Showing papers on "Amorphous silicon published in 1989"

Amorphous silicon and related materials

Abstract: This book presents the most recent important ideas and developments in the field of Hydrogenated Amorphous Silicon and related materials. Each contribution is authored by an outstanding expert in that particular area.

The physics of amorphous-silicon thin-film transistors

Abstract: The basic physics underlying the operation and key performance issues of amorphous-silicon thin-film transistors (TFTs) are discussed. The static transistor characteristics are determined by the localized electronic states that occur in the bandgap of the amorphous silicon. The deep states, mostly consisting of Si dangling bonds, determine the threshold voltage, and the conduction band-tail states determine the field-effect mobility. The finite capture and emission times of the deep localized states lead to a dynamic transistor characteristic that can be described by a time-dependent threshold voltage. The transistors also show longer time threshold voltage shifts due to two other distinct mechanisms: charge trapping in the silicon nitride gate insulator and metastable dangling bond state creation in the amorphous silicon. These two mechanisms show characteristically different bias, temperature, and time dependencies of the threshold voltage shift. Illumination of a TFT causes the generation of electron-hole pairs in the space-charge region leading to a steady-state equal flux of electrons and holes and a reduction in the band-bending. In most applications, the photosensitivity should be minimized. The uniformity of large arrays of transistors for display applications is excellent, with variations in the threshold voltage of 0.5-1.0 V. >

Time and temperature dependence of instability mechanisms in amorphous silicon thin-film transistors

Abstract: We have measured the time and temperature dependence of the two prominent instability mechanisms in amorphous silicon thin‐film transistors, namely, the creation of metastable states in the a‐Si:H and the charge trapping in the silicon nitride gate insulator. The state creation process shows a power law time dependence and is thermally activated. The charge trapping process shows a logarithmic time dependence and has a very small temperature dependence. The results for the state creation process are consistent with a model of Si dangling bond formation in the bulk a‐Si:H due to weak SiSi bond breaking stabilized by diffusive hydrogen motion. The logarithmic time dependence and weak temperature dependence for the charge trapping in the nitride suggest that the charge injection from the a‐Si:H to the nitride is the rate limiting step and not subsequent conduction in the nitride.

The defect density in amorphous silicon

Abstract: The correlation between the density of dangling-bond defects and the slope of the Urbach tail in hydrogenated amorphous silicon is examined. It is shown that this correlation can be explained quantitatively by a spontaneous decay of the weakest bonding orbitals into non-bonding defects during deposition or annealing of a sample and that the same correlation holds for all types of disorder affecting the slope of the Urbach edge. The temperature dependence of the defect density as well as the creation of metastable defects are discussed, and quantitative expressions are derived which can be used to estimate the quality and the stability of a given sample on the basis of the slope of its Urbach tail alone. Possible ways for future improvement of the material are indicated.

High-performance TFTs fabricated by XeCl excimer laser annealing of hydrogenated amorphous-silicon film

Abstract: High-performance staggered a-Si:H and poly-Si thin-film transistors (TFTs) fabricated by XeCl excimer laser annealing of a-Si:H films are discussed. The field-effect mobility of poly-Si TFT is 102 cm/sup 2//V-s, and that of a-Si:H TFT is 0.23 cm/sup 2//V-s. Their drain current on/off ratios are over 10/sup 6/. Except for the crystallization, the fabrication process was the same for both of them. This process appears extremely promising for the integration of matrix elements and peripheral drivers in a single substrate. >

Crystallized Si films by low‐temperature rapid thermal annealing of amorphous silicon

Abstract: Low‐temperature rapid thermal annealing has been used to crystallize both undoped and doped amorphous silicon (a‐Si) films deposited at low temperatures. The polycrystalline films produced are completely crystallized with time temperature budgets such as 4 min at 700 °C. Unlike deposited polycrystalline Si films, the grain size in these crystallized films is not limited by film thickness. In the case of undoped a‐Si films crystallized by this approach, the resulting conductivity is comparable to that achieved in undoped polycrystalline Si films produced by much higher processing temperatures. In the case of doped a‐Si films, the resulting crystallized film yields a conductivity of 160 S/cm, a value which is comparable to the highest reported for doped polycrystalline and microcrystalline silicon. These doped films are found to have mobility values of ∼13 cm2/V s.

Electronic Transport in Hydrogenated Amorphous Semiconductors

Abstract: Elementary treatment of transport in amorphous semiconductors.- Review of transport experiments.- A transport theory for a homogenous model.- Detailed presentation of the theory.- The long-ranged random potential.- Temperature dependent reference energies.- The transport properties of hydrogenated amorphous semiconductors.- Concluding remarks.

Selective area crystallization of amorphous silicon films by low-temperature rapid thermal annealing

Abstract: We report the first demonstration of selective area crystallization of amorphous silicon films using low‐temperature rapid thermal annealing. Crystallization temperatures as low as 500 °C were achieved with the help of a thermally evaporated ultrathin metal layer. The selective area crystallization was accomplished by using this ultrathin metal layer to define the region to be crystallized. The edge between two regions, that which has been crystallized and that which has not, is found to be very sharp.

XeCl Excimer Laser Annealing Used to Fabricate Poly-Si TFT's

Abstract: Polycrystalline silicon thin-film transistors (poly-Si TFT's) with a high carrier mobility were fabricated at low processing temperatures of 150 and 250°C. A hydrogenated amorphous silicon (a-Si:H) film was successfully crystallized at room temperature by multistep irradiation of XeCl-308 nm excimer laser pulses without explosive evaporation of hydrogen. The poly-Si TFT's fabricated by the 250°C process had a carrier mobility of 54 cm2/Vs and a low potential barrier height at a grain boundary of 0.01 eV.

Calorimetric evidence for structural relaxation in amorphous silicon.

Abstract: Differential scanning calorimetry of amorphous silicon (a-Si) prepared by ion implantation shows a one-time low-temperature heat release, equal to one-third of the heat of crystallization. This heat release is direct evidence for structural relaxation of a-Si. It is in agreement with predictions made on the basis of Raman spectrometry.

A new analytic model for amorphous silicon thin‐film transistors

Abstract: We present a new theory describing current‐voltage characteristics of amorphous silicon thin‐film transistors. We calculate the output conductance in saturation by considering channel shortening effects caused by the space‐charge‐limited current in the pinch‐off region. In this model the drain current is expressed through the free‐carrier concentration at the source side of the channel. This allows us to obtain an accurate description of the different operating regimes of a thin‐film transistor using one equation that accounts for the dependence of the free‐carrier concentration in the channel for different regimes. Our model is in good agreement both with experimental data and the results of our two‐dimensional computer simulation. This approach allows one to account for different distributions of localized states in the energy gap. The model has also been developed to be incorporated into a circuit simulator and used for computer‐aided design of amorphous silicon integrated circuits.

Amorphous silicon analogue memory devices

Abstract: Amorphous silicon M-p+ ni-M and M-p+ -M memory devices have been prepared. The characteristics are critically dependent on the metal used for the top contact. Devices with Cr top contacts exhibit the fast digital behaviour reported previously, whereas those using V exhibit fast analogue switching. The paper reports results for these and other metals.

Role of hydrogen in the formation of metastable defects in hydrogenated amorphous silicon

Abstract: This paper presents results of studies on carrier-induced metastable defect creation in hydrogenated amorphous silicon. The metastable defects were studied by measuring the threshold voltage shifts on thin-film amorphous silicon transistors and capacitors as a function of time, temperature, and bias. The kinetics (time, temperature, bias, and doping dependence) of these defects as well as most other metastable-defect processes are quantitatively explained by hydrogen diffusion and the creation of defects due to the presence of excess band-tail carriers.

Radial distribution functions of amorphous silicon

Abstract: Substantial changes in the radial distribution function of amorphous Si films have been observed in neutron-diffraction studies. The spectra indicate changes in short-range order associated with an approx.11% modification in the bond-angle distribution width. The results allow the first direct comparison of structural and vibrational Raman probes of variations in local order in thin-film amorphous solids. Good agreement is obtained between the measured bond-angle variation and that based on Raman estimates.

Band‐gap profiling for improving the efficiency of amorphous silicon alloy solar cells

Abstract: We have developed an amorphous silicon alloy based solar cell with a novel structure in which the optical gap of the intrinsic layer changes in a substantial portion of the bulk. Computer simulation studies show that for a given short circuit current, it is possible with this structure to obtain higher open circuit voltage and fill factor than in a conventional cell design. Experimental cell structures have been made and confirm the theoretical prediction. The new cell design shows a considerable improvement in efficiency. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination.

Present and future applications of amorphous silicon and its alloys

Abstract: Thin films of amorphous silicon (a-Si:H) and its alloys prepared by the glow discharge decomposition of silane and appropriate gases, are presently incorporated into six commercial products and approximately twenty other applications have been proposed for these materials. This presentation will review the present applications and a number of those proposed for the future. Finally, some of the material and technological issues of current importance will be discussed.

Preparation, structure, dynamics, and energetics of amorphous silicon: A molecular-dynamics study.

Abstract: The preparation of amorphous silicon by molecular-dynamics simulations employing the Stillinger-Weber Si potential, via direct slow cooling from the melt, is described. It is shown that previous failures to obtain amorphous Si using these interaction potentials are of kinetic origin, i.e., related to the quench rate employed. The amorphous silicon sample which we prepared exhibits structural and dynamical properties in good agreement with available experimental data for the static structure factor and phonon density of states. Detailed analyses of the structure, including distributions of bond and dihedral angles and ring statistics, and energetics, including the determination of effective temperatures for n-fold-coordinated atoms (n=3--5) and estimates of the formation energy of coordination defects (i.e., n\ensuremath{ e}4) are presented. The lack of medium-range order, measured via correlation between dihedral angles associated with adjacent bonds, is discussed.

Low temperature selective crystallization of amorphous silicon

Abstract: We have used rapid thermal annealing to crystallize PECVD a-Si:H films deposited on glass at low thermal budgets of 700°C/4 min. The a-Si:H films can be selectively crystallized using thermal budgets lower than 700°C/4min. by selectively depositing an ultrathin Pd layer on the silicon surface. We have investigated the electrical and the structural properties of these selectively crystallized films.

Gate dielectric and contact effects in hydrogenated amorphous silicon‐silicon nitride thin‐film transistors

Abstract: The characteristics of glow‐discharge hydrogenated amorphous silicon‐silicon nitride (a‐Si:H/a‐SiNx:H) thin‐film transistors (TFTs) are reported for various deposition conditions. TFTs incorporating a N‐rich nitride gate dielectric, a‐SiN1.6:H, are superior to a‐Si:H TFTs with a Si‐rich gate nitride, a‐SiN1.2:H. In particular, the N‐rich gate nitride TFTs show considerably less interface or near‐interface charging during operation, improved stability, and a higher field‐effect mobility. The average field‐effect mobility μFE is found to be 0.27 and 0.41 cm2/V s for the Si‐ and N‐rich gate nitride TFTs, respectively. A further improvement in mobility, μFE =0.61 cm2/V s, is achieved by increasing the N‐rich gate nitride deposition temperature from 250 to 450 °C. These results suggest that N‐rich a‐SiNx:H, deposited at elevated temperatures, yields a more abrupt or ‘‘cleaner’’ a‐SiNx:H/a‐Si:H interface. We also show, for the first time, that using n+ μc‐Si:H source‐drain contacts in place of n+ a‐Si:H improve...

Study on catalytic chemical vapor deposition method to prepare hydrogenated amorphous silicon

Abstract: A new type of thermal chemical vapor deposition (CVD) method is presented. In the method, material gases are decomposed by catalytic or pyrolytic reaction with a heated catalyzer, so that films can be deposited at temperatures less than 300 °C without any plasma or photochemical excitation, and the method is particularly called ‘‘Catalytic‐CVD.’’ Hydrogenated amorphous silicon films are deposited by this method, and the deposition mechanism is also investigated. It is found that device‐quality amorphous silicon films can be obtained and that inactive species, which are generated at the catalyzer and transported without gas‐phase reactions, are key species to make a high‐quality film by this method.

Saturation of the light‐induced defect density in hydrogenated amorphous silicon

Abstract: We report new experimental results on the saturation of the light‐induced defect density in hydrogenated (and fluorinated) amorphous silicon. The films were illuminated near room temperature up to 5000 h with bandpass filtered red light at a carrier generation rate G of 5×1020 cm−3 s−1, or up to 20 h with Kr+ laser light (λ=647.1 nm) at G=3×1022 cm−3 s−1. The bulk defect densities Ns saturate in both cases in the vicinity of 1017 cm−3. The saturation values are almost independent either of G or of temperature in the range from room temperature to about 70 °C. The illumination time to reach saturation is approximately proportional to 1/G2. These results are discussed within the framework of existing models for the light‐induced defects.

In situ ellipsometry of thin‐film deposition: Implications for amorphous and microcrystalline Si growth

Abstract: Real‐time ellipsometric characterization of the nucleation of hydrogenated amorphous silicon (a‐Si:H) prepared by plasma‐enhanced chemical vapor deposition (PECVD) on smooth, dense metal, and crystalline Si substrates is reviewed. The experimental results for photoelectronic quality a‐Si:H from pure SiH4 on Mo and Cr are consistent with the Volmer–Weber nucleation mode, with a separation of 40–50 A between nucleation centers. For c‐Si substrates, a new interpretation suggests that nucleation occurs on the same scale, but the geometry in the first monolayer is disklike. A well‐defined lobe and cusp in the data can be ascribed to surface smoothening by diffusion that results upon coalescence of these structures. For films from a pure SiH4 plasma, the rates of coalescence and relaxation of substrate‐induced surface roughness on thick films are consistent with a diffusion length of ∼80 A for the adsorbed precursors. For films prepared from a SiH4‐depleted plasma, the average surface diffusion length is reduce...

Electronic transport and recombination in amorphous semiconductors at low temperatures.

Abstract: The problem of simultaneous diffusion and recombination of electron-hole pairs, photoexcited in noncrystalline semiconductors at low temperatures, is reduced to a universal mathematical problem whose solution does not depend on the density-of-states function. We derive a general geminate-recombination function which at low light intensities describes the distribution of radiative recombination times. The low-temperature photoconductivity is found to depend only weakly on material parameters and agrees with experiments on hydrogenated amorphous silicon.

The Influence of the Si-H2 Bond on the Light-Induced Effect in a-Si Films and a-Si Solar Cells

Abstract: The influence of the Si-H2 bond on light-induced degradation and the thermal recovery of a-Si films and a-Si solar cells were studied. The influence of the Si-H2 bond on light-induced degradation depends on the impurity content in a-Si films, and light-induced degradation can be reduced by decreasing the Si-H2 bond density in a-Si films with impurity content of 1018 cm-3. The activation energy of the thermal recovery process was about 1.0 eV, and it did not depend on the Si-H2 bond density. However, an irreversible phenomenon was observed in film properties and solar cell characteristics with high Si-H2 bond density. It is thought that the structural flexibility of the Si-H2 bond is related to this irreversible phenomenon.

Retardation of nucleation rate for grain size enhancement by deep silicon ion implantation of low-pressure chemical vapor deposited amorphous silicon films

Abstract: The effects of silicon ion implantation on the crystallization kinetics and grain size of low‐pressure chemical vapor deposited amorphous silicon on oxidized silicon substrate have been studied by x‐ray diffraction and transmission electron microscopy. The most effective grain size enhancement was achieved by deep silicon ion implantation with the projected range located beyond the bottom interface to allow the maximum kinetic energy transfer at or near that interface. The grain size enhancement was due to a decrease of nucleation rate and an increase of the nucleation activation barrier from 3.9 to 4.9 eV for the Si+‐implanted sample. The amorphous‐to‐crystalline grain growth activation barrier of 3.2 eV was not altered by Si+ implantation, but the growth rate was decreased. Retardation of nucleation and enhancement of grain size are attributable to the implant‐recoiled oxygen. The average grain size increases from ∼0.2 to ∼2.0 μm by using 2×1015 cm−2 of Si+ implantation at 92 keV for 82‐nm‐thick films.

Method for producing amorphous silicon thin film transistor array substrate

Abstract: A method for producing an amorphous silicon thin film transistor array substrate comprising successively coating a gate insulating layer, an amorphous silicon layer and a protective insulating layer on a glass substrate provided with a gate electrode and a gate wiring having a predetermined shape, in such a manner as to not cover the connecting terminal region of the gate wiring. A protective insulating layer is patterned into a predetermined shape. After passing through a predetermined production process to produce an amorphous silicon thin film transistor array, at least a gate wiring and a source wiring are provided. The step of patterning the protective insulating layer comprises covering the connecting terminals of the gate wiring and the exposed region of the glass substrate with a photoresist.