GB2306771A

GB2306771A - Optical storage device

Info

Publication number: GB2306771A
Application number: GB9521995A
Authority: GB
Inventors: Donald Dominic Arnone; Michael Pepper
Original assignee: Toshiba Cambridge Research Centre Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 1995-10-16
Filing date: 1995-10-27
Publication date: 1997-05-07
Anticipated expiration: 2015-10-27
Also published as: GB2306772A; GB9521155D0; GB2306769B; GB2306769A; GB2306771B; GB9521995D0; GB2306772B; GB9521998D0

Description

1 OPTICAL STORAGE DEVICE 2306771 The present invention relates to a

semiconductor optical storage device, which may for example be used as an optically operable memory device or a radiation profile imaging device. The devices according to the present invention are particularly adapted to be sensitive to electromagnetic radiation in the sub-millimetre, far-infrared or mid-infrared wavelength ranges. However, in its broadest sense, the present invention is not limited to operation at any particular microwave, infra-red or visible wavelength or range of wavelengths. Thus, the terms "optical" and "light" are to be interpreted as encompassing all of these.

The device of the present invention utilises the phenomenon of singleelectron charging as it occurs in mesoscopic, low-dimensional structures.

One system which possesses nonlinearities of large magnitude over voltage scales with the photon energy hcole lying in the microwave and infrared region is the single electron charging device. This is a known form of socalled quantum-effect semiconductor device. In this structure, electron motion is confined in all three dimensions. Such confinement is often realised by placing depleting Schottky gates on the surface of a semiconductor heterostructure which itself contains a two dimensional electron gas. In certain instances, single electron charging manifests itself in onedimensional and two-dimensional systems.

It is well known that quantum-effect devices can be made by arranging layers of semiconductor layers of different band gaps, such that it is possible to induce a quantum well ajacent an interface. Carriers can be confined in the well layer so that the current flowing in the well layer, between source and drain regions can be modulated by means of a control potential applied to any overlyinggate electrode.

1 The carriers, usually of high mobility, may exist in two dimensions to behave as a "two-dimensional electron gas" (2DEG) or they may be influenced by means of applied electrode potentials to exist substantially in only one dimension, i.e. as a "onedimensional electron gas" (IDEG) otherwise sometimes referred to as a "quantum wire".

Of course, the majority carriers can be electrons or holes so equivalent devices utilising a two-dimensional hole gas (2DHG) or one-dimensional hole gas (1DHG) can also be realised. However, for simplicity, the generic terms 2DEG or 1DEG or simply "carrier gas" will be used herein and should be understood as encompassing both possibilities, unless specifically indicated to the contrary.

As an extrapolation of such confinement, it is possible to arrange barrier potentials in three dimensions to confine a puddle of 100 or so electrons. This puddle is commonly referred to as a "quantum dot" or quantum box". In this structure, electron motion is confined in all three dimensions.

This kind of quantum dot confinement has previously been realised by placing four or more depleting Schottky gates in a turnstile arrangement on the surface of a semiconductor heterostructure which itself contains a 2DEG. The gates squeeze the electron gas such that the remaining two degrees of freedom are impaired. This additional squeezing or confinement produces tunnelling barriers around the dot through which electrons must pass if they are to enter or exit the dot. Such transport through the dot is then affected by applying an external current or voltage to the dot. This external bias raises the energy of electrons, allowing them to pass over andlor tunnel through the tunnelling barriers.

Figures 1 and I A of the accompanying drawings show the aforementioned known kind of heterostructure 111, in which a substrate 11.3 has formed thereon, a GaAs layer 115, 3 above which is formed an A1GaAs layer 117. A plurality of gates 119, 121, 123, 125, 127, 129 and 131 are situated above the A1GaAs layer. These are shown in more detail in Figure IA.

Electrons in a 2DEG induced in the active layer 115 are squeezed by a gate voltage (negative) applied to the gate electrodes. A negative voltage removes electrons and can electrostatically shape the electron gas to a desirable pattern, in the present case, that of a quantum dot.

Referring specifically to Figure 1A of the accompanying drawings, the black squares and rectangles 119-131 are front gates which deplete the electron gas underneath, as mentioned above. Barriers are formed between all adjacent gates. The arrows show two possible conduction paths through the device. A quantum dot is formed at the centre of the gates.

Single electron charging, commonly referred to as Coulomb Blockade, manifests itself in the current-voltage characteristics of quantum dots when the confinement length is sufficiently small (<300run) and the number of electrons is sufficiently small (several hundred or less). The capacitance (c) of such a dot is C,._ 10- 15F. The single electron charging energy, commonly referred to as the Coulomb energy, is the energy penalty incurred by having to add a whole electron to the dot when electrostatics require only a fractional amount of charge to produce neutrality between the dot and the surrounding reservoir of electrons. This charging energy is -e2/2C, where C is the capacitance of the dot. Under these conditions, it is energetically favourable for an additional electron to enter the dot only when the source drain bias applied across the dot exceeds the Coulomb or charging energy C2/2C. Figure 2 of the accompanying drawings illustrates this concept. Electrostatic barriers are formed by applying a bias voltage Vgate to the gate electrodes in a device such as shown in Figure I A.

4 As shown in Figure 2, the barrier heights exceed the Fermi level and the highest Coulomb state. e2/2C represents the Coulomb energy between the lowest state and that above the Fermi level. Another bias potential (eVsd) is also applied between source and drain contacts on either side of the quantum dot region. If eVsd <e2/2C, electrons cannot leave or enter the quantum dot. Therefore, charge is effectively stored in the dot. This enables the dot to have a potential application for a memory cell. The conventional way of changing the occupancy of the dot is to apply a different source drain bias, or, equivalently, to apply a different bias to the electrostatic barriers (gates) which are used to define the dot. The speed with which such biases can be varied is limited by the capacitance of the dot and by any external circuitry used to induce the bias change.

The applicants have discovered that a more convenient and potentially quicker means of adjusting the number of electrons in a dot or array of dots would be to use absorption of infrared or microwave radiation to change the electron energy, thereby enabling electrons to more easily tunnel through or'hop over' the potential barriers which define the quantum dot. Thus, the present invention now provides an optical storage device comprising a plurality of elements each comprising an active region for induction of a two-dimensional carder gas therein and gate means for confining a pool of carriers from the carrier gas within a confinement region of the active region and optical means for selectively applying electromagnetic radiation of different intensity to different elements.

The device of the present invention employs a plurality of the elements i. e. two or more, although a practical device might contain anything from 3 to 1 -million.elements.

The mechanism for adjusting the number of electrons in a quantum dot by using infrared radiation is shown in Figure 3 of the accompanying drawings. The barrier c heights induced by appropriate gate voltages Vgate are the same relative to the energy levels in the dot and the Fermi level (EF) and the same source-drain bias is assumed. "A" denotes an electron excited over the barrier by infrared radiation. "B" denotes an electron induced to tunnel through a barrier by infrared irradiation. "C" shows an electron cascading to the lower Coulomb level by natural decay (slow). Thus, a small applied bias is necessary to induce the electrons to hop over the barrier or tunnel into the dot, as opposed to tunnel out or hop out of the dot. The value of the bias is critical in determining whether a dot gains an electron under irradiation.

As the radiation is switched on and off by means of electro-optic switches, then this arranaement has considerable time advantages over that associated with changing the source drain bias Vsd. In addition, if it is desired to change the number of electrons in the array, then the infrared irradiation will essentially perform this task for all dots instantaneously. Whilst this can also be accomplished by electrically applying a given source - drain-bias Vsd or gate bias Vgate to all dots simultaneously, the latter method would require more time to accomplish and also requires that each dot in the array gain an additional electron. If it is desired that only certain dots in an array gain an electron, then the bias Vsd or Vgate be applied to each addressed dot individually. By contrast, if irradiation is used to increase the number of electrons, then only those dots with the appropriate bias gain an electron whilst others would not gain any if Vsd or Vgate were set appropriately. Ibis is explained in more detail hereinbelow.

The quantities which are most important in determining whether dot occupancy increases under irradiation are the source-drain bias Vsd, gate bias Vgate, and the photon energy (frequency of irradiation). The sourcedrain bias Vsd is important because it determines the electron energies in the source reservoir at the Fermi level, and therefore how much additional energy from irradiation is required for the electron to jump over the barrier. The gate bias is important because this bias is used to set the barrier heights which confine the electrons in the quantum dot. Thus, a combination of 6 VA and Vgate determines the additional energy eVb which the electron must acquire from the incident photon to hop over the barrier. If tunnelling is important (path B in Figure 3), then these two biases also determine the additional energy the electron needs to tunnel into an empty state in the quantum dot.

The other quantity which determines whether an additional electron will enter the dot is the energy of the infrared photon = hCOIR, where COIR isthe frequency of the radiation and h represents Planck's constant. If hCOIR>eVb, then irradiation will cause electrons to hop over the barrier and occupy the quantum dot under the influence of finite Vsd. The number of electrons which hop over the barrier per second will be proportional to the power of the incident radiation.

Which frequency range is used will depend on eVb. If quantum dots are formed via front gated turnstile devices fabricated on conventional GaAS/A1GaAs HEMTs, then eVb will be on the order of several tens of meV, and far-infrared (FIR) radiation will be most appropriate. lhis radiation can originate from laser or thermal sources, and can be efficiently coupled to the dot by means of antennae.

Infrared radiation may also be used to decrease the number of electrons in quantum dots operating the blockade regime. Figure 4 illustrates the concept- If the confinement width of the dot is changed via application of different gate bias, then the dot energy levels can be arranged such that the last energy level lies near the tops of the barriers, If infrared radiation is used to excite electrons in the dot to this last level, then a small source drain bias may be used to induce the electrons to tunnel or hop into the continuum of stateS lying above the barrier, thus depopulating dot. Therefore, if all the electrons in a dot or array of dots are to be erased, the irradiation at different gate bias may be employed. The proviso is that the source drain bias be such that eVb>hco IR so that additional electrons are not also excited into the dot. This latter requirement 7 dictates that either the Fermi energy EF in the source drain be lowered and/or teh confinement width be reduced.

As shown in Figure 4, the barrier heights are the same as in Figures 2 and 3 and the infrared radiation exciton electron to the last (highest) state near the top of the barriers, a continuum of energy states existing just above the barriers. This is done by applying a different gate voltage Vgate to change the confinement and push the Coulomb energy states closer to the continuum. The electron tunnels or hops out of the dot under the influence of source-drain bias.

In the device of the present invention, it is convenient to form the array of elements on a single wafer. Therefore, in that situation, the active region of each element would be a discrete area of a complete active layer extending across the wafer.

The device according to the present invention may be configured so as to operate as a memory with an optical light capability. In that case, the individual elements will correspond to memory cells. For such a memory, the device may be provided with a radiation source or sources and an optical address means for selectively supplying radiation of predetermined different intensities andlor frequencies to different individual elements. In principle, it would be possible to provide a plurality of sources, each corresponding to an individual element. However, in practice, it may be preferable to provide a single source of radiation and means of scanning or selectively addressing the individual elements.

The radiation source may for example be a thermal source, a laser or laser diode, e.g. a near infrared laser or a thermal source in the mid- infrared range. For far-infrared operation, a Gurin diode may be used, or a Y1G oscillator for microwave operation.

8 The selective scanning or address means may comprise components such as polarisers or electro-optic crystals for the near-infrared range although, the individual sources referred to above may be preferable for lower frequencies. The radiation source and scanning/addrss means etc. may all be formed on the same chip using optical or electron beam lithography.

Moreover, for such an optical light memory, preferably light guide means are provided for channelling the radiation of predetermined different intensities to the different various elements. Examples of these light guide means are fibre optic cables or focusing optics for the nearinfrared range, focusing optics or waveguides for the midinfrared region and waveguides coupled to surface antennae in the far-infrared or microwave regions.

Again, the light guide means may also be formed on the same chip by well known techniques.

Another application of devices according to the present invention is for operation as a radiation intensity profile imaging device, i.e. an analogue of a CCD array, which may for example be used for infrared imaging or the like. Such an imaging device preferably comprises imaging means for causing radiation related to predetermined different pixel areas of an image or radiation intensity profile to be selectively applied to different elements. Such optical means may comprise for example, lenses and/or optical fibres for near-infrared operation, lenses and/or waveguides for mid-infrared operation and lenses, mirrors or condenser cones for the far-infrared and microwave regions.

Imaging devices according to the invention are more sensitive than conventional detector arrays. Using such a dot array with dots operating in the Coulomb regime, it is possible to detect single photons with good signal to noise. This assumes that the 9 quantum efficiency=1. In other words, it assumes that the probability that an incoming photon will be absorbed by an electron, and thereby transfer the electron into the dot, will be 100%. By contrast, conventional infrared CCDs rely on large numbers of electrons being photoexcited by incident radiation, or changes in temperature being induced by incident radiation. Both of these mechanisms require large numbers of incident photons, as opposed to the single photon required for the quantum dot based array.

As indicated above, elements of the device may be provided with antenna means for coupling the electromagnetic radiation to the confinement region.

In described embodiments of the invention hereinbelow, each element is provided with its own source and drain connection so that the active region can provide electrical conduction therebetween.

Each element may be provided with a plurality of gates above the active region, e.g. in the aforementioned known turnstile arrangement (Figures 1 and 1A) for confining the carriers in all three dimensions.

Another suitable element design entails location of the confinement region of the active region within a recessed region of the element. This permits each gate means to be formed as a plurality of "side gates". However, it is also preferred for the gate means to comprise at least one secondary gate for restricting the width of the carrier gas.

With the recessed form of construction, any antenna means can extend into the recessed region of each such element. Such antenna means may comprise a pair of substantially triangular members for impedance matching (in the manner of microwave microstrip). The apex of the triangular members can extend into the respective recessed region. These triangular features can be, e.g. in the form of equila,Cral or isosceles triangles.

Other forms of matched antenna are possible, for example a log-periodic structure (i.e. in the manner of a low-frequency Yagi antenna).

In the described preferred embodiments, the active region of each element is formed on an angled facet of an etched layer structure. The gate means of each element thus comprises a front gate overlying the active region and at least one back gate constituted by a layer or layers or the etched layer structure.

For any element configuration, it is convenient to form the active regions or layer as part of a high electron mobility transistor structure.

Using infrared radiation to change well occupancy is far better than using bandgap radiation which excites both electrons and holes in semiconductors. The reason for this are two-fold. First of all, band-gap radiation will place hole carriers in the valence band which will affect the conductance through the dot. Thus when the conductance through the dot is sampled in order to obtain the number of electrons in the dot, and erroneous electron occupancy may be read. Secondly, if visible radiation is used to increase the electron occupancy in the dot conduction band, then both holes and electrons must be confined in the quantum dot if the energetic electron is to be created by the irradiation. This severely limits the number and type of semiconductor quantum dot systems which may be used to realise such quantum dots.

For quantum dots formed via regrowth in GaAs/A1GaAs, GaInAs/A1InAs or any other semiconductor systems such as silicon/silicon dioxide, then the barriers (and hence eVb) can be larger, and therefore radiation in the mid-infrared (MIR) or near-infrared (NIR) can be used. Other suitable heterostructures are SiGe, InSb, GaSbIlnAs, InAlAs/InP, etc.

The present invention will now be explained in more detail by reference to the following description of a preferred embodiment and with reference to the accompanying drawings in which:-

Figure 1 shows a cross section through a known device utilising coulomb blockade which may be used as an element of a device according to the present invention; Figure IA shows a plan view of the device of Figure 1, showing the turnstile arrangement of gate electrodes; Figure 2 shows an energy band diagram for explaining use of applied potentials for controlling Coulomb blockade in a quantum dot; Figure 3 shows an energy band diagram for explaining use of infrared radiation for controlling Coulomb blockade; Figure 4 is an energy band diagram for explaining use of infrared radiation reducing the occupancy of a quantum dot; Fi aure 5 shows a first embodiment of a device according to the present invention, c capable of functioning as a memory with optical write capability; Figure 6 shows a second embodiment of a device according to the present invention, capable of functioning as a radiation intensity profile imaging device; 12 Figure 7 shows a plan view of another kind a radiation detector element for use in a device according to the present invention; Figure 8 shows a cross-section through the element shown in Figure 7; Figure 9 shows a cross-section through a further kind of radiation detector element for use in a device according to the present invention; and Figure 10 shows a cross-section through yet another kind of radiation detector element for use in a device according to the present invention.

Tuming now to Figure 5, there is shown a first embodiment of a device according to the present invention which is capable of functioning as a memory with optical write capability. This device 201 comprises an array of elements 203 etc. formed lithographically on a single wafer 205. Each element 203 has a source contact 207 and a drain contact 209, as well as a gate arrangement 211. Details of possible element constructions will be described in further detail hereinbelow. A radiation source 213 such as of a kind merItioned hereinbefore, is positioned above the wafer 205. Below the radiation source is used a coupling/scanning or addressing system 215, also of a kind as mentioned as hereinbefore. This system 215 couples the radiatiori to individual interconnects 217 etc., each of which leads to a respective element (memory cell) 203. Each element confinCs a quantum dot containing relatively few electrons.

Figure 6 shows a second embodiment of a device according to the present invention. which is capable of functioning as a radiation intensity profile imaging device. This device 221 also comprises individual elements 223 etc., each lithographically formed on a single wafer 225 and each having a respective source contact 227, drain contact 229 and gate arrangement-23 1. Again, the elements may be of any kind described further hereinbelow. Above the wafer 225 is lwated an imaging/condensing optical 13 system 233, e.g. of a kind as mentioned hereinbefore. This images a radiation intensity profile or image 235 (here shown above the imaging/condensing optic system 233) so that pixel areas of it are imaged onto individual and respective elements 223.

With the structure shown in Figure 6, the incident radiation optically pumps charge only into those dots which have the correct source-drain bias and gate bias. The element array can be read by examining the conductance through each dot. If a 21) array is used with each of the identical dots biased the same manner, one obtains a picture of the transverse intensity profile of the beam, just as in a conventional CCD array operating at visible wavelengths.

The other feature of such a 21) array of quantum dots is that if the source-drain bias and/or gate voltage of the dots are changed, then the intensity profile in a different frequency range may be recorded. This is because optical pumping will occur at a given frequency only if the biases are correct. Thus it is made possible to use an array of quantum dots to fabricate a microwavelinfrared CCD-type camera which is frequency tuneable/wavelength selective.

In this mode, the different intensities, associated with the beam profile is measured by detecting different numbers of electrons in given dots; i. e. the more electrons in a given dot in the array, the larger the intensity in the corresponding part of the beam profile.

As mentioned hereinbefore, each element of the devices of Figures 5 or 6 can be of the general kind described hereinbefore, having gates located over the active region (e.g. in turnstile arrangement), such as shown in Figures 1 and 1A. However, Figures 7 and 8, show another kind of radiation detector element 1 for use in a device according to the present invention. As seen in plan view (Fig. 7), a first antenna member 3 and a second antenna member 5 each are in the shape of an equilateral triangle. One apex 7 of the first antenna member 3 and one apex 9 of the second antenna member 5 point towards 14 one another. Thus, the edge 11 of first antenna member 3, remote from its apex 7 and the edge 13 of the second antenna member 5, remote from its apex 9, are parallel to each other and each is disposed outwardly. The aforesaid apexes 7, 9 are separated by a gap 15.

Such an antenna structure is primarily intended for microwave and farinfrared operation. At higher frequencies, antenna dimensions would be very small and therefore less efficient. Thus, they can be dispensed with for mid-infrared and higher frequencies.

A source region 17 and a drain region 19 are situated either side of the gap 15 along an imaginary line at right angles to an imaginary line joining the two apexes 7, 9 of the antenna members. First, second, third and fourth side gates 21, 23, 25, 27 are buried in the device structure and are each provided with a respective electrical connection 29,31,33,35.

The first and second side gates 21, 23 are both disposed one side of the imaginary line joining the source and drain regions 17, 19 but respectively either side of the imaginary line joining the apexes 7, 9. Similarly the third and fourth side gates 25, 27 are both disposed at the other side of the imaginary line joining the source and drain regions 7, 9, opposite to the first and second side gates 21, 23. However, the third and fourth side gates 25, 27 are also disposed on opposite sides of their source and drain regions 7, 9. Thus, the side gates 21, 23, 25, 27 are in positions (as seen in plan view) roughly corresponding to the corners of an imaginary square bounding the gap 15 between the apexes 7, 9.

Also, within the gap 15 and the aforementioned imaginary square but either side of the aforementioned imaginary source-drain line, are first and second secondary gates 37, 39 or "plungers", also buried in the structure. Each secondary gate 37, 39 is provided with a respective electrical contact 41, 43.

The first and second antenna members 3, 5, are each provided with a respective electrical connection 45, 47, extending from the respective outwardly-facing sides 11, 13 thereof. In use, these may be left open circuit or connected to ground.

Figure 8 shows how, along one axis (the source-drain axis), a recess 49 is etched in a layer structure 51 grown on a GaAs substrate 53. The antenna members 3, 5 are formed on the structure 51 and extend into the recess 49 so that their facing apexes 7, 9 are lowermost. Between the apexes is an uncovered region 55 at the bottom of the recess 49.

The layer 51 is comprised of the following layers formed in turn, upon the substrate 53, prior to selective etching of the recess 49, namely an n+ GaAs back gate layer 57 of 20Onm thickness, an AlAs/GaAs superlattice 59, an undoped GaAs layer 60 of loonffl thickness, a p GaAs side gate layer 61 of 10Onm thickness, and a conventional HEMT structure 63 (which of course includes an active region). The triangular antenna members 3, 5 are deposited on top of the structure, after selective etching to form the recess 49.

The HEMT structure 63 extends into the side gate layer 61 to divide it into the first and second secondary side gates 37, 39. The primary side gates 21, 23, 25, 27 are not visible in this cross-section.

In use, a bias voltage between the source 17 and drain 19 includes a twodimensional electron gas in the active layer of the HEMT 63. The back gate and secondary gates 37, 39 are used to deplete-out all except a narrow conduction region in the centre region 55. Then, bias voltages applied to the primary side gates 21, 23, 25, 27 confine 16 carriers in a quantum dot. Current flowing between source and drain is interrupted until radiation is received via the antenna members 3, 5 to provide the additional energy required to allow conduction across the potential barriers which confine the quantum dot.

Figure 9 shows an alternative kind of detector element 71 for use in devices according to the present invention. It comprises a layeredstructure 73 which has been selectively etched to produce an angled side facet 75 intersecting edges of layers in the structure 73.

A high electron mobility transistor (HEMT) heterostructure 77 is formed by regrowth over the side facet 75. Over the heterostructure 77 is formed an oxide insulating layer 79 and a front gate electrode 8 1.

The layer structure 73 comprises a lower p-Si layer 83 (50run), a middle p-Si layer 85 (10Onm) and an upper p-Si layer 87 (50run) which are respectively separated by lower and upper Si02 insulating layers 89, 91.

Over a region 93 of the HEMT structure 77 opposite the middle p-Si layer 85 is formed a surface-mounted antenna pattern 95 which can have the "bow tie" configuration used in the first embodiment or could consist of concentric circles in log periodic fashion.

In any event, the middle of the antenna 95 corresponds to a "quantum doC confinement region produced in an active layer of the HEMT structure 77. A 2DEG is induced by application of a potential bias between an n-type source region 97 in contact with a lower end of the HEMT structure 77, and an n-type drain region 99 in contact with an upper end 101 of the HEMT structure. The source region 97 and drain region 99 have respective ohmic contacts 103, 105.

17 In use, the confinement potential barriers are induced by applying bias potentials to the front gate 81 and back gates constituted by the p-Si layers 83, 85, 87. Again, radiation received by the antenna 95 enables conduction across the "quantum doC in the confinement region.

As shown in Figure 10, yet another kind of detector element 151 for use in devices according to the present invention is somewhat analogous to that shown in Figure 7.

A recess 153 is etched in a layer structure 155 consisting of a 2000A (100) n+GaAs back gate layer 157 formed on a 500 pm SI substrate 159. The recess divides the n+GaAs layer into respective side gates 161, 163 and to reveal a surface 165 which acts as a regrowth interface.

On the regrowth interface surface 165 are grown as a regrowth structure 167, in order, a 300A GaAs layer 169, a 1500A AlAs/GaAs superlattice 17 1, a 300A active GaAs layer 173, and a 650A top structure 175, consisting of, in order, and A1GaAs spacer layer 177, an n+AlGaAs layer 179 and a GaAs cap layer 18 1.

Optionally, a double -triangular antenna (not shown) is formed over the cap layer 18 1, as in the first embodiment, extending apex-wise into the recess 153. In use, a 2DEG 183 within the active GaAs layer 173 adjacent the A1Ga.As spacer layer is restricted by the applied potentials so that a quantum dot 185 is confined in the 2DEG plane, of the bottom of the recess 153.

In the light of this description, modifications of the described embodiments, as well as other embodiments, all within the scope of the present invention as defined by the appended claims, will now become apparent to persons skilled in this art.

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Claims

An optical storage device comprising a plurality of elements each comprising an active region for induction of a two-dimensional carrier gas therein and gate means for confining a pool of carriers from the carrier gas within a confinement region of the active region and optical means for selectively applying electromagnetic radiation of different intensity to different elements.
2. A device according to claim 1, wherein the active region of each element is a discrete area of an active layer of the device.
3. A device according to either preceding claim, wherein the optical means comprises a radiation source and optical address means for selectively supplying radiation of predetermined different intensities to different elements.
4. A device according to claim 3, wherein the optical means further comprises focusing optics to direct beams of predetermined different intensities to different elements.
5. A device according to claim 3, wherein the optical means further comprises light guide means for channelling radiation of predetermined different intensities to different elements.
6. A device according to claim 1 or claim 2, wherein the optical means comprises imaging means for causing radiation related to predetermined different pixel areas of an image or radiation intensity profile to be selectively applied to different elements.

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7. A device according to any preceding claim, wherein each element is provided with antenna means for coupling electromagnetic radiation to the confinement region of that element.
8. A device according to any preceding claim, wherein each element is provided with source and drain regions so that the active region can provide electrical conduction therebetween.
9. A device according to any preceding claim, wherein the gate means of each element comprises a plurality of gate above the active region.
10. A device according to claim 9, wherein each plurality of gates is in a turnstile configuration.
11. A device according to any of claims 1-8, wherein the confinement region of the active region of each element is located within a recessed region of the element.
12. A device according to claim 11, wherein said gate means of each element comprises a plurality of side gates.
13. A device according to claim 12, wherein said gate means of each element further comprises at least one secondary gate for restricting the width of the carrier gas.
14. A device according to any of claims 11-13 when dependent upon claim 7, wherein the antenna means of each element extends into the recessed region.
15. A device according to any of claims 11-14 when dependent upon claim 7, wherein the antenna means of each element comprises a pair of substantially triangular members.
16. A device according to claim 15, wherein in each element, an apex of each substantially triangular member extends into the recessed region.
17. A device according to claim 15 or claim 16, wherein said substantially triangular members are substantially equilateral.
18. A device according to any of claims 11 - 14, when dependent upon claim 7, wherein the antenna means of each element is configured as a logperiodic antenna.
19. A device according to claim 1, wherein the active layer is formed on an angled facet of an etched layer structure.
20. A device according to claim 11, wherein the gate means comprises a front gate overlying the active layer and at least one back gate constituted by a layer or layers of said etched layer stnicture.
21. A device for electromagnetic radiation, the device being substantially as hereinbefore described with reference to any of Figures 3-9 or either of Figures 1 and 1 A in combination with Figure 5 or 6 of the accompanying drawings.