Abstract:
A photon receptor having a sensitivity threshold of a single photon is readily fabricated on a nanometric scale for compact and/or large-scale array devices. The fundamental receptor element is a quantum dot of a direct semiconductor, as for example in a semiconductor (such as GaAs) isolated from a parallel or adjacent gate electrodes by Nano-scale gap(s). Source and drain electrodes are separated from the photoelectric material by a smaller gap such that photoelectrons created when a photon impinges on the photoelectric material it will release a single electron under a bias (applied between the source and drain to the drain) to the drain electrode, rather than directly to the gate electrode. The drain electrode is connected to the gate electrode by a detection circuit configured to count each photoelectron that flows to the gate electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to the U.S. provisional application having Ser. No. 60/729,147 and entitled “Single Photon Receptor”, filed on Oct. 20, 2005, which is incorporated herein by reference. 

   BACKGROUND OF INVENTION 
   The present invention relates to a photon receptor, which may be configured to detect a single photon. 
   There is a need for a photon receptor that is capable of detecting a single photon. Single photon detection is also useful as a low level light detection means for spectroscopy, medical imaging, military applications or astronomy. An optimum signal to noise ratio is achieved when a photon wave is detected by an array of photon receptors, as the noise is then limited by the shot noise and is independent of noise. 
   Single photon receptors are available in the form of photo multiplier tubes (PMT) and single photon avalanche photo diodes (SPAD). PMTs have the disadvantage of having low quantum efficiency, being expensive, bulky, mechanically fragile, and requiring high biasing voltages and cooling. They can also be damaged and can require a long settling time after exposure to high light levels or stray magnetic fields. On the other hand, SPADs have the disadvantage of having a relatively low gain and high dark count rates, especially when operated at higher repetition rates. They are also expensive and require high bias voltages and external cooling. 
   Prior methods of providing a single photon detection threshold photodetector are described in U.S. Pat. Nos. 6,720,589 and 6,885,023, which are incorporated herein by reference. U.S. Pat. No. 6,885,023 (to Shields , et al., issued Apr. 26, 2005) discloses an optical device and a method of making an optical device, such as a radiation detector or an optically activated memory, that includes a barrier region located between two active regions. One or more quantum dots are provided such that a change in the charging state of the quantum dot or dots affects the flow of current through the barrier region. The charging states of the quantum dots are changed by an optical device. 
   U.S. Pat. No. 6,720,589 (also to Shields, issued Apr. 13, 2004) discloses a semiconductor device, which can be configured as optically activated memories or single photon detectors. The devices comprise an active layer with a plurality of quantum dots and an active layer. The devices are configured so that charge stored in the quantum dots affects the transport and/or optical characteristics etc of the active layer. Hence, measuring such a characteristic of the active layer allows variations in the carrier occupancy of the quantum dots to be determined 
   The devices of the &#39;023 and &#39;589 patents generally require fabricating devices having one or more sheets of semiconductor quantum dots buried within another thin film layer, and generally comprising 4 to 8 total layers to form an active device. As the fabrication of multiple thin films and active semiconductor layers in the structure suggested in the above patents poses technical challenges that generally decrease yield and increase manufacturing cost. 
   SUMMARY OF INVENTION 
   In the present invention, a single photon receptor comprises a direct semiconductor quantum dot a drain electrode disposed adjacent to the first side of the quantum dot being there from separated by a first gap, a source electrode disposed adjacent and opposite the first side of the quantum dot being separated there from by a second gap, a collector region disposed opposite the quantum dot being separated there from by a third gap, the third gap being greater than the first and second gap. A detection unit having electron counting means is disposed between the collector region and the drain electrode, wherein applying a bias between drain and source electrodes causes photoelectrons generated when light impinges on the quantum dot to flow from the quantum dot to the drain electrode, and from the drain electrode to the collector through the detection unit. The unit comprises a switch that opens or closes in response to each electron flowing through it permitting the counting of photons received by the quantum dot. 
   A second aspect of the invention is characterized in that the single photon receptor is fabricated from a planar substrate that has been coated with planar layers of a direct semiconductor, such as Gallium Arsenide and alloys thereof, (GaAs) separated by an insulator such as silicon dioxide (SiO2). Such a substrate would have a layer structure sequences of such as of: GaAs/SiO2/GaAs/bulk substrate of undoped silicon, and may include additional layers of isolated semiconductors, such as N or P doped silicon for forming solid state detection circuitry. 
   The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic illustration showing a first embodiment of the invention 
       FIG. 2  is a plan view of a second embodiment of the invention. 
       FIG. 3  is a schematic illustration showing another embodiment of the invention that includes filters and/or collection optics. 
       FIG. 4  is a plan view illustrating another embodiment of the invention that includes a plurality of sensors organized as an array. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 through 4 , wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved photodetector, generally denominated  100  herein. 
   In accordance with the present invention,  FIG. 1 , the photodetector  100  comprises a direct semiconductor quantum dot  110 , a positive or drain electrode  120  separated from the semiconductor quantum dot  110  by a first gap  115 , a negative or source electrode  130  separated from the semiconductor quantum dot  110  by a second gap  135  and a gate electrode  140  separated from the direct semiconductor quantum dot  110  by a third gap  145 . Circuit trace  121  connects the positive or drain electrode  120  to the gate electrode  140 . Circuit trace  131  connects the negative or source electrode  130  to the gate electrode  140 . 
   Generally, the gate electrode  140  is the same order of size as the direct semiconductor quantum dot  110 . However, the positive or drain electrode  120  is connected to the gate electrode  140  via a detection unit  150  disposed between portion  123  and  122  of circuit trace  121 . The detection unit  150  generally comprises one or more photodiode (n-p-n type). It will be recognized by those of ordinary skill in the art that Single Electron Transistors (SET) technologies can be deployed to intercept generated electron (from absorption of Photon in GaAs and some other materials) and then count them with a digital signal processor and like components. 
   The device is powered by a bias means interposed to bisect circuit trace  131  into a first segment  133  connecting negative or source electrode  130  to the negative pole of bias means  160  and a second segment  132  connecting the positive pole of bias means  160  to gate electrode  140  via a second segment  132 . It should be appreciated that circuit segments  132  and  122  may both connect directly to gate electrode  140 ,or as shown in  FIG. 1  joint at junction  142 , which then connects to gate electrode  140  via circuit segment  141 . 
   The first  115  and second  135  gaps are generally both smaller than the third gap  145  so that when a photon is absorbed by the direct semiconductor quantum dot  110 , creating an electron-hole pair; the bias promotes tunneling of the electron to the drain electrode  120 . This results in the hole remaining in the quantum dot  110 . As the gate electrode  140  is in close proximity to the direct semiconductor  110 , the electron, being negatively charged flows to the gate electrode to balance the charge on the quantum dot  110 . The detection circuit measures this change in current to count each photon incident on quantum dot  110 . 
   Preferably, the bias is about 1 electron volt, or possibly lower to cause the free photoelectron to tunnel through gap  115 . The optimum bias will depend on the shape and effective area of the source and drain electrodes with respect to the quantum dot. In practice, the optimum maximum bias voltage can be readily adjusted when the device is not exposed to any photonic radiation (i.e. a dark state) by reducing the bias until no current flows through the device. Thus, when the device is illuminated, any current generated is due to photoelectrons flowing to the gate electrode  140  to balance the positive charge on the quantum dot  110 . 
   In additional embodiments of the invention, shown schematically in  FIG. 3 , an optical filter  170  is deployed to block, reject or reflect light of wavelengths not of interest that would merely add background noise to the signal, i.e. photons of interest. 
   In an alternative embodiment, shown schematically in  FIG. 3 , collection optics  180  are deployed above the photodetector  100  to focus photons on the smallest possible quantum dot so as to increase the absolute area sensitivity of the device. Although the collection optical element  180  is illustrated as a refractive optical lens, it will be appreciated by one of ordinary skill in the art that any device that acts as an electromagnetic field director, such as without limitation reflective optics, including fresnel optics, as well as hybrid and diffractive optical elements are equally applicable to improve the area sensitivity of the device by collecting and focusing photons from an area much larger than the device itself. 
     FIG. 3  also illustrates an optical mask  190  that blocks, absorbs or reflects all radiation that might be incident on portions of the device other than quantum dot  110 . This is important in embodiments wherein the source, drain and gate electrodes are fabricated from direct semiconductor materials to limit the production of electron-hole pairs to the quantum dot portion  110  of device  100 . 
   Examples of potentially suitable materials for such quantum dots are GaAs, InP, Al x Ga (1-x) As, GaxIn (1-x) AsyP (1-y) , GaInNAs and GaInNAsSb. GaAs, or gallium arsenide is particularly preferred as a direct semiconductor. When the direct semiconductor that forms the quantum dot is GaAs, the quantum dot diameter may be as small as about 1 to 5 nm with a thickness as small as about 2 lattice layers. It should be appreciated that the important distinction on the size of the semiconductor is that it has dimensions that cause it to act as a quantum dot. Thus, to the extent that it may be possible to form quantum dots with much larger molecules, for example considering the possibility of organic direct semiconductor molecules, the physical size of a quantum dot can be significantly larger than 5 nm, and conceivably as larger as several microns. 
   However, in the preferred embodiments utilizing GaAs as the direct semiconductor when the physical gap between each electrode and GaAs photoelectric material is about 1 to 5 nm, then the gap between the GaAs quantum dot and the collector or gate electrode will generally be greater than this first gap, generally by at least about 1 nm. It should also be appreciated that the maximum gap between each of the source and drain electrode with the quantum dot will depend on the bias and electrode shapes, so that the resistance due to the gap will be greater than the thermal energy fluctuation at room temperature. 
   In some embodiments, the collector  140  is a parallel disk of substantially the same size as the quantum dot. This can be readily accomplished by forming the device of either  FIG. 1  or  2  from a multilayer semiconductor substrate. Such a multilayer semiconductor substrate comprises a bulk silicon substrate having thereon a layer of silicon dioxide A layer of doped conductive silicon is disposed on the layer of insulating silicon dioxide. Then another layer of insulating silicon dioxide, or another dielectric material, is disposed on the doped conductive silicon. Finally, a layer of the direct semiconductor that is a photoreceptor, such as gallium arsenide (GaAs), is deposited on the dielectric layer. U.S. Patent Application No. 2004/0232525 (to Ramdani, et al., published Nov. 25, 2004) describes various methods of forming structures comparable to the above wherein a semiconductor structure, from any of the Group  111 A and VA elements (III-V semiconductor compounds), mixed III-V compounds, Group II (A or B) and VIA elements (II-VI semiconductor compounds), and mixed II-VI compounds can be formed on doped, i.e. conductive silicon with an intervening layers dielectric or insulating thin film, material, such as silicon dioxide. 
   A central region of the upper GaAs layer is readily defined by the etching processes to have nanoscale lateral dimensions so as to form a quantum dot. Adjacent layer of GaAs form the source and drain electrodes respectively, with the gap between each of the drain and source electrode being defined by the width of the etched trench that continues down to SiO 2  layer. The third gap is defined by the thickness of the SiO 2  layer which separates the upper most parallel and planar quantum dots of GaAs from the gate electrode of conductive silicon. It should be appreciated that the conductive traces between adjacent portion of the upper layer of GaAs (the source electrode) and the lower GaAs layer that forms the gate electrode or collector can be formed as vias between through the intervening layers. Likewise, the conductive traces between adjacent layer of GaAs (the drain electrode) and the lower GaAs layer that forms the gate electrode or collector can be formed as vias between through the intervening layers that also connect detector electronics. 
   It should also be appreciated that as the photodetector  100  can be fabricated with nano sized detector elements; other embodiments of the invention include arrays of multiple detectors with adjacent thin film detection circuits. An exemplary portion of such an array  400  is shown in  FIG. 4 , showing four photodetectors:  100 ,  100 ′,  100 ″ and  100 ′″. The electron detection circuitry  150  is preferably formed as integrated circuits on the same substrate as each detector  100 . Further, a common bias source  160  may be used to power each of the four detectors  100  in array  400 , being connected in parallel to the source electrode  130  adjacent each semiconductor quantum dot  110 . However, the drain electrode  120  adjacent each semiconductor quantum dot  110  is connected to a separate electron detector  150 . The electron detection circuitry  150  may take the form of any known solid state device that acts as a relay of switch such as transistors and zener diodes. The X and Y traces adjacent each photodetector  100  are connected to the electron detector  150  at switch contacts  151  and  152 . Normally the X and Y traces, labeled X 1 , X 2  and Y 1  and Y 2  are isolated from each other. However, the flow of an electron from drain electrode  120  to electron detector  150  closes the switch shorting poles  151  and  152  between adjacent pair of traces X 1  and Y 1 . Each of the X and Y traces is connected to addressing circuitry to detect such shorts and thus correlate the closing of switch  150  with a specific location on the detector  400  where each photon is received. Such addressing circuitry is well known to one of ordinary skill in the art of photodetector arrays used in imaging devices and the like. 
   While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.