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Electronic Noise Spectroscopy of InGaAs QDs

Electronic Noise Spectroscopy of InGaAs QDs. Tim Morgan. Outline. Motivation Noise Theory Experimental Techniques Discussion & Results Conclusions. Quantum Dots. Quantum Dot Devices. Infrared Detectors. cqd.eecs.northwestern.edu. Single Photon Emitter. byz.org. QD Laser. Biosensors.

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Electronic Noise Spectroscopy of InGaAs QDs

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  1. Electronic Noise Spectroscopy of InGaAs QDs Tim Morgan

  2. Outline Motivation Noise Theory Experimental Techniques Discussion & Results Conclusions Noise in QDs 7.23.08

  3. Quantum Dots Quantum Dot Devices Infrared Detectors cqd.eecs.northwestern.edu Single Photon Emitter byz.org QD Laser Biosensors Igor L. Medintz, et. al. Nature Materials 2003 Noise in QDs 7.23.08

  4. The Nature of Noise in QDs What is the source of noise in QDs? Does noise in QDs differ from bulk? Is there a correlation of noise, defects and QDs? Noise in QDs 7.23.08

  5. The Plan Noise Spectroscopy  Noise Measurements Hall Effect  Carrier Conc. & Mobility Photoluminescence  Electronic structure AFM  Morphology MBE Growth  In0.35Ga0.65As QDs Noise Spectroscopy of QDs

  6. Noise Spectroscopy Vnoise Vbias SV is the average change in voltage squared in a bandwidth of 1 Hz. Noise in QDs 7.23.08

  7. Thermal Noise Due to random fluctuations from Brownian motion of electrons Noise in QDs 7.23.08

  8. Flicker noise Arises from both carrier and mobility fluctuations in the conductivity. The Hooge Parameter (α) is an indicator of crystal quality. Noise in QDs 7.23.08

  9. Why 1/f Dependence? Tail States (Defects) Sum the noise of all the tail states together. Noise in QDs 7.23.08

  10. Generation-Recombination Noise Show picture with deep trap in compared to tail states Noise in QDs 7.23.08

  11. Total Noise • Noise is a useful signal • Probe defects within the sample that cannot otherwise be detected • Tells us about crystal quality • Used to calibrate the resistance of a material Measured Spectra Noise in QDs 7.23.08

  12. Sample Creation MBE Growth Solid Source Riber 32 P RHEED monitoring Noise in QDs 7.23.08

  13. The Structure 1500 Å GaAs: Si ND = 6 × 1016 200 Å GaAs: undoped InGaAs QD layer 200 Å GaAs: undoped 5000 Å GaAs: Si ND = 6 × 1016 5000 Å GaAs buffer GaAs (001) SI RHEED Sample # MLs Bulk S1 0 QW S2 6 QDs S3 9 QDs S4 11 QDs S5 13 Noise in QDs 7.23.08

  14. Morphology 9 ML Height: 33 ± 2.8 Å Density: 3.8 × 1010 cm-2 11 ML Height: 47 ± 2.8 Å Density: 8.4 × 1010 cm-2 13 ML Height: 53 ± 2.8 Å Density: 7.2 × 1010 cm-2 Noise in QDs 7.23.08

  15. AFM Trends Insert Graphs showing how height, density change with MLs Noise Spectroscopy of QDs

  16. PL Width of energy well is the height of the QD. Noise in QDs 7.23.08

  17. PL Trends Insert Graphs of Peak, FWHM, Integral Intensity vs MLs Noise Spectroscopy of QDs

  18. Sample Preparation 270 nm Au 20 nm Ni 75 nm AuGe 20 µm Greek Cross Noise in QDs 7.23.08

  19. Finished Structures 13 ML All samples meet All samples meet Noise in QDs 7.23.08

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

  21. Mobility Noise in QDs 7.23.08

  22. Carrier Concentration 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. 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

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

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

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

  28. Two Views Shoulders Peaks Noise in QDs 7.23.08

  29. GR Analysis • A different expression: • Peaks reveal the activation and ionization energy • ln Smax vs ln ω ionization energy • 1/kBTmax vs ln ω activation energy • Capture cross section: • Trap density: Noise in QDs 7.23.08

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

  31. GR Summary Noise in QDs 7.23.08

  32. Conclusions Sources of Noise in QDs GR Traps with activation energies of 0.74, 0.49, 0.34, 0.18 and 0.1 eV Tail states from crystal imperfections Noise in QDs does differ from bulk Flicker noise decreases with the insertion of In0.35Ga0.65As and the formation of QDs Flicker noise decreases with increase of height and density Additional GR Trap in QD samples not present in bulk Correlation of noise, defects and QDs GR traps give rise to GR Noise QDs lowers flicker noise Noise in QDs 7.23.08

  33. 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

  34. Noise in QDs 7.23.08

  35. What is a Quantum Dot? Single Atom Many Atoms A Few Atoms Confined • Single atom: Discrete energy level transitions • Many atoms: continuum of energy levels • A Few Atoms Confined: lower energy levels discrete because of confinement ~30 nm Noise in QDs 7.23.08

  36. QD Formation InGaAs fluxes When critical thickness is reached, the strain is relaxed and thus 3D islands (quantum dots) are formed. - I’m now happy!! Strain has built up! - I’m very uncomfortable!! 2D InGaAs wetting layer GaAs substrate Used with permission of Jihoon Lee Noise in QDs 7.23.08

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

  38. Photoluminescence Noise in QDs 7.23.08

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

  40. 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 Ohmic contacts made • TLM Measurements: determine contact resistance Noise in QDs 7.23.08

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

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