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)
Semiconductors • SC resistivity between that of conductors and insulators • Resistivity can be tailored over many orders of magnitude by Doping Insulators Conductors higher doping lower doping Semiconductors ρ ~ ∞ ρ ~1 µ Ω -cm
Semiconductors • SC resistivity between that of conductors and insulators • Resistivity can be tailored over many orders of magnitude by Doping Insulators Conductors higher doping lower doping Semiconductors ρ ~ ∞ ρ ~1 µ Ω -cm Nanostructures AlGaAs ~100nm GaAs • Dimension(s) <100nm ~ 20nm AlGaAs ~100nm
Semiconductors • SC resistivity between that of conductors and insulators • Resistivity can be tailored over many orders of magnitude by Doping Insulators Conductors higher doping lower doping Semiconductors ρ ~ ∞ ρ ~1 µ Ω -cm Nanostructures AlGaAs ~100nm GaAs • Dimension(s) <100nm ~ 20nm AlGaAs Spintronics ~100nm e - • 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
Project Objectives n s =IB/(qV H ) n s = Sheet Density • Characterize electrical properties of I = Current semiconductor structures using the B = Magnetic Field Hall Effect q = Charge V H = Hall Voltage B µ =1/(n s eR s ) V H + I R s = Sheet Resistance n s = Sheet Density V H e = Electron Charge - µ = Mobility • Upgrade the PPMS (Physical Properties Measurement System) to allow van der Pauw measurements • Measure samples grown by MBE
Current Source/DMM System Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat Cryostat Sample Puck LN 2 space LHe space Vacuum space Sample space Magnet + 1 2 I + 1 1 2 2 I Resistivity V Hall 3 4 V H 3 4 - -
Current Source/DMM System Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat Cryostat Sample Puck LN 2 space LHe space Vacuum space Sample space Magnet I 1 2 + 1 2 I Resistivity Hall 3 4 V H 3 4 + V - -
Current Source/DMM System Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat Cryostat Sample Puck LN 2 space LHe space Vacuum space Sample space Magnet - 1 2 I + 1 2 I Resistivity Hall V 3 4 V H 3 4 + -
Current Source/DMM System Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat Cryostat Sample Puck LN 2 space LHe space Vacuum space Sample space Magnet V + - 1 2 + 1 2 I Resistivity Hall 3 4 V H 3 4 I -
Current Source/DMM System Control Computer LabView for instrumentation control and data analysis PPMS Electronics Temperature control Magnetic field control Cryostat Cryostat Sample Puck LN 2 space LHe space Vacuum space Sample space Magnet V + - 1 2 - 1 2 Resistivity Hall V H 3 4 3 I 4 I +
Sheet resistivity, sheet density, mobility Spin lifetime depends strongly on carrier concentration
Sheet resistivity, sheet density, mobility Spin lifetime depends strongly on carrier concentration 3 squares 4 squares • Sheet Resistivity (Ohms/square) I I
Sheet resistivity, sheet density, mobility Spin lifetime depends strongly on carrier concentration 3 squares 4 squares • Sheet Resistivity (Ohms/square) I I • Sheet Density (number/cm 2 ) high doping low doping
Sheet resistivity, sheet density, mobility Spin lifetime depends strongly on carrier concentration 3 squares 4 squares • Sheet Resistivity (Ohms/square) I I • Sheet Density (number/cm 2 ) low mobility high mobility • Mobility (cm 2 /V-s) e- e- -
Calculating R s , n s , µ I-V meas. Calculate R s determine R V 0.0006 + - 0.0004 R A = ( R 12 + R 34 )/2 R B = ( R 13 + R2 4 )/2 1 2 0.0002 Voltage (V) exp(- π R A / R S ) + exp(- π R B / R S ) = 1 0.0000 R = V/I -0.0002 3 4 I -0.0004 Calculate R s -0.0006 1 2 3 4 5 6 Longitudinal Current (mA)
Calculating R s , n s , µ I-V meas. Calculate R s determine R V 0.0006 + - 0.0004 R A = ( R 12 + R 34 )/2 R B = ( R 13 + R2 4 )/2 1 2 0.0002 Voltage (V) exp(- π R A / R S ) + exp(- π R B / R S ) = 1 0.0000 R = V/I -0.0002 3 4 I -0.0004 Calculate R s -0.0006 1 2 3 4 5 6 Longitudinal Current (mA) Hall measurement 0.10 0.08 n s = (I/q)*(B/V H ) 0.06 Hall Voltage (Volts) 0.04 1 2 = (I/q)*(1/slope) 0.02 - 0.00 slope -0.02 µ =1/(n s eR s ) -0.04 V H -0.06 -0.08 3 I 4 -0.10 + -4000 -2000 0 2000 4000 Transverse Magnetic Field (Oe)
Sheet resistivity, density, mobility vs. temperature AlGaAs AlGaAs GaAs “multi-2DEG sample” CB VB 120000 -1.25E+012 100000 Sheet Density (cm^-2) -1.30E+012 Mobility (cm^2/v-s) 80000 -1.35E+012 60000 -1.40E+012 40000 -1.45E+012 20000 0 -1.50E+012 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Temperature (K) Temperature (K) • Mobility • Sheet Density Strong function of impurities Generally increases with temp. and temperature (phonons)
Remaining Tasks • Sample puck modifications Faster/Easier 1/f • AC/Lockin measurement I Alternating current More data Signal/Noise V V H • Measure magnetic samples Anomalous Hall Effect B app “hysteresis”
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