Effect of thermal annealing and hydrogen-plasma treatment in boron-doped microcrystalline silicon

We have investigated the effects of temperature (during film growth and post-deposition thermal annealing) and H₂-plasma treatment on the electronic and structural properties of p-type microcrystalline silicon films (p-μc-Si:H) for solar cell applications. The highest dark conductivity is obtained in the thermally annealed p-μc-Si:H prepared at low substrate temperature of 50 °C. This dark conductivity is decreased by two orders of magnitude when the film is exposed to H₂-plasma, being completely restored after thermal annealing. Namely, reversible dual-conductivity cycle is observed between thermally annealed state and H₂-plasma-treated state in p-μc-Si:H. The dual-conductivity cycle is accompanied with the reversible change in the infrared-absorption spectrum at around 1845 cm^? 1 assigned as SiHB complex in p-μc-Si:H network structure. Taking into account of the reversible structural change by H₂-plasma-exposure and thermal-annealing cycles, necessary process-procedure condition has been proposed for obtaining high photovoltaic performance in thin-film-Si solar cells with high quality p-μc-Si:H.     

The interpretation of the electric and optical properties of a-Si:H films produced by rf glow discharge through dark conductivity,photoconductivity and pulse controlled capacitance-voltage measurements

This paper deals with the interpretation of transport properties of amorphous silicon hydrogenated films (a-Si:H) through dark conductivity, photoconductivity and pulse controlled capacitance-voltage measurements. a-Si:H films were produced by rf glow discharge coupled either inductively or capacitively to a 3% SiH₄/Ar mixture at different crossed electromagnetic static fields. The data concerned with the dark activation energy, photoactivation energy, variation of the density of localized states and photosensitivity, (σ_ph/σ_d)_25°C, of a-Si:H films can account for their optoelectronic properties which are strongly dependent on the deposition parameters. We also observed that crossed electromagnetic static fields applied during film formation influences hydrogen incorporation in a different manner than previously proposed.     

Fractional composition of large crystallite grains: A unique microstructural parameter to explain conduction behavior in single phase undoped microcrystalline silicon

We have studied the dark conductivity of a broad microstructural range of plasma deposited single phase undoped microcrystalline silicon (μc-Si:H) films in a wide temperature range (15–450 K) to identify the possible transport mechanisms and the interrelationship between film microstructure and electrical transport behavior. Different conduction behaviors seen in films with different microstructures are explained in the context of underlying transport mechanisms and microstructural features, for above and below room temperature measurements. Our microstructural studies have shown that different ranges of the percentage volume fraction of the constituent large crystallite grains (F_cl) of the μc-Si:H films correspond to characteristically different and specific microstructures, irrespective of deposition conditions and thicknesses. Our electrical transport studies demonstrate that each type of μc-Si:H material having a different range of F_cl shows different electrical transport behaviors.     

Dark conductivity in amorphous undoped silicon films

The temperature dependence of the dark conductivity was investigated in amorphous undoped silicon films deposited by glow-discharge in a SiCl₄H₂ mixture. Different transport processes were indentified according to the investigated temperature range. The dependence of the dark conductivity was also examined as a function of some deposition parameters. The experimental results are discussed in terms of the two-phase structure of the film.     

ESR study of the hydrogenated nanocrystalline silicon thin films

We studied the stability and light-induced paramagnetic centers in hydrogenated nanocrystalline silicon thin films (nc-Si:H) by electron-spin-resonance (ESR) and photothermal-deflection-spectroscopy (PDS). There is no measurable change in defect density upon illumination with white light with a light intensity of 300 mW cm^?2 for 300 h. At low temperatures, upon illumination with sub-bandgap light, a light-induced ESR signal appears. This signal is similar to that in hydrogenated micro-crystalline silicon (μc-Si:H).     

Preparation and properties of microcrystalline silicon films using photochemical vapor deposition

microcrystalline silicon films have been prepared through mercury photosensitized decomposition of monosilane at low gas pressures. The dark and light conductivities of the silicon films tend to increase at reactant pressures lower than 65 Pa and become 10^?2Ω^?1· cm^?1 at 26 Pa. From the Raman scattering and x-ray diffraction, silicon films were found to consist of a mixed phase structure including both microcrystalline and amorphous regions.     

利用快速热退火法制备多晶硅薄膜

为了制备优质的多晶硅薄膜,该论文研究了非晶硅薄膜的快速热退火(RTA)技术.先利用PECVD设备沉积非晶硅薄膜,然后把其放入快速热退火炉中进行退火.退火前后的薄膜利用X射线衍射(XRD)仪、Raman光谱仪及扫描电子显微镜(SEM)测试其晶体结构及表面形貌,利用电导率测试设备测试其暗电导率.研究表明退火温度、退火时间以及沉积时的衬底温度对非晶硅薄膜的晶化都有很大的影响.     

Mechanisms of the growth of nanocrystalline Si:H films deposited by PECVD

Hydrogenated nanocrystalline silicon (nc-Si:H) films were deposited using plasma-enhanced chemical vapor deposition from a SiF₄/SiH₄/H₂ gas mixtures. Properties were examined of nc-Si:H films produced by decreasing the deposition temperature (T_d) under two different hydrogen dilution ([H₂]) conditions. For these films, the X-ray diffraction, the Raman scattering, the Fourier transform infrared absorption and the stress were investigated. Our results show that the decrease in T_d has significant effects in the decrease of the average grain size (〈δ〉), the crystalline volume fraction (ρ) values, and an increase in the density of SiH-related bonds (N_SiH) values. On the contrary, increases in [H₂] decreased the 〈δ〉 and the N_SiH, while the ρ were increased. Our experiments also confirmed that the increase in ρ corresponds with the decrease in N_SiH. In view of these results, it may be concluded that the use of both low T_d and high [H₂] conditions might lead to growth of nc-Si:H films with small grains and high crystallinity. In this context, the surface processes (such as diffusion and etching) for the growth of nc-Si:H films were extensively discussed in this current work.     

Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio (R_H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3-5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.     

Influence of hydrogen and residual disorder on the optical and electrical properties of microcrystalline silicon

The total hydrogen content, Raman diffusion, optical absorption α(hν) and electrical conductivity σ(1/T) of silicon films prepared by glow discharge between 250°C and 500°C were measured. A model is presented according to which α(hν) and σ(1/T) are controlled by the grain boundaries of microcrystalline Si.     

Photostimulated changes of properties of CdTe films

The effect of illumination during the close‐spaced sublimation (CSS) growth on composition, structural, electrical, optical and photovoltaic properties of CdTe films and CdTe/CdS solar cells were investigated. Data on comparative study by using X‐ray diffraction (XRD), scanning electron microscopy (SEM), absorption spectra and conductivity‐temperature measurements of CdTe films prepared by CSS method in dark (CSSD) and under illumination (CSSI) were presented. It is shown that the growth rate and the grain size of CdTe films grown under illumination is higher (by factor about of 1.5 and 3 respectively) than those for films prepared without illumination. The energy band gap of CdTe films fabricated by both technology, determined from absorption spectra, is same (about of 1.50 eV), however conductivity of the CdTe films produced by CSSI is considerably greater (by factor of 10⁷) than that of films prepared by CSSD. The photovoltaic parameters of pCdTe/nCdS solar cells fabricated by photostimulated CSSI technology (J_sc = 28 mA/cm², V_oc =0.63 V) are considerably larger than those for cells prepared by CSSD method (J_sc = 22 mA/cm², V_oc = 0.52 V). A mechanism of photostimulated changes of properties of CdTe films and improvement of photovoltaic parameters of CdTe/CdS solar cells is suggested. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of polymorphous silicon and germanium as thermo-sensing films for infrared detectors

In this work we present a comparative study on the electrical characteristics of polymorphous silicon (pm-Si:H) and polymorphous germanium (pm-Ge:H) thin films deposited by low frequency plasma enhanced chemical vapor deposition (LF-PECVD), aiming to use them as thermo sensing elements in un-cooled microbolometers.We studied the effect of the deposition pressure on the film characteristics that are important for IR detection, as the activation energy (E_a), the thermal coefficient of resistance (TCR), the room temperature conductivity (σ_RT) and the film responsivity with IR radiation.Our results indicate that polymorphous films have advantages over boron doped a-Si:H, material which is currently employed as thermo-sensing element in commercial microbolometer arrays.     

Properties of a-Si : H : F deposited in SiF4/SiH4/Ar Gas mixtures by a glow-discharge technique

Films of silicon-hydrogen-fluorine alloys (a-Si : H : F) were prepared by capacitive coupling rf glow discharge in an SiF₄/SiH₄/Ar gas mixture. Up to 4% fluorine can be incorporated in films prepared with a gas mixture of SiF₄/(SiF₄ + SiH₄) = 0.72. The hydrogen concentration decreases as fluorine concentration increases but there is still more hydrogen than fluorine in films with the highest density of fluorine, which is in sharp contrast to the best films produced by the Madan and co-workers. Both dark conductivity and photoconductivity decrease as fluorine increases, though improvement of doping efficiency was observed in films with moderate concentrations of fluorine. The influence of hydrogen is more important than the influence of fluorine in the present silicon-hydrogen-fluorine alloys.     

Atmospheric aging and thermal annealing effects in a-C:H thin films

Atmospheric aging and thermal annealing effects have been studied in hydrogenated amorphous carbon (a-C:H) thin films deposited using a dc saddle field glow discharge technique with different ion energies (85–225 eV) during deposition. The a-C:H films grown with low ion energies showed aging effects when they were exposed to the ambient atmosphere. Infrared (IR) absorption due to O–H and C=O vibration modes increased while the IR absorption due to C–H vibrations decreased with aging. The absence of absorption due to O–D vibrations in deuterated films indicates that the films reacted with water rather than oxygen when exposed to the atmosphere. The C–H and O–H bond concentrations decreased when the a-C:H films which had been exposed to the atmosphere for a one month period were thermally annealed. The a-C:H films grown with high ion energies exhibited neither atmospheric aging nor thermal annealing effects.     

Study of in-gap defects in intrinsic and B-doped a-Si1−xCx:H by photo-induced optical absorption and photoluminescence

The infrared (IR) absorption dependence on visible light illumination has been measured in doped and undoped hydrogenated amorphous silicon carbide (a-Si_1−xC_x:H) films grown by plasma enhanced chemical vapour deposition. The measurements were made by a highly sensitive technique which exploits properly designed a-Si_1−xC_x:H/ZnO test waveguides for lengthening the interaction region between the IR and visible (VIS) radiations in the material. Experimental data show that boron doping strongly enhances the VIS light induced variation of the IR absorption, whereas the increase in carbon content has a quenching effect on the phenomenon. The a-SiC:H films have been also characterized by photoluminescence measurements. The spectra are dominated by a photoluminescence band, ranging between 1.4 eV and 1.9 eV. This band is enhanced by the increase in carbon content, while is strongly quenched with increasing B-doping level. On the basis of these results, a correlation is found between the measurements of optical absorption, photo-induced absorption and photoluminescence. The type and the density of defects induced in the films by the different growth conditions have been recognized as the origin of the different behaviors observed.     

Optical properties and chemical bonding characteristics of amorphous SiNX:H thin films grown by the plasma enhanced chemical vapor deposition method

Amorphous silicon nitride (SiN_X:H) thin films grown by the plasma enhanced chemical vapor deposition (PECVD) method are presently the most important antireflection coatings for crystalline silicon solar cells. In this work, we investigated the optical properties and chemical bonding characteristics of the amorphous SiN_X:H thin films deposited by PECVD. Silane (SiH₄) and ammonia (NH₃) were used as the reactive precursors. The dependence of the growth rate and refractive index of the SiN_X:H thin films on the SiH₄/NH₃ gas flow ratio was studied. The chemical bonding characteristics and the surface morphologies of the SiN_X:H thin films were studied using the Fourier transform infrared spectroscopy and atomic force microscopy, respectively. We also investigated the effect of rapid thermal processing on the optical properties and surface morphologies of the SiN_X:H thin films. It was found that the rapid thermal processing resulted in a decrease in the thickness, increase in the refractive index, and coarser surfaces for the SiN_X:H thin films.     

Optical and electrical properties of chlorinated and hydrogenated amorphous silicon prepared by glow discharge

Chlorinated and hydrogenated amorphous silicon films were prepared by glow discharge of a SiCl₄/H₂ mixture. Infrared spectra of these films show that, in addition to the hydrogen induced bands, two new modes appear at 545 cm^?1 (SiCl stretching) and 500 cm^?1 (Si TO modes induced by chlorine). Observation of the 545 cm^?1 band proves that chlorine is able to act as a dangling bond terminator in an amorphous silicon matrix. A good agreement is found between the total amount of chlorine determined by electron microprobe analysis and the value estimated from the integrated strength of the SiCl stretching mode. The relatively high value of the optical band gap (1.80 eV) of our material containing only 5 at.% bonded hydrogen shows that chlorine plays a major role in the optical gap value. Electrical conductivity, photoconductivity and luminescence properties are qualitatively similar to that of a: SiH films.     

Isochronal Studies of α‐Si:H Crystallization and the Performance of it's Schottky Barriers

Hydrogenated amorphous silicon films (α‐Si:H) were crystallized employing a metal induced crystalline (MIC) technique. Structural changes during annealing these films at 300 °C for different periods (0‐300 minutes) were obtained by XRD. Al was used as a metal induced crystalline for α‐Si:H produced by ultra high vacuum (UHV) plasma enhanced chemical vapor deposition (PECVD). XRD shows that crystallization of the interacted α‐Si:H film underneath Al initiates at 300 °C for 15 minutes. A complete crystallization was obtained after annealing for 60 minutes. A gold dot was evaporated onto α‐Si:H films, which annealed for different periods to form Schottky barriers. Electrical properties of Au/α‐Si:H were calculated such as the ideality factor, n, barrier height, Φ_B, donor concentration, N_D, and the diffusion voltage, V_d, as a function of the annealing time of α‐Si:H films. All these parameters were carried out through the current voltage characteristics (J‐V) and the capacitance voltage measurements (C‐V). The results were presented a discussed on the basis of XRD performance and the thermionic emission theory.     

Metastable effects in silicon thin films: Atmospheric adsorption and light-induced degradation

The effects of exposure to atmosphere (ageing) and light-soaking on coplanar dark- and photo-conductivity of silicon films of varying crystallinity are examined. Dark conductivity generally increases on ageing in films with significant amorphous fraction and decreases in largely crystalline films, and may be reversed by annealing under vacuum at 130 °C consistent with adsorption and desorption of atmospheric components. Thinner films are more strongly affected by ageing. Boron doping appears to compensate charge introduced by ageing, though there are disagreements in detail. In comparison with ageing, moderate light-soaking affects dark conductivity in transitional microcrystalline silicon films only slightly. Both processes change the majority carrier mu–tau product in line with shifts in Fermi level position.     

DC and AC Conductivity of Barium Hydrogen Phosphate (BaHPO4)

D.C. conductivity of the ferroelectric BaHPO₄ is measured and is found to lie between usual conductivities of semiconductor and insulator. Its temperature dependence shows intrinsic conduction in defective crystals due to H⁺ ions. The a.c. conductivity calculated from the measured dielectric constant follows σ˜ω^1.54, a power law. Frequency of 20 kHz resolves the resistivity peak of a.c. conduction into two Gaussians. The ln sigma vs f plots at different temperatures show an exponential dependence and existence of a focal point.