three ways to detect light - university of arizonaircamera.as.arizona.edu/astr_518/detectors.pdf ·...
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• In photon detectors, the light interacts with the detector material to produce free chargecarriers photon-by-photon. The resulting miniscule electrical currents are amplified to
yield a usable electronic signal. The detector material is typically some form of semiconductor, in which energies around an electron volt suffice to free charge carriers; this energy threshold can be adjusted from ~ 0.01 eV to 10eV by appropriate choice of the detector material.
• In thermal detectors, the light is absorbed in the detector material to produce a minuteincrease in its temperature. Exquisitely sensitive electronic thermometers react to this
change to produce the electronic signal. Thermal detectors are in principle sensitive to photons of any energy, so long as they can be absorbed in a way that the resulting heat can be sensed by their thermometers. Thermal detectors are important for the X-ray and the far-infrared through mm-wave regimes.
• In coherent detectors, the electrical field of the photon interacts with a locally generated signal that downconverts its frequency to a range that is compatible with further electronic processing and amplification. Downconversion refers to a multi-step process in which the incoming photon electrical field is mixed with a local electrical field of slightly different frequency. The amplitude of the mixed signal increases and decreases at the difference frequency, termed the intermediate frequency (IF). This signal encodes the frequency of the input photon and its phase.
Three Ways to Detect Light
We now establish terminology for photon detectors:
Photon absorption and creation of free charge carriers in semiconductors is the basic process behind photo-detection.
Compare the diagram of crystal structure (above) with the band gap diagrams (below). To free an electron in intrinsic material (1) requires a certain energy indicated by the band gap. It takes less energy to free charge carriers from impurities (2) and (3).
The net absorption is characterized by the absorption coefficient, a. Note the difference between direct and indirect absorption (e.g., silicon vs. GaAs) .
)3(,1)(
1
0
1)(
00da
eS
daeSS
ab
The quantum efficiency is:
A decent detector will have close to linear response over some range of signal, and will completely saturate at some high level. In between, it is possible to recover information – the range of signal where useful information can be obtained is the dynamic range.
We characterize the resolution in a number of ways, including the MTF.Although the time response can have complex behavior, we will deal mostly with simple resistance-capacitance (RC) behavior:
)9(0,/
0
tRCt
e
RC
v
outv
The spectral response drops abruptly to zero at the band gap or excitation energy. The responsivity is the amps out per watt of signal in. It rises linearly to the cutoff wavelength for an ideal detector (assuming one charge carrier per absorbed photon).
Detector type #1, Si:X IBC• Physical structure to left, band diagram to right; structure is a thin intrinsic layer, then to right of it a heavily doped absorbing layer, then to right, a contact• An absorbed photon elevates an electron to the conduction band, from which it can migrate to the contact unimpeded. Thermal charges in the impurity band are blocked at the impurity layer, so dark current is low.• Detector type of choice for 5 – 35mm• Notice the separation of zones for electrical properties and photo-response
Use of these detectors in
an array requires some architecture
changes, to allow attaching the
readout (to the left in these drawings).
Also, very high purity must be
achieved in the silicon to allow for
complete depletion of the IR-active
layer, or the quantum efficiency will
suffer (or the bias will have to be set
too high, increasing the noise –
w is the width of the depleted region,
tB the blocking layer thickness, NA the
minority impority concentration, and
Vb the bias).
Here is a state-of-the-art Si:As
IBC array (made by Raytheon for
MIRI on JWST).
It is 1024 X 1024 pixels and at ~
6.7K delivers dark current < 0.1
e/s, read noise of ~ 15 e rms, and
quantum efficiency > 60% from 8
to 26mm (~ 10% of maximum still
at 28.3mm and > 40% at 5mm).
The pixels are 25mm on a side.
A special process is used for the
readout circuit so it works well at
such low temperatures.
Similar devices but somewhat
lower performance have been
made by DRS Technologies for
WISE.
Detector Type #2, Photodiodes
• A depletion region is created by doping to form a junction between n-type
and p-type material
When a photon is absorbed,
the freed charge carriers
diffuse through the material
until one of them
encounters the junction,
which it is driven across by
the internal field to create
the photocurrent. The
diffusion coefficient and
length are:
)17(,q
TkD
m
)18(.DL
where m is the mobility
(characterizes ability of charge
carrier to migrate) and is the
recombination time (goes as
T1/2).
L goes as T3/4
Diodes attached tostrong substrate (readout)and then thinned:SBRC InSb arrays, usedfor SIRTF and largeformat „Aladdin‟ device
Diodes on “back” of transparent substrate:Rockwell HgCdTearrays, used forNICMOS and for largeformat „Hawaii‟ devices
• For operation with low dark current, low temperatures are needed
• Absorbing layer may need to be thinned to 10 - 20mm to collect charge
carriers at these temperatures
• Two approaches are shown below
• The yields in doing this can be low, partly explaining the high prices of these
arrays.
Material Cutoff wavelength mm)
Si 1.1 (indirect)
Ge 1.8 (indirect)
InAs 3.4 (direct)
InSb 6.8 (direct)
HgCdTe ~1.2 - ~ 15 (direct)
GaInAs 1.65 (direct)
AlGaAsSb 0.75 – 1.7 (direct)
• Here are some photodiode materials and their cutoff wavelengths.
HgCdTe has a variable bandgap set by the relative amounts of Hg and Te in
the crystal. AlGaAsSb behaves similarly.
• Indirect absorbers will have poor QE just short of the cutoff
Avalanche Photodiode (APD)
The APD allows gain and
even single photon counting
with a solid-state device. Its
structure has some
characteristics of a BIB
detector, with absorption in a
depletion region maintained
by the bias across the device.
Avalanche multiplication
occurs when the charge
carriers are accelerated to
high enough energy to free
additional charge carriers and
so one can produce an
avalanche of many. The
multiplication occurs in the
junction; the noise is
minimized because of the high
field there and the restricted
depth of the region.
An example: Teledyne HgCdTe
for all the JWST instruments
except MIRI. Here is the
architecture of a pixel. Photons
come in from the right and the
contact to the readout is to the
left. The cap layer has the
Hg/Te ratio adjusted to
increase the bandgap, so free
charge carriers are repelled
without actually having a
discontinuity in the crystal.
These arrays come with either
2.5 or 5mm cutoff. A competing
technology is diodes in InSb,
with a cutoff just beyond 5mm.
An example: the HgCdTe
arrays for NIRCam and
other JWST instruments.
They are 2048 X 2048
pixels in size, with 18mm
pixels. Dark currents at ~
37K are 0.004 e/s for the
short cutoff and 0.01 e/s for
the long. The QE is > 90%
from 1mm to close to the
cutoff and > 70% between
0.5 and 1mm. The read
noise is ~ 6-7e rms.
Similar devices but
somewhat lower
performance have been
made by Raytheon for
VISTA.
The Array Revolution in Infrared Astronomy
1968, 5-m telescope,
3 nights, single
detector
~ 2000, 1.3-m
telescope, 8
seconds, 256 X
256
~ 2006, 6.5-m
telescope, 1 hour,
1024 X 1024
(4 minutes with the
2x2 2048 X 2048
NIRCam mosaic)
Detector Type #3: Image Intensifier
• Vacuum photodiode - similar in physics to semiconductor photodiode
• Lower quantum efficiency because photoelectron has to escape from the
photocathode in to the vacuum space (photoelectric effect)
• Operates well at room temperature (issue is thermal release of charge carriers
from photocathode)
Some old-time image intensifiers
All suffer from signal-induced backgrounds. Their outputs are an amplified
version of the inputs and any leakage back to the input contaminates the signal.
Modern use is in the ultraviolet and with a microchannel array as the amplifier,
with an electronic output.
Infrared Detector Arrays
• Best performance with silicon integrated circuit readout• Cannot manufacture high quality electronics in other semiconductors
• CCD-type readout has charge transfer problems at cold temperatures
• Direct hybrid construction• Fields of indium bumps evaporated on detector array, readout amplifiers
• Aligned and squeezed together - very carefully
The readout: a source follower
simple integrating amplifier
• As charge accumulates on the gate
capacitance of the MOSFET, it
modulates the current in the channel
• To keep from saturating, the charge
is reset from time to time
How should we sample the output?
kTC or Reset Noise
kTVC N2
1
2
1 2 R
kTdfI J
42
• The circuit below has both potential energy (charge on the capacitor) and kinetic energy
(Brownian motion of electrons in the resistor)
• From thermodynamics, we have kT/2 of energy with each degree of freedom, leading to:
)24(.2
2
q
SCkT
NQ From the left expression we get:
For C = 10-13 F and T = 40K, the noise is about 45 electrons rms. A
NIRCam array has a read noise of 6 - 7 electrons. How is this done?
• To avoid reset or kTC noise, we have to use readout strategy (b) or ( c)
• Then, if R = 1019 (say), RC 106 sec
• ( c) lets us take out some forms of slow drift by sampling with the reset
switch closed. However it adds root2 more amplifier noise
• Modern arrays can support strategy (b) even for integrations of 1000 -
2000 seconds
• The kTC noise is then (45e)*(exp(t/RC) - 1) = 0.1e for 2000 seconds
• In general, the reset noise is
RCteq
kTCnoiseread /
21
with additional components from amplifier noise and other sources.
Vout
time
More readout options: “Fowler sampling” to the left and “multi-accum”
to the right. Both address averaging out the high frequency electronic
noise (e.g., from the output amplifier). Fowler sampling has an
advantage in principle for lower net read noise, multi-accum is more
robust against upsets, e.g., cosmic ray hits.
Now consider the operation of array of readout amplifiers:
This approach has
random access and
reads nondestructively.
That is, we can read
out any pixel we want,
and we can read it and
then leave it exactly as
it was with the same
signal. This allows
interesting read out
patterns, like Fowler
sampling (multiple
samples to drive down
amplifier noise) or
sampling up the
integration ramp.
Charge Coupled Devices
(CCDs)
CCDs are actually more
complex than infrared arrays,
so we have saved them for last.
The top shows the physical
arrangement of a pixel, and (b)
and ( c) the situation before and
after exposure to light. The
electrodes deplete the silicon
near them and create potential
wells where free charge carriers
collect.
To keep the photoelectrons
from getting trapped at the back
surface, special coatings are
applied that bend the bands to
repel them.
Reading out a CCD
When it is time to read out the signal, it is transferred along the array by
manipulating the voltages on the electrodes. This illustration is for a 3-phase
CCD, but similar strategies work for 4-phase and, with a bit of a trick on the
electrode design, on 2-phase.
The charges are brought to the output amplifier in (a) in-line transfer, (b )
interline transfer, or (c ) frame transfer architectures. Astronomers
generally prefer (a), in which case the chip continues to collect signal as it
is read out.
)25(.2/1)0
2( NnCTE
N
The charge transfer efficiency must be extremely high. If the proportion of
charge lost in n transfers is , then the noise in transferring N0 charges is
Consider a high performance 2048 X 2048 CCD with 2 electrons read noise.
If it is 3-phase, it takes about 12000 transfers to transfer the most distant
charge package out. Consider a signal of 100 electrons. Then, if the CTE =
0.999999, the CTE noise is 1.5 electrons!
However, as we have drawn the CCD, the photo-electrons collect at the
interface between the electrode and the silicon crystal, where there are lots
of open crystal bonds that trap electrons and degrade the CTE. The noise
achievable with such “surface channel” CCDs is no less than many
hundreds of electrons.
This problem is fixed with a
buried channel. A thin layer of
silicon (about a micron) is
doped oppositely to the rest
of the detector wafer and the
resulting potential minimum
causes the electrons to
collect in the bulk crystal, not
at the electrodes.
This does not work at low
temperatures (< 70K), and if
the wells are over-filled then
the CCD has some electrons
back to surface channel, i.e.,
serious latent images and
bad CTE.
Another issue with over-
filled wells is that charge
can leak from one well to
its neighbor along the
charge transfer direction,
leading to “blooming.”
Some CCDs have extra
thick channel stops to
resist blooming, but this
reduces the quantum
efficiency and is not used
for astronomical detectors.
The buried channel CCD approach is very similar to the one using a
composition change in the HdCdTe cap layer in a Teledyne array to keep
the photo-electrons from getting trapped away from the junction.
Reading the signal out
Finally, the signal gets to the output amplifier. By using a floating gate to put
the matching charge (by capacitive coupling) on the MOSFET gate, we can
read it out while avoiding kTC noise. Since we retain the charge after
readout, we can take it to another amplifier and read it again to reduce the
net noise.
However, this readout is not
random and the charge is
eventually lost (not returned
to the pixel where it
originated).
Some other aspects of CCD performance:
Pixel binning - does
not degrade the noise,
so is a painless way to
reduce the number of
effective pixels in the
array. This can reduce
data rates and also
effective read noise.
The CCD charge transfer process lends itself naturally to clocking charge
in one direction at a set rate. This capability can be useful in applications
where images drift across the detector array at a constant (relatively slow)
rate – the charge generated by a source can be moved across the CCD to
match the motion of the source. As a result, the CCD can integrate
efficiently on the moving scene of sources without physically moving
anything to track their motion.
Time-Delay-Integration (TDI)
Time-delay integration: The CCD can be clocked at the same rate a scene is
moving along its columns (careful alignment is needed to be sure the motion
is purely along the columns). Then, if it is a line-transfer device, the signal
builds up without blurring.
Orthogonal charge transfer:
It is possible to arrange 4-
phase electrodes to allow
transfer of charge in either
direction and backwards
and forwards. As shown
here, transfer downward
goes 3-1-2-3, to the right
goes 4-2-1-4, and so forth.
Deep Depletion
Because silicon is an indirect absorber, CCDs tend to have poor QE near 1 micron.
The curves, in order of increasing absorption, are for 5, 10, 15, 30, and 50mm.
Just making the CCD thick reduces the
free electron collection efficiency
(particularly in the blue where the
electrons are freed in the first 10 – 20
microns). A deep depletion CCD solves
this problem by putting a transparent
contact on the back surface and placing a
bias voltage on it so drive the photo-
electrons into the wells.