Determination of the dislocation density by mphotoluminescence

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Determination of the dislocation density by
m-photoluminescence

• Ramòn Schifano

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The topics of the presentation

• The heteroepitaxial growth and the dislocation
  formation
• Dislocation effects
• Photoluminescence principles
• The m-PL set-up
• Some examples about the correlation of micro-PL
  measurements with the morphology for GaN
• Conclusions

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Why heteroepitaxy ?

• Heteroepitaxy is the growth of one semiconductor
• on another semiconductor
• Motivations of heteroepitaxy
• - Absence of native substrates
  (Ex GaN )
• - Growth of active layers on
  cheaper substrates (Ex GaAs on Si)
• - Realization of heterostructures
  (i.e. band- gap engineering quantum wells,
  quantum dots, HEMTs, .)

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The three main growth mechanism

• In the F-M case the deposited atoms are bound to
  the sub-strate more than to each other,
  pseudomorphic growth
• In the V-W case is the binding in between the
  deposited atoms to prevail, nucleation growth
• The S-K case occurs in the intermediate region

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Pseudomorphic vs. strain relaxed

• In the case of Frank-van der Merwe growth the
  epilayer undergoes through
• (a) a pseudomorphic growth for layers thick
  no more
• than the critical thickness ( )
• (b) followed by a strain relaxed growth with
  creation of misfit dislocation at the interface

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The two energetic contributions

• The strain energy per unit surface is
  an e-
• lastic energy i.e.
• where is the lattice mismatch
• While the dislocation
• energy per unit surface is

• As the thickness is increasing it becomes
  energetically more favorable the creation of
  misfit dislocations generally associated with
  threading dislocations i.e dislocations that
  propagate towards the active region of the device

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The nucleation growth modes

• In the case of growth that occurs through
  nucleation the coalescence of the islands can
  result in the formation of additional
  dislocations which treads along the epilayer i.e
  the active region of the device
• So in the case of large misfit epi-growth, say
  above 5, a high density of threading
  dislocation is ex-pected
• This is the case of the growth of GaN on Sapphire
• ( ) where due to the high mismatch
  15
• misfit dislocations form during islands
  growth

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The effects of dislocations

• Dislocations generally
• - tend to propagate through layers
• -act as a non-radiative recombi-nation
  center or they emit at an e-nergy significantly
  lower than
• the band edge emission
• -limit the carrier mobility due to
• charging
• -act as gettering centres of im-purities

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An example of dislocations in GaN on Sapphire by
HVPE

• Fig. (a) is a cross sec-
• tional TEM image of a
• GaN crystal
• A fraction of dislocations
• propagate from the sub-
• strate to the surface in a
• non perpendicular
• way

() Kyoyeol Lee et al. MRS, 6, 9 (2001)

• TEM analysis cannot by used in the case of a low
• dislocation density

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Basic photoluminescence set-up

• The sample ex-
• cited by the laser
• beam emits lumi
• nescence (PL)
• The PL signal
• is spectrally re-
• solved by a mo-
• nochromator and
• detected

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The monochromator

• The PL signal is dispersed by a grating

• The PL signal is focused on the entrance slit and
  
• reflected by a collimating mirror on the
  grating
• The wavelenght that satisfies the interference
  con-
• dition is collected at the exit slit

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Optical transitions (1)

• Direct band gap vertical transfer of the
  electron (1st order transition)
• Indirect band gap oblique transfer of the
  electron (2nd order transition) a phonon is
  emitted or absorbed

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Optical transitions (2)

• The electron and hole pair created interact so
  in many cases bound states are formed i.e.
  excitons
• According to the mean electron hole
  distance the excitons are classified as
• Wannier excitons
• Frenkel excitons
• Wannier excitons can be de-scribed as an
  hydrogen like system with energy

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Optical transitions (3)

• (1) creation of an electron hole pair
• (2) thermalization of the carriers generated
  in their respective bands and cre-
• ation of an exciton with energy
• (3) recombination of the
• exciton and emission of a phonon with energy
• (4) in addition there can
• be recombinations via lo-

calized centers
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An example of a PL spectra in the case GaN

• An example of a PL spectra in the case of GaN
  (0.3 meV)
• Informations obtained
• 1) energies and charac-
• teristics of the levels
• involved
• 2) energies of the phon-
• ons
• 3) overall crystal quality
• 4) presence of impurities

() M. Mayer et al. Jpn. J. Appl. Phys. 36,
L1634 (1997)
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Beyond the PL

• The linear dimensions of the excited spot in a
  typical PL setup are 100mm and typical final
  powers
• are in the 0.1 -1W range in the case of UV
  excitation
• By adding a microscope objective and correcting
  the aberrations introduced by the cryostat
  windows the spatial resolution can be
  reduced to about the diffraction limit i.e
  l/2
• In addition the evaluation of the effective
  spatial resolution has to take into
  account the carriers diffusion of the
  generated carriers (in high quality Si can be up
  to 100mm) more typically 1mm
• A higher excitation intensity is available in
  this case

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The m-PL setup

• A complete spec-trum can be re-corded at each
  point
• Possible infor-mation to extract
• 1) local spectra
• 2) mapping the
• peak wave-
• lenght
• 3) mapping of the

• intensity of a wavelenght window
• Correlation of morphological and optical
  properties

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An example of dislocations in GaN on Sapphire by
HVPE

• Fig. (a) is a cross sec-
• tional TEM image of a
• GaN crystal
• A fraction of dislocations
• propagate from the sub-
• strate to the surface in a
• non perpendicular
• way

() Kyoyeol Lee et al. MRS, 6, 9 (2001)

• TEM analysis cannot by used in the case of a low
• dislocation density

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Examples of room temperature scanning m-PL on a
GaN surface
a) 300 mm epilayer
b) 500 mm epilayer
c) Dis. density vs. thickness

• In fig (a) the density of dislocations is still
  too high for
• being resolved . In fig (b) the spacing is
  higher than the
• spatial resolution (density
  in (b) )
• In fig. (c) the dislocation density vs. the
  epilayer thick-
• ness is reported

() Kyoyeol Lee et al. MRS, 6, 9 (2001)
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Another example of GaN surface at low temperature
(1)

• Fig. (a) is a mapping of
• a near band edge exciton
• line ( )
• Fig. (b) the position de-
• pendence of the linewidth
• of the same line
• Fig. (c) and (d) energy
• shift of the peak intensity
• that in unstrained condi-
• tions is centered at

N. Gmeinwieser et al. J. Appl. Phys. 98, 116102
(2005)
3.47327 eV
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Another example of GaN surface at low temperature
(2)

• A dipole shift of the energy peak is observed
  around each dislocation
• (expected shape for an edge dislocation)
• The energy shift is ca-used by the tensile and
  compressive strain around the dislocation
• where the tensile strain cause a downward
  shift
• of the peak energy

N. Gmeinwieser et al. J. Appl. Phys. 98, 116102
(2005)
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Conclusions

• PL and m-PL characterization relies on the
  creation of electron-hole pairs in semiconductors
  or quantum stru-ctures and their radiative
  recombination.
• Advantages non-destructive, straightforward
  and contactless. In addition to its traditional
  use (i.e. accurate determination of impurities
  energy levels and in some cases quantitative
  determination of the dopant concentra-tions) it
  can be used for study the surface morphology or
  material stress state
• Spatial mapping can be accomplished with an
  maximun spatial resolution of 1mm
• Disadvantages depends on the radiative
  recombination of carriers, in general low
  temperature is needed

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Thanks for your attention