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UMKB
Defect States Distribution in Microcrystalline Silicon by the Constant Photocurrent Method in the ac-mode
T. Tibermacine1, M. Ledra1, N. Ouhbab1 and A. Merazga2
1Université Mohammed Khider, Laboratoire des Matériaux Semi-conducteurs et Métalliques,2Physics Department, Faculty of Sciences, Taif University, Saudi Arabia.
FIRST INTERNATIONAL CONFERENCE ON NEW MATERIALS AND ACTIVE DEVICES - NMCA’2011 - UNIVERSITY OF OUM EL-BOUAGHI
0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,510-1
100
101
102
103 00c354 c-Si:H
Coeffi
cie
nt
abso
rpti
on
a (c
m-1
)
Photon energy h (eV )
ac-CPM @ 60Hz
0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5
1018
1019
1020
1021
63meV
00c354 c-Si:H
ac-CPM @ 60Hz
Densi
ty o
f st
ate
s D
OS
(c
m-3
eV
-1)
Energy Ec-E (eV )
Measured ac absorption coefficient at 60Hz. Extracted DOS distribution using the measured ac absorption spectrum at 60 Hz.
RESULTS
0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,210-1
100
101
102
103
104
105
106
00c354 c-Si:H
Abso
rpti
on c
oeffi
cien
t
(cm
-1)
Photon energy h(eV )
from optical transmission ac-CPM @ 5Hz c-Si
Absolute ac-CPM (h) spectrum measured at 5 Hz frequency. (h) determined from the transmittance and (h) for crystalline
silicon are both shown for comparison.
Microcrystalline silicon c-Si:H - 00c 354 sample -
CONCLUSION
In the present work, the electronic properties of microcrystalline silicon films have been studied by using constant photocurrent technique in ac regime. By
analyzing the measured absorption coefficients, we have found that the density of valence band-tail states falls exponentially towards the gap with a typical
band-tail width of 63 meV. A schematic picture of the density of states in energetic sense has been obtained. Our study indicates then that the constant
photocurrent method is able to extract the density of localized states in highly-crystalline c-Si:H films.
ABSTRACT
Recently, hydrogenated microcrystalline silicon (µc-Si:H) has attracted much interest in optoelectronic applications. It offers a higher stability against light
degradation and a wider absorption bandwidth extending into the near infrared. The electronic properties and performance of µc-Si:H films are correlated
with the deposition parameters and the structural properties. Therefore, the detailed knowledge of the gap density of states (DOS) in µc-Si:H is of great
importance to understand the transport mechanism. The Constant Photocurrent Method in the ac-mode (ac-CPM) as a powerful technique is used to
determine the defect Density Of States (DOS) in µc-Si:H. The absorption coefficient spectrum, ac- a (hn), is measured under ac-CPM conditions at 60Hz,
then it is converted by the CPM spectroscopy into a DOS distribution covering a portion in the lower energy range of occupied states. We have found that the
density of valence band-tail states falls exponentially towards the gap with a typical band-tail width of 63 meV.
.
Laboratory of Metallic and Semiconducting Materials - LMSM -University of Biskra
PRINCIPLE
Incident photon flux f is adjusted to maintain photocurrent Iph constant over the range of photon energy:
a(hn)=constant/f(hn)
Highly sensitive technique (a(hn) down to 10-2cm-1) Can be configured into DC or AC illumination
WHAT ARE WE GOING TO OBTAIN?
Absorption coefficient spectrum curve both in DC and AC modes
Density of states distribution D(E)
EXPERIMENTAL SET-UP
1.
2.3.
4.
5.
7.
10.11.
6.
7.
8.
PC
DC AC
9.
Dundee Laboratory Scotland - UK -
HT Power Supply
Keithley 485
Picoammeter
Keithley 485
Picoammeter
DC Power Supply PSU 35V -10A-
Opticalcontroller
chopper
Lock -in amplifier 1
Lock -in amplifier 2
-
-
Monochromator
Chopper
Detector 2
Halogen Lamp
Cryostat
+
Sample
+
Detector 1
Filters
Wheel
ComputerComputer
12
2- DC mode1-AC mode
12 12 12
2-1-
12
2-SCPM1-TCPM
12 12 12
2-1-
PERSPECTIVE
WHAT WE WOULD LIKE TO DO IN THE NEAR FUTURE!
Laboratory of Metallic and Semiconducting Materials - LMSM -University of Biskra