Decomposition Of Silane Research Articles

Light‐Emitting Silicon Nanocrystals from Laser Pyrolysis

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NeoSilicon materials and silicon nanodevices

We propose a novel material NeoSilicon, in which both particle size and interparticle distance of nanocrystalline silicon (nc-Si) quantum dots are precisely controlled. New functions in electron transport, photon emission and electron emission are expected due to quantum effects at room temperature and large interaction between dots. The bandgap is determined by the particle size. The conductivity is controlled mainly by tunneling processes. The transport characteristics are also controlled by charge quantization effect. We present fabrication and characterization of NeoSilicon. Based on the idea of separation of the nucleation and the growth processes in plasma decomposition of silane, we have successfully prepared nc-Si particles of 8 nm in diameter with size dispersion of 1 nm, whose surfaces are covered by naturally formed oxide of 1.5 nm thickness. Further reduction of dot size is achieved to 4 nm using self-limiting oxidation processes. Single electron transport characteristics of nc-Si are demonstrated in both planar and vertical transistor structures. Highly efficient electron emission is observed in NeoSilicon samples. Photoluminescence intensity enhancement due to no-phonon-assisted indirect transition is observed in strongly confined nc-Si dots covered by oxide.

An Investigation of Particle Dynamics in a Rotating Disk Chemical Vapor Deposition Reactor

This paper describes a numerical model for the nucleation, growth, and transport of gas-phase particles formed during the chemical vapor deposition (CVD) of epitaxial silicon from silane. These particles can lower the deposition rate by consuming precursor, and contaminate the growing film via diffusion to the surface. This model has been constructed for use with the Sandia SPIN code, which contains a solver for the reacting flow and heat transfer in a vertical, rotating disk CVD reactor. A detailed gas-phase chemical kinetic mechanism for the thermal decomposition of silane was developed to simulate formation of small silicon clusters and the depletion of reactive intermediates through condensation. The particle model uses a moment transport formulation to examine the effects of total reactor pressure, temperature, rotation rate, inlet gas composition, and rate of particle growth via condensation on the characteristics of the particle population. Numerical results are presented in terms of the integral moments of the particle distribution which correspond physically to the particle number concentration, average particle diameter, and particle light scattering intensity. In situ validation experiments have been performed in an optically accessible reactor under conditions typical of silicon CVD. The rate of particle growth via condensation, controlled numerically by a global condensation parameter (GCP), was found to control the characteristics of the particle population. The numerical results were found to compare favorably with experiment if this GCP was properly chosen. © 2003 The Electrochemical Society. All rights reserved.

Optical bandgap and quantum well model in hydrogenated amorphous silicon carbon alloy films

Hydrogenated amorphous silicon carbon alloy (a-SiC : H) films of various composition were prepared by the rf glow discharge decomposition of acetylene and silane as well as ethylene and silane and were also annealed for various annealing temperatures. Careful and detailed IR absorption and the optical bandgap (Eopt) measurements were undertaken for a-SiC : H films of different composition (x) and annealing temperatures (Ta). Interesting variation of Eopt with x and Ta have been observed. Our study demonstrates, for the first time, that the models involving alloying or/and ordering break down and the proposed quantum well model based on heterogeneity is found to be the only model capable of explaining the observed variation of Eopt. This, consequently, may have considerable implications.

Light‐Emitting Silicon Nanocrystals from Laser Pyrolysis

AbstractCrystalline Si nanoparticles with diameters between 2.5 and 20 nm are prepared by CO2‐laser‐induced decomposition of silane in a gas flow reactor. A small portion of the products created in the reaction zone is extracted through a nozzle into a high‐acuum apparatus to form a freely propagating molecular beam of clusters and nanoparticles that can be deposited on suitable substrates. The strong visible photoluminescence (PL) of the Si nanocrystals is studied as a function of their size, and as a function of the time for which they are exposed to air. All observations can be explained on the basis of quantum confinement as the only origin of the PL. Chemical methods are exploited to modify the surface of the Si nanoparticles and to reduce their size, thus shifting their PL to shorter wavelengths. With this technique, the Si nanoparticles, collected in much larger quantities in the filter of the flow reactor, can be made strongly luminescent so that they may be used for various applications.

Investigation of the growth process of Si nanowires using the vapour-liquid-solid mechanism

Silicon nanowires have been grown by the thermal decomposition of silane via the vapour-liquid-solid (VLS) mechanism. Three different stages of VLS growth (eutectic alloy formation, crystal nucleation and unidirectional growth) were studied separately using a scanning electron microscope and a high-resolution transmission electron microscope. Very short silicon nanowires prepared under particular conditions provide direct evidence of the VLS mechanism on a nanometre scale. Our results will be very helpful for the controllable synthesis of Si nanowires.

Crystallization of amorphous hydrogenated silicon films deposited under various conditions

The possibility of using the magnetron-assisted silane decomposition technique for the deposition of a-Si:H films as the basic materials for the production of polysilicon is analyzed. It is shown how specific features of the film structure affect the crystallization process.

Transient detection in infrared multiphoton decomposition of (chloromethyl)silane and 1,3-disilacyclobutane: evidence for cleavage of SiCH 4 intermediates

The transients produced in the infrared multiphoton decomposition (IRMPD) of (chloromethyl)silane—HClSi:, H 2Si: and H 2C:—and of 1,3-disilacyclobutane—H 2Si: and H 2C:—have been identified by laser-induced fluorescence (LIF). The nascent HClSi: and H 2Si: populations are generated with a high degree of excitation and the species produced from (chloromethyl)silane possess more rotational and vibrational energy than those produced from 1,3-disilacyclobutane. The obtained quenching rate and radiative lifetime for HClSi: are (4.6±0.3)×10 −10 cm 3 per molecule s −1 and 369±10 ns, respectively. The formation of H 2Si: and H 2C: in IRMPD of both (chloromethyl)silane and 1,3-disilacyclobutane adds to previous knowledge on decomposition of (chloromethyl)silane and 1,3-disilacyclobutane and suggests that these transients are produced from the same SiCH 4 (H 3Si(H)C: and (CH 3)HSi:) intermediates.

Reaction kinetics in silicon chemical vapor deposition

Gas-phase reactions of simple silicon hydride species underlying many types of chemical vapor deposition processes for silicon-based thin-film growth are reviewed in this paper. Mass spectrometry and laser-based spectroscopy are applied to identify gas-phase intermediates in thermal and hot-wire chemical vapor deposition processes. The mechanism of the thermal decomposition of silanes, including reactions that lead to the formation of hydrogenated silicon clusters, is examined. The gas-phase chemical kinetic mechanism in hot-wire chemical vapor deposition is proposed to explain precursor molecules for the film growth.

Detecting reactive species in hot wire chemical vapor deposition

Major recent advances: A variety of in situ diagnostic techniques have been applied recently to detect reactive species, including H, Si, SiH, SiH 2, SiH 3 and Si 2H 6, in hot wire chemical vapor deposition of hydrogenated silicon films. Several recent studies confirmed silicon radicals as a major silane decomposition product from the hot wire. The dependence of silicon production on filament temperature measured in the different studies appears to be similar; however, the reported apparent activation energies vary across research groups.

Numerical simulation of silane decomposition in an RF plasma

A model of silane decomposition in a radio-frequency argon plasma is constructed. The concentrations of SiH4 decomposition products, as well as products of synthesis (higher silanes), are calculated. The role of metastable argon atoms in the formation of SiH3 radicals and the higher silanes is analyzed.

Correlation between the opto-electronic and structural parameters of amorphous semiconductors

The variation of opto electronic parameters like the optical band-gap (Tauc gap E opt and the isoabsorption gap E 04), and structural parameters like the Tauc slope parameter B, of the hydrogenated amorphous silicon carbon alloy films prepared by r.f. glow discharge decomposition of silane and acetylene as well as silane and ethylene, are discussed for various compositions and also for different annealing temperatures. We observe, for the first time, an interesting new correlation between ( E 04− E opt) and the Tauc slope parameter B of amorphous semiconductors.

Improvement of 1.54 μm luminescence in erbium-doped a-SiO x:H alloys

Erbium-doped amorphous silicon films have been deposited using magnetron-assisted silane decomposition (MASD) technique. The increase of partial concentration of O 2 in the gas phase above 6.5 mol% during the deposition enhances the intensity of the 1.54 μm Er photoluminescence (PL) at room temperature. The films exhibit an inhomogeneous structure containing oxygen and Er enriched nanoclusters, where the Er ions reside in optically active but electrically passive configuration.

Effect of Filament Material on the Decomposition of SiH4 in Hot Wire CVD of Si-Based Films

AbstractThe choice of filament material has an effect on the decomposition of silane during the hot wire chemical vapor deposition (HW-CVD) of amorphous and microcrystalline silicon films. The Si radicals produced from W, Re, Mo and Ta filament materials have been probed by laserbased single photon ionization (SPI) as a function of hot wire temperature. The Si radical profiles are shown to demonstrate two distinct regimes: a regime below 1600°C-1800°C (depending on filament material) limited by surface reaction at the filament in which Si concentration increases monotonically; and a mass transfer limited regime above 1600°C-1800°C where Si intensity saturates. The apparent activation energy of Si radical production in the surface reaction regime from Ta (140-170 kcal/mol) is found to be close to the corresponding Si thermal desorption energy from a Ta surface, suggesting that the Si production is controlled by the desorption process from the bare metal. On the other hand, the Si activation energies from W and Re (30-60 kcal/mol) are lower than the related desorption energies, suggesting that other rate limiting reactions play a role for these materials. The apparent activation energy for the Mo surface (60-90 kcal/mol) is intermediate between the other metal values. In addition to the Si radical study, corresponding film deposition is detected in situ by multiple internal reflection infrared (MIR-IR) spectroscopy. The IR measurements have been used to estimate the growth rate of a-Si:H deposited on a Ge substrate. The results show similar activation energies for both the growth rate and the Si formation from a W filament, implying that Si radical production and subsequent film growth may be dominated by the same elementary reactions within the decomposition and film growth processes at low pressure.

Absence of a Steady State in Hydrogen Diluted Silane Plasmas Due to Mass Dependent Gas Pumping Speeds and Its Consequences

AbstractWe have investigated the cause for continued changes in the silane related species observed using mass spectroscopy during i-layer deposition process lasting several tens of minutes. Silane decomposition is found to influence the mass spectrometer filament and therefore, pressure changes were compared to ascertain differences in pumping speed using a rotary, mechanical pump. The cathode voltage was used as a measure of effective hydrogen to silane ratio in real time and deliberate tailoring of this ratio is seen to result in a discernible difference in the degradation of single junction amorphous silicon solar cells. Some obvious consequences of this subtle, pump-type dependent transient behavior are discussed.

Separation of silicon isotopes by silicon tetrafluoride-silane technology

Silane enriched in silicon isotopes was obtained in high yield by reacting SiF4 with a solution of NaAlH4 in diethylene glycol dimethyl ether in a purpose-designed apparatus. Chemical analyses are presented for isotopically enriched silicon obtained by the thermal decomposition of silane.

Plasma enhanced growth, composition and refractive index of silicon oxynitride films

Silicon oxynitride films of various compositions have been prepared by the glow discharge decomposition of silane (SiH 4) and nitrous oxide (N 2O). The refractive index of the films has been measured using ellipsometry, while the composition of the films has been determined using Rutherford backscattering (RBS) and elastic backscattering (EBS) techniques. This study has demonstrated that the measurement of composition using EBS technique is distinctly superior to RBS technique, particularly for the films like silicon oxynitride containing multicomponent light elements in low concentration.

Chemical kinetics for film growth in silicon HWCVD

Resonance enhanced multiphoton ionization spectroscopy coupled with molecular beam technique has been used to measure H and Si atoms produced by the hot-wire induced decomposition of silane. The catalytic decomposition of silane on the hot filament exhibited different temperature distributions between H and Si atoms. A chemical kinetic model starting with Si+SiH 4 and H+SiH 4 reactions was proposed to explain the chemical kinetic phenomena occurring in the silicon hot-wire chemical vapor deposition (HWCVD). The modeling results suggest that temperature sensitivity of film growth properties is primarily due to the gas composition near the surface.

Donor formation in plasma-deposited amorphous silicon (a-Si:H) by erbium incorporation

Erbium-doped amorphous silicon films have been prepared by plasma decomposition of silane doped with the volatile erbium compound tris(2,4-pentanedionato)-Er(III), which has a more favourable erbium–carbon ratio than previously used material. The films exhibit a rather high n-type dark conductivity ( σ d ≈3×10 −4 S/ cm at T=300 K), a high photoconductivity and a relatively low defect density (N D ≈3×10 17 cm −3) . These properties are similar to those of films doped with approximately 10 ppm phosphine. By co-doping with diborane we obtain compensated (near-intrinsic) a-Si:H(Er) films and p-type samples. These films thus may be suitable for the fabrication of light emitting p–i–n diodes.

Performance of Si-modified Pd catalyst in acetylene hydrogenation: the origin of the ethylene selectivity improvement

Supported Pd catalyst modified with Si, which is deposited selectively on Pd by silane decomposition and subsequently oxidized in oxygen, shows improved ethylene selectivity in acetylene hydrogenation. The origin of the selectivity improvement has been studied by observing the adsorption and desorption behavior of acetylene, ethylene, and hydrogen on the Pd surface. The adsorption strength of ethylene on Pd becomes weak and the amount of adsorbed hydrogen decreases when the Pd catalyst is modified with Si. The Si modification also reduces the amounts of surface hydrocarbons or carbonaceous species that are deposited on the catalyst either during the temperature-programmed desorption (TPD) of ethylene or by surface reactions between co-adsorbed acetylene and hydrogen. The hydrocarbon species deposited on the Si-modified catalyst have a shorter chain length than those produced on the Pd-only catalyst. All these results are consistent with the improvement in ethylene selectivity on the Si-modified Pd catalyst, which has been explained in this study based on the reaction mechanism of acetylene hydrogenation.