Amorphous to Crystalline Structural Transition during Growth of Silicon by Plasma Enhanced Chemical vapor Deposition
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AMORPHOUS TO CRYSTALLINE STRUCTURAL TRANSITION DURING GROWTH OF SILICON BY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION W.J. VARHUE*, S. KRAUSE**, J. DEA** and C.O. Jung** *University of Vermont, Department of Electrical Eng., Burlington, VT 05405 **Arizona State University, Department of Chem. Bio. and Materials Eng., Tempe, AZ 85287
ABSTRACT The growth of thin Si films by RF glow discharge undergoes a transition from amorphous to microcrystalline as power density is increased. This results in a substantial change in the film's electrical conductivity and activation energy for electrical conduction. The RF glow discharge has been characterized in terms of plasma density, plasma potential and electron temperature with emissive Langmuir Probe measurements. The structural transition has been observed with a transmission electron microscope.
INTRODUCTION The possibility of obtaining microcrystalline Si with temperatures below 0 400 C by RF glow discharge has generated considerable interest. This material could be used in a variety of thin film devices and should yield an improvement in carrier mobilities over that of a-Si:H. The doping efficiency of this material has been shown to be considerably higher than that of a-Si:H [1,2]. Y. Mishima, et.al. [3] have proposed a structural model in which microcrystallites of approximately 60 A are imbedded in an amorphous medium. The amorphous medium is hydrogen rich with mono- and dihydride bonding. In the transition from amorphous to microcrystalline type behavior, it has been observed by x-ray diffraction that the volume fraction of microcrystallites increases from 36 to 65%. This appears to occur by an increase in the number of crystallites and not in the size of the crystallites. The size of the microcrystallites is believed to be limited by the total energy balance between microcrystalline and amorphous phases. Electrical conductivity increases significantly with an increase in the microcrystalline volume fraction. This is attributed to a decrease in the The amorphous phase which increases the number of conducting channels. activation energy for electrical conduction decreases from .8 to .25 eV for Although the transition from an amorphous to microcrystalline structure. the activation energy is assumed to be associated with the potential energy barrier that exists between crystalline and amorphous phases, it is not clear what the precise mechanism is. The microcrystalline phase has been found to grow when the plasma generates a high ratio of [H]/[SiH] species. High ratios of these species have been shown by optical emission spectroscopy to occur with high H2 /SiH 4 feed ratios or high RF power densities [3]. Another means would be to confine the plasma with a magnetic field, which increases the plasma density [4]. It is not clear how the increased [H]/[SiH] ratio increases the nucleation probability or the resulting higher microcrystalline volume fraction. This investigation involves the growth of amorphous and microcrystalline thin films by RF glow discharge in a parallel
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