c© 2016 by May-Ling Marie Li. All rights reserved.

THE C33 ELASTIC CONSTANT OF MoS2 AS A FUNCTION OF PRESSURE ANDTHE DEPOSITION OF MULTILAYER THIN FILMS BY SPUTTERING

BY

MAY-LING MARIE LI

THESIS

Submitted in partial fulfillment of the requirementsfor the degree of Master of Science in Materials Science and Engineering

in the Graduate College of theUniversity of Illinois at Urbana-Champaign, 2016

Urbana, Illinois

Adviser:

Professor David G. Cahill

ABSTRACT

The c33 elastic constant of exfoliated MoS2 flakes was measured up to 11 GPa using a diamond anvil cell and

picosecond interferometry. The resulting elastic constants were similar those predicted by hybrid density

functional theory calculations by Peelaers and Van de Walle, but lower than those calculated from lattice

constant measurements by Fan. However, due to the failure of the Lorentz-Lorenz relationship between

index of refraction n and density, the change in n was not compensated for. This means that the actual c33

constants are lower than those that have been reported here.

A two source sputtering chamber for deposition of [Co,Pt] multilayer transducers for use in time-resolved

magneto-optical Kerr effect (TR-MOKE) experiments is also discussed, along with the preliminary film and

transducer characterization. Four-point probe measurements of the thin films indicated high resistivities

corresponding to roughness or small grain size. A multilayer transducer was tested on a SiO2 reference wafer

and had a dθK/dT of the same order of magnitude as similar [Co,Pt] transducers.

ii

ACKNOWLEDGMENTS

I would like to thank my adviser Prof. David Cahill for guiding me through this work and the entire Cahill

research group for their help with my experiments and calculations. I am also deeply grateful to Prof.

Pinshane Huang, Prof. Jay Bass and his students, and Prof. Bill King and his students for lending me their

knowledge and use of their equipment to prepare and characterize my samples.

This work was financially supported by the Carnegie-DOE Alliance through grant DE-FC52-29908NA28554.

iii

TABLE OF CONTENTS

Chapter 1 Elastic Constant of MoS2 Under High Pressure . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Chapter 2 Two Source Sputtering Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2 Characterization of Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3 [Co,Pt] Multilayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

iv

CHAPTER 1

ELASTIC CONSTANT OF MoS2 UNDER HIGHPRESSURE

1.1 Introduction

Molybdenum disulfide (MoS2) is a hexagonal layered material comprised of layers of Mo atoms covalently

bonded between planes of S atoms. These S-Mo-S layers are weakly bound together by Van der Waals forces.

It has long been understood to be a good lubricant due to this weak interlayer bonding[1], and has more

recently has attracted attention for use in optoelectronic[2] and electronic applications[3, 4].

The mechanical and electrical properties of materials are known to change under pressure, which can

be useful for tuning physical properties for different applications. There have been multiple theoretical and

experimental studies investigating crystal and electronic structural changes[5, 6, 7]. One important property

that has received less focus is the elastic behaviour of MoS2.

The elastic constants for a material relate its deformation to the applied stress. This is important

for understanding the mechanical behavior of a material[8] and the thermal transport. The number of

independent elastic constants depend on the structure of the material. For material with hexagonal symmetry

there are five independent elastic constants[9]. While there have been some measurements of the MoS2’s

elastic constants under ambient conditions[10, 11], there has been limited experimental work under pressure.

Here I have measured the out of plane Young’s modulus c33 using Brillouin scattering.

Feldman used neutron and x-ray scattering data to calculate the approximate elastic constants of MoS2,

and found c33 to be 52 GPa[10]. More recently Zhao also measured c33 as 52.0 GPa in few-trilayer MoS2

using Raman spectroscopy[11].

From x-ray diffraction studies of the lattice constants it is known that the lattice constant c is more

compressible than a and that it is most easily compressed when pressure is first applied, but as the interlayer

spacing decreases and the atoms are forced into closer proximity the interaction strength increases and the

rate of change with pressure decreases[5, 6]. The changes in the lattice constants lead to the expectation

that the stiffness of the MoS2 will rapidly increase. The hybrid density functional theory calculations by

Peelaers predict an order of magnitude increase in c33 over 40 GPa[12].

1

1.2 Methods

Diamond Anvil Cell and Sample Preparation

gasket

Pressure mediumAr, H2O, silicone oil, …

Pump/probeDiamond

sample

Figure 1.1: Schematic of a diamond anvil cell showing the diamonds, gasket, and sample. The sample canbe probed through either diamond. Adapted from [13].

A diamond anvil cell (DAC) is a common tool for investigating the properties of materials at high

pressures. There are several types of DACs, but they all have the same basic principle of operation[14].

A DAC holds two diamonds mounted so that their flat culets are aligned and parallel. To use it a soft

metal gasket is first indented by the two parallel diamond faces. A hole drilled is drilled in the center of

the gasket which is then placed back on the diamond face which acts as the bottom of the sample chamber

formed by the hole. The sample and a pressure medium are placed in the chamber and the diamond faces

are brought into contact with the gasket, deforming it, and sealing the chamber as shown in figure 1.1. The

pressure medium is there to ensure that hydrostatic pressure is applied to the sample. The pressure in the

chamber depends on the force F applied and the area of the diamond culets A as P = F/A. The force on

the diamonds can be changed by adjusting the screws holding them together. In this manner it is easy to

generate several GPa of pressure in the chamber of the DAC.

One of the advantages of using diamonds is that they are transparent in the visual, infrared, ultraviolet,

and x-ray regions of the electromagnetic spectrum, which allows a great number spectroscopic techniques to

be performed in situ on a sample inside a DAC, including X-ray diffraction, Brillouin scattering, and Raman

scattering.

I used a Boehler-Almax Plate DAC with diamond culets of 400 µm with 0.01 inch thick rhenium gaskets

from H Cross. The gaskets were indented to 0.073-0.079 inches and a 190 µm diameter hole was drilled using

2

an electrical discharge machining tool (EDM).

The samples were prepared from natural MoS2 from SPI by micromechanical exfoliation onto 500 nm

SiO2 wafer pieces. To exfoliate the flakes, adhesive tape is used to cleave the MoS2 sample until it is very

thin. The clean surface is then pressed onto a substrate and light pressure is applied. When the tape is

removed flakes remain adhered to the substrate though Van der Waals forces. This produces a wide range of

flake size and thickness making it necessary to identify flakes of suitable size and thickness. MoS2 on SiO2

exhibits a color change towards gold as the number of layers increases becoming completely gold at greater

than 50 layers, so an optical microscope was used to identify flakes 100-150 µm wide and at least 50 layers

thick. An optical profilometer was used to determine the smoothness of the flake’s surface and to establish

the approximate thickness. The flakes used were between 1-10 µm.

Loading the MoS2 flake into the diamond anvil cell requires the use of microtools and a stereoscopic

microscope in order to see and manipulate the small flakes. I initially tried to move the flakes using SiC

microtools, but I discovered that the force required to dislodge the flakes from the substrate tended to break

them. In order to avoid damaging the flakes I used isopropanol to detach the flake from the substrate and

a flexible tungsten needle to move the flake into the sample chamber. A silicone oil (octamethyltrisiloxane)

from Sigma-Aldrich was used as a pressure medium. A syringe was used to add it to the sample chamber

after the sample had been place.

(a) Brillouin scattering. Image from [15].

sample

objective lens

spectrometer

notch filters

488 nmlaser

focusing lens

mirrors

aperturebandpass filter

(b) Low frequency Raman scattering.

Figure 1.2: Experimental set ups for measurements

3

Brillouin Spectroscopy and Raman Spectroscopy

Brillouin spectroscopy is a non destructive optical technique that detects the change in frequency of light

inelastically scattered frrom acoustic phonons in a material. It is commonly used for the measurement of

elastic constants and acoustic velocity[16]. There are two basic types of Brillouin scattering: spontaneous

and stimulated. In spontaneous scattering the thermal fluctuations in the intensity of the acoustic wave while

in stimulated scattering an acoustic pulse is generated and intensity of the measurement is proportional to

its amplitude[17]. The signal in stimulated scattering is larger[18].

1 0 0 2 0 0 3 0 0 4 0 0- 1 0

- 5

0

V in (µV

)

T i m e D e l a y ( p s )

Figure 1.3: Periodic oscillations detected using picosecond interferometry on a MoS2 flake inside the DACat 8.8 GPa. The high frequency oscillations at short delay times are from the MoS2, and the lower frequencyoscillations are from the silicone oil pressure medium.

In order to measure the Brillouin frequency f in the diamond cell, a type of stimulated Brillouin spec-

troscopy know as picosecond inteferometry was used[19]. A pulsed femtosecond Ti:sapphire laser mode-locked

at 785 nm with a repetition rate of 80 MHz was split into a pump and probe beam, and the probe beam

was sent through a mechanical delay stage. The pump and probe beam are both focused onto the same

spot on the sample and the signal from the reflected probe beam is measured using a Si photodiode. The

full experimental setup is shown in figure 1.2a. When the pump beam is incident on the sample it causes

4

a rapid rise in temperature which creates a longitudinal strain pulse due to thermal expansion. The probe

beam weakly reflects from the moving acoustic pulse which causes constructive or destructive interference

with the stronger reflections from the rest of the sample[19, 20]. The photodiode detects this as a periodic

oscillation that has the Brillouin frequency.

Inside the DAC the thermal expansion of the sample can also create a strain wave in the pressure medium.

This allows the detection of the Brillouin frequency of the pressure medium and the sample. An example of

this is shown in figure 1.3.

For my experiments a 20x objective lens with a numerical aperature of 0.28 was used to focus the pump

and probe beam onto the MoS2 sample, and data was taken from both sides of the DAC. In order to maximize

the signal from the DAC, pump and probe powers between 9 and 13 mW were used. Multiple sets of data

were taken and used to average out the noise to ensure a very clear signal using the principle that the noise

will vary in each data set while the actual signal will not. Taking the average thus reduces the noise without

changing the signal.

Low frequency Raman scattering was also done using a 488 nm solid state laser. The experimental set

up is shown in figure 1.2b. Raman measurements were only taken from the front side of the DAC

- 5 0 0 0 5 0 00

5

1 0

1 5

2 0

2 5

3 0

Norm

alized

Inten

sity (m

W-1 s-1 )

R a m a n s h i f t ( c m - 1 )

E 22 g

A 1 g

E 12 g

6 . 2 G P a

7 . 5 G P a

8 . 8 G P a

Figure 1.4: Raman peak shift of MoS2 with pressure.

5

Pressure Determination

I tried two methods to determine the pressure in the diamond cell: using the Brillouin signal from the

silicone oil which had previously been calibrated by Greg Hohensee[13] and Raman shift of the MoS2 which

was measured at low pressures by Sugai[7].

The Raman spectra for MoS2 is shown in figure 1.4. From the spectra it can be seen that the peaks all

increased in response to pressure and that the intensities decreased. Although variations in peak intensity

with pressure were seen in the measurements by Chi, it did not consistently decrease as pressure inceased[21].

The Raman system was realigned and calibrated between the lower two measurements in order to reduce

the Rayleigh scattering being detected. In order to avoid any zero off-set calibration errors the Stokes and

Anti-Stokes peaks were measured and averaged to be symmetric. The uncertainty in the measurements

found using Si and bulk MoS2 reference samples was 0.5 cm−1. I used the E22g, E1

2g, and A1g peaks that I

measured to find the pressure by comparing them to the values measured by Sugai[7]. The results of this

and the Brillouin frequencies from the silicone oil are shown in 1.5.

0 1 2 3 4 50123456789

1 01 11 2

Pressu

re (G

Pa)

D i a m o n d C e l l P r e s s u r e

B r i l l o u i n E 2

2 g E 1

2 g A 1 g

Figure 1.5: The DAC pressure measured using the Brillouin scattering from the pressure medium and theRaman shift of the MoS2. The x-axis refers to each time the pressure in the diamond cell was increased.The y-ais shows the pressure calibration of my data to Hohensee and Sugai’s results.

6

The error bars for the Raman shift were calculated by combining the uncertainty in my measurements

with that in the calibration data for each peak. For the uncertainty in the Brillouin frequency I similarly

combined the uncertainty in the frequency measurements with the uncertainty in the calibration data.

However, the calibration done by Hohensee in this pressure range had a lot of data and had a relatively

small uncertainty[13]. At higher pressures the uncertainty is significantly larger due to fewer data points

being available.

- 2 0 2 4 6 8 1 0 1 2 1 4 1 6 1 802 04 06 0

3 8 0

4 0 0

4 2 0

4 4 0

4 6 0

E 12 g S u g a i

E 22 g S u g a i

A 1 g S u g a i E 2

2 g E 1

2 g A 1 g L i t e r a t u r e

Rama

n shif

t (cm-1 )

P r e s s u r e ( G P a )

Figure 1.6: Measured Raman shift compared with data from Sugai[7]. The y-axis is the pressure measuredfrom the Brillouin frequency. Literature values for ambient MoS2 peaks are from [22, 23].

The E12g shift indicated a lower pressure than the A1g shift and the Brillouin scattering from the silicone

oil. However, the pressures indicated by the A1g and Brillouin scattering were very similar. Surprisingly, the

E22g mode is the most dissimilar. However, part of this discrepancy in pressures could be because the Raman

shift I measured is lower at all pressures than Sugai. This is shown in figure 1.6. Even at ambient pressure

I measured all the modes except E22g as being significantly lower. This suggests that there is a systematic

offset in the data. Established literature values for the Raman peaks agree more closely with my data than

with Sugai’s measurements. Given this, combined with the fact that the Brillouin signal from the silicone

oil was very clear, with the exception of the data taken when the DAC was first loaded, and provided a more

precise measure of the pressure than the Raman shift, I chose to use it as my method of determining the

7

pressure.

1.3 Results and Discussion

Brillouin Signal Strength

As the pressure was increased inside the DAC, the Brillouin signal from the MoS2 increased in strength.

This effect exists independently of variations in the probe power used to take the measurements, which

also increase the signal strength. The signal strength, corrected for different pump and probe powers and

normalized to the signal strength of the flake under ambient conditions, is shown in figure 1.7.

- 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2

0 . 1

1

Amplit

ude

P r e s s u r e ( G P a )

Figure 1.7: The normalized amplitude of the Brillouin signal from MoS2 in the DAC at different pressures.The amplitude for the same MoS2 flake at ambient pressure when not loaded in the DAC is shown forcomparison.

Inelastic light scattering is comprised of three basic processes: an incoming photon excites an electron-

hole pair, the electron-hole pair is scattered by the lattice creating a phonon and another electron-hole pair,

this electron-hole pair decays and produces a new photon. The final photon produced will have the energy

of the initial photon shifted by that of the phonon. The first process mentioned dominates the scattering

cross-section[17].

8

MoS2 is an indirect semiconductor with bandgap that has been experimentally measured as 0.8 eV, 1.29

eV, and 1.2 eV[24]. The band-gap in most semiconductors is sensitive to strain, and DFT calculations of the

band structure for MoS2 have shown that the band-gap of MoS2 decreases with pressure[12, 25]. This would

cause an increase in the scattering cross-section by increasing the number of electron-hole pairs excited and

could account for the observed increase in signal strength. The decrease in bang-gap energy is also predicted

to be non-linear. As the band-gap continues to decrease at higher pressures I expect the Brillouin signal to

increase as well, but it will be interesting to see if the rate of change tracks that of the band-gap energy.

Elastic Constant Calculation

The Brillouin frequencies were extracted by taking the FFT of the oscillations. Because the backscattering

geometry was used, the Brillouin frequency (f) is related to the longitudinal speed of sound (vL) in the

material by

f =2nvLλ

(1.1)

where n is the index of refraction and λ is the wavelength of the incident laser[16].

The c33 elastic constant is related to vL by Christoffel’s equation for a hexagonal system along the Z axis

vL =

√c33ρ

(1.2)

where ρ is the density of the material[9].

Combining these equations gives the direct relationship between the Brillouin frequency and c33

c33 = ρ

(λf

2n

)2

(1.3)

To get the change in ρ with pressure I used the bulk density of 5.06 g cm−3[26] and a third order Birch-

Murnaghan fit to Aksoy’s data on the change of unit cell volume with pressur[5]. The change in density also

corresponds to a change in the refractive index. In order to model this I tried to use the Lorentz-Lorenz

relationship

n2 − 1

n2 + 2=

4π

3αρ (1.4)

where α is the polarizability of the material. I used n and ρ at ambient pressure to find α, then assuming

α remained constant, graphically solved for n at different pressures. However, I found that the relationship

does not hold in this situation: n increased rapidly from 4.7[27] at ambient pressure to >10 by 11 GPa. At

only 20.5 GPa it becomes unsolvable.

9

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 50

1 0 0

2 0 0

3 0 0

P e e l a e r s F a n F r o n t B a c k

C 33 (G

Pa)

P r e s s u r e ( G P a )

Figure 1.8: The c33 elastic constant for MoS2 at different pressures. The DFT calculation by Peelaer andthe elastic constants from the pressure derivative of Fan’s measurement of the c lattice constant are shownfor comparison[28]. The c33 error bars do not include the uncertainty from the change in n with pressure.

The resulting elastic constants are shown in figure 1.8. Error bars are shown for the pressure and the

elastic constants. The uncertainty in the pressure in the same as what is shown in figure 1.5. As the

Brillouin signal becomes larger the uncertainty in the Brillouin frequency decreases. The uncertainty shown

for c33 stems from error propagation of the uncertainty in the measurements of the Brillouin frequency, the

index of refraction, and the density. The uncertainty in the density is taken from the uncertainty values

reported in [5] for the volume of the unit cell. The values of the index of refraction for MoS2 reported in

the literature vary widely[29], but there are only two reported values for the light wavelength used in my

experiments[30, 27]. I estimated the uncertainty in the index of refraction from them.

Slightly different MoS2 Brillouin frequencies were measured at different positions on the sample, which

leads to c33 varying by as much at 14 GPa. This indicates that there might be local pressure gradients in the

material. However the overall trend is that c33 increases linearly as the material is compressed and stiffens.

This is what is predicted by the hybrid density functional theory (DFT) calculations performed by

Peelaer and Van de Walle using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof along with

the semiempirical Grimme D2 correction[12]. My results show a similar rate of change, but are higher than

10

those predicted by Peelaers. This could be partially due to the uncompensated change in the index of

refraction. Since the index of refraction will have increased with pressure the elastic constants shown should

be slightly lower, which would make them more similar to those predicted by Peelaers.

I also calculated c33 from the finite element pressure derivative of the c lattice constants reported by

Fan[28] for comparison. These values show a similar trend as the ones I measured and Peelaer’s calculations,

but are significantly higher. I attribute this to the large amount of uncertainty for this type of calculation.

Under ambient conditions c33 has been reported as 52 GPa using neutron scattering data with an uncertainty

of 20%[10]. However, more recent studies of c33 measured with Raman scattering report values of 52.0

GPa[11], and 59.2 GPa[31] for MoS2, which is in agreement with my value of 53.4 GPa.

A way to model the change in n with pressure will be need to be determined in order to more accurately

measure c33, as well as gathering data over a wider range of pressures in order see if the linearity holds.

However, thus far the measured c33 is in agreement with the available work in the literature.

11

CHAPTER 2

TWO SOURCE SPUTTERING CHAMBER

2.1 Introduction

One of the experimental techniques used in the Cahill group is time-resolved magneto-optical Kerr effect

(TR-MOKE), which can be used to measure the thermal properties of materials. A thin ferromagnetic

transducer is deposited on top of the sample and the changes in the Kerr rotation with temperature are

measured. In order to facilitate these experiments I built a two source sputtering chamber to deposit [Co,Pt]

multilayers for use as transducers. A diagram of the chamber is shown in figure 2.1.

turbo pump

V+

V+ V+

P

shutters

heated sample stage

rough pump

Ar

Ar

door

sputter sources

H2O cooling lines

Figure 2.1: Schematic diagram of the sputter chamber. P1 is a convection gauge, P2 is a cold cathode iongauge and P3 is a capacitive manometer.

12

Sputtering is a simple physical vapor depostion method that can be used to easily create thin metal films.

An Ar plasma is used to remove atoms from a target of the desired thin film material. The released material

is then deposited onto the substrate as long as the distance between the target and the substrate is smaller

than the mean free path of the atoms. The deposition rate can be controlled by the distance between the

target and the substrate, the Ar pressure, and the power of the plasma, but in practical terms only the latter

two options are easily changed by the operator.

The chamber I built is similar in design to other sputter chambers used in the Cahill group for thin metal

film deposition, with a few notable differences. The top flange of the chamber has two 1.5” target sputter

sources mounted on it along with a viewing port that allows visual monitoring during deposition. The two

targets makes it possible to deposit a multilayer film without exposing it to contamination. This is very

important for materials that are prone to oxidation and need a capping layer before they are exposed to

atmosphere to prevent the formation of native oxides that could change the properties of interfaces. Each

source has a manual shutter that can be used to alternate deposition of the materials. A fast entry door

allows for easy loading and unloading of samples onto the heated sample stage.

2.2 Characterization of Films

In order to calibrate the deposition rate of the chamber for each material 10 minute depositions were

performed at 20 W and 1 mTorr of Ar with a base pressure better than 2×10−7 mTorr. I attempted to

measure the thickness of the films using picosecond acoustics. This technique uses the same configuration

as the Brillouin scattering discussed earlier, but can only be done using an optically thick thin film on top

of the sample[32]. The film propagates a strain wave through the sample that reflects off of interfaces and

changes the reflectivity of the film when it reaches the surface. This is detected as a series of regularly spaced

“pulses” in the data collected by the photodiode that correspond to twice the thickness of the material. The

deposition times I used were not long enough to create optically thick films and instead these measurements

resulted in Brillouin scattering.

The Brillouin scattering for the 10 minute deposition of the Co layer is shown in figure 2.2. The first set

of oscillations has a period of 6.3 ps and is a breathing mode of the cobalt. This was used with the sound

velocity calculated from the density 8.86 g/cm3[26] and c33 357 GPa[33] in the <001> direction to calculate

the wavelength of the breathing mode as 40 nm, which corresponds to the twice the thickness of the layer.

The breathing mode oscillations are continue until 36.5 ps when the strain wave reaches the SiO2 layer. This

produces a much longer oscillation with a period of 51 ps. However, the layer is not thick enough to see a

13

0 5 0 1 0 0 1 5 0 2 0 0- 1 0

0

1 0

2 0

3 0

4 0

5 0

6 0

V in (µV

)

T i m e D e l a y ( p s )

Figure 2.2: Pump-probe measurement of a 20 nm Co film on 500 nm SiO2 on Si. The breathing modes ofthe Co and Brillouin scattering of the SiO2 and Si can be clearly seen.

full period. At 39 ps the wave reaches the SiO2/Si interface, which results in short oscillations again. As the

delay time increases the Co and Si scattering begin to cause interference with each other leading to mixed

oscillations.

To further confirm the thickness of the layers I also used single wavelength ellipsometry. The Co film

was about 20 nm thick and the Pt film was about 30 nm. Between the two methods of measuring the film

thickness I established a deposition rate of 0.33 A/s for the Co and 0.5 A/s for the Pt. This is a deposition

ratio of 1.51 , which is higher than the ratio of 1.15 that has previously been found for 600 eV Ar+ ions[34].

Part of the reason for this discrepancy is that the targets in this chamber are at slightly different distances

and angles with respect to the substrate, which will change their deposition rate.

The individual Co and Pt films were further characterized by measuring the electrical resistivities using

a four-point probe. The resistivities (ρ) for the Co and Pt films were 18 ×10−8 Ωm and 17 ×10−8 Ωm

respectively. These are higher than the bulk values of 5.2 ×10−8 Ωm and 10.6 ×10−8 Ωm[35]. While this

is not uncommon for very thin films due to scattering effects, previous studies of high quality Pt films have

shown that ρ approaches bulk values in films thicker than 10 nm[36][37]. This indicates that there could be

high surface roughness or small grain size in the films I deposited. The surface roughness could be further

14

Silicon

SiO2

Pt

Co

Figure 2.3: The [Pt (1 nm)/Co (0.5 nm)]x6/Pt(11 nm) transducer. Drawing not to scale.

investigated and quantified using an optical profilometer. In the case of the Co film it is possible that

oxidation has changed the resistivity since the four-point probe measurements were not done in a vacuum.

Co is known to form a native oxide of 8-10 A[38].

2.3 [Co,Pt] Multilayer

A magnetic transducer for TR-MOKE experiments requires a large enough Kerr rotation angle θk to be

detected and needs to be perpendicularly magnetized. Both of these characteristics have been extensively

studied for [Co,Pt] multilayers and depend on the ratio of Co to Pt layer thickness, number of bilayers, and

total thickness of the multilayer as well as various growth parameters for the films[39]. It has been shown

that the θk is increased for films thinner than 300 A[40]. A higher Co content also increases the θk, but too

much Co can cause a loss of perpendicular magnetization in the films; 3-7 A thick layers of Co are generally

considered a good range[41]. An increasing number of bilayers also increases θk [42]. Using a thicker buffer

layer of Pt on the sample can improve the texture of the multilayer, which enhances θk[43]. When balancing

all these factors there are a number of different structures that will work for a [Co,Pt] multilayer transducer.

I chose to deposit a multilayer of [Pt (1 nm)/Co (0.5 nm)]x6/Pt(11 nm) on top of 500 nm SiO2 on Si wafer

(shown in figure 2.3). This structure is not near any of the extremes that can cause the transducer to lose

perpendicular magnetization, which also makes it an appropriate test structure in case the deposition does

not proceed exactly as planned. It has previously been used for TR-MOKE measurements[44], thus it makes

15

a good test of whether or not this sputter chamber can deposit usable [Co,Pt] multilayers.

The resulting TR-MOKE data is shown in figure 2.4. Since the properties of the SiO2 wafer are known,

it was possible to fit for the thickness of the transducer, which was 26 nm. For the fit the volumetrically

averaged heat capactity 2.94 J/K-cm3 of the Co and Pt layers was used. The thickness found was greater

than the planned 20 nm stack, indicating either the deposition rates or the timing during the multilayer

deposition were slightly off. The layers were deposited at 20 W for 15 s for each Co layers and 20 seconds for

the Pt layers. The Pt buffer layer was deposited for 220 s. The deposition rate of the films could be decreased

by lowering the power used in order to increase control. The deposition rate should change linearly with

power, but in reality it would be better to actually measure the film thickness to determine the deposition

rate at each power used. One possible source of the discrepancy is if the substrates used for the thickness

depositions were not placed in the exact same position as the substrate used for the multilayer deposition.

The uniformity of the flux for this chamber has not be tested, and it is possible that there is enough variation

to account for part of the 6 nm difference.

1 0 1 0 0 1 0 0 0

1 . 5

2 . 0

2 . 5

3 . 0

3 . 5

4 . 0

-V in/Vou

t

T i m e D e l a y ( p s )

Figure 2.4: TR-MOKE data and fit for [Co,Pt] transducer on a SiO2 reference sample

This transducer is not optically thick, so it cannot be used for time domain thermo-reflectance (TDTR)

measurements, which utilizes the change in the reflectance of the transducer with temperature to measure

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thermal conductivity[15]. However, future multilayers could be made optically thick by increasing the buffer

layer, which would allow the direct comparison of the the two techniques.

The dθK/dT for this transducer was calculated to be about 5×10−5 K−1. This is the same order of

magnitude for similar [Co,Pt] multilayers that have been previously used by our group. For this calculation

I used the volumetric average of the absorption coefficients and the heat capacities for Co and Pt assuming

the ideal structure shown in figure 2.3.

Further testing and optimization of the [Co,Pt] multilayers will need to be done, but preliminary results

indicate that this sputter chamber is capable of producing usable transducers for TR-MOKE experiments.

It also has the potential to be modified for cosputtering and the deposition of different multilayers in the

future.

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