Figure 1. MFLI lock-in Amplifier and OptistatTMDry cryostat
This article outlines a new process to perform various characterizations of a Quantum Point Contact (QPC) device through a combination of MFLI lock-in amplifier from Zurich Instruments and OptistatDry cryostat system from Oxford Instruments (Figure 1). The Semiconductor Physics Group of the Cavendish Laboratory at University of Cambridge, UK provided the test sample.
Quantum bits, also known as qubits, are the fundamental building blocks of a quantum computing device. There are many proposals and implementations are available for such a device, and they all need to have a read-out mechanism to operate in a consistent manner.
In the case of semiconducting quantum dots (QDs) or GaAs, QPCs provide a way to readout a qubit’s charge state. The functionality of QPC can be easily evaluated at and around 4 K, while the operation of a solid state qubit is performed at relatively lower temperatures (milliKelivin) where the coherence times are longer.
Oxford Instruments' OptistatDry provides a compact and user-friendly cold environment. Integrated tools in the MFLI lock-in amplifier also provide a unique set of electrical measurements. In this experiment, all of the measurements were carried out within a single day, providing advanced integration for high-end research applications.
Description of MFLI lock-in Amplifier
The MFLI is a digital dual-phase demodulator lock-in amplifier that operates between DC and 5 MHz. The front panel of the device is fitted with:
- Four 18 bit auxiliary outputs (AuxOut)
- Two auxiliary inputs (AuxIn)
- Differential voltage input/outputs (V)
- Current input (I)
The two AuxIns connectors serve dual functions: they can be used for external signal referencing and for adding analog DC bias to the signal outputs. In addition, the user interface provides a digital path for integrating the DC component onto the signal output.
The AuxOuts can be employed to output both the automated/manual voltage bias sweeps and the demodulated signal (X, Y, R, theta) components. The multi-demodulator (MF-MD) option is provided with four demodulators.
This enables I and V measurements and/or demodulation at four varying frequencies simultaneously, including DC. Figure 2 shows an example of such a configuration.
The LabOne user interface controls the MFLI, offering a complete analysis toolset including parameter sweeper, spectrum analyzer, oscilloscope, etc. This makes the MFLI lock-in amplifier suitable for the characterization and measurement of devices.
Figure 2. A screenshot of the MFLI LabOne user interface from Zurich Instruments. The top panel provides the instrument configuration settings of the MFLI device with the multi-demodulator (MF-MD) option. The bottom graph shows the Sweeper tool with AC (full) and DC (dotted) current measurements of the forward (blue) and the backward (red) gate voltage sweeps.
Sample Description and Schematics
A 1D quantum channel forms in a QPC device when an electrostatic potential is applied onto a 2D electron gas formed beneath the GaAs top layer. Figure 3a shows the structure of the QPC device.
A QPC is formed when current conducts between the side gate electrodes (with an anti-dot) and source-drain leads. Although another top-gate is present across a crosslinked PMMA, it is not used in the measurements illustrated in this article.
OptistatDry Cryostat Description
The OptistatDry from Oxford Instruments is a Cryofree, closed-cycle refrigerator cooled, table top cryostat that provides a temperature range down to below 3 K, which can be realized within 3 hours from room temperature. Although the sample is kept in a vacuum environment, it can be accessed through a load port easily.
A demountable sample puck is used to mount the sample and this puck can contain a heater for fine temperature control and a thermometer for accurate temperature reading of the sample. The sample space has electrical access through the sample puck and a multi-axis optical access via up to five windows.
After mounting the QPC sample in a chip insert, the PLCC chip carrier is placed on the sample puck using GE varnish. Fine Al wires were used to wire bond the electrical contacts from the PLCC carrier to the sample puck. In order to ensure the sample node equipotential during handling, additional wire bonded links were added.
Sample mounting in the OptistatDry system can be achieved by loading the puck with the help of tweezers or vacuum tool. Users would need to wear an antistatic wrist strap.
Good electrical contact with the spring-loaded pins and good thermal contact with the cooling plate is ensured by tightening the three screws. In the experimental set-up, the cabling features an ESD protection box between the MFLI and the OptistatDry. This makes it possible to ground all of the signal lines, ensuring that the sample is at equipotential through the instrumentation signal lines.
The lines can be separately switched and can also be individually switched to the MFLI or to ground. With all of the lines switched to ground, the ESD protection link wires on the puck can be cut. The signal lines are then switched to the MFLI lock-in amplifier to initiate the device characterization experiment.
Using source-drain electrodes, conductance (I-V) measurements were performed and electrodes transport was altered using the two side gates (Figure 3a). 200 µV was the source drain voltage excitation and the current was determined at the 100 µA input range with complete input bandwidth of up to 5 MHz.
Sample continuity was initially checked at room temperature, followed by cooling down the sample to the base of 2.3 K. The overall noise spectrum at the input was evaluated using the Scope tool of the LabOne user interface.
To select the optimal measurement area, the LabOne Sweeper tool, in the range of 10 Hz to 10 kHz, was used to run a frequency sweep. This showed a flat AC current frequency response (black curve, Figure 3b).
Up to four times higher noise, below the corner frequency of 100 Hz, was revealed by subsequent noise measurements (blue curve, Figure 3b). For these specific tests, the measurement frequency of 237 Hz was selected.
Figure 3. The measurement schematics and lead connection to MFLI Lock-in Amplifier. Figure 3b. AC current (black) and the current noise density (blue) plotted as a function of frequency obtained using the Sweeper tool. AC (Figure 3c) and DC (Figure 3d) current measurements as a function of the side gate voltage swept at temperatures 2.3 K (black) and 10K (red), forwards (full) and backwards (dotted). The side gates are driven simultaneously using one AuxOut of the lock-in amplifier.
The tests involved measuring both DC and AC current components of the source-drain conductance to track the transport of leakage currents.
This can be achieved by using the MFLI MF-MD option, which enables the measurement frequency to be set at 0 Hz for the DC and at the AC frequency using two demodulators at the same time. The AuxOut voltage driving the side gates was swept using the LabOne Sweeper tool.
To setup the sweeps, the side gate voltage was manually changed in small steps of 10 mV. When using the LabOne UI's Plotter tool, it was observed that the currents are relaxing on long time scales so the wait time was set to 60 seconds to track the equilibrium state.
Voltage sweeps were carried out between 0 to -3 V and back. Figures 3c and 3d (dotted curves are backward sweeps) show an example of the finished sweeps of AC and DC currents, at 2.3 K (black) and 10 K (red).
It was observed that the conduction current decreases in the channel as the temperature increased as a result of the activation of extra scattering mechanisms. It was also seen that the pinch off of the channel was at around -3 V, and there were a number of conductance reduction steps along the way.
A large hysteresis was noticed in the back sweep, demonstrating that considerable charge transfer is occurring towards the end of the long sweep. DC and AC currents exhibit sharp changes at the same gate voltages pointing to the device’s current leakage, specifically between the drain electrode and the side gates. The I-V characteristics of source-drain electrodes were also determined at side gate voltage of -1.3 V and various temperatures.
The DC bias is digitally combined with the AC excitation. Figure 4 shows the measurement results at four different temperatures from 5 K to 20 K. The source-drain bias was restricted to 10 mV.
In Figure 4a, the I-V characteristics are linear as validated by the almost flat AC current response in Figure 4b.
Figure 4. The DC (Figure 4a.) and AC (Figure 4b.) current measurements as a function of the side gate voltage bias taken at four different temperatures from 5 K to 20K.
The characterization of a QPC device can be efficiently performed as a function of gate bias and source-drain at various frequencies and temperatures. The OptistatDry system enables rapid and accurate temperature control, whereas the integrated oscilloscope of the MFLI lock-in amplifier can help to address potential noise source issues, including set-up ground loop.
The fast frequency sweep highlights the optimal measurement frequency, while the MF-MD option enables DC and AC current measurements, with high signal to noise ratio as a function of side gate bias, indicating the presence of the leakage currents.
The contacts’ linearity, shown by the source-drain sweeps, can be taken for varying temperatures and side gate voltages. This article shows how the combination of systems from Zurich Instruments and Oxford Instruments allow cost effective and efficient lab setups. The time to first characterization takes less than a single day, which marks a key step for reducing time to publication.
With the combination of two instruments, users can efficiently perform optical and electrical experiments down to 2.3 K. Programming effort is also reduced with the use of sophisticated user interfaces such as LabOne as it offers a comprehensive, unique toolset that is readily available for many instant analysis.
Compared to using several external measurement tools, this reduces the complexity related to setups as well as prospective failure modes.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Nanoscience.
For more information on this source, please visit Oxford Instruments Nanoscience.