Light-sensitive hydrogenated amorphous silicon pixel arrays are now under development for real-time megavoltage and diagnostic fluoroscopic imaging. Such applications place stringent demands upon a variety of array properties including the uniformity of the light-response function of the pixels. It is desirable that the design and fabrication of these imaging arrays maximize such uniformity. The implications of uniformity for imaging are reviewed, and data obtained from small arrays are presented and discussed.

Two-dimensional a-Si photodiode arrays with the a-Si TFT readout scheme were fabricated. Some performance characteristics were investigated for detecting visible light, X-ray and γ ray. Arrays of 64×40 1mm2-pixels have good uniformity with ± 1 % in linearity variation, ± 10 % dark current variation and ± 5 % sensitivity variation. The cross-talk between adjacent pixels was found to be negligible so that the intrinsic spatial resolution was determined entirely by the pixel size. Due to the high resistivity of a-Si photodiodes and TFTs, charges generated by pulsed light illumination were retained in photodiodes for a long period of time. This enables the accumulation-mode operation for detecting low-intensity light, where only one image is obtained over a long integration time. An encapsulated Co-57 source (3 μ.Ci) was successfully imaged with 5 sec integration. Alternatively, the array can be operated by the cine-mode for detecting high-intensity light, where a number of images are obtained successively.

Measurements of the optical absorption spectrum of doped and undoped hydrogenated amorphous silicon (a-Si:H) for photon energies ranging from 0.8 to 2.5 eV are obtained using photo-pyroelectric spectroscopy (PPES). This technique, based upon the pyroelectric effect, has a simpler experimental set-up than photo-thermal deflection spectroscopy, and presently has a sensitivity of ad > 10-3, where d is the sample thickness. In addition, using PPES we have measured the non-radiative quantum efficiency in a-Si:H, and find that it has a strong wavelength dependence for sub-bandgap illumination.

A series of boron doped a-Si:H films have been characterized by PDS, FTIR, Raman, and SIMS in order to evaluate the effects of boron incorporation on structural properties and hydrogen bonding. Doping by B2H6 or B(CH3)3 does not significantly enhance the overall disorder of the silicon network showing up in the TO-like Raman halfwidth whereas remarkable changes in local, defect related structures are evident from PDS. An analysis of the data suggests two bands of defects in the pseudogap at low boron concentration and only one band for higher concentration. To account for Fermi level positions, shifts of the hole transport path well into the valence band tail upon doping must be invoked.

We derive a closed form expression for differential junction capacitance applicable when the density of states (DOS) varies exponentially with energy. Using this expression, we analyze p/n junction capacitance measurements that probe the DOS in boron doped hydrogenated amorphous silicon and silicon carbide alloy. In both materials we find that the p-layer DOS is described by an exponential increase with energy above the Fermi level.

Time-of-flight and charge collection experiments performed on a-Si:H p-i-n structures in the temperature range 40–300K reveal electron drift mobilities which continuously decrease with decreasing temperature and electric field. At 40K, fields ≥ 2×105 V/cm are necessary to collect almost all carriers. A discussion of the data on the basis of a nearest neighbour hopping model shows the significant influence of the applied field on the hopping down process of electrons. Field-induced re-emission of deep trapped carriers back into shallow tail states establishes a narrow band of energy levels where electrons accumulate and propagate. The band width and energy position of the accumulation levels in the tail depends strongly on the applied field and the distribution of localized states. With increasing electric field, it shifts up in energy closer towards the conduction band edge and changes the hopping mobility for orders of magnitude. The calculated and experimental data are in reasonable agreement with the model of a hybrid distribution of localized states.

Depletion-width-modulated (DWM) ESR spectroscopy and junction capacitance techniques were employed to investigate deep-defect states in various metastable states of 10 and 80 Vppm PH3-doped a-Si:H. From the joint application of these methods, we enumerate the change in total spin and charge associated with emission of carriers from states within the mobility gap resulting from a modulated bias voltage. We conclude that the magnitude of the modulated spin-to-charge ratio is nearly unity, consistent with earlier similar investigations of n-type a-Si:H, where the D-/D0 transition was shown to dominate. Furthermore, we present results of a previously unreported “quadrature” DWM-ESR signal of large magnitude under a wide range of experimental conditions. Similar related effects are also reported from spin- and capacitance-transient studies. These results clearly demonstrate that the changes in spin significandy lag corresponding changes in charge.

The photomodulation spectra of a-Ge:H of average photoelectronic quality(ημπ = 1 × 10-10cm2/V) and of improved quality (ημπ = 3 × 10-7cm2/V), produced under different plasma conditions in an r.f. diode reactor by glow discharge, were measured at 80K and are analyzed in analogy with earlier studies of a-Si:H. The spectra of the poorer material are dominated by transitions between dangling bond states and the conduction and valence bands. By contrast, the spectra of the better material require contributions of transitions from the band tail states, indicating that the reduced defect density has resulted in pump-beam induced quasi-Fermi levels reaching near the conduction and valence band edges. A very acceptable fit between plausible density-of-states distributions and the experimental spectra has been found.

We have systematically investigated the formation of electronic states in the region of the conduction and valence band edges of a Si as functions of variations in the bond angle distributions. Local Density of States (LDOS) for Si atoms in disordered environments have been calculated using the cluster Bethe lattice method with a tight-binding Hamiltonian containing both first and second nearest neighbor interaction terms. LDOS for atoms with bond angle dis ortions in the nearest neighbor and second neighbor shells are compared and contrasted, both showing an influence on the LDOS near the gap. We also consider the role of the second neighbor term in the Hamiltonian by comparing the DOS for a distoned infinite Bethe lattice using Hamiltonians with and without the second neighbor interactions. It is found that in this case the second neighbor interaction terms cause greater conduction band tailing than using the nearest neighbor interaction terms alone.

The validity of determining the defect density in a-Si:H and its alloys by integrating the sub-bandgap optical absorption coefficient is examined. It is shown that a formula derived for estimating Si-H bond density has been incorrectly applied to defect related absorption. In addition, the complex defect absorption processes have been oversimplified, by assumption, to the case of a single type of transition. Further, in practice, the defect absorption spectrum can not be unambiguously obtained at energies near and above the low end of the optical absorption edge. It is concluded that die integration method for quantitative defect density determination should be viewed at best as an empirical practice with considerable arbitrariness.

We have presented ESR and PAS measurements for a series of a-SiS:H and a-Si: H films deposited by glow discharge at different parameters. The spin density in a-SiS:H alloys measured by ESR is essentially independent of the sulphur content, while the density of defects measured by PAS increases significantly with the increasing of sulphur content. The ESR signals in a-SiS:H alloys strongly depend on both annealing and illumination. The spin density increases up to 540°C and then decreases with raising annealing temperature for a-SiS:H and a-Si:H alloys. The results suggest that some new defects such as molecular hydrogen and microvoids are appeared when addition of sulphur to a-Si:H films.

This is the first NMR study of a-Si:H which measures the 29Si spin-lattice relaxation time, T1. Measurements of T1 are useful in learning about the silicon network structure and the localized states within the mobility gap. Dangling-bonds are randomly distributed in device quality materials but are inhomogeneously distributed in nondevice quality materials. There are two simultaneously occurring dangling-bond spin-lattice relaxation mechanisms: one is through the spin-orbit coupling modulated by thermal excitation of “two-level systems”, and the other is through hopping conduction between localized states near the Fermi level.

The dependence of the spin density upon temperature and charge depletion is calculated based on the standard defect model in a-Si:H of a D-center with positive, neutral, and negative charge states. The results are compared with recent measurements of depletion width modulated spin densities and temperature-dependent spin densities. It is shown that the initial charge density assumed for the defect system substantially affects conclusions regarding electronic correlation energies drawn from the measurements.

We report the determination of the distribution of occupied defect states in hydrogenated amorphous silicon from the deconvolution of constant photocurrent measurement (CPM) spectra, and the modelling of the distribution with the defect pool.

The CPM spectra were taken on undoped a-Si:H samples either in their as-grown state, in the annealed state, after quenching from high temperature, or after light-soaking. The spectra were deconvoluted to account for transitions from deep levels and from valence band tail states to a conduction band assumed to have a sharp edge. As-grown or annealed-state samples show a peak at 1.0 eV (0.2 eV FWHM) below the conduction band edge. Assuming a mobility gap of 1.9 eV, this peak lies 0.9 eV above the valence band edge. We ascribe this peak to the Do/+ transition. CPM spectra of light-soaked and thermally quenched samples show shifts in the peak position and increases in die peak height in accordance with the defect pool model. The model calculations agree with the CPM results, so that the applicability of CPM spectral analysis to obtaining detailed values of defect pool parameters is demonstrated.

High quality a-Si:H films are deposited by d.c. magnetron reactive sputtering of a Si target in an (Ar + H2) plasma. This paper reports the first comprehensive understanding of the growth process. The incident flux, surface H coverage, H2 release, and bulk H incorporation are determined using four in situ, real time techniques: double modulation mass spectroscopy, isotope replacement experiments, reflection absorption infra-red spectroscopy, and spectroscopic ellipsometry. In addition, the sputtered particle transport is simulated using Monte-Carlo techniques. For conditions which produce electronic quality a-Si:H, the total H flux arriving at the surface varies between 0.5 – 2 times the depositing Si flux; approximately half of this flux appears to reflect from the surface without interaction. The growth surface has excess H varying between 0.5 – 2 × 1015/cm2, and this surface H coverage is uniquely related to the bulk H incorporation.

The growing aSi:H based component industry needs large PECVD machines for high throughput coating of flat substrates up to 1 meter in size. Deposition uniformity can be affected by gas depletion in diffusion and laminar gas flow geometries. Deposition rate is shown to be in conflict with large size uniformity. Main uniformity problems are found to originate from uneven RF voltage distribution, the effect is shown to be amplified near plasma equilibrium transitions. Contamination, a key factor for production quality, is controlled by the hot pressurized plasma box by both reducing unwanted background gas near UHV level and by allowing systematic fast plasma cleaning without machine corrosion or post contamination. Powder formation and dynamic is discussed based on several novel experiments and powder formation threshold is shown to be pushed away if the plasma is made intermittent.

A novel preparation technique termed “Chemical Annealing (CA)” was developed with aim of making a stable and rigid structure of Si-network. The a-Si:H films were made by the alternate deposition of several tens angstrom thick a-Si:H and the treatment with atomic hydrogen or excited novel gases such as Ar* and He*. Hydrogen contents (CH) and optical gap (Eg) in the film prepared by this tecnique were able to reduced by CH of 1.5at%, and Eg of 1.5eV, respectively at substrate temperature:300C. All of them exhibited high photoconductivities in the level of 10-5 10-4 S/cm under illumination of 100mW/cm2. In the films with CH of 3at% or less, in particular, marked improvement was confirmed in the stability after light soaking. In addition, the time-of-flight measurement revealed a non-dispersive transport and a significant enhancement in the drift mobility of holes up to 0.2cmm2/Vs at room temperature in the film with CH : 5at% and Eg:1.65eV prepared at 300C. Advantages of the CA process are summarized together with the discussion of role of atomic hydrogen, excited novel gases such as Ar* and He* in the growing surface.

To understand the deposition process of pulse discharge (PD) CVD, we studied spatial distribution and decay of plasma parameters and those of SiH LIF signals. In addition, we studied film properties of a-SiC:H and a-SiGe:H deposited by the PD-CVD. It was found that the quality of p-type a-SiC:H films deposited by the PD-CVD from SiH4+CH4+B3 was better than those of conventional p-type a-SiC:H films. The p-type a-SiC:H films were applied to the p-layer of p-i-n a-Si solar cells, and a conversion efficiency of 12. 3% was obtained for a 1cm2.

Amorphous silicon of a high electronic quality with Odatk 10-10, σPh ≈ 1·10-4 (Ωcm)-3 and density of gap states of about 0.3 to 2.1016 eV-1cm-3 is deposited at rates up to 17 Å/sec. The rate limiting steps are identified and their control via the plasma parameters is explained. This allows one to establish the fundamental scaling parameters for large-area high-rate deposition of a-Si. Long term stability of the films in terms of postoxidation and photode-gradation is briefly addressed.

We report measurements of the Urbach edge, optical bandgap, and ambipolar diffusion length on a series of hydrogenated amorphous silicon (a-Si:H) films deposited by hot-wire-assisted chemical vapor deposition (HW). We compare the properties of these films to those of a series of a-Si:H films deposited by the traditional radio frequency (rf) glow discharge (GD) technique, where we varied the substrate temperature to change the bonded H content (CH). We show for the first time that, as CH is decreased below the value traditionally associated with device quality GD a-Si:H (∼10 at.%), the electronic properties of the GD films deteriorate in the traditional manner while those for the HW samples remain device quality. Properties of these low CH HW samples will be presented and compared to those of GD films containing comparable CH. Because several indications exist that the structure of the HW films is different than that of the GD films, Raman and Small Angle X-Ray Scattering (SAXS) measurements are presented to illustrate structural differences.