Preferred growth of nanocrystalline silicon in boron-doped nc-Si : H Films

Preferred growth of nanocrystalline silicon (nc-Si) was first found in boron-doped hydrogenated nanocrystalline (nc-Si:H) films prepared using plasma-enhanced chemical vapor deposition system. The films were characterized by high-resolution transmission electron microscope, X-ray diffraction (XRD) spectrum and Raman Scattering spectrum. The results showed that the diffraction peaks in XRD spectrum were at 2theta approximate to 47degrees and the exponent of crystalline plane of nc-Si in the film was (220). A considerable reason was electric field derived from dc bias made the bonds of Si-Si array according to a certain orient. The size and crystalline volume fraction of nc-Si in boron-doped films were intensively depended on the deposited parameters: diborane (B2H6) doping ratio in silane (SiH4), silane dilution ratio in hydrogen (H-2), rf power density, substrate's temperature and reactive pressure, respectively. But preferred growth of nc-Si in the boron-doped nc-Si:H films cannot be obtained by changing these parameters. (C) 2004 Elsevier Ltd. All rights reserved.

Amorphous silicon carbide films prepared by H-2 diluted silane-methane plasma

This paper reports on the preparation and characterization of hydrogenated amorphous silicon carbide films prepared by H-2 diluted silane-methane plasma. Carbon-rich a-SiC:H film with band gap of up to 3.3 eV has been achieved. IR and UV Vis spectra were employed to characterize the chemical bonding and optical properties of as-prepared films. It is shown that hydrogen dilution is crucial in obtaining these wide band gap carbon-rich films. Raman and PL measurements were performed to probe the microstructure and photoelectronic properties of these films before and after annealing. Films with intermediate carbon concentration seem more defective and exhibit stronger photoluminescence and subband absorption than others. Films with different compositions exhibit different annealing behaviours. For silicon rich and carbon rich films, high temperature annealing results in the formation of silicon crystallites and graphite clusters, respectively. (C) 2003 Elsevier B.V. All rights reserved.

Structure, stability and photoelectronic properties of transition films from amorphous to microcrystalline silicon

A series of hydrogenated silicon films near the threshold of crystallinity was prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) from a mixture of SiH4 diluted in H, The effect of hydrogen dilution ratios R-H = [H-2]/[SiH4] on microstructure of the films was investigated. Photoelectronic properties and stability of the films were studied as a function of crystalline fraction. The results show that more the crystalline volume fraction in the silicon films, the higher mobility life-time product (mu tau), better the stability and lower the photosensitivity. Those diphasic films contained 8%-31% crystalline volume fraction can gain both the fine photoelectronic properties and high stability. in the diphasic (contained 12% crystalline volume fraction) solar cell, we obtained a much lower light-induced degradation of similar to 2.9%, with a high initial efficiency of 10.01% and a stabilized efficiency of 9.72% (AM1.5, 100 mW/cm(2)). (c) 2005 Elsevier B.V. All rights reserved.

Transition films from amporphous to microcrystalline silicon and solar cells

A series of hydrogenated silicon films near the threshold of crystallinity was prepared by very high frequency plasmaenhanced chemical vapor deposition (VHF-PECVD)from a mixture of SiH4 diluted in H-2. The effect of hydrogen dilution ratios R = [H-2]/[SiH4] on the microstructure of the films was investigated. The photoelectronic properties and stability of the films were studied as a function of crystalline fraction. The results show that the diphasic films gain both the fine photoelectric properties like a-Si: H and high stability like mu w-Si:H. By using the diphasic silicon films as the intrinsic layer, p-i-n junction solar cells were prepared. Current-voltage (J-V) characteristics and stability of the solar cells were measured under an AM1.5 solar simulator. We observed a light-induced increase of 5.2% in the open-circuit voltage (V-oc) and a light-induced degradation of similar to 2.9% inefficiency.

NIP a-Si : H solar cells on stanless steel with p-type nc-Si : H window layer

The successful application of boron-doped hydrogenated nanocrystalline silicon as window layer in a-Si: H nip solar cells on stainless steel foil with a thickness of 0.05 mm is reported. Open circuit voltage and fill factor of the fabricated solar cell were 0.90V and 0.70 respectively. The optical and structural properties of the p-layers have been investigated by using UV-VIS and Raman spectroscopy. It is confirmed that the p-layer is hydrogenated nanocrystalline silicon with a wide optical gap due to quantum size effect.

Impact of hydrogen dilution on microstructure and optoelectronic properties of silicon films deposited using tirisilane

We explored the deposition of hydrogenated amorphous silicon (a-Si: H) using trisilane (Si3H8) as a gas precursor in a radiofrequency plasma enhanced chemical vapour deposition process and studied the suitability of this material for photovoltaic applications. The impact of hydrogen dilution on the deposition rate and microstructure of the films is systematically examined. Materials deposited using trisilane are compared with that using disilane (Si2H6). It is found that when using Si3H8 as the gas precursor the deposition rate increases by a factor of similar to 1.5 for the same hydrogen dilution (R = [H-2]/[Si3H8] or [H-2]/[Si2H6])- Moreover, the structural transition from amorphous to nanocrystalline occurs at a higher hydrogen dilution level for Si3H8 and the transition is more gradual as compared with Si2H6 deposited films. Single-junction n-i-p a-Si: H solar cells were prepared with intrinsic layers deposited using Si3H8 or Si2H6. The dependence of open circuit voltage (V-oc) on hydrogen dilution was investigated. V-oc greater than 1 V can be obtained when the i-layers are deposited at a hydrogen dilution of 180 and 100 using Si3H8 and Si2H6, respectively.

Characterization of silicon carbide thin films and their use in colour sensor

A series of hydrogenated amorphous silicon carbide (a-Si1-xCx:H) films were prepared by plasma-enhanced chemical vapour deposition (PECVD) using a gas mixture of silane, methane, and hydrogen as the reactive source. The previous results show that a high excitation frequency, together with a high hydrogen dilution ratio of the reactive gases, allow an easier incorporation of the carbon atoms into the silicon-rich a-Si1-xCx:H film, widen the valence controllability. The data show that films with optical gaps ranging from about 1.9 to 3.6 eV could be produced. In this work the influence of the hydrogen dilution ratio of the reactive gases on the a-Si1-xCx:H film properties was investigated. The microstuctural and photoelectronic properties of the silicon carbide films were characterized by Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), and FT-IR spectrometry. The results show that a higher hydrogen dilution ratio enhances the incorporation of silicon atoms in the amorphous carbon matrix for carbon-rich a-Si1-xCx:H films. One pin structure was prepared by using the a-Si1-xCx:H film as the intrinsic layer. The light spectral response shows that this structure fits the requirement for the top junction of colour sensor. (c) 2004 Elsevier B.V. All rights reserved.

Correlation between Er3+ emission and Si clusters in Erbium-doped a-SiOx : H films

Erbium-doped hydrogenated amorphous silicon suboxide films containing silicon clusters (a-SiOx:H) were prepared. The samples exhibited photoluminescence (PL) peaks at around 750nm and 1.54 mu m, which could be assigned to the electron-hole recombination in silicon clusters and the intra-4f transition in Er3+, respectively. We compared annealing behaviors of Si clusters and Er3+ emission and found that Si clusters emission depends strongly upon crystallinity of Si clusters, whereas Er3+ emission is not sensitive to whether it is Si nanocrystals (nc-Si) or amorphous Si (a-Si) clusters. The erbium-doped a-SiOx:H films containing either a-Si clusters or nc-Si have the same kind of Er3+ -emitting centers. Based on these results, it is concluded that a-Si clusters can play the same role on Er3+ excitation as nc-Si. (c) 2004 Elsevier B.V. All rights reserved.

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51 % and a stabilized efficiency of 8.01% (AM 1.5, 100 mw/cm(2)) at room temperature. (c) 2006 Published by Elsevier B.V.

Nanostructure in the p-layer and its impacts on amorphous silicon solar cells

The open circuit voltage (V-oc) of n-i-p type hydrogenated amorphous silicon (a-Si:H) solar cells has been examined by means of experimental and numerical modeling. The i- and p-layer limitations on V-oc are separated and the emphasis is to identify the impact of different kinds of p-layers. Hydrogenated protocrystalline, nanocrystalline and microcrystalline silicon p-layers were prepared and characterized using Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), optical transmittance and activation energy of dark-conductivity. The n-i-p a-Si:H solar cells incorporated with these p-layers were comparatively investigated, which demonstrated a wide variation of V-oc from 1.042 V to 0.369 V, under identical i- and n-layer conditions. It is found that the nanocrystalline silicon (nc-Si:H) p-layer with a certain nanocrystalline volume fraction leads to a higher V-oc. The optimum p-layer material for n-i-p type a-Si:H solar cells is not found at the onset of the transition between the amorphous to mixed phases, nor is it associated with a microcrystalline material with a large grain size and a high volume fraction of crystalline phase. (c) 2006 Elsevier B.V. All rights reserved.

Heterojunction solar cells with n-type nanocrystalline silicon emitters on p-type c-Si wafers

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/1T0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 mu m)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm(2). (c) 2006 Elsevier B.V. All rights reserved.

Study on variable capacitance diode of (p)nc-Si : H/(n)c-Si heterojunction

Hydrogenated nanocrystalline silicon (nc-Si:H) layers of boron-doped increasing step by step was deposited on n-type crystalline silicon substrate using Plasma Enhanced Chemical Vapor Deposition (PECVD) system. After evaporating Ohm contact electrode on the side of substrate and on the side of nc-Si:H film, a structure of electrode/ (p)nc-Si:H/(n)c-Si/electrode was obtained. It is confirmed by electrical measurement such as I-V curve, C-V curve and DLTS that this is a variable capacitance diode. (C) 2003 Elsevier Science Ltd. All rights reserved.

Microstructure characterization of transition films from amorphous to nanocrocrystalline silicon

Hydrogenated silicon (Si:H) films near the threshold of crystallinity were prepared by very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a wide range of hydrogen dilution R-H = [H-2]/[SiH4] values of 2-100. The effects of H dilution R-H on the structural properties of the films were investigated using micro-Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy. The obtained Raman spectra show that the H dilution leads to improvements in the short-range order and the medium-range order of the amorphous network and then to the morphological transition from amorphous to crystalline states. The onset of this transition locates between R-H = 30 and 40 in our case, and with further increasing R-H from 40 to 100, the nanocrystalline volume fraction increases from similar to23% to 43%, and correspondingly the crystallite size enlarges from similar to2.8 to 4.4 nm. The FTIR spectra exhibit that with R-H increasing, the relative intensities of both the SiH stretching mode component at 2100 cm(-1) and wagging mode component at 620 cm(-1) increase in the same manner. We assert that these variations in IR spectra should be associated with the formation of paracrystalline structures in the low H dilution films and nanocrystalline structures in the high H dilution films. (C) 2003 Elsevier Science B.V. All rights reserved.

Preparation and characterization of the stable nc-Si/a-Si : H films

High-quality nc-Si/a-Si:H diphasic films with improved stability were prepared by using the plasma-enhanced chemical vapor deposition technology. In comparison with typical amorphous silicon, the diphasic silicon films possess higher photoconductivity (two orders larger than that of the amorphous silicon film) and fairly good photosensitivity(the ratio of the photo-to dark-conductivity is about 10) and higher stability (the degradation of the photoconductivity is less than 10% after 24h long light soaking with 50 mW/cm(2) intensity at room temperature). In addition, the diphasic silicon film has a better light spectra response in the longer wavelength range. The improvement in photoelectronic properties may be attributed to: the existence of the disorder within the amorphous matrix, which breaks the momentum selection rule in the optical transition and, consequently, results in the large light absorption coefficient and high photosensitivity; the improved medium range order and low gap states density. Excess carriers generated in the amorphous matrix tend to recombine in the embedded crystallites, which suppresses nonradiative recombination within the amorphous matrix and reduces the subsequent defect creation.

Correlation between Er3+ emission and the microstructure of a-SiOx : H < Er > films

Photoluminescence (PL) from Er-implanted hydrogenated amorphous silicon suboxide (a-SiOX:H(x<2.0)) films was measured. Two luminescence bands with maxima at lambda congruent to 750 nm and lambda congruent to 1.54mum, ascribed to the a-SiOX:H intrinsic emission and Er3+ emission, were observed. Peak intensities of the two bands follow the same trend as a function of annealing temperature from 300 to 1000degreesC. Micro-Raman results indicate that the a-SiOX:H films are a mixture of two phases, an amorphous SiOX matrix and amorphous silicon (a-Si) domains embedded there in. FTIR spectra confirm that hydrogen effusion from a-SiOX:H films occurs during annealing. Hydrogen effusion leads to a reconstruction of the microstructure of a-Si domains, thus having a strong influence on Er3+ emission. Our study emphasizes the role of a-Si domains on Er3+ emission in a-SiOX:H films.