(a) Scanning electron micrograph of the surface metallic gates defining a quantum point contact (QPC) between the source and drain Ohmic contacts ( and ) and a lateral electrostatic quantum dot. (b) SEM of pinhole aperture etched in an opaque Al layer, 150 nm thick, acting as a shadow mask to illuminate only the quantum dot region. Gates are buried under layers. (c) Cross section view of the device. The heterolayers consist of a 5 nm Si-doped GaAs cap layer, a 60 nm Si-doped layer, a 30 nm spacer layer, on an undoped GaAs buffer.
(Color) Single electron escape from the dot detected by the QPC transistor. The plunger gate, G4, is swept from to with a scan rate of starting at curve marked (a) and ending at (e) with each curve spanning 0.5 V. Gates G2, G3, and G5 are held at while G1 is changed in between each curve, to reset the QPC current. The curves have been offset along the voltage axis to fit on one graph. The bottom inset shows the step sizes of the last two electrons in the dot seen in curve (c) after subtracting out the background slope [ at the last electron step on curve (c)].
(Color) Hysteresis measured in the current through the QPC transistor, associated with the transition of the dot from the metastable filled state to the equilibrium empty state. The current switches from to as the G4 plunger gate ejects stored electrons in the cycle from to . In the metastable state following (or equivalently ), the dot potential resides above the surrounding Fermi level, as shown in the top right inset. The thick tunnel barriers formed in our geometry when G3 and G5 are at prevent fast tunnelling of the trapped electrons. At , these electrons are forcibly expelled over the thick barriers by a large repulsive potential on the plunger. They do not subsequently reenter when the potential well is recreated at , owing to the thick barriers. When the barriers are reopened and closed in the cycle from to , electrons remain trapped in the dot, restoring the current to . The color of the vertical transitions is coded to the color of the corresponding gate switch for that transition. Level represents the desired empty state of the dot, at which it is ready to accept and trap photoinjected electrons. Such a hysteretic behavior in the QPC current could be observed whenever the plunger gate voltage sweep was begun at a value prior to the removal of the last electron from the dot and no hysteresis was observed upon starting from an empty dot.
(Color) (a) Photoelectron trapping in the quantum dot detected by adjacent point contact transistor. The dot is fully emptied before exposure to pulses, at a flux of into the dot, within a time window. The time traces depict the transistor current, centered on the pulse time window. The traces have been offset for clarity. (b) An expanded view of transistor current for pulses 20, 21, and 22 without any offset. The charge sensitivity per photoelectron is .
(Color) An optical pulse series with an average flux of within the dot area. Occasional positive steps can be attributed to the photoionization of a residual neutral donor, or the annihilation of a photohole within the electrostatic dot.
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