Study of effects of silver incorporation on electrical and optical properties of tco thin films

Effect of silver sputtering time to the transparent of TNO thin films, which were annealed in 15min ..... Effect of silver sputtering time to the transparent of TNO thin films, which wer

VIETNAM NATIONAL UNIVERSITY, HANOI

VJU VIETNAM JAPAN UNIVERSITY

TRAN NGOC LAN

Study of effects of silver incorporation on electrical and optical properties of TCO thin films

MASTER THESIS

Submitted in partial fulfillment of the requirement for the degree of

Master of Nanotechnology

Hanoi, July 2018

VIETNAM NATIONAL UNIVERSITY, HANOI

VJU VIETNAM JAPAN UNIVERSITY

TRAN NGOC LAN

Study of effects of silver incorporation on electrical and optical properties of TCO thin films

Submitted in partial fulfillment of the requirement for the degree of

Master of Nanotechnology

Supervisors: Dr Nguyen Tran Thuat

Dr Hoang Ngoc Lam Huong

Hanoi, July 2018

ACKNOWLEDGEMENTS

I would like to express deep gratitude to my first supervisor Dr Nguyen Tran Thuat, Nano and Energy Center, VNU University of Science, Vietnam National University, Hanoi, who is a teacher – scientist has been directly guiding and helping me to finalize this thesis During the implementation of this thesis, I have received much useful scientific knowledge from him

I especially would like to sincerely thank to my second supervisor Dr Hoang Ngoc Lam Huong, who has enthusiastically helped me to perform experiments to get the most accurate results I am really grateful to her for all her supports and encouragement Her ideas and suggestions were extremely useful to get this thesis to its final shape

I would like to sincerely thank Dr Pham Tien Thanh, Vietnam Japan University, Vietnam National University - Hanoi, who helped me to use the Alpha Step profilometer

I would like to sincerely thank Mr Luu Manh Quynh, Center of Materials Science, VNU University of Science, Vietnam National University, Hanoi - who helped me to use the UV-Vis spectrophotometer

I would like to sincerely thank Ms Le Hac Huong Thu, RIKEN Center for Advanced Photonics, Innovative Photon Manipulation Research Team, Saitama, Japan, who helped me to analyze some results

I would like to thank all the teachers and college students at the Nano and Energy Center, VNU University of Science, those who always create the most favorable conditions to finish this my thesis

I would like to thank all the teachers and students at Vietnam Japan University, Vietnam National University, Hanoi

I would like to thank project Science and Nano Technology, Vietnam National University, Hanoi (VNU) has created favorable conditions on equipment

in the implementation this thesis

Finally, I extend my sincere to thank to all my friends and my family, those who provide support, encouragement and help me during the learning as well as during the research and the completion of this thesis

Hanoi, July 16th , 2018

Table of Contents

1 INTRODUCTION 1

1.1 Transparent conductive oxides 1

1.1.1 Main applications of TCO materials 2

1.1.2 Trends in the development of TCO materials 3

1.1.3 A quantitative figure of merit (FOM) of TCO materials 3

1.2 Niobium-doped titanium oxide 4

1.2.1 Structural properties 5

1.2.2 Electrical properties 8

1.2.3 Optical properties 9

1.3 CuAl x O y 10

1.3.1 Structural properties 12

1.3.2 Electrical properties 15

1.3.3 Optical properties 16

1.4 Effects of silver incorporation on TCO thin films 17

1.5 Aim of work 19

2 EXPERIMENTS 21

2.1 Co-sputtering methods 21

2.2 Preparation of sample before sputtering and sputtering 27

2.3 Incorporation of silver in TNO 31

2.4 Incorporation of silver in CuAl x O y 32

2.5 Characterization methods 33

2.5.1 Thicknesses measurement 33

2.5.2 4-point probe measurement 34

2.5.3 Thermal coefficient of resistance measurement 35

2.5.4 UV-VIS measurement 35

3 RESULTS AND DISCUSSIONS 38

3.1 Effect of silver concentration on TNO properties 38

3.1.1 Thickness of thin films 38

3.1.2 Optical properties 39

3.1.3 Electrical properties 47

3.1.4 FOM 50

3.2 Efffect of silver concentration on CuAl x O y properties 52

3.2.1 Thickness of thin films 52

3.2.2 Optical properties 54

3.2.3 Electrical properties 58

3.2.4 FOM 65

4 CONCLUSION 67

REFERENCES 68

LIST OF FIGURE

Figure 1 The shape and the color of the crystalline anatase (a), rutile (b), brookite

(c) and TiO 2 powder (d) [14] 5

Figure 2 Crystal structures of anatase (a), rutile (b) and brookite (c) [14] 6

Figure 3 Unit cell structure of AgAlO 2 [65] 12

Figure 4 Unit cell structure of CuAlO 2 [66] 12

Figure 5 Schematic representation of the delafossite structure (ABO 2 ) The gray polyhedral and black spheres represent edge-shared B 3+ O 6 distorted octahedral and linearly coordinated A + cations, respectively [67] 13

Figure 6 The general operating principle of the sputtering method [92] 22

Figure 7 Principle of magnetron sputtering [93] 24

Figure 8 Four-gun sputtering machine SYSKEY SP-01 at the cleanroom of Nano and Energy Center 27

Figure 9 The substrate cleaning solution: acetone, ethanol, acid H 2 SO 4 and H 2 O 2 28

Figure 10 The targets for sputtering 28

Figure 11 CG Substrates were hold on the holder 29

Figure 12 Substrates were loaded into chamber 29

Figure 13 Simulation figure of co-sputtering process 30

Figure 14 Simulation diagram of TNO thin films co-sputtering process 32

Figure 15 Simulation diagram of CuAl x O y thin films co-sputtering process 33

Figure 16 The Alpha Step profilometer at Vietnam Japan University’s laboratory 34

Figure 17 The 4-point probe Jandel at Clean Room, Nano and Energy Center, Hanoi University of Science 34

Figure 18 The Self-designed temperature dependent resistance measurement system 35

Figure 19 The Ultraviolet–visible spectroscope or ultraviolet-visible spectrophotometry (UV-Vis) at Center for Materials Science, Hanoi University of Science 36

Figure 20 Schematic UV-Vis measurement of samples 37

Figure 21 The thickness of TNO thin films depended on silver sputtering time 38

Figure 22 Effect of annealing time to the transparent of TNO-0s Ag thin films 39

Figure 23 Effect of annealing time to the transparent of TNO-30s Ag thin films 40

Figure 24 Effect of annealing time to the transparent of TNO-60s Ag thin films 41

Figure 25 Effect of annealing time to the transparent of TNO-90s Ag thin films 42

Figure 26 Effect of silver sputtering time to the transparent of TNO thin films, which were annealed in 15min 43

Figure 27 Effect of silver sputtering time to the transparent of TNO thin films, which were annealed in 30min 44 Figure 28 Effect of silver sputtering time to the transparent of TNO thin films, which were annealed in 60min 45 Figure 29 Effect of annealing temperature to the transparent of TNO-60s Ag thin films 46 Figure 30 Effect of annealing time to the conductivity of TNO thin films 47 Figure 31 Effect of annealing temperature to the conductivity of TNO thin films 48 Figure 32 Effect of annealing temperature to the FOM of TNO thin films 50 Figure 33 Effect of annealing time to the FOM of TNO thin films 51 Figure 34 The thickness of CuAl x O y thin films depended on sputtering power of copper 52 Figure 35 The thickness of CuAl x O y thin films depended on silver sputtering time 53 Figure 36 Effect of sputtering power of copper to the transparent of CuAl x O y thin films 54 Figure 37 Effect of silver sputtering time to the transparent of CuAl x O y -Cu 60W thin films 55 Figure 38 Effect of silver sputtering time to the transparent of CuAl x O y -Cu 80W thin films 56 Figure 39 The resistance depended on temperature of CuAl x O y thin films 58 Figure 40 The resistance depended on temperature of Ag doped CuAl x O y -Cu 60W thin films 59 Figure 41 The resistance depended on temperature of Ag doped CuAl x O y -Cu 80W thin films 60 Figure 42 The TCR and the conductivity of CuAl x O y thin films depended on sputtering power of copper 61 Figure 43 The TCR and the conductivity of CuAl x O y -Cu 60W thin films depended

on silver sputtering time 62 Figure 44 The TCR and the conductivity of CuAl x O y -Cu 80W thin films depended

on silver sputtering time 63 Figure 45 The FOM of CuAl x O y thin films depended on sputtering power of copper 65 Figure 46 The FOM of Ag doped CuAl x O y thin films depended on silver sputtering time 66 Figure 47 Resistivity vs temperature plot for a) Metal b) Semiconductor and c) Superconductor 83 Figure 48 Two-wire resistance measurement schematic 84 Figure 49 Four-wire resistance measurement configuration 85

Figure 50 Diagram of 4-point probe 85

Figure 51 A simplified schematic for the design process and the corresponding temperature diagram 87

Figure 52 Thermal coefficient of resistance measurement system 88

Figure 53 Schematic diagram of the Thermostat and Temperature Measurement 90 Figure 54 Schematic diagram of the Peltier Controller 91

Figure 55 The operation principle and the symbolic meaning of TEC1-12706 93

Figure 56 The TEC1-12706 performance specifications 94

Figure 57 Keithley Model 2400 Series Source Meter specifications 95

Figure 58 Transmission spectrum of a sample thin film 98

LIST OF TABLE

Table 1 Some basic physical parameters of TiO 2 anatase, rutile and brookite

phase [17] 7

Table 2 Optical band gap values estimated from the transmittance and reflectance spectra of TNO thin films [39] 10

Table 3 Some typical values for air at different pressures at room temperature [94] 25

Table 4 The list of TNO samples 31

Table 5 The list of CuAl x O y samples 32

Table 6 Average transmittance of CuAlO 2 thin films in the visible range 57

Table 7 Resistivity of various materials 83

LIST OF ABBREVIATIONS

Ω.cm to 3.7x Ω.cm, accompagned with good optical transmittances, higher than 60% in the visible range The results on CuAlxOy doped Ag thin films showed that CuAlxOy doped Ag they can be hardly applied for transparent conductive layers However, these films exhibited relatively high temperature coefficient of resistance

of about 3%/K, thus being suitable for applications in microbolometers

1 INTRODUCTION

1.1 Transparent conductive oxides

There are many review papers reporting crystalline and physical properties, prospects for further development, and applications of transparent and conducting oxides (TCO) semiconductors The coexistence of optical transparency and electrical conductivity in these materials mainly depends on the nature, number, and atomic arrangements of metal cations in amorphous or crystalline oxide structures It also depends on the resident morphology and on the presence of intrinsic or intentionally introduced defects The important TCO semiconductors are impurity-doped ZnO, SnO2, In2O3 and CdO, as well as the ternary compounds Zn2SnO4, ZnSnO3, Zn2In2O5,

Zn3In2O6, In2SnO4, CdSnO3, and multi-component oxides consisting of combinations of ZnO, SnO2 and In2O3 [1] Among them, Sn doped In2O3 (ITO) and

F doped SnO2 (FTO) thin films are the preferable materials for most present applications However, the expanding use of TCO materials, especially for the production of transparent electrodes for optoelectronic device applications, is endangered by the high price and scarcity of In This situation promotes the search for alternative TCO materials in order to replace ITO materials The electrical resistivity of the novel TCO materials should be about

10-5 cm and the typical absorption coefficient smaller than 104 cm-1 in the near ultra violet and visible range, with optical band gap is about 3eV [2] Recently, ZnO:Ga (GZO) and ZnO:Al (AZO) semiconductors could become good alternatives

to ITO for thin-film transparent electrode applications The AZO thin films have low resistivity of the order of 10−4 .cm, are non-toxic and inexpensive source materials [3] However, development of large area deposition techniques are still needed to make able the production of AZO and GZO films on large area substrates with a high deposition rate In addition, on the requirement of electrical and optical characteristics, applied TCO materials should be also stable in hostile environment containing acidic and alkali solutions, oxidizing and reducing atmospheres and elevated temperature Most of TCO materials are n-type semiconductors, but TCO-

type p materials have been developed and researched such as IZO, ZnO:Mg, ZnO:N, NiO, NiO:Li, CuAlO2, Cu2SrO2 or CuGaO2 thin films [1] However, at present, these materials have not yet found place in actual applications

Most transparent conductive oxides (TCO) are binary or ternary compounds, containing one or two metallic elements Their resistivity could be as low as 10-4

cm, and their extinction coefficient k in the optical visible range (VIS) could be

lower than 10-4 because their wide optical band gap (E g) that could be greater than 3eV This remarkable combination of conductivity and transparency is usually impossible in intrinsic stoichiometric oxides However, it is achieved by producing them with a non-stoichiometric composition or by introducing appropriate dopants

In 1907, Badeker discovered that CdO thin films possess such characteristics[4] Later, it was found that thin films of ZnO, SnO2, In2O3 and their alloys were also TCOs[5] Doping these oxides resulted in improved electrical conductivity without deduction their optical transmission The Aluminum doped ZnO (AZO), tin doped

In2O3 (ITO) and antimony or fluorine doped SnO2 (ATO and FTO) are among the most utilized TCO thin films in modern technology

1.1.1 Main applications of TCO materials

The potential and actual applications of TCO thin films include: (1) transparent electrodes for photovoltaic cells and for flat panel displays, (2) window defrosters, (3) low emissivity windows, (4) light emitting diodes, (5) transparent thin films transistors and (6) semiconductor lasers [6] As the usefulness of TCO thin films depends on both their electrical and optical properties, therefore, both parameters should be considered together with environmental stability, electron work function, abrasion resistance and compatibility with substrate and other components of a given device Besides, the availability of the raw materials and the economics of the deposition method are also significant factors in choosing the most appropriate TCO material Therefore, the selection decision is generally made by maximizing the functioning of the TCO thin film by considering all relevant

parameters, and minimizing the costs If TCO material selection only based on maximizing the conductivity and the transparency can be faulty

1.1.2 Trends in the development of TCO materials

Recently, the scarcity and high price of Indium needed for the fabricating of ITO, the most popular TCO, as spurred R&D aimed at finding a substitute Its electrical resistivity () should be about 10-4 cm or less, with an absorption coefficient () smaller than 104 cm-1 in the near-UV and VIS range, and with an optical band gap higher than 3eV A 100 nm TCO film with these values for  and

will have optical transmission (T) = 90% and a sheet resistance (RS) = 10  At present, AZO and ZnO:Ga (GZO) semiconductors are promising alternatives to ITO for thin film transparent electrode applications because its source materials are inexpensive and non-toxic For example, AZO can have a low resistivity, on the order of 10-4 Ω.cm[7] However, the development of large area, high rate deposition techniques is hardly needed Another objective of the recent effort to develop novel

TCO materials is depositing p-type TCO films Although most of the TCO materials are n-type semiconductors, but p-type TCO materials are required for the

development of solid lasers and some other applications Reviews published on the TCO report comprehensively on the deposition and diagnostic techniques, on the characteristics of the film and on the proposed applications[8][9][10]

1.1.3 A quantitative figure of merit (FOM) of TCO materials

In order to study the optimized deposition conditions, extensive electrical and optical measurements have been carried out on TCO thin films A quantitative figure

of merit (FOM) is usually calculated for the combination of optical and electrical properties Widely used FOMs of this type include 2 different ways:

of the material does not depend on its thickness, the FOM can tend to infinity, leading to failure of this FOM The FOM was calculated by the Eq 2, which has dimensions of Ω-1, has been proposed with a power x = 10 This choice was made based on the stipulation that the maximum FOM, for a given TCO thin film, occurred at 90% transmission without really considering the electronic transport properties of the thin film In this thesis, to overcome the above-presented singularity and balance the effect of electrical transport with optical transmittance, the thickness should be measured exactly and the FOM, which was calculated by the Eq 1, has been used This FOM was consistently calculated in order to determine the optimum synthesis conditions for TCO thin films

1.2 Niobium-doped titanium oxide

A potential candidate for TCOs without Indium was Niobium doped Titanium dioxide (TNO) Furubayashi et al and have discovered that TNO films was grown from epitaxial on SrTiO3 (STO) single crystal or LaAlO3 substrates created by pulsed laser deposition (PLD) and showed good conductivity and high transparent in visible range [13] In the past, many interesting and compelling results related to TNO films have been reported Highly conductive TNO films were prepared by two

methods One is by PLD and one is by sputter deposition Between the two methods, sputtering is a good method to be more suitable for making low cost films with uniform thickness which can be very attractive for high volume device manufacturing

Figure 2 Crystal structures of anatase (a), rutile (b) and brookite (c) [14]

On the Fig 2, rutile has been known to be the most stable phase in TiO2phases Because of its scientific important and practical, TiO2 rutile has been the subject of many theoretical and experimental investigations Opposite to the extensive studies on the rutile phase, rarely knowledge is about the other 2 less stable phases The electronic properties of anatase are usually assumed to be similar to those of rutile Recently, interest in anatase was motivated by its key role in the injection process in a photochemical solar cell with superior conversion efficiency Forro et al investigated the transport properties and revealed a very shallow donor level and high n-type mobility in anatase crystals On the other hand, Chauvet et al studied the magnetic properties and confirmed the existence of the shallow donors [15] In addition, Tang et al have studied photoluminescence in the same anatase crystals [16] Luminescence due to the recombination of self-trapped excitons has been seen on the anatase crystals, but not on the rutile crystals

Table 1 Some basic physical parameters of TiO 2 anatase, rutile and brookite phase [17]

Crystal structure Tetragonal

a=0.4594 c=0.2959

a=0.9184 b=0.5448 c=0.5145 Basic cell volume

0.1937

Bonding angle

Water solubility Insoluble Insoluble

The present researches on anatase thin films are motivated by the distinct properties already displayed by anatase crystals, as well as by the fact that thin films are often required for practical uses (Table 1) On the other hand, thin films have more advantages over crystals for investigation in that thin films can be heat treated

more easily Among the literature about the preparation, characterization and application of TiO2 thin films, less known of the difference in fundamental electronic properties between rutile and anatase thin films The better knowledge of these differences should be beneficial to further exploitation of the electronic properties of TiO2 thin films and particularly in optoelectronic devices [18]

1.2.2 Electrical properties

The vacancy formation is the most important condition for obtaining highly conductive TNO thin films [19] Therefore, crystallization of amorphous films by annealing in reductive atmosphere [20] or using lower-oxide based such as Titanium (Ti) or Ti2O3 based targets [21] are significantly effective methods to obtain highly conductive TNO thin films Oxygen deficient TNO showed metallic conductivity [22]

In sharp contrast to s or p electrons-based conventional TCOs, d-electrons play a major role in TNO, which might provide a new concept for spintronic device For this potential application, the controlled incorporation of transition metal elements with desired amount and intentional defect engineering offer novel approaches to performance improvement On the other hand, doping ions can allow energy band structure engineering to avail visible light absorption as well as to trap photogenerated electrons/holes for effective separation of the electron–hole pairs which depress the electron–hole recombination, leading to high photocatalytic activity Numerous relevant papers have described the growth of TNO films as TCO

at lower doping concentration, while investigation of high doping level is rare In order to achieve high carrier concentration, high dopant concentration is needed Therefore, high solubility of Nb into anatase TiO2 with excellent structure perfection

is desired Also, studies of segregation of Nb inside TiO2 as well as its effect on electron transport property are comparatively limited This is due to the difficulties

of incorporating high level Nb doping into the lattice while maintaining the good structure of anatase phase [23]

All of electrical properties improved significantly when Nb was added to TNO films Resistivity decreased and carrier concentration became higher as co-sputtered time decreased Mobility did not have progressive behavior but also improved The optimized film was obtained; with resistivity of 3.5×10-3 Ω.cm, carrier concentration of 2.4×1020 cm-3, and Hall mobility of 5.0 cm2.V-1.s-1 [24] The resistivity measured by four-point probe method depends strongly on annealing temperatures of TNO thin films Lower resistivity (less than 1×10−2 Ω.cm) in TNO thin films can be obtained by annealing at higher than 450°C The lowest resistivity was 2.4×10−3 Ω.cm and they were obtained at 600°C annealed TNO thin film These resistivity values were almost of the same order as those found in TNO thin films that were prepared and reported by other researchers The lowest resistivity at each annealing temperature of these TNO thin films decreased with the increase in the annealing temperature, i.e., the higher annealing temperature corresponded to lower resistivity of the TNO thin film [25]

1.2.3 Optical properties

As recently, TiO2 has also been taken into account for photovoltaic applications because it is a semiconductor material with long-term stability, broadly low cost availability and non-toxic environmental acceptability Because of optical band gaps above 3 eV (3.0 eV for rutile [26][27], 3.4 eV for anatase [28][29] and 3.3

eV for brookite), natural TiO2 is only photoactive in the UV region of the electromagnetic spectrum and the inefficient active solar cell material However, the material advantages of TiO2 can be used indirectly in technically and economically viable dye-sensitized solar cells [30] where it works as an electron transporting substrate for the chemisorbed photoactive dye

In another interesting direction, Niobium doped anatase TiO2 (Ti1-xNbxO2) thin films in both polycrystalline and epitaxial forms were found recently to exhibit low resistivity ρ of the order of 10-4

Ω.cm and high transmittance (60% to 90%) in the visible light region [31][32][33][34].The important TiO2 properties are other

conventional host materials of TCOs do not possess, such as a large static permittivity [35], high transmittance in the infrared region, high refractive index k [36] and high chemical stability especially in a reducing atmosphere [37] Therefore, TNO shave sufficient potential as a next-generation of TCOs As same as the TCOs properties, TNO has low infrared transparency, hence possibly becomes a promising material for application of heat-resistant glass window and the energy saving solution [38] The transmittance and reflectance spectra of TNO films, annealed at 450°C and 550°C The TNO films exhibited from 60% to 80% transmissions in both the visible and infrared regions

Table 2 Optical band gap values estimated from the transmittance and reflectance spectra of TNO thin films [39]

The estimated values of band gap of TNO thin films are listed in Table 2 This optical band gap expansion is calculated by Burstein–Möss effect, where the lowest energy state in the conduction band is occupied, and electron transitions can occur only to higher than the Fermi energy Larger band gap values (higher than 3.7 eV) can be explained by the higher carrier densities of TNO films (1.5×1021 – 3.35×1021 cm−3) [39]

From these data, it has been proved that the formation of high transparency and highly conducting TNO films by a combination of high speed RF-magnetron sputtering, which depends mostly on the sputtering power and the annealing conditions in vacuum

1.3 CuAl x O y

Many delafossites (A+B3+O2) were found on physical properties They were reported at the first time as part of large research about structure, synthesis, and

electrical transport properties by Rogers, Shannon, and Prewitt [40][41][42] In a short time after, the optical and the electrical properties of certain copper delafossites

in a many papers were reported by Benko and Koffyberg [43][44] In the 1997, the attention to delafossites decreased when a seminal paper by Kawazoe and co-workers found that a 500 nm thick film of CuAlO2 transmitted about 70% in visible

light and exhibited p-type conductivity of 0.95 S.cm-1 [45] From then, delafossites, especially the Ag and Cu elements, have received considerable attention owing to their application as the transparent conducting oxides [46][47] Particular emphasis was placed on optimizing their electrical and optical properties through proper selection of the parent delafossites, aliovalent dopant and control of the oxygen stoichiometry Because of their compositional versatility, delafossites also were studied deeply as catalysts [42–44], luminescent materials [45,46], batteries [53], and thermoelectrics [48–50] Recent photocatalytic applications have researched the sensitivity of delafossites to transmit in the visible region, and the variability in optical band gaps for different A- and B- site cation combinations offers many others applications [51–55] For copper delafossites, the effect of altering the B-site cation

on both the electrical and optical properties was investigated extensively [44][62][63][64] In comparison, the electrical and the optical properties of silver delafossites have not been well understood Therefore, there are some disagreements between researchers on the reported about the optical properties of delafossites

1.3.1 Structural properties

Figure 3 Unit cell structure of AgAlO 2 [65]

Figure 4 Unit cell structure of CuAlO 2 [66]

Figure 5 Schematic representation of the delafossite structure (ABO 2 ) The gray polyhedral and black spheres represent edge-shared B 3+ O 6

distorted octahedral and linearly coordinated A + cations, respectively [67]

“The layered delafossite A+B3+O2 structure, as exemplified by the parent delafossite mineral, CuFeO2, maintains the expected valences for the monovalent A-site cations (Ag+ and Cu+) and trivalent B-site cations [0.535 < r(B3+) < 1.03 Å] (Fig

3 and Fig 4) From Fig 5, the structure consists of alternate layers of dimensional close-packed A cations with linear O–A+–O bonds and slightly distorted edge-shared B3+O6 octahedral Furthermore, each oxygen atom is coordinated by four cations (one A+ and three B3+) in a pseudo tetrahedral arrangement Depending

two-on the stacking of the double layers (close packed A catitwo-ons and BO6 octahedral), two polytypes are possible The 3R polytype consists of “AaBbCcAaBbCc” stacking along the c axis and has rhombohedra symmetry with the space group R3m, whereas the 2H polytype consists of an alternate “AaBbAaBb” stacking sequence in the P63/mmc space group”[67]

Since the first discovery of p-type transparent conductivity of CuAlO2 films with delafossite structure by Kawazoe et al in 1997 [45], many studies have been carried out to develop relevant novel ABO2 p-type semiconductors To date, numerous ABO2 compounds (where A = Ag, Cu; and B = B, Al, Ga, In, Fe, Cr, Sc,

Y, etc.) have been reported with high p-type conductivity (10−2 − 102 S.cm−1) and high optical transparency (50% ∼ 85%), dependent on their chemical compositions and film deposition methods These delafossite oxides could play important roles in diverse photoelectronic and photoelectrochemical applications, such as field electron emitters, light-emitting diodes, solar cells, functional windows, photocatalysts, and

so on [68][69][70][71][72]

Various methods for preparing delafossite oxides have been investigated such

as high-temperature solid-state reactions, cation exchange reactions, hydrothermal reactions, for the powders synthesis, and sputtering, sol−gel, pulsed laser deposition (PLD) for the preparation of thin films Since target material for thin film deposition

is composed of particles, the synthesis of phase-pure powder is the required initial step Some general rules leading to the formation of ABO2 oxides could be found in diverse synthesis methods For example, copper-based delafossite oxides (CuAlO2, CuCrO2, CuFeO2 and CuScO2) could be synthesized readily via high temperature solid-state reactions under an inert atmosphere (N2 or Ar) at ∼ 800−1200°C, since

Cu+ is even more stable than Cu2+ at high temperatures However, for the low temperature hydrothermal synthesis of CuAlO2, CuCrO2, and CuGaO2, the raised difficulty rests with how to reduce the soluble Cu2+ precursor to Cu+ and maintain the valence of Cu+ in the monovalent state in a wet chemical environment For the synthesis of silver-based delafossite oxides, solid-state reactions at high temperature generally encountered practical problems, because of the easy decomposition of

Ag2O to elemental silver at a temperature of ∼300°C Therefore, most reported silver-based delafossite oxides, such as AgInO2, AgCrO2, AgAlO2, and AgGaO2, were synthesized via low- temperature hydrothermal methods in closed reaction systems Moreover, among the various delafossite oxides, CuAlO2 and AgAlO2 are more difficult to synthesize, because of the higher crystal formation energy barrier, which is associated with cleavage and reorganization of the high-energy Al−O bonds Conversely, these two aluminum-based delafossite oxides are superior in chemical and thermal stability than other delafossite oxides; besides, high optical transparency

and a low-cost aluminum source are two other important advantages of these two materials, which are highly desired in many practical applications Although the synthesis of CuAlO2 nanocrystals at 400°C via supercritical hydrothermal methods has been reported since 2004 few reports have followed up such a procedure, which might be hard to reproduce Besides, until now, there have been few systematical studies focusing on the hydrothermal synthesis mechanism of aluminum-based delafossite oxides [73]

1.3.2 Electrical properties

Based on the argument of Nagarajan et al that holes are more mobile in the d states than in the p states, it has been proposed previously that the admixture of oxygen 2p states with the silver 4d states at the VBM limits the mobility of holes and thus the conductivity of silver delafossites [74] Indeed, copper delafossites are more conducting because their VBM comprises isolated copper 3d states Nonetheless, the conductivities of both copper and silver delafossites containing main-group B-site cations are low owing to a diffusion-limited polaron conduction mechanism in which trapped holes and resultant lattice distortions “hop” between sites, which limits room temperature mobility (<1 cm2.V-1.s-1) and therefore conductivity Similar to copper delafossites, acceptor doping via aliovalent substitution or oxygen intercalation could be used to enhance the conductivity of silver delafossites by increasing the carrier concentration [66]

Electronic structure calculations and analysis of the Kubelka–Munk absorption data reveal that, similar to copper delafossites, silver delafossites have a disparity in energy between the “forbidden” fundamental direct and indirect band gaps and optically measured band gap While their optically measured band gaps widen with an increase in the radius of the B-site cation, the decreased fundamental band gaps for larger B-site cations result in some absorption of photons in the visible light range for AgGaO2 and AgInO2, which reduces their optical transparency and thereby colors these delafossites When corresponding copper and silver delafossites

with the same B-site cation are compared, however, silver delafossites have larger band gaps and lower visible light absorption, owing to a shift in the valence band states to lower energy upon replacement of copper 3d states with silver 4d states Thus, the electronic structure prediction is general (i.e., any silver delafossite has a larger band gap than the corresponding copper delafossite) Moreover, while silver delafossites have conductivities lower than those of polycrystalline powders of copper delafossites, this study provides a starting point for the difficult process of improving the conductivity of delafossites through extrinsic doping without significantly compromising their optical properties [75]

Another important electrical property of these materials is temperature coefficient of resistance (TCR) The most important property of a bolometer is its infrared sensitive layer TCR Vanadium oxide is the traditional materials for the sensing layer of a micro bolometer due to the high value of TCR which is in the range of 2%/K to 3%/K However, single crystal vanadium oxide, which has higher TCR value of around 4%/K, is rather difficult to achieve [76] In this thesis, ternary oxide minerals (MIMIIIO2, also known as delafossite oxide materials), or in particular, CuAlO2 and AgAlO2, are chosen to be studied as an infrared sensitive material This type of materials has shown some interesting properties such as thermoelectric effect

or being used as transparent conducting oxide However, not much research has been done on the TCR of this type of materials

1.3.3 Optical properties

D Xiong et al the optical transmittance spectra within the wavelength range of 300−800 nm and the calculated band gaps of CuAlO2 and AgAlO2 films The transmittance for the CuAlO2 film (0.5μm) is 60% ∼ 85% in the visible range The calculated value of the direct band gaps for CuAlO2 is 3.48eV, which is consistent with other reports [73] For the AgAlO2 crystals, the transmittance of the AgAlO2film (∼0.6μm) is even better than that of the CuAlO2 film, which is above 80% in the entire visible range The calculated values of direct band gaps for AgAlO2 is

3.89eV, which is close to the value reported earlier on the band gap (3.6eV) If one compares the band-gap values of CuAlO2 and AgAlO2, the larger optical band gap of AgAlO2 is suggested to be due to a shift of the valence band states toward lower energy, associated with the replacement of Cu 3d states with Ag 4d states [73]

1.4 Effects of silver incorporation on TCO thin films

The nature of the dopant affects the surface morphology, grain structure and nucleation process which ultimately govern the structural, the optical properties and the electrical properties of the samples

In the past decades, metal-doped TiO2 nanoparticles have attracted much attention, because introduction of metal ions could enhance the photocatalytic activity of TiO2 drastically in some cases Generally, the accepted mechanism to explain the enhancement is the formation of shallow charge trapping on the surface

of TiO2 nanoparticles due to the replacement of TiIV by metal ions For example,

FeIII ions, there in, improve the separation of photo induced charge carriers [77] Although these researches have noticed the novel effects of metal ions on the improvement of photo activity of TiO2 nanoparticles, however, the effects of metal ions with special properties, for example, Ag ions, have been less studied As well known, Ag ions and Ag shows good antibacterial activity The Ag ion was also the effective composition of most inorganic antibacterial agents It is rational that introduction of Ag ions into TiO2 could improve antibacterial ability of TiO2 [78]

On the other hand, the radius of Ag ions was 126 pm great larger than TiIV and different from the radius of metal ions used by most previous researchers The investigation on the photocatalytic activity and configuration of the Ag-doped TiO2could also give some useful information to clarify the doping mechanism of metal ions

The use of noble metal/TiO2 nanocomposites is an efficient way to enhance the photocatalytic efficiency of TiO2 owing to the efficient electron hole separation by noble metals [79] Among the metallic species which can be incorporated onto TiO2

surface, Ag has shown an enhanced electron-hole separation and interfacial charge transfer ability, as well as the increase of the visible light and UV light excitation of TiO2 Silver is especially favorable for industrial and environmental applications owing to its easy preparation and low cost The effects of Ag doping on the surface

or lattice of TiO2 have been study TiO2 load with Ag enable the catalyst to implement more effectively and shortens the illumination period Scientists have researched the effect of silver including on the microstructure and photocatalytic activity of TiO2 prepared by various methods [80]

Doping by introducing electron donor or acceptor elements into the host crystal

is a successful approach in thin film devices Usually, TNO exhibits n-type conductivity because of native defects, such as oxygen vacancies and titanium interstitials [81] Among these, Ag, as an element of group IB could also act as an acceptor in TNO, if incorporated on substitutional Ti sites For instance, Ag acted as

an atmospheric dopant, existing both on substitutional Ti sites and in the interstitial sites Besides, Ag is not only a good electric conductor with relatively low optical absorption coefficient in the visible region but also an important optical material in the visible region and the near infrared region While the doping of the different metals was successful for tuning the electrical properties of TNO in thin films and in bulk forms which has been widely reported [82] But the reports on the systematic studies of the optical and electrical properties of Ag doped TNO, either in bulk or in thin film forms are still very scarce

Another type of oxide material, copper aluminate (CuAlO2), which is stable at high temperatures up to 1,400 K and possessing a good thermoelectric power, is expected to be another promising material for thermoelectric devices This type of materials has also gained much attention in the field of optoelectronic applications due to the fact that the CuAlO2 has a direct band gap of 3.5 eV and is a transparent conductor CuAlO2 crystallizes in the rhombohedra, delafossite p-type structure (a=2.85670 Å, c=16.9430 Å) and shows p-type semiconductivity [83] Park et al have investigated the thermoelectric properties of CuAl1-xCaxO2 (0 ≤ x ≤ 0.2) and

found that the substitution of Ca for Al up to x = 0.1 increases both the electrical conductivity and the Seebeck coefficient [84] Lately, the effects of Mg or Fe substitution for Al in CuAlO2 were also reported Among these studied elements, the highest value of power factor (1.1 × 10-4 W/mK) was obtained for the CuAl0.9Fe0.1O2 sample at 1,140 K [85][86] Moreover, the calculation of the electronic structure of Ni or Zn doped CuAlO2 using a full potential linear augmented plane-wave method, reported by Lalic et al., showed that Ni and Zn substituted for Cu-sites act as acceptor and donor impurities, respectively [87] As for delafossite p-type of materials, the effect of Ag substitution for Cu-sites in CuRhO2 has been investigated [88] However, to our knowledge, the effect of element substitution for Cu-sites in CuAlxOy has not been reported to date In this study, we focus on the substitution of Ag to Cu-sites in CuAlxOy and systematically investigate their effects on the high temperature thermoelectric properties of these compounds

Therefore, our first aim was to deposit silver-augmented TNO materials on nano thin films via co-sputtering method Deposition conditions of the preparation were modified in order to reach the best thin films We analyzed the results of conductive properties by the 4-point probe method, results of optical properties characterized on an UV-Vis spectrometer and results of sample thickness on an Alpha step profilometer Besides, FOM of these thin films need to be calculated to

determine the best sample deposited under the optimum condition in order to use for many important applications of TCO

For our second aim, we used co-sputtering method to deposit the Ag-doped delafossite CuAlxOy nano thin films Some synthesizing conditions of the preparation were modified in order to investigate changes in optical and electrical properties of thin films We also analyzed the results of conductive properties by the 4-point probe method, results of optical properties on an UV-Vis spectrometer, results of sample thickness on an Alpha step profilometer Moreover, the thermal coefficient of resistance was determined on a self-designed temperature dependent resistance measurement system For our purposes, the most important properties of CuAlxOy thin films were TCR and resistivity We would like to find out deposition conditions giving highest TCR and good conductivity

2 EXPERIMENTS

2.1 Co-sputtering methods

There are several physical vapor deposition methods for producing coatings

in a vacuum environment and these can be separated into two main groups: (i) those involving thermal evaporation techniques, where material is heated in vacuum until its vapor pressure is greater than the ambient pressure, and (ii) those involving ionic sputtering methods, where high-energy ions strike a solid and knock off atoms from the surface Ionic sputtering techniques include diode sputtering, ion-beam sputtering and magnetron sputtering In physics, the mean free path is the average distance traveled by a moving particle (such as an atom, a molecule, a photon) between successive impacts (collisions) which modify its direction or energy or other particle properties [89]

Sputtering was first observed by Grove in 1852 as an undesired „dirt effect‟ It took more than one century until the industry began to make use of the process The main reason for this delay was, of course, the need for reliable and affordable vacuum equipment The simplest layout of a sputtering apparatus is the DC powered diode A low pressure glow discharge is ignited in a space between two planar electrodes The substrate to be coated represents the anode, while the cathode carries the target, which is eroded due to bombardment by energetic argon ions being created in the gas discharge The atoms move towards the substrate (at a typical working pressure of 100 Pa for diode sputtering the mean free path is around 1 mm) where they form the thin film In order to sputter also high insulating target materials like SiO2 or Al2O3, radio frequency (RF) driven plasma discharges were introduced operating at 13.56 MHz Diode sputtering has turned out to be unsuitable for many industrial applications, since the deposition rates are low and the thermal load to substrates is high It is not possible to deposit films on temperature sensitive materials like plastic discs or foils The invention of the planar magnetron cathode

by Chapin (patent issued in 1979) marked the beginning of a new era in vacuum coating technology [90]

We are specifically concerned here with cathode sputtering techniques where the ions are derived from plasma in a low-pressure gas between two electrodes Sputtering as a phenomenon was first observed back in the 1850s but remained a scientific curiosity until around the 1940s when diode sputtering was first used to any significant extent as a commercial coating process However, diode sputtering suffers from very low deposition rates and in many applications was too slow to make the process economic and was only used in applications where the special benefits of sputtered films were justified Then in the mid-1970s a magnetically enhanced variant of diode sputtering emerged and this became known as magnetron sputtering Magnetron sputtering is a high-rate vacuum coating technique for depositing metals, alloys and compounds onto a wide range of materials with thicknesses up to about 5pm It exhibits several important advantages over other vacuum coating techniques and this has led to the development of a number of commercial applications ranging from microelectronics fabrication through to simple decorative coatings [91]

Figure 6 The general operating principle of the sputtering method [92]

The Fig 6 describes the sputtering process, and in particular the magnetron version: how it is achieved, its technological benefits and how the application of simple physical principles has led to a successful commercial process The discussion begins with the special advantages of magnetron sputtering and is

followed by a description of the basic diode sputtering process, progressing to a description of the development of the magnetron principle

Magnetron sputtering has emerged to complement other vacuum coating techniques such as thermal evaporation and electron-beam evaporation However these techniques show certain disadvantages In particular, alloys and refractory metals cause problems because of differences in alloy constituent vapor pressures and their high melting points (the need to run sources very hot thereby affecting your coated articles) In addition compounds can dissociate into their chemical constituents at the low evaporation pressures used Magnetron sputtering overcomes these problems and has many other advantages

The primary advantages are (1) high deposition rates, (2) ease of sputtering any metal, alloy or compound, (3) high-purity films, (4) extremely high adhesion of films, (5) excellent coverage of steps and small features, (6) ability to coat heat-sensitive substrates, (7) ease of automation, and (8) excellent uniformity on large-area substrates, e.g architectural glass These points will be discussed later but it is immediately clear that sputtering is a very powerful technique which can be used in

a wide range of applications

Several terms may be met describing the sputtering process - cathode sputtering Diode sputtering, RF or DC sputtering, ion-beam sputtering, reactive sputtering - but all these are variants of the same physical phenomenon Sputtering is the process whereby atoms or molecules of a material are ejected from a target by the bombardment of high-energy particles More significantly, cathode sputtering is the process discussed here and in this case the bombardment is by positive ions derived from an electrical discharge in a gas Materials are ejected from the target in such a way as to obtain usable quantities of material which can be coated directly onto substrates To obtain sputtering as a useful coating process a number of criteria must be met Firstly, ions of sufficient energy must be created and directed towards the surface of a target to eject atoms from the material Secondly, ejected atoms must

be able to move freely towards the object to be coated with little impedance to their movement This is why sputter coating is a vacuum process: low pressures are required (i) to maintain high ion energies and (ii) to prevent too many atom-gas collisions after ejection from the target The concept of mean free path (MFP) is useful here This is the average distance that atoms can travel without colliding with another gas atom

Figure 7 Principle of magnetron sputtering [93]

The magnetron uses the principle of applying a specially shaped magnetic field

to a diode sputtering target (Fig 7) The principle is that the cathode surface is immersed in a magnetic field such that electron traps are created so that E x B drift

currents close in on themselves The principle was discovered as far back as the 1930s by Penning but has only been used in the magnetron coating context for about fifteen years In essence the operation of a magnetron source relies on the fact that primary and secondary electrons are trapped in a localized region close to the cathode into an endless „racetrack‟ In this manner their chance of experiencing an ionizing collision with a gas atom is vastly increased and so the ionization efficiency

is increased too This causes the impedance of the plasma to drop and the magnetron source operates at much lower voltages than diode systems (500-600 V as compared with several kV) This greater ionization efficiency leads directly to an increase in ion current density onto the target which is proportional to the erosion rate of the target The following table lists some typical values for air at different pressures at room temperature

Table 3 Some typical values for air at different pressures at room temperature [94]

Vacuum

range

Pressure

in hPa (mbar)

Pressure in mmHg (Torr)

Molecules/cm3 Molecules/m3 Mean free

path Ambient

high

vacuum

< 10-12 < 8x10-13 < 104 < 1010 > 105 km

From the table 4, we can see that if the distance between guns and substrates

is longer than mean free path corresponding to the sputtering pressure, the ability of atoms to adhere to the substrate will be greatly reduced

There are many different methods used to deposit the thin films from simple

to complex, including physical methods such as vacuum evaporation or sputtering; chemical methods such as chemical deposition methods or sol-gel and many other methods However, because many advantages and suitable with the conditions of the Cleanroom of Nano and Energy Center – Hanoi University of Science – Hanoi National University (Fig 8), in this thesis, the only sputtering method was used to deposit thin films

Figure 8 Four-gun sputtering machine SYSKEY SP-01

at the cleanroom of Nano and Energy Center

2.2 Preparation of sample before sputtering and sputtering

Step 1: Step to clean the Corning Glass (CG)

Corning glasses, which were about 1.5x1.5 cm, were cleaned in the following process:

i Ultrasonic vibration in 10 minutes with acetone solution and 10 minutes with isopropanol (CH3C2H5OH) solution

ii Soak piranha (with ratio H2SO4:H2O2 = 3:1) within 2 hours

iii Discharge with deionized water and then ultrasonic vibrate once again with acetone turn and isopropanol turn, the duration of each vibration are

15 minutes

iv Finally, the corning glasses were dried by nitrogen gun and could be used

Figure 9 The substrate cleaning solution: acetone, ethanol, acid H 2 SO 4 and H 2 O 2

Step 2: Preparing target and loading target to the gun

Prepare targets: The targets are 2 inches in diameter and have a purity of about 99.95% (Fig 10) In TNO experiment, TNO (TiO2 doped 9%wt Nb) target and Ag target were used In CuAlxOy experiment, Cu target, Ag target and Al2O3 target were sputtered

Figure 10 The targets for sputtering

Load the target into the gun: In TNO experiment, the TNO target was loaded

on the RF2 gun and the Ag target was loaded on the RF1 gun In CuAlxOy experiment,