The preparation of μc-Si films from SiH4-H2 mixtures by electron-cyclotron resonance (ECR) CVD at deposition temperatures ≤ 400°C on foreign substrates is reported. Deposition conditions were identified for which Si films with a high degree of crystallinity were grown as was confirmed by Raman spectroscopy. A factorial analysis was carried out, for which the influence of deposition temperature, microwave power, hydrogen dilution and total pressure on film growth were investigated. Samples of optimized crystallinity were prepared in a lowpressure and high-hydrogen dilution regime. In-plane grain sizes were measured by TEM and found to be on the order of 10 - 12 nm. Next to the optimization of crystallinity several sources of impurity contamination during film deposition were identified and eliminated. Intrinsic μc-Si layers could be prepared under these conditions that exhibited a dark conductivity σd of 2 × 10-7 S/cm and photosensitivity σph/σd of 150. It is concluded that ECR CVD is capable of producing intrinsic layers with electronic properties as necessary for use in state-of-the-art n-i-p μc-Si solar cells.

We report on that plasma damage on SnO2 can be suppressed by using surface termination by chlorine. It was found that the darkening of SnO2 is decreased and a wider gap p-a-Si material is obtained by using SiH2Cl2 especially at the higher reaction pressure and at the lower substrate temperature. The suppression of darkening of SnO2 and wide optical gap is correlated to chlorine contents in the film. It is demonstrated that SiH2Cl2 is also beneficial for boron-doped material, indicating a suitable material for a window layer of solar cells.

We have characterized a-Si:H initial layers for μc-Si:H nucleation by real time spectroscopic ellipsometry (SE) and infrared attenuated total reflection spectroscopy (ATR) to investigate the μc-Si:H formation mechanism. By performing Ar plasma treatment of a-Si:H layers, we confirmed a presence of a 2 monolayer thick sub-surface in a-Si:H layers. In the a-Si:H sub-surface that leads to the μc-Si:H nucleation, an important peak at ∼1937 cm−1 assigned to the SiHn complex was found in the ATR spectra. From H2 plasma treatment experiments, we proposed that this SiHn complex is formed by H insertion into strained Si-Si bonds. The SiHn complex formed in the a-Si:H sub-surface showed a clear relationship with the μc-Si:H nucleation. From these results, we conclude that the μc-Si:H nucleation occurs by the formation of the chemically active and flexible SiHn complexes in the 2 monolayer thick a-Si:H sub-surface.

MicroElectroMechanical Systems (MEMS) are used in a wide variety of applications such as accelerometers [1], gyroscopes [2], infrared detectors [3],…etc. For high volume applications, fabrication costs can be possibly reduced by monolithic integration of MEMS with the driving electronics. The easiest approach for monolithic integration is post processing MEMS on top of the driving electronics, as this does not introduce any change into standard fabrication processes used for realizing the driving electronics. On the other hand, post processing imposes an upper limit on the fabrication temperature of MEMS in order to avoid any damage or degradation in the performance of the driving electronics. Polycrystalline silicon (Poly Si) has been widely used for MEMS applications [4], but the main disadvantage of this material is that it requires a high processing temperature (higher than 800°C [5]) to achieve the desired physical properties. In particular, a low tensile stress is needed for MEMS. Polycrystalline silicon germanium (Poly SiGe) seems to be an attractive alternative to poly Si as it has similar properties, while the presence of germanium reduces its melting point. Hence, the desired physical properties are expected to be realized at lower temperature. Depending on the germanium concentration and the deposition pressure, the transition temperature from amorphous to polycrystalline can be reduced to 450°C [6], or even lower, compared to 580°C for LPCVD poly Si. Also, the residual mechanical stress in poly SiGe is lower than that in poly Si [7].

In order to understand the kinetics of formation of interface/surface states and its correlation on the final device performance, a preliminary study was performed on MIS structures, before and after surface oxidation/passivation, using different oxidation techniques and oxides: thermal (in air), chemical (in H2O2) and oxygen plasma.

The devices used in this work are based on a glass/Cr/a-Si:H(n+)/a-Si:H(i)/SiOx/Pd structures, where the amorphous silicon intrinsic layer (i a-Si:H) with a photosensitivity of 107 was deposited by a modified plasma enhanced chemical vapour deposition (PECVD) triode system.

The electrical properties of a-Si:H MIS structures were investigated by measuring their diode current-voltage characteristics in the dark and under illumination as well as the spectral response, as a function of the various oxidation techniques. Infrared spectroscopy and spectroscopic ellipsometry were used as a complementary tool to characterise the oxidised surface.

We grow hydrogenated amorphous-silicon (a-Si:H) by the hot-wire chemical vapor deposition (HWCVD) technique. In our standard tube-reactor we use a single filament, centered 5 cm below the substrate and obtain deposition rates up to 20 Å/s. However, by adding a second filament, and decreasing the filament-to-substrate distance, we are able to grow a-Si:H at deposition rates exceeding 167 Å/s (1 µm/min). We find the deposition rate increases with increasing deposition pressure, silane flow rate, and filament current and decreasing filament-tosubstrate distance. There are significant interactions among these parameters that require optimization to grow films of optimal quality for a desired deposition rate. Using our best conditions, we are able to maintain an AM1.5 photoconductivity-to-dark-conductivity ratio of 105 at deposition rates up to 130 Å/s, beyond which the conductivity ratio decreases. Other electronic properties decrease more rapidly with increasing deposition rate, including the ambipolar diffusion length, Urbach energy, and the as-grown defect density. Measurements of void density by small-angle X-ray scattering (SAXS) reveal an increase by well over an order of magnitude when going from one to two filaments. However, both Raman and X-ray diffraction (XRD) measurements show no change in film structure with increasing deposition rates up to 144 Å/s, and atomic force microscopy (AFM) reveals little change in topology.

The optical quantum efficiency and spectral response of p-i-n thin film amorphous silicon (a-Si:H) solar cells have been modeled using software based on optical admittance analysis. The optical constants of a-Si:H and Indium Tin Oxide (ITO) thin film layers have been measured by Variable Angle Spectroscopic Ellipsometry (VASE) and used as inputs into the optical admittance analysis program in order to model cells constructed from these films.

Amorphous silicon thin films and p-i-n assemblies have been deposited by glow discharge and reactive sputtering techniques. The optical constants of doped and intrinsic a-Si:H thin films were determined by VASE and the film thickness verified by Scanning Electron Microscopy studies. The optical constants of commercially available transparent conducting oxide (TCO) coated substrates have also been determined by VASE.

The experimental transmission spectra of p-i-n assemblies are compared with transmission spectra that have been modeled using the measured optical constants. Results of modeling different a-Si:H solar cell structures using these materials are presented, including a study of the optimal TCO layer thickness for p-i-n a-Si:H solar cells. This work shows that optical admittance modeling gives a good prediction of the optical behavior of p-i-n assemblies, but that accurate measurements of the optical constants of the component films are required in order to model effectively the optical quantum efficiency and photocurrent.

Time evolution of Si dangling bonds (dbs) was monitored during atomic hydrogen treatment of a-Si:H films using an in-situ electron-spin-resonance (ESR) technique. A high diffusion coefficient (>10−10 cm2s−1) of free atomic H in a-Si:H was detected at the very initial stage of H exposure. Atomic H diffuses into the bulk of the film (∼100 nm) and creates additional metastable dbs. The spatial distribution of such metastable dbs becomes deeper at lower treatment temperatures. An activated type of db creation reaction determines the distribution of these dbs.

The microstructure of polysilicon and Si0.69Ge0.31 thin films, grown by chemical-vapordeposition (CVD) on oxidized silicon wafers covered with a very thin polysilicon seed layer, was investigated using high-resolution electron microscopy (HREM). The plan-view HREM images showed that polysilicon films contained less substructure inside grains and had fewer multiple twins and more extended twin bands than Si0.69Ge0.31. On the other hand, only SiGe contained multiple twins with five-fold symmetry. The atomic model of the second-order symmetric twin boundary proposed for Si and based on the insertion of five-member and seven-membered rings was found to describe the atomic structures of second-order symmetric twin boundaries in Si0.69Ge0.31 as well. Within the accuracy of HREM, the repeat unit of the boundary was the same in Si0.69Ge0.31 and Si.

Large grain poly-silicon film (poly-Si) with high material quality and uniformity can have numerous novel applications such as providing a low cost alternative to form silicon-on-insulator (SOI) substrates and a breakthrough technology to ultra-dense 3-dimensional multi-layer SOI like devices and circuits. Nickel Induced Lateral Crystallization (NILC) of amorphous Si (a-Si) has been studied intensively, yet the grains are still small (∼ 1 μm). Recently, we have reported a novel method by combining NILC and a new annealing (at above 900 °C) to form poly-Si film with very large grains ranging from 10 μm to 100 μm. The film has good quality and the TFTs formed are highly comparable to SOI TFTs. This work further reports the effect of Ni to the new large-grain poly-Si film.

One very important problem concerning erbium-doped silicon is the electronic structure of the Er3+ impurities. In particular, it is still not clear if the 4f levels can be treated as frozen core levels or their overlap with s and p states of their neighbors must be considered explicitly. For crystalline Si, the 4f levels have been supposed to be anywhere between 20 eV below the valence band and within the energy gap. In this paper we report on the first ultraviolet photoemission spectroscopy (UPS) measurements on Er-doped a-Si:H. Samples of a-Si:H<Er> with different Er contents (up to 1 at. % Er) were prepared by co-sputtering from a Si target partially covered with metallic Er platelets. In order to enhance the Er states relative to the Si and H states, the excitation energy was tuned between 40 and 140 eV with a synchrotron light source. At 140 eV excitation energy the cross-section of the Er 4f and 5p states is more than an order of magnitude higher than the cross section of the Si 3s or 3p states. As the Er concentration increases, a shoulder and then a peak appears at 10.0±0.5 eV binding energy. The intensity and width of this peak is well correlated with the Er concentration, and with the Er 5p and 5p½ levels at 26 and 32 eV binding energy, respectively. We attribute the peak at 10.0±0.5 eV binding energy to the Er 4f level. These are the only occupied states that can be related to the presence of Er, indicating that these levels are not valence states and consequently can be treated as frozen core levels.

Colour characterisation of a three-terminal three-channel colour detector in the assembly glass/TCO1/PI1NI2P/TCO2/PI3N/Al is performed, using a method of polynomial regression. The metameric errors of the detector calculated and investigated for different orders of polynomial colour correction are presented and discussed. The benefits of the first order colour correction are shown and graphically illustrated. The final transformation matrix for the first order colour correction and conversion to the output R, G and B signals is given. At the end, a demonstrational application with the a-Si:H-based detector and corresponding read-out and colour correction electronics is presented.

The surface and subsurface processes occurring during the growth of a-Si:H are analysed to understand why dangling bond defects and weak Si-Si bonds form. We argue that hydrogen elimination to form the Si-Si network is the rate limiting process at low temperature, and this leads to the creation of weak Si-Si bonds. Dangling bonds form subsequently from weak bonds by a defect pool type process. Plasma processes, such as ion bombardment, which dehydrogenate the surface layers, can reduce the weak bond density.

This paper discusses the use of wide bandgap a-Si:H prepared by reactive magnetron sputtering (RMS) in low temperature and hydrogen control process deposition for multijunction cell applications. Primarily technological experiments are presented. The studies were carried out on a-Si:H films and TCO / n+ a-Si:H / i a-Si:H / Nickel Schottky barrier structures with i layer thicknesses ranging between 0.1 and 0.5 μm, using AFM, X-ray small angle and Raman spectroscopy, and optical transmission and reflection measurements after a degraded steady state. The properties exhibited by the films obtained in various ways are compared. In particular, fill factor dependence on light, field, and thickness is presented for RMS films.

The high-density and low temperature microwave plasma utilizing a spokewise antenna was successfully applied to fast deposition of highly crystallized and photconductive microcrystalline silicon (μ-Si:H) films at low temperatures. The deposition rate and film crytstallinity significantly depend on the axial distribution of the plasma parameters. Best crystallinity was obtained at the axial distance Z from the quartz glass plate, where the spread of the ion beam energy impinging to the growing surface was minimum. By optimizing the axial distance Z and total pressure, highly crystallized μ-Si:H films could be fabricated with a high deposition rate of ∼47Å/s in the SiH4 and Ar mixture plasma with no use of the H2 dilution. An intentional control of the ion beam energy is also attempted using a mesh grid electrode to suppress the ion bombardment to the growing surface.

We report on the growth of microcrystalline Si and (Si,Ge) cells on stainless steel substrates. The devices were grown using a remote, low pressure, ECR growth technique at low temperatures (250-350 C). The precursor gases were silane, germane and hydrogen. The devices were of the p-i-n type, with light incident on the p layer. The p layer was a-(Si,C). A novel interface buffer layer, consisting of an amorphous alloy whose bandgap was graded from 1.3 eV to 1.9 eV was used to match the crystalline base layer with the higher gap p layer. It was found that the properties of this buffer layer were critical in determining the properties of the resulting device. The buffer layer was found to increase the voltage by almost 20%. Cells with high fill factors were made using this technique. The quantum efficiency data indicated that the base layers had absorption characteristic of crystalline materials.

We have characterized by fluctuation electron microscopy the medium-range order of hydrogenated amorphous silicon thin films deposited by a variety of methods. Films were deposited by reactive magnetron sputtering, hot-wire chemical vapor deposition, and plasma enhanced chemical vapor deposition with and without H2 dilution of the SiH4 precursor gas. All of the films show the signature of the paracrystalline structure typical of amorphous Si. There are small variations in the degree of medium-range order with deposition method and H content. The PECVD film grown with high H2 dilution contains Si crystals ∼5 nm in diameter at a density of ∼109 cm−2. The amorphous matrix surrounding these crystals shows no difference in mediumrange order from the standard PECVD film. This supports explanations of the resistance of the H-dilution material to light-induced degradation that depend only on the presence of crystalline grains without modifications of the amorphous matrix.

Practical use of amorphous silicon stacked-junction color detectors in large-area arrays requires periodic readout of the photo-charge stored in the capacitance of the device by a transient technique of sensing. In any stacked-junction devices, color information is obtained by the “self-biasing” process: during an integration time, the three junctions independently lose charge; during the readout pulse, the capacitances of the three junctions in electrical series are re-charged. Equilibrium is reached after a few cycles, when the charge integrated in a cycle by each junction is the same, and equals the readout charge. The amount of charge is determined by the reverse biased junction and accounts for the light intensity.

Dimensioning the amorphous silicon Thin Film Transistor (TFT) used as a pixel switch for the detector is a critical part of the project of a color imager. The actual design determines the self-bias process duration and the readout accuracy. The typical large thickness difference between the detector junctions makes the constraints for the switching process extremely demanding: since a greater capacitance is expected in the thinner top junction detecting blue radiation, the on-resistance must be reduced. Since the front junction does not ensure full rejection of green and red light, a calculation must be performed to extract the information on blue radiation. This requires further precision in the readout process.

In this work we present a simulation study of the self-biasing process. Both a-Si:H TFT and the a-Si:H p-i-n-i-p two-color detectors are simulated by a finite-elements two-dimensional simulator ensuring a correct modeling of both the devices. Simulations allow to study in detail the timing and the accuracy of the self-biasing process. Including electrostatic capacitance and trapped charge, a set of design rules for the TFT is achieved in terms of on-state design. Similar considerations can be extended to the case of ATCD three-color detectors.

Proton-29Si double resonance NMR measurements on high quality plasma-enhanced chemical vapor deposition a-Si:H deposited from SiH4 show that more than one third of the contained hydrogen is present as H2 molecules residing in the amorphous equivalent of T sites. The NMR signal from these trapped H2 appears in the narrow 4 kHz proton line, which arises from the less clustered hydrogen population. Very little of the molecular component is in the broad ~24 kHz line, which arises mostly from clustered hydrogen tightly bonded to silicon.

We report on electrically detected magnetic resonance (EDMR) studies in three different types of thin-film silicon solar cells: (i) c-Si absorbers with epitaxially grown silicon thin-film emitters (ii) c-Si absorbers with hydrogenated amorphous silicon (a-Si:H) emitters, and (iii) microcrystalline silicon (µc-Si:H) pin diodes. Although cells of type (i) and (ii) are of a similar structure their EDMR spectra are completely different. We identify surface recombination via Pb0 centers in cells with c-Si emitters but hopping transport through conduction bandtail states in the 30 nm thin a-Si:H emitter layer. No signals related to interface recombination are detected in either cell. The EDMR signals of cell (iii) are identified as hopping among tail states of the conduction band with a subsequent nonradiative tunneling transition to neutral dangling bonds. At elevated temperature recombination is suggested to be dominated by direct capture. The EDMR signal of the dark current can be described with a simple diode model only involving diffusion and recombination currents.