V OLUME 93, N UMBER 3
2.3 Sample Processing
by a final Si doping layer. To complete the DQW wafer, a cap structure is then grown by depositing 400 ˚A of Al0.3Ga0.7As and then 100 ˚A of GaAs. Gallium arsenide has a high density of surface states in a narrow band near the middle of its band gap.
These surface states pin the Fermi level at the surface within the gap and help to prevent parallel conduction layers beyond the DQW [14].
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beyond this limit requires the use of electron beam lithography, in which high energy electrons are used to expose sections of resist as small as 10–30 nm.
We now outline the steps used to fully process DQW wafers. These sample- processing steps are described in further detail in Appendix C. First, we cleave off a 5×5 mm2 piece of DQW wafer. Pieces from the center of the wafer generally have higher mobility and lower tunneling conductance. Then we use photolithography to define an etch mask made of photoresist in a particular shape near the center of the wafer piece. By dunking the sample in an acid solution for a few minutes, we can etch away the uncovered GaAs. The bilayer 2DESs are then confined to the resulting mesa underneath the remaining photoresist. Once the acid etch is done, the photoresist is removed in warm n-butyl acetate.
A fresh coat of photoresist is applied and exposed in a pattern to create small uncovered squares overlapping with certain regions of the mesa. By thermally evap- orating Ni/AuGe onto those exposed squares and then removing the unexposed pho- toresist (along with the metal on top of it), we can deposit squares of Ni/AuGe in desired locations. Heating the sample at 440◦ within a flow of H2 and N2 gas will cause the AuGe to anneal down through the heterostructure and produce an electrical contact with both 2DESs directly underneath it. Because these electrical contacts are generally ohmic in behavior, they are known as ohmic contacts. Once the rest of the fabrication steps are done, we can solder wires directly to the ohmic contacts in order to permit electrical transport measurements of the 2DESs.
To tune the density of the 2DESs in the bilayer, we can deposit thin films of aluminum in various shapes on the upper or lower surface of the DQW wafer. These films of aluminum are known as top and bottom gates. Metals such as aluminum will form a Schottky barrier with the semiconducting GaAs [14] that prevents direct conduction between the metal and the 2DES. The aluminum and 2DES together act like a parallel plate capacitor. By applying a negative voltage bias to the aluminum while keeping the 2DESs grounded, one can reduce the electron density of the 2DES within a region that overlaps with the gate.
Annealed Ni/AuGe ohmic contacts will diffuse down to both 2DESs in the bilayer
Ohmic contact 1 Ohmic contact 2
Back arm gate
Top arm gate
VBG
Figure 2.7: Selective depletion technique. Voltage biases VT G and VBG are applied to the top and bottom arm gates, respectively. These biases are set in order to deplete the appropriate region of the 2DES that is closest to the corresponding gate. Thus, ohmic contact 1 becomes effectively connected only to the top layer while ohmic contact 2 is connected only to the bottom layer.
system, shorting them together. Thus, any wire soldered to a given ohmic contact will be electrically connected to both layers simultaneously. The most spectacular transport properties of the νT = 1 QH system require current leads and voltage probes that are connected to only one layer at a time, however. To achieve this, we uses the selective depletion technique [26], which we illustrate in figure 2.7. We first note that the upper 2DES will almost totally screen the lower 2DES from the electric field from a top gate, leaving the lower 2DES essentially unaffected by the top gate until the upper layer is completely depleted of electrons. Similarly, the lower 2DES will screen the upper 2DES from the electric field of a bottom gate. We can take advantage of this by creating a mesa where a number of “arms” extend from the central region, which is the region of interest. At the end of each arm, a Ni/AuGe ohmic contact is formed. Each arm is usually also overlapped with both a top gate and a bottom gate; these specialized gates are generally referred to as arm gates.
By applying the proper bias to a top arm gate (generally −0.5 V in the traditional
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DQW wafers), we can fully deplete the upper 2DES directly underneath that gate.
Thus, the ohmic contact in that arm will only be connected to the central region via the bottom layer. By applying a large bias to the bottom arm gate for another arm, we will similarly deplete the bottom layer within a localized region in that arm.
The ohmic contact associated with that arm will then be connected to the central region only through the top layer. An ohmic contact can be fully disconnected from the central region by applying a large bias to its matching top arm gate (generally
−1.2 V) to deplete both layers.
The central region itself is covered by one or more top and bottom gates to in- dependently control the electron density in either layer within this central region.
Through proper application of biases to the arm gates, we can for example measure interlayer tunneling within the central region using one contact to the upper layer and another contact to the bottom layer. In order to reduce fringe fields to a tolerable level, we must bring our bottom gates to within 50 µm of the bilayer system. To do this, we thin the wafer piece to a thickness of 50µm using a bromine-methanol etch.
Once the sample is thin, the bottom gates can be deposited. We must then carefully solder and epoxy wires to the contact pads for each gate and ohmic contact. The wires are then soldered to an 18-pin header, which can be plugged into the sample holder of a dipping stick or dilution refrigerator.