ZnS nanocrystals doped with Mn2+ ions are prepared by a solution process and subsequent UV irradiation to produce the samples with different S/(Zn+Mn) ratios and/or surface modification by acrylic acids. Coordination states around Mn2+, ions were examined at 9 and 35 GHz by electron paramagnetic resonance spectroscopy. The Mn2+ sites in the vicinity of 2- vacancies or carboxyl groups are observed at the frequencies more than 9 or 35 GHz, respectively, for nanocrystals, but are not for the bulk sample of 250 nm diameter. Such Mn2+ sites enhance the photoluminescence due to d-d transition of Mn2+ ions through energy transfer from S2- vacancies or carboxyl groups, excited simultaneously by a light of 350 nm for exciting ZnS.

The powders of nanocrystalline tin dioxide were prepared by two different methods: conventional hydrolysis of SnCl4 in aqueous solution and novel cryosol technique. The microstructure, composition, and electrical properties of the samples were investigated. The sintered pellets obtained by means of the cryosol method are characterized by significantly higher values of electrical resistance as compared to those prepared by conventional technique. A significant effect of the microstructure on the sensing properties of nanocrystalline SnO2 has been found. The sensitivity to H2S of the samples synthesized by cryosol method was shown to be higher than that of the samples obtained by traditional precipitation.

Copper doped CdSe quantum dots (QDs) and larger nanocrystals were deposited by chemical solution deposition at two different temperatures (5°C and 80°C) and their morphological, optical and surface properties compared with similarly deposited undoped CdSe. Crystal sizes and structure were investigated by X-ray diffraction and transmission electron microscopy. All the samples exhibited cubic (sphalerite) structure except for the 80°C Cu-doped ones which were substantially hexagonal (wurtzite). For the 5°C samples, the crystal sizes were similar for the doped and undoped samples (ca. 3.4 nm). For the 80°C samples, the crystal size of the doped CdSe was much larger (84 nm) than the undoped one (6.3 nm) and extensive twinning occurred. Optical measurements on the doped samples showed the presence of surface states due to the copper seen as an increase in absorbance in the long wavelength region. Xray photoemission spectroscopy similarly showed tails in the valence band spectra of the doped samples. These tails were dominant in the 5°C samples and less so for the 80°C ones. XPS also showed that the Cu was present predominantly in the univalent state. The copper was found to be located on the QD surface for the 2–3 nm QDs and distributed both on the surface and in the bulk (or at twin boundaries) for the larger nanoparticles.

ZnSe nanoparticles with an average size of 15 nm were prepared using the ultrasonic radiation method. The characterization was carried out by means of XRD, TEM, XPS and Raman scattering spectroscopy. The experimental results indicate that the as-prepared powders are composed of ZnSe with zinc-blende structure. The high purity of ZnSe particles was confirmed by XPS analysis. In the Raman spectra, TO and LO phonon modes were observed at 205 and 257 cm−1 in the ZnSe nanoparticles.

Our team has been investigating the use of particle-based contacts in CdTe solar cell technologies. Toward this end, particles of Cu-doped HgTe (Hg-Cu-Te) and Sb-Te have been applied as contacts to CdTe/CdS/SnO2 heterostructures. These metal telluride materials were characterized by standard methods. Hg-Cu-Te particles in graphite electrodag contacts produced CdTe solar cells with efficiencies above 12% and series resistance (Rse) of 6 Ω or less. Metathesis preparation of Cu(I) and Cu(II) tellurides (i.e., Cu2Te and CuTe, respectively) were attempted as a means of characterizing the valence state of Cu in the Hg-Cu-Te ink. For Sb-Te contacts to CdTe, open circuit voltages (Vocs) in excess of 800 mV were observed, however, efficiencies were limited to 9%; perhaps a consequence of the marked increase in the Rse (i.e., >20 Ω) in these non-graphite containing contacts. Acetylene black was mixed into the methanolic Sb-Te colloid as a means of reducing Rse, however, no improvement in device properties was observed.

Rutherford backscattering spectroscopy (RBS) has been applied to determine the constitution of prototypical CdSe nanocrystals synthesized by the high temperature pyrolysis of organometallics in trioctylphosphine oxide (TOPO). The diameter of the nanocrystals was varied from 22 Å to 58 Å. For all nanocrystal sizes the nanocrystals are Cd rich with an average Cd:Se ratio of 1.2 ± 0.1. The Cd:Se stoichiometery is independent of the starting Cd:Se ratio used for nanocrystal preparation, indicating the excess Cd is not associated with the initial abundance but is an intrinsic property of nanocrystals prepared by this method. The size dependence of excess Cd indicates the extra Cd is on the surface of the crystallite. The coverage of the surface passivating TOPO ligands has also been determined and is larger than reported in previous X-ray photoelectron spectroscopy (XPS) studies of Bowen Katari et al. The origin and structural implications of non-stoichoimetric Cd are discussed.

The effect of the adsorption of an electron donor (thiophenol, TP) on the surface of CdSe nanoparticles (NPs) on the emission and electron-hole dynamics is studied. It is found that while the emission is completely quenched, the effect on the transient bleach recovery of the band gap absorption is only slight. This is explained by a mechanism in which the hole in the valence band of the NP is rapidly neutralized by electron transfer from the TP. However, the excited electron in the conduction band is not transferred to the TP cation, i. e. the electron does not shuttle via the organic moiety as it does when naphthoquinone is adsorbed [1]. The excited electron is rather trapped by surface states. Thus the rate of bleach recovery in the CdSe NP system is determined by the rate of electron trapping and not by hole trapping. Comparable conclusions resulted previously [2] for the CdS NP when the CdS-MV2+ system is studied. A comparative discussion of the electron-hole dynamics in these systems (CdSe-NQ, CdS-MV2+ and CdSe-TP) is given.

Fundamental properties of silicon films crystallized by a 30-ns-pulsed XeCI excimer laser were discussed. Although crystallization of 50-nm thick silicon films formed on quartz substrates occurred through laser hearing at the crystalline threshold energy density of 160 mJ/cm2, a higher laser energy density at 360 mJ/cm2 was necessary to crystallize silicon films completely. Analyses of free carrier optical absorption revealed that phosphorus-doped silicon films with a carrier density about 2×1020 cm−3 had a high carrier mobility of 20 cm2/Vs for irradiation at the crystallization threshold energy density, while Hall effect measurements gave a carrier mobility of electrical current traversing grain boundaries of 3 cm2/Vs. This suggested that the crystalline grains had good electrical properties. As the laser energy density increased to 360 mJ/cm2 and laser pulse number increased to 5, the carrier mobility obtained by the Hall effect measurements markedly increased to 28 cm2/Vs because of improvement of grain boundary properties, while the carrier mobility obtained by analysis of free carrier absorption increased to 40 cm2/Vs. A post annealing method at 190°C with high-pressure H2O vapor was developed to reduce the density of defect states. Increase of carrier mobility to 500 cm2/Vs was demonstrated in the polycrystalline silicon thin film transistors fabricated in laser crystallized silicon films.

We investigate nucleation densities in UHV-CVD of Si on ultrathin SiO2 layers (0.2-2 nm) which were prepared by three different oxidation methods: thermal, UV-ozone, and plasma oxidation. The experiments changing the Si2H6 pressure in UHV-CVD indicate that these oxide surfaces have preferred sites for nucleation. Among the three oxidation methods, the nucleation density, Ns, on the thermal oxide is the lowest, while the plasma oxide shows the highest Ns. These results suggest that strained bonds and ion-induced damages in the oxide layers assist nucleation. For UV-ozone and plasma oxides Ns is independent of orientation, reconstruction, and morphology of the initial Si surface.

Ordered GaAs micro crystal growth on a As-terminated Si (001) surface was demonstrated using a low energy focused ion beam. Si (001) surface was terminated by Arsenic. The surface showed a (2×1) structure with As dimers. The As layer was sputtered periodically with low energy focused Ga ion beam. Supplied Ga atoms migrated on the surface and trapped at the As removed region, forming Ga droplets. GaAs micro crystals were grown from Ga droplets by As molecule supply. The proposed method was shown to be effective as a fabrication method.

Real time optical studies have provided insights into the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) thin films by plasma-enhanced chemical vapor deposition as a function of the H2-dilution gas flow ratio Research-article=[H2]/[SiH4], the accumulated film thickness db, and the substrate material. Results pertinent to the optimization of a-Si:H-based solar cells have been obtained in studies of Si film growth at moderate to high R on dense amorphous semiconductor film surfaces. For depositions with 15≤Research-article≤80 on freshly-deposited a-Si:H, initial film growth occurs in the amorphous phase. Upon continued growth, however, a transition is observed as crystallites begin to nucleate from the amorphous film. The thickness at which this amorphous-to-microcrystalline (a→μc) transition occurs is found to decrease with increasing R.Based on these results, a deposition phase diagram has been proposed that describes the a–gc transition as a continuous function of R and db. We find that the optimum stabilized a-Si:H p-i-n solar cell performance is obtained in an i-layer growth process that is maintained as close as possible to the phase boundary (but on the amorphous side) versus film thickness.

We have deposited thin B- and P-doped Si layers by electron cyclotron resonance CVD on c- Si (4 Ωcm, CZ) and on quartz glass substrates at T=325°C. Films grown on quartz glass are of microcrystalline nature with crystalline volume fractions of about 70 % and a resistivity ranging from 0.01 - 10 (Ωcm)−1 depending on doping concentration. The doping efficiency is close to unity with the carrier mobility being independent of doping concentration for both B- and Pdoping. Films grown on c-Si, on the other hand, exhibit perfect homoepitaxial morphology when the gas phase doping concentration exceeds 1000 ppm and 5000 ppm for P- and B-doping, respectively. The quality of the films is tested by preparing thin film emitter solar cells. We find efficiencies above 11 % for cells without ARC. The result are compared to cells with diffused emitters, otherwise prepared with the same technological steps.

The dynamic change of the dangling bond (db) intensity in hydrogenated amorphous silicon (a-Si:H) during H2 and Ar plasma treatments was observed using an in-situ electron-spinresonance (ESR) technique. The experimental results show that the time to reach the steady state between gas-phase H atoms and the a-Si:H surface is less than 1 sec, and Ar plasma treatments create a top-surface region with an extremely high db density.

We present low cost near infrared photodetectors based on polycrystalline Ge film thermally evaporated on a silicon substrate. We demonstrate that, by proper choice of deposition conditions and device configuration, a responsivity of 16mA/W and a response time of a few nanoseconds can be achieved at the wavelength of 1.3micron. The device can operate up to 1.55micron. We also describe the fabrication and the operation of a 16 pixel linear detector array, with pitch of about 100 micron.

We report the first observation of ultraviolet lasing in ZnO powder and ZnO polycrystalline films grown on amorphous fused silica substrates. In the absence of any fabricated mirrors, laser action occurs in the closed loops formed by multiple optical scattering of light.

A characterization of Semi-insulating Polycrystalline Silicon (SIPOS) layers deposited from SiH4 on SiO2 is presented, as a function of growth and annealing conditions (time and temperature), in order to better understand the processes involved in nucleation of silicon nanocrystals. Correlation between optical and XPS measurements allows determination of the starting composition of the amorphous material. After annealing, Fourier transform infrared (FTIR), ultraviolet-visible and Raman spectroscopies have been used to determine the structural and optical characteristics of the resulting material. Thermal treatment promotes a phase separation, modifying the layer properties and degrading the electrical insulation characteristics. Concentration of the silicon dioxide phase increases, whereas elemental silicon precipitates into nanocrystals which nucleate near the interface with the underneath SiO2. Their density depends of the initial silicon content in the SIPOS layer, and some directions such as <1I1> and <220> grow preferentially whereas other directions such as <311> show a slower growth. As the percentage of oxygen increases, the formation of precipitates is less marked.

Hot wire (HW) cell method has been newly developed and successfully applied to grow polycrystalline silicon films at a low temperature with a relatively high growth rate. In the HWcell method, silane is decomposed by reaction with a heated tungsten wire placed near the substrate. It is found that polycrystalline silicon films can be obtained at substrate temperatures of 175-400°C without hydrogen dilution. The film crystallinity is changed from polycrystalline to amorphous with decreasing the total pressure. The X-ray analysis clearly showed that the films grown at the filament temperature of 1700°C have a very strong (220) preferential orientation. The films consist of large grains as well as small grains, and it was found from cross-sectional SEM that the films have columnar structure. These results suggested that the HW-cell method would be a promising candidate to grow device-grade polycrystalline silicon films for photovoltaic application.

Hydrogenated microcrystalline silicon (μc-Si:H) thin films have been obtained by plasma decomposition of SiF4-H2-He mixtures at low temperature (120 °C). The size of crystalline grain and their volume fraction with respect to the amorphous phase have been found dependent on the r.f. power as evaluated by grazing incidence X-ray diffraction, microRaman and ellipsometry measurements. Chemical and electrical properties change according to the microcrystallinity. Pure and/or highly microcrystalline silicon has been obtained at temperature and r.f. power as low as 120 °C and 15 Watt.

A study of Si:H layers formed by high dose hydrogen implantation (up to 3x107cm-2) using pulsed beams with mean currents up 40 mA/cm2 was carried out in the present work. The Rutherford backscattering spectrometry (RBS), channeling of He ions, and transmission electron microscopy (TEM) were used to study the implanted silicon, and to identify the structural defects (a-Si islands and nanocrystallites). Implantation regimes used in this work lead to creation of the layers, which contain hydrogen concentrations higher than 15 at.% as well as the high defect concentrations. As a result, the nano- and microcavities that are created in the silicon fill with hydrogen. Annealing of this silicon removes the radiation defects and leads to a nanocrystalline structure of implanted layer. A strong energy dependence of dechanneling, connected with formation of quasi nanocrystallites, which have mutual small angle disorientation (<1.50), was found after moderate annealing in the range 200-500°C. The nanocrystalline regions are in the range of 2-4 nm were estimated on the basis of the suggested dechanneling model and transmission electron microscopy (TEM) measurements. Correlation between spectroscopic ellipsometry, visible photoluminescence, and sizes of nanocrystallites in hydrogenated nc-Si:H is observed.

Low-temperature crystallization of a-Si is important for display and Silicon-On- Insulator (SOT) technologies. We present optical characterization (Raman scattering and photoluminescence) of H2 and O2 plasma enhanced crystallization of a-Si:H films. H2 plasma treatment is shown to be the most efficient, leading to larger grain sizes, and both H2 and O2 plasma lead to visible photoluminescence (PL). Recently, the PL of re-crystallized a-Si films has been explained in terms of quantum confinement [1]. The mean size of the crystallites in our re-crystallized films is determined by Raman scattering for different treatments parameters. No correlation between size and the photon energy of the visible emission is found. However, we can clearly distinguish between the PL from purely amorphous and re-crystallized a-Si:H films: Their PL temperature dependence and spectra are very different. The origin of the visible PL in re-crystallized thin Si films is discussed.