Rockwell International's Solid State Photomultiplier (SSPM) is an impurity-band avalanche device which can count individual photons with wavelengths between 0.4 and 28 micrometers. Its response to a photon is a pulse of between 10(exp 4) and 10(exp 5) conduction electrons, making it an important device for use in phenomenology. The characteristics of the SSPM make it a potentially important device for use in astronomical applications. Contract NAS2-12400 was initiated in June 1986 to conduct modeling and characterization studies of the SSPM to provide a basis for assessing its use in astronomical systems. Some SSPM models and results of measurements which characterize the group of SSPMs recently fabricated on this contract are discussed

Hays, K. M.

Laviolette, R. A.

Petroff, M. D.

Stapelbroek, M. G.

English

NASA Technical Reports Server

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https://ntrs.nasa.gov/search.jsp?R=19900012003 2020-03-19T23:17:50+00:00Z
The Solld State Photomultiplier -
Status of Photon Counting Beyond the Near-Infrared*
Rockwell International Science Center
Anaheim, CA 92803
Rockwell International's Solid State Photomultiplier (SSPM)[ I]
is an impurity-band avalanche device which can count individual
photons with wavelengths between 0.4 and 28 micrometers.[ 2] Its
response to a photon is a pulse of between 104 and 105
conduction electrons, making it an important device for use in
phenomenology.
The characteristics of the SSPM make it a potentially important
device for use in astronomical applications. Contract NAS2-
12400 was initiated in June 1986 to conduct modeling and
characterization studies of the SSPM to provide a basis for
assessing its use in astronomical systems. This paper discusses
some SSPM models and results of measurements which characterize
the group of SSPMs recently fabricated on this contract.
*K. M. Hays, R. A. LaViolette, M. G. Stapelbroek and M. D.
Petroff, Third Infrared Detector Technology Workshop, NASA-Ames
Research Center, February 7-9, 1989.
I. M. D. Petroff, M. G. Stapelbroek and W. A. Kleinhans, U. S.
Patent Number 4,586,068, Filed Oct. 7, 1983, Granted Apr. 29,
1986.
2. M. D. Petroff, M. G. Stapelbroek and W. A. Kleinhans, Appl.
Phys. Lett. 51 (1987) 406.
6O
STRUCTURE OF IMPURITY BAND CONDUCTION IR DETECTORS
The Solid-State Photomultiplier (SSPM) is the only device known
which can continuously count single photons in the i to 28 _m
wavelength range. The detector is a Si:As impurity band
conduction device. SSPMs are fabricated by growing doped and
undoped epitaxial layers on silicon substrates and then defining
the detectors using established silicon processing techniques.
Area and line arrays of SSPMs, as well as discrete detectors,
have been fabricated. Epitaxy for back-illuminated detectors
must be grown on intrinsic silicon substrates. Arrays of this
type can be connected to multiplexers or other signal processors
with indium columns to form hybrids. An etch to the buried
contact is required as a common detector biasing lead.
Front illuminated SSPMs have also been fabricated. Heavily-
doped substrates can be used for front-illuminated devices to
reduce optical crosstalk and to eliminate the need for a
transparent buried contact.
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SOLID STATE PHOTOMULT!PLIER OPERATING PRINCIPLE
The Solid State Photomultiplier is a unique device in that it
can continuously count single photons of wavelengths up to 28
_m. A pulse of about 40,000 electrons is the immediate and
direct consequence of the absorption of a single photon.
An electric field profile through the SSPM is shown in (b), with
the blocking layer on the left side of the cross-section. The
slope of the field in the (depleted) gain region is proportional
to the acceptor concentration. The higher fields toward the
left end of the gain region produce most of the dc bias current
by field-assisted thermal ionization (Poole-Frenkel effect).
The dc bias current is responsible for the non-zero field
throughout the undepleted part of the IR-active layer.
The detection of a single photon is shown schematically in (a).
The photon ionizes a neutral donor in the drift region. The
resultant ionized donor charge travels toward the substrate,
while the conduction electron moves more quickly toward the gain
region. At some point in the gain region the electric field is
strong enough for impact ionization to occur. The primary
photo-electron then ionizes additional neutral donors which, in
turn, ionize other neutral donors, causing an avalanche. The
produced pulse is a few nanoseconds wide and contains an average
of about 4xlO 4 electrons. The ionized donor charges produced in
the avalanche move slowly and contribute a low tail to the
pulse.
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THEORY OF SSPM PULSE AMPLITUDE DISTRIBUTION
The SSPM pulse amplitude distribution is strikingly narrow, a
feature which cannot be predicted with conventional avalanche
photodiode (APD) theory.[ 3] A new phenomenological theory of
the SSPM avalanche successfully predicts the shape of the
observed pulse amplitude distribution by including the history-
dependent effects present in the electron transport.[ 4] These
effects appear as the presence of a finite mean distance _ in
the threshold distribution.[ 5] In conventional APDs, the mean
distance-to-threshold is so small (i.e. R<O.05_m) that its
neglect is justified. For a simplified SSPM model, Monte-Carlo
calculations show that RzO.15-O.25_m. [4] Although such
distances are only about 5 percent of the SSPM gain region
thickness, they are not negligible. The magnitude of the
distance is due to the low (relative to the APD) electric field
in the SSPM gain region; the variations are strongly correlated
with variations in the field. As the accompanying figures show,
for the SSPM model employed in Ref. 4, the theory provides
pulse-amplitude distributions which are both realistic and
dramatically different from the history-independent prediction.
3. G. E. Stillman and C. M. Wolfe, in Semiconductors and
Semimetals, edited by R. K. Willardson and A. C. Beer, Vol. 12
(Academic Press, NY, 1977)
4. R. A. LaViolette and M. G. Stapelbroek, J. Appl. Phys. 65
(1989) 830-6.
5. The threshold distribution is the distribution of distances
between the birth of an electron via impact ionization and its
subsequent achievement of the threshold energy required for
impact ionization.
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SSPM PROVIDES HIGH QUANTUM EFFICIENCY FROM 0.4 TO 28 MICROMETERS
The photon-counting quantum efficiency of an SSPM is affected by
illuminating different faces of the device. The quantum
efficiency changes because of differences in optical path
lengths and the absorption coefficients of various layers.
Back-illuminated SSPMs make high-efficiency photon detectors in
the infrared wavelength ranges, out to 28 _m. Photons that pass
once through the active region of the SSPM without being
absorbed are reflected at the front surface of the detector by
an aluminum layer, so that the active thickness of the device is
effectively doubled. Back-illumination is also the best
approach for building hybrid focal plane arrays.
Front-illuminated SSPMs make high-efficiency photon counters in
the visible and infrared wavelengths. The quantum efficiency in
the visible range is extremely high, due to the generation of
electron-hole pairs in the blocking layer and the subsequent
impact-ionization of neutral donors in the IR-active layer by
holes moving toward the substrate.
Finally, edge-illumination offers a significant advantage over
front- or back-illumination in the near-to-mid IR wavelength
ranges. With edge-illumination, the optical path length is no
longer limited by the thickness of the IR-layer epitaxy, so that
the probability of detecting these IR wavelengths, with their
long attenuation lengths, can be greatly increased.
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DETECTORS ARE TESTED IN VERSATILE CRYOGENIC DEWARS
SSPMs are characterized in custom-built low background test
dewars. The extremely high sensitivity of these detectors
requires that the flux be at or below ix10 6 photons/cm2-s for
accurate dark pulse rate measurements. Flux levels can be
measured using an integral calibrated arsenic-doped extrinsic
photoconductor mounted in close proximity to the detector cold
stage. The temperature of the cold stage can be regulated
between 4.3 and 2OK.
The dewar contains a 3.2_m LED which can be used continuously or
pulsed for intervals shorter than I microsecond. Other
available illumination sources include pinhole shutter and hot
resistor sources which are filtered by an interchangeable
bandpass filter. Finally, a plastic optical fiber which
transmits visible light and IR up to about l.O_m wavelength is
included for flood illumination studies of spectral response in
the short wavelength range.
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QUANTUM EFFICIENCY OF SSPMs FABRICATED FOR NASA-AMES
A fabrication lot of SSPMs was fabricated for NASA-Ames under
contract NAS2-12400. One epitaxy set of wafers contained
devices which showed quantum efficiencies of over 60 percent at
I = 20 _m.
The figures show photon counting quantum efficiencies gathered
as a function of bias voltage and operating temperature. The
quantum efficiencies were calculated using:
Uillum-Udark
,_ =
®Adet
where u was the average number of SSPM pulses counted per second
and Ade t = 150x150 _m _ was the detector surface area. The
incident flux density, ®, was on the order of 108 photons/cm2-s
at both wavelengths.
The improvements in _ with higher bias voltages and temperatures
were due, in part, to increases in the drift region's electric
field. Field-assisted thermal ionization near the high-field
end of the gain region causes the bias current to increase at
higher temperatures, resulting in a higher drift-region field.
At IIK, the maximum quantum efficiencies were lower than those
measured at 1OK. This was due to the high Udark, which was
strongly temperature dependent. The recommended operating
temperature for these SSPMs was 1OK.
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BIAS CURRENT OF SSPMs FABRICATED FOR NASA-AMES
The bias currents of the SSPMs whose quantum efficiencies
appeared on the previous chart were measured as a function of
temperature and bias voltage. It is important to characterize
and minimize bias current for applications in which SSPM output
current is integrated on a capacitor and periodically read out.
The bias currents were approximately given by:
(C 2 V+C_/T)
Ibias(Vbias, T) = C Adete
1
where Ade t is the SSPM's area and C I, C2, and C3 are constants
which depend on the doping levels used in a particular device.
The effect of field-assisted thermal ionization and the doping
levels can cause changes in the detector's electric field
profiles at different biases. This can result in a departure
from the exponential dependence at lower bias voltages.
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UNSURPASSED RESULTS OBTAINED FROM RECENT DEVICES
Efforts to produce SSPMs with lower dark count rates and higher
operating temperatures have been successful. Since their
invention, SSPM dark count rates at 1OK have been reduced by
almost two orders of magnitude. The success was due, in part,
to improvements in epitaxy control and characterization. Lot
PM-79 was produced under contract NAS2-12400 with NASA-Ames for
Dr. C. McCreight. At their operating temperature, devices from
lot PM-79 have higher quantum efficiencies and lower dark count
rates than any previously produced SSPMs.
Further improvements in materials control are expected to result
in further improvements in SSPM performance. The goal is a
level of understanding and control sufficient to design SSPMs
with gain, gain dispersion, dark count rates and operating
temperatures matched to the needs of specific applications.
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SSPM HAS UNPRECEDENTED SENSITIVITY AND SPEED
Due to its high gain, low gain dispersion and high speed, the
SSPM does not require an ultra-low noise amplifier in order to
resolve pulses due to single photons. As a result, SSPM
performance is not limited by amplifier noise, even at extremely
low flux levels. When it is used as a photon counter at low
flux densities, the noise is given by:
an = (ndark+nsignal)1/2
where on is the uncertainty in the number of counts, and ndark
and nslgna I are the numbers of dark and signal counts,
respectively. If the best illumination approach (e.g. front,
back, or edge-illumination) is used at each wavelength, the low
dark count rate and sensitivity of the SSPM result in a noise
equivalent power that is orders of magnitude better than what is
typically obtained from other common infrared detectors.
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The solid state photomultiplier: Status of photon counting beyond the near-infrared

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