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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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Presentation Transcript
outline
Outline

Motivation

Noise Theory

Experimental Techniques

Discussion & Results

Conclusions

Noise in QDs 7.23.08

quantum dot devices

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

the nature of noise in qds
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

the plan
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

noise spectroscopy
Noise Spectroscopy

Vnoise

Vbias

SV is the average change in voltage squared in a bandwidth of 1 Hz.

Noise in QDs 7.23.08

thermal noise
Thermal Noise

Due to random fluctuations from Brownian motion of electrons

Noise in QDs 7.23.08

flicker noise
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

why 1 f dependence
Why 1/f Dependence?

Tail States (Defects)

Sum the noise of all the tail states together.

Noise in QDs 7.23.08

generation recombination noise
Generation-Recombination Noise

Show picture with deep trap in compared to tail states

Noise in QDs 7.23.08

total noise
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

sample creation
Sample Creation

MBE Growth

Solid Source Riber 32 P

RHEED monitoring

Noise in QDs 7.23.08

the structure
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

morphology
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

afm trends
AFM Trends

Insert Graphs showing how height, density change with MLs

Noise Spectroscopy of QDs

slide16
PL

Width of energy well is the height of the QD.

Noise in QDs 7.23.08

pl trends
PL Trends

Insert Graphs of Peak, FWHM, Integral Intensity vs MLs

Noise Spectroscopy of QDs

sample preparation
Sample Preparation

270 nm Au

20 nm Ni

75 nm AuGe

20 µm Greek Cross

Noise in QDs 7.23.08

finished structures
Finished Structures

13 ML

All samples meet

All samples meet

Noise in QDs 7.23.08

hall measurements
Hall Measurements

Hall Measurements

Mobility

Carrier concentration

Resistance measurements

Noise in QDs 7.23.08

mobility
Mobility

Noise in QDs 7.23.08

carrier concentration
Carrier Concentration

Noise in QDs 7.23.08

dlns setup experiments
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

noise curves
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

flicker noise1
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

qd height comparison
QD Height Comparison

300 K

82 K

Noise in QDs 7.23.08

qd density comparison
QD Density Comparison

300 K

82 K

Noise in QDs 7.23.08

two views
Two Views

Shoulders

Peaks

Noise in QDs 7.23.08

gr analysis
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

analysis plots
Analysis Plots

11 ML

11 ML

Ionization Energy

Activation Energy

Noise in QDs 7.23.08

gr summary
GR Summary

Noise in QDs 7.23.08

conclusions
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

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

Noise in QDs 7.23.08

what is a quantum dot
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

qd formation
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

atomic force microscopy
Atomic Force Microscopy

Surface data

  • Height
  • Diameter
  • Density

Noise in QDs 7.23.08

photoluminescence
Photoluminescence

Noise in QDs 7.23.08

hall effect
Hall Effect
  • Transport Info
  • Mobility
  • Carrier
  • Concentration
  • Hall Coefficient
  • Conductivity
  • Primary Carrier

Noise in QDs 7.23.08

contact optimization
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

hooge comparison
Hooge Comparison

300 K

82 K

Noise in QDs 7.23.08

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