J.M. Asensi

Stability of hydrogenated nanocrystalline silicon thin-film transistors

Abstract Hydrogenated nanocrystalline silicon thin-films were obtained by catalytic chemical vapour deposition at low substrate temperatures (150°C) and high deposition rates (10 A/s). These films, with crystalline fractions over 90%, were incorporated as the active layers of bottom-gate thin-film transistors. The initial field-effect mobilities of these devices were over 0.5 cm2/V s and the threshold voltages lower than 4 V. In this work, we report on the enhanced stability of these devices under prolonged times of gate bias stress compared to amorphous silicon thin-film transistors. Hence, they are promising candidates to be considered in the future for applications such as flat-panel displays.

P-doped polycrystalline silicon films obtained at low temperature by hot-wire chemical vapor deposition

Abstract P-doped polycrystalline silicon films were deposited over Corning 7059 substrates at a moderate temperature, 330°C, in a hot-wire reactor. The films were obtained from the decomposition of silane and hydrogen (10% SiH4, 90% H2) and different amounts of diborane. The structure and morphology of the samples were studied with X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). X-ray diffraction spectra show sharp diffraction peaks corresponding to silicon reflections, and Raman spectra show no evidence of amorphous phases and present a high intensity and narrow peak at 520 cm−1, which is the typical feature of crystalline silicon structure. The efficiency of boron incorporation was studied by secondary ion mass spectrometry (SIMS). The electrical properties of doped samples were also studied.

Analysis of the role of mobility-lifetime products in the performance of amorphous silicon p-i-n solar cells

An analytical model of an amorphous silicon p-i-n solar cell is presented to describe its photovoltaic behavior under short-circuit conditions. It has been developed from the analysis of numerical simulation results. These results reproduce the experimental illumination dependence of short-circuit resistance, which is the reciprocal slope of the I(V) curve at the short-circuit point. The recombination rate profiles show that recombination in the regions of charged defects near the p-i and i-n interfaces should not be overlooked. Based on the interpretation of the numerical solutions, we deduce analytical expressions for the recombination current and short-circuit resistance. These expressions are given as a function of an effective μτ product, which depends on the intensity of illumination. We also study the effect of surface recombination with simple expressions that describe its influence on current loss and short-circuit resistance.

Structure of a-Si: H/a-Si1−xCx: H multilayers deposited in a reactor with automated substrate holder

Abstract This paper deals with the structural properties of a -Si:H/ a -Si 1− x C x : H multilayers deposited by glow-discharge decomposition of SiH 4 and SiH 4 and CH 4 mixtures. The main feature of the rf plasma reactor is an automated substrate holder. The plasma stabilization time and its influence on the multilayer obtained is discussed. A series of a -Si:H/ a -Si 1− x C x : H multilayers has been deposited and characterized by secondary ion mass spectrometry (SIMS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). No asymmetry between the two types of interface has been observed. The results show that the multilayers present a very good periodicity and low roughness. The difficulty of determining the abruptness of the multilayer at the nanometer scale is discussed.

Hot Embossing of Polymer Substrates for Thin Silicon Cell Applications

The use of polymer substrates for thin film solar cells is becoming an issue of great interest, as they facilitate monolithic interconnection of the cells to produce modules and can be used in continuous roll-to-roll processes. However, to reach high efficiencies of thin film silicon cells on polymer substrates, the development of efficient light confinement strategies has to be improved. In this work, hot embossing is used to produce a suitable surface morphology on PEN substrates. Three morphologies have been studied by using three selected masters. To evaluate the quality of the embossing, the morphology of the masters and that of the stamped PEN samples have compared. Finally, the stamped polymers have been covered with thin Ag and transparent conductive oxide (TCO) layers and whole reflectance experiments have been performed to asses the efficiency of the fabricated back reflectors

Hydrogen related effects in a-Si:H studied by photothermal deflection spectroscopy

Abstract A study of thermal annealing of a-Si:H samples between 300 and 600°C has been carried out. At increasing annealing temperatures, the sub-gap absorption measured by PDS increases showing two inflections, centered at 375 and 550°C. The hydrogen content measured by thermal desorption spectroscopy evolves in the same temperature range, whereas the evolution of the hydrogen content deduced from the IR transmission spectra differs, decreasing sooner and vanishing already at about 450°C.

Substrate influence on the properties of doped thin silicon layers grown by Cat-CVD

We present structural and electrical properties for p- and n-type layers grown close to the transition between a-Si:H and nc-Si:H onto different substrates: Corning 1737 glass, ZnO:Al-coated glass and stainless steel. Structural properties were observed to depend on the substrate properties for samples grown under the same deposition conditions. Different behaviour was observed for n- and p-type material. Stainless steel seemed to enhance crystallinity when dealing with n-type layers, whereas an increased crystalline fraction was obtained on glass for p-type samples. Electrical conduction in the direction perpendicular to the substrate seemed to be mainly determined by the interfaces or by the existence of an amorphous incubation layer that might determine the electrical behaviour. In the direction perpendicular to the substrate, n-type layers exhibited a lower resistance value than p-type ones, showing better contact properties between the layer and the substrate.

Comparative study of microcrystalline silicon films prepared in low or high pressure regime by hot-wire chemical vapor deposition

Abstract A comparison of structural, optical and electronic properties between undoped hydrogenated microcrystalline silicon films deposited by hot-wire chemical vapor deposition using either a high vacuum reactor or an ultra-high vacuum reactor has been made. The growth process has been assessed by changing the dilution ratio of silane in hydrogen and the process gas pressure, under the high pressure range (∼10−1 mbar) and the low pressure range (∼10−3 mbar) respectively. Structural properties (crystalline, amorphous and void fractions) of the films are analyzed through ultra-violet (UV)–visible ellipsometry and Raman spectroscopy data whereas electronic properties (majority carrier mobility-lifetime product and activation energy of the conductivity) are analyzed through dark conductivity and photoconductivity. All these results have been interpreted in terms of the flux of atomic hydrogen arriving on the growing surface, compared with that of radicals. This flux ratio is very sensitive to the gas pressure. This study gives better insight into the hot-wire deposition mechanisms.

Structural characterization of a-SiC:H by thermal desorption spectroscopy

Abstract The structure of a-Si1−xCx:H thin films prepared from different mixtures of SiH4 and CH4 was studied by thermal desorption spectroscopy. The influence of the gas phase composition and the thickness on the hydrogen evolution spectra is presented and discussed. A third hydrogen evolution was observed in a-Si1−xCx films with higher carbon content. The results show that the surface controlled desorption processes become dominant when increasing the carbon content.

Light-induced defects in thermal annealed hydrogenated amorphous silicon

Abstract The metastable defects of a-Si:H samples annealed at temperatures in the 300–550°C range have been studied by photothermal deflection spectroscopy (PDS). The light-soaked samples show an increase in optical absorption in the 0.8 to 1.5 eV range. The metastable defect density decreases when the annealing temperature increases, while the defect density increases. This decrease in the metastable defect density shows an almost linear correlation with the decrease in the hydrogen content of the samples, determined by IR transmission spectroscopy and thermal desorption spectroscopy.