We investigate the formation of defect states in the gap of a-Si arising from deviations from the ideal tetrahedral bond angles. The local density of states for Si atoms in disordered environments is calculated using tight-binding parameters for the cluster-Bethe lattice method. The Hamiltonian for a-Si with bond angle distortions is taken as an average over many configurations associated with a random choice of bond angles, weighted by Gaussian distributions with standard deviations between 2°.and 10°. Bond angle deviations in this range generate a density of defect states at the valence band edge that: 1) increases as the average bond angle deviation increases; and 2) is significantly larger than the density of band tail states generated at the conduction band edge. We obtain a shift of the absorption edge from the joint density of states (DOS) as a function of bond angle deviations. In addition, a calculation of the DOS for a distorted tetrahedral cluster embedded in an idealized Bethe lattice yields a threshold bond angle distortion of ±20° for the appearance of a discrete state in the gap near the valence band edge.

Based upon the thermal-equilibration theory and the annealing of defects introduced at the growing surface of amorphous silicon, the distribution of defect-states in energy and in space is calculated for a comprehensive set of deposition and post-deposition treatment parameters. We include the growth temperature, growth rate, illumination time, illumination intensity, annealing temperature, and annealing time. We compare the theoretical results with experimental data for the dark-conductivity-activation energy and for the fill factor of a solar cell. Agreement between the modelled and the experimental results reconfirms the validity of the assumptions made, and encourages further application of the model to analyses of a wide variety of amorphous silicon devices.

The drift of electrons generated by a 3 ns illumination impulse was studied in several undoped a-Si:H specimens by using transient photocharge measurements from 10 ns to 50 sec. The transients show four stages of evolution: band transport, deep-trapping and subsequent re-emission, and recombination. From the temperature dependence of the emission time we estimated the mean energy of the trap distribution and the attempt-to-escape frequency associated with this trap to be 0.6 eV below the conduction band and 5×1012 sec−1 respectively; this trap has been identified in previous work with the D-center found in electron spin resonance. In addition the data resolve the two orders of magnitute discrepancy between mobility-lifetime estimates from earlier charge-collection experiments and from steady-state photoconductivity.

A general photoconductivity formula is derived for the case of a semiconductor steady state photocarrier grating (SSPG)1. It is shown that, under the condition of weak applied electric field, the ambipolar diffusion length can be determined by the SSPG technique1without the lifetime-regime restriction2,3 if the lifetime of photocarriers is known. The general formula presented here is reduced to the simple lifetime-regime formula1–3 under the condition of fast dielectric relaxation.

The transient forward bias current of a-SI:H p-i-n diodes has been measured as a function of temperature and voltage. The transient shows an initial decay followed by a substantial rise which suggests a large value for the ratio of the carrier lifetime to the transit time as demonstrated by Shapiro. We found that both the time at which the rise begins and the ratio of the final saturated current to the minimum current increase with decreasing temperature for temperature greater than a characteristic temperature, Tc. For T < Tc, both the time and the ratio abruptly decrease with temperature. We attribute this change in temperature dependence to a change in the transport kinetics from ballistic to hopping controlled recombination.

Thermally stimulated current (TSC) and Time of flight (TOF) experiments have been performed on the same set of Pt/a-SiGe:H Schottky diodes in order to compare the band tail characteristics obtained from the two methods. For the silicon rich alloys, good agreement is found in the values of the band tail width. However, the values of the attempt-to-escape frequencies obtained from TSC are typically orders of magnitude smaller than those obtained from TOF. To analyse the origin of this discrepancy we propose another kind of TSC experiment denoted Pulsed TSC: short voltage pulses are applied to the sample while it is heated and the resulting current is monitored. Preliminary results are presented and discussed .

Spin dependent photoconductivity measurements provide evidence for the existence of tunneling transitions from localized tail states into the dangling bond recombination centers. These processes can be included approximately into the conventional model based on rate equations. Satisfactory values for the activation energy of the photoconductivity at low temperatures are obtained. Our model produces thermal quenching and an enhancement of the photoconductivity with rising dark Fermi level.

Amorphous silicon p-i-n diodes with intrinsic layer thickness exceeding 50 microns have been produced. As a probe of the carrier collection, reverse bias collection efficiency measurements using weakly absorbed light were performed. The results have shown for the first time that amorphous silicon diodes of such thickness can be fully depleted. By analyzing the shape of the collection efficiency curve, profiles of the light induced space charge within the intrinsic layer are obtained. The space charge density in thick devices is found to vary strongly with position in the intrinsic layer as well as with excitation intensity.

The effect of the depletion region in amorphous silicon Schottky diodes on the mobility-deep trapping lifetime product μτ of electrons has been investigated using transient photoconductivity techniques. We varied the laser wavelength of the exciting laser in time-of-flight (TOF) measurements. The results are correlated with measurements of the internal field by means of the charge collection efficiency, and with measurement of the deep-trapping time by the Delayed-Field TOF technique (DFTOF). We find that the electron μτ decreases from the bulk toward the top surface.

a-Si:H/a-Si1−XCX:H single and double barrier structures imbedded at different locations of the i layer of a p-i-n structure were used to study the carrier transport properties of amorphous QW. The shapes of the observed I-V curves of both structures are similar. The bound states of the well layer of double barrier structures do not play a role in the carrier tunneling process. The resonant tunneling time is larger than the inelastic scattering time by 5 to 12 orders of magnitude. When the tunneling process of amorphous QW is in the completely incoherent tunneling regime, the peak-to-valley ratio of NDR and the density of states in the well go to zero. From the analysis of I-V characteristics of both structures under different monochromatic light illumination, the observed current dips are due to the collection of photogenerated carriers in two regions of the i layer separated by the imbedded barrier structure.

Electrical noise measurements have been performed on amorphous semiconductor films under irradiation with monochromatic light at different wavelengths. The Power Spectrum Densities (PSDs) of highly photoconductive undoped a-Si:H and a-SiC:H (deposited by glow discharge) in the frequency range .1 – 20 kHz are obtained at room temperature. The analysis is performed upon metal/semiconductor/ITO Schottky barriers. The PSDs vs frequency follow l/fn law and show a change of magnitude when the wavelength of the incident light is varied around the value corresponding to the energy gap.

We present experimental results showing that the ON state of amorphous silicon memory structures exhibits ballistic electron transport associated with a quantised resistance, h/2ie2, where i is the number of occupied one dimensional conducting channels (sub-bands) and the spin degeneracy is two (in the case when no magnetic field is applied). Conduction in the memory ON state is restricted to a narrow conducting channel through which the electrons can travel ballistically i.e. no collisions occur. As the applied voltage is increased, the width of the conducting channel is broadened. This results in additional conducting channels (sub-bands) passing through the Fermi energy and consequently the resistance drops by quantised values. In the presence of a magnetic field additional steps occur corresponding to the split levels at values of h/2(i + ½)e2. A particular feature of this quantised resistance is that the effect can be observed at relatively high temperatures effect can be observed at relatively high temperatures (from 4.2 K up to ∼190K).

We have developed a means to speed up the recovery of both the threshold voltage shift of hydrogenated amorphous silicon (a-Si:H) transistors and the Vx shift of high voltage a-Si devices. This is accomplished by placing a lightly doped compensated layer adjacent to the active layer in these transistors. This proximity recovery layer does not alter the initial characteristics of a-Si:H transistors and is completely process compatible with standard fabrication procedures.

The effect of Fermi level position on the annealing of defects created by electron accumulation is investigated. Results indicate that holes accelerate the annealing of defects created by electrons confirming the theoretical understanding of the proximity compensated layer. Capacitance-voltage curves indicate that hole accumulation tends to create defect states located closer to the conduction band than electron accumulation–a result in agreement with energetics of defect formation.

Amorphous-silicon thin-film transistors (a-Si TFTs) have been fabricated by using an a-Si layer deposited by low-temperature thermal-CVD method using higher silanes. The TFT with thermally grown SiO2 gate was operated in both the n-channel mode and the p-channel mode. The maximum field-effect mobility and typical on/off current ratio were 1.5 cm2/Vs and 107 for the n-channel operation, and 0.2 cm2/Vs and 106 for the p-channel operation, respectively. Free electron mobility in the conduction band and free hole mobility in the valence band have been estimated, using the temperature dependence of the field-effect mobilities. It was found that the hole mobility is as high as the electron mobility.

Hydrogenated amorphous silicon (a-Si:H) thin-film-transistors (TFT’s) and logic circuits have been fabricated using a simple 3-mask process. Inverted staggered TFT structures were prepared in a multichamber UHV-compatible system that provides: 1) sequential deposition of dielectrics, and intrinsic and doped a-Si:H by remote plasma-enhanced chemical-vapor deposition (Remote PECVD) in a single deposition chamber; 2) in-situ surface analysis by Auger Electron Spectroscopy (AES); and 3) dry etching of deposited thin films by Remote plasma-enhanced etching, Remote PEE. A field-effect electron mobility of 0.74 cm2/V-s, and a threshold voltage of 2.5 Volts have been measured at room temperature in the TFTs. The on/off current ratio of the TFT exceeds 106. The a-Si:H TFT logic circuits, 2-transistor inverters and addressable 6-transistor static memory cells, operate at a supply voltage of as low as 8 Volts. The experimental results indicate that high performance a-Si:H thin film devices can be fabricated by sequential Remote plasma processing in a multichamber integrated system.

Effects of residual phosphorus in the channel region of amorphous silicon thin film transistors(a-Si TFTs) on the TFT characteristics were quantitatively investigated. Concentration and the depth profile of the residual phosphorus were measured by high resolution secondary ion mass spectroscopy(SIMS). The OFF characteristics of a-Si TFTs were also measured.

The SIMS data showed that the phosphorus exists about 100nm deep into intrinsic a-Si(i-a-Si), but the OFF characteristics showed that the activity of the residual phosphorus is 4 order of magnitude lower than that of heavily phosphorus doped a-Si(+-a-Si). The residual phosphorus is found to be inactive and stable, and has little effect on a-Si TFT characteristics.

These results enabled us to fabricate inverted staggered a-Si TFTs by the simplest process using only 2 photo-mask steps and 1 self-aligned exposure.

Through the time and temperature dependence measurements on threshold voltage shifts (Δ VT) in amorphous silicon thin film transistors, it has been found that two separate instability mechanisms exist; within short stress time ranges Δ Vτ increases as log t and this behavior corresponds to charge trapping in SiN. On the other hand, in long stress time ranges Δ VT increases as t t/4 and can be explained by time-dependent creation of trap in a-Si.

We measured the equivalent noise charge of a-Si:H pin diodes (5 ∼ 45 μm i-layer) with a pulse shaping time of 2.5 μ.sec under reverse biases up to 30 V/μm and analyzed it as a four component noise source. The frequency spectra of 1/f noise in the soft-breakdown region and of the Nyquist noise from contact resistance of diodes were measured. Using the conversion equations for a CR-RC shaper, we identified the contact resistance noise and the 1/f noise as the main noise sources in the low bias and high bias regions respectively. The 1/f noise of a-Si:H TFTs with channel length of 15 μm was measured to be the dominant component up to ∼100kHz for both saturation and linear regions.

The capability of tailoring the field profile in reverse-biased a-Si:H diodes by doping and/or manipulating electrode shapes opens a way to many interesting device structures. Charge collection in a-Si:H radiation detectors is improved for high LET particle detection by inserting thin doped layers into the i-layer of the usual p-i-n diode. This buried p-i-n structure enables us to apply higher reverse-bias and the electric field is enhanced in the mid i-layer. Field profiles of the new structures are calculated and the improved charge collection process is discussed. Also discussed is the possibility of field profile tailoring by utilizing the fixed space charges in i-layers and/or manipulating electrode shapes of the reverse-biased p-i-n diodes.