mep jdg photodetectors ir-nir [11-12] · detection to define the parameters (figure of merit) which...

IPEQ - ICMP - SB - EPFLStation 3CH - 1015 LAUSANNE

J-D GaniereEPFL - SB - ICMP - IPEQCH - 1015 Lausanne

Credit: www.national.com

Experimental Methods in Physics

[2011-2012]

Photodetectors IR-FIR

InterestsClassificationNoiseFigures of meritExamples

1

J-D Ganiere MEP/2011-2012

References

Photodiode technical information [Hamamatsu]

The Physics of Semiconductors: An Introduction Including Devices and Nanophysics by Marius Grundmann / ISBN-13: 978-3540253709

Photodetectors: Devices, Circuits and Applications by Silvano Donati ISBN-13: 978-0130203373

Photodetection and Measurement: Maximizing Performance in Optical Systems (Hardcover) by Mark Johnson ISBN-13: 978-0071409445

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Objectives of this course

To classify photodetectors according to the physical mechanism by which they respond to light

To identify the sources of noise in optical detection

To define the parameters (figure of merit) which characterize the optical detectors (i.e. how to choose the right detector ?)

3


Interests

IR spectroscopy

identification of compounds, teledetectionresearch and development

Thermal infrared imaging camera

industrialmedicalsecurity, military

Night vision systems

Remember that:

Highlighting a difference in temperature is relatively easy.Measuring the absolute value of temperature is very difficult.Spatial resolution of an IR image is still limited by diffraction !

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Interests IR molecular spectroscopy

UV-VIS spectroscopy deals with electronic transitions.

IR radiation is to low an energy to excite electronic transitions ... IR spectroscopy is limited to the study of the rotational and vibrational structure.

Rotovibrational spectrum is like a molecular fingerprint.

IR spectroscopy is a very important tool in qualitative analysis

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Spectral bands

25–200 nm Vacuum ultraviolet VUV

200–400 nm Ultraviolet VUV

400–700 nm Visible VIS

700–1000 nm Near infrared NIR

1–3 μm Short wavelength infrared SWIR

3–5 μm Medium wavelength infrared MWIR

5–14 μm Long wavelength infrared LWIR

14–30 μm Very long wavelength infrared VLWIR

30–100 μm Far infrared FIR

100–1000 μm Submillimeter SubMM

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Interests

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Interests Teledetection

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Interests Thermography

fom Infratec

from www.dias-infrared.com

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Interests Nightvision

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IR arrays .... boosted by CCD and MEMS

CCD

Microbolometer array1970 1980 1990 2000 2010

ROICReadOut Integrated Circuit

Micro-electro-mechanical systems [MEMS]

Coupled Charged Devices [CCD]

IR detector arrays

from www.flir.com/ch

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Classification of the photodetectors

Thermal detectors

Quantum detectors

Photodiodes

Photoconductors

Photomultipliers

Photographic plates

Imaging Plates

Pyroelectric detectors

Bolometer detectors

Golay detectors

Thermopile

Phot

oelect

ric

effe

ctBl

ackb

ody

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Quantum detectors

Type Transition Electrical output Example

Intrinsic Interband

PhotoconductivePhotovoltaicCapacitancePEM

AlGaN, Si, GaAs, PbSe, InSb, HgCdTeAlGaN, Si, InGaAs, InSb, HgCdTeSi, GaAs, InSb, HgCdTeInSb, HgCdTe

Extrinsic Impurity to band Photoconductive Si:In, Si:Ga, Ge:Cu, Ge:Hg

Free carriers Intraband

Photoemissive

PhotoconductivePhoton-drag

PtSi, Pt2Si, IrSi Schottky barriersGaAs/CsOInSb electron bolometerGe

Quantum wellsTo and/or from spatiallyquantised levels

PhotoconductivePhotovoltaic

GaAs/GaAlAs, InSb nipiInAs/InGaSb SLS

from Opto-Electron. Rev., 12, no. 2, 2222004

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Thermal Detectors

Detector Method of operation

Bolometer - Metal - Semiconductor - Superconductor - Ferroelectric - Hot electron

Change in electrical conductivity

Thermocouple/Thermopile

Voltage generation, caused by change in temperature of the junction of two dissimilar materials

Pyroelectric Changes in spontaneous electrical polarization

Golay cell/Gas microphone Thermal expansion of a gas

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Photodetectors Quantum efficiency

The quantum efficiency, η, is defined as the

percentage of the electrons produced for every photon incident on the photosensitive surface

The carriers are produced via a photoelectric effect. There is production of electrons only if the energy of the photons verifies:

E= h $ o$ Egap= h $ ogap

h= h phe

h=mgapm

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Quantum and thermal detectors Noise

Shot noise (or quantum noise) is associated to the random fluctuations in a measurement signal due the random arrival time of the signal carriers (electrons, photons, ...)

Johnson-Nyquist noise (thermal noise) is generated by thermal agitation of the charge carriers inside an electrical conductor.

Flicker noise is an electronic noise with a 1/f, or pink spectrum

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Figure of merit Noise Equivalent Power [NEP]

The NEP (Noise Equivalent Power)* is defined as the radiant flux which produces an output signal equal in magnitude to that produced by noise signal.

The NEP gives a measure of the minimum amount of light which can be detected.

the unit of NEP is [W]

The NEP varies with the active area of the detector and the temperature.

lower the NEP ... better the detector !

Some manufacturers and authors define NEP as the minimum detectable power per square root bandwidth, in that case the unit is of NEP is [W Hz-1/2]

*

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Figure of merit Normalized Detectivity [N*]

A figure of merit also used to characterize performance, equal to the reciprocal of noise equivalent power (NEP), normalized to unit area and unit bandwidth.Specific detectivity, D * (synonym D-Star), is given by:

where A is the area of the photosensitive region of the detector and Δf is the effective noise bandwidth. Its

common units are [cm W-1Hz1/2]

Higher the detectivity .... better the detector

D *= NEPA $ Df

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Normalized Detectivity D* [cmHz1/2/W]

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NTED [Noise Equivalent Temperature Difference]

Noise Equivalent Temperature Difference (NETD) is a measure of the sensitivity of a detector of thermal radiation in the infrared, terahertz radiation or microwave radiation parts of the electromagnetic spectrum.

It is the scene temperature difference equal to either the internal noise of the detector (detector NETD) or the total electronic noise of a measurement system (system NETD). The signal-to-noise ratio is thus equal to one.

If a detector is limited by either shot noise or Johnson noise then the NETD can be decreased by using an increased integration time. The NETD of flicker noise limited detectors can not be reduced by increased integration time.

Typically uncooled bolometric detectors have NETD figures of 80-200 mK. Cooled photon detecting infrared detectors using materials such as HgCdTe (LWIR or MWIR) or InSb (MWIR) can approach a NETD figure of 10 mK.

from Wikipedia.org

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Thermal radiation summary

Planck’s law - Spectral Radiance for a blackbody at temperature T

Stefan Boltzmann law

Wien displacement law

I(m,T) =m52hc2

$ekTho

-1

1< F

J= v $T 4

mmax nm^ h$T= 2897

v=5.67 $ 10-12 Js-1m-2K-46 @

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Thermal Detectors

I(m,T)dm=H(T)m1

m2

#

λ1 λ2

if the emissivity is unknown ... a precise estimation of T is problematic !

estimation of T

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IR Detectors

Detector Method of operation cooled spectral range

Bolometer - Metal - Semiconductor - Superconductor - Ferroelectric - Hot electron

Change in electrical conductivity no 0.2 !m - 1000!m

Thermocouple/Thermopile Voltage generation, caused by change in temperature of the junction of two dissimilar materials

no 0.2 !m - 1000!m

Pyroelectric Changes in spontaneous electrical polarization no 0.2 !m - 1000!m

Golay cell/Gas microphone Thermal expansion of a gas no 0.2 !m - 1000!m

PhotodiodeInduced current (photoconductor mode) or voltage (photovoltaic mode) yes material dependant

QWIPPhotoexcitation of electrons between ground and first excited state subbands of multi-quantum wells (MQWs)

yes material dependant

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Infrared detectors - figures of merit

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Bolometers

A bolometer consists of an absorber connected to a heat sink through an insulating link. Any absorption of radiation absorbed by the absorber raises its temperature above that of the heatsink. Temperature changes can be measured directly (if the electrical resistance of the absorber changes with temperature) or indirectly via an attached thermometer. The spectral response of bolometers is flat over a broad spectral range and can, in principle, be used from UV to FIR.

absorber

insulating link

heatsink

thermometer

EM radiation

+

R1

R2

D

A

R3

B

C

Vg

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Bolometer

the two arms form the two branches of a wheatstone bridge which was fitted with a sensitive galvanometer and connected to a battery.

Electromagnetic radiation falling on the exposed strip would heat it, and change its resistance, the circuit thus effectively operating as a resistance temperature detector.

Room Temperature Operation

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Microbolometer

http://upload.wikimedia.org/wikipedia/en/b/bb/Cross-sectional_microbolomter.jpg

Individual sensor elements use the change in electrical resistance of a resistive layer deposited onto the tiny “platelets” fabricated by silicon micro-machining. Incoming target radiation heats the IR absorbing layer causinga change in electrical resistance, which is readout by measuring the resulting change in bias current. 80’000 and more sensors can be fabricated together into a two-dimensional array. The structure can be dimensioned to operate at 30 Hertz.


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Microbolometer Room Temperature Operation

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Microbolometer Room Temperature Operation

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Microbolometer array

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Bolometer ... the next step

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Photoconductors

The material (usually a semiconductor) becomes more conductive due to the absorption of EM radiation. When light is absorbed , le number of electrons and holes increases and raises the electrical conductivity of the material. The energy of the incident photon must be higher than the bandgap energy.

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p-n Photodiodes

The high electric field present in the depletion region causes photo-generated carrriers (generated via an internal photoelectric effect) to be separated and collected accross the (reversed biased) junction. The energy of the incident photon must be higher than the bandgap energy.

p n+

-

- +

RL

depletion region

Bias Voltage

Load resistor

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Photodiode Equivalent Circuit

Il: Current generated by the incident light (proportional to the amount of light)

ID: Diode current

Cj: Junction capacitance

Rsh: Shunt resistance

Rs: Serie resistance

I’: Shunt resistance current

VD: Voltage across the diode

VD: Voltage across the diode

I0: Output current

V0: Output voltage

Is: Photodiode reverse saturation current

e: Electron charge

k: Boltzmann’s constant

T: Absolute

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Photodiode Equivalent Circuit

The output current, Io, is given by:

I0 = Il- ID- I ' = Il- IS exp kTeVD -1b l- I '

with:

Il = h $U $ hcme

quantum efficiency

with:with:

light flux

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Photodiode I-V characteristics

from Hamamatsu

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Photodiode open circuit mode (photovoltaic)

VOC = ekTln ISIl- I ' +1; E

the open circuit voltage VOC is the output voltage when I0 equal to 0:

Usually Il >> I’ and Il/Is >> 1:

VOC = ekTln ISIl; E= e

kTln hcIShUem; E

In photovoltaic mode, the net current, I0, is zero and the voltage developped across an open circuit diode is a logarithmic function of the light flux

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Photodiode - Photoconductive mode

In photoconductive mode, the junction is operated under reverse potential and it is the photon generated current which constitutes the measured output signal and not the voltage drop across the diode:

I0. Il = h $U $ hcme

The ouput signal is a linear function of the light flux. Photoconductive operation results in a higher response speed than photovoltaic.

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Photodiodes

PV logarithmic: Rl >> Rsch

PV “linear”: Rl << Rsch

PC load line

Photoconductive mode Photovoltaic mode

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IR semiconductor photodetectors

HgCdTe [3-14!m]

PbSe [1.5 - 5 !m]

InGaAs [0.9 - 3 !m]

PbS [1.3 - 1.6 !m]

InAsB [8 - 14 !m]

Work as photoconductors (change in the electrical resistance under illumination) or as p-n photodetectors (± photovoltaic mode).

Most of the time, photovoltaic HgCdTe array are hybridized to a full custom silicon CMOS readout circuit.

Lower the temperature ... better the performances !

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HgCdTe - MCT (Mercury Cadmium Telluride) detector

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The retina of the detector is based on a photovoltaic HgCdTe array hybridized to a full custom silicon CMOS readout circuit. The readout circuit is versatile so that it answers several system requirements for hyperspectral applications.

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QWIP

How QWIP Works

QWIPs operate by photoexcitation of electrons between ground and first excited state subbands of multi-quantum wells (MQWs) which are artificially fabricated by placing thin layers of two different, high-bandgap semiconductor materials alternately. The bandgap discontinuity of two materials creates quantized subbands in the potential wells associated with conduction bands or valence bands. The structure parameters are designed so that the photo-excited carriers can escape from the potential wells and be collected as photocurrent.

from http://qwip.jpl.nasa.gov/tutorial.htm

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QWIP Characteristics

DQWIP* = 2.67Dideal*

spectral range 3-20 !m (Δλ=6 !m)

array size 600 x 500

QE 60%

NEDT 15 mK

Uniformity < 6%

D* max

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Thermopile

A thermopile is a number of thermocouples connected in series. A thermocouple is a junction of dissimilar metals which produce voltage when one side of the junction has a different temperature to the other. The so called cold side of the junction is kept close to ambient temperature by bonding it to a temperature stable mass. The hot side of the junction is exposed to incident radiation.Connecting many thermocouples in series produces higher voltage.

Thermopile detector output is proportional to incident radiation while the pyroelectric detectors output is proportional to rate of change of incident radiation. In other words, the thermopile detector is DC coupled while the pyroelectric detector is AC coupled.


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Thermopile

negativethermistorpositive

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Thermopile imaging array Dexter Research Center Inc.

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Thermopile imaging array Dexter Research Center Inc.

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Pyroelectric detector FUJI & CO. [Piezo Science]

Pyroelectric Infrared Detectors (PIR) convert the changes in incoming IR light to electric signals. Pyroelectric materials are characterized by having spontaneous electric polarization, which is altered by temperature changes as infrared light illuminates the elements. They can be used at ambient

temperature even in the presence of thermal noise. By choosing appropriate IR receiving electrodes, they serve a wide range of applications.

Characteristics

High SensitivityVersatile selection of IR wavelength filtersRoom temperature operationLow costRobust under severe environmental conditionsStable against ambient temperature and atmospheric changes

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Pyroelectric detector FUJI & CO. [Piezo Science]

Array (2005)

64x64 QVGA (320x240)

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Golay detector

This detector is based on the deformation of a membrane mirror (due to the thermal expansion of a gas when EM radiation is absorbed. This is a rugged broadband detector and is still used in FIR spectrometers.

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