Noise spectroscopy of InGaAs/GaAs

1. Noise spectroscopy of InGaAs/GaAs 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 • Discuss MBE technique w/ RHEED monitoring • Discuss SK growth mode and mechanism Noise in QDs 7.23.08

9. Morphology Surface data • Height • Diameter • Density Noise in QDs 7.23.08

10. Photoluminescence Noise in QDs 7.23.08

11. Hall Effect • Show how effect works • Show what info can be obtained • Discuss mobility • Discuss carrier concentration Noise in QDs 7.23.08

12. Deep Level Noise Spectroscopy • Discuss overview of noise • Types • Put example of spectra and how main types are represented Noise in QDs 7.23.08

13. Thermal Noise • Discuss origin • Show forumla Noise in QDs 7.23.08

14. Flicker noise • Discuss different models • Discuss formula Noise in QDs 7.23.08

15. Generation-Recombination Noise • Origin • formula Noise in QDs 7.23.08

16. GR Analysis • Show td spectra at fixed frequencies • Show comparitive between samples • Talk about assumptions and method • Show what can be calculated under conditions Noise in QDs 7.23.08

17. Sample Creation • MBE Growth • Structure design • Growth conditions • RHEED • Post Growth • AFM for morphology • PL Noise in QDs 7.23.08

18. Sample Preparation • Discuss photolithography & packaging Noise in QDs 7.23.08

19. Contact Optimization • Annealing • IV Testing • TLM Measurements Noise in QDs 7.23.08

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

21. DLNS • Understand measurement • Describe setup & equipment • Experiments • Room temperature @ different biases • 82K with different biases • Temperature dependence from 82-390K Noise in QDs 7.23.08

22. Morphology Noise in QDs 7.23.08

23. PL • 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 Noise in QDs 7.23.08

24. Sample Prep (??) • IV & TLM • Ohmic contacts • Contact resistance much lower than sample • Show pics of structures Noise in QDs 7.23.08

25. Mobility • Show mobility curves • 6ML behaves like QW • QD samples slightly lower at lower temperatures Noise in QDs 7.23.08

26. Carrier Concentration • Show carrier concentration • More ionization at higher temperatures • Small bumps indicate defects Noise in QDs 7.23.08

27. Flicker Noise • Hooge parameter doesn’t indicate change in flicker noise between samples at RT & LT • Show comparisons and trends Noise in QDs 7.23.08

28. GR Noise • G-R defects visible in spectra • G-R peaks move • Show TD spectra (comparitive) • Show analysis Noise in QDs 7.23.08

29. GR Summary • 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 Noise in QDs 7.23.08

30. 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

31. 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