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TRANSCRIPT
International Journal of Semiconductor Science & Technology (IJSST) ISSN 2250-1576 Vol.2, Issue 2 June 2012 1-17 © TJPRC Pvt. Ltd.,
INVESTIGATION ON MILD CONDITION
PREPARATION AND STRUCTURAL, OPTICAL AND
THERMAL PROPERTIES OF PVP CAPPED CDS
NANOPARTICLES
N.S. NIRMALA JOTHI, G. RAMALINGAM, R. GUNASEELAN, A.R.
BABY SUGANTHI AND P. SAGAYARAJ
Department of Physics, Loyola College, Chennai- 600 034, India.
ABSTRACT
Polyvinlypyrrolidone (PVP) capped cadmium sulphide (CdS)
nanoparticles are synthesized using a simple hydrothermal method. The
powder X-ray diffraction (XRD) result indicates that the nanoparticles are
crystallized in hexagonal phase. The optical properties are characterized
by Ultraviolet-Visible (UV-Vis) absorption and Photoluminescene (PL)
spectra. The transmission electron microscope (TEM) reveals that the
nanoparticles of CdS posses well defined morphology and high
crystallinity. The d-spacing measured from well resolved lattice fringes of
HRTEM ascertains the structure of CdS nanocrystals. The morphology
and composition of CdS nanoparticles are investigated using Scanning
electron microscope (SEM) and Energy dispersive x-ray analysis (EDAX)
respectively. The thermal behavior of the as prepared nanopowder has
been studied by Thermo gravimetric analysis (TGA).
2 N.S. Nirmala Jothi, G. Ramalingam, R. Gunaseelan, A.R. Baby Suganthi and P. Sagayaraj
KEYWORDS: Nanostructure; Chemical synthesis; Electron
microscopy; Powder diffraction; Optical properties;
1. INTRODUCTION
Recently, the synthesis of inorganic nanocrystals has attracted
much interest due to their strong size dependent, special optical,
electronic properties and potential applications in solar cells, light
emitting diode, nonlinear optical materials, optoelectronic and electronic
devices, biological labelling, thermoelectric coolers, thermo-electronic
and optical recording materials, etc. [1, 2]. These properties and the
applications are largely dependent on the size, shape and the impurities of
the nanomaterials.
*CORRESPONDING AUTHOR
Dr. N.S.Nirmala Jothi, Assistant Professor of Physics, Loyola College, Chennai – 600 034, India
Email: [email protected]
As a typical semiconductor material of the II-VI group, cadmium
sulphide (CdS) nanocrystals have been widely investigated [3, 4]. CdS is
a direct band gap semiconductor with the band gap of 2.4 eV for bulk
hexagonal (Wurtzite) structure and 2.38 eV for bulk cubic (zinc blend)
structure [5, 6]. CdS nanopartilces are considered to be one of the model
systems for investigating the unique optical and electronic properties of
quantum confined semiconductors. However, the lack of adequate
synthetic methods for producing the desired high quality nanoparticles is
Investigation on mild condition preparation and structural, 3 optical and thermal properties of PVP capped CdS nanoparticles
currently a bottleneck in this field. Over the past two decades, numerous
colloidal chemistry (or solution chemistry) methods have been developed
for the preparation of cadmium sulfide nanocrystals [7]. The solution
chemistry synthesis of CdS nanocrystals utilizes the organic stabilizers to
cap surface atoms of nanoparticles in order to control the growth process.
The kind of stabilizers is of great importance, since, it affects the
chemical as well as the physical properties of the semiconductor
nanocrystals, from stability to solubility to light emission. Modifying the
surfaces of nanoparticles with various organic, inorganic, species is
projected to remove their surface defects and subsequently, influence their
property. Organic capping of nanoparticles with surfactants would give
rise to a barrier to aggregation and electronic passivation of the particles.
The ability to segregate between the zinc blend (metastable phase) and
wurtzite structures is considered as the main key.
In aqueous phase synthesis of CdS nanoparticles, the
conventionally homogeneous phase arrested precipitation with the use of
phosphates, various thiols or hydrophilic polymer as capping reagents are
usually adopted [8]. Among the various traditional synthesis approaches
of the nanomaterials, the solvothermal/hydrothermal methods have been
widely applied to improve the crystallinity of nanosized particles.
Solvothermal synthesis is one of the most efficient methods used to
synthesize CdS with different morphologies. It is simple, convenient and
inexpensive. Some polymers such as PEG, PAA, PAN, PVP and PVA
have been used to modify the surface chemistry of the crystals and the
concentration of soluble species for crystal growing [9-12]. The
complexing agent (PVP), can cap the particle surface to prevent the
colloidal particle from agglomeration and play an important role in the
formation of the nanoparticles. The adsorption of PVP onto the
nanoparticles can solubilise the formed nanostructure into water. This
4 N.S. Nirmala Jothi, G. Ramalingam, R. Gunaseelan, A.R. Baby Suganthi and P. Sagayaraj
may be attributed to the hydrophilicity of the PVP molecules and the
repellence between positively charged PVP molecules adsorbed onto
separated nanoparticles. Competitive reaction kinetics between the growth
of CdS and termination of growth by capping the surface is considered to
induce the growth of CdS nanoparticles [13].
Recently, a wide range of cadmium sulfide (CdS) 3D
polycrystalline walnut-like nanocrystals were prepared by solvothermal
method with polyvinylpyrrolidone (PVP) as stabilizer [14]. The large
surface area-to-volume ratio, along with the ability to tune the band gap
makes the semiconductor nanoparticles like CdS to be used as sensitizers
and catalysts in photochemical reaction being universally accepted.
In spite of the concentrated efforts to develop high quality CdS
nanoparticles, there are still challenges to further simplify the synthesis
procedures so as to encourage the mass production and the use of
hydrothermal synthesis route is one of the best options to move towards
this goal. Further, the use of water as a solvent offers several advantages,
compared with non-aqueous synthesis, aqueous is more reproducible, low
cost, environment friendly and the “as-prepared” samples are more water
soluble and bio-compactable [8]. In-situ surface modification under
hydrothermal conditions helps significantly in preparing nanocrystals
with a highly controlled size, shape and dispersibility. This article deals
with the preparation of CdS nanoparticles by a simple hydrothermal
method and the influence of water soluble polymeric capping agent like
polyvinylpyrrolidone (PVP) on the crystalline quality, morphology, size
and crystalline phase is investigated. The sample was systematically
characterized by powder XRD, UV-Vis absorption and
photoluminescence spectroscopy, SEM and TEM, and thermal analysis.
Investigation on mild condition preparation and structural, 5 optical and thermal properties of PVP capped CdS nanoparticles
2. EXPERIMENTAL
2.1. Materials
Cadmium nitrate (Cd (NO3)2.4H2O) Merck and thiourea
(H2NCSNH2) are used as the starting materials. All chemical are of high
purity and no further purification is done. Polyvinylpyrrolidone (PVP) is
the capping ligand.
2.2. Synthesis of CdS nanoparticles modified with
polyvinylpyrollidone
The stoichiometric ratio of the starting materials (Cd (NO3)2.4H2O
and H2NCSNH2) was kept as 1:3. Polyvinylpyrollidone (PVP) is a water
soluble polymer used here as the capping ligand. Initially, 2 g of PVP was
taken and dissolved in 20 ml of water and then cadmium nitrate solution
was added and mixed thoroughly in 75 ml of water. In a similar way
thiourea was dissolved in 75 ml of water and stirred magnetically. These
two solutions were mixed together for about 1 h. Ammonia was directly
added in the solution until the pH of the solution reaches 12. The final
solution was transferred into a 200 ml Teflon coated autoclave. The
autoclave was placed in an electrical oven and maintained at 180 °C for 6
hours. After that the autoclave is removed from the oven and naturally
allowed to cool down to room temperature. The yellow precipitate was
washed continuously with water and ethanol several times so as to
remove the excessive thiourea and other by products. The as prepared
powder is grounded in a mortar and dried with a temperature of 80 °C
nearly for 4 hours.
2.3. Characterization
The room temperature powder XRD pattern for the as prepared
sample was done using RICH SEIFER with monochromatic nickel
6 N.S. Nirmala Jothi, G. Ramalingam, R. Gunaseelan, A.R. Baby Suganthi and P. Sagayaraj
filtered CuKα (λ=1.5461 Å) radiation. The UV-Vis absorption spectral
studies were carried out using VARIAN CARY 5E UV-Vis-NIR
SPECTROPHOTOMETER in the spectral region of 200 and 800 nm. The
photoluminescence spectra of the samples were recorded with a VARIAN
CARY 5E UV-Vis-NIR SPECTROPHOTOMETER. Particles size and
distribution analyses were carried out with TEM model JEOL JEM 3010
at an accelerating voltage of 200 kV. For the TEM observations, the
sample was dispersed in ethanol and ultrsonicated for 30 minutes and then
it was kept on a carbon coated grids. Scanning electron microscope
(SEM) was employed for morphological study using JEOL JSM 6310
operated at 10 kV with Energy Dispersive X-ray analyzer (EDX).
Thermogravimetric (TG) and Differential thermo gravimetric analysis
(DTG) for the air dried sample was performed on a SDT Q600 with a
heating rate of 20 °C min-1.
3. RESULTS AND DISCUSSION
3.1 Powder XRD analysis
Fig. 1 shows typical XRD pattern of obtained CdS nanocrystals
with Millipore water as a solvent and polyvinylpyrrolidone (PVP) as
capping reagent. All the diffraction peaks are conveniently indexed to
hexagonal structure. Compared with the standard card (JCPDS card (PDF
No.80-0006), the (0 0 2) diffraction peak, the second strongest peak in
bulk hexagonal CdS, were unusually strong and narrow, which may be
ascribed to the preferential growth along [0 0 1] hexagonal CdS
crystallites. The relatively broad peaks probably resulted from the smaller
dimensions of the other surfaces [15]. The corresponding lattice constants
are a = b = 4.121 Å and c = 6.682 Å. There are reports on the role of
Investigation on mild condition preparation and structural, 7 optical and thermal properties of PVP capped CdS nanoparticles
different dosages of PVP on the resulting structure and orientation growth
[15]. In the present study, the preparation carried out with 2 g of PVP/(20
ml) resulted in good quality nanoparticles with best orientation growth.
Fig1:XRD pattern of PVP capped CdS nanocrystals.
3.2 Energy dispersive X-ray analysis (EDAX)
EDAX is an important technique to analyze the composition of
elements quantitatively and solve the chemical identity of any
nanomaterial. It is inferred from the result of the EDAX spectrum (Fig. 2)
obtained for nanoparticles prepared using PVP, that the sample is
composed of only Cd and S which are exactly CdS nanoparticles and no
trace of other elements is observed. From the Fig. 2 it is clear that the
sample is generally cadmium rich even though a relatively higher
concentration of sulphur was used than the cadmium to synthesize CdS
nanoparticles. The fact that the nanoparticles are richer in cadmium than
8 N.S. Nirmala Jothi, G. Ramalingam, R. Gunaseelan, A.R. Baby Suganthi and P. Sagayaraj
in sulphur suggests that their surface should be mainly composed of
cadmium atoms. From the EDAX and XRD analyses, it is clear that the
obtained product is pure cadmium sulphide in wurtzite phase.
Fig 2.EDAX pattern of CdS nanoparticles modified with
PVP capping agent.
3.3 UV-Vis absorption spectroscopy
The controlling and tuning of band edge emission and surface traps
state emission of CdS nanocrystals are obviously very important to realize
the tunable optical properties and laser emission [16]. The UV-Vis
spectral analysis was carried out between 200 nm and 800 nm. Fig. 3
shows the absorption spectrum of CdS nanoparticles prepared with
surfactant PVP. The absorption band edge was shifted to 490 nm and the
Investigation on mild condition preparation and structural, 9 optical and thermal properties of PVP capped CdS nanoparticles
corresponding band gap is 2.53 eV which is higher compared to bulk CdS
band gap (2.4 eV). Thus it is clear from the optical absorption study that
the capping of CdS with PVP modifies the band gap of the CdS
nanoparticles and the sample is blue shifted when compared with the bulk
CdS (512 nm).
200 300 400 500 600 700 800
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Abs
orba
nce
W ave length (nm )
Fig 3.Uv-vis absorption spectrum of PVP capped CdS nanoparticles.
3.4 Photoluminescence (PL) study
One of the interesting aspects of the photo-physical properties is
the photoluminescence (PL) of CdS nanocrystallites. The PL studies on
CdS nanocrystallites have been investigated by various research groups
[17]. In general, the reported emission spectrum consists of two broad
bands in the range 400-520 nm and then 520-800 nm. Usually, the
peaking is observed at around 480 and 650 nm respectively [18].
Recently, Li et al investigated the room temperature luminescence of CdS
10 N.S. Nirmala Jothi, G. Ramalingam, R. Gunaseelan, A.R. Baby Suganthi and P. Sagayaraj
nanostructured materials prepared with various sulphur sources and
studied the influence of S sources on the green and red emission spectra
of the nanopowder [19]. The PL property is influenced by the structure,
composition, particle size and morphology of the CdS nanoparticles. In
addition, the method of preparation has marked influence. Fig. 4 shows
the PL spectrum of CdS nanoparticles prepared with PVP. It is evident
from the PL spectra that the emission peak at 505 nm for the sample can
be assigned to the surface trap induced fluorescence which involved the
recombination of electrons trapped inside a sulphur vacancy with the hole
in the valence band of CdS nanoparticle. The emission peak observed at
505 nm for CdS synthesized by PVP assisted solvothermal method goes
well with the earlier report [15].
Fig4. PL spectrum of PVP capped CdS nanoparticles.
Investigation on mild condition preparation and structural, 11 optical and thermal properties of PVP capped CdS nanoparticles
3.5 SEM analysis
The SEM image of CdS nanocrystals with water as a solvent and
polyvinylpyrrolidone as capping reagent is shown in Fig. 5. The
solvothermal temperature along with the capping agent can influence the
nanoparticle size. At the lower temperature around 80oC only irregular
nanospheres of large diameter are usually formed. A close observation of
the SEM image of the present case suggests that the surfaces of the
nanospheres are relatively smooth and there are few prolated spheres as
well and this could be attributed to the relatively high reaction
temperature employed. It reveals that PVP played an important role in
controlling the size and mono-dispersion of the CdS nanocrystals in this
process. The absence of agglomerates is attributed to the role played by
PVP. In the formation process of the shape evolution of CdS architectural
structure, the capping agent PVP is adsorbed onto the different planes of
the incipient CdS nuclei and it not only prevents the particles from
agglomeration, but also influences the growth of these planes [14].
Fig5. SEM image of PVP capped CdS nanoparticles.
12 N.S. Nirmala Jothi, G. Ramalingam, R. Gunaseelan, A.R. Baby Suganthi and P. Sagayaraj
3.6 Transmission electron microscopy (TEM)
The transmission electron microscopic analysis allows one to
visualize particles at nanosize regime with high degree of accuracy, it
offers better understanding about growth aspects and helps to analyze the
actual size of the particles, shape and growth pattern.
The TEM micrograph of the PVP capped CdS nanopowder is
shown in Fig. 6, which suggests the formation of spherical as well as
short rods of nanoparticles. Further, the side faces of the product are not
smooth and the particles and the level of agglomeration are slightly on the
higher side and the short rods are not clearly visible. In the present case,
the aspect ratio of the nanorods formed is only 3-4 and this is possibly due
to the short duration of the reaction time employed. In the HRTEM
image, a representative nanorod of width 6 nm and 25 nm length is clearly
visible. There are reports on the capping mechanism of PVP in growth
process of CdS nanoparticles and also the influence of PVP dosage in
tailoring the shape and size of the CdS nanostructures. The best dosage of
PVP for the orientation growth of CdS nanocrystal was optimized as 0.8 g
for 50 ml by Qingqing et al [15]. The HRTEM image shows the presence
of few short nanorods which are seen crosslinked due to agglomeration, in
spite of this, we notice in Fig. 6 (iv), the formation of well arranged lattice
fringes in a single nanorods. Thus the problem of agglomeration and
improving the aspect of the nanorods are the issues yet to be addressed.
The SAED pattern of the CdS nanoparticles is presented which confirms
the hexagonal (Wurtzite) phase of CdS. The pattern consists of diffraction
rings corresponding to particle size and morphology.
Investigation on mild condition preparation and structural, 13 optical and thermal properties of PVP capped CdS nanoparticles
Fig.6. TEM and HRTEM images of CdS nanoparticles capped with
polyvinylpyrrodoline (PVP). (i) Nanoclusters with small CdS
nanoparticles (Inset SAED pattern), (ii, iii and iv) HRTEM images of one
dimensional CdS with lattices fringe
14 N.S. Nirmala Jothi, G. Ramalingam, R. Gunaseelan, A.R. Baby Suganthi and P. Sagayaraj
3.7 Thermal analysis
The thermal behavior of the semiconductor nanoparticles of CdS
was studied by employing TG/DTA analysis. Since the temperature plays
an important in the formation of nanostructured materials, temperature
induced phase changes are important for the utility of these nanoparticles
for various applications. The TG analysis of the sample CdS obtained
with PVP was done at the heating rate of 20°C/min. The TGA graph (Fig.
7) of the sample synthesized using PVP as capping agent shows the sharp
weight loss of about 3.277 % at 525 °C.A gradual weight loss of about
1.246 % was obtained between 525 °C and 840 °C.
Fig.7. TG/DTA traces of PVP capped CdS nanoparticles
4. CONCLUSION
Semiconductor nanoparticles of CdS are successfully prepared
under solvothermal/hydrothermal conditions in surfactant assisted
Investigation on mild condition preparation and structural, 15 optical and thermal properties of PVP capped CdS nanoparticles
synthesis. The powder XRD result show that particles are purely
crystallized in hexagonal phase with the broadening of diffraction peaks is
attributed to nanoscale size of the particles. From the optical absorption
spectra, the blue shift of 490 nm as compared to bulk counterpart is due to
the quantum confinement effect. The broad emission band observed in the
PL spectrum for PVP capped CdS nanoparticles is assigned to the surface
trap induced fluorescence which involved the recombination of electrons
trapped inside a sulphur vacancy with the hole in the valence band of CdS
nanoparticle. From TEM, it is evident that the nanoparticles of CdS
exhibit well defined morphology and high crystallinity. The HRTEM
result reveals the well resolved lattice fringes of CdS nanoparticles. Thus,
the present study demonstrates that the hydrothermal synthesis method
with capping ligand is one of the successful routes for obtaining good
quality CdS nanoparticles.
ACKNOWLEDGMENTS
The authors are grateful to UGC for the instrumentation facility
provided at Loyola College through a project (F38-119/2009(SR)). The
authors thank Prof. B.S.Murty, Department of Metallurgy and Materials
Science, Indian Institute of Technology, Madras for TEM facility and for
useful suggestions.
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Investigation on mild condition preparation and structural, 17 optical and thermal properties of PVP capped CdS nanoparticles
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FIGURE CAPTION
Fig.1. Powder XRD pattern of PVP capped CdS nanoparticles
Fig.2. EDAX pattern of CdS nanoparticles modified with PVP capping
ligand
Fig.3. UV-Vis absorption spectrum of PVP capped CdS nanoparticles
Fig.4. PL spectrum of PVP capped CdS nanoparticles
Fig.5. SEM image of PVP capped CdS nanoparticles
Fig.6. TEM and HRTEM images of CdS nanoparticles capped with
polyvinylpyrrodoline (PVP). (i) Nanoclusters with small CdS
nanoparticles (Inset SAED pattern), (ii, iii and iv) HRTEM
images of one dimensional CdS with lattices fringes.
Fig.7. TG/DTA traces of PVP capped CdS nanoparticles
International Journal of Semiconductor Science & Technology (IJSST) ISSN 2250-1576 Vol.2, Issue 2 June 2012 19-41 © TJPRC Pvt. Ltd.,
STUDY AND MODELING OF THE TRANSPORT MECHANISM IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING.
MMrr..AA..RREESSFFAA ** .. BOURZIG Y.SMAHI. BRAHIMI.R.MENEZLA.
Corresponding Author Tel:213 6 67 60 17 80 Email address: [email protected]
Laboratory of modeling and conception of the circuits electronic,
department of electronics. University Djillali Liabès. BP89, Sidi Bel Abbes 22000.ALGERIA.
ABSTRACT:
The current through a metal–semiconductor junction is mainly due
to the majority carriers. Three distinctly different mechanisms exist in a
schottky diode: diffusion of of the semiconductor carriers in metal,
thermionic emission-diffusion (TED) of carriers through a Schottky gate,
and a mechanical quantum that pierces a tunnel through the gate. The
system was solved by using a coupled Poisson-Boltzmann algorithm.
Schottky BH is defined as the difference in energy between the Fermi
level and metal band carrier majority of the metal - semiconductor
junction to the semiconductor contacts. The insulating layer converts the
MS device in an MIS device and has a strong influence on its current–
voltage (IV) and the parameters of a Schottky barrier from 3.7 to 15 eV.
There are several possible reasons for the error that causes a deviation of
the ideal behaviour of Schottky diodes with and without an interfacial
insulator layer. These include the particular distribution of interface
states, the series resistance, bias voltage and temperature. The GaAs and
20 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
its large concentration values of trap centers will participate in an increase
of the process of thermionic electrons and holes, which will in turn the
IV characteristic of the diode, and an overflow maximum value
[NT=3×1020] is obtained. The I–V characteristics of Schottky diodes are
in the hypothesis of a parabolic summit.
KEYWORDS: The electrocstatic potentiel and density of carriers, The
current thermionic emission-diffusion (TED) and The current tunnel
through the gate.The current-voltage(IV) characteristics of Schottky
diodes,and the temperature.
1.1. INTRODUCTION
The operation of semiconductor components at high temperature
and high frequency such as Schottky diodes and PN is usually described
by a set of features for implementation of voltage (IV) whose analysis
provides some information on electricity from the transport mechanism.
The determination of the model parameters that are fundamental to
the Schottky diode enable us ; to know the height of the gate, the factor of
ideality, and the resistance set, whish plays an important role in the
conception and the manufacture of the semiconductor devices such as
photopiles.
The goal of this thesis is to contribute to the mathematical
modeling and the simulation of greatly
inhomogeneous semiconductor devices.
Study of the physical origin of currents in the I–V to low
temperature of Schottky diode on the basis of a GaAs. One finds that
these excess currents are due to the generation of network shortcomings
STUDY AND MODELING OF THE TRANSPORT MECHANISM 21 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
close to the metal–semiconductor interface at the time of irradiation, One
warming heat or of an external distortion. These shortcomings produce
levels of trap in some parts of the space load region.
This paper investigate the diffusive limit of the Boltzmann
equations, to get a second order approximation of the concentration of the
carriers. The drift-diffusion equations are unchanged, but a correction of
the boundaries of layers, which is proportional to current flow, appears in
the boundary conditions for concentration. The proportionality coefficient
is calculated by solving the spectral method.
These limiting and classical conditions are compared numerically
on a physical problem. Thi paper is also dedicated to the extention of the
particle simulation programs for the treatment of thes inhomogeneous
structures. In these devices, the dynamics are governed by the limiting
conditions that need to be taken precisely into account. Geometry is one-
dimensional in space and three-dimensional with axisymmetry in a wave
vector. It is therefore about solving the system coupled of Boltzmann–
Poisson for the modeling of a Schottky diode, in the cases united and
multi-dimensional respectively. What will permit to explain the numeric
results obtained are explained later.
The system was solved by using a coupled Poisson–Boltzmann
algorithm. Schottky BH is defined as the difference in energy between the
Fermi level and the metal band majority carrier of a metal semiconductor
junction to the semiconductor contacts[1-4]. These include the particular
distribution of interface states[2,5], the series resistance[6-8], bias
voltage[6-10] and temperature[1, 2, 6,9,11].
22 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
1.1.1. SCHOTTKY CURRENT DIODE
The theory of diffusion supposes that the driving force is
distributed olong the length of the layer of the depletion. The theory of the
thermionic emission-diffusion (TED), only applies to energetic carriers,
which have energy equal to or bigger than the energy of the conduction
strip to the interface of the metal–semiconductor. Quantum mechanics
that pierces a tunnel through the gate, into account the wave nature of the
electrons. In a given junction, a combination of all three mechanisms
could exist[12-14]. However, typically there is only one dominant current
mechanism. The analysis reveals that the diffusion and thermionic
emission-diffusion (TED) can be written in the following form[5,7,15]:
( )( )1expexp... −⎟⎠⎞
⎜⎝⎛−= Vt
VaVtBNcqJ n
φν . (1)
This expression affirms that the current is the product of the
electronic load, q, a speed, v, and the available carrier density in the
semiconductor located next to the interface. Speed equal to the mobility
multiplied by the electric field at the interface, the diffusion current and
the speed of Richardson and the current of the thermionic emission-
diffusion (TED) [16-18]. It ensures that the current is zero so no voltage is
applied in thermal equilibrium.
The current tunnel is of a similar shape, to know:
Θ= nqJn Rν , (2)
where the vR is the Richardson velocity, q the electronic charge, and n is
the density of carriers in the semiconductor. The current tunnel has the
term of the probability Θ, is added since the total current depends on the
STUDY AND MODELING OF THE TRANSPORT MECHANISM 23 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
carrier' flux arriving at the tunnel gate multiplied with the probability, Θ,
that it pierces a tunnel through the gate.
1.1.2. Diffusion current
This analysis supposes that the depletion layer is big compared to
the middle free trajectory, so that the concepts of movement and diffusion
are valid. The density currents are thus obtained as.
( ) ( )[ ]1expexp22
−⎟⎠⎞
⎜⎝⎛−−= Vt
VaVt
NdVaqVtDnNc
Jn B
s
iq φε
φ . (3)
The current depends on the applied voltage, Va, and exponentiality
on the height of the gate, φB, therefore the prefactor can be consisted more
easily if one rewrites it as function of the electric field on the interface of
the metal–semiconductor max:
( )
s
NdVaiqε
φε −= 2max , (4)
( )[ ]1expexp. max −⎟⎟
⎠
⎞
⎜⎜
⎝
⎛−= Vt
VaVtNcnqJn Bφεμ , (5)
so that the prefactor equals the current of the movement to the interface of
the metal–semiconductor.
24 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
1.1.3. Current thermionic emission-diffusion
The theory of thermionic emission-diffusion (TED) supposes that
electrons, with energy bigger than the summit of the gate, will cross the
well stocked gate that is displaced toward the gate. The real shape of the
gate is ignored by this. The current can be expressed as:
⎟⎠⎞⎜
⎝⎛ −= − 12* eeTAJ Vt
VaVt
MS
Bφ, (6)
where h
KmAq
3
2** ...4π= is the effective Richardson constant, q the
electronic charge, k Boltzmann constant,
T the absolute temperature, and φB is the height of the Schottky gate.
The expression for the current due to TED can also be written as function
of the middle speed with which the electrons approach the gate interface.
This speed is known as the Richardson speed and is given by:
mKT
R πν 2= . (7)
So that the current density becomes:
( )[ ]1expexp... −⎟⎟
⎠
⎞
⎜⎜
⎝
⎛−= Vt
VaVtNcqJn B
R
φν . (8)
1.1.4. Current tunnel
The current tunnel is obtained with the product of the velocity and
density. The velocity is the Richardson velocity. The carrier's density
STUDY AND MODELING OF THE TRANSPORT MECHANISM 25 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
equals the available electron density, n, multiplied with the probability of
piercing a tunnel, Θ, to give: ... Θ= nqRnJ ν (9)
Here the probability tunnel is obtained by:
( )⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛−=Θ ε
φ 2/3*..234exp Bmq
h, (10)
and the electric field equals ε = φB/L. Therefore the current tunnel
exponentially depends on the height of gate, φB, to the 3/2.
1.2. The ohmic contact
A metal–semiconductor junction results in an Ohmic contact (a
contact with voltage independent resistance) if the Schottky gate height,
φB, is zero. In such case, the carriers are free to flow in or out of the
semiconductor so that there is minimal resistance through the contact. For
an n-type semiconductor, it means that the workfunction of metal must be
close to or smaller than the electron affinity of the semiconductor. A p-
type semiconductor, requires that the workfunction of the metal must be
close to or bigger than the sum of the electron affinity and the bandgap
energy. It can be problematic to find a metal that provides a p type Ohmic
contact with a semiconductor with a large bandgap such as GaN or SiC.
1.2. The ohmic contact
A metal–semiconductor junction results in an Ohmic contact (a
contact with voltage independent resistance) if the Schottky gate height,
φB, is zero. In such case, the carriers are free to flow in or out of the
semiconductor so that there is minimal resistance through the contact. For
an n-type semiconductor, it means that the workfunction of metal must be
26 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
close to or smaller than the electron affinity of the semiconductor. A p-
type semiconductor, requires that the workfunction of the metal must be
close to or bigger than the sum of the electron affinity and the bandgap
energy. It can be problematic to find a metal that provides a p type an
Ohmic contact with a semiconductor with a large bandgap such as GaN
or SiC.
1.2.1. The tunnel contact
A more convenient contact is a tunnel contact. Such contacts have
a positive gate to the metal semiconductor interface. If the width of the
region of the depletion to the metal–semiconductor interface is very thin,
of the order of 3 nm or less, the carriers can pierce a tunnel comfortably
through such a gate. The required doping density for such contact is a 1019
cm–3 or higher.
1.2.2. Resistance of contact
All sample or semiconductor structures are inevitably joined to
metallic lines of current transportation. It is indispensable that contacts
between the lines of transportation and the semiconductor allow the
current in the two direction to pass and present the weakest resistances
possible.
The resistance of a contact is defined by:
R=RC/S Ω.
Rc = specific resistor of contact (Ω·cm2 ) (resistor of contact) ; S = surface
of contact.
One can decrease this resistance while increasing the contact surface.
STUDY AND MODELING OF THE TRANSPORT MECHANISM 27 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
Fig. 1. Ohmic contact on semiconductor N.
Fig. 2. Ohmic contact on semiconductor P.
28 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
An ohmic contact on a SC'N' and SC'P' (Figs. 1, 2) is theoretically
possible with a metal working less than the output of a semiconductor.
Unfortunately this ideal situation is rarely achieved. In practice, one
decreases the resistance of a contact superficially by overdoping the
region where one wants to achieve contact: we obtain a degenerate buffer
layer (of 1019 to 1020 cm–3). An ohmic contact on a SC'N' and SC'P' (Figs.
1, 2) is theoretically possible with a metal working less than the output of
a semiconductor. Unfortunately this ideal situation is rarely achieved. In
practice, one decreases the resistance of a contact superficially by
overdoping the region where one wants to achieve contact: we obtain a
degenerate buffer layer (of 1019 to 1020 cm–3).
22.. RESULTS AND DISCUSSION
SSttrruuccttuurree ddiiooddee SScchhoottttkkyy ooff ttyyppee nn
We have used an SIM 3D simulator in our study,which is designed
the study of devices with small geometry. The I–V characteristics of
Schottky diodes in the hypothesis of a parabolic summit of the potential
and of a distribution of Boltzmann of the electrons.
2.1.1. Potential electrostatic and electric field
The electric field generated by a voltage of polarization presents an
intensity and a specific direction in every point in the ZCE. However it is
important to first determine all points of the electrostatic potential,
potential interest is the fact that the value of the electric field strength at a
point in drifts directly affects the variation of the potential (Fig. 3).
STUDY AND MODELING OF THE TRANSPORT MECHANISM 29 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
Fig. 3. Distribution of the potential to the out thermodynamic balance of Schottky diode of N type in GaAs (Plan XOY, Z=0.65μm))..
TThheerrmmooddyynnaammiicc bbaallaannccee rreeggiimmee
Fig. 4. Distribution of the potential to the thermodynamic balance of a
Schottky diode of N type in GaAs (Plan XOY, Z=0.65μm).
30 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
2.1.2. Distribution of the potential lines
Fig. 5. Distribution of the potential lines of the thermodynamic balance of
an N type Schottky diode in GaAs (Plan XOY, Z=0.65μm).
The electric field is characterized in every point of the domain by a
vector E(x, y, z) with a direction and an intensity (Fig. 4). In a three-
dimensional reference mark, it is marked by its three components scalar
Ex(x, y, z), Ey(x, y, z), Ez (x, y, z). The potential lines are generally given
by closed lines (Fig. 5). They include the loads and are perpendicular to
the lines of field.
The ZCE due to the metallurgic contact presents a width that
varies from inversely proportional to the concentration the doping of the
integrated layer. This zone is therefore important since it also presents a
intensity considerable electric field intensity due to the density of state of
the center traps condensed at the surface of the metallurgic diode.
STUDY AND MODELING OF THE TRANSPORT MECHANISM 31 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
2.1.3. The density of state of the center traps on I–V characteristics
Fig. 6. Variation of the NTD according to the voltage of polarization.
Fig. 7. Variation of the NTD according to the voltage of polarization.
32 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
Fig. 8. Variation of the NTD according to the voltage of polarization.
The simulation of the equilibrium state, shows the influence of
deep centers on I–V characteristics.
The density of electrons evolving between [of 6×1016 to 3×1020] is
inversely proportional JTED that evolves between [2.1×10–8 and 8.1×10–8
A] (Fig. 6/Fig. 7/Fig. 8).
The deep centers involved in a recombination mechanism such as
Shockley-Read is characterized by four new parameters that can vary
independently from each other: n1t (and p1t) (cm–3), which are functions of
the energy level in the forbidden gap; τnt and τpt (s–1) are related to the
capture efficient sections for electrons and holes, and also to the density
Nt (cm–3) of the centers.
The application of a forward biais voltage shows a reduction of the
transistion region ZCE, its variation depends to the deep trapping level
STUDY AND MODELING OF THE TRANSPORT MECHANISM 33 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
Δnt. Because N type semiconductors are always fully ionised, the second
trap center has a negligible influence on the capture of carriers due to its
low density. It can interfere in the recombination process, because of its
large capture coefficients due to its short lifetime (τnt and τpt: 10–8 to 10–10
s). An increase of the density of electrons through the N+ contact leads to
an increase of Δnt and consequently to an increase of the density of free
holes through the P+ contact leads to an increase of Δpt.
To achieve contact: one achieves a degenerated layer buffer (of
8×1016 to 3×1020 cm–3). The ZCE of the gate formed between the layer
buffer and the contact metal is so fine that the carriers can cross it by a
tunnel effect. The contact is no longer a rectifier and the characteristic
I(V) is symmetrical.
2.2. The current thermionic emission-diffusion (TED)
Fig. 9. Variation of the JTED current according to
the voltage of polarization.
34 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
Fig. 10. Variation of the JTED current according to
the voltage of polarization.
Fig. 11. Variation of the JTED current according to the
voltage of polarization.
STUDY AND MODELING OF THE TRANSPORT MECHANISM 35 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
The current through a metal–semiconductor junction is mainly due to majority carriers. Three distinctly different mechanisms exist in a Schottky diode: diffusion of the semiconductor carriers in metal, thermionic emission-diffusion (TED equation (6)) and a mechanical quantum that pierces a tunnel through the gate.
We conclude that the I–V characteristics of Schottky diodes in the hypothesis of a parabolic summit of potential.
Two mechanisms can cause breakdown, namely avalanche multiplication or impact ionization of carriers in the high electric field. Neither of the two breakdown mechanisms is destructive. However heating is caused by the breakdown voltage and diode may be destroyed unless sufficient heat sinking is provided. The breakdown in silicon can be predicted.
The introduction of deep centers in a semiconductor causes a disturbance of the characteristic I (V)
2.2.1. The current tunnel
Fig. 12. Variation of the JTunnel current according to the
voltage of polarization.
36 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
The GaAs and its large concentration values of traps centers, will participate in an increase of thermionic electrons and holes, which will in turn act on the I (V) characteristic of the diode, and it is the overflow
maximum value [NT=3×1020]. The characteristic I (V) is shown in Figs.
9–11. One notices that the Jtunnel current varies between 1×10–13 and
5×10–13 .This range of variation is very small in relation to the one of
JTED.
The current of thermionic emission-diffusion JTED is between
[6×10–8 A and 5×10–13 A]. The JTunnel current figure 12 (Eq. (9)) is
inversely proportional to the density of traps NT, whose values are
located between [8×1016 and 3×1020 cm-3 ] and is going to influence the
region of zone of desertion of carriers, act very quickly under the electric field effect the effect of the mechanism of transport of the Schottky diode. The electrons pulled out of the crystalline structure will be filled by other pairs of electrons holes created by the same phenomenon, this process repeats itself several times, until thermal saturation occurs, which leads to the straining of the diode.
Fig. 13. Variation of the JTED current according to the temperature.
STUDY AND MODELING OF THE TRANSPORT MECHANISM 37 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
3. THE TEMPERATURE
The ln(I)– V plots are generated at various temperatures and
ideality factors, and are estimated by a (of 1 to 2) fitting of simulated at
all temperatures (for 50 to 250 K) in Figure13, this is the range of
temperature that the thermionic emission-diffusion JTED current [of
6×10–8A and 9.5×10–8 A] remained steady at for small values. Beyond
this temperature, the current progresses in a brutal way until the Schottky
diode are destroyed. Electron transport may occur via shallow traps,
which are fewer in number, thus leading to relatively low ideality factor.
The interesting observation that the ideality factor increases above
unity depending on decreasing temperature is almost identical to the
reported variation of ideality factor obtained from the experimental data
on MIS contacts[19, 20]. Thus, an increase in ideality factor with decreasing
temperature is only possible if the interface state density is assumed to
have inverse temperature dependence. Inverse temperature dependence
implies that the interface states are effective at low temperature.
It can be further related to the available energy levels of interface
states. At higher energies there is a lower density of states, while at low
energies more states are available.
Thus, more of the energy levels of the interface states will be at
low energies. At low temperature, electron transport may occur through
deep level, traps states, whereas at high temperature due to the high
energies of electrons, shallow traps may participate in the conduction
process at the interface.
Therefore, at low temperatures electron transport occurs via deep traps states, of which there are more, so the effective density of states is higher and hence the resulting ideality factor arising due to potential drop
38 MMrr..AA..RREESSFFAA,, BOURZIG Y.SMAHI AND BRAHIMI.R.MENEZLA
across the layer increases. On the other hand, at high temperature electron transport may occur via shallow traps, which are fewer in number, thus leading to relatively low ideality factor. The similar inverse temperature dependence of interface state density derived from the experimental work on MIS Schottky diodes is also reported in Refs[21-24]. The actual temperature dependence of interface state density at the MIS junction governs the rising trend of ideality factor with decreasing temperature. The exact distribution of interface state density will shed more light on under standing the behaviour of MIS Schottky diodes, which requires more investigation and is open for future discussion.
4. CONCLUSIONS
The I–V characteristics of Schottky diodes in the hypothesis of a parabolic summit of the potential.The I–V characteristics of Schottky diodes with an interfacial insulator layer are studied by numerical simulation. The I–V data of the MIS Schottky diode are generated using the TED equation considering an interfacial layer parameter. The calculated I–V data are fitted into an ideal TED equation (6) to see the apparent effect of an interfacial layer on barrier parameters. It is shown that mere presence of an interfacial layer at the MS interface makes the Schottky diode behave as an ideal diode of high apparent BH. The apparent BH is shown to decrease linearly with decreasing the temperature. However, the ideality factor and series resistance remains the same as considered for a pure Schottky contact without an interfacial layer. It is shown that the bias coefficient of the tunneling barrier, however, increases the ideality factor, but makes the ideality factor decrease with decreasing temperature. It is considering that the potential drop across an interfacial layer also gives rise to high ideality factor, which remains constant at all temperatures. The inverse temperature dependence of interface states is suggested to be a possible reason for causing an increase in ideality factor with decreasing device temperature.
STUDY AND MODELING OF THE TRANSPORT MECHANISM 39 IN A SCHOTTKY DIODE ON THE BASIS OF GaAs SEMI INSULATING
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