(Invited) Challenges and Developments in GeSn Process Technology for Si Nanoelectronics
In these several years, we have investigated the crystal growth of Ge1-xSnx and Ge1-x-ySixSny thin films and the crystalline and electronic properties [2-5]. In addition, we have recently investigated and developed Ge1-xSnx technologies for electronic applications; those are (1) crystalline and electronic properties of defects, (2) strain and energy band engineering, (3) impurity doping, (4) metal/insulator/semiconductor (MOS) gate-stack structure, (5) metal/semiconductor (MS) contacts. In this report, we will present recent achievements in our research of Ge1-xSnxtechnology.
Defects in Ge1-xSnx alloy
It is known that a vacancy in Ge is an acceptor-like defect and undoped Ge grown at low temperature often shows p-type conduction. The formation and behavior of defects in Ge1-xSnx alloy should be essentially understood for electronic and optoelectronic applications. We investigated the influence of Sn on the electronic properties of Ge1-xSnx by preparing metal/Ge Schottky diodes with Sn-ion implantation and measuring the deep-level transient spectroscopy (DLTS) . The Sn-ion implantation with a dose of 1014 cm-2 and a acceleration voltage of 370 kV generates acceptor-like defects in Ge substrate with a hole-concentration of 1017 cm-3, and these defects is hardly annihilated even after annealing as high as 500 °C. Also, we found by DLTS measurement that the concentration of vacancy-related deep-level defects can be reduced with Sn-implantation, although some additional Sn-related defects are formed.
Oxidation of Ge1-xSnx and gate-stack structures
It is essentially important to investigate the oxidation behavior of Ge1-xSnx for engineering the Ge1-xSnx gate-stacks. We previously reported the electrical properties of Al2O3/Ge1-xSnx/Ge MOS capacitors . Recently, we examined the oxidation of Ge1-xSnx layers pseudomorphically grown on Ge substrate and investigated the behavior of Sn segregation during oxidation . We found the surface segregation of Sn atoms near the Ge1-xSnx surface by oxidation at 400 °C. Thermal oxidation of Ge1-xSnx causes the Sn diffusion from Ge1-xSnx and the formation of Sn oxide on the surface. The capacitance-voltage measurement of Al/Al2O3/Ge1-xSnx capacitors also reveals that the oxidation of Ge1-xSnx increases in the concentration of holes unintentionally generated from acceptor-like defects in Ge1-xSnx layers. We need to develop the defect control technology for gate-stack formation of Ge1-xSnxwith considering Sn behavior during oxidation and annealing.
Metal/Ge1-xSnx contact formation
The formation of metal/Ge contacts with low parasitic resistance is required to improve on the performance of Ge devices. We examined the formation of NiGe/Ge1-xSnx contacts with germanidation processes of Ni/Ge1-xSnx system . A single layer of Ni mono germanostanane, Ni(Ge1-xSnx) can be formed by annealing Ni/Ge1-xSnx/Ge samples at 350~550 °C. We found that the Sn content in Ni(Ge1-xSnx) increases with the Sn content of Ge1-xSnx and that decreases with the annealing temperature due to the Sn segregation from Ni(Ge1-xSnx). Smooth surface of Ni(Ge1-xSnx) layers on Ge1-xSnx can be successfully obtained with annealing at as low as 350 °C even for a Sn content as high as 6.5%, although the agglomeration of Ni(Ge1-xSnx) grains severely occurs after annealing at higher than 550°C.
In summary, the incorporation of Sn promises additional key factors for engineering strain, energy band structure, interface, and defects in Ge nanoelectronics. We need to develop the crystal growth, process, and device technologies based on deeply understanding the crystalline and electronic properties of Sn-related group-IV semiconductor materials.
This work was partially supported by a Grant-in-Aid for Specially Promoted Research of MEXT and an ALCA Program of JST in Japan.
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