Engineering
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Scalable Quantum Integrated Circuits on Superconducting Two-Dimensional Electron Gas Platform
Chapters
Summary August 2nd, 2019
Quantum integrated circuits (QICs) consisting of array of planar and ballistic Josephson junctions (JJs) based on In0.75Ga0.25As two-dimensional electron gas (2DEG) is demonstrated. Two different methods for fabrication of the two-dimensional (2D) JJs and QICs are discussed followed by the demonstration of quantum transport measurements in sub-Kelvin temperatures.
Transcript
To form a coherent quantum transport in hybrid superconductor-semiconductor superconductor Josephson junctions, the formation of the homogeneous and barrier-free interface between two different material is necessary. Here we introduce a novel two-dimensional material platform, and then study the proximity-induced superconductivity in two-dimensional electron gas indium gallium arsenide that is a basis of a hybrid quantum integrated circuit. To design the JJ's and QIC device layout, first clean an indium gallium arsenide wafer with acetone and isopropyl alcohol.
Then dry the device with nitrogen gas. Spin the photoresist on top of the indium gallium arsenide wafer. Bake the device on a hot plate for a few seconds.
Following this, place a photomask in a mask aligner, and place the device under the appropriate pattern. Expose the device to UV light through the photomask of mesa and QIC layouts. Then develop the resist in MF-319 developer for a few minutes.
Etch the mesa to act as the active region by placing the device in a solution of water, sulfuric acid and hydrogen peroxide. Rinse the device with deionized water for thirty seconds, and dry with nitrogen gas. Now ensure an etch depth of around 150 nanometers using a DEKTAK surface profiler.
Clean the device with acetone and isopropyl alcohol. Next, form ohmic pad to make electrical contact between the metal and two-dimensional electron gas by spinning photoresist on top of the device. Bake the device on a hot plate for a few seconds.
Place a photomask in the mask aligner, and place the device under the appropriate pattern. Expose the device to UV light through the photomask of ohmic patterns. Then develop the resist in MF-319 developer for a few minutes.
Following this, deposit a thin layer of gold germanium nickel alloy on the resist-patterned sample in an evaporator machine. After performing the liftoff in acetone, anneal the device as 430 degrees Celsius for a few seconds. Spin photoresist on top of the device.
Then bake the device on a hot plate for a few seconds. Wet-etch a 130 nanometer-deep trench on top of the active region to form two-dimensional JJ's by photolithographically patterning and wet-etching in acid, as previously described. Cut the device into small chips.
Load the chip that contains an array of two-dimensional JJ's on a standard leadless chip carrier by using GE varnish. Then make the electrical contacts between the device and leadless chip carrier pads. Finally, load the device into a dilution refrigerator for transport measurements.
The SEM image of one junction on the circuit of Device 2 is shown here. The distance between two Niobium films in each side of the junction is 550 nanometers at the shortest path. The SEM image of one junction of Device 1, which is photolithographically fabricated, shows that the two Niobium electrodes are separated by a distance of 850 nanometers.
Normal and Andreev reflections in hybrid superconducting-semiconducting junctions are depicted here. The temperature dependence-induced superconducting gap with pronounced subharmonic energy gap structures, peaks, and dips for Device 1 are shown here. At the lowest temperature, subharmonic energy gap structures appear with 3 peaks and 3 dips.
The temperature evolution of the peaks and dips due to the suppression of the induced superconductivity with temperature increase are shown here. All features are significantly temperature-dependent, and the strongest subharmonic energy gap peaks are observed at 50 millikelvin. The superconducting gap as a function of applied source drain voltage and temperature of Device 2 is shown here.
The temperature and magnetic field dependences transport measurements of Device 2 do not show any sign of in-gap or sub-gap oscillations, which are observed for Device 1. The most important things when performing this procedure is to obtain the right H level to access the two-dimensional electron gas in the device and form the nanojunction. I believe two-dimensional Josephson junctions can be studied with different lengths and widths to investigate the effect of junctions'dimensions on the topological phase of observation.
This technique make it possible to measure hundreds of quantum devices in one fridge cooldown, paving the way for realization of scalable hybrid quantum devices.
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