Schottky Barrier Height Engineering in NiGe/n-Ge(001) Contacts by Germanidation Induced Dopant Segregation
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0994-F08-01
Schottky Barrier Height Engineering in NiGe/n-Ge(001) Contacts by Germanidation Induced Dopant Segregation S. L. Liew, C. T. Chua, D. H. L Seng, and D. Z. Chi Institue of Materials Research and Engineering, 3 Research Link, Singapore, 117602, Singapore
ABSTRACT Effective Schottky barrier height engineering of NiGe/Ge(001) contacts was achieved through germanidation induced dopant segregation on As-implanted Ge substrates. For NiGe contacts formed through solid state reaction of Ni with Ge via 550oC-60s RTA on 55 KeV As+implanted and subsequently annealed p-Ge(001), it was found that I-V characteristics change from typical ohmic behavior to rectifying one when the implant dose was increased from 0 to 1×1014 cm-2, indicating the effective barrier height modulation by germanidation induced dopant segregation or redistribution. INTRODUCTION Studies on future high speed MOSFETs have increasingly focused on using Ge channel due to its higher carrier mobilities than for Si. The continual scaling of MOSFETs however limits the applications of the conventional ion implantation for source/drain (S/D) junctions in these Ge-based MOSFETs. The dopants commonly used in Ge (e.g. B, As, P) have lower solubility limits in Ge than for Si [1] while diffusivities are higher in Ge [2], rendering the realization of ultra-shallow S/D junctions difficult. Several metal germanides were developed as Schottky barrier contact materials for the S/D to overcome the limitations of ion implantation. Of these, NiGe is highly attractive due to its low formation temperature (~300 oC), electrical resistivity (~13 ñ 15 µΩ.cm) and small Schottky barrier height ΦB to p-Ge(001) [3-5]. However a high ΦB of NiGe on n-Ge(001) implies a high contact resistance to n+ S/D (contact resistance is an exponential function of Schottky barrier height), thus limiting its application. Considering the difficulty in forming Schottky contacts on p-Ge(001) even with low work function metals/germanides due to the serious Fermi level pinning to the valence band edge of Ge [reference], it is essential therefore to reduce the ΦB of NiGe on n-Ge(001) in order that it can be used as well in n-Ge(001) as an electrical contact in Schottky S/D. The idea of modulating effective barrier height in a Schottky diode by introducing a thin layer of (~10nm or less) high concentration dopants was first proposed and demonstrated by Shannon [6]. The principle of this technique is rather straight forward it can be explained as follows. When there is a thin layer (with a thickness Xa) of high concentration (Nd1) uniformly dopants (same type) present within the depletion region (with a depletion width of W) of the semiconductor side with normal doping concentration (Nd2), it can be shown that the maximum electrical field m at the metal-semiconductor interface is given by m ≈ qNd1Xa /εs for Nd1Xa >> Nd2W [7], which is much larger than the normal maximum electrical field m = qNd2W/εs without the thin high concentration doping layer. Due to the extremely high electrical filed in the hi
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