Supporting Information Topological surface transport properties of ...
Supporting Information Topological surface transport properties of single-crystalline SnTe nanowire Muhammad Safdar‡, Qisheng Wang‡, Misbah Mirza, Zhenxing Wang, Kai Xu and Jun He* National Center for Nanoscience and Technology, Beijing 100190, China ‡These authors contributed equally to the work
Email: [email protected]
Synthesis of SnTe nanowires. SnTe nanowires were synthesized via CVD in a horizontal vacuum tube furnace. Si substrate was dipped in HF to remove native oxide layer then 10 nm Au layer deposited using DC magnetron sputtering system. SnTe powder was loaded (99.99 %, alfa aesar) in the center of quartz tube and Au thin film covered Si substrates were placed at 10-20 cm downstream area from source for CVD. Quartz tube was sealed well and evacuated, then high purity Ar gas was used to flush the quartz tube five times to provide oxygen free environment. After that Ar gas was fed with a constant flow rate 100 sccm by maintaining tube pressure of 150 Torr. The furnace temperature raised up to 900 °C at a ramp rate 15 °C /min. The temperature for growth time was maintained for 1hr to complete reaction in atmosphere of Ar gas and then furnace was cooled down to room temperature. Characterizations. FESEM S4800 (Tokyo, Japan) at 10 KV equipped with EDX was used to characterize the SnTe morphologies. XRD (Philips X’Pert Pro Super with Cu Kα radiation) at room temperature was done at a tube voltage of 40 kV and a tube current of 200 mA. TEM and HRTEM information were collected by FEI Tecnai F20 at an accelerating voltage of 200 kV. AFM was carried out by Veeco Multimode. Device Fabrication. Firstly, SnTe nanowires were mechanically separated in acetone by 10 seconds sonication and transferred onto a cleaned Si substrate which has SiO2 dielectric layer of 300 nm. Then FEI Nanolab 600i
1
SEM/FIB dual bean system was used to fabricate the Pt-electrodes onto an individual SnTe nanowire. In order to prevent the contamination from ions, a two-step electrode fabrication process was adopted: electron beam induced deposition (EBID) and focused ions beam deposition (FIBD). It is known EBID process is clean due to that electrodes made by this technique only cover the surface of nanowire. While FIBD process will introduce contaminations since ions can permeate into the sample. The four parallel electrode leads on the nanowire were firstly fabricated by EBID. And then FIBD was used to connect these electrode leads to the gold pads. The nanowire was not irradiated by Ga+ ion beam during the entire process. Transport measurements. Magneto resistance (MR) measurement was carried out in Quantum design PPMS9 instrument and Keithley 4200 semiconductor characterization system. The resistance was measured using a standard four-terminal device to eliminate the contact resistance. The temperature ranged from 2 to 45 K with highest magnetic field up to 8T.
Figure S1. Schematic diagram of electronic structure of SnTe (001) surface around X. Thick red lines represent topological surface states and thin blue lines denote bulk bands. The Dirac point appears between Ƭ and X.
2
a
b
Figure S2. Statistical data of nanowire density and diameter: the nanowire density of (a) is estimated to be 1.7x109/cm2, deposition temperature of substrate (a) is about 500 oC. (b) The diameters of nanowires in substrate (a) mainly distribute in 50-99 nm (75%), the diameters of the rest nanowires are in the range of 100-199 nm (25%), the average diameter in this substrate is 98 nm.
Figure S3. Magnified TEM shows top gold particle connected very well with SnTe nanowire.
3
a
c
b
179 nm
125 nm
49 nm
Figure S4. (a), (b) and (c) are AFM images of three SnTe nanowires, inset shows cross-section shape of SnTe nanowire.
1600
b
a 2.0
T=2K
T=10 K
1400
B (T)
R (GΩ)
1.5 1.0 0.5
1200
T=2 K 0.0
0.5
1.0
∆B=0.169 T
0.0 0
1.5
B (T)
2
4
6 n
8
10
12
Figure S5. (a) AB oscillation of SnTe nanowire at 2 K and 10 K. (b) The plot of peak position B versus peak index n suggests the period of AAS interference at 2 K is 0.169 T.
124 nm
Figure S6. AFM of SnTe nanowires used in SdH measurements. Near rectangular cross section with height of 124 nm offers sufficient large top and bottom surface for SdH oscillations.
4
R (MΩ)
6.470
T=2 K
6.465
6.460
0.4
0.6
0.8
1.0
1.2
1.4
-1
1/B (T ) Figure S7. MR versus 1/B at 2 K with the magnetic field applied up to 4 T, the red solid line shows the periodicity of SdH oscillation.
5
Email: [email protected]
Synthesis of SnTe nanowires. SnTe nanowires were synthesized via CVD in a horizontal vacuum tube furnace. Si substrate was dipped in HF to remove native oxide layer then 10 nm Au layer deposited using DC magnetron sputtering system. SnTe powder was loaded (99.99 %, alfa aesar) in the center of quartz tube and Au thin film covered Si substrates were placed at 10-20 cm downstream area from source for CVD. Quartz tube was sealed well and evacuated, then high purity Ar gas was used to flush the quartz tube five times to provide oxygen free environment. After that Ar gas was fed with a constant flow rate 100 sccm by maintaining tube pressure of 150 Torr. The furnace temperature raised up to 900 °C at a ramp rate 15 °C /min. The temperature for growth time was maintained for 1hr to complete reaction in atmosphere of Ar gas and then furnace was cooled down to room temperature. Characterizations. FESEM S4800 (Tokyo, Japan) at 10 KV equipped with EDX was used to characterize the SnTe morphologies. XRD (Philips X’Pert Pro Super with Cu Kα radiation) at room temperature was done at a tube voltage of 40 kV and a tube current of 200 mA. TEM and HRTEM information were collected by FEI Tecnai F20 at an accelerating voltage of 200 kV. AFM was carried out by Veeco Multimode. Device Fabrication. Firstly, SnTe nanowires were mechanically separated in acetone by 10 seconds sonication and transferred onto a cleaned Si substrate which has SiO2 dielectric layer of 300 nm. Then FEI Nanolab 600i
1
SEM/FIB dual bean system was used to fabricate the Pt-electrodes onto an individual SnTe nanowire. In order to prevent the contamination from ions, a two-step electrode fabrication process was adopted: electron beam induced deposition (EBID) and focused ions beam deposition (FIBD). It is known EBID process is clean due to that electrodes made by this technique only cover the surface of nanowire. While FIBD process will introduce contaminations since ions can permeate into the sample. The four parallel electrode leads on the nanowire were firstly fabricated by EBID. And then FIBD was used to connect these electrode leads to the gold pads. The nanowire was not irradiated by Ga+ ion beam during the entire process. Transport measurements. Magneto resistance (MR) measurement was carried out in Quantum design PPMS9 instrument and Keithley 4200 semiconductor characterization system. The resistance was measured using a standard four-terminal device to eliminate the contact resistance. The temperature ranged from 2 to 45 K with highest magnetic field up to 8T.
Figure S1. Schematic diagram of electronic structure of SnTe (001) surface around X. Thick red lines represent topological surface states and thin blue lines denote bulk bands. The Dirac point appears between Ƭ and X.
2
a
b
Figure S2. Statistical data of nanowire density and diameter: the nanowire density of (a) is estimated to be 1.7x109/cm2, deposition temperature of substrate (a) is about 500 oC. (b) The diameters of nanowires in substrate (a) mainly distribute in 50-99 nm (75%), the diameters of the rest nanowires are in the range of 100-199 nm (25%), the average diameter in this substrate is 98 nm.
Figure S3. Magnified TEM shows top gold particle connected very well with SnTe nanowire.
3
a
c
b
179 nm
125 nm
49 nm
Figure S4. (a), (b) and (c) are AFM images of three SnTe nanowires, inset shows cross-section shape of SnTe nanowire.
1600
b
a 2.0
T=2K
T=10 K
1400
B (T)
R (GΩ)
1.5 1.0 0.5
1200
T=2 K 0.0
0.5
1.0
∆B=0.169 T
0.0 0
1.5
B (T)
2
4
6 n
8
10
12
Figure S5. (a) AB oscillation of SnTe nanowire at 2 K and 10 K. (b) The plot of peak position B versus peak index n suggests the period of AAS interference at 2 K is 0.169 T.
124 nm
Figure S6. AFM of SnTe nanowires used in SdH measurements. Near rectangular cross section with height of 124 nm offers sufficient large top and bottom surface for SdH oscillations.
4
R (MΩ)
6.470
T=2 K
6.465
6.460
0.4
0.6
0.8
1.0
1.2
1.4
-1
1/B (T ) Figure S7. MR versus 1/B at 2 K with the magnetic field applied up to 4 T, the red solid line shows the periodicity of SdH oscillation.
5
