The reliability of the a-Si drum has been investigated via environmental tests and durability copy testing, and its electrophotographic properties have been confirmed to be very stable. Regarding imaging properties, a-SiC surface layer plays an important part suppressing the noise within the image. It has been discovered that the blurring of the image in high humidity after many copying cycles is caused by the deterioration of the surface layer which occurs through exposure to corona discharge. This blurring is eliminated by preventing absorption of moisture at the surface, such as heating the drum. In this manner, it has been observed that a-Si drum maintains a high quality image up to 1,000,000 copy sheets.

New types of multilayered photoreceptors using photosensitive a-SiC:H and a-Si:H have been investigated. The a-Si1-xCx:H was deposited at high rate of 1–6 µm/h from SiH4-C2H2 by a plasma CVD and used as a carrier generation layer (CGL) or a carrier transport layer (CTL). The type with the a-SiC:H(x=0.1) as CGL for positive charging shows higher sensitivity at λ<700 nm than the a-Si:H photoreceptors, reflecting the higher photosensitivity and wider band gap of the a-SiC:H. Furthermore, the negative charging type using the a-SiC:H(x=0.1) as CTL shows excellent sensitivity over full visible rangq compared with other positive charging types, because the transport ability of electron is superior to that of hole in both a-Si:H and a-SiC:H.

To solve an image degradation problem of blurring for a-Si:H photoreceptors during the repetitions of electrophotographic process, effect of a-C:H:F overcoat layer has been investigated. The film is deposited by glow discharge decomposition of C2 F6 and H2 mixture gases. The hydrophobic property of the film is estimated by contact angle of water drop. The contact angle of a-C:H:F is larger than that of a-SiC:H especially after 5 hours corona exposure treatment. In the printing test, the blurring life time of a-C:H:F overcoated a-Si:H photoreceptor becomes longer about 9 times than that of a-SiC:H overcoated one.

The electroluminescent characteristics of multilayer structures involving Si C as the light emitting material are reported. The structures studieA have two types of multilayer stacks a) Si rich Si02/ Si02/- Six Cl v and b) Si rich SiO2 /Six C1-x, Both type of structures were deposite on an n-type silicon surstrt-e and conductive indium oxide was deposited on top of the structure to define the electroluminescent device. The light emitted spectra present two broad peaks at ∼530 and 670nm when an undoped Si C layer is integrated in the structure. However, when a boron dopedX SixC1-x is used, the light emission at larger wavelengths is dominant.

We measured the response of reverse-biased a-Si:H diodes, in the p-i-n geometry, to charged particles as a function of ionization, detector bias (V) and angle of incidence, in the ionization range of 10–300 keV/µm. In the two cases where the amount of signal was weak, namely at low biases or at low ionizations, the signal was extracted from the noise using a coincidence technique. The response was found to be roughly proportional to , at all values of dE/dx, in the bias range of 7 V to 50 V. The dependence of the charge collection efficiency on the ionization density indicates a strong plasma effect in the detector. This collection efficiency increases steeply when the ionization is decreased but no plateau has yet been reached, indicating that the plasma effect is still important at 10 keV/µm.

Recent progress in the range of applications of amorphous silicon (a-Si:H) for large area electronics has led to a better understanding of the requirements for stable circuit operation. In particular, the application of doped a-Si:H as a material for thin film resistors has highlighted the instability of this material both due to its thermal history and its relatively high activation energy. In this paper, these limitations are addressed and alternative materials studied.

The properties of thin film resistor materials for a-Si:H integrated circuits are driven by the requirements of long term stability, a high sheet resistance, a weak temperature dependence, and processing compatibility with a-Si:H.Cermet films are a promising material as the nature of the transport results in a low activation energy, though the sheet resistance of these materials is usually limited to 10–50 kohms/square. In this paper, the suitability of rf sputtered Cr/SiO2 cermet films for thin film resistors is addressed. Its electrical properties are studied as a function of target composition and its structure investigated by transmission electron microscopy. A sheet resistance of 40 Mohms/square with an 0.05 eV activation energy for a 500A thick film is demonstrated. The carrier transport is interpreted in terms of a tunneling model of electrons between Cr islands.

Indium tin oxide (ITO) film properties and hydrogenated amorphous silicon (a-Si:H) photo diode characteristics are described as a function of the oxygen partial pressure during ITO magnetron sputtering deposition. The photo diode has a Cr/a-Si:H/ITO structure. The a-Si:H and ITO bulk properties along with the a-Si:H/ITO interface properties are important to device performance.

The ITO film deposited at high oxygen partial pressures had a smooth surface and the target did not tarnish during sputtering deposition. Therefore, with respect to stability and reproducibility, ITO deposition at high oxygen partial pressures seems to be quite effective. A high dark photo diode reverse current due to presence Of SiOx (x<2) at the a-Si:H/ITO interface was observed. Portions of the SiOx (x<2) formations were changed to SiO2, by thermal annealing, causing the dark current to decrease remarkably.

We report a study of the transient photoconductivity of a-Si,Ge:H,F in reversebiased Schottky barrier detectors. The barriers were used to detect light pulses from commercial (AI,Ga)As laser diodes. The photodetectors work at a modulation frequency of up to 10MHz. Lowering the optical gap, Eopt, reduces the optical absorption length but raises the dark current and thus reduces the specific detectivity D*. The charge collection efficiency is determined by deep trapping of electrons in the conduction band tail. The frequency response is controlled by the electron drift mobility. The performance of the photodetector is discussed in terms of the optical and transport properties of the a- Si,Ge:H,F alloys.

Large variations in barrier height of a-Si:H Schottky diodes can be achieved by introducing thin inversion layer at the interface. Electronic density of states in the basic film depends upon the growth and surface preparation conditions.However, the high density of surface states of the order of 1013 cm-2eV -1usually found at a-Si:H film masks the role of bulk density of states which are ∼1016 cm-3 eV -1. As the density of surface states increases, the barrier height increases. At very large density (∼ 5 × 1014 cm-2 eV-1), Fermi level of the film pins to the neutral level and the barrier height becomes independent of the metal work function. Poisson's equation has been employed to illustrate the role of various charges contributed by the basic semiconductor, the inversion layer and the eletronic states. The concept of interfacial layer of 2 to 3 atomic dimensions with the permittivity of free space is essential to understand the mechanism of barrier height formation in a-Si:H Schottky diodes like similar devices in other semiconductors.

We show our results in detecting particles of various linear energy transfer, including minimum ionizing electrons from a Sr-90 source with 5–12 micron thick n-i-p and p-i-n diodes. We measured W (average energy to produce one electron-hole pair) using 17keV filtered Xray pulses with a result W=6.0+0.2eV. This is consistent with the expected value for a semiconductor with band gap of 1.7–1.9eV. With heavily ionizing particles such as 6 MeV alphas and 1–2 MeV protons, there was some loss of signal due to recombination in the particle track. The minimum ionizing electrons showed no sign of recombination. Applications to pixel and strip detectors for physics experiments and medical imaging will be discussed.

Drift-mobility measurements in undoped amorphous hydrogenated silicon (a-Si:H) are reviewed with emphasis on the effects of deep levels (principally the D or dangling bond defect) on the electron drift mobility. An outline of several techniques for measuring drift mobilities is also given to establish their relationships to the transient drift-mobility function. µ(t). Three aspects of the electron µ(t) in undoped a-Si:H are described in detail: (i) anisotropy at long times, requiring a distinction between axial electric fields (parallel to the growth axis) and planar fields normal to it, (ii) the D center deep-trapping cutoff observed in the axial µ(t), and (iii) D center multiple-trapping at long times in the planar drift-mobility. Microstructure effects which might account for the electron drift-mobility are discussed.

The transport of a pulse of photogenerated excess electrons has been investigated in a-Si p+-i-n+ junctions under forward bias in the presence of strong double injection. It is found that the electron drift mobility is completely independent of forward current densities of up to 0.33 Acm−2, in disagreement with the work of Silver et al. We suggest that the experimental disagreement arises from problems inherent in the voltage step technique of mobility measurement.

A new technique to measure the electron drift mobility, µD, in n-type doped a-Si:H is described. µD is defined from the d. c. conductivity, σ, and the density of electrons in the band tail, nBT, by the expression σ = nBT e µD. The temperature dependence of the mobility was measured from 200K to 450K for both the thermal equilibrium and the frozen in states at different doping levels, using sweep out measurements to obtain nBT for the same thermal conditions. Doping up to 10-2 PH3/SiH4 suppresses µD by about an order of magnitude at room temperature, and increases its activation energy. Numerical modelling shows that a shift of the mobility edge into the conduction band by about 100 meV gives the best fit to the data, and is attributed to potential fluctuations induced by charged dopant and defect states.

The drift mobility in compensated a-Si:H as measured by standard T.O.F. is much less than in intrinsic material; presumably due to charge defects. In order to determine other differences in behavior between compensated and intrinsic material, we have studied the transient forward bias current of each in double injecting devices. We have found that: a) a much higher voltage is required in compensated material to obtain the same current as in the intrinsic; b) intrinsic and compensated samples show similar transient current responses but the compensated devices are much slower; c) the steady state current degrades in the compensated current but not in the intrinsic; and d) mobilities obtained by the voltage pulse technique in compensated material are at least three times larger than by the standard T.O.F. technique.

Time-of-flight (TOF) and charge collection measurements are evaluated to determine electron transport quality of a-Si1−x Gex:H for 0 ≤ x ≤ 0.3. The drift mobility data are used to calculate the tail state distribution at the conduction band, which turns out to be of hybrid structure (flat linear followed by a steep exponential decay). By incorporation of Ge additional localization introduced by chemical disorder broadens the band tail. The dangling bond density, calculated from electron µDTE products, also dramatically increases. Both effects contribute to the drop of photoelectronic quality of a-Si1−xGex: H alloys.

In addition it is shown that the extended state mobility deduced from TOF experiments reflects a tunneling transport mechanism in localized states above a dominant transport level that separates states with high tunnel probability from states where carriers propagate via thermal release from and capture into localized states.

We use the self-consistent pseudopotential method with a supercell model of a-Si:H to study the electronic structures of substitutional fourfold- coordinated P and fourfold-coordinated B at the second neighbor of a dangling bond. Partial transfer of charge from the impurity state to the dangling bond state is examined. The influence of passivating the dangling bond by a single H atom on the impurity state is also discussed.

Transient photoconductivity can provide extensive information concerning the characteristics and energy distribution (g(E) ) of trapping centres in disordered solids. In this paper, we use both pre-transit (extraction free) and post-transit current decays, from the TOF experiment, to probe g(E) in a-Si:H of intentionally reduced quality.

Cr-metallised amorphous silicon samples have been studied as a function of temperature using a dielectric technique with probe frequencies in the range 10 mHz to 10 KHz. The data is presented in the form logC′, logC″ versus log frequency, where C′, C″ represent, respectively, the real and imaginary components of the complex capacitance. An accurate analysis of the data provides evidence of Schottky barrier-like behaviour in which the signature from the response of two deep centres is also observed. These represent the existence of two defects lying at energies of 0.61 and 0.74eV from a band edge. Additional information is shown in which the parallel barrier conductance is activated with an energy of 0.71eV. Similarities and differences due to different contact materials are discussed.

A general purpose simulator has been developed for transient experiments on amorphous semiconductors. Simulated time-of-flight (TOF) experiments have been used to study the effects of band tails of the form . Values of m from 0.5 to 2.0 are compared to the pure exponential of m = 1.0. Quantitative results demonstrate that dispersive transport as observed in TOF experiments on a-Si:H requires a value of m very close to 1.0.

Hydrogenated amorphous silicon (a-Si:H) films have been deposited by dc magnetron reactive sputtering in an Ar+H2 working gas. In this process, the hydrogen content A controlled by two independent variables: the substrate temperature and the hydrogen partial pressure in the deposition chamber. For a wide range of deposition conditions studied, it is shown that total hydrogen content of the films, as inferred from IR absorption, is uniquely related to the hydrogen bonding and the microstructure. We report films of high electronic quality, deposited between 200–300 °C, in which hydrogen bonding varies from predominantly SiH to mixed SiH and SiH2 modes as the hydrogen content is increased. This struciral change is reflected in hydrogen evolution spectra. In addition to peaks at ˜500 and ˜600 ° C, a low-temperature peak (350–400 C) has been observed for the films which have high hydrogen content and consequently are rich in SiH2. The low temperature peak is absent in SiH dominant films. However, the low-temperature peak is not associated with a columnar morphology because SEM images of fracture cross-sections are featureless for SiH dominant films and have a “bumpy” texture for high SiH2 content films. We propose that in our SiH2 rich films, excess hydrogen leads to a void rich but non-columnar morphology. Preliminary measurements of the Staebler-Wronski effect using AM-1 light show a low susceptibilty to degradation in both types of films. The different electronic properties of the films indicate that the quality of magnetron sputtered films does not necessarily correlate with the IR absorption peaks in a simple manner.