2. Experimental methods 43
2.2. Electrical measurements
Similarly to the above mentioned fabrication schemes (vdW-heterostruc-tures and suspended samples), the same nanofabrication processes were used to finalize the CVD devices. More details can be found in the appendicesA andB.
2.2. Electrical measurements
Sufficiently low temperatures are needed to study various phenomena in grap-hene based devices. The lattice temperature but also the electron tempera-ture has to be sufficiently low to observe quantum mechanical effects such as coherent transport phenomena or superconductivity. Throughout this thesis several cryogenic measurement set-ups have been used with base temperatures from 20 mK over 4.2 K up to room temperature. Liquid4He has a temperature of 4.2 K (at a pressure of 1 bar) and can directly be used to immerse a sample to cool it to 4.2 K. By evaporation cooling, i.e. pumping on the liquid He, latent heat is removed due to the evaporation and temperatures around 1.4 K can be reached. In combination with a heater, various temperatures can easily be achieved. Lower temperatures can only be achieved by using3He instead of4He. Similarly, evaporation cooling leads to base temperatures on the order of 220 mK. Since3He is very rare and therefore expensive, only closed systems exist and therefore continuous operation is not possible. The3He has to be recondensed once in a while. A continuous operation and even lower tempera-tures are possible by3He/4He dilution refrigerators. Their working principle rely on the fact that a mixture of 3He and4He spontaneously separates into a 3He-rich and 3He-poor phase at ∼870 mK [152]. Essentially the dilution of3He from the3He-rich to the3He-poor phase generates the cooling power [152]. Magnetic fields up to 9 T were applied with superconducting magnets.
In some experiments a vector magnet was used to align the magnetic field to the in-plane direction of the sample. At room temperature an electro mag-net was used to apply fields up to 500 mT in the in-plane and 800 mT in the out-of-plane direction.
It is not straightforward to connect a nanoscale device to a macroscopic me-asurement unit. Obviously, commercial, standardized components were used whenever possible. Along the line from the table-top measurement unit toward the device, the last commercial and standardized piece was the chip-carrier, as shown in Fig.2.5on the left. The sample chip was glued with silver paint into this 20 terminal chip-carrier that can be plugged into the corresponding sockets in the measurement set-ups. Electrical connections from the chip-carrier to the metallic structures on the Si/SiO2 wafers were established in a ultrasonic wedge bond process. The on-chip metallic lines then connected to the active device region as shown in Fig.2.5on the right for example. In the two experiments were high frequency signals were involved (spin pumping, see
2. Experimental methods
chapter6, and quantum capacitance and dissipation measurements, see chap-ter8) the chip carrier and chip-socket was replaced by custom circuit boards with RF and DC connectors.
1 µm 9 mm
Figure 2.5. How to connect nanoscale samples to the real world:The left shows a wire bonded sample in a chip-carrier. The images to the right show scanning electron micrographs of a typical device at different magnifications.
Most of the time, the active device region is on the order of 1 µm with feature sizes down to ∼100 nm.
The device in the chip-carrier, mounted into the corresponding chip-socket in the measurement set-up, is then connected through twisted pairs to a break-out box at room temperature where BNC cables were used to connect it to measurement electronics. It is well known that the electron temperature de-couples from the lattice temperature at low temperatures if not appropriate thermal anchoring and electrical filtering is performed [152]. Therefore, all me-asurement lines are well thermalized to the coldest spot in the cryostat (e.g.
the mixing chamber in case of a dilution refrigerator). Several filter stages are employed to shield from (thermal) high frequency radiation. We typically use a two-stage filter set-up with a first, commercial filter-stage (cut-off fre-quency around 1 MHz) directly mounted on the break-out box. A home-built tape-worm filter with a cut-off frequency of 10 MHz is implemented directly at the cold-finger in dilution refrigerators and in the3He-system. The sample is shielded from thermal radiation by a Faraday cage in all set-ups. We typically reach electron temperatures below 100 mK in dilution refrigerators.
Standard low-frequency lock-in techniques7were used to measure differential conductance and resistance. A typical schematics of a measurement set-up is shown in Fig.2.6. Home-built8 low-noise and low-drift voltage amplifiers and I/V-converters were used in the detection chain. DC voltages were sourced either by commercially available sources (Yokogawa YK7651) or by a home-built low-noise/high-resolution DAC. AC voltages were superimposed on top of a DC voltage by the usage of a tansformer. Additionally, the AC signal
7Using Standford SR830 lcok-ins.
8Designed and built by the electronics workshop at the Department of Physics, University of Basel[153].
52
2.2. Electrical measurements
Break-out box
105-109 V/A
I/V converter
R
Sample room temperature
DC sources VAC Lock-In
π ters
10 kΩ
10 Ω
voltage divider 1:1000
-+
rf ters
Backgate transformer 1:4
Tbase= 20 mK/230 mK/1.5 K / 4.2 K - RT
Vin
CryostatMeasurement electronics 1MΩ
Figure 2.6. Schematics of a cryogenic measurement set-up: Schematic of a typical set-up for voltage biased differential conductance measurements at low temperature indicating the most important components. Image adapted from [84].
could also directly be applied to the device while the DC voltage required for biasing was applied to the offset voltage of the IV-converter. Small magnetic fields as required for localization measurements presented in chapter 7were generated by replacing the standard current sources at the superconducting magnets with source meters from Keithley (2400). All measurement electronics were controlled by LabView routines or Igor Pro scripts that communicated with the instruments over RS232 or GPIB interfaces.
3 Investigation of building blocks:
Ferromagnetic contacts and CVD hBN
1In this chapter, we investigate essential building blocks for spin valve devices.
Ferromagnetic contacts are widely used to inject spin polarized currents into non-magnetic materials such as semiconductors or 2-dimensional materials like graphene. The shape of the nanomagnets as well as their composition can be engineered to tailor their properties for specific applications. However, oxidation of ferromagnetic contacts poses an intrinsic limitation on device performance. In this chapter we characterize nanomagnets with magnetic force microscopy, X-ray magnetic circular dichorism imaging and we study the role of ex-situ transferred chemical vapour deposited (CVD) hexagonal boron nitride (hBN) as an oxidation barrier for nanostructured cobalt and permalloy electrodes. For efficient spin injection tunnel barriers are needed, for which we have used CVD hBN. In this chapter we investigate the quality of CVD hBN from several sources with different imaging techniques.2
1Parts of this chapter have been published in similar form in Ref. [154].
2A photo emission electron microscopy image of an iron foil on which single layer hBN has been grown partially. The image is recorded at the L3edge and the hBN covered regions (e.g. the triangle in the middle) appear brighter since there the iron is less oxidized.
3. Investigation of building blocks: Ferromagnetic contacts and CVD hBN