GB2352087A

GB2352087A - Single photon detector

Info

Publication number: GB2352087A
Application number: GB0010582A
Authority: GB
Inventors: Andrew James Shields
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 1999-07-10
Filing date: 2000-05-02
Publication date: 2001-01-17
Anticipated expiration: 2020-05-02
Also published as: GB0010582D0; GB9916182D0; GB2352087B

Abstract

A photon detector configured to detect a single photon comprising first 311 and second 307 active layers separated by a first barrier layer 309 and detecting means 325, 327 for detecting a change in a characteristic of the first active layer 311, wherein the first active layer 311 is a quantum well layer capable of supporting a two dimensional carrier gas and the second active layer 307 comprises at least one quantum dot, the device also comprises means for separating a photo-excited electron-hole pair 321, 323. The detector may be used in an optical memory device.

Description

1 2352087 Photon Detector

The present invention relates to the field of so-called low dimensional semiconductor devices. More specifically, the present invention relates to the field of single photon detectors.

There is a need for an optical detector which is capable of detecting a single photon. Recently, this need has been heightened by the advent of quan'turn cryptography of optical signals. In essence, quantum cryptography relies upon the transmission of data bits as single particles, in this case photons, which are indivisible. One way in which the data can be encoded is via the polarisation of the electric field vector of the photons. The key component of such a system is a detector which can respond to individual photons. It has been proposed that quantum cryptography can be used to transmit the key for the encryption of data. Examples of information which might be encrypted in this way are internet data or data from automatic teller machines.

Single photon detection is also useful as a low level light detection means for spectroscopy, medical imaging or astronomy. An optimum signal to noise ratio is achieved when a single photon is detected, as the noise is then limited by the source and it is completely independent of noise arising due to the amplifier or detector itself.

A single photon detector could also be used for time-of-flight ranging experiments where the distance of an object from a fixed point is measured by calculating the time over which a single photon takes to return to a detector. This technique can also be used to scan the surface of an object, even a distant object, to form a spatial image of its surface depth, thickness etc.

2 The present invention is primarily concerned with a single photon detector which is cheap, compact and mechanically rugged. It operates using standard voltages (less than 5V) and can operate a low or room temperature. It is also suitable for fabrication into a multichannel array of detectors, which is useful for spatial imaging and spectroscopy. It also has a fast time response and so can be used to measure the time of the arrival of the photon. Unlike a SPAD it can measure more than I photon in each reset cycle.

Single photon detectors are available in the form of photo multiplier tubes (PMT) and single photon avalanche photo diodes (SPAD). PMTs have the disadvantage of having low quantum efficiency, being expensive, bulky, mechanically fragile, requiring high biasing voltages and cooling. They can also be damaged and can require a long settling time after exposure to high light levels or stray magnetic fields. On the other hand, SPADs have the disadvantage of having a relatively low gain and high dark count rates, especially when operated at higher repetition rates. They are also expensive and require high bias voltages and external cooling.

The present invention is concerned with a photon detector which belongs to a class of so-called low dimensional semiconductor device. The electrons and holes in an ideal bulk semiconductor have a continuous spectrum of energy states. Confinement of the carriers in one or more dimensions modifies this energy spectrum by a quantisation of the k- vector along the confinement direction(s). In a quantum dot, the motion of the carriers is restricted in all three spatial dimensions. Consequently, the energy spectrum of the dots consists of a series of discrete levels. As the, size of the quantum dot reduces, the energy spacing between these discrete levels increases. The maximum number of electrons which can occupy each electron level is two, corresponding to the up and down spin states. Similarly, each hole level has an occupancy of two. Optical transitions can occur between the discrete electron and discrete hole levels.

In a first aspect, the present invention provides a photon detector configured to detect a single photon, the detector comprising first and second active layers separated by a first barrier layer, and detecting means for detecting a change in a characteristic of the first 3 active layer, wherein the first active layer is a quantum well layer capable of supporting a two dimensional carrier gas and the second active layer comprises at least one quantum dot, the device ftirther comprises means for separating a photo-excited electron-hole pair.

Preferably the means for separating an electron-hole pair will be provided by a means for applying an electric field normal to the active layers. However, the device may be fabricated such that the internal field of the device allows separation of photo-excited electron-hole pairs.

The device is capable of detecting a single photon. This is because optical illuminat-13L of the device leads to a change in the charge occupancy of the quantum dots and this, in turn induces a change in a transport or optical characteristic of the first active layer. Absorption of a single photon by the device results in a change in the occupancy of a quantum dot by one carrier and this in turn induces a change in a transport or optical characteristic of the first active layer. A single photon incident on the device will photoexcite one electron-hole pair within the device. One of these photo-excited carriers is trapped by a quantum dot and induces a change in a characteristic of the first active layer. For simplicity, it will be assumed that the photo-excited hole is trapped within the quantum dot. However, it will be appreciated by those skilled in the art that the electron can be the photo-excited carrier which is trapped within the dot.

The present invention is configured to detect the presence of a single photon either by the size of the device, the total number of dots in the second active layer, the layer structure of the device or in the actual detection mechanism of the device.

Preferably, to reliably detect single photons irradiating the device, the number of active dots in the device is less than 100,000, more preferably less than 10,000. To avoid confusion, the term active dot is intended to mean a dot in the active area of the device which is capable of trapping carriers during normal operation of the device. Dots which cannot trap charge or which are outside the active area are not active dots. Also, the 4 term active area is intended to mean the area of the device which is subjected to the separating means and which contributes to a change in the characteristic of the first active layer. In some cases, the field will be applied by a gate and a mesa will be etched to define the device boundaries, the active area here will be the area of overlap between the gate and the mesa. In this case the size of the gate is chosen so that the number of active dots in the device is less than 100,000, more preferably less than 10,000. The active area defined by the gate can be less than 10-8M2. In alternative examples, the size of the active region is limited by reducing the size of the conduction channel within the first active layer. This may be done by using a split gate, which defines a conduction channel in the first active layer. Here the defined conduction channel will define the active area and not the area of overlap between the gate and the wesa. Preferably, the length of the conduction channel will not be more than I OOgm, more preferably not morethanlOgm. Another method to limit the size of the conduction channel is by etching or damaging regions of the first active region.

The detecting means can either measure a change in the transport characteristics of the first active layer or the optical characteristics of the layer. For example, a change in the optical characteristics due to a change in the carrier concentration or electric field across the layer could be detected. Changing the carrier density in the first active layer alters its absorption or emission spectrum. A change in the electric field across the first active layer will cause a change in the energy or intensity of its absorption or emission spectrum. Changes in the carrier concentration, mobility or field across the layer also manifest themselves in the transport characteristics of the layer.

Preferably, the means for detecting a change in a transport characteristic is configured to detect a change in a transport characteristic over a length of I 00gin in the transport direction of the first active layer.

The photon detector can operate to detect photons over a wide range of wavelengths. The number of photons absorbed in each of the barriers and active layers depends on the incident photon f1ux, the absorption coefficient of the layers and their thicknesses.

While not wishing to be bound by any theory, it is believed that the following mechanisms contribute to photon detection by the device.

If the incident radiation has a photon energy higher than that of the barrier layers of the device, electron-hole pairs will be photo-excited in the barriers of the device, in addition to either of the active layers. The internal field of the device, or the applied field, disassociates the electron-hole pair and sweeps them in opposite directions, one type of carrier will be trapped by the dot and this will produce a change in the transport characteristic of the first active layer.

In the situation where the photon energy of -the incideni radiation is lower than the band gap of the barrier layers, but higher than that of the first active layer and also higher than that of the second active layer, electron-hole pairs will be excited in both the first active layer and the second active layer. Essentially no carriers will be excited in the barrier.

Since the first active layer typically has a higher absorption coefficient than the second active layer, most of the carriers will be excited within the first active layer. Due to the internal or applied electric field, one of the carriers can tunnel into the quantum dot leaving the other carrier in the first active layer. This will also cause a change in the characteristics of the first active layer.

If the photon energy of the incident radiation is less than the band gap of the first active layer but larger than the band gap of the quantum dot layer, then electron-hole pairs are excited in the quantum dot layer. One of these carriers can tunnel out of the first active layer leaving the other carrier trapped within the quantum dot layer. This also causes a change in the characteristic of the first active layer.

The device can also detect illumination which has a photon energy lower than the band gap of the quantum dot, if the quantum dot is designed to contain excess charge. In this situation, a single photon can excite excess charge out of the dot by an intra-band transition. It is then swept away from the quantum dot by applied electric field or an

6 internal electric field. This will also lead to a change in the characteristic of the first active layer.

The applicant does not wish to be bound by a particular theory or explanation. However, it is believed that characteristics of the first active layer can be affected by the trapped carriers through two main mechanisms. In the first mechanism, the excess carriers in the first active layer have an opposing polarity to the carriers which are stored in the dots. In the second mechanism, the carriers in the first active layer and the carriers trapped in the dots are of the same type.

The first mechanism will be explained using holes as the stored carrier and a detecting mean consisting of a 2DEG. However, it will be apparent to a person skilled in the art either electrons or holes can be stored in the quantum dots depending on the layer thickness and composition, doping polarities and applied biases.

A quantum dot in the second active layer is illuminated with a beam of radiation. The structure is biased so that it is energetically favourable for the electron to tunnel through the barrier to a 2DEG in the quantum well layer, leaving behind a hole stored in the quantum dot. This effects a change in the conductivity of the 2DEG.

Without wishing to be bound by any particular theory or explanation, the applicant believes that the change in conductivity is predominantly due to the positive charge stored in the barrier which alters the band bending and hence persistently alters the conductivity of the 2DEG. Thus, depending on the actual configuration of the device, the conductivity of the 2DEG can either increase or decrease. It is also believed that where the stored carriers are electrons, the negative charge stored in the barrier will change the conductivity of the quantum well layer. In the case where the stored charges are electrons, the quantum well will support a two dimensional hole gas.

In the second mechanism, the carriers stored in the dots are of the same type as the carriers stored in the well. In the situation where the carriers in the quantum well are 7 electrons, the stored carriers in the quantum dots are electrons. Carriers are preferably provided to the dots via a doped barrier situated on an opposing side of the second active layer to the first active layer. This layer may also be a modulation doped barrier layer, with an undoped spacer layer adjacent the second active layer.

The applicant does not wish to be bound by any theory or explanation. However, it is believed that the dots contain excess electrons prior to illumination. The charged dots act as scattering centres for the electrons in the 2DEG, which consequently has a relatively low mobility. After illumination, the number of excess electrons in the quantum dots is reduced and the number of electrons in the 2DEG increases. A decrease in the negative charge in the dots results in an increase in the 2DEG conductivity. Also, the increase in the carrier concentration of the 2DEG causes an increase in the conductivity of the 2DEG.

The inverse structure may also be realised where excess holes populate the quantum dots prior to illumination and the quantum well supports a two dimensional hole gas.

To detect the presence of charge in the first active layer, it is preferable if at least two ohmic contacts are provided to the first active layer. A voltage will be measured between these two contacts, or a current will be measured flowing between them. For an accurate reading, it is more preferable if a four-terminal voltage measurement is used.

The detection means preferably differentiates (with respect to time), the measured characteristic of the first active layer in order to detect small changes in the characteristics e.g. the conductivity or resistivity of the first active layer. The detection means may also comprise means for detecting a change in the characteristic of the first active layer with respect to time. More preferably, the differentiated signal is fed through a pulse counter to count the number of detected photons. This pulse counter is ideally provided with a discriminator which determines the minimum signal level which will be registered by the counter. This helps to discriminate the pulses due to photons from the noise on the signal.

8 Preferably, the device is configured so that electrons are supplied to the first active layer. This is preferably done by means of a doped second barrier layer which is preferably provided adjacent the first active layer. More preferably, this doped barrier layer is a modulation doped barrier comprising a doped barrier layer and an undoped spacer which lies adjacent the first active layer.

Devices according to a first aspect of the present invention either with or without a doped barrier layer overlying the layer of quantum dots, can also be used where the stored carriers provided to the first active layer are holes.

It should be noted that in some cases, it is preferable to provide a thin growth smoothing layer, say for example, GaAs, prior to forming the dot layer to aid growth of the dot layer. Often, the growth smoothing layer will be provided between a barrier layer and the second active layer.

The field normal to the active layers can be generated by providing a front-gate overlying the structure. The front gate may be a metal front gate or a doped semiconductor gate. More preferably, a back-gate is also provided. The back-gate may be a metal back-gate or a doped semiconductor back-gate. It is preferable if the frontgate is semi- transparent to radiation with an energy close to that of the quantum dot band-gap. Semi transparent is taken to mean that the gate transmits at least about 50% of the radiation incident on the gate. The front and/or back gate may be a full gate, alternatively one or both of them may be a split gate. A fall gate is preferable when the detection of photons causes a decrease in the resistance of the first active layer, a split gate is used to define a conduction channel and is preferably used when illumination causes an increase in the resistance of the first active layer.

The active area of the full gate is preferably no more than 10-8M2. The split gate preferably defines a conduction channel of no more than 1 00[tm, more preferably no more than I OWn.

9 The means for applying electric fields may also comprise a p-type terminal and n-type terminal located on opposite sides of the first and second active layers. In other words, the structure is sandwiched between doped p and n-type layers.

The detector preferably comprises an anitreflection coating provided on the surface of the device which is to be illuminated.

The energy spectrum of the quantum dots is dependent on its size, shape and local environment. Hence, different quantum dots possess different ground state energies and different optical transition energies. The device may comprise quariturn dots of different sizes which require radiation of different frequencies to resonantly excite an electronhole pair.

A convenient method of forming a layer of quantum dots is by using the StranskiKrastanow growth mode wherein a first layer is grown on a layer with a different lattice constant to the first layer. The first layer proceeds by three dimensional island growth and small quantum dots can be produced which typically have lateral dimensions of less than 50 mn. A preferable material system for producing this device uses the growth of InAs, InGaAs or InAlAs quantum dots with GaAs or (AlGa)As barriers.

The device may be formed such that the 2DEG layer is grown before the quanturn dot layer. However, the ordering may be reversed i.e. the 2DEG layer formed overlying the dot layer. Other lattice mismatched systems can be used such as InGaN or AlGaN. Another possible system for producing the dots uses strained SiGe heterostructures.

The device may also conveniently be formed from silicon. Here, the dots would be formed from an amorphous layer of silicon which forms dots after annealing at 8000. It will also be appreciated by a man skilled in the art that germanium could also provide another possible material for fabricating the device.

Detection of single photons is also enhanced if the device further comprises an absorption layer. Such an absorption layer can be a relatively thick layer, for example greater than I 00nm, which forms a barrier layer to the quantum dots. Photons are absorbed within the absorbing layer, creating electron-hole pairs within the absorbing layer. An applied electric field, or internal electric field, within the absorbing layer separates the electron and holes which are swept in opposing directions by the field. One polarity of carrier is swept into the quantum dot layer. Generally, the absorption layer would be provided outside the active region of the device i.e. the absorption layer would not be placed in between the first or second active layers. Such a device will generally also comprise a semiconductor substrate.

The photon detector of the present invention is not limited to a device which has just a single layer of dots. Two or more layers of dots may be provided to trap charge to affect the conductivity of the first active layer. Alternatively, a detector may comprise a plurality of first and second layers separated by a barrier. This device can be thought of as a plurality of photon detector arranged on top of one another.

A photon detector array may also be fabricated comprising a plurality of pixels, each pixel comprising a photon detector as previously hereinbefore described. Such a photon detector array may be provided with a grid of bit-lines and word-lines, wherein each pixel is addressable by applying an appropriate voltage to a word-line and/or a bit-line. Preferably, the bit-lines and word-lines are configured to apply a control signal to the means for separating a photon-excited electron-hole pair.

In a second aspect, the present invention provides a method of operating the device of the first aspect of the present invention, the method comprising the steps ofilluminating the device with a beam of radiation to excite at least one electronhole pair such that at least one carrier becomes trapped in the second active layer; detecting a change in the transport characteristics in the first active layer.

Preferably, a external field will be supplied to separate the electronhole pairs. This, in some circumstances, will be provided by the internal field of the device.

The device of the first aspect of the present invention may also be used as a memory device which is sensitive to data delivered by a single photon.

Recently, a semiconductor device which can be configured as a memory device has been developed by Shields et al. Appl Phys Lett Vol 74, pp735 (1999) and UK patent application number 9820192.4.

The memory de-vice of the above UK patent application uses a large number of quantum dots. In the example given in the accompanying paper, over a hundred million dots were used. Such a device is inappropriate for single photon operation.

For a write operation, the device is illuminated with a light energy of near to that of the quantum dot band gap. An electron-hole pair will be excited and one of the charged carriers will be stored in the quantum dot.

For a read operation, the device is again illuminated with light energy of near the quantum dot band gap, but now with a weaker intensity. If two holes have already been stored in the dot during the write operation, another hole cannot be stored. Hence, there will be no further change in the conductivity of the 2DEG. If no holes are trapped in the dot, a change in the conductivity of the 2DEG will be detected.

For both read and write modes of the first mechanism, the conduction band edge of the 2DEG lies below the first confined conduction band level of the quantum dot. Thus, electrons transfer from the dot to the 2DEG, as the 2DEG is energetically more favourable. To reset the device, the bias across the device is changed so that the energy of an electron in the 2DEG lies above that of the conduction band level of electrons in the dot. Thus, electrons are transferred to the quantum dots which can then combine 12 with the holes. This resets the device as the electron-hole pairs relax back to the ground state.

To produce a memory structure with a good retention time, it is preferable -if there is a large confining potential for the trapped carriers. This is equally applicable to devices which are believed to operate by either the first or the second mechanism. The confining potential is largely dependent on the characteristics of the first barrier layer. For the avoidance of doubt as used hereinafter, a barrier layer is a layer with a larger band-gap than that of the active layers.

ro produce a large confining potential, it is advantageous to maximise the carrier potential discontinuity between the quantum dot and the barrier layer. The 'size' of the confining potential is dependent on both the potential height of the barrier layer and the width of the barrier layer itself. A large barrier is taken to mean a barrier which has a large carrier potential discontinuity with respect to the quantum dot layer and/or a barrier which is relatively wide.

For example, if the holes are stored in InAs quantum dots, a large trapping potential can be created by choosing AlAs as the barrier material both above and below the dots.

The electric field normal to the layers can be varied to modulate the band structure of the device.

More preferably, the first and second active layer are coupled layers. For the memory structure, the layer are preferably weakly coupled. The coupling between the layers needs to be sufficiently strong to allow tunnelling of some carriers from the second active layer to the first active layer. However, the coupling must also be sufficiently weak to suppress the tunnelling of carriers from the first active layer back to the second active layer.

13 The quantum well can be thought of as a sheet of charge located within the first active layer. The position of the quantum well within the first active layer, with respect to the adjacent layers of the first active layer is dependent on the band structure. In order to achieve sufficiently weak coupling between the quantum well and the plurality of dots, it is preferable if the separation between the quantum well and the dots is between 10 nin and 500 nm.

For example, if the plurality of quantum dots are InAs (or AlInAs) and the first active layer is InGaAs (or GaAs), the tunnel barrier layer could be AlAs or Al.,Gal-.,As (or GaAs) with a width of between 10 nm and 500 nin. More preferably between 10 nm and 200 rim.

A particularly useful example of the device is provided by a InyAl1yAs/InGa1,As system. This system allows the first active layer to be fabricated from InGaAs and a barrier region, comprising a InAlAs layer provided adjacent the first active layer. The barrier region is preferably the first barrier layer. The large conduction band discontinuity between InGaAs and InAlAs allows the device to operate at much higher temperatures than those of the other systems previously referred to in this document. The conduction band discontinuity where x = 0.53 and y = 0.52 has been measured between 500 and 550 meV. The system also had advantages in that it results in a low Schottky barrier height and a high mobility.

As previously explained, the device preferably comprises an absorption layer. This absorption layer is typically provided as one of the barrier layers and is preferably provided outside the active region of the device i.e. it would not be placed between the first or second active layers. In a particularly preferred configuration, the absorption layer is provided on the opposing side of the second active layer to the first active layer and even more preferably directly in contact with the second active layer.

A InGaAs absorption layer has a lower band gap than GaAs. Therefore, this absorption layer is able to absorb radiation further into the infrared region. For example, 14 In0.53Ga0.47As can absorb at the important wavelengths of 1.3 and 1.55 microns. These wavelengths are commonly used for fibre optic communication. In such systems, the dots will typically comprise InAs. Any Indium containing absorption layer is advantageous for example, the absorption layer could be formed from InGaAs, InGaAlAs or InGaAsSb.

The above system does not have the same lattice constant as a GaAs substrate. Therefore, the preferred substrate is InP. This will allow the growth of a InO-53Gao.47As/Ino.52AI0.48As structure without lattice strain.

However, it is not mandatory to use a lattice ma't(,hed ziabstrate. For example, it is possible to form the above system on a GaAs substrate or any other substrate for that matter if means are provided for lattice matching the lattice constant of the second active layer with that of the substrate. Such means may comprise a compositionally graded buffer wherein the composition of the buffer is graded such that the lattice constant of the buffer matches that of the substrate at its lower interface and that of the first active layer at its upper surface. Such a compositionally graded buffer may comprise In,,Gal-,As where W changes from W = 0 to W = X = 0.52 gradually throughout the buffer layer. This gradual changegradually alters the lattice constant from that of the substrate to that of the second active layer. Other compositionally graded buffer layers could be used, for example, AlGaAs,,,Sbl, where x is varied.

The use of a lattice matching means such as a compositionally graded buffer allows a free choice of the indiurn content in the first active layer. This is because the lattice constant of the compositionally graded buffer can be tuned to any desired value.

It is also possible to use a strain relaxed buffer layer, for example a quartenary such as AlGaAsSb. Again, the strain relaxed buffer layer can be used to provide any lattice constant for subsequent growth. The composition of the quartenary can be varied in order to match the lattice constant of the first active layer. However, it should be noted that the composition of the quartenary does not change in the same way as described for the compositional graded buffer layer. The strain relaxed buffer layer can accommodate dislocations.

The width of such a buffer layer is typically at least I [im.

The structure may be provided with upper and lower cladding layers to channel light in a direction parallel to the plane of the first active layer. The structure may also be provided with guide means to confine light to a region of the first active layer, for example, a strip type waveguide could be used.

The device may also be illuminated in the plane of the active layers instead of or in addition to illuminating generally perpendicular to the layers.

The memory device could also be used as a holographic type of optical memory device. Here, the optical beam is split into a signal beam and a reference beam. The signal beam is passed through a spatial light modulator in order to encode the information to be stored. The signal and reference beams are focused onto the surface of the sample where they produce an interference pattern. This creates a spatial variation in the carrier occupancy of the dots which acts to store information in the dots. The information can be recalled by illuminating the same area of the sample by the reference beam. The stored variation in the dot occupancy acts to diffract the reference beam and thereby to recover the signal beam which is detected by a suitable means, such as a charge coupled device array.

Therefore, a memory structure may be fabricated comprising a plurality of pixels, each pixel containing a memory device as described above. Preferably, the memory structure comprises a plurality of bit-lines and word-lines for selectively addressing each pixel. More preferably, the bit-lines and word-lines are configured to apply a control voltage for the separating means for separating a photon-excited electron-hole pair.

16 The present invention will now be described a way of example and with reference to the accompanying figures in which; Figure I shows a schematic band structure or part of a device in accordance with an embodiment of the present invention; Figure 2 shows a plan view of a device in accordance with an embodiment of the present invention; Figure 3 shows a plan view of a device in accordance with another embodiment of the present invention; Figure 4 shows a layer structure of a single photon detector in accordance with the present invention; Figure 5 shows a schematic band structure of the device of Figure 4; Figure 6 shows a variation on the device of Figure 4; Figure 7 shows yet another variation on the layer structure of Figure 4; Figure 8 shows a schematic band diagram of the device of Figure 7; Figure 9 shows another variation on the layer structure of the device of Figure 4; Figure 10 shows a schematic band structure of the device of Figure 9; Figure 11 shows a detection circuit incorporated in a device in accordance with an embodiment of the present invention; Figure 12 shows a variation of the detection circuit of Figure 11; I Figure 13 shows a simplified plan view of the device in accordance with an embodiment of the present invention; Figure 14 shows results taken from the device of Figure 13; Figure 15 shows possible layer structures of fabricating the single photon detector of the present invention; Figures 16A and 16B show a device in accordance with an embodiment of the invention fabricated from Silicon; Figure 17 is a schematic band structure of the first active layer region of the device fabricated using the InAlAs/InGaAs system; Figure 18 is a schematic layer structure of a device fabricated using the InAlAs/InGaAs system; Figure 19 is a variation of the device of Figure 18 fabricated on a GaAs substrate; Figure 20 is a further variation on the device of Figure 18; Figure 21 shows a single photon detector in accordance with an embodiment of the present invention being used in a selectively addressable optical memory; Figure 22 is a plot showing the contribution of and individual quantum dot to the absorption of the ensemble; Figure 23 shows a band structure of the embodiment of Figure 2 1, in a "write mode"; Figure 24 shows a band structure of a device of Figure 21, in a "read mode"; 18 Figure 25 shows a band structure of the device of Figure 21, with the conduction band edge of the 2DEG below the first conduction band level of the quantum dot; Figure 2 6 shows a band diagram of the device of Figure 2 1, where the conduction band edge of the 2DEG is located higher than the first confined conduction band level of the quantum dot; Figure 27 shows a MODFET in accordance with an embodiment of the present invention; Figure 28 shows a layer structure of the device band structure of Figure 27; Figure 29 shows a heterojunction bi-polar transistor in accordance with an embodiment of the present invention; and Figure 30 shows a band structure of the device of Figure 29.

Figure I shows a schematic partial band diagram of a single photon detector having a quantum dot layer 1, a quantum well or 2DEG layer 3. The quantum dot layer I and the 2DEG layer 3 are separated by a tunnel barrier 5. The device can be irradiated with illumination of almost any wavelength. When the device is illuminated, a single photon will excite an electron-hole pair. The probability of absorption of an electron-hole pair at any point in the device will depend on the wavelength of the incident radiation, the specific band structure of the device and the position of the illuminated surface of the device with respect to the 2DEG and/or dot layer.

The layers inside the device are subject to electric fields. This can either be an internal electric field due to the internal layer structure of the device or it may be an applied electric field. The electric field causes the photo-excited electron hole pair to separate. Dependent on the band structure and applied electric field, the electron will either

19 become trapped in the conduction band of the quantum dot, or the hole will become trapped in the valence band of quantum dot 1. The presence of a trapped carrier in the quantum dot causes a change in the characteristics of the 2DEG layer 3. Hence, the change in the characteristic of the 2DEG provides a means for detecting a photon.

This discussion will now concentrate on detecting a transport characteristic of the 2DEG. However, it will be appreciated by those skilled in the art that an optical change in the characteristics of the 2DEG could also be used to measure single photons.

It has been mentioned above that the device can be illuminated with radiation of virtually any wavelength. The number of carriers excited in each of the barriers, wells and dots depends on the incident photon flux, the absorption coefficient of the layers and their thicknesses. While not wishing to be bound by any theory, it is believed that the following mechanisms contribute to photon detection by the device.

If the radiation has a photon energy higher than the barrier layer 5 or the second barrier layer 7, then electron hole pairs are photo-excited in either the barriers 5 or 7, the quantum well 3 or the dot 1. The internal electric field of the device, or the applied electric field, disassociates the electron hole pair and sweeps them in opposite directions, one type of carrier (either the electron or the hole) will be trapped by the dot and hence produce a change in the transport characteristic of the 2DEG.

If the photon energy of the incident radiation is lower than the band gap of the barrier layers 5 or 7, but higher than the band gap of the 2DEG 3 and also higher than the bandgap of the dot layer 1, then electron hole pairs are excited in both the quantum well layer 3 and the dot layer 1. Essentially no carriers will be excited in the barrier. Since the absorption co-efficient of the quantum well 5 is typically higher than that of the dot layer 1, most of the carriers will be excited in the quantum well. Due to the internal field or the applied electric field, one of the carriers (i.e. either the electron or the hole) can tunnel into the quantum dot 1, leaving the other carrier in the quantum well 5. This causes a change in the characteristic of the 2DEG in the quantum well 3.

If the photon energy of the incident radiation is less than the band gap of the 2DEG (and also the barrier layers) but it is larger than the band gap of the quantum dot 1, then electron hole pairs are excited in the quantum dot 1. One of the carriers can tunnel into the 2DEG layer leaving the other carrier trapped within the dot 1. This also causes a change in the characteristic of the 2DEG 3.

The device can also detect illumination which has a photon energy lower than the band gap of the quantum dot 1, if the quantum dot I is designed to contain excess charge. In this situation, a single photon can excite excess charge out of the dot I by an intra-band transition. It is then swept away frorii the quantum dot I by applied electric field or an internal electric field. This will also lead to a change in the characteristic of the 2DEG3.

It should be understood that the quantum well can support either a two dimensional electron gas or a two dimensional hole gas. The polarity of the trapped carriers in the quantum dot I can either be the same as that of the two dimensional carrier gas or it can have a different polarity. Regardless of the relative polarities of the carriers trapped in the dot and the carriers in the two dimensional carrier gas, carriers trapped in the dot still affect the characteristics of the two dimensional carrier gas. The applicant does not wish to be bound by a particular theory or express explanation. However, it is believed that the device can operate via two different mechanisms. In the first mechanism, carriers in the quantum well have an opposing polarity to the carriers which are stored in the dots 1. In the second mechanism, the carriers in the quantum well and the carriers stored in the dots are the same type.

The first mechanism will be explained using holes with a stored carrier. However, it will be apparent to a person skilled in the art that either electrons or holes can be stored in the quantum dots depending on the layer thickness and composition, doping polarities (n- or p-type) and applied biases.

21 In this situation, it is believed that the positive charge stored in the dot 1, alters the band bending and hence persistently alters the characteristics (for example the conductivity of the two dimensional electron gas).

In the second mechanism the carriers stored in the dots are of the same type as the carriers stored in the well. It is believed that the dot I contains excess carriers prior to illumination. The charged dots act as scattering centres for the 2DEG which consequently has a relatively low mobility. After illumination, the number of excess carriers in the quantum dots increases and the number of electrons in the 2DEG increases. A decrease in the negative charge of the dots results in an increase in the 2DEG conductivity. Also, an increase in the carrier concentration of the 2DEG causes an increase in the conductivity of the 2DEG.

When a transport characteristics of the two dimensional carrier gas is measured and where the electric field is applied via gates to the device, there are two prefer-red gate designs. Figure 2 shows the preferred gate for the case when illumination leads to a decrease in the resistance (or an increase in the conductivity) of the two dimensional carrier gas. The active area of the device is defined by a small full gate 11. The gate is contacted by gate contact 13. To fabricate this device, the layers to produce the quantum dot layer and the active layer are grown in the way described later. The layers are then etched to form a mesa 15. The shape of the mesa is such that four-terminal resistance measurements can be made. Four ohmic contacts are shown in the figure, ohmic contacts 17 and 19 are the source and drain. Ohmic contacts 21 and 23 are voltage probes which allow a four-terminal measurement to be performed. Of course, it will be understood by a man skilled in the art that these two voltage probes 21 and 23 can be omitted to allow a two-terminal resistance or conductivity measurement. The gate I I is then formed over the mesa 15. The overlap area of the gate I I and the mesa 15 defines the active area of the device.

Figure 3 shows a plan view- of the device structure which is preferably used when illumination of the structure leads to a increase in the resistance of the carrier gas (or 22 decrease in the conductivity). In this situation, the gate shown is not a full gate. Instead, it is a split gate 31 is defined on the top surface of the structure. As explained for Figure 2, a mesa 15 is formed by etching the dot layer and preferably the 2DEG layer. Source and drain contacts 17 and 19 are provided on either side of the mesa and voltage probes 21 and 23 are provided to allow four-terminal resistance measurements.

In Figure 2, the active area is defined by the region of the device where the gate I I overlapped with the mesa 15. This is not the case here. The active area of the device is a small conduction channel which is defined in the two dimensional carrier gas by the depleting split gates 3 1. For this device, the electric field which separates the photoexcited electrons and holes may also be supplied by biasing a back gate ccntacct ( not shown in Fig. 3) with respect to an Ohmic contact to the carrier gas, or by the internal field due to the back gate. The trapped photo- excited charge in the quantum dots may also be removed by applying a bias to the back gate.

It will be apparent to those skilled in the art that a narrow channel in carrier gas may also be defined in a number of other ways. For instance photolithography could be used to etch the carrier gas layer so as to define a narrow conduction channel. Alternatively a narrow channel may be defined by damaging regions of the carrier gas layer outside the channel by ion-bombardment.

Figure 4 shows an example of a semiconductor layer structure for a single photon detector. An absorbing barrier layer 303 is formed on an upper surface of a P+ substrate 305. A layer of quantum dots 307 is then formed on an upper surface of the absorbing layer. A first barrier layer 309 is formed on an upper surface of the layer of dots 307. A layer capable of supporting a two dimensional electron gas 3 11 is formed on an upper surface of the barrier layer 309, an upper barrier layer 313 is then formed on the upper surface of the 2DEG layer 311. The upper barrier layer is a modulation doped barrier layer comprising an undoped barrier layer 315 formed overlaying the 2DEG barrier layer 311 and a doped barrier layer 317 formed overlaying the undoped barrier layer 315. A capping layer 319 overlies the structure. On top of the capped layer is formed a front- 23 gate:321. This gate needs to be able to pass radiation of certain wavelengths. Typically, the gate is made from a thin layer of NiCr with a thickness of about 8nin. The gate may also be provided by a doped semiconductor layer. A back gate contact 323 is then formed on the p+ buffer/substrate layer 305. The p+ back gate 305 and the front gate 321 serve as a means for applying the an electric field. A source ohmic contact 325 and a drain ohmic contact 327 are made to the 2DEG 311 in the conventional manner. The back gate 305 and front gate 321 may be biased with respect to an Ohmic contact 327 to the 2DEG.

Figure 5 shows the band structure of the device of Figure 4. For clarity, the same reference numerals are used in figures 4 and 5. The devicc. zan be illuminated from the front (i.e. through the gate 321) or from the back. When the device is illuminated from the back with light with a photon energy larger than the bandgap of the substrate, part of the substrate within an active window defined by lithography should be removed by etching. When the device is illuminated, an electron hole pair is photo-excited in the absorbing layer 303. Due to the electric field induced between the two gates, the electron is swept into the quantum dot 307 and the hole is swept towards the P+ buffer/substrate 305. The electron will become trapped in the dot. For the band structure shown here, the trapped electron in the dot increases the resistivity of the 2DEG 311. It can either reduce the 2DEG density and/or the mobility. To monitor this increase in resistance due to single electrons, the structure is preferably provided with a split gate (as shown in Figure 3).

If the device is used with a split gate, then the device requires a backgate to modulate the field across the layers (i.e. the field to separate the photo-excited hole pairs) or the band structure of the device needs to be configured such that there is an internal electric field.

Figure 6 shows the structure of Figure 4 inverted. For clarity, the same reference numerals will be used as those for Figure 4. A substrate forms the base of the device 305. A P+ back gate layer is formed overlying the upper surface of the substrate 305.

24 An undoped layer 308 is formed overlying the back-gate 306. An undoped barrier layer 3 10 is formed overlying the undoped layer 306. A modulation doped barrier layer 313 comprising a doped barrier layer 317 and a spacer layer 315 formed overlying the doped barrier layer is formed overlying the undoped barrier layer 3 10.

A 2DEG layer 311 is formed on an upper surface of the spacer layer 315. A first barrier layer 309 is formed overlying the 2DEG layer 311. A dot layer 307 is formed overlying the barrier layer 309. An absorbing layer 303 is then formed overlying the dot layer 307. Here, a p+ front gate is provided 33 1. Overlying the p' front gate 331 is formed an antireflective coating 333. This anti-reflective coating can be formed on the illuminated surface of any of the dcvices discussed in this application.

As described with reference to Figure 4, a source ohmic contact 325 and a drain ohmic contact 327 are made to the 2DEG layer 311. This device will work in the same way as Figure 4 as described with reference to Figure 5.

Figure 7 shows a variation on the structure of Figure 4. Here, an undoped barrier layer 407 is provided overlaying a buffer/substrate 405. A modulation doped barrier layer comprising a doped barrier layer 409 and an undoped spacer layer 411 overlying the doped barrier layer 409 is provided overlying barrier layer 407. A 2DEG layer 311 is provided overlying the modulation doped barrier layer 411, 409. A first barrier layer 309 is provided overlying the 2DEG layer 311. A quantum dot layer 307 is provided overlying the first barrier layer 309. An undoped spacer barrier layer 413 is provided overlying the dot layer 307 and a doped barrier layer 415 is provided overlying the spacer layer 413. A capping layer 417 is provided overlaying the doped barrier layer 415. A gate contact 321 is provided overlying the capping layer 417. Source and drain ohmic contacts 325 and 327 are made to the 2DEG layer 311 in the same manner as with Figure 4. This device does not have a back gate. The means for separating the photo-excited electron hole pairs will either be provided by the front gate 321 or by the internal field inherent in the device.

Figure 8 shows a band structure of the device of Figure 7. For easy comparison, the reference numerals have been kept the same. In the device with the biasing conditions shown in Figure 8, excess electrons are trapped within the quantum dot 307. If the device is illuminated with radiation with a photon energy greater than that of the band gap of the quantum well 311, electron hole pairs are photo-excited in the quantum well 311. Photo-excited holes tunnel from the quantum well into the dots. These photoexcited holes recombine with the excess electrons already present in the dot 307 reducing the total number of trapped electrons in the dots. The lowering of the number of electrons in the dot 307 decreases the resistance of the 2DEG 311. Therefore, this device will ideally be used with the small full gate structure of Figure 2.

It should also be noted that if the photon energy of the illuminating radiation is larger than the band gap of the barrier layers, photoexcited holes will be captured by the dots. These will also recombine with the excess electrons reducing the total number of electrons in the dots and decreasing the resistance of the 2DEG 311.

If the photon energy of thexadiation is smaller than the bandgap of the quantum well 311, it will tend to photo-excite electrons trapped in the dot 307 into the surrounding barrier layers. The electric fields in the device will tend to sweep these liberated electrons away from the dot 307. The reduction in the negative charge in the dot causes a decrease in the resistance of the 2DEG 311.

Figure 9 shows a variation on the device of Figure 4. For the sake of clarity, the same reference numerals will be used for Figure 9 (and its accompanying band structure which is shown in Figure 10). The relative positions of the 2DEG layer 311 and the quantum dot layer 307 have been interchanged so that the dot layer 307 is grown after the 2DEG layer 311. Hence, in order of growth; the layers follow the pattern of a 2DEG layer 3 11 being formed on the upper surface of the absorbing layer, a barrier layer 3 09 is. formed on an upper surface of the 2DEG layer 311 and a dot layer 3 07, is formed on the upper surface of the barrier layer 309.

26 Figure 10 shows a schematic band structure of the device of Figure 11. Again to aid easy comparison, the same reference numerals will be used as for Figure 11. Again as in Figure 10, the conduction band-edge of the 2DEG layer 3 11 and the quantum dot layer 307, both lie below the Fermi level and hence, they both have excess electrons.

The device of Figure 9 can work in both ways. Electrons or holes can be trapped in the quantum dot 307. The absorbing layer increases the absorption and the quantum efficiency of the device. This device can either be used with the full gate of Figure 2 or the split gate of Figure 3 depending on whether the device is configured to increase or decrease the resistance of the 2DEG 311.

Figure I I shows a detection circuit using the detection device previously described. The detection device 501 is controlled by a control voltage applied to point 503 which is connected to the front gate of the device 501. The conductivity of the first active layer of the device is measured by passing a current from source 505 through the first active layer of device 501.

The measured potential will increase or decrease due to the detection of single photons. The circuit shown is designed to detect this change in the resistance of the first active layer501. The signal 507 from the first active layer is passed through a low pass filter arrangement comprising a capacitor 509 in series with the device 501 and a resistor 511 in parallel with the device 501.

The high pass filter arrangements acts as a differentiator. The output of the high pass filter arrangement is a plurality of spikes, each spike corresponding to a single photon. The output from the high pass filter unit is fed into a pulse counter to count the number of photons detected. The pulse counter (not shown) will be provided with a discriminator which will allow the pulse counter only to count peaks higher than a fixed level. This allows the counter to discriminate between spikes due to photons and noise.

27 Figure 12 shows a more sophisticated version of the detection circuit of Figure 11. The signal derived from output 507 is passed into differentiating circuit 521. The differentiating circuit comprises an op amp 523 with a resistor R2 and a capacitor C2 connected in parallel with the op-amp 523. The capacitor C2 and the resistor R2 are also connected in parallel with one another. A resistor RI and a capacitor Cl are connected in series with one another and between the inverting input of the op-amp and the device 501.

The basis differentiating circuit is provided by the op-amp 523 and C I and R2. Differentiators tend to be unstable at high frequencies, therefore, components R2 and C I are provided to block high frequency components. Again, as for Fig-= 11, the output of the differentiator is a series of pulses, each pulse being caused by a single photon. The output of the differentiator is read into a pulse counter.

Figure 13 shows the device of the present invention configured for single photon detection. In this specific geometry the mesa has been etched with a narrow central portion which has a width of 2 microns. NiAuGe Ohmic contacts (not shown) are annealed to the first active layer. A constant current I is allowed to flow between the source and drain contacts and the voltage between Ohmic contacts 201 and 203 to the first active layer is measured. A gate contact layer 205 consisting of a 7nm thick NiCr layer is defined intersecting the centre of the mesa. The width of the gate 205 is 8 microns, thus defining an active area of 2 x 8 microns, which will cover roughly 3000 quantum dots.

Figure 14 shows results from the device of Figure 13, the detection of single photons can be seen in the data. The source drain resistance of the first active layer (measured between points 201 and 203 in Figure 13). As the device is illuminated, the resistance is seen to drop as the sample is illuminated. During the course of this experiment, the sample was under constant illumination. The resistance of the first active layer drops due to electrons tunnelling from the second active layer into the first active layer. A single electron tunnels when it is excited by a photon. The steps 207 indicated on the 28 data show the effects of discharging by a single dot due to a single photon. This figure proves the single photon detection.

The actual layer structure of the device from. which the results in Figure 14 were taken is listed in the table. The layers are listed in order of growth by MBE on a (100) oriented GaAs substrate.

Material thickness (=)doving substrate temp GaAs 500 undoped 5900C AIO.33GaO.67As 250 undoped 590 AIO.33GaO.67As 40 Si IM crin-3 AlO.33GaO.67As 15 undoped 490 AIO.33GaO.67As 25 undoped 590 GaAs.20 undoped 590 AIO.33GaO.67As 10 undoped 590 GaAs 2 undoped 590 InAs 1.7 monolayers undoped 520 AIO.33GaO.67As 10 undoped 520 AIO.33GaO.67As 30 undoped 590 AIO.33GaO.67As 40 Si loll cm-1 590 GaAs 10 undoped 590 The substrate temperature is lowered during the growth of the InAs layer to facilitate the fon-nation of quantum dots via the Stranskii-Krastinow growth mode. The substrate temperature must be 530 T or less so that Indiurn does not segregate above the growth surface. For this structure 1. 7 monolayers of InAs were deposited. However, a higher dot density can be achieved by depositing slightly more Indium (1.7 - 3 monolayers). This would be advantageous for increasing the storage capacity of the memory. It is preferable for the layer grown immediately after the quantum dots is also grown at a lower temperature (520 'C) to prevent Indiurn segregation destroying the dots. The growth temperature can be raised again after 10 nm of growth. The 2nm. of GaAs grown 29 before the InAs layer were deposited in order to smooth the growth surface. The lower Si doped layer and its overlayer were also grown at a lower substrate temperature toprevent the Si impurities segregating and thus being incorporated into subsequently grown layers.

The wafer was etched into a mesa using standard photolithographic techniques. NiAuGe Ohmic contacts to the electron gas were deposited and annealed. In addition to the source and drain contacts at the ends of the mesa. A semitransparent NiCr layer (7mn thick) was evaporated over a central portion of the mesa to act a Schottky front contact.

Figure 15 shows typical layer structures which may be used to fabricate a photon detector in accordance with an embodiment of the present invention. There are many methods for producing quantum dots. For example, they can be produced by photolithographic techniques, electron lithography methods, etc. A preferable method is to use the Stranski-Krastanow growth mode. When a first layer is grown on top of a substrate which has a different lattice constant from the first layer, the first layer proceeds by island growth. In other words, 3D islands are formed over the top of the substrate, which when capped with a barrier material produce a plurality of quantum dots. In Figure 15a, a doped barrier layer 41 of AlGaAs is formed on a substrate or buffer layer 43. An undoped spacer layer is then formed on an upper surface of the doped barrier 41. This undoped spacer layer 45 and the doped barrier layer 41 together form a modulation doped barrier layer. A 2DEG layer 47 of GaAs is formed overlying the undoped spacer layer 45. An undoped AlGaAs layer 49 is fortned overlying the GaAs 2DEG layer 47. A doped layer 51 is then epitaxially growth on an upper surface of the barrier layer 49. This layer is InAlAs which has a substantially different lattice constant from AlGaAs. Thus, a plurality of quantum dots are formed. The structure is then finished with a barrier layer of AlGaAs 53.

A schematic band structure for Figure 15a is shown on the right of the diagram, carriers can tunnel from the 2DEG layer 51 through the tunnel barrier 49, to the 2DEG layer 47 and vice versa.

Figure 15b shows a similar structure to that of Figure 15a. For comparison, the layers are numbered as Figure 15 a. Here, an AlAs (or AlGaAs) tunnel barrier layer 49 is present between the dot layer 51 and the 2DEG layer 47. The schematic band structure shows that the tunnel barrier 49 is higher in this case than the structure in Figure 15a. The AlGaAs tunnel barrier layer 49 has a different Al content to layer 49 in Figure 15a or layer 41 and 53 in Figure 15b.

Figure 15c shows a variation on the structure of 15a. The barrier layers 41, 49 and 53 and the space layer 45 are GaAs and the 2DEG layer 47 is InGaAs. The dot layer 51 is formed from either InAlAs or InAs.

Figure 15d shows a slight modification of the structure in Figure 15c. Here, the tunnel barrier 49 is either AlAs or AlGaAs. This provides a larger tunnel barrier 49 as can be seen by comparing the band structures of Figure 15c and 15d.

Figure 15e again shows a slight modification on the structure as shown in Figure 15c. Here, the doped barrier layer 41 and undoped spacer layer 45 are both AlGaAs. These layers have a larger band gap than GaAs. Therefore, it can be seen in the band structure of Figure l5e that the conduction band edge is higher for these layers.

Figure 15f shows a further modification on the structure shown in Figure l5e. Here, the tunnel barrier 49 is also AlGaAs. The band structure shows a larger tunnel barrier 49 than that shown for the structure of Figure l5e.

Also, the structure can be made using an Si/Si02 based system. Silicon provides an attractive material system for fabrication of the single photon detector because it is a relatively mature technology and cheap.

31 An example of such a structure is shown in Figure 16. First a thermal oxide 803 is formed on the Si wafer 801 using well know techniques. Next an amorphous layer of Si 805 is deposited on the oxide using UHV-CVD for instance. Silicon quantum dots are formed from this layer by annealing the sample at 800 oC under high vacuum conditions. Alternatively other semiconductor materials, for example Germanium, can be used to form the quantum dot layer in a similar manner. After this a further Si02 807 is deposited on top of the quantum dots.

The wafer is processed into field effect transistor structures as shown in Figure 16B. First a semitransparent gate metal 809 is defined on a region of the top surface uc---.9 well known lithographic techniques. The Si02 in areas outside of this gate layer 809 are etched so as to expose the Si wafer beneath. N-type ohmic contacts 811 are formed to the exposed Si region on either side of the gate/oxide region.

t A particularly useful material system for fabricating the device is the ln,Gaj,As/InyAlj-yAs system. The conduction band discontinuity AE, 601 (shown schematically in Figure 17) is much larger for this type of system than for GaAs/AIO.3Ga.0.7As. A value of between 500 and 550 meV has been measured for the situation where x--0.53 and y--0.52. The larger band discontinuity results in a larger potential barrier between the first and second active layers. Therefore, it is less likely that carriers can be thermally excited between the first and second active layers. Hence, the operating temperature of the device is increased. In Figure 17, the second active layer 603 is located between the first barrier layer 605 and barrier layer 607.

Figure 18 shows a schematic layer structure of a device fabricated using the InGaAs/InAlAs system. The structure is as follows:

An InyAll.yAs doped (n+) growth initiation. layer 611, having a thickness of 300nm is formed overlying and in contact with an InP substrate 609.

32 0.5 microns of Ino.53GaO.47As doped buffer layer 613 is formed overlying and in contact with the growth initiation layer 611. An n+ Ino.53Gao.47M back-gate layer 615 having a thickness of 0.2 microns is formed overlying and in contact with said buffer layer 613. In some cases, a doped buffer layer will be grown, this avoids the need for the extra back-gate layer 615.

Photon absorption layer 617 is then formed overlying and in contact with said back-gate 615. The photon absorption layer comprises Ino.53Gao.47As and has a thickness of 1.5 microns. The band gap of Ino.53Gao.47M allows the absorption of photons at the important wavelengths of 1.3 and 1.55 microns which are commonly used in fibre optic communications. A quantum dot layer 619 which comprise-z a fe- -, monolayers (e.g. 3) of InAs is then deposited overlying the absorption layer 617. The quantum dot layer 619 is strained so that self-organised dots are formed.

An InAlAs barrier layer 621 is then formed overlying and in contact with said quantum dot layer 619. The barrier layer in this example of a thickness of 10 rim. Quantum well layer 623 is then formed overlying and in contact with said barrier layer 62 1. The quantum well layer comprises 20 nm of InO.53GaO.47As. An undoped spacer layer of Ino.52AI0.48As 625 is then formed overlying and in contact with said quantum well layer 623. A doped barrier layer having a dopant density of 10 1 8 cm 3 and a thickness of 40 rim is formed overlying and in contact with said spacer layer 625. The structure is finished with a InO.53GaO.47As cap layer 629 having a thickness of 10 rim which is formed overlying the doped barrier layer 627.

A variation on the structure of Figure 18 is shown in Figure 19. To avoid unnecessary repetition, where possible, like numerals denote like features. The structure is identical to that of Figure 18 from layer 615 upwards. However, the structure is formed on a GaAs substrate 609. Such a substrate is not lattice matched to the first active layer 623. A growth initiation layer 611 of GaAs is formed overlying the substrate 609. The layer may be doped such that it can act as a contact to a back gate. A graded buffer layer comprising In,,Gal-,,As is formed overlying and in contact with the growth initiation 33 layer 611. The value of W of the buffer layer 613 changes from 0 to x (where x is the indium. content in the second active layer 623) throughout the buffer layer. Therefore, lattice matching is achieved. It is possible to grow layers one on top of one another without fulfilling lattice matching requirements. However, without lattice matching dislocations would be set up within the structure which would seriously degrade the mobility of the second active layer 623. Potentially, such dislocations could render the device completely useless.

In this particular example, W changes uniformly from 0 to 0.53 over a thickness of 2 microns. A further growth of 0.5 microns InO.53GaO.47As is then performed to complete the buffer layer. The graded buffer layer 612 is formed on the growth initiation layer 611 to ensure good growth. Growth directly onto the substrate usually results in a high dislocation density.

Figure 20 shows a further variation on the structure of Figure 18. Again, where possible, like numerals denote like features. In the same manner as Figure 19, the layers from the back-gate 615 upwards are identical to those of Figure 18. Also, the substrate, 609, is GaAs. Here, two growth initiation layers, the first being 5 nm of AlAs and 35 mn of AlSb are grown by molecular beam epitaxy at 500'C. A strain relaxed quaternary buffer layer 613 is then formed overlying and in contact with growth initiation layer 611. The strain relaxed layer comprises 2 microns of Alo. sGaO.5AsO.55Sbo.45- Again, as in the case as Figure 19, the buffer layer 613 can be used to allow growth of the InGaAs/AlInAs heterostructure on a GaAs substrate.

The structure can be processed as described in relation to Figure 14. However, it would be appreciated by those skilled in the art that variations on the structure could be used in order to aid processing. For example, contact must be made to the active layers and also to the back gate. The back gate can be formed by a regrowth method in which the backgate is partially etched so that it is completely removed from certain parts of the structure. The remaining layers (617 onwards) are then grown overlying the back-gate. In reality, probably growth initiation layers and/or buffer layers will be formed overlying 34 the back-gate in order to provide a smooth growth interface. This allows the use of deep ohmic contacts, for example, NiGeAu, to be used to make contact to the active layer 623. If these contacts are made in a region where the back-gate is etched away, there is no danger of these contacts shorting to the back-gate. Contact can then be made to the back- gate well away from the active area of the device so that the active layer can be etched away above the back gate.

Ion-beam damage, which destroys the conductivity of the back-gate within certain regions, could also be used to facilitate the use of deep ohmic contacts.

Alternatively, ap-iypeback-gate could be used. Such agate could not be contacted by the n-type contacts which would preferably be used to contact the quantum well. Such a gate would require p-type contacts, for example, BeAu contacts. BeAu contacts would not contact the second active layer 623 if the second active layer was n-type.

Alternatively, a shallow ohmic technique could be used to contact the second active layer 623. Typical shallow ohmic contacts comprise PdGe. Such contacts. would not short to the back-gate. Contact could then be made to the back-gate well away from the active area of the device. The quantum well layer could be removed by etching from this region.

Figure 21 shows an outline structure of a selectively addressable detector, which can be used as an optical memory operating on single photon detection. A first active layer which is a 2DEG layer 3 is provided overlying a substrate or buffer layer 1. A barrier layer 5 is provided overlying the 2DEG layer 3. The barrier layer 5 has a larger band gap than the 2DEG layer 3. A second active layer comprising a plurality of quantum dots 7 overlies the barrier layer 5. An upper undoped barrier layer 8, is provided overlying the quantum dot layer 7. For simplicity, front and back gates or other means of applying an electric field across the device are not shown in this diagram. Two ohmic contacts 9 and I I are provided to the 2DEG layer 3. A current I can be measured between the two ohmic contacts.

For a "write" operation, a laser beam 13 is scanned across the sample. The laser beam 13 has an energy close to the quantum dot band gap, i.e. it is capable of exciting an electron-hole pair in a predetermined dot. If the laser beam illuminates such a dot, an electron 10 tunnels from the dot layer 7 through the tunnel barrier 5 to the 2DEG layer 3. This causes a change in the conductivity of the 2DEG.

The laser can be focused on an area of the device so as to excite all the dots in that area. Alternatively, the wavelength of the laser can be used to write to different dots within the laser spot size. The enlarged portion of Figure I shows part of the layer of dots 7 in detail. The dots are inhomogeneously broadened at each position. Thus, the plurality of dots comprise dots with different transitions energies. Dots with different transition energies can be individually addressed by changing the laser wavelength.

Figure 22 shows a graph of the absorption of the dot layer against laser wavelength. The dots have a plurality of different transition energies.

A possible mechanism to explain the operation of the device will be described with reference to the band structure in Figure 23. A field is applied across the device such that, in the write operation, the conduction band edge 21 of the 2DEG 23 lies below the first conduction band confined energy state 25 of the quantum dot 27. Also, the conduction band edge 29 of the 2DEG 23 lies below the first confined conduction band state 31 of the quantum dot 27. Upon illumination, an electron hole pair is excited in the quantum dot 27. The electron transfers to the quantum well at first excited energy state 25 and the hole is trapped in the first confined valence band state 3 1. The electron trapped in the first confined energy state 25 tunnels through the barrier 33 into the 2DEG 23. The hole in the first confined valence band state of 31 is trapped here as it is not energetically viable for it to transfer to the 2DEG 23.

The "read" operation is shown in Figure 24. The biasing of the structure remains the same as for Figure 3. Here, the device is illuminated but an electron/hole pair cannot be 36 separated as there is no free energy level to accommodate the electron hole pair. Therefore, an electron is not transferred to the 2DEG 23 and no change in the conductivity of the quantum well is observed by illuminating a dot which already contains holes.

Figures 25 and 26 shows how the excess charges in the quantum dot can be erased. In Figure 25, the conduction band edge 21 of the 2DEG 23 is at a lower energy than the first confined conduction band level of the quantum dot 27. Therefore, electrons trapped in the first confined conduction band level 25 tunnel to the energetically more favourable 2DEG.

Figure 24 shows the situation where the field normal to the active layers is adjusted so that confined level 25 of the quantum dot 27 is below the conduction band edge of the 2DEG 23. This results in electrons from the 2DEG 23 tunnelling through the tunnel barrier 33 into the vacant state 25. A dot located in the first confined valance band level 31 can then recombine with the electron in level 25. Once the holes trapped in level 31 have combined with electrons, the biasing can be returned to the levels shown in Figure 21 so that the excess electrons tunnel through barrier 33 back into the 2DEG 23.

The possible device operation described in relation to figures 19 to 22 has assumed that an undoped barrier layer overlies the dot layer 27. If barrier layer is doped, the observed external operation of the device is similar to that described above, i.e. a change in the conductivity of the 2DEG 23 is observed in response to a write operation. The applicant does not wish to be bound by a particular theory or explanation. However, it is believed that if an n-doped barrier layer is provided overlying the quantum dots 27, the quantum dots are populated with electrons prior to illumination. - These occupied dots 27 act as scattering centres for transport in the 2DEG 23. When the dots 27 are illuminated, the number of excess electrons in the quantum dots 27 are reduced and the number of electrons in the 2DEG 23 is increased. The increase in the carrier concentration of the 2DEG 23 causes the conductivity of the 2DEG 23 to increase. Also, the decrease in the negative change of the dots 27 results in an increase in the conductivity of the 2DEG 23.

37 Figure 27 shows another embodiment of the present invention used as a memory device. Here, the field normal to the active layers is modulated by a front-gate and a back-gate. Essentially, the structure is a field effect transistor. A p+ back-gate 77 is formed on an upper surface of a buffer layer or substrate 71. An undoped barrier layer 75 is formed overlying an upper surface of the back-gate 73. A modulation doped barrier layer 77 is formed overlying an upper surface of the undoped barrier layer 75. The modulation doped barrier layer has a doped barTier layer 79 and an undoped spacer layer 8 1. A quantum well layer is formed on the upper surface of the spacer layer 8 1. A tunnel barrier layer 85 is then formed on the upper surface of the quantum well layer 83. The dot layer 87 is then formed on an upper surface of the taruxl barrier layer 85. A capping layer 89 is then formed overlying the dot layer. Then an n+ front-gate 91 is formed on the upper surface of the capping layer 89.

Two ohmic contacts 93 and 95 which form a source and drain respectively are made to both the quantum well layer 83 and the dot layer 87. A backgate contact 97 is made to the back-gate 73 and a front-gate contact 99 is made to the front-gate 91. A bias can be applied between the front- gate 91 and the back-gate 73 so as to modulate the electric field normal to the quantum well layer 83 and the dot layer 87.

Figure 28 shows the band structure of the device of Figure 27. For simplicity, the layers in the band structure have retained the same reference numerals as for Figure 18. For the bias condition shown, the conduction band edge 10 1 of the quantum well layer 83 lies below that of the first excited state 103 of the quantum well layer 87 and the Fermi level of the system (Ef). Therefore, electrons in the quantum well form a 2DEG.

Decreasing the voltage on the front-gate 91 causes the potential separation 105 to decrease. This pulls down the conduction band edge and hence, under appropriate biasing conditions, the first excited energy level 103 of the quantum dot 87 can be pulled below the conduction band edge 101 of the quantum well level 83. Similarly, decreasing the backgate energy separation 107 can have a similar effect.

38 Increasing the back-gate energy separation (and/or decreasing the front- gate energy separation 105) causes the separation between the conduction band edge 10 1 of the quantum well level and the first confined energy state 103 of the quantum dot layer to increase.

Looking at the valence band edge 109, the first confined valence band state 111 lies above the valence band level in the quantum well. Therefore, it is not energetically favourable for the hole trapped in the valence band state 111 to transfer to the quantum well layer.

In the optical memory device the tunnel barrier 83 needs to allow tunnelling from the second active layer 85 to the first active layer 81 as the device is illuminated. However, the barrier needs to be large enough to prevent tunnelling back from the first active layer 81 to the second active layer 85 during the charge storage operation.

Figure 29 shows a heterojunction bipolar transistor structure. The device can be used a single photon detector or a memory structure. The simplified structure shown in Figure 25 has a semi-insulating substrate or buffer 120 with an emitter layer 122 formed on its upper surface. A base 124 comprising a plurality of layers (which will be described with reference to Figure 12) formed on an upper surface of the emitter layer 122. A lightly doped p-type collector 126 and a heavily doped p+ collector 128 are formed on an upper surface of the base 124.

Two ohmic contacts 130 and 132 corresponding to a source and drain are provided to the base. A collector contact 134 is made to the P+ collector 128 and an emitter contact 136 is made to the emitter layer 122.

The structure of the device is shown in more detail in Figure 26. The collector layers 126 and 128 are p-type, the base layer 124 is n-type and the emitter layer is p-type. The base section 124 comprises an undoped section 138, an n+ barrier region 140, a quantum 39 well layer 142 which is separated from a quantum dot layer 144 via a barrier layer 146. The field across the base can be modulated by appropriate biases across the emitter and collector. Thus, the relative separations of the conduction band edge 142 of the 2DEG and the first excited confined energy level of the quantum dot 144 can be modulated.

Claims

CLAIMS:

1. A photon detector configured to detect a single photon comprising first and second active layers separated by a first barrier layer, and detecting means for detecting a change in a characteristic of the first active layer, wherein the first active layer is a quantum well layer capable of supporting a two dimensional carrier gas and the second active layer comprises at least one quantum dot, the device further comprising separating means for separating a photo-excited electron-hole pair.

2. A photon detector according to claim 1, wherein the second active layer comprises no more than 100,000 active quantum dots.

3. A photon detector according to either of claims I or 2, wherein the active area of the second active layer is no more than 10-8m2.

4. A photon detector according to any preceding claim, wherein the means for separating a photo-excited electron-hole pair comprises means for applying an electric field normal to the active layers.

5. A photon detector according to any preceding claim, wherein the means for detecting a change in the characteristic of the first active layer comprises means for detecting a transport characteristic of the carriers in the first active layer.

6. A photon detector according to claim 5, wherein the means for detecting a change in a transport characteristic is configured to detect a change in a transport characteristic over a length of I OOgm in the transport direction of the first active layer.

7. A photon detector according to any preceding claim, wherein the separation between the first and second active layers is thin enough to allow coupling between the layers.

41 9. A photon detector according to any preceding claim, wherein the means for measuring the transport characteristic of the first active layer comprises at least two ohmic contacts provided to the first active layer.

9. A photon detector according to any preceding claim, wherein excess carriers are provided to the quantum well layer.

10. A photon detector according to claim 9, wherein the carriers are provided to the quantum well by means of a doped second barrier layer.

11. A photon detector according to claim 10, wherein the secZ.nd bamier laVer is a modulation doped barrier layer comprises a doped layer and a spacer layer, the spacer layer being provided adjacent the quantum well layer.

12. A photon detector according to any of claims 9 to 11, wherein the excess carriers are electrons.

13. A photon detector according to any preceding claim, wherein the device further comprises a third barrier layer provided on an opposing side of the second active layer to the first active layer.

14. A photon detector according to claim 13, wherein the third barrier layer is a doped barrier layer.

15. A photon detector device according to claim 14, wherein the barrier is n-doped.

16. A photon detector according to any preceding claim, wherein the means for applying a field normal to the active layers comprises a front gate provided overlying the second active layer.

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17. A photon detector according to claim 16, wherein the front-gate is semitransparent to radiation of a predetermined frequency.

18. A photon detector according to any preceding claim, wherein the means for applying a field normal to the active layers further comprises a backgate located underneath the first active layer.

19. A photon detector according to any of claims I to 15, wherein the means for applying a field normal to the first and second active layers comprises a p-type terminal on one side of the active layers and an ntype terminal located on the opposite side of the first and second active layer.

20. A photon detector according to any preceding claim, wherein the second active layer comprises quantum dots with a distribution of optical transition energies.

21. A photon detector according to any preceding claim, wherein the quantum dots are made by depositing InAs or InGaAs.

22. A photon detector according to any preceding claim, wherein the first barrier layers is AlAs or AlGaAs or GaAs.

23. A photon detector according to any preceding claim, wherein the first active layer is InGaAs and a barrier region comprising an InAlAs layer is provided adjacent the first active layer.

24. A photon detector according to claim 23, wherein the InAlAs layer comprises the first barrier layer.

25. A photon detector according to either of claims 23 or 24, formed on an InP substrate.

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26. A photon detector according to either of claims 23 or 24, formed on a GaAs substrate.

27. A photon detector according to claim 26, wherein strain reducing means are provided to reduce the strain in the second active layer due to the lattice mismatch between the first active layer and the substrate.

28. A photon detector according to claim 27, wherein the strain reducing means are provided by a layer of quaternary material, the lattice constant of which can be configured to match that of the first active layer.

29. A photon detector according to claim 27, wherein the strain reducing means is provided by a layer having a graded change in composition in the direction of formation of the layer.

30. A photon detector according to any preceding claim, wherein a beam of radiation is incident on a surface of the device.

31. A photon detector according to any preceding claim, comprising an antireflection coating on the surface if the detector which is to be illuminated.

32. A photon detector according to any preceding claim, wherein a beam of radiation is incident on the device parallel to the plane of the layers.

33. A photon detector according to any preceding claim, wherein the device further comprises upper and lower cladding layers, wherein the first active layer overlies the lower cladding layer and the upper cladding layer overlies the second active layer.

34. A photon detector according to any preceding claim, wherein the device further comprises guide means for confining light in a predetermined region of the first active layer.

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35. A photon detector according to any preceding claim, further comprising an absorption layer for absorbing incident radiation

36. A photon detector according to claim 35, wherein the absorption layer is a barrier layer.

37. A photon detector according to either of claims 35 or 36, wherein the absorption layer comprises Indium.

38. A photon detector according to claim 3 7, wherein the absorption layer is InGaAs.

39. A photon detector according to any preceding claim, comprising a full gate with an area of up to 10-8M2.

40. A photon detector according to any of claims I to 38, comprising a split gate.

41. A photon detector according to any preceding claims being irradiated with radiation with a frequency higher than that of the band gap of the first active layer.

42. A photon detector according to any of claims 3 6 to 4 1, when dependent on claim 35, wherein the device is irradiated with radiation with a frequency higher than that of the band-gap of the absorption layer.

43. A photon detector according to any preceding claim, wherein the detecting means comprises means for differentiating a signal with respect to time obtained from a change in the characteristic of the first active layer or means for detecting a change with time of a characteristic of the first active layer.

44. A photon detector according to claim 43, wherein the detecting means further comprises means for counting pulses from the differentiated output from the first active layer.

45. A photon detector according to any preceding claim comprising two or more layers of quantum dots.

46. A photon detector according to any preceding claim, comprising a plurality of first and second active layers separated by a barrier layer, each provided with means for detecting a change in a characteristic of the first active layer.

47. A photon detector array comprising a plurality of pixels, each pixel comprising a photon detector according to any of claims I to 46.

48. A photon detector array according to claim 47, further comprising a grid of bitlines and word-lines, wherein each pixel is addressable by applying an appropriate voltage to a word-line and/or a bit-line.

49. A photon detector array according to claim 48, wherein the bit-lines and wordlines are configured to apply a control signal to the means for separating a photonexcited electron-hole pair.

50. A memory device comprising a photon detector according to any of claims I to 46.

51. A memory structure comprising a plurality of pixels, each pixel containing a memory device in accordance with claim 50.

52. A memory device according to claim 5 1, comprising a plurality of bitlines and word-lines for selectively addressing each pixel.

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53. A memory device according to claim 52, wherein the bit-lines and word-lines are configured to apply a control voltage for the separating means for separating a photonexcited electron-hole pair.

54. A method of operating a photon detector according to any of claims 1 to 46, the method comprising the steps ofilluminating the device to photo excite electron-hole pairs; and measuring a change in the characteristic of the first active layer.

55. A photon detector as substantially hereinbefore described with reference to any of the accompanying figures.

56. A photon detector array as substantially hereinbefore described with reference to any of the accompanying figures.

57. A memory device as substantially hereinbefore described with reference to any of the accompanying figures.

58. A method for operating a photon detector as substantially hereinbefore described with reference to any of the accompanying figures.