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