Ternary Semiconductor Detectors
When three elements are in play, say HgCdTe or InGaAs, the
ratio of elements provides a tunable bandgap.
InGaAs Research at UVa
The IR lab has an NSF-funded research program to evaluate
InGaAs arrays produced by Sensors Unlimited, Inc.
The communications industry uses InGaAs with 1.7um cutoff
for a variety of applications.
The peak of transparency of optical fibers is at about 1.55um
InGaAs is grown on top of an InP substrate.
These two materials have a common crystal lattice spacing for a
53% Indium 47% Gallium mixture.
Identical lattice spacing is important in preventing crystal defects
(dislocations) from creating artificial energy levels with low binding
energy – producing excessive dark current.
As a consequence “standard” InGaAs
has a wavelength cutoff of 1.7um.
epitaxy
n : growing a crystal layer of one mineral on the crystal base
of another mineral in such a manner that its crystalline
orientation is the same as that of the substrate
Astronomical InGaAs
Astronomers would prefer a detector sensitive to at least
2.32 um (the cutoff of the astronomical Ks band).
The required mixture is 82% Indium 18% Gallium
The lattice constant of this mixture is 2% different from that of
InP. Simple direct growth will produce defects.
Minimizing Defects with Buffer Layers
Growing a gradient buffer structure on
the InP substrate will minimize
stress/defects.
The buffer structure gradually transitions
from the 53% Indium (lattice matched to
InP) to the 82% Indium alloy.
Defects tend to propagate parallel to the
layers rather than through them, further
minimizing defect induced dark current.
Sensors has iteratively grown test diodes
with the aim of producing dark currents
of order 100 electrons/s or less in a
25um pixel
dark currents at this level would be lower
than the airglow and thermal backgrounds in
the H and Ks astronomical bands.
only natural backgrounds would then limit
observational sensitivity
Low Dark Current Extended-Wavelength InGaAs
Observation of dark current vs.
temperature will reveal the onset
of defect-induced dark current.
Dark current should initially fall
exponentially with inverse
temperature with an e-folding
determined by the bandgap energy
of the intrinsic semiconductor
material.
E
Dark Current ∝ e
−
2kT
Defects will produce energy states
with energies lower than the
bandgap energy.
At some temperature the decline of
dark current will become less steep,
and ultimately flatten, since these
energy states will remain accessible
at lower temperatures.
Low Dark Current Extended-Wavelength InGaAs
Observation of dark current vs.
temperature will reveal the onset
of defect-induced dark current.
Dark current should initially fall
exponentially with inverse
temperature with an e-folding
determined by the bandgap energy
of the intrinsic semiconductor
material.
E
Dark Current ∝ e
−
2kT
Defects will produce energy states
with energies lower than the
bandgap energy.
At some temperature the decline of
dark current will become less steep,
and ultimately flatten, since these
energy states will remain accessible
at lower temperatures.
Extended InGaAs Array Testing
Sensors Unlimited applied the second generation of
extended wavelength InGaAs material to a 320x240
multiplexer.
Extended InGaAs Array Testing
Sensors Unlimited applied the second generation of
extended wavelength InGaAs material to a 320x240
multiplexer.
Extended InGaAs Array Testing
The long-wavelength cutoff is a function of temperature due to
changing lattice spacing and freezing out of population of carriers
thermally excited across the bandgap.
Extended InGaAs Array Testing
The current generation of material has a dark current of <10
electrons/s at 78K.
A new (and presumably improved) array is due any day.
Hybrid Arrays
Photodiodes can have near-optimal performance even if they
are microns in size.
Lithography techniques permit the manufacture of large format
detector arrays by epitaxial growth.
For silicon detectors, the electronic multiplexer and the silicon
detectors can be grown on the same substrate since the
electronic components are also made from dopped silicon.
Infrared array materials must be joined to silicon multiplexers
via mechanical indium “bump bonding” - one minsicule soft
metal bump per detector
The different coefficients of thermal expansion of the infrared and
multiplexer layers produces destructive forces upon cooling to
cryogenic temperature
Detectors/multiplexers can be made thin to permit “stretching”
– Mechanically strong layers can be introduced to take up the
stresses.
The current generation of 2Kx2K HgCdTe devices suffers from
occasional destructive failure due to differential expansion – the
“exploding array” problem.
–
Detector Arrays
As opposed to CCD devices,
which drag charge around
with varying potentials,
detector unit cells may be
wired to transistor switch
''multiplexers'' to permit
direct addressing of vast
numbers of detectors
arrayed in rows and
columns.
In this case, since the
charge stays with the
detector, readouts are “nondestructive” and any pixel,
or the entire array, can be
read out multiple times.
Outputs
Each individual detector in an array must be connected
to circuitry which reports the collected photon induced
charge in Volts.
Photocurrents are miniscule. Ideally, readout
electronics should be sensitive to the charge of an
individual electron
Electronic circuitry must amplify these week signals without
adding noise beyond that which is intrinsic to the detector
(Johnson noise and photon Poisson noise).
Array detectors must have one amplifier per detector or
switching schemes to hook up detectors to shared output
amplifiers.
The capacitance in these switches, for example, can eat up much
of the photon generated carriers if one is not careful.
In the case of the arrays all of this amplification and switching
must happen at high speeds in order to address a million pixels in
a second
–
typical “dwell” times are a few microseconds per pixel.
Transimpedance Amplifiers
Back to single detector circuits for a moment...
Read Chapter 3 in Horowitz and Hill to understand Op Amps.
Photovoltaic diodes produce a current, I, in response to light
Shoving this current through a resistance, R, will produce a
voltage V=IR will be produced in response.
An operational amplifier configured as an inverting amplifier
will do just this.
Used as a current to voltage converter, this device is called a
''transimpedance'' amplifier.
Capacitive Transimpedance (Charge
Integrating) Amplifiers
The feedback resistor in a transimpedance amplifier can be replaced
by a capacitor.
A fixed current supplied to a capacitor will charge (or discharge) it
linearly with time.
V t =
Q t
C
=
I t
C
The accumulated voltage on the capacitor can be read at the end
of the integration period.
A switch must reset the capacitor's voltage after each integration
cycle otherwise it will saturate by reaching the voltage limit of the
power supply.
Capacitive Transimpedance (Charge
Integrating) Amplifiers
The feedback resistor in a transimpedance amplifier can be replaced
by a capacitor.
A fixed current supplied to a capacitor will charge (or discharge) it
linearly with time.
V t =
Q t
C
=
I t
C
The accumulated voltage on the capacitor can be read at the end
of the integration period.
A switch must reset the capacitor's voltage after each integration
cycle otherwise it will saturate by reaching the voltage limit of the
power supply.
Amplifying the Photocurrent in a Charge
Integrating Configuration
The detector current can also be amplified to charge/discharge a more
robust capacitor.
The circuit below is the ''unit cell'' in the Sensors Unlimited 320x240
InGaAs arrays under test in the IR Lab.
Integration using the Detector's Capacitance
A photodiode junction has a capacitance and can serve to integrate
the photocurrent.
A typical array element (20um x 20um) has a capacitance of order
<0.1 picoFarad - 600,000 e-/V
Since kTC noise is proportional to capacitance, the smaller the
detector capacitance the better
and thus the smaller the detector area the better
–
–
–
–
however, the detector size must be consistent with the focal plane
optics
however, small capacitances imply small well depths to saturation.
however, the variation in pn junction properties as a function of bias
will lead to non-linearity (from varying capacitance).
nevertheless... it works pretty well overall.
In practice, the detector is “reset” by charging it to a larger reverse
bias voltage
photocurrent discharges the capacitor
–
when the capacitor/detector is completely discharged the detector
has “saturated”
Array Readout Electronics - Multiplexers
Single detector unit cells
may be wired to
transistor switch
''multiplexers'' to permit
addressing of vast
numbers of detectors
arrayed in rows and
columns.
Integrated circuit
technology
(semiconductor
lithography) permits the
incorporation of millions
of detectors/transistors
in a single device.
The current generation of
computer CPU contains
100 million transistors
for comparison.
Integration using the Detector's
Capacitance
Sampling strategies
Resetting the detector is a
“violent” event. The opening
and closing of the transistor
switch can lead to the
deposition of an unknown
charge on the detector.
The post-reset image is thus
noisy.
Taking advantage of the nondestructive nature of array
readouts
–
–
–
if one samples the image
just after reset one captures
the random reset values for
future reference
if the array is readout later,
but prior to another reset,
this reference image can be
subtracted, removing the
random reset component
Integration using the Detector's
Capacitance
Since there is no cost in reading
the array multiple array readouts
can be used to drive down the
overall read noise of the device.
–
–
–
since read noise arises from
the statistical non-ideal noise
of the amplifiers, it can
potential be driven down as
the square root of the number
of samples.
In practice, non-ideal noise
behavior limits the
improvement of read noise to
a factor of 2 or 3.
These multiple samples can be
obtained in bunches at the
beginning and end of an
integration (Fowler Sampling)
or uniformly during an
integration (sample-up-theramp).
Correlated Double Sampling
Array read out at beginning and end of integration period.
22
slide stolen from Dan Clemens' Mimir instrument presentation
Fowler Sampling
Array read multiple times at beginning and end of integration.
23
slide stolen from Dan Clemens' Mimir instrument presentation
Sample Up the Ramp
Array read continuously
In addition to noise supression, this method permits the unsaturated
extraction of stars that would have saturated in the full exposure
time.
24
slide stolen from Dan Clemens' Mimir instrument presentation
Infrared Array Frame Calibration
Construct difference
image from the
independent readouts
contains “bias” pattern
of the array, plus
structure from thermal
radiation from the
telescope.
Subtract dark/bias frame
to remove bias.
Divide by flat field to
correct pixel-to-pixel
sensitivity variation
Construct a median
image of the sky and
subtract to remove
thermal structure.
Does not work well in
crowded/diffuse regions.