Nearinfrared NIR Single Photon Counting Detectors SPADs Chong

Near-infrared (NIR) Single Photon Counting Detectors (SPADs) Chong Hu, Minggou Liu, Joe C. Campbell & Archie Holmes ECE Department University of Virginia

Outline ü Introduction to Single Photon Counting Detectors (SPADs) ü Current States of near-infrared SPADs ü Summary and Future Goals

Mature PMT Novel APD Superconductors 2000 V • High gain • Low dark current • Low noise • Low quantum efficiency • Large, bulky • Expensive • High voltage • Fragile • Ambient light catastrophic t • Electron & Hole avalanche multiplication • Good efficiency • Acceptable dark counts • Afterpulsing • Hotspot Generation and Resistive Barrier • High efficiency • Low dark counts • No afterpulsing • T < 1 K!!

Geiger mode - APD functions as a switch Analog Digital Responsivity Single photon detection efficiency Dark current Probability of dark count Geiger-mode operation Single photon input Concept of excess noise does not apply! APD output time Discriminator level Digital comparator output time Photon absorbed Successful single photon but insufficient gain – missed detection count Dark count – from dark current

Linear on Geiger mode on Linear Geiger quench mode avalanche Current off arm Vdc + DV V Voltage br

Performance Parameters Single photon input ü Photon detection efficiency (PDE) Ø The probability that a single incident photon initiates a current pulse that registers in a digital counter ü Dark count Rate (DCR)/Probability (DCP) Ø The probability that a count is triggered by dark current instead of incident photons APD output time Discriminator level Digital comparator output time Photon absorbed Successful single photon but insufficient gain – missed detection count Dark count – from dark current

Photon Detection Efficiency h. PDE = hexternal x hcollection x Pavalanche In. Ga. As. P In. Ga. As 1. 06, 1. 3 m 1. 55 m

40 m-diameter In 0. 53 Ga 0. 47 As/In. P Dark current • 0. 18 n. A at 95% of Vbr at 297 K • 0. 15 p. A at 95% of Vbr at 200 K Good enough?

1. 06 μm SPADs: DCR vs. PDE ü In. Ga. As. P absorber lower generation-recombination dark current ü DCR approaching Si SPAD DCR with greatly increased PDE Ø Si SPADs have PDE < 2% at 1. 06 μm 105 Dark Count Rate (Hz) 300 K * 104 273 K – 295 K 259 K 250 K 103 237 K 80 μm dia. In. Ga. As. P SPAD 1 -ns gated operation 500 k. Hz repetition rate 0. 1 photon/pulse AP probability < 10 -4 per gate 102 101 0% 10% 20% 30% Photon Detection Efficiency 40% 50%

Dark Count Probability versus Photon Detection Efficiency Dark Count Probability 10 -2 In. Ga. As/In. P, 300 K 1550 nm (2007) 10 -3 10 -4 10 -5 In. Ga. As/In. P (2007) 268 K 240 K 200 K 10 -6 10 -7 215 K 0 5 10 15 20 25 30 35 40 45 Single Photon Detection Efficiency (%) 50

Linear on Geiger mode on Linear Geiger quench mode avalanche Current off arm Vdc + DV V Voltage br

Quenching Techniques Quenching circuit – Quench avalanche current – Reset the device • Passive Quenching – Quenched by discharging capacitance – Slow recharge • Active Quenching – Raise the anode voltage – Quick recharge • Gated Quenching Vq>VEX

Afterpulsing Number of trapped carriers Biasing scheme Initial avalanche Released carriers from traps

Reduction of Afterpulsing: Decreasing Charge Flow Passive quenching The total charges flowing through device: Q=(Cs+Cd)Vex Cd: device capacitance Cs: stray capacitance

Passive Quenching with Active Reset (PQAR)

PQAR at 230 K Measured counts x 1000 (/s) 70 50 Vex=2. 6 V Vex=2. 0 V Vex=1. 6 V 40 Hold-off = 15 s 60 30 20 10 0 1. E-01 1. E+00 1. E+01 1. E+02 1. E+03 1. E+04 CW laser power (f. W) Compare with gated mode results Vb 10 -4 34 s 15 s Dark count probability (/ns) Voltage on device NEP ~ 10 -16 W/Hz 1/2 10 -5 10 -6 16

Gated Quenching of a SPAD AC pulse width Total bias on APD Excess bias (V ex ) Vbr V dc Time Laser pulse Time Avalanche pulse due to incident photon Missed photon (false negative) Photon Detection Efficiency Avalanche pulse due to dark carriers (false positive) ß Dark Count Rate

Gated-PQAR Compared to PQAR Rs=50Ω Amplifier A Counter • Suppressed dark counts by gated bias Cac • Reduced complexity Cag 10 MΩ • Array operation: recharge together, quench separately Vgate Vbias The transistor can be HBT monolithically integrated on the SPAD, and the its gate/base input can be shared over the whole array.

Gated Quenching and Gated PQAR Dark Count Rate (k. Hz) 10 8 6 4 220 K 200 K 180 K 2 1 0 5 10 gated-quenching gated-PQAR 15 20 25 30 Photon Detection Efficiency (%) 35

Dark Count Probability Gated Quenching and Gated PQAR 220 K, Vex = 5. 6% 200 K, Vex = 6. 0% 180 K, Vex = 6. 2% -3 10 gated-quenching gated-PQAR -4 10 -5 10 0. 01 0. 1 1 Repetition Rate (MHz) 10

Conclusions ü Performance of Geiger-mode APDs is improving rapidly ØAcceptable detection efficiencies and dark count probability levels ØGetting a better control over the afterpulsing problem

Future Goals ü Move closer to quantum limited detection ØDark Current 0 ØQuantum Efficiency 100% ØRead Noise 0 ü Move to longer wavelengths ü Do photon number resolving

High QE Structure

Type-II Quantum Wells (QWs) Formed between materials with staggered band line-ups ü Electrons and holes are confined in adjoining layers ü Spatially indirect absorption and emission Ø Smaller effective bandgap for long-wavelength operation Ga. As 0. 5 Sb 0. 5 ΔEc=0. 236 e. V 0. 79 e. V CB 0. 49 e. V ≈ 2. 5 μm VB ΔEv=0. 247 e. V Ga 0. 47 In 0. 53 As 0. 77 e. V x E

Where we are now (pin devices) Rogalski, A. , Progress in Quant. Elec. , 27(2 -3), pp 59 (2003) -2 V bias (200 K) -2 V bias (RT)

Gated-PQAR for Synchronized Detection Gated Quench photon AC Gated-PQAR photon Vgate Vdiode output • Comparable circuit complexity • Wider AC pulses: easier to generate and synchronize output Vbias Compared to gated quench photon • Uniform output pulse shape, good for photon-numberresolution with multiplexing Transistor on: low resistance for fast reset Transistor off: high resistance for fast passive quench 26

Questions? ?

Simulated Breakdown Probabilities J. P. R. David, University of Sheffield In. P Al. In. As Decreasing thickness: 0. 2 m, 0. 5 m, 1. 0 m

• 20 x 20 Array – 98% yield • 4 x 4 Subarray – uniform single-photon response

Afterpulsing Probability vs. Total Charge AC pulse Delay 1 s Period Laser pulse 30

Total Charge Flow 31

“Effective Excess Noise Factor” 32

Pixel Level Monolithic Integration of Active Switching Elements • Reduced parasitics Rs =50 W Counter • Faster quenching • Reduced afterpulsing • Increased transmission and Active switching element Vbias sampling rates • Packaging advantages