LINEAR-MODE SINGLE-PHOTON-SENSITIVE AVALANCHE PHOTODIODES FOR GHZ-RATE NEAR-INFRARED QUANTUM COMMUNICATIONS
Andrew Huntington, Madison Compton, Sam Coykendall, George Soli, and George M. Williams Voxtel, Inc. Beaverton, Oregon
Photon counting in Geiger mode is possible with an ABSTRACT InGaAs APD, but Geiger-mode operation is undesirable We present design and performance data for a high- for high-count-rate applications because of a fundamental speed telecom-band (1.3 μm) single-photon-sensitive tradeoff between DCR and MCR. If a Geiger APD is receiver based on a new class of multi-stage InGaAs cooled to reduce its DCR, then it must be quenched below avalanche photodiode (APD) operated in the proportional its breakdown voltage in between detection events to mode (linear mode). Peak photon detection efficiency of avoid afterpulsing. This substantially reduces its MCR. 70% was measured at 1.064 μm. Unlike Geiger-mode Development of a telecom-band linear APD with very single-photon-sensitive APDs (SPADs), a multi-stage high gain and very low noise eliminates the trade between linear SPAD is operated below its avalanche breakdown DCR and MCR because the receiver need not be gated off voltage and need not be gated to suppress afterpulsing, between detection events if the APD is operated in linear permitting operation at a much higher maximum count mode. rate (MCR) without an associated increase in dark count A basic linear-mode APD receiver consists of the APD rate (DCR). Further, the linear-mode APD preserves detector element and a transimpedance amplifier (TIA). signal amplitude information, whereas the response of a The APD converts incident photons to primary Geiger APD is binary. The multi-stage APDs photocarriers, and amplifies the resulting primary demonstrated can be operated with a linear gain in excess photocurrent through internal avalanche gain. The TIA of M = 8000, and have silicon-like multiplication noise converts the APD’s current signal into a voltage signal; if characterized by an effective ionization coefficient ratio of a capacitive feedback TIA (CTIA) is used, then the k = 0.03 out to M = 1000. voltage is proportional to the total multiplied charge delivered by the APD. INTRODUCTION When the receiver is configured as a thresholded Detection of a single photon is required in many detector — i.e., where a count is registered if the voltage quantum communication applications. Much research exceeds a programmed threshold — then a direct activity (both theoretical and experimental) on single- comparison to Geiger SPADs can be made. Like a photon detectors has been reported.1,2 Though excellent Geiger-mode SPAD, a thresholded linear-mode photon- low-noise single-photon-detection performance has been counter can be configured to discard signal amplitude reported for superconducting devices, low operating information. Such a thresholded receiver can be temperature requirements (normally <4 K) limit their considered single-photon-sensitive if it has a reasonably practical use. In real applications, APDs are used more high probability of registering a count in response to a often due to their lower cost, smaller package, and ease of single-photon signal. Alternatively, the signal amplitude employment. data may be recorded when a count is registered, GHz-rate APD-based photon counting systems are preserving that information. However, even in the case of attractive for quantum communication applications, but a very low-noise APD, the statistical distribution of the currently available high-speed telecom-band APDs suffer APD’s gain is wide enough to limit the accuracy of the from high multiplication noise (k ≈ 0.4) and low gain amplitude measurement for weak signals. (M < 30), preventing their application to photon counting. These limitations are a function of the InGaAs-on-InP LINEAR APDS, GEIGER APDS, AND material system required to efficiently detect telecom- AFTERPULSING band photons in the 1.3–1.5 μm spectral range. The noisy In linear mode, the reverse bias applied to the APD is multiplication process is a function of the bulk properties held constant, and the primary photocurrent generated in of the alloys (typically InP or InAlAs) that are compatible the APD’s absorber is amplified by a proportional with InGaAs, and the noisy multiplication process makes multiplication factor that is independent of signal strength stable operation at high gain infeasible. (below saturation). The output of a linear APD is
978-1-4244-2677-5/08/$25.00 ©2008 IEEE 1 of 6 proportional to the level of illumination it receives, and so In contrast, the current that flows in a linear-mode APD a linear APD can read signal amplitude. Linear APDs are is too small to fill any appreciable population of traps, so typically used in optical receivers to boost weak signals linear APDs recover from detection events as soon as the above the noise floor of the receiver’s amplifier. In Geiger current pulse clears the diode junction — typically mode, the reverse bias is modulated, and the APD’s in ≈1 ns. In principle, a linear-mode APD can be cooled to response is binary. Geiger-mode operation of an APD reduce its DCR without lowering its MCR. Thus, linear- involves momentarily biasing the diode above its mode photon-counting APD technology is one way to avalanche breakdown voltage Vbr. The excess voltage address the twin requirements of low DCR and high MCR. applied is called the overbias. In this active state, In the absence of afterpulsing, the MCR of a linear- avalanche breakdown of the diode junction can be mode APD is determined by its impulse response. This is triggered by as little as a single primary carrier, which is the time it takes for the avalanche of carriers in its why the technology has been used for photon counting. multiplication region to complete (called the “avalanche However, the current that flows during avalanche buildup time”) and for the last of the secondaries to clear breakdown is determined by the characteristics of the the junction (related to the saturation drift velocity of the external circuit rather than the number of primary carriers slowest carrier, which for holes in InGaAs is about that initiated the breakdown, so the breakdown current of 6 × 106 cm s–1). The fastest InGaAs telecommunication a Geiger APD is essentially the same for all signal APDs can operate at an avalanche gain of M = 10 with a strengths — and it is very large. 10-GHz bandwidth. This is achieved by minimizing the Geiger APDs suffer long reset times following each thickness of the junction, and operation at lower gain (to detection event, due to the afterpulse phenomenon. That is, minimize the avalanche buildup time). Photon-counting the very large breakdown current that flows in a Geiger APDs that must be sensitive to 1.55 μm light when APD during a detection event populates traps in the operated cold require thicker absorption layers (perhaps detector that release their trapped carriers over time 2 μm of InGaAs or more), and must be operated at (Figure 1). The sooner a Geiger APD is returned to considerably higher gain (M > 400). Nonetheless, a service after it fires, the more likely it is to trigger off of a bandwidth of hundreds of MHz is achievable. carrier that was trapped during the previous detection The possibility of circumventing the fundamental event, registering a spurious count. Quench times >1 µs tradeoff between DCR and MCR using linear-mode APD are generally necessary, and the lower the temperature the technology is exciting, but requires sensitive amplifiers. Geiger APD is operated at to suppress its dark count rate, InGaAs APDS WITH BULK MULTIPLIERS the longer the required quench duration. This is the physical origin of the tradeoff between DCR and MCR Manufacturing low-noise APDs with high responsivity encountered with Geiger APDs. in the telecom wavelengths is a challenge because the For quantum communication systems (e.g. quantum III-V compound semiconductor alloys that are compatible cryptography), today’s low-speed single-photon Geiger- with efficient InGaAs absorbers have fairly high bulk mode APDs are a bottleneck. values of k, which is the ratio of the material’s ionization coefficient for holes to that for electrons. For instance, InP Extra Dark Carriers Released by Traps — the most common alloy used in telecommunications APDs — has a k value of 2.5. The inverse ratio (k = 0.4) Dark Carrier is often quoted to enable side-by-side comparison with Genera on Rate electron-avalanche materials like Si or InAlAs, for purposes of calculating the excess noise factor, F(M). Geiger event A erpulses (fills traps) (Dark counts triggered by carriers released from traps) Lower k is better: ⎡ − 2 ⎤ APD = − − ⎛ M 1⎞ 3 Current F(M ) M ⎢1 (1 k)⎜ ⎟ ⎥ . (1) ⎣⎢ ⎝ M ⎠ ⎦⎥ Quench Gate ON On that basis, conventional silicon APDs (with (when pulse detected) k ≈ 0.02) greatly outperform SWIR APDs made from bulk Bias Overbias Hold InGaAs/InP (k ≈ 0.4) or InGaAs/InAlAs (k ≈ 0.3). V OFF br DESIGN OF MULTI-STAGE APD INGAAS τ (Maximum count rate = 1/τ) APDS Time Figure 1: Illustration of the afterpulse phenomenon that limits the APDs and other optoelectronic devices such as lasers count rate of a Geiger APD. are commercially manufactured in both the InGaAs/InP
2 of 6 700 Electron-ini ated ioniza on dp = 50 nA Hole-ini ated ioniza on R = 0.875 A/W Electric field profile
600 / k = 0.40 200 InAlAs Al0.24Ga0.24In0.52As ] 500 1/2 k = 0.20 150
400 100 (arbitrary units) (arbitrary 300 NEP [fW/Hz k = 0.02 50 Field Number of Ioniza ons 200 0 01020304050 020406080100 Avalanche Gain (M) Loca on (arbitrary units) Figure 2: Effect of k on NEP calculated for APDs. Figure 3: Monte Carlo simulation of a multiplication layer in which a change in alloy composition has been used to localize impact and InGaAs/InAlAs alloy systems. Although both alloy ionization. systems span a similar band gap range (InP has a room- gain — through spatial localization of the ionization temperature band gap of 1.34 eV; for InAlAs it is 1.46 events. eV), the band offset ratio is higher in AlGaInAs than in Ionization events tend to be localized inside thin InGaAsP (0.7 versus 0.4) and the impact ionization rate multiplication regions because, following each ionizing for electrons in InAlAs is higher than that for holes, but collision, carriers must pick up energy across a certain vice versa for InP. When grown as bulk alloys, InP and distance — the dead space — before they are capable of InAlAs have high values of k, and consequently poor causing subsequent ionizations. By effectively eliminating excess noise performance in accordance with Equation (1) some of the places where impact ionization can occur in above. InP has a natural k value of 0.4; InAlAs is only the structure, the dead-space effect reduces the number of slightly better, with k ≈ 0.3. possible ionization chains; this is the source of the noise The problem, then, is that the InGaAs alloy which reduction observed for such APDs. absorbs SWIR efficiently is not compatible with an A thin multiplication layer introduces little noise avalanche material that naturally has low excess noise. because the number of possible ionization chains is small, Our multi-stage APD design is an engineered but by the same token, the long ionization chains InAlAs/InGaAs structure in which the k value is closer to necessary to produce high gain do not fit inside a thin that of silicon (k ≈ 0.02–0.04), so that a near infrared multiplication layer. Higher gain can be obtained from a (NIR)-sensitive APD can be made with low excess thin multiplication layer by increasing the field strength, multiplication noise. The impact of a reduction in k from but in doing so, feedback from hole-initiated ionizations is 0.4 to 0.02 on the noise-equivalent power (NEP) of an intensified and noise suppression is lost, precisely because APD receiver is illustrated in Figure 2. The desirability of a larger number of ionization chains can now fit into the lower k should be apparent. same space. And not only do stronger fields degrade As noted earlier, conventional InGaAs APDs typically excess noise performance, they enhance dark current have bulk InP multiplication layers, characterized by an leakage through mechanisms such as band-to-band ionization coefficient ratio of k ≈ 0.4. We manufacture tunneling and thermionic field emission, so increasing advanced low-noise APDs that have thinner InAlAs field strength is counterproductive. multiplication layers, characterized by k ≈ 0.2. We applied Any technique that acts to localize impact ionization the same engineering principles to achieve k = 0.03 at a and eliminate some of the structure’s possible ionization gain of M ≈ 1000. We accomplished this through an chains will also reduce its excess noise. The practice of advanced application of impact-ionization engineering building heterostructure multiplication regions from (I2E).4 2 materials with dissimilar ionization thresholds falls into The goal of I E is to reduce excess multiplication noise this category. A typical structure of this variety has a by designing semiconductor structures in which the wide-band gap region on the p side of the multiplication impact ionization events will naturally be correlated. In layer and a narrower-band gap region on the n side; general, two tools are used: (1) the so-called “dead-space” electrons that pick up energy in the wide-gap material do effect and (2) localized enhancement of the ionization rate. not trigger ionizations until they hit the lower-threshold Both reduce the number of possible ionization chains — narrow-gap material, whereas the holes generated by and, hence, narrow the distribution of the multiplication those collisions in the narrow-gap material fail to gain
3 of 6 300 abc InAlAs Al0.24Ga0.24In0.52As / 800 250 /
Electron-ini ated ioniza on 200 600 Hole-ini ated ioniza on Electric field profile 150 400
(arbitrary units) (arbitrary Electron-ini ated ioniza on
(arbitrary units) (arbitrary 100 Hole-ini ated ioniza on 200 Electric field profile Field Number of Ioniza ons Field 50 Number of Ioniza ons 0 0 020 40 60 80 100 0 200 400 600 800 1000 Loca on (arbitrary units) Loca on (arbitrary units) Figure 4: Monte Carlo simulation of a multiplication layer in which Figure 5: Monte Carlo simulation of a 10-stage APD in which the the electric field profile has been used to localize impact ionization. multiplication layer design from Figure 4 is cascaded. Hole relaxation layers between each stage prevent hole feedback. sufficient energy to ionize when they drift through the M ≈ 4.5 This limitation is a consequence of the fact that wide-gap region (Figure 3). Unfortunately, use of narrow- they derive their low-noise properties by eliminating the gap alloys for part of the multiplication region is limited longer impact ionization chains; short chains produce low by the commensurate increase in dark current from gain. If individual I2E multiplication layers cannot be tunneling. This consideration limits the contrast in operated at high gain and still preserve their low noise ionization coefficients that can be practically achieved character, one possible solution is to operate I2E through material selection alone. multiplication layers at low gain and cascade them in A new concept we developed for the multi-stage stages. To our knowledge, William Clark was the first to structure is I2E through electric field control (Figure 4). propose a multi-stage design.6 The impact ionization rate in a semiconductor typically Figure 5 depicts a Monte Carlo simulation of our has an exponential dependence upon the local electric multi-stage I2E APD. The purpose of the first repeating field. Just as alloys of different ionization threshold can layer, marked ‘a’, is to have a field that (1) heats electrons be ordered so as to enhance the ionization rate of one traveling to the right, and (2) is insufficient to cause hole- carrier type over the other, the electric field profile inside initiated ionizations from holes traveling to the left. The the junction can be shaped to achieve the same effect. second repeating layer, marked ‘b’, having both a higher Used in conjunction with material selection, this field and a smaller gap, is designed to cause the hot technique is very powerful. electrons to ionize; it is too thin for cold holes to ionize in Due to the physics of the gain process described above, it. Finally, the hole relaxation layer, marked ‘c’, has an I2E APDs have been reported on in the literature with electric field so low that carriers can lose any extra kinetic k ≈ 0 at very low gain, though they have not been able to energy gained in neighboring high-field regions. Note that sustain this level of performance much above a gain of ‘c’ is much physically thicker than either ‘a’ or ‘b’ in the 100 10-Stage APD #1 10-Stage APD #2 –4 10 Light #1 Gain #1 12000 10-Stage APD #3 Dark #1 k = 0.4 (bulk InP) k = 0.3 (bulk InAlAs) Light #2 Gain #2 10000 10–5 Dark #2 k = 0.04 k = 0.03 Light #3 8000 k Dark #3 Gain #3 = 0.02 (Si) 50 k 10–6 = 0 (HgCdTe) 6000 Gain
Current [A] Current 4000 10–7 Excess Noise Factor Excess 2000 10–8 0 0 70 72 74 76 78 80 82 0 200 400 600 800 1000 1200 Reverse Bias [V] Avalanche Gain Figure 6: I-V characteristics (left axis) and avalanche gain (right axis) Figure 7: Excess noise measurements for 10-stage APDs compared measured for 10-stage Voxtel APDs. to relations characteristic of other technologies.
4 of 6 Figure 9: Raw count data collected from a 10-stage APD receiver Figure 8: Photon detection efficiency measured from the count operated at M = 1800, both in the dark and under illumination by a data of Figure 9. sparse stream of single photons. layer structure, but does not appear this way in the Monte photon-counting measurements were taken on 50-μm Carlo model due to the use of an uneven grid spacing. The devices. purpose of the hole relaxation layer is to prevent feedback The 10-stage I2E APDs were shown to operate at gains between stages by making sure that hot holes from the as high as M = 8000 (Figure 6), and maintain silicon-like ionization events in subsequent gain stages lose their shot noise levels up to at least M = 1000 (Figure 7). energy through collisions, thus preventing them from Photon-counting thresholded linear-mode receivers ionizing as they stream back through the gain stages. were assembled from APD die hybridized to commercial The range over which a multi-stage APD consisting of 2-GHz TIA chips. Raw count rate data measured under cascaded I2E multiplication stages can maintain low-noise continuous-wave illumination from a stabilized 1.064-μm operation is limited by the gain-per-stage at which the diode laser source, attenuated down to a sparse stream of individual multiplication stages lose their low-noise single photons, is shown in Figure 8; the error bars character. represent the standard deviation across multiple samples. Photon detection efficiency (PDE) was calculated as the MEASUREMENTS ON 10-STAGE APDS AND ratio of the excess counts above the dark count tally to the RECEIVERS total number of photons delivered during the sample We grew the 10-stage InGaAs-based APD design period (Figure 9). A maximum PDE of 70% was presented above in Figure 5 using solid-source molecular measured, but the DCR was quite high, at nearly 100 beam epitaxy (MBE). MBE techniques were required to MHz. Comparison of the DCR of a receiver both with and achieve the doping precision necessary to implement the without the APD powered has shown that the great structure, but the low-noise/high-gain results reported majority of these dark counts originate in the TIA. We here have proven repeatable across four different process estimate that band-limiting the TIA to 1 GHz will reduce lots. Our initial results for 5-stage APDs with a maximum these dark counts by between 2 and 4 orders of magnitude, gain of M = 2000 and silicon-like noise (k < 0.02) up to a depending upon the chosen threshold; work is underway gain of M = 20 have been reported elsewhere.7 Since then, to test this. we have manufactured a second lot of 5-stage APDs with RADIATION EFFECTS identical characteristics, one lot of 7-stage APDs with silicon-like noise beyond M = 100, and the 10-stage APDs We made a preliminary assessment of the radiation described here. hardness of the multi-stage APDs to test their suitability The 2" wafers were processed into back-illuminated for a NASA optical communications application. 8 etched-mesa APDs encapsulated with benzocyclobutene Displacement damage by protons, quantified by non- (BCB) resin using contact photolithography. Standard ionizing energy loss (NIEL), can be modeled by the detector sizes produced include APDs 30-, 50-, 75-, and Stopping and Range of Ions in Matter (SRIM) program 200 μm in diameter. The gain and noise data presented based on the work of Ziegler et al.9 The SRIM program here were measured from 75-μm diodes, whereas the was used to calculate NIEL for 1- and 2-MeV protons.
5 of 6 –5 –5 10 2 MeV, 1011 cm–1 predic on 10 2 MeV, 1011 cm–1 measurement Post-Irradia on, Calculated 2 MeV, 1010 cm–1 predic on Post-Irradia on, Measured 10–6 10–6 2 MeV, 1010 cm–1 measurement
10–7 10–7
10–8 10–8 Dark Current [A] Dark Current Dark Current [A] Dark Current
10–9 10–9
30 35 40 45 50 55 60 30 35 40 45 50 55 60 Reverse Bias [V] Reverse Bias [V] Figure 10: Calculated and measured dark current curves for 5-stage Figure 11: Average measured dark current for a 5-APD sample after APDs exposed to 2-MeV protons. Note that the data obtained for irradiation by a 5 × 1010 cm–2 fluence of 63.5-MeV protons, 1-MeV protons, which was used to predict 2-MeV damage, was compared to a calculation for this dose extrapolated from data on a truncated above 35 V for the 1010 cm–2 fluence; this made prediction similar sample that received a 1010 cm–2 dose of 2-MeV protons. of the 2-MeV dark current for this fluence impossible above 35 V.
The calculation of NIEL is simplest — and probably most planned to determine the activation energy of the aging accurate — for purely elastic collisions mediated by the process. electromagnetic force, so these low proton energies were CONCLUSION chosen to exclude the possibility of inelastic scattering involving nuclear reactions. By comparing the increase in We have presented design and performance data for a dark current measured after irradiation to the difference in new class of multi-stage InGaAs with high gain and low NIEL that was calculated by SRIM, it was confirmed that excess noise. When operated with a TIA as a thresholded displacement does account for nearly all of the radiation linear-mode photon counter, high PDE and very fast MCR damage observed for fluences in the 1010–1011 cm–2 range are obtained. DCR is presently limited by amplifier noise, (Figure 10). but substantial improvement is possible by band-limiting Displacement damage from elastic scattering also the TIA. appears to dominate for higher proton energies. A 5-device sample of 5-stage APDs was irradiated under 1 S. Cova, M. Ghioni, A. Lotito, I. Rech, and F. Zappa, “Evolution bias by a 5 × 1010 cm–2 fluence of 63.5-MeV protons, and prospects for single-photon avalanche diodes and quenching circuits,” Journal of Modern Optics 51, pp. 1267–1288, 2004. resulting in a 23% increase in gain-normalized dark 2 G. Ribordy, J. D. Gautier, H. Zbinden, and N. Gisin, current, measured at room temperature at a gain of M = 20. “Performance of InGaAs/InP avalanche photodiodes as gated-mode SRIM predicts that NIEL of a 63.5-MeV proton should be photon counters,” Applied Optics 37, pp. 2272–2277, 1998. 4.94% that of a 2-MeV proton. The NIEL-based 3 R. J. McIntyre, “Multiplication noise in uniform avalanche diodes,” IEEE Transactions on Electron Devices ED-13, pp. 164–168, calculation underpredicts damage by ≈20% (Figure 11). 1966. LIFETIME TESTING 4 J. C. Campbell, “Recent Advances in Telecommunications Avalanche Photodiodes,” Journal of Lightwave Technology, vol. 25, An 11-device sample of 5-stage APDs was sealed no. 1, pp. 109–121 (2007). hermetically and aged under bias at 50 °C for a 5 S. Wang, J. B. Hurst, F. Ma, R. Sidhu, X. Sun, X. G. Zheng, A. L. Holmes, Jr., A. Huntington, L. A. Coldren, and J. C. Campbell, “Low- cumulative duration of 717 hours. Because dark current Noise Impact-Ionization-Engineered Avalanche Photodiodes Grown on leakage is greatly increased at elevated temperature, each InP Substrates,” IEEE Photonics Technology Letters, vol. 14, no. 12, device was wired in series with a 500-kΩ resistor to pp. 1722–1724 (2002). protect it from burning out. The average current through 6 W. Clark, U.S. Patent No. 6,747,296 B1 (2004). the devices during aging was approximately 100 µA, 7 A. S. Huntington, M. A. Compton, and G. M. Williams, “Linear- which is well above the level during normal illuminated mode single-photon APD detectors,” Proceedings of SPIE, vol. 6771, 67710Q (2007). operation. After an initial burn-in period during which 8 NASA SBIR contract NNG05CA28C device-to-device variation decreased significantly, no 9 J. F. Ziegler, J. P. Biersack, and U. Littmark, The Stopping and overall rise in dark current was observed during this time Range of Ions in Solids (New York: Pergamon), 1985. period. A longer-term study at higher temperatures is
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