PL spectra of Quantum Wells

1
PL spectra of Quantum Wells
n 3
n 2
n 1
n 1
n 2
n 3

• The e1-h1 transition is most probable and
  observed with highest intensity
• At higher temperature higher levels can be
  populated, and e2-h2 transition can also be seen

2
PL spectrum for non-polar QW

• With increasing well width
• The intensity increases due to increased
  confinement
• The peak position shifts to lower energy due to
  reduction in quantum size effect (QSE) i.e.
  splitting of energy levels in a QW
• The full width at half maximum (FWHM) ? also
  decreases

3
Nitride QW PL spectrum
Electron and hole wave-functions for non-polar
material
Electron and hole wave-functions for polar
material due to built-in electric field

• With increase in well thickness
• Intensity decreases due to reduced overlap due to
  quantum confined stark effect (QCSE)
• Energy decreases due to quantum size effect
  (QSE), and by lowering of energy gap between the
  energy states

4
PL as indicator of material quality

• Better quality of epilayer means higher intensity
  and narrower FWHM
• Also true for quantum wells where the interface
  fluctuations controls the FWHM of PL peaks
• AlGaN epilayers grown on superlattice (SL)
  buffered GaN layers produces the best quality

5
Summary of Photoluminescence

• Information on bandgap and hence material
  composition (peak position). Direct or indirect
  bandgap (from intensity)
• Information on dopant density and their energy
  levels (FWHM and peak position)
• Information on the quality of material, both
  substrate and epitaxial layers (poor quality
  material has more states giving rise to
  non-radiative recombination, or radiative
  recombination at a different wavelength)
• Information on material properties such as phonon
  energies, effective mass, and dielectric constant
  (from spectra of the hydrogenic model of the
  impurities)
• Information on the energy levels of quantum
  wells, their interface roughness, alloy disorder,
  and built-in electric field (from the peak
  position, FWHM, and the variation of lineshape
  with width of the QWs)