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Defect analysis of GaAs/InGaAs heterostructures. Tim Morgan. Outline. Introduction to Quantum Structures Theory of Electrical Transport and Noise Experimental Techniques Discussion & Results Conclusions. Introduction. Short History of QDs Types of QDs Uses of QDs

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Presentation Transcript
outline
Outline
  • Introduction to Quantum Structures
  • Theory of Electrical Transport and Noise
  • Experimental Techniques
  • Discussion & Results
  • Conclusions

QDs & Noise 6.27.08

introduction
Introduction
  • Short History of QDs
  • Types of QDs
  • Uses of QDs
  • Motivation for Noise Study of QDs

QDs & Noise 6.27.08

theory
Theory
  • QD Formation
    • SK growth
    • RHEED studies
    • AFM
    • Other techniques
  • Photoluminescence
    • Physical understanding of what happens
    • Peak Position
    • Integral Intensity
    • Spectral Shape

QDs & Noise 6.27.08

theory1
Theory
  • Hall Measurements
    • Brief description of scenario
    • Mobility
    • Electron Concentration
  • Deep Level Noise Spectroscopy
    • Types of Noise
      • Thermal
      • Flicker
      • Generation-Recombination
    • QD contributions to noise
  • Our experiment

QDs & Noise 6.27.08

experimental techniques
Experimental Techniques
  • MBE Growth
    • Structure design
    • Growth conditions
  • Preparing & Processing Samples
    • Van der Paaw Measurements
    • AFM
    • Photolithography
    • IV Testing
    • TLM Measurements
    • Packaging

QDs & Noise 6.27.08

experimental techniques1
Experimental Techniques
  • PL
  • Hall Measurements
    • Temperature dependent measurements
      • Mobility
      • Carrier concentration
  • Resistance measurements

QDs & Noise 6.27.08

experimental techniques2
Experimental Techniques
  • Deep Level Noise Spectroscopy
    • Understand measurement
    • Describe setup & equipment
    • Room temperature @ different biases
    • 82K with different biases
    • Temperature dependence from 82-390K

QDs & Noise 6.27.08

discussion results
Discussion & Results
  • AFM shows QDs for 9, 11 & 13 ML samples
    • Anisotropy
      • Elongation in [01-1]
    • Shape
      • Height increases from 9 to 11 to 13 ML samples
      • Histograms of Height & diameter
  • PL
    • Red shift in energy from 9 to 11 to 13 ML samples
    • Single size distribution for all QD samples
    • Decrease in integral intensity from 9 to 11 to 13 ML
    • All results correlate well with AFM data

QDs & Noise 6.27.08

discussion results1
Discussion & Results
  • IV & TLM
    • Ohmic contacts
    • Contact resistance much lower than sample
  • Hall Measurements
    • Mobility
      • 6ML behaves like QW
      • QD samples slightly lower at lower temperatures
    • Carrier concentration
      • More ionization at higher temperatures
      • Small bumps indicate defects

QDs & Noise 6.27.08

discussion results2
Discussion & Results
  • DLNS
    • Room temperature
      • Hooge parameter doesn’t indicate change in flicker noise between samples
      • G-R defects visible in spectra
    • Low temperature (82K)
      • Hooge parameter doesn’t indicate change in flicker noise between samples
    • Temperature dependence
      • G-R peaks move

QDs & Noise 6.27.08

discussion results3
Discussion & Results
  • Defect Analysis
    • 3 defects in all 5 samples
    • Defect D in just QD samples
      • 2 different activation energies: 0.18 eV for 9 & 11 ML, 0.1 eV for 13 ML
      • 0.18 eV defect known as M1 in GaAs; other is suspected to be associated just with QD samples
    • Defect characteristics
      • Capture cross sections determined
      • Trap density
      • Energy level below conduction band for defect A only

QDs & Noise 6.27.08

conclusions
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

QDs & Noise 6.27.08

future work
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

QDs & Noise 6.27.08