1/41 Electronic Noise Spectroscopy of InGaAs QDs Tim Morgan

2/41 Noise Spectroscopy in QDs Outline Motivation Noise Theory Discussion & Results Conclusions Future Work

3/41 Quantum Dot Devices Noise Spectroscopy in QDs Infrared Detectors cqd.eecs.northwestern.edu Igor L. Medintz, et. al. Nature Materials 2003 Biosensors Single Photon Emitter byz.org QD Laser

4/41 Infrared Photodetectors cqd.eecs.northwestern.edu Passmore, B.S.; Jiang Wu; Manasreh, M.O.; Kunets, V.P.; Lytvyn, P.M.; Salamo, G.J.; Electron Device Letters, IEEE Volume 29, Issue 3, March 2008 Page(s): Quantum Dot Devices Noise Spectroscopy in QDs

5/41 The Nature of Noise in QDs What is the source of noise in QDs? Does noise in QDs differ from bulk? Can noise be lowered by optimization of QDs? Noise Spectroscopy in QDs

6/41 The Plan Noise Spectroscopy in QDs MBE: Grow In 0.35 Ga 0.65 As QDs AFM: How does noise depend on morphology? Photoluminescence: How noise depend on Electronic structure? Hall Effect: How does Noise depend on Carrier Conc. & Mobility? Noise Spectroscopy: Noise Measurements & Analysis

7/41 Noise Spectroscopy in QDs Noise Spectroscopy

8/41 Thermal Noise Noise Spectroscopy in QDs Due to random fluctuations of electron motion from lattice vibrations. No Bias S V,thermal

9/41 1/ f noise Noise Spectroscopy in QDs Arises from both carrier and mobility fluctuations in the conductivity. The Hooge Parameter (α) is an indicator of crystal quality. 1/f Bias Added

10/41 Generation-Recombination Noise Noise Spectroscopy in QDs g-r, τ 1 g-r, τ 2 Bias Added

11/41 Total Noise Noise Spectroscopy in QDs

12/41 The Plan Noise Spectroscopy in QDs MBE: Grow In 0.35 Ga 0.65 As QDs

13/41 Noise Spectroscopy in QDs Sample Creation MBE Growth –Solid Source Riber 32 P –RHEED monitoring

14/41 The Structure Noise Spectroscopy in QDs GaAs (001) SI 5000 Å GaAs: Si N D = 6 × Å GaAs: undoped InGaAs QD layer 200 Å GaAs: undoped 1500 Å GaAs: Si N D = 6 × Å GaAs buffer 13S5 11S4 9S3 6S2 0S1 # MLsSample QDs QW Bulk RHEED

15/41 The Plan Noise Spectroscopy in QDs MBE: Grow In 0.35 Ga 0.65 As QDs AFM: How does noise depend on morphology?

16/41 Noise Spectroscopy in QDs Morphology 9 ML Height: 33 ± 2.8 Å Density: 3.8 × cm ML Height: 47 ± 2.8 Å Density: 8.4 × cm ML Height: 53 ± 2.8 Å Density: 7.2 × cm -2

17/41 AFM Trends Noise Spectroscopy in QDs QD Height increases with number of monolayers. QD Density increases with number of monolayers.

18/41 The Plan Noise Spectroscopy in QDs MBE: Grow In 0.35 Ga 0.65 As QDs AFM: How does noise depend on morphology? Photoluminescence: How noise depend on electronic structure?

19/41 Photoluminescence Noise Spectroscopy in QDs Width of energy well is the height of the QD.

20/41 PL Trends Noise Spectroscopy in QDs Peak position decreases in energy with increase in height. FWHM shows increase Integral Intensity shows decrease with increase in height 9 ML 11 ML 13 ML

21/41 The Plan Noise Spectroscopy in QDs MBE: Grow In 0.35 Ga 0.65 As QDs AFM: How does noise depend on morphology? Photoluminescence: How noise depend on electronic structure? Hall Effect: How does Noise depend on carrier conc. & mobility?

22/41 Sample Preparation Noise Spectroscopy in QDs 20 µm Greek Cross 75 nm AuGe 20 nm Ni 270 nm Au

23/41 Noise Spectroscopy in QDs Finished Structures 13 ML # MLsR C (Ω) R S (Ω) All samples meet

24/41 Mobility Noise Spectroscopy in QDs

25/41 Carrier Concentration Noise Spectroscopy in QDs

26/41 The Plan Noise Spectroscopy in QDs MBE: Grow In 0.35 Ga 0.65 As QDs AFM: How does noise depend on morphology? Photoluminescence: How noise depend on electronic structure? Hall Effect: How does Noise depend on carrier conc. & mobility? Noise Spectroscopy: Noise Measurements & Analysis

27/41 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 Spectroscopy in QDs

28/41 Noise Curves Noise Spectroscopy in QDs Series of spectra at fixed temperatures and various biases Fit each specturm with all components of noise Extract fit parameters for component breakdown analysis 0 ML 300 K

29/41 Curve Fittings Noise Spectroscopy in QDs

30/41 Flicker Noise Fit Parameter: Determine the Hooge Parameter at 300 K and 82 K Noise Spectroscopy in QDs 0 ML Sample at 300 K

31/41 Hooge Comparison Noise Spectroscopy in QDs 300 K

32/41 QD Comparisons Noise Spectroscopy in QDs 300 K 9 ML 11 ML 13 ML 9 ML 11 ML 13 ML

33/41 Two Views Noise Spectroscopy in QDs Shoulders Peaks 0 ML

34/41 Bulk to QDs Noise Spectroscopy in QDs

35/41 GR Analysis A different expression: Peaks reveal the activation and ionization energy –ln S max vs ln ω  ionization energy –1/k B T max vs ln ω  activation energy Capture cross section: Trap density: Noise Spectroscopy in QDs

36/41 Analysis Plots Noise Spectroscopy in QDs 11 ML Position of Fermi Energy compared to Trap Position Activation Energy

37/41 Noise Spectroscopy in QDs GR Summary S0S6S9S11S13 Ref6 ML9ML11 ML13 ML defect A S max slope E 1 (eV) E 0 (eV) σ 0 (cm 2 )2.09E E E E E-11 N t (cm -3 )3.68E E E E E+14 defect B S max slope E 1 (eV) σ 0 (cm 2 )2.86E E E E E-12 N t (cm -3 )7.05E E E E E+13 defect C S max slope E 1 (eV) σ 0 (cm 2 )1.95E E E E E-13 N t (cm -3 )4.71E E E E E+12 defect D S max slope E 1 (eV) σ 0 (cm 2 ) 2.67E E E-18 N t (cm -3 ) 9.42E E E+11

38/41 Noise Spectroscopy in QDs Conclusions Sources of Noise in QDs –GR Traps with activation energies of 0.74, 0.49, 0.34, 0.18 and 0.1 eV Noise in QDs does differ from bulk –Flicker noise decreases with the insertion of In 0.35 Ga 0.65 As and the formation of QDs Optimization of QDs to lower noise –Increase in height and density lowers flicker noise –QDs do have an additional trap associated with them

39/41 Noise Spectroscopy in QDs Impact Reveal additional unknown defect associated with QDs QDs leads to lower flicker noise Lateral noise technique is sensitive tool to feel nanoscale objects MRS Fall 08: “Low-frequency noise and lateral transport studies of In 0.35 Ga 0.65 As/GaAs quantum dot heterostructures” Pending Publication: “Noise spectroscopy of deep traps in GaAs/InGaAs heterostructures: transition from quantum well to quantum dots.”

40/41 Noise Spectroscopy in QDs Future Work Study Gated QD samples –Change where current flows to determine which layer noise arises from 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 limitations because of the noise

41/41 Noise Spectroscopy in QDs Thanks Dr. Greg Salamo Dr. Vasyl Kunets Professor Ken Vickers Dr. Bill Brown Dr. Huaxing Fu Rob Sleezer

42/41 Noise Spectroscopy in QDs

43/41 What is a Quantum Dot? Noise Spectroscopy in QDs Single AtomMany 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

44/41 QD Formation Noise Spectroscopy in QDs GaAs substrate 2D InGaAs wetting layer Strain has built up! - I’m very uncomfortable!! When critical thickness is reached, the strain is relaxed and thus 3D islands (quantum dots) are formed. - I’m now happy!! InGaAs fluxes Used with permission of Jihoon Lee

45/41 Atomic Force Microscopy Noise Spectroscopy in QDs Surface data Height Diameter Density

46/41 Noise Spectroscopy in QDs Photoluminescence

47/41 Hall Effect Noise Spectroscopy in QDs Transport Info Mobility Carrier Concentration Hall Coefficient Conductivity Primary Carrier

48/41 Contact Optimization Annealing: minimize barrier to create Ohmic contacts IV Testing: Verify Ohmic contacts made TLM Measurements: determine contact resistance Noise Spectroscopy in QDs AuGe/Ni/Au Dopant lTlT d dx cc cc x 0-l rsrs RsRs RsRs

49/41 Noise Spectroscopy in QDs Hall Measurements –Mobility –Carrier concentration Resistance measurements

50/41 Noise Spectroscopy in QDs Noise Spectroscopy V bias V noise S V is the average change in voltage squared in a bandwidth of 1 Hz.

51/41 Why 1/ f Dependence? Noise Spectroscopy in QDs Sum the noise of all the tail states together. Tail States (Defects)

52/41 Hooge Comparison Noise Spectroscopy in QDs 82 K

53/41 Noise Spectroscopy in QDs Noise Spectroscopy V bias V noise