Nonalloyed Ohmic Contacts to N-Si using a Strained Si 0.5 Ge 0.5 Buffer Layer
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ABSTRACT Nonalloyed shallow ohmic contacts to n-Si have been fabricated by using an 80 A thick strained Si 0.5Ge 0 .5 buffer layer grown by molecular beam epitaxy. X-ray photoelectron spectroscopy was employed to investigate the Si 2p and Ge 3d core level binding energies of the strained and the relaxed Si0 .5Geo. 5 and to determine their relative Fermi level positions. It was found that the surfaces of strained Si0 5. Ge0 .5 exhibit pinning very close to the conduction band. Rutherford backscattering and Auger depth profiling were employed to determine the contact reactions using Ti, W or Pt as contact metals. In the case of Pt, a 500 A W diffusion barrier can protect the ohmic behavior up to 550 °C for 30 min. The specific contact resistance of the Pt/W/Si 05.Ge 0 .5 /n-Si contact extracted from the D-type cross-bridge Kelvin resistor was 3.5x 10-5 Q. cm 2 .
INTRODUCTION Ohmic contact resistance is a serious issue affecting device performance in the submicron regime. For Si-based devices, the usual contact materials are Al alloys (with some Si and/or Cu) and titanium silicides. These contacts need an annealing process in order to break down the natural oxide on the Si. In the Si/SiGe material system, the epitaxial strain determines the energy band gaps and their line-up at the interface1 -3 . By controlling the strain, two-dimensional electron and hole gases 4.5 have been demonstrated in this material system and their contact behavior is of interest. Since the conventional schemes of contact must be annealed, it is possible that the epitaxial strain will relax during the annealing. In addition, the interfacial reaction between a metal and SiGe is a ternary system; it is difficult to control the interfacial composition as in the contacts to GaAs 6 -8. Therefore a stable ohmic contact to Si/SiGe without affecting the strain and the chemical composition of SiGe alloys is important. The specific contact resistance of a contact interface is proportional to the exponential of its Schottky barrier height. It is usually difficult to form ohmic contacts to a semiconductor with a high Schottky barrier height without increasing the semiconductor surface doping concentration and annealing process. In such case, a low Schottky barrier height semiconductor which has a small conduction band offset with respect to the high barrier semiconductor can be used as a buffer layer between the contact metal and the adjacent high barrier semiconductor to facilitate ohmic contact formation, with the ultimate goal that nonalloyed shallow ohmic contacts can be formed. Based on this principle, Hafich et al.9 have made nonalloyed ohmic contacts to n+-Si using a 3500 A thick arsenic doped n+-Ge as a buffer layer. This contact thin film structure is similar to that used to contact n-GaAs 10 ,11. However, the conduction band discontinuity between Ge and Si is 0.28 eV 2 ,3 , it will have some contributions to the contact resistance. On the other hand, the conduction band discontinuity between a strained SilxGex and Si is so small (about 20 meV)
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