Characterization of silicon avalanche photodiodes for photon correlation measurements. 3: Sub-Geiger operation Robert G. W. Brown and Matthew Daniels We continue examination of the photon correlation properties of siliconavalanche photodiodes operated in the single-photon counting mode by extending their operation from that of passive [Appl. Opt. 25, 4122-4126 (1986)] and active [Appl. Opt. 26, 2383-2389 (1987)] quenching to the sub-Geiger mode, with potential for high quantum efficiency and very low afterpulsing. 1. Introduction Single-photon counting and correlation is of central importance in many quantum optical experiments. Traditional detectors have been photomultiplier tubes, specially selected for low noise, low afterpulsing, and adequate quantum efficiency.' Recently, wepub- lished results using silicon avalanche photodiodes (APDs) operated in the Geiger mode (reverse bias in excess of breakdown) capable of high quality photon counting and correlation performance. They proved to be an excellent replacement for photomultiplier tubes in our laboratories because of their small size and cost, low noise, relatively small bias voltages, superior ruggedness, and greater quantum efficiency at close- to-room temperature operation. 2 ' 3 Such detectors are now gaining acceptance in quantum optics experi- ments 4 and other fields such as photon correlation spectroscopy and velocity measurements 56 for detec- tion of very weak light fields. This paper addresses the use of silicon APDs in the sub-Geiger mode, moti- vated by the need to increase quantum efficiency, re- duce afterpulsing, and move toward photon counting detector array capability. II. Review Silicon APDs can be arranged to yield single-photon detection at quantum efficiencies exceeding 20% when operated just a few volts in excess of breakdown re- verse bias, the so-called Geiger mode. This can be The authors are with Royal Signals & Radar Establishment, St. Andrews Road, Malvern, Worcs WR14 3PS U.K. Received 24 April 1989. 0003-6935/89/210000-06$02.00/0. achieved with noise dark-counts of only a few hundred per second when operated at -273 K (0 0 C) with the most recently available devices, far superior to our earlier experiments. 23 In addition, for actively quenched Geiger operation, 3 photon counting at rates in excess of 106 s1 is now possible with negligible distortion, and afterpulsing is of little consequence when operated on quench-reset time scales greater than -40 ns. However, these specifications apply only to single-element detectors. In wishing to extend operational capability to 1-D and 2-D detector arrays, the Geiger mode ceases to be valid because photons are generated in the pn junction during the avalanche process. 7 Sufficient optical iso- lation of closely adjacent detectors (pixels) seems im- possible. Thus when one detector detects a photon, some or all of the adjacent detection elements also do so almost simultaneously. This destroys the photode- tection (statistical) integrity of the array of elements. Silicon APDs operated in the sub-Geiger mode offer a possible solution to these problems, through greatly reduced probability of interelement crosstalk. In this mode we find the potential of high quantum efficien- cies, truly negligible afterpulsing, low noise, and feasi- ble interelement isolation in detector array applica- tions, while retaining single-photon counting and correlation capability. The penalties for operating in this mode are in- creased performance demands in high stability and very low noise peripheral electronics; also, at present photon count rates of < = 105 photodetections per pixel per second, unless further limited by peripheral electronics.8 Nevertheless, such a single-photon counting rate per element in an array photon counting detector is exciting when compared with other detec- tors used today, MAMA, PAPA, CCD, etc., 9 and at relatively high temperatures when compared to 4 K impact ionization devices. 10 4616 APPLIED OPTICS / Vol. 28, No. 21 / 1 November1989
Transcript
Characterization of silicon avalanche photodiodes for photoncorrelation measurements. 3: Sub-Geiger operation
Robert G. W. Brown and Matthew Daniels
We continue examination of the photon correlation properties of silicon avalanche photodiodes operated in
the single-photon counting mode by extending their operation from that of passive [Appl. Opt. 25, 4122-4126
(1986)] and active [Appl. Opt. 26, 2383-2389 (1987)] quenching to the sub-Geiger mode, with potential for
high quantum efficiency and very low afterpulsing.
1. Introduction
Single-photon counting and correlation is of centralimportance in many quantum optical experiments.Traditional detectors have been photomultipliertubes, specially selected for low noise, low afterpulsing,and adequate quantum efficiency.' Recently, we pub-lished results using silicon avalanche photodiodes(APDs) operated in the Geiger mode (reverse bias inexcess of breakdown) capable of high quality photoncounting and correlation performance. They provedto be an excellent replacement for photomultipliertubes in our laboratories because of their small size andcost, low noise, relatively small bias voltages, superiorruggedness, and greater quantum efficiency at close-to-room temperature operation.2'3 Such detectors arenow gaining acceptance in quantum optics experi-ments4 and other fields such as photon correlationspectroscopy and velocity measurements5 6 for detec-tion of very weak light fields. This paper addressesthe use of silicon APDs in the sub-Geiger mode, moti-vated by the need to increase quantum efficiency, re-duce afterpulsing, and move toward photon countingdetector array capability.
II. Review
Silicon APDs can be arranged to yield single-photondetection at quantum efficiencies exceeding 20% whenoperated just a few volts in excess of breakdown re-verse bias, the so-called Geiger mode. This can be
The authors are with Royal Signals & Radar Establishment, St.Andrews Road, Malvern, Worcs WR14 3PS U.K.
Received 24 April 1989.0003-6935/89/210000-06$02.00/0.
achieved with noise dark-counts of only a few hundredper second when operated at -273 K (00C) with themost recently available devices, far superior to ourearlier experiments. 2 3 In addition, for activelyquenched Geiger operation,3 photon counting at ratesin excess of 106 s1 is now possible with negligibledistortion, and afterpulsing is of little consequencewhen operated on quench-reset time scales greaterthan -40 ns. However, these specifications apply onlyto single-element detectors.
In wishing to extend operational capability to 1-Dand 2-D detector arrays, the Geiger mode ceases to bevalid because photons are generated in the pn junctionduring the avalanche process.7 Sufficient optical iso-lation of closely adjacent detectors (pixels) seems im-possible. Thus when one detector detects a photon,some or all of the adjacent detection elements also doso almost simultaneously. This destroys the photode-tection (statistical) integrity of the array of elements.Silicon APDs operated in the sub-Geiger mode offer apossible solution to these problems, through greatlyreduced probability of interelement crosstalk. In thismode we find the potential of high quantum efficien-cies, truly negligible afterpulsing, low noise, and feasi-ble interelement isolation in detector array applica-tions, while retaining single-photon counting andcorrelation capability.
The penalties for operating in this mode are in-creased performance demands in high stability andvery low noise peripheral electronics; also, at presentphoton count rates of < = 105 photodetections perpixel per second, unless further limited by peripheralelectronics.8 Nevertheless, such a single-photoncounting rate per element in an array photon countingdetector is exciting when compared with other detec-tors used today, MAMA, PAPA, CCD, etc.,9 and atrelatively high temperatures when compared to 4 Kimpact ionization devices. 10
Use of the sub-Geiger mode for silicon APD opera-tion has been described in detail by McIntyre and hiscolleagues.78 The APD is reverse biased at just lessthan the breakdown voltage. Very low noise (<100selectrons rms) charge-sensitive preamplification, andthresholding are used to detect the current changecaused by a photodetection. Because the APD opera-tion is below breakdown, a complete avalanche doesnot occur following detection and the gain may be ofthe order of 200, compared to -105 found in the Geigermode. Thus, semiconductor traps are filled to a muchlesser extent and afterpulsing (due to trap decay) isgreatly reduced. The probability of secondary photongeneration in the gain region, as outlined earlier, islikewise reduced. Further, the photodetection rate isnot limited by the detector but by the bandwidth of itsperipheral electronics. Detection probability statis-tics as a function of threshold level have been quanti-fied previously by McIntyre.7
In the experiments presented here, we have not triedto optimize the peripheral electronics for sub-Geigerphotodetection but have concentrated on obtainingconditions suitable for high quality operation so thatthe inherent photon statistical properties of APDs un-der sub-Geiger operation may be examined. We haveemployed discrete off-the-shelf electronic componentsto examine single-element detectors and their funda-mental performance properties.
We chose to examine selected RCA C30921S, reach-through style7 APDs on the basis of our previous suc-cessful experiences in the Geiger mode,23 anticipatinglowest noise performances with these devices. Prior tosub-Geiger operation it is necessary to determine thereverse-bias breakdown voltage for the device undertest. This was achieved using the circuit in Fig. 1 inRef. 2, in passive quenching conditions. The break-down voltage (typically -195 V at room temperature)varies not only from diode to diode, but also withoperational temperature. It is also statistical in na-ture, i.e., there is not an exact breakdown voltage be-cause of dopant variations in the p-n interface region,etc.11"12 Nevertheless, it is important to know the biasvoltage at which Geiger pulses can occur because largepulses of this kind can destroy charge-sensitive devicesused peripherally. In sub-Geiger operation the aim isto operate with a bias voltage as close to breakdownvoltage as possible, to achieve the largest gain andquantum efficiency, potentially exceeding 50% ormore.8
IV. Experiments
In these experiments the C30921S APD was fol-lowed by an AMPTEK type A225 charge-sensitiveamplifier ( 280 electrons rms noise at room tempera-ture) to feed voltage pulses to an AMPTEK A206 leveldiscriminator and pulse generator. Output pulsesrepresenting photodetections were directed to a digitalphoton correlator (Malvern Instruments K7026) andDG30 computer for probability and correlation dataanalysis.' The arrangement is shown schematically in
Fig. 1. Schematic arrangement of the sub-Geiger experiment cir-cuitry.
Fig. 1. Applied bias voltages were stable to ±1 mV.Considerable care was taken to eliminate stray capaci-tances and to shield the electronics from extraneouselectrical noise. The A206 comparator had an internaltime delay which caused saturation for an input pulsefrequency of 0.16 MHz.
Full details of circuit design, construction, testing,experimental sequencing, results, and data analysisare recorded in a thesis.13 From that work we selectand discuss the most important characteristic results.We choose results obtained when the APD was operat-ed at a sufficiently stable temperature of 275 + 0.05 K(2°C) while the AMPTEK devices were operated atroom temperature, -293 K, yielding very stable count-ing rates. In these conditions the applied bias voltagewas a few volts less than the previously determinedbreakdown voltage, so as to operate far from Geigeroperation with consequent reduction in quantum effi-ciency. Without illumination it was necessary to setthe A206 threshold level some three times greater thanthat value needed to remove all the spurious electricalpulses with the bias circuitry switched off. Thus, thebias circuit was our limiting noise source even thoughextensive precautions had been taken in circuit con-struction. Such athreshold caused a significantloss ofphoton counts and a major loss in effective quantumefficiency (a prime motivation). However, the funda-mental statistical properties of sub-Geiger detectionwere still capable of being studied. The threshold wasfinally reduced slightly so that about one dark countper second was counted on average, being <0.1% of theminimum light-induced count rates to be used in sub-sequent experiments.
V. Factorial Moments
Photodetection probability density function (pdf)measurements were made using a constant intensityPoisson light source and the K7026 photon correlatoroperated in the pdf mode.' Measurements were takenfor a series of reverse-bias voltages applied to the pho-todiode and for a number of different imposed countrates. The value of effective dead time used for sys-tematic correction of the observed factorial moments14was obtained for each data set by a manual fit, main-taining the value of the corrected second moment with-in theoretical error. Preferable alternative meth-
ods2"15 of obtaining dead time were unusable because ofthe very low count rate and long experiment timesinvolved. Using the pdf measurements and the effec-tive dead time we examined the normalized factorialmoments,"4 as is customary for first-order photon sta-tistics. For a Poisson source, all factorial momentsshould be unity. Error of observation is calculatedfrom an equation given previously.'6
A table of results corrected to second order for deadtime effects is shown in Table I. Divergence from theexpected value (unity) of the higher factorial momentsincreases with increasing count rate, using an effectivedead time evaluated at a count rate of 1500 s'1. How-ever, the results compare well to data published else-where for photomultipliers and Geiger-mode APDs.'-3At count rates of -10,000 s-1 the divergence was signif-icant; the peripheral circuitry (whose effective deadtime was 30 Ms) started to exhibit momentary satura-tions and violations of dead time assumptions (discon-tinuous model) that affect the factorial moments inthis highly sensitive test. Thus, the maximum usablecount rate for this particular peripheral electronic con-figuration was determined. The close correspondenceof the measured factorial moments to the theoreticallyexpected value showed that circuit performance (e.g.,spurious oscillation) was not significantly distortingthe results at the lower count rates. It also showedthat the afterpulsing levels must be small, a fact con-firmed and quantified in the autocorrelation measure-ments to be discussed in Sec. VI.
Using the K7026 correlator in the autocorrelationmode,' the output photodetection pulse train was usedto form autocorrelation functions over a series of countrates and sample times. Correlograms for sampletimes of 10, 100, and 1000 ,s are to be seen in Figs. 2,3,and 4, respectively. The correlograms in Figs. 2 and 3show clearly through the rising curve the effect of thefinite dead time and saturation of the system where theprobability of detection for delay times is less than-400 As. In Fig. 2 the first delay channel is far belowthe average level at longer delays. This is caused bydetections occurring within the output pulse length ofthe discriminator, which, therefore, cannot themselvesbe registered by a discriminator pulse. The reason thefirst correlogram delay channel is not zero in Fig. 2 isthat the sample time was 10 Ms, just a little longer than
1.0
00
T = 10-5s
Rate = 10kHz
AS61
50. . . . . . 1 TFig. 2. Measured autocorrelation functions of a constant intensityPoisson source for a sample time of 10 js and photon count rates per
second as indicated.
Table I. Factorial Moments
Eff.Bias dead
Count rate voltage time Factorial momentsper second V Ps First Second Third Fourth
0 50 100 TFig. 3. Measured autocorrelation functions of a constant intensityPoisson source for a sample time of 100 iAs and photon count rates per
second as indicated.
92 (T)
1.0[. ................ - -. ...... -.-
T = 10-3s
Rate = 10kHz
AS53
00 50 100 T
Fig. 4. Measured autocorrelation functions of a constant intensityPoisson source for a sample time of 1000 jus and photon count rates
per second as indicated.
the pulse length of 6 s from the discriminator. Be-yond the early delay channels, but within the deadtime of the amplifier, the probability of detection de-pends on the size of the detector's current pulse; hence,the average probability is reduced with respect to thefar point (very long delay time value).
Data quantifying the correlograms in Figs. 2-4 aresummarized in Table II. In this table, afterpulsing(oc) is defined from the correlogram g2 (T) as oc =[g2(y) -1] (n), where (n) is the mean count rate per
sample time T. It can be seen that estimated after-pulsing rarely exceeds the far point statistical error[j2g2(T)] of the random correlation process, and at theshort sample time of 10,us it is insignificant because ofthe dead time effect.
To improve on this result and look for an afterpuls-ing level within statistical error, we re-measured g2 (T)in some very long experiments, -60 min per correlo-gram, and at longer sample times. Table III shows theresults of these experiments. The afterpulsing is sat-
isfactory for the intended applications such as photonlevel imaging.
Vil. Discussion and Conclusions
These first tests of the photon statistical propertiesof silicon APDs operated in the sub-Geiger mode haveyielded encouraging data, a necessity before proceed-ing further with their development. While these ex-periments have been neither optimal nor ideal, theyhave shown many ways of improving experimental anddeployment techniques. Importantly, the resultsshow no defects in this mode of operation which funda-mentally preclude using the APDs in the sub-Geigermode. Satisfactory single-photon counting operationhas been observed with respect to factorial momentsand correlation properties and over a useful range ofcount rates.
Numerous improvements to these experiments canbe implemented, but they will take time. First, thebias voltage noise should be minimized to allow fullreduction of the threshold level and thereby maximizethe effective quantum efficiency. An integrated elec-tronic unit with very short well-shielded connections isdesirable.
Second, the A225 amplifier is too noisy at 280 elec-trons rms. The A250 or another amplifier at 100 elec-trons rms would improve the situation, as would cool-ing of the amplifier to achieve _10 electrons rms noise.While this is standard practice in the nuclear instru-mentation field, the instrumentation is presently toobulky, and integrated electronic technology meetingsuch demanding specifications has yet to becomereadily available.
Third, completion of these improvements will thenallow sub-Geiger operation with bias voltages muchcloser to breakdown voltage, enhancing gain and quan-tum efficiency, thereby making possible the expected
Table II. Correlogram Analysis
Count Sample Errorrate time Afterpulsing percentage
Plot (n) T percentage g2(7)%na1name s-1 ius oc, % %
improvements in performance. With increased gain,the noise level from the detector will also increase,requiring some additional cooling (a few degrees Kextra) to maintain performance.
Fourth, not only did bias electronics cause noise, butsignificant extraneous noise was encountered associat-ed with the discrete amplifiers and discriminators.This problem could be substantially alleviated withhybrid amplifiers and discriminators of the type pro-duced by REL-LABS, if they can be obtained both at-10 electrons rms noise and integrally packaged with-in the detector housing. This will become mandatoryin the move toward array devices for photon levelimaging.
While pursuing these experimental improvements,we are also evaluating potential application areas. Itis already possible to make some progress in specificapplications in astronomical and optical aperture syn-thesis experiments,9 confocal scanning microscopy,bioluminescent imaging, absolute absorption mea-surements, and nonclassical light sources.4 All theseapplications require the potential of high quantumefficiency to be realized together with a single-photoncounting capability to provide a significant enhance-ment of experimental capability and to compete suc-cessfully with established detectors.
Linear arrays of reach-through APDs are alreadyavailable'7"18 and currently subject to investigations ofthe type described here, particularly adjacent pixelcrosstalk statistics. Two-dimensional arrays of APDsare also desirable, but no array will be fully usable inthe suggested applications until, for reasons of sheersize, cost, and noise, all the essential peripheral elec-tronics can be integrated inside the detector encapsu-lation.
We are encouraged by these preliminary results andexpect eventually to extend the sub-Geiger techniquenot only to array devices but also to APDs of differentsemiconductor materials, e.g., InGaAs, when inherentdevice noise is reduced sufficiently.
We are most grateful to J. G. Rarity of RSRE forvaluable comments during the course of these investi-gations and P. Mather for assistance with preliminaryexperiments. We thank R. J. McIntyre of RCA, Vau-dreuil, Canada, for introducing us to sub-Geiger opera-tion and A. W. Lightstone and A. D. MacGregor ofRCA for valuable discussions.
References
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SOCIETY OF VACUUM COATERS33rd ANNUAL TECHNICAL CONFERENCE
April 29-May 4, 1990Fairmont Hotel, New Orleans
Sessions on the following topics will be presented by the Technical Advisory Committees.Optical Coating Compact Disc and Media Technology
Vacuum Web Coating Industrial Thin FilmsProcess Technology Vacuum Technology
Contamination Control Vendor PresentationsA PLENARY talk and Technical Session on Coatings on Polymers will be a highlight of this Conference.
The deadline for the CALL FOR PAPERS is December 1, 1989.An Equipment Exhibit and Education Program of short courses will complement the Technical Conference. For
further information concerning the program, contact the SVC Administrative Office at (505) 298-7624.
