Nonlinear bleaching and induced absorption effects in CuS microcrystals with modified (after oxidation) surface have been studied. Relaxation times of these processes demonstrate two components - fast (˜ 80 ps) and slow (>>500 ps). Energy level scheme for explanation of these phenomena was proposed. Appearance of bleaching effect in CuS microcrystals due to surface oxidation makes them as perspective materials for saturable absorbers for pico- and femtosecond lasers.

Silicon multiple quantum wells confined by ZnS barriers have been grown by MOCVD. Calculations indicate that well widths must be less than 15Å for visible light emission, basically independent of the band offsets. No near-infrared photoluminescence (except bulk Si band-edge emission) was observed from samples with 70Å or greater Si layers, using stimulation at 325 nm with a He-Cd laser; hole trapping in ZnS may play a role. However, we found evidence of yellow light emission from samples which possibly contain Si quantum dots embedded in ZnS.

Persistent spectral hole-burning (PSHB) phenomena of semiconductor quantum dots enable to perform the precise, high-resolution spectroscopy for the quantum dots of a certain size. We succeeded in the observation of the higher-excited states of excitons in CuCI quantum dots by means of the PSHB phenomena. Moreover, the exciton-state-splitting is observed and in agreement with the theoretical results on the assumption of platelet quantumdots.

Heteroepitaxy of semiconductor materials has introduced whole new classes of electronic and optical devices as well as new physical phenomena. Heteroepitaxy always introduces stress in some or all of the deposited material. The resulting strain changes the bandgap of the materials in which it occurs, and changes the conduction and valence band offsets relative to neighboring materials. In all instances studied so far, the effects of strain can be calculated accurately using continuum stressstrain relationships developed for homogeneous, bulk materials.

Incorporation of N-atoms at the Si-Si02 interface in field effect transistors, FETs, with ultrathin dielectrics (≤ 5.5 nm) improves device reliability. Four aspects of our recent research on nitrided Si-Si02 interfaces are discussed in this paper: i) the low-temperature/low-thermal budget process by which interface chemistry is controlled, and optimized; ii) the use of on-line and off-line diagnostics to determine the spatial confinement and concentration of interfacial N-atom incorporation; iii) comparisons of device properties for non-nitrided and nitrided interfaces; and iv) the proposal of an atom-scale model for the role that interfacial N-atoms play in improving device reliability.

Due to hydrolysis by atmospheric water, the first atomic layer of silicon carbide contains oxygen atoms. The oxygen content on the surface dramatically increases for nanosized SiC powder and strongly affects the overall properties. The presence of SiH groups on the SiC surface, already proven by Fourier transform infrared spectrometry, can be used to investigate the oxidation mechanism. Indeed, the v(SiH) stretching frequency known to strongly depend on the surrounding electronegativity of the silicon atom is directly related to the oxidation degree. Based on our infrared spectroscopic experimental data, ab initio calculations were performed on models representing the SiH surface species in different environments. Using the Hartree-Fock method with 3–21G and 6–31G basis sets, the v(SiH) frequencies were calculated for each model versus the number of substituted oxygen atoms. The comparison with the experimental values led to a discussion of our proposed models and of the SiC surface oxidation mechanism.

Characteristics of ultrathin silicon oxynitride (15–25Å) and its interface with Si have been investigated. Oxynitride films with thickness down to 15Å can be grown reproducibly in a conventional furnace. The leakage currents through these films can be well explained by the direct tunneling mechanism and can be fit by the same equation as that for pure oxide. This result indicates that incorporation of nitrogen atoms does not seriously affect the basic properties of the film and its interface such as the effective mass and the barrier height. A p-type poly gate MOS structure with 22Å oxynitride has also been fabricated successfully without boron penetration even using BF2+ ion implantation and a conventional furnace. Since the leakage current thorough oxynitride with this thickness is acceptable for circuit operation, thickness of the gate insulator in the dual poly-Si process can be scaled down at least to 22Å.

A multi-interface solar cell design exploiting the parts of solar spectrum heretofore never converted by single-crystal silicon devices seems to be possible with local material modifications combined with a superimposition of hetero-interface transition zones. Possible structural modifications by implantation of a silicon single-crystal target causes a series of “secondary” effects of basic importance from the photovoltaic conversion point of view. The 1800 nm divacancy infrared band activity has revealed totally unknown behavior in the built-in strain field of the inserted α-Si/c-Si hetero-interface. First, even an annealing temperature of 770 K is not enough to quench the divacancy absorption. Next, the elimination of useful band-tail and useless divacancy activities is not coincident, i.e. divacancy absorption can be quenched without too much reduction of the band-tail activity. A relatively important infrared current could be observed experimentally up to 2500 nm and by extrapolation up to about 3500 nm.

Proximity rapid thermal diffusion (RTD) using arsenic has been used for doping high aspect ratio trench capacitors which may be used in high density dynamic memories. Nonuniformity of doping has been observed in SEM analyses at the trench comers after preferential etching used for junction delineation. Severe thinning was seen at the bottom comers of the junctions produced at low temperatures for long times (90 sec). This non- uniformity was alleviated at high diffusion temperatures for short time resulting in very uniform junctions. We modeled diffusion processes for trench doping using a process simulator, TSUPREM-IV. Default program data produced large discrepancies with experimental results and a new set of parameters was derived. The stress model in the oxide growth was incorporated in our calculations of the diffusion profiles to simulate formation of the dopant glass and obtain match with the experimental results.

We investigated effects of surface preparation in doping silicon wafers using proximity rapid thermal diffusion (RTD). This process uses a spin-on-dopant (SOD) as an evaporating source for doping processed wafers. Various cleaning processes were used to study formation of the dopant glass on the silicon wafers during RTD. Doping was evaluated by sheet resistance measurements, dopant profiling using SIMS analyses, carrier distributions using spreading resistance profiling (SRP) or anodic oxidation, and was complemented by the composition analyses of the SOD deposited on the source wafers and of the dopant glass formed on the target wafers using Fourier Transform Infrared Spectroscopy (FTIR) in the transmission and reflection modes, respectively. It was found that the influence of the cleaning steps is much stronger in low temperature doping processes than at high temperatures. In addition, the temperature effect related to the wafer size was also an important factor in determining the doping efficiency in this RTDs.

We have studied GaAs/AlAs periodic quantum dot arrays using high resolution x-ray diffraction (reciprocal space mapping) around the (004) and (113) reciprocal lattice points. From the distribution of the diffracted intensities we deduced the average strain status of the dots. From the numerical simulations it is evident that random elastic strain fields are present, which extend through almost the entire volume of the quantum dot. The simulations of the x-ray measurements revealed that the crystalline part of the dots is considerably smaller as scanning electron micrographs would indicate.

It is shown that an x-ray interference phenomenon, Coherent Grating X-ray Diffraction (CGXD), can be used to study lateral nanostructure arrays on crystal surfaces and interfaces. Compared to Fraunhofer grating diffraction of visible light, x-ray grating diffraction contains information not only about geometric profiles of a surface, but also about the internal crystalline structures and lattice strain distributions in grating features. Grating diffraction patterns can also be measured in a white-beam Laue method using highly collimated polychromatic synchrotron radiation. This provides a way for parallel data collection and is useful in in-situ studies of structural evolution of nanostructure arrays.

We describe a technique for measuring thin film stress using wafer curvature that is robust, compact, easy to setup, and sufficiently sensitive to serve as a routine diagnostic of semiconductor epilayer strain in real time during MBE or CVD growth. We demonstrate, using growth of SiGe alloys on Si, that the critical thickness for misfit dislocation can clearly be resolved, and that the subsequent strain relaxation kinetics during growth or post-growth annealing are readily obtained.

Bulk GaAsP, 20Å-500Å GaAsP/GaInP single quantum wells and 70 Å period GaAsP/GaInP superlattices were deposited on GaAs substrates by Flow Modulation Epitaxy. In these structures, the disordered GaInP is lattice matched while the GaAsP is in tension with the As mole fraction varying from 0.6 to 1. The structures were studied using asymmetric x-ray diffraction, 1K photoluminescence, Raman scattering, transmission electron microscopy and atomic force microscopy. Raman and x-ray diffraction are used to assess the structural quality of the superlattices, especially with regard to the presence/absence of superlattice x-ray satellites and disorder activated longitudinal acoustic phonons. A model including the effects of composition, strain, and confinement on longitudinal optic phonons is described and used to estimate the composition, using Raman scattering, in the thin, pseudomorphic GaAsP layers in the superlattices. Photoluminescence is used to assess the composition of the interfacial layers in the single quantum wells and to determine transition energies in the superlattices. In addition, analysis of the heterostructure luminescence, including prediction of the energy band alignment as calculated with the model solid theory corrected for strain, is found to suggest the presence of a type II band alignment in the heterostructures for some values of GaAsP composition. Finally, Raman scattering and x-ray diffraction are used to compare arsenide to phosphide interfaces in GaAs and InP-based heterostructures.

A novel method for determining thin film density by energy dispersive x-ray reflectivity is demonstrated for a polymer-derived spin-on-glass dielectric intended for microelectronics applications. The effects of sample misalignment limit the accuracy of x-ray reflectivity as typically practiced. These effects may be properly accounted for by measuring the critical angle for reflection at many different x-ray wavelengths simultaneously. From this measurement, thin film density can be ascertained with much improved accuracy. The results of the x-ray reflectivity measurement are compared to those derived from MeV ion scattering.

Spectroscopic Ellipsometry (SE) and Grazing X-ray Reflectance (GXR) techniques are applied on different III-V epitaxial structures in order to extract accurately their structural information. Thickness information are obtained directly from Fourier transformation of the GXR spectra and confirmed by simulation of the reflectance curve. Compositions are deduced from SE regression using GXR thickness as input values. Examples on monolayer and bilayer structures are first remembered. Then the potentiality of the method is demonstrated to more complex systems (multilayer mirrors and HEMT structures).

A new technique is presented for the determination of concentration depth profiles. Surface atoms are sputtered by an ion beam and deposited on a clean silicon wafer. The wafer is rotated behind a slit in step with the sputtering progress. In this way the depth profile of the sample is transferred into a lateral distribution of the sputtered atoms on the target wafer. Subsequently the wafer is scanned by Total Reflection X-ray Fluorescence Spectrometry (TXRF) which is capable of detecting traces of metallic impurities on wafers down to 10-4 of an atomic monolayer. The sequence of ion-beam sputtering, deposition of the sputtered atoms and TXRF analysis results in an excellent depth resolution in the case of areal structures. Using an ion source of the Kaufmann type, an extrapolated perpendicular resolution better than 0.1 nm was obtained for a 1500 mm2 surface. For a surface area of 3 mm2 a depth resolution of 1 nm is expected. 1.4 nm was actually measured to be the width of a coherent Ti/Al-interface within a layered structure.

We have achieved spatially resolved room temperature photoluminescence (PL) from MOVPE GaAs (001) surfaces by Near-Field Scanning Optical Microscopy (NSOM). The PL intensity variation was related to the change of surface state density. Using this technique, the uniformity of surface passivation after (NH 4)2S treatment has been studied. We have performed the topography, reflection and PL measurements by NSOM as well as the topography measurements by Atomic Force Microscopy (AFM) on the as grown, etched and sulfur passivated GaAs samples. The uniformity of GaAs with a thin Al0 65Ga0.35As cap layer has also been studied and compared with the (NH 4)2S treatment. We found the sub-micron scale variations in PL intensity which were not correlated to the topographic features. Theoretical modeling has been used to obtain semi-quantitative analysis of the experimental results.

A new GaAs surface passivation method, CS2 treatment at moderate temperature was developed for effective passivation of GaAs surfaces. The CS2 treatment of GaAs surfaces at 350 °C and 10 atm leads to deposition of a homogeneous film, with a thickness of several hundred A. The passivation layer thus produced causes a significant enhancement in room temperature photoluminescence intensity and the passivation effect of the sulfide film was confirmed by Raman spectroscopy. The passivation layer remained electrically and chemically stable over a period of nine months under ambient atmospheric conditions. In-depth Auger electron spectroscopy (AES) revealed that the carbon and oxygen content in the film was negligible, whereas sulfur was uniformly distributed throughout the film. A metal-insulator-semiconductor diode whose insulating layer is produced by the CS2 treatment shows well-defined accumulation and depletion regions in its capacitance-voltage (C-V) characteristics with low hysteresis.

We describe studies of luminescence and lateral transport properties of excited carriers in GaAs- AIGaAs multiple quantum well (MQW) structures by cathodoluminescence measurements in a scanning electron microscope. We examine the effect of in-plane, etch-defined feature size on MQW luminescence efficiency and variability, and determine the diffusion length and its temperature dependence from ˜ 8K to 250K. Our measurements also provide information about nonradiative surface recombination velocity at the side walls of etch-defined MQW structures.