Effect of strain on Ni-(GeSn) x contact formation to GeSn nanowires
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Effect of strain on Ni-(GeSn)x contact formation to GeSn nanowires M. Noroozi1, M. Moeen2,3, A.Abedin2, M. S. Toprak1 and H. H. Radamson2 Department of Materials and Nano Physics, 2Department of Devices and Circuits 1,2 KTH Royal Institute of Technology, Isafjordsgatan 22-26, 16640 Kista, Sweden 3 Nocilis Materials, Isafjordsgatan 39, 16440 Kista, Sweden 1
ABSTRACT In this study, the formation of Ni-(GeSn)x on strained and relaxed Ge1−xSnx (0.01≤x≤ 0.03) nanowires in contact areas has been investigated. The epi-layers were grown at different temperatures (290 to 380ºC) by RPCVD technique. The strain in GeSn layers tailored through carefully chosen of growth parameters and virtual substrate. The nanowires were fabricated through both I-line and dry-etching. 15 nm Ni was deposited either on the contact areas or whole length of nanowires. The wires went through rapid thermal annealing at intervals of 360 to 550ºC for 30s in N2 ambient. The results show the thermal stability and amount of particular phases were strain-dependent. The formation of Ni-GeSn was eased when GeSn layers were strain-free. When the Sn content is high the epi-layers suffer from Sn segregation. The Sn-rich surface impedes remarkably the Ni diffusion. The electrical conductivity measurement of nanowires shows low resistivity and Ohmic contact are obtained for Ni-GeSn. INTRODUCTION GeSn is a promising material in electronics and photonics devices due to its higher carrier mobility for CMOS channels, its capability to act as a stressor in source and drain, and its tunable band gap [1-2]. Large lattice mismatch between Sn and Ge and low solid solubility of Sn in Ge (1%) make it difficult to grow epitaxial layers with low defect density. Recent developments of new Sn precursors e.g. SnCl4 and SnD4 give the possibility to grow Sn-alloys by using CVD at temperatures as low as 290 C [3-5]. Low temperature epitaxy suppresses the Sn segregation where highly strained GeSn can be grown. Integration of GeSn layers in devices brought the attention to assessment of reliable metal contact and contact resistance issues since these may degrade the performance of the devices [6]. It has been demonstrated that Ohmic contact and contact resistance are major issues for low dimensional materials like nanowires (NWs). NiSi is an example of an intermetallic phase which is commonly used for Si in CMOS technology with low resistivity and high thermal stability [7]. For GeSn layers, situation may differ since the strain in these layers can possibly has impact on their reaction with Ni. The strain has a significant role on the transport properties of GeSn layers, hence the device characteristics. The reaction of Ni with GeSn alloys is reported previously, however, a detailed study which takes strain into account is still missing. This work presents the effect of strain on thermal reaction (360-550ºC) of Ni with GeSn material and the resulting electrical properties. The GeSn layers were strain-free or having either compressive or tensile strain. The results presented in this
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