Transcript Slide 1

PL spectra of Quantum Wells
n=3
n=2
n=1
 n

En 
* 
2m  LZ

2
2

 , n  1, 2, 3...

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
Slide # 1
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
Slide # 2
– The full width at half maximum (FWHM)  also decreases
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
Slide # 3
PL as indicator of material quality
P L in te n sity (a .u .)
• 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
50000
B1098
B1025
B1001
40000
S L b u ffe re d A l .2 G a .8 N : 4 .5  m
30000
20000
S L b u ffe re d A l .2 G a .8 N : 2 .1  m
10000
co n ve n tio n a l A l .2 G a .8 N : 1 .2  m
0
260
280
300
320
340
360
380
400
420
W a ve le n g th (n m )
Slide # 4
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)
Slide # 5