Showing papers on "Amorphous silicon published in 1991"

Hydrogenated amorphous silicon

Abstract: 1. Introduction 2. Growth and structure of amorphous silicon 3. The electronic density of states 4. Defects and their electronic states 5. Substitutional doping 6. Defect reactions, thermal equilibrium and metastability 7. Electronic transport 8. Recombination of excess carriers 9. Contacts, interfaces and multilayers 10. Amorphous silicon device technology.

Deposition of device quality, low H content amorphous silicon

Abstract: Device‐quality hydrogenated amorphous silicon containing as little as 1/10 the bonded H observed in device‐quality glow discharge films have been deposited by thermal decomposition of silane on a heated filament. These low H content films show an Urbach edge width of 50 mV and a spin density of ∼1/100 as large as that of glow discharge films containing comparable amounts of H. High substrate temperatures, deposition in a high flux of atomic H, and lack of energetic particle bombardment are suggested as reasons for this behavior.

Hydrogen in semiconductors

Abstract: :N. H. Nickel, Introduction to Hydrogen in Semiconductors II. Noble M. Johnson and Chris G. Van de Walle, Isolated Monatomic Hydrogen in Silicon. Yu. V. Gorelkinskii, Electron Paramagnetic Resonance of Hydrogen and Hydrogen-Related Defects in Crystalline Silicon. N. H. Nickel, Hydrogen in Polycrystalline Silicon. W. Beyer, Hydrogen Phenomena in Hydrogenated Amorphous Silicon. Chris G. Van de Walle, Hydrogen Interactions with Polycrystalline and Amorphous Silicon-Theory. K. M. McNamara Rutledge, Hydrogen in Polycrystalline CVD Diamond. R. L. Lichti, Dynamics of Muonium Diffusion, Site Changes and Charge-State Transitions. Matthew D. McCluskey and Eugene E. Haller, Hydrogen in III-V and II-VI Semiconductors. S. J. Pearton and J. W. Lee, The Properties of Hydrogen in GaN and Related Alloys. Jorg Neugebauer and Chris G. Van de Walle, Theory of Hydrogen in Ga N.

Structural relaxation and defect annihilation in pure amorphous silicon

Abstract: Thick amorphous Si layers have been prepared by MeV self-ion-implantation and the thermodynamic and structural properties examined by calorimetry, Raman-spectroscopy, and x-ray-diffraction techniques. Defects have been introduced into well-annealed amorphous and single-crystal Si by He, C, Si, and Ge bombardment. The defect structures are examined by these techniques and by transmission electron microscopy. The structure of amorphous Si in intermediate states of relaxation or annealing have been determined. It is shown that amorphous Si formed by either implantation or deposition contains a large population of point defects and point-defect clusters. Amorphous Si formed by laser quenching cannot be distinguished from well-annealed amorphous Si. Structural relaxation, also known as short-range ordering, can be understood as annihilation of a large fraction of these defects. Both structural relaxation in amorphous Si and defect annihilation in crystalline Si obey bimolecular reaction kinetics. The defect-formation and -annihilation processes are similar in amorphous and crystalline Si. Defect saturation occurs in amorphous Si at estimated defect concentrations of about 1 at. %. These formation and annihilation properties are intrinsic to pure amorphous Si. For hydrogenated amorphous Si, it is pointed out that the metastable-defect-creation and -annealing processes are essentially different from the annihilation processes in pure amorphous Si.

A fully automated hot-wall multiplasma-monochamber reactor for thin film deposition

Abstract: We present a study on the development and the evaluation of a fully automated radio‐frequency glow discharge system devoted to the deposition of amorphous thin film semiconductors and insulators. The following aspects were carefully addressed in the design of the reactor: (1) cross contamination by dopants and unstable gases, (2) capability of a fully automated operation, (3) precise control of the discharge parameters, particularly the substrate temperature, and (4) high chemical purity. The new reactor, named ARCAM, is a multiplasma‐monochamber system consisting of three separated plasma chambers located inside the same isothermal vacuum vessel. Thus, the system benefits from the advantages of multichamber systems but keeps the simplicity and low cost of monochamber systems. The evaluation of the reactor performances showed that the oven‐like structure combined with a differential dynamic pumping provides a high chemical purity in the deposition chamber. Moreover, the studies of the effects associated with the plasma recycling of material from the walls and of the thermal decomposition of diborane showed that the multiplasma‐monochamber design is efficient for the production of abrupt interfaces in hydrogenated amorphous silicon (a‐Si:H) based devices. Also, special attention was paid to the optimization of plasma conditions for the deposition of low density of states a‐Si:H. Hence, we also present the results concerning the effects of the geometry, the substrate temperature, the radio frequency power and the silane pressure on the properties of the a‐Si:H films. In particular, we found that a low density of states a‐Si:H can be deposited at a wide range of substrate temperatures (100 °C≤Ts≤300 °C).

Interference-Free Determination of the Optical Absorption Coefficient and the Optical Gap of Amorphous Silicon Thin Films

Abstract: A new method to determine the optical absorption coefficient (α) of thin films is presented. α of hydrogenated amorphous silicon (a-Si:H) based alloys can be accurately determined from transmittance (T) and reflectance (R) by using T/(1-R), which almost completely eliminates disturbance from the optical interference effect. The method is applicable to any thin films, as long as the film is a single layer. Based on the interference-free α, various methods to determine the optical gap (EOPT) of a-Si:H, a-SiC:H, and a-SiGe:H films are discussed. The (nαhν)1/3 plot and the (αhν)1/3 plot are most suitable for characterizing these films. The well-known (αhν)1/2 plot is less suited for detailed discussion of the EOPT than the cube root plot, because the plot includes a large ambiguity in the EOPT. The effect of the optical interference effect on the determination of the EOPT is also discussed.

Electrically reprogrammable nonvolatile memory device

Abstract: An electrically plastic device comprising an amorphous silicon semiconductor layer including movable dopant formed between a pair of electrodes and; at least one gate electrode formed on said amorphous silicon semiconductor layer through an insulation layer or a high resistance layer; whereby the operation of said gate electrode controls the dopant distribution of said amorphous semiconductor layer, thereby varying the electrical conductivity thereof.

Analogue memory and ballistic electron effects in metal-amorphous silicon structures

Abstract: We present experimental results showing that p+ amorphous silicon memory structures exhibit polarity-dependent analogue memory switching. The effect is non-volatile and we propose that it is associated with changes in a tunnelling barrier within the structure. It is also observed that conduction in the memory ON state is restricted to a narrow conducting channel through which the electrons can, under certain conditions, travel ballistically. As a conseauence, auantized resistance levels associated with ballistic electron transport are observed under certain circumstances. In the presence of a magnetic field, additional steps in the auantized resistance levels occur. A particular feature of this auantized resistance is that the effect can be observed at relatively high temperatures (up to about 190 K).

Amorphous silicon studied by ab initio molecular dynamics: Preparation, structure, and properties

Abstract: We present a first-principles molecular-dynamics study of pure amorphous silicon obtained by simulated quench from the melt. A cooling rate of ${10}^{14}$ K/s is sufficient to recover a tetrahedral network starting from a well-equilibrated metallic liquid having average coordination larger than 6. Dramatic changes in physical properties are observed upon cooling. In particular, a gap forms in the electronic spectrum, indicating a metal-to-semiconductor transition. The as-quenched structure has average coordination very close to 4, but contains several coordination defects as well as a large fraction of distorted bonds. Subsequent annealing reduces the amount of strain and the number of defects present in our system. The average structural, dynamical, and electronic properties of our sample are in good agreement with the available experimental data. We report a detailed analysis of the structural relaxation processes accompanying annealing and compare our findings with recent experiments.

Study of back reflectors for amorphous silicon alloy solar cell application

Abstract: The role of back reflectors in enhancing the absorption of weakly absorbing, long‐wavelength light has been investigated as applied to amorphous silicon alloy solar cells. The reflectance and scattering properties of various types of back reflectors have been studied. The performance of p‐i‐n amorphous silicon alloy solar cells deposited on different back reflectors has been analyzed. The studies elucidate the role of back reflectors in improving the short‐circuit current density and thereby the efficiency of the cell.

Molecular-dynamics simulation of thermal conductivity in amorphous silicon

Abstract: The temperature-dependent thermal conductivity ~(T) of amorphous silicon has been calculated from equilibrium molecular-dynamics simulations using the time correlations of the heat flux operator in which anharmonicity is explicitly incorporated. The Stillinger-Weber two- and three-body Si potential and the Wooten-Weaire-Winer a-Si model were utilized. The calculations correctly predict an increasing thermal conductivity at low temperatures (below 400 K). The ~(T), for T) 400 K, is affected by the thermally generated coordination-defect states. Comparisons to both experiment and previous calculations will be described. I. INTRODUCTION In spite of extensive studies, a number of outstanding problems do remain in understanding the lowtemperature properties of amorphous materials. Among the intriguing features have been the linear specific heat of glasses, which is generally believed to be due to the presence of localized two-level states. The nature of the vibrational modes in glasses and the excess vibrational density of states at low frequencies is also an interesting aspect. In this paper we study the thermal conductivity of amorphous materials, using amorphous silicon as a prototype. The temperature-dependent behavior of the thermal conductivity is amorphous materials has the following three characteristic regimes. (1) At low temperatures (T( 1 K), tc( T) is proportional to T', which was explained by Anderson, Halperin, and Varma' as being due to the scattering from localized two-level states; (2) at intermediate temperature (1 ( T ( 30 K), a plateau was seen, which has attracted many theoretical investigations; (3) at high temperature (T ~ 30 K), where tc(T) increases smoothly to a limiting value, in contrast to the crystalline insulators where it decreases with 1/T. Birch and Clark and Kittel gave qualitative explanations of regime (3) using the kinetic formula tc=Cvl/3 that is applicable only in the Boltzmann regime where one can assign velocities U to propagating modes. Allen and Feldman (AF) have examined the validity of the kinetic formula in their recent calculation of the thermal conductivity in amorphous silicon. Cahill and Pohl argued, based on Einstein model, that regime (3) can be explained by locally uncorrelated harmonic oscillators with relaxation times of the order of the period of vibration. However, this model is only valid in the limit of highly disordered crystals. In this study we will study regime (3) with a different approach from previous works. There exist many theoretical models for the structure of amorphous Si. A convenient theoretical model is the four-coordinated Wooten-Winer-Weaire (WWW) a-Si model. Biswas et al. obtained a vibrational density of states for the WWW model that agreed well with experiment. They have also obtained amorphous silicon configurations with molecular-dynamics (MD) simulations by cooling the molten silicon configuration. It is also known that Stillinger-Weber (SW) potential is valid for a wide range of properties of a-Si. Since the WWW model has only four-coordinated silicon atoms, this will be a more convenient starting point for the thermal-conductivity calculations than molecular dynamics models of a-Si (Ref. 8) that have coordination defects. In addition, the use of the WWW model together with the SW Si potential allows a direct comparison with the recent calculation of Allen and Feldman. Allen and Feldman have used the novel procedure of using the Kubo formula for calculating thermal conductivity in a-Si. Matrix elements of the heat-current operator were calculated between harmonic vibrational states. Although the zero-temperature WWW structure was used, the temperature entered through the quantum occupation of the vibrational states. AF obtained a tc(T) that increases in temperature up to 300 K and then saturates at a value close to experiment. Low-temperature values (T (300 K) for tc(T) were significantly smaller than experiment. We present in this paper an alternate approach that explicitly incorporates anharmonicities and accounts for temperature-dependent structural changes. Theoretical techniques developed by AF have served as a useful guide in the present work.

Recrystallization of amorphous silicon films deposited by low‐pressure chemical vapor deposition from Si2H6 gas

Abstract: This paper investigated the recrystallization of low‐pressure chemical vapor deposition amorphous silicon (a‐Si) films deposited using Si2H6 gas at various substrate temperatures. The grain size of recrystallized films formed from Si2H6 is larger than that formed from SiH4. The maximum grain size is obtained at the substrate temperature of 460 °C, where the nucleation rate is minimum due to the maximum structural disorder of the Si network. The structural disorder is increased not only by lowering the substrate temperature but also by increasing the deposition rate. The field effect mobility of thin‐film transistors (TFTs) using the recrystallized films reaches 120 cm2 V−1 s−1, even though the highest temperature during the TFT fabrication process is only 600 °C.

Amorphous and microcrystalline semiconductor devices : optoelectronic devices

Abstract: Solar cells made of amorphous and microcrystalline semiconductors flat panel displays using amorphous and monocrystalline semiconductor devices amorphous silicon charge-coupled devices amorphous superlattices and multilayer structures amorphous and microcrystalline silicon carbide alloys light emitting diodes - physics and properties amorphous materials image sensors - physics, properties and performance charged particle, gamma ray and light detection in amorphous silicon devices application of amorphous silicon position sensitive detectors amorphous materials and physics issues in electrophotography emorphous silicon for optically addressed spatial light modulators experimental studies of artificial neural networks using amorphous silicon.

Performance of thin hydrogenated amorphous silicon thin‐film transistors

Abstract: In this paper we have analyzed the influence of the mask channel length (LM) on the performance of the 55‐nm‐hydrogenated amorphous silicon (a‐Si:H) thin‐film transistors (TFTs), incorporating nitrogen‐rich hydrogenated amorphous silicon nitride gate dielectric and phosphorus‐doped microcrystalline silicon (n+μc‐Si:H) source/drain (S/D) contacts. In our TFTs the n+μc‐Si:H S/D contacts have a specific contact resistance around or below 0.5 Ω cm2. We have shown that in our TFTs a field‐effect mobility and threshold voltage are dependent on LM, and this dependence is most likely due to the influence of the S/D contact series resistance on TFTs characteristics. Finally, we have demonstrated that if the mask channel length is extended by a ΔL (which is a distance from the S/D via edge at which the electron injection/collection is taking place) the field‐effect mobility and threshold voltage are independent of the channel length. In such a case μFE, VT, and ON/OFF current ratio around 0.76 cm2/V s, 2.5 V, and 1...

Hydrogen effusion: a probe for surface desorption and diffusion

Abstract: Hydrogen effusion results are discussed for hydrogenated amorphous silicon (a-Si:H) and related alloys as well as for crystalline silicon (c-Si). It is demonstrated that depending on the microstructure of the material, hydrogen effusion gives information on hydrogen diffusion or surface desorption. The results suggest for compact a-Si:H and for ion implanted c-Si a similar hydrogen diffusion process, which is a trap limited motion of atomic hydrogen. Hydrogen effusion from defect-free c-Si and from void-rich amorphous semiconductors is limited by surface desorption. Both hydrogen diffusion and desorption depend on the Fermi energy if hydrogen bonds to the host material are broken.

Hot spot susceptibility and testing of PV modules

Abstract: To probe the sensitivity for localized heating of commercial amorphous silicon and crystalline modules, several intrusive and nonintrusive experiments were performed. In the intrusive experiments, each cell in several commercial amorphous silicon modules was evaluated separately and in groups for localized heating effects. Damage in amorphous silicon modules occurred under reverse-bias conditions in the dark above a 5-20 mAcm/sup -2/ cell current density at the interconnection between cells. Shading can cause a larger temperature rise than current mismatch. For the monolithic amorphous silicon modules investigated, the current mismatch between each cell was substantial, but the temperature rise was negligible because of the rather low shunt resistance. >

Light-enhanced hydrogen motion in a-Si:H.

Abstract: We report the first direct observation of light-enhanced hydrogen motion in hydrogenated amorphous silicon. Diffusion enhancement increases with illumination intensity in undoped material and is suppressed in doped and in compensated material. The enhancement is attributed to an increased release rate of hydrogen from silicon-hydrogen bonds in the presence of photogenerated carriers. The implications of the effect for metastable defect formation are discussed.

A novel preparation technique for preparing hydrogenated amorphous silicon with a more rigid and stable Si network

Abstract: A novel preparation technique, termed ‘‘chemical annealing,’’ was developed with the aim of making a stable and rigid Si network structure. The hydrogen content (CH) in the films and the optical gap could be reduced gradually without any change in substrate temperature by alternating deposition of a hydrogenated amorphous silicon layer several tens of A thick and treatment with atomic hydrogen. These films showed CH of 1.5–10 at. %, and exhibited high photoconductivities in the level of 10−5–10−4 S/cm. In the films with CH of 3 at. % or less, in particular, improvement was observed in stability against illumination with light. Their photoconductivity remained at about 65% of the initial value even after illumination with white light (AM1, 100 mW) for 60 h. In addition, time‐of‐flight experiments revealed a significant enhancement in hole drift mobility to a value of 0.2 cm2/V s at 300 K.

Thin film semiconductor device having inverted stagger structure, and device having such semiconductor device

Abstract: TFTs with an inverted stagger structure are fabricated according to the invention as follows; a glass substrate after depositing amorphous silicon (a-Si) thereupon is transferred to a laser annealing chamber which is kept in non-oxidation ambient and provided with a sample holder and a substrate heating mechanism. The substrate is fixed on the sample holder, then subjected to laser annealing while being heated from the glass substrate side, thereby growing polycrystalline silicon having substantially improved crystallinity, on which a-Si is further deposited. According to this process of the invention, it is capable of forming TFTs having a higher mobility and a smaller leakage current in the periphery of the substrate, with addition of almost no changes to the process and device structures of conventional TFTs which constitute pixels, and even more the peripheral drive circuitry is capable of being integrated in the display substrate.

Hydrogen diffusion and electronic metastability in amorphous silicon

Abstract: Hydrogen diffusion and its role in the many electronic metastability phenomena in hydrogenated amorphous silicon (a-Si:H) is reviewed. A-Si:H contains about 10 at% hydrogen, most of which is bonded to silicon. The hydrogen diffuses at relatively low temperatures by releasing hydrogen from the Si-H bonds into interstitial sites. The reactions of hydrogen with the silicon dangling bonds and the weak bonds provide a hydrogen-mediated mechanism for electron-structural interactions, which are manifested as electronic metastability. The annealing of light-induced defects, the equilibration of defects and dopants, the stretched exponential relaxation kinetics, and the atomic structure formed during growth, are all attributed to hydrogen diffusion.

Pulsed laser‐induced amorphization of silicon films

Abstract: The reversible transition from crystalline to amorphous silicon was observed on films deposited on quartz substrates through laser‐induced melting using a 30‐ns pulsed XeCl excimer laser. A 20‐nm‐thick silicon film was completely melted and then amorphized. The melt duration exceeded 70 ns when the film was amorphized. The transition from liquid to the amorphous state would occur homogeneously throughout the film because the temperature gradient in molten silicon could be reduced to 1.0×105 K/cm (0.2 K/20 nm) at 70 ns after initiation of melt. The laser‐amorphized film had a large mid‐gap density of states of 5.3×1019 cm−3 eV−1. The density of states was remarkably reduced using a hydrogen plasma treatment at 250 °C for only 1 min. Thin‐film transistors fabricated in a laser‐amorphized film showed good characteristics with a carrier mobility of 0.6 cm2/V s after hydrogenation.

In situ observation of fractal growth during a‐Si crystallization in a Cu3Si matrix

Abstract: We observe that the crystallization of amorphous Si thin films in contact with a copper silicide layer occurs at a temperature of around 485 °C in the form of dendrites with a fractal dimension of 1.7. The in situ observation of both the silicidation reaction, forming Cu3Si, and the subsequent crystallization of the remaining amorphous silicon in the silicide matrix, were observed during annealing in a transmission electron microscope. We estimate the radial growth rate of these crystallites at 5 nm/s at this temperature. The fractal dimension of the dendrites indicates a growth process similar to one known as diffusion‐limited aggregation.

Role of Atomic Hydrogen During Growth of Hydrogenated Amorphous Silicon in the “Chemical Annealing”

Abstract: In our previous paper, we proposed a novel preparation technique termed "Chemical annealing" to make a rigid and stable Si-network. In this letter, with the aim of the understanding the role of atomic hydrogen on the growing surface, systematic studies were made on the concentrations of H and D for the chemically annealed films made by SiH4 and atomic deuterium system as a function of the deposition time in one cycle, the annealing time and the substrate temperature. In the chemically annealed film, the structural relaxation is thermally activated with an activation energy of 0.3 eV. The role of atomic hydrogen is the creation of dangling bonds in the top surface for the enhancement of a cross linking reaction; passivation of dangling bond and break of weak Si-Si bonds for the rearrangement of Si-network. In comparison with the post-deutrization results, the role of atomic hydrogen in the "Chemical annealing" is discussed.

Characterization of instability in amorphous silicon thin‐film transistors

Abstract: Instability mechanism of amorphous silicon‐silicon nitride thin‐film transistors (TFTs) is examined. By investigating double‐layer insulator TFTs, it is demonstrated that the instability is caused by an electrical charge stored at the interface between amorphous silicon and silicon nitride. The amount of stored charge at the interface (Q) does not depend on either drain voltage or drain current. Study on TFTs with several insulator thicknesses has shown that Q strongly depends on the band bending in the amorphous silicon that is related to the gate electric field (E) through the gate insulator. The Q‐E relationship is found to be a more general expression of the dependence of threshold voltage shift on gate voltage, and is incorporated into a formula suitable for examining the interface quality.

Finite-temperature properties of amorphous silicon.

Abstract: Ab initio local-orbital quantum molecular dynamics is used to study the finite-temperature properties of amorphous silicon. Using two structural models of a-Si with 63 and 216 atoms, we examine the structure and temperature dependence of conduction and valence band tails. An explanation is suggested for the recent experimental observations of an asymmetry in the temperature dependence of the band-tail widths of the conduction and valence bands in a-Si. Finally, we conect these simulations to other theoretical descriptions of band tailing.

Reactive ion etching of monocrystalline, polycrystalline, and amorphous silicon carbide in CF4/O2 mixtures

Abstract: Reactive ion etching of monocrystalline and polycrystalline β‐SiC and hydrogenated amorphous a‐SiC:H in CF4/O2 mixtures was investigated. The a‐SiC:H films, deposited by plasma‐enhanced chemical vapor deposition, had the highest etch rate while monocrystalline β‐SiC had the lowest etch rate at all compositions of the CF4/O2 mixture. The etch rates for the three materials increased with the addition of oxygen to CF4 and reached a maximum, but the maxima occurred at different compositions due to different rates of oxide formation.

Plug contact with antifuse

Abstract: An antifuse particularly suitable for submicron geometries is presented. The antifuse is formed between a silicon layer, which could be a doped region of the semiconductor substrate, an epitaxial layer or a polysilicon layer, and an upper metal interconnection layer. In contact holes in a silicon dioxide layer insulating the silicon and metal interconnection layers from each other, the antifuses have a thick refractory metal layer having a top surface approximately at the same level as the top surface of the insulating layer. Depending upon the process used to deposit the refractory metal layer, a thin adhesion layer may be located immediately below the refractory metal layer. Between the underlying silicon layer and upper interconnection layer, a thin semiconductor material layer of amorphous silicon may be located either below the refractory metal layer or above it. At its bottom, the interconnection layer also has a barrier layer to prevent any intermixing between the amorphous silicon layer and the metal interconnection layer.

Fluid flow sensor with thermistor detector

Abstract: An airflow sensor formed on a silicon chip comprises a silicon base covered with an insulating polyimide layer, a lineal resistance heater on the chip energized with current pulses to propagate thermal waves, and a thermistor on the chip downstream of the heater to detect the arrival of each thermal wave. Circuitry determines flow rate as a function of the measured propagation time of the thermal wave. The thermistor may be replaced by a bridge of four resistive elements of which only one or two are sensitive to the thermal wave. The thermistor material is platinum, polycrystalline silicon or amorphous silicon which exhibit high temperature coefficients of resistance.