Thin Films of Semiconducting SnSi Alloys Grown by Pulsed Laser Deposition
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ABSTRACT Semiconducting SnxSil-x (0•_x_ 0.6) thin-film alloys have been grown by pulsed laser deposition (PLD). These new materials are amorphous to X-rays and display small positive optical band gaps, suggesting potential applications in solar cells. The tin silicide films were grown by depositing very thin (1-30 A) alternating atomic layers from individual Sn and Si targets utilizing an automated multi-target holder coupled to a conventional PLD system. The value of x was selected by controlling the thickness of the atomic layers. The films were characterized by X-ray diffraction, optical absorption, Rutherford backscattering spectroscopy, temperature-dependent resistivity, and X-ray photoelectron spectroscopy. Tin segregation is prevented by keeping the Sn layer thickness below a critical value. Compositions beyond x >0.6 led to semimetallic SnxSilx films with tin crystallites. INTRODUCTION The technological need to develop new Infrared (IR) detector materials is well documented. One candidate material is SnSi. SnxSilix would ideally have a range of band gaps between those of Sn and Si, 0 and 1.5 eV, respectively. Several groups have attempted to prepare SnxSil-x, with limited success. Previous attempts, described in the literature, to make amorphous, semiconducting tin-rich alloys (x>0.5) by co-evaporation and by sputtering from mixed targets were unsuccessful [1,2,3,4,5,6]. In the sputtered and evaporated films, the tin would segregate and crystallize into (metallic) Is-tin causing the films to become semimetallic. This paper describes how it is now possible to make tin-rich SnSi alloys by depositing the films as superlattices. Tin segregation is prevented by keeping the Sn layer thickness below a critical value. EXPERIMENTAL The films were grown by pulsed laser deposition (PLD) [7] at a base pressure of 10-7 Torr by ablating individual tin and silicon targets held on an automated target changer mounted in a high-vacuum chamber. The number of laser pulses per atomic layer and the number of layers per film were prescribed and the target positioning and laser triggering were computer controlled. The KrF excimer laser (248 nm) was operated at 10 Hz using a fluence of -5 J/cm 2 . The films were deposited on room-temperature Pyrex substrates. The film structure was characterized by X-ray diffraction (XRD) on a standard 0-20 diffractometer using Cu Ka radiation. The film compositions were found by Rutherford backscattering spectroscopy (RBS) and by X-ray photoelectron spectroscopy (XPS). Depth profiling of deposited film compositions was determined by repeated sputtering and XPS analysis. The optical transmission measurements were performed on a dual beam spectrometer with a wavelength range of 340 to 2400 nm with an uncoated substrate used as a reference. Electrical properties were determined using temperature-dependent resistivity (R(T)) measurements between 8 and 350 K. Gold stripes were sputtered onto the films for the four-point R(T) measurements. 775
Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Researc
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