J. Cárabe

Hg1−xCdxI2/CdTe heterostructures for nuclear radiation detectors: Effect of epitaxial growth on substrate properties

We demonstrate the possibility to fabricate nuclear radiation detectors operating at room temperature from CdTe substrates affected by the vapor phase epitaxy (VPE) growth of Hg1−xCdxI2 layers. The VPE layers with the thickness 10–30 μm were grown using an α-HgI2 polycrystalline source at 220 °C and time in the range of 30–100 h. The as-grown heterostructures were chemically etched to remove the epilayers, and Au–CdTe–Au detectors were made. The substrates were characterized by synchrotron x-ray topography before and after the VPE growth, and the current–voltage (I–V) and spectroscopic measurements of the detectors were carried out. The effect of the VPE growth on the substrates and detectors has been studied and on the basis of this it has been possible to fabricate γ-ray detectors with Ohmic I–V characteristic and good spectral response.

Effect of preparation conditions on the properties of glow-discharge intrinsic amorphous silicon

Abstract The influence of the preparation conditions (process pressure, substrate temperature, RF-power density and deposition time/thickness) on the optical and electrical properties of intrinsic hydrogenated amorphous silicon (a-Si:H) has been investigated with the aim of optimising such films to be used as absorbent layers of a-Si:H-based p-i-n solar cells. Highly photosensitive films have been obtained at high growth rates (6.2 A s−1) in the depletion regime using a high process pressure (1000 mTorr), a moderate substrate temperature (250°C) and a relatively high RF-power density (35.2 mW cm−2). These films have excellent properties for the application in question.

Microstructure of thin films prepared by plasma-enhanced chemical vapour deposition of helium-diluted silane

The present paper describes an investigation on the microscopic structure of silicon thin films made by plasma-enhanced chemical vapour deposition (PECVD) in conditions different from those typically applied in the preparation of hydrogenated amorphous silicon. Gas mixtures containing helium, silane, and diborane have been used. High radiofrequency power densities have been applied, so that a quasi-equilibrium is reached between film growth and selective etching due to ion bombardment. The specimens have been analysed using infrared-absorption spectroscopy, atomic-force microscopy, X-ray diffraction, Raman spectroscopy and photothermal-deflection spectroscopy. The results clearly indicate two phases in the material: microcrystalline and amorphous. The preparation approach can, thus, be considered an alternative to hydrogen dilution for making microcrystalline-silicon thin films.

Influence of interface treatments on the performance of silicon heterojunction solar cells

So-called heterojunction silicon solar cells are generating increasing interest. They differ from conventional crystalline- or multicrystalline-silicon devices in that, instead of forming the emitter/s by thermal-diffusion doping of the absorber, they are deposited as silicon thin films by plasma-enhanced chemical vapour deposition (PECVD) or some other thin-film technique. The approach has important advantages such as good uniformities, low temperatures, excellent thickness control, etc., making the procedure essential for the development of new-generation cells based on ribbon silicon and similar materials. The performance of heterojunction silicon cells is dramatically influenced by the thin-film/absorber interface. Impurities in the junction, passivation of defects, lattice matching and band structure are absolutely crucial. The present paper describes a number of investigations on the preparation of n-p cells made by depositing n-type thin-film silicon by PECVD onto p-type crystalline-silicon wafers. The work is focused on the dependence of cell performance on the treatments applied to the crystalline-silicon surface prior to thin-film deposition. The results show that a treatment with a hydrogen plasma prior to emitter deposition can lead to important efficiency improvements attributed to defect passivation.

Carbon nanotube–amorphous silicon hybrid solar cell with improved conversion efficiency

We report a hybrid solar cell based on single walled carbon nanotubes (SWNTs) interfaced with amorphous silicon (a-Si). The high quality carbon nanotube network was dry transferred onto intrinsic a-Si forming Schottky junction for metallic SWNT bundles and heterojunctions for semiconducting SWNT bundles. The nanotube chemical doping and a-Si surface treatment minimized the hysteresis effect in current-voltage characteristics allowing an increase in the conversion efficiency to 1.5% under an air mass 1.5 solar spectrum simulator. We demonstrated that the thin SWNT film is able to replace a simultaneously p-doped a-Si layer and transparent conductive electrode in conventional amorphous silicon thin film photovoltaics.

Optoelectronic properties of doped layers for a-Si solar cells prepared using helium dilution

Abstract This paper is the first part of a work about the preparation and characterisation of doped layers for hydrogenated-amorphous-silicon (a-Si:H) thin film solar cells. An approach for RF-glow discharge deposition of a-Si consisting of dilution of silane (SiH 4 ) in helium and application of high RF-power densities, has been tested. In this first part the optimisation of n-type layers has been accomplished. The influence of preparation conditions on the optical and electrical properties of the films has systematically been studied. It has been found that the use of high RF-power densities and high dilution levels of SiH4 in He favour the doping efficiency and film quality when the substrate temperature is 300°C. As a result of these investigations, n-type layers with thicknesses between 250 and 360A, an optical gap about 1.95 eV, a dark-conductivity of 0.1 (Ωcm) −1 and an extended-state conductivity activation energy of 0.1 eV have been prepared. Such properties make them suitable for their use as n-type layers for a-Si:H thin-film solar cells.

Alternative doping of p-type amorphous silicon films using boron trifluoride

P-type amorphous hydrogenated silicon thin films have been prepared using plasma-enhanced chemical vapour deposition with boron trifluoride instead of diborane as the dopant gas. The influence of the substrate temperature (Ts), the radio frequency (r.f.) power density (RFP) and the dopant gas concentration (C(BF3)) have been investigated. The applicability of boron triflouride to make device-quality p-type amorphous silicon has been demonstrated. The incorporation of fluorine to the lattice has been deduced from the analysis of the infrared transmission spectra. The experimental results show interesting features such as the inhibition of bandgap degradation in a wide doping range, which has been associated with the presence of fluorine. P-type thin films with 1.82 eV optical gap, 4.8 × 10−4 (ω cm)−1 conductivity and 0.43 eV activation energy, have been produced.

A simple amorphous‐silicon photodetector for two‐dimensional position sensing

A simple two‐dimensional position‐sensitive detector is proposed and initial tests are reported. The device, aimed to be high‐energy‐radiation resistant and reliable under magnetic fields, is based on a set of small p‐i‐n amorphous silicon photodiodes acting as pixels sandwiched between two perpendicular strip electrode structures. No thin‐film transistors are used. A simplified prototype has been made and tested. The results show that the position of a laser spot on its surface can be deduced from the signals it generates. Further work proposed, includes tests on possible cross talk and behavior under radiation and magnetic fields, as well as the development of methods for accurate position calculation and for appropriate response calibration.

PECVD deposition of device-quality intrinsic amorphous silicon at high growth rate

Abstract The combined influence of RF-power density (RFP) and silane flow-rate (Φ) on the deposition rate of plasma-enhanced chemical vapour deposition (PECVD) intrinsic amorphous silicon has been investigated. The correlation of the results obtained from the characterisation of the material with the silane deposition efficiency, as deduced from mass spectrometry, has led to an interpretation allowing to deposit intrinsic amorphous-silicon films having an optical gap of 1.87 eV and a photoconductive ratio (ratio of ambient-temperature conductivities under 1 sun AM1 and in dark) of 6 orders of magnitude at growth rates up to 10 A/s, without any structural modification of the PECVD system used. Such results are considered of high relevance regarding industrial competitiveness.

Parameterization of a-Si crystallization by continuous-wave green laser irradiation: from single spot to large area

Abstract. An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. In the present work, thin-film amorphous-silicon samples were irradiated with a continuous-wave green laser source. Laser irradiated spots were produced by using different laser powers and irradiation times. Micro-Raman spectroscopy was used to study the crystallization induced on the irradiated surface. Both laser peak power density and irradiation time are identified as key variables in the crystallization process, but within the parametric window considered, the enhancement of the crystalline factor, is more sensitive to the power density than to the irradiation time. The optimum parameters are then used for crystallizing a large sample area by means of overlapped laser scanned lines. Ellipsometric data experimentally show that the whole volume of a micron-thick sample is crystallized.