Abstract:
A high speed and miniature detection system, especially for electromagnetic radiation in the GHz and THz range comprises a semiconductor structure having a 2D charge carrier layer or a quasi 2D charge carrier layer with incorporated single or multiple defects, at least first and second contacts to the charge carrier layer, and a device for measuring photovoltage between the first and second contacts. System operation in various embodiments relies on resonant excitation of plasma waves in the semiconductor structure.

Description:
BACKGROUND 
     The region of electromagnetic waves lying in the giga-terahertz (THz) frequency range has recently become of increasing interest in various fields of science and technology. In part, such interest is caused by the upcoming need for higher frequency computer communication channels and systems. In addition, large toxic molecules of biological and chemical agents have resonant absorption lines in the THz region, thereby enabling, for example, tomography of different human tissues. Also detection of certain (chemical) weapons and explosives could be accomplished. Other potential applications can encompass detection of structural and other defects in materials, food inspection, and investigation of astronomical objects. 
     SUMMARY 
     The following is a summary description of illustrative embodiments of the invention. It is provided as a preface to assist those skilled in the art to more rapidly assimilate the detailed design discussion which ensues and is not intended in any way to limit the scope of the claims which are appended hereto in order to particularly point out the invention. 
     Embodiments disclosed hereafter provide a new fast, portable, miniature electromagnetic radiation detector based on a resonant excitation of plasmons in semiconductor systems containing two-dimensional (2D) charge carrier (electron or hole) layers with an incorporated defect. The operation frequency of the detector lies in the millimeter/submillimeter ranges (corresponding frequencies between 1 GHz and 10 THz). The device can comprise one or more semiconductor structures that each comprise at least one two-dimensional charged layer (electron or hole) with at least one intentionally incorporated defect and at least two potential contacts to said layer or layers. A “defect” may comprise any inhomogeneity introduced into the two-dimensional charged layer. For example, such a defect could be realized in the following forms: an etched area, a restriction or expansion, metallic coverage, impurity doping, dielectric environment defect, structural defect, etc. In materials with strong anisotropy, charge carriers may be allowed to move in three dimensions, but predominantly move in two dimensions, thus comprising a “quasi” two dimensional charge carrier layer. 
     Optionally, the device may include apparatus for applying a magnetic field perpendicular to the charge carrier layer and/or a means for tuning the electron density in the charge carrier layer. The radiation to which the device is exposed can be detected by measuring a voltage/current induced by the radiation. Matrix cameras can be easily created on the basis of the disclosed basic detector because of its small size (normally of the order of a few micrometers) and lack of moving components. 
     An output signal of the measuring device or detector provides information about at least one of the presence of electromagnetic radiation and the intensity of the incident electromagnetic radiation. A detection system embodiment can include matrices of detectors for giving information about the spacial distribution of at least one of the presence of electromagnetic radiation and the intensity of the incident electromagnetic radiation. 
     An illustrative method of detecting electromagnetic radiation as disclosed hereafter includes the steps of: directing radiation on the device, thereby causing excitation of plasmons in the presence of the electromagnetic radiation; detecting the excitation of said plasmons by measurement of photovoltage or photocurrent related to it; and forming/evaluating the result of said measurement to obtain information about the electromagnetic radiation. 
     Operation of illustrative embodiments according to the principles hereafter disclosed may further include the following features:
         1. Incident electromagnetic radiation is coupled to the potential probes and/or to the two-dimensional charged layer and/or to the antenna structure evaporated on top of the crystal, thereby inducing on them an alternating potential.   2. The alternating potential gives rise to plasma waves, which propagate in the cavity formed by the two-dimensional charged layer.   3. The plasma waves are partially reflected by the defects and resonate in the cavity formed by at least one potential probe and at least one defect and/or in the cavity formed by at least two defects. This generates a complicated oscillating electric field inside the device. The amplitude of this field is determined by the ratio of the cavity size to the wavelength of the plasmons which, in turn, is a function of the radiation frequency, applied magnetic field, and the carrier density in the device.   4. The oscillating electric field inside the device is rectified by non-linear behavior of the device, resulting in a de voltage between different pairs of potential contacts. The non-linear behavior may be caused by non-linear volt-ampere characteristics of the transition between contact and charged layer and/or by the presence of at least one defect. The amplitude of the measured signal contains information on the intensity of the radiation.       

     The illustrative detector operation just discussed has been demonstrated in GaAs/AlGaAs quantum-well devices, fabricated in the form of a stripe with two contacts at the ends with a defect or a series of defects introduced across the stripe. The photovoltaic effect produced an easily detectable signal at temperatures up to 200 K. It would appear that embodiments operating at higher temperatures up to and above ambient can be configured according to the principles set forth herein. Successful detector operation of illustrative embodiments has been verified in the frequency span from 1 GHz to 600 GHz. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side schematic view of a detector according to an illustrative embodiment; 
         FIG. 2  is a top schematic view of the detector of  FIG. 1 ; 
         FIG. 3  is a side schematic view of an alternative embodiment; 
         FIG. 4  is a top schematic view of the embodiment of  FIG. 3 ; 
         FIG. 5  is a side schematic view of an alternative embodiment; 
         FIG. 6  is a top schematic view of the embodiment of  FIG. 5 ; 
         FIG. 7  is a side schematic view of an alternative embodiment; 
         FIG. 8  is a top schematic view of the embodiment of  FIG. 7 ; 
         FIG. 9  is a top schematic view of an alternate embodiment comprising a charge layer formed as a heterojunction; 
         FIG. 10  is a side schematic view of the embodiment of  FIG. 9 ; 
         FIG. 11  is an energy band diagram illustrative of one embodiment of a device according to  FIGS. 9 and 10 ; 
         FIG. 12  is a side schematic view of an alternate embodiment comprising a restriction defect; 
         FIG. 13  is a top schematic view of the device of  FIG. 12 ; 
         FIG. 14  is a side schematic view of an alternative embodiment comprising an etched area defect; 
         FIG. 15  is a top schematic view of the embodiment of  FIG. 14 ; 
         FIG. 16  is a side schematic view of an alternate embodiment comprising a step defect; 
         FIG. 17  is a top schematic view of the embodiment of  FIG. 16 ; 
         FIG. 18  is a side schematic view of an alternate embodiment comprising two gate defects; 
         FIG. 19  is a top schematic view of the embodiment of  FIG. 18 ; 
         FIG. 20  is a side schematic view of an alternate embodiment comprising six gate defects; 
         FIG. 21  is a top schematic view of the embodiment of  FIG. 20 ; 
         FIG. 22  is a side schematic view of an alternate embodiment employing interleaved defects; 
         FIG. 23  a top schematic view of the embodiment of  FIG. 22 ; 
         FIG. 24  is a graph of photovoltage vs electron density for an illustrative embodiment of the device of  FIG. 10 ; 
         FIG. 25  is a graph depicting mixing measurements (normalized response vs frequency in Giga Hertz) for an illustrative embodiment of the device of  FIG. 10 ; 
         FIG. 26  is a graph of detector voltage vs magnetic field strength; 
         FIG. 27  is a graph of detector voltage vs magnetic field strength; 
         FIG. 28  is a graph of detector signal amplitude vs temperature in degrees Kelvin (K); 
         FIG. 29  is a graph of detector voltage vs neck width for illustrative embodiments of the device of  FIGS. 12 and 13 ; 
         FIG. 30  is a graph of detector voltage vs magnetic field for an illustrative embodiment of the device of  FIGS. 14 and 15 ; 
         FIG. 31  is a graph of detector voltage vs magnetic field for an illustrative embodiment of the device of  FIGS. 18 and 19 ; and 
         FIG. 32  is a graph of photovoltage vs. magnetic field. 
     
    
    
     DETAILED DESCRIPTION 
     There has been an increased interest in the behavior of elementary excitations such as plasma excitations (plasmons) in low-dimensional electron systems. The impetus for such interest comes both from scientific interest and the many potential applications in the field of millimeter and submillimeter radiation detection. Plasmons are elementary excitations in solids associated with oscillations in charge density relative to the background of screened impurities. Plasmons in a two-dimensional electron system were first reported and observed in liquid helium in 1976, and later in silicon inversion layers (1977) and GaAs heterostructures (1979). The spectrum of 2D plasmons in the long-wavelength limit (k F &gt;&gt;q&gt;&gt;w/c) was calculated as early as in 1967 by Stem as follows: 
     
       
         
           
             
               
                 
                   
                     
                       w 
                       p 
                       2 
                     
                     ⁡ 
                     
                       ( 
                       q 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           n 
                           s 
                         
                         ⁢ 
                         
                           ⅇ 
                           2 
                         
                       
                       
                         2 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           m 
                           * 
                         
                         ⁢ 
                         
                           ɛ 
                           0 
                         
                         ⁢ 
                         
                           ɛ 
                           ⁡ 
                           
                             ( 
                             q 
                             ) 
                           
                         
                       
                     
                     ⁢ 
                     q 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Here, q is the wave vector of the plasmon, while ns and m* are the density and the effective mass of the two-dimensional electrons. The permittivity of vacuum and the effective permittivity of the surrounding medium are denoted as ∈ 0  and ∈(q), respectively. The 2D plasmon spectrum possesses two features: (i) it is gapless, i.e., the 2D plasmon frequency approaches zero as q approaches zero, and (ii) the plasmon frequency is perturbed by the geometry and dielectric properties of matter in the immediate vicinity of the 2D electron system via the effective permittivity ∈(q) in Equation (1). For instance, for a real case of a silicon MOS (metal-oxide-semiconductor), the 2D plasmon spectrum reads: 
                       w   p   2     ⁡     (   q   )       =           n   s     ⁢     ⅇ   2           m   *     ⁢     ɛ   0         ⁢     q       ɛ     0   ⁢           ⁢   X       +       ɛ   Si     ⁢   coth   ⁢           ⁢     qd   ′                     (   2   )               
where ∈ 0X  and ∈ Si  are the effective permittivities of the oxide and silicon layers, and d is the thickness of the oxide layer. For most of the experimentally demonstrated cases in GaAs/AlGaAs heterojunctions and quantum wells ∈(q)=(∈+1)/2 with ∈=12.6 being the dielectric constant of GaAs.
 
When introducing an external perpendicular magnetic field, Eq. (1) no longer describes the plasmon excitations in the two-dimensional electron system and modifications to the plasmon spectrum are bound to occur. The magnetic field evokes plasma waves, which are confined to the edge of the electron system and propagate along the edge in the direction determined by the orientation of the field. The dispersion of these edge-magnetoplasmons has been calculated by Volkov and Mikhailov in 1988 under the assumption of a uniform conductivity tensor across the sample and an abrupt drop in the conductivity at the sample edge:
 
                       ω   emp     ⁡     (   q   )       ⁢   α   ⁢       σ     ∞   ⁢           ⁢   y         2   ⁢           ⁢     ɛ   0     ⁢     ɛ   ⁡     (   q   )           ⁢   q           (   3   )               
The Hall conductivity is denoted as σ ∞y  α n s /B. Edge magnetoplasmons are observable if the applied magnetic field B satisfies the condition ω c τ&gt;1. Here, ω c  is the cyclotron frequency.
 
     With respect to embodiments of tunable detectors of electromagnetic radiation, a special device geometry is created. This geometry restricts plasmon propagation to a certain space—plasmonic cavity. If the geometric resonator length amounts to L, then due to interference, only plasma waves with wave numbers q=mπ/L (m=1, 2, 3 . . . ) are excited. The plasmon frequency may be easily derived from the dispersion laws considered heretofore. 
     The photovoltaic effect, induced by incident giga-terahertz radiation has been observed in a number of device embodiments. In particular, the illustrative embodiments use two-dimensional electron and/or hole systems, where plasma waves are excited by the incident radiation. For simplicity, an electron system is used below, however it is to be understood that these results apply to a hole system as well. A device family is shown schematically in the  FIGS. 1-22 . 
       FIGS. 1 and 2  show an illustrative device  11  according to an illustrative embodiment. The two-dimensional (2D) electron gas  13  is formed on the interface between the substrate  15  and the barrier layer  17 . The electrons are attracted to the interface by the electric field arising from the potential at gates  23 . The corresponding energy band diagram  29  is shown at the right of  FIG. 1 , where E f  represents the Fermi level and E c  and E v  represent the borders of the conduction and valence bands, respectively. In the device  11  of  FIGS. 1 and 2 , the two-dimensional electron gas  13  is confined in a triangular potential well that is formed on one side due to band banding (banding of electron energy levels) and on the other side by conduction band discontinuity. 
     The electron system of the device  11  comprises a defect which is formed by the gate slit  27 . The width of slit  27  should be of the order of oxide thickness to maintain the device channel. The device  11  terminates at each end in respective contacts  19 ,  21 . The defect  27  and the contacts  19 ,  21  restrict the regions of the electron system to lengths L 1  and L 2 . Under the incident radiation these regions act as resonant plasmon cavities tuned by the gate potential or/and magnetic field. 
     Additional embodiments are depicted in  FIGS. 3-22 . All of these embodiments differ in the number and type of involved defects. For example, the device  31  of  FIG. 3  comprises a step-like defect  32 , while the device  33  of  FIG. 5  includes two step-like defects at 34, demarcating three plasmon cavities of lengths L 1 , L 2 , and L 3 . Finally, the device  35  of  FIG. 7  includes two slit-shaped defects  36 ,  37 . 
       FIGS. 9-11  show an alternative embodiment in which the device  41  comprises a charge layer  43  formed as a heterojunction. The device  41  includes a substrate  47 , a two-dimensional charge layer  43 , and two edge contacts  49 ,  51 . Further, the device  41  includes a single metallic gate defect  53 . 
     As an example, the energy band diagram  45  for the case of AlGaAs/GaAs heterojunction in a device  41  is illustrated in  FIG. 11 . The two-dimensional electron gas originates in a potential well that is formed at the boundary between materials with different band gaps. As shown in  FIG. 11 , an illustrative device structure comprises an initial 10 nanometer (nm) region of GaAs; a 50 mm region of Al 0036 Go 0.36  As with delta doping; an 18 nm region of Ga As as quantum well; a “superlattice” region of AlAs, Al 0.36 Ga 0.64  As and GaAs; and a subsequent region of GaAs substrate. 
     The photovoltaic effect, induced by incident giga-terahertz radiation has been experimentally observed in a number of device embodiments. The experiments have been performed on a quantum-well GaAs/AlGaAs heterostructure  41  as shown in  FIG. 11 . The first device embodiment which was verified is further depicted in the inset of  FIG. 24 . The device comprises a two-dimensional electron layer  43  processed into a stripe geometry with a single metallic gate defect  53 . The device geometries were as follows: the stripe width W (0.1 mm and 0.05 mm) and different distances “L” between potential probe  51  and the gate  53  (200 μm, 100 μm, 50 μm, 30 μm, 10 μm). Different gate positions on the stripe  43  were examined, but these cases do not contribute much to physical understanding of device operation. The device response was a combination of signals from two cavities formed by the defect and two subsequent contacts. The density of electrons in the sample was about n=2×10 11  cm −2  and the low-temperature (4.2 K) mobility about 1×10 6  cm 2 /Vs. The sample was placed either in an oversized 16×8 mm waveguide or in an optical cryostat behind the window. In the case of the cryostat, terahertz radiation was focused at the sample by means of quasi-optical reflectors and lenses. Generators covered the frequency range from 1 GHz to 1 THz with output power levels ranging from 10 to 0.1 mW. 
       FIG. 24  shows the dependence of device signal (mV) on electron density n 1  beneath the gate  53  under microwave irradiation with frequency f=43 GHz. Electron density n 1  was tuned by applying a voltage to the gate  53 , in an experiment carried out in zero magnetic field. The device  41  has the width of W=0.1 mm and the distance L=0.2 mm. It should be noted that, according to Equation (1), at the frequency 43 GHz, the first plasmon mode is excited in the resonance cavity formed between the gate  53  and the potential probe  51 . 
     Turning back to  FIG. 24  we see that the device response is greatly affected by the boundary condition formed by the gate defect  53 . The detector signal mV is raised several thousand times by changing n 1  from n 1 =n=2×10 11  cm −2  to 0.2×10 11  cm −2 . From the experimental data, an estimate for the device responsivity R and noise equivalent power N E P has been done with the resultant R=10 3  V/W, N E P=10 −13  W/Hz 0.5  at an operating temperature of 200 K. These values of R and N E P are comparable to those of commercial GHz-THz detectors such as Schottky diodes, Golay cells, pyroelectric detectors, and microbolometers. However, in comparison to the aforementioned commercial detectors, the disclosed plasma wave detector has an advantage of a much lower response time (up to 10 ps). For example,  FIG. 25  illustrates results for mixing measurements on the device  41  with gate defect  53 . Radiation from two microwave generators with different frequencies was mixed and directed onto the device. Due to the non-linear response of the device, the output signal comprises a harmonic at the differential frequency of the two generators. The  FIG. 25  represents the amplitude of this harmonic versus the differential frequency at two temperatures. The mixing bandwidth amounts to 50 GHz, corresponding to the device response time τ=20 ps. 
     The resonant frequency of the plasmon cavity could be readily tuned by changing its electron density or/and by applying external magnetic field (see Eq. (1)-Eq. (3)).  FIG. 26  displays how the magnetic field (T) influences the device signal (mV). The consequent maximums correspond to the excitation of different plasmon modes with q=(π/L) N (where N=1, 2, . . . being integers) in the cavity. From the figure, it becomes obvious that the cavity size greatly affects the B-spacing between the resonances. This feature confirms the very concept of the plasmon cavity. According to Eq. (3), dispersion of plasma waves in external magnetic field B has the following form: 
     
       
         
           
             ω 
             ⁢ 
             
                 
             
             ⁢ 
             α 
             ⁢ 
             
               n 
               B 
             
             ⁢ 
             q 
           
         
       
     
     The consequent resonances correspond to the excitation of plasmon modes with q (π/L)N (where N=1, 2, . . . being integers). That is, combining with Eq. (3) we get the following expression for the spacing between the adjacent resonances: 
     
       
         
           
             Δ 
             ⁢ 
             
                 
             
             ⁢ 
             B 
             ⁢ 
             
                 
             
             ⁢ 
             α 
             ⁢ 
             
               n 
               L 
             
           
         
       
     
     This formula explains the relation between ΔB and L in  FIG. 26 . Successful detector operation was verified in the frequency span from 1 GHz to 0.6 THz. In  FIG. 27 , magnetic field dependent detector signal traces are depicted for different frequencies. First, from  FIG. 27  it is apparent that plasmon cavities with different sizes cover different frequency ranges. Second, the higher device operation frequency the smaller the plasmon cavity which is needed. The following table shows experimentally obtained approximate maximum working frequencies F for different plasmon cavity sizes L. 
                                                                                             Cavity size                L = 400   L = 200                       μm   μm   L = 100 μm   L = 50 μm   L = 20 μm                        Working   &lt;40 GHz   &lt;80 GHz   &lt;140 GHz   &lt;0.25 THz   &lt;0.6 THz       frequency                    
The table illustrates the fact that terahertz frequencies are achievable with no need for complicated and high cost sub-micron technologies.
 
     All the experiments previously discussed were conducted at a temperature of 4.2 K. The temperature dependencies of the photovoltaic effect for cavities with different sizes is presented in  FIG. 28 . The experiments were carried out at zero magnetic field when n 1 =1×10 11  cm −2 . Plots for different L were scaled up to start from one point at T=4.2 K. In general, the responsivity of the detector drops with temperature increase. However, The signal amplitude is only weakly decreased up to a critical temperature T c  for each cavity size. For example, for L=0.1 mm the critical temperature is T c =125 K, but the signal is still observable even at T=200 K. The depicted signal behavior could be ascribed to decrease of the plasmon coherence length with temperature. An abrupt drop in signal amplitude occurs when the coherence length amounts to the cavity size. From the theoretical point of view (see for example S. A. Mikhailov, Appl. Phys. Lett. 89, 042109 (2006)), the plasmon coherence length depends linearly on the electron concentration. Therefore, an increase in concentration and decrease in cavity size could elevate the operation temperature to ambient point. 
     The device embodiment  65  ( FIGS. 12 ,  13 ) which has been tested experimentally is presented in  FIG. 29 . The tested structure has a stripe-shape charge layer  67  with incorporated restriction defect  69 . Sizes used in the experiment are as follows L=0.15 mm, W=50 μm. The radiation frequency was chosen to satisfy the first plasmon cavity mode condition. The detector signal (mV) at zero magnetic field is greatly affected by the restriction geometry. For the simplest rectangular geometry shown in  FIG. 29 , the detector signal (mV) is sufficiently raised by decreasing the neck width W 1 . 
       FIG. 30  shows operation of an alternative embodiment in which the device comprises the etched area defect  70  ( FIGS. 14 ,  15 ). Electron density n 1  in the defect area was equal to n 1 =n/2=10 11  cm −2 . Two geometries have been examined with L=0.2 mm and L=0.1 mm. The device photovoltage displays a magnetic field T tunable resonant response due to the plasmon cavity between contact  51  (??) and defect  70 . If the size of the plasmon cavity is doubled, the distance between resonant peaks is decreased two times. This agrees well with Eq. (4) and reflects the plasmon dispersion peculiarities.  FIGS. 16 and 17  illustrate an alternate embodiment including a step defect  71 . 
       FIG. 31  illustrates the case when two gate defects  81  and  83  (e.g.  FIGS. 18 ,  19 ) are implemented on a single device  72 . The device  72  under consideration has a width of W=50 μm. The length L 1 =150 μm of the plasmon cavity  85  formed by contact  49  and defect  81  is equal to the length of the second plasmon cavity formed by the other contact  51  and the defect  83 . The length of the third plasmon cavity formed by defect  81  and defect  83  is equal to L 1 =40 μm. The photovoltage device response ( FIG. 31 ) reveals a combination of the signals coming from the three independent plasmon cavities  85 ,  86  and  87 . The device embodiment discussed with respect to  FIG. 30  provides an opportunity to measure photo-response from the separate plasmon cavity restricted by two easily-tuned defects, as opposed to a defect and non-tunable contact boundary. 
     The tunability of the device embodiment of  FIG. 31  is well illustrated in  FIG. 32 , where the device under investigation has the following dimensions to L 1 =300 μm, L=50 μm and stripe width W=50 μm. The electron density in the device is equal to n=1.4*10 11  cm −2 . Curve  107  corresponds to the case when only defect  81  is active, that is a voltage is applied to the gate with respect to contact  49  and the electron density in the defect  81  region equals to n=1.4*10 11  cm −2 , and the electron density in the defect  83  region is not changed n 1 =n. For the curve  108 , the opposite situation is realized, only defect  83  is active. The photovoltage device response which originates from the plasmon cavity formed by defects  81  and  83  changes polarity when the active defect is altered. Thus, the device signal symmetry can be easily controlled by the dual gate defects. Curve  109  corresponds to the symmetric case when both defects function. It is apparent that asymmetry introduced into the system greatly increases the device response. The inset to  FIG. 32  represents the dependence of photovoltage oscillation amplitude calculated at the magnetic field B=1 Tesla on the electron density under the working gate defect. The upper dots correspond to the active defect  83 , and the bottom dots correspond to the active defect  81 . 
       FIGS. 20 and 21  illustrate a further alternate embodiment employing six gate defects  92 , while  FIGS. 22 and 23  illustrate an alternate embodiment employing alternating metallization  101 ,  102 ,  103 ,  104 ,  105 ,  106  wherein alternating defects are commonly connected. 
     It should be understood that the experimental data has been set forth only for purposes of clarity and illustration. It is not intended to be exhaustive or to restrict the invention to the precise disclosed examples. While various aspects of the invention have been discussed in terms of an electromagnetic wave detection method, it is understood that the disclosed findings and discoveries apply to all aspects of radiation managing. These aspects encompass generation, mixing, and/or frequency multiplication of radiation having a particular frequency. 
     Thus, the foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. Therefore, it must be understood that many modifications and variations are possible. Such modifications and variations are intended to be included within the scope of the appended claims.