Brad Gussin John Romankiewicz 12/1/04 Quantum Dots: Photon Interaction Applications.

Brad GussinJohn Romankiewicz

12/1/04

Quantum Dots: Photon Interaction

Applications

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Semiconductor Structures

Bulk Crystal (3D) 3 Degrees of Freedom (x-, y-, and z-axis)

Quantum Well (2D) 2 Degrees of Freedom (x-, and y-axis)

Quantum Wire (1D) 1 Degree of Freedom (x-axis)

Quantum Dot (0D) 0 Degrees of Freedom (electron is confined in all directions)

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Structure vs. Energy

Quantum Dots are sometimes called “artificial atoms”

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Infrared Photodetection QWIPBulk Crystal

Wavelength

Inte

nsi

tyQuantum Well

IP

+ -

Photons

CB

VB

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QWIP Drawbacks

• High Intensity / Low Temp

• Polarization scatter grating

• Detector only works when photon hits semiconductor perpendicularly to the two unconfined axis

Grating

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Quantum Dots(Structure and Formation)

Self-Assembly (a.k.a Stranski-Krastanow Method): Mismatched lattice constants cause surface tension which results in Qdot formation with surprisingly uniform characteristics.GaAs 5.6533 Å InAs 6.0584 Å

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QDIPs

• 3D e- Confinement: Sharper wavelength discrimination

E = n / R2 E controlled by dimensional parameter R

• No need for Polarization

• “Photon Bottleneck” : e- stays excited for a longer time (i.e. less recombination), resulting in a more efficient detector and resistance to temperature.

• Higher temperatures and lower intensity.

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The Future of QDIPsSelf-assembly techniques still unstable: tune photodetection properties by manipulating the shape and size of Qdots

Possible Solutions:• Different Material Combinations

• More precise control of parameters (T, pressure, physical setup)

• Combine self-assembly with lithography and etching techniques

• For example: create crevices or pre-etched holes to encourage qdot growth in specified positions.

* Dr. Bijan Movaghar estimates 5-10 years before commercially practical QDIPs are in use.

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QDIP ApplicationsIncrease detectivity Increase number of

applicationsMedical (Thermal Imaging) Weather Military

Astronomy: Infrared Image

of the Milky Way

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Currently, quantum wells and Currently, quantum wells and quantum dots are being researched quantum dots are being researched for use in solar cells.for use in solar cells.

Factors that affect solar cell efficiency:Factors that affect solar cell efficiency:– Wavelength of lightWavelength of light– RecombinationRecombination– TemperatureTemperature– ReflectionReflection– Electrical ResistanceElectrical Resistance

Solar Cell ApplicationsSolar Cell Applications

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On an I-V curve On an I-V curve characterizing the characterizing the output of a solar cell, output of a solar cell, the ratio of maximum the ratio of maximum power to the product of power to the product of the open-circuit voltage the open-circuit voltage and the short circuit and the short circuit current is the fill factor. current is the fill factor. The higher the fill factor, The higher the fill factor, the “squarer” the shape the “squarer” the shape of the I-V curve.of the I-V curve.

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Quantum Well ApplicationQuantum Well Application Photocurrent and output voltage can be

individually optimized– Absorption edge and spectral

characteristics can be tailored by the width and depth of QWs.

In GaAs/AlxGa1-xAs p – i – n structure with inserted QWs, researchers have observed enhancement in the short-circuit current and thus efficiency in comparison with control samples that are identical except without the QWs.

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Quantum Dot ApplicationQuantum Dot Application Have potential to improve efficiencyHave potential to improve efficiency

– Reducing recombinationReducing recombination Channeling the electrons and holes through the

coupling between aligned QD’s Photon bottleneckPhoton bottleneck

– Increasing the amount of usable incident lightIncreasing the amount of usable incident light– Can be used at higher temperaturesCan be used at higher temperatures

DrawbacksDrawbacks– Size is harder to controlSize is harder to control

Thus harder to control the light absorption spectrumThus harder to control the light absorption spectrum

Solutions for quantum dot solar cells are Solutions for quantum dot solar cells are similar to QDIP solutions similar to QDIP solutions

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Structure and Energy Structure and Energy DiagramDiagram

P-type, intrinsic, P-type, intrinsic, n-type structuren-type structure

Based on a self-Based on a self-organized organized InAs/GaAs InAs/GaAs systemsystem

Quantum dots Quantum dots used in active used in active regionsregions

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ConclusionConclusion

Qdot technology will help optimize the photon Qdot technology will help optimize the photon detection and photovoltaic industries by making detection and photovoltaic industries by making devices more efficient and functionally devices more efficient and functionally effective.effective.

Possible other areas for commercial Possible other areas for commercial development include use in the automobile or development include use in the automobile or robotics industriesrobotics industries

Detect object or humans in the vicinity of the device.Detect object or humans in the vicinity of the device. Power generation for device.Power generation for device.

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SourcesSources

Interview with Interview with Dr. Bijan Movaghar, visiting professor (November 19, 2004).

Aroutounian, V. et al. Journal of Applied Physics. Vol. 89, No. 4. February 15, 2001.

Razeghi, Manijeh. Fundamentals of Solid State Engineering. Kluwer: Boston. 2002.

Center For Quantum Devices. <http://cqd.ece.northwestern.edu/>

Quantum Dots Introduction <http://vortex.tn.tudelft.nl/grkouwen/qdotsite.html>