AlGaAsGaAs Quantum Wells: Optical Characteristics and Dynamic Nuclear Polarization Phenomena

1
AlGaAs/GaAs Quantum WellsOptical
Characteristics and Dynamic Nuclear Polarization
Phenomena

• Presenter Shu-chen Liu
• University of Florida
• Department of Physics
• December 1st, 2003

2
De Haas-van Alphen Effect
?De Hass-van Alphen Effect can be explained by
the 2-D (or 3-D) electron gas in the external
magnetic field with quantized Landau levels.
3
Landau Levels
4
Two-Dimensional Electron Gas (2DEG) in silicon
MOSFET (Metal Oxide Semiconductor Field Effect
Transistor)
5
Two-Dimensional Electron System (2DES)in
GaAs/AlGaAs Heterostructures
Ionized donors
6
Band Structure of Multiple Quantum Wells (MQW)
7
Integer Quantum Hall Effect
? Discovered by Klaus von Klitzing in 1980
At filling factor ? integer 1, 2, 3, Hall
resistance RH Rxy UH/I Uxy/I h/?e2 ?
plateau (transverse resistance) Magnetoresistance
Rxx Uxx/I ? minima (longitudinal resistance)
( Rxx 0 at T 0 K ) The plateau value at ?
1 h/e2 25.813k? (in 1980)
8
Fractional Quantum Hall Effect
? Discovered by Dan Tsui and Horst Stormer in
1981.
9
Topic II.Optical Properties of Bulk
Semiconductors and Semiconductor Quantum Wells
10
Excitons -- electron-hole pairs
11
Absorption Coefficient v.s. Penetration Depth
Example When the infrared light incident on the
bulk GaAs sample with the wavelength 794 nm
(photon energy 1.566 eV, larger than the band gap
of GaAs 1.52eV at 0K 1.43eV at 300K Kittel,
1996), the penetration depth0.3?m300nm. For
comparison, the band gap of AlAs 2.228eV
(Tlt4.2K) and 2.14eV(T300K) W. Schäfer, Martin
Wegener, 2002, smaller than the photon energy,
so the penetration depth is much larger.
12
Experimental Measurement of Penetration Depth on
InP and GaAs Samples
13
Optical Penetration of GaAs/AlxGa1-xAs MQW
14
Photoluminescence of Bulk GaAs and GaAs/AlGaAs
Quantum Well
Advancements in Optically Detected Nuclear
Magnetic Resonance Applied to Nanoscopic
GaAs/AlGaAs Heterostructures, Dissertation of
Björn Lenzmann, 2001
15
Photoluminescence Spectra of the Ga Interstitial
in AlxGa1-xAs/GaAs Heterostructure

• Four negative resonance lines in 0.8-1.2eV range
  
• Gai signals are negative down to 0.7eV since
  Ga2/Ga3 energy level 0.56eV above acceptor
  level
• Slittings due to hyperfine interaction with Ga69
  and Ga71 of spin 3/2
• Ga point defect could be Ga interstitial and Ga
  antisite, but only Gai has a strong hyperfine
  splitting
• Alloy disorder (local symmetry for interstitial
  sites in AlGaAs)gt weak anisotropy (g//?g?)

16
Topic III.Solid State Effect versus
Overhauser Effect
17
Energy Levels for the Two Spin System
18
(No Transcript)
19
Energy Level Diagram for the Electron and
Nuclear Spin System
Population distribution at thermal equilibrium
20
Rate Equations for Overhauser effect
Slichter 1992
21
Rate Equations for Solid-State effect
Slichter 1992
22
Overhauser Effect(Electron-Nucleus Cross
Relaxation)
23
Solid State Effect (I)
24
Solid State Effect (II)
25
Examples of Overhauser Solid-state effect
26
Distinction between Overhauser Effect and
Solid-State effect

• Overhauser effect
• Occurs in metals, heavily doped samples, or near
  the paramagnetic impurity centers
• Unresolved hyperfine coupling (one single
  electron resonance line)
• Electron-nucleus cross-relaxation forbidden
  transition induced by spin-lattice coupling
• Solid-State effect
• Case I electron line width , the
  frequencies
  inside the electron resonance line, so
  the hyperfine coupling is unresolved.
• Case II , there are homogeneous or
  inhomogeneous broadening of the electron line. In
  a system of like interacting spins, the electron
  line broadening is homogeneous with unpredictable
  results. In a system of narrow spin packets with
  a small range of Larmor frequencies, which can be
  saturated independently, the electron line
  broadening is inhomogeneous. The hyperfine
  coupling is well-resolved, especially when the
  frequencies for maximum and opposite enhancements
  were separated
  by
  (well-resolved hyperfine structure)
  for low doping concentrations, while in heavily
  doped samples, the Overhauser effect dominates
  resulting only one single electron resonance
  line. Or with strong exchange coupling between
  the electronic spins of fixed paramagnetic
  impurity centers, the electron resonance line is
  narrowed and the Overhauser effect is observed.
• Occurs in semiconductors and paramagnetic
  impurities with low donor concentrations
• Electron-nucleus cross-relaxation forbidden
  transition driven by applying r.f. fields

Abragam 1985
27
The distinction between the mechanism of dynamic
nuclear polarization in metals, semiconductors,
and paramagnetic defects where hyperfine
couplings are resolved