Name: nanofabrication of gate-defined gaas/algaas lateral quantum dots
Uploaded: 2013-11-01T19:15:00
Description: Watch this Scientific Journal Video about Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots at JoVE.com

The authors thank Michael Lacerte for technical support. M.P.-L. acknowledges the Canadian Institute for Advanced Research (CIFAR), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovations (CFI) and Fonds de Recherche Qu&eacute;bec - Nature et Technologies (FRQNT) for financial support. The device presented here was fabricated at CRN2 and IMDQ facilities, funded in part by NanoQu&eacute;bec. The GaAs/AlGaAs substrate was fabricated by Z.R. Wasilewski from the Institute of Microstructural Sciences at the National Research Council Canada. J.C.L and C.B.-O. acknowledge CRSNG and FRQNT for financial support.

The fabrication process described below is done on a GaAs/AlGaAs substrate with dimensions of 1.04 x 1.04 cm. Twenty identical devices are fabricated on a substrate of this size. All steps of the process are done in a cleanroom and appropriate protective gear must be used at all times. Deionized water is used throughout the process, but is simply referred to as &#34;water&#34; in the protocol below.
1. Etching of the Mesa
The result of this fabrication step is sho...

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The process presented above describes the fabrication protocol of a double quantum dot able to reach the few-electron regime. However, the parameters given may vary depending on the model and calibration of the equipment used. Therefore, parameters such as the doses for exposures during the e-beam and photolithography steps will have to be calibrated before the fabrication of devices. The process can easily be adapted to the fabrication of gate-defined quantum dots on other types of substrates, such as Si/SiGe, that also...

Quantum information science has drawn a lot of attention ever since it was shown that quantum algorithms can be used to solve certain problems exponentially faster than with the best known classical algorithms1. An obvious candidate for a quantum bit (qubit) is the spin of single electron confined in a quantum dot since it is a two-level system. Numerous architectures have been suggested for the implementation of quantum dots, including semiconducting nanowires2, carbon nanotubes3, self-assembled quantum dots4, and semiconductor vertical5 and lateral quantum dots6. Gate-defined lateral quantum dots in GaAs/AlGaAs heterostructures have been very successful because of their versatility and their fabrication process is the focus of this paper.
In lateral quantum dots, the confinement of electrons in the direction perpendicular to the sample surface (z direction) is achieved by choosing the proper substrate. The GaAs/AlGaAs modulation-doped heterostructure presents a two-dimensional electron gas (2DEG) confined to the interface between the AlGaAs and the GaAs layers. These samples are grown by molecular beam epitaxy to obtain a low impurity density which, combined with the modulation-doping technique, leads to high electron mobility in the 2DEG. A schematic of the different layers of the heterostructure as well as its band structure are shown in Figure 1. A high electron mobility is needed in the 2DEG to ensure the coherence of electronic states over the entire surface of the quantum dot. The substrate used for the fabrication process described below was purchased from the National Research Council of Canada and presents an electron density of 2.2 x 1011cm-2 and an electron mobility of 1.69 x 106 cm2/Vsec.
The confinement of electrons in the directions parallel to the sample surface is achieved by placing metallic electrodes on the surface of the substrate. When these electrodes are deposited on the surface of the GaAs sample, Schottky barriers are formed7. Negative voltages applied to such electrodes lead to local barriers in the 2DEG below which only electrons with sufficient energy can cross. Depletion of the 2DEG occurs when the voltage applied is negative enough that no electrons have enough energy to cross the barrier. Therefore, by carefully choosing the geometry of the electrodes, it is possible to trap a small number of electrons between depleted regions of the sample. Control of the number of electrons on the dot as well as the tunneling energy between the dot and the 2DEG in the rest of the sample can be achieved by fine-tuning the voltages on the electrodes. A schematic of the gate electrodes and the depleted electron gas is shown in Figure 2. The design for the gate structures forming the dot is inspired by the design used by Barthel et al.8
To control and read out information regarding the number of electrons on the dot, it is useful to induce and measure current through the dot. Readout can also be done by using a Quantum Point Contact (QPC), which also requires a current through the 2DEG. The contact between the 2DEG and voltage sources is ensured by ohmic contacts. These are metallic pads that are diffused from the surface of the sample all the way down to the 2DEG using a standard rapid thermal anneal process7 (see Figures 3a and 4b). To avoid short circuits between the source and the drain, the surface of the sample is etched so that the 2DEG is depleted in certain regions and the current is forced to travel through certain specific channels (see Figures 3b and 4a). The region where the 2DEG still remains is referred to as the &#34;mesa&#34;.&#160;
The following protocol details the entire fabrication process of a gate-defined lateral quantum dot on a GaAs/AlGaAs substrate. The process is scalable since it remains the same regardless if the device being fabricated is a single, double, or triple quantum dot or even an array of quantum dots. Manipulation, measurement, and results for double quantum dots fabricated using this method are discussed in further sections.

<table border="1">
		<tbody>
 <tr>
 <td>Name of the reagent/material</td>
 <td>Company</td>
 <td>Product number</td>
 <td>CAS number</td>
 </tr>
 <tr>
 <td>Acetone - CH3COCH3</td>
 <td>Anachemia</td>
 <td>AC-0150</td>
 <td>67-64-1</td>
 </tr>
 <tr>
 <td>Isopropyl Alcohol (IPA) - (CH3)2CHOH</td>
 <td>Anachemia</td>
 <td>AC-7830</td>
 <td>67-63-0</td>
 </tr>
 <tr>
 <td>1165 Remover</td>
 <td>MicroChem Corp</td>
 <td>G050200</td>
 <td>872-50-4</td>
 </tr>
 <tr>
 <td>Microposit MF-319 Developer</td>
 <td>Shipley</td>
 <td>38460</td>
 <td>75-59-2</td>
 </tr>
 <tr>
 <td>Sulfuric Acid - H2SO4</td>
 <td>Anachemia</td>
 <td>AC-8750</td>
 <td>766-93-9</td>
 </tr>
 <tr>
 <td>Hydrogen Peroxide (30%) - H2O2</td>
 <td>Fisher Scientific</td>
 <td></td>
 <td>7722-84-1</td>
 </tr>
 <tr>
 <td>LOR 5A Lift-off resist</td>
 <td>MicroChem Corp</td>
 <td>G516608</td>
 <td>120-92-3</td>
 </tr>
 <tr>
 <td>Microposit S1813 Photo Resist</td>
 <td>Shipley</td>
 <td>41280</td>
 <td>108-65-6</td>
 </tr>
 <tr>
 <td>Microposit S1818 Photo Resist</td>
 <td>Shipley</td>
 <td>41340</td>
 <td>108-65-6</td>
 </tr>
 <tr>
 <td>PMMA LMW 4% in anisole</td>
 <td>MicroChem Corp</td>
 <td></td>
 <td>100-66-3, 9011-14-7</td>
 </tr>
 <tr>
 <td>PMMA HMW 2% in anisole</td>
 <td>MicroChem Corp</td>
 <td></td>
 <td>100-66-3, 9011-14-7</td>
 </tr>
 <tr>
 <td >GaAs/AlGaAs wafer</td>
 <td>National Research Council Canada</td>
 <td>See detailed layer structure in Figure 1.</td>
 <td></td>
 </tr>
 <tr>
 <td>Ni (99.0%)</td>
 <td>Anachemia</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Ge (99.999%)</td>
 <td>CERAC inc. </td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Au (99.999%)</td>
 <td>Kamis inc. </td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Ti (99.995%)</td>
 <td>Kurt J Lesker</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Al</td>
 <td>Kamis inc.</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Silver Epoxy</td>
 <td>Epoxy Technology</td>
 <td>H20E</td>
 <td></td>
 </tr>
	 </tbody>
 </table>

nanofabrication of gate-defined gaas/algaas lateral quantum dots

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and entanglement, to solve certain problems exponentially faster than with the best known algorithms on a classical computer. Gate-defined lateral quantum dots on GaAs/AlGaAs are one of many avenues explored for the implementation of a qubit. When properly fabricated, such a device is able to trap a small number of electrons in a certain region of space. The spin states of these electrons can then be used to implement the logical 0 and 1 of the quantum bit. Given the nanometer scale of these quantum dots, cleanroom facilities offering specialized equipment- such as scanning electron microscopes and e-beam evaporators- are required for their fabrication. Great care must be taken throughout the fabrication process to maintain cleanliness of the sample surface and to avoid damaging the fragile gates of the structure. This paper presents the detailed fabrication protocol of gate-defined lateral quantum dots from the wafer to a working device. Characterization methods and representative results are also briefly discussed. Although this paper concentrates on double quantum dots, the fabrication process remains the same for single or triple dots or even arrays of quantum dots. Moreover, the protocol can be adapted to fabricate lateral quantum dots on other substrates, such as Si/SiGe.

One of the critical steps in the process described above is the etching of the mesa (step 1). It is important to etch enough to remove the 2DEG below while avoiding overetching. Therefore, it is recommended to use a bulk GaAs dummy sample to test the etching solution before performing the etch on the GaAs/AlGaAs sample. The etch rate of the GaAs/AlGaAs heterostructure is larger than that of GaAs, but the etching of the dummy can give an indication to whether the solution is more or less reactive than usual and the etch t...

Watch this Scientific Journal Video about Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots at JoVE.com

Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots

This paper presents a detailed fabrication protocol for gate-defined semiconductor lateral quantum dots on gallium arsenide heterostructures. These nanoscale devices are used to trap few electrons for use as quantum bits in quantum information processing or for other mesoscopic experiments such as coherent conductance measurements.

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and ...

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