Measurements are reported of metastable defect states in undoped a-Si: H, with the aim of understanding the relation between the different types of metastability. The temperature dependence of the thermal equilibrium defect density agrees well with a proposed thermodynamic model and their relaxation time varies with deposition conditions. The rate of light induced defect creation and annealing in samples deposited at low temperature and with a large initial defect density, decreases progressively as the irreversible defects are removed by annealing.

The time dependence of the dc conductivity of Li- and Be-doped a-Si:H after thermal quenching is reported. The equilibration kinetics, similar to that observed in substitutionally-doped a-Si:H, is characterized by stretched exponential decays and activated time constants. The equilibration times for Bedoped a-Si:H were too short (< 1 min) for the kinetics of equilibration to be accurately measured. In Li-doped a-Si:H, the equilibration time constants were longer than those in 1% P-doped a-Si:H, which is due to the smaller carrier concentrations in the Li-doped a-Si:H films.

Both the temperature dependence at elevated temperature and the increase after fast cooling from elevated temperature of the density of dangling bonds are measured by ESR for undoped hydrogenated amorphous silicon–based alloy films, a–Si:H, a–Si1−xCx:H, a–Si1−xNx:H and a–Si1−xOx:H. Both a–Si:H and a–Si1−xCx:H clearly show the increase in the density of dangling bonds at elevated temperature, while the increase is less prominent for a–Si1−xNx:H and a–Si1−xOx:H. The observed results for both a–Si:H and a–Si1−xCx:H are fairly well reproduced by the model recently proposed by Smith et al. The results of CPM measurements combined with those of ESR measurements suggest that the density of charged dangling bonds present in undoped a–Si:H also increases after fast cooling from elevated temperature.

The density of gap states in the bulk of intrinsic a-SiH has been probed using depletion width modulation electron spin resonance spectroscopy (DWM ESR) as well as complementary capacitance techniques. Our sample consists of a 2.85 micron thick layer of undoped a-Si:H sandwiched between a large area Pd Schottky barrier contact and a p+ c-Si substrate, allowing charge-counting capacitance and spin-counting ESR measurements to be performed on the same sample. DWM ESR measures the spin change that occurs when electrons are removed from gap states by increasing reverse bias on a Schottky barrier. The phase of the signal indicates whether unpaired spins are created or destroyed with increasing bias, thus revealing the defect charge state at the Fermi level The DWM ESR signal from our sample indicates that removing electrons from gap states leads to a reduction of the spin signal. This is qualitatively consistent with a band of Do states near EFo. However, for all metastable states and at all temperatures studied, the magnitude of the DWM ESR signal account for only about 14% of the responding charge as determined by junction capacitance. This would occur only if the majority of the filled bulk gap states were doubly occupied with a small correlation energy Ueff. In that case the majority of transitions caused by depletion width modulation would be D−/D+ which would produce no spin change. Model calculations show that the small observed Do/D+ DWM ESR signal would indeed result from the occupation statistics for a near zero value of Ueff at the moderate temperatures of the experiment. Comparison with our data sets the overall limits If −20meV < Ueff < 50 meV. Implication for recent models of the density of states in intrinsic a-Si:H are discussed.

Space Charge Limited Current (SCLC) measurements are used to obtain the density of gap states of intrinsic a-Si:H. If an electron is trapped by a positively charged defect, then the electron can be released by a high electric field which disturbs the SCLC measurement. A correction for this effect, which is called Poole-Frenkel emission, in SCLC measurements is derived and used to analyze current-voltage measurements performed on n-i-n a-Si:H devices. It is shown that the Poole-Frenkel emission is absent, and that this is in accordance with the contemporary models for the gap states in a-Si:H.

We also studied the characteristic temperature which is based on the concept that the distribution of gap states is exponential, and concluded that this distribution is not exponential.

We use first principles pseudopotential method to study the electronic structures of compensated dopants in a realistic model of hydrogenated amorphous silicon. Boron and phosphorus atoms are treated as dopants. Two states in the gap are identified as the boron-phosphorus pair related states. The lower energy state, strongly localized around the boron atom, is obtained when the atom has a hydrogen atom as a nearest neighbor. The local environment does not affect the localization feature of the state. Other environment around the boron atom cause the charge distribution to be more extended in space. The higher energy state having its charge around the phosphorus atom does not show, strong localization at the phosphorus atom.

The effect of carbon incorporation into a-Si:H has been investigated in terms of gap states. The shallow and deep states are measured by a combination of two methods of the time-of-flight and the depletion discharge transient spectroscopy. The results show that the carbon incorporation of 10Z into a-Si:H leads to a slight increase in the conduction-band tail states and an extention of the deep states distribution. However, the increase in the shallow states affects little change in the electron transport properties. The origin of the deep states is also discussed.

Device quality undoped hydrogenated microcrystalline silicon has been prepared by plasma enhanced chemical vapor deposition under different conditions. The dependence of physical, chemical, structural, and electrical properties on the deposition conditions has been investigated. Conductive (conductivity above 10−3Ω−1 cm−1) and resistive (conductivity around 10−9Ω−1cm−1) layers having approximately the same grain size, at a given substrate temperature, have been deposited between 200 and 500°C at two different hydrogen dilutions. Independently of the hydrogen dilution, the average grain sized is dependent on the deposition temperature and the film thickness; and a maximum average grain size of about 40 nm has been achieved for a thick film deposited at 500°C. The density of paramagnetic defects also increases with increasing deposition temperature, which indicates that more dangling bond defects are introduced as the total area of the grain boundaries increases. The etch rate decreases with increasing deposition temperature, and for the films deposited at 250 and 500°C the etch rate has been measured to be 6.6 and 2.7 nm/min, respectively. Thin film transistors incorporating a microcrystalline channel have been fabricated and evaluated. The best device had the following properties: field effect mobility, threshold voltage, and on/off current ratio of about 0.8 cm2/V sec, below 5 V, and around 106, respectively.

The effects of fluorine incorporation on the microstructural and electronic properties of a-Si:H:F with 1–7 at.% F have been systematically studied. Films were prepared by direct photo-CVD of disilane with xenon difluoride as the fluorine source. The fluorine content was measured by infrared spectroscopy, xray photoelectron spectroscopy (XPS) and electron microprobe. Infrared spectra show that as the fluorine content increases, silicon dihydride bonding increases. Density measurements confirm that this is associated with an increase in microvoid content. With the increase in F and SiH2, the photoconductivity decreases over two orders of magnitude. A review of the literature shows that the appearance of SiH2 is a universal result of >1% F incorporation and is not limited to the present study. This suggests that fluorination of amorphous silicon is not beneficial for photovoltaic application.

Glow discharge a-Ge:H films produced at substrate temperatures (Tδ) between 50°C and 350°C, with and without a top a-Si:H capping layer, have been studied. The uncapped samples produced at Tδ < 250°C suffer severe post-deposition atmospheric contamination, resulting in orders of magnitude of unstable increase in both the photoresponse and dark conductivity. The capped samples, which have very much reduced immediate post-deposition contamination, show only small increases in the efficiency-mobility-lifetime product (ŋμτ) with increasing Tδ. This contrasts with the results of earlier similar studies on uncapped samples, which showed a peak in either the photoconductivity1 or the ratio of photoconductivity to dark conductivity2 for 150°C < Tδ < 2000C. We have also observed a decrease in the bandgap, a narrowing of the band-tails, an increase in the index of refraction, and a reduction of hydrogen content of the films with increasing Tδ.

An analytical expression for the thermal activation energy of the steady-state photoconductivity is shown to agree with experimental data in a range of temperature and generation rate for undoped hydrogenated amorphous silicon (a-Si:H). This agreement supports our suggestion that the commonly observed small activation energy of the photoconductivity in undoped a-Si:H originates in the strong temperature dependence of the quasi-Fermi level for electrons.

The dependence upon emission energy of the transient photoresponse of photoluminescence in amorphous silicon in the time range 100 ns to 20 μs, and for temperatures between 100 K and 160 K has been the subject of a detailed study. Analysis of this dependence, as a function of temperature, is carried out in terms of band tail carrier thermalisation and recombination. By using the simple multiple trapping treatment for the thermalisation of photoexcited carriers in their respective band tails, and by allowing for a fast time- and temperaturedependent recombination through defects deep in the gap, we are able to reproduce the correct energy and time dependence, as well as the temperature behaviour. On the basis of this analysis, the significance of the dispersive parameters deduced from transient photoluminescence decays is discussed. Comparison with the results of transient experimental techniques such as time of flight will be given.

We have prepared device quality hydrogenated amorphous silicon (a-Si:H) films by DC magnetron reactive sputtering in an Ar+H2 atmosphere. The hydrogen content (CH) of these films ranges from ∼10 to ∼28 at.% (determined from IR absorption) and is controlled mainly by the hydrogen partial pressure in the discharge. The electron and hole transport, and the density of defect states show that these films are suitable for solar cell applications. In this report we will emphasize the carrier transport properties of the films. The electron drift mobilities, determined by time-of-flight experiments, are between 0.5 and 2 cm2/V-sec. The minority carrier (hole) mobility-recombination lifetime product, (μτrec)p, has been determined on Schottky barrier structures from a fit of the voltage dependence of the photocurrent to the Hecht expression. This measurement gives results of (μτrec)p ∼ 0.9- 3×10−8 cm2/V. The density of deep-level defect states for these films (10≤CH≤28), measured with CPM, is between ∼1×1015 and ∼1×1016 cm−3. The AM-1 photoconductivity is in the range from 0.8−3.5×10−5 (Ω-cm)−1 and the dark conductivity decreases from ∼1×10−9 to ∼1×10−12 (Ω-cm)−l as the hydrogen content (CH) increases. These results show that device quality a-Si:H films can be grown by the DC magnetron reactive sputtering technique.

The first major progress towards complex LCD's has been made using arrays of thin film transistors (TFT's) fabricated using amorphous silicon (a - Si) technology. However, apart from other two and three terminal devices, there are also two new LCD technologies which directly improve the performance of the LCD without the need for extra switching elements. These are the supertwist (or STN) LCD and displays based on ferroelectric liquid crystals (FELC). These two alternative technologies are described and teir performance compared with that of displays fabricated using α - Si TFT's.

This paper describes the performance of both an 8 dots/mm and a 400 DPI contact-type linear image sensors which integrate poly-Si TFT scanning circuits and a-Si/a-SiC heterojunction photodiodes on the same substrate. Each of these image sensors provides excellent cost and performance for their applications. In particular, the 400 DPI image sensor has achieved a higher scanning speed of 1 msec/line by using the integrated scanning circuits with hydrogenated poly-Si TFTs having high carrier mobility.

In this paper we describe a new analytic model for both the current-voltage and capacitance-voltage characteristics of amorphous-silicon thin-film transistors. This analytic model has been incorporated into a circuit simulation program (SPICE) to provide an accurate comprehensive three terminal model for amorphous-silicon thin-film transistors. We present results showing good agreement between circuit simulations based on this new device model and experimental data. The development of amorphous silicon SPICE simulation tools increases the design accuracy of advanced analog and digital circuits.

Device quality phosphorous (P) doped hydrogenated microcrystalline silicon (n+μc - Si:H) has been prepared by using the plasma enhanced chemical vapor deposition technique. The dependence of physical, chemical, structural and electrical properties on substrate temperature have been investigated. Conductivities for thick films up to 12 Ω−lcm−1 and 40 Ω−1cm−1 have been achieved for layers deposited at 300°C and 500°C, respectively. For films 50 nm thick deposited at 300°C a conductivity of about 5 Ω−1cm−1 has been obtained. A maximum average grain size around 30 nm was obtained. The etch rates of P-doped microcrystalline silicon have been found to be between 8 and 10 times higher than that of undoped hydrogenated amorphous silicon (a-Si:H) films deposited at the same temperature. Thin film transistors incorporating heavily P-doped amorphous and microcrystalline layer between source/drain metal and the a-Si:H channel have been fabricated. We show that an n+μc - Si:H source/drain contacts in thin film transistors provides very good characteristics, yielding an average effective field effect mobility, threshold voltage, and on/off current ratio of about 0.9cm2V−1 sec−1, below 4 V, and above 107, respectively.

This paper describes the experimental and theoretical results of Distributed Threshold Voltage FETs. Based on these results important advantages of DTV FET are clarify and a novel device structure has been proposed.

We present a comparison of the properties of films of amorphous silicon nitride, amorphous silicon oxynitride, and amorphous fluorinated silicon nitride deposited by plasma enhanced chemical vapor deposition. The properties of fluorinated silicon nitride films degrade when exposed to air. TFT devices fabricated with silicon nitride and silicon oxynitride insulators show thteshold voltages ≤3 V and source drain current ON/OFF ratios exceeding 107 for gate voltages smaller than 20 V, whereas TFTs with fluorinated silicon nitride insulators show an inferior performance. We also present ideas on the possible relation between the stress in the insulator film and the reliability of TFTs fabricated using these layers.

We describe a vertical amorphous silicon thin-film transistor which is easy to fabricate and has a very short channel length that is determined by deposition, not lithography. Our vertical TFTs are compatible with large-area processing techniques andd have suitable terminal characteristics for use in practical circuits. Unlike a conventional thin-film transistor, the current path is primarily parallel to the electric field created by an insulated gate electrode. A two-dimensional computer program is used to analyze these devices and guide their design and optimization. We show how to suppress excessive leakage currents and improve the saturation of the output characteristics by a novel current-blocking technique.