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STScI Course on Infrared Instrumentation & Observing Techniques

Detectors
Bernar d J. Rauscher March 9, 2000
In living black & white for easier printing

Aims & Objectives
Aim : To begi n to develo p quanti tative u nderstandi ng of in frared array detectors. Rather than f ocusin g on specif ic techn ologi es, our emp hasis wil l be on und erstandin g the u nderly ing physi cal prin cipl es. Objectiv es: Ґ Photon detection i n PN jun ctions Ґ Review CCDs Ґ Intro duction to IR arrays Ґ Noise Sour ces, Sampli ng , & non-Id eal Behav ior

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Outline
Ґ Photon Detection in PN Junctions - Review semiconductors - The PN Junction - Charge collection in PN junctions Review of CCDs - CCD construction - Charge collection in CCDs - Charge transfer

Ґ

Outline Continued
Ґ Introduction to IR arrays - IR array construction - Differences to the CCD - Charge collection is similar to that in a CCD.

2

Outline Continued
Ґ Noise Sources & non-Ideal Behavior
п п п п п п Dark curren t kTC and FE T noi se Correlated Dou ble Sam pli ng Fowl er Sam plin g, рUp the ram pс, etc. Persistence Optical Crosstal k

1. Photon Detection in PN Junctions

3

Valence & Condu ction Ban ds in Sem icon ductors
ҐWhen atoms (a) come together to form a crystal, the outer energy levels overlap and blend to create bands (b). ҐThe outermost filled band is called the valence band (c). ҐAbov e the valence band, one finds a forbidden energy gap -the рband gapс, and (at higher energies) conduction bands populated by thermally excited electrons. ҐIn metals, the valence and conduction bands overlap resulting in conduction. In insulators, the band gap is wider resulting in very poor conduction.

Periodic Table

Semiconductors occupy column IV of the Periodic Table

4

Periodic Table Continued
Ґ The colu mn nu mber g iv es the num ber of v alence electron s per atom. Pri mary semi conductor s have 4. Ґ Compo unds i nclu ding elem ents fro m n eigh borin g colum ns can be form ed. These alloy s have semi conductor properties as well ( e.g. HgCdTe & InSb) . Ґ Mercury -cadmi um -tell uri de (HgCdT e; used in NICMOS) and i ndi um -anti mon ide (In Sb; used in SIRTF ) are the dom in ant detector technologi es in the near-I R.

P & N Type Semiconductors
ҐIn a semiconductor, some electrons are promoted from the valence band into conduction by thermal excitation at room temperature. ҐThese promoted electrons leave behind positively charged рholesс. ҐBoth electrons in the conduction band, and holes in the valence band, contribute to conduction.

5

P & N type Semiconductors Continued
Ґ One can р dopeс the sem iconductor by addi ng im purities to the crystal . Addin g an im purity wi th more vale nce electrons than the crys tal wil l donate negativ e charges to the conduction band, thereby creating an р n-typeс semi conductor. Ґ If the im purity as f ewer val ence el ectrons than the cry stal, it wil l donate holes to the valen ce band gi vi ng rise to a рptypeс sem iconductor. Ґ When p-type material is butted again st n-type material , the result is a PN j unction. In CCDs an d most IR arrays that are in use today, photo-excited charge is col lected in PN jun ctions.

PN Junctions

ҐIn a PN junction, positively charged holes diffuse into the n-type material. Likewise, negatively charged electrons diffuse in the the p-type material. ҐThis process is halted by the resulting E field. ҐThe effected volume is known as a рdepletion regionс. ҐThe charge distribution in the depletion region is electrically equivalent to a 2-plate capacitor.

r

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Photon detection in PN junctions

Ґ A photon can interact with the semiconductor to create an electronhole pair. Ґ The electron will be drawn to the most positively charged zone in the PN junction, located in the depletion region in the n-type material. Ґ Likewise, the positively charged hole will seek the most negatively charged region. Ґ Each photon thus removes one unit of charge from the capacitor. This is how photons are detected in both CCDs and most IR arrays.

The Band Gap Determines the Red Limit

E G = hc =

hc . c

(1)

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Longer Red Wavel ength Cutoffs Can be Obtained by Doping Ґ Up until now, we have been discussing рintrinsic semiconductorsс. Ґ If the band gap is too large, one can dope the semiconductor to provide intermediate energy рdonorс and рacceptorс levels. Ґ These allow photoexcitation by lower energy photons. Ґ The resulting material is called an рextrinsic semiconductorс

Som e properties of ex trin sic semi condu ctors

Ґ Doping creates in termedi ate energy l evel s that permit lower energy photons to make photoexcited electron-hole pairs.

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Some Extrinsic Semiconductor Properties

2. A Brief Review of CCDs
Consider a 3-ph ase CCD. ҐColum ns are separated by non -condu ctin g chann el stops. ҐRows are defin ed by electrostatic poten tial. ҐCharge i s physi cally mov ed withi n the detector duri ng r eadout.

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CCD Vertical Structure

Ґ In the vertical direction, one sees a PN jun ction and control electrodes. Ґ Depleti on regions f orm under both the metal gate and at the PN juncti on. Ґ Charge i s collec ted where these depl etion regions overl ap.

Charge moves in a CCD

Ґ By ch angi ng el ectrode voltag es, charge can be mov ed to the outpu t ampl ifi er. Ґ Thi s process is called charg e transfer. Ґ In an IR arr ay, this does not h appen. Charg e is sensed in pl ace.

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3. Infrared Arrays

This 2048x2048 pixels mercury cadmium telluride ( HgCdTe) Rockwell array is the largest IR array that is available today.

IR Arrays are рHybridс Sensors
Ґ A photosensitive array of PN junctions is рbump bondedс to a silicone readout multiplexer (MUX). Ґ This is done because silicon technology is much more advanced than any other semiconductor electronics technology. A modern MUX has about as many transistors as the most advanced Pentium CPU. Ґ The рbump bondsс are made of indium, a very so ft metal used for рweldingс dissimilar materials.

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Charge is Collected in PN junctions

Ґ Like a CCD, ch arge is coll ected in the depleti on region formed b y PN j unctions. Ґ Unl ike a CCD, charge i s sens ed in pl ace by the MUX. As a result, IR arrays hav e dif feren t noise characteristi cs to those of a CCD.

Schematic View of an IR Array

Ґ Note that each pixel has only one electrode.

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NICMOSуs MUX Shows Many Com mon Featur es

4. Noise, Sampling, & non-Ideal Behavior
Ґ Like a CCD, IR array s are effe cted by dark current and a vari ety of nois e mechan is ms. Ґ Dark current is the sign al that is seen in the ab sence of any li ght. For the near-IR (1-2.5 _m), the domi nant components are dif fusi on, thermal gen eration-recombi nation (G-R) of charges wi thin the sem iconductor, and leakage currents. Combi ni ng these, it can be sh own that
idark kT = (exp eR0 diff
eV / kT

2 kT - 1) + eR0 GR

V 2 1 - (exp Vbi

1

eV / 2 kT

- 1) + ileak , (2)

where V is the vol tage across the detector and R0dif f and R0GR are the detector impende nces at zero bias for dif fusi on and G-R.

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Noise Continuedи
Ґ As can be seen from equati on 2, dark curren t dim ini shes rapidl y with decreasing t emperatu re. Thi s is the reason why IR arrays are cooled to th e temper atures that th ey are. Ґ рkTCс noise occurs in both CCDs and IR array s when th e detector capacitor is recharg ed. As we will see shortly , this goes as
kTC electrons, (3) e where k is the B oltzm ann con stant, T is temper ature, C is capacitance, and e is the elem entary ch arge.

kTC

=

kTC Noise
ҐConsi der the therm al equi partition of charge on a capacitor.
Q = CV dQ = CdV dV = dE = QdV = dQ C

QdQ Q2 d( ) C C

ҐBoltzman n statistics appl y
E N ( E) , exp - kT N where N is the number of capacitors and n( E )e 2 1 = exp - . Solving for n( E ), kTC 2 n( E ) kTC . e2

ҐWe are counting ch arges, so the vari ance is gi ven b y Poiss on statistics.

n( E )

=

kTC e

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Noise Continuedи Ґ 1/f noise in the multiplexerуs output field effect transistor (FET).
п In th e mid -1980s, readout noi se ~300 electron s was comm on. Better out put FE Ts increased the num ber of m icrov olts per electron and thereby helped to redu ce readout noi se to <10 electrons seen today.

Ґ Other 1/f components are seen. e.g.,
п The рpedestal eff ectс seen in NICMOS whereby t he entir e exposureуs bias is seen to vary by ~75 electron s. There is som e consensus that these 1/f com ponents are ther mal ly driven .

Sampling Methods Ґ By being a bit clever about reading out the array, one can minimize or eliminate some of these noise modes. Ґ During an exposure, typically each pixel is sampled several times. Ґ The most common approaches are correlated double sampling (CDS), multiple non-destructive reads (aka рFowle r Samplingс), & fitting a line (aka рup the rampс). We discuss these briefly.

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Correlated Double Sampling
VRST voltage is l eft on when sam pli ng

V = (Vf - VRST ) - (Vi - VRST )
ҐEli mi nates kTC noise as the capaci tor is onl y reset once. ҐSusceptibl e to 1/f noi se.

Fowle r Sampling (or Multiple non-Destructive Reads)
N N

V=

1 N

j =1

Vi j -

1 N

j =1

Vf

j

ҐReduces noi se by av eragin g. ҐIn practice, this i s 3-5x quie ter than CDS. ҐNobody reall y kn ows why the gai ns stopи

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рUp the Rampс

Ґ Ґ Ґ Ґ

Fi t best line to m ulti ple non -destructi ve sampl es. Sam ple spacing does not need to be uni form . Not clear wheth er this or F owler sam plin g is best. Thi s is what is done i n NICMOS MUL TIACCUM mod e.

Non-Ideal Behavior
Ґ We briefly consider
п persistence and п pixel -to-pi xel crosstal k

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Persistence Ґ Persistence is the fruit of charge traps. Ґ A trap can be modelled as a square well with a lip. Ґ This is analogous to Fermiуs model of beta decay. Ґ One robust conclusion is that the trap will decay with an exponential timescale. This behavior was seen in NICMOS cosmic ray hits.

Persistence Continued

Here a is the inter-atomic spacing. Well depth is set by the uncertainty principle.

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Ґ Ґ Ґ Ґ

Pixel-to-pixel Crosstalk Crosstalk occurs when a pixel value is influenced by its neighbors. Charge diffusion is an important crosstalk mechanism in IR arrays and CCDs. Once charge carriers are created, their motion is governed by charge diffusion. Holloway (1986) has studied charge diffusion in detail. We discuss some of his results.

Crosstalk Continued
Ґ The continui ty equation for excess carri ers is

2 n - n / L2 + g / D = 0

(4)

Ґ Where L is the di ff usi on length, D is the di ff usion coeff ici ent, & g is the excess (i .e., non-therm al) generati on rate. Ґ We can sol ve this to fi nd the crosstalk (shown on next page). Ґ The amount of crosstalk depends on wav elen gth. Short wavel engths wi ll be abs orbed further from the chargecoll ecting depl etion region than wi ll long wav elen gth photons.

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Crosstalk Continued

Ґ The cur ves are semi conductor size, and C are most pr actical

for v arious v alues of thickn ess (C). Both A, =1/2 pix el in un its of th e diff usio n leng th. For detectors, C~1.

Summary Ґ Charge collection on both CCDs and IR arrays happens in the depletion region near a PN junction. Ґ Unlike a CCD, charge does not move in IR arrays. It is sensed in situ. Ґ Most of the practical differences in noise between CCDs and IR arrays arise from this difference.

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To know more aboutи
Ґ Array detectors in general, see
п Mc Lean, I.S. 1997, рEle ctr onic Imagi ng i n A stro nom y: Dete cto rs an d Ins trume ntati onс, J ohn Wile y & Son s: New York п Rieke , G.H. 1994, рDet ect ion of Ligh t: f rom the Ult raviol et t o th e Sub mill ime ter с, ed . K. Vis nov sky , Camb ridg e U niv ers ity P res s:Ne w Yo rk

Ґ Ґ

Charge diffusion, optical crosstalk, & blurring, see
п Hollo way , H. 1986, J. A ppl. Phy s., 60, 1091

Measuring noise in CCDs and IR arrays, see
п Janes ic k, J.R., E llio tt, T., Co llin s, S., Bl ouke, M .M., & F ree man, J. 1987, Optic al Eng ine erin g, 26, 692

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