the effects of electric fields on the atomic layer

THE EFFECTS OF ELECTRIC FIELDS ON THE ATOMIC LAYER DEPOSITION OF THIN FILMS Geoff McConohy Research Mentor: Professor Xudong Wang

A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science (Engineering Physics) at THE UNIVERSITY OF WISCONSIN- MADISON

Abstract Atomic Layer Deposition (ALD) is becoming an increasingly important topic as more emphasis is placed on nanoscale materials. Currently, researchers have some parameters available to control when using ALD, such as temperature, precursor material, and pulse time. However, applying electric fields during the growth of thin films could add an additional degree of control over material properties. The results of this study show that uniform electric fields have little-to-no effect on the coverage or morphology of ALD coatings, even for electric fields on the order of 106 V/m. The crystal structure of electric field ALD coatings was also examined. It was found that for TiO2 thin films, electric fields do not have an influence on the crystal structure or orientation as measured with X-Ray Diffraction (XRD).

Acknowledgements I would like to first thank my family for their unwavering support throughout my undergraduate education. In addition, thank you to Professor Wang for his support and use of laboratory equipment and supplies. Special thanks to Matthew Starr for inspiring this project, Yanhao Yu for assistance with SEM imaging, and Zhaodong Li for continuing assistance with ALD equipment.

Nomenclature ALD-Atomic Layer Deposition CVD-Chemical Vapor Deposition XRD-X-ray Diffraction SEM-Scanning Electron Microscope EDS-Energy Dispersive X-ray Spectroscopy FTO-Functionalized Tin-Oxide TMA-Trimethylaluminum

1

Table of Contents Abstract ......................................................................................................................................1 Acknowledgements .....................................................................................................................1 Nomenclature ..............................................................................................................................1 Table of Contents ........................................................................................................................2 1.

Introduction and Background ...............................................................................................3

2.

Literature Review.................................................................................................................5

3.

4.

5.

2.1

Literature Involving Electric Fields and CVD ...............................................................5

2.2

Literature Involving Electric Fields and ALD ................................................................5

Experimental Methods .........................................................................................................7 3.1

Apparatus......................................................................................................................7

3.2

Substrate Fabrication .....................................................................................................8

3.3

Experimental Procedure ................................................................................................9

3.4

Characterization Techniques .........................................................................................9

Results ............................................................................................................................... 10 4.1

Electric Fields and Film Morphology/Coverage .......................................................... 10

4.2

Crystal Structure Results ............................................................................................. 14

Discussion.......................................................................................................................... 15 5.1

Vapor Phase Electric Field Effects .............................................................................. 15

5.2

Surface Electric Field Effects and Morphology ........................................................... 16

5.3

Crystal Orientation Discussion .................................................................................... 16

6.

Future Work....................................................................................................................... 17

7.

Conclusion ......................................................................................................................... 20

8.

References ......................................................................................................................... 20

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1. Introduction and Background

Number of Publications

As more emphasis is placed on nanoscale materials research, new methods have been developed to precisely control properties down to the atomic level. During the 1960s and 1970s, a new growth technique was developed called Atomic Layer Deposition (ALD) [1]. ALD was originally termed Atomic Layer Epitaxy, which implies the deposited material has some order with respect to the underlying substrate. However, many materials deposited using ALD are amorphous and do not have an ordered structure relative to the substrate, so the name atomic layer deposition is used most frequently to refer to this technique. While it was originally developed for depositing thin films for flat panel displays [2], ALD is also used to grow or coat nanostructures in order to enhance various properties [3]. This method has come to be a widely studied topic due to the high quality, pinhole free films which can be deposited down to a single monolayer of atoms, even in confined spaces and complex geometries. ALD has become increasingly popular among academic researchers and industry due to its use in semiconductor applications such as depositing dielectric materials for transistor gates and solid state memory [4]. This interest is reflected in the number of papers published involving ALD. As shown in Figure 1, the number of published papers per year involving ALD has risen steadily over the past decade. 2000 1800 1600 1400 1200 1000 800 600 400 200 0

2000

2002

2004

2006

2008

2010

2012

Year

Figure 1: Number of Papers Published Per Year with Atomic Later Deposition in the Title or Topic as Listed in Web of Science: Thomson Reuters

ALD is an advancement on a previous deposition technique called Chemical Vapor Deposition (CVD) in which gas phase materials are deposited continuously onto a substrate using high temperature reactions. ALD refines this process by using a cyclic deposition process to offer more control over the properties of deposited films. A complete cycle consists of multiple pulses and purges. During a pulse, a precursor is allowed to flow into the reaction chamber, which is held in medium vacuum. Most precursors are stored in the liquid phase. Therefore, the amount of precursor which enters the chamber is dependent on the vapor pressure- and hence the temperature3

of the precursor. This first precursor adsorbs onto the surface of a substrate via surface controlled chemical reactions. After a specified length of time (on the order of 60 seconds), the first precursor pulse stops and the chamber is purged using an inert gas such as nitrogen. This removes all precursor material which is not adsorbed to the substrate, leaving behind a monolayer. A second pulse follows of a different type of precursor, allowing the two precursors to react and form the desired material. This cycle is repeated to create the desired thickness of thin film. A schematic of the ALD process is illustrated in Figure 2. Users have control over a variety of parameters in the deposition process such as chamber temperature, precursor temperature, and pulse time. Chamber temperature is a particularly important parameter. If the temperature of the chamber is too high, thermal energy will be enough to eject adsorbed molecules from the substrate surface, leaving nonuniform films [1]. However, if the temperature is too low, chemical reactions may not proceed quickly enough or may allow gases to condense non-uniformly on the substrate. In addition to controlling various parameters, users can also select different variants of ALD, one of which is termed Plasma Enhanced ALD (PE-ALD). This technique uses ionized gasses accelerated through an electric field to enhance various properties of the thin films [1]. While this process may seem similar to the experiments discussed in this thesis, the experiments described here do not utilize ionized gasses as in PE-ALD.

Figure 2: Schematic of one ALD Cycle [5]

Various materials can be deposited using ALD, most commonly metals and metal-oxides. One such material is TiO2, which is researched for use in applications such as photo-catalysts, gate dielectrics, and even antibacterial coatings [6]. Professor Wang’s group at UW-Madison is actively studying the growth of TiO2 nanostructures for use in photoelectrochemical water splitting. Recently, a ZnO nanorod substrate served as a template for the growth of highly dense TiO 2 nanostructures [7]. In general, the most common precursors for the deposition of TiO2 are water and TiCl4. With these precursors, TiO2 can be grown in both amorphous and crystalline phases depending on the reaction temperature. At temperatures above 300°C the crystalline phase, anatase, can be deposited using the apparatus described in this thesis. Due to its crystalline 4

structure, TiO2 was used to examine crystallographic changes in the experiments described in this thesis. A second, very commonly used material in ALD is Al2O3. It is arguably the most well understood ALD material as it is used as a model for ALD deposition in the literature [4]. In addition, Al2O3 is of great importance to the semiconductor industry as a high K gate dielectric material. This material was used in these studies due to low precursor reactivity and low electrical conductivity so that the electric field strength can be increased without significant side effects occurring. The precursor materials used to deposit Al2O3 are water and Trimethylaluminum (TMA). At the temperatures discussed in this thesis, the deposited Al2O3 films are amorphous.

2. Literature Review 2.1 Literature Involving Electric Fields and CVD Due to the small amount of literature available, and because Chemical Vapor Deposition (CVD) shares many commonalities with ALD, some effects of electric fields on CVD processes will now be discussed. Beginning in the early 1990s, researchers began to use electric fields in conjunction with CVD [8]. Electric fields of magnitude 0.78 MV/cm were used to align polymer molecules during film growth without having to use poling techniques following the film growth. It has also become common to use electric fields to align carbon nanotubes during CVD growth [9, 10, 11]. In another study, Naik et al. have showed that the growth rate and crystal structure of various metal oxide thin films can be affected by electric fields in aerosol assisted CVD [12]. This study applied electric fields parallel to the substrate using electrodes deposited on the substrate itself. Authors of this article report that X-Ray Diffraction (XRD) peaks showed broadening for VO2 thin films using both AC and DC fields. For TiO2 and WO3 thin films, both peak broadening and narrowing was found for AC and DC fields respectively. Peak broadening/narrowing indicates that more/less crystal orientations are present in the films. Fewer crystal orientations are indicative of single crystal thin films- a desirable property for many applications. Interestingly, the results differ between the VO2 films as compared to the other two metal oxide thin films. Therefore, it seems that the electric field effects are a function of the material deposited in combination with the frequency of the applied field. In addition to crystal orientation, electric fields also had a marked impact on the morphology of the thin films. The TiO2 thin film showed a decrease in the size of spherical-like particles which make up the film from 10 µm to 5 µm. The electric fields produced changes in the films such that when used in a gas sensor, the sensitivity of the sensor increased significantly. 2.2 Literature Involving Electric Fields and ALD While a great deal of research is conducted regarding ALD, the effects of electric fields on the materials grown using ALD remains largely unexplored. Only a few experiments have been conducted in this area and all tests utilized ZnO as the deposited material. Both studies discussed 5

here used diethyl zinc and water as the precursor gasses. In the first study, Liu et al. examined how electric fields influence the crystal orientations of ZnO growth [13,14]. They found that an electric field can create thin films characteristic of epitaxial films, as compared to polycrystalline films. More specifically, XRD measurements show an increase in the intensity of one peak, while suppressing other peaks. In addition, they found that nanowire-like structures were formed when the electric field was high enough. They hypothesize the reason for these effects is due to the polarization of precursor molecules in an external field. More recently, Lu et al. also examined the effect of electric fields on ZnO ALD growth. In their study, the polarization of the electric field was switched depending on which precursor gasses were entering the chamber [15]. The results show that both crystal structure and morphology can be altered by the application of electric fields. Specifically, the most aligned crystals were found to grow when a negative bias was applied to the substrate while water entered the chamber, and a positive bias when diethyl zinc entered the chamber. The roughness of the films was examined as well. The smoothest films were produced with a negative bias during the water pulse and positive bias during the diethyl zinc pulse. The results of both studies indicate that a single crystal orientation can be induced in the thin films, depending on the electric field configuration and temperature. This result agrees with the experiments conducted on CVD and electric fields discussed above. One characteristic of both studies is that a non-uniform electric field was used during the deposition. More specifically, both groups applied a voltage to a substrate holder while using the surrounding chamber as a second electrode or ground. This was done due to the fact that molecules will feel a force in a non-uniform electric field, due to both their atomic polarizability and permanent dipole moment. If this is the case, then experiments utilizing a uniform electric field would produce negligible results compared to a non-uniform electric field, as measured by XRD. Liu and Chang conducted an experiment using parallel plates and indeed found a notable decrease in the mono-crystalline nature of the film [14]. However, this experiment still bears the same qualitative results as the tests performed with non-uniform electric fields. Furthermore, the electric field strength (or inter-plate distance) for the parallel plate experiment is not given. Therefore, additional verification of this effect would be beneficial. In addition, an aspect of the two sets of experiments seem to be conflicting. Liu and Chang claim that the polarity of the electric field had insignificant effects on the XRD data produced by the thin films. However, Lu et al. seem to find significant differences when a different field polarity is used [15]. Additional experiments will help to clarify this discrepancy and confirm effects observed by previous research.

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3. Experimental Methods 3.1 Apparatus The experimental apparatus consists of a home-built ALD system, DC power supply, capacitive sample holder, and electrical feedthrough. Specifically, the ALD system is constructed from a programmable tube furnace with stainless steel tube as shown in Figure 3. A vacuum pump keeps the tube under medium grade vacuum (~350 mTorr) throughout the deposition process. To control the amount of precursor chemicals entering the chamber, a series of valves are opened and closed using a microcontroller based timing system, allowing vapor from the precursors to flow into the deposition chamber with microsecond accuracy.

Figure 3: Photograph of ALD system used in experiments

Three types of sample holders were used in this experiment. All configurations used copper plates as the counter electrode and copper wiring from the sample holder to the electric feedthrough. The first configuration used a large electrode spacing (referred to as D spacing) and electrical connection was made from the sample to electrode via wire clamp. This configuration was used for low strength electric field experiments and is shown in Figure 4A,B. A modification of this apparatus was used for high strength electric fields and is shown in Figure 4C,D. To drastically decrease the D spacing, 0.15 mm glass slides were used as spacers and where pressed between a conductive substrate and the counter electrode with screws. A small D spacing allowed for electric field strengths of above ~1 MV/m. Electric field strengths above this order of magnitude caused electrical breakdown in the apparatus and were not examined in these experiments. The third

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A

B

C

D

Figure 4: (A) and (B) photograph and diagram of sample holder used for low strength electric field studies. (C) and (D) photograph and diagram of sample holder used for high strength electric field studies.

iteration of the apparatus allowed for the application of non-uniform electric fields. To create nonuniform fields, a 4 mm hole was drilled into the counter electrode to decrease the electric field strength in the middle of the sample. When depositing TiO2, care was taken to ensure that no electrical shorts occurred during the deposition process. Thin films of the material were found to be conductive and would create a system resistance of ~104 Ohms after 200 cycles. Due to this phenomena, the glass spacers separating the plates were replaced after each deposition. However, with high strength electric field experiments, the glass spacers would still short circuit after a moderate number of cycles due to the very small spacing between the electrodes. The power source used for the study is a HP DC high voltage source which can produce potentials ranging from 0 to 6kV. A fine adjustment to the voltage allows for a precision of ±1 V. It is connected to the deposition chamber via an electrical feedthrough, which allows the vacuum to be maintained while electrical contact is made. 3.2 Substrate Fabrication Two types of substrates were used in these experiments. The first was a single crystal silicon wafer orientated in the direction. This substrate was used primarily to examine any crystallographic changes in the thin films. A flat surface allows grains to orient in a specific direction, compared to the second substrate: randomly orientated ZnO nanowires. To more easily examine the morphology changes in this study, an array of ZnO nanowires was grown on either FTO glass or silicon substrates. All ZnO nanowires were grown using a hydrothermal method. To seed the growth of the nanowires for Si substrates, a thin layer (~5 nm) 8

of ZnO was sputtered onto the surface prior to growth in solution. The solution used to grow the nanowires consisted of a mixture of 25 mM sodium nitrate and 25 mM hexamethylenetetramine (HMT). Nanowires were grown for two hours at a temperature of 90°C. The resulting nanowires had a diameter of 100 ± 20 nm and showed good vertical alignment. 3.3 Experimental Procedure Two types of coatings were deposited in this study: Al2O3 and TiO2. TiO2 was used to examine crystallinity changes, as it deposits as a crystalline phase at temperatures above 300°C (cite). Al2O3 was used for the highest strength electric field experiments because it is a good electrical insulator and will not cause an electrical short at such small scales. To deposit the coatings, common precursors were used. Specifically, TiCl4 and water were used to deposit TiO2, and Trimethyl Aluminum (TMA) and water were used for Al2O3 deposition. The reactions for both depositions are given below. 𝑇𝑖𝐶𝑙4 (𝑔 ) + 2 𝐻2 0(𝑔) → 2 𝑇𝑖𝑂2 (𝑠) + 4 𝐻𝐶𝑙(𝑔)

(1)

2 𝐴𝑙(𝐶𝐻3 )3 (𝑔) + 3 𝐻2 0(𝑔) → 𝐴𝑙2 𝑂3 (𝑠) + 6 𝐶𝐻4 (𝑔)

(2)

The deposition was performed at various temperatures depending on the desired outcome. These temperatures spanned from 300 to 450°C. Typical experiments involved 200 ALD cycles which resulted in films 10-25 nm thick. An electric potential was applied to the two plates of the sample holder for the entirety of the coating process. This potential ranged from 100 to 400V. Depending on the electrode spacing, the electric field strength could reach a maximum of ~5x106 V/m before electrical breakdown occured. 3.4 Characterization Techniques The primary method of characterization for this project was Scanning Electron Microscopy (SEM) in conjunction with Energy Dispersive X-ray Spectroscopy (EDS). Using SEM imaging, the morphology of the films can be examined. SEM fires electrons accelerated through a large potential difference (typically 5-10kV) at a given sample. These electrons interact with the surface of a sample and eject what are termed secondary electrons. These secondary electrons are then detected and converted to electrical signals, which are displayed on a computer. The resulting images can allow for examination of features as small as 10 nm. To compliment SEM images, EDS can be used to determine the elemental composition of given regions of a sample. Typically EDS uses higher electron energies (8-15 keV) than SEM imaging. While EDS is effective for determining which elements are present in a sample, it does not give a truly quantitative measurement of the thickness or amount of material deposited. To supplement the SEM and EDS results, ellipsometry measurements were taken. Ellipsometry measures the 9

thickness of thin films by comparing the amplitude and phase of light before and after reflecting off a sample with known refractive index. This technique can measure film thicknesses as small as 1 nm and is commonly used for ALD thin film measurements. In addition to SEM and EDS, X-ray diffraction (XRD) was used to examine how the crystallographic properties of the thin films change with different electric field strengths. XRD examines the atomic level structure of crystalline solids by measuring reflected X-Rays from a sample as a function of angle. For a material that is ground into powder, XRD will display multiple peaks which correspond to different crystallographic planes. XRD can also be used to examine the relative amounts of each phase by comparing peak heights in a given sample. For this study, the incident angle was fixed at 5° while the area detector collected data between 10 and 40°. Scans were performed in order to examine changes in peak height caused by electric field effects. Epitaxial-like films will show fewer peaks than polycrystalline peaks because fewer crystallographic planes will fulfill the Bragg condition for diffraction. In XRD measurements, most epitaxial films still show the characteristic peaks of a polycrystalline sample, but the relative intensity of some peaks decreases.

4. Results 4.1 Electric Fields and Film Morphology/Coverage From numerous experiments using both TiO2 and Al2O3 coatings, it seems that a uniform electric field has no effect on the coating coverage or morphology as indicated by SEM and EDS. Figure 5 shows four SEM images of TiO2 coatings on ZnO nanowires. Electric field strengths in these images are on the order of 0.1 MV/m. These images clearly show how the morphology of the coating does not seem to change when uniform electric fields are applied. Any changes are easily accounted for in variations of the sample morphology without electric fields. In addition to SEM images, EDS data given in Figure 6 shows that the Ti to Zn peak height ratio does not change by a significant margin with the application of electric fields. Graphs in Figure 7 show very small variations in peak height for various electric field strengths which are insignificant compared to variations within the sample. Similar results were found using Al2O3 thin films. Al2O3 films could be placed in a higher strength electric field than TiO2 films because Al2O3 films are less conductive. Even at the highest strength attainable before electrical breakdown, there was no notable difference between the morphology of the films when using electric fields. The SEM images in Figure 8 show how the coating is nearly identical to the control sample without electric fields. In general, Al2O3 films are much smoother than TiO2 films because Al2O3 is amorphous at the temperatures deposited. Therefore, any subtle morphology change may be difficult to detect.

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A

B

C

D

Figure 5: SEM images of TiO2 coatings on ZnO nanowires. Samples coated at 300°C for 200 cycles. The electric field strengths are 0(control),+0.15,-0.15, and -0.1 MV/m for A, B, C, and D respectively. Negatively signed electric field strengths indicate the substrate is negatively charged and vice versa.

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A

B

C

D

Figure 6: EDS data for samples described in Figure 3 taken over a 318x200 μm area. Y axis is counts, X axis is X-Ray energy in keV. Vertical line indicates peak used for Ti/Zn height ratio data found in graph below. 0.16

Ti/Zn Peak Height Ratio

0.14 0.12 0.1

0.08 0.06 0.04 0.02 0

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

Electric Field Strength (MV/m) Figure 7: Plot of EDS peak ratios from samples measured in Figure 6. Titanium peak height was measured at 4.5 keV. Zn peak height was measured at 1.0 keV.

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A

B

Figure 8: SEM images of ZnO nanowires coated with Al2O3 at 450°C for 200 cycles. (A) control sample. (B) sample coated under 2 MV/m electric field.

The EDS results discussed above provide some evidence that electric fields do not impact the thickness of the coatings. To further this evidence, flat substrates were coated and their thickness measured using ellipsometry. These measurements also indicate that there is no significant change in film properties when an electric field is applied during deposition. Table 1 highlights these results. The thickness difference between the two samples is approximately 1 nm, much too small to have any statistical significance. Table 1: Thickness of Al2O3 thin films for various electric field strengths. Substrate is polished Si. Electric Field Strength (MV/m) Al2O3 Thickness after 200 cycles at 300°C (nm) 0 2.0

23.60 22.65

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Further experiments were conducted to examine the effect of non-uniform electric fields on the coverage and morphology of thin films. Using the third iteration of sample holder, macroscopic, non-uniform fields were applied during deposition of Al2O3. This also resulted in no change to the coating coverage or morphology when viewed with SEM. 4.2 Crystal Structure Results In order to expand upon the crystallographic work discussed in the literature review, coatings of TiO2 were deposited on a polished Si substrate. XRD data was taken for these coatings to determine if any preferred orientation developed in the presence of electric fields. In order to have sufficient XRD signal, these films were much thicker than films coated in the previous section. 1000 cycles were used when coating the TiO2 films used for XRD studies. The XRD results show that there is very little change, if any, in the crystallographic orientation of the sample. FIGURE shows the difference between a control sample coated at 350°C for 1000 cycles, and a sample coated under a large electric field strength of 1.3 MV/m. These graphs show that the same peaks for both the control and the electric field sample. Very close examination of the graphs shows that the anatase to rutile peak ratio might be slightly higher in the electric field case. However, the high level of noise and variation within a sample makes this conclusion questionable.

Rutile Anatase

Blue (Top): Control TiO2 Sample Red (Bottom): Sample Coated With 1.3 MV/m electric field.

Figure 6: XRD graphs for TiO2 coated at 350°C for 1000 cycles on flat Si wafers. Top curve shifted in the +y direction for clarity.

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5. Discussion 5.1 Vapor Phase Electric Field Effects Results from section 4.1 are consistent with classical electrostatic theory. There are multiple ways in which the applied electric fields could impact the amount of material deposited. While in the gas phase, such mechanisms include molecule polarization, alignment of polar molecules with the electric field, and acceleration of polar molecules in a non-uniform field. However, for each of these cases, the degree in which the electric fields impact the molecular motion is insignificant compared to the thermal energy in the gas. Induced dipoles and molecular geometry changes will occur for any molecule in an electric field. However, simple models indicate that these changes are far too small to have any significant effect on molecular motion or reactivity for gas phase species. One such model is to assume that a molecule such as TiCl4 is composed of a slightly positive inner atom and the outer atoms are slightly negative, forming a roughly spherical shell of charge. Given these assumptions, it is quite simple to calculate the displacement of the center atom. The equation for the displacement, d, is given as 𝑑=

𝐸 𝑟3 𝑘𝑞

Where E is the electric field magnitude, r is the radius of the charged shell, k is Coulomb’s constant, and q is the charge magnitude. For this study, the maximum value of E is less than 107 V/m. The radius, r, of the outer shell of charge can be taken as the Ti-Cl bond length. Finally, q can be taken to be between 0 and 1 electron charge. Using these values, the charge displacement is ~10-16 m. Therefore, the induced dipole for a TiCl4 molecule will be qd ~10-35 Cm. This displacement and induced dipole are small enough that they will not have any measureable effect on the system. A second mechanism to consider is alignment of molecules in an electric field. Alignment of molecules occurs because a potential energy difference exists between different orientations of polar molecules. This energy difference is given by 𝑈 = 2 𝐸𝑝 where E is the magnitude of the electric field and p is the magnitude of the molecular dipole moment. Considering the only polar molecule in this study is water, with a dipole moment on the order of 10-30 C m, even a large electric field strength of ~107 V/m would result in an energy difference on the order of 10 -23 J (10-9 eV) which is much smaller than the rotational thermal energy of the gas, kT= 0.05 eV. This means that the gas phase molecules will not experience any preferred orientation due to electric field effects.

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A third gas phase mechanism could be due to accelerated polar molecules in a non-uniform electric field. The force, F acting on a dipole in a non-uniform electric field can be derived from basic physics as F = ∇E·P, where E and P are the electric field and polarization vectors, respectively. This would suggest that molecules with a dipole should be accelerated in a non-uniform electric field. However, as discussed above, the gas phase molecules will not have any preferred orientation as they move through space. Therefore, the time averaged force on the particle will be near zero, because P will continuously change direction over time. 5.2 Surface Electric Field Effects and Morphology The previous section illustrates how electric fields have no significant effect on the motion of vapor phase molecules. Therefore, the discussion turns to the potential effects electric fields could have on the thin films once molecules begin to interact with the surface. In general, this is a much more complicated system because as precursor molecules interact with a surface, chemical reactions and surface diffusion occur. However, there is some evidence from the literature which suggests that electric fields can affect the morphology of a material via surface diffusion. A paper by Gill et al. found that surface diffusion can be induced in materials via electric fields to form islands of material hundreds of nanometers tall [16]. This effect is driven by the reduction in electrical potential energy upon forming such hills. In addition, the energy difference must be larger than the energy required to form such a surface (i.e. the material must have a low surface energy). In ALD processes, precursor molecules first adhere to the substrate very loosely via interactions such as Van Der Waals forces. These week interactions likely form a sort of pseudo surface in which the surface energy is very low. By this logic, it seems possible that electric fields could induce surface diffusion in ALD processes. However, nanoscale island growth was not observed in these experiments. There are many reasons which could explain this uninteresting behavior. The most obvious explanation is that the adsorbed molecules do not form a conductive layer and therefore do not allow for any changes in energy upon rearrangement of the molecules. Another possible explanation is that the timescales at which diffusion takes place are much longer than the ALD cycle times. In this way, the adsorbed monolayer of molecules will react with the second precursor before any particles have the opportunity to diffuse to a lower energy state. 5.3 Crystal Orientation Discussion The results from section 4.2 demonstrate that the results reported by Liu [13,14] and Lu [15] may only be specific to ZnO ALD films. The question remains why this effect was prominent for previous studies, but not observable to a significant degree in this study. Both authors of prior papers claim that gas phase electric field effects are responsible for the crystalline changes observed. However, from the arguments presented in section 5.1, it seems obvious that gas phase 16

electric field effects will be negligible compared to any thermal gas motion. Therefore, the discussion of crystalline orientation turns to either surface effects or bulk crystal effects. One interesting observation is that ZnO (wurtzite) is piezoelectric and exhibits electrical asymmetry. Both TiO2 structures of anatase and rutile are not piezoelectric. Therefore, it seems reasonable to suspect that ZnO will have preferential orientation due to electrical asymmetries in the crystal structure. For ZnO, the dipole moment of one unit cell is about 1/6 that of water’s dipole moment [17]. This is significantly larger than other wurtzite structures and may be one possible reason for the prominent changes in crystal structure observed in previous studies.

6. Future Work While these experiments show quite clearly that non-ionizing electric fields do not have any effect on either the coverage or morphology for the given system, there are still many experiments which may produce interesting results. Some experiments conducted here yielded interesting results, but were outside the scope of the study. For example, if an aluminum counter electrode was used instead of copper while depositing TiO2 under electric fields, the precursor chemicals would react with the counter electrode and eject aluminum atoms. Upon examination of the substrate, it was found that aluminum was coated throughout the substrate in addition to titanium and oxygen. An extremely diverse range of morphology was observed on this sample including aluminum microwires, and jagged aluminum-titanium nanostructures shown in Figures 8 and 9.

Figure 7: Al/Ti micro-wires deposited on Si substrate after 200 cycles of TiO2 deposition at 350°C. Electric field strength was 2.7 MV/m. Counter electrode was an aluminum block.

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Figure 8:Al/Ti nano-structures deposited on Si substrate after 200 cycles of TiO2 deposition at 350°C. Electric field strength was 2.7 MV/m. Counter electrode was an aluminum block.

This experiment indicates that electric fields may provide an interesting hybrid between chemical and physical deposition techniques. Specifically, it may serve as a method to deposit materials typically difficult to deposit using ALD. By using chemicals that react with or dissolve away a particular electrode, a material could be deposited directly from a bulk material. A carefully selected combination of electrode materials and vaporous chemicals may allow for a novel way to deposit materials without using complicated precursor molecules. Beyond novel deposition mechanisms, there were also interesting morphology changes observed which may or may not be attributed to electric field effects. In many of the high magnitude electric field experiments, changes in morphology were noticed around the edge of the glass spacer (See Figure 10). While it seems obvious that these changes are due to inhomogeneous fields found in that region, experiments deliberately inducing inhomogeneous fields do not replicate this effect. Therefore, other effects may be in play near the spacer edges such as localized heating from leakage electric currents. More experiments may provide explanation of the observed changes.

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Figure 9: Unusual structures found near the edge of glass spacers. Characteristic of most high strength electric field experiments. Image shown is Al2O3 deposited on ZnO nanowires at 450°C under 2 MV/m electric field.

One important feature to note about these experiments is that no morphology or thickness changes were noted within the accuracy of the measurement methods. There may be some effects that are more subtle, but may simply require higher accuracy measurements such as Tunneling Electron Microscopy (TEM). Furthermore, subtle thickness changes may be visible after an in-depth study using ellipsometry for a wide range of ALD cycles. In this way, the thickness could be tracked over an increasing number of cycles and samples with and without electric fields could be compared, showing more subtle changes in thickness. While this thesis has focused on the effects of non-ionizing fields on ALD processes, one of the most obvious ways to induce more exotic effects is to use strong enough electric fields to ionize precursor gases. Using ionizing fields would likely produce similar effects as plasma enhanced ALD such as improved film density, lower deposition temperatures, and increased growth rate [18]. Plasma enhanced ALD ionizes precursor gases far away from the substrate and then flows the ionized gas over the substrate. In future studies with electric fields, other effects may emerge. For example, effects could be induced locally if arcing could be generated at a specific point on the substrate. Finally, an extremely useful tool for further probing the effects of electric fields on ALD would be a detailed physical model of substrate surfaces in ALD. Computation of the effect that electric fields have on either the reactivity or diffusion of precursor molecules would be a huge asset for designing experiments. Some other helpful computational studies could include thermodynamic 19

calculations of the lowest energy state of thin films in an electric field. Is a given phase or orientation of material more stable at a specific electric field strength? In addition, determine how energetically favorable different orientations of water molecules are for different surface charge densities.

7. Conclusion The previous research has documented the effects of non-ionizing electric fields on the morphology of TiO2 and Al2O3 thin films and also the crystal structure of TiO2 thin films deposited by ALD. After characterization by SEM and ellipsometry, it was found that the morphology and thickness of the thin films did not change after treatment with electric fields. Furthermore, it was found that the crystal structure or orientation of TiO2 thin films did not change appreciably under electric fields. Basic theory was applied to gas phase precursor molecules in order to rule out gas phase effects contributing to any effects observed. The results of these experiments indicate that previous studies performed on ZnO ALD with electric fields may only be specific to that material system. Despite these results, there are many more exotic experiments which could be performed with electric fields including ionizing fields, or hybrid physical-chemical film deposition. In the future, more theory must first be done in order to help explain what mechanisms cause the effects reported in previous literature and what experiments could be designed to test such mechanisms.

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