A novel method of UV-assisted metal-induced-crystallization is introduced to grow polysilicon films on ordinary glass at temperatures as low as 400°C. Annealing is accomplished in the presence of an ultra-violet exposure, leading to high crystallinity of the silicon film as confirmed by XRD, TEM and SEM analyses. A back-reflecting chromium layer is incorporated to further trap UV photons and enhance their absorption in the silicon film. This results in a significant increase in the crystallization rate as studied by XRD spectroscopy. A growth rate of 2 µm/hr is observed at 400 °C, when employing this method for lateral crystallization. Thin-film transistors fabricated using the proposed UV-assisted MILC show a threshold voltage of 1V and hole mobility of about 50 cm2/V.s.

This paper is concerned with the phenomenon of the increase of the holes lifetime with the increase of the dangling bond concentration in a-Si:H. This rather surprising phenomenon that was observed, but not discussed, previously is shown to be a non-trivial effect which is based on the charged nature of the dangling bonds and a special scenario of the concentrations of the various defect states in the material. The most important implication of our study is that the charged dangling bonds can sensitize the valence band tail states, in contrast with the accepted roles of these types of states. The present understanding suggests that many new interesting phototransport phenomena can be found in a-Si:H.

Steady-state space-charge limited current (SCLC) measurements are used to investigate the density of states (DOS) in the mobility gap of hydrogenated amorphous silicon (a-Si:H). The density of states is calculated by different methods based on both continuous DOS and discrete traps assumptions. The density of states found by the SCLC measurements is used to set the trap densities and trap energy levels to model a vertical a-Si:H thin-film transistor (TFT) using the Medici device simulation package. The effect of different sets of traps in the bulk of a-Si:H and variation of the physical dimensions of the device on the characteristics of the vertical TFT is studied. The simulation on the space-charge limited current is performed to verify the validity and accuracy of the SCLC method.

Carrier transport was investigated in two different types of ultra-thin silicon films, polycrystalline silicon (poly-Si) films with large grains > 20 nm in size and hydrogenated nanocrystalline silicon (nc-Si:H) films with grains 4 nm – 8 nm in size. It was found that there were local non-uniformities in grain boundary potential barriers in both types of films. Single-electron charging effects were observed in 30 nm × 30 nm nanowires fabricated in 30 nm-thick nc-Si:H films, where the electrons were confined in crystalline silicon grains encapsulated by amorphous silicon. In contrast, the poly-Si nanowires of similar dimensions showed thermionic emission over the grain boundary potential barriers formed by carrier trapping in grain boundary defects.

We fabricated self-aligned polycrystalline silicon (polysilicon) thin film transistors on flexible steel substrates. The polysilicon was formed by furnace crystallization of hydrogenated amorphous silicon at 950°C/20sec or 750°C/2min. The TFTs made from these polysilicon films have hole field effect mobilities in the linear regime of 22 cm2·V−1s−1 (950°C) and 14 cm2·V−1s−1 (750°C). The OFF current at 10 V drain-source voltage is 10−10A and the drain current ON/OFF ratio is ∼106.

In this paper, we present a comprehensive study of microcrystalline silicon thin film samples deposited by a novel growth process intended to maximize their grain size and crystal volume fraction. Using Atomic Force Microscopy, Raman spectroscopy, and x ray diffraction the structural properties of these samples were characterized qualitatively and quantitatively. Samples were grown using a Hot-Wire Chemical Vapor Deposition process with or without a post-growth hot-wire annealing treatment. During Hot-Wire Chemical Vapor Deposition, SiF4 is used along with SiH4 and H2 to grow the thin films. After growth, some samples received an annealing treatment with only SiF4 and H2 present. These samples were compared to each other in order to determine the deposition conditions that maximize grain size. Large microcrystalline grains were found to be aggregates of much smaller crystallites whose size is nearly independent of deposition type and post-annealing treatment. Thin films deposited using the deposition process with SiF4 partial flow rate of 2 sccm and post-growth annealing treatment had the largest aggregate grains ∼.5 µm and relatively high crystal volume fraction.

A modified very high frequency (MVHF) glow discharge technique is used to deposit amorphous silicon (a-Si) and amorphous silicon-germanium (a-SiGe) alloy solar cells at high deposition rates. High quality a-Si alloy solar cells have been obtained by using MVHF at deposition rates up to ∼10 Å/s. The cells show good initial and stabilized efficiencies comparable to those obtained from conventional radio-frequency (RF) glow discharge deposition at low rates (∼1 Å/s). However, high quality a-SiGe alloy solar cells are more difficult to achieve at high deposition rates. In this paper, we present the progress made on a-SiGe alloy solar cells by incorporating bandgap profiling and appropriate buffer layers. Using the improved a-SiGe alloy solar cells, a-Si/a-SiGe tandem configurations are made and results presented.

We report studies of the image-blur effects caused by lateral cross-talk between neighboring pixels of large-area amorphous silicon (a-Si:H) image sensors. The lateral conduction is attributed to three effects: conduction along the interface between the a-Si:H film and the underlying passivation; field-dependent electron injection at the edge of the sensor; and a field enhancement of the interface conduction due to the bias applied to the address lines. We show that the cross-talk can be controlled by choice of the operating conditions and optimization of the materials.

Recent experiments suggest that the chemistry responsible for the growth of amorphous and microcrystalline Si and (Si,Ge) alloys is more complicated than suggested by the standard model of growth. In particular, unexpected phenomena related to the density of ion flux, magnitude of ion energy and the presence of inert gas ions in the discharge suggest that both inert and reactive ion flux profoundly influence the growth of amorphous and microcrystalline films and devices. The influence of the ion flux is particularly important in understanding the growth of high quality a-(Si,Ge):H films. By carefully controlling this flux, we can change significantly the H bonding environment in a-(Si,Ge):H and produce high quality materials and devices. These observations suggest that the standard model of growth, which postulates that the growth is primarily limited by surface diffusion of radicals, and that H is eliminated by spontaneous bond breaking between neighboring Si-H bonds and formation of H2 molecule, does not describe the actual growth chemistry very well. Rather, the growth is limited by both surface diffusion of radicals and more fundamentally, by how rapidly H desorbs from surface and subsurface bonds. Ion induced desorption of excess H is shown to be very important in controlling the growth of both a-Si and a-(Si,Ge):H.

High quality amorphous silicon germanium (a-SiGe:H) alloys have been obtained using the hot wire chemical vapor deposition (HWCVD) from a gas mixture of SiH4, GeH4, and H2 at a deposition rate of ∼10 Å/s. Solar cells in a SS/n-i-p/ITO configuration are evaluated in which the n- and i-layers are deposited by HWCVD at NREL and the microcrystalline p-layer by conventional RF glow discharge in a separate reactor by United Solar. Effects of hydrogen dilution and step-wise bandgap profile have been studied and optimized. The best cell has an average optical bandgap of 1.6 eV and incorporates multi-bandgap steps where the narrow-most bandgap is near the p-i interface. J-V characteristics are measured under AM 1.5 illumination with a λ>530 nm filter. The best initial power output obtained exceeds 4 mW/cm2, which is usually used as an indicator for a good quality middle-gap cell. Double-junction cells are made on textured Ag/ZnO back reflectors. The bottom cell uses the optimized a-SiGe:H alloy cell by HWCVD, and the top cell uses an optimized a-Si:H cell near the amorphous-to-microcrystalline transition by PECVD at ∼1 Å/s. The best double-junction cell made to date exhibits an initial AM 1.5 active-area efficiency of 11.7%, and a stable efficiency after 1000 hours of one sun light soaking of 9.6%.

The effects of random noise on density of states determination from transient photocurrent data are examined by superimposing noise levels similar to those found experimentally (1% to 20%) on computer-simulated current-time data. Mathematically approximate methods based on Fourier and Laplace transformations are found to operate effectively at noise levels of up to 20%. Mathematically exact methods offer higher resolution, but this is compromised by greater susceptibility to noise. A Tikhonov regularisation method yields both high resolution and good noise tolerance.

Nano-crystalline Si films were prepared by plasma enhanced chemical vapor deposition (PECVD) and rf-magnetron sputter techniques at various deposition conditions, and the relations of the photoluminescence (PL) phenomena of the films with process variables are discussed. The phase of the films prepared at R.T. by PECVD techniques is somewhere between amorphous and crystalline states, consisting of nano-crystallites of size ranging from 3.2 to 5.3 nm. These films exhibit significant PL intensities near blue light region; the PL peaks shift from 510 to 460 nm with decreasing the reaction gas (SiH4) fraction from 4.7 to 2.0%. The films prepared at 500 °C by PECVD are composed of about 5 and 150 nm crystallites, exhibiting little PL phenomena. The PL intensity of the films prepared by sputter techniques was observed to increase with raising the sputter power and the post-deposition heat-treatment temperature.

Using ab initio density functional calculations we investigate the energetics of hydrogen in a-Si:H starting from a basic supercell that contains 142 atoms and is completely free of defects. The study includes isolated H atoms bonded to dangling bonds, H atoms bonded to dangling bonds in regions of clustered H, bond centered hydrogen, and molecular hydrogen. In particular, the difference between clustered and isolated hydrogen has been largely ignored in the past. Energetics of doped as well as undoped cells are considered. The results are discussed with particular emphasis on H diffusion and the replacement of atoms in molecular hydrogen with those bonded.

The effect of moderate hydrogen dilution of the process gas, F(H2)/F(SiH4) = 0 to 3, on the properties of amorphous silicon is discussed for material and solar cells deposited by Hot-Wire CVD. Dielectric properties were obtained from spectroscopic ellipsometry and are related to stability and hydrogen bonding configuration of films deposited with varying hydrogen dilution at different substrate temperatures. The stability was determined by comparing defect densities obtained from photoconductivity spectroscopy in the constant photocurrent mode (CPM) before and after pulsed-light soaking. At low substrate temperatures, which are relevant for the prepara- tion of pin-type solar cells (160-200°C), moderate hydrogen dilution (∼0.3) improves material quality regarding density and network disorder (oscillator bandwidth) as obtained from spectro-scopic ellipsometry, resulting in a higher stability. At higher substrate temperatures (300°C), stability and hydrogen bonding configurations are generally better, but moderate hydrogen dilu-tion already deteriorates these properties compared to material prepared without dilution. The incorporation of Hot-Wire-a-Si:H into pin-type solar cells is also discussed and a good correlation of ellipsometric results with bulk-related properties of solar cell performance is observed. The optimum hydrogen dilution is found to be 0 to 0.3 for i-layer deposition yielding initial efficiencies of up to 8.9% for solar cells entirely fabricated by Hot-Wire CVD.

Three Laplace transform methods for recovering the density of electronic states from transient photocurrent data are evaluated through a study of light-induced defect creation in PECVD a-Si:H films. A mathematically approximate method is shown to be sufficient to resolve the deep defects, whose density is estimated to increase by a factor of five from the annealed state after 3 hours' exposure to simulated AM1 illumination. An exact method, and a method employing Tikhonov regularisation, are found to give very similar results, provided the current-time data are smoothed beforehand in the former case. The increased resolution available is, however, unnecessary here, and these methods are shown to be more suited to the study of discrete levels or narrow distributions.

The study and characterization of the (opto)electronic properties of a-Si:H and µSi filmsby contactless transient photoconductivity measurements is presented. The importance ofminority carrier trapping is shown for the example of a-Si:H films prepared with different dopinglevels. It is shown that the microwave mobility determined by these measurements is a versatiletool for the characterization of the films. Examples are given by the study of µ Si filmsproduced by laser crystallization of a-Si:H films and the optimization of the substratetemperature for the Hot Wire deposition of µ Si films.

In order to investigate microcrystalline silicon (μc-Si:H) nucleation from the hydrogenated amorphous silicon (a-Si:H) phase, we performed a H2-plasma treatment of a-Si:H layers deposited at different temperatures. In the H-treatment experiment, the formation process of an infrared peak at ∼1937 cm-1, assigned to SiHn (n=1∼2) complex, is studied, as the SiHn complex is proposed to be a precursor for the μc-Si:H nucleation. With increasing the a-Si:H deposition temperature, the total amount of the SiHn complex formed by the H treatment increased. Nucleation of μc-Si:H under high H2-dilution conditions showed a clear relationship with the SiHn complex formed by the H treatment. The SiHn complex formation process, in terms of strained Si-Si bond breaking by H, is discussed.

Hydrogen diffusion and solubility effects in hydrogenated amorphous and microcrystalline silicon films are reviewed. Various diffusion-related effects have been observed which need to be considered in models of hydrogen diffusion. Hydrogen solubility is found to affect hydrogen incorporation and hydrogen transport.

This paper reports on the first full realization and characterization of a two-dimensional array of amorphous silicon (a-Si:H) color sensors, addressed by integrated amorphous silicon-based thin-film transistors (TFTs). The array includes 512 × 512 pixels with 75-µm pitch, or about 340 dpi. Each pixel features a color sensor realized by a p-i-n-i-p stack of doped and undoped a-Si:H layers, and the TFT. The color sensors are made of two back-to-back p-i-n diodes, which selectively sense the illumination according to the polarity of the applied bias voltage. The sensor layers are grown on top of the TFTs to improve the array fill factor. The p-in-i-p sensor stack is mesa-isolated into single sensors to reduce cross-talk.

Images are acquired using two bias voltages and yield the red and blue/green components of the original with a good color separation. A color image is reconstructed using the information from the two images acquired. Aside from a color bias, which is expected for a two-color reconstruction, the imaging system works well. In particular, the array shows very low leakage currents, which enable a very large dynamic range and sensitivity. In the response of the array to a light pulse, the bottom thick diode ensures a fast drop in the signal after the flash, while the top thin diode exhibits some residual image lag.

We propose a mesh-type PECVD to minimize the hydrogen concentration in this study. Since in this system deposition rate is very slow, so for increasing a deposition rate, we suggest an applied DC bias enhanced sputtering process. We investigated several conditions to compare with conventional PECVD. Excimer-laser melting and regrowth of thin a-Si films is for fabricating polycrystalline-Si (poly-Si). Furthermore, we fabricate poly-Si thin-film transistor(TFTs) and measure threshold voltage (V), field-effect mobility (cm2 /Vs) and on/off current ratio