Hydrogenated microcrystalline silicon films have been obtained by hot-wire chemical vapor deposition (HWCVD) in a silane and hydrogen mixture at low pressure (<5 × 10-2 mbar). The structure of the samples and the residual stress were characterised by X- ray diffraction (XRD). Raman spectroscopy was used to estimate the volume fraction of the crystalline phase, which is in the range of 86 % to 98%. The stress values range between 150 and -140 MPa. The mechanical properties were studied by nanoindentation. Unlike monocrystalline wafers, there is no evidence of abrupt changes in the force-penetration plot, which have been attributed to a pressure-induced phase transition. The hardness was 12.5 GPa for the best samples, which is close to that obtained for silicon wafers.

The electronic transport properties of a series of samples prepared by hot-wire chemical vapor deposition with a transition from a-Si:H to μc-Si:H were measured applying the photoconductive frequency mixing technique. We found both improved stability against light-soaking and different values for the photomixing electron lifetime and mobility close to the onset of microcrystallinity as compared to the amorphous state. In particular, the mobility-lifetime product of charge carriers in some of the μc-Si:H samples turns out to lie about two orders of magnitude higher than that of a-Si:H films. The mobility on the other hand, is shown rather to decrease in the transition to the μc-region. Additional measurements of the range and the depth of long range potential fluctuations yield a possible explanation for our results in that grain boundaries may serve as scattering centers and barriers against recombination.

Entirely μc-Si:H p-i-n structures presenting an enhanced sensitivity to the near infrared region and a positive spectral response under forward bias higher than the open circuit voltage are analysed under different external voltage bias and illumination conditions.

A two phase model to explain the transport properties is proposed using as input parameters the measured experimental data. The results suggest that the transport is preferentially concentrated inside the crystalline grains. The conduction within the amorphous regions is poor. The percolation path is different for electrons and holes and is determined by the local fields at the boundaries. These local fields are independent of the externally applied condition, and they can be related to the persistence of the small photocurrent observed when a bias voltage higher than the open circuit voltage is applied.

Silicon films were grown on (100) n-Si with an electron-cyclotron resonance chemical vapor deposition (ECRCVD) system by decomposition of SiH4 at 325°C. Structure and composition of thin films were investigated by SEM, Raman spectroscopy, elastic recoil detection analysis (ERDA) and TEM. Excellent epitaxial growth was achieved for some hundred nm thickness. For more than 1 μm thick films, however, SEM revealed the occurrence of conical structures orientated upside-down with their basal plane in the film surface. Depth-profiling of the elemental composition of thin films by means of ERDA showed the hydrogen content CH to exhibit a pronounced increase with increasing film thickness. Raman spectroscopy evidenced the coexistence of c-Si and a-Si:H by the occurrence of two bands at 520 and 480 cm-1, the ratio of which was found to depend sensitively upon the position of the laser spot on the sample. All experimental results could be consistently explained by assuming the conical precipitates to consist of a-Si:H which was finally proven by coherent electron beam diffraction (CEBD).

We report on the preparation and properties of microcrystalline (Ge,C) alloys grown using a remote, reactive H plasma beam deposition technique. The plasma beam was generated using an ECR reactor. The films were grown at low temperatures (300 - 400°C) on glass, stainless steel and c-Si substrates. The optical properties of the films were measured using spectrophotometer and two-beam photoconductivity techniques. The C content was measured using XPS techniques. We find up to 3% C incorporation in the lattice. The degree of crystallinity determined using Raman spectroscopy was very good. X-ray diffraction measurements indicated grain size in the range of a few tens of nm. The best crystallinity was obtained on conducting substrates, indicating the importance of H ion bombardment in promoting crystallinity. We find that the absorption curves for increasing C content remain similar to the curves for c-Ge, but are shifted to higher energies. Thus, the absorption curve is sharper than for c-Si in the same energy range. The defect densities remain low for the range of C content measured. Because of its sharper absorption curve compared with c-Si, the material may be attractive for photovoltaic energy conversion.

Using a Gas Jet thin film deposition technique, microcrystalline silicon (μc-Si) materials were prepared at rates as high as 15-20 Å/s. The technique involves the use of a gas jet flow that is subjected to a high intensity microwave source. The quality of the material has been optimized through the variation of a number of deposition conditions including the substrate temperature, the gas flows, and the applied microwave power. The best films were made using deposition rates near 16 Å/s. These materials have been used as i-layers for red light absorbing, nip single-junction solar cells. Using a 610nm cutoff filter which only allows red light to strike the device, pre-light soaked currents as high as 10 mA/cm2 and 2.2-2.3% red-light pre-light soaked peak power outputs have been obtained for cells with i-layer thicknesses near 1 micron. This compares with currents of 10-11 mA/cm2 and 4% initial red-light peak power outputs obtained for high efficiency amorphous silicon germanium alloy (a-SiGe:H) devices. The AM1.5 white light efficiencies for these microcrystalline cells are 5.9-6.0%. While the efficiencies for the a-SiGe:H cells degrade by 15-20% after long term light exposure, the efficiencies for the microcrystalline cells before and after prolonged light exposure are similar, within measurement error. Considering these results, the Gas Jet deposition method is a promising technique for the deposition of μc-Si solar cells due to the ability to achieve reasonable stable efficiencies for cells at i-layer deposition rates (16 Å/s) which make large-scale production economically feasible.

Two types of poly-Si:H thin films made by Hot Wire CVD have been evaluated with respect to utilisation in solar cells. Poly-Si:H films made at high hydrogen dilution are highly porous and have large interconnected voids. The void density is 25000/μm-3 as determined by XTEM. On the other hand, poly-Si:H layers made at low hydrogen dilution have a compact structure and a much smaller density of voids. In these films, two types of voids exist: globular voids smaller than 15 nm, and elongated voids, often located between columns of large crystals of 150-250 nm wide at the top. The density for the 5 - 15 nm spherical voids is usually -50/μm3, but larger concentrations often occur locally, up to 1000/pm3, i.e., 0.05% volume fraction. High oxygen content in the poly-Si films made at high hydrogen dilution is largely due to post deposition intrusion of water vapour through the interconnected voids. Profiled layers are made by depositing device quality poly-Si:H layers (low hydrogen dilution) on top of a seed layer (high hydrogen dilution) of high nucleation density. Cells incorporating profiled poly-Si:H films as i-layers at a deposition rate of 0.5 nm/s were made on stainless steel substrates in the configuration SS/n-μc-Si:H(PECVD)/i-poly-Si:H(HWCVD)/p-μc-Si:H(PECVD)/ITO. For our n-i-p solar cell with poly-Si i-layer we obtained an efficiency of 4.41% and a FF of 0.607. Due to native surface texture a current density of 19.95 mA/cm2 is generated in only ~1.22 μm thick i-layer without back reflector.

The formation of tunnel junctions for applications in amorphous silicon (a-Si:H) based multijunction n-i-p solar cells has been studied using real time optics. The junction structure investigated in detail here consists of a thin (~200 Å) layer of n-type microcrystalline silicon (μc-Si:H) on top of an equally thin layer of p-type μc-Si:H, the latter deposited on thick (~2000 Å) intrinsic a-Si:H. Such a structure has been optimized in an attempt to obtain single-phase μc-Si:H with a high crystallite packing density and large grain size for both layers of the tunnel junction. We have explored the conditions under which grain growth is continuous across the p/n junction and conditions under which renucleation of n-layer grains can be ensured at the junction. One important finding of this study is that the optimum conditions for single-phase, high-density μc-Si:H n-layers are different depending on whether the substrate is a μc-Si:H p-layer or is a H2-plasma treated or untreated a-Si:H i-layer. Thus, the top-most μc-Si:H layer of the tunnel junction must be optimized in the multijunction device configuration, rather than in single cell configurations on a-Si:H i-layers. Our observations are explained using an evolutionary phase diagram for a-Si:H and μc-Si:H film growth versus thickness and H2-dilution ratio, in which the boundary between the two phases is strongly substrate-dependent.

The development of a hybrid heterojunction fabricated by growing ultrathin amorphous silicon by Plasma Enhanced Chemical Vapor Deposition using temperatures below 250°C offers the potential of obtaining high efficiency solar cells deposited on glassy substrates. The surface preparation represents one of the most critical steps. The first aim of etching is to remove the native oxide layer from the surface of the crystalline wafer, before amorphous layer deposition. The possibility of obtaining this goal with a dry procedure that reduces the exposure of the sample to the environment is not trivial.

We performed several dry etching processes but the best results were obtained using an etching process involving CF4/O2 gases. We have found evidence that plasma etching acts by removing the native oxide and the damaged surface of textured silicon and by leaving an active layer on silicon surface suitable for the emitter deposition. SEM analysis has confirmed that it is possible to find plasma process conditions where no appreciable damage and changes in surface morphology are induced. Detailed investigation was performed to find compatibility and optimization of amorphous layer deposition both on flat and textured cast silicon by changing the plasma process parameters. By using this process we achieved on cast silicon for solar applications photovoltaic conversion efficiencies of 12.9% on 51 cm2 and 9.2% on 45 cm2 active areas for amorphous crystalline heterostructure devices realized on monocrystalline and polycrystalline silicon respectively. We also investigated the compatibility of the process with industrial production of large area devices.

The optical absorption of microcrystalline silicon germanium alloys (μc-Si1-xGex:H) increases in the near infrared region with increasing germanium content. Therefore, these alloys are promising candidates for the application as intrinsic absorber material in thin film solar cells with tandem and triple structure or IR-detectors. The material properties for a germanium content (x) up to 0.6 and the performance of solar cells based on this material were investigated. Graded bandgap structures are used to optimize the cell performance. For cells with x > 0.3 a continuously graded bandgap in the rear 20 nm of the i-layer (at the i/n interface) results in an enhancement of the open circuit voltage by 40-80 mV compared to an abrupt bandgap discontinuity. When the design of the p/i interface region is changed in a similar way no effect on Voc is observed.

The performance of an entirely microcrystalline p-i-n solar cell was monitored during a long-term outdoor test in Lisbon starting in September 1998. A small decrease of the short circuit current was observed after 5 months of operation. The open-circuit voltage remained stable around 400 mV. From the analysis of the I-V characteristic in dark and under illumination we could identify the weak points of the test structure, like large series resistance, high recombination rate, and intensity-dependent collection efficiency.

Hydrogenated boron-doped microcrystalline silicon-carbide (p-μc-SiC:H) films were grown by a photo chemical vapor deposition (photo-CVD) method from silane (SiH4), hydrogen (H2), diborane (B2H6), and ethylene (C2H4) gases. Since the photo-CVD is a mild process (~10mW/cm2), we can avoid the ion damage of the film, which is inevitable during the deposition of μc-SiC:H employing conventional PECVD technique. A dark conductivity as high as 5 × 10-1 S/cm, together with an optical bandgap of 2 eV, was obtained by the C2H4 addition, which is the first approach in photo-CVD systems. From the Raman and FTIR spectra, it is clear that our p-μc-SiC:H films are made up of crystalline silicon grains embedded in amorphous silicon-carbide tissue. We investigate the role of the hydrogen dilution and ethylene addition on the electrical, optical, and structural properties of p-μc-SiC:H films.

Microcrystalline silicon layers have been deposited by PECVD using a resonance plasma source (Helix) operating at frequencies of 46, 68, 113 and 163 MHz. The plasma discharges in hydrogen and various hydrogen/silane mixtures were investigated by optical emission spectroscopy (OES) and mass spectroscopy (MS). Growth rate, crystalline fraction and hydrogen content of the layers were studied for different gas compositions, excitation frequencies and plasma powers.

Plasma monitoring revealed preferably the formation of positive ions. The density of positive hydrogen ions increased steadily 15 times by raising the frequency from 46 MHz to 163 MHz, whereas the ion energy was reduced from 75 eV to 23 eV and the radiation from hydrogen decreased to 50 %. Growth rates up to 1.5 μm/h have been achieved for microcrystalline layers at 230 °C deposition temperature. The content of hydrogen was below 15 at.%. Raman spectroscopy measurements reveal that 20 to 70 % of the silicon was crystalline depending on the silane concentration.

Three-dimensional photonic lattices have been fabricated in the infrared using a combination of advanced silicon processes. The structures display bandgaps centered at 12μ. and 1.55μ.

In this work polysilicon TFI's were fabricated by excimer laser crystallization of active channel silicon which was deposited by three different methods: I) LPCVD at 550 °C; 2) PECVD at 225 °C; and 3) PVD at room temperature. CMOS devices were produced with the same low temperature (less than 600 °C) top gate process and the laser anneal condition was optimized for the material type and thickness. For PECVD material a pre-anneal step of 450 °C for 1 hour was required before crystallization to avoid bubbling and ablation due to hydrogen evolution, but no such anneal was required for either LPCVD or PVD material due to their low hydrogen and Ar content. For 50 nm films, laser energy densities were typically in the range of 300-400 miJ/cm2. Excellent device results were obtained for both LPCVD and PECVD material with n-channel field effect mobilities greater than 100 cm2/Vs and on/off ratios greater than 108 at 5 V drain bias. Good results were also obtained for PVD films that can be further improved by optimizing deposition and anneal conditions. In moving toward very low temperature polysilicon processing (less than 230 °C) both PECVD and PVD channel films appear to be viable candidates.

Selective exposure of an a-Si:H film to a room temperature hydrogen plasma using a patterned SiNx, capping layer and a subsequent anneal at 600°C, resulted in polycrystalline and amorphous silicon regions in a single silicon layer on the same glass substrate. Top-gate non-self-aligned TFTs were fabricated in both the amorphous and polycrystalline regions with all shared processing steps and no laser processing using a re-hydrogenation step. The TFTs had good characteristics, with field-effect mobilities upto 1.2 cm2/Vs and 15 cm2/Vs for the a-Si:H and the poly-Si TFTs, respectively, and ON/OFF ratios >105 in either case.

Hydrogen passivation of grain boundary and in-grain defects is a key process step in the fabrication of high quality poly-Si TFTs. The sensitivity of the hydrogenation process to device geometry is therefore an important consideration. The effects of rf-plasma hydrogenation on the operating performance of a range of self-aligned and offset drain (Loff = 5 to 40 μm) poly-Si TFT configurations is reported. The hydrogenation of offset drain structures results in a predictable increase in the pre-threshold slope and a reduction in the device threshold voltage. However, extended hydrogenation (up to 12 h) can result in a significant reduction in the device drive current (by up to 2 orders). A similar effect is observed in metal field plate HVTFTs in which some portion of the offset region is un-modulated by the additional electrode. The on state conduction in the offset region is examined as a function of hydrogenation time, temperature and planar electric field. The increase in the on resistance is attributed to a reduction in the poly-Si defect density, which moderates carrier transport through the offset region.

Most of the models for Thin Film Transistors take no account of the way that electrons move across grain boundaries. The purpose of this work is to produce an analytical model, physically based, that takes account of both the change in height of the potential barriers that separate the grains, and the change of carrier density in the grains with gate voltage. The high current and subthreshold regions are treated. The model enables the change of field effect mobility with grain size and temperature to be determined. Two closed form expressions are provided which should be of value to both device and circuit designers, as well as providing insight to the physical processes occurring in such devices. They form a close analogy with the electrical characteristics of MOSFETs on crystalline silicon, diffusion and drift being replaced by quasi-diffusion and primary quasi-drift.

We have successfully used self-aligned Amorphous Si Thin-Film Transistors, fabricated by a laser doping/annealing process, to construct dynamic shift register circuits, which can be used as gate-line drivers or in other peripheral circuits for flat-panel displays and imagers. Taking advantage of easily scaling down the TFT channel length in a self-aligned process, much higher circuit speeds can be achieved compared to that of circuits using conventional TFTs. We have successfully demonstrated a four-phase dynamic shift register, operating at a clock speed higher that 250 kHz (1 μs for each clock phase) built on 3 μm channel length TFTs. This new technology opens up possibilities for integrating peripheral circuits in flat-panel displays and imagers based on a-Si TFTs.

Much of the mechanical strain in semiconductor devices can be relieved when they are made on compliant substrates. We demonstrate this strain relief with amorphous silicon thin-film transistors (a-Si:H TFTs) made on 25-μm thick polyimide foil, which can be bent to radii of curvature R down to 0.5 mm without substantial change in electrical characteristics. At R= 0.5 mm the channel area of the TFTs is strained by ~ 1%. The reduction in bending radius, from R ≠ 2 mm on steel foil of the same thickness, agrees with the theoretical prediction that changing from a stiff to a compliant substrate can reduce the bending strain in the device plane by a factor of up to 5.