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Research on semiconducting films is examined in detail, with an assessment of optical and electrical characterization of single crystals and films,

Research was conducted to investigate the photoluminescence (PL) properties of Tb ions introduced into a thin oxide layer on a silicon wafer. The visible luminescence at 543 nm was also examined. Results indicate that Tb concentrations exceeding 0.5 at.% Tb in SiO2 will always favor green emission over blue emission. It was observed that Tb PL lifetime for well-annealed samples amounts to 3.5 ms and remains constant for all investigated concentrations.

The intrinsic microcrystalline absorber layer growth in thin-film silicon solar-cells is investigated by in-situ Raman spectroscopy during plasma enhanced chemical vapor deposition. In-situ Raman spectroscopy enables a detailed study of the correlation between the process settings, the evolution of the Raman crystallinity in growth direction, and the photovoltaic parameters η (solar cell conversion efficiency), JSC (short circuit current density), FF (fill factor), and VOC (open circuit voltage). Raman spectra were taken every 7 nm of the absorber layer growth depending on the process settings. The Raman crystallinity of growing microcrystalline silicon was determined with an absolute error of approximately ±5% for total absorber layer thicknesses >50 nm. Due to this high accuracy, inherent drifts of the Raman crystallinity profiles are resolvable for almost the entire absorber layer deposition. For constant process settings and optimized solar cell device efficiency Raman crystallinity increases during the absorber layer growth. To compensate the inhomogeneous absorber layer growth process settings were adjusted. As a result, absorber layers with a constant Raman crystallinity profile — as observed in-situ — were deposited. Solar cells with those absorber layers show a strongly enhanced conversion efficiency by ~0.5% absolute. However, the highest FF, VOC, and JSC were detected for solar cells with different Raman crystallinity profiles. In particular, fill factors of 74.5% were observed for solar cells with decreasing Raman crystallinity during the later absorber layer growth. In contrast, intrinsic layers with favorable JSC are obtained for constant and increasing Raman crystallinity profiles. Therefore, monitoring the evolution of the Raman crystallinity in-situ provides sufficient information for an optimization of the photovoltaic parameters with surpassing depth resolution. [ABSTRACT FROM AUTHOR]

The formation of microcrystalline silicon was studied employing the layer-by-layer technique with very high frequency plasma excitation. Microcrystalline growth occurred when the thickness of the deposited layer was reduced to a certain value in each cycle. Etching of the deposited layers was found to be a dominant effect during hydrogen plasma treatment. The etching stopped when a noticeable crystalline volume fraction was produced.

Strained Si nanowires (NWs) are attractive for deeply-scaled complementary metal-oxide-semiconductor devices due to the combination of enhanced carrier mobility and excellent electrostatic control as was demonstrated with trigate metal-oxide-semiconductor field effect transistors. The challenge in using strained Si NWs for devices is to preserve the elastic strain during the required processing steps. In this work we investigated the influence of fundamental processing steps like patterning and dopant ion implantation on the structural and transport properties of strained Si layers and NWs on silicon-on-insulator (SOI) substrates. NWs with widths down to 35 nm, fabricated on 25 nm strained SOI and implanted to doses ranging from 5×1014 to 2×1015 ions/cm2 were investigated. We show that strain conservation and a low sheet resistivity of 6.2×10-4 Ω cm, close to the layer resistivity, can only be obtained if the NWs are patterned on doped layers. For NWs directly implanted to doses above 1×1015 ions/cm2, complete strain relaxation and structural disorder by solid phase recrystallization were observed. In both cases, NWs with widths smaller than 55 nm exhibit an increased specific resistivity. [ABSTRACT FROM AUTHOR]

The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (μc-Si:H) has been systematically investigated. Single μc-Si:H layers and complete μc-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of ∼1.2 μm thick μc-Si:H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2×1019 to 2×1019 cm-3; in the case of nitrogen contamination the critical impurity level is lower ([N]critical=6×1018–8×1018 cm-3). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of μc-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the μc-Si:H deposition process, the cell performance drops at an air leak fraction from 140 to 200 ppm compared to the total gas flow during i-layer deposition. It is demonstrated that oxygen and nitrogen impurities close to the p/i-interface have a stronger effect on the cell performance compared to impurities close to the n/i-interface. Moreover, thick μc-Si:H solar cells are found to be more impurity-sensitive than thinner cells. [ABSTRACT FROM AUTHOR]

The influence of atmospheric contaminants oxygen and nitrogen on the performance of thin-film hydrogenated amorphous silicon (a-Si:H) solar cells grown by plasma-enhanced chemical vapor deposition at 13.56 MHz was systematically investigated. The question is addressed as to what degree of high base pressures (up to 10-4 Torr) are compatible with the preparation of good quality amorphous silicon based solar cells. The data show that for the intrinsic a-Si:H absorber layer exists critical oxygen and nitrogen contamination levels (about 2×1019 atoms/cm3 and 4×1018 atoms/cm3, respectively). These levels define the minimum impurity concentration that causes a deterioration in solar cell performance. This critical concentration is found to depend little on the applied deposition regime. By enhancing, for example, the flow of process gases, a higher base pressure (and leak rate) can be tolerated before reaching the critical contamination level. The electrical properties of the corresponding films show that increasing oxygen and nitrogen contamination results in an increase in dark conductivity and photoconductivity, while activation energy and photosensitivity are decreased. These effects are attributed to nitrogen and oxygen induced donor states, which cause a shift of the Fermi level toward the conduction band and presumably deteriorate the built-in electric field in the solar cells. Higher doping efficiencies are observed for nitrogen compared to oxygen. Alloying effects (formation of SiOx) are observed for oxygen contaminations above 1020 atoms/cm3, leading to an increase in the band gap. [ABSTRACT FROM AUTHOR]

Hydrogenated microcrystalline silicon (μc-Si:H) thin-film solar cells were prepared at high rates by very high frequency plasma-enhanced chemical vapor deposition under high working pressure. The influence of deposition parameters on the deposition rate (RD) and the solar cell performance were comprehensively studied in this paper, as well as the structural, optical, and electrical properties of the resulting solar cells. Reactor-geometry adjustment was done to achieve a stable and homogeneous discharge under high pressure. Optimum solar cells are always found close to the transition from microcrystalline to amorphous growth, with a crystallinity of about 60%. At constant silane concentration, an increase in the discharge power did hardly increase the deposition rate, but did increase the crystallinity of the solar cells. This results in a shift of the μc-Si:H/a-Si:H transition to higher silane concentration, and therefore leads to a higher RD for the optimum cells. On the other hand, an increase in the total flow rate at constant silane concentration did lead to a higher RD, but lower crystallinity. With this shift of the μc-Si:H/a-Si:H transition at higher flow rates, the RD for the optimum cells decreased. A remarkable structure development along the growth axis was found in the solar cells deposited at high rates by a “depth profile” method, but this does not cause a deterioration of the solar cell performance apart from a poorer blue-light response. As a result, a μc-Si:H single-junction p-i-n solar cell with a high efficiency of 9.8% was deposited at a RD of 1.1 nm/s. [ABSTRACT FROM AUTHOR]

Semiconducting iron disilicide (β-FeSi[sub 2]) precipitates in silicon were fabricated by ion beam synthesis. The samples were characterized by Raman spectroscopy, transmission electron microscopy, and photoluminescence spectroscopy. By comparison with a β-FeSi[sub 2] single crystal, the silicide precipitates are found to be unstrained in all cases, so there is no correlation between strain and photoluminescence efficiency. Our results indicate that carrier recombination at silicon dislocations is sufficient to explain the photoluminescence in our samples. [ABSTRACT FROM AUTHOR]

Green photoluminescence and electroluminescence (EL) from Tb implanted A1N films have been observed at room temperature. The AlN films of 180 nm thickness were grown on n-type Si(111) by metalorganic chemical vapor deposition. X-ray diffraction shows that the AlN is polycrystalline. The A1N films were doped by ion implantation of Tb to a peak concentration of 1 at. %. A postimplantation annealing step was required to obtain optically active Tb ions. A dc EL device was fabricated using a transparent ZnO:Al top electrode. The strong room-temperature green light emission was observable with the naked eye. It was obtained with a drive current density of 2-70 mA/cm² at a drive voltage of 70-100 V. The emission lines between 490 and 650 nm originate from Tb[sup 3+] transitions from the [sup 5]D[sub 4] level to ground state multiplets. The observed luminescence lifetimes are approximately 0.5 ms. [ABSTRACT FROM AUTHOR]

Recently, Ru[sub 2]Si[sub 3] has been predicted to be a direct semiconductor with a band gap of approx. 0.8 eV. Since the corresponding wavelength of this potential light emitter coincides with the absolute absorption minimum of glass fibers of 1.5 μm, considerable attention has been attracted. Measurements of the temperature dependence of the electrical resistivity of silicide films on insulating substrates were carried out in van der Pauw geometry. The results were explained by assuming carrier hopping over grain boundaries. The optical absorption coefficient was measured on thin films grown on various substrates, on self-sustaining films, where the substrate was partly removed and on a single crystal by photothermal deflection spectroscopy. A direct band gap at 0.84 eV was found. The absorption coefficient is very low up to approx. 1.5 eV, likely due to a low density of states, and then strongly increases at higher energies. The experimental results qualitatively confirm the predictions of the band structure calculations. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The optical and structural investigations of beta-FeS(sub 2)/Si layers, where the silicide microstructure was varied from small precipitates to a nearly continuous layer was carried out. The results show the suitability of iron disilicide precipitate layers for optical emitter is least controversial.