The density of states in a-Six C1-x:H and a-Cy Sn1-y:H (F) semiconducting materials has been investigated by phototheTmal deflection spectroscopy (PDS) and their photoconductive properties have been related to the gap states distribution parameters, the Urbach energy Eo and the localized states density gm. We found that an Eo of 200 meV is the highest value above which no photoconductivity can be detected. This threshold corresponds to a density of gap states of about 1019 eV−1 cm−3 for optical gaps ranging between 1.1 and 2.5eV. Fourier spectral analysis of the long photoresponse decay time observed in our fluorinated a-Cy, Sn1-y samples shows the existence of well defined trapping levels and randomly distributed defect states.

The theory developed by Coulson, Rushbrooke and Longuet-Higgins (CRLH) on the electron-charge density and the atom-bond polarisability in hetero-atom conjugated systems is used to show that n-electron charge transfer by Sn substitutional incorporation in aC:H would promote the tetrahedral structure. An agreement with Phillips stability criterion is found. The effective medium theory is then applied for predictions on the dielectric properties of narrow optical band-gap (0,6–1,5 eV) C-Sn alloys. It is suggested that the incorporation of electronegativeelements (F, Cl) would reduce the high sub-bandgap absorption.

The synthesis of crystalline diamond in the Soviet Union, Japan, and the United States is reviewed. A comparison of the various growth techniques is presented. Hydrogen and its role in the synthesis process has been controversial; a model is presented illustrating its essential, but limited role. Another controversial aspect of diamond synthesis concerns the nature of the carbon bond in the reactant gas and how this influences the resultant crystal; an heuristic model is provided. Non-thermal equilibrium growth techniques are generic to the recent advances in diamond growth; the separation of the process for the disassociation of the reactant gas from that of the “surface cleaning” and the artificial stimulation of the surface migration velocity may be absolutely necessary for defect-free nucleation and growth. While the various properties of diamond have been known long before the advent of artifact diamond, its potential for use in devices has only recently been modelled and calculated; numerical figures of merit are compared with those of other semiconducting materials. Charge carrier velocity profiles of diamond are compared with those of more conventional semiconductors. Strengths and weaknesses accruing from the diamond velocity profile are addressed.

Growth of filmy diamond crystals has been strongly desired, and the plasma CVD is expected as one of the growth techniques which provide high quality crystalline diamond films. This study deals with the growth of uniform crystalline diamond films by means of the plasma CVD. The main stress was laid on the effects of the surface treatments of the silicon substrate and the substrate potential on composition of deposited materials as well as on the morphology of diamond crystals grown on the substrate. A mixture of methane (1%)/hydrogen with a pressure to 400 Pa was flown at a rate of 150 cc/min and was discharged by applying 200 W, 2.45 GHz microwave power. The Si substrates were heated to 850 ° C. Nuclei for the diamond crystal growth were preferentially generated on the defect sites caused by the surface treatments. With increasing the substrate potential in the negative direction, the growth of silicon carbide was observed to increase. When the substrate was positively biased, filmy diamond layers were found to be deposited.

Thermoelectric energy conversion utilizing nuclear heat sources has been employed for several decades to generate power for deep space probes. In the past, lead telluride and, more recently, silicon-germanium alloys have been the prime choices as thermoelectric materials for this application. Currently, a number of refractory semiconductors are under investigation at the Jet Propulsion Laboratory in order to produce power sources of higher conversion efficiency and, thus, lower mass per unit of power output. Included amongst these materials are improved Si-Ge alloys, rare earth compounds and boron-rich borides. The criteria used to select thermoelectric materials, in general, and the above materials, in particular, will be discussed. The current state of the art and the accomplishments to date in thermoelectric materials research will be reviewed.

Silicon-germanium alloys doped with GaP are used for thermoelectric energy conversion in the temperature range 300°C - 1000°C. The conversion efficiency depends on Z - S2/ρΛ, a material's parameter (the figure of merit), where S is the Seebeck coefficient, ρ is the electrical resistivity and Λ is the thermal conductivity. The annealing of several samples in the temperature range of 1100°C - 1300°C resulted in the power factor P (=S2/ρ) increasing with increased annealing temperature. This increase in P was due to a decrease in ρ which was not completely offset by a drop in S2 suggesting that other changes besides that in the carrier concentration took place. SEM and EDX analysis of the samples indicated the formation of a Ca- P-Ge rich phase as a result of the annealing. It is speculated that this phase is associated with the improved properties. Several reasons which could account for the improvement in the Power factor of annealed GaP doped SiGe are given.

The electrical resistivities and the Seebeck coefficients of SmS1−xAsx alloys (0 ≤ x ≤ 1) have been measured between room temperature and 1273 K as part of a continuing search for high temperature materials having a high thermoelectric figure of merit and suitable for space power applications. SmS is an n-type semiconductor with an energy gap measured to be 0.16 eV. SmAs and SmS0.2As0.8 are itinerant electron conductors. Hysteresis between high and low resistivity states has been observed in our SmS0.9As0.1 and SmS0.8As0.2 samples. Alloys with x ≤ 0.05 appear to be the most promising high temperature thermoelectric materials.

The phase relationships and the important structural, electrical and thermal properties of the R3X4-R2X3 (where R = lanthanides and X = S, Se and Te) phases having the Th3P4 -type structure are reviewed. The room temperature electrical resistivity and Seebeck coefficient of these materials are independent of R and only slightly dependent on X, but critically dependent on the X:R ratio. The long term stability of these phases is also reviewed. Although these materials have good thermoelectric properties there are some problems which need to be solved before these phases can be utilized in thermoelectric devices. These problems include long term stability, higher than desirable thermal conductivities, and low electron mobilities.

The system Ba-Fe-S contains a phase Ba1+xFe2S4 whose structure consists of infinite chains of FeS4 edge sharing tetrahedra with Fe-Fe distances of 2.7Å. The chains are 6 Å apart and Ba2+ fills the space between the chains. Each value of x designates a distinct phase and such structures have been labeled “infinitely adaptive.” The Ba1+xFe2S4 compounds are anisotropic semiconductors. The room temperature electrical conductivity for the x = 0 and x = 0.086 phases changes by nearly 6 orders of magnitude as increasing x populates the conduction band. The preparation of these materials requires very careful control of sulfur vapor pressure in closed ampules at elevated temperatures. A similar effect is observed in rare earth sulfides with the Th3P4 structure. Intrinsic La2S3 and BaLa2S4 are predicted to be high resistivity semiconductors. As the composition changes to a stoichiometry such as Ba0.8La2.2S4 the behavior becomes semi-metallic. Control of the sulfur vapor pressure will be a critical parameter for designing physical parameters into these compounds.

The Th3P4-type lanthanum sulfides are n-type semiconductors and a continuous series of solid solutions is found between the compositions of La3S4 and La2S3. These materials are attractive for use in high temperature thermoelectric applications because of their high melting points, low thermal conductivities and their inherent ability to vary the electron concentration from a maximum of 6.02 × 1021 cm−3 for La3S4 to zero for La2S3 (i.e., self-doping). In this study, La3S4 compounds in which trivalent La has been partially substituted by divalent Sm, Eu or Yb (La3−xMxS4 , x = 0.1 to 0.9) have been prepared by the pressure assisted reaction sintering method and were found to have the Th3P4 structure. The thermoelectric properties (Seebeck coefficient and electrical resistivity) were measured as a function of temperature and the optimum composition was found for x between 0.2 and 0.3 in which the electrical power factor was a maximum at 1000°C.

Small amounts of second phase materials can have important effects on the thermoelectric properties of polycrystalline γ-La3-xX4 (X-S, Te; 0<x<l/3). Microscopic examination by SEM of hot pressed La3–xTe4 samples has revealed from 1–5 vol. % of La202Te, an amount which is not detected by x-ray powder diffraction measurements. This amount of La202Te resulting from oxygen contamination can reduce the concentration of electrons by as much as 10% to 75% below the electron concentration calculated for single phase La3–xTe4 in the composition range of greatest interest. Small amounts of second phase materials can also lower the lattice thermal conductivity by scattering low frequency phonons These results indicate that microstructural effects should be considered when electrical and thermal properties of polycrystalline materials are analyzed.

The use of A2 Q/Q melts (A - alkali metal, Q - S or Se) for the synthesis of new one-dimensional solid-state materials is found to be of general utility and is illustrated here for the synthesis of K4 Ti3 SI4. Reaction of Ti metal with a K2 S/S melt at 375°C for 50 h affords K4 Ti3 SI4. The structure possesses one-dimensional chains of seven and eightcoordinate Ti atoms with each chain isolated from all others by surrounding K atoms. There are six S-S pairs (dave - 2.069(3) Å) so that the compound is one of TiIV and may be described as K4 [Ti3 (S)2 (S2)6]. Electrical conductivity measurements indicate that this material is a semiconductor.

Oxygen uptake by the rare earth chalcogenides takes place through a series of ordered compounds. At high oxygen concentrations the distinct and nearly stoichiometric oxychalcogenides, Ln202X (X = S, Se, Te) appear. For the chalcogenides of the larger rare earths there appears an ordered oxygen-containing beta structure, Ln10X14OxX1-x (X = S, Se). The vibrational spectrum of the trigonal oxysulfide structure contains four infrared and four Raman bands (2 A2u + 2 Eu + 2 Alg + 2 Eg). Band wavenumbers across the La to Lu series vary linearly with unit cell volume. The Raman bands are sharp indicating a high degree of order in the intermediate compounds. The Raman bands of the beta structure are remarkably sharp indicating that this compound also has a highly ordered structure. Known data plus synthesis data are combined to form not-impossible phase diagrams for the larger rare earth sulfide systems. The effect of oxygen in both series of compounds is to produce small wavenumber shifts rather than high wavenumber oxygen impurity bands.

Polyacrylonitrile (PAN) films have been spin-cast and pyrolyzed to produce thin (500–1500 Å) carbon films. These films have higher electrical conductivities than carbon films produced by other methods at similar temperatures. The conductivity can be varied by at least four orders of magnitude by changing the pyrolysis temperature. UV, IR, and Raman spectroscopies were used to investigate the chemical structure of the films during different stages of processing.

Gallium nitride has many useful properties that include a large direct gap, high electrical and thermal conductivities, and nearly the hardness of sapphire. GaN decomposes at -100°C, can sustain high electron velocities and exhibit acoustoelectric effects. But two challenges remain: to make it conducting p-type and to synthesize the cubic phase in a large single crystal instead of the usual hexagonal structure.