Hydrogenated Amorphous Silicon by Infrared Multiphoton Absorption with a Pulsed CO₂‐Laser

Abstract

The following work gives a discussion on hydrogenated amorphous silicon (a‐Si:H) film preparation by laser induced chemical vapor deposition (LICVD) using a pulsed CO2‐laser in a parallel configuration. Deposition rate and initiation of polymerization were studied as a function of total pressure, gas flow, gas mixture (buffer gas, silane and/or disilane), substrate temperature, and radiation flux. The results lead to a reaction model where higher silane homologues play an important role for the film production. The film properties strongly depend on the substrate temperature Ts. The activation energy Ea of the dark conductivity decreases from 1.1 eV to 0.7 eV for films prepared at Ts = 230°C to 450°C. In the same temperature range, the dark conductivity σd increases from ∼10−14 to ∼10−10 (Ω cm)−1, whereas the photo conductivity σph shows a maximum of ∼10−6 (Ω cm)−1 at Ts = 320‐330°C. These values, especially their temperature behaviour, are compared with the results of other CO2‐LICVD works and further production methods. It is shown that the film formation depends on the gas phase chemistry as well as on the substrate temperature.

Ab Initio Chemical Kinetics for SiH2 + Si2H6 and SiH3 + Si2H5 Reactions and the Related Unimolecular Decomposition of Si3H8 under a-Si/H CVD Conditions

“…Mechanistically, SiH 2 is known to be a primary decomposition product in the decomposition of silanes. − The interaction of the SiH 2 radical with Si 2 H 6 is a subject of interest in the present work because Si 2 H 6 is one of the most commonly employed reagents and silylene is known to be a very reactive diradical that inserts into Si–H bonds very efficiently, forming excited adduct Si 3 H 8 . − The excited adduct can dissociate via intramolecular 1,2-H shifts, producing various products. − Inoue et al detected SiH 2 by laser-induced fluorescence and found the rate constants of the SiH 2 reaction with Si 2 H 6 to be 5.7 × 10 –10 cm 3 molecule –1 s –1 in 1 Torr of helium at room temperature. Later, Jasinski et al generated SiH 2 by using laser absorption spectroscopy and found the overall rate constant of the SiH 2 with disilane reaction from 1 to 10 Torr of He pressure at 298 K; the rate constant measured at 1 Torr of pressure was 1.5 × 10 –10 cm 3 molecule –1 s –1 .…”

“…Roth et al investigated the gasphase parameters such as the flow, temperature, and pressure of participating reactants; the energy input for dissociation reactions, the size, and the structure of Si−H compounds have an important influence on the properties of the surface layer. 13 Mechanistically, SiH 2 is known to be a primary decomposition product in the decomposition of silanes. 1−19 The interaction of the SiH 2 radical with Si 2 H 6 is a subject of interest in the present work because Si 2 H 6 is one of the most commonly employed reagents and silylene is known to be a very reactive diradical that inserts into Si−H bonds very efficiently, forming excited adduct Si 3 H 8 .…”

“…The chemical processes of the amorphous material formation thus involve gas-phase and surface reactions, which are affected by the system’s temperature and have a strong influence on the quality of the film. Roth et al investigated the gas-phase parameters such as the flow, temperature, and pressure of participating reactants; the energy input for dissociation reactions, the size, and the structure of Si–H compounds have an important influence on the properties of the surface layer …”