Growth and Properties of Micro-Crystalline (Si,Ge):H Films
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ABSTRACT This paper reports on the growth and characterization of micro-crystalline (Si,Ge) films deposited on glass and polyimide substrates. The films were grown using a hydrogen diluted remote ECR plasma process. The feedstock gases were silane and germane. The entire range of composition from 100% Si to 100% Ge was studied. A low-pressure, low-substrate temperature, and high-power environment was used to change the morphology from amorphous to micro-crystalline. The films deposited at 5 mT pressure are generally micro-crystalline. The film's structure was studied using Raman spectroscopy. Raman spectra show clear, sharp crystalline-type Si and Ge peaks. Electronic properties of the films, such as activation energy and absorption constant down to ax = 1 cm-1 were also measured. The sub-gap absorption data also show crystalline absorption behavior, with the absorption shifting to lower energies as the Ge content is increased. This absorption data shows that the materials have low defect densities.
INTRODUCTION Microelectronic devices such as solar cells, thin film transistors, infrared detectors, flat panel displays, image sensors, etc., are constantly looking for new materials that will lead to increased performance. Understanding the growth processes for these thin film semiconductor
devices and their physical properties are critical for making smaller, faster, and higher quality devices. Micro-crystalline (Si:Ge):H alloys are one such material under investigation. A number of groups have reported characteristics on (Si:Ge):H alloys deposited by RF sputtering and parallel plate, DC glow discharge, chemical vapor deposition, laser and plasma-assisted CVD, laser photochemical vapor deposition, and ECR-CVD growth techniques [1 - 4]. Most of these films have been deposited on glass substrates or [100] Si wafers; however, deposition on flexible polyimide substrates has also been explored [4]. This paper reports on the morphology of (Si:Ge):H alloys as the silane to germane ratio changes, while using the hydrogen diluted remote ECR technique [5].
EXPERIMENT The films are deposited with an ECR plasma reactor in a remote, reactive mode as shown in Fig. 1. An intense ECR plasma consisting of H ions, radicals, and electrons is generated in the source region. The plasma is directed toward the substrate in a line-of-sight fashion by tuning the two magnets to achieve the ECR condition. The magnetic field also establishes a plasma resonance zone about 30 cm from the substrate. Decreasing the pressure in the chamber causes an increase in the H ion flux and energy [6]. Process gases such as silane and germane are introduced near the substrate where they are dissociated into radicals by the electrons and H radicals coming from the plasma. With sufficient hydrogen dilution, the a priori radicals that are 987 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
produced are Sill3, GeH 3, and H2. Hydrogen dilution along with the increased ion bombardment due to the low-pressure provides for homogenous growth b
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