Growth and properties of hydrogenated amorphous silicon carbon alloys by DC reactive magnetron sputtering

Yang, Siyuan

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Description

Title

Growth and properties of hydrogenated amorphous silicon carbon alloys by DC reactive magnetron sputtering

Author(s)

Yang, Siyuan

Issue Date

1995

Doctoral Committee Chair(s)

Abelson, John R.

Department of Study

Materials Science and Engineering

Discipline

Materials Science Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Engineering, Materials Science

Language

eng

Abstract

• We deposited amorphous hydrogenated silicon-carbon (a-Si$\rm\sb{1-x}C\sb{x}$:H) alloy films by dc reactive magnetron sputtering (RMS) of a silicon target in a plasma of (Ar + H$\rm\sb2$ + CH$\sb4).$ Films with optical bandgaps of 1.8-2.0 eV and x $\le$ 0.2 have a compact microstructure; no CH$\rm\sb{x}$ groups are detected by their stretching mode infrared absorption, and small angle X-ray scattering indicates a microvoid volume of $\le$2.5% for x $\le$ 0.3. The electronic properties are also excellent in terms of density of sub-bandgap defect states DOS, Urbach energy E$\rm\sb{U},$ ambipolar diffusion length L$\rm\sb{h},$ photoconductive mobility-lifetime product $\mu\tau$ and dark conductivity $\rm\sigma\sb{D}$ for Tauc bandgap E$\rm\sb{g}\approx$ 1.90 eV. We attribute the quality of these films to the translational energy of the RMS growth flux, which leads to a dense and relatively homogeneous structure.
• Substrate cleaning and ion bombardment play important roles in the adhesion, morphology and structure of hydrogenated amorphous silicon and silicon-carbon alloys grown on c-Si by dc reactive magnetron sputtering. Scanning electron microscopy is used to study the surface morphology and fracture cross section of films. The hydrogen bonding and microstructure are studied by IR absorption, thermal evolution of hydrogen. The best adhesion is obtained by (i) cleaning the substrate with combination of cycles of UV/ozone oxidation and HF etching and by (ii) reducing plasma ion bombardment through electrically floating the c-Si substrate. The reduced ion bombardment has a significant influence on the hydrogen bonding configurations and hydrogen evolution process.
• The film composition is a function of the substrate temperature, growth rate, and the partial pressures of methane and hydrogen. We show which combination of parameters leads to the best film properties, and provide empirical evidence for the mechanisms of C and H incorporation. We then use the compositional control provided by RMS to resolve several long standing debates in the literature. (i) We deposit films of constant composition from 150 to 310$\sp\circ$C, and find that higher substrate temperature yields the best electronic properties. (ii) The hydrogen content is primarily responsible for the bandgap increase. (iii) Carbon incorporation with x $\le$ 0.3 suppresses microvoid formation in sputtered material. (iv) The relative intensity of SiH$\rm\sb{x}$ to SiH stretching mode absorptions (the IR microstructure factor) and the amount of microvoids is not monotonically correlated with the photoconductive mobility-lifetime product. Considering the influence of film compositions on microstructure observed in this study, we propose that similar compositions should be the premise before the comparison of microstructure difference and the study of its influence on electronic properties.
• Our results indicate that reactive magnetron sputtering is an excellent and industrially suitable technique for the deposition of electronic-grade a-Si$\rm\sb{1-x}C\sb{x}$:H thin films.

Type of Resource

text

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Copyright 1995 Yang, Siyuan

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