STRUCTRAL, ELECTRICAL AND THERMOELECTRIC PROPORTIES OF CrSi 2 THIN FILMS by Makram Abd El Qader Candidate for Master of Science in Electrical Engineering Department of Electrical and Computer Engineering Department
STRUCTRAL, ELECTRICAL AND THERMOELECTRIC PROPORTIES OF
CrSi2 THIN FILMS
by
Makram Abd El Qader
Candidate for Master of Science in Electrical Engineering
Department of Electrical and Computer Engineering Department
Dept. of Electrical and Computer Engineering
University of Nevada, Las Vegas
Thermoelectric Materials and Application
Clean source of energy – Power generation upon application of heat gradient
2
PN couple used as TEG (a)-Seebeck effect, and TEC (b)-
Peltier effect
(a) (b)
Thermoelectric phenomena and coefficients
In Solid state thermoelectric devices
Diffusion Principle in materials Mobile charge carriers Thermal gradient
Charge build up (e-) & (h+) Electrostatic potential (voltage) Seebeck effect-
thermoelectric generation (TEG)
The efficiency of power generation in thermoelectric devices is
determined by its dimensionless figure of merit (ZT):
ZT=α2σ/κ
α is the Seebeck coefficient µV/K,
σ is the electrical conductivity Ωm, and
κ is the thermal conductivity W/m-K.
The thermoelectric performance can also be evaluated by the
power factor
P=α2/ρρ is the resistivity Ωm
Dept. of Electrical and Computer Engineering
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Dept. of Electrical and Computer Engineering
University of Nevada, Las Vegas
Background and Thermoelectric Phenomena
1. Approximately 90% of the world’s electricity is produced by heat energy
as a result of burning fossil fuel
2. Production plants typically operate at 30-40 per cent efficiency, loosing
around 15 terawatts of power in the form of heat to the environment.
3. Waste Heat sources are found almost in every process and electronic
devices (Residential heating, automotive exhaust, and industrial
processes
4. Thermoelectric power generators can convert some of this waste heat
into useful power
5. Thermoelectric devices are potential power source due to their direct
conversion of thermal gradients into electric current.
6. Electronic devices, International space station and Satellites, Automobile
companies, Power plants
4
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Potential thermoelectric materials
CrSi2, FeSi2, CoSi2,…. …
Characteristics of Silicides
Partially filled d- orbital's-
Seebeck value much higher
High melting point and
chemical stability at high
temperatures
Relatively low thermal
conductivity values
Transition
metal silicides
5
Materials with highest figure of merit
BiT2 and SbTe hold the highest ZT values of 3
A good thermoelectric material should
have low electrical resistivity, low thermal
conductivity, and a large Seebeck
coefficient.
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Motivation
6
The electrical and thermal properties of a material are determined by the same crystal
and electronic structure
Usually:
They cannot be controlled independently. The challenge is to find ways to decouple the
electrical and thermal properties
keys:
Study thermoelectric materials in Thin Film form. This may cause a change in the
material thermal and electrical properties
2-D dimensions
Precise controlled composition
Easy to create defects-doping, process conditions..
Scalable for small/large devices
Theoretical studies predict better enhanced ZT with low dimensional structures.
The issue
Study the structural, electrical, and thermoelectric properties of CrSi2 thin films to better
enhance the ZT.
Literature data on CrSi2
Physical property Value
Energy gap indirect band gap 2.7eV
Carrier type P type 4×109 cm3
Bulk electrical resistivity at (RT) 0.9 mΩcm
Bulk Seebeck coefficient at (RT) 96µV/K
Bulk thermal conductivity at (RT) 10W/mK
Thin film crystallization temperature 300˚C
Crystal structure Hexagonal structure
Space group P6222
Lattice parameters a= b= 4.4220Å, c=6.351Å
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Structural, thermal, and electrical properties of bulk CrSi2 are well studied.”
“Structural, thermal, and electrical properties of CrSi2”, by T. Dasgupta, J. Etourneau,.”
electrical and structural properties of ( 50nm) thin film of sputtered CrSi2” Electrical and
structural properties of thin films of sputtered CrSi2”, by S.F. Gong a, X.-H. Li a..”
Electrical, structural, and transport properties of CrSi2/ Si (111)
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Outline
PART 1
Thin Film Preparation - Experiments on Thin Film samples
Thin film processing
Energy Dispersive X-ray diffraction (EDAX)
X-ray Diffraction (XRD)
Four probe point resistivity measurement
Seebeck coefficient measurement
Power factor measurement
Results and discussion drawn on thin film samples
Final conclusions and Future work
8
PART 2
Design and assembly of three gun sputtering system
Design motivation
Design methodology
Results and discussion drawn from system pump down
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quartz glass substrates( κ=1.38W/mK, R=1018Ωm) were prepared by:
Aquasonic deionized water bath, methyl alcohol, dried out with nitrogen
gas, and heated.
1µm and 0.1µm CrSi2 thin films were prepared by RF sputtering
Thin film processing
9
Process condition Value
Base pressure (torr) 1.2×10-7
Ar gas pressure (mtorr) 1
RF power supplied (W) 200
Target substrate
distance (inch)
3
Pre- sputtering time
(min)
10
Deposition time (min) 7 min for 0.1µm, 37min
for 1µm
Sputtering Process chamber
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Thin film thickness measurement
Surface Profiler Veeco Dektak 6M Stylus Profilometer
S.No
Deposition Time
(min)
Thickness (µm)
1. 5 0.08
2 10 0.12
3 30 0.75
4 45 1.2
5 60 1.4
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The obtained thin films have a step profile
similar to the one show below
Step Profile
Deposited CrSi2 material
Glass substrate
11
Thin film annealing
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In order to find out the effect of temperature, the thin film samples were
annealed under argon gas (Ar) ambience.
Annealing Temperature (T) = 300˚C, 400 ˚C, 500 ˚C, 600 ˚C
Argon gas Pressure (P) = 695 torr
Duration time (t) = 60 and 120 minutes
12
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Thin film - Compositional Analysis
The compositions of processed thin filmsamples were verified by
performing Energy Dispersive X-ray Analysis (EDAX).
JOEL JSM – 5600 Scanning Electron Microscope, Energy = 15keV
13
Thin film samples with 0.1µm thickness have
shown an atomic composition of Cr=37.64%
and Si=62.36%.
Thin films samples with 1µm thickness have
shown an atomic composition of Cr=39.27%
and Si=60.73%
The obtained results show that the discrepancy between the
compositions of the target material and thin films are less than
5%.
Thin film microstructure images-Scanning Electron Microscope (SEM)
0.1 µm thin film as sputtered 0.1 µm thin film after annealing at 300˚C
1 µm thin film as sputtered 1 µm thin film after annealing at 300˚C
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Dept. of Electrical and Computer Engineering
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Thin film – Structural Analysis
15
X-ray diffraction pattern were taken using a Bruker-AXS D8 Vario Advance
using a Johansson-type primary monochromator with Cu kα1 emission
λ=1.54063Å
Incident beam angle θ= 5˚
Reflected angle 2θ=10-90˚
The Rietveld structure refinement allows peaks
fitting by calculating the structure factors for
each lattice plane by applying :
pseudo-Voigt type profile functions (Thompson-
Cox-Hastings)
fundamental parameter approach.
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Thin film structural analysis
16
The obtained results from the Rietveld refinement for all samples regarding their
Bragg residuals:
indicates the difference
between the calculated and
measured intensities
Scaling factor: gives an
indication about amount of
the phase in the material
The refined lattice
parameters
Sample ID R-Bragg
Refinement
Residual (<< 5%)
Scaling Factor Refined cell
parameters, a and c
(Å)
CriS2 as-sputtered NA NA NA
CriS2 300C 1h1.103
0.0002094.449, 6.293
CriS2 400C 1h 1.292 0.0002354.4331, 6.317
CriS2 500C 1h 1.705 0.00016374.4152, 6.3359
CriS2 600C 1h 1.309 0.0002804.443, 6.244
CriS2 300C 2h1.249
0.0002584.445, 6.285
CriS2 400c 2h 1.353 0.0002994.4289, 6.304
CriS2 500c 2h1.891
0.00022484.4127, 6.3382
CriS2 600C 2h 1.388 0.00027504.4304, 6.2981
X-ray diffraction refinement values for CrSi2 1µm thin films
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Thin film structural analysis
17
R-Bragg Refinement Residual much less than 5%, thus fit is excellent. lattice parameters
obtained for various thin films are in the within the expected values for CrSi2.
X-ray diffraction refinement values for CrSi2 0.1 µm thin films
Sample ID R-Bragg Refinement
Residual (<< 5%)
Scaling Factor Refined cell
parameters, a and c
(Å)
CriS2 as-sputteredNA
NANA
CriS2 300C 1h0.646
0.0002194.438, 6.280
CriS2 400C 1h 0.814 0.0002654.439, 6.253
CriS2 500C 1h 0.625 0.0002644.425, 6.262
CriS2 600C 1h 0.538 0.0004524.435, 6.272
CriS2 300C 2h0.512
0.0001934.420, 6.286
CriS2 400c 2h 0.602 0.000263 4.433, 6.260
CriS2 500c 2h0.581
0.0002254.423, 6.265
CriS2 600C 2h0.691
0.0002344.439, 6.271
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Thin film structural analysis- diffraction patterns
18
1µm thin film- 1 hour annealing time- 300˚C-
600˚C
1µm thin film- 2 hour annealing time-
300˚C- 600˚C
Crystallization of the hexagonal modification
of CrSi2 was observed at 300˚C
Crystallization became better at higher
annealing temperatures.
The diffraction pattern for 1 hr. is dominated
by the (111) and (112) peak intensities, and for
2 hr. is dominated by the (111),(112), and (003)
peak intensities.
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Thin film structural analysis- diffraction patterns
0.1µm thin film- 1 hour annealing time-
300˚C- 600˚C
0.1µm thin film-2 hour annealing time-
300˚C- 600˚C
Crystallization of the hexagonal modification
of CrSi2 was observed at 300˚C
Crystallization became better at higher
annealing temperatures.
There is no change in the peak intensities
between 1 hr. and in the 2 hr. annealed samples
This indicates that 0.1µm CrSi2 thin films are
fully crystallized at 1 hr.
Dept. of Electrical and Computer Engineering
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Seebeck coefficient measurement
20
Seebeck voltages of 1µm and 0.1µm thin films were measured for various annealing
temperatures in the range of 100˚C-600˚C for two different annealing times, 1hr and 2 hr.
A Seebeck voltage measurement device was designed and built to measure the Seebeck
coefficient of the CrSi2 films at room temperature
The estimated accuracy of the seebeck coefficient measured was ±5%, and was verified by
measuring the Seebeck coefficient of Ni samples in both bulk and thin Film form with
known Seebeck coefficient values
Seebeck coefficient measurement apparatus at 20˚C ΔT
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Seebeck coefficient results
0
10
20
30
40
50
60
70
80
90
0 200 400 600 800Seeb
eck c
oeff
cie
nt
(µV
/K)
Annealing temperatures(C˚)
Seebeck coefficient (µV/K)-1hr annealing
Seebeck coefficient (µV/K)-2 hr annealing
0
10
20
30
40
50
60
70
0 100 200 300 400 500 600 700
Seeb
eck c
oeff
cie
nt
(µV
/K)
Annealing temperatures(C˚)
Seebeck coefficient (µV/K)-1hr annealing
Seebeck coefficient (µV/K)-2 hr annealing
1µm thin film
0.1µm thin film
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2d
1
22
Seebeck coefficient discussion
Seebeck coefficients in general increase with the annealing temperature for both
thicknesses and annealing times up to 400oC. This behavior is directly related to the
better crystallinity of the thin films at higher annealing temperatures.
In the temperature range of 400 to 500oC, all plots show a sudden change in Seebeck
coefficient
Seebeck coefficient saturates at around 60µV/K for 0.1 µm thin films
.For 1 µm thin films annealed for 1 hr. the Seebeck coefficient shows a plateau
between 400 and 500oC and then increases and reaches 81µV/K close to the reported
bulk value of 96µV/K, whereas the 2 hr. annealed thin film shows a decrease
This difference behavior of the 1 µm thin films can be related to the degradation of the
thin film micro- structurally with the creation of voids and cracks at higher annealing
temperature and longer annealing times.
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Thin Film resistivity measurement
23
Resistivity of 1µm and 0.1µm thin films for various annealing temperatures in the range
300oC-600oC for two different annealing time, 1hr and 2hr.
Four probe point resistance measurement apparatus (ASU-Newman Group) was used
at room temperature
Thin film resistivity values were calculated using
with
t is the thin film thickness
s is the spacing between the probes
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Thin Film resistivity results
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600 700
Resis
tivit
y (
mΩ
-cm
)
Annealing temperatures(C˚)
Resistivity (mΩ-cm)- 1 hr annealing
Resistivity (mΩ-cm)- 2 hr annealing
0.1µm thin film
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Thin Film resistivity results
Resistivity of 1µm thin films couldn’t be measured due to their high resistance values
which exceeded the limitation of the measurement system
It is estimated that 1µm thin films have a resistance value larger than 1MΩ. Based this
estimate, the resistivities of the annealed 1µm thin films were calculated to be larger than
0.000453 MΩ-cm, while the as deposited show to have resistivity of 1.197mΩ-cm.
For both annealing times, 1hr. and 2hr., 0.1 µm thin films show that the resistivity
increases with annealing temperature till 300oC and reaches a value of 0.9 mΩ-cm,
which is close to the reported bulk value and then decreases till 400o C and then saturates
The increase in resistivity is consistent with the film become more crystalline with
temperature. Decrease of resistivity beyond 400oC cannot be explained. This needs to be
investigated further.
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Thermoelectric power factor measurement
The thermoelectric power factors, P, of 0.1µm thin films was calculated and
plotted for various annealing temperatures in the range of 300˚C-600˚C for two
different annealing times, 1hr. and 2 hrs.
The thermoelectric power factor, P for 1µm thin films could not be calculated as
resistivity, which is necessary for the calculation could not be measured due to the
limitation instrument.
The calculations of the power factor were done using the following equation:
P=α2/ρ (W/K2 m)
where
α is the Seebeck coefficient
ρ is the resistivity
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Thermoelectric power factor results
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
0 100 200 300 400 500 600 700
Po
wer
Fac
tor
( W
/K2
m)
Annealing temperature C˚
power factor 0.1 1hour
annealed
power factor 0.1 2hour
annealed
Thermoelectric power factor increases with annealing temperature from 300oC to
400oC and saturates at about 0.9 x 10-3 W/(K2.m) beyond 400oC for 0.1µm thin films
annealed for 2 hrs
0.1µm thin films annealed for 1 hr, thermoelectric power factor increases with
annealing temperature from 300oC to 500oC and saturates at about 1.1 x 10-3 W/(K2.m)
beyond 500oC
This behavior can be attributed to increase in crystallinity in the higher annealing
temperature range.
0.1µm thin film
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PART 1-Results and discussion
28
Seebeck coefficient and resistivity increases linearly, between 100˚C to 300˚C
this correlates well with the observation of increased crystallinity of the deposited
thin films.
The difference measured Seebeck coefficients between 0.1 µm and 1 µm thin
films annealed in this temperatures range is very minimal.
The resistivity results show a marked difference with 0.1 µm exhibiting
measurable values in the range of 0.2 to 0.9 mΩ-cm, and 1 µm thin films have
resitivities larger than 0.000453 MΩ-cm
This difference is related to the drastic difference in the mictrostructure between
the two thicknesses. Annealed 1 µm thin films exhibit a large density of pores,
where as 0.1 µm thin films exhibit a smooth texture.
Both 0.1 and 1 µm thin films show a transition in Seebeck coefficient between
300oC and 400oC
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PART 1- Results and discussion
0.1 µm thin film showing a plateau beyond the transition temperature and 1 µm thin film
showing a plateau for about 100 C range and then increasing further for shorter anneal times
and a peak at the transition temperature for longer anneals.
Degradation of properties for 1 µm thin films with longer duration of anneal may be related
to degradation of the thin films microstructurally. In other words, cracks and voids may
cause the degradation.
0.1 µm thin films show a peak in resistivity around 300oC
Decrease of resitivities beyond 300˚C anneal is unclear
1 µm thin films have resistivity larger than the limits of the instrument. Such high
resistance may be a result of porosity observed in the annealed films.
Thermoelectric power factors for 0.1 µm thin films with respect to annealing temperatures
show a behavior similar to that of Seebeck coefficients, increasing with temperature and
reaching a plateau value of 1.0 x 10-3 W/(K2 m) at around 400o C to 450o C
Results and discussion
Due to highly resistive nature of 1 µm thin films, the thermoelectric power factor for
these films has an upper estimate of 6.403×10-6 W/(K2 m)
These results suggest that annealed 400˚C thin films of thicknesses in the range of
0.1µm are more suitable for device applications when glass substrates are employed.
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PART 2-Design of Three Gun Sputtering System
Design motivation
limitation of the current sputtering system in the solid
state fabrication laboratory at
UNLV.
Investigate ternary and higher order
thermoelectric alloys
Better control over process conditions ( gas
input, heat, rotation, vacuum level, etc….)
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Design of Three Gun Sputtering System
Design considerations
Precise inert gas control
Deposition yield
monitoring
Heating capability for
substrate oxide remove
99% pure films
High vacuum level ( 10-9
sale)
multiple target materials / DC,
RF power
Ion beam etching and
cleaning capability
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Design of Three Gun Sputtering System
Three gun sputtering system building blocks:
Oil sealed rotary mechanical pump (MP)
Molecular drag pump (MDP)
Turbo-molecular pump
CTI Cryogenic pump
Vacuum process chamber
Convectron gauge
Ionization gauge
Capacitance manometer gauge
Mass flow controller
Crystal thickness monitor (QCM)
Substrate table- heat and rotation
Residual gas analyzer (RGA)
Sputter sources
Ion gun
Gate valves
Water chiller
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Design of Three Gun Sputtering System
Solid works design
A drawing of the stainless steel 6 way
cross chamber
A schematic diagram showing top
flange-housing for sputter guns and
shutters
A schematic diagram of top flange with sputter
sources and shutters installedschematic diagram of the three sputter sources-guns used
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Design of Three Gun Sputtering System
Solid works design
A schematic diagram illustrating the focus of the three guns
to the location of the substrate
A drawing of the of the deposition chamber
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Design of Three Gun Sputtering System
System assembly
A photograph showing the three gun sputtering system
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Design of Three Gun Sputtering System
photograph showing an inside look of the chamber
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Design of Three Gun Sputtering System
A symbol representation of the 3 gun sputtering system
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Three gun sputtering system results
Residual gas analyzer results
The quadrupole gas analyzer spectra's are plots of versus partial pressure
It is observed from above spectra that when the system was turned on for the first
time, high Nitrogen (N) at of 28 and Oxygen (O2) of 32 peaks, were observed
making the vacuum level to stay in 10-05 Torr scale.
Quadrupole gas analyzer spectrum after initial pump down
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39
Three gun sputtering system results
Before (Yellow) and after (Green) RGA spectrum showing effect of reducing the
foreline pressure of the turbopump by adding a molecular drag pump
It was observed from the green RGA spectrum that the vacuum level in the chamber
gets much better (10-7 torr) after solving the problem of compression ratio by installing
the molecular drag pump between the turbopump and mechanical pump
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c
40
Three gun sputtering system results
Quadrupole gas analyzer spectrum of ratio versus partial pressure-
At the present
The system pumped overnight to the mid 10-09 Torr range, leaving the water
peak of 18 as the major one as expected
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c
41
PART 2- Results and discussion
In order a deposit ternary and higher order alloys, a three gun sputtering
system was designed, built and tested for its level of vacuum levels and
cleanliness.
The tests showed that the three-gun sputtering system is of vacuum levels of
10-9 torr and shows extremely low level of impurities and is ready for future
sputtering works in this area.
Dept. of Electrical and Computer Engineering
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Dept. of Electrical and Computer Engineering
University of Nevada, Las Vegas
Conclusion
43
CrSi2 films of two different thicknesses were prepared by rf sputtering.
As deposited and annealed (300˚C to 600˚C) were characterized for their structural,
electrical, and thermoelectric transport properties
As-sputtered CrSi2 film is amorphous at room temperature and crystallizes around
300˚C independent of thickness.
The Seebeck voltage of the1µm films increase sharply with annealing temperatures
and reaches a value of 81µV/K, which close to that of bulk CrSi2, and 62µV/K for
0.1µm films
These results suggest that annealed thin films of thicknesses in the range of 0.1µm
round 400˚C are more suitable for device applications when glass substrates are
employed.
Dept. of Electrical and Computer Engineering
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Recommendation and Future work
Based on our experience with CrSi2 deposition and characterization, and also the design
and assembly of the three gun sputtering system, the following issues are recommended
for future investigation:
Investigation of the structural behavior of the 1µm CrSi2 thin films at annealing
temperatures greater than 300C. In other words, identify the reasons for the film to crack
with annealing.
Study of the electrical and thermoelectric properties as a function of thin film
composition before and after annealing.
Measurement of the thermal conductivity of all deposited thin films before and after
annealing, to allow us calculate the thermoelectric figure of merit ZT.
Use of the designed three gun sputtering system to better sputter CrSi2 thin films.
44
Dept. of Electrical and Computer Engineering
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Acknowledgment Committee members:
Dr. Rama Venkat
Dr. Ravhi Kumar
Dr. Thomas Hartmann
Dr. Nathan Newman
Group members:
Stan Goldfarb
Dr.Paolo Ginobbi
Brandon blackstone
Nirup Bandaru
Jorge Reynaga
Eric Knight
Mike Shappie
Friends and family:
I would like to thank my parents, my
family, and my freinds for their great
support. I would like to thank my brothers
Charbel Azzi and Charles Azzi on their
great support too.
Dept. of Electrical and Computer Engineering
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I would like to thank the following companies on their support for making the design of the 3
gun sputtering system possible:
Engineering college-Electrical and computer engineering Department
College of sciences- Physics Dept- High pressure center
UNLV Graduate College
Ron Powell; Novellus
Steve Schwartz and Steven Michaud; Brooks Automation
Dan Watt
John Brooks and Tom Bogdan; MDC;
Fred Van der Linde
Craig Hall; Ferrofluidics
Dave Mahoney; Rigaku
Richard Osburn NCCAVS
Ralph Brogan; Pumps International
Mike Ackeret; Transfer Engineering
Neal Ely; Las Positas College
Todd Johnson and Harry Grover; MeiVac
Larry Lu; CLuLab
Will Hale; AJA International
Mark Bernick; Angstrom Sciences
Chris Malocsay; Semicore
Paul Becker; Fil-Tech
Neil Peacock and Dick Jacobs; MKS
Doug Schatz; Advanced Energy
Mark Bernick; Angstrom Sciences
Don Sarrach; Plasmaterials
Dept. of Electrical and Computer Engineering
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THANK YOU ALL