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Noise spectroscopy of InGaAs / GaAs Heterostructures

Noise spectroscopy of InGaAs / GaAs Heterostructures. Tim Morgan. Outline. Introduction to Quantum Dots Theory of Electrical Transport and Noise Experimental Techniques Discussion & Results Conclusions. Quantum Dot Devices. Optoelectronics. Infrared Pictures. Optimized Device.

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Noise spectroscopy of InGaAs / GaAs Heterostructures

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  1. Noise spectroscopy of InGaAs/GaAsHeterostructures Tim Morgan

  2. Outline • Introduction to Quantum Dots • Theory of Electrical Transport and Noise • Experimental Techniques • Discussion & Results • Conclusions Noise in QDs 7.23.08

  3. Quantum Dot Devices Optoelectronics Infrared Pictures Optimized Device InAs/InGaAs QDs http://cqd.eecs.northwestern.edu/research/qdots.php Biosensors Igor L. Medintz, et. al. Nature Materials 2003 Noise in QDs 7.23.08

  4. How to Probe for Noise Capacitance-Voltage Spectroscopy Deep Level Transient Spectroscopy NOISE Deep Level Noise Spectroscopy Admittance Spectroscopy Noise in QDs 7.23.08

  5. The Goal What noise is present in QDs? What is the source of noise in QDs? Where are the defects within QD samples? Noise in QDs 7.23.08

  6. The Structure Sample # MLs 1500 Å GaAs: Si T=1133ºC S1 0 200 Å GaAs: undoped S2 6 S3 9 InGaAs QD layer S4 11 200 Å GaAs: undoped S5 13 5000 Å GaAs: Si T=1133ºC 5000 Å GaAs buffer GaAs (001) SI Noise in QDs 7.23.08

  7. The Plan Processing & Packaging Structures & Contacts MBE Growth Hall Measurements Mobility & Carrier Concentration AFM Morphology Studies PL Optical Studies DLNS Noise Properties What noise is in QDs? Noise in QDs 7.23.08

  8. QD Formation • MBE Growth • Lattice mismatch between materials • Layers added until critical thickness reached • Surface relaxes, forming QDs Noise in QDs 7.23.08

  9. Atomic Force Microscopy Surface data • Height • Diameter • Density Noise in QDs 7.23.08

  10. Photoluminescence Noise in QDs 7.23.08

  11. Hall Effect • Transport Info • Mobility • Carrier • Concentration • Hall Coefficient • Conductivity • Primary Carrier Noise in QDs 7.23.08

  12. Deep Level Noise Spectroscopy • Noise Types • Thermal Noise • Flicker Noise • Generation-Recombination Noise • Shot Noise Noise in QDs 7.23.08

  13. Thermal Noise • Frequency Independent • Dominant at high frequencies • Due to random fluctuations from Brownian motion of electrons Noise in QDs 7.23.08

  14. Flicker noise • Dominant at lower frequencies • Due to conductivity fluctuations • Can arise from both carrier and mobility fluctuations Noise in QDs 7.23.08

  15. Generation-Recombination Noise • Lorentzian with characteristic frequency • Apperas as a shoulder in the noise spectrum • Due to electrons being emitted and recombining from traps Noise in QDs 7.23.08

  16. Temperature Dep. Spectra • Noise spectrum over temperature • Frequency Range (5-2560 Hz) • Peaks represent traps • Characterize traps • activation energy • ionization energy • capture cross section • trap density Noise in QDs 7.23.08

  17. GR Analysis • A different expression: • Peaks reveal the activation and ionization energy • lnSmaxvslnω ionization energy • 1/kBTmaxvslnω activation energy • Capture cross section: • Trap density: Noise in QDs 7.23.08

  18. Sample Creation • MBE Growth • Solid Source Riber 32 P • RHEED monitoring • Post Growth • AFM • PL • 10 K, 532 nm • YAG laser 20 W/cm2 • ~ 20 μm spot size Noise in QDs 7.23.08

  19. Sample Preparation 270 nm Au 20 nm Ni 75 nm AuGe 30 µm Greek Cross • Wet Etch • Metallization Noise in QDs 7.23.08

  20. Contact Optimization lT c c AuGe/Ni/Au d rs Rs Rs dx Dopant 0 -l x • Annealing: minimize barrier to create Ohmic contacts • IV Testing: Verify Ohmiccontacs made • TLM Measurements: determince contact resistance Noise in QDs 7.23.08

  21. Hall Measurements • Hall Measurements • Mobility • Carrier concentration • Resistance measurements Noise in QDs 7.23.08

  22. DLNS: The Measurement • Measure voltage perpendicular to bias direction • The fluctuations in voltage are measured over a time period • Filtered  Squared  Averaged  Fourier Transform Noise in QDs 7.23.08

  23. DLNS: Setup & Experiments • Setup • Shielded sample • Power supply: battery pack and series of resistors • Low noise preamplifier with band filter • Noise spectrum analyzer • Experiments • Temperature dependence: 82 K – 390 K, fixed bias • Room temperature: several biases • Low temperature (82 K): several biases Noise in QDs 7.23.08

  24. Morphology • No QDs with 0 or 6 ML samples 9 ML Height: 33 ± 0.5 Å Density: 3.8 × 1010cm-2 11 ML Height: 47 ± 0.3 Å Density: 8.4 × 1010cm-2 13 ML Height: 53 ± 0.4 Å Density: 7.2 × 1010cm-2 QDs & Noise 9.5.07

  25. PL • Red shift in energy • Single size distribution • Decrease in integral intensity • All results correlate well with AFM data • Increase in height • Same FWHM trend Noise in QDs 7.23.08

  26. Sample Prep All samples meet 13 ML QD sample Noise in QDs 7.23.08

  27. Mobility Noise in QDs 7.23.08

  28. Carrier Concentration Noise in QDs 7.23.08

  29. Noise Curves 0 ML 300 K • Series of spectra at fixed temperatures and various biases • Fit each specturm with all components of noise • Extract fit parameters for component breakdown analysis Noise in QDs 7.23.08

  30. Flicker Noise • Fit Parameter: • Determine the Hooge Parameter at 300 K and 82 K 0 ML Sample at 300 K Noise in QDs 7.23.08

  31. Hooge Comparison 300 K 82 K Noise in QDs 7.23.08

  32. QD Height Comparison 300 K 82 K Noise in QDs 7.23.08

  33. QD Density Comparison 300 K 82 K Noise in QDs 7.23.08

  34. Two Views Shoulders Peaks Noise in QDs 7.23.08

  35. Analysis Plots 11 ML 11 ML Ionization Energy Activation Energy Noise in QDs 7.23.08

  36. GR Summary Noise in QDs 7.23.08

  37. Conclusions • Additional defect found due to QDs • Flicker noise doesn’t appear to change with the addition of QDs • Good news for QD devices as it is the main component of noise, especially at lower frequencies where it dominates over thermal noise • Proved that DLNS is a viable technique for defect detection in nanostructures as results agree with other well established techniques (e.g. DLTS) • Lateral technique versus vertical Noise in QDs 7.23.08

  38. Future Work • Study Gated QD samples • Change where current flows to determine which layer noise arises from • Study QDs with vertical biasing • Vary doping to change Fermi level • Enhance noise when in resonance with traps • Inject minority carriers with light into QD samples • Determine energy positions relative to conduction band • QDIPs • Look at noise in a QD device and show its detection limit because of the noise Noise in QDs 7.23.08

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