A study of transparent p type semiconducting oxide cu al o

Chapter Introduction Transparent conducting oxides (TCOs) or transparent oxide semiconductors (TOS) are widely used as coating films for IR reflection and transparent electrodes in flat panel displays, functional glass, solar cells, touch panels [1-3] and other optoelectronic applications, with the purpose of producing “invisible circuits” [4]. TCOs always possess wide-gap (> eV) properties and show optical transparency in visible wavelength (400 to 800 nm) region. New materials with high electrical conductivity and optical transparency are still being explored. Most of the TCOs show n-type electrical conductivity. N-type TCOs, for example, zinc oxide (ZnO), and tin oxide (SnO2) have promising electrical and optical properties with and without dopants. However, p-type TCOs are needed for the fabrications of devices, especially p-n junction. The availability of p-type TCOs is still limited with the absence of optimised electrical and optical properties. Therefore, fabrications of p-type TCOs have been the focus of researchers recently. To date, TCO with delafossite structure has been identified to be an important candidate for p-type TCO. P-type CuAlO2 thin film, with a delafossite structure and an optical band gap of 3.5 eV, was produced by Kawazoe et al. in 1997 using pulsed laser deposition (PLD) [5]. Preparations of CuAlO2 films by sputtering [6], chemical vapour deposition (CVD) [7, 8], and sol-gel [9] have also been tried. However, sputtering deposition of CuAlO2 was not successful. Although CuAlO2 was successfully deposited by PLD, small area coverage has been the disadvantage of PLD for industrial production. Hence, the current research work concentrates on depositing CuAl-O films with optimised optoelectronic properties by using sputtering technique with large area coverage. Effect of substrate temperature during deposition is the main parameter to affect the properties of Cu-Al-O system. This work will therefore, specifically address the effect of substrate temperature on the Cu-Al-O thin films deposited by PLD and sputtering, the effect of sputtering modes and parameters on the properties of as-deposited films, and the effect of post-deposition annealing (PDA) on the physical and electrical properties, including phase transformation, crystallinity, and electrical conductivity of the Cu-Al-O films. In order to give a comprehensive overview of the Cu-Al-O system, the effect of temperature was studied through the solid state reaction of CuO and Al2O3 powders. In addition, a review of current work and literature in the TCO area is included in this chapter. Subsequently, a brief introduction of PLD and sputtering will also be presented. 1.1 1.1.1 P-type Transparent Conducting Oxides (TCOs) The Need for P-type TCOs For the fabrications of transparent devices, especially p-n junction, p-type and n-type TCOs with optimised electrical and optical properties are required. Most of the useful oxide-based materials are n-type conductors that ideally have a wide band gap (> eV), and the ability to be doped to degeneracy through the introduction of native or substitutional dopants, and a conduction band shape (dictating electron effective mass) that ensures that the plasmon-absorption edge lies in the infrared range [10]. The most popular n-type TCOs are tin oxide (SnO2), indium tin oxide (ITO), zinc oxide (ZnO), doped ZnO (dopants: Al, Ga, In, etc.), and cadmium stannate (Cd2SnO4). Among these n-type TCOs, ITO has been widely employed as electrodes in device fabrications [10]. The high electrical conductivity of the n-type TCOs results mainly from stoichiometric deviation, where excess metal ions or oxygen vacancies supply the conduction electrons. Considerable interests exist in developing p-type TCOs. With the aim of fabricating transparent electronics, focus has been placed in exploring new p-type TCOs with comparable electrical and optical properties as existing n-type TCOs. 1.1.2 Candidates for P-type TCOs Currently, many experimental efforts are underway to search for new p-type TCOs. Sato et al. reported the NiO films possessed p-type characteristic in 1993 with a resistivity of 1.4×10-1 Ω-cm [11]. Doping method was also employed in producing ptype TCOs. P-type ZnO, a potential candidate for the fabrication of p-n homojuction, was produced by simultaneous addition of NH3 in carrier gas and excess Zn in the source ZnO [12]. The reported resistivity was approximately 100 Ω-cm, which was too high for application in devices. Another p-type TCO was reported in 1997 by Kawazoe et al. [5], CuAlO2, a ternary oxide with delafossite structure. The p-type CuAlO2 film was prepared by laser ablation with a resistivity of 10.5 Ω-cm. The discovery of CuAlO2 has placed the focus in producing new p-type TCOs with delafossite structure from other elements (through substituting or doping). The difficulty in fabricating p-type TCO is due to the electronic structure of oxides. The valence band in oxide materials is mainly constituted by the oxygen 2p energy band. However, the 2p energy levels of oxygen ions are generally lying much lower than the valence orbitals of metallic atoms [13]. As a result, a positive hole, if it is successfully introduced by doping, localizes on a single oxygen ion and cannot migrate within the crystal lattice, even under an applied electric field. In other words, the positive hole constitutes a deep acceptor level, which could result in the low mobility of holes. In order to delocalize these positive holes and reduce the strong Coulomb force by oxygen ions, the valence band edge of oxide should be modulated by the covalency in the metal-oxygen bonding to induce the formation of an extended valence-band structure. This can be achieved by mixing orbital of appropriate cations that have similar energy filled levels with O 2p [14]. Fig. 1.1 Schematic illustration of the chemical bond between an oxygen ion and a cation that has a closed shell electronic configuration. (adapted from reference 14) Fig. 1.1 shows a schematic illustration of the necessary electronic configuration of the cationic species. The cation is expected to have a closed shell electronic configuration in order to avoid coloration. Transition-metal cations with an open d shell (partially occupied) are not appropriate because of the intra-atomic excitations (d-d transitions) [14]. If the energy level of the uppermost closed shell on the metallic cation is almost equivalent to those of the 2p levels of the oxygen ions, chemical bonds with considerable covalency can be formed between the metallic cations and the oxygen ions. Both of the atomic orbitals are occupied by electron pairs, and the resulting antibonding level becomes the highest occupied level, which is the valence band edge. The closed shell electronic configuration of d10s0 is reported to be a successful candidate for the construction of p-type TCOs. There are numerous monovalent cations that fulfil d10s0 configuration but Cu+ and Ag+ cations are reported to be the species that satisfy the desired electronic structure. Furthermore, the energy of the d10 closed shell electrons for Cu+ and Ag+ cations is the highest. They are expected to have higher probability to overlap with the 2p electrons on oxygen ions. The overlapping can lead to the formation of covalent bond, which will give rise to a large dispersion in the valence band or reduce the localization of positive holes. As for the trivalent cation M, almost every trivalent cation (Al, Sc, Cr, Fe, Co, Ga, In, La, Y, Rh, Pr, Nd, Sm, Eu, and Tl) [15] is suitable for the fabrication of p-type AMO2. In this project, delafossite CuAlO2 will be the focus of discussion as it represents the prototype of p-type TCOs. Furthermore, an appropriate crystal structure will improve the covalent bonding between the cations and oxygen ions. Tetrahedral coordination of oxygen ions leads to no antibonding level on an oxygen ion and reduces the localization behaviour of 2p electrons on oxygen ions. Cu2O is a p-type semiconducting oxide, with the oxygen ions exhibit the tetrahedral coordination. The tetrahedral coordination of oxygen ions in the structure is advantageous for p-type conductivity as the localization of the valence band edge can be reduced. The valence state of the oxygen ions is sp3 in this conformation and hence the eight electrons (including 2s2) on an oxygen ion contribute to the formation of four σ bonds with the four coordinating Cu+ cations. However, the three-dimensional interactions between 3d10 electrons of neighbouring Cu+ ions may lead to the band gap narrowing effect. This is the reason why Cu2O has a small band gap value of 2.1eV [16] and it is not transparent in visible range. The band gap can be widened by lowering the three-dimensional cross-linking of the Cu+ network (in the form of dumbbell units) in Cu2O to two-dimensional in delafossite structure of CuMO2 where M is a trivalent cation. Delafossite crystal structure of CuAlO2 consists of a hexagonal and layered structure. The layers of Cu cations and AlO2 are stacked alternately, perpendicular to the c-axis. There is no oxygen within the Cu cation layer and only two oxygen ions are linearly coordinated to each Cu cation in axial positions. The AlO2 layers consist of AlO6 octahedral, sharing edges. Each oxygen ion is in pseudo-tetrahedral coordination, as Al3CuO. Another expression of the crystal structure is CuAlO2, consisting of stacked layer of -O-Al-O-Cu-O- along the c-axis built by three different structural units: octahedral AlO6, linear O-Cu-O units, and hexagonal Cu layers. Local symmetries around Cu+ and O2- ions in CuAlO2 phase are the same as in Cu2O, except that the nearest neighbouring cations of the oxygen ions in this structure are one Cu+ and three Al3+. Four Cu cations coordinate with each O anion in Cu2O, whereas only one Cu cation coordinates with each O anion in CuAlO2. As a result, it is considered that mixing and interaction between Cu 3d and O 2p orbital in Cu2O is greater than in CuAlO2. However, the repulsive interactions between the Cu cations in Cu2O could reduce the band gap energy. As abovementioned, the larger dispersion of the valence band results in large mobility. Lower dimensional structure of Cu cations is found in CuAlO2. Consequently, CuAlO2 has an appropriate dispersion in the valence band and a wide band gap, which gives rise to the optical transparency in the visible light region. The delafossite crystal structure of CuAlO2 is presented in Fig. 1.2. Fig. 1.2 The delafossite crystal structure of CuAlO2 where Cu+ cation is in two-fold linear coordination to oxygen and the Al3+ cation is in octahedral coordination. (adapted from reference 17) 1.1.3 P-type Conduction Mechanism The major charge carriers in p-type conventional semiconductors (Si and GaAs) are the positive holes introduced by doping process. There is no exception for p-type TCOs. The most probable origin of the positive holes is the excess oxygen. There are two possible models for this excess oxygen: cation vacancy and interstitial oxygen. Porat and Riess [18] proposed that the origins of the positive holes in Cu2-yO were possibly due to the ionized Cu vacancies and ionized interstitial oxygen. The admixed state of Cu 3d and O 2p primarily constitutes the upper valence band, which controls the transport of positively charged holes. For delafossite CuAlO2, the possible origins of positive holes of this p-type TCO are also Cu vacancies and interstitial oxygen ions as proposed by Kawazoe et al. [14]. 1.1.4 Recent Developments on P-type TCOs Fabrication of transparent devices is a major focus in recent years. N-type TCOs with excellent electrical and optical properties have been extensively achieved. However, due to the lack of compatible p-type TCOs, research of new p-type transparent semiconducting oxide materials has been the focus in recent years. In 1997, Kawazoe et al. [5] reported the discovery of CuAlO2 thin film, which has better optical transmittance than NiO discovered by Sato et al. [11]. The films of 500 nm thick showed 80% transmission and a conductivity of 0.95 Scm-1. Furthermore, p-type CuAl-O films prepared by CVD showed better electrical conductivity [7, 8]. Since then, p-type delafossite TCOs such as CuGaO2 [19], CuFeO2, CuCrO2, CuScO2+x and bipolarity of CuInO2 [20] have been extensively developed. Doping in p-type TCO has been tried by several research groups but there is no satisfactory result for doping of CuAlO2 and CuGaO2. Furthermore, new p-type TCOs with new crystal structure such as LaCuOS [21] and ScCu2O2 [22] have been developed with promising electrical and optical properties. All these developments show a promising future of TCO materials. Good quality of transparent optoelectronic devices can be produced in future. Although p-type CuAlO2 has been produced successfully by PLD [5] and Cu-Al-O has been studied by CVD [7, 8], Cu-Al-O system by PLD and sputtering techniques is not well studied. Furthermore, the effects of substrate temperature and the sputtering parameters on the growth behaviour of the Cu-Al-O films are not understood. The effect of post-deposition annealing on the properties of Cu-Al-O films is also not wellstudied. This work will therefore, specifically address these effects on the Cu-Al-O films produced by PLD and sputtering. 1.2 Deposition Techniques Thin films are thin layers of material, usually less than µm thickness and sometimes as thin as nm [23]. Due to their extreme thinness, thin films possess unique properties that are different from their bulk counterparts. Properties such as electrical conductivity and density may show significant differences from the bulk materials. Furthermore, thin films might even be regarded as ‘all surface’ as the surface properties of thin films are also significant and unique. They can be produced by thinning down from the bulk materials (aluminum foil). However, it has become customary to refer to specimens prepared by thinning down specimens as thin foils and to reserve the name thin films for specimens made by building up the material molecule by molecule. Throughout the building up process, thin films are generally formed on and supported by a more substantial base or substrate since they are so thin and fragile. There are numerous methods of depositing thin films. Basically, the techniques can be classified into two categories, chemical vapour deposition (CVD) and physical vapour deposition (PVD). CVD has been widely employed for the thin film deposition in the semiconductor industry. Thin film deposition by CVD techniques involves the molecular species in the gas phase chemically reacting at a film surface, resulting in the formation of a condensed film as well as the emission of volatile by-products. For PVD, there are a lot of techniques such as thermal evaporation, sputtering, and electrodeposition. Recently, pulsed laser deposition (PLD) has emerged as a novel PVD technique. These techniques are generally atomic in nature, in which the films are deposited from single atoms or small clusters. Any reaction (such as oxidation or nitridization) that occurs on the film surface is independent of the source process. Since the p-type Cu-Al-O films were prepared by pulsed laser deposition and sputtering, more information of these two techniques will be introduced in the following sections. 1.2.1 Pulsed Laser Deposition (PLD) Pulsed laser deposition (PLD) belongs to the physical vapour deposition (PVD) category as it fulfils the three basic requirements of PVD: a source material, a substrate, and an energy supply to transport material from the source to the substrate during film deposition. The form of energy and its associated mass transfer mechanism are the unique characteristics for a particular technique. For PLD, as the name suggests, a high-energy pulsed laser is used as an external power source to ablate the source or target material [24]. The interaction is short but intensive, and introduces ablation via a cascade of complex events. In a PLD system, a laser beam is focused onto the target surface. The type of laser used for PLD has been evolving with time. Ruby and Nd: Glass lasers were used in early studies [24]. They were replaced by the more reliable Nd: YAG and TEA-CO2 lasers. In recent development, short-wavelength lasers have been chosen for PLD application to produce ablation plumes with higher plasma temperature and thin films with better crystallinity. UV excimer lasers such as 248 nm KrF excimer laser and 193 nm ArF excimer laser have been employed in most of the PLD systems. In order to avoid deep surface pitting caused by repetitive ablation, the focused laser beam changes location constantly. This can be done by target rotation, or beam rastering, or a combination of both. The most common configuration uses rotating target holder attached to a motor that provides constant rotation speed. Furthermore, a 10 12]. Therefore, it is believed that the p-type conduction could be contributed by the formation of CuO phase. Fig. 6.10 XRD patterns of the Cu-Al-O films deposited by reactive co-sputtering of Cu and Al metallic targets at 100°C after PDA treatments at 480°C in the air for hours. _ The numbered peaks correspond to: (1) CuO (1 1); (2) CuO (1 1); (3) CuO (0 0); _ (4) CuO (0 2); and (5) CuO (2 0), respectively. Furthermore, it can be seen that the structure of the films with higher Al contents (Cu/Al~3.0, 2.0, 1.0, and 0.9) had changed from amorphous-like structure (Fig. 5.13) to polycrystalline structure with the formation of crystalline CuO phase after heat treatments. This could also elucidate the improvements of electrical properties of the films after PDA processes. Fig. 6.11 shows the AFM images of the films after PDA treatments. As shown, grain growth effect was observed after heat-treatment, particularly for the films with higher Al contents (Cu/Al~1.0, and 0.9). The estimated particle sizes of the films with Cu/Al 147 ratio of 10.0, 7.0, 1.0, and 0.9 were 40, 51, 80, and 107 nm, respectively (as-deposited values: 42 nm, 46nm, 59 nm, and 58 nm, Fig. 5.14). Although the grain growth effect was also observed in the annealed films deposited at room temperature, the improvement of electrical properties of this series of films after PDA was greater. Furthermore, the films with higher Al contents (Cu/Al~1.0, and 0.9) showed lower electrical resistances than the films with lower Al contents (Cu/Al~10.0, and 7.0). Due to the larger particle size of the films with higher Al contents after annealing, the amount of scattering centers (grain boundaries) was reduced compared to the smaller particle size, leading to the lower electrical resistance of the films. (a) (b) (c) (d) Fig. 6.11 AFM images of the Cu-Al-O films deposited at 100°C after PDA treatments at 480°C in the air for hours. (a) Cu/Al: 10.0; (b) Cu/Al: 7.0; (c) Cu/Al: 1.0; and (d) Cu/Al: 0.9. The normalized transmittance (with a film thickness of 100 nm) and Tauc’s plots are shown in Fig. 6.12(a) and (b), respectively. 148 (a) (b) Fig. 6.12 (a) Transmittance; and (b) Tauc’s plots of the Cu-Al-O films deposited at 100°C after PDA treatments at 480°C in the air for hours. Transmittance of up to 90% from the range of 400 to 800 nm was obtained. It decreased gradually with the decrease of Al content in the films. A drop of 5% in transmittance was observed compared to the as-deposited results discussed in Chapter (Fig. 5.15). This result is identical to the previous batch of samples deposited at room temperature. It was noticed that the temperature had little effect on the transmittance of the films. The drop could be due to the formation of CuO. 149 Table 6.7 Fundamental absorption edge values of the films deposited at 100°C after annealing at 480°C in the air for hours. Sample Fundamental Absorption Edge (eV) C1 2.83 C2 2.80 C3 2.76 C4 2.82 C5 2.74 C6 2.93 Furthermore, a reduction in the optical absorption edge was observed as shown in Fig. 6.12(b). The fundamental absorption edge values were around 2.8 to 2.9 eV, compared to the values of 2.9 to 3.3 eV for the as-deposited films. The values of the fundamental absorption edge are tabulated and shown in Table 6.7. The reduction could be related to the phase formation of CuO in the films after PDA treatments. As shown earlier in Fig. 6.10, CuO phase was the prominent crystalline phase existing in the films after annealing. Moreover, the amorphous-like structure in the as-deposited films had been replaced by the polycrystalline structure after the annealing treatments. The CuAlO2 phase (band gap ~ 3.2 eV) could have been decomposed and replaced by the formation of CuO (band gap ~ 1.5 eV) [14], which could eventually lower down the absorption edge of the films after annealing. However, the coexistence of Al2O3 and other phases (could be in amorphous state) could have led to higher estimated absorption edges compared to pure CuO. Therefore, it is believed that such reduction could be mainly caused by the phase transitions during the PDA process. The PDA processes at 480°C in the air for hours did not produce any crystalline ternary phase such as CuAlO2 or CuAl2O4. In term of structure, the Cu-Al-O film deposited by sputtering of CuAlO2 target was also heat-treated under the same conditions (480°C in air for hours). The electrical resistance was approximately 900 MΩ. Similar diffraction pattern (compared to Fig. 6.7 and Fig. 6.10) was obtained as shown in Fig. 6.13. 150 Fig. 6.13 XRD pattern of the Cu-Al-O film deposited by sputtering of CuAlO2 target after PDA treatment at 480°C in the air for hours. The numbered peaks correspond to: _ _ _ (1) CuO (1 1); (2) CuO (1 1); (3) CuO (0 0); (4) CuO (1 3); (5) CuO (3 1); _ and (6) CuO (2 0), respectively. It is believed that no chemical reaction will occur at this temperature in the films deposited by different modes of sputtering. The possible phases are CuO and Al2O3 (could be amorphous in the films), as discussed in the DTA results in Chapter 3. Furthermore, it was noticed that the CuAlO2 phase in the films deposited at room temperature had been decomposed after annealing. This could be due to the weak bonding and low crystallinity of the phase. The structural change from amorphous to polycrystalline was observed after annealing, which could have improved the electrical properties of the films. From these evidences, the possible mechanisms of the annealing at 480°C could be the diffusions, rearrangements of atomic positions, and structural changes. It was also observed that the electrical resistance of the films with higher Al contents was lower than those with lower Al contents. There was no linear 151 relationship between the reduction in resistance and Al content in the films as discussed previously for the films deposited at room temperature and 100°C. The possible factor could be that the formation of CuO is thermodynamically favoured at 480°C [11]. However, crystalline CuO may not be uniformly distributed on the samples and therefore lead to different electrical resistances among the samples. With the formation of crystalline CuO, rearrangement of atomic positions (thermally activated) could occur. This could lead to the formation of amorphous structure in the films. Furthermore, the films with higher Al contents can cause the formation of amorphous Al2O3. It is also believed that the amorphous CuAlO2, CuAl2O4, and other possible combinations of Cu-Al-O could also be formed. If the crystalline CuO phase is accompanied by these possible amorphous candidates, the conduction path of charge carriers (holes) could be interrupted, leading to the high resistance of films. Moreover, these crystalline and amorphous phases could be randomly distributed, resulting in the inconsistency of electrical properties of the films. The distributions of these phases are still not well understood. This was noticed in the XRD patterns shown in Fig. 6.7, 6.10, and 6.13, respectively for the reactive sputtered films grown at room temperature and 100°C, and the films grown by sputtering of CuAlO2 target. Amorphous phase could increase the resistance of the samples as amorphous phase always shows insulating characteristics. Distribution of amorphous phase among the films could act as scattering centers that disturb the conduction path of the charge carriers. This phenomenon is still not well understood. If the amorphous phase is segregated due to the diffusion, the conduction path could be enhanced and the resistance could be lowered compared to the scattered amorphous phases among the crystalline CuO phase. It was also noticed that better crystallinity of CuO was achieved with higher Al contents. The improved crystallinity of CuO could also improve the electrical 152 properties. There may be other factors for the low resistance of the films with higher Al contents after annealing. Since the PDA processes were carried out in air, more oxygen interstitials incorporated in CuO crystallites during the diffusions could have increased the carrier (holes) concentrations that eventually lead to lower electrical resistance. However, further experimental studies such as depth profile XPS is required. A common phenomenon was observed for both series of films and the films deposited by using the CuAlO2 target after PDA process. CuO phase was formed after treatment at 480°C for hours as shown in the XRD patterns. This implies that low annealing temperature favours the formation of Cu2+ compound. In this case, CuO was formed. According to Ghosh et al. [11] and Mathew et al. [15], Cu+ state is stable at low temperature and low oxygen level. Hence, CuAlO2 peaks/humps were observed for the as-deposited films. However, when the films were heat-treated at 480°C, the decomposition occurred and led to the formation of CuO, at which Cu2+ state is more stable. The improvement of electrical properties was thus contributed by CuO. The transition route is summarized in the following reaction [11]: 2Cu2O + O2 Æ 4CuO (above 300°C and high O2 level) (6.5) Although Cu2+ state was stable at temperature of 480°C, the electrical properties are not as good as the films annealed at high temperature that led to the formation of CuAlO2. Furthermore, the crystallinity of CuAlO2 phase in the films deposited at low temperature was not compatible with the films annealed at 1000°C and hence resulted in poor electrical conductivity. The films after annealing showed good optical properties with high optical transparencies and wide absorption edges. 153 6.4 Comparisons By comparing the three PDA conditions discussed above, it was noticed that high temperature annealing (above 700°C) can trigger the formation of CuAlO2 and CuAl2O4 whereas low temperature annealing (< 500°C) involves the diffusions, structural changes, and position rearrangements of the atomic species. Annealing at high temperature in an inert ambience could lower the decomposition temperature of CuAl2O4, which eventually leads to the formation of CuAlO2 and thus decreases the electrical resistivity. It is believed that annealing at high oxygen level could favour the formation of CuAl2O4 at high temperature (> 700°C). Furthermore, inert gas (Ar) flow during the high temperature annealing could suppress the evaporation of the films because high pressure could be generated with the gas flowing. For the annealing at low temperature (480°C), formation of CuO phase was noticed. No other crystalline ternary phases, such as CuAlO2 and CuAl2O4, were observed. Therefore, CuO could have contributed to the p-type conduction of the films after annealing. Diffusions, structural changes, and rearrangements of atomic positions could be the main mechanisms for the annealing at this temperature. The effects of these possible mechanisms on the electrical properties are still not understood. It was also noticed that grain growth effect could have caused the decrease of resistance. 6.5 Summary Post-deposition annealing (PDA) temperature could affect the electrical properties of the Cu-Al-O films mainly by phase transformations. Through the post-deposition annealing processes on the Cu-Al-O system prepared by PLD and sputtering, improvements of electrical properties had been achieved. It was found that PDA temperatures above 700°C favoured the improvements of the electrical properties. 154 Furthermore, CuAlO2 films could be produced when the PDA temperature was at 1000°C under atmospheric pressure with Ar flow for longer time (more than hours). Even though the high temperature annealing improved the electrical properties, annealing temperature at 480°C was also found to improve the conductivity. However, the low annealing temperature did not help to form the CuAlO2 phase. Instead, the formation of CuO, one of the p-type candidates, had improved the electrical properties. Moreover, grain growth effect could also be one of the factors as a reduction of grain boundaries after annealing helps to lower the resistance of the films after annealing. As a result, PDA process is helpful in improving the electrical properties of the Cu-Al-O system through phase transformation and grain growth effect. 155 References [1] J. F. Chang, H. L. Wang, and M. H. Hon, J. Cryst. Growth 211 (2000) 93. [2] D. V. Morgan, A. Salehi, A. H. Aliyu, and R. W. Bruce, Renew. Energ. (1996) 205. [3] E. Nishimura, M. Ando, K. Onisawa, M. Takabatake, and T. Minemura, Jpn. J. Appl. Phys. 35 (1996) 2788. [4] R. E. Stauber, J. D. Perkins, P. A. Parilla, and D. S. Ginley, Electrochem. Solid St. (1999) 654. [5] S. Yin, Q. W. Zhang, F. Saito, and T. Sato, Chem. Lett. 32 (2003) 358. [6] G. S. Shahane, B. M. More, C. B. Rotti, and L. P. Deshmukh, Mater. Chem. Phys. 47 (1997) 263. [7] D. M. Mattox, Handbook of Physical Vapour Deposition (PVD) Processing: Film Formation, Adhesion, Surface Preparation and Contamination Control (Noyes Publications, New Jersey, 1998), pp. 260-268. [8] F. A. Cotton, and G. Wilkinson, Advanced Inorganic Chemistry, 5th edition (John Wiley and Sons, New York, 1988), pp. 755-775. [9] S. Ishizuka, T. Maruyama, and K. Akimoto, Jpn. J. Appl. Phys. 39 (2000) L786. [10] R. C. Ropp, Solid State Chemistry (Elsevier Science B. V., Amsterdam, 2003), pp. 129-189. [11] S. Ghosh, D. K. Avasthi, P. Shah, V. Ganesan, A. Gupta, D. Sarangi, R. Bhattacharya, and W. Assmann, Vacuum 57 (2000) 377. [12] B. Balamurugan, and B. R. Mehta, Thin Solid Films 396 (2001) 90. [13] F. P. Koffyberg, and F. A. Benko, J. Appl. Phys. 53 (1982) 1173. [14] R. D. Shannon, D. B. Rogers, and C. T. Prewitt, Inorg. Chem. 10 (1971) 713. [15] X. Mathew, N. R. Mathews, and P. J. Sebastian, Sol. Energy Mater. Sol. Cells 70 (2001) 277. 156 Chapter Conclusion 7.1 Concluding Remarks This project has investigated several aspects of Cu-Al-O thin films, including the influence of various deposition techniques and deposition conditions, the effect of Cu/Al atomic ratios, substrate temperatures as well as the annealing effects on the CuAl-O films. Throughout the project, it was found that temperature plays an important role in affecting the electrical properties of Cu-Al-O system, which is related to the phase transitions at different temperatures. The study of Cu-Al-O was carried out by solid state reaction. The fabrication of CuAlO2 powder and targets for physical vapour deposition (PVD) was successfully achieved through solid state reaction of CuO and Al2O3 mixed powder in a 2-to-1 molar ratio at 1200°C for more than 10 hours. It was noticed that the formation of CuAlO2 could be achieved at temperature of 750°C but consisting of other phases such as CuO, Al2O3, and CuAl2O4. These phases were stable at temperature lower than 1100°C, especially for CuAl2O4, as the formation was time dependent as discussed in Chapter 3. The formation of CuAlO2 dominated at temperatures of 1100°C and above. Formation of CuAlO2 leads to lower electrical resistivity compared to the multiple phases of CuAlO2, CuAl2O4, CuO, and Al2O3. Cu-Al-O thin films were deposited by pulsed laser deposition (PLD). Even though stoichiometry was maintained during the depositions, the high electrical resistivity was 157 detected in the films deposited at different substrate temperatures and frequencies. It was noticed that the formation of CuAlO2 phase was favoured at high temperatures (450°C to 710°C) during the deposition. However, multiple phases were incorporated in the as-deposited films. This suggests that pure CuAlO2 could not be obtained directly through deposition at high temperatures. Furthermore, the high electrical resistivity could be due to the existence of multiple phases such as CuAlO2, CuAl2O4, CuO, and Al2O3, as indicated in the discussions of bulk materials. However, wide-gap properties were determined for the as-deposited films with high optical transparency in the visible wavelength region (400 to 800 nm). Wide optical absorption edges (> 3.5 eV) were also determined. Due to the incorporation of multiple phases, existence of two absorption edges was observed for the films deposited at higher substrate temperatures (600°C, 625°C, 650°C, and 710°C). Cu-Al-O thin films were also deposited by RF sputtering. Two sputtering modes were employed: (1) sputtering of single CuAlO2 target produced by solid state reaction, and (2) reactive co-sputtering of metallic Cu and Al targets in the environment of Ar/O2. (1) Sputtering of single CuAlO2 target The sputtering process was rather slow for the oxide target, leading to low sputtering yield of the product. The depositions were carried out at different O2 concentration environments (0%, 5%, 10%, and 15%) at room temperature for the purpose of compensating the loss of oxygen during the depositions, which could lead to the formation of oxygen vacancies that degrade the p-type electrical conduction. However, high electrical resistivity was obtained for the as-deposited films. Amorphous-like diffraction patterns were determined for all the as-deposited films, which could be the reason of the high electrical resistivity of the films, as amorphous materials are always 158 insulating. Formation of amorphous-like structure could be due to the low substrate temperature. However, wide-gap properties were detected for the as-deposited films with high optical transparency and wide absorption edges of approximately 3.3 eV. From the results obtained for the films deposited by this mode and PLD, similarity in electrical properties was observed. The amorphous structure and poor crystallinity of CuAlO2 phase could be the main reasons that lead to the high electrical resistivity of the films deposited at different substrate temperatures. Furthermore, the incorporation of other phases could degrade the electrical properties, even though high temperature is favoured for the growth of CuAlO2. (2) Reactive co-sputtering of metallic Cu and Al targets in the environment of Ar/O2 This mode increased the sputtering yield of the films with shorter time required for the depositions. However, the sputtering yield of Al was affected by the presence of O2 (5%) during the depositions, also known as target poisoning for reactive metallic elements. Stoichiometric Cu/Al atomic ratio (1-to-1) could still be achieved with a varying incident power of Al target (60 to 110 W) and a constant Cu target power (30 W) at room temperature. The as-deposited films with measurable electrical resistances and p-type characteristic were produced. The Cu/Al atomic ratio was larger than 1. For the films with Cu/Al ratio close to 1, the electrical resistances were not measurable even though the CuAlO2 phase was detected with poor crystallinity. The electrical conduction could be contributed by the CuO phase formed in the films with lower Al contents whereas the high electrical resistance of the films with higher Al contents could be caused by the amorphous-like structure and incorporation of Al2O3 that lead to the poor electrical conduction. Furthermore, poor crystallinity of CuAlO2 for the films with stoichiometric Cu/Al atomic ratio could also lead to the poor electrical 159 conduction. However, the as-deposited films still possessed wide-gap properties with high optical transparency and wide absorption edges (2.9 to 3.3 eV). Similar trend was observed for the films deposited at 100°C. Deposition of Cu-Al-O films was also carried out at different oxygen concentrations at room temperature by reactive co-sputtering with constant powers of 30 W and 80 W for Cu and Al targets. Stoichiometric Cu/Al atomic ratio was achieved for the films deposited at 5% and 10% O2 concentrations with high electrical resistances beyond the measurable range. However, improvement of electrical resistivity (16.7 Ω-cm) of the films was achieved by the deposition at 15% O2 environment. Stoichiometric Cu/Al atomic ratio was interrupted (approximately 3-to-1) by the presence of higher O2 content during the deposition. Amorphous structure was formed during the deposition as evidenced by both XRD pattern and HRTEM image. XPS result showed that Cu+ and Cu2+ were incorporated in the films with a ratio of approximately 3-to-1. Codoping effect could be the factor that leads to the improvement of electrical properties as Cu2+ could be n-type dopant and oxygen interstitials as p-type dopants. Oxygen interstitials could be increased due to higher O2 concentrations was introduced during the deposition, leading to higher conductivity. Wide-gap properties were achieved for the films deposited at various O2 contents. The films with stoichiometric Cu/Al atomic ratio possessed absorption edges of approximately 3.3 eV whereas the conducting film had a value of 2.9 eV. Post-deposition annealing (PDA) was performed on the Cu-Al-O films deposited by PLD and sputtering. High (above 700°C) and low (480°C) annealing temperatures were employed for the PDA processes. High temperature annealing was performed in the air and with Ar flow. It was found that annealing in the air at 750°C for hours led 160 to the formation of multiple phases of CuAlO2, CuAl2O4, and CuO with measurable electrical resistances. Annealing in air at temperatures higher than 750°C caused the evaporation of the films. Annealing in the environment with Ar flow was carried out under atmospheric pressure. It was found that annealing at 1000°C for hours with Ar flow led to the formation of CuAlO2 phase with good crystallinity. No evaporation was observed at 1000°C, which could be due to the higher pressure upon the surface of the films with Ar gas flow. Furthermore, inert environment could cause the decomposition of CuAl2O4 at lower temperature, leading to the formation of CuAlO2 with better electrical properties. Absorption edge value of 3.2 eV was determined for the annealed films. For the annealing at 480°C in air for hours, improvement of electrical properties could be caused by the formation and improvement of crystallinity of CuO phase, and the grain growth effect during the annealing processes. Furthermore, the film with higher Al contents showed lower electrical resistance. This could be due to the formation of scattering centers of amorphous phases (could be amorphous Al2O3, CuAlO2, and CuAl2O4) for the films with lower Al contents. For the films with higher Al contents, the formation of segregated amorphous phases may occur during the annealing process and improve the conduction path of charge carriers. This could lead to less amorphous phases scattering among the CuO matrix that results in lower resistance. The annealed films possessed high optical transparency and wide absorption edges of 2.8 to 3.0 eV after annealing. 7.2 Suggestions for Future Work Direct synthesis of conducting Cu-Al-O through deposition technique is important for further studies. Reactive deposition of single Cu-Al metallic target with stoichiometric 161 atomic ratio could be employed. This may enhance the depositions of Cu-Al-O films with higher sputtering rate and yields. The deposition parameters such as substrate temperature, working pressure, and so on can be closely monitored without the distraction of Cu/Al ratio. Furthermore, the analysis of chemical compositions of the films should be verified with more precise equipments/techniques such as Rutherford backscattering spectroscopy (RBS). Depth profile studies by XPS could also enhance the understanding of composition effects on the properties of the films. Although most of the Cu-Al-O thin films in this work were characterized by XRD on the structural properties, transmission electron microscopy could be employed for further analysis on the crystallite sizes, and diffraction patterns of the films. Post-deposition annealing at high pressure could be employed. It is believed that the formation of CuAlO2 at lower temperature can be achieved due to the decrease of the formation temperature of CuAlO2 at high pressure. Furthermore, low temperature annealing (< 500°C) could enhance the understanding of the relationship between phase transitions and electrical properties at different temperatures. 162 [...]... Chapter 5 covers the discussions of physical and electrical properties of Cu- Al- O thin films prepared by sputtering of single oxide target of CuAlO2 and 2 metallic targets of Cu and Al (also known as reactive sputtering) and different deposition parameters Effects of post-deposition annealing on the properties of the films prepared by PLD and sputtering are presented in Chapter 6 Finally, Chapter 7 concludes... Dry powder was then obtained and used for further processing and characterizations 2.1.2 Calcination of Mixed CuO and Al2 O3 Powder Copper aluminium oxide (CuAlO2) was produced by calcining the mixture of CuO and Al2 O3 powder of 2-to-1 molar ratio The completeness of the reaction and the uniformity of the product depend on the particle sizes, size distribution, mixedness of reactants, duration, temperature,... face configuration; and 5) shadow-masked configuration [24] Material ablation induced by high-power laser radiation is a very complex process and can be described as a series of heterogeneous events The sequential steps are: (a) photon absorption; (b) molten surface layer formation; (c) vaporization and plasma formation; (d) plasma heating and recoil force induced splashing; and (e) plume expansion... the workspace is evacuated so as to avoid the collisions Inside the ion beam sources, ions are normally produced by collisions between electrons and atoms Practically Ar gas is always used as the source for plasma generation An atom consists of a nucleus containing positively charged protons and is surrounded by an equal number of negatively charged electrons which make the atom electrically neutral... to cause significant atomic diffusion for solid-state sintering Sintering rate is approximately proportional to the inverse of the particle size Hence, good control of the preparation and compaction processes is very important [4] Sintering temperature of CuAlO2 was 1200°C with heating and cooling rate of 5°C per minute The sintering duration was set for 10 hours The cooling process is important for... developing the proper oxidation states and annealing differential strain in pressed products If the cooling rate is too fast, the sintered product could crack easily Finally, all the preparation processes can be summarized in the flow diagram shown in next page 21 CuO + Al2 O3 powder 1) Ball Milling Slurry of mixed powder 2) Heat at 80°C Dry powder mixture 3) Characterizations 4) Calcination in an alumina... by radiation of heat, hence high temperature region (up to 1400°C) can be investigated The position (on the temperature or x-axis), shape, and number of peaks in a DTA curve are used for qualitative identification of a substance The areas of the peaks, which are related to the enthalpy of the reaction, are used for quantitative estimation of the reactive substance [6] The DTA curves are always reproducible... for the mass flow controllers (MFCs) and to provide ratio-ed set points for multiple gas control The gas ratio of the ambience inside the sputtering system was calculated by adjusting the relative flow rate of each type of gas: Flow Rate of Gas 1 Ratio of Gas 1 = (2.1) Flow Rate of Gas 1 + Flow Rate of Gas 2 • Substrate temperature The substrates were placed on a rotating stage (0 – 40 rpm) with diameter... size and shape, as well as pore size and shape As a result, the originally porous powder was consolidated with the powder particles joining together into an aggregate with higher strength This was the reason accountable for the requirement of longer time for mechanical grinding and ball milling of the powder after calcinations than the initial batch of powder in order to produce a well-mixed powder with... deposition and multilayer growth are applicable in PLD PLD is also known for its fast turnaround time for growing a thin film of a new material starting from its powder form However, the difficulty in achieving large-area uniformity has always been PLD’s main drawback This is due to the narrow angular profile of the plume and the narrow separation between the target and substrate Both limitations pose . suitable for the fabrication of p- type AMO 2 . In this project, delafossite CuAlO 2 will be the focus of discussion as it represents the prototype of p- type TCOs. Furthermore, an appropriate. electrical and optical properties. All these developments show a promising future of TCO materials. Good quality of transparent optoelectronic devices can be produced in future. Although p- type. by sputtering of single oxide target of CuAlO 2 and 2 metallic targets of Cu and Al (also known as reactive sputtering) and different deposition parameters. Effects of post-deposition annealing