Controlling uniformity of an array of quantum dots
A method of controlling random charge effects originating from local defects ( 202 - 1, 202 - 2, 202 - 3 ) above a quantum dot in a quantum dot array is described, wherein the method comprises: selecting one or more electrodes (Vg, Vb) configured to control one or more quantum structures formed in one or more semiconductor layers arranged on a substrate; and, applying one or more first voltage pulses (inset) to the one or more selected electrodes, the amplitude of the one or more first voltage pulses being selected to induce a shift in one or more charge states ( 210 - 1, 210 - 2, 210 - 3 ) of one or more offset charges in one or more dielectric, semiconductor and/or interface layers between the one or more selected electrodes (Vg, Vb) and the one or more semiconductor layers in which the one or more quantum dots are formed by the application of voltages on one or more electrodes (Vg, Vb).
1 . A method of controlling random electrostatic potential fluctuations in on an array of quantum dot structures comprising:
selecting one or more gate electrodes from a plurality of gate electrodes configured to electrostatically control quantum dot structures of the array of quantum dot structures formed in one or more semiconductor layers arranged on a substrate, each of the one of more gate electrodes being associated with an initial threshold voltage; and,
applying stress voltage pulses to the one or more gate electrodes to progressively modify a threshold voltage of each of the one or more gate electrodes from an initial threshold voltage towards a target threshold voltage, wherein amplitude of the stress voltage pulses are selected to modify the random electrostatic potential fluctuations so that one or more threshold voltages of the one or more gate electrodes respectively are modified towards the target threshold voltage, the random electrostatic potential fluctuations being associate one or more random traps, defects, donor implants or acceptor implants, in one or more dielectric layers, semiconductor layers or interface layers between the one or more gate electrodes and the one or more semiconductor layers in which the array of quantum dot structures are formed.
2 . The method according to claim 1 wherein the method further includes:
measuring a modification in the electrostatic potential fluctuations in response to the application of the stress voltage pulses.
3 . The method according to claim 2 wherein the modification is measured by determining a change in a threshold voltage associated with the one or more gate electrodes.
4 . The method according to claim 2 wherein the modification is measured using a charge sensor.
5 . The method according to claim 4 wherein measuring the modification in the electrostatic potential fluctuations includes:
controlling the charge sensor to measure a first charge state of the quantum dot structures of the array of quantum dot strutures before applying the stress voltage pulses;
controlling the charge sensor to measure a second charge state of the quantum dot structures of the array of quantum dot strutures after applying the one or more stress voltage pulses; and
determining based on the first and second charge state the modification in the electrostatic potential fluctuations.
6 . The method according to claim 5 , wherein the method further includes:
if a response of the charge sensor indicates a modification of the electrostatic potential fluctuations, applying one or more further stress voltage pulses and determining one or more further sensor responses of the quantum dot structures of the array of quantum dot strutures until the one or more further sensor responses indicate that the electrostatic potential fluctuations are equalized.
7 . The method according to claim 1 wherein the plurality of gate electrodes include one or more plunger gates, each-plunger gate being arranged to form a quantum well in the array of quantum dot structures.
8 . The method according to claim 1 wherein the plurality of gate electrodes include one or more barrier electrodes for controlling a barrier region between neighboring quantum dot structures in the array of quantum dot structures.
9 . The method according to claim 8 wherein at least two barrier electrodes are configured to locally control a barrier region between the neighboring quantum dot structures.
10 . The method according to claim 4 wherein the charge sensor comprising a single electron tunneling (SET) transistor, the SET transistor comprising a source and a drain connected by tunneling junctions to a conductive island.
11 . The method according to claim 1 wherein the array of quantum dot structures include quantum dot regions, nano wires or hut wires.
12 . A controller for controlling random electrostatic potential fluctuations in an array of quantum dot structures, the controller being configured to:
selecting one or more gate electrodes from a plurality of gate electrodes configured to electrostatically control quantum dot structures of the array of quantum dot strutures formed in one or more semiconductor layers arranged on a substrate, each of the one or more gate electrodes being associated with an initial threshold voltage; and
applying stress voltage pulses to the one or more gate electrodes to progressively modify a threshold voltage of each of the one or more gate electrodes from an initial threshold voltage towards a target threshold voltage, wherein amplitudes of the stress voltage pulses are selected to modify the random electrostatic potential fluctuations so that one or more threshold voltages of the one or more gate electrodes respectively are modified towards the target threshold voltage, the random electrostatic potential fluctuations being associated with one or more random traps, defects, donor implants or acceptor implants, in one or more dielectric layers, semiconductor layers of interface layers between the one or more gate electrodes and the one or more semiconductor layers in which the quantum dot structures of the array of quantum dot strutures are formedmed.
13 . The controller according to claim 12 wherein the controller is further configured to:
measure a modification in the random electrostatic potential fluctuations in response to applying stress voltage pulses.
14 . The controller according to claim 12 wherein the controller is further configured for:
controlling a charge sensor to determine a first-sensor response in response to the applying stress voltage pulses, the first-sensor response being indicative of a modification in the random electrostatic potential fluctuations.
15 . The controller according to claim 14 wherein controlling the charge sensor to determine the sensor response further comprises:
controlling the charge sensor to measure a first charge state of the quantum dot structures of the array of quantum dot strutures before applying the stress voltage pulses;
controlling the charge sensor to measure a second charge state of the quantum dot structures of the array of quantum dot strutures after applying the one or more stress voltage pulses; and
determining based on the first and second charge state a modification in the random electrostatic potential fluctuations.
16 . The controller according to claim 14 , wherein the controller is further configured to:
if the sensor response indicates a modification in the random electrostatic potential fluctuations, applying one or more further stress voltage pulses and determining one or more further sensor responses of the quantum dot structures of the array of quantum dot strutures until the one or more further sensor responses indicate that the random electrostatic potential fluctuations are equalized.
17 . A method for equalizing electrostatic potential fluctuations in an array of quantum dots comprising:
determining initial threshold values for gate electrodes or sets of gates electrodes of the array of quantum dots;
applying stress voltages to the gate electrodes or sets of gate electrodes wherein the stress voltages are selected to be higher than initial threshold values in order to increase threshold voltages for the gate electrodes or sets of gate electrodes towards a target threshold voltage; and,
repeating the application of the stress voltages to the gate electrodes or sets of gate electrodes, wherein the stress voltages are selected to be higher than the initial threshold values of the gate electrodes or set of gate electrodes, until the threshold voltages for the gate electrodes or sets of gate electrodes have reached the target threshold voltage.
18 . The computer program product comprising software code portions configured for, when run in memory of a computer, executing the method steps according to claim 1 .
19 . The method according to claim 3 wherein the threshold voltage defines a minimum voltage value at which conductance through the quantum dots in the array of quantum dots is measured.
20 . The method according to claim 10 wherein the SET transistor is formed in the one or more semiconductor layers.