EYES Internship Program

Bias tunable DWELL structure Infrared Photodetector

Presented by Guided byAmit Siroya Prof. Sanjay Krishna Microelectronics,IITB ECE, UNM

Date:7/05/2007

Presentation Outline

Infrared Detector

Quantum Dot

(Quantum)Dot in (Quantum) Well (DWELL) structure

My Work: Single pixel characterization of DWELL Infrared Photodetector

Infrared Rays

Range Wavelength Energy (photon)IR Spectrum 0.75 µm -1 mm 1.7-0.001 eVNear IR Spectrum 0.75-1.4 µm 1.7-.89 eVMid IR Spectrum (MWIR) 3-5 µm 0.4-0.2 eVLong IR Spectrum (LWIR) 8-12 µm 0.15-0.10 eV

4 6 8 10 12 14 18

H20H20C02

Atmospheric Window

Wavelength (μm)Tr

ansm

issi

on4 6 8 10 12 14 18

H20H20C02

Atmospheric Window

Wavelength (μm)Tr

ansm

issi

on

ApplicationsNight vision: Enabling the police and military to distinguish warm targets, such as

human beings and automobiles.

Meteorology: Satellite record data about the amount of infrared light reflected or emitted from the Earth's surface.

Astronomy: Stars emit a smaller portion of their energy in the infrared spectrum, so nearby cool objects such as planets can be more readily detected.

Applications

Communication: In short-range communication among computer peripherals and personal digital assistants. It is also used in Free Space communication.

Thermography: It provides a non-contact, non-destructive test method to record thermal patterns and temperatures across the surface of an object.

6

Infrared detectors

Photovoltaic (PV) Photoconductive (PC)

IR detectors

Thermal Photon

Thermal detectorsLong response timeLong response time

Less dependence on wavelengthRoom temperature operation possibleRoom temperature operation possibleBolometer, Thermistor, DTGS etc.Bolometer, Thermistor, DTGS etc.

Photon detectorsPhoton detectorsFast Response Wavelength dependentLow temperature operationQWIPs, QDIPs, MCT detectors etc.

Existing Technology

Drawbacks: Single Color. No spectral variation.

Images from 320*256 FPA with one color.

Quantum Dot

What is Quantum Dot: A quantum dot is a semiconductor nanostructure with the dimension of the order of De-Broglie wavelength of electron (in that material) that confines the motion of carriers in all three spatial directions.

Analogy:3-D potential Well: Confine the electrons in volume and have discreet energy spectrum.

Atom: Both have a discrete energy spectrum and bind a small number of electrons

5 nm5 nm

QD Infrared Photodetector(QDIP)

IR radiation Generation of Carriers Change Conductivity by intersuband transition

Detection

Transition Energy WavelengthDot to Dot 50-60 meV ~25μm

Dot to continuum 250 meV 5μm

Advantages and Issues: QDIP

Advantages

• Low dark current

• Normal incidence operation possible

Issues

• Limited voltage tunability

• Control of operating wavelength

• Low operating temperature

• Low Quantum efficiency and Responsivity

Dot in Well structure

* Krishna et al, IEEE Circuits and Devices, p.14, Jan. 2002; Appl. Phys. Lett.,79, 21, 2001.

Raghavan et al, Appl. Phys. Lett., 81, 1369, 2002Krishna et al, Appl. Phys. Lett., 80, 3898, 2002

Merits of DWELL Structure

• Placing the dots in an InGaAs well (DWELL) is expected to lead to reduced thermionic emission → lower dark current → higher operating temperature

•The operating wavelength and nature of transition can be tailored by varying the width and the composition of the InGaAs QW

• Tailoring asymmetry in bandstructure for exploiting Quantum Confined Stark Effect

• Novel Physics: transitions between carriers with different degrees of confinement

DWELL IR detector

QCSE can be exploited for bias tunabilty of DWELL detector

Transition Energy WavelengthDot to Dot 100meV 10 μm

Dot to Well 160meV 7.5 μm

Dot to continuum 320 meV 3.9 μm

13

ΔE ~250 meV ( 5 μm)

ΔE ~ 50-60 meV ( 25μm)

InAs QD

InGaAs QW

GaAs Barrier

Device 1781

Schematic of 30-period InAs/AlAs dot-in-well (DWELL) quantum dot infrared

photodetector

GaAs

GaAs

InAs QDs (n = Undoped – 6 x 1010 cm-2) 2.4 MLs

AlGaAs 500 Å

GaAs (n = 2 x 1018 cm-3) 0.2 µm

AlGaAs (n = 2 x 1018 cm-3) 200nm

GaAs S.I. Substrate

x30

GaAs-AlGaAs Quantum well: Less strain

30 Stacks: More Quantum efficiency

More QCSE More bias tunability

Device 1781

GaAs-AlGaAs Quantum well: Less strain

30 Stacks: More Quantum efficiency

More QCSE More bias tunability

Single Pixel Characterization

Spectral ResponseNoise CurrentResponsivityDetectivityPhotoconductive GainQuantum efficiency

Characterization

•• Optical chopper to differentiate between noise and signalOptical chopper to differentiate between noise and signal

•• Blackbody source for emitting radiation of different temperaturBlackbody source for emitting radiation of different temperatures es

•• Radiation incident on the sample in the cryostat cooled to 78KRadiation incident on the sample in the cryostat cooled to 78K

•• Current amplifier to amplify the output signal from the sampleCurrent amplifier to amplify the output signal from the sample

•• FFT analyzer to measure signal and noiseFFT analyzer to measure signal and noise

BlackbodyBlackbodySourceSource

Chopper Chopper

CryostatCryostat CurrentCurrentAmplifierAmplifier

SpectrumSpectrumAnalyzer/FTIRAnalyzer/FTIR

Spectral Response

The relative response of device at different wavelength.

5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 00

5 0

1 0 0

1 5 0

2 0 0

2 5 0

Rel

ativ

e re

spon

se

W a v e le n g th (μm )

Noise Current

• Generation-Recombination Current :

G - photoconductive gain Eq - photon irradiance gth - thermal generation factor lx - thickness of the detectorη – quantum efficiencyAd – Area of the detectorΔf – noise equivalent bandwidth

xdthdqRG flAgfAEqGi Δ+Δ=− η2

Noise dueto signal

Noise dueto thermal

Noise Current

-6 -4 -2 0 2 4

1E-12

1E-11

1E-10

-6 -4 -2 0 2 4

1E-12

1E-11

1E-10

Noise (100)

Noi

se(A

mps

/Sqr

tHz)

Volts

Responsivity

••Peak Responsivity Peak Responsivity

••Blackbody radianceBlackbody radiance

••II00 –– Total output current measuredTotal output current measured

••R(R(λλ)/R()/R(λλcc)) –– Normalized spectral responseNormalized spectral response

••AAss -- Area of blackbody apertureArea of blackbody aperture

••AAdd -- Photoresponse area of the detector Photoresponse area of the detector

••rr -- Distance between the detector and the blackbody Distance between the detector and the blackbody

••t t –– transmittance of the cryostat windowtransmittance of the cryostat window

••FFF F -- Form factor (depends on the geometries of the chopper and blackForm factor (depends on the geometries of the chopper and blackbody)body)

∫=

2

1

2),()()(λ

λ

λλλλ dFt

rAA

TLRR

IR

Fds

ec

oi

⎥⎦

⎤⎢⎣

⎡−

−=

msrcmwatt

e

hcTLTk

hce μλ

λλ

25

2

)1(

2),(

Responsivity

-6 -4 -2 0 2 40.01

0.1

1

-6 -4 -2 0 2 4

0.01

0.1

1

Responsivity (100)

Res

pons

ivity

(A/W

)

Volts

Detectivity

Detectivity:

pd

RN

fAD

Δ∗=*

Ad – Area of the detector N – Noise currentΔf – Noise equivalent bandwidth

-6 -4 -2 0 2 41E7

1E8

1E9

1E10

1E11-6 -4 -2 0 2 4

1E7

1E8

1E9

1E10

1E11

Detectivity (100)

D*

(Jon

es)

Volts

Photoconductive Gain

Photoconductive Gain:

fqIigdc

n

Δ=

4

2

in – noise current Idc – total currentΔf – frequency bandwidth

-6 -4 -2 0 2 40.01

0.1

1

10

100

1000

10000-6 -4 -2 0 2 4

0.01

0.1

1

10

100

1000

10000

G (100)

G

Volts

Conclusion

Bias dependence of Responsivity

Noise Power spectral density: 9*10-11 -2*10-12 A/√Hz

Peak Responsivity: 4.3 A/W

Detectivity( D*): 1* 109 Jones

Photoconductive Gain: 10-12

Future work

To document and format the results to submit in Applied Physics Letters(APL) or other scientific journals.

Key Parameters Reported for QDIPs

•Highest Detectivity ( 3 x 1011 cm Hz½ / W) at 78 K and a bias of 1.4 V and at a wavelength of 9.3 µm

Eui-Tae Kim et al., Appl. Phys. Lett., 84(17), 2004.

•Highest Operating Temperature (300 K, 6.7 x 107 cm Hz½ / W, λ~4.1 µm)H. Lim et al., Appl. Phys. Lett., 90(13), 2007.

•Demonstration of first LWIR 320 x 256 FPA using InAs/In0.15Ga0.85As DWELL detectors (80K, λ~8-10 µm, NEΔT~ 100mK)

S. Krishna et al., Appl. Phys. Lett., 86, 2005.

•Demonstration of first MWIR/LWIR 640 x 512 InAs/GaAs/AlGaAs DWELL FPA (60K, NEΔT~ 40mK, λ~8.1 µm)

S. D. Gunapala et al., Proceedings of SPIE,Volume 6206, pp. (2006).

•High Operating temperature MWIR 320 X 256 FPA QDIP (200 K, λ~4 µm, NEΔT~ 344mK)

S. Tsao et al., Appl. Phys. Lett., 90(13), 2007.

Acknowledgments

Prof. Sanjay Krishna for believing in me and for insightful discussions on broader aspects of ongoing research.

Dr. Yagya Sharma for mentoring and guidance.

Dr.J.B.Rodriguez, Greg Bishop, Rajeev Shenoi, Wooyong(Eric) Jang, Michael C Lenz for all support, discussions and troubleshooting throughout my experiments.

Prof. Sudharman K. Jayaweera and Ms. Anna Royball for making our stay a memorableone.

Thanks!!Questions?