Ion-Assisted Deposition of Silicon Epitaxial Films with High Deposition Rate Using Low Energy Silicon Ions
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Ion-Assisted Deposition of Silicon Epitaxial Films with High Deposition Rate Using Low Energy Silicon Ions Lars Oberbeck, Thomas A. Wagner, and Ralf B. Bergmann Institute of Physical Electronics, University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany ABSTRACT Ion-assisted deposition (IAD) enables low temperature (≥ 435°C), high-rate (≤ 0.5 µm/min) epitaxial growth of silicon films. Therefore, IAD is an interesting deposition technique for microelectronic devices and thin film solar cells. The Hall-mobility of monocrystalline epitaxial layers increases with deposition temperature Tdep and reaches values comparable to those of bulk Si at Tdep ≥ 540°C. Polycrystalline epitaxial layers exhibit inhomogeneous electrical properties, as shown by Light Beam Induced Current measurements. Recombination within the grains dominates over recombination at grain boundaries. Secco etching identifies an inhomogeneous density of extended structural defects in the polycrystalline epitaxial layers and in the substrate. A major part of the extended defects in the epitaxial layers originates from defects in the substrate. INTRODUCTION Low temperature silicon epitaxy is important not only for electronic circuit applications [1,2] but also for thin film silicon solar cells [3]. Low epitaxial deposition rates in the temperature regime below Tdep = 600°C limit the use of chemical vapor deposition (CVD). Various CVD methods such as electron cyclotron plasma enhanced CVD [4] or hot-wire CVD [5] only allow deposition rates below 0.12 µm/min. Furthermore, the thickness of epitaxial layers at Tdep < 400°C is limited [6]. Ion-assisted deposition (IAD), an advanced physical vapor deposition technique developed about a decade ago [7], overcomes this limitation in deposition rate. Ion-assisted deposition is based on electron-gun evaporation and subsequent ionization of a small fraction of evaporated silicon atoms. An applied voltage of 20 V accelerates the silicon ions towards the substrate. The hyperthermal kinetic ion energy allows lower deposition temperatures and higher deposition rates as compared to other low temperature epitaxial growth techniques. Figure 1 shows, that IAD enables epitaxial deposition rates as high as 0.5 µm/min at a deposition temperature Tdep = 600°C. This deposition rate is significantly higher than those published for CVD methods in the low temperature regime and makes IAD very interesting for rapid epitaxial deposition processes of Si layers. In this study, we report on room-temperature Hall-effect measurements to investigate the dependence of hole Hall-mobility on deposition temperature. X-ray diffraction (XRD) measurements reveal the lowest deposition temperature for ion-assisted deposition of several µm thick epitaxial Si layers. Furthermore, we grow Si epitaxial layers on polycrystalline Si substrates and investigate the homogenity of the electrical and structural properties of the epitaxial layer. While Light Beam Induced Current (LBIC) measurements demonstrate the inhomogeneous current collection in pol
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