David D. Allred received Alumni Professorship Award

Abstract: Silicon films were deposited by pyrolytic decomposition of silane on substrates held at various temperatures, View the MathML source, in the range 550 to 800°C. The absorption coefficient, refractive index, anf X-ray diffraction pattern of these films were determined. The films deposited at temperatures, View the MathML source are amorphous, and their absorption profile resembles that reported in the literature for sputtered or evaporated amorphous films after long-time anneal. Films deposited on substrates at or above 670°C are partially crystallized, with particle size increasing gradually with substrate temperature. When the amorphous films are annealed, the resulting changes depend on length and temperature of the anneal. After a temperature-dependent induction period, the samples crystallize rapidly. The volume shrinks by ≈3% as determined from the decrease in film thickness. The onset of crystallization is indicated first by a red shift of the absorption edge, which after further anneal is overcompensated by a blue shift. The results demonstrate that the superior solar absorptance of amorphous silicon can be utilized in photothermal solar energy converters of sufficient stability without sacrificing the advantages of CVD fabrication.

Abstract: By pyrolytic decomposition of silane in the presence of dopant gases, a set of amorphous silicon films was prepared that contains various concentrations of carbon, nitrogen, boron or germanium. The effect of these dopants on the crystallization process and the optical properties is investigated. Films containing aböut 18 at % carbon show the properties most favorable for solar absorbers. The crystallization is retarded to temperatures near 1000°C, and the solar absorptance is greater than that of non-intentionally doped CVD amorphous silicon. From the experimentally determined activation energy of crystallization, the structural lifetime for such absorber films is extrapolated to be in excess of several decades for continuous operation at 700°C. For identical thicknesses of absorber layers, spectrally selective stacks of stabilized amorphous silicon deposited on top of a molybdenum reflector have higher solar absorptance than stacks composed of polycrystalline silicon on a silver reflector, amorphous silicon on molybdenum having been tested at temperatures in excess of 500°C.

Abstract: High infrared reflectance, coupled with high solar absorptance, is required for efficient photothermal conversion. Converters can be fabricated by depositing an absorber on a highly reflecting metal. The absorber functions in the visible, yet becomes transparent in the near infrared, allowing the metal to suppress the thermal emittance. Economic considerations demand the use of thin films, rather than bulk materials. The thin film reflector must be capable of withstanding high temperatures of operation. Compatibility of the re-flector with the substrate below, and the absorber above, is required for long-time service. Highly reflective silver films suffer reflectance losses by agglomeration, and require stabilization layers. Refractory materials such as molybdenum avoid agglomeration at temperatures of operation of photothermal converters. Unlike other deposition methods, chemical vapor deposition (CVD) can produce molybdenum films with an infrared reflectance rivaling that of bulk molybdenum. CVD is a non-vacuum based technology with potential for sequential throughput fabrication. Studies are being undertaken to determine how sensitively the reflectance reacts to inclusions of impurities into the molybdenum. Thin film passivators deposited on the molybdenum prevent reflectance losses induced by oxidation, and insure high temperature survival of optimal reflectance. Complete converter stacks have been annealed at 550°C for over 1000 hours in air.

Abstract: Efficient photothermal conversion requires surfaces of high solar absorptance and low thermal emittance. This can be accomplished by the tandem action of a good infrared reflector overlaid by a film of sufficient solar absorptance that is transparent in the infrared. Crystalline silicon is a suitable candidate for the absorber layer. Its indirect band gap, however, results in a shallow absorption edge that extends too far into the visible. In contrast, the absorption edge of amorphous silicon is steeper and located farther into the infrared, resulting in a larger solar absorptance. We report on the fabrication of amorphous silicon absorbers by chemical vapor deposition (CVD). Their optical and structural properties are determined as a function of the deposition temperature. We describe the effects of a progressive crystallization during anneal above 650 C and report the performance of converter stacks that are identical "twins" except for the use of a polycrystalline silicon absorber in one and an amorphous absorber in the other.

Abstract:

The application of nuclear reaction techniques to hydrogen analysis problems in metallurgical, mineralogical and semiconductor areas is described. Hydrogen analyses and profiles obtained with both the ¹H(¹⁹F, αγ)¹⁶O and ¹H(¹⁵N, αγ)¹²C reactions are presented. The advantages and disadvantages of the two techniques are discussed. Particular emphasis will be given to interpretive problems associated with analyzing the data. Various corrections to the data will be discussed, including off-resonance cross-section corrections and lower energy resonance corrections. Both crystalline and amorphous materials are examined. The hydrogen content of electrodeposited hard gold films has been determined as a function of plating conditions. Hydrogen contents as high as 9 atom % have been measured. The hydrogen profile of natural and synthetic SiO₂ samples was determined. Hydrogen was found to be quite stable in amorphous silica samples but highly mobile in crystalline quartz samples under the analysis conditions. A hydrogen depth profile for a film of glow discharge deposited amorphous silicon (∼4500 Å thick) has been obtained and will be compared with a profile measured by secondary ion mass spectrometry (SIMS) on the same sample.