Patent Publication Number: US-7902570-B2

Title: Apparatus comprising a single photon photodetector having reduced afterpulsing and method therefor

Description:
STATEMENT OF RELATED CASES 
     This case is a division of co-pending U.S. patent application Ser. No. 11/277,562 filed Mar. 27, 2006, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to avalanche photodetectors in general, and, more particularly, to single-photon avalanche photodetectors. 
     BACKGROUND OF THE INVENTION 
     A semiconductor photodetector generates a free-carrier pair (electron-hole) when it absorbs a photon. When the photodetector is subjected to an electric field (by the application of a bias voltage to the photodetector), free-carriers generated in the photodetector give rise to a macroscopic electric current. 
     A useful photodetector is characterized by high overall efficiency and high sensitivity. Efficiency can be defined as the number of free carriers that are generated per incident photon. Consequently, high efficiency implies a high generated current for a given incident optical power. Sensitivity is characterized by the minimum optical signal that gives rise to a current that can be distinguished from the background current due to noise (e.g., dark current, thermal noise, Johnson noise, 1/F noise, etc.). 
     One widely-used type of photodetector is the avalanche photodetector. Avalanche photodetectors have high sensitivity and, in fact, can be made sensitive enough to detect even a single photon. Avalanche photodetectors are so named because of the “avalanche” of free-carrier pairs that is generated by the detector. The “avalanche” is the result of a multiplication of the free-carrier pairs, The multiplication occurs when the free-carrier pairs that were generated by incident photons are accelerated to high energies by an applied reverse bias voltage. As the accelerated free carriers travel through the multiplication region of the avalanche photodetector, they collide with bound carriers in the atomic lattice of the multiplication region, generating more free carriers through a process called “impact ionization.” 
     The current flow in the avalanche photodetector is directly related to the number of free carriers generated from electron-hole pairs. The gain of a photodetector (i.e., the increase in the number of free carrier pairs) is a function of the reverse bias voltage applied to the photodetector. 
     An avalanche photodetector is characterized by a “breakdown voltage.” When the avalanche photodetector is biased above its breakdown voltage, carrier generation can become self-sustaining and result in run-away avalanche. In order to function as a single-photon detector, an avalanche photodetector is biased above its breakdown voltage. This is referred to as “arming” the avalanche photodetector. Once the detector is armed, the single free carrier created by the absorption of a single photon can create a runaway avalanche resulting in an easily detectable macroscopic current. It is also possible for a free carrier to be created by mechanisms other than photon absorption (e.g., thermal excitation and carrier tunneling). These “dark” carriers can give rise to the same easily detectable macroscopic current, in this case referred to as false counts, or “dark counts.” Dark counts constitute noise in a single-photon avalanche detector, and therefore reduce its sensitivity. 
     After a photon (or dark count) is detected, it is necessary to stop the self-sustained avalanche in order to make further use of the detector. In order to halt the avalanche process, the bias voltage of the avalanche photodetector is reduced below its breakdown voltage. This process is referred to as “quenching” the avalanche photodetector. Although quenching stops the avalanche process, not all free carriers are swept out of the avalanche region. Instead, some carriers become trapped in trap states that exist in the multiplication region due to crystalline defects or other causes which create energy levels within the semiconductor band gap of the multiplication region material. 
     At a later time, trapped carriers “detrap,” again becoming free carriers. These detrapped carriers can become an additional source of dark counts. The creation of additional dark counts caused by spurious, uncontrolled emission of trapped charges after quenching is referred to as “afterpulsing.” Afterpulsing raises the total dark count rate above the baseline dark count rate established by thermal carrier emission and carrier tunneling in the absence of afterpulsing. Since any increase of dark count rate degrades the performance of a single-photon detector, elimination of afterpulsing is of great interest. 
     Several strategies exist in the prior art for reducing afterpulsing. Trapped charges will generally become free carriers in random fashion due to their thermal emission from the trap. Therefore, one approach used is to simply wait a sufficient period of time after quenching to allow trapped charges to detrap on their own (i.e., the inherent “detrapping time.” If the inherent detrapping time is long, this approach leads to an undesirably long period of time when the single-photon detector is inoperable. In gated-mode operation, wherein a bias voltage pulse is periodically applied to arm the avalanche photodetector (i.e., a “gating pulse”), simply waiting for thermal emission of trapped carriers reduces the repetition rate at which single photons can be measured. 
     A second prior-art approach for reducing afterpulsing is to operate the single-photon detector at an elevated temperature to promote detrapping. But operating at an elevated temperature results in an increase in the baseline dark count rate due to an increase in thermal carrier emission and carrier tunneling processes. 
     In a third prior-art approach for reducing afterpulsing, trapped carriers are photoionized via “sub-band illumination.” In this approach, the photodetector is illuminated by a beam of light. The energy of the photons in this beam of light is a function of the wavelength of the light. Photons in longer-wavelength light have relatively lower energy than photons in shorter-wavelength light. The wavelength of light used in this prior-art approach is selected to provide photoionization energy sufficient only to detrap carriers, but insufficient to liberate carriers that are not in trapped states. 
     To avoid the detection of the sub-band illumination by the photodetector, the wavelength of light used for sub-band illumination must be longer than the detection limit, or “cutoff wavelength,” of the absorbing material in the photodetector. In the case of an indium-phosphide-based avalanche photodetector with an indium-gallium-arsenide absorbing material, the wavelength of light used for sub-band illumination is greater than 1700 nanometers. 
     There exists a need, therefore, for a single-photon detector with reduced afterpulsing that overcomes some of the limitations of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention enables single photon detection without some of the costs and disadvantages for doing so in the prior art. For example, embodiments of the present invention enable high-repetition-rate gated-mode operation of a single-photon detector. 
     In accordance with the illustrative embodiment of the present invention, trapped carriers in the photodetector are excited by a pulse of stimulating energy, such as a thermal pulse. This proactive excitation of trapped carriers clears energy trap states. This reduces the probability of random emissions of these carriers, as might otherwise occur in the absence of the stimulus (e.g., during conventional operation as a single-photon detector, etc.). As a result, the dark count rate of the photodetector during single-photon operation is reduced. 
     Embodiments of the present invention, like the prior art, use an avalanche photodetector that is biased above its breakdown voltage to detect the incidence of a single photon. Like the prior art, some embodiments of the present invention proactively excite carriers that are in energy trap states, thereby reducing afterpulsing. But unlike the prior art, in the illustrative embodiment of the present invention, trapped carriers are only temporarily excited. 
     More particularly, in the prior art, photodetectors have been operated at an elevated temperature to stimulate the emission of trapped carriers from energy trap states. These photodetectors remain at this elevated temperature during operation as a single-photon detector. As a result, prior-art single-photon photodetectors are subject to high dark count rates. In contrast, the present invention applies only a Pulse of thermal energy to the photodetector to temporarily raise the temperature of a photodetector. The photodetector is not operated in single-photon detection mode during the application of the stimulating pulse. The thermal pulse substantially clears energy trap states of carriers so that the photodetector has a reduced dark count rate when it is subsequently operated as a single-photon photodetector. 
     An embodiment of the present invention comprises: an avalanche photodetector having a multiplication layer comprising a multiplication region; and a stimulator, wherein the stimulator provides a stimulus pulse, and wherein the stimulus pulse detraps electrical carriers in the avalanche photodetector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of the salient components of single-photon detection system  100  according to an illustrative embodiment of the present invention. 
         FIG. 2  depicts a schematic diagram of the salient components of a photodetector according to an illustrative embodiment of the present invention. 
         FIG. 3  depicts a representative timing diagram for photodetector gating and thermal pulsing, according to the illustrative embodiment of the present invention. 
         FIG. 4  depicts a method of operating a photodetector as a single-photon detector in gated-mode operation. 
     
    
    
     DETAILED DESCRIPTION 
     The following terms are defined for use in this Specification, including the appended claims:
         Monolithically-integrated means formed either: (1) in the body of a layer or substrate, typically by etching into the layer or substrate or; (2) on the surface of the layer or substrate, typically by patterning layers disposed on the surface.   Thermal-cycling means temporarily changing a temperature to cause an effect. An example of thermal-cycling is the rapid-thermal annealing of a substrate to induce crystal growth or stress relaxation.   Pulse means a brief sudden change in a normally constant quantity. Examples of pulses include, without limitation: (1) a thermal pulse, wherein a short rapid increase in the temperature of an element or portion of an element is produced; and (2) a voltage pulse, wherein a short rapid increase in a voltage is produced.   Multiplication region means a region of an avalanche photodetector wherein avalanche gain predominantly occurs.       

       FIG. 1  depicts a schematic diagram of the salient components of single-photon detection system  100  according to an illustrative embodiment of the present invention. Single-photon detection system  100  comprises photodetector  102 , stimulator  104 , stimulus sink  106 , and controller  108 . Single-photon detection system  100  is a system that provides a macroscopic current in response to the incidence of a single photon on photodetector  102 . 
     Photodetector  102  is an indium-phosphide-based avalanche photodetector having separate absorption and multiplication regions. Photodetector  102  will be described in detail below and with respect to  FIG. 2 . In some embodiments, photodetector  102  is an avalanche photodetector formed using material systems other than indium-phosphide. It will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that comprise avalanche photodetectors that are based on any suitable material system. 
     Stimulator  104  is a stimulator for providing a pulse of stimulating energy to electrical carriers that have become trapped in energy level traps in the multiplication region of photodetector  102 . The energy provided by stimulator  104  proactively excites the trapped carriers out of their energy trap states. In other words, the pulse of stimulating energy “detraps” the trapped carriers. During the time when stimulating energy is being applied to photodetector  102 , the photodetector is biased below breakdown and can not detect either incident photons or dark carriers. Therefore, photodetector  102  is inoperative for single-photon detection during the duration of the stimulus pulse. After the pulse of stimulating energy ends, however, photodetector  102  can again be made operative for single-photon detection. In some embodiments, stimulator  104  provides stimulating energy to photodetector  102  as a whole. In some other embodiments, stimulator  104  provides stimulating energy to a selected portion or portions of photodetector  102 . 
     Stimulus sink  106  is an element for drawing stimulating energy away from the active area of photodetector  102 . Stimulus sink  106  facilitates a rapid decay of the stimulating energy in the active area of photodetector  102 , which thereby shortens the time during which photodetector  102  is inoperative for single-photon detection. In some embodiments, stimulus sink  106  is a passive element, such as a high stimulus-conductivity path that facilitates the conduction of the stimulus away from the active area of photodetector  102 . In some other embodiments, stimulus sink  106  is an active element that draws the stimulus away from the active area of the photodetector  102 . 
     Controller  108  is a general purpose processor and power supply. Controller  108  provides a bias voltage to photodetector  102 , receives electrical signals from photodetector  102 , stores and processes data, and provides power and control signals to stimulator  104  and stimulus sink  106 . In the illustrative embodiment of the present invention, controller  108  controls the sequence of arming photodetector  102 , quenching photodetector  102 , and stimulating trapped carriers in photodetector with a stimulus pulse, and actively sinking stimulating energy from photodetector  102 . In some embodiments of the present invention, stimulus sink  106  is a passive element that does not require control by controller  108 . 
       FIG. 2  depicts a schematic diagram of the salient components of a photodetector according to an illustrative embodiment of the present invention. Photodetector  102  comprises substrate  202 , absorption layer  204 , grading layer  206 , field control layer  208 , layer  210 , and passivation layer  212 . Layer  210  comprises active region  218 , which comprises diffused-region  214  and multiplication region  216 . 
       FIG. 2  depicts avalanche photodetector  102  integrated with heater  220  and Peltier cooler  222 . 
     Substrate  202  is a substrate suitable for use in the formation of an avalanche photodetector, as is well-known in the art. 
     Absorption layer  204  is a lightly-doped layer of indium gallium arsenide (InGaAs) with low band-gap energy. It will be clear to those skilled in the art how to make and use absorption layer  104 . 
     Grading layer  206  is an n-doped indium gallium arsenide phosphide (InGaAsP) layer that smoothes the interface between absorption layer  204  and field control layer  208 . 
     Field control layer  208  is a moderately n-doped layer of indium phosphide. Field control layer  208  enables maintenance of a low electric field in absorption layer  204 , while supporting a high electric field in multiplication region  216 . It will be clear to those skilled in the art how to make and use field control layer  208 . 
     Multiplication layer  210  is an intrinsic layer of indium phosphide. Within multiplication layer  210  is active region  218  which includes diffused-region  214  and multiplication region  216 . Active region  218  is formed by diffusing a high level of p-type dopant into multiplication layer  210  to form diffused region  214 . The extent of diffused region  214  forms a p-n junction. The undoped portion of active region  218  forms multiplication region  216 . Avalanche multiplication occurs substantially in multiplication region  216 . In some embodiments of the present invention, multiplication layer  210  is a lightly n-doped layer of indium phosphide and diffused region  214  is heavily doped with a p-type dopant. In some other embodiments, multiplication layer  210  is a lightly p-doped layer of indium phosphide and diffused-region  214  is heavily doped with an n-type dopant. It will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention in which multiplication layer  210  is other than an intrinsic layer of indium phosphide. 
     Passivation layer  212  is a layer of silicon nitride that has a thickness of 100 nanometers. In some other embodiments, passivation layer  212  has a thickness other than 100 nanometers. In some embodiments, passivation layer  212  comprises thinned regions on which heater  220  and/or Peltier cooler  222  are disposed to facilitate thermal conduction to and from heater  220  and/or Peltier cooler  222 . It will be clear to those skilled in the art, after reading this specification, how to make and use passivation layer  212 . 
     Heater  220  is a thin-film resistive heater having a semi-annular shape, which is deposited on top of passivation layer  212  just outside the lateral extent of active region  218 . Stimulator  104  comprises heater  220 , as described above and with respect to  FIG. 1 . When an electric current is provided to heater  220  by controller  108 , heater  220  rapidly heats active region  218 . In the illustrative embodiment, the duration of electric current pulses is less than 50 nanoseconds. In some embodiments, the duration of electric current pulses can be as long as several hundred nanoseconds. In some embodiments, electric current pulses as short as 1 to 5 nanoseconds are used. The duration of the current pulse (and, therefore, the heat pulse in active region  218 ) influences the repetition rate at which detrapping can occur. It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention that utilize electric current pulses to heater  220  that are other than 50 nanoseconds in duration. 
     In the illustrative embodiment, heater  220  and photodetector  102  are monolithically-integrated. In some alternative embodiments of the present invention, heater  220  and photodetector  102  are not monolithically-integrated. In yet some further embodiments of the present invention, heater  220  radiates heat to at least a portion of photodetector  102 . It will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention wherein heater  220  and photodetector  102  are not monolithically-integrated. 
     In the illustrative embodiment, thermal energy is the stimulating energy for detrapping trapped carriers in photodetector  102 . In some alternative embodiments, thermal energy is imparted to active region by pulsing optical energy onto photodetector  102 . The wavelength of this optical energy is chosen such that active region  218  will absorb a significant amount of the optical energy and convert it to thermal energy. Suitable wavelengths for this optical energy include those equal to or less than 900 nanometers when the active region comprises indium-phosphide. For embodiments that include a different active region material, suitable wavelengths will be those for which the photon energy is greater than the band gap energy of the active region material. 
     In some alternative embodiments, thermal energy is imparted to active region  218  by pulsing optical energy onto an absorption layer deposited on photodetector  102 . Suitable materials for use in absorption layers include, without limitation, silicon, silicon dioxide, silicon nitride, silicon carbide, tungsten, titanium, titanium-tungsten, titanium nitride, and organic materials. In some embodiments, the wavelength of the optical energy can be greater than 900 nanometers, since suitable wavelengths for the optical energy will be dependent upon the absorption characteristics of the materials used in the absorption layer. 
     In the illustrative embodiment, stimulator  104  comprises heater  220 , which stimulates trapped charges with pulses of thermal energy. In some other embodiments, stimulator  104  stimulates trapped charges with pulses of other forms of energy. It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention that comprise stimulators that stimulate trapped charges with pulses of such other forms of energy. 
     Peltier cooler  222  is a thermo-electric cooler having an annular shape, as is well-known in the art. Peltier cooler  222  facilitates the rapid removal of heat from active region  218  after heater  220  has turned off. Peltier cooler  222  composes stimulus sink  106  as described above and with respect to  FIG. 1 . The speed at which Peltier cooler  222  removes heat from active region  218  influences the repetition rate at which detrapping can occur and the percentage of time in which single-photon detection system  100  is operative. 
     In some alternative embodiments of the present invention, a passive stimulus sink is used rather than an active stimulus sink. For example, in some of these embodiments, a thick-film metallization is used, rather than Peltier cooler  222 , to provide a low thermal resistance path to conduct heat away from active region  218 . 
     In still some other embodiments of the present invention, heater  220  and Peltier cooler  222  are combined into a single element. For example, a Peltier device can be used to either heat or cool, depending on the flow of current through it. Therefore, in these embodiments, Peltier device  222  provides both heating and cooling functions. 
       FIG. 3  depicts a representative timing diagram for photodetector gating and thermal pulsing, according to the illustrative embodiment of the present invention. Timing diagram  300  depicts the relationship between gate pulse train  302  and thermal pulse train  304 . Gate pulse train  302  is a graphic representation of the bias voltage applied to photodetector  102 . 
     In order to more clearly demonstrate the present invention, operation of photodetector  102  as a single-photon detector is described here, with reference to  FIGS. 2 ,  3  and  4 . 
       FIG. 4  depicts a method of operating a photodetector as a single-photon detector in gated-mode operation. Since gated-mode operation comprises a repetitive cycle, only one such cycle is described here. In the gated-mode operation of photodetector  102 , photodetector  102  is biased at a baseline voltage below its breakdown voltage, V br . In order to arm photodetector  102 , a gate pulse that increases the bias voltage above V br  is applied to photodetector  102 . The gate pulse has a gate-pulse period T p  and a gate-pulse width of T g . At the end of the gate pulse (i.e., at T g ), photodetector  102  is quenched by reducing the bias voltage once again below V br . Therefore, when photodetector  102  detects a photon during a gate pulse, the resulting avalanche current signal is limited to the remainder of that gate pulse. At the end of that gate pulse, photodetector  102  is quenched by reducing the bias voltage back below V br  to stifle the avalanche current. 
     Method  400  begins with operation  401 , wherein photodetector  102  is armed for single-photon detection. To arm photodetector  102 , processor  108  provides a bias voltage to photodetector  102  that is above its breakdown voltage, V br . Operation  401  is depicted in  FIG. 3  as the rising edge the first pulse of gate pulse train  302 . 
     At operation  402 , photodetector  102  is quenched by reducing the bias voltage below V br . To quench photodetector  102 , processor  108  provides a bias voltage to photodetector  102  that is below its breakdown voltage, V br . This is depicted in  FIG. 3  as the falling edge of the first pulse of gate pulse train  302 . The time between operations  401  and  402  is equal to gate pulse width T g , which in some embodiments of the present invention is approximately equal to 1 nanosecond. 
     At operation  403 , active area  218  of photodetector  102  is provided with a stimulus to excite trapped carriers from their trapped states (i.e., cause them to detrap). In the illustrative embodiment, the stimulus comprises application of heat to active area  218 . To heat active area  218 , controller  108  provides electric current to heater  220 . Operation  403  is depicted in  FIG. 3  as the rising edge of the first pulse in thermal pulse train  304 . 
     At operation  404 , the stimulus applied to photodetector  102  is removed. To remove the stimulus, controller  108  stops the flow of electric current to heater  220  and provides electric current to Peltier cooler  222  to cause it to begin cooling active area  218 . Operation  404  is depicted in  FIG. 3  as the falling edge of the first pulse in thermal pulse train  304 . The time between operations  403  and  404  is equal to thermal pulse width T t , which in some embodiments of the present invention is in the range of sub-nanosecond to tens of nanoseconds, and in some embodiments is approximately equal to 1 nanosecond. In some embodiments of the present invention, which employ a passive stimulus sink, operation  404  does not include the provision of electric current to Peltier cooler  222 . 
     After operation  404 , photodetector  102  is ready to be armed for single-photon detection again. Since each gate pulse is followed by a thermal pulse to excite trapped carriers in photodetector  102  from their trapped states, the periodicity of method  400  is also equal to T p . In some embodiments of the present invention, the sequence of temporally interleaved gate pulses and thermal pulses can be non-periodic. 
     Immediately after quenching photodetector  102 , a thermal pulse is applied to photodetector  102  to heat active region  218 . When active region  218  is heated, trapped carriers are stimulated to detrap. The rate at which trapped carriers detrap is a function of the temperature of active region  218  (i.e., they detrap more quickly at higher temperatures). The duration of the thermal pulse, T t , is a function of the detrap rate, but can be as short as one nanosecond. Once the thermal pulse has ended (i.e., after T t ), photodetector  102  can be armed again. It should be noted that it is not necessary to wait until the temperature of active region  218  has dropped all the way to its base temperature before arming photodetector  102 . Significant improvement in afterpulsing performance can be obtained from inducing a sufficient temperature swing between detrapping and application of a gate pulse. 
     In the prior art, T p  is typically limited to at least tens of microseconds due to the time required to passively detrap trapped carriers. In contrast, the present invention utilizes active detrapping of trapped carriers by stimulating them to detrap. The present invention, therefore, enables gated-mode operation having a gate-pulse period, T p , as short as 1 to 5 nanoseconds. 
     Therefore, photodetector  102  is armed during gate-pulse width, T g . Timing diagram  300  depicts gated-mode operation with periodicity, T p , equal to 50 nanoseconds. 
     Although the illustrative embodiment describes operation of photodetector  102  in gated-mode operation, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention that comprise operation of photodetector  102  in non-gated-mode operation. For example, in some alternative embodiments of the present invention, photodetector  102  remains armed until a photon is detected. Once photon detection has occurred, photodetector  102  is quickly quenched and a thermal pulse is applied to photodetector  102  to detrap trapped carriers. Subsequent to the thermal pulse, photodetector  102  is armed again to await detection of another photon. 
     In some embodiments of the present invention, photodetector  102  remains armed until controller  108  quenches and detraps it. In some embodiments of the present invention, controller  108  will quench, detrap, and arm photodetector  102  in response to the receipt of a stimulus by controller  108 . In some embodiments of the present invention, controller  108  will quench, detrap, and arm photodetector  102  at the request of an operator. In some embodiments of the present invention, controller  108  will quench, detrap, and arm photodetector  102  after satisfaction of a preset condition, such as a duration between photon detections that exceeds a maximum time-period. 
     It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
     Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.