It is shown that Auger and electron energy loss spectroscopies can provide a well defined picture of the local bonding and structure of a-Si1−x Cx:H alloys. The samples were found macroscopically homogeneous but microscopically heterogeneous on a scale of few Angstroms. A simple model where the tetrahedral network is chemically ordered at low x values and carbon tends to segregate in C-C microclusters at high x values seems to reproduce the data very well. By fitting the C and Si Auger spectra it was possible to have a quantitative determination of the percentage of different configurations.

The local environment and diffusion of hydrogen in hydrogenated amorphous silicon (a-Si:H) depend on both temperature and doping. Previous studies of hydrogen evolution indicate that the manner in which hydrogen diffuses and desorbs from the material depends on doping and may occur at relatively low temperatures over extended time frames. We have examined the microstructure of hydrogen in intrinsic, boron doped and phosphorous doped a-Si:H as a function of annealing temperature and annealing time using hydrogen NMR. Changes in both the H NMR spectrum and spin-lattice relaxation times occur. We find annealing time has only a small effect on these parameters, whereas the annealing temperature has a substantial effect. The bonded-H content drops and the molecular-H2 content is seen to decrease slightly as the samples are annealed to higher temperatures. However the bonded-H remains essentially constant for long time, low-temperature anneals, while the molecular-H2 content is also seen to diminish slightly. The changes are more profound for B-doped samples than for P-doped or intrinsic material, consistent with the conclusions of other studies.

a—Si1−xGex:H alloy films with 0 > × > 1 have been prepared by reactive co— sputtering (rf—magnetron) from separate targets. The preparation conditions which have been optimized for each composition x must be changed from a—Si:H—like (0 > × > 0.4) to a—Ge:H—like (× < 0.7) to obtain the best photoelectrical quality of the alloy material. The preferential attachment of hydrogen was quantitatively investigated by detailed ir—spectrometric studies. Its monotonic change with x is attributed to fundamental preferential attachment of hydrogen to silicon superimposed by a further preferential attachment to the respective minority alloy partner. The preferential attachment of hydrogen essentially influences the microstructure of the film and thus the material properties.

We have studied the local structure of hydrogenated amorphous silicon-carbon alloy films, a-Si1−xCx:H, by measuring the extended x-ray absorption fine-structure (EXAFS) at the Si K edge.

We find that first coordination shell average bond lengths are 2.35 Å for Si-Si and 1.86 Å for Si-C and are constant with concentration to within × 0.015 Å.

By comparing the composition of the first coordination shell around Si with the average concentration we show that the alloy tends to be chemically ordered, in that heteroatomic bonds are preferred.

A thorough investigation of plasma-CVD amorphous hydrogenated boron (a-B:H) has been conducted with the main emphasis on how in this amorphous semiconductor the variation of the coordination number in comparison to a-Si:H influences the structural disorder and density of states in the band tails. a-B:H was deposited from different concentrations of B2 H6 diluted in H2 by both DC- and RF-plasma-CVD. The influence of the change of substrate temperature, pressure, flow and deposition power on the structural, optical and electronic properties of the material was examined. Raman-scattering and IR-absorption reveal the clear non-crystallinity of the deposited films. Almost all samples show some photoconductivity with a σphoto/σdark ratio from 10-1 to 3×101. Although a strong influence of some of the deposition parameters on bandgap and refractive index, hydrogen content, dark and photoconductivity was observed, the density of states in the band tails as measured by PDS was relatively high, showing always a rather flat Urbach slope of about 180 – 220 meV. An explanation for this unexpected almost uniform huge Urbach slope might be that even in the amorphous state the behaviour of boron is still dominated by its electron deficiency character which leads to a certain amount of three centre bonds (as seen in IR-absorption) and results in comparison to amorphous hydrogenated silicon in even stronger constraints of the amorphous network and obviously in more potential fluctuations.

We have used real time, in situ infrared reflectance spectroscopy to study the evolution of Si-H bonding during the growth of hydrogenated amorphous silicon (a-Si:H) by dc magnetron reactive sputtering. The surface Si-H stretching mode (2100 cm-l) is observed in films 5 Å thick and the 2000 cm-l mode, which has been attributed to isolated monohydrides in the bulk, appears for thickness (d) < 10–15 Å For larger thicknesses the intensity of both modes increases approximately linearly. These trends are interpreted in terms of the nucleation and coalescence of islands for d> 15 Å followed by bulk-like growth.

A new self-aligned a-Si TFT has been developed. Ion doping and chromium silicide (CrSix) formation technique was used to fabricate source and drain, which are self-aligned to the gate electrode, instead of using the previously reported lift-off process. The fabricated TFT mobility is about 0.5cm2/V' sec and threshold voltage is about 3V. The ON/OFF ratio is over 106. The actions of as short as 2μm channel TFT have been confirmed, using a large area TFT process. These results show that this technique can be applicable to manufacturing high quality TFT-LCDs in a large area.

Thin—film transistors (TFTs) were prepared by the glow—discharge deposition of amorphous silicon nitride (a—SiNx:H) and amorphous silicon (a—Si:H). The properties of these TFTs were varied in two ways: a) doping of the amorphous silicon film with phosphine or diborane and b) exposure of the a—SiNx:H film to an oxygen plasma prior to the deposition of the a—Si:H layer. The TFTs are characterized by measurements of the transfer characteristic, ISD(VG) and of the effective density of interface states, Ni(E), using a transient current spectroscopy (TCS). The dependence of Ni(E) on the Fermi—level position in the a—Si:H film suggests that for EC—EF > 0.6eV this quantity is mainly determined by interface related defect states whereas for Ec,—EF <0.6eV it is determined by doping—induced defect states. The exposure to the oxygen plasma results in a reduction of Ni in both the upper and lower half of the gap and in an improvement of the characteristic, in particular in p—channel TFTs. These changes are discussed in terms of the chemical-equilibrium or defect—pool concept.

We have developed a numerical simulation program (MANIFEST) which calculates the time-dependent behaviour of one and two dimensional amorphous silicon devices. Our model solves the complete set of transport equations for both electrons and holes and fully includes the appropriate time dependent occupation functions for the traps in the band gap of the amorphous silicon. These traps are assumed to obey Shockiey-Read-Hall kinetics. We have simulated the transient behaviour of pin and nin diodes, both in the dark and under illumination, as well as the characteristics of thin film transistors. Our initial results are in good agreement with experimental data.

The influence of hydrogenation on OFF current of TFTs with a bottom gate staggered structure has been investigated. The hydrogenation is done by exposing the surface of the a-Si:H channel layer to H2 plasma. The hydrogenation decreases the OFF current by more than one order of magnitude. The decrease in the OFF current is attributed to the increase in the density of states at the interface between the a-Si:H channel layer and the SiN passivating layer.

The OFF current in a-Si TFTs is an important parameter, especially for applications such as active matrix liquid crystal displays. In some demanding applications operation at 70°C or higher is required. This paper reports studies of the OFF current limiting mechanisms at room temperature and above. It is shown that the limiting mechanisms are hole injection from the drain junction at room temperature and electron conduction at higher temperatures. The importance of silicon thickness and interface state density at the passivation interface is stressed.

We show that bias annealing modifies the rate of threshold voltage shift under bias stress in the regime where state creation dominates. Previously we have shown that the deep state distribution near the interface of a-Si:H TFTs can be altered by bias annealing. In particular the energy position of the defects can be made high or low in the energy gap, consistent with the defect pool model.

In this paper, we relate the kinetics of new state creation at room temperature to a particular equilibrated defect distribution. In particular we show that the rate of state creation can be reduced by prior bias annealing.

A theoretical study of the properties of a-Si:H thin-film transistors (TFTs) has been performed using uniform and non-uniform distributions of gap states in the a-Si:H layer. The potential profile in the semiconductor is calculated in a straightforward way by solving Poisson's equation numerically. From the band bending the source-drain current ISD(VG) of the TFT is predicted. In addition we calculate the transient discharge current I(t) generated by switching the gate voltage from positive to negative values. In experimental studies such transients are analyzed to obtain an effective density of interface states, Ni(E). We find that N;(E) depends little on the spatial variation of gap states, N(E,x), but sensitively on the energy distribution of N(E,x). It is shown that experimental N;(E) curves and ISD(VG) characteristics from the same sample can be fitted by the same N(E,x) which however cannot be unequivocally determined.

The experimental and analytical results regarding to the effects of temperature and electrical stress on the characteristics of amorphous silicon thin-film transistors (a-Si TFT's) have been presented. The variations in the device parameters of a-Si TFT, such as threshold voltage and field-effect mobility, have been examined under various operating temperatures and electrical stress conditions. The hysteresis in the transfer characteristics and the trapped charges at the a-Si/silicon nitride interface were measured at the operating temperature ranges. From the experimental results, it has been found out that the increase of the interface charge trapping may be responsible for the degradation in the a-Si TFT characteristics. Also, an analytical formulation, employing the interface charge trapping, is presented to clarify the instability phenomena and verified successfully with the experimental results.

We present a new analytical model of amorphous silicon thin-film transistor (a-Si TFT) suitable for circuit simulators such as SPICE. The effects of localized gap state distributions of a-Si as well as temperatures on the a-Si TFT performances have been fully considered in the presented model. The parameters used in SPICE, such as transconductance, channel-length modulation, and power factor of source-drain current, are evaluated from the measured current-voltage and capacitance-voltage characteristics by employing the proposed extraction method. It has been found out that the analytical model is in good agreement with experimental data at both room temperature and elevated temperature and successfully implemented in a widely used circuit simulator.

A novel two step reactive ion etch (RIE) process is described for the etching of bilayer Mo/Cr source-drain metallization on hydrogenated amorphous silicon (a-Si:H) inverted-staggered thin film transistors. The Cr acts as an etch stop during Mo etching, and is thin enough (∼3 0 nm) that only a small thickness of underlying a-Si is removed during the Cr etch. The resulting Mo/Cr profile is sloped, compared to the more vertical and somewhat uncontrolled slope that is achieved with Mo wet etch. Very similar transistor behavior was observed for both Mo wet etched and Mo/Cr reactive ion etched source-drain metallization. The major advantage of this process over wet etching of Mo source-drain is improved step coverage of subsequently deposited layers due to the less vertical sidewall slope.

We report on the illumination time dependence of the generation of positive charge in gate-quality nitrogen-rich amorphous silicon nitride films subjected to sub-bandgap illumination at different temperature in vacuum. The influence of film thickness and gate bias applied during illumination on the generation of positive charge is also described. We have found that a stretched-exponential function, which characterizes dispersive charge transport in silicon nitride, gives the best description of our experimental results.

We analyse the temperature dependence of the forward characteristics of a-Si:H nip diodes, which are characterised by a well defined exponential region at low bias and space charge limited current at higher bias. At each bias, the temperature dependence of the diodes shows a well defined activation energy and a linear dependence on bias exists for both the activation energy and exponential prefactor. We describe these phenomena with a first order temperature expansion of the diode quality factor n and present a physical interpretation in terms of electron and hole recombination in the i-layer.

An analysis of the room-temperature dark conductivities and activation energies for doped μc-Si and μc-Si,C is used to develop a band alignment model which shows that the maximum attainable dark conductivities in these materials are determined by transport through, or over interfacial potential barriers between Si crystallites, c-Si, and the intervening amorphous regions: a-Si:H or a-Si,C:H, respectively. For all levels of doping in μc-Si,C, the transport is limited by thermionic emission over interfacial barriers at the c-Si/a-Si,C:H interface, placing a significant constraint on applications requiring both high optical transparency and high conductivity.

In doped and undoped microcrystalline silicon prepared with Very High Frequency Glow Discharge, hydrogen is found to be mainly located at the grain boundaries from where it desorbs easily at low annealing temperatures. In undoped material hydrogen evolution peaks are between 400°C and 500°C. Upon doping, a new major peak appears at 300°C and a strong reduction of the typical Si-H infrared absorption bands are found for doped samples when annealed up to 300°C. This is accompanied by an increase of the conductivity due to either de-passivation of dopants in the crystallites or a favourable reconstruction at the grain boundaries. Hydrogen profiles show a hydrogen depletion at the film/air interface that is more profound in doped material, thus correlating with the appearance of the low temperature evolution peak. The high free carrier density in the crystallites of the doped material gives rise to strong optical absorption. Although correlating nicely with conductivity, the free carrier absorption cannot be evaluated simply in terms of die Drude theory. In view of the high conductivities and the dominance of Si-H surface bonds we argue that our material does not contain a large amount of amorphous tissue.