Abstract:
A spin coherent, single photon detector has a body of semiconductor material with a quantum well region formed in barrier material in the body. The body has a first electrode forming an isolation electrode for defining, when negatively energized, an extent of the quantum well in the body and a second electrode positioned above a location where an electrostatic quantum dot is defined in said quantum well when positively energized. The quantum well occurs in three layers of material: a central quantum well layer and two outer quantum well layers, the two outer quantum well layers having a relatively low conduction band minimum and the barrier having a relatively high conduction band minimum while the central quantum well layer having a conduction band minimum between the relatively high and relatively low conduction band minimums.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The present invention was made with support from the United States Government under Contract No. DAAD19-01-C-0077 for a Scalable Quantum Information Processing and Applications awarded by the Army Research Office and the Defense Advanced Research Projects Agency (DARPA). The U.S. Government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     This invention relates to the design of a spin-coherent, single-photon detector and a spin-resonant transistor, devices that will allow the next generation of quantum computers to talk to each other using fiber-optic light beams. 
     BACKGROUND INFORMATION 
     A single-photon detector absorbs photons (particles of light), while faithfully transmitting quantum information onto individual, spinning electrons. In order for this data transfer to be efficient, electrons in the devices must reside within a thin layer referred to as a quantum well and have the property that, in a magnetic field, the energy for “spin-up” and “spin-down” electrons are equal. Through a process known as g-factor engineering, it is well-known that the magnetically-induced energy difference between “spin-up” and “spin-down” electrons can be controlled by varying the composition and thickness of the quantum well and the surrounding material (the barrier). For additional information see Kiselev, Kim and Yablonovitch “Designing a heterostructure for the quantum receiver”  Applied Physics Ltrs.  vol. 80, num. 16, pg. 2857-2859, 22 Apr. 2002, which is hereby incorporated herein by reference. 
     A prior art spin-coherent, single-photon detector is described in some detail already in R. Vrijen and E. Yablonovitch, “A spin-coherent semiconductor photo-detector for quantum communication” Physica E., vol. 10, pg. 569 (2001), which is hereby incorporated herein by reference. 
     The present disclosure suggests an improvement, namely, adding a third material, located within the quantum well itself, which should relax certain constraints on the design of so-called zero g-factor devices. The new material should possess a different g-factor and a different band alignment than the quantum well material and typically also different than the barrier material. 
     The addition of a third material to the design of zero g-factor devices is expected to provide the following advantages: 
     1. The quantum well should be less susceptible to fluctuations in its thickness because the presence of the third material within the well will allow it to be much thicker. 
     2. The spin-resonant transistor is a device for which the individual electrons are held in position by applying a positive voltage to a gate electrode. In some situations, it is desirable to modulate the g-factor by varying the bias on the gate electrode. The new quantum well disclosed herein will allow for the g-factor of the trapped electrons to be more sensitive to applied gate bias and therefore, will require less modulation to change the g-factor by a given amount. This is an advantage because too large a change in gate bias can either cause a trapped electron to leave or attract a second electron to enter the device. Both of these alternatives can be detrimental to the performance of the device. 
     3. The new quantum well is more flexible with respect to engineering tradeoffs between well thickness, g-factor, and band gap energies of the well and barrier materials. The added flexibility in the design makes the devices more easily compatible with commercial, networking technologies. 
     The term “zero g-factor devices” embraces, of course, devices which have a zero effective, weighted g-factor. But since devices whose effective, weighted g-factor is dose to zero are also satisfactory, one issue becomes how dose to zero is close enough? For a spin-coherent detector to work properly, the device is put in a magnetic field, which results in an energy difference between “spin-up” and “spin-down” electrons. This energy difference (referred to a the Zeeman energy) is directly proportional to both the strength of the aforementioned magnetic field and the g-factor. The choice of the magnetic field can vary significantly. The g-factor must be close enough to zero to make the Zeeman energy less than a linewidth (expressed in terms of energy) of the photons to be detected. 
     The disclosed technology is expected to be important to the development of novel, quantum information processing devices. Such devices will have applications in secure communications and quantum computing, currently active areas of research funded by the US Government (notably DARPA and ARDA). 
     The prior art is described in the aforementioned paper by Kiselev et al. (Applied Physics Letters, v. 80, p. 2857 (2002)) in which the authors thereof discuss how to design a heterostructure to achieve zero g-factor in a quantum receiver application. The results show that by varying the quantum well thickness and Ga concentration, for an InGaAs quantum well in InP, the g-factor can be tuned from large and negative, to somewhat positive. For a g-factor dose to zero, however, the thickness of the quantum well becomes very small and the Ga composition becomes large, resulting in films that are highly strained and metastable. In addition, single monolayer fluctuations in quantum well thickness can cause large variations (percentagewise) in the performance of these devices. 
     Modifying the structure of a quantum well to change its properties is not a new idea; however, this is believed to be the first time that anyone has applied these concepts to g-factor engineering. We have experimented with very thin InGaAs quantum wells in InP at HRL Laboratories LLC in Malibu, Calif., and know from experience that electrons within these layers are extremely difficult to control. The advantages of using a different material for the barrier and a third material within the well became clear to us during the course of our experimental work. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect the present invention provides a spin coherent, single photon detector comprising a body of semiconductor material having a quantum well region formed in barrier material in said body, said body having first and second electrodes formed thereon, the first electrode forming an isolation electrode for defining, when negatively energized, an extent of the quantum well in said body and the second electrode being positioned above a location where an electrostatic quantum dot is defined in said quantum well in response to positive energization of said second electrode, the quantum well being defined by at least three layers of material, a central quantum well layer and two outer layers, the two outer layers of the quantum well having a relatively low conduction band minimum and the barrier having a relatively high conduction band minimum while the central quantum well layer having a conduction band minimum between the relatively high and relatively low conduction band minimums, and wherein an effective, weighted g-factor for the detector is sufficiently close to zero that the Zeeman energy is less than a linewidth, expressed in terms of energy, of photons to be detected by the detector. 
     In another aspect the present invention provides spin coherent, single photon detector comprising a body of semiconductor material having a quantum well region formed in barrier material comprising AlInAs in said body, said body having first and second electrodes formed thereon, the first electrode forming an isolation electrode for defining, when negatively energized, an extent of the quantum well in said body and the second electrode being positioned above a location where an electrostatic quantum dot is defined in said quantum well in response to positive energization of said second electrode, the quantum well occurring in at least three layers of material: a central quantum well layer comprising InP and two outer layers each comprising InGaAs. 
     In still yet another aspect the present invention provides a method of reducing the susceptibility of a quantum well to fluctuations in well thickness during manufacture, the method comprising forming a central layer with a quantum well, the central layer being sandwiched between two adjacent layers, the two adjacent layers of the quantum well having a relatively low conduction band minimum and a barrier having a relatively high conduction band minimum while the central layer has a conduction band minimum between the relatively high and relatively low conduction band minimums, and wherein an effective, weighted g-factor for the quantum well is sufficiently close to zero that the Zeeman energy is less than a linewidth, expressed in terms of energy, of photons to be detected by the detector. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1   a,    1   b  and  1   c  depict the band structure of a quantum well under different bias conditions. Electrons occupying states in the well can be made to oscillate under an applied bias between two layers composed of Material B. Material C forms a small barrier within the well which helps modulate the effective g-factor. 
         FIG. 2  is a graph of g eff  vs the gate voltage (V gate ). For an appropriate choice of materials and thicknesses, the effective g-factor (g eff ) of an electron in a quantum well can be made to vary from negative to positive values. In principle, one could design the structure to have two bias points where the g-factor could be equal to zero (see the Xs). The voltage at which the g-value reaches a maximum can also be engineered. 
         FIG. 3  is a side elevation view of the layer structure of a quantum well in accordance with the present disclosure. 
         FIGS. 4   a  and  4   b  are exploded perspective views showing a gated Hall bar device being used with my improved quantum well, which can be used to help measure the g-factor of different material systems, different geometries and well sizes as well as with different gate voltages applied thereto. 
         FIGS. 5   a  and  5   b  are exploded perspective views showing a spin-coherent, single photon detector in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention involves the use of three semiconductor materials, A, B, and C possessing the following properties. Material A is a quantum well barrier material which preferably has a positive g-factor and also has a relatively high conduction band minimum compared with Materials B and C. Materials B and C define the quantum well. Material B has a negative g-factor and a low conduction band minimum. Material C is preferably disposed in the middle of Material B and has a positive g-factor and a moderate conduction band minimum. In one embodiment Material A preferably comprises AlInAs, Material B preferably comprises InGaAs and Material C preferably comprises InP. Advantageously, these particular materials can be selected so that they are all lattice matched to InP. 
     The situation is depicted schematically in  FIGS. 1   a,    1   b  and  1   c.  In  FIG. 1   a,  a negative gate bias is applied to the quantum well (formed by Materials B and C) causing electrons to be driven away from the gate (to the right in  FIG. 1   a ). This causes the average electron in the well to experience primarily the g-factor associated with material B (there is little penetration of the wavefunction into material A because it has such a high conduction band minimum). Therefore, under high, negative biases, the effective g-factor is negative. 
     Similarly for high positive biases (see  FIG. 1   c ), the electrons are driven closer to the gate (to the right in  FIG. 1   c ), and the average electron experiences primarily the g-factor of Material B once again. Therefore, the effective g-factor is also expected to be negative for a high, positive gate bias. 
     At flatband (close to V GATE =0) as shown in  FIG. 1   b,  the average electron has a significant likelihood of being in material C and experiencing its positive g-value. For an appropriate choice of materials and thicknesses, the effective g-factor for an electron at or near flatband can be made positive. 
     These assertions imply a qualitative behavior of the effective g-factor as a function of applied gate bias similar to what is depicted in  FIG. 2 . At least  2  bias points should be available for which the effective g-value is equal to zero. The g-factor should also be a strong function of gate bias, particularly near the flat band condition. A great deal of flexibility in the design should enable control over the gate voltage for which the g-factor is a maximum. 
     The most desirable choices of materials satisfying the conditions detailed above is AlInAs for material A, InGaAs for material B, and InP for material C, as indicated above. One can also use compositions for the AlInAs and InGaAs so that these materials can be grown strain-free on an InP semi-insulating substrate. 
       FIG. 3  is side elevation view of the layer structure of a quantum well formed by materials A, B and C. The device disclosed herein is preferably fabricated on a semiconducting heterostructure consisting of a quantum well (Materials B and C) and surrounding barrier material (Material A), with doped n-type material in thin layers possibly both above and below the quantum well (see  FIG. 3 ). The doped layers  10  may have a thickness of about 10 nm and doping concentrations in the range of 1×10 16  to 1×10 18  cm −3 . The figures show the doped layers  10  and the quantum well  20 , itself comprised of three layers containing materials B and C. The best values to use for the doping concentrations in the layers  10  above and below the quantum well  20 , and the composition and thickness of the layers of materials B and C in the quantum well are not known. The thickness of the three layer stack shown in  FIG. 3  can be about 5 to 15 nm. The thickness of the individual layers A, B and C can be allowed to vary, depending on the desired response of the g-factor to applied bias. The barrier layer A may be formed on a substrate  32 , which may be formed of the same material as Material C, for example. 
       FIGS. 1   a - 1   c  and  2  depict a possible response of the quantum well band structure (and the wavefunction associated with a trapped electron within the quantum well) to applied bias for a device geometry referred to as a gated Hall bar. A drawing of a gated Hall bar, with the added layer of material C in the quantum well, is shown in  FIGS. 4   a  and  4   b.  Such a device is not a spin-coherent, single photon detector but rather, a useful diagnostic tool to understand how the g-factor varies under an applied bias. Experiments are currently underway at HRL Laboratories on devices like this to gather data on the bias dependence of the g-factor for InGaAs quantum wells in InP. The data allows one to refine 3-D models of the electrostatics of such and similar device geometries including more complex gate arrangements. 
       FIG. 4   a  and  4   b  are exploded, perspective views of the Hall bar with the added layer of material C in the quantum well.  FIG. 4   a  shows the Hall bar with metal electrode structure thereon, including a source  22 , drain  24  and a pair of gates  26 . The metal electrode structure is disposed on or over a semiconductor mesa  28  and the quantum well is formed as thin layer  30  within the mesa  28 . The quantum well layer  30  is bounded on its top and bottom sides by Material A previously described with reference to  FIGS. 1   a,    1   b,    1   c,    2  and  3 . The mesa  28  is preferably disposed on substrate  32  and preferably contains layers of Material A, B and C. The magnetic field is also depicted.  FIG. 4   b  shows the quantum well  30  in greater detail which is formed of layers of Material B on either side of a central layer of Material C. 
     The quantum well  30  is preferably designed so that the effective overall g-factor (with the gate voltage applied to gate  26 ) weighted by the electron probably factor that an electron will be in a given layer is approximately equal to zero (as defined above and mentioned below). The gated Hall bar of  FIGS. 4   a  and  4   b  help one to determine what the effective g factor is for various possible quantum well sizes, materials and applied gate biases. 
     As indicated above, if AlInAs is used for material A, and InGaAs is used for Material B and InP is used for Material C, these material choices allow for the effective weighted g factor for the device to be approximately zero. Furthermore, if both AlInAs and InGaAs are grown lattice-matched (i.e. strain-free) on InP for x Ga =0.47 (Ga fraction in InGaAs) and x Al =0.48 (Al fraction in AlInAs), a strain-free device will result, which has certain advantages. Some very small amount of strain in a heterostructure will not hurt it, but increasing the strain leads to defect formation and performance degradation. These problems become more extreme should high process temperatures be required during device fabrication. So it is best practice to reduce the strain and it can be eliminated altogether using the preferred materials for Materials A, B and C and by adjusting their constituent concentrations so that these materials have the same crystalline lattice constants. 
     One possible embodiment of a spin-coherent, single photon detector is shown in  FIGS. 5   a  and  5   b.  For a negative bias applied to the circular gate electrode  34  and a positive bias applied to the air-bridged gate electrode  36 , one can create a situation for which an incident light particle (a photon)  38  can create an electron-hole pair in the quantum well  40 . These voltages would typically be less that a few volts. In  FIG. 5   a,  the circular gate electrode  34  defines the area or region within which an absorbed photon can be detected. The quantum well layer  30  is quantized in the direction normal to the plane of the ring of the circular gate electrode  34  by selecting its thickness (the thickness of materials B and C) sufficiently thin that quantization of energy levels come into play, as is well known in the art. The quantum well layer  30  is sandwiched between layers of Material A in the mesa  28  on which gate  34  is formed and is preferably formed of three layers, two layers of Material B with a single thin layer of Material C centered therein as shown by  FIG. 5   b.  Because the quantum well  40  is designed to have an effective, weighted g factor≈0 for electrons, quantum information stored in photon polarization states can be faithfully transmitted into electron spin states (see Vrijen et al., Physica E, vol. 10, pg. 569 (2001), mentioned above). The electron is then allowed to drift to the electrostatic quantum dot  42  formed at the center of the quantum well  40  and detected by the spin-resonant transistor thereat. 
     As indicated above, for a spin-coherent detector to work properly, the device is put in a magnetic field, which results in an energy difference between “spin-up” and “spin-down” electrons. This energy difference (referred to the Zeeman energy) is directly proportional to both the strength of the aforementioned magnetic field and the g-factor. The choice of the magnetic field can vary significantly. Preferably, its direction occurs in a plane parallel to the major planes of layers B, C, B, as shown. But its direction can be varied, although a magnetic field which is perpendicular to the major planes of layers B, C, B would typically be the least desirable choice. The g-factor must be close enough to zero (g factor≈0) to make the Zeeman energy less than a linewidth (expressed in terms of energy) of the photons to be detected. 
     The basic concept of a spin-coherent, single-photon detector is described in some detail already in Vrijen&#39;s 2001 paper mentioned above, but as already indicated this disclosure takes that prior art further by incorporating layer Material C into the quantum well, thereby producing advantage(s) discussed above. The circular region defined by the isolation gate  34  can be made as large as about 1 μm in diameter and still allow the electron (from the electron-hole pair formed by the detected photon) to drift to the quantum dot  42  in a reasonable amount of time (&lt;1 μs). As is shown in  FIGS. 5   a  and  5   b,  the spin-resonant transistor gate spans several μm. A tiny post  36   p,  which may be 5-10 nm in diameter, associated with gate electrode  36 , is located in the interior of the isolation gate  34  and usually also in the center of the device (however, it need not be located precisely at the center of the device). The spin-resonant transistor gate, centered on the post  36   p,  continues to a point outside of a mesa  28  which is preferably etched in the heterostructure. Finally, the quantum dot  42  is located several tens of nm below the spin-resonant transistor gate post  36   p.    
     As previously indicated Materials A, B and C should preferably exhibit the following relationships: 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Material A 
                 Material B 
                 Material C 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 bulk g factor 
                 &gt;0 
                 &lt;0 
                 &gt;0 
               
               
                   
                 E c   
                 highest 
                 lowest 
                 middle 
               
               
                   
                   
               
             
          
         
       
     
     As indicated above, the overall weighted (by the electron probability factor) g factor should be at or close to zero. The probability of finding an electron in Materials A, B and C will depend somewhat on their conduction band minimums (E c ). So it could be in certain embodiments that the effective g factor for the barrier material A could be zero or somewhat negative, if its influence is not great upon the overall weighted g factor (which can occur when it has a large E c  compared to the E c  of Material B, giving it a low weighting factor). 
     A small electrostatic gate  36   p  applied to the surface of structures can produce quantum dots  42  under an applied positive bias. The electrons trapped in quantum dots  42  formed in the layered structures of Materials B and C discussed here would make excellent candidates for electron spin qubits. Quantum dot devices such as these can be designed to work alternatively as spin-resonant transistors. These devices ultimately have applications in quantum information processors such as the quantum repeater or in a quantum computer. 
     Having described the invention in connection with a preferred embodiment, modification will now suggest itself to those skilled in the art. For example, so practicing the present invention might consider adding a backside gate in order to control the carrier density in the quantum well  42 . As such the invention is not to be limited to this preferred embodiment except as specifically required by the appended claims.