Ion-Beam Synthesized Semiconducting pd-FeSi 2 Controlled By Annealing Procedures And Phase-Transitions
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A). These large differences in the structure between pfFeSi 2 and Si suggest that crystalline frFeSi 2 nucleates in the Si matrix with difficulty. FeSi 2 grows in two different characteristic phases, a high temperature metallic tetragonal phase (rz-FeSi 2 at T>937 C, a=b=2.695 A, c=5.390 A) and a metallic cubic low temperature metastable phase y-FeSi 2 (a=5.431 A)[13]. Recent band structure calculations shows that the direct band-gap of f3-FeSi 2 is related to a lattice distortion from the tetragonal symmetry to the orthorhombic one due to a Jahn-Teller effect. Therefore, the synthesis of high quality crystalline f-FeSi2 must control such structural-sensitive optical and electrical properties. Ion-beam synthesis (IBS) has been employed to make good quality P3-FeSi 2 /Si heterojunctions[2-9,14]. The IBS using mass separated 56Fe+ ions controls the amount of impurities from the source (usually 99.9% pure). The IBS is an appropriate method to make a high quality heterojunction with Si substrates because of its wide variety of implantation profiles controlled by the ion energy and dose. In this study, we examine the formation of ion-beam synthesized frFeSi 2 in two samples prepared in different implantation conditions as a function of the annealing temperature or the surface concentration of Fe. They show good optical properties necessary for optoelectronics devices. EXPERIMENT For the sample preparation mass separated 56Fe+ ions were implanted into n-type FZ grown Si(100) wafers with the resistivity of 2-8 S2cm. Sample #1 was prepared by triple implantation 329 Mat. Res. Soc. Symp. Proc. Vol. 486 ©1998 Materials Research Society
50 and 30 keV with the dose of lxlO17 at 100, cm- 2 and sample #2 by single energy implantation at 100 keV with the dose of 5x10 16 cm-2. The ion beam fluency was about 1 mA/cm2. After the implantation, the samples were subsequently annealed by a halogen lamp furnace at 500 - 800 "C for 2 h in a flowing Ar ambient to form the pFeSi 2 grains and remove the implantation damage. o The samples were analyzed before and after annealing by RBS in both aligned and random orientations using 2 MeV Het ions at the backscattering angle of 15W". The crystal structures and phase assignment of the sample were analyzed by Raman spectroscopy using a 5145 A-Ar ion laser and a Seeman-Bohlin XRD using a Cu anode at the incident angle of 1". The
b Fig.I The unit cell ofp-FeSi 2 .
sample surface morphology was observed by SEM. Optical properties of the sample were characterized by reflectance and absorption spectra measurements.
RESULTS Figure 2 shows the Fe-depth profile derived from the RBS data of the (a) sample #1 and (b) sample #2 before and after annealing. The sample #1 in Fig.2(a) showed little change in the Fedepth profiles before and after annealing even at 800 *C. The concentration near the surface is close to stoichiometric composition of FeSi 2 (Si-33atFe). In the sample #1, precipitation of pFeSi2 might not require long-range diffusion of Fe atoms and rapid precipitation takes place on the surface,
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