Formation of ultra-shallow Ohmic contacts on n-Ge by Sb delta-doping
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Formation of ultra-shallow Ohmic contacts on n-Ge by Sb delta-doping K. Sawano1, Y. Hoshi1, K. Kasahara2, K. Yamane2, K. Hamaya2,3, M. Miyao2, Y. Shiraki1 1
Research Center for Silicon Nano-Science, Advanced Research Laboratories,Tokyo City University, 8-15-1 Todoroki, Setagaya-ku, Tokyo 158-0082, Japan 2 Department of Electronics, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan 3 PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan
ABSTRACT We demonstrate low-resistivity Ohmic contacts for n-Ge with ultra-shallow junction. Using the impurity δ-doping techniques with Ge homoepitaxy on Ge(111) below 400 ºC, we can achieve a very abrupt doping profile within a nanometer-scale width. By introducing the δdoping to atomically controlled metal/Ge contacts, the current-voltage characteristics clearly show Ohmic conductions owing to the effective tunneling through the Schottky barrier. This approach is promising for a formation technology of ultra-shallow source/drain contacts for scaled Ge devices.
INTRODUCTION Ge has been recognized as one of the most important candidates for next-generation channel material in complementary metal oxide semiconductor (CMOS) circuits since carrier mobilities for both electron and hole are intrinsically very high. Several technological challenges have been imposed, however, for realization of scaled Ge MOS devices. One of the critical issues is formation of ultra-shallow junction with highly reduced contact resistance. Low contact resistance is generally obtained with high-concentration impurity doping in the source/drain (S/D). For Ge, however, ion implantation of n-type dopants has several problems, such as large diffusion and low solubility of the dopants. Relatively high temperature annealing is required to sufficiently activate the implanted dopants, but it causes the enhanced dopant diffusion [1-6]. Employment of the dopant segregation during metal germanidation was reported [7-9] as new methods which enable relatively low temperature activation, where metal-induced crystallization plays an important role. This approach, however, also needs ion implantation for inducing the dopant into Ge, making it difficult and complicated to control the depth profile precisely and attain ultra-shallow junctions. High-density impurity doping without using ion implantation is, therefore, favorable especially for Ge n-MOS applications. For this purpose, we attempt impurity δ-doping and following Ge homo-epitaxy. Unlike ion implantation, the δ-doping is basically free from defect introduction, and hence, the thermal activation annealing, which unavoidably causes the unfavorable dopant diffusion, is not necessary. Moreover, arbitral and precise controlling of the dopant depth distribution is possible within a several nm range. These superior advantages make this approach quite attractive for realization of ultra-shallow junctions for Ge. Practically, however, dopant atoms are known to highly segregate up to a growing surface during the Ge growth strongly depending on the gro
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