GB2440569A

GB2440569A - A photon detector and a method of fabricating the detector

Info

Publication number: GB2440569A
Application number: GB0615198A
Authority: GB
Inventors: Beata Ewa Kardynal; Andrew James Shields
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2006-07-31
Filing date: 2006-07-31
Publication date: 2008-02-06
Anticipated expiration: 2026-07-31
Also published as: GB0615198D0; GB2440569B

Abstract

A photon detector comprises at least one quantum dot structure (103) configured to capture a photo-carrier, the device being configured such that said photo-carrier affects an electrical sensing current through a transport region (104-107). A photon collecting region (102) configured to absorb photons and supply the photo-carrier to said at least one quantum dot structure (103) has a photon collecting area substantially perpendicular to the incident direction of photons to be detected. The cross section of said transport region substantially perpendicular to the incident direction of photons is smaller than the photon collecting area, wherein the cross section is defined as that of the emitter (107) or alternatively the tunnel barrier layers. By keeping the electrically active area of the device as small as possible noise is reduced in the device.

Description

1 2440569 A Photon Detector and a Method of Fabricating a Photon

Detector The present invention relates to the field of detectors for detecting weak photon pulses and a method for the fibrication of such a detector. More specifically, the present invention relates to the field of photon detectors which are capable of detecting a signal from a single photon.

There is a need for an optical detector which is capable of detecting a single photon.

This need has been heightened by the continually expanding field of quantum 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.

Single photon detection is also useful as a low level light detection means for spectroscopy, medical imaging or astronomy. Both in medical and astronomical applications the high energy photons (X-ray etc) or high energy particles are converted in scintillators into many (10-100) low energy photons. These low energy photons are then detected by avalanche photodiodes or photomultiplier tubes. As the low energy photons that are produced are scattered in space there is a need for large area detectors which are very sensitive. Also arrays of such detectors allow the spatial distribution of low energy photons to be obtained in order to gain information about the original photon.

The other possible application of single photon detectors is in molecular spectroscopy, where detection of a photon can mean that certain reaction took place. In this case again, because the emission of the photon cannot be directed with very high precision, larger area detectors are necessary.

Single photon detectors have been proposed which operate using quantum dots to capture photo-carriers, such as photo-electrons or photo-holes, resulting from absorption of photons. Photo-carriers trapped within the quantum dots can affect the electric current in a transport region of the device which allows the presence of photons to be detectet Structures of this type are described in GB 2365210 which describes a photon detector based on a resonant tunneling diode structure and GB 2341 722 which describes a transistor structure.

These types of detectors suffer from the problem that increasing the electrically active area of the device causes the noise level of the device to increase. Therefore, there is a need to keep the electrically active area of the device as small as possible. However, this then causes problems if a large area detector is required.

The present invention addresses the above problem, and in a first aspect provides a photon detector comprising: at least one quantum dot configured to capture a photo-carrier, the device being configured such that said photo-carrier affects an electrical sensing current through a transport region; a photon collecting region configured to absorb photons and supply photo-carriers to said at least one quantum dot, said photon collecting region having a photon collecting area substantially perpendicular to the incident direction of photons to be detected; wherein the cross section of said transport region substantially perpendicular to the incident direction of photons is smaller than the photon collecting area.

The photon detector may be configured such that an clecirical sensing current through the transport region occurs substantially parallel to the direction of incident radiation or in a direction substantially perpendicular to the direction of incident radiation.

The transport region may comprise a resonant tunnelling diode structure. The structure may be configured such that absorption of a photon causes a tunnelling current to flow.

In other words, the tunnel structure is off resonance unless a photo-carrier has been captured. AJternatively, the structure may be configured such that a tunnel current flows if a photo-carrier has not been captured.

The tunnel barriers of said tunnel structure may be provided on the opposite side of said absorption region to said incident photons or on the same side of the absorption region to the incident photons.

The cross section of the transport region may be determined by the cross sectional area of the emitter and/or collector. Also, it may additionally or alternatively be determined by the cross sectional area of the tunnel barriers through which a tunnel current flows.

The said cross sectional area of the transport region may also be determined by a current aperture layer, said current aperture layer comprising a current blocking region and a current aperture which allows current to flow through said layer.

Said quantum dot is preferably provided within said photon absorption region.

The at least one quantum dot is preferably part of a quantum dot layer and said quantum dot layer extends across the cross section of the absorption region. However, the structure may be configured so that the quantum dot layer is also patterned with the emitter/collector and/or tunnel barriers.

Preferably, the cross section of the tunnel region is 1-2 pin2 or less The photon collecting area is preferably between S pm2 and 1mm2.

In an alternative embodiment, the transport region comprises a transistor structure.

In a second aspect, the present invention provides a method of fabricating a photon detector, the method comprising: forming a transport region, where an electrical sensing current flows; forming at least one quantum dot configured to capture a photo-carrier, the device being configured such that said photo-carrier affects an electrical sensing current through a transport region; forming a photon collecting region configured to absorb photons and supply photo-carriers to said at least one quantum dot, said photon collecting region having a photon collecting area substantially perpendicular to the incident direction of photons to be detected, patterning said photon absorption region; and patterning said transport region such that the cross section of said transport region substantially perpendicular to the incident direction of photons is smaller than the photon collecting area.

Preferably, forming said at least one quantum dot comprises using strained layer epitaxy.

The cross section of the transport region may be defined by etching at least one layer of said transport region. The etched part of at least one layer is preferably filled with an insulator.

In an alternative embodiment, the cross section of the transport region is defined by oxidisation.

The device may be fabricated by a regrowth technique. Such a technique comprises: forming at least some of the layers of said transport region in an epitaxiai growth chamber; removing the layers from the growth chamber; patterning at least one of the said layers; and re-inserting said patterned layers back into said growth chamber to allow growth of subsequent layers.

Patterning may comprise etching and filling said etched regions with an insulator.

In a further embodiment, patterning of the photon absorber and patterning of the transport region is perfonned after epitadal growth has been completed.

The present invention will flow be described with reference to the following non-limiting embodiments in which: Figure 1 is a schematic band structure illustrating a possible mode of operation of a photon detector based on a resonant tunnelling structure, figure la shows the device in an on-resonance state and figure lb shows the device when it is taken off resonance due to absorption of a photon; Figure 2 schematically illustrates a device in accordance with a first embodiment of the present invention where a photon absorption region is provided above a resonant tunnelling diode (RTD) structure only the emitter of the RTD structure is patterned; Figure 3 schematically illustrates a variation on the device of figure 2 where the photon absorption region is located above the RTD structure and the tunnel barriers of the R.TD are patterned as well as the emitter; Figure 4 schematically illustrates a variation on the device of figure 3 where a quantum dot layer in addition to the tunnel bathers and the emitter of the RiD layer are patterned; Figure 5 shows a plan view of the structure of figures 2 to 4; Figure 6 schematically illustrates a variation on the device of figure 2 where the photon absorption region is located below the RTD structure and just the emitter of the transport region is patterned; Figure 7 shows a plan view of the device of figure 6; Figure 8 is a schematic cross-section of a variation on the device of figure 6 where the photon absorption region is located below the RTD. The emitter and the tunnel bathers of the RTD are patterned; Figure 9 shows a variation on the device of figure 8 where the dot layer is patterned in addition to the tunnel barriers RTD; Figure 10 is plan view of the devices of figures 8 and 9; Figure 11 shows a variation on the device of figure 2 where the current aperture is defined by modi1'ing the tunnel barriers of the RTD; Figure 12 shows a variation on the device of figure 11 comprising a current aperture defining layer provided within the photon absorption region; Figure 13 shows a variation on the device of figure 12 where the current aperture defining layer is provided within the emitter; Figure 14 schematically illustrates a cross section of a device in accordance with a further embodiment of the present invention where the photon detector is based on a transistor structure; Figure 15 shows a further cross section of the structure of figure 14 which has been rotated by 900; Figure 16 shows a plan view of the structure of figures 14 and 15.

Figure IA and Figure lB are schematic band structures which are used to illustrate a mode of operation for a photon detector.

A conduction band 1 and a valence band 3 are shown. Figure IA shows the detector prior to illumination. Figure lB shows the detector after illumination.

An emitter 5 and a collector 7 are provided at either end of the detector. An emitter-collector bias V is applied across the detector such that the potential of the collector 7 is more positive than that of the emitter 5, thus inducing the flow of electrons from the emitter to the collector. In this example, the carriers will be electrons. However, it will be appreciated by those skilled in the art that detector could be configured with holes as the majority carriers.

The detector comprises a first low dimensional system 9 which is located between the emitter 5 and the first barrier layer 11. Electrons in the low dimensional layer 9 have energy of the first energy level 13 (this level can be seen more clearly on Figure 1B).

Adjacent the barrier layer 11 and on the opposing side to the first low dimensional system 9 is a second low dimensional system 15. The second low dimensional system I5is capable of confining electrons with a second energy level 17. In the detector shown in Figure IA (before illumination), the first energy level 13 and the second energy level 17 align.

Adjacent the second low dimensional system 15 is a second barrier layer 19. The second barrier layer 19 ii thin enough so that when the first 13 and the second 17 energy levels align, resonant tunnelling takes place through the first barrier layer 11 and the second barrier layer 19. An absorption layer 23 is then provided between the second barrier layer 19 and the collector 7. A quantum dot layer 21 is then provided in said absorption layer 23, on the opposing side of the second barrier layer to that of the second low dimensional system 15.

Due to the alignment between the first and second energy levels 13, 17, charge flows freely from the emitter 5 to the collector 7 when a bias V is applied. The alignment of the first and second energy levels will be dependent on the magnitude of the applied bias. The magnitude V is chosen such that the energy levels 13 and 17 align.

Figure lB shows the same device as that of Figure IA. However, the device has been illuminated. To avoid unnecessary repetition, like numerals have been used to denote like features. On absorption of a photon, an electron-hole pair is excited, here shown as electron 25 in the conduction band and hole 27 in the valence band. The bias V causes the electron 25 to be swept towards the collector. However, the hole 27 is swept in the opposite direction and is swept into dot 21 where it is trapped. The change in the charging state of' dot 21 causes a change in the alignment of the first energy level 13 and the second energy level 17 near the dot. As these two levels do not now align, the detector is brought "off-resonance" and tunnelling through the first barrier layer is suppressed locally near the dot. Therefore, the charge cannot flow freely from the emitter 5 to the collector 7. This change in current can easily be detected and signifies the absorption of a single photon.

In the above device, the photon detector is switched on by applying collector/emitter bias V across the detector. When a single photon is absorbed, the emitter/collector current is reduced.

In figure 1, the device is configured so that resonant tunnelling occurs unless a photon has been absorbed. However, the device may also be configured in the reverse manner where current flow from the emitter to the collector is blocked unless a photon is absorbed. In other words, a photon brings the device onto resonance.

Further variations on a resonant tunnelling diode single photon detector are described in GB2365 210.

Figure 2 shows a schematic cross-section of a device in accordance with an embodiment of the present invention.

In figure 2, an emitter contact layer 108 is formed overlying and in contact with a substrate 114. The emitter contact layer 108 may comprise n-doped GaAs if the system is built around the GaAs/A1(Ga)As material system or n-doped InGaAs if the device is fabricated around the InGaAs/InAlAs system.

Overlying and in contact with said emitter contact layer 108 is the emitter layer 107.

Emitter layer 107 comprises n-type GaAs or graded AIGaAs in a GaAs/Al(Ga)As system or n-doped InGaAs in an InGaAs/JnAlAs system. The emitter layer 107 is formed as a narrow pillar. Typically, the pillar will have a cross-sectional diameter measured in the plane of the substrate of approximately 1m or less. How this pillar is defined will be described later. The emitter pillar 107 is then surrounded by an insulator 109 which provides insulation and mechanical support. A suitable material for insulating layer 109 is polyimide.

Overlying the emitter pillar 107 and the polyimide surrounding layer 109 is first tunnel barrier 106. First tunnel barrier 106 comprises AIM or AIGaAs in GaAs/AI(Ga)As systems or AlAs or InAlAs in InGaAsfLnAlAs systems.

Quantum well layer 105 is then provided overlying and in contact with said first tunnel barrier 106. Said quantum well layer comprises GaAs in GaAs/Al(Ga)As systems or InGaAs in InGaAs/JnAlAs systems.

Second tunnel barrier 104 (equivalent to second tunnel barrier 19 in figures 1A and 1B) is then formed overlying and in contact with said quantum well layer 105. Said second tunnel barrier layer 104 comprises the same material as the first tunnel barrier layer 106.

Next, photon absorption region 102 is formed overlying and in contact with said second tunnel barrier layer 104.

The photon absorption region 102 comprises either insulating GaAs or insulating lnGaAs dependent on whether the material system of the photon detector is GaAs/Al(Ga)As based or InGaAs/InAIAs based respectively. The photon absorption region acts to collect photons and supply photocarriers for capture by quantum dots.

A layer of quantum dots for example biAs quantum dots 103 is provided in the absorption region close to the second tunnel barrier layer 104. Quantum dot layer 103 may be formed by a variety of techniques. For example, it may be formed using strained layer growth where 1 to 10 monolayers of a material having a different lattice constant of that of a photon absorption region 102. lnAs quantum dots in GaAs or InGaAs will provide a strained growth system. Due to the strained layer growth, the InAs layer will form as quantum dots. Alternatively, the quantum dots may be formed by interface fluctuations where a few monolayers of a material having a narrow band gap are embedded in-between layers of semiconductors with a wide band-gap. This forms quantum well, fluctuations in the thickness of the well act as quantum dots.

Layer 103 forms quantum dot 21 of figures 1A and lB.

Collector contact layer 101 is then formed overlying and in contact with said photon absorption region 102. Collector contact layer comprises n-doped GaAs in a GaAs/Al(Ga)As system or n-doped InGaAs in InGaAsfInAlAs system.

In this particular embodiment, the layers from the first barrier layer 106 to the collector contact layer 101 are patterned together to form a structure having a circular cross section with an approximate cross-sectional diameter of between 10pm and 100pm. It should be noted that this diameter is larger than the diameter of the emitter pillar 107.

The device of figure 2 is intended to be illuminated from the top so that top of device provides the surface of the device which is exposed to incident radiation.

An ohmic contact 110 is made to the emitter contact layer 108 and ohmic contact 111 is made to the collector contact layer 101. In order to allow illumination of the top of the device 115, collector ohmic contact 111 is provided close to the edge of collector contact layer 101. It is difficult to electrically bond to contact 111. Therefore, a further insulating layer 116 is provided overlying the side of the device. A contact layer 112 is then provided overlying insulating layer 116. Contact layer 112 extends to bonding pad 113 which allows electrical bonding away from the absorption region of the device. By electrically bonding to area 113, an electrical contact can be made to ohmic collector contact 117.

The device of figure 2 may be fibricated in two main ways. In both ways, it is important that there is a separate patterning step for the photon absother layer 102 and for the emitter 107.

In a first fabrication method, the emitter contact layer 108 and the emitter layer 107 are grown using a epitaxial growth technique, for example, molecular beam epitaxy (MBE).

The device is then removed from the growth chamber and emitter pillar 107 is defined using a standard technique such as an electron beam lithography coupled with either wet etching or a dry etching technique e.g. reactive ion etching.

An insulator is then spun over the layers and etched back to expose the top of emitter pillar 107 such that there is a flat growth surface fonned by the top of emitter pillar 107 and insulating layer 109. The device is then placed back into the growth chamber for growth of the layers from the first tunnel barrier layer 106 to the collector layer 101.

It should be noted that the layers grown on an insulator are not going to be epitaxial but rather amorphous. This may work but there may also be many traps for the photo-carriers. The layers 101 to 106 are then patterned using standard photolithography to define the cross section of the photon absorption region.

In an alternative fabrication technique, the layers from the emitter contact layer 108 to the collector contact layer 101 are formed one after each other in a growth chamber.

The device is then patterned to form the patterned photon absorption layer 102 and layers 101 to 106.

Emitter pillar 107 is then patterned during a second selective etching step where the etch is used to undercut layer 106. Polyimide or another insulator 109 is then spun onto the structure in order to give the overhanging layers 106 to 101 some structural support.

Ohmic emitter contact 110 and ohmic collector contact 111 are then patterned and defined in the standard manner. Next, bonding pad 113 is formed to the substrate 114 well away from the part of the device which is to be illuminated. A further insulator 116 is then spun over the device. The insulator is patterned and etched so that it provides an insulator 116 for contact metal 112. Contact metal 112 is then provided between the bonding pad 113 and the ohmic contact 111.

In the previous structure, the emitter 107 is patterned on its own. The first tunnel barrier layer 106, the quantum well layer 105, the second tunnel barrier layer 104 are patterned together with a photon absorber layer 102. However, it is possible to pattern the emitter 107 together with the first tunnel barrier layer 106, the quantum well layer and the second tunnel barrier layer 104 so that layers 107, 106, 105 and 104 together form a narrow pillar with a cross section of 1pm or less. Patterning of the emitter together with both barriers and the well provides better definition of the tunneling area. It may also result in electrically less noisy devices due to removal of a source of possible charge traps on oxidised large exposed surface of AlAs or A1GaAs in GaAs/AlGaAs devices or AlAs or InAlAs in JnGaAs/InAlAs devices.

The layer structure of such a device is identical to that described with reference to figure 2. Therefore, to avoid any unnecessary repetition, like reference numerals will be used to denote like features.

The difference between figures 2 and 3 occurs during the patterning of the layers. In figure 3, the emitter contact layer 108, the emitter layer 107, the first tunnel bather layer 106, the quantum well layer 105 and the second tunnel barrier layer 104 are formed simultaneously. These layers are then removed from the growth chamber and patterned to form a pillar. Insulator 109 which is preferably polyimide is spun onto the structure and the structure is etched to expose the top of second tunnel barrier layer 104 which has been now defined as a small pillar. The structure is then placed back into the growth chamber for growth to occur as previously discussed.

The remaining fabrication is identical to that described with reference to the device of figure 2.

Figure 4 schematically illustrates a further variation on the devices of figures 2 and 3.

In figure 4, the layer of quantum dots 103 is also patterned to form part of the emitter pillar. This may have an advantage of removing dots, which can potentially capture photo-carriers but being removed from the electrically active area of the device not result in measured signal.

The device of figure 4 is fabricated in the same manner as described with reference to figure 2. However, in the first growth stage, emitter contact layer 108 is formed followed by emitter 107, second tunnel barrier 106, quantum well layer 105, second tunnel barrier layer 104, part of photon absorption region 102 containing quantum dot layer 103. These layers are then patterned as described with reference to figure 2 and an insulator is spun on to surround the pillar.

The device is then put back into the growth chamber for the rest of photon absorption region 102 to be formed. The remainder of the device fabrication is the same as that described with reference to figure 2.

Figure 5 shows a plan view of the device described with reference to figures 2 to 4. The plan view of the device of figures 2 to 4 is the same.

As described with reference to figure 4, the device is formed on substrate 114. In the centre of the device is the top of the device 115 which is formed by the collector contact layer 101 which lies above the patterned electrical area (not shown). The patterned electrical area is not shown because it has a smaller cross sectional area of the top of the device 115.

The cross sectional area of the photon absorption region 102 on the top of the device is the photon collecting area.

The device is patterned so that emitter contact layer 108 extends away from the collector/emitter active region in one direction and a first ohmic contact 110 is provided to the collector contact layer. A contact to the top of the collector is provided under contact metal 112. Contact metal 112 is insulated from the device via insulating layer 116 and takes the signal away from.the collector ohmic contact 111 to bonding pad 113.

Figure 6 shows a variation on the device of figure 2. The device of figure 6 is similar to that of figure 2 but fabricated in a reverse order with the emitter being provided above the collector.

Collector layer 207 is provided overlying and in contact with substrate layer 213.

Collector layer 207 comprises n-type GaAs or graded A1GaAs in GaAs/Al(Ga)As systems or InGaAs in InGaAS/InAIAS systems. A photon absorption region 202 is then formed overlying and in contact with said collector layer 207. Said photon absorption

I

region 202 comprises insulating GaAs or insulating InGaAs. As before, a quantum dot layer 203 is formed within the photon absorption region 202. The quantum dot layer 203 is formed in the same way as described with reference to figure 2.

Then first tunnel barrier layer 204 is formed overlying and in contact with said photon absorption region 202, first tunnel barrier region comprises AlAs or AIGaAs in GaAs/Al(Ga)As systems or the barriers may be formed of AlAs or InAlAs in lnGaAs/InAlAs system. Quantum well layer 205 is then formed overlying and in contact with said first barrier layer 204. Said quantum well layer 205 comprises GaAs if the system is a GaAs/Al(Ga)As system InGaAs if the system is a InGaAsIInAlAs system. Second tunnel bather layer 206 is formed overlying and in contact with said quantum well layer 205. It is made out of the same material as first tunnel barrier layer 204.

Next, emitter 201 is formed. Emitter layer comprises n-doped GaAs or graded AlGaAs in a GaAs/A](Ga)M system or InGaAs in a InGaAs/InAIAs system. The emitter layer 201 is patterned to form a narrow pillar which has a cross sectional diameter of approximately lj.tm or less.

An ohmic contact 209 is provided overlying and on top of said emitter layer 201. The ohmic contact 209 and emitter layer 201 may be surrounded by an insulator such as polyimide or be surrounded by air. Contact metal 211 is then provided from the top of ohmic contact 209 to bonding pad 212. A collector bonding pad 210 is provided to the collector layer 207.

When fabricating the device, first, the layers 207 to 201 are fabricated. Next, the photon absorption region 202 and layers 204,205 and 206 are patterned in a first patterning step to form an area with a cross section of approximately l0zm to lOOjim.

The region is then patterned again to define emitter pillar 201. The emitter pillar has a cross sectional diameter of approximately ljim. Patterning of the emitter pillar may be achieved by electron beam lithography.

The ohmic contact 209 may be used as a mask in order to provide a sell aligned technique for the etch which defines emitter pillar 201 and ohmic contact 209. An insulator such as polyimide is then spun over the structure and etched back to reveal the top of ohmic contact 209. A contact metal 211 is then provided overlying said ohmic metal 209 and the insulator 208 to provide a path from bonding pad 212 to emitter contact209. If it is desired to form an air bridge, the insulator 208 may be dissolved.

Figure 7 shows a plan view of the device of figure 6. The photon collection area 215 is seen in the centre of substrate 214 and is defined by the cross sectional area of the second tunnel barrier layer 206. The emitter pillar 201 is hidden underneath insulator 208 in figure 7.

To one side of the photon collecting area (and underneath the collecting area) extends collector contact 207 which reaches ohmic contact 210. In the opposite direction,, contact metal 211 overlies insulator 208 to provide a contact to the top of emitter 201 (not shown) via bonding pad 212.

Figure 6 shows a structure where only the emitter 201 is patterned separately to the photon absorption area 202. However, it is possible to pattern further electrically conductive layers. Figure 8 shows such a variation on the structure where the first tunnel barrier 204, the quantum well layer 205 and the second tunnel barrier 206 are all patterned with emitter 201 to form a pillar with a cross sectional diameter of less than 1gm.

The layer structure of the device is fabricated in exactly the same way as that of figure 6. Therefore, to avoid any unnecessary repetition, like reference numerals would be used to denote like features.

However, where the etch to form emitter 201 just etches emitter layer 201 in figure 6, in the device of figure 8, this etch is taken down through the first tunnel barrier layer 204 such that the pillar is formed from layers 201, 206, 205 and 204.

The remainder of the contacts are fabricated in the same manner as described with reference to figure 6.

Etching barriers and the well of the device together with the emitter of the device defines better the tunneling area of the device. In addition removing easily oxidising AlAs layers may reduce number of traps in the device leading to less noisy photon detection.

Figure 9 shows a yet further variation on the structure of figure 6 and figure 8. Where the structure of figures shows patterning of the first and second tunnel barrier layers 204,206 along with the emitter layer 201, the structure of figure 9 also shows patterning of a part of the photon absorption area 202 which contains quantum dot layer 203 along with emitter layer 201, first and second tunnel barrier layers 204 and 206 and quantum well layer 205.

This structure is fabricated in the same way as described with reference to figure 6.

However, the etch to form the pillar of emitter layer 201 i this time taken down into the photon absorption layer 202 past the quantum dot layer 203.

Comparing with device in Figure 8, pattering of InAs dots layer removes dots, which can capture photo-carriers, but which are too far from the electrically active area of the detector to result in electrical response.

Figure 10 shows a plan view of the structures of figures 8 and 9. The plan view of figure 10 is similar to that described with reference to figure 7. However, whereas in figure 7, the upper surface of the photon collection region was provided by second barrier layer 206, in figure 10, the upper surface is formed by photon absorption region 202.

The remainder of the contacts remain the same as those described with reference to figure 7.

Figure 11 shows a yet further variation on the device of figure 2. As for the device of figure 2, the photon absorption area is provided overlying the resonant tunnelling diode structure. In the structure, patterning of the transport region is achieved by patterning the tunnel barrier layers.

As previously described, emitter contact layer 309 is formed overlying and in contact with substrate 315. Emitter contact layer 309 comprises n-doped GaAs if the structure is a GaAs/A1(Ga)M system or n-doped InGaAs in InGaAs/JnAIAs systems.

An emitter 308 is formed overlying and in contact with said emitter contact layer 309.

The emitter layer may be n-type GaAs or graded AlGaAs in GaAs/Al(Ga)As systems or n-doped lnGaAs in InGaAs/InAlAs systems. The emitter layer 308 and the emitter contact layer 309 may be formed of same material.

Next, the first tunnel barrier layer 306 is formed. The first tunnel barrier layer 306 comprises AlAs or AIGaASGaAs/AJ((3a)As systems or AlAs or InAlAs in InGaAs/InAlAs systems. Next, a quantum well layer 305 is formed overlying and in contact with said first tunnel barrier layer 306. Said quantum well layer 305 comprises GaAs in GaAs/Al(Ga)As systems or InGaAs in InGaAs/lnAlAs systems. Next, second tunnel barrier 304 is formed overlying and in contact with said quantum well layer 305.

The second tunnel bather layer may be formed from the same material as the first tunnel barrier layer 306.

The tunnel bather layers 306,304 are both patterned so that they have a central area which allows tunnelling to occur and circumferential areas 307 which have been modified to increase the bather height to prevent a sizeable tunnel current from flowing.

The modified areas may comprise oxidized AlAs or oxidized InAlAs.

The section of tunnel barrier layers 304, 306 which allows the tunnel current to pass are approximately 1pm or less in cross sectional diameter. These tunnel layers 306,304 then define the cross section of the electrically active area.

Next, photon absorption region 302 is formed overlying and in contact with said second tunnel bather layer 304. As described with reference to figure 2, quantum dot layer 303 is formed within photon absorption region 302. Finally, the structure is finished with collector contact layer 301. Collector contact layer 301 comprises n-doped GaAs in GaAs/Al(Ga)As systems or n-doped InGaAs in InGaAs/InAIAs systems.

First, the photon absorption region 302 is patterned as described with reference to figure 2. The etch to define the photon absorption region is taken down to the emitter contact layer 309. This etch defines a region having approximately a circular cross-section with a diameter of between 10pm to 100pm.

Next, the first and second tunnel barriers 306 and 304 are patterned. This is achieved by oxidizing the structure. The AlAs or InAlAs will oxide starting from the outside of the barrier layer inwards to define high resistance regions 307. This will leave a small tunnelling region or current aperture region in the centre of the structure.

The remainder of the device is then fabricated as described with reference to figure 2 with an ohmic contact 311 being provided to emitter contact layer 309. A small ohmic contact 312 is provided to collect or contact layer 301, the collector contact 312 being connected to a bonding pad 314 by contact material 313 which is provided overlying an insulator 310.

A variation on the structure of figure 11 is shown in figure 12. Unlike the structure of figure 11, the structure of figure 12 does not have patterned first and second tunnel barrier layers 304,306. Integral, in figure 12, a separate current aperture layer is formed.

As previously described, emitter contact layer 410 is formed overlying and in contact with substrate 416. Emitter contact layer 410 comprises n-doped GaAs if the structure is a GaAsIA1(Ga)As system or n-doped InGaAs in InGaAs/InAJAs systems.

An emitter 409 is formed overlying and in contact with said emitter contact layer 410.

The emitter layer may be n-type GaAs or graded AlGaAs in GaAs/Al(Ga)As systems or n-doped InGaAs in InGaAs/InAlAs systems. The emitter layer 409 and the emitter contact layer 410 may be formed of same material.

Next, the first tunnel barrier layer 406 is formed. The first tunnel barrier layer comprises AlAs or AlGaAsGaAs/A1(Ga)As systems or AlAs or InAlAs in InGaAsIlnAIAs systems. Next, a quantum well layer 405 is formed overlying and in contact with said first tunnel barrier layer 406. Said quantum well layer 405 comprises GaAs in GaAs/A1(Ga)As systems or InGaAs in lnGaAsflnAlAs systems. Next, second tunnel barrier 404 is formed overlying and in contact with said quantum well layer 405.

The second tunnel barrier layer may be formed from the same material as the first tunnel barrier layer 406.

Next, the photon absorption 402 is formed overlying and in contact with said second tunnel barrier layer 404. As before, the photon absorption layer comprises either GaAs or InGaAs. As before, a quantum dot layer 403 is formed within the photon absorption region 402. The quantum dot layer 403 is formed in the same manner as described with reference to figure 2.

In said photon absorption region overlying said quantum dot layer 403, a current aperture layer 408 is formed. Said current aperture layer 408 comprises a current aperture region 407 which defines the cross-section of the electrical transport region.

The cross section of the electrical transport region is approximately circular and has a diameter of 1i.un or less.

The current aperture layer 408 functions to define the area over which elecliical current flow occurs. The current aperture layer 408 may be achieved in two ways.

Current aperture 408 may be formed from AlAs or InAlAs which has been oxidized in at its edges to envisage current flow through aperture 407. Alternatively, the structure of the current aperture layer 408 may be formed by n-doped GaAs or n-type lnGaAs which has been etched away at the edges to define aperture 407.

The remainder of the photon absorption region 402 is then formed overlying and in contact with said current aperture layer 408. The collector contact layer 401 is then formed overlying and in contact with the photon absorption region 402.

Where the current aperture layer 408 is fonned by oxidization, the stack of layers 410 to 401 are formed in one continuous growth. The device is then etched to define the photon absorption region 402 and the etch is taken down to emitter contact layer 410.

The device is then oxidized as previously described in order to define current aperture 407 in current aperture layer 408. During oxidization, it is important to mask first and second tunnel barrier regions 406 and 404 in order to avoid unwanted oxidization of these layers. However, if oxidization of these layers does occur, it is still possible to define a current aperture using these layers as described with reference to figure 11.

The device is then fabricated as described with reference to figure 11.

If the device is formed using a current aperture region comprising either n-doped GaAs or n-type InGaAs, the layer can be formed in two ways. The device may be fabricated with all layers grown in one growth and then patterned as previously described to define photon absorption region 402. The etch for defining a photon absorption region 402 being taken down to emitter contact layer 410. A selective etch is then used in order to etch away either the n-type GaAs or n-type InGaAs in current aperture layer 408 in order to define current aperture 407. It is important to ensure that other layers of this composition are masked during the selective etch.

Alternatively the structure may be grown up to and including current aperture layer 408. The structure is then etched to remove current aperture 408 in all areas except for the actual aperture region 407. An insulator is then provided and etched back to expose a flat surface exposing current aperture 407 and the top of the insulator which forms the remainder of layer 408.

An ohmic contact 412 is made to emitter contact layer 410. A collector ohmic contact 413 is made to collector 401. Collector contact 413 is connected to bonding pad 415 by contact metal 414. Contact metal 414 is provided overlying insulator 414.

Figure 13 is closely based on figure 12. However, here, current aperture layer 408 is provided underneath the first and second barrier regions 406 and 404. To avoid any unnecessary repetition, like reference numerals will been used to denote like features.

As described with reference to figure 12, the emitter layer 409 is formed overlying and in contact with the emitter contact layer 410. Growth of the emitter layer 409 is interrupted to form current aperture layer 408. Current aperture layer 408 is formed in the same manner as described with reference to figure 12. After growth of the current aperture layer 408, growth of the emitter layer 409 resumes. Next, the first tunnel barrier layer 406 and the remaining layers are formed as previously described.

Figure 14 shows a completely different type of device to that described with reference to figures Ito 13.

The structure is based on a field effect transistor comprising a quantum well. A layer of quantum dots is provided parallel to the quantum well layer. Photon absorption causes carriers to be trapped in the layer of quantum clots which, in turn, affects transport within the quantum well. This is described in more detail in GB 2341 722.

The structure of figure 14 comprises a substrate 515. A back gate layer comprising, for example n-type GaAs in GaAs/A1GaAs systems 508 is provided overlying and contact with said substrate 515. A back gate barrier layer 507 is then formed overlying and in contact with said back gate layer 508. Back gate barrier layer 507 comprises AlAs or A1GaAs or AlAs/GaAs superlattice.

Overlying and in contact with said back gate barrier layer 507 is photon absorption layer 505. Photon absorption layer 505 comprises GaAs. Quantum dot layer 506 is then formed within said photon absorption region 505. Quantum dot layer 506 comprises InAs and is formed in the same way as described with reference to figure 2.

A barrier layer 504 is then provided overlying and in contact with the top of said photon absorption region 505. Barrier layer 504 comprises AIGaAs or AlAs.

Overlying and in contact with said barrier layer 504 is quantum well layer 503.

Quantum well layer 503 comprises GaAs. The channel of the transistor is formed at the intethce between barrier 504 and quantum well layer 503.

Overlying and in contact with said quantum well layer 503 is quantum well layer doped barrier 502. Quantum well layer doped barrier comprises n-type doped A1GaAs. A thin layer of undoped AlGaAs is provided between the doped barrier layer and the quantum well layer in order to provide a modulation doped barrier layer 502. The structure is finished with a GaAs cap layer 501.

As previously explained, during operation, the absorption region will collect photons. If a photon is collected and the resulting photocarrier is trapped within a quantum dot in quantum dot layer 506, transport within the quantum well region 503 is modified.

Therefore, by measuring the transport through the quantum well region 503, it is possible to determine if a carrier were to become trapped in a quantum dot due to photon absorption in the photon absorption region.

Figure 14 shows a cross section of the structure from one direction. Figure 15 shows a cross section of the structure rotated through 900 (in the direction of source-drain current flow) and figure 16 shows a plan view of the structure.

Turning next to figure 16, layers 503, 502 and 501 of the structure are patterned to form a narrow transistor region 501 which is used to measure transport through quantum well layer 503 using source and drain contacts 509. The transport region 501, 502,503 is considerably smaller that the absorption region which is seen as a large square on figure 16.

Returning to figure 14, first, the absorption region 505 is patterned by etching a square (see figure 16) through the structure right down to back gate region 508. Contacts 513 are then made to back gate region 508.

A second etch is then perfoimed to isolate transistor region into a narrow strip running into the plain of the paper in figure 14 and across the plain of the paper as shown in figure 15. Ohmic contacts 509 are made to either end of the transistor channel 503.

As the quantum well region 503 will be relatively small, it is preferable to make small ohmic contacts 509 to the region and then provide an insulator 510 overlying the edges of the structure and provide a further contact metal 511 which connects ohmic contact 509 tobonding contact 512. Therefore, measurement via contacts 512 will allow the resistance of the quantum well 503 to be determined.

By modulating the bias on the back gate using contacts 513, it is possible to reduce the carrier concentration within the quantum well 503 and hence modify the device to the appropriate sensitivity.

Normally the channel would be 1 jim or less in width and length.

Claims

CLAIMS: 1. A photon detector comprising: at least one quantum dot

configured to capture a photo-carrier, the device being configured such that said captured photo-carrier affects an electrical sensing current through a transport region; a photon collecting region configured to absorb photons and supply photo-carriers to said at least one quantum dot, said photon collecting region having a photon collecting area substantially perpendicular to the incident direction of photons to be detected; wherein the cross section of said transport region substantially perpendicular to the incident direction of photons is smaller than the photon collecting area.

2. A photon detector according to claim 1, wherein an electrical sensing current through the transport region flows substantially parallel to the direction of incident radiation.

3. A photon detector according to claim 1, wherein electrical sensing current through the transport region occurs in a direction substantially perpendicular to the direction of incident radiation.

4. A photon detector according to any preceding claim, wherein the transport region comprises a resonant tunneling diode structure.

5. A photon detector according to claim 4, configured such that absorption of a photon causes a tunneling current to flow.

6. A photon detector according to either of claims 4 or 5, wherein the tunnel barriers of said tunnel structure are provided on the opposite side of said absorption region to said incident photons.

7. A photon detector according to any of claims 4 to 6, wherein the said cross section of the transport region is determined by the cross sectional area of the emitter and/or collector.

8. A photon detector according to any of claims 4 to 7, wherein the said cross section of the transport region is determined by the cross sectional area of the tunnel barriers through which a tunnel current flows.

9. A photon detector according to any of claims 4108, wherein the said cross sectional area of the transport region is determined by a current aperture layer, said current aperture layer comprising a current blocking region and a current aperture which allows current to flow through said layer.

10. A photon detector according to any preceding claim, wherein said quantum dot is part of a quantum dot layer and said quantum dot layer extends across the cross section of the absorption region.

11. A photon detector according to any of claims I to 3, wherein the transport region comprises a transistor structure.

12. A photon detector according to any preceding claim, wherein said quantum dot is provided within said photon absorption region.

13. A method of fabricating a photon detector, the method comprising: forming a carrier transport region; forming at least one quantum dot configured to capture a photo-carrier, the device being configured such that said captured photo-carrier affects an electrical sensing current through a transport region; forming a photon collecting region configured to absorb photons and supply photo-carriers to said at least one quantum dot, said photon collecting region having a photon collecting area substantially perpendicular to the incident direction of photons to be detected, patterning said photon absorption region; and patterning said transport region such that the cross section of said transport region substantially perpendicular to the incident direction of photons is smaller than the photon collecting area.

14. A method according to claim 13, wherein forming said at least one quantum dot comprises using strained layer epitaxy.

15. A method according to either of claims 13 or 14, wherein the cross section of the transport region is defined by etching at least one layer of said transport region.

16. A method according to claim 15, wherein the etched part of at least one layer is filled with an insulator.

17. A method according to either of claims 13 or 14, wherein the cross section of the transport region is defined by oxidisation.

18. A method according to any of claims 1!3 to 17, comprising: forming at least some of the layers of said transport region in an epitaxial growth chamber, removing the layers from the growth chamber; patterning at least one of the said layers; and re-inserting said patterned layers back into said growth chamber to allow growth of subsequent layers.

19. A method according to claim 18, wherein patterning comprises etching and filling said etched regions with an insulator.

20. A method according to any of claims 13 to 1 7 wherein patterning of the photon absorber and patterning of the transport region is performed after epitaxial growth has been completed;