in a 17 nickel maraging steel

Doppler broadening measurements of the γ-ray emmision from positron annihilations have been made on a 17 nickel maraging steel, heat treated to Rockwell hardness, C Scale 49, and subsequently cathodically charged with hydrogen. Changes in the positron annihilation γ-ray spectra were measured using a line shape parameter (S) based on the ratio of central to total spectra area. The recovery of the S parameter as a function of time after hydrogen charging is correlated with hydrogen egress from the metal. The observed sensitivity of this Doppler broadening positron annihilation technique to hydrogen content is attributed to hydrogen compensation of lattice defects (dislocations) in the maraging steel.