Patent Description:
Sources of single photons, containing a photon emitter in the form of a two-dimensional semiconductor transition metal dichalcogenide (2D TMD) layer excited by laser light, are applied widely in telecommunication for quantum key distribution in secure encrypted connections. Emitters of single photons in the form of a two-dimensional semiconductor layer of tungsten diselenide (WSe<NUM>), which, when excited by an external light with the wavelength of <NUM>-<NUM>, can emit single photons with the wavelength of <NUM>-<NUM>, are disclosed, among others, in the article by <NPL>)] and in the article by <NPL>)].

The article by<NPL>)] reveals that it is possible to fabricate an optoelectronic device emitting single photons, which is based only on an electric supply of the TMD layer. However, the fabrication of such device requires a very advanced technology of transferring thin layers of graphene, hBN or TMD. Such a procedure is very sensitive to external conditions, e.g. vibrations.

The existing sources of single photons, which are based on light emission from quantum dots placed in resonant cavities, still require a complicated system that transmits the light of the exciting laser. Examples of such sources are disclosed in the publication by <NPL>] and in the publication by <NPL>]. The devices described in these publications are offered on the market by Quandela, and their descriptions can be found on the websites: https://optoecomponents. com/single-photon-source-from-quandela/ and http://quandela.

<CIT> discloses a LED device comprising a WSe2 layer as a LED pumped nano cavity laser as a nanocavity laser.

Optoelectronic devices based on gallium nitride and its InAlGaN type alloys, such as light emitting diodes and laser diodes which emit light within the range from ultraviolet to red, have been known for many years. Nitride diodes achieve very high efficiencies and are mass produced as sources of white light and a light source for telecommunication.

Object of the invention is to develop a compact single-photon source that makes use of a nitride light emitting diode as a source of light that excites a photon emitter.

This object is achieved by a hybrid diode according to the invention, characterised in that it contains a photon emitter in the form of a transition metal dichalcogenide (TMD) monolayer and a source of light exciting the emitter, in the form of an epitaxial layer structure of a light emitting diode made of group-III nitrides on a crystalline substrate of gallium nitride. The aforementioned layer structure is composed of, counted from the crystalline substrate, a lower n-type conductivity layer, a tunnel junction, a hole injection layer, an active region, an electron injection layer, and an upper n-type conductivity layer. A photon emitter is placed on the surface of the upper n-type conductivity layer, in a light emission region surrounded by a region of an epitaxial structure that is made electrically inactive by the ion implantation method. The implanted region extends vertically from the upper surface of the electron injection layer to the inside of the lower n-type conductivity layer. The surface area of the light emitting region on the upper surface of the electron injection layer ranges from <NUM> to <NUM><NUM> and can be inscribed into a circle with the diameter of at least <NUM>. The active region of the light emitting diode is composed, counted from the crystalline substrate, of a lower barrier layer, a quantum well layer and an upper barrier layer. The lower and upper barrier layers of the active region are made of an undoped InxGa<NUM>-xN indium gallium nitride alloy, wherein the indium content x ranges from <NUM> to <NUM>. The quantum well layer is made of an undoped InyGa<NUM>-yN indium gallium nitride alloy, wherein the indium content y ranges from <NUM> to <NUM>.

In one of the variants of a hybrid diode according to the invention, a photon emitter is composed of a tungsten diselenide (WSe<NUM>) monolayer, a tungsten disulfide (WS<NUM>) monolayer or a molybdenum disulfide (MoS<NUM>) monolayer.

In another variant of a hybrid diode according to the invention, a tunnel junction is composed, counted from the crystalline substrate, of a first sublayer, a second sublayer, a third sublayer and a fourth sublayer, all made of doped InyGa<NUM>-yN indium gallium nitride alloys. The indium contents y in the first and in the fourth sublayer of the tunnel junction are identical and range from <NUM> to <NUM> and said sublayers are doped with silicon and magnesium, respectively, at the level from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>. The indium contents y in the second and in the third sublayer of the tunnel junction are identical and range from <NUM> to <NUM>. The second sublayer of the tunnel junction is silicon- or germanium-doped with the doping level ranging from <NUM> × <NUM><NUM>cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>, while the third sublayer of the tunnel junction is magnesium-doped with the doping level ranging from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>.

The method according to the invention relates to the fabrication of a hybrid diode, that is, an optoelectronic device containing a photon emitter in the form of a transition metal dichalcogenide (TMD) monolayer and a light source exciting the photon emitter. The invention consists in that first an epitaxial layer structure of a light emitting diode, fulfilling the role of an exciting light source, is fabricated. This structure is fabricated by applying layers of group-III nitrides on a crystalline substrate made of gallium nitride in the process of nitrogen plasma-assisted molecular beam epitaxy (PAMBE) growth. A lower n-type conductivity layer, a tunnel junction, a hole injection layer, an active region and an electron injection layer are deposited successively on the aforementioned crystalline substrate. Subsequently, the layer structure growth process is interrupted and the light emission field on the upper surface of the electron injection layer is limited by surrounding the original light emission field with an epitaxial structure region that is made electrically inactive by the ion implantation method. The layer structure region that is electrically inactive extends vertically from the upper surface of the electron injection layer to the inside of the lower n-type conductivity layer such that the surface area of the limited light emission field ranges from <NUM> to <NUM><NUM> and can be inscribed into a circle with the diameter of at least <NUM>. After the aforementioned limitation of the light emission field, the epitaxial growth is ended by applying a subsequent upper n-type conductivity layer. In the next step, a photon emitter is placed on the surface of the upper n-type conductivity layer, in the previously defined light emission field of the light emitting diode. During the fabrication of the active region of said diode, a lower barrier layer, a quantum well layer and an upper barrier layer are applied successively, counted from the crystalline substrate. The lower and upper barrier layers are made of an undoped InxGa<NUM>-xN indium gallium nitride alloy, wherein the indium content x ranges from <NUM> to <NUM>, while the quantum well layer is made of an undoped InyGa<NUM>-yN indium gallium nitride alloy, wherein the indium content y ranges from <NUM> to <NUM>.

In one of the variants of the method according to the invention, a tungsten diselenide (WSe<NUM>) monolayer, a tungsten disulfide (WS<NUM>) monolayer or a molybdenum disulfide (MoS<NUM>) monolayer is used as a photon emitter.

In another variant of the method according to the invention, the application of a tunnel junction consists in applying successively, counted from the crystalline substrate, a first sublayer, a second sublayer, a third sublayer and a fourth sublayer, which are made of doped InyGa<NUM>-yN indium gallium nitride alloys. The indium contents y in the first and in the fourth sublayer of the tunnel junction are identical and range from <NUM> to <NUM>, wherein said sublayers are doped with silicon and magnesium, respectively, with the doping level from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>. The indium contents y in the second and in the third sublayer of said tunnel junction are identical and range from <NUM> to <NUM>. The second sublayer of the tunnel junction is doped with silicon or germanium with the doping level ranging from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>, while the third sublayer of the tunnel junction is doped with magnesium with the doping level ranging from <NUM> × <NUM><NUM> cm-<NUM> to <NUM> × <NUM><NUM> cm-<NUM>.

A hybrid diode according to the invention is characterised by a high emission quality because the g<NUM> correlation coefficient determined for said diode is significantly lower than <NUM>. This is an effect of the spatial limitation of the exciting light emission region, which made it possible to limit the impact of the background radiation in the wavelength range corresponding to TMD layer emission. Said diode successfully replaces a traditional single photon emitter. The low operating voltage of such a diode allows harmful heating of the epitaxial structure, which might lead to deterioration of emission quality, to be avoided. It enables an effective conversion of the current into exciting light. It is possible to adjust the size of the exciting emission region in a simple manner by adapting it to the applied size of the photon emitter. The small dimensions of a hybrid diode according to the invention enable its placement in a standard TO5. <NUM> type diode mount, which allows easy integration with the existing electronic circuits. A hybrid diode according to the invention does not require positioning (aligning) the exciting light beam, which essentially limits the impact of vibrations and shocks on the stability of the operation of this device. The small dimensions enable also the application of efficient and economical cooling systems, such as Peltier devices, in order to achieve the required operating temperature. The invention makes it possible to build miniature and portable single photon emission circuits, which significantly extends the area of their possible applications, for example in miniature signal transmitters for quantum key distribution in telecommunication (for example in <NUM> networks).

An embodiment of the invention is reflected in a drawing, with <FIG> presenting schematically a vertical profile of a hybrid diode according to the invention, <FIG> presenting a tunnel junction structure of the diode from <FIG> presenting a hole injection layer structure of the same diode. <FIG> presents a spectrum of electroluminescence of a source of light exciting the photon emitter from <FIG> for eight different temperatures ranging from <NUM> to <NUM>. <FIG> presents a spectrum of an excited photon emitter from <FIG> at the temperature of <NUM>, while <FIG> presents a function of the g<NUM> correlation of the photons emitted by said emitter at the temperature of <NUM>.

The present invention is presented in more detail in the embodiment described below. The exciting light source of an embodiment of a hybrid diode according to the invention was fabricated in the process of nitrogen plasma-assisted molecular beam epitaxy (PAMBE) growth. The substrate <NUM> of gallium nitride with the thickness of <NUM> was baked in a prechamber at the temperature of <NUM> for <NUM> hours. Subsequently, the baking was continued in a preparatory chamber at the temperature of <NUM> for <NUM> minutes. The baked sample was transferred to a growth chamber and epitaxial growth was performed in metal-rich conditions. The nitrogen used for the growth was supplied in the form of plasma generated by a power supply unit with the power of <NUM> W and the nitrogen flow of <NUM> sccm. At the temperature of <NUM> a lower n-type GaN conductivity layer <NUM>, with the thickness of <NUM> and the silicon doping level of <NUM> × <NUM><NUM> cm-<NUM>, was fabricated. Subsequently, the growth temperature was reduced to <NUM> and a tunnel junction <NUM> with the thickness of <NUM> was fabricated. The tunnel junction <NUM> was composed of four sublayers: 3A, 3B, 3C and 3D (<FIG>). The first, counted from the substrate <NUM>, sublayer 3A with the thickness of <NUM> was made of a silicon-doped In<NUM>Ga<NUM>N alloy, with the doping level of <NUM> × <NUM><NUM>cm-<NUM>. The second sublayer 3B with the thickness of <NUM> was made of a silicon-doped In<NUM>Ga<NUM>N alloy, with the doping level of <NUM> × <NUM><NUM> cm-<NUM>. The third sublayer 3C with the thickness of <NUM> was made of a magnesium-doped In<NUM>Ga<NUM>N alloy, with the doping level of <NUM> × <NUM><NUM> cm-<NUM>. The fourth sublayer 3D of the tunnel junction <NUM> had the thickness of <NUM> and was made of a magnesium-doped In<NUM>Ga<NUM>N alloy, with the doping level of <NUM> × <NUM><NUM> cm-<NUM>. Subsequently, the growth temperature was increased to <NUM> and a hole injection layer <NUM> with the thickness of <NUM> was made of magnesium-doped GaN, with the first <NUM> of the thickness (4A) of the layer <NUM> was doped with magnesium with the doping level of <NUM> × <NUM><NUM>cm-<NUM>, while the successive <NUM> of the thickness (4B) of the layer <NUM> was doped with magnesium with the doping level of <NUM> × <NUM><NUM> cm-<NUM> (<FIG>). Subsequently the temperature was reduced to <NUM> and an active region was fabricated, composed of a lower barrier layer <NUM> with the thickness of <NUM>, made of an undoped In<NUM>Ga<NUM>N alloy, of a quantum well layer <NUM> with the thickness of <NUM>, made of an undoped In<NUM>Ga<NUM>N alloy, and of an upper barrier layer <NUM> with the thickness of <NUM> made of an undoped In<NUM>Ga<NUM>N alloy. Subsequently the temperature was increased and an electron injection layer <NUM> with the thickness of <NUM> was fabricated of GaN. At the next stage the sample was removed from the molecular beam epitaxy (MBE) machine and subjected to the photolithography process for the purpose of creating target regions of emission of light that excites the emission of single photons. After masking the regions with the diameter D (<FIG>), with the size ranging from <NUM> to <NUM>, with a photoresist with the thickness of <NUM> on the upper surface of the electron injection layer <NUM>, the sample was subjected to the process of He+ implantation with the energy of <NUM> keV and the dose of <NUM> × <NUM><NUM> cm-<NUM>. It is important that the implantation depth <NUM> is so deep enough for ions to penetrate the strongly doped tunnel junction <NUM>. After removing the photoresist away, the sample was placed in the prechamber of the MBE machine at the temperature of <NUM> for <NUM> hours. Subsequently, the baking was continued in a preparatory chamber at the temperature of <NUM> for <NUM> minutes. The baked sample was transferred to a growth chamber and epitaxial growth of the upper n-type conductivity layer <NUM> with the thickness of <NUM>, made of silicon-doped GaN with the doping level of <NUM> × <NUM><NUM> cm-<NUM>, was performed in metal-rich conditions. After removing from the MBE machine again, metallisation <NUM> (upper contact) with the thickness of <NUM>/<NUM> was made on the gallium side outside the light emitting region and metallisation <NUM> (lower contact) was made on the whole nitrogen surface of the substrate <NUM>. Subsequently, devices with contacts <NUM> and <NUM> with the dimensions of <NUM> x <NUM> were separated. The surface areas of the light emitting regions, limited by the regions <NUM>, which were made electrically inactive as a result of ion implantation, ranged from <NUM> to <NUM><NUM>, arising from the diameters D of photoresist mask, with which the layer structure was shielded from the impact of the aforementioned ion implantation. In the case of a different shape of the mask that shields LED structure from the impact of ion implantation that results in electric inactivity, it is important that the target light emitting region can be inscribed into a circle with the diameter D of at least <NUM>. Subsequently, a photon emitter <NUM> in the form of a WSe<NUM> monolayer was placed on the aforementioned light emitting region with the use of a micromanipulator. After applying the monolayer <NUM>, the instrument was fastened in a TO5. <NUM> type semiconductor package. Because the upper surface of the layer <NUM> is flat regardless of the LED light emitting region size, it is possible to place the emitter <NUM> layer thereon accurately. Also, a tungsten disulfide (WS<NUM>) monolayer or a molybdenum disulfide (MoS<NUM>) monolayer can be used as an emitter <NUM> of a hybrid diode according to the invention. A LED, exciting the emitter <NUM>, fabricated according to the aforementioned method, emits light only in the region which was not subjected to ion implantation (<NUM>). <FIG> presents the current flow <NUM> from the lower <NUM> to the upper <NUM> contact by means of a dashed line. A high-conductive n-type layer (<NUM>) makes it possible to place an upper contact <NUM> outside the light emitting region with the diameter D. The limitation, by the aforementioned ion implantation <NUM>, of the light emitting region in the LED, reduces the luminous power, and as a consequence essentially reduces the undesirable impact of photons, generated by said LED, with the energy comparable to photons from the emitter <NUM>. This is very important due to be able to distinguish single photon emission from WSe<NUM>.

<FIG> presents an electroluminescence spectrum of said diode for the current density of 25A/cm<NUM> for eight different operating temperatures ranging from <NUM> to <NUM>. Individual spectra are shifted along the vertical axis in the diagram to separate them for the purpose of a better presentation of the results. As shown by said diagram, the amplitude of the electroluminescence exciting the emitter <NUM> does not decrease in line with the reduction of the operating temperature, which makes it possible to effectively excite the TMD layer. The diagram shown in <FIG> presents a spectrum of excitation of a WSe<NUM> monolayer to emit photons by the aforementioned LED operating at the temperature of <NUM>. This diagram shows a very narrow emission line in the wavelength of <NUM>. <FIG> presents the correlation function (g<NUM>) of photons at the temperature of <NUM> emitted by the aforementioned hybrid diode according to the invention for the aforementioned wavelength of <NUM>. The minimum of the function is only <NUM>, while in the known devices for single photon emission a minimum of <NUM> is acceptable. The behaviour of the g<NUM> correlation function was determined in the Hanbury Brown and Twiss interferometer, wherein detailed information about the methods of quality assessment of devices that emit single photons, a description of the g<NUM> correlation function used for this purpose, and a description of the interferometer used in this embodiment are disclosed in the publication by <NPL>)].

Claim 1:
A hybrid light emitting diode for emitting single photons, wherein it contains a photon emitter (<NUM>) in the form of a transition metal dichalcogenide (TMD) monolayer and a source of light exciting said emitter (<NUM>), in the form of an epitaxial layer structure of a light emitting diode made of group-III nitrides on a crystalline substrate (<NUM>) of gallium nitride, with said structure being composed of, counted from the crystalline substrate (<NUM>), a lower n-type conductivity layer (<NUM>), a tunnel junction (<NUM>), a hole injection layer (<NUM>), an active region (<NUM>, <NUM>, <NUM>), an electron injection layer (<NUM>), and an upper n-type conductivity layer (<NUM>), wherein the photon emitter (<NUM>) is placed on the upper surface of the upper n-type conductivity layer (<NUM>), in a light emitting region surrounded by a region (<NUM>) of an epitaxial structure that is made electrically inactive by the ion implantation method, extending vertically from the upper surface of the electron injection layer (<NUM>) to the inside of the lower n-type conductivity layer (<NUM>), the surface area of the light emitting region on the upper surface of the electron injection layer (<NUM>) ranges from <NUM> to <NUM><NUM> and can be inscribed into a circle with the diameter (D) of at least <NUM>, while the active region of the light emitting diode is composed, counted from the crystalline substrate (<NUM>), of a lower barrier layer (<NUM>), a quantum well layer (<NUM>) and an upper barrier layer (<NUM>), wherein the lower (<NUM>) and upper (<NUM>) barrier layers are made of an undoped InxGa<NUM>-xN indium gallium nitride alloy, wherein the indium content x ranges from <NUM> to <NUM>, while the quantum well layer (<NUM>) is made of an undoped InyGa<NUM>-yN indium gallium nitride alloy, wherein the indium content y ranges from <NUM> to <NUM>.