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