SupplementaryData
Gate-tunable superconducting quantum interference devices of PbS nanowires
Hong-Seok Kim1, Bum-Kyu Kim1,Yiming Yang2, Xingyue Peng2,Soon-Gul Lee1,Dong Yu2, Yong-Joo Doh1,*
1Department of Applied Physics, Korea University Sejong Campus, Sejong, 339-700, Republic of Korea
2Department of Physics, University of California, Davis, CA 95616, USA
*E-mail:
1. PbS nanowire growth
The PbS nanowires (NWs) were synthesized via a chemical vapor deposition (CVD) method in a horizontal tube furnace (Lindberg Blue M). Lead chloride (PbCl2, 99.999%, Alfa Aesar) and Sulfur (S, 99.9999%, Alfa Aesar) powders were employed as precursors, and placed in the center and outside the heating zone of the furnace, respectively. The growth substrate was prepared by e-beam evaporation (CHA e-beam evaporator) of 100 nm Ti thin film onto a SiO2 coated Si wafer. The substrate was placed 5 cm from the center of the heating zone, where the temperature is measured to be around 550 °C. In a typical synthesis, the system was first evacuated to a base pressure of 15 mTorr, and then filled with N2 (99.999%) to atmosphere pressure. A 150 sccm N2 flow was maintained while the furnace temperature was quickly ramped to 630 °C at 60 °C/min. Immediately after the peak temperature was reached, the quartz boat containing S was transferred into the heating zone to trigger the growth. The growth duration varies from 30 minutes to 2 hours. After the growth, the furnace was naturally cooled down to room temperature over approximately 3 hours. The N2 flow was kept during the entire cooling down process to remove any S residue.
The doping of the PbS NWs was modulated by varying the weight ratio of PbCl2 and S. In this work, three individual ratios are adopted to generate n-type (mPbCl2 : mS = 3:1), intrinsic (1:1), and p-type (1:3) NWs, respectively.
2. Device fabrications
Grown PbS NWs are mechanically transferred to a highly p-doped silicon substrate covered by a 300 nm-thick oxide layer with pre-patterned Cr/Au bonding pads. The conductive silicon substrate is used as a back gate. Metal contact electrodes are patterned using standard electron beam lithography followed by e-beam evaporation of Pb0.5In0.5 (250 nm)/Au (15 nm) to connect the NWs to the bonding pads. Prior to metal deposition, oxygen plasma treatment is performed to remove resist residues. Then, the NW is treated using 6:1 buffered oxide etch (BOE) for 10 s to remove native oxide layer from the NW surface. Diameters of PbS NWs range from d = 97 to 120 nm, and the lengths of the weak links L = 235 to 310 nm.
Fig.S1. (a) SEM image of the PbS NW SQUIDs. (b) Resistance vs. temperature curve of PbIn electrode. Line is to guide the eye. Superconducting transition temperature is obtained to be Tc = 6.75 K.
Fig. S2. (a) Temperature dependence of Ic1 and Ic2 in D2. (b) Temperature-dependent ratio of Ic2/Ic1. The Ic asymmetry increases at higher temperatures.
References
1.M. Octavio, M. Tinkham, G. E. Blonder and T. M. Klapwijk, Phys. Rev. B 27, 6739 (1983).