GB2252870A

GB2252870A - Quantum dot structure

Info

Publication number: GB2252870A
Application number: GB9102582A
Authority: GB
Inventors: David A Williams
Original assignee: Hitachi Europe Ltd
Current assignee: Hitachi Europe Ltd
Priority date: 1991-02-06
Filing date: 1991-02-06
Publication date: 1992-08-19
Anticipated expiration: 2011-02-06
Also published as: GB9102582D0; GB2252870B

Abstract

A quantum dot structure comprising semiconductor material has a controllable well configuration (10) formed in a two dimensional electron gas utilising a potential applied to a surface electrode (7). The lateral extent of the well (in direction x) is controlled by means of an electrode (8) which applies a potential to the two dimensional electron gas. <IMAGE>

Description

QUANTUM DOT STRUCTURE DESCRIPTION This invention relates to a quantum dot structure capable of providing a potential well for constraining small numbers of electrons in a semiconductor device.

Several fabrication methods have been used hitherto in order -to produce a quantum dot, which is a structure for confining small numbers of electrons in a semiconductor device. [Reed & Kink "Nanostructure Physics and Fabrication - Academic Press 1989 ] . A quantum dot utilises a potential well which permits a quantum mechanical standing wave to be established for e.g. a single electron. Quantum dots have been formed by mesa etching, the resulting dots having a well defined structure. However, the size of the confining potential well cannot be altered. Electrostatic confinement has also been used to provide a range of quantum dot structures. However, the use of only one electrode limits the configuration of quantum dot that can be achieved and the resulting dot is not readily tunable in size.

An object of the invention is to provide an improved quantum dot structure having a potential well which is tunable both in terms of energy depth and lateral extent.

In accordance with the present invention there is provided a quantum dot structure comprising a semiconductor material, means associated with the semiconductor material to produce an essentially two dimensional electron gas therein, first electrode means for forming a potential well in the semiconductor material for retaining an electron therein, and second electrode means for forming an electrical connection with said electron gas, for permitting a lateral bias field to be applied via said electron gas to said potential well for modifying the width thereof.

Thus, in accordance with the invention, the quantum dot structure can be tuned in terms of its potential energy depth by applying a variable bias potential to the first electrode, and the well can be tuned in terms of its lateral extent by applying a variable bias field to the second electrode.

Preferably, a third electrode is disposed centrally of the first electrode for modifying the depth of the potential well in order to provide further control of the shape thereof.

In order that the invention may be more fully understood an embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic sectional view of a semiconductor including a quantum dot structure according to the invention; Figure 2 is a schematic perspective view of the top surface of the device shown in Figure 1; Figure 3 is a schematic diagram of the potential well established by the arrangement shown in Figures 1 and 2; and Figure 4 shows a modification to the device shown in Figure 1 wherein a third central electrode is provided.

Referring to Figure 1, the quantum dot structure is shown fabricated in GaAs semiconductor. The device comprises a GaAs substrate 1 on which is grown a GaAs hetero layer 2 which forms a hetero junction with an overlying GaAlAs layer 3. The hetero junction is overlaid with a dielectric passivation layer, typically undoped GaAs 4. As well kndwn in the art, the hetero junction produces a two dimensional electron gas 5 in the GaAs layer 2.

In order to form the quantum dot structure, a circular ring 6 is cut into the overlying layers 3, 4 so as to provide a central disc shaped portion thereof 3a, 4a.

The metalisation layer 7 is then laid over the annular ring and the layers 4, 4a. A perspective view of the top of the device, showing the metalisation layer 7, is shown in Figure 2. The metalisation layer 7 constitutes a first electrode for the quantum dot structure in the form of a ring. A second metallic electrode 8 is formed in contact with the top surface of the GaAs layer 2 so as to make electrical connection with the two dimensional electron gas 5.

In use, bias potentials are applied to the first and second electrodes 7, 8 with a result that a potential barrier and well is formed in the semiconductor beneath the ring 6. Equipotentials for the well are shown in dotted outline in Figure 1 and a more detailed perspective schematic view of the equal potential configuration is shown in Figure 3. Thus, it will be seen that the potential well is in the shape of an atol having a central well portion 10 and outlying flank portions 11. The advantage of the structure shown in Figure 1 is that by increasing the bias potential applied to the first electrode 7, the depth of the well portion 10 can be controlled i.e. increased or decreased. Furthermore, by increasing the bias potential applied to the second electrode 8, an increased "pinching" field can be applied in the direction of arrows X so as to reduce the diameter of the potential well.Thus, the lateral extent of the well can be controlled by means of the second electrode 8. By increasing the potential on electrode 8, both the equipotentials 9 and 10 are squeezed inwardly.

Similarly, by reducing the potential on electrode 8, the well is permitted to relax laterally outwardly.

A modification to Figure 1 is shown in Figure 4 wherein the central disc portion 3a, 4a of Figure 1 is cut away centrally to form an annulus and the resulting central portion is filled with a third electrode 11. The third electrode 11 provides an additional control for the depth of the potential well 10 independently -of the potential applied to ring electrode 6, which can then be used separately to control the overall shape of the atol shaped equipotential configuration.

It will be appreciated that many such quantum dot structures can be integrated into a single semiconductor device with a high packing density. Furthermore, since the size of the potential well 10 is tunable and thus could be used as a tunable laser or as a device having resonant tunnelling operation. The structure should operate at room temperature for a double vertical well resonant tunnelling structure. The structure also permits vertical electron transport measurements to be made.

Claims

1. A quantum dot structure comprising a semiconductor material, means associated with the semiconductor material to produce an essentially two dimensional electron gas therein, first electrode means for forming a potential well in the semiconductor material for retaining an electron therein, and second electrode means for applying an electrical potential to said electron gas, for permitting a lateral bias field to be applied via said electron gas to said potential well for modifying the width thereof.

2. A structure according to claim 1 wherein said first electrode is in the form of a circular ring whereby to form a potential well of circular cross section whereby the bias potential applied to the second electrode modifies the diameter of said potential well.

3. A structure according to claim 1 or 2 including a third electrode disposed centrally of the first electrode for modifying the depth of the potential well.

4. A structure according to any preceding claim including a GaAs substrate, hetero junction means on the substrate for establishing said electron gas, dieletric means covering said hetero junction means, and a ring cut in said dielectric, the first electrode being formed in said cut ring.

5. A structure according to claim 4 including a central opening formed in said dielectric within the ring, said third electrode being formed in said central opening.

6. A quantum dot structure substantially as hereinbefore described with reference to the accompanying drawings.