Large area thin film position sensitive detectors based on amorphous silicon technology have been prepared on polyimide substrates using the conventional plasma enhanced chemical vapour deposition technique. The sensors have been characterised by spectral response, illuminated I-V characteristics and position detectability measurements. The obtained one dimensional position sensors with 5 mm wide and 60 mm long present a maximum spectral response at 600 nm, an open circuit voltage of 0.6 V and a position detectability with a correlation of 0.9989 associated to a standard deviation of 1×10−2, comparable to those ones produced on glass substrates. The surface of the sensors at each stage of fabrication was investigated by Atomic Force Microscopy.

In this study, we optimize the plasma-enhanced chemical vapor deposition (PECVD) process to achieve high-density nucleation of single-phase microcrystalline silicon (µc-Si:H) p-type layers on zinc oxide (ZnO) surfaces at 200 °C for applications in amorphous silicon (a-Si:H) based p-i-n solar cells. The phase evolution of the Si:H p-layers on specular ZnO-coated glass substrates is characterized using real time spectroscopic ellipsometry (RTSE). The resulting evolutionary phase diagram depicts the accumulated film thickness at which the amorphous-to- microcrystalline (→µc) transition occurs versus the H2-dilution ratio, with all other parameters fixed. Guided by this diagram, we find that high-density microcrystallite nucleation and fully- coalesced µc-Si:H p-layers ∼100 Å thick can be obtained on specular ZnO at 200 Å using a B(CH3)3 doping gas flow ratio of D=[B(CH3)3]/[SiH4]=0.02 and an optimized H2-dilution ratio of R=[H2]/SiH4]=200. Lower H2-dilution levels (R<160) generate purely amorphous or mixed (a+µc) phases, and higher dilution levels (R>200) generate longer induction periods, low-density nucleation, and incomplete coalescence of microcrystallites even after ∼100 Å. The time evolution of the microstructure and the resulting dielectric functions as determined by RTSE are similar for optimized µc-Si:H p-layers ∼200 Å thick prepared on specular and textured ZnO surfaces, indicating that the substrate texturing does not necessitate process reoptimization.

Experimental results on the electronic properties of conditioned Cr/hydrogenated amorphous silicon (a-Si:H)/V thin film devices are presented. The devices under test were electro-formed, and had resistances in the range from several hundred ohms to several kiloohms. The current of conditioned devices varied non-linearly with the bias at low voltages, but exhibited ‘jumps' at a threshold voltage (Vth) (typically 2∼3V ), leading to a resistance change of 1∼3 orders of magnitude. Above Vth the current increased almost linearly with bias, and the carrier transport changed from a semiconducting behaviour into a more conducting (metallic) state. The threshold voltage Vth was found to decrease with increasing temperature and disappear at 340 ∼ 350K, but recovered on reducing the temperature. The transition at Vth has been analysed in terms of electrothermal mechanism. The calculated turnover temperature is about 346K, close to that for the disappearance of Vth. We suggest that the observed transition could involve vanadium oxides such as VO2.

Amorphous hydrogenated silicon (a-Si:H) solar cells need efficient light trapping structures to achieve high efficiency. To this end, aluminum doped zinc oxide (ZnO:Al) as a transparent front contact was periodically structured. Solar cells with grating periods between 390 and 980 nm were realized. The structures were characterized by Atomic Force Microscopy (AFM) and optical reflection. A simple formula for the wavelength where total internal reflection starts is deduced for each diffraction order. Solar cells with a periodic grating show a significant reduction in the overall reflectance which is comparable to cells with an optimized statistical texture.

This contribution investigates the electronic properties of a-Si:H/c-Si solar cells and explains their electrical output parameters open circuit voltage, short circuit current, and fill factor. Our device analysis is based on measurements of the internal quantum efficiency, of current/voltage and capacitance/voltage curves. We find carrier recombination within the crystalline silicon absorber material to be responsible for the limitation of the open circuit voltage. The short circuit current is restricted by collection losses in the absorber material and by absorption in the electrically inactive a-Si:H emitter. Resistive losses affecting the fill factor originate from the transport of minority carriers across the interface. The I/V curves measured at low temperatures reveal a characteristic S-shaped behavior. This effect increases with decrasing temperature and stems from the minority carrier transport, which is hindered by the band offset between a-Si:H and c-Si. We propose a new analytical model to describe this anomalous behavior.

Amorphous silicon is an ideal system for optical engineering since its optical properties can drastically be changed through its ability to absorb such impurities as hydrogen whose contents range from zero to beyond twenty percent. In this work we report light-induced changes in its optical properties which may add potential in active optical engineering. Considerable changes in the phases in reflected polarized lights from transparent-substrate side of amorphous silicon films are observed both after prolonged illumination of intense light at the light-soaking temperature range from 40 to 250 °C. Most of the changes are localized to amorphous silicon region near substrate-film interfaces. Illumination time dependencies and annealing characteristics are examined, and physics behind these observed changes are discussed.

Microcrystalline silicon germanium alloys (ν-Si1−xGex:H) exhibit an enhanced optical absorption in the near infrared region with increasing germanium content. Therefore, the employment of this material as intrinsic absorber is a promising challenge for thin film technology in photovoltaics and sensorics. The influence of hydrogen dilution on the material and the performance of pin diodes prepared in a PECVD process with 10 % GeH4 in the gas phase is discussed. These results are compared with the characteristics of diodes prepared with higher germane content in the gas phase. With increasing germane content the solar cell parameters Voc and FF decrease and the deposition regime where good optoelectronic properties are observed is narrowed.

The influence of boron and phosphorous doping and the effect of structural changes on hydrogen bonding in a-Si:H and µc-Si are investigated using Raman spectroscopy. Information on H bonding is obtained from Si-H stretching local vibrational modes (LVM's). In n-type and undoped a-Si:H the LVM's of isolated Si-H near 2000 cm−1 and clustered Si-H around 2100 cm−1 can be clearly distinguished. In heavily B doped a-Si:H, however, the LVM at 2100 cm−1 disappears. A structural change from a-Si:H to µc-Si gives rise to an additional mode centered at 1911 cm−1. Similar LVM's are observed in poly-Si containing platelets.

We present a new design of micro-photodiodes for in-vitro tests to electrically stimulate the ganglion cells of chicken and rat retinae upon light exposure of the photodiodes. Based on amorphous silicon, our laterally series connected double-stacked micro-photodiodes provide an open circuit voltage of 2.3 volts. Photolithographic steps as well as etching procedures for patterning the back contact, the amorphous silicon layers and the front contact are described. We analyse current- voltage-measurements performed with direct contact of the metal needles of a micro-positioning system to the device's electrodes. In order to test the performance of an individual micro-photodiode in an electrolyte environment, the stimulation electrode of the device is also contacted with a micro-droplet of buffer solution. Further improvement is needed, mainly addressing the problem of long-term stability of the device in electrolyte environments.

The advantage of using very high frequencies for preparation of a-Si:H materials at high rates (above 5 Å/s) for intrinsic layers (i-layer) of solar cells has been well documented. In an effort to identify film properties which may be related to this superior device performance, a study of the structural, optical and electrical properties of films made at various deposition rates between 1 and 15 Å/s using rf frequencies of 13.56 and 70 MHz has been made. The films were characterized using a number of techniques including small-angle x-ray scattering, infrared absorption spectroscopy, and scanning electron microscopy. For the films made using the 70 MHz frequency, the amount of nanovoids with sizes of < 100Å increases systematically as the deposition rates increases beyond 5 Å/s. Accompanying the increase in void fraction in the films are increases in the hydrogen content and the amount of 2070 cm-1 mode in the infrared absorption spectra. In addition to an increase in the amount of nanovoids in the films as the deposition rate exceeds 5 Å/s, the films made using the 13.56 MHz and high deposition rates have large amounts of SAXS related to scattering features with sizes > 200 Å. This scattering is associated with large bulk density fluctuations and/or enhanced surface roughness. None of the films in the study displayed signs of having columnar-like microstructures. The nanovoids are not related to changes in the solar cells with increasing i-layer deposition rate for both fabrication processes, perhaps due to the relatively small volume fractions of less than 0.2% and/or good void-surface passivation. However, the larger-scale structures detected in the films made using the 13.56 MHz technique could cause poorer performance in cells prepared at high growth rates.

A comparison of the performance of aluminium (Al)-gated thin film transistors (TFTs) is presented in which we varied its sputter deposition conditions, such as deposition temperature, process pressure, and power. Gate films deposited at 30°C/5mTorr/300W yield TFT characteristics with low leakage current (~ 10 fA at low VDS), an ON/OFF ratio better than 108, and a mobility of 1.1 cm2/Vs. In contrast, films deposited at 150°C/10mTorr/400W, yield a significant degradation in leakage current (~ 1 pA) and mobility (0.77 cm2/Vs). The degradation stems from the high surface roughness of the a-SiNx:H gate insulator, and hence the TFT channel, caused by hillock formation on the Al gate. In addition, the high roughness leads to a correspondingly large shift in threshold voltage. After one-hour bias stress of +25 V applied to the gate, the shift in threshold voltage is ΔVT ~ 5 V, as compared to the small shift of ΔVT ~ 2.3 V associated with the smoother gate. Also included in our comparison is a TFT whose Al gate is now capped with 20 nm of molybdenum (Mo) to minimize propagation of the gate surface roughness to the active channel. Its cross sectional topography shows the interface smoothness to be as good or better, to yield improved leakage and stability characteristics.

The fabrication of 250 Å thick, undoped, single crystal silicon on insulator by lateral solid phase epitaxial growth from amorphous silicon on oxide patterned (001) silicon substrates is reported. Amorphous silicon was grown by low pressure chemical vapor deposition at 525°C using disilane. Annealing at temperatures between 540 and 570°C is used to accomplish the lateral epitaxial growth. The process makes use of a Si/Si1-xGex/Si stacked structure and selective etching. The thin Si1-xGex etch stop layer (x=0.2) is deposited in the amorphous phase and crystallized simultaneously with the Si layers. The lateral growth distance of the epitaxial region was 2.5 μm from the substrate seed window. This represents a final lateral to vertical aspect ratio of 100:1 for the single crystal silicon over oxide regions after selective etching of the top sacrificial Si layer. The effects of Ge incorporation on the lateral epitaxial growth process are also discussed. The lateral epitaxial growth rate of 20% Ge alloys is enhanced by roughly a factor of three compared to the rate of Si films at an anneal temperature of 555°C. Increased random nucleation rates associated with Ge alloy films are shown to be an important consideration when employing Si1-xGex to enhance lateral growth or as an etch stop layer.

Crystallization behavior of 100nm amorphous silicon film with and without a 160nm poly-Ge layer on top was investigated. Ge was observed to increase the nucleation rate as well as to increase the incubation time for nucleation in a-Si. Activation energy for nucleation was 2.3eV for Ge-covered Si films as compared to 2.7eV for the control a-Si films with no poly-Ge. Activation energy for incubation is almost unchanged for both the films (-3.3eV). An alternative seeding technique for a-Si films using the increase in the incubation time for nucleation due to presence of poly-Ge is proposed.

A new process of surface micromachining has been developed, where amorphous silicon (a-Si) and oxynitride films are used as structural and sacrificial layers on the glass substrate, respectively. Due to glass as the substrate material, the temperatures for all process steps need to be lower than 600°C. Some generic mechanical microstructures such as cantilever beams, bridges, and membranes have been fabricated. The stress changes of a-Si films with annealing temperatures are studied. It is found that the residual stress can be minimized using thermal annealing at 430°C for a few hours. In addition, some process issues such as a-Si film bubbling, the film adhesion of a-Si to the glass substrate, and stiction during structure release are discussed.

The compatibility of inorganic materials with living tissues and biological compounds is crucial in many areas, including medical diagnostics, biosensors, drug delivery, etc. In this work, we are interested in the interaction of living mammalian cells with semiconductor surfaces for novel thin-film biosensor devices. Amorphous silicon may give advantages over crystalline silicon for some devices because of its large-area, low-temperature compatibility, and its large optical absorption coefficient in the visible spectrum. Amorphous silicon thin films (500Å) deposited on quartz glass were cleaned using ambient UV/O3 treatment, leaving the surface largely OH-terminated and hydrophilic. The hydrophilic surface was then exposed to a vapor of octyltrichlorosilane (CH3(CH2)7SiCl3, OTS). The resulting surface was strongly hydrophobic, with advancing contact angles with water <106°. This organic surface was masked to reserve areas of uncoated hydrophilic substrate, and placed in a cell culture (BHK-21 cells) to observe cell adhesion and proliferation. A high degree of cell attachment was observed on the UV/O3-treated surfaces (~400 cells/mm2) compared to ~450 cells/mm2 on the culture dish control surface, indicating cell proliferation and growth. Little cell adhesion occurred on the hydrophobic organiccoated surface (~40 cells/mm2), and the cells remained round and only minimally attached. On masked surfaces, the organic-free areas showed dense, well-adhered cell growth while the coated areas showed much fewer and rounded cells. On all samples as well as control surfaces, cell death was <1%. These results suggest a means for selectively controlling cell adhesion to thin film electronic device surfaces, through the patterning of hydrophobic surface coatings.

Higher crystalline Si volume fractions in hydrogenated microcrystalline silicon ( µc-Si:H) films have been achieved by the hot-wire assisted plasma enhanced chemical vapor deposition (HWA-PECVD) method compared with those in films by conventional PECVD. µc-Si:H films can also be prepared by HWA-PECVD under typical conditions used for preparing hydrogenated amorphous silicon (a-Si:H) films by PECVD, in which the hydrogen-dilution ratio (H2 / SiH4) is ∼ 10. The hot wire seems to produce hydrogen radicals. As a result, the HWA- PECVD method can control hydrogen-radical densities in the RF plasma, and this method can also control the ratio of hydrogen coverage at the surface of the film.

The mechanism (or mechanisms) of hydrogen transport in a-Si:H is of great intrinsic interest but is also important for the understanding of a number of phenomena such as light induced defects. We find that the literature includes many calculations for the activation energy for hydrogen diffusion that are at best misleading and often interpreted incorrectly. In this paper we summarize a number of calculations that apply to particles hopping in complex media and apply the results to a-Si:H. These calculations, coupled with existing estimates of the energy of various H configurations in a-Si:H, show that the standard picture is likely to be incorrect. However, previous work on the mobility of molecular hydrogen in crystalline Si coupled with some molecular dynamics simulations presented in this paper show that the mobile species of hydrogen in a-Si:H may well be molecular.

To form crystalline Si dots embedded in SiO2, we have deposited thin films of silicon rich oxide (SRO) by plasma-enhanced chemical vapor deposition of SiH4 and O2. Then the materials wereannealed in N2 ambient at temperatures between 950 and 1100 °C. Under such processing, the supersaturation of Si in the amorphous SRO film produces the formation of crystalline Si dots embedded in SiO2. The narrow dot size distributions, analyzed by transmission electron microscopy, are characterized by average grain radii and standard deviations down to about 1 nm. The memory function of such structures has been investigated in metal-oxidesemiconductor (MOS) capacitors with a SRO film sandwiched between two thin SiO2 layers as insulator and with an n+ polycrystalline silicon gate. The operations of write and storage are clearly detected by measurements of hysteresis in capacitance-voltage characteristics and they have been studied as a function of bias.

In the present paper regular systems of silicon and platinum assemblies have been fabricated in a three-dimensional (3D) void sublattice of synthetic opal. The detailed TEM and HREM structure study of ‘opal-Si’ and ‘opal-Pt-Si’ composites was carried out. It was found that in regular composites ‘opal-Si’ the silica spheres were covered uniformly with a nanocrystalline silicon layer of up to 25-30 nm in thickness. To form the Pt-Si contact the silica spheres were coated with platinum layer before embedding silicon. The results obtained demonstrate a possibility of creating 3D multilayer semiconductor structure (p-n junctions, Schottky barriers etc.) on the inner surface of opal voids.

Dopant segregation in atmospheric-pressure, chemically vapor deposited (APCVD), ~300 nm thick, polycrystalline Si0.95Ge0.5 and Si0.9Ge0.1 thin films, implanted at 80 KeV with 6×1013 to 5×1014 P/cm2 and annealed at 800 °C for 1 hr, was investigated using a combination of Hall and conductivity vs. temperature measurements. Hall measurements, feasible only in heavier doped films, showed that 29% of the phosphorus in Si0.9Ge0.1 and 42% of phosphorus in Si0.95Ge0.05 was electrically inactive. The loss was attributed to dopant segregating to grain boundaries. The density of grain boundaries states was also found to increase slightly with increasing Ge content, from 3.6×1012/cm2 in Si0.95Ge0.05 to 4.4×1012/cm2 in Si0.9Ge0.1.