Nanocrystalline silicon as multifunctional material for optoelectronic and photovoltaic applications

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Nanocrystalline silicon as multifunctional
material for optoelectronic and photovoltaic
applications

• S. Pizzini, M. Acciarri, S. Binetti, D.
  Cavalcoli, A. Cavallini, D. Chrastina, L.
  Colombo, E. Grilli, G. Isella, M. Lancin, A. Le
  Donne, A. Mattoni, K. Peter, B. Pichaud, E.
  Poliani, M. Rossi, S. Sanguinetti, M. Texier, H.
  von Känel

e-MRS/IUMRS 2006 Spring Meeting
 May 29-June 02, 2006, Nice, France  
Symposium V ADVANCED SILICON FOR THE
21st CENTURY
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OUTLINE

• Introduction aims why nc-Si as
  multifunctional material? promises, challenges
  and problems
• growth procedures why the LEPECVD?
• experimental characterization techniques
• preliminary modeling results and comparison with
  experimental results
• conclusions

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INTRODUCTION

• nc-Si is a two phase material, consisting of Si
  nanocrystals embedded in an (higher energy gap)
  a-Si matrix (quantum confining and energy gap
  tuning potentialities)
• already proven to be a good candidate for PV
  applications, but its use in optoelectronic
  demands further knowledge acquisition
• tuning the electronic properties is a difficult
  task, due to the number of growth parameters to
  be controlled and driven to get
• a) the desired nanomorphology (crystal size)
• b) and, thus, the optimized conversion
  efficiency of light in carriers and carriers in
  light

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AIMS

• the development of models to be used for the
  simulations of the Low Energy Plasma Enhanced
  Chemical Deposition (LEPECVD) process,
• the growth of undoped and doped nc-Si layers on
  convenient substrates by the LEPECVD process
• the quantitative experimental determination of
  the correlation between the crystallinity
  fraction, the film microstructure, the grain
  size/shape/orientation, the hydrogen content, the
  density of the recombination centres, the optical
  absorption coefficient the strain/stress state
  and the carrier mobility and diffusion length in
  undoped and n-type and p-type films, in view of
  the optimization of the minority carrier
  generation and carrier collection.

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Why LEPECVD?
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LEPECVD advantages among other PECVD methods

• Intense, but low energy, ion bombardment of the
  surface during growth limited surface damage
• high growth rates already demonstrated for nc-Si
  (gt 1 nm/s)
• an expected effective dissociation of the silane
  molecules into reactive radicals
• an expected enhanced desorption of hydrogen by
  the low energy plasma.

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Characterization techniques

• TEM, HR-TEM, AFM (in contact and non-contact
  mode) and SEM for microstructure and topography
• XRD for grain size, crystal preferential
  orientation and local strain determination
• Raman spectroscopy for crystallinity
• FTIR for hydrogen content
• Resistance Microscopy (SSRM) or Conductive AFM
  (c-AFM)
• Photoluminescence measurements

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Typical features of the samples studied so far

• Deposition temperatures from 210C to 280 C
• (100) Cz-Si and oxidized (100) Cz Si substrates
• Silane dilution d( ? SiH4/ (? SiH4 ? H2) from
  1 to 50

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MODELING STUDIES

• The nucleation of nc-Si in a matrix of a-Si is
  modeled by inserting strings (nuclei) of
  crystalline silicon oriented along lt100gt in the
  matrix and then performing an accelerated crystal
  growth test by heating for 2 ns at 1200K.
• A MD code is used and atomic forces are
  calculated with EDIP potentials.

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Modeling the nucleation process
Sample cell size x16.2 nm, y 14.1 nm z axis
corresponds to the lt100gt orientation
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Comparison between model and experiments
Sample cell size x16.2 nm, y 14.1 nm
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comments

• The resulting material is polycrystalline with
  the residual a-Si localized at GBs, in good
  agreement with experimental results

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EXPERIMENTAL RESULTSSTRUCTURE AND DEFECTS
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Section of the sample 6956 T230C, d
4.2
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Section of the sample 56172 T280C, d 3 a
domain structure is evident
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Domains and disorder (a-Si?) at domain
interfaces TEM plane view
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HR-TEM image of an individual nanograin within a
domain (plane view HR-TEM image using a JEOL
2010F Field Emission TEM)
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(No Transcript)
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Sample 6733 AFM topography 3D-image shows
surface roughness
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comments

• In the range of d up to 20 the films are
  columnar in the entire T deposition range the
  section of the columns remains ? constant along
  the growth axis
• a-Si localized at the domain interfaces
• surface roughness of the order of 10-20 nm

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Defects in nc-Si
Stacking faults in nc-silicon grains from HR-TEM
(plane view HR-TEM image) using a JEOL 2010F
Field Emission TEM
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Nanometric grains in domains interface disorder
(plane view HR-TEM image) using a JEOL 2010F
Field Emission TEM
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Grain boundaries inside the
domains
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lt111gt planes (d 3.1354 Å) orthogonal to the
surface populate the domain
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56172 Twinned grain, domain- large
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56172 Twin defects
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XRD Studies results

• On Si/SiO2 substrates (Tdep 280C)
• Llt111gt( 20.7 5.5 nm) Llt220gt(24.17.4nm)
  Llt311gt(18.8 3nm)
• On Si substrates (Tdep 210C)
• Llt111gt( 20.7 2.2 nm) Llt220gt(14.14.7 nm)
  Llt311gt(16.5 3.6 nm)

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comments

• The nc-Si films present a typical two-level local
  organization, with relatively large domains
  (?10-12 nm) presenting inter-domain disorder and
  nanometric grains inside (size around 3 nm).
• Some domains consist of twinned grains
• Nanometric grains might present interface
  disorder and highly defective regions.
• From TEM the (111) orientation preferred but
  non systematic
• From XRD elongated grains along (220) on
  Si/SiO2 and along (111) on Si substrates
• XRD data are consistent with the presence of
  domain-large twinned grains or of mosaic
  structures

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EXPERIMENTAL RESULTSfilm crystallinity patterns
by Raman spectroscopy

• C. Smit, R. van Swaaij, H. Donker, A. Petit, W.
  Kessels, M. van de Sanden, J.Appl.Phys. 94 (2003)
  358

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Crystallinity map for the sample
7653 (d10)
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Crystallinity map for the sample 7666 (d20)
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Crystallinity map for the sample 7664 (d30)
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Crystallinity of the samples grown at 280C
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comments

• Films prepared in the 210-280 C range present
  high crystallinity features (?cgt 70 )
• crystallinity decreases with the decrease of
  dilution (increase of d)
• good crystallinity uniformity of the samples up
  to d values of 20 atomic hydrogen
  preferentially etches silicon with unsaturated
  bonds, leaving nuclei of crystalline silicon
  (R.E. Hollingsworth APL 64(1994) 616)
• large deviations from uniformity at dgt 20 due to
  plasma non- uniformity nucleation processes are
  enhanced at the centre of the sample
• In this range a careful control of the process
  parameter is needed

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EXPERIMENTAL RESULTSELECTRONIC PROPERTIES
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c-AFM map of the sample 7365 (d4.2 , Tdep
210C) percolation of carriers at the perifery
of the domains
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SPS spectrum of crystalline
silicon
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Taucs plot for amorphous silicon
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SPS spectrum at RT of the nc-sample 6956
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Normalized PL spectra in the IR region (T 14
K) the emission is absent in the five digits
samples.
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Normalized PL spectra in the visible region (T
14 K) the intensity and the blue shift of the
band at ?1.3 eV increases with d (with the
increase of a-Si content)
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comments

• On undoped samples 3D percolation is the dominant
  carrier migration process
• nc-Si films present (feeble) emissions in both
  the IR and visible range
• SPS measurements show a systematic blue shift of
  the optical gap towards 1.5 eV
• PL measurements show a systematic blue shift of
  the 1.3 eV emission with the increase of the a-Si
  content band tails are responsible of the
  difference between the optical gap from SPS and
  PL band to band emissionemission
• sub-gap defect levels and band tails are
  detected both by SPS and PL

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CONCLUSIONS structure

• The growth process of nc-Si films in the range of
  low values of d was succesfully modelled
• The effect of silane dilution on crystallinity
  was evidentiated
• A domain structure is typical of our nc-Si films,
  sub- domain grains have a size of 3 nm
• Domains are the envelope of multiple grains or
  consist of a single twinned grain
• The domain interface consists of a-Si
• Grains present several structural defects (SF,
  grain boundaries

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CONCLUSIONS optoelectronic properties

• The blue shift of the optical gap and PL emission
  shows some quantum confining effect

The relatively low intensity of the emission is
presumibly associated to the low density of
active grains and to multiple defects. Structure
improvements are needed
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Acknowledgments

• This work was entirely granted by the European
  Commission (Contract 013944)
• the colleagues M.Guzzi, L.Miglio and C.Cavallotti
  are warmly acknowledged for their continous
  support.