P. D. Persans

Raman scattering as a compositional probe of II‐VI ternary semiconductor nanocrystals

We show how Raman optic‐mode peak positions and relative intensities can be used in a straightforward way to determine crystallite composition in CdSxSe1−x nanocrystals embedded in glass. These Raman techniques are particularly useful for low‐concentration or small‐crystallite‐size composites where x‐ray diffraction is not a viable technique for structural characterization of crystallites.

Extended infrared photoresponse and gain in chalcogen-supersaturated silicon photodiodes

Highly supersaturated solid solutions of selenium or sulfur in silicon were formed by ion implantation followed by nanosecond pulsed laser melting. n+p photodiodes fabricated from these materials exhibit gain (external quantum efficiency >3000%) at 12 V of reverse bias and substantial optoelectronic response to light of wavelengths as long as 1250 nm. The amount of gain and the strength of the extended response both decrease with decreasing magnitude of bias voltage, but >100% external quantum efficiency is observed even at 2 V of reverse bias. The behavior is inconsistent with our expectations for avalanche gain or photoconductive gain.

Room-temperature sub-band gap optoelectronic response of hyperdoped silicon

Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 10(20) cm(-3) into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.

Electrical transport in amorphous hydrogenated Ge/Si superlattices

Electrical conductivity measurements on a‐Ge:H/a‐Si:H superlattices parallel and perpendicular to the layers are explained by a simple quantum well model which yields effective electron masses 0.4m0 for a‐Ge:H and 0.3m0 for a‐Si:H. Space‐charge‐limited currents observed at high fields are used to determine the density of deep gap states in the a‐Ge:H layers.

Bandgap and resistivity of amorphous semiconductor superlattices

Abstract Measurements of the electrical resistivity and optical bandgap of amorphous semiconductor superlattices consisting of alternating layers of amorphous silicon with amorphous silicon carbide and amorphous silicon nitride are compared with the predictions of a one dimensional quantum-well model.

Crystallization kinetics of amorphous Si/SiO2 superlattice structures

Thermal annealing of amorphous silicon/silicon oxide multilayers leads to crystallization of silicon layers clad by an amorphous SiO2 matrix. Isothermal crystallization kinetics and microstructure of the crystallized layer are strongly dependent upon sub-layer thickness for a thickness less than 200 A.

Effect of adsorbates and insulating layers on the conductance of plasma deposited a-Si:H☆

Becauseof the low density of gap states in a-Si:H the planar cinductance G is strongly affected by the space charge layers near surface and the substrate. Several surface layer substances, such as H2O and NH3, act as donors and increase or decrease G of highly resistive n-type or p-type a-Si:H films, respectively, by many orders of magnitude. Other substances such as Se act as acceptors and thus produce the opposite effect. • †G • increses with Se thickness and saturates near 1000A of Se. This is a measure of the space charge width in Se. Changes in G are accompanied by corresponding changes in thermal activation energy. The changes in G can be reversed by removing the adsorbed or deposited layers by heating in vacuum. H2O and NH3 adsorption causes a rapid rise of G of n-type a-Si:H followed by a slow decrease over many hours. The magnitude and time constants of these effects depend on the initial oxide layer thickness. The slow decrease of G is essentially absent for adsorption in situ on freshly deposited a-Si:H.

Simulation of Titan haze formation using a photochemical flow reactor The optical constants of the polymer

Solar UV is the principal energy source impinging the atmosphere of Titan while the energy from the electrons in Saturn’s magnetosphere is less than 0.5% of the UV light. Titan haze analogs were prepared by the photolysis of a mixture of gases that simulate the composition of its atmosphere (nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene). The real ( n) and imaginary (k) parts of the complex refractive index of haze analogs formed from four different gas mixtures were calculated from the spectral properties of the solid polymer in UV-visible, near infrared and infrared wavelength spectral regions. The value of n was constant at 1.6 ± 0.1 throughout the 0.2–2.5 µm region. The variation of k with wavelength for the values derived for Titan has a lower error than the absolute values of k so the more significant comparisons are with the slopes of the k(λ) plots in the UV-VIS region. Three of the photochemical Titan haze analogs had slopes comparable to those derived for Titan from the Voyager data (Rages and Pollack, 1980, Icarus 41, 119–130; McKay and Toon, 1992, in: Proceedings of the Symposium on Titan, in: ESA SP, Vol. 338, pp. 185–190). The slopes of the k(λ) plots for haze analogs prepared by spark discharge (Khare et al., 1984, Icarus 60, 127–137) and plasma discharge (Ramirez et al., 2002, Icarus 156, 515–529) were also comparable to Titan’s. These finding show that the k(λ) plots do not differentiate between different laboratory simulations of atmospheric chemistry on Titan in the UV-VIS near IR region (0.2–2.5 microns). There is a large difference between the k(λ) in the infrared between the haze analogs prepared photochemically and analogs prepared using a plasma discharges (Khare et al., 1984, Icarus 60, 127–137; Coll et al., 1999, Planet. Space Sci. 47, 1331–1340; Khare et al., 2002, Icarus 160, 172–182). The C/N ratio in the haze analog prepared by discharges is in the 2–11 range while that of the photochemical analogs is in the 18–24 range. The use of discharges and UV light for initiating the chemistry in Titan’s atmosphere is discussed.

A doping-precipitated morphology in plasma-deposited a-Si: H

A grossly inhomogeneous, noncolumnar morphology has been observed in doped, plasma‐deposited hydrogenated amorphous silicon (a‐Si:H) within critical ranges of the diborane to silane ratio in the gas phase. The diborane level can be as low as 5 ppm and depends both on the electrode self‐bias potential and the growth rate. Undoped specimens prepared under the same deposition conditions have properties typical of device‐grade a‐Si:H. These morphology observations suggest a structural origin for some doping‐dependent properties in a‐Si:H.

Relationship between bond angle disorder and the optical edge of aGe:H

Abstract We report changes in optical absorption and Raman spectra of a-Ge:H when H content is increased from 0 to 16 atomic %. Increasing H content increases the optical gap by 0.4eV, sharpens the absorption tail by a factor of two and decreases bond angle fluctuations by 2 0 . We conclude that H can affect the optical edge by decreasing network disorder.