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M y Su m m e r Va c a ti on

M y Su m m e r Va c a ti on. or. “ Intersubband Electroluminescence from Silicon-based Quantum Cascade Structures, Light Emission from the Silicon Materials System, and Excited-State QCLs ”. Silicon-Based Quantum Cascade Structures. Research and Experimentation.

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M y Su m m e r Va c a ti on

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  1. My Summer Vacation or “Intersubband Electroluminescence from Silicon-based Quantum Cascade Structures, Light Emission from the Silicon Materials System, and Excited-State QCLs”

  2. Silicon-Based Quantum Cascade Structures Research and Experimentation

  3. Current Quantum Cascade Lasers • Materials system derived from groups III and V of periodic chart (i.e. GaInAs/AlInAs) • Operation in conduction band • Charge carriers = electrons (negative 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.

  4. Silicon-based Quantum Cascade Structures • Materials system derived from group IV of periodic chart (Si1-xGex/Si) • Operation in valence band • Charge carriers = holes (positive 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.

  5. The Big Picture Advantages of group IV: • Integration with microelectronics • High-temperature operation • Low cost • Widely available, easy to handle and manufacture • Advanced processing techniques • Negligible polar optical phonon scattering • Possibility of surface-emission without a grating using Light Hole (LH)  Heavy Hole (HH) transitions for TE-polarized light

  6. The Big Picture Disadvantages: • Indirect band gap • Need to work in valence band • Larger effective masses • In between HH bands where transitions take place are LH bands • Reduces non-radiative lifetime • Strain from lattice mismatch • Critical Thickness • Limits # of cascades and # of wells per cascade • Group IV QCL’s must be “drastically simplified version” of III/V’s.

  7. The Big Picture Disadvantages: • Primary scattering mechanism of SiGe is nonresonant: deformation potential scattering with optical phonons. • Limited quantum efficiency (~10-5 for EL) • No population inversion • Auger recombination • Free-carrier absorption • No stimulated emission or optical gain

  8. Sample structure From Dehlinger, Diehl, Gennser, Sigg, Faist, Ensslin, Grutzmacher, Muller. Intersubband Electroluminescence from Silicon-Based Quantum Cascade Structures. Science, v290 12/22/2000. Emission energy = 125 meV EL at peak ~ 0.073 pW/meV FWHM = 22meV First EL in the 10 µm range with narrow linewidth

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

  10. Experimental Results • SiGe C • Side emission • Top emission • All • Voltage Current Characteristics • SiGe B • Side emission • SiGe A • Side emission

  11. SiGe C (side emission): Noise Possible causes: • Improper alignment • Wire Bonding problem • Design Flaw • Etching not deep enough • Side emission obstructed

  12. SiGe C (top emission): Noise Possible causes: • TE-polarized emission usually much less intense than TM • Initial setup: wires soldered directly to pads, sample attached with double-sided tape to cryostat rod • With new cryostat: new sample holder accommodates top emission Top Side Note: figure fromDiehl 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.

  13. Voltage-Current Characteristics • Current gradually increased from ~40mA to ~800mA (Voltage varies up to 20V) • SiGe A and C give (approximately) linear V-I graphs • SiGe B gives one sub-linear graph, repeat gives linear graph • This sample chosen for emission testing • The characteristic deteriorates somewhat above currents of 400mA, so current was thereafter kept at or below this value.

  14. SiGe B (side emission): Detection at last • Sample spectrum: 700ns pulse at 400mA • Linewidth≈650cm-1 • Low output: Lock-in sensitivity = 100µV • Spectra taken with varying currents: pulse width=500ns, currents vary from 300 to 400mA • Spectrum broadens with increasing current, but noise also becomes a lesser factor Sample Spectrum Varying Currents

  15. SiGe B (side emission): Detection at last • Spectra taken with varying pulse widths • Current =400mA, pulse widths vary from 300 to 700ns • Spectrum broadens with increasing pulse widths, but noise also becomes a lesser factor • Signal becomes much clearer with lower sensitivities (bottom spectrum has 50µV sensitivity, the rest have 100µV) Varying Pulse Widths

  16. SiGe A (side emission): More Detection • Just a few spectra taken • 500ns pulse at 400mA • Linewidth≈550cm-1

  17. Is It Heat? • Very broad spectrum, while Science paper reports “narrow linewidth” of 22meV with lower current but much higher pulse width • Emission does have polarization dependence in both emitting samples

  18. The Next Step • Regain SiGe B emission capabilities • Rebond to different mesas • Continue taking spectra with varying current and pulse length • Document characteristics • Find ideal spectrum • Present to Sturm lab, say “good job,” request another structure(s) • Work with Princeton-grown samples • Create new designs

  19. Light Emission from Other Silicon-Based Structures A brief overview

  20. Light Emission from Other Silicon-Based Structures Difficulties • Indirect bandgap • Fast nonradiative recombination • Slow radiative routes • Low quantum efficiency • Processing techniques must be consistent with microelectronics (CMOS)

  21. Light Emission from Other Silicon-Based Structures Methods employed: • Bulk Silicon • Solar cell based • Dislocation based • Silicon Nanocrystals (Si-nc) • Porous silicon • Silica matrix • Rare earth doping • Er-doping • Other gain materials grown on Si substrates • SiO2/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.

  22. Bulk Silicon #1:Solar-cell based Reduce non-radiative rates using: • high-quality intrinsic silicon substrates • passivation of surfaces by high quality thermal oxide • small metal areas • limiting the high doping regions to contact areas NOTE: figures from Green et al. Efficient silicon light-emitting diodes. Nature v412 p805-809. 8/23/2001.

  23. Bulk Silicon #1:Solar-cell based Problems: • The need for both high purity (low doping concentration) and surface texturing makes the device processing incompatible with standard CMOS processing • The strong and fast free-carrier absorption typical of bulk Si still prevents population inversion • Integration of the active bulk Si into an optical cavity can be a problem • The modulation speed of the device can be limited by the long lifetime of the excited carriers (milliseconds) and by the need for a large optical cavity.

  24. Bulk Silicon #2:Dislocation Loops • external quantum efficiency of about 1% • high injection efficiency into the confined regions • efficiency increases with temperature exploits the strain produced by localized dislocation loops to form energy barriers for carrier diffusion. NOTE: Figures from Ng et al. An efficient room-temperature silicon-based light-emitting diode. Nature v410 p192-196 3/8/2001. Picture from http://www.tec.ufl.edu/~avci/proposal.pdf

  25. Bulk Silicon #2:Dislocation Loops Problems: • Does not remove the two main problems preventing population inversion: Auger recombination and free-carrier absorption • Emission wavelength of these bulk silicon LEDs is resonant with the silicon band gap: difficult to control the region where the light is channeled

  26. Silicon Nanocrystals #1:Porous Silicon Turns silicon into a low dimensional material and exploits quantum confinement effects to increase the radiative probability of carriers. High luminescence efficiency due to: (i) quantum confinement, leading to a larger bandgap and an increased recombination probability (ii) spatial confinement of free carriers , preventing them from reaching non-radiative recombination points (iii) reduction of the refractive index of the material, increasing the extraction efficiency. NOTE: Figures from Hirschman et al. Silicon-based visible light-emitting devices integrated into microelectronic circuits. Nature v384 p338-341. 11/28/1996. Picture from http://www.bios.el.utwente.nl/pubs/2000pubs(50-58)/55JMEMSmultiwalled2000.pdf

  27. Silicon Nanocrystals #1:Porous Silicon Problems: • high reactivity of the texture, causing uncontrollable variation of the LED performance with time • No optical gain in bulk PS

  28. Silicon Nanocrystals #2:Silica Matrix • 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 between the Si-nc and the oxide or within the oxide matrix. • Balance between Auger recombination and stimulated emission: optical 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. Optical gain in silicon nanocrystals. Nature v408 p440-444. 11/23/2000. Picture from http://www.berlin.ptb.de/8/82/821/ferro02/pdffiles/pridoehl_b.pdf

  29. Silicon Nanocrystals #2:Silica Matrix Remaining Issues: • Role of the Si-nc and embedding medium in optical gain • Parameters determining the presence of gain • Influence of nanocrystal interaction on gain • Plausibility of low-loss active waveguides

  30. Rare Earth Doping:Er-doped Silicon • Nonradiative de-excitation processes are reduced by widening the Si bandgap • Reduced free-carrier concentration, limiting Auger processes • Active material (Er3+ in SiO2) 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. Electroluminescence at 1.54µm in Er-doped Si nanocluster-based devices. Applied Physics Letters v81 No17 p3242-3244. 10/21/2002. Picture from http://ej.iop.org/links/q97/inAA7g8+Gz5ZE4MYzkBQYw/d4_11_006.pdf

  31. Rare Earth Doping:Er-doped Silicon Problems: • Carrier Injection • System Reliability

  32. Other Gain Materials Grown on Silicon Substrates Take advantage of high optical gain of other materials while still growing on Si substrate Microdisks made of SiO2, thin layer of ZnO deposited on top as gain medium Whispering Gallery modes Advantage: higher quality factor, easier to fabricate Disadvantage: lack of directional output High lasing threshold Note: info, picture and figure from Liu et al. Optically pumped ultraviolet microdisk laser on a silicon substrate. Applied Physics Letters v84 No14 p2488-2490. 4/5/2004.

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

  34. Excited-State Quantum Cascade Lasers The beginnings of an investigation

  35. Excited-State QCL • Transitions between excited states have increased optical dipole matrix elements • Could a QC structure take advantage of this fact without incurring too much loss? • This question examined using the following Figure Of Merit (FOM): 6 5 4 3 2 1 (expanded)

  36. E3 E2 E1 Active Region: Desired Parameters • 3 lasing states: E1, E2, E3 • states 3,4, and 5 found to work better than states 2,3, and 4 or states 4, 5, and 6 • E3 should be 100-200meV above E2 for desired wavelength • E2 should be a phonon (40meV) above E1 • E1 and E2 should be anticrossed 6 5 100-200meV 4 40meV 3 2 1

  37. Injector: Desired Parameters • Highest injector state anticrossed with E1 of active region • States slope downward to guide electrons from upper active region to lower one • Lowest injector state anticrossed with E3 of next active region

  38. E3 Ground Strategies • 2-well active region: smaller well followed by larger one • 3-well active region: similar structure to 2-well followed by a third, much thinner well • Relatively high bias (~58kV/cm) pushes upper AR ground state above lower AR E3 • 5-well injector found to work better than 4-well

  39. Difficulties • Upper energy states creep into injector region • Energy gap between E3 and E2 is low; this leads to a higher wavelength laser (≤13µm) • Injector must collect from 3 energy states instead of one • Ground state of upper AR is below E3 of lower AR at lower biases Upper AR Lower AR

  40. Final Products 2-well: 10µm laser FOM ≈ 860

  41. Final Products 3-well: 13µm laser FOM ≈ 1050

  42. The Next Step • 2-well sample is being grown at Bell Labs • Make waveguide for 3-well design and have it grown • If designs needs improvement: • Try to anticross AR states 1 and 2 with injector • Alter bias and number of injector wells • Reduce anticrossing between upper AR states and continuum states • 2-well design is especially weak in these areas • Alter energy program to allow investigation of state behavior with more repetitions • Improve waveguide

  43. It’s Been Awesome!

  44. Special Thanks to: • Dr. Sturm and Keith for processing the samples and agreeing to grow more • Dan for giving me stuff to do and trusting me to figure out the rest • Gary for getting MATLAB on my computer and consequently teaching me how to steal University software • Alvaro for not killing me when I hogged the FTIR every freakin’ day • Scott for fixing MATLAB and acting interested in my excited-state QCL design • Afusat for teaching me how to use the energy program and for having a cool accent • Alvaro, Scott, Dan and Gary for their bonding time • Rakib for making me get out of my chair every once in a while to click a button for him • Jon and Yisa for collaboration, conversation, and generally keeping me sane down in the lab • Dr. Gmachl, of course, for giving me a job and making it great

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