Optical Property profiles of SiO2 films Containing Si Nanocrystals Formed by Si+
Implantation
Y. Liu*1, T. P. Chen1, M.S. Tse1, P.F. Ho1, A.L.K. Tan2and Y.C.Liu 2
1
School of Electrical and Electronic Engineering,
Nanyang Technological University, Singapore 639798
2
Singapore Institute of Manufacturing Technology, Singapore 638075
Abstract
Experiment
Introduction
Silicon, due to its indirect band gap
(which makes Si a poor light emitter) and to the
absence of linear electro-optic effects, has been
considered for a long time not suitable for
optoelectronic applications. In recently years, it is
suggested that Si nanocrystal (Si-nc)/SiO2 system
would be the best one to couple the information
processing capabilities of microelectronics with
the interconnection properties of optoelectronics
because of its mature technology in
microelectronics and its electroluminescence
property. As the Si nc is introduced into the SiO2
films, the optical properties of the films will
change. For applications such as light-emitting
structures, single-electron memories, optical
storage devices [1-4], it is essential to have
detailed information on the exact depth profiles of
optical properties of the Si-doped SiO2 films. In
this paper, we report an approach to determination
of depth profiles of optical constants at various
wavelengths of Si-nc embedded in SiO2 films.
The refractive index and extinction coefficient (n
and k) of the films are obtained by spectral fittings
to the experimental ellipsomitric data using a
MATLAB program developed in this work.
Based on the depth profiles of optical constants
obtained, we believe a light wave-guide may exist
in the Si-implanted SiO2 films.
* Corresponding author. Email: P150531616@ntu.edu.sg
Silicon nanocrystals were prepared by ion
implantation of Si+ into SiO2 layers (550nmthick) grown by 1000C on p-type Si (100). The
SiO2/Si samples were implanted with doses
ranging from 21016 to 11017 atoms/cm2 of Si+ at
50 keV. Then the wafers were annealed in
nitrogen ambient at 1000C for 1 hour to induce
Si crystallization, namely the formation of Si nc.
Fourier transform infrared spectroscopy (FTIR)
measurements on the as-implanted samples
showed the characteristic absorption due to the
stretching vibration of the Si-O-Si bonds at about
1010 cm-1; this frequency was remarkably lower
than that of a stoichiometric SiO2 film (around
1080 cm-1), due to the induction effect resulting
from the replacing of O atoms with Si atoms in
the network of the sub-stoichiometric SiOx ( x<2)
films [5]. A weak dependence of the Si-O-Si
stretching frequency on the composition of SiOx
films in the dose range used in this study was
observed (for example, the frequency for the dose
of 21016 atoms/cm2 is 1012 cm-1 while it is 1008
cm-1 for the dose of 11017 atoms/cm2), as shown
in Fig.1. SE measurements before and after Si-nc
formation were carried out in the wavelength
range of 400 to 1200 nm.
0.20
0.15
Absorbance (a.u.)
A quantitative approach to determination
of depth profiles of optical properties of Siimplanted SiO2 films based on spectroscopic
ellipsometry (SE) is presented. From the SE
measurements, the depth profiles of the complex
refractive index of SiO2 films containing Si
nanocrystals (nc) are obtained with an effective
medium approximation (EMA) in the wavelength
range of 400-1200nm.
The optical profiles
obtained imply the existence of a wave-guide in
the Si-doped SiO2 films.
0.10
0.05
0.00
dose=1e17/cm2
dose=8e16/cm2
dose=2e16/cm2
-0.05
-0.10
850
900
950
1000
1050
1100
1150
1200
1250
Wavenumber (cm-1)
Fig1. IR spectra of Si-doped SiO2 films with different
implantation doses (pure SiO2/Si substrate was taken as
background).
SE Analysis and Results
2.4
 i   SiO2

  SiO2
 vi nc Si
 i  2 SiO2
 nc Si  2 SiO2
(4)
where i (=Ni2, i=1, 2,…m) is the complex
dielectric function of the ith sub-layer, sio2 is the
complex dielectric function of SiO2, nc-Si is the
complex dielectric function of nc-Si in the SiO2
film, and i is the volume fraction of nc-Si.
Therefore, (2) and (3) now can be written as
= f1( v1 , v2 , …vm , Nsi , Nnc-Si ,  )
(5)
= f2( v1 , v2 , …vm , Nsi , Nnc-Si ,  )
(6)
A spectral fitting based on (5) and (6) to
experimental  and  can yield one set of (v1 , v2 ,
…vm ). Then i (i=1, 2, …m) can be calculated
with (4), and thus the depth profile of the complex
refractive index, i.e. (N1, N2, ... Nm) (note that Ni
=
i ) is obtained.
Wavelength
Refractive Index n
2.2
400nm
800nm
1200nm
2.0
1.8
Pure SiO2
1.6
1.4
0
50
100
150
200
250
300
Depth (nm)
0.030
Wavelength
0.025
Extinction coefficient k
As the Si-nc concentration in the SiO2
films is a function of the depth, the optical
properties of the films will vary with the depth.
We divided the SiO2 film into m sub-layers with
equal thickness d = Tox / m where Tox is the total
thickness of the films. Each sub-layer has its own
complex refractive index. In the ellipsometric
formalisms used in this study, ‘‘characteristic
equations’’ are used to describe the individual
sub-layers. For the total system, the ratio  of the
complex reflection coefficients for the p and s
polarizations is given by [6]
  Rp / Rs  tan( ) exp( j ) ,
(1)
where  and  are the ellipsometric angles. Each
sub-layer can be characterized by its complex
refractive index Ni =ni+ jki (i= 1,2, … m) where ni
and ki are the refractive index and extinction
coefficient of the ith sub-layer, respectively. For a
fixed incident angle and a given d,  and  can be
written as
= f1( N1 , N2 , …Nm ,NSi,  )
(2)
= f2(N1 , N2 , …Nm ,NSi,  )
(3)
where NSi is the complex refractive index of Si
substrate,  is the wavelength, and f1 and f2 are
two complicated functions which can not be
shown analytically. As NSi is known,  and  can
be calculated with (2) and (3) for one set of (N1,
N2, …Nm) at a given wavelength. The parameters
(N1, N2, …Nm) can be converted to the depth
profiles of nc-Si concentration in the SiO2 film
through the following effective medium
approximation (EMA)
400 nm
800nm
1200nm
0.020
0.015
0.010
Pure SiO2
0.005
0.000
-0.005
0
50
100
150
200
250
300
Depth (nm)
Fig.2. Depth profile of complex refractive index as a
function of wavelength. The Si implant dose is 11017
atoms/cm2.
Fig.2 shows the depth profiles of the
refractive index and extinction coefficient as
functions of wavelength for an implant dose of
11017 atoms/cm2, respectively. For all the
wavelengths, there are dramatic changes in both
the refractive index and the extinction coefficient
in the depth range of about 50 - 150 nm with the
maximum changes occurring in the depth of about
90nm. In the region deeper than about 200nm, the
optical constants are basically the same as those
of pure SiO2. Actually, the depth profile of the
optical constants is just an analogous of that of the
excess Si atoms. The depth profiles can be
roughly divided into three regions including the
surface region (depth < ~ 50nm), the active region
(depth of ~ 50 - 150nm) and the bulk region
(depth > ~ 150 nm). As shown in Fig.2, the
changes of the optical constants due to the excess
Si atoms depend on the wavelength with larger
changes at shorter wavelengths, and the
wavelength dependence of the change of the
extinction coefficient is much stronger than that of
the refractive index. For all wavelengths, the
refractive index in the active region is
significantly larger than that in the surface and
bulk regions (the maximum difference could be as
large as about 0.45). For short wave lengths, the
extinction coefficient (and thus the absorption
coefficient) in the active region is much higher
than that in the surface and the bulk regions;
however, for long wavelengths (near-infrared to
infrared), the extinction coefficient is almost zero
and thus there is almost no light absorption in all
the three regions. This could provide a possibility
of using the Si implanted SiO2 film as an optical
wave-guide for long-wavelength applications as
the light could be confined and transmitted in the
active region. And importantly, the refractive
index and the extinction coefficient and their
profiles can be accurately controlled through Si
implantation. Fig.3 shows the influence of Si
implantation dose on the refractive index and the
extinction coefficient, respectively. Obviously, a
higher dose leads to higher refractive index and
extinction coefficient in all the three regions and
also a broader depth profile. Based on the above
discussions, an optical wave-guide could be
formed in the Si-doped SiO2 films, as shown in
Fig4.
based on the SE. In the SE analysis, a Si-doped
SiO2 film is divided into m sub-layers with equal
thickness. The depth profiles of excess Si are
obtained from the spectral SE fittings first, and
then the depth profiles of the complex refractive
index are determined with the effective medium
approximation. In such a Si-doped SiO2 film
structure, light-wave trapping may exist in the
middle active region of the films.
Fig.4 Schematic illustration of light trapping in the Sidoped SiO2 film on Si substrate.
2.0
@ wavelength 400nm
Acknowledgment
Refractive Index n
1.9
Si+ Dose
2x1016 / cm2
1.8
6x1016 / cm2
1.7
1.6
Pure SiO2
1.5
1.4
0
50
100
150
200
250
Depth (nm)
@ Wavelength 400nm
0.010
Extinction Coefficient k
This work has been financially supported by the
Academic Research Fund from Nanyang
Technological University under project No. RG
8/01. The Authors wish to record their sincere
thanks to the technical staff of Micro-Fabrication
Laboratory Ms. Yang Xiao Hong, Mr. Li Wen,
and Mr. Liu Kai Yu for their skillful technical
support.
0.008
References:
Si+ dose
2x1016 /cm2
6x1016 /cm2
0.006
0.004
0.002
Pure SiO2
0.000
0
50
100
Depth
150
200
250
(nm)
Fig.3. Influence of Si implant dose on the complex
refractive index and its depth profile. The wavelength
is 400nm.
Conclusion
In conclusion, we have developed an
approach to determination of depth profiles of the
optical properties of SiO2 films containing Si nc
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