The photo-field effect has been measured between 4.2K and 300K using an a-Si thin film transistor. The data are fit self-consistently taking full account of changes in space charge, photo carrier drift, diffusion and recombination. Gap state density and capture cross sections are fitting parameters. Evidence for tunnel recombination is found. Electron localization radius was found to be 13Å.

Measurements of coplanar conductance fluctuations in undoped a-Si:H are described. Statistical tests show that the 1/f noise is non-Gaussian and has a power-law frequency dependent second spectrum, as observed in n-type a-Si:H. By careful consideration of the thermal history of the sample, the noise statistics are found to be different above and below the equilibration temperature, which has been associated with hydrogen diffusion. These results suggest that the non-Gaussian noise in a-Si:H is influenced by the motion of bonded hydrogen and is not significantly dependent upon doping.

The physical origin of the blue shift of the optical Taue bandgap of compositional multilayers of a-Si:H and its alloys with Ge, C or N is analysed. It is shown that for multilayers with bandgap differences up to 0.7 eV between well and barrier Material, the effect can be understood completely classical in terms of the effective medium approximation, when the mean composition of the multilayer films is not kept constant. For the variation of both the electronic well and barrier widths a shift of the Taue bandgap is theoretically expected. It is of the same order of magnitude as the effect which is experimentally observed. Interfacial transition layers can account for a blue shift also for higher bandgap differences, as usually the case in a-Si:H/a-SiNx:H Multilayers, and when the mean composition of the multilayers is kept constant.

An analytic model is developed for differential charge measurements which avoids the use of the depletion approximation. In this experiment the incremental charge in the device is calculated as the difference between the total steady-state junction charge at two nearly equal voltages, similar to a very low frequency capacitance Measurement. All of the calculations, including an iteration to find the bulk Fermi level, can easily be performed using a spreadsheet program on a personal computer. It is shown that the results of these calculations agree extremely well with simulations performed with a complete numerical device simulator. Thus, the model useful for the analysis of experimental data

Steady-state photoconductivity, sub-bandgap absorption and electron spin resonance (ESR) Measurements were carried out on annealed and light soaked intrinsic hydrogenated Amorphous silicon (a-Si:H) films. The experimental results were modeled using detailed numerical Model. The defect densities derived from the sub-bandgap absorption in the light soaked films were correlated with the ESR spin densities. Selfconsistent fitting of the data was obtained using a gap state distribution which consists of positively charged defect states above, negatively charged defect states below and neutral defect states at about Midgap. Both the annealed and the light degraded states are modeled using the same distribution of gap states whose densities increase upon light soaking with a slight increase in the ratio of the neutral to charged defect densities. These results on intrinsic a-Si:H are consistent with those of charged defect Models.

Results from numerical calculations with a recombination model involving one class of correlated dangling-bond states and exponential band tails, in a-Si:H, are reported. Fermi level, light intensity and temperature dependences of the μτ products are studied. The results are consistent with experimental data. It is found that photo-enhancement of (μτ)e, or superlinear photoconductivity, as well as thermal quenching, are associated with a capture cross section of the band tails smaller than the capture cross sections of the dangling-bond states.

A comparative study of the deposition temperature (Ts) dependence of the Mobility-lifetime (μτ) products of the charge carriers in glow-discharge and rf sputter-deposited a-Si:H is described and discussed. The Ts-dependence of the μτ's the majority carrier light-intensity exponents of the two types of films are strikingly similar. These observations lead to the conclusion that the structure of the recombination levels as well as the recombination processes are in accord with the “defect pool” Model, in contrast to previous suggestions. The differences between the two types of films thus appear to be limited to the differences in the concentrations of dangling bonds.

We used the transient infrared photoconductivity to measure the rise and fall of the non-equilibrium photocarrier concentration n at helium temperatures and the conditions for recombination. The growth rate of n was found to be only a few percent of the electron-hole pair generation rate G in agreement with LESR data of Street and Biegelsen. The saturation value of n is about 1017cm−3 and nearly independent of G. After turning off the generation light, the residual n requires an hour to decrease by a factor 2 but it can be made to recombine in seconds by infrared light.

Fast transient photocurrent measurements have been performed on a-Si:H based pin diodes with thicknesses down to 0.5 μm. On a thick sample drift mobilities derived in either the small-signal or the space-charge limited current mode are compared. On real solar cell devices in SCL voltage (integral) Mode the electron lifetimes τ are determined from the extraction time of the photo-generated charge in dependence on the degradation state of the sample. A clear and reproducible correlation between lifetime and degradation state has experimentally been established. Lifetimes τ, for which an intensity dependence has been observed in the SCL voltage Mode, are compared to the μτ-product from a small-signal charge collection Measurement. The influence of the real absorption profile on the measurements is discussed.

The space-charge-limited current mode in the post-transit time-of-flight method was used to determine the density of states (DOS) in the band gap of a-Si:H. SCLC conditions result in an enhancement of the occupancy of deep states, leading consequently to an improved sensitivity. In comparison with the small signal case, the emission time of deep traps changes as a function of the time dependent occupancy of traps. The change in the emission time of traps is explained by consideration of the deep defect relaxation concept.

The low temperature thermally stimulated conductivity (TSC) in a-Si:H film has been investigated in the temperature range from 20 to 150 K. Unlike the results of the high temperature TSC, the low temperature peak value and position (Tm) of TSC do not depend on the starting temperature T0 at low temperatures. This new phenomenon can not be explained by TSC theory [1]. Based on the multiple trapping (MT) Model, TSC theory limits its application to the intermediate and high temperature range. In this paper, a model of the hopping conduction with the transport energy Et in the band tail is proposed to understand the behavior of the low temperature TSC in which the thermal emission energy Em does not depend on the starting temperature of TSC.

Junction capacitance measurements were employed to study the thermal emission of electrons after application of a voltage filling pulse on a 80 Vppm PH3 doped a-Si:H sample on p+ c-Si substrate. We show that these data can be explained in terms of the relaxation Model. In addition, the time dependence of the charge flow into the depletion region during the filling pulse is investigated by current transient Measurements. Finally, we present charge transient data for a 9 Vppm a-Si:H sample on n+ c-Si substrate and compare the results to those obtained on samples on p-type substrates.

Multilayer systems (MLS) consisting of hydrogenated Amorphous silicon (a-Si:II) and a transition Metal (Mo, Ti) were prepared by alternating dual magnetron sputtering. In this study we investigate the electrical performance of multilayer films containing discontinuous metal sublayers separated by a-Si:H sublayers of 80 nm thickness. A voltage of typical 8 V applied to the coplanar electrodes switches the samples from a semiconducting to a metallic state. The switching effect, which is reversible by thermal annealing, is accompanied by chaotic current oscillations. The temperature dependence of the conductivity in the metallic state is described by Inσ α T−1/2. The asymmetric illumination of the interelectrode spacing in coplanar configuration resulted in a generation of a lateral photovoltage.

We have studied the effect of ion doping on the electrical properties for atmospheric pressure chemical vapor deposition (APCVD) Amorphous silicon (a-Si) films. The room temperature conductivities after ion doping at optimum doping temperatures for n- and p-type a-Si films were found to be > 10−2 and > 10−4 S/cm, respectively. The unintentional hydrogen incorporation into a-Si during ion doping enhances the quality of ion doped APCVD a-Si as compared to that of plasma enhanced CVD (PECVD) a-S.i.H. We obtained the field effect mobility of > 1 cm2/Vs for APCVD a-Si TFT using ion doped n+-layer.

A novel technique has been proposed for selective and in-situ excimer-laser crystallization and doping to thin poly-Si films. Dopant atoms are supplied, during the Si laser crystallization process, to the Si film on glass from the doped SOG (spin-on-glass) film coated on the top. Conductivity of the processed film was increased to more than 10S/cm from about 10−8S/cm of the starting film. This technique has been applied to the bottom gate Amorphous-Si TFTs with self-aligned poly-Si source and drain. The electron field-effect mobility was 1.0cm2/Vs and the on/off current ratio was more than 106. No parasitic effects were observed, and the hole conduction was effectively suppressed. This in-situ crystallization and doping technique can also be applied to the top gate a-Si TFT process.

The Very High Frequency (70 MHz) PECVD has recently proven its ability to produce Amorphous silicon with high deposition rates (10 Å/s) without affecting the quality of the Material. A comparative study of the optoelectronic properties of undoped silicon carbon alloys produced by Very High Frequency and by conventional RF (13.56 MHz) is carried out. Conductivity, infrared absorption, optical transmission and deep defect densities via PDS and ESR have been Measured. Deposition parameters under study aie methane fraction and hydrogen dilution. In contrast to conventional PECVD, we observe for VHF depositions an increase of the deposition rate with the addition of Methane. Larger energy gap and smaller Urbach energy values seem to indicate a better incorporation of carbon in the VHF case. A study of the degradation induced by light is also presented.

The film properties and solar cell performance of a-SiGe:H samples with the same optical gap and different combinations of hydrogen content (CH) and germanium content (CGe) have been compared. The optimum composition for the initial properties, such as the tail characteristic energy, defect density and conversion efficiency of the solar cell, was determined, and the differences could be explained by the difference in H bonding configuration. The degradation ratio of the conversion efficiency becomes larger in higher CH samples. This suggests that hydrogen or Si-H2 participates in light-induced degradation. As a result, the optimum CH for an efficient solar cell is believed to shift to the lower CH region after light soaking. Based on these findings, the stabilized conversion efficiency of 3.3% under red light (γ>650nm) for an a-SiGe:H single-junction solar cell (1cm2) and 10.6% under lsun light for an a-Si/a-SiGe double-junction stacked solar cell (1cm2) have been achieved. The degradation ratio is only 8.6% for the double-junction solar cell.

We have carried out a detailed study of the energy distribution of deep defects for high quality glow discharge a-Si,Ge:H alloys using both thermal emission and optical Methods: drive-level capacitance profiling, transient photocapacitance and photocurrent plus modulation photocurrent spectroscopy. Four distinct bands of transitions involving defect states have been identified: two associated with thermal transitions, and the other two related to optical transitions. We have, for the first time, observed a negative signal in the photocapacitance spectra at photon energies near 1.2eV. This striking aspect verifies the presence of a distinct defect band above Ep from which electron thermal emission is greatly suppressed. Our Measurements also disclose a fairly narrow defect band below the Fermi level which contrasts with the defect properties observed in a-Si:H. Time resolved photocapacitance spectra indicate that this defect band exhibits configuration relaxation.

Time resolved mass spectrometry and the measurement of appearance potentials of various GeH -cationic fragments combined with selfconsistent theoretical modelling have been used to study the mechanism of plasma induced deposition of germanium. It is shown, that the high rate deposition occurs via germylene, GeH as the direct precursor. Digermane May play some role only at low deposition rates. Although we could reproduce the best values of theμτ product reported so far, no further improvement could be achieved.

We have prepared a series of hydrogenated Amorphous silicon-carbon alloys a-Si1-xCx:H in conditions where the power delivered to the plasma is below the threshold of primary decomposition of CH4 (“low-power regime”). Carbon is therefore incorporated into the growing film in the form of methyl groups -CH3, thus preserving the sp3 hybridization in the solid. Since the carbon is deposited only by chemical reaction with the radicals produced by the decomposition of SiH4, its concentration in the alloys cannot exceed the value x=0.5.

The variations of the optical gap and of the “average gap” with x, as measured by optical transmission, indicate as expected the predominantly tetrahedral coordination of the carbon, with a record value of EQ4=3.6 eV for x=0.42 (3.95 eV if corrected for the Urbach-tail absorption). Since the Si-C network does not relax to a more stable lower sp2 coordination, the material is highly strained, with an Urbach tail larger than that of the standard Material.