Near-Infrared Detector Arrays M. Robberto (with several slides grabbed from J. Beletic, K. Hodapp et...

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Near-Infrared Detector Arrays M. Robberto (with several slides grabbed from J. Beletic, K. Hodapp et al.)

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  • Near-Infrared Detector Arrays

    M. Robberto(with several slides grabbed from J. Beletic, K. Hodapp et al.)

  • Intrinsic materialsThe bandgap depends on the temperaturee.g. for InSb: Eg = 0.24 eV and = -2x10-4eV

    materialc(mm)Eg(eV)AgCl0.393.20CdS0.522.40GaP0.552.24CdSe0.691.80CdTe0.711.45GaAs1.350.92Si1.111.12Ge1.850.67PbS2.950.42InAs3.180.39PbTe5.00.25PbSe5.400.23InSb5.400.23Pb1-xSnxTe0.10Hg1-xCdxTe0.10

    NASA CDR 05-08-01

    Extrinsic materialsP-type N-type

    materialc(mm)Eg(eV)Ge:Au8.270.15Ge:Hg13.80.09Ge:Cd20.70.06Ge:Cu30.20.041Ge:Zn37.60.033Ge:Be400.03Ge:B119.20.0104Ge:Ga1200.01Ge:Li1400.009Si:In8.000.165Si:Mg14.30.087Si:Ga17.10.0723Si:Bi17.60.0706Si:Al18.10.0685Si:As23.10.0537

  • Homework 1Small fractional changes in x lead to large fractional changes in the gap energy. How well we need to control x at room Temperature to have a 2% uncertainty in response at cutoff forHgCdTe 1.72micron cutoff at 145K [WFC3]HgCdTe 2.5micron cutoff at 77K [ground based]HgCdTe 5micron cutoff at 35K [JWST]HgCdTe 10micron cutoff at 35K [NEOCAM]

    Among these, that is the most demanding material to grow?

  • Cross-section of HgCdTe detectorp-on-nP-on-N design

  • PN junctionEF = Fermi Level => occupancy at high T

  • PN junctionN-typeP-typeDoped semiconductorsImpurities (doping) move the EF closer to the valence (P-type) or conduction (N=type) bands.

  • PN junctionN-typeP-typeP-N JunctionWhen the two materials are brought into electrical contact, the electrons and hole diffuse. Recombination occurs until the Fermi levels are in equilibrium.Depletion or Space Charge regionE

  • Depletion RegionNot Neutral: there is an electric field from the N-type (+ charged) to the P-type (- charged)Free (depleted) of mobile carriers: extremely low conductivity, or high resistivity. An insulator between two charge distributions is a capacitance.The development of the electric field eventually stops the diffusion: built-in voltage or contact potential The electric field facilitates the flow of charges in one direction and prevents in the other: diodeE

  • PN junctionReverse biased P-N JunctionReverse bias: apply voltage with the same polarity of the contact potential + Voltage to the N-type - Voltage to the P-typemakes depletion region wider and increases the resistance of the junction. (but do not exagerate! => breakdown)

    Forward bias: smaller depletion region, eventually no E: high conductivityN-typeP-typeE+Eb

  • PN junction illuminatedReverse biased P-N JunctionAssume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is kept constant, or recombines with a hole and discharges the junctionIf the bias is floating, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the reverse direction with respect to the original one that set the junction.(Same is true for photogenerated holes in N-type material).N-typeP-type

  • PN junction illuminatedReverse biased P-N JunctionAssume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is kept constant, or recombines with a hole and discharges the junctionIf the bias is floating, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the reverse direction with respect to the original one that set the junction.(Same is true for photogenerated holes in N-type material).N-typeP-type

  • PN junction illuminatedReverse biased P-N JunctionAssume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is kept constant, or recombines with a hole and discharges the junctionIf the bias is floating, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the reverse direction with respect to the original one that set the junction.(Same is true for photogenerated holes in N-type material).N-typeP-type

  • Back to zero bias and beyondN-typeP-typeP-N JunctionEventually the junction is discharged but photons are still absorbed. The diffusion current pushes back to maintain the built-in bias. Dark and photocurrent therefore work in different directions. An equilibrium is reached: saturation.

  • Reset

  • Photon detectionDo you see the cross-talk/MTF problem?

  • End of integration

  • Reading out the generated chargesHybrid CMOS sensorsIndium bumps are aligned, squeezed and distorted to establish electric contact between detector layer and multiplexer: COLD-WELDINGThe addressing and readout electronics is built on Silicon. More standard technology (still >107 transistors).

  • HAWAII-2: Photolithographically Abut 4 CMOS Reticles to Produce Each 20482 ROICTwelve 20482 ROICs per 8 Wafer20482 Readout Provides Low Read Noise for Visible and MWIR

  • RSC ApproachH A W A I I - 2 R GHgCdTe detector substrate removed to achieve 0.6 m sensitivity HgCdTe Astronomy Wide Area Infrared Imager with 2k2 Resolution, Reference pixels and Guide ModeSpecifically designed multiplexerhighly flexible reset and readout options optimized for low power and low glow operationthree-side close buttableTwo-chip imaging system: MUX + ASICconvenient operation with small number of clocks/signalslower power, less noise

  • HAWAII-2RG

  • Block DiagramAll pads located on one side (top)Approx. 110 doubled I/O pads (probing and bonding)Three-side close buttable18 m pixelsTotal dimensions: 39 x 40.5 mm

  • Output Options

  • Output Options (2)

  • Interleaved readout of full field and guide windowGuide windowFull fieldFPASwitching between full field and guide window is possible at any time any desired interleaved readout pattern can be realizedThree examples for interleaved readout: 1. Read guide window after reading part of the full field row 2. Read guide window after reading one full field row 3. Read guide window after reading two or more full field rows

  • Reset Schemes

    Pixel by pixel reset

    Line by line reset

    Global reset

    Full field

    Guide window

  • MIRI Detectors: Si:As IBCExtrinsic (vs. HgCdTe, intrinsic)Blocked Impurity Band (BIB) extrinsic (vs. Bulk)

  • READOUT INTEGRATED CIRCUIT (ROIC)1024 1024 / 25 m pixels 7 K Operation Source-Follower-per-Detector (SFD) PMOS input circuit Low Noise: 10 12 e- rms with Fowler-8 Low Read Glow Low Power: < 0.5 mW 4 outputs with interleaved columns Reference pixels on all outputs mimic "dark" detectors Reference output averages noise from 8 "dark" reference pixels 2.75 second read time at 10 sec per sample (100 kHz pixel data rate)

  • TimeDiode Bias Voltage0.5 V0 VResetReadoutResetkTC NoiseReset-Read Sampling

  • Reset Noise in Capacitors

    Energy stored in a capacitor:

    Noise floor energy: E_n = kT

    Noise Charge: E=En

    Problem:Calculate the Reset noise for JWST detectors, assuming: C= 50 fF, T=37 K

  • TimeDiode Bias Voltage0.5 V0 VResetOpen ShutterClose ShutterReadoutResetReadoutkTC noiseCDS SignalDouble Correlated Sampling

  • TimeDiode Bias Voltage0.5 V0 VResetReadoutResetReadoutkTC noiseMCS SignalFowler (multi) Sampling

  • TimeDiode Bias Voltage0.5 V0 VResetResetUp-the-ramp ReadoutkTC noiseMCS SignalUp-the-Ramp Sampling

    *Intrinsic materials are excellent absorbers of radiation (every atom can do it). The QE can be high with minimal amount of material (low thickness).**Elements from group I and V can act as acceptors in HgCdTe if they substitute metallic (column II) sites and nonmetallic (column VI) sites, respectively