Resistivity and Hall Voltage Investigation of Phosphorus Segregation in Polycrystalline Si 1-x Ge x Thin Films
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Resistivity and Hall Voltage Investigation of Phosphorus Segregation in Polycrystalline Si1-xGex Thin Films W. Qin, D. G. Ast, and T. I. Kamins+ Department of Materials Science and Engineering, Bard Hall, Cornell University Ithaca, NY 14853-1501 + Hewlett Packard Laboratories, Palo Alto, CA 94304 ABSTRACT Dopant segregation in atmospheric-pressure, chemically vapor deposited (APCVD), ~300 nm thick, polycrystalline Si0.95Ge0.5 and Si0.9Ge0.1 thin films, implanted at 80 KeV with 6×1013 to 5×1014 P/cm2 and annealed at 800 °C for 1 hr, was investigated using a combination of Hall and conductivity vs. temperature measurements. Hall measurements, feasible only in heavier doped films, showed that 29% of the phosphorus in Si0.9Ge0.1 and 42% of phosphorus in Si0.95Ge0.05 was electrically inactive. The loss was attributed to dopant segregating to grain boundaries. The density of grain boundaries states was also found to increase slightly with increasing Ge content, from 3.6×1012/cm2 in Si0.95Ge0.05 to 4.4×1012/cm2 in Si0.9Ge0.1. INTRODUCTION Scanning Transmission Electron Microscopy (STEM) and Hall and resistivity measurements have shown that n-type dopants, such as As and P, segregate to grain boundaries in elemental polysilicon [1,2,3]. A recent STEM study found that P also segregates to grain boundaries in the polycrystalline alloy Si0.87Ge0.13 [4]. While STEM microanalysis provides unique information on dopant segregation to individual grain boundaries, it is a cumbersome technique to determine average values of segregation. Furthermore, understanding the effect of segregation on electrical properties - important in device applications - requires knowledge of the electrical activity of P incorporated into grain boundaries. In this study, we report on the electrical properties of polycrystalline SiGe alloys. The segregation of P to the grain boundaries in SiGe films is discussed, and the trap density due to dangling bonds at grain boundaries in these films is also investigated. If the resistivity ρ0 of a polycrystalline material at temperature T0 is known, the resistivity at temperature T is given by [see 5]: ρ T E 1 1 ρ = 0 exp[ a ( − )] (1) T0 k T T0 where Ea is the barrier height. In the Seto model [6], a critical dopant concentration, denoted N*, exists above which the activation energy for conduction, Ea, decreases inversely with dopant concentration, N: q 2 NT2 Ea = (2) 8εN where ε is the permittivity of Si, and NT is the number of grain boundary trapping states per unit area. The critical concentration, N*, increases linearly with NT and decreases inversely with grain size as N*=NT/L. Dopant was added so that N ≥ N* in all samples considered in this study.
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EXPERIMENTAL APCVD grown, ~300 nm thick polycrystalline Si1-xGex thin films with x=0.05 and 0.1 were patterned into van der Pauw samples [7,8] and implanted at 80 KeV with phosphorus corresponding to volume concentrations of 2×1018, 3×1018 and 1.5×1019 P/cm3. After implantation, the samples were capped with PECVD oxide and annealed under N2 for 1 hr at 80
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