35 ps time resolution at room temperature
with large area single photon
avalanche diodes
A. Gulinatti, P. Maccagnani, I. Rech, M. Ghioni and S. Cova
It is demonstrated that remarkable timing performance is achievable
with large area single photon avalanche diodes (SPADs), provided that
the avalanche current is sensed at very low level, when the multiplication process is still confined within a small area around the
photon absorption point. By employing a suitable current pick-up
circuit, an unprecedented time resolution of 35 ps was obtained with a
100 mm active area diameter SPAD.
Introduction: Specially designed avalanche photodetectors operated
in Geiger-mode, i.e. biased at voltage higher than the breakdown
level, can detect single photons. These devices are called single
photon avalanche diodes (SPADs). SPADs have been successfully
used in photon counting and timing measurements for a wide range of
emerging scientific applications such as fluorescence analysis in the
life sciences [1, 2], quantum cryptography [3], and profilometry of
remote objects with optical radar techniques [4].
Time resolution is a key parameter for SPAD devices employed in
photon timing measurements; it is determined by the precision with
which the arrival instant of the incident photon on the photodetector is
identified. Single-photon counting modules based on SLIK avalanche
diodes with thick depleted region (20–25 mm) and large active area
(180 mm diameter), are commercially available from PerkinElmer
Optoelectronics [5]: they have very good quantum efficiency and low
noise, but they have inadequate time resolution. In fact, their typical
time resolution full-width at half maximum (FWHM) is about 400 ps
while the intrinsic limit of SLIK devices lies around 150 ps FWHM [6].
However, SPAD devices with thin depleted region (1–2 mm) and
small active area (10–20 mm diameter), fabricated in planar silicon
technology on epitaxial silicon [7] can attain time resolution down
to about 30 ps FWHM at room temperature [6]. This performance is
comparable with that of the best micro channel plate (MCP) photomultipliers. It was shown by Lacaita and Mastrapasqua [8] that the time
resolution of planar SPADs is strongly dependent on the detector’s
diameter, due to the propagation of the avalanche multiplication process
from the point where the photon is absorbed (seed) to the whole active
area. In fact, the risetime of the avalanche current corresponds to the
time taken by the current to propagate and fill the whole detector area.
This time systematically depends on the position of the initial seed. A
fast comparator is normally used for sensing the avalanche leading
edge. Therefore, if the position of the initial seed within the area is
random, also the delay from the photon absorption to the threshold
crossing time is randomly distributed. It follows that the larger is the
active area the worse the timing performance. It was suggested in [9]
that the trade-off between active area diameter and time resolution may
be overcome by detecting the avalanche current during the initial part of
its rise (at a 100 mA level), where the multiplication process is still
confined within a small area around the seed point.
In this Letter, we demonstrate that time resolution down to 35 ps
FWHM can be easily obtained at room temperature with SPAD
detectors having thin depletion layer and active area diameter up to
100 mm, by employing a suitable current pick-up circuit consisting in a
linear network that feeds a fast comparator. The linear network makes
it possible to extract the avalanche current from the detector without
affecting the initial rise of the signal. This is mandatory for allowing
a true low-level discrimination of the avalanche current. The current
pick-up can be added to any of the quenching circuit configurations
described in the technical and scientific literature [6].
Experimental results: Double-epitaxy SPAD devices were fabricated
in collaboration with CNR – IMM (Sezione di Bologna), using the
process described in [7]. The tested devices have an active area
diameter of 20, 50 and 100 mm and a breakdown voltage of 24 V at
room temperature. Suitable gettering processes were used to avoid the
steep increase of the dark counting rate with the detector diameter that
was observed in the previous generations of SPAD devices [10]. In the
past, this effect prevented the exploitation of SPADs with active area
diameter larger than 20 mm. The fabricated devices were biased 5 V
above breakdown and operated at room temperature using the active
quenching circuit (AQC) reported in [11]. They show a remarkably
good dark counting rate, ranging from about 300 c=s for the 20 mm
devices to about 6000 c=s for the 100 mm devices. The photon
detection efficiency peaks at 38% around 550 nm and it is about
10% at 820 nm wavelength.
Time resolution measurements were performed in a conventional
time-correlated single photon counting (TCSPC) setup using an ultrafast laser diode (Antel MPL-820 laser module) emitting 10 ps FWHM
optical pulses at 820 nm wavelength. The pick-up network (Fig. 1) was
connected to the high voltage terminal of the SPAD in order to extract
the avalanche current pulse and to generate the timing signal. Details
about the design of the timing pick-up can be found in [12].
Fig. 1 Simplified block diagram of timing pick-up circuit that can be added
to any of the existing active quenching circuits (AQCs) for improving
photon timing resolution
Fig. 2 shows the time resolution FWHM of the 50 mm SPAD
measured with unfocused light against the threshold voltage of the
timing discriminator. As expected, the time resolution improves by
reducing the threshold voltage, showing that a minimum time resolution
of about 35 ps FWHM is obtained with a threshold voltage of 8 mV,
corresponding to an avalanche current level of about 100 mA.
Fig. 2 FWHM time resolution of SPAD having active area diameter of
50 mm against threshold voltage of timing discriminator
Table 1: Time resolution FWHM measured with threshold voltage
of 8 mV
Active area diameter (mm) Time resolution FWHM (ps)
20
32
50
34
100
35
As shown in Table 1, the time resolution FWHM measured with the
lower threshold voltage is almost independent of the detector size, thus
confirming that the discrimination level of the comparator is sufficiently
low to detect the avalanche current before it spreads all over the active
area of the device.
ELECTRONICS LETTERS 3rd March 2005 Vol. 41 No. 5
Fig. 3 shows the time resolution curve of the 100 mm SPAD detector.
The curve shows a prompt peak with a FWHM of 35 ps and a clean
exponential diffusion tail with 80 ps lifetime.
A. Gulinatti, I. Rech, M. Ghioni and S. Cova (Dipartimento di
Elettronica e Informazione, Politecnico di Milano, Piazza Leonardo
da Vinci 32, 20133 Milano, Italy)
E-mail: ghioni@elet.polimi.it
P. Maccagnani (CNR - IMM Sezione di Bologna, Via P. Gobetti 101,
40129 Bologna, Italy)
References
1
Fig. 3 Time resolution curve of SPAD having active area diameter of
100 mm, measured with threshold voltage of 8 mV
Conclusion: We have shown that the trade-off between sensitive area
and time resolution in SPAD devices with thin depleted region can be
overcome by detecting the avalanche current during the initial part of
its rise (at a 100 mA level). By employing a suitable current pick-up
circuit, we achieved an unprecedented time resolution of 35 ps with a
100 mm active area diameter SPAD. This performance is comparable
with that of small-area SPAD devices with the same structure. This
result may open the way to a widespread use of SPAD detectors in
high-resolution photon timing applications such as high-rate shortwavelength quantum cryptography.
Acknowledgments: This work was supported by the Italian Ministry
of University and Research (MIUR-FIRB Program) and by the
European Community under the 6th framework programme (contract
no. 506813, SECOQC project).
# IEE 2005
Electronics Letters online no: 20047445
doi: 10.1049/el:20047445
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