MMy Suy Summmmeerr VaVaccaatitionon

MMy Suy Summmmeerr VaVaccaatitionon

““Intersubband Electroluminescence from Intersubband Electroluminescence from Silicon-based Quantum Cascade Structures, Silicon-based Quantum Cascade Structures,

Light Emission from the Silicon Materials Light Emission from the Silicon Materials System, and Excited-State QCLsSystem, and Excited-State QCLs” ”

or

Silicon-Based Quantum Silicon-Based Quantum Cascade StructuresCascade Structures

Research and Research and ExperimentationExperimentation

Current Quantum Cascade LasersCurrent Quantum Cascade Lasers

Materials system Materials system derived from derived from groups III and V of groups III and V of periodic chart (i.e. periodic chart (i.e. GaInAs/AlInAs)GaInAs/AlInAs)

Operation in Operation in conduction bandconduction band

Charge carriers = Charge carriers = electrons (negative electrons (negative charge)charge)

NOTE: diagram from Sirtori et al. Quantum cascade laser with plasmon-enhanced waveguide operating at 8.4 µm wavelength. Appl. Phys. Lett. v66 p3242. June 12, 1995.

Silicon-basedSilicon-based Quantum Cascade Structures Quantum Cascade Structures

Materials system Materials system derived from group derived from group IV of periodic chart IV of periodic chart (Si(Si1-x1-xGeGexx/Si)/Si)

Operation in Operation in valence bandvalence band

Charge carriers = Charge carriers = holes (positive holes (positive charge)charge)

NOTE: Diagram from Diehl et al. Intersubband quantum cascades in the Si/SiGe material system. Physica E: Low-dimensional Systems and Nanostructures, v13 p829-834. 2002.

The Big PictureThe Big Picture

Advantages of group IV:Advantages of group IV: Integration with microelectronicsIntegration with microelectronics High-temperature operationHigh-temperature operation Low costLow cost Widely available, easy to handle and Widely available, easy to handle and

manufacturemanufacture Advanced processing techniquesAdvanced processing techniques Negligible polar optical phonon scattering Negligible polar optical phonon scattering Possibility of surface-emission without a Possibility of surface-emission without a

grating using Light Hole (LH) grating using Light Hole (LH) Heavy Heavy Hole (HH) transitions for TE-polarized lightHole (HH) transitions for TE-polarized light

The Big PictureThe Big Picture

Disadvantages:Disadvantages: Indirect band gapIndirect band gap

– Need to work in valence bandNeed to work in valence band Larger effective massesLarger effective masses In between HH bands where transitions take place are In between HH bands where transitions take place are

LH bandsLH bands– Reduces non-radiative lifetimeReduces non-radiative lifetime

Strain from lattice mismatchStrain from lattice mismatch– Critical ThicknessCritical Thickness

Limits # of cascades and # of wells per cascadeLimits # of cascades and # of wells per cascade Group IV QCL’s must be “drastically simplified Group IV QCL’s must be “drastically simplified

version” of III/V’s.version” of III/V’s.

The Big PictureThe Big Picture

Disadvantages:Disadvantages: Primary scattering mechanism of SiGe is Primary scattering mechanism of SiGe is

nonresonant: deformation potential nonresonant: deformation potential scattering with optical phonons.scattering with optical phonons.

Limited quantum efficiency (~10Limited quantum efficiency (~10-5-5 for EL) for EL) No population inversionNo population inversion

– Auger recombinationAuger recombination– Free-carrier absorptionFree-carrier absorption

No stimulated emission or optical gainNo stimulated emission or optical gain

Sample structureSample structureFrom From Dehlinger, Diehl, Gennser, Sigg, Faist, Ensslin, Grutzmacher, Muller.

Intersubband Electroluminescence from Silicon-Based Quantum Cascade Structures. Science, v290 12/22/2000.

First EL in the 10 µm range with narrow linewidth

Emission energy = 125 meVEL at peak ~ 0.073 pW/meVFWHM = 22meV

Experimental samplesExperimental samples Grown at Bell Labs, processed at PrincetonGrown at Bell Labs, processed at Princeton 3 samples: SiGe A, B, and C3 samples: SiGe A, B, and C Each etched to a different depth Each etched to a different depth (unsure which is which)(unsure which is which) Ridges

Mesas

Experimental ResultsExperimental Results

SiGe CSiGe C– Side emissionSide emission– Top emissionTop emission

AllAll– Voltage Current CharacteristicsVoltage Current Characteristics

SiGe BSiGe B– Side emissionSide emission

SiGe ASiGe A– Side emissionSide emission

50muV 500ms. 1.5mus pulse, 300mA

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50muV, 200ms. 1mus pulse, 215mA

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Wavenumbers (cm-1)

SiGe C (side emission): NoiseSiGe C (side emission): Noise

Possible causes:Possible causes: Improper Improper

alignmentalignment Wire Bonding Wire Bonding

problemproblem Design FlawDesign Flaw Etching not deep Etching not deep

enoughenough Side emission Side emission

obstructedobstructed

SiGe C (top emission): NoiseSiGe C (top emission): Noise

Possible causes:Possible causes: TE-polarized emission TE-polarized emission

usually much less usually much less intense than TM intense than TM

Initial setup: wires Initial setup: wires soldered directly to soldered directly to pads, sample attached pads, sample attached with double-sided tape with double-sided tape to cryostat rodto cryostat rod

With new cryostat: With new cryostat: new sample holder new sample holder accommodates top accommodates top emissionemission

Note: figure from Diehl et al. Electroluminescence from strain-compensated Si0.2Ge0.8/Si quantum-cascade structures based on a bound-to-continuum transition. Applied Physics Letters, v81 No25 p4700-4702. 12/16/2002.

Top

Side

Voltage-Current CharacteristicsVoltage-Current Characteristics Current gradually Current gradually

increased from ~40mA to increased from ~40mA to ~800mA (Voltage varies up ~800mA (Voltage varies up to 20V)to 20V)

SiGe A and C give SiGe A and C give (approximately) linear V-I (approximately) linear V-I graphsgraphs

SiGe B gives one sub-linear SiGe B gives one sub-linear graph, repeat gives linear graph, repeat gives linear graphgraph

This sample chosen for This sample chosen for emission testingemission testing

The characteristic The characteristic deteriorates somewhat deteriorates somewhat above currents of 400mA, above currents of 400mA, so current was thereafter so current was thereafter kept at or below this value.kept at or below this value.

SiGe B Test 1

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)

SiGe B Test 2

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)

SiGe B (side emission): SiGe B (side emission): Detection at lastDetection at last

700ns 400mA (-) 100muV 500ms

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500ns 400mA (-) 100muV 500ns

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500ns 360mA (-) 50muV 500ms

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500ns 340mA (-) 50muV 500ms

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Wavenumbers (cm-1)

Sample spectrum: 700ns Sample spectrum: 700ns pulse at 400mApulse at 400mA

Linewidth≈650cmLinewidth≈650cm-1-1

Low output: Lock-in Low output: Lock-in sensitivity = 100µVsensitivity = 100µV

Spectra taken with varying Spectra taken with varying currents: pulse currents: pulse width=500ns, currents width=500ns, currents vary from 300 to 400mAvary from 300 to 400mA

Spectrum broadens with Spectrum broadens with increasing current, but increasing current, but noise also becomes a noise also becomes a lesser factorlesser factor

Sample Spectrum

Varying Currents

Spectra taken with varying Spectra taken with varying pulse widthspulse widths

Current =400mA, pulse Current =400mA, pulse widths vary from 300 to widths vary from 300 to 700ns700ns

Spectrum broadens with Spectrum broadens with increasing pulse widths, but increasing pulse widths, but noise also becomes a lesser noise also becomes a lesser factorfactor

Signal becomes much Signal becomes much clearer with lower clearer with lower sensitivities (bottom sensitivities (bottom spectrum has 50µV spectrum has 50µV sensitivity, the rest have sensitivity, the rest have 100µV)100µV)

700ns 400mA (-) 100muV 500ms

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600ns 400mA (-) 100muV 500ms

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Wavenumbers (cm-1)

Varying Pulse Widths

SiGe B (side emission): SiGe B (side emission): Detection at lastDetection at last

SiGe A (side emission): SiGe A (side emission): More DetectionMore Detection

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Wavenumbers (cm-1)

Just a few spectra takenJust a few spectra taken 500ns pulse at 400mA500ns pulse at 400mA Linewidth≈550cmLinewidth≈550cm-1-1

Is It Heat?Is It Heat?

Very broad spectrum, while Very broad spectrum, while ScienceScience paper reports “narrow linewidth” of paper reports “narrow linewidth” of 22meV with lower current but much 22meV with lower current but much higher pulse widthhigher pulse width

Emission does have polarization Emission does have polarization dependence in both emitting dependence in both emitting samplessamples

The Next StepThe Next Step

Regain SiGe B emission capabilitiesRegain SiGe B emission capabilities– Rebond to different mesasRebond to different mesas

Continue taking spectra with varying Continue taking spectra with varying current and pulse lengthcurrent and pulse length– Document characteristicsDocument characteristics– Find ideal spectrumFind ideal spectrum

Present to Sturm lab, say “good job,” Present to Sturm lab, say “good job,” request another structure(s)request another structure(s)

Work with Princeton-grown samplesWork with Princeton-grown samples Create new designsCreate new designs

Light Emission from Other Light Emission from Other Silicon-Based StructuresSilicon-Based Structures

A brief overviewA brief overview

Light Emission from Other Light Emission from Other Silicon-Based StructuresSilicon-Based Structures

DifficultiesDifficulties

Indirect bandgapIndirect bandgap

Fast nonradiative recombinationFast nonradiative recombination

Slow radiative routesSlow radiative routes

Low quantum efficiencyLow quantum efficiency

Processing techniques must be consistent Processing techniques must be consistent with microelectronics (CMOS)with microelectronics (CMOS)

Light Emission from Other Light Emission from Other Silicon-Based StructuresSilicon-Based Structures

Methods employed:Methods employed:

Bulk SiliconBulk Silicon Solar cell basedSolar cell based Dislocation basedDislocation based

Silicon Nanocrystals (Si-nc)Silicon Nanocrystals (Si-nc) Porous siliconPorous silicon Silica matrixSilica matrix

Rare earth dopingRare earth doping Er-dopingEr-doping

Other gain materials grown on Si substratesOther gain materials grown on Si substrates SiOSiO22/ZnO/ZnO NOTE: Information on various methods taken

from: Pavesi. Will silicon be the photonic material of the third millennium? Journal of Physics: Condensed Matter v15 pR1169-R1196. 2003.

Bulk Silicon #1:Bulk Silicon #1:Solar-cell basedSolar-cell based

Reduce non-radiative rates using:Reduce non-radiative rates using:1.1. high-quality intrinsic silicon substrateshigh-quality intrinsic silicon substrates2.2. passivation of surfaces by high quality thermal oxidepassivation of surfaces by high quality thermal oxide3.3. small metal areassmall metal areas4.4. limiting the high doping regions to contact areaslimiting the high doping regions to contact areas

NOTE: figures from Green et al. NOTE: figures from Green et al. Efficient silicon light-emitting diodesEfficient silicon light-emitting diodes. Nature v412 p805-809. . Nature v412 p805-809. 8/23/2001.8/23/2001.

Bulk Silicon #1:Bulk Silicon #1:Solar-cell basedSolar-cell based

Problems:Problems:The need for both high purity (low doping The need for both high purity (low doping concentration) and surface texturing makes the concentration) and surface texturing makes the device processing incompatible with standard CMOS device processing incompatible with standard CMOS processingprocessingThe strong and fast free-carrier absorption typical of The strong and fast free-carrier absorption typical of bulk Si still prevents population inversionbulk Si still prevents population inversionIntegration of the active bulk Si into an optical cavity Integration of the active bulk Si into an optical cavity can be a problemcan be a problemThe modulation speed of the device can be limited by The modulation speed of the device can be limited by the long lifetime of the excited carriers (milliseconds) the long lifetime of the excited carriers (milliseconds) and by the need for a large optical cavity.and by the need for a large optical cavity.

Bulk Silicon #2:Bulk Silicon #2:Dislocation LoopsDislocation Loops

external quantum efficiency of about 1%external quantum efficiency of about 1%

high injection efficiency into the confined regionshigh injection efficiency into the confined regions

efficiency increases with temperatureefficiency increases with temperature

exploits the strain produced by localized dislocation loops to form energy barriers for carrier diffusion.

NOTE: Figures from Ng et al. NOTE: Figures from Ng et al. An efficient room-temperature silicon-based light-emitting diodeAn efficient room-temperature silicon-based light-emitting diode. Nature . Nature v410 p192-196 3/8/2001.v410 p192-196 3/8/2001.

Picture from Picture from http://www.tec.ufl.edu/~avci/proposal.pdfhttp://www.tec.ufl.edu/~avci/proposal.pdf

Bulk Silicon #2:Bulk Silicon #2:Dislocation LoopsDislocation Loops

Problems:Problems:

Does not remove the two main problems Does not remove the two main problems preventing population inversion: Auger preventing population inversion: Auger recombination and free-carrier absorptionrecombination and free-carrier absorption

Emission wavelength of these bulk silicon Emission wavelength of these bulk silicon LEDs is resonant with the silicon band gap: LEDs is resonant with the silicon band gap: difficult to control the region where the light is difficult to control the region where the light is channeledchanneled

Silicon Nanocrystals #1:Silicon Nanocrystals #1:Porous SiliconPorous Silicon

High luminescence efficiency due to:High luminescence efficiency due to:(i) quantum confinement, leading to a larger bandgap and an increased recombination (i) quantum confinement, leading to a larger bandgap and an increased recombination

probabilityprobability(ii) spatial confinement of free carriers , preventing them from reaching non-radiative (ii) spatial confinement of free carriers , preventing them from reaching non-radiative

recombination pointsrecombination points(iii) reduction of the refractive index of the material, increasing the extraction efficiency.(iii) reduction of the refractive index of the material, increasing the extraction efficiency.

Turns silicon into a low dimensional material and exploits quantum confinement effects to increase the radiative probability of carriers.

NOTE: Figures from Hirschman et al. NOTE: Figures from Hirschman et al. Silicon-based visible light-emitting devices integrated into Silicon-based visible light-emitting devices integrated into microelectronic circuitsmicroelectronic circuits. Nature v384 p338-341. 11/28/1996. . Nature v384 p338-341. 11/28/1996.

Picture from Picture from http://www.bios.el.utwente.nl/pubs/2000pubs(50-58)/55JMEMSmultiwalled2000.pdfhttp://www.bios.el.utwente.nl/pubs/2000pubs(50-58)/55JMEMSmultiwalled2000.pdf

Silicon Nanocrystals #1:Silicon Nanocrystals #1:Porous SiliconPorous Silicon

Problems:Problems:

high reactivity of the texture, causing high reactivity of the texture, causing uncontrollable variation of the LED uncontrollable variation of the LED performance with timeperformance with time

No optical gain in bulk PSNo optical gain in bulk PS

Silicon Nanocrystals #2:Silicon Nanocrystals #2:Silica MatrixSilica Matrix

Optical gain due to localized state recombinations either in the form of Optical gain due to localized state recombinations either in the form of silicon dimers or in the form of Si=O bonds formed at the interface silicon dimers or in the form of Si=O bonds formed at the interface between the Si-nc and the oxide or within the oxide matrix.between the Si-nc and the oxide or within the oxide matrix.Balance between Auger recombination and stimulated emission: optical Balance between Auger recombination and stimulated emission: optical gain not possible in all Si-nc samples.gain not possible in all Si-nc samples.

Produce silicon nanocrystals (Si-nc) in a silica matrix to exploit the quality and stability of the SiO2/Si interface and the improved emission properties of low

dimensional silicon.

NOTE: figures from Pavesi et al. NOTE: figures from Pavesi et al. Optical gain in silicon nanocrystalsOptical gain in silicon nanocrystals. Nature v408 p440-444. . Nature v408 p440-444. 11/23/2000.11/23/2000.

Picture from Picture from http://www.berlin.ptb.de/8/82/821/ferro02/pdffiles/pridoehl_b.pdfhttp://www.berlin.ptb.de/8/82/821/ferro02/pdffiles/pridoehl_b.pdf

Silicon Nanocrystals #2:Silicon Nanocrystals #2:Silica MatrixSilica Matrix

Remaining Issues:Remaining Issues:

Role of the Si-nc and embedding medium in Role of the Si-nc and embedding medium in optical gainoptical gain

Parameters determining the presence of gain Parameters determining the presence of gain

Influence of nanocrystal interaction on gainInfluence of nanocrystal interaction on gain

Plausibility of low-loss active waveguidesPlausibility of low-loss active waveguides

Rare Earth Doping:Rare Earth Doping:Er-doped SiliconEr-doped Silicon

Nonradiative de-excitation processes are reduced by widening the Nonradiative de-excitation processes are reduced by widening the Si bandgapSi bandgapReduced free-carrier concentration, limiting Auger processesReduced free-carrier concentration, limiting Auger processes

Active material (ErActive material (Er3+3+ in SiO in SiO22) has already shown lasing properties) has already shown lasing properties

Strong enhancement of Er luminescence when Er is implanted or deposited in a SiO2 matrix where Si-nc have been formed

NOTE: figure from Iacona et al. NOTE: figure from Iacona et al. Electroluminescence at 1.54µm in Er-doped Electroluminescence at 1.54µm in Er-doped Si nanocluster-based devicesSi nanocluster-based devices. Applied . Applied Physics Letters v81 No17 p3242-3244. Physics Letters v81 No17 p3242-3244. 10/21/2002.10/21/2002.Picture from Picture from hhttp://ej.iop.org/links/q97/inAA7g8+Gz5ZE4ttp://ej.iop.org/links/q97/inAA7g8+Gz5ZE4MYzkBQYw/d4_11_006.pdfMYzkBQYw/d4_11_006.pdf

Rare Earth Doping:Rare Earth Doping:Er-doped SiliconEr-doped Silicon

Problems:Problems:

Carrier Injection Carrier Injection

System ReliabilitySystem Reliability

Microdisks made of SiO2, thin layer of ZnO deposited on top as gain mediumWhispering Gallery modesAdvantage: higher quality factor, easier to fabricateDisadvantage: lack of directional outputHigh lasing threshold

Other Gain Materials Grown on Other Gain Materials Grown on Silicon SubstratesSilicon Substrates

Take advantage of high optical gain of other materials while still growing on Si substrate

Note: info, picture and figure from Liu et al. Liu et al. Optically pumped ultraviolet microdisk Optically pumped ultraviolet microdisk laser on a silicon substratelaser on a silicon substrate. Applied . Applied Physics Letters v84 No14 p2488-2490. Physics Letters v84 No14 p2488-2490. 4/5/2004.4/5/2004.

Conclusions, orConclusions, or

The theory, materials, and processing The theory, materials, and processing needed for these lasers are all currently needed for these lasers are all currently outside of this lab’s main focus.outside of this lab’s main focus.In addition, all of these methods are In addition, all of these methods are currently several orders of magnitude currently several orders of magnitude away from an efficient laser.away from an efficient laser.There could be future work for the lab There could be future work for the lab here, but that is in the somewhat distant here, but that is in the somewhat distant future.future.

My Uninformed OpinionsMy Uninformed Opinions

Excited-State Quantum Excited-State Quantum Cascade LasersCascade Lasers

The beginnings of an investigationThe beginnings of an investigation

Excited-State QCLExcited-State QCL Transitions between excited states have increased Transitions between excited states have increased

optical dipole matrix elementsoptical dipole matrix elements Could a QC structure take advantage of this fact without Could a QC structure take advantage of this fact without

incurring too much loss?incurring too much loss? This question examined using the following Figure Of This question examined using the following Figure Of

Merit (FOM):Merit (FOM):

254

41424354

51525354

111

11

11111

dFOM

254

54

45 1 dFOM

(expanded) -350 -300 -250 -200 -150 -100 -50 0 500

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Active Region: Desired ParametersActive Region: Desired Parameters 3 lasing states: E1, E2, E3 3 lasing states: E1, E2, E3 states 3,4, and 5 found to work better than states 2,3, and 4 or states 3,4, and 5 found to work better than states 2,3, and 4 or

states 4, 5, and 6states 4, 5, and 6 E3 should be 100-200meV above E2 for desired wavelengthE3 should be 100-200meV above E2 for desired wavelength E2 should be a phonon (40meV) above E1E2 should be a phonon (40meV) above E1 E1 and E2 should be anticrossedE1 and E2 should be anticrossed

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40meV

Injector: Desired ParametersInjector: Desired Parameters Highest injector state anticrossed with E1 of active regionHighest injector state anticrossed with E1 of active region States slope downward to guide electrons from upper active region to States slope downward to guide electrons from upper active region to

lower onelower one Lowest injector state anticrossed with E3 of next active regionLowest injector state anticrossed with E3 of next active region

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StrategiesStrategies 2-well active region: 2-well active region:

smaller well followed by smaller well followed by larger onelarger one

3-well active region: 3-well active region: similar structure to 2-well similar structure to 2-well followed by a third, much followed by a third, much thinner wellthinner well

Relatively high bias Relatively high bias (~58kV/cm) pushes upper (~58kV/cm) pushes upper AR ground state above AR ground state above lower AR E3 lower AR E3

5-well injector found to 5-well injector found to work better than 4-wellwork better than 4-well

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DifficultiesDifficulties Upper energy states Upper energy states

creep into injector regioncreep into injector region Energy gap between E3 Energy gap between E3

and E2 is low; this leads and E2 is low; this leads to a higher wavelength to a higher wavelength laser (laser (≤≤1313µµm)m)

Injector must collect from Injector must collect from 3 energy states instead of 3 energy states instead of oneone

Ground state of upper AR Ground state of upper AR is below E3 of lower AR is below E3 of lower AR at lower biasesat lower biases -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100

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Final ProductsFinal Products2-well: 102-well: 10µµm laserm laser

FOM FOM ≈≈ 860 860

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Final ProductsFinal Products3-well: 133-well: 13µµm laserm laser

FOM FOM ≈ 1050≈ 1050

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The Next StepThe Next Step

2-well sample is being grown at Bell Labs2-well sample is being grown at Bell Labs Make waveguide for 3-well design and have it Make waveguide for 3-well design and have it

growngrown If designs needs improvement:If designs needs improvement:

Try to anticross AR states 1 and 2 with injectorTry to anticross AR states 1 and 2 with injector Alter bias and number of injector wellsAlter bias and number of injector wells Reduce anticrossing between upper AR states and Reduce anticrossing between upper AR states and

continuum statescontinuum states 2-well design is especially weak in these areas 2-well design is especially weak in these areas Alter energy program to allow investigation of state Alter energy program to allow investigation of state

behavior with more repetitions behavior with more repetitions Improve waveguideImprove waveguide

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It’s Been Awesome!It’s Been Awesome!

Special Thanks to:Special Thanks to:

Dr. Sturm and Keith for processing the samples and agreeing to Dr. Sturm and Keith for processing the samples and agreeing to grow moregrow moreDan for giving me stuff to do and trusting me to figure out the restDan for giving me stuff to do and trusting me to figure out the restGary for getting MATLAB on my computer and consequently Gary for getting MATLAB on my computer and consequently teaching me how to steal University softwareteaching me how to steal University softwareAlvaro for not killing me when I hogged the FTIR every freakin’ dayAlvaro for not killing me when I hogged the FTIR every freakin’ dayScott for fixing MATLAB and acting interested in my excited-state Scott for fixing MATLAB and acting interested in my excited-state QCL designQCL designAfusat for teaching me how to use the energy program and for Afusat for teaching me how to use the energy program and for having a cool accenthaving a cool accentAlvaro, Scott, Dan and Gary for their bonding timeAlvaro, Scott, Dan and Gary for their bonding timeRakib for making me get out of my chair every once in a while to Rakib for making me get out of my chair every once in a while to click a button for himclick a button for himJon and Yisa for collaboration, conversation, and generally keeping Jon and Yisa for collaboration, conversation, and generally keeping me sane down in the labme sane down in the labDr. Gmachl, of course, for giving me a job and making it greatDr. Gmachl, of course, for giving me a job and making it great