# Field Effect on Crystal Phase of Silicon in Si/CeO2/SiO2 Structure

Authors: Dmitry E. Milovzorov
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

The structural, optical, and conductivity properties of silicon film deposited on cerium dioxide buffer layer were studied. The electronic structure of system consists of various defect levels inside band gap. The temperature spatial distribution plays a great role in silicon crystallization. The field destruction of crystal phase and its restoration, after annealing, were investigated.


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## Body

## 1. Introduction

In recent years, thin silicon films
deposited on glass are widely used in various fields of electronic industry
such as thin film transistors [1], integration circuits,
photodetectors, and
light-emitting devices. The main problem to improve parameters of electronic
devices is the  preparation of the crystalline
homogenious thin film. However,
the thin film (with thickness around 100 nm)
deposition on glass substrate does
not open possibility to check surely its properties, from one side, and to get
crystalline material, from another side. In some instances, an amorphous film
is deposited, and after annealing its crystallinity is improved. The other way
is to deposit a buffer layer to prevent randomization in bonding by the
deposition of silicon thin film on silicon oxide substrate. Cerium dioxide is a
suitable dielectric (Ebg=5.5 eV) with a crystal lattice parameter
similar to silicon crystal structure to design the ultra-large-scale
integration devices. Its dielectric constant is higher than 25, because by thin film transistors,
making the leakage current for cerium dioxide thin dielectric layer with
thickness of 2.8 nm is four orders of magnitude lower than ordinary SiO2 dielectric layer [2] at an applied bias of 2 V.In our work, we studied the
field-stimulated destruction of crystal phase in silicon thin film deposited on
ultrathin layer of cerium dioxide, and the recrystalization by annealing. The
results of this research work can be
applied to memory devices design and creation of silicon cluster structures inside amorphous
phase, including silicon nanowires. It is seen that only the silicon bonding as
crystal structure can be made easily by preparing the preliminary temperature
field of substrate. In this case, we can easily obtain the fine crystal
structures on glass substrate with significant density of packaging for
integration circuit. In addition, such crystal structure can be easily
destroyed and restored because of memory effect.Due to the possibility of large area
deposition of thin silicon film on glass, we can achieve the large amount of
elements in one unit. The magnetron sputtering of cerium dioxide combined with plasma-enhanced chemical vapor deposition (PECVD) technique results in potential industrial application of product.


## 2. Experiments

## 2.1. Film Preparation

Thin silicon film was deposited by
using PECVD method. The gas mixture of silane and hydrogen was used. The temperature of deposition was in the
range of 480–570 K. The buffer layer of
cerium oxide was deposited on glass directly by using magnetron sputtering
system. Its thickness was varied from 1 to 10 nm. Figure1  shows the Raman
spectrum of silicon film deposited on CeO2 buffer layer. The
measurements were done by means of probe Ar laser beam with wavelength
of 488 nm, and were detected by double monochromator (Jobin Ivon, Kyoto,
Japan) coupled with photo multiplier tube.Figure 1
Raman spectrum of silicon film deposited on cerium oxide
buffer layer with thickness of 1 nm.


## 3. Field Effect on Silicon Film

## 3.1. Raman Spectra

Figure2 shows the scanning electron
microscopic photo of silicon film deposited on glass substrate with buffer CeO2
thin layer. Also, we studied the Raman spectra (Figure  3) by applying the
electrical field 3–6 μ V/A. It is seen that the crystal phase-related
peak with position around 520cm−1 has disappeared.Figure 2
Scanning electron
microscope data of silicon film (thickness being around 80 nm) deposited on
cerium oxide layer and glass substrate (Corning 1737).Figure 3
Raman spectrum of silicon film by electric field
applying. There is only amorphous phase, and no crystal. Crystal  silicon bonding was destructed.By annealing the sample with silicon film deposited on cerium
dioxide layer, we obtained
Raman data on the appearance of crystalline-related spectral peak around520cm−1 (see Figure 4), which were measured without application of
external electric field. Silicon film was annealed in 30 minutes at a
temperature of 473 K. The Raman scattering at elevated temperatures can be
explained by quantum Raman susceptibility [3].Figure 4
Raman spectrum of silicon film after 30-minute annealing
at a temperature of 473 K.


## 3.2. Conductivity Measurements

Conductivity measurements as a function of annealing temperature are shown in Figure5. The silicon film deposited
on Corning 7059 substrate has activation energy of 0.2 eV and high value of conductivity due to the high
concentration of defects and high crystallinity. The lower conductivity corresponds to the
silicon film deposited on thin buffer layer of cerium dioxide because of low
content of crystal phase (Ebg=1.12 eV) and high fraction of
amorphous phase with energy band gap of Ebg=1.77 eV.Figure 5
Conductivity measurements of silicon films
deposited on glass substrates, Corning 7059 and Corning
1737, with cerium
oxide buffer layer of 1 nm.


## 3.3. Local Factors on Micro- and Nanoscales

The equations for local factors determination on nanoscopic and microscopic scales
are strictly different. For microscopic scale, the dipole local field
factors surrounding is
significant as for nonlinear [4] as well as linear optical response: (1)L(ω,ρ)=ρ4π(εc(ω)−εa(ω))1+(εc(ω)−εa(ω))(Λ−βρ),where εc and εa are
dielectric functions of crystalline and amorphous silicon, respectively.
For sphere depolarization factor Λ is equal to 1/3, β is the Lorentz constant (β=1/3
for a homogeneous spherical surrounding).The crystal phase homogeneity on the microscope is low, compared to silicon films with large
thickness. The crystallization of silicon here mainly depends on the
crystallization of cerium dioxide layer. Figure6 shows the Raman spectrum for
silicon film deposited on cerium dioxide buffer layer by high-temperature (600°C)
conditions. It is seen that the crystal phase-related spectral peak around 520cm−1 and wide spectra peak extended from 440cm−1 to 500cm−1. The spectral deconvolution illustrates eight main oscillators
with defined eigenfrequencies. One of them with position 456cm−1 is
related to the Ce–O dipoles. Silicon nanocrystal bonds that are weaker than
bulk crystal bonding cause the spectral peak around 500cm−1.Figure 6
Raman spectrum deconvoluted by using Gauss and Lorentz
peaks. The Ce–O bonding is a
related 456–460cm−1 spectral peak.For nanoscopic scale, the local field factor (see Figure8) is determined by tensor
of polarizability [5]. Dielectric susceptibility tensor can be written as (2)χαβ=∂Pα/∂Eβ,where Pα is the component of polarization along
the Cartesian coordinates, and Eβ is the component of external field. The
film polarization mainly contributed to various components of dipole moments
along the coordinates, particularly the dipoles Si–O–Si on film surface and
intergrains boundary. We can estimate
the value of dipole moment by the angle between the two Si–O bonds being around
90° using formula for quantum
dipole: (3)μSi–O–Si2pO–3pSi=22πe∫0ξ∫02π∫0πΨ3pxSiΨ3pySirΨ2px–OΨ2py–Or2drdφdθ.The dipole moment is equal to the 1.4 D. The orientation of dipoles inside silicon film plays a great role
in  optical response because of strong asymmetrical properties and sufficient
polarization charge. It is
necessary to note that the electron wave function in Si–O–Si configuration is
distributed among the 12 silicon atoms [6], because, to calculate the tensor
value for Si/CeO2/SiO2 structure for thin silicon film,
we can use the linear combination of all components along the coordinates:
(4)χSilicon-film(1)=χ(1)Si–O Intergrain+χfilm-surface(1)+χ(1)CeO–Si Interface.  Figure 7 shows the microscopic homogeneity of crystal phase
for silicon film. The crystal phase is inhomogeneously laterally distributed according to
surface temperature fields’ variations. Figure  8 illustrates the morphology of
silicon film surface. It is clear that using dipole-surrounding calculations
can combine the local factor field. The Si–O bonding on silicon film surface
plays a great role in local factor value determination. For more detailed
description to calculate local factor field value, we use the silicon
nanocrystal orientation, oxygen atoms incorporation on silicon surface, as well.Figure 7
Raman mapping measurements of silicon crystal phase for silicon film deposited
onCeO2 buffer layer. Thickness of buffer layer is 90 nm.Figure 8
The microscopic photo of silicon film.


## 4. Model of Field Destruction

## 4.1. Silicon Clusters and Nanowires

The structural properties of silicon film deposited on crystalline buffer layer of cerium dioxide strictly depend on
the cerium dioxide crystallinity lateral distribution. The cerium dioxide
nanocrystals are deposited by magnetron target sputtering at determined
temperature of substrate. Increasing the temperature of substrate causes the
increase in cerium dioxide nanocrystal lateral density, because, by using the
definite silicon film preparation, the appearance of silicon crystal
nanostructures with single bonding is observed. Such single-bonding
one-dimensional structure can extend through the silicon film. Silicon
nanoclusters and nanowires made by applying nanoscopic temperature field
consist of silicon bonding with surely one-dimension scale. The
field-stimulated destruction can only be achieved by silicon-silicon bond elimination
and defect creation such as dangling bond. The memory effect for nanostructured
silicon crystals can be observed by annealing theSi/CeO2/SiO2 structure. It is clear that the appearance of crystal phase every time by
annealing depends on the minimums of potential energy that are determined by
cerium dioxide nanocrystals’ spatial positions.


## 4.2. Oxygen Incorporated into Silicon

For thin silicon films (thickness being less than 100 nm) deposited on silicon
oxide, the concentration of oxygen atoms is significant. Usually, the oxygen
atoms are incorporated with silicon in grain boundary of nanocrystals by
complicated surface morphology of film, because the proposed model [7] of
stabilized silicon-oxygen incorporation in the grain boundary after transformation
of the silanol groups into strong siloxane bonds was calculated for two types
of structures: 13 silicon atoms incorporated with 14 atoms of oxygen (SiOx,
x≈1) by the Si–O bond angle being equal to 130°, the
size of glass cluster is around 8.86 Å; 29 silicon atoms incorporated with 39 oxygen atoms (x≈1.34) create the
glass cluster size being equal to 14.77 Å.
The consequence of the saddle points in potential energy of oxygen atom
was changed by the stress [8].


## 4.3. Role of Cerium Dioxide

For the silicon crystallization in thin
silicon film, the role of crystallized clusters of cerium dioxide is great. The
crystallization conditions of cerium oxide layer are different from the silicon
film deposition conditions. Because of the high temperature of cerium silicide
creation, we suppose that the main role in silicon crystallization is played by a
temperature field on substrate.



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*Source: 712985-2008-06-16.xml*