Low-Noise, Low-Temperature and High-Speed Characterization of Semiconductor Nanodevices

The Nanonis Tramea quantum transport measurement system (QTMS) in combination with an Oxford Instruments HelioxVL refrigerator (Figure 1), have been utilized to successfully measure the energy levels of a qubit, shaped in a Germanium nanowire.

Nanonis Tramea, and HelioxVL

Figure 1. Nanonis Tramea, and HelioxVL

As a result of the low-noise and high-speed of this measurement system, it is possible to produce high resolution conductance measurements in brief acquisition times.

Introduction

The successful running of quantum computers is reliant on the creation of what are known as ‘qubits’, where the state is not restricted to merely zero and one, as in traditional binary computers.

In contrast, each piece of data is characterized by the state of a quantum particle, which can be in a superposition of multiple states at the same time. An extensive range of methods to form qubits are being examined.

In this work, qubits are created naturally, through the spatial confinement of holes. The limitation is provided by nanowires with dimensions near to the natural wavelength of the carrier, as calculated by quantum mechanics.

The Nanonis Tramea is a low-noise, fast QTMS, and is also safe to use, using a single ground. It removes the possibility of spoiling fragile samples when disconnecting and connecting cables and ground lines to a selection of single-function instruments in a measurement rack.

Experiment

Figure 2 demonstrates a simple diagram of the typical device. A wire made up of Germanium is developed on a Silicon substrate and electrical contact is generated lithographically. In order to allow a potential to be applied to move the energy levels of the confined hole up or down, or to block or permit hole-flow through the wire, a gate is added.

Schematic layout of device.

Figure 2. Schematic layout of device.

In this study, all the room temperature measurement electronics requirements were satisfied using the Nanonis Tramea instrument. The Nanonis Tramea system is a completely digital, integrated package that offers the functions of an oscilloscope, spectrum analyzer, voltage sweepers, voltmeters, lock-in amplifiers, and function generator, in a single, efficient package.

The Nanonis Tramea can control eight inputs and eight outputs (expandable to 24 and 24) at the same time in a single user interface. As all these functions are based in a single unit, the communication speeds and thus, the data procurement times are much quicker.

To be specific, it is not necessary to make use of the slow bus connections between individual devices, upon which traditional instruments are reliant. Figure 3 demonstrates the connection scheme between the device and the electronics.

Connection diagram between Nanonis Tramea and device.

Figure 3. Connection diagram between Nanonis Tramea and device.

A current amplifier is connected to the drain contact and data from this is transmitted to an input on the electronics. One output is linked to the source end of the wire, while a further output is linked to the top gate along the wire.

Rather than being sourced from a variety of different instruments, possibly even from a variety of suppliers, these connections are all thus tied to a single unit, without the requirement for custom software to bind.

Results

The device was attached to the 3 He pot of an Oxford instruments’ HelioxVL 3 He system and measured at a temperature of 270 mK. Figure 3 illustrates the early results, in which the familiar Coulomb diamonds can be seen.

Differential conductance of the nanowire device as a function of the gate (V2) and source-drain (V1) potentials. Note the fine details (yellow ovals) that emerge when using the Nanonis Tramea measurement electronics. Note that the excited states (yellow ovals) were not visible with our previous measurement electronics.

Figure 4. Differential conductance of the nanowire device as a function of the gate (V2) and source-drain (V1) potentials. Note the fine details (yellow ovals) that emerge when using the Nanonis Tramea measurement electronics. Note that the excited states (yellow ovals) were not visible with our previous measurement electronics.

As the source-drain potential is swept and a hole can enter the dot, Coulomb repulsion stops further carriers from tunneling into the dot, until a supplementary surge in potential results in another energy level becomes available for occupation.

The fine structure which can be seen in the lower left of the diagram is where the energy levels are arranged, formed as a result of the stimulated states of the qubit, which enable the holes to travel through the device. This fine structure was only detected in the use of the Tramea system, and could not be seen on the older, multiple-instrument system.

Additionally, as all of the measurement boundaries are stored within the Tramea, the whole acquisition can run automatically and at a high speed, in comparison to older methods in which software runs on a PC, transmitting order sequentially to each part of the measurement system.

As part of plans to scale up activity and incorporate multiple qubits, the Nanonis Tramea has become an important device in experimental design. As this instrument can manage 24 inputs and outputs using a single software interface, further equipment is not required.

Conclusion

In this study, the Nanonis Tramea has been utilized to examine the energy levels of a qubit shaped within a nanowire. Initial findings uncover novel details in the conduction through the device, previously disguised by the noise level of traditional measurement systems.

Furthermore, the single instrument nature of the Nanonis Tramea allows for a significant increase in acquisition speed by eradicating the slow bus protocol relied up in the past when it was necessary to combine individual components (DC voltage sources, lock-in amplifiers, etc.) with a custom software package.

The next research step will be to carry out these tests at ultra-low temperatures on a Triton Cryofree dilution refrigerator (down to 50 mK), which will necessitate the very low-noise features of the Nanonis Tramea.

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.

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