Characterization of Green Laser Crystallized GeSi Thin Films
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Characterization of Green Laser Crystallized GeSi Thin Films Balaji Rangarajan1, Ihor Brunets1, Peter Oesterlin2, Alexey Y. Kovalgin1 and Jurriaan Schmitz1 1 MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands. 2 INNOVAVENT GmbH, Bertha-von-Suttner Str. 5, 37085 Gottingen, Germany. ABSTRACT Green laser crystallization of a-Ge0.85Si0.15 films deposited using Low Pressure Chemical Vapour Deposition is studied. Large grains of 8x2 ȝm2 size were formed using a locationcontrolled approach. Characterization is done using Scanning Electron Microscopy, Atomic Force Microscopy, X-Ray Photoelectron Spectroscopy and X-Ray Diffraction. INTRODUCTION Germanium-silicon (GeSi) alloys deposited at lower temperatures (400-450 °C) can be incorporated in various 3D integration schemes (e.g. monolithic integration) [1, 2]. Making the temperature CMOS-back-end compatible has generated renewed interest in such films in recent years [3]. Electrical characteristics of the devices (e.g. transistors) fabricated on such films can however be affected by the randomly positioned grain boundaries as shown earlier for Si [4, 5]. In order to control the positioning of the grain boundaries, green laser crystallization was used in our study to crystallize a-Ge0.85Si0.15 films using a location-controlled approach involving pre-patterned a-GeSi lines. The context of the present work is in infrared detection and therefore the choice of this particular film composition. EXPERIMENT The a-Ge0.85Si0.15 and poly-Ge0.85Si0.15 films were deposited using Low Pressure Chemical Vapour Deposition (LPCVD). The GeSi films were deposited on top of 450-nm oxide which was grown on Si substrate using wet oxidation. The custom built LPCVD system has a horizontal furnace for batch deposition and uses resistive heating (to heat the furnace walls). A quartz wafer-boat is used in order to load the 4-inch wafers directly into the furnace. The system has a base pressure of 10-3 mbar. The precursor gases used for the deposition were SiH4 and GeH4. Deposition at 430 °C using 75 sccm of SiH4 flow and 37 sccm of GeH4 flow, with a total pressure of 6 mbar, resulted in the formation of a-Ge0.85Si0.15 films. On the other hand, deposition performed using the same set of process parameters except for a total pressure of 0.2 mbar resulted in poly-Ge0.85Si0.15 films. In order to form nucleation sites for GeSi film deposition on oxide, a few nanometers of a-Si were previously deposited using the same LPCVD system at 430 °C using 88 sccm of SiH4 flow with a total pressure of 0.5 mbar. The variation in the composition of LPCVD poly-GeSi films with respect to different GeH4 partial pressures has also been explored in this study. The as-deposited poly-Ge0.85Si0.15 films acted as the reference material while a-Ge0.85Si0.15 films were used for laser crystallization.
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A green laser (Ȝ = 515 nm) pulsed at a frequency of 10 kHz with a pulse duration of 285 ns was used for the laser crystallization of a-GeSi films. The laser beam had a Gaus
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