Photocarrier Excitation and Transport in Hyperdoped Planar Silicon Devices
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Photocarrier Excitation and Transport in Hyperdoped Planar Silicon Devices Peter D. Persans1, Nathaniel E. Berry1, Daniel Recht2, David Hutchinson1, Aurore J. Said2, Jeffrey M. Warrender3, Hannah Peterson1,3, Anthony DiFranzo1, Christina McGahan1, Jessica Clark1, Will Cunningham1, and Michael J. Aziz2 1
Rensselaer Polytechnic Institute, 110 8th Street, Troy NY 12180 Harvard, School of Engineering and Applied Science, 29 Oxford Street, Cambridge, MA 02138 3 US Army – ARDEC, Benet Laboratories, Watervliet, NY 12189 2
ABSTRACT We report an experimental study of photocarrier lifetime, transport, and excitation spectra in silicon-on-insulator doped with sulfur far above thermodynamic saturation. The spectral dependence of photocurrent in coplanar structures is consistent with photocarrier generation throughout the hyperdoped and undoped sub-layers, limited by collection of holes transported along the undoped layer. Holes photoexcited in the hyperdoped layer are able to diffuse to the undoped layer, implying ( μτ ) h ~ 5 × 10−9 cm2/V. Although high absorptance of hyperdoped silicon is observed from 1200 to 2000 nm in transmission experiments, the number of collected electrons per absorbed photon is 10-4 of the above-bandgap response of the device, consistent with ( μτ )e < 1 × 10−7 cm2/V. INTRODUCTION Pulsed laser irradiation enables the production semiconductors that have compositions not readily attainable with other methods and that exhibit interesting and potentially useful optical properties[1, 2]. Recently, optically smooth sulfur-supersaturated crystalline silicon has been fabricated in thin film form using ion implantation followed by nanosecond pulsed laser melting [3]. This new material exhibits an unexpectedly high sub-bandgap absorption coefficient [2, 3]. The nature of the transitions that give rise to this absorption has not yet been determined, however free carrier absorption has been ruled out. The present work is an attempt to elucidate the excitation and photoconduction mechanisms in hyperdoped silicon devices. EXPERIMENT Hyperdoped layers were prepared by implantation of sulfur into a thin crystalline siliconon-insulator (SOI) layer, followed by pulsed laser melting to rapidly recrystallize the implanted layer while retaining a high dopant concentration [3]. For the current study, the SOI layer was 260 nm thick with a 1000 nm oxide. The peak implant density of ~1020 cm-3 is about 110 nm deep. After laser melting of the top 200 nm, the peak S density is about 2.5x1019 cm-3. The back 50 nm of the SOI layer is not melted in order to act as a seed layer for crystalline regrowth. Au surface contacts for coplanar measurements were evaporated with a 0.8 mm gap.
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The optical absorption coefficient for a hyperdoped layer prepared by implantation of 1015 S ions per cm2 followed by pulsed laser melting is plotted against wavelength in Fig. 1 [4, 5]. Dilute sulfur doping leads to donor impurity levels 0.3 eV below the conduction band [6, 7]. It has been proposed that hyper-doping with S leads to an impurity
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