The novel version of the field effect conductivity method for the direct density-of-states (DOS) determination has been developed. The combination of both, a new analytical solution, in the form of integral equation which relates the surface potential to the gate voltage, and new data-analysis algorithms allows one to solve the problem by using the results of measurements of TFT quasi-static transfer characteristics. This sensitive, and easy to use, method provides the detailed information on the defect density in the gap of hydrogenated Si. The ability of this technique to extract parameters of the TFT, including the flat band voltage, from the dependence of the surface potential on the gate voltage, and to resolve fine-scale features in the midgap DOS of a-Si:H has been demonstrated.

We have shown that the dependence of the DOS on the energy in the range 0–0.15 eV from the conduction band is close to the exponent, while deeper in the gap, the DOS can be described as a series of localized defect bands with amplitude values in the range 1016 cm−3 eV−1-2×1018 cm−3 eV−1.

The integrated charge density and the surface potential as functions of gate voltage have been investigated. The difference between the surface potential and activation energy from temperature dependence of drain current has been discussed. The short-time current relaxation and metastable defect transformations have been studied and analyzed.

Thin(<20nm) p-type microcrystalline silicon f'dms have been deposited by PECVD in an unusual parameter regime, specifically optimized for extremely thin films. High conductivity (>10.2 Scm−1) and low activation energy(<0.08eV) of thin films have been achieved on various metal oxide substrates i.e., Coming 7059 glass, SnO2:F, TiO2 and Ta2O5. Crystallinity was confirmed by Raman spectroscopy and UV reflectance. Deposition of p-μc-Si:H is possible on void rich films (a-SiC:H and low temperature deposited a-Si:H) but not on device quality a-Si:H. Cells made in a superstrate structure using p-μc-Si:H as the window layer directly on top of SnO2:F coated glass yielded 9.63% efficiency, with an improvement in the blue spectral response compared to the cell made with a- SiC:H(B) as window layer. Tandem cells (a-Si:H/a-Si:H) incorporating p-μc-Si:H along with n-μc- Si:H in the tunnel junction yielded 8.8% efficiency.

The plasma deposition of boron doped microcrystalline films was optimized with respect to crystallinity and doping efficiency. High room temperature conductivities up to 39 Scm−1 were achieved under condition when the energy of positive ions impinging on the growth surface is minimized.

The low temperature hydrogen induced phase transformations are the basis for the layer-by-layer micro-crystalline silicon growth technique which involves the sequential deposition of thin amorphous silicon layers followed by hydrogen treatment. Very high frequency-PECVD layer-by-layer microcrystalline film growth was studied as a function of frequency. The highest crystalline content films were prepared on glass substrates at 49.7 MHz with deposition cycle times of 12 seconds and hydrogen treatment times of 120 seconds or longer. At lower frequencies, 13.56 and 27 MHz, no micro-crystalline content was observed. At higher frequencies (94.7 MHz) crystalline content was reduced compared to that of the 49.7 MHz case. Crystalline content is not correlated with the hydrogen etch rate as the highest etch rates occurred at 94.7 MHz while etch rates at 49.7 and 27 MHz were lower. It is possible to prepare thin micro-crystalline films using the VHF layer-by-layer growth technique.

Textured polycrystalline silicon films with columnar structure have been deposited on glass at low temperature (400–550°C) and high deposition rate (10 to 15 Å/s) by hot-wire chemical vapour deposition using SiH4-H2 gases. The homogeneity of the deposited layer is ± 5% on a 8 cm diameter. As deposited films have a poor photoconductivity. However hydrogen confinement in the films during the deposition or after the deposition is found to be the key for obtaining g.tc/poly-Si with a significant diffusion length. Eventually reasonable values of the mobility lifetime product (> 10−7 cm2/V) are obtained by in situ hydrogen passivation of poly-Si films after deposition. Efficient shifting of the Fermi level is achieved by in situ B or P doping. The incorporation of boron in poly-Si network strongly influences the morphology and the crystalline structure. Undoped films have a Hall mobility of 14 ± 5 cm2/V.s which decreases versus the carrier concentration.

The growth of amorphous (a-Si:H) and microcrystalline (pc-Si) silicon by hot-wire chemical vapor deposition (HWCVD) has been studied by combining in-situ ellipsometry, atomic force microscopy (AFM), and Raman spectroscopy. Generally a dense nucleation layer is formed during a-Si:H HWCVD, containing nuclei about 0.8 nm high and 10 to 20 nm in diameter. The surface roughness gradually increases with film thickness and settles at a root mean square (RMS) value of 1.6 nm at about 200 nm thickness. For hydrogen dilution at gas flow ratios x=[H2]/[SiH4] of 15 to 120 microcrystalline material was obtained. The grain size and nucleation layer, however, are strongly dependent on x. Low H2 dilution enhances the formation of an amorphous-like interface layer from which the μc-Si:H growth eventually starts. Increasing x promotes the etching of amorphous regions and the surface diffusion of precursors, resulting in larger nuclei. X = 30 yields extended μc-Si nuclei (30 nm height, 90 nm diameter) and a pronounced increase in surface roughness for thicker films, but suppresses the formation of the amorphous-like nucleation layer. A further increase in x remarkably lowers the growth rate, but smoother surfaces at comparable film thickness and larger lateral dimensions of the grains occur. This is interpreted as incipient etching of the crystallites.

We report new results on microstructures and electronic properties of nanocrystalline silicon (nc-Si:H) films prepared by a triode PECVD method with a hydrogen diluted silane plasma. The transition from a-Si:H to nc-Si:H was obtained by varying the ratio of H2 to SiH4 while the bias of the grid electrode was fixed at 100.V with respect to the substrate. The threshold value of [H2]/[H2 ]+[SiH4] is about 93.3%. With increasing value of [H2]/[H2]+[SiH4], the volume fraction of crystallinity in nc-Si:H films increased from 12% to 50%, while the average grain size remaind constant. This is quite different from the structures of samples prepared by the conventional diode PECVD with H2 dilution. The traveling-wave technique was applied to study the transport properties of nc-Si:H films, and experimental results confirmed percolation transport in nc-Si:H films.

We studied the effect of RF deposition power and temperature on the electrical and structural properties of plasma enhanced chemical vapor deposited (PECVD), heavily doped microcrystalline silicon films (n+μC-Si:H). The film growth process was found to be CVDlike at low powers and PVD-like (Physical Vapor Deposition) at high powers. We show that the film properties strongly depend on the nature of the growth process. We observed that at low temperatures the microcrystalline formation is mainly governed by the presence of hydrogen. This can be improved by increasing the substrate temperature. However, a further increase in substrate temperature tends to reduce hydrogen incorporation into the film and hence decreases the role of hydrogen in the formation of microcrystallites. Resistivities as low as 0.1 Ω.cm were achieved for 500Å thin layers deposited at a relatively low temperature of 220°C and power density of 40mW/cm2.

The technique of preamorphization is a well-known method to avoid channeling tails of implanted dopants when fabricating devices. This is achieved by implanting a nondopant atom to produce the amorphous layer. We studied the recrystallisation of silicon preamorphized by Si, Ge and Sn implants. The recrystallisation was investigated by RBS and the y-y perturbed angular correlation (PAC) technique. RBS provides information about the thickness and the perfection of the recovered layer. Additional experimental data are supplied by the PAC technique, which used the postimplanted radioactive isotope 111Ina s a probe atom. The recovery of the lattice structure in the vicinity of the probe atoms is monitored on a microscopic scale via the electric field gradient produced at the site of the In nucleus by the surrounding lattice defects. The results are discussed and compared to the RBS data.

The performance of polysilicon thin film transistors fabricated by two-step annealing, which consists of furnace annealing and subsequent excimer laser annealing, is described. It was found that the average grain size of low temperature furnace annealed polysilicon films was several times larger than that of excimer laser annealed polysilicon films while the density of in-grain defect in low temperature furnace annealed films was much higher than that of excimer laser annealed film. The device characteristics of the low temperature furnace annealed polysilicon thin film transistors were improved significantly by postannealing, such as high temperature furnace annealing and excimer laser annealing, due to the effective elimination of in-grain defects. The density of trap states, which was extracted from the transfer curves of polysilicon thin film transistors, was used to demonstrate the effects of modifying the deep and tail trap levels by two-step annealing.

The fluorinated amorphous and microcrystalline silicon (a,μc-Si:H;F) films have been prepared by rf plasma enhanced chemical vapor deposition (PECVD) with SiH 4 and SiF 4 gas mixtures. The stretching Si-O (1085 cm-1) and SiH2 (2100 cm-1) bands estimated from infrared (IR) spectroscope data have related to the evolution of crystallinity and the optical band gap was shifted by introducing Si-O bonds. The sub-band gap absorption coefficient in a,μc-Si:H;F films was about one order lower than that in hydrogenated amorphous silicon film (a-Si:H). The subband gap absorption in a-Si:H;F film was comparable to that in tic-Si:H;F films. The lightinduced degradation of a,μc-Si:H;F films were also suppressed.

The modification of surface reactions with an inert gas plasma treatment method has been investigated for the first time in order to improve the properties of a-Si:H for solar cells. The deposition of a thin a-Si:H layer and exposure to inert gas plasma such as He, Ar or Xe were repeated by using RF plasma CVD at a substrate temperature of 200 °C. It has been found that the hydrogen content (CH) can be controlled in a wide range from about 18 atomic% to about 35 atomic%, although it is a hydrogen-free process. Experimental results show that the change in CH of a-Si:H films is mainly determined by the decrease in CH of the treated a-Si:H surface and the increase in CH of the a-Si:H deposited on the treated surface. Furthermore, the plasma treatment probably promotes the surface reaction, which reduces SiH2ISiH and CH. Consequently, wide-gap (1.64 eV by ( a h ν )1/3 versus h ν plots, 1.75 - 1.85 eV by Tauc's plot) a-Si:H films with high stabilized photoconductivity (> 10-5 Ω-1cm-1 under AM-1, 100 mW/cm2 irradiation) have been obtained by the inert gas plasma treatment method.

High quality wide gap hydrogenated amorphous silicon has been prepared using the chemical annealing technique. It was possible to prepare materials with band gaps ranging 1.8 to 2.1 eV by varying the preparation parameters. Low defect densities less than (3–8) x 1015 cm-3 could be maintained over the entire band gap range. Improved stability for light soaking was also observed in the wide gap materials.

Fast (7 nm/s) deposition of amorphous hydrogenated silicon with a midgap density of states less than 1016 cm-3 and an Urbach energy of 50 meV has been achieved using a remote argon/hydrogen plasma. The plasma is generated in a dc thermal arc (0.5 bar, 5 kW) and expands into a low pressure chamber (20 Pa) thus creating a plasma jet with a typical flow velocity of 103 m/s. Pure silane is injected into the jet immediately after the nozzle, in a typical flow mixture of Ar:H2:SiH4=55:10:10 scc/s. As the electron temperature in the recombining plasma is low (typ. 0.3 eV), silane radicals are thought to be produced mainly by hydrogen abstraction.

Material quality in terms of refractive index, conductivity, microstructure parameter and optical bandgap was found to increase monotonously with substrate temperature, even up to 350 °C; for practically all low growth rate deposition schemes an optimum around 250 °C is observed. It will be argued that this behavior is consistent with a simple kinetic model involving physisorption and hopping, growth on dangling bonds and thermal desorption of hydrogen.

We have previously shown that the carrier drift mobility in amorphous silicon can be enhanced by optimizing the ion-bombardment energy during growth on conducting substrates. However, there exists a lack of reproducibility of samples exhibiting high mobility which we attribute to the rf field induced fluctuation of the plasma potential in a conventional (Te ≈ 2eV) silane plasma. Here we introduce an enclosed plasma configuration that allows us to confine the effect of the rf field and therefore obtain a low-electron-temperature (Te ≈ 0.1 eV) silane plasma as determined from Langmuir probe measurements. The measured ion-energy distributions correlate with those for electrons and the mean ion-energy can be controlled by biasing the substrate which allows us to reproducibly fabricate high drift mobility amorphous silicon.

Dc-magnetron assisted silane decomposition technique has been tested for deposition of undoped a-Si:H at substrate temperature Ts=300–400°C. In the optimized conditions device-quality a-Si: H films were deposited independently of Ts. A low hydrogen content CH (up to 2 at.°) and microstructure variations are characteristic of the MASD films.

The effect of hydrogen dilution and substrate temperature on the optoelectronic and structural properties of thin films deposited by hot-wire chemical vapor deposition with filament temperatures between 1900 and 2500 °C from silane and hydrogen are studied. Amorphous silicon films are obtained at high deposition rates for hydrogen dilutions below 90%. The deposition rate scales approximately linearly with the filament temperature in this regime. Microcrystalline films are obtained for hydrogen dilution above 90%, independently of the filament temperature and substrate temperature, with much lower growth rates. The Raman spectrum of these films shows high crystalline fraction and small grain size. High conductivity films, typical of microcrystalline silicon, with high growth rates were achieved by either increasing the substrate temperature at low hydrogen dilution, or by using a hydrogen dilution just at the amorphous to microcrystalline transition point.

We have applied the Drive-Level Capacitance Profiling (DLCP) method to n-i-p a-Si:H diodes to characterize the mid-gap defect densities in the i layer. Our results show that there are no significant changes in the drive-level densities, Ndl, in n-i-p diodes and p+-i-m as well as n-i-m Schottky diodes, which indicates that DLCP can directly provide reliable energy distribution and spatial distribution of the mid-gap defects in the n-i-p device. We have found that the ratio of Ndl to ND*, the ionized defect density determined by hole onset measurement, is changed with the deposition conditions; it is 3 for standard samples, 2 for helium diluted samples and 6 for hydrogen diluted samples. These results indicate that there may be different defect distributions in these materials, which suggest the ratio of charged (D-) density to the neutral defect (D0) density may be altered when growth conditions are varied.

High deposition rate (up to 5 nm/s) a-Si:H films suitable for recrystallization were deposited using a λ/4 helical resonator source. Refractive index, Tauc-gap, photo- and dark conductivities were measured for film characterization. The metastable behaviour was characterized by the light- induced degradation of the photoconductivity. Hydrogen content and bonding configuration were analyzed by IR absorption and mass separated thermal effusion transients, film microstructure was studied by intentionally incorporating carbon and oxygen. Most of the hydrogen is located on internal surfaces in the otherwise dense material. Differences between the deposition from our highly excited plasma and the conventional remote PECVD process are discussed.

We present a dc saddle-field glow-discharge deposition procedure which combines the positive attributes of the conventional dc and rf glow-discharge techniques. Preliminary mass spectra analyses of both silane and methane glow-discharges demonstrates that ions constitute a significant fraction of the species reaching the film surface. Growth rate analyses suggest that ions play a significant role in the saddle-field glow-discharge deposition of amorphous semiconducting films.