Photodetectors - Santa Cruz Institute for Particle Physicssirius.ucsc.edu/.../supporting_docs/Photodetectors.pdf · · 2009-05-25Photodetectors • The side is on the order of less

Photodetectors

• Convert light signals to a voltage or current.• The absorption of photons creates electron hole pairs.• Electrons in the CB and holes in the VB.• A type junction describes a heavily doped p-type

material(acceptors) that is much greater than a lightlydoped n-type material (donor) that it is embedded into.

• Illumination window with an annular electrode for photonpassage.

• Anti-reflection coating ( ) reduces reflections.

np+

43NSi

Photodetectors

• The side is on the order of less than amicron thick (formed by planar diffusioninto n-type epitaxial layer).

• A space charge distribution occurs aboutthe junction within the depletion layer.

• The depletion region extendspredominantly into the lightly doped nregion ( up to 3 microns max)

+p

p+

SiO2

Electrode

!net

–eNa

eNdx

x

E (x)

R

Emax

e–h+

Iph

h" > Eg

W

E

n

Depletion region

(a)

(b)

(c)

Antireflection

coating

Vr

(a) A schematic diagram of a reverse biased pn junctionphotodiode. (b) Net space charge across the diode in thedepletion region. Nd and Na are the donor and acceptor

concentrations in the p and n sides. (c). The field in thedepletion region.

Electrode

Vout

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

PhotodetectorsShort wavelengths (ex. UV) areabsorbed at the surface, and longerwavelengths (IR) will penetrate intothe depletion layer.

What would be a fundamental criteriafor a photodiode with a wide spectralresponse?

Thin p-layer and thick n layer.

What does thickness of depletion layerdetermine (along with reverse bias)?

Diode capacitance.

What does capacitance dictate? Response time.

Photodetectors• Assume a reverse bias condition

(Vr) applied to the device.

• The depletion layer presents a highresistance under this condition and avoltage of Vr + Vo is developedacross W. (Vo is a built in voltage).

• An E field develops in the depletionregion and can be determined byintegration of the net space chargedensity pnet.

• The field is not uniform. It ismaximum at the junction and tapersinto the n region.

p+

SiO2

Electrode

!net

–eNa

eNdx

x

E (x)

R

Emax

e–h+

Iph

h" > Eg

W

E

n

Depletion region

(a)

(b)

(c)

Antireflection

coating

Vr

(a) A schematic diagram of a reverse biased pn junctionphotodiode. (b) Net space charge across the diode in thedepletion region. Nd and Na are the donor and acceptor

concentrations in the p and n sides. (c). The field in thedepletion region.

Electrode

Vout


Regions outside the depletion region (neutral regions) hold majority carriers. These neutralregions can be considered resistive extensions of electrodes to the depletion region.When a photon with an energy greater than bandgap ( ) is incident, the photon isabsorbed and generates a free EHP in the depletion layer.

Photodetectors

gE

e–h+

iph(t)

Semiconductor

(a)

V

x

(b)

(a) An EHP is photogenerated at x = l. The electron and the hole drift in opposite

directions with drift velocities vh and ve. (b) The electron arrives at time te = (L ! l)/ve and

the hole arrives at time th = l/vh. (c) As the electron and hole drift, each generates an

external photocurrent shown as ie(t) and ih(t). (d) The total photocurrent is the sum of hole

and electron photocurrents each lasting a duration th and te respectively.

E

l L ! l

t

vevhvh

0 Ll

t

e–h+

th

te

t

0

th

te

iph(t)

i(t)

t

0

th

te

evh/L + eve/Levh/L

ie(t)

ih(t)

(c)

(d)

Charge = e

evh/L eve/L


Photodetectors • The E field separates theelectron and hole andcauses them to drift inopposite directions untilthey reach neutral regions.

• The drifting carriersgenerate a photocurrent inthe external circuitdeveloping an electricalsignal.

• The photocurrent exist fora time frame equal to thetime it takes for theelectron and hole to crossthe depletion layer andarrive at the neutral region.

• The magnitude of thephotocurrent is dependanton the number of EHPsgenerated and the driftvelocities of the carrierswhile moving across thedepletion layer.

Photodetectors

• Note: the absorption of photons occurs over a distance thatis dependant on wavelength. Remembering that thedistribution of the field is not uniform tells us thatdetermining the time dependence of the photocurrentsignal is difficult.

• The resultant photocurrent is a result of electron flow onlynot hole migration.

• Integrating the hole current to calculate the Q charge willshow that the total photogenerated electrons is(electrons) and not (electrons and holes).

eN

eN2

PIN Photodetectors• Lower doping levels cause depletion region

to become thicker which in turn reducesdiode capacitance.

• PIN photodiode implements this concept byinsertion of a thick, high Z low doped n-typelayer (middle layer) between the p and nlayers of the original model.

• The middle layer is called the intrinsic layeror I-layer.

• A moderate quantity of reverse bias canextend the depletion layer to the bottom ofthe I-layer.

Result: 1. Faster response time. 2. Improved (wider) spectral response.

p+

i-Si n+

SiO2

Electrode

!net

–eNa

eNd

x

x

E (x)

R

Eo

E

e–h+

Ip h

h" > Eg

W

(a)

(b)

(c)

(d)

Vr

The schematic structure of an idealized pin photodiode (b) The netspace charge density across the photodiode. (c) The built-in fieldacross the diode. (d) The pin photodiode in photodetection isreverse biased.

Vout

Electrode


PIN Photodetectors

h! > Eg

p+i-Si

e– E

h+

Wl

Drift

Diffusion

A reverse biased pin photodiode is illuminated with a shortwavelength photon that is absorbed very near the surface.The photogenerated electron has to diffuse to the depletionregion where it is swept into the i-layer and drifted across.

Vr


Responsivity (R ) vs. wavelength (!) for an idealphotodiode with QE = 100% (" = 1) and for a typicalcommercial Si photodiode.

0 200 400 600 800 1000 12000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (nm)

Si Photodiode

!g

Responsivity (A/W)

Ideal Photodiode

QE = 100% ( " = 1)


Drift velocity vs. electric field for holes and electrons in Si.

102

103

104

105

107106105104

Electric field (V m -1)

Electron

Hole

Drift velocity (m s -1)


Silicon Photodetectors -- Interdigitated Lateral Trench

Interdigitated electrodes areoften used to increase theactive region area whileoptimizing the electric fieldsin the carrier collectionregion. Electrode can eitherbe p+/n+ or just metal.

Finger space = 3.3 µmTrench depth = 8 µmFinger size = 0.35 µmFor λ=845 nm,BW=1.5 GHz, Responsivity = 0.47A/W at 5V

Silicon Photodetectors -- Resonant-cavity-enhanced

Why? High Speed

Uses three pair of quarter wavelengthSiO2 and polysilicon at bottom(LPCVD). SiO2 Side-wall to preventdefects at the edge of poly.Two pairs of ZnSe-MgF on top(evaporated).

Silicon Photodetectors -- Schottky Barrier

300,000 PtSi/p-SiSchottky barrier IRdetector focal planearrays have beendeveloped and usedon Air Force B-52

Silicon Photodetectors -- Schottky Barrier

• High dark current, has to operate at low temperature (40 ~ 80 K).• Low quantum efficiency (QE).

( )2

1

2

1

1124.1 !!

"

#$$%

&'=

'=

C

B Ch

qhCQE

(((

)

*)

High λC gives high QE. To expand the spectrum, need to decrease the barrier height.

s p+

SiO2Electrode

!n et

x

x

E (x)

R

E

h" > Eg

p

Ip h

e– h+

Absorption

region

Avalanche

region

(a)

(b)

(c)

(a) A schematic illustration of the structure of an avalanche photodiode (APD) biasedfor avalanche gain. (b) The net space charge density across the photodiode. (c) Thefield across the diode and the identification of absorption and multiplication regions.

Electrode


n+

Avalanche Photodiodes

h+

E

sn+ p

e–

Avalanche region

e–

h+

Ec

Ev

(a) (b)

E

(a) A pictorial view of impact ionization processes releasing EHPs andthe resulting avalanche multiplication. (b) Impact of an energeticconduction electron with crystal vibrations transfers the electron'skinetic energy to a valence electron and thereby excites it to theconduction band.


SiO2

Guard ring

Electrode

Antireflection coating

nn n+

p+

s

p

Substrate

Electrode

n+

p+

s

p

Substrate

Electrode

Avalanche breakdown

(a) (b)

(a) A Si APD structure without a guard ring. (b) A schematic illustration of thestructure of a more practical Si APD


InP

InGaAs

h+

e–

E

Ec

Ev

Ec

Ev

InP

InGaAs

Ev

Ev InGaAsP grading layer

h+

!Ev

(a) Energy band diagram for aSAM heterojunction APD wherethere is a valence band step !Ev

from InGaAs to InP that slowshole entry into the InP layer.

(b) An interposing grading layer(InGaAsP) with an intermediatebandgap breaks !Ev and makes it

easier for the hole to pass to the InPlayer

(a)

(b)


E

N n

Electrode

x

E (x)

R

h!

Ip h

Absorption

region

Avalanche

region

InP InGaAs

h+

e–E

InP

P+ n+

Simplified schematic diagram of a separate absorption and multiplication(SAM) APD using a heterostructure based on InGaAs-InP. P and N refer top and n -type wider-bandgap semiconductor.

Vr

Vout


P+–InP Substrate

P+–InP (2-3 µm) Buffer epitaxial layer

N–InP (2-3 µm) Multiplication layer.

Photon

n–In0. 5 3

Ga0 . 4 7

As (5-10µm) Absorption layer

Graded n–InGaAsP (<1 µm)

Electrode

Electrode

Simplified schematic diagram of a more practical mesa-etched SAGM layeredAPD.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)h!

h+

e–

n+

Ec

Ev

10–20 nm

p+

E

Eg 1

Eg 2

"Ec

Energy band diagram of a staircase superlattice APD (a) No bias. (b) Withan applied bias.

(a) (b)


Photodetectors in High Energy Physics

• Calorimeters (measure energy and position)- Scintillation light detected by photodetector

- Cherenkov light radiation• Time-of-flight

- Fast scintillators used to determine speed of particle• Readout of electronics in large hermetic detectors• Fiber backbone for local and wide area networks

• First use for detection of α particles:Geiger & Marsden (1909) using ZnS (Ag)

Solid State Photodetectors for HEP -- Issues

• Silicon is not cheaper per unit are than vacuum photodetectors(for areas greater than a few mm2)

• Really large devices cannot be made

• Problem with damage from high neutron flux in hadron colliderexperiments (such as the LHC)

• Need low noise (I.e. expensive) pre-amplifiers

• Hard to do photon counting

Hybrid Photodetectors• Generate free photelectrons in a vacuum (like a photomultiplier tube)

• Accelerate photelectrons to a high energy (10 to 20 kV)

• Use a silicon diode as an electron detector, with approximately 2500eh-pairs for each 10 kV photoelectron

Applications

• Calorimetry: conversion of particle energy into light by eitherscintillation or Cherenkov effect

Applications

• Cherenkov: when a charged particle travels in a dense medium faster than thespeed of light in that medium then Cherenkov light is produced.

For a given medium, there is a minimum velocity below which no light is produced.

Light is emitted in a cone around the particle trajectory, with a yield ~ λ-2

Appliction: Photodetectors for LIGO

• Material: InGaAs based family• Pattern: Single element• Diameter > 2 mm• Frequency response: ~100 MHz• Packaging: rf operable• Cooling: Possible TEC• Optical power: ~1 W• Quantum efficiency target: 70%

Imaging Photodetectors & Biology Applications

Image Sensors

Image Sensors

Image Sensors

Image Sensors

• Retina is a light sensitive neural network• Diseases such as Retinitis Pigmentosa (RP) and Age-related

Macular Degeneration (AMD) primarily affect the photoreceptors,are both presently incurable, and render 100,000s blind each year

Webvision, Kolb, Fernandez, and Nelson, 2003.

• Epiretinal– Less disruptive to the

retina.– More flexibility in

component placement– More complex stimulus

algorithms required• Subretinal

– In natural position ofphotoreceptors

– Disruptive to retina– Devices relying on incident

light for power cannotgenerate effective stimulus

Retinal Prosthesis – Epiretinal vs. Subretinal

Retinal Prosthesis – State of the Art

• Epiretinal and Subretinal at Investigational DeviceExemption Stage

• Epiretinal - encouraging results, but better technologyrequired

• Subretinal – No direct evidence demonstratingfunctional electrical stimulation, but patients reportsubjective improvements in vision

Optobionics ASRTM

ArgusTM II

Artificial Silicon Retina (ASR)The ASR contains about 3,500

microscopic solar cells that are able toconvert light into electrical pulses,mimicking the function of cones androds. To implant this device into theeye, surgeons make three tiny incisionsno larger than the diameter of a needlein the white part of the eye. Throughthese incisions, the surgeons introducea miniature cutting and vacuumingdevice that removes the gel in themiddle of the eye and replaces it withsaline. Next, a pinpoint opening ismade in the retina through which theyinject fluid to lift up a portion of theretina from the back of the eye, whichcreates a small pocket in the subretinalspace for the device to fit in. The retinais then resealed over the ASR.

Photo courtesy OptobionicsARCC will give blind patients the ability to see 10 by 10 pixelimages, and are developing a version of the chip that wouldallow 250 by 250 pixel array

Retinal Prosthesis – MEMS component

microelectronics

electroplated or assembledelectrodes

surface micromachinedsprings(polymer) frame

flexible frame forattachment

micromachined electrodearray (silicon substrate)

retina

posts forassemblyand electricalinterconnect

electrodes

flexibleinterconnecttack

antenna

inner-eyeelectronics

Documents

Photodetectors - Santa Cruz Institute for Particle Physicssirius.ucsc.edu/.../supporting_docs/Photodetectors.pdf · · 2009-05-25Photodetectors • The side is on the order of less