THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE
DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING
INVESTIGATION OF METAL THIN-FILMS ON BORON ARSENIDE AND HEXAGONAL BORON NITRIDE
RAJEH SALAH R. ALSAADI SPRING 2020
A thesis submitted in partial fulfillment of the requirements for a baccalaureate degree in Materials Science and Engineering with honors in Materials Science and Engineering
Reviewed and approved* by the following:
Suzanne E. Mohney Professor of Materials Science and Engineering and Electrical Engineering Thesis Supervisor
Robert A. Kimel Associate Teaching Professor of Materials Science and Engineering Honors Adviser
* Electronic approvals are on file. i
ABSTRACT
Boron arsenide (BAs) and hexagonal boron nitride (h-BN) are of great interest for the development of next-generation electronic and optoelectronic devices due to their potential for superior performance for heat management and as insulating two-dimensional layered materials, respectively. Studies of metal electrical contacts to BAs, or heterojunctions involving metals and h-BN, are scarce in the literature. A device with BAs as an active semiconducting layer would require electrical contacts, so reactivity at metal/BAs interfaces were assessed based on condensed phase equilibria for metal-B-As systems. A MATLAB program was utilized to calculate the ternary phase diagrams for BAs with Pt, Mo, Cr, Ti, Au and Ag at room temperature. Platinum, Mo, Cr and Ti demonstrate are predicted to have exhibit a thermodynamic drive to react with BAs, while Au and Ag are in thermodynamic equilibrium with BAs. Assessment of stability of thin metal films on h-BN involved collection of condensed phase equilibria for metal-B-N systems from the literature, which showed that the early transition metals Cr, V, Ti, Zr, and Al have a thermodynamic driving force to react, while later transition metals Fe, Co, Ni, Mn, Au, Ag, Cu and Cd are expected to be stable on h-BN. Additionally, a gold thin film was deposited at room temperature on h-BN via electron beam evaporation and was analyzed by x-ray diffraction (XRD). No reaction products were detected between Au and h-
BN. The (111) plane of Au was detected in a Bragg-Brentano geometry, but no symmetric or asymmetric Au peaks were observed from preliminary phi-scans to confirm epitaxy. Since room- temperature epitaxy has been observed for many metals on MoS2 and WSe2, future work could study similarities or differences in the behavior of metals on h-BN. ii
TABLE OF CONTENTS
LIST OF FIGURES ...... iii
LIST OF TABLES ...... v
ACKNOWLEDGEMENTS ...... vi
Chapter 1 Introduction ...... 1
1.1 Motivation ...... 1 1.2 Boron Arsenide: Structure, Properties and Applications ...... 3 1.3 Hexagonal Boron Nitride: Structure, Properties and Applications ...... 3 1.4 Metal Thin-Films ...... 5 1.4.1 Physics of Metal-Semiconductor Interfaces ...... 5 1.4.2 Epitaxy of Metal Thin-Films ...... 6 1.4.3 Phase Equilibria and Ternary Phase Diagrams ...... 7 1.5 Accreditation Board for Engineering and Technology (ABET) Considerations ...... 9
Chapter 2 Computational and Calculation Methods ...... 11
2.1 Ternary Phase Diagrams ...... 11 2.1.1 Calculation Method ...... 11 2.1.2 Thermodynamic Data Collection and Selection Criteria ...... 14 2.1.3 MATLAB Code ...... 15
Chapter 3 Boron Arsenide: Results and Discussions ...... 18
3.1 Condensed Phase Equilibria and Ternary Phase Diagrams...... 18 3.1.1 Literature Review of Thermodynamic Data for M-B-As Systems...... 18 3.1.2 Calculated Ternary Phase Diagrams for M-B-As Systems ...... 36
Chapter 4 Hexagonal Boron Nitride: Results and Discussions ...... 39
4.1 Condensed Phase Equilibria and Ternary Phase Diagrams...... 39 4.2 Experimental Procedures ...... 41 4.2.1 Deposition of Metal Thin-Films on Boron Nitride...... 41 4.2.2 Characterization of Metal Contacts on Boron Nitride ...... 42 4.2.3 Safety Considerations ...... 42 4.3 Epitaxy of Metal Thin-Films ...... 43
Chapter 5 Conclusions ...... 48
BIBLIOGRAPHY ...... 50
iii
LIST OF FIGURES
Figure 1.1 Hexagonal boron nitride (h-BN) crystal structure [7]...... 4
Figure 1.2 Energy band diagram of Ohmic contact [23]...... 6
Figure 1.3 Energy band diagram of an n-type Schottky diode [23]...... 6
Figure 1.4 3-D ternary phase diagram [31]...... 9
Figure 1.5 Isothermal Plot [30]...... 9
Figure 2.1 An example of a ternary phase diagram with all possible tie lines...... 12
Figure 2.2 Final ternary phase diagram with only stable tie lines...... 14
Figure 2.3. A screen shot of the Excel sheet for B-As-Mo system...... 16
Figure 3.1 Pt-B binary phase diagram [50]...... 21
Figure 3.2 Pt-As binary phase diagram [52]...... 22
Figure 3.3 Mo-B binary phase diagram [57]...... 24
Figure 3.4 Gibbs free energy of formation of Mo-B [57]...... 25
Figure 3.5 Mo-As binary phase diagram [60]...... 26
Figure 3.6 The Cr-B binary phase diagram [64]...... 28
Figure 3.7 Cr-As binary phase diagram [71]...... 30
Figure 3.8 The Ti-B binary phase diagram [72]...... 32
Figure 3.9 Au-B binary phase diagram [84]...... 33
Figure 3.10 Au-As binary phase diagram [86]...... 34
Figure 3.11 Ag-B binary phase diagram [87]...... 35
Figure 3.12 Ag-As binary phase diagram [88]...... 36
Figure 3.13 Calculated Pt-Ba-As ternary phase diagram...... 37
Figure 3.14 Calculated Mo-Ba-As ternary phase diagram...... 37
Figure 3.15 Calculated Ti-Ba-As ternary phase diagram...... 37
Figure 3.16 Calculated Cr-Ba-As ternary phase diagram...... 37 iv
Figure 3.17 Calculated Au-Ba-As ternary phase diagram...... 37
Figure 3.18 Calculated Ag-Ba-As ternary phase diagram...... 37
Figure 4.1. Bragg-Brentano XRD measurement for the Au/h-BN/Al2O3 sample, which indicates the presence of Au(111) and Al2O3(0001) basal plane peaks...... 45
Figure 4.2. Asymmetric phi-scan at the Al2O3(024) peak, which shows no Au film peaks. ... 46
v
LIST OF TABLES
Table 2.1 Example reaction matrix for Gibbs energies of reaction in kJ/mol of atoms...... 13
Table 3.1 Gibbs free energy of formation of the B-As system...... 20
Table 3.2 Gibbs free energy of formation of Pt-B binary system...... 21
Table 3.3 Gibbs free energy of formation of Pt-As binary system...... 22
Table 3.4 Gibbs free energy of formation of Mo-B binary system...... 24
Table 3.5 Gibbs free energy of formation of Mo-As binary system ...... 26
Table 3.6 Gibbs free energy of formation of Cr-B binary system...... 28
Table 3.7 Gibbs free energy of formation of Cr-As binary system...... 29
Table 3.8 Gibbs free energy of formation of Ti-B binary system...... 31
Table 3.9 Gibbs free energy of formation of Ti-As binary system...... 33
Table 4.1. Thermodynamic stability of various metals with BN [34]...... 40
Table 4.2 Classifications of M-W-Se Systems [11]...... 41
Table 4.3. Homologous temperatures of various metals in an ascending order with stable metal with BN being bolded...... 47
vi
ACKNOWLEDGEMENTS
First and foremost, I would like to thank my thesis supervisor Professor Suzanne Mohney for giving me the opportunity to conduct research and be part of her group, and for offering support and mentorship during the last three years. This experience has improved my technical and analytical skills, which will help me to succeed in my academic career in the future.
Additionally, I would like to express my greatest gratitude to my graduate mentor Kayla Cooley for providing help and guidance through my journey in this group, and for her kind encouragement and thoughtful advice. My thanks also go to my dear fellows in Mohney group for offering help and support. I would like to thank Anushka Bansal and Nichole Wonderling for providing help with my experimental work. Moreover, I would like to acknowledge King
Abdullah University of Science and Technology for a generous scholarship. Finally, I thank my family for their love and support.
1
Chapter 1
Introduction
1.1 Motivation
Computers and electronic devices have shrunk in size and their performances have improved dramatically. Over the past decades, the gate length of transistors has been minimized to nearly 10 nm, allowing packing of billions of field effect transistors (FETs) on a single 1-inch chip [1]. With this scaling of device dimensions and creation of more complex structures, a major drawback that follows this development is the amount of heat produced by these devices.
The heat flux produced by the modern FETs at the nanoscale is comparable to that of the Sun
[1]. This degrades the performance of electronic devices and requires additional supporting cooling systems like fans or cooling water, which is not cost-effective. A feasible solution is the use of high thermal conductivity (k) materials that efficiently conduct the heat flux and dissipate it. One of the currently used materials for heat management is diamond, which has a thermal conductivity near 2200 W m-1 K-1 [2], the highest value reported in the literature. However, high cost and difficulty of synthesis prevent diamond from being widely used. Searching for alternative less expensive candidates requires consideration of the criteria for high thermal conductivity, which includes strong bonds that act like stiff springs and small mass contrast of constituent atoms [3]. Additionally, with the major development of high-speed RF switching materials like graphene and the scaling of FETs, finding excellent insulating substrates for packaging and high-k gate dielectrics remains a demanding requirement to enhance their 2 performances [4-5]. For instance, SiO2 substrates have been found to degrade intrinsic transport properties of graphene due to scattering of charge carriers at the interface [5]. Addressing this issue requires pristine and inert interfaces free of dangling bonds and charge traps.
In this thesis, boron compounds are considered to address the technological needs described above. Boron arsenide (BAs) [2] and hexagonal boron nitride (h-BN) [5-7] have attracted huge attention due to their potential for superior performances in heat management and as insulating two-dimensional layered materials, respectively. In particular, this thesis considers the integration of these compounds with metals as electrical contacts. In order to integrate BAs and h-BN in electronic devices and to optimize their performances, high-quality metal thin-films are needed; metal-semiconductor and metal-insulator junctions have an immense impact on power consumption and dissipation [4]. Ideally, metal contacts to a semiconductor like BAs need to be Ohmic and have low resistance and not consume the BAs and not control the BAs in an uncontrolled reaction [4]. Understanding the physical and chemical interactions at these interfaces is crucial to improve performances of electronic devices [8-9].
Up-to-date, studies of metal electrical contacts to BAs, or heterojunctions involving metals and h-BN, are scarce in the literature. This situation motivates a search for high-quality metal thin-films to BAs and h-BN, and to investigate the interactions that take place at their interfaces. The study of electrical metal contacts to BAs follows the same thermodynamic method used by the former undergraduate student in our research group Yitian Zeng that is described in his published paper on WSe2 [10]. The investigation of epitaxy of metal films on
BN follows the same approach as in prior collaborative work in which I was involved on epitaxy of metal contacts to the layered material WSe2, as described by our published paper [11]; this approach involves both experimental studies and thermodynamic predictions. 3 1.2 Boron Arsenide: Structure, Properties and Applications
A promising candidate material for heat management applications that has attracted huge attention in recent years is boron arsenide (BAs), which has a predicted thermal conductivity similar to diamond at room temperature [2]. BAs exhibits high thermal conductivity due to the tight spacing between the acoustic phonon branches, large frequency gap between acoustic and optic phonon branches and weak phonon-isotope scattering [2]. Kang et al. (2018) [12] have successfully obtained the largest measured thermal conductivity reported in the literature of 1300
Wm-1 K-1 through a modified chemical vapor transport growth process. BAs has a zinc-blende face-centered cubic (FCC) crystal structure with a lattice parameter of 4.777 Å and bond length of 2.069 Å [13-14]. Boron arsenide can exist in two phases, boron mono-arsenide (BAs) or a sub-arsenide (B12As2); further explanation of their stabilities will be discussed in Chapter 4.
1.3 Hexagonal Boron Nitride: Structure, Properties and Applications
Boron nitride (BN) is an insulating material that exists in three crystalline forms: hexagonal boron nitride (h-BN) that is similar to graphite, sphalerite boron nitride (β-BN) that is similar to cubic diamond, and wurtzite boron nitride (γ-BN) that is to hexagonal diamond [15].
Among these allotropes, h-BN is the most thermodynamically stable form [3], and it is the one of interest in this thesis. h-BN exhibits a 2D layered structure with strong and directional covalent bonds connecting boron and nitrogen atoms within the plane of each layer, and the planes are held together by weak van der Waals forces as shown in Figure 1. With such a crystal structure, it is important to note that its properties are anisotropic. 4
Figure 1.1 Hexagonal boron nitride (h-BN) crystal structure [7].
Similar to other 2D materials, h-BN is flexible and transparent, which makes it attractive for flexible optoelectronic applications [16]. h-BN has a strong covalent bond with length of
1.45Å and a small nitrogen to boron mass ratio, making it an excellent thermally conductive material with a predicted thermal conductivity (k) range of 300-2000 W/m-1 K-1 and an experimental value of 390 W/m-1 K-1 [2,7,16-17]. Nonetheless, h-BN has a wide band gap of 5.9 eV, dielectric constant of 6 and a breakdown voltage of around 8-10 MV/cm, which makes it an excellent candidate for gate dielectrics and insulating applications [18-20]. It has been proven that it is reliable and stable against voltage stress and current leakage [21]. With an analogous structure and a small lattice mismatch of 1.7% to that of graphene, h-BN outperforms SiO2 as a dielectric substrate due to the smooth dangling bond free surface [22]. Other applications including thermal radiators, field emitters and UV emitters have also been reported [5]. 5 1.4 Metal Thin-Films
1.4.1 Physics of Metal-Semiconductor Interfaces
When a metal and semiconductor are brought into contact, they form either a Schottky diode or an Ohmic contact depending on the energy barrier at the metal/semiconductor interface.
One model for predicting the barrier height is the Schottky-Mott model, which often applies best for ionic semiconductors. In this model, the work function (Φ), defined as the difference between vacuum energy and Fermi level of a material, is important. For simplicity, the following explanation is for the example of an n-type semiconductor, and it is only true for this type. A rectifying Schottky diode is formed when the work function of the metal is greater than that of the semiconductor (Φm > Φs), creating a depletion region as shown in Figure 1.2. In contrast, a non- rectifying Ohmic contact is formed when the work function of the metal is smaller than the work function of the semiconductor (Φm < Φs), creating an accumulation region as shown in Figure 1.3.
In the first case, a Schottky barrier is formed, which acts as a potential energy barrier for charge carriers to move across the metal/semiconductor junction. The Schottky barrier height (Φb) is predicted to be the difference between the work function of the metal (Φm) and the electron affinity of the semiconductor (χ). In the second case, current is able to flow freely in both forward and reverse bias. From a power-consumption standpoint, high operating current densities at the lowest voltage drop across the metal-semiconductor junction are desired. 6
Figure 1.2 Energy band diagram of Ohmic contact [23].
Figure 1.3 Energy band diagram of an n-type Schottky diode [23].
The previous explanations hold for ideal metal-semiconductor interfaces. However, in
real-life electronic devices, defects and metal induced gap states at the interface can pin the
Fermi level at particular energies, making the barrier height more independent of the metal work
function and the semiconductor electron affinity [23]. Epitaxy at a metal/semiconductor interface
is also interesting since it has been shown that atomic arrangement affects the barrier height at
the interface [24-26]. These factors give rise to the need for high quality metal-semiconductor
junctions and a thorough understanding of the factors that yield such junctions.
1.4.2 Epitaxy of Metal Thin-Films
Growth of epitaxial metal thin-films are desired to reduce defects at the metal-
semiconductor interface. Forming epitaxial metal contacts can be affected by several factors 7 including lattice mismatch. For 3D semiconductors like BAs, low lattice mismatch with the metal film is desired; otherwise, the metal film will be strained and then relaxed to form misfit dislocations [27]. On the other hand, 2D semiconductors like h-BN with van der Waals forces and no dangling bonds at their surfaces, the lattice matching constraint is relaxed [28]. Indeed, it has been shown that epitaxial metal films can be grown on 2D semiconductors such as transition metal dichalcogenides (TMDs) with lattice mismatches exceeding 30% [11, 28-29]. The epitaxial layer (epilayer) orients itself to the plane with symmetry that matches that of the substrate [27]. Cooley et al. (2019) [11] showed that only metals with FCC structure formed epitaxial metal films on WSe2 with the (111) plane of FCC metals on the (0001) plane of WSe2.
Homologous temperature is another important factor for epitaxy, which is the ratio of deposition temperature (Td) to the melting temperature of the deposited metal (Tm), both in
Kelvin. Higher homologous temperature favors the formation of epitaxial films as the metal atoms become more mobile and therefore able to reach to more energetically favorable positions.
Domask et al. (2018) [29] have shown that metals with high homologous temperatures tend to be epitaxial with the 2D MoS2. When discussing epitaxy of metal films later in the thesis, lattice mismatch, planar orientation and homologous temperatures will be considered.
1.4.3 Phase Equilibria and Ternary Phase Diagrams
Thermodynamics is another critical factor to understand chemical reactions at metal- semiconductor interface. It helps to predict whether metals would interact with the semiconductor or would they both exist in equilibrium. Upon chemical reactions at the interface, new phases would exist between the metal and the semiconductor; this process might be 8 undesired in many cases as these new phases can consume the semiconductor. Phase diagrams are thermodynamic tools that describe phases that exist in equilibrium under certain conditions including compositions of constituents, pressure and temperature.
Ternary phase diagrams are composed of three pure elements with four variables: temperature, pressure and compositions of two of the components [30]. To construct a ternary phase diagram with the four variables, a complex four-dimensional representation is required.
However, for simplicity, pressure is typically held constant at atmospheric pressure in most of the cases in the literature. This results in a 3D triangular prism model with the temperature being plotted on the vertical axis, and the composition plotted on the base of the prism as shown in
Figure 1.4. The vertical sides of the prism in the case of Figure 1.4 represent the three binary systems, AC, CB and BA namely.
Ternary phase diagrams calculated and discussed in this thesis are at room temperature
(298 K), which reduces the 3D diagrams to 2D isothermal plots as shown in Figure 1.5. The equilateral triangle isothermal plot is a cross-section cut of the 3D triangular prism. Points on the sides of the equilateral triangle represent the binary alloys, and the points within the area of the triangle represent the ternary alloys. When two compounds of the ternary phase diagram are in equilibrium, a straight tie line connects them together, and it represents a two-phase equilibria region. At equilibrium, tie lines cannot intersect, except in special cases, because it violates the phase rule 9
Figure 1.4 3-D ternary phase diagram [31]. Figure 1.5 Isothermal Plot [30].
1.5 Accreditation Board for Engineering and Technology (ABET) Considerations
The Accreditation Board for Engineering and Technology (ABET) promotes engineering technologies and designs that address and take into consideration global challenges including economic effects, environmental issues, sustainability, manufacturability, ethical issues, health and safety, and social and political impacts [32]. As stated earlier, modern electronic devices generate high amounts of heat that require additional cooling systems like fans or cooling water.
The use of high thermal conductivity material like BAs emerges as a promising solution, and this thesis facilitates the implementation of BAs in next-generation electronic devices by considering the stability of interfaces between metals and BAs, where heat and possibly even current must be 10 conducted. Such a solution takes into account some of the ABET considerations by promoting sustainability and addressing environmental concerns. Lab safety considerations are discussed in
Section 4.2.3.
In line with the shrinking dimensions and increasing of power density of modern electronics, operation at high temperatures requires fast and efficient heat sinking solutions to prevent melting of the electrical components [4]. Cooling systems like fans in computers, for example, are relatively large and are not suitable for small and portable electronic devices. BAs serves as a sustainable solution that could be integrated in next-generation electronics.
Due to its crystal structure, h-BN is an attractive dielectric to integrate into emerging electronic devices prepared from 2D semiconductors, which may improve our day-to-day life with inexpensive and possibly flexible electronics [5]. Moreover, with the growing concerns of harmful environmental impacts of fossil fuels, the development of safe, sustainable and efficient energy-generating sources is an essential next-generation technology, and electrochemical-based fuel cells are attractive solutions to fulfill these requirements [33]. High-performing electrocatalysts that exhibit high activities and long-term durability are critical in the development of fuel cells. A study showed that h-BN could serve as an excellent electrocatalyst for oxygen reduction reactions (ORR) when supported by conducting materials like Au [34]. The electrocatalyst performance depends critically on the h-BN/metal interface and the stability of the conducting metal [34]. Thus, the study of metal/h-BN interfaces in this thesis promotes for the development of alternative energy-generating sources.
11 Chapter 2
Computational and Calculation Methods
2.1 Ternary Phase Diagrams
2.1.1 Calculation Method
The calculation of ternary phase diagrams for metal-semiconductor systems follows the work of Klingbeil and Schmid-Fetzer [35]. The first step is to construct an equilateral triangle with elements set at the corners and all existing intermediate stoichiometric compounds, which represent the binary phases between any two elements, set along the sides. There were no known ternary phases to consider in our study. All possible lines connecting elements and stoichiometric compounds are drawn; each line represents a possible two-phase equilibrium between the phases at the endpoints. An example is illustrated below in Figure 2.1 with A, B and C as the elements and AC, AB and BC as the intermediate stoichiometric compounds. 12
Figure 2.1 An example of a ternary phase diagram with all possible tie lines.
Not all of the possible lines can be tie lines, since no intersecting lines are allowed as it violates the phase rule except under special conditions. For intersecting lines, the dominant line is the most thermodynamically stable line against the formation of other lines. If two lines intersect, a chemical reaction can be written with the phases at the endpoints of one line as the reactants and the other line’s phases as the products. For instance, the balanced chemical reaction of the A-BC and AC-AB intersecting lines from Figure 2.1 can be written as:
2A + BC → AC + AB
With known Gibbs free energy of formation (ΔGf°) of each phase in the chemical reaction, Gibbs free energy of reaction (ΔGR) can be calculated and compared. A positive ΔGR means that the reactant line, A-BC, dominates and is more stable, whereas a negative ΔGR means that the product line, AC-AB, dominates and is more stable. The unstable lines then are deleted and removed, resulting in a ternary phase diagram with no intersecting lines, only tie lines. 13 ΔGR needs to be calculated for every possible combination of intersecting lines, and then compared against each other. To simplify this comparison, Klingbeil and Schmid-Fetzer suggest constructing a reaction matrix in which rows represent reactants and columns represent products.
Each cell in the reaction matrix is filled with ΔGR of the corresponding reaction of the intersecting lines. If a system of reactions cannot be solved, and the reactants and products lines do not intersect, the cell is then filled with two asterisks (**). Table 2.1 represents an exemplary reaction matrix that corresponds to the ternary phase diagrams in Figure 2.1.
Table 2.1 Example reaction matrix for Gibbs energies of reaction in kJ/mol of atoms.
A-BC AC-AB B-AC C-AB AC-BC AB-BC Remarks
A-BC ** -9 -5 8 ** ** Rejected
AC-AB 5 ** ** ** ** ** Stable
B-AC 3 ** ** 12 ** -3 Rejected
C-AB -20 ** -7 ** -23 ** Rejected
AC-BC ** ** ** 1 ** ** Stable
AB-BC ** ** 13 ** ** ** Stable
In each row, there is a minimum value of ΔGR, and it is underlined. This is called row minimum value (RMV), and it determines the stability of the reactant in each row. If the RMV value is negative, this indicates that there is at least one product line that is more stable, and so the reactant in that row is not stable and cannot be a tie line; it is then marked “rejected” in the remarks column. But if the RMV is larger than zero, no product line is more stable against the formation of the reactant line, thus the reactant line is a tie line, and it is marked “stable”. After 14 all stable tie lines are determined, the final ternary phase diagram of the above reaction matrix can be drawn as shown in Figure 2.2.
Figure 2.2 Final ternary phase diagram with only stable tie lines.
2.1.2 Thermodynamic Data Collection and Selection Criteria
For all ternary systems in this thesis, the literature has been reviewed extensively to find any ternary phase diagrams at room temperature (298 K). In the case of unavailability, the calculation method described in the previous section will be utilized for the construction of ternary phase diagrams. For any M-X-Y ternary system, the first step is to determine the intermetallic compounds that exist at room temperature in the three binary systems X-M, Y-M and X-Y. If the binary phase diagrams are only available at elevated temperatures, it will be
0 assumed that they also exist at 298 K. Secondly, Gibbs free energies of formation (ΔG f) for each 15 0 of the intermetallic compounds is collected or calculated if not found. ΔG f is calculated by the following equation:
0 0 0 ΔG f = ΔH f -TΔS f (Eqn.1)
0 0 where ΔH f is the enthalpy of formation and ΔS f is the entropy of formation. Upon searching for these thermodynamic data, units may be expressed differently, and it is important to convert
0 0 them to consistent units. In this thesis, ΔH f and ΔS f are expressed per mole of atoms.
Sources of the thermodynamic data and phase diagram are primarily from the online ASM
Alloy Phase Diagram Database, Phase Diagrams of Ternary Boron Nitride and Silicon Nitride
Systems, Springer Materials and many other research papers in the literature [36-38]. Whenever experimental or theoretical values of the enthalpy of formation were not available, the online
Miedema’s calculator was utilized [39]; the theoretical description of this method is summarized and analyzed by Zhang et al. (2016) [40]. Finally, when the entropy of formation was not available in the literature, it was assumed to be zero as it is small when only condensed phases are involved.
2.1.3 MATLAB Code
A MATLAB program was utilized to compute all possible reactions of intersecting lines as described in the previous section. The PhD candidate in our group Kayla Cooley has developed the MATLAB program and thankfully allowed the use of it for the completion of this thesis. The program serves to perform the reactions quickly and accurately, and it outputs the stable tie lines to construct the ternary phase diagrams of any system of interest.
16 Main tasks of the program are described as follows:
A. The program defines the corners and axes of the ternary phase diagram M-X-Y, where M
is the metal and X and Y are the constituent atoms of the semiconductor compound.
B. The program registers information of the intermetallic compounds including chemical
formulas, stoichiometries and ΔGf°, which it reads from an Excel file. Once the user runs
this program, a prompt in the command window will ask the user to input the Excel file
name then the sheet name. An example of the Excel sheet is shown below in Figure 2.3.
Figure 2.3. A screen shot of the Excel sheet for B-As-Mo system. 17 The chemical formulas of the M-X compounds will always be inserted in row
number 1, M-Y in row number 8 and X-Y in row number 15. The following rows 2, 9
and 16 represent the ratios of X/M, Y/M and Y/X atoms for each of the intermetallic
compounds, respectively. Rows 3, 10 and 17 contain ΔGf° (in kJ/mol of atoms) for the
corresponding columns. The next three rows represent the number of X, Y and M atoms
in each of the intermetallic compounds, respectively. Lastly, rows 22-26 define the metal
M in the same order as the intermetallic compounds. All Excel sheets of the different
metals must follow the same order for the program to run them properly.
C. The program initializes structural arrays for all of the intermetallic compounds defined in
step (2). These arrays define chemical formulas, stoichiometries, Gibbs free energies of
formation ΔGf° and coordinates of the binary phases on the ternary plot defined in step
(1).
D. The program generates structural arrays for all possible tie lines between constituent
atoms and intermetallic compounds and between intermetallic compounds themselves.
The arrays include the same information as in step (3).
E. The program generates a “virtual” reaction matrix as described in the previous section,
and it assigns each possible tie line as a reactant. For any given reactant, the program
determines if it intersects with any other possible tie line. If it does and for every
intersection, the program calculates Gibbs energies of reaction ΔGR. Once the program
finds a ΔGR that is less than 0, the product line is rejected, and the program moves to the
next possible tie line. Otherwise, the program accepts and stores ΔGR values in an array if
they are greater than 0. After ΔGR values for all of the reactions were calculated, the 18 program compares them and determines the minimum value to store it in an array for the
final stable tie lines.
F. The program outputs all reactions with their ΔGR values and prints a list for the final
stable tie lines.
Chapter 3
Boron Arsenide: Results and Discussions
3.1 Condensed Phase Equilibria and Ternary Phase Diagrams
To date, no ternary phase diagrams have been found in the literature for metal-boron- arsenic systems. Thus, the calculation method described in Chapter 2 was utilized. In this thesis, the ternary phase diagrams of BAs with Pt, Mo, Cr, Ti, Au and Ag at 298 K were calculated.
Below is a detailed analysis for the criteria for choosing the thermodynamic data and phases for each of the ternary systems.
3.1.1 Literature Review of Thermodynamic Data for M-B-As Systems
B-As
Boron arsenide can exist in two phases, boron mono-arsenide (BAs) or a sub-arsenide
B12As2. One of the earliest attempts to synthesize BAs was carried out by Williams et al. (1960) 19 [41] by chemical vapor transport (CVT). Formation of each phase depends on the temperature of the reaction and the arsenic pressure. At temperatures between 973-1193 K and arsenic pressure larger than 1 atm, boron mono-arsenide (BAs) is the product. BAs undergoes phase decomposition to the orthorhombic B12As2 at temperatures between 1273-1373 K and arsenic pressure less than 1 atm. Multiple studies followed the conclusions of Williams et al. (1960) [41] to synthesize BAs at temperatures below 1193 K [42-45]. In these experimental studies, various defects were detected in the grown BAs crystals including twin plane defects [42], As-vacancies
[43] and antisite defects (As-atom on B and the opposite) [44]. However, a recent density functional theory (DFT) study indicated that B12As2 is the only stable phase in the B-As system at temperatures below 1073 K, and that BAs phase is either a metastable phase or it is stabilized by high concentration of defects [46]. To test the hypothesis proposed by the DFT study [46],
Kang et al. (2018) [45] synthesized single crystalline BAs with no observable defects at 1083 K; to confirm the stability of the BAs phase, grown crystals were kept in air for 10 months and heated up to 600 K with no degradation in thermal conductivity; this rejects the claim that BAs phase is not a stable phase under 1083 K. Therefore, both BAs and B12As2 phases will be considered in this thesis.
Alikhanyan et al. (1975) [47] calculated enthalpies of formation of -17,782 and -9,922
J/mol-at for the BAs and B12As2 phases respectively. In addition, Dumont et al. (2006) [48] estimated a value of Gibbs free energy of formation for the BAs phase of -20,000 J/mol-at; this estimation was done by comparing the Gibbs free energies of formation for other boride and arsenide compounds in the literature and then extrapolate an estimate value, which is close to the value of Alikhanyan et al. (1975). In addition, Chae et al. (2018) [49] calculated a value of -11,481.5 J/mol-at for the BAs phase by DFT first-principles method. No other available 20 thermodynamic data were found in the literature for the phase B12As2. Therefore, the experimental enthalpies of formation measured by Alikhanyan et al. (1975) will be used in this project. Table 3.1 below summarized all enthalpies of formation and shows used values in bold.
Table 3.1 Gibbs free energy of formation of the B-As system. 0 0 0 ΔH f ΔG f ΔH f
(J/mol-at) (J/mol-at) (J/mol-at)
BAs -17782 [47] -20000 [48] -11481.5 [49]
B12As2 -9922 [47] ------
Pt-B-As
Pt-B
Thermodynamic data for the binary system of Pt-B in the literature are very limited.
Figure 3.1 below shows the binary phase diagram with two low-temperature phases, Pt2B and
Pt3B namely. Even thought that the phase diagram goes as low as 673 K, it will be assumed that these phases exist at room temperature. Kleppa et al. (1985) [51] calculated the enthalpy of formation for the Pt2B phase to be -20,000 J/mol-at. No thermodynamic data for the Pt3B phase have been found in the literature. Thus, Miedema’s calculator will be used to find the enthalpy of formation. To apply relatively equal measurements, Miedema’s enthalpy of formation will be also used for the Pt2B phase, which is close to the experimental value. Table 3.2 below summarizes the enthalpy of formation for the Pt-B system. 21 Table 3.2 Gibbs free energy of formation of Pt-B binary system.
0 ΔG f 0 ΔG f (J/atom*mol) (J/atom*mol) Miedema’s
Pt2B - 20000 [51] - 28587
Pt3B --- - 21438
Figure 3.1 Pt-B binary phase diagram [50].
Pt-As
Figure 3.2 below illustrates the binary phase diagram of the Pt-As binary system with a single intermediate phase. The phase diagram only goes down to 473 K, but it will be assumed that the PtAs2 exists at room temperature. This phase diagram agrees with an assessment for the binary system performed by Li et al. (2007) [53]; the authors used the PtAs2 phase in the ternary 22 phase diagram for the Pt-GaAs system at room temperature. Murray et al. (1967) [54] experimentally calculated an enthalpy of formation of 58576 J/mol-at for the PtAs2 phase, which will be used in this project. Table 3.3 below summarizes the enthalpy of formation for the Pt-B system.
Table 3.3 Gibbs free energy of formation of Pt-As binary system.
0 ΔG f
(J/atom*mol)
PtAs2 - 58576 [54]
Figure 3.2 Pt-As binary phase diagram [52].
23
Mo-B-As
Mo-B
No available full temperature range phase diagram of B-Mo system has been found in the literature. Spear and Liao (1988) [55] performed an assessment on the Mo-B system based on data from literature and their assessment. They proposed the presence of the following phases:
Mo2B, MoB, MoB2, Mo2B5 and MoB4. In addition, they reported that the stability of Mo3B2 is doubtable. Values of Gibbs free energies of formation are shown in Table 3.4 below. Tojo et al.
(2010) [56] performed a thermodynamic analysis of the Mo-B system in part by analyzing the
Cr-Mo-B system using first principles calculations and the CALPHAD method. They agreed with Spear and Liao on the existence of the phases below, but they suggested that Mo3B2 is a stable phase; however, it does not exist below a temperature around 2200 K, hence, it will not be used in our calculations. They described MoB2 and Mo2B5 using a single function since they are separated by a miscibility gap; both phases exist at temperatures above 1800 K. Gibbs free energies of formation are shown below in Table 3.4. Recently, Witusiewicz et al. (2016) [57] performed a thermodynamic refinement of the binary phase B-Mo. They suggested that all analyses in the literature including Tojo et al. (2010) are not reliable since they describe boride phases as stoichiometric compounds, whereas available experimental data show that they display rather broad (from ~2 to ~5 at% B) homogeneity ranges. With available experimental data in the literature in addition to first principle calculations and assessments, the phase diagram of the binary B-Mo system is shown below in Figure 3.3. Even though the lower limit of the temperature is 1000 K, it will be assumed that Mo2B, MoB, Mo2B5 and MoB4 exist at room temperature. This optimized phase diagram agrees with the findings of Spear and Liao (1988) 24 [55]. Figure 3.4 represents the Gibbs free energy of formation of B-Mo phases; values of Spear and Liao (1988) seem to align with the refined Gibbs of formation value (shown in green kite).
Thus, these values will be used in this project (include both enthalpy and entropy). Other experimental data for the phase MoB agree with Spear’s [58-59]. Table 3.4 below summarizes the enthalpy of formation for the Mo-B system.
Table 3.4 Gibbs free energy of formation of Mo-B binary system.
0 0 ΔG f ΔG f
(J/atom*mol) [55] (J/mol*atom) [47]
MoB4 -33174 -29151
Mo2B5 -49668 -47086
MoB - 52949 -50401
Mo2B -42136 -32985
Figure 3.3 Mo-B binary phase diagram [57].
25
Figure 3.4 Gibbs free energy of formation of Mo-B [57].
Mo-As
Very limited data on Mo-As system was found in the literature. Brewer et al. (1980) [60] have determined the Mo-As binary phase as shown in Figure 3.5. The phase diagram only covers up to 693 K; it will be assumed that the following phases exist at room temperature: Mo5As4,
Mo2As3, MoAs2 and MoAs5. Moreover, Han et al. (1992) [61] performed an experimental assessment of the ternary phase of Mo-Ga-As. In the Mo-As binary phase diagram, Mo5As4,
Mo2As3 and MoAs2 were found to be stable, agreeing with compounds found in the literature and
Brewer [60]. The MoAs compound reported by Boller et al. (1964) [62] was not observed, agreeing with Brewer (1980). In addition, the MoAs5 phase found by Brown (1965) [63] and
Brewer (1980) was neglected as it does not affect ternary phase equilibria, but it will be included in the Mo-As binary system in this thesis to show the full phase diagram. Therefore, Mo5As4,
Mo2As3, MoAs2 and MoAs5 phases will be used without neglecting MoAs5 phase. Table 3.5 below summarizes the enthalpy of formation for the Mo-B system. 26
Table 3.5 Gibbs free energy of formation of Mo-As binary system 0 ΔG f
(J/atom*mol) [60]
Mo5As4 -34442
Mo2As3 -34669.4
MoAs2 -34336.8
MoAs5 -16877.4
Figure 3.5 Mo-As binary phase diagram [60].
27 Cr-B-As
Cr-B
Liao and Spear (1986) [64] have assessed the binary phase diagram based on experimental data. The phase diagram is shown below in Figure 3.6, and the phases with Gibbs free energy of formation are presented in Table 3.6. The phase diagram only shows values above
1700 K. Moreover, Campbell et al. (2002) [65] performed an assessment of the binary phase diagram, describing the work of Liao and Spear (1986) to lack treating B as an interstitial; Gibbs free energies are shown in Table 3.6. In addition, Yamada et al. (2011) [66] analyzed the Cr-B system by a combination of experimental data and first-principles calculations; they referred to the phases found by Liao and Spear (1986). In addition, Yamada et al. (2011) stated that the parameters for the CrB phase found by Campbell et al. (2002) cause this phase to be unstable at lower temperatures. Thus, it needed a re-assessment. An experimental value of -39.8 kJ/mol*atoms for CrB2 was obtained by Topor et al. (1985) [67], which is close to the value
Campbell et al. have obtained. Therefore, Yamada’s values will be used as shown in Table 3.6 in bold.
28 Table 3.6 Gibbs free energy of formation of Cr-B binary system.
0 0 0 ΔG f ΔG f ΔG f
(J/mol-at) [64] (J/mol-at) [65] (J/mol-at) [66]
Cr2B -28383.3 -30800 -36573
Cr5B3 -37385.4 -34200 -40863
CrB -39802.3 -37800 -52488
Cr3B4 -40962 -42900 -49349.7
CrB -41188.2 -39720 -36488 2
CrB4 -24603.8 -24000 -22557.7
Figure 3.6 The Cr-B binary phase diagram [64].
29 Cr-As
Data for the Cr-As are very limited in the literature. Deal et al. (1985) [68] analyzed the binary system and found the following phases experimentally: CrAs, Cr4As3, Cr2As and Cr4As.
Schmid-Fetzer (1988) [69] referred to the phases of Deal et al. (1985), except for substituting
Cr4As to Cr5As, to calculate ternary phase diagrams of Cr-Ga-As; he used Miedema’s values.
Williams et al. (1985) [70] have deposited Cr on GaAs and predicted the Gibbs free energy of formation from bulk data of the following compounds: Cr3As, Cr2As, CrAs and CrAs2. They have not indicated how they found these phases. Thus, they will not be used. Venkatraman et al.
(1990) [71] have performed a detailed assessment of the binary phase of Cr-As and have found the following phases in literature: Cr3As, Cr2As, Cr5As3, Cr3As2, Cr4As3, CrAs Cr2As3, and
CrAs2. They indicated that the existence of Cr2As3 and Cr5As3 phases is questionable, and it is only reported by one author; thus, they were not considered. Moreover, they included the CrAs2 phase in their phase diagram calculations as shown below in Figure 3.7, which covers temperatures down to 273 K. Since thermodynamic data on this binary system is scarce,
Miedema’s values of heat of formation will be used in this project as shown below in Table 3.7.
Table 3.7 Gibbs free energy of formation of Cr-As binary system. 0 ΔG f
(J/mol*atom)
Cr3As -36756.7
Cr2As -45097
Cr4As3 -49003.1
CrAs -47916.6
CrAs2 - 35603 30
Figure 3.7 Cr-As binary phase diagram [71].
Ti-B-As
Ti-B
An assessment of the binary phase diagram of the Ti-B system was performed by Murray et al. (1986) [72]. The phase diagram, as shown in Figure 3.8, indicates the presence of three intermediate phases, TiB2, Ti3B4 and TiB. A recent assessment performed by Ma et al. (2004)
[73] agrees with the previous phases. Even though the lowest temperature of the phase diagram is 873 K, it will be assumed that the previous intermediate states exist at room temperature.
Gibbs free energies of formation from both assessments are listed below in Table 3.8.
The TiB2 phase was the only phase that had experimental thermodynamic data; measured enthalpy of formation reported Topor et al. (1985) [74] and Guzman et al. (1995) [75] and Gibbs 31 free energy of formation reported by Yurick et al. (1979) [76] are shown below in Table 3.8.
Furthermore, Han et al. (2007) [77] calculated an enthalpy of formation of 108766.4 J/mol-atom using first-principles calculations, which agrees with previous experimental data. Therefore, the optimized Gibbs free energy of formation of the TiB2 phase calculated by Ma et al. (2004) [73] will be used in this project as it is the latest one.
Experimental enthalpy of formation of the TiB phase reported by Schissel et al. (1962)
[78] and Yamamoto et al. (1997) [79] are shown in Table 3.8. Clearly, they disagree with the reported value of Murray et al. (1986) [72], which is expected as it was not based on experimental data. Therefore, the enthalpy of formation of Yamamoto et al. (1997) [79] will be used since it is the most recent one and it is in an excellent agreement with Materials Project’s value.
No experimental thermodynamic data was found for Ti3B4 phase. A recent first-principles calculation of the Ti3B4 phase performed by Wang et al. (2015) [80] resulted in an enthalpy of formation as shown in Table 3.8. This value is in excellent agreement Materials Project’s value and similar to the assessed value of Ma et al. (2004) [73]. Values in bold in Table 3.8 are the energies of formation that will be used for the binary system of Ti-B.
Table 3.8 Gibbs free energy of formation of Ti-B binary system.
0 0 0 0 0 ΔG f ΔG f ΔH f ΔH f ΔG f (J/atom*mol) (J/atom*mol) (J/mol*atom) (J/mol*atom) (J/mol*atom) -105109 [80] -107151.8 [73] -109460 [74] -107936 [74] -104.220.3 [76] TiB2
Ti3B4 -110414.2 [80] -98042.8 [73] -90694 [80] ------
TiB -105778.5 [80] -83000* [73] -80123.5 [78] - 81000 [79] ---
32
Figure 3.8 The Ti-B binary phase diagram [72].
Ti-As
Data on the binary system of Ti-As are very limited in the literature. Maex et al. (1989)
[81] and Klingbeil et al. (1994) [82] constructed ternary phase diagrams which included the Ti-
As binary system. The following phases for the Ti-As binary system at room temperature were used: Ti4As, TiAs and TiAs2, which are the same phases acknowledged by Wenglowski et al.
(1964). Due to the scarcity of experimental data, Maex et al. (1989) [81] used Miedema to calculate the enthalpies of formation. Thus, Miedema’s enthalpies of formation will be used in this project as shown in Table 3.9 below.
33 Table 3.9 Gibbs free energy of formation of Ti-As binary system. 0 ΔG f
(J/atom*mol)
Ti4As - 61683.7
TiAs -110501
TiAs2 -86609.5
Au-B-As
Au-B
Figure 3.9 shows the binary phase diagram of the Au-B system with an intermediate of
AuB2. However, Massalski et al. (1990) [84] described this phase as a metastable phase; this agrees with the findings of a first-principles study performed by Geest et al. (2014) [85] that this phase does not exist. In addition, Miedema’s enthalpy of formation for the AuB2 phase is a positive value of 12531 J/mol-at, indicating its instability. Therefore, no intermediate phases will be used for the Au-B binary system.
Figure 3.9 Au-B binary phase diagram [84]. 34 Au-As
Figure 3.10 below indicates no intermediate phases in the Au-As binary system.
Therefore, no intermediate phases will be used for the Au-As binary system.
Figure 3.10 Au-As binary phase diagram [86].
Ag-B-As
Ag-B
Figure 3.11 shows the binary phase diagram of the Ag-B system with an intermediate of
AgB2. However, similar to gold, Massalski et al. (1990) [84] questioned the stability of this phase; this agrees with the findings of a first-principles study performed by Geest et al. (2014)
[85] that this phase does not exist. In addition, Miedema’s enthalpy of formation for the AgB2 phase is a positive value of 25605 J/mol-at, indicating its instability. Therefore, no intermediate phases will be used for the Ag-B binary system. 35
Figure 3.11 Ag-B binary phase diagram [87].
Ag-As
Figure 3.12 below shows the Ag-As binary system. An assessment of this binary phase performed by Baren (1990) [88] indicates the absence of any intermediate phases. Therefore, no intermediate phases will be used for the Ag-As binary system.
36
Figure 3.12 Ag-As binary phase diagram [88].
3.1.2 Calculated Ternary Phase Diagrams for M-B-As Systems
0 Based on the accepted Gibbs free energies of formation (ΔG f) from the previous section, the MATLAB program was used to calculate and construct the ternary phase diagrams of BAs systems with Pt, Mo, Cr, Ti, Au and Ag at 298 K, which are depicted below in Figures 3.13-
3.18.
37
Figure 3.13 Calculated Pt-Ba-As ternary phase diagram. Figure 3.14 Calculated Mo-Ba-As ternary phase diagram.
Figure 3.16 Calculated Cr-Ba-As ternary phase diagram. Figure 3.15 Calculated Ti-Ba-As ternary phase diagram.
Figure 3.17 Calculated Au-Ba-As ternary phase diagram. Figure 3.18 Calculated Ag-Ba-As ternary phase diagram. 38 These results can be categorized into two main groups: ternary phase dominant systems and BAs dominant systems. For the former the group, no tie lines exist between BAs and the transition metals, and ternary phases are favored to form; this is observed for the cases of early transition metals Ti, Cr and Mo (groups 4 & 6) and for Pt (group 10). These metals are predicted to not be in equilibrium with BAs, and interfacial reactions are likely to occur. On the other hand, the group 11 noble metals Au and Ag indicate complete immiscibility; they both show
BAs dominant systems, and they are predicted to be in thermodynamic equilibrium with BAs.
As described in Chapter 1, achieving high-quality electrical contacts to BAs requires pristine and defect-free interfaces to operate electronic devices with high current densities at the lowest voltage drop across the metal-semiconductor junction. Reactive metal contacts like Ti, Cr,
Mo and Pt are expected to form MxBy or MxAsy intermetallic compounds at the metal-BAs interface. The presence of new compounds will add new interfaces at the junction, which could change the electrical and thermal properties of the junction. Additionally, the reactions at the interface will consume some amount of the metal film upon forming new compound. Thus, in comparison to stable metal films, which could deplete the BAs. However, stable metal contacts like Au and Ag would not form intermetallic compounds at their interfaces with BAs. 39 Chapter 4
Hexagonal Boron Nitride: Results and Discussions
In this chapter, thermodynamic predications and experimental results for metal thin-films to h-BN are discussed. Additionally, the results are compared to previous published work by our research group on metal thin films to the layered materials WSe2 and MoS2 [11, 29].
4.1 Condensed Phase Equilibria and Ternary Phase Diagrams
Ternary phase diagrams for metal-boron-nitrogen systems were collected and adapted from the reference Phase Diagrams of Ternary Boron Nitride and Silicon Nitride Systems [34].
While some systems were only available at elevated temperature, their ternary phase diagrams were assumed to be the same at room temperature. Table 4.1 summarizes the results for the ternary phase diagrams of BN with Al, Cr, V, Ti, Zr, Fe, Co, Ni, Mn, Au, Ag, Cu and Cd.
Additionally, in our published paper of electrical metal contacts on WSe2 [11], Ramya
Gurunathan calculated ternary phase diagrams for WSe2 with Au, Ag, Co, Cu, Al, Ni and Pd at
298 K using the same calculation method described in Chapter 2. These metals were classified into three main groups as depicted in Table 4.2: WSe2 dominant systems (thermodynamically stable), metal selenide (Al2Se3) dominant systems (reactive) and indeterminant systems that lacked thermodynamic data in the literature. Similarly, metal-B-N could be classified into two main categories: BN dominant systems (thermodynamically stable) or mixed systems (systems that are dominated by both metal-borides and metal-nitrides). Early transition metals (groups 4-
6) in addition to Al are predicted to be reactive and thermodynamically unstable with BN. On the 40 other hand, later transition metals (groups 7-12) show complete immiscibility and are predicted to be in thermodynamic equilibrium with BN.
Table 4.1. Thermodynamic stability of various metals with BN [34].
Metal Equilibrium Stability with BN
Al Unstable
Cr Unstable
V Unstable
Ti Unstable
Zr Unstable
Fe Stable
Co Stable
Ni Stable
Mn Stable
Au Stable
Ag Stable
Cu Stable
Cd Stable
41 Table 4.2 Classifications of M-W-Se Systems [11].
WSe2 Dominant Metal Selenide Dominant Indeterminant
Au Al Ni
Ag Pd
Cu
Co
4.2 Experimental Procedures
Experimental work was carried out only on Hexagonal boron nitride. h-BN films were prepared and provided by the PhD candidate Anushka Bansal advised by Prof. Joan Redwing. A few layers of h-BN were deposited on an α-Al2O3(0001) substrate via chemical vapor deposition
(CVD) technique by using B2H6 and NH3 as B and N precursors, respectively. In plane epitaxy of the h-BN film was confirmed using reflection high-energy electron diffraction (RHEED).
4.2.1 Deposition of Metal Thin-Films on Boron Nitride
The sample of h-BN film on the sapphire substrate was cleaned to remove any contaminants that might have accumulated on the surface. The cleaning procedure is as follows: the sample was submerged in acetone, IPA and DI water for five minutes each, respectively, then it was left to dry in air for approximately 30 minutes. Next, electron beam evaporation was utilized to deposit 50 nm of Au using an Edwards FL 400 system. After loading the sample into 42 the chamber, the system pressure reached 7×10-7 Torr using both roughing and turbomolecular pumps, and it was reduced to 2×10-7 Torr using liquid nitrogen. To further improve the vacuum system, 20 nm of Ti was deposited on the walls of the chamber while the shutter on the sample was closed. The deposition of the gold film was performed under a rate of 1 Å/s, and the thickness was measured by a quartz crystal monitor. Lastly and after deposition, the sample was left in the chamber for 30 minutes to cool down.
4.2.2 Characterization of Metal Contacts on Boron Nitride
X-ray diffraction (XRD) measurements were performed to characterize the epitaxy of Au film using a PANalytical MRD diffractometer. The system operated at 45kV voltage and 40mA filament current to generate a standard Cu-Kα x-ray. Various configurations were used to find the orientational relationship between the Au film and the sapphire substrate, and that includes
Bragg-Brentano and off-axis phi-scan configurations. Since B and N are very light elements, and the layer was very thin, XRD peaks were expected to be too low in intensity to detect.
4.2.3 Safety Considerations
The thin film preparation and metal deposition were performed in the Mohney group laboratory that is equipped with smoke detectors, an eyewash, a safety shower station in the hall, and telephones with emergency contact numbers. Standard operating procedures (SOP) and
Safety Data Sheets (SDS) are compiled in an easy-to-access binder at the lab’s entrance. Initial safety training was required before conducting any experiment in addition to an annual refresher training. The cleaning procedure was performed in a fume hood with a maximum operating sash 43 window height of 18 inch, and all chemicals were safely stored in the cabinet beneath the fume hood. Personal protective equipment (PPE) are required to be worn at all times, which includes safety goggles or glasses, long pants and closed-toe shoes. For my experiment, gloves and a lab coat were also required. In thin film deposition, liquid nitrogen was handled with extra precautions including the use of safety cryogenic apron, a face shield and cryo-gloves. Prior to independently operating any instrument in the lab, two hands-on trainings by a super user are required. Each instrument has its own log book to keep record of operating details.
The x-ray diffraction (XRD) analysis was performed in a shared user facility that requires the registration in the Research Instrumentation Management System (RIMS) system, which requires an initial safety training of the facility. Before working on the XRD instrument, in- person trainings in radiation safety are required. For operating any instrument in the shared user facility, hands-on initial and solo trainings are performed with the supervision of lab staff member. For my XDR analysis experiment, PPE included a lab coat, safety glasses, long pants and closed-toe shoes.
4.3 Epitaxy of Metal Thin-Films
For heterojunctions involving h-BN, thermodynamically stable metals are favored to prevent the formation of undesired intermetallic or amorphous phases at the interface where epitaxy is the goal. Additionally, and roughly speaking, for thin films, higher homologous temperatures favor the formation of epitaxial films as the metal atoms become more mobile and therefore able to reach to more energetically favorable positions on the surface. Homologous temperatures of Al, Cr, V, Ti, Zr, Fe, Co, Ni, Mn, Au, Ag, Cu and Cd metals are calculated and 44 presented in Table 4.3 in an ascending order with thermodynamically stable metals with BN being bolded. The trend in Table 4.3 indicates that metals with higher homologous temperatures
(>0.15) are expected to be in thermodynamic equilibrium with BN, except for Al. Domask et al. showed that metals with homologous temperatures larger than 0.22 (except for an anomalous case of 0.15 for Pd) were epitaxial on MoS2 [29].
For the previous purposes. Gold (Au) film was chosen for its matching symmetry of the close-packed plane (111) with the (0001) of h-BN, favorable homologous temperature, and thermodynamic stability with BN. 50 nm of Au was deposited on h-BN(0001) film on a sapphire
(Al2O3) substrate (0001). XRD diffractogram in Bragg-Brentano (2θ-omega) configuration is shown below in Figure 4.1. This result indicates the presence of Au(111) and Al2O3(0001) basal plane peaks. Additionally, phi-scans were performed to look for epitaxy of the Au film on h-BN; for an epitaxial Au film, both Au and Al2O3 peaks should appear in symmetric or asymmetric
(off-axis) phi-scans. For the symmetric phi-scan at the Al2O3(0001) peak, no Au peaks were observed. Asymmetric phi-scan at the Al2O3(024) peak also showed no Au peaks as depicted in
Figure 4.2. Despite the similar close-packed plane with 6-fold symmetry of the Au(111) with the h-BN basal plane, no epitaxial relationship was observed. The Au is at least textured, but epitaxy cannot be confirmed. It could be that the Au peaks were too weak to be detected relative to the intense peaks from the perfect sapphire crystal beneath the BN; or the Au might not be epitaxial.
In comparison to other 2D materials reported in the literature like WSe2 and MoS2, Au films were epitaxial upon deposition at room temperature. In our previous work on WSe2, Pd was epitaxial only after annealing the sample at 673K for 4 hours, and Ni was epitaxial at room temperature but showed improved alignment after annealing [11]. Thus, Au might show epitaxial 45 relationship with h-BN after annealing as atoms move to more energetically favorable spots at higher temperatures.
Al2O3(0001)
Au(111)
Figure 4.1. Bragg-Brentano XRD measurement for the Au/h-BN/Al2O3 sample, which indicates the presence of Au(111) and Al2O3(0001) basal plane peaks. 46
Figure 4.2. Asymmetric phi-scan at the Al2O3(024) peak, which shows no Au film peaks.
47
Table 4.3. Homologous temperatures of various metals in an ascending order with stable metal with BN being bolded.
Ts/Tm Metal Tm (k)
Cr 2180 0.14
V 2183 0.14
0.14 Zr 2127
Ti 1943 0.15
Fe 1811 0.16
Co 1768 0.17
Ni 1728 0.17
0.20 Mn 1519
Cu 1357.6 0.22
Au 1337 0.22
Ag 1235 0.24
Al 933.3 0.32
Cd 594 0.50 1
Chapter 5
Conclusions
Boron arsenide (BAs) and hexagonal boron nitride (h-BN) have attracted huge attention due to their potential for superior performances in heat management and as insulating two- dimensional layered materials, respectively. Particularly, this thesis investigated the integration of these compounds with metals as electrical contacts and studied the interactions that take place at their interfaces. Metal/BAs interfaces were assessed based on condensed phase equilibria for metal-B-As systems. Using a MATLAB program, Ternary phase diagrams of BAs with Pt, Mo,
Cr, Ti, Au and Ag at room temperature were calculated by collecting thermodynamic data from the literature. The results predict that Pt, Mo, Cr and Ti to be unstable and to form intermetallic compounds at the interface, while Au and Ag are predicted to be in thermodynamic equilibrium with BAs.
Additionally, thin-films to h-BN were assessed based on thermodynamic stability and their potential for epitaxy. Based on ternary phase diagrams found in the literature, early transition metals Cr, V, Ti and Zr in addition to Al were predicted to have a thermodynamic driving force to react, while later transition metals Fe, Co, Ni, Mn, Au, Ag, Cu and Cd were predicted to be stable with h-BN at room temperature. Despite x-ray diffraction studies showing the presence of the close-packed Au(111) plane parallel to the surface of the h-BN film, epitaxy of the Au film was not confirmed upon phi-scans. It is possible that the Au peaks were too weak to be detected in comparison to the strong Al2O3 signals, or that Au might not be epitaxial. In our previous work on WSe2, Pd was epitaxial only after annealing the sample, and Ni was epitaxial 49 at room temperature but showed improved alignment after annealing. Thus, Au might show an epitaxial relationship with h-BN after annealing as atoms move to more energetically favorable spots at higher temperatures. To further investigate and predict the epitaxy of metals with h-BN, homologous temperatures were calculated, and the results indicated a positive correlation between higher values of homologous temperatures and thermodynamic stability of metals with h-BN.
Thermodynamic stability of metal thin-films to both BAs and h-BN serves as an important prediction tool to determine if intermetallic compounds will form at their interfaces.
The formation of such compounds is often not desired since it would consume the thermally conductive BAs and ultra-thin h-BN, or add new interfaces at the junction, which could change the electrical and thermal properties of the junction. Experimental work on the predicted condensed phase equilibria of metal films to BAs and h-BN is a potential study in the future to determine the validity of these predictions.
50 BIBLIOGRAPHY
[1] P. Ball, “Computer engineering: Feeling the heat”, Nature, 492, 174-176 (2012). [2] L. Lindsay, D. A. Broido, T. L. Reinecke, “First-principles determination of ultrahigh thermal conductivity of boron arsenide: A competitor for diamond?”, Phys. Rev. Lett., 111, 025901 (2013). [3] S. Trolier-McKinstry and R. E. Newnham, “Materials Engineering”; chapter 12, pp. 412-433. Cambridge University Press, Cambridge, UK (2017). [4] International Technology Roadmap for Semiconductors, “Heterojunction Integration”, ITRS 2.0, 2015 Edition. Retreived from web on Jan. 29, 2020.
[9] S. Das, H.Y. Chen, A. V. Penumatcha, J. Appenzeller, “High performance multilayer MoS2 transistors with scandium contacts”, Nano Lett., 13, 100-105 (2013). [10] Y. Zeng, A. C. Domask and S. E. Mohney, “Condensed phase diagrams for the metal–W–S systems and their relevance for contacts to WS2”, Materials Science and Engineering B, 212, 78- 88 (2016) [11] K. A. Cooley, R. Alsaadi, R. L. Gurunathan, A. C. Domask, L. Kerstetter, W. A. Saidi, S. E. Mohney, “Room-temperature epitaxy of metal thin films on tungsten diselenide”, Journal of Crystal Growth, 505, 44-51 (2019) [12] J. Kang, M. Li, H. Wu, H. Nguyen and Y. Hu, “Experimental observation of high thermal conductivity in boron arsenide”, Science, 361, 575-578 (2018). 51 [13] F. Tian and Z. Ren, “High Thermal Conductivity in Boron Arsenide: From Prediction to Reality”, Angew. Chem., 131, 5882-5889 (2019). [14] G. L. W. Hart and A. Zunger, “Electronic structure of BAs and boride III–V alloys”, Phys. Rev. B, 62, 13522 (2000). [15] A. Pakdel, Y. Bando, D. Golberg, “Nano boron nitride flatland”, Chem. Soc. Rev, 43, 934- 959 (2014). [16] G. H. Lee, Y. J. Yu, X. Cui, N. Petrone, C. H. Lee, M. S. Choi, D. Y. Lee, C. Lee, W. J. Yoo, K.Watanabe, T. Taniguchi, C. Nuckolls, P. Kim, and J. Hone, “Flexible and transparent
MoS2 field-effect transistors on hexagonal boron nitride-graphene heterostructures”, ACS Nano., 7, 7931-6 (2013). [17] T. Ouyang, Y. Chen, Y. Xie, K. Yang, Z. Bao, J. Zhong, “Thermal transport in hexagonal boron nitride nanoribbons”, Nanotechnology, 21, 245701 (2010). [18] G. H. Lee, Y. J. Yu, C. Lee, C. Dean, K. L. Shepard, P. Kim, “Electron tunneling through atomically flat and ultrathin hexagonal boron nitride”, Appl. Phys. Lett., 99, 243114 (2011). [19] L. Britnell, R. V. Gorbachev, R. Jalil, B. D. Belle, F. Schedin, M. I. Katsnelson, “Electron tunneling through ultrathin boron nitride crystalline barriers”, Nano Lett., 12, 1707-1710 (2012). [20] K. Watanabe, T. Taniguchi, and H. Kanda, “Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal”, Nat. Mater., 3, 404-409 (2004). [21] Y. Ji, C. Pan , M. Zhang, S. Long, X. Lian, F. Miao , F. Hui, Y. Shi, L. Larcher, E. Wu, and M. Lanza, “Boron nitride as two dimensional dielectric: Reliability and dielectric breakdown”, Appl. Phys. Lett., 108, 012905 (2016). [22] C.R. Dean, A.F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K.L. Shepard, and J. Hone, “Boron nitride substrates for high-quality graphene electronics”, Nature Nanotechnology, 5, 722-726 (2010). [23] D. A. Neamen, “Semiconductor Physics and Devices: Basic Principles”, 4th ed., New York, NY: McGraw-Hill (2012). [24] R. T. Tung, “Chemical Bonding and Fermi Level Pinning at Metal-Semiconductor Interfaces”, Phys. Rev. Lett., 84, 6078 (2000). [25] R. T. J. Tung, “Schottky barrier height—do we really understand what we measure?”, Vac. Sci. Technol. B, 11, 1546 (1993). 52 [26] R. T. Tung, “Schottky-Barrier Formation at Single-Crystal Metal-Semiconductor Interfaces”, Phys. Rev. Lett., 52, 461 (1984). [27] T. Zheleva, K. Jagannadham, and J. Narayan, “Epitaxial growth in large-lattice-mismatch systems”, Journal of Applied Physics, 75, 860 (1994). [28] A. Koma, “Van der Waals epitaxy—a new epitaxial growth method for a highly lattice- mismatched system”, Thin Solid Films, 216, 72-76 (1992). [29] A. C. Domask, K. A. Cooley, B. Kabius, M. Abraham, and S. E. Mohney, “Room temperature van der Waals epitaxy of metal thin films on molybdenum disulfide”, Cryst. Growth, 18, 3494-3501 (2018). [30] F. C. Campbell, “Phase diagrams-Understanding the basics”, ASM International, Materials Park, Ohio (2012). [31] D. R. F. West and N. Saunders, “Ternary phase diagrams in materials science”, 3rd edition, Maney Publishing for the Institute of Materials, London, UK (2002). [32] Accreditation Board for Engineering and Technology. Accessed on April 18, 2020. < https://www.matse.psu.edu/undergraduate/ABET-considerations>. [33] K. Khan, A. K. Tareen, M. Aslam, Y. Zhang, R. Wang, Z. Ouyang, Z. Gou and H. Zhang, “Recent advances in two-dimensional materials and their nanocomposites in sustainable energy conversion applications”, Nanoscale, 11, 21622-21678 (2019). [34] K. Uosaki, G. Elumalai, H. Noguchi, T. Masuda, A. Lyalin, A. Nakayama, and T. Taketsugu, “Boron nitride nanosheet on gold as an electrocatalyst for oxygen reduction reaction: theoretical suggestion and experimental proof”, J. Am. Chem. Soc., 136, 6542-6545 (2014). [35] J. Klingbeil and R. Schmid-Fetzer, “TerQuat: Approximate prediction of ternary and quaternary phase diagrams comprising stoichiometric phases”, Journal of Phase Equilibria, 13, 522-531 (1992). [36] ASM Alloy Phase Diagram Database. Accessed on Sept. 2019. < https://matdata- asminternational-org.ezaccess.libraries.psu.edu/apd/index.aspx>. [37] P. Rogl and J. C. Schuster, “Phase diagrams of ternary boron nitride and silicon nitride systems”, ASM International, Materials Park, Ohio (1992). [38] Springer Materials. Accessed on Sept. 2019. < https://materials.springer.com>. [39] Miedema Calculator, Institute of Metallurgy and Materials Science. Accessed on Sept. 2019.
[76] T.J. Yurick, K.E Spear, “Thermodynamics of TiB2 From Ti-B-N studies”, Thermodynamics of Nuclear Materials, 1, 73-90 (1980).
[77] Y. Han, Y. Dai, D. Shu, J. Wang, B. Sun, “Electronic and bonding properties of TiB2”, Journal of Alloys and Compounds, 438 327-331 (2007). [78] P.O. Schissel, O.C. Trulson, J. Phys. Chem. 66, 1492-1496 (1962). [79] T. Yamamoto, A. Otsuki, K. Ishihara, P.H. Shingu, “Synthesis of near net shape high density TiB:Ti composite”, Materials Science and Engineering, 239, 647-651 (1997). 56 [80] G. Wang, Y. Li, Y. Gao, Y. Cheng, S. Ma, “Theoretical study of structural, mechanical, thermal and electronic properties of Ti3B4 with Ta3B4 structure under high pressure”, Computational Materials Science, 104, 29-34 (2015). [81] Maex, K., Ghosh, G., Delaey, L., Probst, V., Lippens, P., Den hove, L., & De Keersmaecker, R., “Stability of As and B doped Si with respect to overlaying CoSi2 and TiSi2 thin films”, Journal of Materials Research, 4, 1209-1217 (1989). [82] J. Klingbeil and R. Schmid-Fetzer, “Phase Equilibria in some In-As-Metal and Al-As-Metal Systems”, CALPHAD, 18, 429 (1994). [83] S. Wenglowski, G.B. Bokii and E.A. Pobedimskaya, Zhurnal Stmkturnoi Khimii, 5, 1, 64-69 (1964). [84] T.B. Massalski, H. Okamoto, and L. Kacprzak, “The B-Au (Boron-Gold) System”, Binary Alloy Phase Diagrams, 1, II Ed., Ed. T.B. Massalski, 340-342, ASM International, Materials Park, OH, USA (1990). [85] A.G. Van Der Geest, A.N. Kolmogorov, “Stability of 41 metal–boron systems at 0 GPa and 30 GPa from first principles”, CALPHAD, 46, 184-204 (2014). [86] H. Okamoto and T.B. Massalski, “The As-Au (Arsenic-Gold) System”, Bulletin of Alloy Phase Diagrams, 5, No.1, 56-59 (1984). [87] H. Okamoto, “Ag-B (Silver-Boron)”, Journal of Phase Equilibria, 13, 2, 211-212 (1992). [88] M.R. Baren, “The Ag-As (Silver-Arsenic) System”, Bulletin of Alloy Phase Diagrams, 11, No. 2, 113-116 (1990).
ACADEMIC VITA
Rajeh Salah R. Alsaadi Education The Pennsylvania State University | Schreyer Honors College University Park, PA Bachelor of Science in Materials Science and Engineering | Minor in Electronic and Photonic Materials Dean’s List 7/7 Class of 2020 Research Experience King Abdullah University of Science and Technology | Prof. Husam Alshareef Research Group | Thuwal, Saudi Arabia June 2019 – August 2019 • Synthesized and optimized the growth parameters of epitaxial MoO2 precursor for CVD conversion to Mo2S. • Utilized pulsed laser deposition, atomic force microscopy, Raman spectroscopy and x-ray diffraction. Prof. Suzanne Mohney Research Group, the Pennsylvania State University | University Park, PA May 2017 – May 2020 • Studied diffusion mechanism of epitaxial metals intercalations in 2-dimensional semiconductor MoS2. • Investigated epitaxial interfaces of metal contacts with WSe2 at room and elevating temperatures. • Deposited metal thin films using physical vapor deposition and analyzed TEM diffraction patterns. • Utilized atomic force microscopy, physical vapor deposition and tube furnace. Cornell Center for Materials Research | Prof. Gregory Fuchs Lab, Cornell University | Ithaca, NY June 2017 – August 2018 • Developed a temperature and resistance characterization set-up for magneto-thermal microscopy. • Designed a 3D model using Inventor and programmed the set-up using LabVIEW. Prof. Ismaila Dabo Research Group, the Pennsylvania State University | University Park, PA January 2017 – May 2017 • Simulated phonon modes of nanostructured silicon using finite element method. • Studied effects of periodicity on altering the vibrational modes. Skills • Proficient in PLD, AFM, sputtering, e-beam evaporation, XRD, Raman spectroscopy, tube furnace and optical microscope. • Intermediate in LabVIEW and MATLAB. Publications Kayla A. Cooley, Rajeh Alsaadi, Ramya L. Gurunathan, Anna C. Domask, Lauren Kerstetter, Wissam A. Saidi, Suzanne E. Mohney, “Room-Temperature Epitaxy of Metal Thin Films on Tungsten Diselenide”, Journal of Crystal Growth, 505, 2019, Pages 44-51. Poster Presentations Alsaadi, R., Cooley, K., Mohney, S.; “Room-Temperature Metal Epitaxy on WSe2”, Emerging Researchers National Conference, Washington DC (February 2018). Awards • King Abdullah University of Science and Technology (KAUST) Gifted Students Program Thuwal, Saudi Arabia August 2014 – Present KAUST grants scholarships to top high school graduates to get a bachelor degree of science abroad. • Donald W. Hamer Scholarship in Electronic and Photonic Materials Scholarship Fall 2017 and 2018 • Matthew J. Wilson Honors Scholarship Fall 2017 and 2018