LINEAR-MODE SINGLE-PHOTON-SENSITIVE AVALANCHE …

1 978-1-4244-2677-5/08/$ 2008 IEEE 1 of 6 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 ABSTRACT We present design and performance data for a high-speed telecom-band ( m) SINGLE-PHOTON-SENSITIVE receiver based on a new class of multi-stage InGaAs AVALANCHE photodiode (APD) operated in the proportional mode (linear mode). Peak photon detection efficiency of 70% was measured at m. Unlike Geiger-mode SINGLE-PHOTON-SENSITIVE APDs (SPADs), a multi-stage linear SPAD is operated below its AVALANCHE breakdown voltage and need not be gated to suppress afterpulsing, permitting operation at a much higher maximum count rate (MCR) without an associated increase in dark count rate (DCR). Further, the LINEAR-MODE APD preserves signal amplitude information, whereas the response of a Geiger APD is binary.

2 The multi-stage APDs demonstrated can be operated with a linear gain in excess of M = 8000, and have silicon-like multiplication noise characterized by an effective ionization coefficient ratio of k = out to M = 1000. INTRODUCTION Detection of a single photon is required in many quantum communication applications. Much research activity (both theoretical and experimental) on single - photon detectors has been ,2 Though excellent low-noise single - photon -detection performance has been reported for superconducting devices, low operating temperature requirements (normally <4 K) limit their practical use. In real applications, APDs are used more often due to their lower cost, smaller package, and ease of employment. GHz-rate APD-based photon counting systems are attractive for quantum communication applications, but currently available high-speed telecom-band APDs suffer from high multiplication noise (k ) and low gain (M < 30), preventing their application to photon counting.

3 These limitations are a function of the InGaAs-on-InP material system required to efficiently detect telecom-band photons in the m spectral range. The noisy multiplication process is a function of the bulk properties of the alloys (typically InP or InAlAs) that are compatible with InGaAs, and the noisy multiplication process makes stable operation at high gain infeasible. photon counting in Geiger mode is possible with an InGaAs APD, but Geiger-mode operation is undesirable for high-count-rate applications because of a fundamental tradeoff between DCR and MCR. If a Geiger APD is cooled to reduce its DCR, then it must be quenched below its breakdown voltage in between detection events to avoid afterpulsing. This substantially reduces its MCR. Development of a telecom-band linear APD with very high gain and very low noise eliminates the trade between DCR and MCR because the receiver need not be gated off between detection events if the APD is operated in linear mode.

4 A basic LINEAR-MODE APD receiver consists of the APD detector element and a transimpedance amplifier (TIA). The APD converts incident photons to primary photocarriers, and amplifies the resulting primary photocurrent through internal AVALANCHE gain. The TIA converts the APD s current signal into a voltage signal; if a capacitive feedback TIA (CTIA) is used, then the voltage is proportional to the total multiplied charge delivered by the APD. When the receiver is configured as a thresholded detector , where a count is registered if the voltage exceeds a programmed threshold then a direct comparison to Geiger SPADs can be made. Like a Geiger-mode SPAD, a thresholded LINEAR-MODE photon -counter can be configured to discard signal amplitude information. Such a thresholded receiver can be considered SINGLE-PHOTON-SENSITIVE if it has a reasonably high probability of registering a count in response to a single - photon signal.

5 Alternatively, the signal amplitude data may be recorded when a count is registered, preserving that information. However, even in the case of a very low-noise APD, the statistical distribution of the APD s gain is wide enough to limit the accuracy of the amplitude measurement for weak signals. LINEAR APDS, GEIGER APDS, AND AFTERPULSING In linear mode, the reverse bias applied to the APD is held constant, and the primary photocurrent generated in the APD s absorber is amplified by a proportional multiplication factor that is independent of signal strength (below saturation). The output of a linear APD is 2 of 6 proportional to the level of illumination it receives, and so a linear APD can read signal amplitude. Linear APDs are typically used in optical receivers to boost weak signals above the noise floor of the receiver s amplifier. In Geiger mode, the reverse bias is modulated, and the APD s response is binary.

6 Geiger-mode operation of an APD involves momentarily biasing the diode above its AVALANCHE breakdown voltage Vbr. The excess voltage applied is called the overbias. In this active state, AVALANCHE breakdown of the diode junction can be triggered by as little as a single primary carrier, which is why the technology has been used for photon counting. However, the current that flows during AVALANCHE breakdown is determined by the characteristics of the external circuit rather than the number of primary carriers that initiated the breakdown, so the breakdown current of a Geiger APD is essentially the same for all signal strengths and it is very large. Geiger APDs suffer long reset times following each detection event, due to the afterpulse phenomenon. That is, the very large breakdown current that flows in a Geiger APD during a detection event populates traps in the detector that release their trapped carriers over time (Figure 1).

7 The sooner a Geiger APD is returned to service after it fires, the more likely it is to trigger off of a carrier that was trapped during the previous detection event, registering a spurious count. Quench times >1 s are generally necessary, and the lower the temperature the Geiger APD is operated at to suppress its dark count rate, the longer the required quench duration. This is the physical origin of the tradeoff between DCR and MCR encountered with Geiger APDs. For quantum communication systems ( quantum cryptography), today s low-speed single - photon Geiger-mode APDs are a bottleneck. In contrast, the current that flows in a LINEAR-MODE APD is too small to fill any appreciable population of traps, so linear APDs recover from detection events as soon as the current pulse clears the diode junction typically in 1 ns. In principle, a LINEAR-MODE APD can be cooled to reduce its DCR without lowering its MCR.

8 Thus, LINEAR-MODE photon -counting APD technology is one way to address the twin requirements of low DCR and high MCR. In the absence of afterpulsing, the MCR of a LINEAR-MODE APD is determined by its impulse response. This is the time it takes for the AVALANCHE of carriers in its multiplication region to complete (called the AVALANCHE buildup time ) and for the last of the secondaries to clear the junction (related to the saturation drift velocity of the slowest carrier, which for holes in InGaAs is about 6 106 cm s 1). The fastest InGaAs telecommunication APDs can operate at an AVALANCHE gain of M = 10 with a 10-GHz bandwidth. This is achieved by minimizing the thickness of the junction, and operation at lower gain (to minimize the AVALANCHE buildup time). photon -counting APDs that must be sensitive to m light when operated cold require thicker absorption layers (perhaps 2 m of InGaAs or more), and must be operated at considerably higher gain (M > 400).

9 Nonetheless, a bandwidth of hundreds of MHz is achievable. The possibility of circumventing the fundamental tradeoff between DCR and MCR using LINEAR-MODE APD technology is exciting, but requires sensitive amplifiers. InGaAs APDS WITH BULK MULTIPLIERS Manufacturing low-noise APDs with high responsivity in the telecom wavelengths is a challenge because the III-V compound semiconductor alloys that are compatible with efficient InGaAs absorbers have fairly high bulk values of k, which is the ratio of the material s ionization coefficient for holes to that for electrons. For instance, InP the most common alloy used in telecommunications APDs has a k value of The inverse ratio (k = ) is often quoted to enable side-by-side comparison with electron- AVALANCHE materials like Si or InAlAs, for purposes of calculating the excess noise factor, F(M). Lower k is better: =21)1(1)(MMkMMF.

10 3 (1) On that basis, conventional silicon APDs (with k ) greatly outperform SWIR APDs made from bulk InGaAs/InP (k ) or InGaAs/InAlAs (k ). DESIGN OF MULTI-STAGE APD INGAAS APDS APDs and other optoelectronic devices such as lasers are commercially manufactured in both the InGaAs/InP Extra Dark CarriersReleased by TrapsA erpulses(Dark counts triggered by carriers released from traps)Geiger event(fills traps)Quench(when pulse detected)Gate ONBiasAPDC urrentDark CarrierGenera on RateHoldOFFVbrOverbiasTime(Maximum count rate = 1/ ) Figure 1: Illustration of the afterpulse phenomenon that limits thecount rate of a Geiger APD. 3 of 6 and InGaAs/InAlAs alloy systems. Although both alloy systems span a similar band gap range (InP has a room-temperature band gap of eV; for InAlAs it is eV), the band offset ratio is higher in AlGaInAs than in InGaAsP ( versus ) and the impact ionization rate for electrons in InAlAs is higher than that for holes, but vice versa for InP.