Localized Characterization of GaAs/AlGaAs Quantum Well Devices
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Transcript of Localized Characterization of GaAs/AlGaAs Quantum Well Devices
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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