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Spin-flip Raman scattering of sub-monolayer CdSe

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Title: Spin-flip Raman scattering of sub-monolayer CdSe


1
Spin-flip Raman scattering ofsub-monolayer CdSe
  • Daniel Wolverson
  • Department of Physics
  • University of Bath

2
Team and collaborators
  • Oleg Karimov (CdSe)
  • Ivan Griffin (AlGaInP)
  • Catherine Orange (ZnSe)
  • J. John Davies
  • Stephen Bingham
  • AF Ioffe Institute
  • AN Reznitsky, AA Klochikin, SY Verbin, LN
    Tenishev, SA Permogorov, SV Ivanov, EL Ivchenko
  • MPI-FKF Stuttgart
  • T Ruf, M Cardona
  • U. Bremen
  • U. Wuerzburg
  • U. Heriot-Watt
  • EPI Ltd
  • Sharp Ltd

3
Contents
  • Introduction to spin-flip Raman scattering
  • Zn1-xCdxSe epilayers
  • CdSe fractional monolayers
  • Related behaviour of other systems

4
Raman scattering
  • Raman scattering is inelastic light scattering
  • Usually, the scattering (creation or
    annihilation) of light by phonons is implied
  • The processes illustrated on the left are
    resonant their cross-section is very large when
    the excitation energy equals an excitonic
    transition energy

5
PL of cubic Zn1-xCdxSe
6
SFRS of Zn1-xCdxSe
7
Measured electron g-factorsfor Zn1-xCdxSe
8
kp model and parameters
9
Calculation of ge for Zn1-xCdxSe
10
CdSe fractional monolayers
  • Large lattice mismatch between CdSe, ZnSe
  • CdSe cubic on ZnSe (normally wurtzite)
  • Self-organised dots form at around 3 monolayers
  • Growth of 0.5 ML of CdSe on ZnSe gives islands
    probably of Zn1-xCdxSe
  • Transition energies close to those of ZnSe
  • Exciton localised at island rather than confined

11
M Strassburg et al., APL 72 (1998) 942
12
Photoluminescence of CdSe FMs
  • Large blue shift compared to CdSe
  • Large lh-hh splitting of around 30meV
  • Broad PL band with long low-energy tail
  • Stokes shift of PL band of around 10meV from hh

13
SFRS of CdSe FMs
  • Stokes and anti-Stokes signals present
  • Faraday geometry
  • Selection rules obeyed
  • Excitation in resonance with PL band shown
    previously

14
Varying the direction of B
  • Several new signals appear
  • Raman shifts vary with angle B, z
  • Anisotropy suggests valence band states are
    involved and thus excitonic levels

15
B in-plane
  • Faraday geometry
  • Two Raman signals
  • Signal DE depends on excitation photon energy
  • Signal DE shows both Stokes and anti-Stokes
    lines.

16
Resonance behaviour ofFM signals
  • solid line PL
  • triangles PLE
  • open circles E
  • solid circles DE

17
Varying the magnitude of B
  • DE signal not linearly dependent on magnetic
    field
  • Non-zero splitting d0 at zero magnetic field
  • Splitting d0 is a function of excitation photon
    energy

18
Excitons localised at CdSe FMs
  • Should consider effects of
  • strain and confinement
  • electron and hole Zeeman splitting (linear in B,
    J)
  • electron-hole exchange (linear in J)
  • crystal field, terms cubic in J, higher powers of
    B
  • PL shows that 1. is required
  • B field dependence shows that 2. and 3. are
    required 3. gives splitting at zero field
  • No evidence for need for 4.

19
Simplest model
  • neglect terms in J3
  • electron gc isotropic

20
Exciton spin and zero-field splittings
In zincblende Td symmetry
  • Optical emission from the exciton spin state F2
    is forbidden, hence dark exciton, DE.
  • Electron-hole exchange splits the F1 (G5) and
    F2 (G3,G4) states (even at zero magnetic field).
  • Strain splits the lh and hh components of these.
  • Crystal field could also split G3 and G4.

21
Comparison of model and data
  • 4 hh energy levels (given the large lh-hh
    splitting) two F2 (DE), two F1
  • At least 4 of 6 possible transitions are observed
  • These are described by the model with one set of
    parameters for all angles between B, z and all
    values of B.

22
Exchange and localisation
  • Large exchange energy compared to bulk binary
    materials (around 10)
  • Linear variation of exchange splitting with
    localisation energy
  • SFRS studies of alloys show similar effects

23
Alloy SFRS
  • p-type doped (Ga,In)P alloys show signals
    identified as electron (e) and hole (h) SFRS.
  • Hole signal becomes anisotropic when a biaxial
    strain is applied.
  • Calculated electron g-factor (3-band kp) agrees
    well with experiment.

24
Resonance of SFRS signals
  • High-energy side of high-energy PL band
  • Similar for electron and hole but hole at
    slightly lower energy
  • No signals observed with excitation at low-energy
    PL band

25
Effects of increasing Al content
  • Increasing Al content increases disorder and,
    therefore, potential fluctuations
  • New signals develop (in addition to electron and
    hole-like signals)
  • New signals show zero-field splittings

26
Exciton localisation in alloys
  • (Al,Ga)0.5In0.5P and also (Zn,Mg)(S,Se) alloys
    studied by SFRS
  • Electron and hole signals identified in doped
    layers
  • Exciton signals seen in undoped layers
  • Exchange splitting present
  • Exchange again correlated with excitation energy

27
Differences between alloys FMs
  • For (Al,Ga)0.5In0.5P
  • Lattice-matched to substrate, therefore very
    small (or zero) strain
  • Must include lh and hh.
  • Potential fluctuations are due only to alloy
    disorder
  • Exciton radius is larger than in the II-VI
    selenides
  • Exchange splitting smaller

28
Summary
  • Spin-flip Raman scattering is a versatile
    technique for the investigation of excitonic fine
    structure
  • Excitonic localisation occurs in a wide variety
    of systems and leads to similar SFRS spectra
    (III-V and II-VI alloys and FM structures)
  • Fractional monolayer structures are model systems
    for the study of exciton localisation with the
    useful features of (i) a large lh-hh splitting
    and (ii) a significantly enhanced exchange energy
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