Three Ways to Detect Light In photon detectors, the light interacts with the detector material to produce free charge carriers 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 minute increase 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. 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 ) ( 0 0 d a e S d a e S S 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: v out v 0 t / e RC, t 0 RC 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). (9)
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, t B the blocking layer thickness, N A the minority impority concentration, and V b 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: D m kt q L, D. (17) (18) where m is the mobility (characterizes ability of charge carrier to migrate) and is the recombination time (goes as T 1/2 ). L goes as T 3/4
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. Diodes on back of transparent substrate: Rockwell HgCdTe arrays, used for NICMOS and for large format Hawaii devices Diodes attached to strong substrate (readout) and then thinned: SBRC InSb arrays, used for SIRTF and large format Aladdin device
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 Material Si Ge InAs InSb HgCdTe GaInAs AlGaAsSb Cutoff wavelength mm) 1.1 (indirect) 1.8 (indirect) 3.4 (direct) 6.8 (direct) ~1.2 - ~ 15 (direct) 1.65 (direct) 0.75 1.7 (direct)
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. Avalanche Photodiode (APD)
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?
1 2 1 C V N kt 2 2 ktc or Reset Noise 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: 2 4kTdf I J R From the left expression we get: Q 2 N kt C S q 2. (24) 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 = 10 19 (say), RC 10 6 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 read noise ktc q 2 1 e t / RC with additional components from amplifier noise and other sources.
V out 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.
The charge transfer efficiency must be extremely high. If the proportion of charge lost in n transfers is, then the noise in transferring N 0 charges is N CTE ( 2 n N ) 1/ 2. 0 (25) 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 overfilled 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.
Time-Delay-Integration (TDI) 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: 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 photoelectrons into the wells.