Showing papers on "Amorphous silicon published in 1993"

Phase transformation mechanisms involved in excimer laser crystallization of amorphous silicon films

Abstract: We have investigated the phase transformation mechanisms and the resulting microstructures of excimer laser‐induced crystallization of amorphous Si films on SiO2. It is shown that the process can be characterized into two major regimes, based on the dependence of the grain size and the melt duration as a function of the incident energy density. It is found that at the transition between the two regimes, exceedingly large grain‐sized polycrystalline films can be obtained. We call this the super lateral growth phenomenon, and propose a model based on liquid‐phase regrowth from the residual solid seeds when near‐complete melting of the Si film occurs.

Bias‐stress‐induced stretched‐exponential time dependence of charge injection and trapping in amorphous thin‐film transistors

Abstract: The threshold voltage instabilities in nitride/oxide dual gate dielectric hydrogenated amorphous silicon (a‐Si:H) thin‐film transistors are investigated as a function of stress time, stress temperature, and stress bias. The obtained results are explained with a multiple trapping model rather than weak bond breaking model. In our model, the injected carriers from the a‐Si:H channel first thermalize in a broad distribution of localized band‐tail states located at the a‐Si:H/aSiNx:H interface and in the a‐SiNx:H transitional layer close to the interface, then move to deeper energies in amorphous silicon nitride at longer stress times, larger stress electric fields, or higher stress temperatures. The obtained bias‐stress‐temperature induced threshold voltage shifts are accurately modeled with a stretched‐exponential stress time dependence where the stretched‐exponent β cannot be related to the β=TST/T0 but rather to β≂TST/T0*−β0 for TST≤80 °C; for TST≥80 °C, the β is stress temperature independent. We have al...

Silicide formation and silicide‐mediated crystallization of nickel‐implanted amorphous silicon thin films

Abstract: The nucleation and growth of isolated nickel disilicide precipitates in Ni‐implanted amorphous Si thin films and the subsequent low‐temperature silicide‐mediated crystallization of Si was studied using in situ transmission electron microscopy. Analysis of the spatial distribution of the NiSi2 precipitates strongly suggested the occurrence of site saturation during nucleation. NiSi2 precipitates were observed in situ to migrate through the amorphous Si thin films leaving a trail of crystalline Si at temperatures as low as ∼484 °C. Initially, a thin region of epitaxial Si formed on {111} faces of the octahedral NiSi2 precipitates with a coherent interface which was shown by high‐resolution electron microscopy to be Type A. Migration of the NiSi2 precipitates led to the growth of needles of Si which were parallel to 〈111〉 directions. The growth rate of the crystalline Si was limited by diffusion through the NiSi2 precipitates, and an effective diffusivity was determined at 507 and 660 °C. A mechanism for the enhanced growth rate of crystalline Si is proposed.

Improved defect-pool model for charged defects in amorphous silicon.

Abstract: We have developed an improved defect-pool model for the calculation of the density of dangling-bond states in amorphous silicon. The results of this improved defect-pool model are contrasted with earlier work, from which we have eliminated some errors and approximations. We show that the calculated energy-dependent density of states depends on the specific microscopic reaction involving hydrogen, in contrast to previous conclusions. We calculate the bulk density ofstates, using the best input parameters drawn from experiment, and conclude that the best agreement with experimental results is found for a rather wide defect pool and for a microscopic model where two Si-H bonds break for every weak bond converted into two dangling bonds

Thermal conductivity and localization in glasses: Numerical study of a model of amorphous silicon.

Abstract: Numerical calculations of thermal conductivity κ(T) are reported for realistic atomic structure models of amorphous silicon with 1000 atoms and periodic boundary conditions. Using Stillinger-Weber forces, the vibrational eigenstates are computed by exact diagonalization in harmonic approximation. Only the uppermost 3% of the states are localized. The finite size of the system prevents accurate information about low-energy vibrations, but the 98% of the modes with energies above 10 meV are densely enough represented to permit a lot of information to be extracted. Each harmonic mode has an intrinsic (harmonic) diffusivity defined by the Kubo formula, which we can accurately calculate for ω>10 meV

Low temperature crystallization and patterning of amorphous silicon films on electrically insulating substrates

Abstract: A fabrication process polycrystalline silicon thin film transistors commences with the deposition of an ultra-thin nucleating-site forming layer onto the surface of an insulating substrate (e.g., 7059 glass). Next, an amorphous silicon film is deposited thereover and the combined films are annealed at temperatures that do not exceed 600° C. By patterning the deposition of the nucleating site forming material on the glass substrate, the subsequently deposited amorphous film can be selectively crystallized only in areas in contact with the nucleating-site forming material.

Electrical Properties of Amorphous Silicon Transistors and MIS‐Devices: Comparative Study of Top Nitride and Bottom Nitride Configurations

Abstract: The electrical properties of silicon nitride/amorphous silicon structures were investigated using thin film transistors (TFTs) and metal insulator semiconductor (MIS) devices employing either a top nitride (TN) or bottom nitride (BN) as gate insulator. The density of states (DOS) deduced from the subthreshold transfer characteristic of the TFTs is one to two orders of magnitude higher than that obtained from quasistatic C(V) measurements on the MIS structures. This difference is discussed by considering the different thickness of the a‐Si:H layers of the two devices and the role of a fixed charge at the rear interface. Both techniques indicate a DOS in BN devices which is only slightly lower than in TN devices, by less than a factor of two. The measured field effect mobility of BN TFTs is about 70% higher. The differences in the measured field effect mobility for TN and BN configuration are discussed and ascribed to the source and drain parasitic resistances. The conclusion is verified by the fabrication of a TN TFT with a pure phosphine rear surface treatment, which exhibits performance comparable to BN TFTs.

Polythienylenevinylene thin‐film transistor with high carrier mobility

Abstract: A thin‐film transistor (TFT) with high carrier mobility has been fabricated using precursor‐route poly(2,5‐thienylenevinylene) (PTV) as semiconductor. The carrier mobility has been determined to be 0.22 cm2/V s, which is in the same level of that of amorphous silicon TFT. It has also been made clear that the carrier mobility is linearly proportional to the conversion ratio from the insulated precursor polymer to π‐conjugated PTV. The π‐conjugation length is crucial to obtain high carrier mobility in π‐conjugated polymer TFT.

Origin of luminescence from porous silicon deduced by synchrotron-light-induced optical luminescence

Abstract: FOLLOWING reports of intense optical luminescence from porous silicon1,2, the opportunity for engineering optoelectronic devices using this material3,4 has attracted considerable attention. At present, however, the question of the origin of the luminescence has not been fully resolved5. The quantum-confinement model6–8 suggests that a quantum size effect gives optical transitions, and hence luminescence, in the visible range—this idea gains support from the wavelength dependence of the luminescence on porosity. An alternative model9,10 attributes the luminescence to siloxene-like compounds11 formed on the silicon surface. A third model, which invokes hydrogenated amorphous silicon as a possible source12,13, seems to be contradicted by X-ray absorption fine structure (XAFS) studies14–16. Here we report optical luminescence in porous silicon and siloxene induced by soft X-rays with energies near the silicon K-edge (1,839 eV). Using the luminescence together with the total electron yield, we can obtain the XAFS spectra for the luminescent sites in both materials. Our results show that the luminescence from porous silicon does not derive from siloxene (either freshly prepared or annealed), and thus suggest that the quantum-confinement model seems to provide the only viable explanation.

Defect formation during growth of hydrogenated amorphous silicon.

Abstract: The defects believed to limit the performance of devices fabricated using hydrogenated amorphous silicon are identified as singly occupied dangling bonds. The formation of these defects can be explained on the basis of two distinct models which attribute their occurrence to bulk thermodynamic equilibrium and surface diffusion of precursors during film growth, respectively. We present experimental results on the deposition-rate dependence of electronic and structural properties that are shown to be inconsistent with bulk thermal equilibration. The results, which were anticipated from the surface-diffusion model, are shown to be consistent with the underlying defect-formation mechanism. Simple isotopic substitution of hydrogen is shown to systematically alter the defect-formation rate, providing clear evidence of a surface-controlled defect-formation process during growth of hydrogenated amorphous silicon.

Hydrogenated Amorphous Silicon Alloy Deposition Processes

Abstract: Introduction material characteristics of amorphous silicon-based alloys film diagnostic measurements conventional glow discharge deposition processes for amorphous silicon-based alloys design of glow discharge deposition reactors glow discharge deposition parameters for hydrogenated amorphous silicon glow discharge deposition reaction chemistry for hydrogenated amorphous silicon modifications of conventional glow discharge remote-plasma-assisted chemical vapour deposition methods photochemical vapour deposition thermally-induced chemical vapour deposition physical vapour deposition methods etching properties of amorphous silicon-based alloys comparison of alternative deposition methods microcrystalline silicon and silicon carbide safety.

Multiple-order Raman scattering in crystalline and amorphous silicon.

Abstract: Raman-scattering measurements have been performed on c-Si and a-Si over a wide range of frequencies, including Stokes and anti-Stokes sides, and up to fourth order. All the features are accounted for by using the same physical parameters in both phases. In particular, it is shown that multiple-order scattering processes are not negligible, but rather of the same order of magnitude as first-order processes. In amorphous materials, light-scattering excess, spurious background, Boson-peak or hot-luminescence processes, which have been recently put forward, turn out to be mainly caused by high-order Raman-scattering processes.

Method of making a thin film transistor by overlapping annealing using lasers

Abstract: The present invention provides a method of making a thin film transistor for driving a liquid crystal display comprising the steps of forming a gate electrode on a glass substrate and forming an insulating layer and an amorphous silicon layer in turn on said glass substrate and said gate electrode, and scanning laser beams on the surface of said amorphous silicon layer with the end portions of the respective scanned laser beams being overlapped. According to the method of making a thin film transistor for driving a liquid crystal display of the present invention, a thin film transistor suitable for HDTV, the field effect mobility of which is high, is achieved. Further, in making a thin film transistor, a separate processing step is not required and the number of processing steps can be reduced because constructional features of a TFT are utilized.

Plasma deposition of hydrogenated amorphous silicon: Studies of the growth surface

Abstract: This review focusses on the plasma-surface interactions and surface processes involved in a-Si: H thin film growth. We restrict our discussion of growth fluxes to a summary, and do not address plasma kinetics. In recent years, powerful in situ experiments have been carried out on the growing film surface, which reveal the adsorption, penetration, reaction, and elimination of precursor species, as well as the atomic-scale morphology and composition of the growth zone. Good data sets are available both for PACVD and reactive magnetron sputter deposition. These form an interesting comparison, since the former process is dominated by the hydrogen-rich radical SiH3 at low energy, and the latter by energetic atomic Si and H. We review the key experiments and conclusions, underlining those aspects which are well established and those which remain qualitative; and we discuss the transition from amorphous to fine-grained polycrystalline film growth at high hydrogen pressures in terms of the surface mechanisms. This field is now entering a scientific stage where a detailed theory of low-temperature, plasma-assisted growth can be developed.

Negative hydrogenated silicon ion clusters as particle precursors in RF silane plasma deposition experiments

Abstract: Stable negative ions containing up to sixteen silicon atoms have been measured by mass spectrometry in RF power-modulated silane plasmas for amorphous silicon deposition. These hydrogenated silicon cluster ions reach much higher masses than the positive ions, which have no more then six silicon atoms. This supports the view that negative ions are the precursors to particulate formation in silane plasmas. The time-dependent fluxes of positive and negative ions from the plasma are shown with a 5 mu s time resolution. Possible cluster reaction sequences are discussed and the effect of visible light on the negative ion signal is commented upon.

Room temperature reactions involving silicon dangling bond centers and molecular hydrogen in amorphous SiO2 thin films on silicon

Abstract: Exposing thin films of amorphous SiO2 to molecular hydrogen at room temperature converts some silicon dangling bond defects, E’ centers, into two hydrogen coupled complexes These reactions may play important roles in radiation and hot carrier instabilities in metal/oxide/silicon devices

Manufacture of semiconductor

Abstract: PURPOSE: To obtain a polycrystalline silicon semiconductor film which has an excellent electric characteristic by a method wherein a hydrogenated amorphous silicon film is formed at low temperatures and is heat-treated in a vacuum and then it is dehydrogenated to generate a dangling bond in the film and the excimer laser is cast on the film in a vacuum-unbroken state. CONSTITUTION: An SiO 2 film or silicon nitride film is formed as a base protective film 12 on a glass substrate 11. Nextly, an intrinsic hydrogenated amorphous silicon semiconductor layer 13 is formed in the thickness of 100nm by the plasma CVD method. At that time, by setting the film formation temperature low, the formed amorphous silicon film is allowed to have in it a good quantity of water and bonds of silicon are neutralized with hydrogen as much as possible. Nextly, a device separation patterning is conducted and the sample is heated in a vacuum at 450°C for one hour to be dehydrogenated completely and dangling bonds (unpaired bonds) are generated in high density in the film. With the vacuum state being maintained, the excimer laser is cast on the sample to crystallize it. COPYRIGHT: (C)1993,JPO&Japio

Method for manufacturing a liquid crystal display substrate

Abstract: An amorphous silicon film is formed on a glass substrate by a CVD method, and then the island regions of the amorphous silicon film is changed to a plurality of polycrystalline silicon regions which are arranged in a line and apart with each other in a predetermined distanced by intermittently irradiating laser pulses each having the same dimensions as those of the island region onto the amorphous silicon film, using a laser beam irradiating section. Switching elements including the island regions as semiconductor regions are formed by etching and film-forming process to constitute a driving circuit section. The section is divided to gate driving circuit sections and source driving circuit sections for driving thin film transistors formed in a pixel region.

Method of forming polycrystalline silicon film in process of manufacturing LCD

Abstract: In a method of forming a polycrystalline silicon film in a process of manufacturing an LCD, a hydrogenated amorphous silicon film is formed on a glass substrate by plasam CVD throughout areas serving as the pixel portion and driver unit of the LCD. A laser beam is radiated on a selected region of the film on the area serving as the driver unit. The energy of the laser beam is set such that hydrogen in the film is discharged without crystallizing the film and damaging the film. The energy of the laser beam is gradually increased to gradually discharge hydrogen from the film. The energy of the laser beam is finally set such that the film is transformed into a polycrystalline silicon film. The amorphous silicon film can be poly-crystallized without damaging the film by the discharge of hydrogen.

Conducting filament of the programmed metal electrode amorphous silicon antifuse

Abstract: Antifuses in PROM and FPGA applications have used silicon and/or polycrystalline silicon electrodes. Metal electrode antifuses have the lowest resistance and lowest capacitance among programmable interconnect structures. The ViaLink, a metal electrode amorphous silicon antifuse, has been used as a programmable interconnect device for a FPGA. This paper describes for the first time, the composition, structure, electrical characteristics, and temperature dependence of the conducting filament in the programmed TiW electrode amorphous silicon antifuse. >

Pulsed laser‐induced melting followed by quenching of silicon films

Abstract: Amorphization and crystallization were studied through laser‐induced melting of silicon films formed on quartz substrates induced by irradiation with a 30 ns XeCl excimer laser. Homogeneous and rapid solidification occurs and amorphous solid state can be formed when the melt duration is long enough to make the temperature gradient in liquid silicon lower than 1×105 K/cm at the Si/quartz interface. The solid state after homogeneous solidification is governed by recalescence caused by latent heat released at solidification. A completely amorphous state is formed when film thickness is thinner than 24 nm because latent heat reduces as film thickness decreases. Both crystalline and amorphous states were observed for film thickness above 24 nm because recalescence can cause crystalline grain growth. Complete crystallization occurs through interface controlled growth when the temperature gradient is higher than 1×105 K/cm. The velocity of liquid/solid interface is 0.6 m/s, which is too low to cause amorphization.

Optimization of ZnO Films for Amorphous Silicon Solar Cells

Abstract: ZnO films prepared by the metalorganic chemical vapor deposition (MOCVD) method were applied to amorphous silicon (a-Si) solar cells as a transparent conductive oxide (TCO). The B2H6 flow rate and the thickness of the film were optimized at 0.5 µmol/min and 2.0 µm, respectively. We also proposed a preannealing technique of the ZnO film before fabrication of a-Si solar cells, and it was shown that the open circuit voltage was increased with use of this technique. Furthermore, we fabricated a novel p-i-n a-Si solar cell structure, which had a 0.1-µm-thick ZnO layer between a-Si and the metal electrode. Theoretical calculation and experimental results proved that the reflectance of the ZnO/Ag back reflector was higher than that of the Ag electrode at wavelengths between 600 nm and 800 nm. As a result, a marked enhancement of collection efficiencies in the long-wavelength region was achieved, resulting in the increase of the short-circuit current of about 1 mA/cm2. Using both the preannealing technique of ZnO film and a ZnO/Ag/Al back reflector, a conversion efficiency of 11.9% (Voc=0.893 V, Isc=18.6 mA/cm2, FF=0.715) was obtained.

Principles for controlling the optical and electrical properties of hydrogenated amorphous silicon deposited from a silane plasma

Abstract: The optical, electrical, and structural properties of hydrogenated amorphous silicon (a‐Si:H) films are systematically investigated as functions of the substrate temperature (Ts) and plasma parameters, such as the rf power, gas pressure, and electrode dimensions. The films are deposited by the plasma chemical vapor deposition method. The properties of a‐Si:H can be controlled over a wide range by varying the plasma parameters at fixed Ts. Reducing the film deposition rate and raising Ts have the same effect on the properties of a‐Si:H. A unified relationship is found to exist among those properties of a‐Si:H in the range of deposition conditions in this study, which includes ‘‘device‐quality’’ conditions. No apparent effects of gas‐phase polymerization or ion bombardment are observed. The experimental results suggest that during device‐quality a‐Si:H film deposition under conventional plasma conditions, the film properties are governed by a competition between the rate of film growth and the rate of thermally activated surface reactions at or near the film‐growing surface. The limitations on the controllability of plasma‐deposited a‐Si:H, especially at low Ts, can be surmounted by adding hydrogen or helium to the plasma, and by treating a‐Si:H with the hydrogen plasma.

Experimental test of kinetic theories for heterogeneous freezing in silicon.

Abstract: The crystallization rate of liquid silicon has been measured during epitaxial explosive crystallization of amorphous silicon. The measurements, together with numerical temperature calculations indicate that freezing in silicon saturates at 15.8 m/s for large undercooling (g130 K) below the equilibrium melting temperature. These data, as well as a variety of experimental results of other investigators, are used to test two models describing the kinetics of heterogeneous freezing. A transition-state theory in which the phase transformations are assumed to go through an intermediate state at a rate limited by the sound velocity is not consistent with the data. A theory in which the rate-limiting factor in freezing of liquid silicon is atomic diffusion in the liquid close to the interface describes the data well. The activation energy for self-diffusion of atoms in the liquid near the interface is found to be 0.7--1.1 eV.

Strained-layer van der Waals epitaxy in a Langmuir-Blodgett film

Abstract: Atomic force microscope images of Langmuir-Blodgett films of lead and manganese fatty acid salts show that these monolayers have long-range order and are oriented with respect to the mica substrate, although the lattice symmetries of the monolayers and substrate are dramatically different. The surface lattice of sequentially thicker films evolves toward the bulk structure while retaining the substrate alignment. This behavior is in distinct contrast to films of cadmium fatty acid salts on mica, or all films on amorphous silicon oxide, in which the monolayer structure is disordered and a three-layer-thick film displays the bulk structure.

Low temperature process for laser dehydrogenation and crystallization of amorphous silicon

Abstract: A low temperature process for dehydrogenating amorphous silicon using lasers Dehydrogenation occurs by irradiating one or more areas of a hydrogenated amorphous silicon layer with laser beam pulses at a relatively low energy density After the multiple laser pulse irradiation at a relatively low energy density, the laser energy density is increased and multiple irradiation at a higher energy density is performed If after the multiple irradiation at the higher energy density the amorphous silicon hydrogen content is still too high, dehydrogenation proceeds by multiple irradiations at a yet higher energy density The irradiation at the various energy densities can result in the formation of polysilicon due to melting of the amorphous silicon layer As irradiation may be selectively applied to the amorphous silicon, an integral amorphous silicon-polysilicon structure may be formed

Density changes and viscous flow during structural relaxation of amorphous silicon

Abstract: Structural relaxation of amorphous silicon (a‐Si) surface layers made by ion irradiation has been studied during heating using wafer curvature measurements. These measurements, which determine the stress in the amorphous layer, are sensitive to both plastic deformation and density changes. The amorphous layer first expands (0.1%) on heating from room temperature to 250 °C and then densifies (0.1%) on heating further to 500 °C. A larger expansion (≥0.3%) is observed on heating liquid‐nitrogen‐temperature irradiated a‐Si to room temperature. This behavior reveals the existence of two distinct relaxation regimes, and is explained in terms of the annihilation of complementary features of the amorphous covalent network. In addition to density changes, shear deformation was observed during heating the a‐Si layers. This deformation was characterized by a Newtonian shear viscosity of roughly 3×1012 N s/m2. The thermal‐expansion coefficient of the a‐Si was determined to be roughly 6.5% smaller than that of crystalline Si. Stress changes due to crystallization by epitaxial regrowth were observed between 600 and 700 °C and revealed evidence for the existence of large compressive stresses at the amorphous‐crystalline interface.

Electrically programmable interconnect structure having a PECVD amorphous silicon element

Abstract: In one method for forming amorphous silicon antifuses with significantly reduced leakage current, a film of amorphous silicon is formed in a antifuse via between two electrodes. The amorphous silicon film is deposited using plasma enhanced chemical vapor deposition, preferably in an silane-argon environment and at a temperature between 200 and 500 degrees C., or reactively sputtered in a variety of reactive gases. In another method, an oxide layer is placed between two amorphous silicon film layers. In yet another method, one of the amorphous silicon film layers about the oxide layer is doped. In another embodiment, a layer of conductive, highly diffusible material is formed either on or under the amorphous silicon film. The feature size and thickness of the amorphous silicon film are selected to minimize further the leakage current while providing the desired programming voltage. A method also is described for for forming a field programmable gate array with antifuses.