J. Puigdollers

Surface passivation of p-type crystalline Si by plasma enhanced chemical vapor deposited amorphous SiC x :H films

Excellent passivation properties of intrinsic amorphous silicon carbide (a-SiCx:H) films deposited by plasma enhanced chemical vapor deposition on single-crystalline silicon (c-Si) wafers have been obtained. The dependence of the effective surface recombination velocity, Seff, on deposition temperature, total pressure and methane (CH4) to silane (SiH4) ratio has been studied for these films using lifetime measurements made with the quasi-steady-state photoconductance technique. The dependence of the effective lifetime, teff, on the excess carrier density, ?n, has been measured and also simulated through a physical model based on Shockley–Read–Hall statistics and an insulator/semiconductor structure with fixed charges and band bending. A Seff at the a-SiCx:H/c-Si interface lower than 30?cm?s-1 was achieved with optimized deposition conditions. This passivation quality was found to be three times better than that of noncarbonated amorphous silicon (a-Si:H) films deposited under equivalent conditions.

Polycrystalline silicon films obtained by hot-wire chemical vapour deposition

Silicon films were deposited at moderate substrate temperatures (280–500° C) from pure silane and a silane-hydrogen mixture (10% SiH4, 90% H2) in a hotwire CVD reactor. The morphology, structure and composition of the samples were studied with scanning electron microscopy, transmission electron microscopy, transmission electron diffraction, X-ray diffraction, Raman spectroscopy and secondary ion mass spectrometry. The sample deposited at 500° C with pure silane has an amorphous structure, whereas the samples obtained from silane diluted in hydrogen have a polycrystalline structure, even that grown at the lowest temperature (280° C). Polycrystalline samples have a columnar structure with 0.3–1 μm crystallite sizes with preferential orientation in [220] direction. Deposition rates depend on the filament-substrate distance and range from 9.5 to 37 Å/s for the polycrystalline samples. The high quality of the polycrystalline samples obtained makes the hot-wire technique very promising. Moreover, it is expected to be easily scaled up for applications to large-area optoelectronic devices and to photovoltaic solar cells.

Molecular order of air-stable p-type organic thin-film transistors by tuning the extension of the π-conjugated core: the cases of indolo[3,2-b]carbazole and triindole semiconductors

Charge transport in organic devices depends strongly on the molecular order and morphology of the organic semiconductor thin films. In the design of new organic semiconductors, the selection of the appropriate core plays a key role in the molecular packing and charge transport characteristics of the organic device. Four derivatives of carbazole that mainly differ in the extension of the π-conjugated core, including indolo[3,2-b]carbazole and triindole derivatives, exhibited hole mobilities ranging from 10−5 to 10−2 cm2 V−1 s−1 as active layers in organic thin-film transistors (OTFTs). X-ray analyses of the single crystals and evaporated thin films gave insights into the molecular packing of the compounds that justified their OTFTs characteristics.

Pentacene thin-films obtained by thermal evaporation in high vacuum

Pentacene thin-films were obtained by thermal evaporation in high vacuum of a pure (98%) commercially available source. All the samples were grown at room temperature (25 °C) and high deposition rates (>20 A/s) on Corning glass substrates. The microstructure of the pentacene thin-films evidences the coexistence of thin-film and bulk triclinic crystalline phases. The optical absorption edge close to 1.7 eV, together with the high optical absorption in the visible range, make pentacene a promising candidate for low cost solar cells. Doping series of samples were obtained by dipping pieces cut from the same pentacene sample into a solution of iodine in acetonitrile for different times (<1 h). Thereby, room temperature dark conductivity changes from 3×10-7 to 4×10-5 O-1 cm-1. Finally, glass/SnO/pentacene/Al and glass/ZnO:Al/pentacene/Au structures were fabricated showing rectifying characteristics with on/off ratios of approximately 3 orders of magnitude.

Surface passivation of n-type crystalline Si by plasma-enhanced-chemical-vapor-deposited amorphous SiCx:H and amorphous SiCxNy:H films

Excellent passivation of n-type crystalline silicon surface is demonstrated by means of intrinsic amorphous silicon carbide (a-SiCx:H) thin films. An optimum CH4/SiH4 ratio is determined, leading to an effective surface recombination velocity, Seff, lower than 54 cm s−1. By adding a constant flow of N2 to the precursor gases, the surface passivation is improved to Seff⩽16 cm s−1. From infrared spectroscopy measurements of these films, it can be deduced that the N2 flow increases the carbon content of the layers for a constant CH4/SiH4 ratio. The dependence of the effective lifetime, τeff, on the excess charge carrier density, Δn, is measured using the quasisteady-state photoconductance technique, and these curves are simulated through an electrical model based on an insulator/semiconductor structure.

Trimethylboron doping of CVD diamond thin films

Abstract Trimethylboron (B(CH3)3) has been used to obtain p-doped CVD diamond films in a microwave CVD reactor. Diamond films were grown from mixtures of methane, hydrogen and variable quantities of B(CH3)3 diluted in helium. We obtained samples with boron contents in the range 0.03–9 at.%. Raman analysis showed that samples with boron levels up to 0.2 at.% present an increase of film quality in terms of preserving diamond phase and decreasing the graphitic content. For higher boron concentrations the diamond Raman peak vanishes, and X-ray diffraction analysis shows an important expansion of the diamond crystalline network. Electrical measurements showed that, in samples with a boron content up to 0.2 at.%, the electrical conductivity increases by five orders of magnitude. For higher boron concentrations, the conductivity does not increase further. Using the temperature dependence of conductivity an activation energy of 0.1 eV and 0.17 eV was calculated in films with boron contents of 0.15 at.% and 0.03 at.% respectively. C–V measurements of the lowest doped sample, containing 0.03 at.% of boron, gave an acceptor density of 3 × 1016cm−3.

Experimental observation of oxygen-related defect state in pentacene thin films

The authors report on a metastable defect observed in pentacene thin films. The defect, which is characterized by a hole trap at Ev+0.6 eV and attempt-to-escape frequency of 5x1012 s−1, can be reversibly created/removed under a negative/positive bias voltage applied to the aluminum/ pentacene Schottky diode at room temperature in air. Annealing the sample in vacuum at 360 K removes the defect and prevents its creation by application of any bias voltage in vacuum. Considering recent calculations of defects in pentacene the authors assume that the defect is formed by replacing one of the hydrogen atoms by an oxygen atom (C22 H13 O).

Optoelectronic properties of CuPc thin films deposited at different substrate temperatures

Structural and optical characterization of copper phthalocyanine thin film thermally deposited at different substrate temperatures was the aim of this work. The morphology of the films shows strong dependence on temperature, as can be observed by atomic force microscopy and x-ray diffraction spectroscopy, specifically in the grain size and features of the grains. The increase in the crystal phase with substrate temperature is shown by x-ray diffractometry. Optical absorption coefficient measured by photothermal deflection spectroscopy and optical transmittance reveal a weak dependence on the substrate temperature. Besides, the electro-optical response measured by the external quantum efficiency of Schottky ITO/CuPc/Al diodes shows an optimized response for samples deposited at a substrate temperature of 60 °C, in correspondence to the I–V diode characteristics.

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.

Analysis of bias stress on thin-film transistors obtained by Hot-Wire Chemical Vapour deposition

The stability under gate bias stress of unpassivated thin film transistors was studied by measuring the transfer and output characteristics at different temperatures. The active layer of these devices consisted of in nanocrystalline silicon deposited at 125°C by Hot-Wire Chemical Vapour Deposition. The dependence of the subthreshold activation energy on gate bias for different gate bias stresses is quite different from the one reported for hydrogenated amorphous silicon. This behaviour has been related to trapped charge in the active layer of the thin film transistor.