Chapter 1 Introduction
1.9 Dissertation Organization
The content in each chapter is summarized below:
In chapter 1, we introduce the fundamental of thermoelectric effect, the thermoelectric in thin films, the basic of ALD and its progress on thermoelectricity;
besides, we also introduce the other applications of ALD-deposited superlattices. Next, we state our research motivation and objectives and show the research approaches we used in this dissertation.
In chapter 2, we state the experimental details in this dissertation, e.g. processing equipment and parameters, and the measurement and analysis techniques.
In chapter 3, we focus on enhancing the thermoelectric performance of ZnO by doping ZnO with dopants and forming ZnO inorganic superlattices. In this study, the following topics are systematically investigated, including the selection of dopants, the distribution patterns of doping layer, and the incorporation of isotopes.
In chapter 4, we seek to further enhance the thermoelectric performance of the doped ZnO by the incorporation of conducting polymers to form the metal
oxide/polymer superlattice. Hence, the properties of novel MLD-deposited conducting polymers we develop are stated first. Next, the essential of interface-engineering at doped ZnO/polymer interfaces during depositing metal oxide/polymer superlattice is discussed. Finally, the structure of doped ZnO/polymer superlattice, such as periodicity
and thickness of each layer, i optimized to achieve the best thermoelectric performance.
In chapter 5, we state the progress of developing anti-hydrolysis polymers by MLD technique first, in which includes the deposition of polyester, hydrophobic polyamide; besides, the performance of gas barrier with anti-hydrolysis polymer is presented also.
In chapter 6, the conclusion of the dissertation is stated.
Chapter 2
Experimental methods
2.1 Equipment and experiment details 2.1.1 ALD and MLD deposition systems
In this study, a commercial ALD system (Cambridge NanoTech Savannah 100) was utilized to deposit metal ion doped ZnO and isotope incorporated ZnO thin films.
For MLD conducting polymer thin films and metal oxide/polymer superlattice, they were deposited by a home-made system with similar configuration to Savannah 100.
For ALD system and MLD system, the camber pressures were kept around 0.1 Torr and 0.3 Torr, respectively, with both constant 20 sccm high-purity N2 gas flow throughout process. Precursors were introduced and purged with the assistance of N2 flow gas.
As for the sample preparations, alkali-free glass and silicon wafer were utilized as substrates. The substrates were ultrasonically cleaned for 10 minutes in each of the following baths in order: acetone, methanol, isopropanol and deionized water. Before ALD or MLD deposition process, substrates are dried by N2 blow and then were treated with an oxygen plasma treatment in a plasma cleaner (Harrick Scientific, Model PDC-32G, USA) for 5 minutes to remove residual surface contamination and increase surface hydroxyl groups for ALD nucleation.
2.1.2 Conventional and mixed ALD metal ion doping process
In this study, the precursors we utilized to deposit ZnO thin films were diethyl zinc (DEZn, purchased from Sigma-Aldrich with 97% purity) and deionized water (H2O). About metal ion dopants, for controlling the reactivity of precursor, we used the organometallic compounds with same functional groups as precursors, i.e.
tetrakis(dimethylamido)titanium (TDMATi, purchased from Sigma-Aldrich with ≧ 99.99% purity), tetrakis(dimethylamido)zirconium (TDMAZr, purchased from Sigma-Aldrich with ≧99.99% purity) and tetrakis(dimethylamido)hafnium (TDMAHf,
purchased from Sigma-Aldrich with ≧99.99% purity) were the source of Ti, Zr and Hf, respectively. Also, all the organometallic compound precursors were used as received.
During deposition processes, DEZn and deionized water were kept at room temperature;
however, TDMATi, TDMAZr and TDMAHf were heated to 75°C for acquiring
sufficient vapor pressure. Besides, the deposition temperature was set at 150°C, which is a normal ALD ZnO process temperature. The ALD cycle settings including precursor pulse time, precursor soaking time and N2 purge time are listed in Table 2.1, Table 2.2 and Table 2.3. All the samples were composed of around 500 cycles of deposition. As for the nomenclature, doped ZnO with 49 cycles of ZnO and 1 cycle of conventional process HfO2 layer is called as con 49:1 Hf:ZnO. In contrast, the one with 24 cycles of ZnO and 1 cycle of mixed ALD doping process is called as mix 24:1 Hf:ZnO.
Table 2.1: ALD parameters of depositing undoped ZnO.
Table 2.2: ALD parameters of depositing conventional process metal ion doped ZnO.
Precursors Pulse
Table 2.3: ALD parameters of depositing mixed ALD doping process metal ion doped
2.1.3 Oxygen isotope incorporated superlattice
For the deposition of oxygen isotope incorporated superlattice, H218O (water-18O with 97 atom% 18O was purchased from Sigma-Aldrich and used as received) and was utilized as the source of oxygen isotope. As same as the normal H2O, H218O is still ease to volatilize at room temperature, so the pulse time of H218O was set at 0.03s, which was close to the lower limit pulse time of our system for all process. However, prolonged purge time was required to remove excess physisorbed H218O molecules owing to slightly low vapor pressure compared to normal H2O. For convenience, we prolonged the purge time from 5s to 8s arbitrarily.
2.1.4 MLD conducting polymer process
About development of novel MLD conducting polymers, the oxidants we selected were vanadium oxychloride (VOCl3) (mp:-77°C; bp:126°C)and antimony chloride (SbCl5) (mp:2.8°C; bp:90°C), which are both much volatile than molybdenum
pentachloride (MoCl5) (mp: 194°C; bp: 268°C). As for monomers, we used thiophene, aniline and 3, 4-ethylenedioxythiophene (EDOT) to deposit polythiophene, polyaniline and PEDOT, respectively. All the precursor were used as received and purchased from Sigma-Aldrich with ≧99% purity except for EDOT whose purity was 97%. The temperature of each precursor and the setting parameters for depositing polythiophene, polyaniline and PEDOT are summarized in Table 2.4, Table 2.5 and Table 2.6,
respectively. With regard to the deposition temperature, the chamber temperature was set at 150°C to match up the ALD doped ZnO deposition process for developing metal oxide/polymer superlattice.
Table 2.4: MLD parameters of depositing polythiophene.
Precursor Temperature (°C)
Table 2.5: MLD parameters of depositing polyaniline.
Precursor Temperature
Table 2.6: MLD parameters of depositing PEDOT.
Precursor Temperature (°C)
2.1.5 Anti-hydrolysis polymers and metalcone process
In the development of anti-hydrolysis polymers deposited by MLD technique, malonyl chloride (MC, purchased from Sigma-Aldrich with ≧97% purity) and ethylene glycol anhydrous (EG,≧99.8% purity) were utilized as the monomers to deposit polyester thin films. The detailed deposition parameters of polyester are listed in Table 2.7. As for the deposition of polyamide thin films, three combinations were tried:
malonyl chloride and piperazine (≧99% purity); malonyl chloride and 1,8-diaminooctane (≧98% purity); terephthaloyl chloride (TC,≧99% purity) and
ethylenediamine (ED,≧99% purity). The detailed deposition parameters of polyamide are presented in Table 2.8. Regarding the deposition of metalcone films (alucone we deposited only), the precursors were trimethylaluminum (TMA, with ≧99.99% purity) and EG. The detailed parameters are summarized in Table 2.9. It is noteworthy that all precursors were purchased from Sigma-Aldrich and utilized as received.
Table 2.7 MLD parameters of depositing polyester.
Precursor Temperature (°C)
Table 2.8: MLD parameters of depositing polyamides.
Precursor Temperature
Table 2.9: MLD parameters of depositing alucone at 105°C.
Precursor Temperature (°C)
2.2 Thin film characteristics analysis
2.2.1 Measurements of electrical conductivity and Seebeck coefficient
The electrical conductivity, carrier concentration and carrier mobility of the deposited films were determined by Hall effect system (ECOPIA HMS-3000, USA).The design of system is based on Van der Pauw four-point probes method. As for Seebeck coefficient, a home-made system was set up to conduct the measurement. The scheme illustration of home-made system is shown in Figure 2.1. The system was composed of a thermoelectric heating apparatus with a controllable power supply PROVA8000 (TES, Taiwan) to adjust output power and create consistent temperature gradient, and two type-T thermocouples which were connected to the data acquisition switch unit (Keysight 34970A, USA) to measure the temperature at the two ends of the sample. Also, the copper wires of the thermocouples were utilized to measure the electrical voltage between two ends by the data acquisition switch unit. The
measurement of Hall effect and Seebeck coefficient were conducted four times (i.e. four sides or four corners) and then were averaged.
Figure 2.1: The scheme illustration of home-made system for measuring Seebeck coefficient.
2.2.2 Measurement of thermal conductivity by the time-domain thermoreflectance method (TDTR)
Time-domain thermoreflectance (TDTR) is an ultrafast optical pump-probe technique. This method is conducted by using the output of a femtosecond, mode-locked Ti:sapphire laser to heat the metal transducer film which is coated onto the under-test material and probe the temperature evolution by measuring temperature-dependence reflectance (thermoreflectance) of metal transducer film. Typically, the Al film with ~80nm thickness is selected as metal transducer film owing to its great thermoreflectance response at the wavelength of Ti:sapphire laser (~785nm). In this
study, the coated Al films are deposited by thermal evaporation.
In measurements, the output of the Ti:sapphire laser is split into two beams, pump beam and probe beam. The pump beam is modulated at 8.7 MHz by an electro-optical modulator and the probe beam is mechanically chopped at around 200 Hz. Both beams are focused on the Al film by an objective lens. The probe beam heats up the Al
transducer film and then the temperature change of Al films is detected as a function by probe beam. Next, a Si photodiode and a radio-frequency lock-in amplifier are utilized to measure the intensity variant of reflected probe beam caused by
temperature-dependence reflectance of Al film. The output signal 𝑉𝑉(𝑡𝑡) is composed of two parts, the in-phase component 𝑉𝑉𝑖𝑖𝑖𝑖(𝑡𝑡) and the out-of-phase component 𝑉𝑉𝑡𝑡𝑜𝑜𝑡𝑡(𝑡𝑡), and can be described as V(t) = 𝑉𝑉𝑖𝑖𝑖𝑖(𝑡𝑡) + 𝑖𝑖𝑉𝑉𝑡𝑡𝑜𝑜𝑡𝑡(𝑡𝑡), where t is the delay time between two beams.
The thermal conductivity of materials can be determined by compared the measured ratio 𝑉𝑉𝑖𝑖𝑖𝑖(𝑡𝑡)/𝑉𝑉𝑡𝑡𝑜𝑜𝑡𝑡(𝑡𝑡) to the calculations from simulation of thermal transport in
samples. The multilayered structure used for simulation is shown in Figure 2.2 and one of the determined thermal conductivity by matching the measured ratio and simulation is shown in Figure 2.3. This set of measuring apparatus is supported by The Extreme Conditions Laboratory at IES, Academia Sinica and the schematic layout can be seen in Figure 2.4. The detailed description of thermal model used in simulation can be seen in Ref 96.
In order to evaluate the uncertainty of obtained thermal conductivity, the
sensitivity parameters of each term utilized in simulation are calculated. The sensitivity parameter is defined as: 𝑆𝑆𝛼𝛼= 𝜕𝜕 ln (−𝑉𝑉𝜕𝜕 ln𝛼𝛼𝑖𝑖𝑖𝑖⁄𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜), where 𝑉𝑉𝑖𝑖𝑖𝑖⁄𝑉𝑉𝑡𝑡𝑜𝑜𝑡𝑡 is the ratio of measured in-phase and out-of-phase voltages. α is one of parameters utilized in simulation, such as thickness of Al, interface thermal conductance between Al film and under-test material and thermal conductivity of under-test material. Figure 2.5 shows the
calculated 𝑆𝑆𝛼𝛼 for each parameters in simulation. Theoretically, the ratio 𝑉𝑉𝑖𝑖𝑖𝑖⁄𝑉𝑉𝑡𝑡𝑜𝑜𝑡𝑡 is more sensitive to the thermal conductivity of under-test material at 100 ps≤ t ≤ 500 ps.
Therefore, the influential parameters in our simulation are the thickness and heat capacity of Al film, thickness of ALD deposited material and the interface thermal conductance between ALD deposited material with Al film and Si substrate. Assuming 3% uncertainties of these influential parameters, the total error in simulated thermal conductivity of ALD deposited material is ~7%.
Figure 2.2: The multilayered structure used for simulation.
Figure 2.3: The result of ZnO thermal conductivity by TDTR method.
Figure 2.4: The schematic layout of TDTR apparatus.
Figure 2.5: The calculated 𝑆𝑆𝛼𝛼 for each parameters in simulation.
2.2.3 Quartz crystal microbalance (QCM)
To monitor the surface reaction of precursors (e.g. adsorption and desorption) during process in real time, ALD and MLD deposition systems are equipped with customized in-situ QCM system. The QCM signals were recorded and analyzed by STM-2 (INFICON, Switzerland).
2.2.4 Spectral characterization
As for the thickness and refractive index measurement of the inorganic ALD films, they were determined by ellipsometry (Elli-SE-U, Ellipso Technology, Korea). In order to compare the doping concentrations between conventional process and mixed
ALD doping process, determining the composition of the deposited films was conducted with X-ray photoelectron spectroscopy (XPS) by PHI 5000 VersaProbe (ULVAC-PHI, Japan) using an Al Kα X-ray source. Before depth profile XPS measurement, Ar+ sputtering pre-treatment was applied to remove surface
contamination. The element concentrations were determined by averaging the depth-profiling results. For crystallinity and crystal structure, X-ray diffraction (XRD) patterns were collected by Rigaku TTRAX3 X-ray diffractometer (Rigafu, Japan) with Cu Kα radiation. The characteristic IR absorption peaks and the UV-Vis absorption of
deposited polymer films were collected by Perkinelmer spectrum 100 FT-IR operated in transmission mode at 4 cm−1 resolution and Jasco V-770 UV-Visible/NIR
spectrophotometer with wavelength in the range of 200-1300 nm, respectively.
2.2.5 Transmission electron microscopy (TEM)
To clearly observe the microstructure of superlattices along the direction of film growth, the cross-sectional TEM images were obtained by the field emission TEM, JEOL 2010F system (JEOL, Japan). The preparation of the cross-sectional TEM samples were implemented by Gatan Precision Ion Polishing System II, 695.B (Gatan, USA).
2.2.6 Micro-figure measurement (Alpha-step)
As for determining the thickness of MLD conducting polymer thin films, it is hard to conduct by ellipsometry since the refractive indexes of deposited materials are
uncertain, which arises the uncertainty of the measured thickness. For this reason, micro-figure measurement (MICROFIGURE MEASURING
INSTRUMENT-Surfcorder ET3000) was utilized to determine the thickness of deposited films. Before measurement, we removed parts of the deposited films either physically or chemically depending on the characteristic of the films. Hence, a sharp step was created and then the height of the step, i.e. film thickness, was able to be measured.
2.2.7 Gas barrier performance measurements
In this dissertation, two types of measurements, water vapor transmission rate (WVTR) and helium transmission rate (HeTR), were implemented. In the measurement of WVTR, the gas barrier was deposited on a PI substrate (thickness=75 µm) and then the WVTR was measured by MOCON AQUATRAN Model 1 at 38°C and 100% RH.
As for the HeTR of the ALD/MLD deposited gas barrier with PI substrate, it was measured by a home-made setup developed by our group with the helium leak detector (Alcatel, ASM, 142Graph) as the helium sensor. For the detailed description of home-made setup, it can be found in the doctoral dissertation of Ming-Hung Tseng, Ph.D.
2.2.8 Thermogravimetric analysis (TGA)
To evaluate effect of low-temperature pyrolysis process we developed,
thermogravimetric analysis (TGA) was utilized to measure the weight of carbon-residue inside the ALD/MLD deposited superlattice. In this measurement, it was conducted by TA Instruments SDT-Q600 with a 100 sccm (standard cubic centimeter per minute) O2
flow.
Chapter 3
Metal oxide superlattice
3.1 Selection of dopants
For verifying the effects of Ti, Zr and Hf doping on thermoelectric properties of ZnO, we deposited four types of doped ZnO films by conventional process, i.e. con 49:1, con 24:1, con 19:1 and con 9:1 for each dopant, and the characteristics of them are summarized in Table 3.1.
Starting with the carrier concentration, it is noteworthy that the carrier
concentration in Ti:ZnO system was higher than those in the other two systems. This phenomenon could be originated from the actual dopant concentration and the distance between ionized dopants. The elemental composition measured by XPS and calculated atomic number density are listed in Table 3.2. Although we chose the precursors with the same functional groups for Ti, Zr and Hf, the reactivity of these precursors were still different owing to the formation energy difference of chemisorbed precursors. Because the formation of a chemisorbed precursor involved breaking a metal/ligand bond within precursor itself and then forming a metal/oxygen bond between precursor and surface hydroxyl group. Therefore, the reactivity of a precursor might be described by the difference between the bond energies (ΔBE) of metal/ligand and metal/oxygen. The bond energy of each bond is shown in Table 3.3. It could be discovered that the actual
dopant concentration indeed followed the trend of ΔBE, i.e. Hf > Zr > Ti, and so did the number density of each dopant. Based on effective field theory, the close-packed dopants are not able to donate free electrons freely owing to the Coulomb repulsion force between adjacent ionized dopants68,97,98; thus, Ti:ZnO system had a higher doping efficiency. As for the decreasing electrical mobility for all structures, it was attributed to dopant scattering effect and the suppression of crystallinity. The XRD patterns of undoped ZnO and all con 9:1 doped ZnO are shown in Figure 3.1. The reduction of intensity for all peaks implies the suppression of crystallinity with the incorporation of dopants. Especially, (002) peak, which is parallel to direction of film growth and periodicity of dopants, decreases obviously and shifts to higher angle due to lattice shrinkage in z-axis caused by the smaller ionic radius of dopants compared to Zn, which are 56 pm, 73 pm, 72 pm and 74 pm for Ti, Zr, Hf and Zn, respectively. With great radius difference, the degree of change in the diffraction pattern for Ti:ZnO was most severe, so the electrical mobility reduced seriously as well. For the overall influence on electrical conductivity, it could be found that the electrical conductivity rose as the dopant concentration increased except for con 9:1 structure. Also, this result could be explained by effective field theory. Therefore, the carrier concentration only rose slightly even decrease when the concentration of dopant rose beyond a certain level, such as con 9:1 Ti:ZnO.
In the case of Seebeck coefficients, they basically followed Mott formula, which states the trade-off characteristic of the absolute value of Seebeck coefficient and electrical conductivity. Furthermore, it could be discovered that the Seebeck coefficient was less sensitive to the electrical mobility than the carrier concentration by comparing con 24:1 Zr:ZnO with con 19:1 Hf:ZnO and con 9:1 Ti:ZnO and con 9:1 Hf:ZnO.
Overall, the highest PF value occurred at con 49:1 structure in these three systems, which was only slightly doped, the carrier concentration was around 10 × 1019 and the electrical mobility was maintained as high as possible.
Referring to thermal conductivity, we initially assumed that Hf:ZnO might has a lower thermal conductivity attributed to the large mass difference. However, this hypothesis only held for Zr:ZnO and Hf:ZnO. With the almost same ionic radius, the effect of dopants on crystallinity was almost about the same; thus, the mass difference played a crucial factor in suppressing thermal conductivity. Thus, the thermal
conductivity of Hf:ZnO was lower than Zr:ZnO regardless of doping concentrations. As for Ti:ZnO, the effect of reducing crystallinity overwhelmed the influence of mass difference, so the thermal conductivity of Ti:ZnO showed a lower value when doping concentration rose. Especially, con 9:1 Ti:ZnO had the lowest thermal conductivity even the concentration of dopant was lower than Zr:ZnO and Hf:ZnO. For the overall
enhancement on ZT, although the degree of reduction for thermal conductivity was
almost 15-fold, the decreasing PF compensated the outstanding effort; thus, the ZT was only enhanced by the factor of 6.44 in con 9:1 Ti:ZnO.
In summary, these three tetravalent dopants were able to promote the electrical conductivity of ZnO by the factor of around 2.5 and suppress thermal conductivity by the factor of near 15, but the decreasing absolute value of Seebeck coefficient retarded the further enhancement in ZT, which was just about 6-fold enhancement. Although the degree of enhancement in ZT was about the same in these three system, we could still conclude that Hf might be the best dopant among them. Because the incorporation of Hf didn’t decrease the electrical mobility severely. This advantage ensured high electrical conductivity without increasing carrier concentration greatly, and thus the high absolute value of Seebeck coefficient could be held. Besides, the great mass difference
suppressed the thermal conductivity effectively.
Table 3.1: The thermoelectric characteristic of undoped ZnO, Ti:ZnO, Zr:ZnO and Hf:ZnO. n: carrier concentration and the unit is ×1019 cm-3; μ: electrical mobility and the unit is cm2 V-1 s -1 ; σ: electrical conductivity, S cm-1 ;κ: thermal conductivity, W m-1
Table 3.2: The concentration of dopant measured by XPS and atomic number density of dopants. The number density is calculated by using composition ratio, film thickness and density41,99.
O:Zn:dopant Number density of
dopant (nm-2)
Con 9:1 Ti:ZnO 40.33 : 56.57 : 3.30 3.51
Con 9:1 Ti:ZnO 40.33 : 56.57 : 3.30 3.51