GB2261112A - Electronic device with phase selection - Google Patents

Abstract

A phase selecting device for selecting charge carriers such as electrons that have a quantum mechanical wavefunction with a particular phase characteristic, includes a source of charge carriers and a phase selection structure. In one example. the phase selection structure consists of an interdigitated gate structure 6, 7 and an applied magnetic field which causes carriers to pass along a helical path 8 between the interdigitated gates. Only electrons of a particular wavefunction will pass along the path 8. In another embodiment interdigitated electrodes (9, 10) pass electrons between potential wells. In a further embodiment, electrons are phase locked using a crystal lattice. In a still further embodiment an energy selector comprising a well (19) between barriers (19a, 19b) is used. <IMAGE>

Description

CHARGE CARRIER SOURCE DESCRIPTION The present invention relates to a device for producing charge carriers with a particular phase characteristic.

Recently, it has been proposed to utilise nanofabrication techniques to construct electronic circuits wherein information is expressed by the phase of an electron wavefunction. Such circuits are referred to as quantum interference circuits and form the subject of our copending GB patent application, No. 9103083.3. In order for these circuits to operate it is necessary for them to be supplied with electrons with a set phase characteristic.

It is an aim of the present invention to provide a device suitable for fabrication as part of an integrated circuit, which produces a supply of electrons for use in a quantum interference circuit.

According to the present invention there is provided a phase selecting device for selecting charge carriers that have a quantum mechanical wavefunction exhibiting a particular phase characteristic, comprising a source means for said charge carriers, selecting means for selecting particular charge carriers with said characteristic, and output means for. said selected charge carriers.

In a first embodiment, the selecting means comprises a control region having an input and an output, charge carrier supply means for supplying charge carriers to the control region input, and means for applying a magnetic field to the control region for controlling the trajectory of charge carriers from the charge carrier supply means within the control region, wherein the control region is arranged such that only charge carriers having a trajectory which achieves conformity with a predetermined trajectory reach the control region output.

Advantageously, charge carriers are caused to move through the control region by means of an electric field.

Preferably, the trajectory is a helix.

Preferably, the control region comprises a semiconductor including one or more barrier regions, e.g. depletion regions, the or each barrier region defining at least in part a predetermined charge carrier trajectory. However, the control region may comprise a conducting path through an insulating region, the geometry of the conducting path defining the predetermined charge carrier trajectory.

If the barrier regions comprise depletion regions, the or each depletion region may be conveniently formed by means of a gate located on a surface of the control region. The gates may be independantly controllable.

The phase selecting means may, however, include a region of semiconductor material, input means for inputting charge carriers into said region, output means for outputting charge carriers from said region, and means for defining alternate potential wells and barriers between the input means and the output means, the potential barriers being selectively lowerable for transferring a charge carrier between the input means and the output means. The first and second potentials may be the same or different.

Preferably, the potential wells are selectively deepenable.

Preferably, the means for defining said potential wells and barriers comprise gate electrodes.

Advantageously, this form of selecting means includes a clocking circuit for sequentially lowering said potential barriers to enable a charge carrier to transfer between adjacent potential wells, so as to effect transport of a charge carrier from the input means to the output means. Although, clocking signals may be derived from another device.

Preferably, the clocking circuit includes means for deepening a receiving potential well for said transfer of a charge carrier.

In another embodiment, the phase selecting device is a device for producing coherent charge carriers, comprising selecting means for selecting charge carriers according to their energy, and charge carrier wavefunction phase locking means for locking the phase of a charge carrier wavefunction to a reference.

Preferably, the selecting means and the phase locking means are arranged such that the selecting means selects charge carriers from the phase locking means, preferably including means for causing net propagation of the phase-locked charge carriers through the phase locking means in a predetermined direction.

In one example, the energy selecting means is a resonant tunnelling diode. An example of an alternative is an asymmetric pair of quantum well waveguides.

The phase locking means may comprise a region of monocrystalline semiconductor material and the reference is the position of the atoms within the crystal lattice. An alternative is a superlattice.

These embodiments of the present invention will now be described in more detail, by way of example, with reference to the acompanying drawings, in which: Figure 1 is a diagrammatic representation of a first embodiment according to the present invention; Figure 2 is a diagrammatic representation of a second embodiment according to the present invention; Figure 3 is a diagrammatic representation of a third embodiment according to the present invention; Figure 4 is a skeleton view of the stucture shown in Figure 3; Figures 5a is a diagrammatic representation of a fourth embodiment according to the present invention and Figures 5b and 5c are schematic cross-sections of the device in Figure 5a; Figure 6 is a schematic cross-section of a fifth device according to the present invention;; Figure 7 is a diagrammatic representation of the device shown in Figure 6; Figure 8 shows the relationship between a crystal lattice and the wavefunction of a valence band electron at the Brillouin zone boundary; Figures 9a and 9b show phase-locked electrons tunnelling through a barrier from respectively the conduction band and the valence band of a semiconductor; Figure 10 is a diagrammatic representation of a sixth embodiment according to the present invention; Figure 11 is a diagrammatic representation of a seventh embodiment according to the present invention; and Figure 12 is a diagrammatic representation of an eighth embodiment according to the present invention.

Referring to Figure 1, a phase selector comprises a helical channel 1 extending across a trough 2 in a semiconductor substrate 3. The helical channel 1 has a submicron radius and is several microns long. A magnetic field E and a magnetic field B lie parallel to the axis of the helix.

An electron subjected to parallel electric and magnetic fields will describe a helical path. The radius of the helix so described is the cyclotron radius of the electron, whilst its pitch is determined by the velocity of the electron which is a function of the applied electric field.

A stream of electrons, having a range of values of energy, flowing into one end of the helical channel 1 will come under the influence of the electric E and magnetic B fields. Individual electrons will then begin to describe helical trajectories dictated by their energies. Those electrons whose trajectory matches the helical channel 1 will pass out from the other end of the helical channel 1. The remaining electrons will be reflected or scattered. Some of the reflected or scattered electrons will have their wavefunction changed by these interactions in such a way that their trajectories come to conform to the helical channel 1.

By varying the electric E and magnetic B fields it is possible to vary the wavefunction of electrons emerging from the helical channel 1. If electrons of a known energy are introduced into the helical channel 1, the phase separator may be made to function as a switch by suitable variation of the electric E or magnetic B fields.

A known phase shift may be introduced into an electron wavefunction by selecting a suitable length of trajectory.

An alternative structure is shown in Figure 2 which is much easier to fabricate than the structure shown in Figure 1. A linear conductive channel 4 formed in a semiconductor substrate is diagonally crossed by a series of spaced depletion regions 5. The depletion regions 5 are formed by means of a pattern of gates on the surface of the channel 4 but do not extend to the full depth of the channel 4. Individual helix loops are constrained between pairs of adjacent depletion regions 5 the loops being connected via the lower region of the channel 4. This structure is capable of selecting electrons having related energies, for instance an electron following a helical trajectory which loops between alternate pairs of depletion regions 5 would pass easily through the device.

A third structure is shown in Figures 3 and 4. A series of interdigitated depletion regions 7, extend partially across the channel 6 and parallel to each other, alternately from opposite sides of a channel 6.

The channel 6, is similar to that in Figure 2.

However, the depletion regions 7 extend to the bottom of the channel 6. Figure 4 shows the helical trajectory 8 defined by the depletion regions 7.

Referring to Figures 5a, 5b and 5c, a plurality of gates 9, 10, extending transversely across a channel 11 similar to that in Figure 3, are initially biased to set up a series of potential wells 12a-f, separated from each other and from an input region 13 and an output region 14 by potential barriers 15a-g. An electron 16 is transferred from from the input region 13 to the first potential well 12a by applying a signal to the set of gates 9 which causes the first potential barrier 15a to drop to a level approaching that of the bottom of the first potential well 12a. With the first potential barrier down, there is a finite probability that an electron 16 will be in the first potential well 12a when the first potential barrier 15a is brought back up again.

If the second potential barrier 15b is then dropped, the electron 16 in the first potential well 12a will spread out into the second potential well 12b. When the second potential barrier 15b is raised again, there is a finite probability that the electron 16 will be in the second potential well 12b. Thus by correct sequencing of the signals on the sets of gates 9, 10 it is possible to transfer an electron 16 along the length of the channel 11 against a potential gradient.

Referring to Figures 6 and 7, the probability that an electron will transfer between potential wells can be increased by increasing the depth of the receiving well when a potential barrier is lowered. The potential well can then be returned to its normal state after the potential barrier has been raised again. Two addition sets of gates 17, 18 are arranged interdigitally with the sets of gates 9, 10 for controlling the depth of the potential wells 12a-f.

These arrangements would have advantages for use with a quantum interference circuit, where it may be desirable to increase the potential of electrons without taking leads directly to an external supply. Such leads would tend to dephase the electrons. Suitable combinations of electric and magnetic fields enable devices based on geometries other than a helix to be constructed.

Referring now to Figure 8, an alternative embodiment will now be described. At a Brillouin zone boundary, with a finite lattice potential P, the electron wavefunction is iS phase-locked to the lattice by the Coulomb interaction. The charge density Y t * corresponds to the lattice periodicity and the lowest energy state, i.e. at the top of the valence band, has the maxima of charge density coincident with the lattice sites S. The corresponding wavefunction at the bottom of the conduction band is tic/2 out of phase, with the charge density in anti-phase. Although with no bias applied across the lattice, there is no charge propagation, a small applied bias electric field will result in net electron propagation.

In accordance with the invention, an energy selector is used to select electrons close to the band edge energies associated with a Brillouin zone boundaries, since these electrons have a phase that has been determined by phase-locking with the crystal lattice. A suitable energy selector is a resonant tunnelling diode as will now be explained with reference to Figures 9a and 9b.

In Figures 9a and 9b, electrons e having energy levels matching the bottom of the conduction band 20a or the top of the valence band 20b of a semiconducting crystal 20 may tunnel through a first barrier 19a of a resonant tunnelling diode 19. The characteristics of a potential well 19b between the first barrier 19a and a second barrier 19e are arranged such that only those electrons e from the bottom of the conduction band 20a (Figure 9a) or from the top of the valence band 20b (Figure 9b) may tunnel into it. Once in the potential well 19b the electrons e can tunnel out into a region which may lead to a phase sensitive circuit.Thus the device selects electrons of an energy known to be phase locked to the crystal lattice of the semiconductor material 20, and as a result only electrons with a particular phase corresponding to the lattice spacing can pass through the diode. A resonant tunnelling diode is particularly suitable as it introduces a constant phase shift and thus does not destroy the phase coherence produced by phase locking with the crystal lattice.

Figure 10 shows the structure of a device operating on the aforementioned principle. The device is fabricated using molecular beam epitaxy. A mesa structure 21 is formed on a substrate 22, formed, for example, from GaAs. A truncated right angle wedge portion 23 of the mesa structure 21 is provided on its inclined face with an array of transverse guide electrodes 24. The aforementioned resonant tunnelling diode includes a first barrier layer 25 formed on the top of the wedge portion 23, a potential well layer 26 formed on the first barrier layer 25 and a second barrier layer 27.

A layer of Brillouin zone-band edge material 28 is deposited on the second barrier layer 27 and has a metal gate electrode 28 on its upper surface.

The gate elecrode 29 is used to produce an electric field for injecting electrons into the BZ-band edge material 28 and for causing propagation of charge down through the BZ-band edge material 28. Electrons having momenta associated with the edge of the Brillouin zone of the BZ-band edge material 28 are phase-locked to its crystal lattice. The first and second barrier layers 25, 27 and the potential well layer 26 are arranged to selectively allow phase-locked electrons to tunnel out of the BZ-band edge material 28 and to inject the selected electrons into the mesa structure 21. Control voltages applied to the transverse electrodes 24 generate electric fields which redirect the electrons so that they propagate parallel to the surface of the substrate 22.

The electron selection could be by band edge or Fermi level overlap, rather than by the double barrier resonant tunnel diode structure hereinbefore described.

If a natural crystal lattice is used to phase-lock electrons, the device is limited to producing coherent electrons having a wavelength equal to twice the lattice spacing or to a lesser extent multiples thereof. However, the same phase locking mechanism occurs in a regular superlattice. Since the wavelength of the phase-locked electrons will be determined by the dimensions of the superlattice, the superlattice may be fabricated so as to produce electrons of a desired wavelength. If the superlattice and a double barrier energy selector are defined by surface electrodes, the device may be tuned to selectively produce electrons of different wavelengths and phases.

Referring to Figure 11, a mesa waveguide 30 is formed on a substrate 31. A first set of gate electrodes 32, running transversely across the waveguide 30, define a superlattice region 33. A second set of gate electrodes 34, parallel to the first set 32, define the barriers of a double barrier energy selector.

In operation, control voltages are applied to the first set of gate electrodes 32 to define a superlattice for phase locking electrons of a desired wavelength.

Further control voltages are applied to the second set of gates 34 to define the energy selector barriers such that phase-locked electrons of the desired phase are selected. The selected electrons then propagate along the waveguide 30 to one or more phase sensitive devices.

An alternative structure is shown in Figure 12. This structure is similar to that shown in Figure 10 but with the BZ-band edge material layer 28 replaced by a superlattice 35. Additionally, the device is based on a square having a ramp running up to each side. As a result the device produces four streams of coherent electrons, one from each side of the square.

In the aforegoing description, embodiments have been described exclusively with reference to electrons.

However, the principles of their operation apply equally to holes.

Claims (19)

1. A phase selecting device for selecting charge carriers that have a quantum mechanical wavefunction exhibiting a particular phase characteristic, comprising a source means for said charge carriers, selecting means for selecting particular charge carriers with said characteristic, and output means for said selected charge carriers.

2. A phase selecting device according to claim 1, wherein the phase selecting device is a device for producing coherent charge carriers, comprising energy selecting means for selecting charge carriers according to their energy, and charge carrier wavefunction phase locking means for locking the phase of a charge carrier wavefunction to a reference.

3. A phase selecting device according to claim 2, wherein the energy selecting means and the phase locking means are arranged such that the selecting means selects charge carriers from the phase locking means.

4. A phase selecting device according to claim 2 or 3, wherein the energy selecting means is a resonant tunnelling diode.

5. A phase selecting device according to claim 2, 3 or 4, wherein the phase locking means comprises a region of monocrystalline semiconductor material.

6. A phase selecting device according to claim 2, 3 or 4 wherein the phase locking means comprises a superlattice.

7. A phase selecting device according to claim 1, including a control region having an input and an output, charge carrier supply means for supplying a flow of charge carriers to the control region input, and means for applying a magnetic field to the control region for controlling the trajectory of charge carriers from the charge carrier supply means within the control region, wherein the control region is arranged such that only charge carriers having a trajectory which achieves conformity with a predetermined trajectory reach the control region output.

8. A phase selecting device according to claim 7, wherein charge carriers are caused to move through the control region by means of an electric field.

9. A phase selecting device according to claim 7 or 8, wherein the trajectory is a helix.

10. A phase selecting device according to claim 7, 8 or 9, wherein the control region comprises a semiconductor including one or more barrier regions, the or each barrier region defining at least in part a predetermined charge carrier trajectory.

11. A phase selecting device according to claim 7, 8 or 9, wherein the control region comprises a conducting path through an insulating region, the geometry of the conducting path defining the predetermined charge carrier trajectory.

12. A phase selecting device according to claim 10, wherein the barrier regions comprise depletion regions.

13. A phase selecting device according to claim 1 including a region of doped semiconductor material, input means for inputting charge carriers into said region, output means for outputting charge carriers from said region, and means for defining alternate potential wells and barriers between the input means and the output means, the potential barriers being selectively adjustable, for transferring a charge carrier from the input means to the output means.

14. A phase selecting device according to claim 13, wherein the output means is at a higher potential than the input means.

15. A phase selecting device according to claim 13 or 14, wherein the potential wells are selectively deepenable.

16. A phase selecting device according to claim 13, 14 or 15, wherein the means for defining said potential wells and barriers comprise gate electrodes.

17. A phase selecting device according to any one of claims 13 to 16 including a clocking circuit for sequentially lowering said potential barriers to enable a charge carrier to transfer between adjacent potential wells, so as to effect transport of a charge carrier from the input means to the output means.

18. A phase selecting device according to claim 17, wherein the clocking circuit includes means for deepening a receiving potential well for said transfer of a charge carrier.

19. A phase selecting device substantially as hereinbefore described with reference to the accompanying drawings.