Showing papers on "Amorphous silicon published in 1992"

In situ transmission electron microscopy studies of silicide‐mediated crystallization of amorphous silicon

Abstract: The silicide‐mediated phase transformation of amorphous to crystalline silicon was observed in situ in the transmission electron microscope. Crystallization of nickel‐implanted amorphous silicon occurred at ∼500 °C. Nickel disilicide precipitates were observed to migrate through an amorphous Si film leaving a trail of crystalline Si. Growth occurred parallel to 〈111〉 directions. High resolution electron microscopy revealed an epitaxial NiSi2/Si(111) interface which was Type A. A diffusion‐controlled mechanism for the enhanced crystallization rate was determined.

Novel technique for preparing porous silicon

Abstract: We have performed photoluminescence studies on porous, p‐type as well as n‐type silicon wafers which have been prepared in air or in a dry nitrogen atmosphere, utilizing a spark‐erosion technique. This sample preparation, which does not involve aqueous solutions or fluorine contaminants, yields similar photoluminescence spectra as those obtained by anodic etching in HF or unbiased etching in various HF‐containing reagents. The wavelength of the photoluminescence peaks are somewhat shifted into the blue region compared to porous silicon obtained by anodic etching. We have also taken photoluminescence spectra on amorphous silicon, SiO2, and oxidized, annealed porous silicon. Our results are interpreted in the light of the presently suggested theories.

Photoluminescence studies on porous silicon

Abstract: We have measured the temperature dependence of the photoluminescence of porous silicon and have found that it disagrees with the expected behavior of crystalline or amorphous silicon. We also found that soaking the samples in oxygen and simultaneously illuminating them with light results in the quenching of the photoluminescence. We propose that luminescence in porous silicon may actually be luminescence from molecules attached to the Si surface, rather than any previously assumed quantum size effect.

Direct evidence for the amorphous silicon phase in visible photoluminescent porous silicon

Abstract: We report on micro‐Raman spectroscopy studies of porous silicon which show an amorphous silicon Raman line at 480 R cm−1 from regions that emit visible photoluminescence. A Raman line corresponding to microcrystalline silicon at 510 R cm−1 is also observed. X‐ray photoelectron spectroscopy data is presented which shows a high silicon‐dioxide content in porous silicon consistent with an amorphous silicon phase.

Crystallization of silicon in aluminium/amorphous-silicon multilayers

Abstract: The crystallization of amorphous Si (a-Si) in Al/a-Si multilayer thin films has been investigated by ex situ and in situ transmission electron microscopy (TEM), X-ray diffraction and calorimetry. The a-Si crystallizes at about 200°C, a significantly lower temperature than for the pure elemental state, with a heat of crystallization of about 12 kJ (mol Si)−1. We show that crystalline Si (c-Si) nucleates within the Al layers and penetrates the Al as the c-Si grows. The speed of the growth of c-Si observed by in situ TEM was a few angstroms per second at 220°C. Al grains are separated and the layered structure is destroyed, while the Al(111) film texture is enhanced. The overall activation energy of the reaction, determined by calorimetry, is 1·2 ± 01 eV. We propose a model in which diffusion of Si through the Al grains and rearrangement of the Al grains occur simultaneously.

Thin film transistor and semiconductor device including a laser crystallized semiconductor

Abstract: A semiconductor material and a method for forming the same, the semiconductor material having fabricated by a process comprising irradiating a laser beam or a high intensity light equivalent to a laser beam to an amorphous silicon film containing therein carbon, nitrogen, and oxygen each at a concentration of 5×10 19 atoms·cm -3 or lower, preferably 1×10 19 atoms·cm -3 or lower, without melting the amorphous silicon film. The present invention provides thin film semiconductors having high mobility at an excellent reproducibility, the semiconductor materials being useful for fabricating compact thin film semiconductor devices such as thin film transistors improved in device characteristics.

Switching in amorphous devices

Abstract: Electrical switching of a kind can be observed in a great variety of materials in many different forms and structures. Much the most reproducible switching is observed, however, in small and geometrically well-defined devices fabricated from thin films of certain amorphous semiconductors. This paper is concerned with such devices. The general features of electrical switching are first reviewed, followed by a more detailed account of recent work in the authors' own laboratories on digital and analogue switching in hydrogenated amorphous silicon.

Method for producing a roughened surface capacitor

Abstract: A new method to produce a microminiturized capacitor having a roughened surface electrode is achieved. The method involves depositing a first polycrystalline or amorphous silicon layer over a suitable insulating base. The silicon layer is either in situ heavily, uniformly doped or deposited undoped and thereafter heavily doped by ion implantation followed by heating. The structure is annealed at above about 875° C. to render any amorphous silicon polycrystalline and to adjust the crystal grain size of the layer. The polysilicon surface is no subjected to a solution of phosphoric acid at a temperature of above about 140° C. to partially etch the surface and cause the uniformly roughened surface. A capacitor dielectric layer is deposited thereover. The capacitor structure is completed by depositing a second thin polycrystalline silicon layer over the capacitor dielectric layer.

Determination of diffusion mechanisms in amorphous silicon.

Abstract: We have established experimentally and theoretically that transition metals in amorphous Si undergo direct interstitial diffusion that is retarded by temporary trapping at the defects intrinsic to the amorphous structure. The diffusion of Cu, Zn, Pd, Ag, Pt, and Au has been investigated by means of Rutherford-backscattering spectrometry and that of Au tracer atoms by neutron-activation and sputter-sectioning analysis. The data can be fitted using the foreign-atom interstitial diffusion coefficients in crystalline Si modified due to the presence of traps with concentrations between 0.2 and 1 at.% and trapping enthalpies of about 0.9 eV.

Pulsed gas plasma-enhanced chemical vapor deposition of silicon

Abstract: A substrate having silicon receptive surface areas is maintained in a plasma enhanced chemical vapor deposition (PECVD) chamber at a temperature, and under sufficient gas flow, pressure and applied energy conditions to form a gas plasma. The gas plasma is typically made up of hydrogen, but may be made up of mixtures of hydrogen with other gasses. A discontinuous flow of silane gas of predetermined duration and predetermined time spacing is introduced to produce at least one timed pulse of silane gas containing plasma, whereby a thin layer of silicon is deposited on the receptive areas of the substrate. The thin layer of silicon is exposed to the hydrogen gas plasma between the brief deposition time cycles and may result in the modification of the silicon layer by the hydrogen plasma. The surface modification may include at least one of etching, surface hydrogenation, surface bond reconstruction, bond strain relaxation, and crystallization, and serves the purpose of improving the silicon film for use in, for example, electronic devices. Repeated time pulses of silane gas and subsequent hydrogen plasma exposure cycles can result in selective deposition of silicon on predetermined receptive areas of a patterned substrate. Selective deposition of silicon can serve the purpose of simplifying electronic device manufacturing, such as, for example, the fabrication of amorphous silicon thin film transistors with low contact resistance in a single PECVD pump-down procedure.

Semiconductor substrate having a gettering layer

Abstract: A silicon wafer having a low concentration of oxygen and a silicon wafer having a high concentration of oxygen are joined and polished to prescribed thicknesses to form a semiconductor substrate according to the present invention. A region formed of the wafer having a low concentration of oxygen is used as a region where an element is formed, and a region formed of the wafer having a high concentration of oxygen produces a gettering effect on metal impurities and defects. As a DZ layer having a low concentration of oxygen, a wafer manufactured by an MCZ method or a wafer manufactured by a CZ method is used after being heat-treated at high temperature to diffuse oxygen outward. In another example, a damage layer, a polycrystalline silicon layer, an amorphous silicon layer or the like is formed between a DZ layer and an IG layer.

Anti-fuse structures and methods for making same

Abstract: An anti-fuse structure characterized by a substrate, an oxide layer (46) formed over the substrate having an opening (48) formed therein, an amorphous silicon material (52) disposed within the opening and contacting the substrate, and oxide spacers (64) lining the walls of a recess formed within the amorphous silicon. The spacers prevent failures of the anti-fuse structures by covering cusps formed in the amorphous silicon material. The method of the present invention forms the above-described anti-fuse structure and further solves the problem of removing unwanted spacer material from areas outside of the anti-fuse structure locations.

Structure of As‐Deposited LPCVD Silicon Films at Low Deposition Temperatures and Pressures

Abstract: In this work we studied the effect of the deposition temperature, total pressure, source gas dilution, and deposition rate on the structure of the as‐deposited silicon films. Depositions were performed by low pressure chemical vapor deposition (LPCVD) in the temperature range of 530 to 600°C and in the pressure range of 2 to 300 mTorr. For a fixed deposition temperature a phase transition from polycrystalline to amorphous silicon was shown to occur when the deposition rate exceeded a critical value. The critical value for the deposition rate was found to depend only upon the deposition temperature and to decrease as the temperature was decreased. By controlling the rate, as‐deposited polycrystalline silicon was obtained by conventional LPCVD at temperatues as low as 530°C. A relationship between the deposition rate and the partial pressure of the source gas was established via a kinetic model for the decomposition of silane and used to provide a simple model for the dependence of the structure of the as‐deposited silicon films upon the deposition parameters. This model was subsequently used to provide guidelines for both the expected structure of the as‐deposited films and the grain size of the as‐deposited polycrystalline silicon films over an extensive range of deposition conditions.

Effect of microvoids on initial and light‐degraded efficiencies of hydrogenated amorphous silicon alloy solar cells

Abstract: Using a combination of infrared absorption and small‐angle x‐ray scattering on hydrogenated amorphous silicon alloy films and efficiency measurements of solar cells with intrinsic layers prepared under nominally identical conditions to those for the deposition of the films, we observe a correlation between microstructure in the films and solar cell performance. With increasing microvoid density, both the initial and light‐degraded performance of solar cells are found to deteriorate.

Influences of a high excitation frequency (70 MHz) in the glow discharge technique on the process plasma and the properties of hydrogenated amorphous silicon

Abstract: Hydrogenated amorphous silicon has been prepared at a plasma excitation frequency in the very-high-frequency band at 70 MHz with the glow discharge technique at substrate temperatures between 280 and 50-degrees-C. The structural properties have been studied using hydrogen evolution, elastic recoil detection analysis, and infrared spectroscopy. The films were further characterized by dark and photoconductivity and by photothermal deflection spectroscopy. With respect to films prepared at the conventional frequency of 13.56 MHz considerable differences concerning the electronic and structural properties are observed as the substrate temperature is decreased from 280 to 50-degrees-C. Down to a substrate temperature of 150-degrees-C the electronic film properties change only a little and the total hydrogen content c(H) and the degree of microstructure that can be directly correlated to c(H) increase only moderately. Below 150-degrees-C the electronic properties deteriorate in the usual manner but still the total hydrogen content does not exceed 21 at.% even at a substrate temperature of 50-degrees-C. It is argued that the influence of the higher excitation frequency on the plasma and on the growth kinetics plays a key role in this context by allowing a highly effective dissociation of the process gas with the maximum ion energies remaining at low levels. It is concluded that deposition processes at higher excitation frequencies can have important technological implications by allowing a decrease of the deposition temperature without losses in the material quality.

Determination of complex dielectric functions of ion implanted and implanted-annealed amorphous silicon by spectroscopic ellipsometry

Abstract: Measuring with a spectroscopic ellipsometer (SE) in the 1.8–4.5 eV photon energy region we determined the complex dielectric function (ϵ = ϵ1 + iϵ2) of different kinds of amorphous silicon prepared by self‐implantation and thermal relaxation (500 °C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanted a‐Si (i‐a‐Si) differs from that of relaxed (annealed) a‐Si (r‐a‐Si). Moreover, its ϵ differs from the ϵ of evaporated a‐Si (e‐a‐Si) found in the handbooks as ϵ for a‐Si. If we use this ϵ to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the Davis–Mott plot based on the relation: (ϵ2E2)1/3 ∼ (E− Eg). The results are: 0.85 eV (i‐a‐Si), 1.12 eV (e‐a‐Si), 1.30 eV (r‐a‐Si). We attribute the optical change to annihilation of point defects.

High quality photovoltaic semiconductor material and laser ablation method of fabricating same

Abstract: A high quality, narrow band gap, hydrogenated amorphous germanium or amorphous silicon alloy material characterized by a host matrix in which all hydrogen is incorporated therein in germanium monohydride or silicon monohydride form, respectively; their mobility-lifetime product for non-equilibrium charge carriers is about 10-8 and about 10-7, respectively; their density of defect states in the band gap thereof is less than about 1 x 1017 and about 2 x 1016/cm3, respectively; and their band gap is about 1.5 and about 0.9 eV, respectively. There is also disclosed a structure formed from a plurality of very thin layer pairs of hydrogenated amorphous germanium and amorphous silicon alloy material, each layer pair of which cooperates to provide narrow band gap material. From about 3 to about 7 atomic percent fluorine is added to the germanium and/or silicon alloy material so as to provide a strong bond (as compared to hydrogen) so as to provide reduced sensitivity to Stabler/Wronski degradation. The preferred method of fabricating such improved narrow band gap materials is through a laser ablation process in which hydrogen or fluorine gas is introduced for incorporation into the germanium or silicon host matrix, thereby eliminating the reliance on the zoo of precursor species present in r.f. or microwave plasma process. The apparatus (10) employed includes an excimer laser (1) which produces pulsed UV light (2) which passes through a focusing lens (3) and a quartz window (4) in the vacuum chamber (5) and strikes a silicon or germanium target (6) which is mounted on an axle (7). A plasma zone (9) is created within which one or more heated substrates (8) are mounted.

35% efficient nonconcentrating novel silicon solar cell

Abstract: An energy conversion efficiency of 35% was obtained at 1‐sun, air mass 1.5 for a novel silicon cell. The critical feature of the cell structure is the inclusion of local defect layers near a p‐n junction. The local defect layers were proven to hold the key to achieving the exceptionally high efficiency of the novel cell fabricated via noncomplex processing.

Process for producing a large area solid state radiation detector

Abstract: A process for producing an array of solid state radiation detectors includes depositing on a substrate one or more layers of silicon-based materials and then depositing a metal layer overlying silicon-based substance. The metal layer is formed into an array of metal layer regions, and then the metal layer is used as a mask to remove exposed adjacent silicon-based substance layers thereby forming an array of silicon-based substance layers that are aligned with the array of metal layers for forming an array of photosensitive sensing devices. The process of the present invention reduces the number of microlithography steps that are used in forming an array of layered amorphous silicon photosensitive devices.

Method of manufacturing polysilicon film including recrystallization of an amorphous film

Abstract: A method of fabricating a polysilicon film whose crystal grain size can be controlled in a wide range and which has a large surface area and an application thereof to a DRAM are disclosed. In polycrystallizing an amorphous silicon film having a substantially clean surface, nucleation and crystal growth are performed under different conditions. With this method, crystal grain density and crystal grain size can be controlled easily, causing a polysilicon film having finer grains to be formed concomitant with reduction of capacitor area due to increase of integration density of DRAM.

The properties of free carriers in amorphous silicon

Abstract: The properties of free carriers photogenerated in the extended states of hydrogenated amorphous silicon have been investigated using the techniques of femtosecond time-resolved spectroscopy. The optical susceptibility of free carriers can be described by a Drude model with a relaxation time shorter than 1 fs. This relaxation time seems to remain constant for various experimental situations. It implies a very small mobility (∼ 6 cm 2 /V s) in the extended states. The hot carriers thermalize quickly to the mobility edge by emission of phonons. The thermalization rate is found to be ≥ 1 eV/ps. In addition, the decay time of optic phonons into acoustic phonons is ≤ 100 fs. The photogenerated carriers recombine non-radiatively in a time that can be as short as 1 ps at very large injected density ( N ≤ 10 21 cm −3 ). Several regimes are distinguished, depending on the value of N . In general, the characteristic times for all these processes are much shorter in a-Si:H than in a typical direct-gap crystalline semiconductor such as GaAs. The difference can be traced to the lack of momentum conservation in amorphous semiconductors.

A study of the effect of composition on the microstructural evolution of a–SixC1−x: H PECVD films: IR absorption and XPS characterizations

Abstract: Amorphous silicon carbide films (a–SixC1−x :H) deposited by the argon- or helium-diluted PECVD technique were studied as a function of their composition. Microstructural investigations were mainly achieved by means of FTIR and XPS techniques. Nuclear techniques were used to obtain precise information on the film hydrogen content. The Si–H IR-absorption band was deconvoluted in different monohydride and dihydride silicon environments. The existence of SiH2 bonds in the Si-rich composition was evidenced. From the analysis of the C–H and Si–H absorption bands it is shown that hydrogen atoms are preferentially bonded to carbon atoms. The deconvolution of the Si2p core level peak suggests that above a composition of x ∊ 0.5, the noncarburized (Si, Si, H) local environment contribution increases to the detriment of the hydrocarburized (Si, C, H) environments. From the evolution of the C1s peak, it can be deduced that there is a change in the carbon atom bonding states when the film composition is varied. These results are correlated and discussed in terms of the local bonding environments and their evolution with film composition.

Crystallization and doping of amorphous silicon on low temperature plastic

Abstract: A method or process of crystallizing and doping amorphous silicon (a-Si) on a low-temperature plastic substrate using a short pulsed high energy source in a selected environment, without heat propagation and build-up in the substrate. The pulsed energy processing of the a-Si in a selected environment, such as BF3 and PF5, will form a doped micro-crystalline or poly-crystalline silicon (pc-Si) region or junction point with improved mobilities, lifetimes and drift and diffusion lengths and with reduced resistivity. The advantage of this method or process is that it provides for high energy materials processing on low cost, low temperature, transparent plastic substrates. Using pulsed laser processing a high (>900° C.), localized processing temperature can be achieved in thin films, with little accompanying temperature rise in the substrate, since substrate temperatures do not exceed 180° C. for more than a few microseconds. This method enables use of plastics incapable of withstanding sustained processing temperatures (higher than 180° C.) but which are much lower cost, have high tolerance to ultraviolet light, have high strength and good transparency, compared to higher temperature plastics such as polyimide.

Modeling of amorphous-silicon thin-film transistors for circuit simulations with SPICE

Abstract: A static and dynamic model for amorphous silicon thin-film transistors is presented. The theory is based on an assumed exponential distribution of the deep states and the tail states in the energy gap. Expressions are derived that link the density of the localized states and the temperature to the drain current and the distribution of the charge in the transistor channel. In addition the authors take into account parasitic effects such as channel length modulation, off-resistance, drain and source resistances, mobile and free charges in the insulator, surface states, and overlap capacitances. The model is incorporated into the circuit simulation program SPICE. Charge conservation problems are overcome by using a charge-oriented dynamic transistor model. Simulated and measured current-voltage characteristics agree well. A 96-b gate line driver for addressing liquid-crystal displays, which was successfully designed and optimized with the model, is introduced. >

Method of fabricating a polycrystalline silicon film having a reduced resistivity

Abstract: Disclosed is a method of forming a polycrystalline silicon film on a silicon oxide film in which the polycrystalline silicon film includes crystal grains having a large size, typically 4 micrometers, thereby permitting the resistivity of the polycrystalline silicon film to effectively be reduced. An amorphous silicon film is deposited on the silicon oxide film by using a chemical vapor deposition in which the flow rate of impurity gas remains at zero during an initial deposition, after which the flow rate is gradually increased from zero to a predetermined value during a final deposition. Thus, the amorphous silicon film comprises double layers, or an impurity unmixed region abutting the silicon oxide film and an impurity mixed region. After that, by a heat treatment, the amorphous silicon film is crystallized to form a polycrystalline silicon film. Concurrently, the impurity diffusion is accomplished.

Thin-film ROM devices and their manufacture

Abstract: A thin-film ROM device includes an array of open circuit and closed-circuit cells (5 to 8) formed from a stack of thin films (12,21,22,23,11) on a glass or other substrate (10). The semiconductor films (21,22,23) may be of hydrogenated amorphous silicon. At least one of the semiconductor films (21,22,23) is removed from some of the closed-circuit cell areas (5,7,8) before depositing the next film. In this way, at least a second type of thin-film diode (MIM, MIN, MIP) is formed having a different conduction characteristic to that of a first type (NIP), so increasing the information content of the ROM array. A lower semiconductor film (23) can be readily etched away from the lower electrode film (11) by a selective etching treatment in which the electrode film (11) acts as an etch stop. By monitoring emissions during plasma etching, an upper semiconductor film (21 or 22) can be removed from a lower semiconductor film (22 or 23). PN diodes and back-to-back PNP or NPN or PINIP or NIPIN diodes may be formed for some of the closed-circuit cells.

Bond selectivity in silicon film growth.

Abstract: Hydrogen atoms can selectively eliminate strained bonds that form during the growth of amorphous silicon films. By periodically interrupting the growth and exposing the grown material to hydrogen, the film composition can be varied continuously from a non-equilibrium amorphous structure to that of a crystalline solid. Furthermore, by tuning the hydrogen exposure it is possible to discriminate between Si-Si bonds formed on different substrates, thereby allowing substrate-selective growth. The evolution of the film structure during hydrogen exposure is directly observed by scanning tunneling microscopy, and a model describing the role of hydrogen is presented.

Method of epitaxial growth of semiconductor

Abstract: A method of epitaxially growing semiconductor crystal by which a single crystal region which is superior in quality can be selectively formed at a high throughput without employing the lithography technique. A shield mask is formed on an upper face of an amorphous semiconductor layer formed on substrate, and excimer laser light is irradiated upon the amorphous semiconductor layer using the shield mask to produce, in the amorphous semiconductor layer, a core from which crystal is to be grown. After the shield mask is removed, low temperature solid phase annealing processing for the amorphous semiconductor layer is performed to grow crystal from the core to form a single crystal region in the amorphous semiconductor layer. Alternatively, the silicon core is formed by irradiating an energy beam, which is capable of being converged into a thin beam and being used to directly draw a picture, at a predetermined position of the amorphous silicon film.