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Electronic Noise Spectroscopy of InGaAs QDs

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Electronic Noise Spectroscopy of InGaAs QDs

Tim Morgan

Motivation

Noise Theory

Experimental Techniques

Discussion & Results

Conclusions

Noise in QDs 7.23.08

Quantum Dots

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

What is the source of noise in QDs?

Does noise in QDs differ from bulk?

Is there a correlation of noise, defects and QDs?

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Noise Spectroscopy Noise Measurements

Hall Effect Carrier Conc. & Mobility

Photoluminescence Electronic structure

AFM Morphology

MBE Growth In0.35Ga0.65As QDs

Noise Spectroscopy of QDs

Vnoise

Vbias

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

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Due to random fluctuations from Brownian motion of electrons

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Arises from both carrier and mobility fluctuations in the conductivity.

The Hooge Parameter (α) is an indicator of crystal quality.

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Tail States (Defects)

Sum the noise of all the tail states together.

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Show picture with deep trap in compared to tail states

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- 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

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MBE Growth

Solid Source Riber 32 P

RHEED monitoring

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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

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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

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Insert Graphs showing how height, density change with MLs

Noise Spectroscopy of QDs

Width of energy well is the height of the QD.

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Insert Graphs of Peak, FWHM, Integral Intensity vs MLs

Noise Spectroscopy of QDs

270 nm Au

20 nm Ni

75 nm AuGe

20 µm Greek Cross

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13 ML

All samples meet

All samples meet

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Hall Measurements

Mobility

Carrier concentration

Resistance measurements

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- 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

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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

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0 ML Sample at 300 K

- Fit Parameter:
- Determine the Hooge Parameter at 300 K and 82 K

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300 K

82 K

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300 K

82 K

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Shoulders

Peaks

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- 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:

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11 ML

11 ML

Ionization Energy

Activation Energy

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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

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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

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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

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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

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Surface data

- Height
- Diameter
- Density

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- Transport Info
- Mobility
- Carrier
- Concentration
- Hall Coefficient
- Conductivity
- Primary Carrier

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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

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300 K

82 K

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