Electrical Characterization of Semiconductor Nanostructures for - - PowerPoint PPT Presentation

Electrical Characterization of Semiconductor Nanostructures for Spintronics Applications

Intern : Jan Rehorik Major : Computer Engineering Mentor : Jason Stephens Faculty Advisor : David Awschalom

Funded By : Defense Advanced Research Projects Agency (DARPA)

• SC resistivity between that of conductors and insulators
• Resistivity can be tailored over many orders of magnitude by Doping

Semiconductors

Conductors ρ~1 µΩ-cm

Insulators ρ~∞

higher doping lower doping

Semiconductors

• SC resistivity between that of conductors and insulators
• Resistivity can be tailored over many orders of magnitude by Doping

Semiconductors Nanostructures

GaAs AlGaAs AlGaAs

~100nm ~100nm ~ 20nm

• Dimension(s) <100nm

Conductors ρ~1 µΩ-cm

Insulators ρ~∞

higher doping lower doping

Semiconductors

• SC resistivity between that of conductors and insulators
• Resistivity can be tailored over many orders of magnitude by Doping

Semiconductors Nanostructures Spintronics

GaAs AlGaAs AlGaAs

~100nm ~100nm ~ 20nm

• Dimension(s) <100nm
• Concerned with the generation, manipulation, and detection of spin

polarization

• Technological example: HD read heads- “Spin Valve”
• Semiconductor Spintronics : No real world devices yet

How spin behaves in semiconductor material is currently being studied Conductors ρ~1 µΩ-cm

Insulators ρ~∞

higher doping lower doping

Semiconductors e-

Project Objectives

• Characterize electrical properties of

semiconductor structures using the Hall Effect

• Upgrade the PPMS (Physical Properties Measurement System)

to allow van der Pauw measurements

• Measure samples grown by MBE

I

Rs = Sheet Resistance

e = Electron Charge

µ = Mobility

ns=IB/(qVH) ns = Sheet Density

I = Current B = Magnetic Field q = Charge

VH = Hall Voltage

VH + VH

• B

ns = Sheet Density

µ=1/(nseRs)

VH +

• I

Hall

1 2

I +

• V

Resistivity

1 2 1 2 3 4 3 4

System Cryostat

Current Source/DMM Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat LN2 space LHe space Vacuum space Sample space Magnet

Sample Puck

I +

• V

VH +

• I

Resistivity Hall

1 2 3 4 3 4

System Cryostat

Current Source/DMM Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat LN2 space LHe space Vacuum space Sample space Magnet

Sample Puck

1 2

I +

• V

VH +

• I

Resistivity Hall

1 2 3 4 3 4

System Cryostat

Current Source/DMM Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat LN2 space LHe space Vacuum space Sample space Magnet

Sample Puck

1 2

I +

• V

VH +

• I

Resistivity Hall

1 2 3 4 3 4

System Cryostat

Current Source/DMM Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat LN2 space LHe space Vacuum space Sample space Magnet

Sample Puck

1 2

I +

• V

VH + I

Resistivity Hall

2 1 3 3 4 4

System Cryostat

Current Source/DMM Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat LN2 space LHe space Vacuum space Sample space Magnet

Sample Puck

1 2

Sheet resistivity, sheet density, mobility Spin lifetime depends strongly

• n carrier concentration

Sheet resistivity, sheet density, mobility Spin lifetime depends strongly

• n carrier concentration
• Sheet Resistivity (Ohms/square)

I I

3 squares 4 squares

Sheet resistivity, sheet density, mobility Spin lifetime depends strongly

• n carrier concentration
• Sheet Density (number/cm2)
• Sheet Resistivity (Ohms/square)

I I

3 squares 4 squares low doping high doping

• Mobility (cm2/V-s)

Spin lifetime depends strongly

• n carrier concentration
• Sheet Density (number/cm2)
• Sheet Resistivity (Ohms/square)

I I

3 squares 4 squares high mobility low mobility

e- e-

Sheet resistivity, sheet density, mobility

1 2 3 4 5 6

• 0.0006
• 0.0004
• 0.0002

0.0000 0.0002 0.0004 0.0006

Voltage (V) Current (mA)

I-V meas.

Calculating Rs, ns, µ

R = V/I

determine R Calculate Rs

I +

• V

2 1 3 4 Longitudinal exp(-π RA/RS) + exp(-π RB/RS) = 1 RA = (R12 + R34)/2 RB = (R13 + R24)/2 Calculate Rs

• 4000
• 2000

2000 4000

• 0.10
• 0.08
• 0.06
• 0.04
• 0.02

0.00 0.02 0.04 0.06 0.08 0.10

Hall Voltage (Volts)

Magnetic Field (Oe)

1 2 3 4 5 6

• 0.0006
• 0.0004
• 0.0002

0.0000 0.0002 0.0004 0.0006

Voltage (V) Current (mA)

I-V meas.

Calculating Rs, ns, µ

R = V/I

determine R Calculate Rs

Hall measurement ns = (I/q)*(B/VH)

I +

• V

2 1 3 4

VH +

• I

1 2 3 4 Longitudinal Transverse

µ=1/(nseRs)

exp(-π RA/RS) + exp(-π RB/RS) = 1 RA = (R12 + R34)/2 RB = (R13 + R24)/2 Calculate Rs

= (I/q)*(1/slope) slope

Sheet resistivity, density, mobility vs. temperature

• Mobility

Strong function of impurities and temperature (phonons)

50 100 150 200 250 300

• 1.50E+012
• 1.45E+012
• 1.40E+012
• 1.35E+012
• 1.30E+012
• 1.25E+012

Sheet Density (cm^-2) Temperature (K)

50 100 150 200 250 300 20000 40000 60000 80000 100000 120000

Mobility (cm^2/v-s) Temperature (K)

• Sheet Density

Generally increases with temp.

“multi-2DEG sample”

AlGaAs AlGaAs GaAs

CB VB

Remaining Tasks

• Sample puck modifications

Faster/Easier

I V

1/f

• AC/Lockin measurement

Alternating current More data Signal/Noise

• Measure magnetic samples

Anomalous Hall Effect “hysteresis” VH Bapp