MEASUREMENT
Application note

Transport and RF-reflectometry measurements ofCMOS nanodevices
Simon Schaal, John Mansir, Anasua Chatterjee & John J. L. Morton – UCL, London, UK
Stefan Kubicek, Thomas Chiarella & Iuliana P. Radu – IMEC, Leuven, Belgium

Introduction
Pioneering work on semiconductor quantum dots has shownthat quantum dots are promising candidates as a building
block (qubit) for quantum information and computation.
Quantum dots are quasi-zero-dimensional nanostructures
which can confine single electrons, whose spin or charge
degree of freedom can then be used to represent quantum
bits (qubits).
Quantum computing approaches based on semiconductors
can build upon mature micro and nano-fabrication
technologies, which will be invaluable in scaling up to a large
number of reproducible qubits with practical yields, andintegrated electronics. As part of the classical information
technology industry, CMOS transistors have reachedsufficiently small feature sizes that quantum effects canbegin to play a dominant role. This motivates the explorationof quantum effects in such transistors fabricated using
CMOS processes.

Equipment used
Triton 200 Dilution Refrigerator, Nanonis Tramea with lock-in
module, SR 830 lock-in amplifier.

Triton and Nanonis Tramea

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Application note

Experimental set-up
In this note, we describe experiments on quantum dots
created within CMOS finFET (fin Field Effect Transistor)
devices.
The transistor consists of multiple silicon fins, sitting on an
insulating oxide layer, sharing a common source and drain
as well as a wraparound top-gate which is used to control
conduction through the fins. Further control is possible by
the bulk silicon substrate which can be used as a back-gate.
The field effect of the top-gate is greatest in the cornersof the fin cross-section, due to its geometry. This leads toformation of quantum dots when operating the finFET subthreshold, which can be observed at cryogenic temperatures(see transfer curves at 300 K and 30 mK in Figure 1).
As an alternative to measurements based on transport
through the FET, it is possible to characterise, with highsensitivity, the charge transitions of quantum dots evenwhen no signal in transport can be detected. This isachieved by using the top-gate as a dispersive sensor
measuring small changes in the capacitance of the device
originating from the tunnelling of single electrons, detectedby the response of an LC resonator at radio-frequency (RF).
The high sensitivity and bandwidth of this measurement
makes it ideal for studying the transistor in a regime wheretransport is pinched off.
In such gate-based RF-reflectometry, weak RF modulation
is applied to the top-gate via a bias tee, close to theresonance frequency of an LC circuit consisting of a surface
mount inductor and the parasitic capacitance of the PCB
and device itself (see Figure 1). The reflected RF signal isamplified at multiple stages followed by IQ-demodulation
to obtain the amplitude and phase of the reflected signal.
A change in phase of the reflected signal relates to aneffective change in device capacitance.

Figure 1. Measurement setup for RF reflectometry
and finFET device.

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Application note

Experimental results
Transport measurements of a quantum dot rely on electrons
being able to tunnel on and off the dot. Detecting such electron
tunnelling requires the ability to measure very small currents (onthe order of picoamps). Furthermore, as tunnel barriers becomeless transparent, transport measurements become more andmore challenging, and substantial integration may be required toachieve adequate signal-to-noise.
In contrast, RF-reflectometry allows fast and sensitive
measurements even when tunnel barriers are in-transparent.
However, such measurements are often more complicated tointerpret as the signal originates from any change in device
capacitance.

Figure 2: Transport measurement using
GPIB interfaced lock-in (SR 830).

The Nanonis Tramea system is able to improve current
measurement procedures in various ways. The integrated
solution of very fast logic, data acquisition and storage
combined with low-noise inputs allow for an enhancement in
the speed of transport measurements, for example by reducing
the communication time present when separate instruments(e.g. lock-in amplifiers) are used. Furthermore, transport andreflectometry measurements can be performed simultaneously,
allowing for direct comparison between the two methods. Thiscan be particularly useful when exploring a new device wherespecific operating regions need to be identified by large scans
within the parameter space.
A large gate map from a device studied in transport, taken with aGPIB-interfaced lock-in amplifier controlled using a measurement
computer, can take several hours. An example is given inFigure 2 which shows Coulomb oscillations as a function ofbulk and top-gate voltage. With the Tramea system a similarscan can be obtained in only 10 minutes at the same resolution.
Additionally, by comparing the 2 hour and 10 minute scans
we observe a drift or instability in the 2 hour scan which is notapparent in the fast scan. Stable and precise voltage sources areother key factors for such measurements as measurements canlast several days and quantum devices have small tolerance levels.
By feeding the I and Q outputs from the IQ-demodulator
into two other Tramea inputs, we can obtain a reflectometry
scan simultaneously with the direct transport measurement.
Comparing the reflectometry with the transport scan we observe
many additional oscillations at low gate voltage, where tunnel
barriers have become opaque.

Figure 3: Transport measurement using
Nanonis Tramea system.

Figure 4: Reflectometry measurement
using Nanonis Tramea system.

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Application note

Experimental results

Figure 5: Coulomb diamonds intransport using Nanonis Tramea system.

Figure 6: Coulomb diamonds inreflectometry using Nanonis Tramea system.

For further quantum dot characterisation we take a scan varying the source-drain and top-gate voltage. We observe
Coulomb diamonds compatible with multiple quantum dots in transport and reflectometry.

Conclusion
In summary, we have studied a CMOS fin-FET device. At cryogenic
temperatures we observed signatures consistent with multiple quantum
dots. Using the Nanonis Tramea system we were able to significantly speed
up the characterisation process and simultaneously obtain transport andRF-reflectometry data which is useful when exploring the parameter spaceof a new device.

Further reading“Spins in a few-electron quantum dots”, R. Hanson, L. P. Kouwenhoven, J. R. Petta, S. Tarucha
and L. M. K. Vandersypen – Reviews of Modern Physics 79 (2007)“Fast single charge sensing with an rf Quantum Point Contact”, D. J. Reilly, C. M. Marcus, M. P.
Hanson and A. C. Gossard – Appl. Phys. Lett. 91 (2007)“Dispersive Readout of a Few-Electron Double Quantum Dot with Fast rf Gate Sensors”, J. I.
Colless, A. C. Mahoney, J. M. Hornibrook, A. C. Doherty, H. Lu, A. C. Gossard and D. J. Reilly –Phys. Rev. Lett. 110 (2013)“Charge Dynamics and Spin Blockade in a Hybrid Double Quantum Dot in Silicon”, M.
Urdampilleta, A. Chatterjee, C. C. Lo, T. Kobayashi, J. Mansir, S. Barraud, A. C. Betz, S. Rogge,
M. F. Gonzalez-Zalba, and John J. L. Morton – Phys. Rev. X 5 (2015)“Probing the limits of gate-based charge sensing”, M. F. Gonzalez-Zalba, S. Barraud, A. J.
Ferguson & A. C. Betz – Nature Communications 6 (2015)“A CMOS silicon spin qubit”, R. Maurand, X. Jehl, D. Kotekar-Patil, A. Corna, H. Bohuslavskyi,
R. Lavieville, L. Hutin, S. Barraud, M. Vinet, M. Sanquer & S. De Franceschi – NatureCommunications 7 (2016)

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July 2017, Issue 1