High performance planar germanium-on-silicon single-photon avalanche diode detectors

Abstract

Single-photon detection has emerged as a method of choice for ultra-sensitive measurements of picosecond optical transients. In the short-wave infrared, semiconductor-based single-photon detectors typically exhibit relatively poor performance compared with all-silicon devices operating at shorter wavelengths. Here we show a new generation of planar germanium-on-silicon (Ge-on-Si) single-photon avalanche diode (SPAD) detectors for short-wave infrared operation. This planar geometry has enabled a significant step-change in performance, demonstrating single-photon detection efficiency of 38% at 125 K at a wavelength of 1310 nm, and a fifty-fold improvement in noise equivalent power compared with optimised mesa geometry SPADs. In comparison with InGaAs/InP devices, Ge-on-Si SPADs exhibit considerably reduced afterpulsing effects. These results, utilising the inexpensive Ge-on-Si platform, provide a route towards large arrays of efficient, high data rate Ge-on-Si SPADs for use in eye-safe automotive LIDAR and future quantum technology applications.

Fig. 1

Ge-on-Si single-photon avalanche diode (SPAD) structure. Cross-section of a planar Ge-on-Si SPAD showing the Ge top contact and absorption layers, the Si charge sheet, multiplication and bottom contact layers, metallisation and the planarization, passivation and anti-reflection (AR) coating

Fig. 2

Single-photon avalanche diode modelling and current-voltage behaviour. a Electric field profile at 5% excess bias for the planar single-photon avalanche diode (SPAD). b Electric field profile at 5% excess bias for the mesa etched SPAD. c Current-voltage measurements on 100 µm diameter planar (blue line) and mesa etched (green line) structures at a temperature of 100 K under dark conditions. d Current-voltage measurements on a 100 µm diameter planar device at a temperature of 78 K. The graph shows dark current (blue line) and photocurrent when illuminated with a 1310 nm laser (red line). Punchthrough is evident at 20 V

Fig. 3

Ge-on-Si single-photon avalanche diode (SPAD) performance. Single-photon detection efficiency (SPDE) and dark count rate (DCR) as a function of excess bias for a 100 µm diameter SPAD at temperatures of a 78 K, b 100 K and c 125 K. The measurements were taken using the time-correlated single-photon counting (TCSPC) technique with a 50 ns electrical gate applied to the detector to bias it above avalanche breakdown. For the SPDE measurements the gate was synchronised with the arrival of the 1310 nm wavelength attenuated laser pulse. The detector was gated at a repetition rate of 1 kHz

Fig. 4

Ge-on-Si single-photon avalanche diode (SPAD) jitter. Timing histogram for a 100 µm diameter SPAD at an excess bias of 5.5% and a temperature of 78 K with incident radiation at λ = 1310 nm. The jitter full-width-at-half maximum (FWHM) was 310 ps

Fig. 5

Single-photon detection efficiency (SPDE) as a function of wavelength. a Normalised SPDE as a function of incident wavelength for a 100 µm single-photon avalanche diode (SPAD) at temperatures of 125 K (black squares), 150 K (red circles) and 175 K (blue triangles). b Experimental (squares) and theoretical (line) Ge cut-off wavelength as a function of temperature. c Estimated cut-off wavelength and fitting for a Ge-on-Si SPAD with a 2-µm-thick Ge absorption region. The red lines in b and c indicate the temperature required to reach a cut-off wavelength of 1550 nm

Fig. 6

Single-photon avalanche diode afterpulsing. a Afterpulsing probability as a function of gate delay time for a 100 µm diameter Ge-on-Si single-photon avalanche detector (SPAD) (open squares) compared to a 25 µm diameter commercially available InGaAs/InP SPAD (closed squares) when measured at λ = 1310 nm and operated at a single-photon detection efficiency (SPDE) of 17% and a temperature of 125 K. b Activation energy for the Ge-on-Si SPAD as a function of excess bias. The activation energies were acquired by fitting Arrhenius plots to the afterpulsing lifetime time constants. c The band structure at the Ge-Si heterointerface at 125 K showing the trapped defect states from dislocations (dashed lines) and the Γ valley (red line), L valley (blue line), Δ valley (black line), Light Hole (LH) band (grey line), Heavy Hole (HH) band (green line) and Split-Off (SO) band (yellow line)

Fig. 7

Single-photon characterisation setup. Schematic diagram of the experimental setup used for time-correlated single-photon counting (TCSPC) characterisation. The black lines denote electrical connections, the red solid lines denote optical fibre connections and the red dotted line denotes the free-space optical connection. The laser spot was directed onto the optical area of the SPAD using a broadband imaging system (not shown). The cryostat housing the SPAD has been removed for clarity