John H. Flint

Laser‐induced chemical vapor deposition of hydrogenated amorphous silicon. I. Gas‐phase process model

In the laser‐induced chemical vapor deposition (LICVD) process, a CO2 laser beam impinges on a gas mixture parallel to the substrate upon which the film is deposited. Since heating of the reactant gases is accomplished only via the absorption of infrared photons, the reaction zone can be controlled precisely. The LICVD technique is a cold‐wall thermal process allowing independent control of both the gas and substrate temperatures. In this paper, we propose a model for LICVD of silane (SiH4) and growth of hydrogenated amorphous silicon (a‐Si:H) thin films in which the film growth is controlled by gas‐phase homogeneous thermal decomposition of the SiH4. The peak gas temperature Tg depends on many process parameters, namely, gas partial pressures, laser power, substrate temperature, and cell geometry. Due to the extreme sensitivity of the growth rate G to the values of the partial pressures and laser power, these parameters must be fixed to within ±1% variation in order to control G to ±50% and prevent powde...

A Model for the Growth of Silicon Particles from Laser-Heated Gases

Laser-synthesized silicon powder is spherical and nonagglomerated when produced under proper conditions. Otherwise it consists of agglomerates of very small particles or fused aggregates of larger particles. A synthesis model has been developed that includes silane decomposition kinetics, interdiffusion of reactant and annular gases, particle growth by collision coalescence of Si droplets, and a measured temperature distribution. Predicted particle size distributions agree closely with a measured distribution. Variation in the growth time for different flow streams primarily broadens the mass distribution, whereas intermixing of the silane flow with the annular gas primarily broadens the number distribution.

Laser‐induced chemical vapor deposition of hydrogenated amorphous silicon. II. Film properties

Properties of hydrogenated amorphous silicon thin films prepared by the laser‐induced chemical vapor deposition (LICVD) of silane gas are described. We report the results of measurements of hydrogen concentration, infrared absorption, unpaired‐spin density, optical and mechanical properties, electrical conductivity, and photoconductivity experiments. We conclude that the film properties are controlled primarily by the substrate temperature Ts. LICVD films are superior to those produced by conventional CVD because of the permissibly low values of Ts. This results in an increased hydrogen content (up to 30 at. %) and a reduced defect density (∼1016 spins/cm3). The hydrogen concentration is determined by the surface chemistry for Ts 20 at. %) and by H2 evolution for Ts>300 °C([H]<20 at. %). The hydrogen is incorporated primarily in the SiH2 configuration and for Ts<300 °C, the films contain some polysilane (SiH2)n regions. All the physical properties of the films are discussed in conjunction with...

Doped hydrogenated amorphous silicon films by laser-induced chemical vapor deposition

We report the growth and characterization of both n‐type and p‐type doped hydrogenated amorphous silicon films prepared by laser‐induced chemical vapor deposition. For both doping types, the activation energy for electrical conduction has been reduced to below 0.2 eV and controlled doping has been achieved. Phosphine lowers the growth rate, while diborane has essentially no effect on the laser‐induced growth but enhances thermal growth. Diborane also decreases the hydrogen concentration of the films, resulting in reduced optical gaps.

Powder Temperature, Size, and Number Density in Laser-Driven Reactions

A technique to accurately measure the temperature of powders in a laser-driven reaction has been developed. Particles are formed by heating reactant gases with a 150-W CO2 laser. The brightness temperature of the particulate cloud was measured with a micro-optical pyrometer. The emissivity was determined from scattering and transmission measurements. A correction for high turbidity was derived. The scattering and transmission measurements also allow the determination of the size and number density of the particles. The temperature and particle size as a function of height are reported for five silicon powder reaction conditions, and for one silicon carbide reaction. The measurements indicate that particles are often nucleated before the reactant gas has reached the CO2 laser beam. The reaction zone temperature decreases once most of the reactant gas is consumed, unless the produced powder absorbs 10.6 μm radiation as does SiC. The silicon particles nucleate as amorphous silicon, and then crystallize as th...

Ceramic Powders From Laser Driven Reactions

A process for synthesizing Si, Si3N4 and SiC powders from laser heated gas phase reactants has been developed and modelled. The superior process control and the inherent process attributes achievable with laser heating permits powders having nearly ideal characteristics to be produced. Projected manufacturing costs for this process indicate that resulting powders should be lower in cost than conventional powders.

Hydrogenated amorphous silicon produced by laser induced chemical vapor deposition of silane

Abstract a-Si:H films deposited by laser induced CVD (LICVD) have been characterized and the growth process modelled. Growth rates are exponentially dependent on gas temperature and film properties follow the equilibrium hydrogen content, exponentially dependent on substrate temperature.

Laser‐induced chemical vapor deposition of silicon nitride films: Film and process characterizations

We have deposited hydrogenated amorphous silicon‐nitride (a‐SixN1−x: H) films from NH3‐SiH4‐Ar gas mixtures, heated by gas‐phase absorption of CO2 laser radiation. For the first time, stoichiometric (Si/N=0.75) a‐Si3N4: H films were obtained for NH3/SiH4 flow ratios of the order of 1000 and substrate temperatures Ts of about 500 °C. Growth rates as high as 13 A/min were observed, depending on the partial pressure of SiH4, P(SiH4), the gas temperature Tg and Ts. Tg was calculated from a derived energy‐balance equation, which depends sensitively on process parameters. To model the deposition process, the experimental film growth rate is separated into the Si growth rate G(Si) and the N growth rate G(N). Film stoichiometry is interpreted as the ratio G(Si)/G(N). The rate‐limiting steps for Si growth are the gas‐phase decomposition of SiH4 for Tg below about 750 °C and the SiH4 flow rate at higher Tg. G(N) is affected by both Ts and Tg. The NH3/SiH4 flow ratio must be kept large to ensure a sufficient concent...

Laser-induced chemical vapor deposition of hydrogenated amorphous silicon: Photovoltaic devices and material properties

Abstract We report the growth by laser-induced chemical vapor deposition (LICVD) of hydrogenated amorphous silicon p-i-n and n-i-p photovoltaic devices. Properties of the LICVD material and devices were studied by examining the light and dark current density vs. voltage curves and the spectral quantum efficiencies with and without applied reverse bias. We also report steady state photoconductivity measurements of the product of the majority carrier mobility and lifetime in undoped films grown at various substrate temperatures. Although they have less hydrogen and higher neutral dangling bond densities than films grown at lower temperatures, LICVD films grown at a substrate temperature of 400 °C have better photovoltaic and photoconductive properties. Device characterizations show that, in the present geometry, film homogeneity is good along the laser beam axis and extends laterally across a strip 7 mm wide.

Model for gas–laser interaction: application to thermally activated laser-induced chemical vapor deposition

We present a model for the interaction between a laser and gas mixture that can be directly applied to the case of thermally activated laser-induced chemical vapor deposition (LICVD). The model involves the values of specific parameters, particularly gas pressure, laser intensity, detuning frequency, and rotational and vibrational relaxation rates, relevant to absorption, saturation, and heat-transfer processes and their interrelation. We adopt a semiclassical phenomenological approach, considering vibrational energy levels with accompanying rotational energy manifolds and both radiative and nonradiative transition processes. The model is applied to the experimental NH(3) absorption results at the R(6), R(14), and P(20) lines of a cw CO(2) laser in the 10-microm region.