Nanodiamond formation by hot-filament chemical vapor deposition on carbon ions bombarded Si

Nanocrystalline diamond films were grown by a two-step process on Si(1 0 0) substrate, which was first pretreated by pure carbon ions bombardment. The bombarded Si substrate was then transformed into a hot-filament chemical vapor deposition (HFCVD) system for further growth. Using the usual CH4/H-3 feed gas ratio for micro crystalline diamond growth, nanodiamond crystallites were obtained. The diamond nucleation density is comparable to that obtained by biasing the substrate. The uniformly distributed lattice damage is proposed to be responsible for the formation of the nanodiamond. (C) 2002 Elsevier Science B.V. All rights reserved.

Impact of wide bandgap p-type nc-Si on the performance of a-Si solar cells

This paper reports the impact of a wide bandgap p-type hydrogenated nanocrystalline silicon (nc-Si:H) on the performances of hydrogenated amorphous silicon (a-Si:H) based solar cells. The player consists of nanometer-sized Si crystallites and has a wide effective bandgap determined mainly by the quantum size-confinement effect (QSE). By incorporation of this p-layer into the devices we have obtained high performances of a-Si:H top solar cells with V-infinity=1.045 V and FF=70.3 %, and much improved mid and bottom a-SiGe:H cells, deposited on stainless steel (SS) substrate. The effects of the band-edge mismatch at the p/i-interface on the I-V characteristics of the solar cells arc discussed on the bases of the density-functional approach and the AMPS model.

Transient photoconductivity and its light-induced change of lightly boron-doped a-Si : H films

Transient photoconductivity and its light-induced change were investigated by using a Model 4400 boxcar averager and signal processor for lightly boron-doped a-Si : H films. The transient photoconductivities of the sample were measured at an annealed state and light-soaked states. The transient decay process of the photoconductivity can be fitted fairly well by a second-order exponential decay function, which indicates that the decay process is related with two different traps. It is noteworthy that the photoconductivity of the film increases after light-soaking. This may be due to the deactivity of the boron acceptor B-4(-), and thus some of the boron atoms can no longer act as acceptors and drives E-F to shifts upward. Consequently, the number of effective recombination centers may be reduced and so the photoconductivity increases.

The microstructure and its high-temperature annealing behaviours of a-Si : O : H film

The microstructure and its annealing behaviours of a-Si:O:H film prepared by PECVD are investigated in detail using micro-Raman spectroscopy, X-ray photoelectron spectroscopy and Infrared absorption spectroscopy. The results indicate that the as-deposited a-Si:O:H film is structural inhomogeneous, with Si-riched phases surrounded by O-riched phases. The Si-riched phases are found to be nonhydrogenated amorphous silicon (a-Si) clusters, and the O-riched phases SiOx:H (x approximate to 1. 35) are formed by random bonding of Si, O and H atoms. By high-temperature annealing at 1150 degreesC, the SiOx:H (x approximate to 1.35) matrix is shown to be transformed into SiO2 and SiOx ( x approximate to 0.64), during which all of the hydrogen atoms in the film escape and some of silicon atoms are separated from the SiOx:H ( x approximate to 1.35) matrix; The separated silicon atoms are found to be participated in the nucleation and growth processes of solid-phase crystallization of the a-Si clusters, nano-crystalline silicon (ne-Si) is then formed. The microstructure of the annealed film is thereby described with a multi-shell model, in which the ne-Si clusters are embedded in SiOx (x = 0.64) and SiO2. The former is located at the boundaries of the nc-Si clusters, with a thickness comparable with the scale of nc-Si clusters, and forms the transition oxide layer between the ne-Si and the SiO2 matrix.

Gap states of hydrogenated amorphous silicon near and above the threshold of microcrystallinity with subtle boron compensation

The effects of hydrogen dilution, subtle boron compensation, and light-soaking on the gap states of hydrogenated amorphous silicon films (a-Si:H) near and above the threshold of microcrystallinity have been investigated in detail by the constant photocurrent method and the improved phase-shift analysis of modulated photocurrent technique. It is shown that high hydrogen dilution near the threshold of microcrystallinity leads to a more ordered network structure and to the redistribution of gap states; it gives rise to a small peak at about 0.55 eV and a shoulder at about 1.2 eV below the conduction band edge, which are associated with the formation of microcrystallites embedded in the amorphous silicon host matrix. A concurrent subtle boron compensation is demonstrated to prevent excessive formation of microcrystallinity, and to help promote the growth of the ordered regions and reduce the density of gap defect states, particularly those associated with microcrystallites. Hydrogen-diluted and appropriately boron-compensated a-Si:H films deposited near the threshold of microcrystallinity show the lowest density of the defects in both the annealed and light-soaked states, and hence, the highest performance and stability. (C) 2001 American Institute of Physics.

Hydrogenated amorphous silicon films with significantly improved stability

A new regime of plasma-enhanced chemical-vapor deposition (PECVD), referred to as "uninterrupted growth/annealing" method, has been proposed for preparation of high-quality hydrogenated amorphous silicon (a-Si:H) films. By using this regime, the deposition process no longer needs to be interrupted, as done in the chemical annealing or layer by layer deposition, while the growing surface is continuously subjected to an enhanced annealing treatment with atomic hydrogen created in the hydrogen-diluted reactant gas mixture at a relatively high plasma power. The intensity of the hydrogen plasma treatment is controlled at such a level that the deposition conditions of the resultant films approach the threshold for microcrystal formation. In addition, a low level of B-compensation is used to adjust the position of the Fermi level close to the midgap. Under these conditions, we find that the stability and optoelectronic properties of a-Si:H films have been significantly improved. (C) 2001 Elsevier Science B.V. All rights reserved.

Photoluminescence and microstructure of the Erbium-Doped hydrogenated amorphous SiOx(0 < x < 2)

The hydrogenated amorphous SiOx films (a-SiOx:H) with various oxygen contents have been prepared using plasma enhanced chemical vapor deposition technique. The films were implanted with erbium and annealed by rapid thermal annealing. An intense photoluminescence (PL) of Er at 1.54 mum has been observed at 77 K and at room temperature. The PL intensity depends strongly on both the oxygen content of the film and the rapid thermal annealing temperature and reaches its maximum if the ratio of O/Si in the film is approximately equal to 1.0 at 77 K and to 1.76 at room temperature. The microstructure of the film also has strong influences on the PL intensity. The PL intensity at 250 K is slightly more than a half of that at 15 K. It means that the temperature quenching effect of the PL intensity is very weak.

Light and annealing induced changes in Si-H bonds in undoped a-Si : H

Light and annealing induced changes in Si-H bonds in undoped a-Si:H have been investigated by a differential infrared spectroscopy method. The light-induced changes in Si-H bonds are not monotonic, quite different from the usual Staebler-Wronski effect in electronic properties, and involve more complicated physics. The magnitude of the light-induced changes in Si-H bonds is proportional to the hydrogen content in the film. There may exist more than one microscopic process which determine the light-induced changes in Si-H bonds. Almost the whole a-Si:H network is affected when a-Si:H is subjected to Light-soaking or to annealing. The light-induced changes in Si-H bonds may be an independent light-induced phenomenon or an auxiliary process of the metastable SWE defect creation. (C) 2000 Elsevier Science Ltd. All rights reserved.

Study of microstructure of high stability hydrogenated amorphous silicon films by Raman scattering and infrared absorption spectroscopy

The microstructure, hydrogen bonding configurations and hydrogen content of high quality and stable hydrogenated amorphous silicon (a-Si:H) films prepared by a simple ''uninterrupted growth/annealing" plasma enhanced chemical vapor deposition technique have been investigated by Raman scattering and infrared absorption spectroscopy. The high stability a-Si:H films contain small amounts of a microcrystalline phase and not less hydrogen (10-16 at. %), particularly, the clustered phase hydrogen, Besides, the hydrogen distribution is very inhomogeneous. Some of these results are substantially distinct from those of conventional device-quality n-Si:H film or stable cr-Si:H films prepared by the other techniques examined to date. The stability of n-Si:H films appears to have no direct correlation with the hydrogen content or the clustered phase hydrogen concentration. The ideal n-Si:H network with high stability and low defect density is perhaps not homogeneous. (C) 1998 American Institute of Physics.

Dielectric response and its light-induced change in undoped alpha-Si : H films below 13 MHz

The low frequency (<13 MHz) dielectric response and its light-induced change in undoped a-Si:H were investigated in detail. The dielectric constant epsilon (the real part) in this range decreases with illumination time: following a stretched exponential law similar to that found for other light-induced changes. The saturation relative change was about 0.1-0.2 % for the measured samples. The change is fading away either after repeated illumination-annealing training or by aging at room temperature. The present results indicate some rearrangement of the whole Si network caused by light soaking.

The effect of interposing nanocrystalline Si(B) P plus layer on the photovoltaic properties of a-Si: H tandem solar cells

Tandem amorphous silicon solar cells have attracted extensive interest because of better performance than single junction counterpart. As n/p junctions play an important role in the current transportation of tandem solar cells, it is important to design and fabricate good n/p junctions.The properties of the n/p junction of amorphous silicon (a-Si) were studied. We investigate the effect of interposing a nanocrystalline p(+) layer between n (top cell) and p (bottom cell) layers of a tandem solar cell. The crystalline volume fraction, the band gap, the conductivity and the grain size of the nanocrystalline silicon (nc-Si) p(+) layer could be modulated by changing the deposition parameters.Current transport in a-Si based n/p ("tunnel") junctions was investigated by current-voltage measurements. The voltage dependence on the resistance (V/J) of the tandem cells was examined to see if n/p junction was ohmic contact. To study the affection of different doping concentration to the properties of the nc-Si p(+) layers which varied the properties of the tunnel junctions, three nc-Si p(+) film samples were grown, measured and analyzed.

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.

Hydrogenated p-type nanocrystalline silicon in amorphous silicon solar cells

A wide bandgap and highly conductive p-type hydrogenated nanocrystalline silicon (nc-Si:H) window layer was prepared with a conventional RF-PECVD system under large H dilution condition, moderate power density, high pressure and low substrate temperature. The optoelectrical and structural properties of this novel material have been investigated by Raman and UV-VIS transmission spectroscopy measurements indicating that these films are composed of nanocrystallites embedded in amorphous SiHx matrix and with a widened bandgap. The observed downshift of the optical phonon Raman spectra (514.4 cm(-1)) from crystalline Si peak (521 cm(-1)) and the widening of the bandgap indicate a quantum confinement effect from the Si nanocrystallites. By using this kind of p-layer, a-Si:H solar cells on bare stainless steel foil in nip sequence have been successfully prepared with a V c of 0.90 V, a fill factor of 0.70 and an efficiency of 9.0%, respectively. (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.

Improved diphasic nc-si/a-si : H I-layer materials using PECVD

Two series of films has been prepared by using a new regime of plasma enhanced chemical vapor deposition (PECVD) in the region adjacent to the phase transition from amorphous to crystalline state. The photoelectronic properties of the films have been investigated as a function of crystalline fraction. In comparison with typical a-Si:H, these diphasic films with a crystalline fraction less than 0.3 show a similar optical absorption coefficient, higher mobility life-time product ( LT) and higher stability upon light soaking. By using the diphasic nc-Si/a-Si films as the intrinsic layer, a p-i-n junction solar cell has been prepared with an initial efficiency of 9. 10 % and a stabilized efficiency of 8.56 % (AM 1.5, 100 mW/cm(2)).