Abstract:
A photon source includes a substrate, an active region formed above the substrate, and a pair of electrodes configured to provide an injection current which passes through the active region. The active region includes a quantum dot layer including one or more Al y Ga x In 1-x-y N quantum dots, where 0≦x≦1 and 0≦y≦&lt;1, and an AlInN current confinement layer adjacent the quantum dot layer. The current confinement layer has an aperture which defines a low resistance path for the injection current to flow through the active region between the pair of electrodes. The quantum dot layer includes less than 50 quantum dots within the aperture as projected onto the quantum dot layer.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to the field of light emitting devices which are capable of emitting a predetermined number of photons at predetermined times, and more specifically which are capable of emitting a single photon on demand. 
       BACKGROUND OF THE INVENTION  
       [0002]    Quantum information in the form of quantum communications and quantum computing is currently an exceedingly active field. A single source that efficiently produces photons with antibunching characteristics or an entangled-pair of photons is one such pivotal hardware element for quantum information technology. Using a single photon source, secure quantum communication will prevent any potential eavesdropper from intercepting a message without the receiver noticing. Indeed, it works on the fact that measurement of a quantum state causes a disturbance which can be detected by the sender and the intended recipient of the bits. 
         [0003]    There are four main possible sources for single photons; these are a single atom in a cavity (Rempe, PRL 2002), a single nitrogen vacancy in diamond (H. Weinfurter PRL 2000, Grangier PRL 2002), a single molecule at room temperature (B. Lounis and Moerner, Nature 2000), and a single quantum dot. But the first three sources require optical excitation of carriers using an external laser source, which is cumbersome and expensive. However, semiconductor quantum dots can be easily incorporated in an electric device for electrical injection of the carriers. 
         [0004]    Electrically injected single photon emitters have been demonstrated by D. J. P. Ellis et al in Appl. Phys. Lett. 88, 133509 (2006) based upon a quantum dot structure with an aperture in an insulating oxide layer to restrict current injection into a single quantum dot. However, the device used self-assembled InAs quantum dots, thus limiting the working temperature to 10-100° K. Moreover, an annulus of insulating aluminium oxide was used to limit the current injection to a single quantum dot. But uniformity control of the oxidation process is known as an issue which can result to current apertures with different dimensions over the wafer, leading to poor manufacturing yield. Other art which is principally of interest in that it deals with single photon source is U.S. Pat. No. 6,864,501 issued on Apr. 9, 2003, to Shields. 
         [0005]    However, the main obstacle for commonly used epitaxially grown III-V quantum dots as single photon devices, such as InAs quantum dots, is the requirement of liquid helium cryogenic temperatures. II-VI materials have been investigated for overcoming this problem, such as CdSe/ZnS quantum dots. But the quantum efficiency and multi-phonon reduction efficiency at high temperatures were low. 
         [0006]    Nitride quantum dots could allow for high-temperature operation because of strong quantum confinement effects, large optical-phonon energies and large exciton binding energies. 
         [0007]    For example, S. Kako et al., Nature material vol. 5, p 887, November 2006, observed single quantum dot emission up to 250° K. However, they used optical excitation. Electrical injection of the carriers is preferred for practical devices. 
         [0008]    Finally, it can be noted that Castiglia et al., Appl. Phys. Lett. 90, 033514, 2007, described the use of an oxidised AlInN interlayer lattice-matched to GaN to confine the injected current in an InGaN light emitting diode. However, the AlInN was oxidised to be insulating, and there was no mention about using quantum dots in the active region, which is essential for getting single photon emission. 
         [0009]    Though there have been many demonstrations of single photon sources using quantum dots with electrical injection of the carriers, they all required liquid-helium cryogenic temperatures, which is the main obstacle for industrial application. 
       SUMMARY OF THE INVENTION  
       [0010]    So it is an object of the present invention to address the above problems, by providing a single photon source with high efficiency working at room temperature or temperatures easily reachable with thermo-electric cooling. 
         [0011]    In accordance with the principles of the present invention, a new class of single photon sources with electrical injection is provided which offers the possibility of room temperature or easily reachable thermoelectric cooling temperature operation. 
         [0012]    This advantage is realised by using epitaxial nitride quantum dots which provide strong confinement to the carriers. Single photon emission is achieved by restricting the injected current to a single quantum dot. It is proposed to use a layer of single crystal AlInN, the AlInN layer having one aperture such that the dimension of the aperture is tailored to the density of the quantum dots in order to restrict the current injection to a single quantum dot. 
         [0013]    Moreover, the use of high resistance layer of AlInN avoids the need to oxidise the AlInN layer in order to increase its electrical resistance, and avoids the disadvantages mentioned above. 
         [0014]    Thus it is an object of the present invention to produce a new class of single photon source which is capable of emitting predetermined number of photons at predetermined times at room temperature or for temperatures easily reachable with thermo-electric cooling (e.g. above 200° K). These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
         [0015]    According to an aspect of the invention, a photon source includes a substrate, an active region formed above the substrate, and a pair of electrodes configured to provide an injection current which passes through the active region. The active region includes a quantum dot layer including one or more Al y Ga x In 1-x-y N quantum dots, where 0≦x≦1 and 0≦y≦1, and an AlInN current confinement layer adjacent the quantum dot layer. The current confinement layer has an aperture which defines a low resistance path for the injection current to flow through the active region between the pair of electrodes. The quantum dot layer includes less than 50 quantum dots within the aperture as projected onto the quantum dot layer. 
         [0016]    According to another aspect, the quantum dot layer includes less than  10  quantum dots within the aperture as projected onto the quantum dot layer. 
         [0017]    In accordance with another aspect, the quantum dot layer includes only a single quantum dot within the aperture as projected onto the quantum dot layer. 
         [0018]    According to still another aspect, the one or more quantum dots are epitaxial nitride quantum dots. 
         [0019]    According to yet another aspect, the one or more quantum dots have a maximum dimension of less than 50 nm. 
         [0020]    In still another aspect, the one or more quantum dots have a height between 1 nm and 5 nm. 
         [0021]    With regard to another aspect, the current confinement layer has a resistivity greater than 1·10 2  Ω·cm. 
         [0022]    In yet another aspect, the current confinement layer has a resistivity greater than 1·10 4  Ω·cm. 
         [0023]    According to another aspect, the current confinement layer has an In ratio between 0.15 and 0.2. 
         [0024]    According to still another aspect, the current confinement layer has an In ratio of approximately 0.18 so as to maintain a small lattice mismatch to GaN. 
         [0025]    In accordance with another aspect, the current confinement layer is a single crystal AlInN layer. 
         [0026]    According to another aspect, a capping layer disposed on the quantum dot layer is provided, the band gap of the capping layer being higher than the band gap of the one or more quantum dots. 
         [0027]    In accordance with another aspect, the current confinement layer and the capping layer are formed on a same side of the quantum dot layer. 
         [0028]    According to another aspect, the current confinement layer and the capping layer are formed on opposite sides of the quantum dot layer. 
         [0029]    In accordance with yet another aspect, a photon source includes a substrate, a buffer layer formed on the substrate, and a mesa structure formed on the buffer layer. The mesa structure includes an active region with a quantum dot layer including one or more Al y Ga x In 1-x-y N quantum dots, where 0≦x≦1 and 0≦y≦1, a capping layer disposed on the quantum dot layer, the band gap of the capping layer being higher than the band gap of the one or more quantum dots, and an AlInN current confinement layer adjacent the quantum dot layer, the current confinement layer having an aperture which defines a low resistance path for the injection current to flow through the active region, wherein the quantum dot layer includes less than 50 quantum dots within the aperture as projected onto the quantum dot layer. 
         [0030]    According to another aspect, the mesa defines the area of the quantum dot layer. 
         [0031]    In accordance with another aspect, the mesa is circular. 
         [0032]    According to another aspect, the diameter of the mesa is within the range of 1 μm to 20 μm. 
         [0033]    To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0034]      FIG. 1  is a sectional view showing a nitride single photon emitting device according to a first embodiment of the invention. 
           [0035]      FIG. 2  is a sectional view showing the active region of the single photon emitting device containing a layer of (Al,In,Ga)N quantum dots and a AlInN current confinement layer with an aperture. 
           [0036]      FIG. 3  is a sectional view showing a nitride single photon emitting device according to a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    The preferred embodiments of the invention will be described with reference of the drawings. 
         [0038]    A device of the present invention may be grown by any suitable means and on any suitable substrate, which include but is not limited to any orientation of: sapphire, GaN or SiC. 
         [0039]    The first embodiment of the present invention is described with reference to  FIG. 1 . According to the first embodiment of this invention,  FIG. 1  shows a schematic of a single photon emitting device fabricated in the (Al,In,Ga)N material system. The single photon emitting device of  FIG. 1  may contain a sapphire substrate  101 . An n-type buffer layer  102  made in the (Al,In,Ga)N material system, and preferentially in the (Al,Ga)N material system may be disposed on top of the substrate  101 . A non-intentionally doped (Al,Ga)N layer  103  may be disposed on top of the buffer layer  102  to improve injection efficiency of the carriers. The active region  104  is then disposed on top of the layer  103 . A p-type (Al,Ga)N layer  105  may be disposed on top of the active region  104 . On the top surface of the contact layer  105  is a p-electrode  106   a  and on the rear surface of the buffer layer  102  is an n-electrode  106   b.    
         [0040]    The single photon light emitting device of  FIG. 1  may contain an active region  104  shown also in  FIG. 2 . The active region may comprise Al y Ga x In 1-x-y N quantum dots  104   a  disposed on layer  103 . The Al y Ga x In 1-x-y N quantum dots  104   a  may have the composition wherein 0≦x≦1 and 0≦y≦1, such that they may be comprised from GaN, InN, InGaN, and AlGaInN. The quantum dots  104   a  may have the size wherein all three dimensions are each less than 50 nanometers (nm). The quantum dots  104   a  may have a size wherein the height is less than 12 nm. The quantum dots  104   a  may preferably have a height between 1 nm and 5 nm. 
         [0041]    The quantum dots  104   a  may be not intentionally doped. 
         [0042]    An Al y Ga x In 1-x-y N capping layer  104   b  may be disposed on top of the quantum dots  104   a.  The Al y Ga x In 1-x-y N capping layer  104   b  may have a composition wherein 0≦x≦1 and 0≦y&gt;1, such that the band gap of the Al y Ga x In 1-x-y N capping layer  104   b  is higher than the band gap of the quantum dots  104   a.    
         [0043]    The capping layer  104   b  may have a thickness between 1 nm and 100 nm. The capping layer  104   b  may have a thickness between 1 nm and 10 nm. Preferably, the capping layer  104   b  is not intentionally doped or may be p-type doped or n-type doped. In this embodiment, the capping layer  104   b  is preferably not intentionally doped. 
         [0044]    According to the invention, an AlInN layer  104   c  is provided on top of the capping layer  104   b  to act as a current confinement layer. The AlInN layer  104   c  has one aperture defined therethrough, to provide a low resistance path for current to flow between the upper electrode  106   a  and the lower electrode  106   b.  The AlInN preferably has a resistivity higher than 1·10 2  ohm centimeter (Ω·cm) and preferably has a resistivity higher than 1·10 4  Ω·cm. 
         [0045]    In this embodiment, the AlInN current confinement layer  104   c  may have one aperture such that only a few quantum dots are present under the aperture. Preferably, the number of quantum dots under the aperture is less than 50. Preferably, the number of quantum dots under the aperture is 1. For example, if the quantum dot density of the quantum dot layer  104   a  is 1·10 10  cm −2 , a circular aperture size of the AlInN layer  104   c  may have a diameter which is not exceeding 350 nm, i.e. less than 10 quantum dots are present under the aperture, and preferably have a diameter which is around 110 nm, i.e. only 1 quantum dot is present under the aperture. 
         [0046]    The aperture size in the AlInN layer  104   c  is function of the quantum dot density. In order to have only one quantum dot under the aperture, the aperture size can be calculated function to the quantum dot density d as follow: for a square aperture with a side size a, a=squareroot (1/d). For a circular aperture of diameter D, D=2×squareroot (1/(pi×d)). 
         [0047]    The current confinement layer  104   c  is preferably made of AlInN having an In ratio between 0.15 (or In content close to 15%) and 0.2 (20%), and preferably having an In ratio of 0.18 (18%) in order to maintain a small lattice mismatch to GaN. 
         [0048]    A mesa structure  100  is preferably formed. The mesa  100  defines the area of the quantum dot layer. The mesa  100  can be up to  100  micrometers (μm) in diameter, but the preferred diameter is 1-20 μm. A circular mesa is preferred, but alternatively the mesa may take any geometrical shape. 
         [0049]    Mention is next made of an operation of the present invention. 
         [0050]    When a voltage is applied between the two electrodes  106   a  and  106   b,  electrons and holes travel across the active region. Because of the aperture in the AlInN current confinement layer  104   c,  electrons and holes travel only across one quantum dot or a very limited number of quantum dots in the active region  100 . 
         [0051]    At low injection currents, the quantum dot may capture no more than a single electron and a single hole. The single electron and the single hole form an exciton in the quantum dot. The radiative recombination of the electron-hole pair occurs on the timescale of the radiative lifetime. Once the photon is emitted, the quantum dot can capture another electron and another hole. 
         [0052]    For higher injection currents, the quantum dot may capture two electron-hole pairs, which form a bi-exciton. The output spectrum of the quantum dot thus consists of two single lines emitting at two different energies. Moreover, for higher injection currents, the quantum dot may also capture two electrons and a single hole, or two holes and a single electron. In this case, the output spectrum of the quantum dot thus consists of a single line but emitting at a different energy than for the emission of a single electron-hole pair, due to the Coulomb interaction of the extra carrier with the recombining electron-hole pair. Moreover, more than one quantum dot may be present under the aperture of the AlInN current confinement layer  104   c.  In this case, the emission may exhibit extra spectral lines at different energies arising form the emission of different quantum dots. 
         [0053]    Thus, in order to remove photons from the output emitting at unwanted emission energies, the single photon emitting device of the present invention may comprise spectral filtering. The filter may comprise a spectrometer such as a grating or prism spectrometer. An interference filter or a fibre optic device may also be used or any other suitable filtering devices. The filter means may be integral with the single photon emitting device body, or may not be integral with the single photon emitting device body. 
         [0054]      FIG. 3  shows a cross-sectional structure of a single photon emitting device according to a second embodiment of the present invention. The structure of the single photon emitting device of the present embodiment is similar to the one of the first embodiment presented in  FIG. 1 . For example, the device includes a substrate  301 , buffer layer  302 , non-intentionally doped (Al,Ga)N layer  303 , p-type contact layer  307  and electrodes  308   a  and  308   b.    
         [0055]    In the embodiment of  FIG. 3 , however, the AlInN current confinement layer  304  is located under the quantum dot active layer  306   a  and capping layer  306   b,  i.e. the AlInN layer  304  is placed on the side of the n-type doped region of the single photon emitting device of the present invention. A (Al,Ga,In)N layer  305  may be placed between the AlInN layer  304  and the quantum dot layer  306   a.  The (Al,Ga,In)N layer  305  may be preferentially undoped. The (Al,In,Ga)N layer  305  may have a thickness between 1 nm and 100 nm. The (Al,In,Ga)N layer  305  may have preferably a thickness between 1 nm and 10 nm. 
         [0056]    Alternatively, two AlInN current confinement layers may be placed on each side of the quantum dot layer  306   a,  with their apertures being aligned vertically. 
         [0057]    Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.