Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/015—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06N—COMPUTING ARRANGEMENTS BASED ON SPECIFIC COMPUTATIONAL MODELS
  • G06N10/00—Quantum computing, i.e. information processing based on quantum-mechanical phenomena
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06N—COMPUTING ARRANGEMENTS BASED ON SPECIFIC COMPUTATIONAL MODELS
  • G06N10/00—Quantum computing, i.e. information processing based on quantum-mechanical phenomena
  • G06N10/40—Physical realisations or architectures of quantum processors or components for manipulating qubits, e.g. qubit coupling or qubit control
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals

Definitions

• the present invention relates to apparatus and a method for generating single photons.
• Preferred embodiments disclosed herein provide for a reliable source of single photons, for use for example in quantum optics, quantum communication and quantum imaging. This is typically carried out by hot carrier, that is electron or hole injection across a lateral p-n or n-p junction, or across p-i-n junctions.
• Such sources require the controlled preparation of an excited state in order to avoid accidental emission of secondary photons, either simultaneously or within a delay shorter than the detector jitter duration.
• N. Tomm et al. demonstrated up to 57% on-demand generation of photons at the output of the final optical fibre [N. Tomm et al., Nature Nanotechnology 16, 399 (2021 )).
• Non-adiabatic single-electron pumps in a dopant-free GaAs/AIGaAs 2DEG discusses quantized charge pumping using non-adiabatic single-electron pumps in dopant-free GaAs two-dimensional electron gases. It is suggested that the dopant free 111— V platform allows for ambipolar devices, such as p-i-n junctions, which could be combined with such pumps to form electrically driven single photon sources.
• the present invention seeks to provide an improved source of individual photons and an improved method of generating individual photons.
• apparatus for generating singular light photons comprising: an electron pump operable to provide a source of conduction band electrons; a p-n junction stage comprising a terminal coupled to the electron pump, the terminal being located at an n-type region of the p-n junction stage; a p-type region adjacent the n-type region and remote from the electron pump; a boundary between the n-type and p-type regions, wherein the boundary presents an energy step from the n-type region to the p-type region; a source of valence band holes coupled to the p-type region; an energy-relaxation suppression device disposed across the n-type region, wherein the energy-relaxation suppression device is actuatable to suppress the energy relaxation of an electron disposed in the n-type region and travelling from the electron pump to the p-type region; wherein the energy-relaxation suppression device is operable to keep sufficient excitation energy of an electron to enable the electron to pass across
• the teachings herein provide a mechanism, system or method, which is configured in one embodiment to enable an electron to be carried from a single-electron source to the lateral p-n junction (or a hole to be carried from a single-hole source to the lateral n-p junction).
• the lateral p-n (on n- p) junction is set under-biased (that is below the threshold of forward bias current).
• the energy step created at the junction is significantly larger than the thermal energy, in order to avoid the electrons (or holes) in the Fermi sea to cross the p-n junction by thermal excitation.
• the energy step needs to be set around 10 meV or larger, even when the device is cooled down to liquid helium temperature or below, therefore, the electrons (holes) emitted from the singleelectron (hole) source are in a high energy state in order to overcome the energy step to enter the p-type (n-type) region.
• high-energy (hot) carriers electrospray carriers
• the teachings herein solve this issue by a verified method to carry hot electrons (or holes) from a source to a lateral p-n (or n-p) junction emitting photons by recombination process.
• the p-n junction stage is preferably a lateral p-n junction.
• the lateral p-n junction consists of or comprises two-dimensional electron gas and two-dimensional hole gas in the same plane.
• apparatus for generating singular light photons comprising: a hole pump operable to provide a source of valence band holes; a n-p junction stage comprising a terminal coupled to the hole pump, the terminal being located at a p-type region of the n-p junction stage; an n-type region adjacent the p-type region and remote from the hole pump; a boundary between the p-type and n-type regions, wherein the boundary presents an energy step from the p-type region to the n-type region; a source of electrodes coupled to the n-type region; and an energy-relaxation suppression device disposed across the p-type region, wherein the energy-relaxation suppression device is actuatable to keep the excitation energy of a hole disposed in the p-type region and travelling from the hole pump to the n-type region; wherein the energy-relaxation suppression device is operable to keep sufficient excitation energy of a hole to enable the hole to
• the p-n junction stage is preferably a lateral p-n junction.
• the lateral p-n junction consists of or comprises two-dimensional electron gas and two-dimensional hole gas in the same plane.
• the energy-relaxation suppression device comprises a magnet configured to generate a magnetic field having at least a magnetic component extending in a direction perpendicular to a direction between the hole pump and the n-p junction stage (preferably the plane of two-dimensional holes and electrons) , wherein the magnetic filed is operable to guide holes from the hole source across the n-p junction stage.
• the n-p junction stage provides a lateral n-p junction.
• the magnet preferably is configured to guide an electron from the electron pump to the p-type region of the p-n junction stage or a hole from the hole pump to the n-type region of the n-p junction stage.
• the magnet may be an electromagnet.
• the n-type region of the p-n junction stage has an energy (conduction band) below an energy (conduction band) of the p-type region of the p-n junction stage, wherein the n-type region of the p-n junction stage provides an electron bath, or
• the p-type region of the n-p junction stage has an energy (valence band) above an energy (valence band) of the n-type region of the n-p junction stage, wherein the p-type region of the n-p junction stage provides a hole bath.
• the electron/hole pump may be a controllable source of singular electrons/holes.
• the electron pump or the hole pump is a tunable-barrier quantum dot pump.
• the energy-relaxation suppression device comprises a depletion gate disposed across the n-type region of the p-n junction stage or disposed across the p-type region of the n-p junction stage.
• the depletion gate is operable to be maintained at a controllable voltage.
• the apparatus may comprise an inducing gate as the source of holes or electrons.
• the holes or electrons are induced by voltage operation of the inducing gate.
• the p-n junction stage is part of a two-dimensional electron gas and two- dimensional hole gas, or
• the two-dimensional electron gas system comprises a GaAs/AIGaAs hetero-structure.
• the apparatus may comprise a pump control unit connected: (i) to the electron pump and operable to tune an exit barrier height to fix an electron emission energy, or (ii) to the hole pump and operable to tune an exit barrier height to fix a hole emission energy.
• the pump control unit in practical embodiments may comprise an AC voltage source and a DC voltage source.
• a voltage is applied to the source to raise the source potential to emit hot electrons over the barrier (depicted in Figure 3, described below).
• electron pump depicted in Figure 1 , described below
• a radiofrequency signal on the entrance gate barrier lifts up the energy of an electron captured in a pump, and emits the electron as "hot electron” over the exit-barrier potential.
• the electron emission energy is controlled by the height of exit barrier.
• the electron guiding unit could be chiral one-dimensional edge channels such as quantum-Hall edge states formed by an application of magnetic field or chiral transport channels in topological insulator, in which case an external magnetic field may not be needed.
• the apparatus is configured to maintain the Fermi energy of the n-type region adjacent to the p-n junction below the energy of the conduction band in the p-type region by an amount much larger than the thermal energy BT, where the AB is the Boltzmann constant and T is the device temperature, such that the electrons in the n-type Fermi sea cannot travel across the junction.
• a method of generating singular light photons comprising the steps of: supplying from an electron pump a source of conductance band electrons to an n-type region of a p-n junction stage, wherein the p-n junction stage comprises a p-type region adjacent the n-type region and remote from the supply of electrons; generating a boundary between the n-type and p-type regions, wherein the boundary presents an energy step from the n-type region to the p-type region; providing a source of valence band holes to the p-type region; suppressing energy relaxation of an electron disposed in the n-type region and travelling from the electron pump to the p-type region; wherein the energyrelaxation suppression is operable to keep sufficient excitation energy of an electron to enable the electron to pass across the boundary and into the p-type region; the energy-relaxation suppression device comprising a magnet, the method including the step of configuring the magnet to generate a magnetic field having
• the p-n junction stage is a lateral p-n junction.
• a method of generating singular light photons comprising the steps of: supplying from a hole pump a source of valence band holes to a p-type region of an n-p junction stage, wherein the n-p junction stage comprises an n- type region adjacent the p-type region and remote from the supply of holes; generating a boundary between the p-type and n-type regions, wherein the boundary presents an energy step from the p-type region to the n-type region; providing a source of conduction band electrons to the n-type region; suppressing energy relaxation of a hole disposed in the p-type region and travelling from the hole pump to the n-type region; wherein the energy-relaxation suppression is operable to keep sufficient excitation energy of a hole to enable the hole to pass across the boundary and into the n-type region; the energy-relaxation suppression device comprising a magnet, the method including the step of configuring the magnet to generate a magnetic
• the n-p junction stage is a lateral n-p junction.
• the electron or hole may be guided by means of an electromagnet.
• the method may include the step of controlling the source of electrons or holes.
• the step of providing excitation energy may utilise a depletion gate disposed across the n-type region of the p-n junction stage or across p-type region of the n-p junction stage.
• the method includes controlling the voltage of the depletion gate.
• the method preferably includes:
• the two-dimensional electron gas comprises a GaAs/AIGaAs hetero-structure.
• the method includes tuning an exit barrier height to fix an electron or hole emission energy.
• apparatus for generating singular light photons comprising:
• an electron pump operable to provide a source of conduction band electrons
• an n-i junction stage comprising a terminal coupled to the electron pump, the terminal being located at an n-type region of the n-i junction stage, an intrinsic region adjacent the n-type region and remote from the electron pump, a boundary between the n-type and intrinsic regions, wherein the boundary presents an energy step from the n-type region to the intrinsic region;
• a hole pump operable to provide a source of valence band holes, a p-i junction stage comprising a terminal coupled to the hole pump, the terminal being located at a p-type region of the p-i junction stage, an intrinsic region adjacent the p-type region and remote from the hole pump, a boundary between the p-type and intrinsic regions, wherein the boundary presents an energy step from the p-type region to the n-type region; and an energy-relaxation suppression device disposed across the n-type region of the n-i junction stage and the p-type region of the p-i junction stage, wherein the energy-relaxation suppression device is actuatable to keep the excitation energy of an electron disposed in the n-type region of the n-i junction stage and travelling from the electron pump to the intrinsic region and of a hole disposed in the p-type region and travelling from the hole pump to the intrinsic region of the p-i junction stage; wherein the energy-relaxation suppression device is oper
• the n-i junction stage provides a lateral n-i junction
• the p-i junction stage provides a lateral p-i junction
• the magnet is preferably configured to guide an electron from the electron pump to the intrinsic region and a hole from the hole pump to the intrinsic region.
• the magnet is an electromagnet.
• the n-type region has an energy (conduction band) below an energy (conduction band) of the intrinsic region, wherein the n-type region provides an electron bath
• the p-type region has an energy (valence band) above an energy (valence band) of the intrinsic region, wherein the p-type region provides a hole bath.
• the electron pump is a controllable source of singular electrons
• the hole pump is a controllable source of singular holes.
• One or both of the electron pump and the hole pump may be a tunable- barrier quantum dot pump.
• the energy-relaxation suppression device may comprise a depletion gate disposed across at least one of the n-type region of the n-i junction stage or the p-type region of the p-i junction stage.
• the depletion gate is operable to be maintained at a controllable voltage.
• the apparatus may comprise at least one inducing gate as the source of holes or electrons, in which holes or electrons are preferably induced by voltage operation of the inducing gate.
• the n-i junction stage is part of a two-dimensional electron gas and an intrinsic quantum well
• the p-i junction stage is part of a two-dimensional hole gas and an intrinsic quantum well.
• the two-dimensional electron gas comprises a GaAs/AIGaAs hetero-structure.
• the apparatus may comprise a pump control unit connected: (i) to the electron pump and operable to tune an exit barrier height to fix an electron emission energy, and (ii) to the hole pump and operable to tune an exit barrier to fix a hole emission energy.
• a method of generating singular light photons comprising the steps of:
• the intrinsic region provides what could be described as a pocket to which the two electron and hole are trapped and recombine to emit a photon.
• the method may comprise the steps of:
• the electron and hole are guided by means of an electromagnet.
• the method may include the steps of:
• It preferably includes the step of controlling the sources of electrons and holes.
• the step of providing excitation energy utilises depletion gates disposed across the n-type region of the n-i junction stage and the p-type region of the p-i junction stage.
• the method preferably includes the step of controlling the voltages of the depletion gates.
• the method may include:
• the includes tuning exit barrier height to fix electron and hole emission energies.
• Figure 1 is a schematic energy and circuit diagram showing the preferred electrical set-up of the p-n junction stage
• Figure 2 is scanning electron microscope image of the device in false colour with circuit diagram components superimposed on the image, providing a top view of the p-n junction stage matching the schematic view of Figure 1 ;
• Figure 3 is a schematic diagram similar to Figure 1 , showing the effect of controlled operation of an energy-relaxation suppression device of the apparatus of Figure 2;
• Figures 4(a) and (b) show the injection of hot electrons into p-type region against depletion voltage and a bias applied across the p-n junction;
• Figure 6 is a cross-sectional view of an example of device structure that realises the energy structure shown in Figure 1 and Figure 3 and the device geometry shown in Figure 2;
• Figure 7 is a cross-sectional view of another example of device structure formed as GaAsZAIo.33Gao.67As p-type and configured to pump holes into an n-region;
• Figure 8 is a cross-sectional view of another example of device structure formed of Si/SiojGeo.s as an n-type embodiment and using different materials compared to the embodiment of Figure 6;
• Figure 9 shows an embodiment of wafer structure for the GaAsZAIo.33Gao.67As lateral p-n junction
• Figures 10A to 10G shows an example of device fabrication stages, with Figure 10A showing an n-type dopant removal stage;
• Figure 10B shows the isolation etch stage
• Figure 10C shows the creation of the p-type ohmic contact
• Figure 10D shows the creation of the n-type ohmic contact
• Figure 10E shows the creation of the gate electrodes for an n-type single electron pump
• Figure 10G shows the creation of the inducing gate electrode
• Figure 11 shows another embodiment of the present invention, which provides for pumping holes rather than electrons as in the embodiment of the preceding Figures, and specifically is a schematic energy and circuit diagram showing the preferred electrical set-up of the n-p junction stage;
• Figure 12 shows another embodiment of the present invention, and specifically is a schematic energy and circuit diagram showing the preferred electrical set-up of an electron and hole pumping device
• Figure 13 is a schematic illustration of the device with circuit diagram components superimposed on the image, providing a top view of the diode junction stages matching the schematic view of Figure 12.
• the preferred embodiments disclosed herein provide for a reliable source of single photons, preferably generated by use of an electron pump which is coupled to feed electrons one by one to a p-n junction stage.
• the apparatus includes a mechanism to cause an electron ejected from the electron pump into the p-n junction stage to maintain its energy, in effect keeping the electron as a hot electron, as it travels across the n-type region and until it enters the p-type region.
• the electron being the only electron in the conduction band in the p-type region, can radiatively recombine with a hole in the valence band, so as to cause a single photon matching the band-gap energy to be emitted.
• the p-n junction is preferably biased below threshold so that any other electrons/holes cannot travel across the junction.
• the apparatus includes a mechanism to direct the singular electron across the p-n junction, which can optimise the efficacy of the generation of the desired single photon.
• the preferred embodiments as well as providing very reliable sources of single photons, are also able to generate single photons on demand.
• An example of an electron pump suitable for the embodiments disclosed herein is a tunable-barrier quantum dot pump, for instance of a type described by: M.D. Blumenthal, B. Kaestner, L. Li, S. Giblin, T.J.B.M. Janssen, M. Pepper, D. Anderson, G. Jones, and D.A. Ritchie, in Gigahertz quantized charge pumping, Nature Physics 3, 343 (2007), the disclosure of which is incorporated herein by reference.
• the disclosed tunable-barrier quantum dot pump comprises two modulated barriers and operates in a nonadiabatic pump mode. Transmission modulation of the two barriers is achieved by monochromatic sinusoidal signals, the narrow frequency spectrum of which allows effective impedance matching of the various transmission lines between a radio-frequency source and the device gates, generating a clean and controllable signal.
• the device comprises two Ti/AI finger gates, all of which are over an etched channel and provide a time-dependent potential, the remaining three being earthed.
• a static quantum dot over the wire is induced by applying negative de voltages to both gates. These gates provide the entrance and exit barriers.
• the sinusoidal signal from the output of the radio frequency source is fed to the entrance-barrier gate such that the ac sinusoidal signals ride the static voltages.
• an AC signal could be applied to one gate (for instance the entrance barrier).
• the p-n junction stage preferably comprises a lateral p-n junction with an n-type region and a p-type region, in which holes are induced by gate voltage operation.
• the p-n junction stage is part of a two dimensional electron gas (2DEG) system of GaAs/GaAIAs hetero-structure.
• 2DEG two dimensional electron gas
• Other embodiments can use instead a two dimensional electron gas structure of, for example, indium arsenide, or indium aluminium arsenide, or silicon or silicon germanium.
• Other two dimensional electron gas systems will be apparent to the person skilled in the art on the basis of the teachings herein.
• the solution disclosed herein biases the p-n junction below the threshold, meaning that there is a barrier or step in the electrostatic potential that the incoming electrons (from the single-electron pump) must overcome.
• This potential step is set below the energy of pumped electrons ( ⁇ 100 meV) and is higher than the thermal excitation of the Fermi sea ( ⁇ 0.1 meV at 1 K). Setting the potential step to a few tens of meV has been found to meet this requirement efficiently. As a consequence, the incoming electrons must travel as “hot electrons” maintaining a threshold amount of energy without completely relaxing onto the Fermi energy before reaching the p-type region.
• the p-n junction stage 102 comprises an n-type region 104 and a p-type region 106.
• the n-type region is held at a lower energy level (conduction band) 108 compared to the energy level (conduction band) 110 at the p-type region.
• This energy differential creates an energy step 112 from the n-type region up to the p-type region.
• the electron sea, or Fermi sea, in the n-type region acts as a sink and traps any electrons that fail to have the required energy to reach and enter the p- type region. This assists in ensuring that only the desired electrons reach the p- type region at any time.
• Figure 1 also depicts the passage of electron 100 through the n-type region.
• the electron is made to maintain its excitation energy across the gap formed by the n-type region, in a manner as described below.
• the electron 100 in this state is hereinafter referred to as a hot electron.
• Its energy state across the n-type region is depicted by the arrow 120 in Figure 1 .
• the hot electron 100 is maintained at a high energy state, it is able to cross the energy barrier or step 112 between the n-type and p-type regions and thereby enter the p-type region 106.
• any other electrons in the n-type region are kept there by being at a lower excitation energy 108.
• the individual electron 100 once in the p-type region, can couple to a hole 122, at which it releases a photon 124 of energy. Given the individual electron 100 passing into the p-type junction 106, only a single photon 124 is generated.
• Further photons 124 can be generated by feeding further electrons 100 from the source across the n-type region of the p-n junction stage.
• the generation of the electrons 100 is, in the preferred embodiment, an individual electron at a time, and since the maintenance of the excitation energy 120 can be controlled, the generation of further photons can be precisely controlled, a single photon at a time.
• the preferred embodiments also provide a mechanism to direct or guide the electron 100 across the n-type region.
• this achieved by the provision of a magnetic field, generated by an external coil disposed around the p-n junction stage and which in practice powered to generate a magnetic field a direction pointing into the paper in the view of Figure 2. This is described in further detail below.
• Figure 2 is scanning electron microscope image of the device in false colour with circuit diagram components superimposed on the image, to present a top view of the p-n junction stage 102 matching the schematic view of Figure 1 .
• Ohmic contacts are indicated with cross mark in a box and magnetic field B is applied perpendicular to the substrate.
• the p-n junction stage comprises a two-dimensional electron gas 150 (depicted in blue in Figure 2).
• An entrance gate 152 (depicted in pink in Figure 2) is positioned at one end of n-type region 154 and is coupled to an electron source such as a pump as described above.
• the entrance gate 152 controls the input energy of the (hot) electron.
• the exit gate 153 the green gate on the right
• the combination of the entrance gate and bias applied from the current source 180 controls the hot-electron energy.
• the p-n junction stage also includes a depletion gate 160 (depicted in green in Figure 2), which preferably extends across the entire extent of the edge of the n- type region 104. However, as shown in this embodiment, it may cover the edge of a part of the n-type region 104 of the p-n junction, as long as the gate 160 remains sufficient to maintain the excitation energy of the hot electron 100 across the entire extent of the n-type region 104.
• the depletion gate 160 in practice controls the rate of energy relaxation due to phonon emission and electron-electron interactions for hot electrons 100, as described below.
• the edge-depletion gate 160 is not an essential component of the apparatus. It is not necessary when a strong magnetic field is used. It becomes useful in the cases of weaker magnetic fields, where the provision of edgedepletion gate 160 assists in maintaining the trajectory of the electron.
• the p-type region 106 is located adjacent the n-type region 104 and adjacent the depletion gate 160. Disposed within the zone of the p-type region is an inducing gate 170 (depicted in yellow in Figure 2), which acts to induce holes in the p-type region 106 from a p-type ohmic contact 184.
• the white arrow 172 depicts the hot electron trajectory making across the p-n junction while guided by a negative high magnetic field generated by a surrounding magnet.
• the white arrow 174 depicts the trajectory of an electron that has lost energy and cannot enter the p-type region.
• the n-type region 104 may be defined by conventional lithography techniques, etching with acids, metal deposition with vacuum evaporator and alloying of Ge/Au/Ni to get electric contacts to the two dimensional electron gas system.
• the p-type region 106 is in the preferred embodiment a p-type metal- oxide semiconductor field effect transistor (p-MOSFET).
• p-MOSFET p-MOSFET
• the dopant layer of n-AIGaAs in p-type region is etched away to eliminate the two-dimensional electron gas (2DEG).
• 2DEG two-dimensional electron gas
• a layer of AuBe is deposited and annealed to establish ohmic contacts to the induced holes.
• applying a negative voltage Vdep to the depletion gate 160 modifies the optical phonon emission rate and electron-electron scattering rate, which enables hot electrons to travel long distances while maintaining their energy, at least above the energy level required to enter into the p-type region 106.
• a hot electron 100 which overcomes the p-n junction step passes into the p-type region 106 and relaxes into the valence band with the emission of a photon.
• Holes in the p-type region 106 are induced by applying a gate voltage Vind to the inducing gate 170.
• a hot electron passes across the p-n junction stage along with the bottom edge in Figure 2, along the line of the arrows 172. Initially, a hot electron is injected through the potential barrier created at the entrance gate 152, its gate voltage being noted Vent.
• a source measure and/or control unit 180 connected to the entrance contact 152 tunes Vinj to fix the injection current nj.
• a hot electron travels along with the bottom edge 172 losing energy, in dependence upon the voltage applied to the depletion gate, as described below.
• any hot electron intruding into p-type region combines with a hole and emits a photon of light. If the hot- electron injection source is replaced by a single-electron pump, the emission of photon can be controlled one by one. The sequence can be repeated with multiple electrons, each supplied on demand from the electron source, and are able to generate a photon of light at the rate at which photons are induced.
• the emission of one or more photons of light in sequence can be controlled: by the rate of supply of electrons using the entrance gate 152, by the generation of holes by means of the inducer gate 170, by the energy retained in an electron passing through the n-type region 104.
• the device is operated in a large magnetic field ( ⁇ 10 T) having at least a component that is perpendicular to the plane of the lateral p-n junction stage, so that the electrons 100 from the single-electron pump travel along the edge of the two-dimensional electron gas in the n-type region 104.
• a magnet disposed or operable to generate a magnetic field that is perpendicular to the plane of the lateral p-n junction stage.
• the apparatus and system will function with a magnet that does not generate its field directly perpendicular to the plane of the lateral junction, as long as there is a component of that magnetic field that is along the perpendicular line.
• a magnetic field of around 10 Tesla provides optimum results, although the field need only be strong enough to guide the electrons along the edge of the junction. Magnetic fields of 3-4 Tesla have been found to work, though preferably the magnetic fields are between 5 to 10 Tesla. At lower magnetic fields there is a risk of electron scattering. In practice, the minimum magnetic field that can provide reliable operation of the system can be determined by routine experimentation.
• the edge-depletion gate 160 that covers over this electron path controls the electrostatic potential, which in turn controls the rate of energy relaxation by phonon emission.
• the semiconductor device design and device fabrication topology disclosed herein allows the integration of a single-electron pump, an edge-depletion gate 160, and a lateral p-n junction stage.
• the generated light can be detected by a spectrometer through a lens assembly and optical fibre.
• a source measure unit 182 connected to p-type contact 184 can detect the number of electrons that contribute to photon emission, as current l p . Any other electrons that did not enter the p-type region will follow the path 174 and will flow into the top contact 186 and their current l n can be detected by a digital multimeter 190 via a current-voltage converter).
• the magnitude of the current l n depends on the voltage Vdep applied to the depletion gate 160 and on the voltage V p applied to the p-type contact 184.
• Depletion gate voltage Vde P modifies the device edge potential and optical phonon emission rate, which allows hot electrons 100 to maintain their energy for longer distances.
• the p-type contact voltage V p modifies the step size of the p-n junction potential barrier 112. Were the step height 112 lowered significantly, electrons even at ground level can intrude into the p-type region 106.
• the measured voltage thresholds for this embodiment are:
• the blue regions indicate a very low l p and therefore low or no photon generation.
• the red regions depict a high l p and photon generation.
• FIG 3 shows in schematic form similar to Figure 1 the effect of applying a differing depletion voltage Vdep to the depletion gate 160.
• the section of the graph in red shows the energy loss of an electron 100 as it passes across the n-type region 104 of the p-n junction stage.
• Vdep is close to 0 V, specifically below the required threshold
• the electron 100 will lose excitation energy and will relax towards the electron pool 108. They will not maintain enough energy to overcome the barrier 112 between the n-type and p-type regions.
• Vdep when Vdep is sufficiently negative, this will contribute to maintaining the energy of the electron 100 and while this may drop off shortly before the p-n junction 112, depending on the physical arrangement and extent of the depletion gate 160 as well as the voltage Vdep at the gate 160, the electron 100 will still maintain sufficient energy to overcome the barrier 112 and reach the p-type region for emission of a photon of light.
• the amount of current l p is enhanced by applying optimised negative voltage on Vdep.
• Applying negative voltage on the depletion gate electrode 160 separates the orbital of electron wavefunction before and after optical phonon emission, which suppresses phonon emission rate and assists hot electrons 100 to maintain their energy. A majority of hot electrons 100 would lose their energy along the trajectory between the deletion gate 160 and inducing gate 170 in cases where phonon emission is not suppressed.
• the depletion of the background two-dimensional electron gas under the gate also suppresses the energy relaxation by electron-electron interactions. When the maximum energy is smaller than the p-n junction barrier step height 112, no electrons can intrude into the p-type region.
• Figure 5 shows magnetic field dependence of spectrum for electroluminescence from the p-n junction caused by (a) hot electrons 100 and (b) electrons at ground level, which shows that injected hot electrons 100 travelling along a quantum Hall edge channel are converted into photons.
• a spectrum at each magnetic field consists of averaged spectrum offset to make the background signal level equal to zero.
• Raw spectra are filtered to eliminate strong peak signals generated by cosmic rays and are averaged 30 times to improve noisesignal ratio.
• electroluminescence is observed only at high negative magnetic field where injected hot electrons can intrude into the p-type region thanks to the chiral transport through quantum Hall edge channel following the path 172. When the direction of the magnetic field is reversed, the chirality is reversed, and electrons emitted from the source 152 travel upwards in Fig. 2 and sink into the contact 186.
• the perspective provides circular polarisation measurement.
• Photons generated by transitions between Landau levels are expected to have circular polarisation depending on the spin states of the electron and hole due to the conservation law on angular momentum.
• Circular polarisation can be detected by inserting a quarter-wave plate into the device in order to convert them into linear polarisation and by using a half wave plate to resolve the linear polarisation. Conversion is advantageously carried out in a refrigerated environment in which photons converted into linear polarisation a dilution refrigerator can be guided to the spectrometer through linear polarisation maintaining fibres.
• Figure 6 shows an example of an integrated circuit lay-out or topography for the circuit arrangement of the p-n junction stage of Figure 2.
• the n-type contact 104 and the p-type contact 106 are as previously described and disposed at opposing sides of the chip.
• the two-dimensional electron gas is in this embodiment is created by a GaAs/AIGaAs heterostructure 150.
• the layer 150’ is Si-doped Alo.33Gao.67As, and the layer 150” is intnnsic-Alo.33Gao.67As.
• the upper layer 150’ is covered by a layer 200 which is a GaAs cap layer.
• a GaAs quantum well 201 contains two-dimensional electron gas and two-dimensional hole gas 122.
• An insulating layer 190 insulates the p-type contact 106 and the inducing gate 170 from one another.
• an electron 100 will travel along a channel from the n-type region to the p-type region, assuming it has sufficient excitation energy, to hole 122, through the GaAs quantum well 201 .
• FIG. 7 A complementary embodiment to the embodiment using a lateral p-n junction is shown in Figure 7, which comprises a p-type region 302 with a two dimensional hole gas (2DHG) and n-type region 304 in which electrons (instead of holes) are induced by gate 306 voltage operation.
• the structure also includes gate electrodes 307, similar to the gate electrodes 160 of the first described embodiment.
• the teachings herein can be implemented on any semiconductor material system and structure with a 2DEG (or 2DHG) and also induce holes (or electrons) by gate voltage operation.
• Figure 8 shows another embodiment of structure, having a lateral p-n junction, implemented in a different semiconductor material system such as Si/SiGe.
• Other examples include InAs/lnP, GaN/lnGaN, and so on.
• Figure 9 illustrates a wafer in accordance with an embodiment of the present invention.
• variables including layer thickness, number of matrix repeat, doping concentration, and so on, are readily determinable by the skilled person on the basis of their common knowledge in the art and are typically tailored for a particular designed emission wavelength or energy band structure.
• the values set out below in the description of an example structure are therefore to be understood as being examples only and not determinative of essential characteristics of the structure.
• the semiconductor material illustrated in this embodiment comprises a AIGaAs/GaAs heterostructure 310 on a GaAs substrate 312.
• Semiconductor material for the device can be produced by molecular beam epitaxy (MBE) on a semi-insulating GaAs substrate.
• MBE molecular beam epitaxy
• Other common crystal growth techniques on lattice-matched substrates may also be employed, such as metal-organo chemical vapour deposition or vapour phase epitaxy.
• Epitaxial growth commences with a 500 nm intrinsic GaAs buffer on the substrate.
• a graded 250 nm Al x Gai- x As (x increased from 0.05 to 0.33), interspaced with 0.6 nm GaAs and repeated several times to form the buffer.
• a distributed Bragg reflector stack of multiple GaAs/Al y Gai- y As (typically y > 0.5) or GaAs/AIAs layers can be included to enhance the output photon emission efficiency.
• a 15 nm intrinsic i-GaAs quantum well is sandwiched between a lower 50 nm and upper 40 nm intrinsic i-Alo.33Gao.67As barrier.
• the 2DEG will form preferably in this narrow 15 nm i-GaAs quantum well.
• a 40 nm n-type Alo.33Gao.67As doped with Si concentration of 1x10 18 cm -3 followed by an intrinsic 10 nm i-GaAs is grown on the underlying structure.
• a 2DHG version can be produced in the 15 nm i-GaAs quantum well by using a p-type dopant such as Be or C in the 40 nm Alo.33Gao.67As layer.
• Figure 6 depicts a schematic cross-sectional view of the final device structure in accordance with an embodiment of the present invention. It represents the principal elements along the central red axis 185 of Figure 2, although Figure 6 does not show the n and p-type ohmic contacts that provide electrical connections located away from the main active region shown in Figure 2.
• FIGS 10A to 10G illustrate the preferred embodiment of device fabrication sequence.
• the first step is usually to restrict the bulk 2DEG across the whole wafer substrate into a smaller mesa region.
• This step can be done with a photoresist mask defined using photo or electron beam or any other compatible lithographic techniques. Areas left unprotected by photoresist masked areas will be removed by chemical etching with a combination of diluted acid and oxidising solvents or dry plasma based processes using the appropriate gases. Etching is stopped partway in the buffer region.
• the n-type dopants in the n-type Alo.33Gao.67As are removed from the areas where the induced p-regions will be formed, for example by employing a similar resist defined mask and etching technique discussed above.
• the narrower single electron pump channel on the large mesa is also formed concurrently with this step (or separately if a different etch depth is required or preferred).
• another, optional, deeper isolation etch stage can then be performed to reduce potential electrical and optical leakage paths.
• AuBe (or AuZn) p-type ohmic contacts are then deposited, preferably by the standard resist masked lift off process, in the areas where the n-type dopants were removed and then annealed into the substrate.
• FIG 10D shows the AuGeNi n-type ohmic contacts being formed and alloyed, preferably using a similar technique.
• thin ( ⁇ 50nm) sub-micrometre scale metal gate electrodes are defined and then evaporated onto the 2DEG channel in the n- type region, typically using electron beam lithography and a resist lift off process.
• Examples of typical gate electrode metals can be a combination of Ti/Au or Cr/Au or Pt/Au. Assuming similar metal thickness, this gate electrode on the n-side can be delayed and done concurrently with the final stage with the p-side gate electrodes contacts.
• an insulator in then formed on the p-type region.
• This layer can be laid down using atomic layer deposition, for example with AI2O3 or HfO2, or sputtered or chemical vapour deposition, for example with SiO2 or SisN4, or cross linked PMMA or any other compatible material and associated method, such as by etching or lift off or multiple electron beam exposures.
• the inducing gate electrode for the p-region is then defined preferably using the standard resist lift off process.
• this gate electrode is transparent, such as indium tin oxide (or similar metal oxide based), or semi-transparent, such as very thin Ti ( ⁇ 5nm).
• metal interconnects e.g. Ti/Au or Cr/Au or Pt/Au
• a hole pumping system in which the particle that is moved is a hole rather than an electron. This can generate a photon of light, in an analogous manner to the electron pumping embodiment.
• this is an energy and circuit diagram showing the preferred electrical set-up of the n-p junction stage of the system according to this embodiment.
• the n-p junction stage 402 comprises an p-type region 404 and an n-type region 406.
• the p-type region is held at a higher energy level (valence band) 408 compared to the energy level (conduction band) 410 at the n-type region.
• This energy differential creates an energy step 412 from the p-type region up to the n-type region.
• the hole sea, or hole Fermi sea, in the p-type region acts as a sink and traps any holes that fail to have the required energy to reach and enter the n-type region. This assists in ensuring that only the desired holes reach the n-type region at any time.
• Figure 11 also depicts the passage of hole 400 through the p-type region 404.
• the hole is made to maintain its excitation energy across the gap formed by the p-type region, in a manner as described below.
• the hole 400 in this state is hereinafter referred to as a hot hole.
• Its energy state across the p-type region is depicted by the arrow 420 in Figure 11 .
• any other holes in the p-type region are kept there by being at a lower excitation energy 408.
• the individual hole 400 once in the n-type region, can couple to an electron 422, at which it releases a photon 124 of energy. Given the individual hole 400 passing into the n-type junction 406, only a single photon 124 is generated.
• Further photons 124 can be generated by feeding further holes 400 from the source across the p-type region of the n-p junction stage.
• the generation of the holes 400 is, in the preferred embodiment, an individual hole at a time, and since the maintenance of the excitation energy 420 can be controlled, the generation of further photons can be precisely controlled, a single photon at a time.
• the preferred embodiments also provide a mechanism to direct or guide the hole 400 across the p-type region.
• this achieved by the provision of a magnetic field, generated by an external coil disposed around the n-p junction stage and which in practice powered to generate a magnetic field a direction perpendicular to the plane of the two-dimensional hole gas and two- dimensional electron gas.
• the invention also provides for a system which pumps both electrons and holes, so that a single electron can combine with a single hole to generate a single photon.
• a system which pumps both electrons and holes, so that a single electron can combine with a single hole to generate a single photon.
• Such an embodiment can be precisely controlled, thereby to control the generation of single photons.
• a series of electrons and holes can be pumped, with each electron/hole pair in the sequence generating a photon.
• Figure 12 depicts an energy and circuit diagram showing the preferred electrical set-up of the n-i and p-i junction stages of the system according to this embodiment.
• the excitation energy of the electrons and the holes can be maintained sufficiently high by a magnetic field and as appropriate by an edge depletion gate 160 as taught above.
• Figure 13 is a view similar to Figure 2, that is a schematic illustration of the device of Figure 12 with circuit diagram components superimposed on the image, illustrating how such a device can be formed to pump both electrons and holes. Electrons and holes have different chirality when a perpendicular magnetic field is applied, and hence they can move towards each other. In the embodiment shown in Figure 13, a pocket 500 is created in the middle (in the intrinsic region) of the device, where both particles 100, 400 are trapped, find each other to form an exciton, and then emit a photon.
• Figure 13 represents a top view of the n-i junction and p-i junctions stages matching the schematic view of Figure 12.
• Ohmic contacts are indicated with cross mark in a box and magnetic field B is applied perpendicular to the substrate.
• the n-i junction stage comprises a two-dimensional electron gas 154 (depicted in blue in Figure 13).
• An entrance gate 152 is positioned at one end of n-type region 154 and is coupled to an electron source such as a pump as described above.
• the entrance gate 152 controls the input energy of the (hot) electron.
• the exit gate 156 controls the energy of hot-electrons.
• the combination of the entrance gate and bias applied from the current source 180 controls the hot- electron energy.
• the n-i junction stage may also include a depletion gate 160, which preferably extends across the entire extent of the edge of the n-type region 104. However, as shown in this embodiment, it may cover the edge of a part of the n- type region 104 of the n-i junction stage, as long as the gate 160 remains sufficient to maintain the excitation energy of the hot electron 100 across the entire extent of the n-type region 104.
• the depletion gate 160 in practice controls the rate of energy relaxation due to phonon emission and electron-electron interactions for hot electrons 100, as described above.
• the p-i junction stage 402 comprises a two-dimensional hole electron gas 454.
• An entrance gate 452 is positioned at one end of p-type region 454 and is coupled to a hole source such as a hole pump as described above.
• the entrance gate 452 controls the input energy of the (hot) hole.
• the exit gate 460 controls the energy of hot-holes.
• the combination of the entrance gate and bias applied from the current source 480 controls the hot-electron energy.
• the stage 402 may also include a depletion gate 460, which preferably extends across the entire extent of the edge of the p-type region 404. However, as shown in this embodiment, it may cover the edge of a part of the p-type region 404 of the p-i junction 402, as long as the gate 460 remains sufficient to maintain the excitation energy of the hot hole 400 across the entire extent of the p-type region 404.
• the depletion gate 460 in practice controls the rate of energy relaxation due to phonon emission and hole-hole interactions for hot holes 400, as described above.
• the edge-depletion gates 160, 460 are not essential components of the apparatus. They are not necessary when a strong magnetic field is used. They become useful in the cases of weaker magnetic fields, where the provision of edge-depletion gates 160, 460 assist in maintaining the trajectory of the electron and hole.
• the regions of the device depicted in Figure 13 may be formed in the same way and with the same materials as described above in relation to the other embodiments.
• a hole layer can be unreliable due to a charging effect (potentially caused by the holes leaking out from the quantum well confinement and piling up at the interface between the insulator and semiconductor layers).
• a large positive bias can be applied, which then induces trapped negative charges, most likely at the same insulator/semiconductor interface.
• the trapped negative charge can be used to induce holes, rather than using a negative voltage on the inducing gate. So, a sequence can be: 1 ) initially apply a large positive voltage on the inducing gate, 2) suddenly drop the voltage to zero. This induces holes, which decay over (roughly) one second.
• steps 1 and 2) can be repeated. This process was published in “Formation of a lateral p-n junction light-emitting diode on an n-type high-mobility GaAs/AI0.33Ga0.67As heterostructure - lOPscience”.
• the apparatus and method disclosed herein can be used to generate multiple photons in order to create, for example, brighter emissions. In all cases, a single photon can be generated at any point in time, with multiple photons being generated in sequence.
• the apparatus and method can operate at up to very high frequencies, of 1 GHz or more.
• the benefit of this single-photon source technology is the ability to generate photons on demand. It is expected that the timing of photon emission can be controlled with a precision of ⁇ 100 picoseconds (limited by the radiative recombination time). Since the error of electron emission from the single-electron pumps can be less than 1 part in 10 7 , we also expect that the chance of two-photon emission will be very low.

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