STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF HELIUM DILUTED aSi1xCx:H FILMS DEPOSITED BY PECVD

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STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF
HELIUM DILUTED a-Si1-xCxH FILMS DEPOSITED BY
PECVD
ICANS 21 (Lisbon 2005)
STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF
HELIUM DILUTED a-Si1-xCxH FILMS DEPOSITED BY
PECVD
STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF
HELIUM DILUTED a-Si1-xCxH FILMS DEPOSITED BY
PECVD
M.Loulou 1, R. Gharbi 1, M. Fathallah 1, G.
Ambrosone2, U. Coscia2, G. Abbate2, A. Marino2,
S. Ferrero3 and E. Tresso 3
1 Ecole Supérieure des Sciences et Techniques de
Tunis, 5 avenue Taha Hussein , 1008 Tunis,
Tunisia. Mohamed.Fathallah _at_ esstt.rnu.tn
2 INFM-Coherentia and Dipartimento di Scienze
Fisiche. Universita di Napoli Federico II,via
Cintia, 80126, Napoli, Italy.
3 Dipartimento di Fisica ed unita Istituto
Nazionale per la Fisica della Materia.
Politecnico di Torino, Corso Dica degli Abruzzi
24, 10129 Torino, Italy.

• Aim Systematic study of structural, optical
  and electrical properties of Helium diluted
  a-Si1-xCxH
• films deposited by PECVD for x 0
  - 0.28.
• Motivation Device qualities a-Si1-xCxH has
  been produced only at low carbon fraction.

Experimental conditions and results
Optical absorption
Spectroscopic ellipsometry
For gt 104 cm-1, The curves are nearly
parallel and shift towards higher energy as the
carbon content increases.
The Urbach energy increases from 73 meV to 170
meV in the 0.02 0.28 range.
.
The defect density varies from 7.18 1016 cm-3 to
2.46 1017 cm-3.
The refractive index decreases from 3.6 to 2.36.
Conductivity
PL. Measurements
Raman spectroscopy
Presence of two bands when x increases are
attributed to interference effect.
For x 0.06, The spectrum exhibits three
characteristic features of a-SiH the SiSi TO
and LO vibration at around 480 cm-1, the Si-H
wagging vibration at 640 cm-1 and the second
order Si-Si mode at 980 cm-1.
When x increases the PL spectra shift towards
higher energies with increase of the bandwidths
and the efficiency.
Two thermally conduction regimes which can be
described by
If x increases, the band at 480 cm-1 enlarges and
shifts to lower energies due to an increase of
the tetrahedral bond angle fluctuation and local
strain enhancement.
In the high temperature range, the activation
energies is close to Eg/2 The conduction
takes place in extended states above the mobility
gap.
For x 0.28 the band at 780 cm-1 is observable
and is attributed to optical-like modes of Si-C
bonds.
In the low temperature range the electron
conduction is dominated by phonon assisted
hopping below Ec.
Absence at these concentration of C-C vibration
mode.
Conclusion
Two conduction regimes are observed extended
state conduction and hopping conduction in the
conduction band tail.
The PL intensity increases of more than one
order of magnitude when E04 varies from 2.09 to
2.47 eV is due to the increased topological
disorder.
At this low concentration, the carbon is mainly
incorporated in Si-C form, however the Si-C
characteristic band is revealed on the Raman
spectra of the samples only for x gt 0.18.
The bandwidth of the luminescence emission
spectrum is due to a contribution of both
phonon-assisted and zero-phonon processes.
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STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF
HELIUM DILUTED a-Si1-xCxH FILMS DEPOSITED BY
PECVD
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STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF
HELIUM DILUTED a-Si1-xCxH FILMS DEPOSITED BY
PECVD

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The peak of a.SiH sample (x 0),
corresponds to the splitting of bonding and
antibonding states of Si-Si bonds located at 3.8
eV.
The peak moves to higher energy with increase of
carbon content, its height decreases as function
of x and leads to a decrease of the refractive
index.
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