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LOCALIZED CHARACTERIZATION OFGaAs/AlGaAs QUANTUM WELL DEVICES

Imee Rose Tagaca

25 March 2008

Adviser: Dr. Arnel SalvadorCondensed Matter Physics Laboratory

National Institute of Physics

University of the Philippines-Diliman

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Introduction. TYPICAL LIFE CYCLE OF A p-i-n LAYER AT

CMPL

MBEgrowth

PrimaryCharacterization

Device Fabrication SecondaryCharacterization

orGOOD sample!

Scenario 1Scenario 2

BAD sample!

strong emissionweak emission no emission

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Objectives of the Thesis

To find other means of evaluating the quality of a sample (good vs. bad) apart from using optical emission and therefore to explain the performance of the diodes

To know what is happening locally in the device while it is in operation (in terms of efficiency, electric field distribution, quantum well effects)

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Outline Theory

Photocurrent What makes a good device? QE*FF*ECE Electric field effects in CQW

Methodology SQW and CQW wafers Device Fabrication Characterization

Results SQW diodes: spectra, I-V, images CQW diodes: spectra, images

Abstract

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THEORY

PhotocurrentWhat makes a good device? QE*FF*ECEElectric field effects in CQW

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photocurrentP

N

I

conduction band

valence band

P-I-N diode

-

-+

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Theory. What makes a good device?

Quantum Efficiency ηQE (photodetector)- is the number of carriers (electron-hole pairs) that are generated in the depletion region per incident photon. [13, 17, 18]

Fill Factor FF (solar cell) - describes the congruence of the I-V measurements to the ideal

“square” I-V curve. [6]

Energy Conversion Efficiency ηECE (solar cell)- is the percentage of power converted from absorbed light to electrical energy. [6]

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Theory. What makes a good device? QE*FF*ECE

Quantum Efficiency ηQE (photodetector)

Fill Factor FF (solar cell)

Energy Conversion Efficiency ηECE (solar cell)

0),(, IyxIPe

hcI

Pe

hcyx rev

incph

incQE

yxVyxI

VyxI

VyxI

yxPyxFF

ocsc

mm

ocsc ,,

,

,

,),( max

inc

mm

incECE P

VyxI

P

yxPyx

,,, max

DefinitionsIph = photocurrentIrev = reverse currentI0 = dark currentΛ = illumination wavelengthPinc = incident illumination powerPmax = maximum diode output powerIm = current at Pmax

Vm = voltage at Pmax

Isc = short-circuit currentVoc = open-circuit voltageh, c, e universal constants

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Theory. Electric field effects in Coupled Quantum Well

WW

NW

UnbiasedVbias = 0

ResonanceVbias = -Vres

beyondResonanceVbias > І-VresІ

NW = narrow wellWW = wide well

P

N

I

conduction band

valence band

N

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METHODOLOGY

SQW and CQW recipeDevice FabricationCharacterization

Photocurrent SpectroscopyOptical-Beam Induced Current Imaging

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Methodology. SQW and CQW recipe

P33 and P254 with 90Ǻ QW P295 with 110Ǻ/25Ǻ/50Ǻ CQW

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Methodology. Device Fabrication (old design).

Apply photoresist (PR)After UV lithographyand PR development

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Methodology. Device Fabrication (old design).

After mesa etchingand PR removalApply PRAfter UV lithographyand PR developmentDeposit metal contactAfter PR removaland metal lift-off

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Methodology. Device Fabrication (old design).

SEM Mounting: needle probes

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Methodology. Device Fabrication (new design).

Apply polyimide

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Methodology. Device Fabrication (new design).

After UV lithographyand polyimide developmentApply PRAfter UV lithographyand PR development Deposit metal contactAfter PR removalAnd metal lift offDeposit bottom metal contact

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Methodology. Device Fabrication (new design).

Mounting: clipSEM

Easier-to mount the sample-to align with optics-to integrate with the OBIC set-up

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circuit

Methodology. Characterization Set-ups

choppercontrol

Photocurrent Spectroscopy Optical-Beam Induced Current Imaging

circuit

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RESULTS

SQW diodes spectra, I-Vimages: reverse bias, forward bias, gradient maps, derived quantities

CQW diodesspectraimages: reverse bias, ratio maps

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Results. Photocurrent (dotted) and Photoluminescence (solid)

Spectra

P33 (good SQW)Injection current ~ 200µA

P254 (bad SQW)Injection current ~ 5000µA

Both samples behave as 90A SQW

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Results. Current-Voltage Curves

P33 (good SQW) P254 (bad SQW)

P254 has better I-V properties because of better metal contacts

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Results. Current maps for different reverse bias values

Increasing reverse bias

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For the same voltage, P33 produced higher OBIC

Results. Current maps for different reverse bias values

P33 (good SQW) P254 (bad SQW)

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R. How to construct the gradient maps

x

x

x

x

x

Get SLOPEat each point (x, y)

V = -1.0V

V = -2.0V

V = -3.0V

V = -4.0V

V = -5.0V

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Results. Positive gradient maps (reverse bias)

P33 (good SQW) P254 (bad SQW)

All OBIC values increase with bias no negative gradient map

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Results. Positive gradient maps (forward bias)

P33 (good SQW) P254 (bad SQW)

All OBIC values increase with bias no negative gradient map

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Results. Fill factor

P33(good)

P254(bad)

yxVyxI

VyxIyxFF

ocsc

mm

,,

,),(

FF average ~ 52% ± 3%

FF average ~ 79% ± 2%

P254 has better metal contacts

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Results. Energy conversion efficiency

P33 (min = 0, max ~ 2.3%) ± 0.3%

P254 (min = 0, max ~ 6.9% )± 0.7%

inc

mmECE P

VyxIyx

,,

P254 is more efficient in converting optical to electrical energy

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Results. quantum efficiency

P33 (min~76% max~100%) ± 9%

P254 (min~17%, max~42%) ± 3%

0),(, IyxIPe

hcyx rev

incQE

P33 is more efficient in producing e-h carriers

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Results. Summary of SQW data

P33 (good) vs. P254 (bad) P254 has better metal contacts than P33 P254 has higher FF and

ηECE P33 has better growth conditions or compositionally close to the

design wafer P33 has higher ηQE

For both P33 and P254 Nonuniform ηECE suggests nonuniform diode output power;

nonuniform ηQE suggests nonuniform optical absorption coefficient of the material

In determining the emission, ηQE is more influential than FF and ηECE. The performance of P33 can further be improved by optimizing the device fabrication (to raise FF and ηECE). However, P254 has already reached its maximum potential as a device, limited by its low ηQE

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Results. Photocurrent Spectra

750 760 770 780 790 800 810 820 830 840 850 860 870

0.4

0.6

0.8

1.0

0.4

0.6

0.8

1.0

0.2

0.4

0.6

0.8

1.0

750 760 770 780 790 800 810 820 830 840 850 860 870

PC

Wavelength (nm)

0V

PC

neg1p4V

PC

neg3V

1

2

3

1

2

3

Band diagram [27]

1-unbiased2-at resonance3-after resonance

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Results

Theoretical calculation Vres~ -1.4V, coincides with experimental Vbias ~1.4V where λ (803 and 850) transitions have closest responsivity values

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Results

λ = 850nm

λ = 803nm

0V -0.8V -1.4V -2.0V -3.0V

Ratio(850/803)

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Results. Summary of CQW data

PC spectra proves that electric field effect is more pronounced in CQW, as expected.

Tuning the laser into the two associated intrawell transition λs, the 2D current maps are acquired. The consistent ratio values across the device at any bias indicate that the electric field applied is the uniform.

The ratio images follow the same trend exhibited by PC spectra wherein there is a relative intensity change at the two direct well transition λs. The voltage at the peak ratio coincides with the calculated resonance voltage.

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Conclusion

Room-temperature spatially-resolved characterization for different bias voltages was performed on GaAs-based diodes. From this, we derived the two-dimensional (2D) topographies of external quantum efficiency, fill factor and energy conversion efficiency that give insight on the homogeneity of the characteristics at microscopic resolution for three operating functions: light-emitting, photovoltaic and photoconductive. Thus, we were able to compare the optoelectric properties of good and defective diodes. The samples used were GaAs/AlxGa1-xAs single quantum well (SQW) p-i-n and coupled quantum well (CQW) p-i-n grown by molecular beam epitaxy and fabricated into 300 μm circular devices.

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Conclusion

In coupled quantum wells, the variation of the applied bias shifted the position of the QW energy levels with respect to each other, and hence, altered the relative carrier population densities in each QW. This was manifested as relative change in the spectral absorption of the 2 QW wavelength peaks. Ratio imaging revealed that the behavior is consistent across the device, indicative of the uniformity of electric field. This also translated to microscopic mapping of the interwell tunneling effect on the direct transitions, which is greatest at resonance.

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Acknowledgment

Raymund Sarmiento, Dr. Vernon Julius Cemine, and Dr. Carlo Mar Blanca for the use of the OBIC set-up and helpful discussions

Dr. Alipio Garcia for the growth of CQW and Jennifer Constantino for the deposition of metal contacts