CRYOFREE

Application note

Efficient electrical nanodevice characterisation withOptistatDry cryostat and MFLI Lock-in Amplifier
Introduction
In this application note we demonstrate a procedure to carry out
various characterisations of a QPC (Quantum Point Contact) device
Zurichin an OptistatDry cryostat from Oxford Instruments and using anInstruments
MFLI Lock-in Amplifier from Zurich Instruments. The sample wasprovided by the Semiconductor Physics Group in the Cavendish Laboratory at University of Cambridge, UK.

Background
Qubits, or quantum bits, are basic building blocks of a quantum computing device.
There are many implementations and proposals for such a device and all of themneed a read-out mechanism to work reliably. For GaAs or semiconducting QDs(quantum dots), QPCs are a way to readout the charge state of a qubit. Solid statequbit operation is carried out at much lower (milliKelivin) temperatures where thecoherence times are longer, but the QPC functionality can be readily checked at andaround 4 K. The OptistatDry provides a compact and easy to use cold environment.
The MFLI features integrated tools that provide an out-of-the box set ofelectrical measurements. All the measurements described here have been
performed in less than one day, providing a state-of-the-art integration andcombination for high-end research.

Experimental set-up

Figure 1. SampleProtect measurement system
set-up including OptistatTMDry cryostat, MFLI
Lock-in Amplifier and an ESD break-out box –used for this application.

MFLI Lock-in Amplifier description
Operating between DC and 5 MHz, the MFLI is a digital dual-phase demodulator Lock-in Amplifier. The front panel
is equipped with current input (I) and differential voltage input/outputs (V), two auxiliary inputs (AuxIn) and four 18
bit auxiliary outputs (AuxOut). The two AuxIns connectors can be used for external signal referencing but also as away to add analog DC bias to the signal outputs. Furthermore, a digital path for adding the DC component onto
the signal output is available through the user interface. The AuxOuts can be used to output the demodulated signal(X, Y, R, theta) components as well as for the manual or automated voltage bias sweeps. Furthermore, with themulti-demodulator (MF-MD) option, a total of four demodulators are available. This allows for simultaneous I andV measurements and/or demodulation at four different frequencies at the same time, including DC. An example
of such configuration is shown in Figure 2. The MFLI is controlled by the LabOne user interface which provides acomprehensive analysis toolset including oscilloscope, spectrum analyser, parameter sweeper and several more. This
makes the MFLI an excellent choice for device characterisation and measurements.

Sample description and schematics
In a QPC, a 1D quantum channel is formed by applying an electrostatic potential onto a
2D electron gas formed below the GaAs top layer. The QPC device structure can be seenin Figure 3a. The current conductance between the source-drain leads and side gateelectrodes (with an anti-dot) form a QPC. Another top-gate is available over a crosslinked PMMA; however, this is not used in the measurements described in this note.

CRYOFREE C
Application noteFigure 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.

OptistatDry cryostat description
The OptistatDry is a Cryofree®, closed-cycle refrigerator cooled, table top cryostat offering a temperature range
down to below 3 K, which can be achieved in less than 3 hours from room temperature. The sample is in a vacuumenvironment but is easily accessed via a load port. Sample mounting is achieved via a demountable sample puck.
The puck can include a thermometer for accurate sample temperature reading and a heater for fine temperature
control. The sample space has multi-axis optical access via up to 5 windows and electrical access via the sample puck.
The QPC sample was mounted in a chip insert, and the PLCC chip carrier was mounted on the sample puck using
GE varnish. The electrical contacts were wire bonded from the PLCC carrier to the sample puck using fine Al wires.
Extra wire bonded links were added to ensure sample node equipotential during handling.
Mounting in the OptistatDry is simply a matter of loading the puck using the vacuum tool or tweezers. An antistatic wrist strap must be worn. Tightening three screws ensures good thermal contact with the cooling plate andgood electrical contact with the spring-loaded pins. The cabling of the experimental set-up includes an ESD
break-out box between the OptistatDry and the MFLI, see Figure 1. This enables all signal lines to be grounded toensure the sample is at equipotential via the instrumentation signal lines. The lines are individually switchable andcan be separately switched to ground or to the MFLI. With all the lines switched to ground it is possible to cut theESD protection link wires on the puck. To begin the device characterisation experiment the signal lines are switchedto the MFLI.

Experimental results
The conductance (I-V) measurements are done via source-drain electrodes and transport
is modified via the two side gates electrodes, as shown in the Figure 3a. The source drainvoltage excitation was 200 μV and the current is measured at the 100 μA input range
with full input bandwidth up to 5 MHz. The side gates are driven simultaneously using
one AuxOut of the Lock-in Amplifier.

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Application note

The continuity of the sample was first checked at the room temperature and it was cooled down to the base of
2.3 K. Using the Scope tool of the LabOne user interface the overall noise spectrum at the input has been checked(not shown). In order to choose the optimal measurement region a frequency sweep was run using the LabOne
Sweeper tool in the range from 10 Hz to 10 kHz, revealing flat AC current frequency response (black curve, Figure3b). Subsequent noise measurements (blue curve, Figure 3b) reveal up to four times higher noise below the cornerfrequency of 100 Hz, so the measurement frequency of 237 Hz was chosen for these particular tests.
The tests carried out consisted of measuring both AC and DC current components of the source-drain conductance tomonitor the transport for leakage currents. This is done using the MFLI MF-MD option which allows the measurement
frequency to be set at the 0 Hz for the DC and at the AC frequency using two demodulators simultaneously. TheLabOne Sweeper tool was used to sweep the AuxOut voltage driving the side gates. In order to setup the sweeps wemanually change side gate voltage in small steps of 10 mV. It was noticed, using the Plotter tool of LabOne UI (notshown), that the currents are relaxing on long time scales so we chose to set the wait time 60 s in order to monitor
the equilibrium state. Voltage sweeps were done between 0 to -3 V and back. An example of the finished sweeps ofAC and DC currents, at 2.3 K (black) and 10 K (red), are shown in Figures 3c and 3d respectively (dotted curves arebackward sweeps). We see that overall the conduction current drops in the channel as the temperature increased dueto the activation of additional scattering mechanisms. We note further that the pinch off of the channel is at about-3 V and that there are several conductance reduction steps along the way. A large hysteresis is also observed in theback sweep revealing that significant charge transfer is taking place upon the end of the long sweep. We also notethat AC and DC currents have sharp changes at the same gate voltages pointing to the current leakage in the device,
most prominently between the side gates and the drain electrode.
Furthermore, we measured the I-V characteristics of source-drain electrodes at various temperatures and side gate
voltage of -1.3 V. The DC bias is digitally added together with the AC excitation. The measurement results at four
different temperatures from 5 K to 20 K are shown in Figure 4. The source-drain bias was limited to 10 mV. We seethat the I-V characteristics, Figure 4a, are linear as confirmed by the practically flat AC current response, Figure 4b.

Figure 3a

Figure 3b

Figure 3c

Figure 3d

Figure 3a. 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 offrequency obtained using theSweeper tool.
AC (Figure 3c) and DC(Figure 3d) current
measurements as a function of
the side gate voltage swept attemperatures 2.3 K (black) and10 K (red), forwards (full) andbackwards (dotted).

CRYOFREE

Application note

Figure 4a

Figure 4b

Figure 4. The DC (Figure 4a.) and AC (Figure 4b.) current measurements as a function of theside gate voltage bias taken at four different temperatures from 5 K to 20 K.

Conclusion and outlook
We show that characterisation of a QPC device can be done efficiently as
a function of source-drain and gate bias, at multiple temperatures andfrequencies. OptistatDry allows for fast and accurate temperature control
whilst the built-in oscilloscope of the MFLI can help solve set-up groundloop and various potential noise source issues. The fast frequency sweep
points to the optimal measurement frequency. The MF-MD option allows
for simultaneous AC and DC current measurements, with high signal tonoise ratio, as a function of side gate bias quickly revealing the presenceof the leakage currents. The source-drain sweeps show the linearityof the contacts that can be taken for different side gate voltages andtemperatures.

OptistatDry Cryofree cryostat
from Oxford Instruments

MFLI Lock-in Amplifier
from Zurich Instruments

In this note, it was demonstrated how combining the equipment from
Oxford Instruments and Zurich Instruments leads to efficient and costeffective laboratory setups. The time to first characterisation is below one
day and this is a major step towards the objective of reducing time topublication.
The combined expertise of the two instrument vendors enables customers
who need to efficiently harness electrical and optical experiments
performed as low as 2.3 K. The use of advanced user interfaces such
as LabOne reduces the programming effort for users as it provides anextensive out-of-the-box toolset ready for many immediate analyses. This
reduces setup complexity and potential failure modes compared to using
multiple external measurement tools.

Contact us at:
Oxford Instruments NanoSciencenanoscience@oxinst.com
www.oxford-instruments.com

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November 2016, Issue 2