Abstract
Electrical, structural and optical properties of annealed transparent strontium copper oxide (SCO) films were studied in this paper. These SCO films were first deposited by reactive radio frequency magnetron sputtering technique on glass substrates at room temperature with ultra highly pure oxygen and a 100 W sputtering power, and then annealed at different temperatures ranging from 373 to 723 K in an atmosphere of oxygen controlled at 101 Pa. Results showed that the resistivity increased first as the annealing temperature raised from 373 to 473 K, and then decreased when annealed at 623 K. Carrier density of an annealed film increased from 7:09 Â 10 20 cm À3 to 1:21 Â 10 21 cm À3 as the annealing temperature increased. Results also showed that the resistivity of a SCO film was highly correlated to the carrier mobility. The highest carrier mobility observed by annealing a SCO film at 623 K was 4:0 Â 10 À5 m 2 V À1 S À1 . The optical transmittance of an annealed SCO film in the visible range at 550 nm fell between 52.6% and 58.2%. The Hall coefficient measured at room temperature indicated the nature of the annealed films was p-type.
Introduction
Transparent conductive oxides, generally referred as TCOs, are semi-conducting materials that have extensive applications in optoelectronics such as panel displays, solar cells and coatings for anti-static electricity shielding. Common n-type TCOs have good optical and electrical properties, such as zinc oxide (ZnO), indium tin oxide (ITO) and tin oxide doped (SnO 2 :F) 1) which have been extensively studied. However, there have only been a few studies [2] [3] [4] done on transparent p-type TCO semi-conductor devices due to the lack of available materials.
The difficulties in designing a material with positive holes that are not localized on oxygen atoms have contributed to the lack of p-type transparent oxides. 5) This localization phenomenon affects and limits the movement of carriers, i.e. positive holes in p-type TCOs, and also leads to low carrier mobility and high resistivity. Nevertheless, some oxides, such as CuAlO 2 , 6-9) CuInO 2 , 10) CuGaO 2 11) and CuScO 2 , 12) have been developed by so-called ''chemical modulation of valence bound''(CMVB).
5) The oxides mentioned above carry the same chemical formula of AMO 2 and a preferred delafossite structure. With the CMVB strategy, an extended valence band can be formed by introducing covalence into the metal-oxygen bonding which hence improves the carrier mobility. These oxides also exhibit p-type conductivity and good optoelectrical properties.
Another potential candidate for making multi-layer devices is strontium copper oxide (SCO) due to its low deposition temperature. Bobeico et al. has demonstrated the SCO films can be deposited at 623 K by the pulse laser deposition technique (PLD). 13) In the previous article, 14) the feasibility of deposition of SCO films with radio frequency reactive magnetron sputtering at room temperature was proven valid. However, the carrier mobility of a p-type TCO film is much inferior to that of a n-type TCO film. 15) Undergoing a post-annealing treatment for a sputtered SCO film might lower the grain boundary density and carrier scattering and hence further improves the carrier mobility and electrical property of the sputtered SCO film. In this paper, the structural, electrical and optical properties of SCO films annealed at different temperatures were studied.
Experiment Details
SCO films were first deposited by radio frequency (RF) magnetron sputtering onto microscopic carrier glasses with an optical refractive index of 1.5. Big pieces of glass were cut into 1 Â 1 cm 2 substrates for follow-up deposition processes. The deposition rate was 20.5 nm/h, with a 100 W sputtering power, in an atmosphere of pure oxygen at 4 Â 10 À1 Pa. The oxygen gas used in this experiment was of 99.999% ultra high purity. All substrates were ultrasonically cleansed in acetone and alcohol solution. The organic contaminants were removed by de-ionized water before the deposition process began. The SCO target of 99.9% purity was purchased from GUV Team International Co., Ltd. The X-ray diffraction patterns indicated that the target was poly-crystallized SrCuO 2 . Prior to the sputtering process, the background pressure of the vacuum chamber was reduced to below 1:5 Â 10 À4 Pa. There were three sputtering targets in our sputtering system with target diameter of 76.2 mm. The substrate rotated at a speed of 10 rpm. The actual time for the film's growth was only a quarter of the sputtering duration. The substrate was kept at room temperature throughout the sputtering process. Film thickness was measured using an alpha-step surface profiler (TENCOR MA-1450). The films were typically 100 nm-thick. Deposited samples were taken from the same sputtering batch and then annealed at different temperatures ranging from 373 to 723 K. Annealing treatments were performed in the atmosphere whose oxygen * 1 Graduate Student, National Cheng Kung University * 2 Corresponding author, E-mail: brianna41@example.com purity was as in the sputtering process, under a pressure of 101 Pa. All the SCO films were annealed for 3 hours. The changes in structure with corresponding annealing treatment variations were measured by a grazing incident angle X-ray diffractormeter (GIAXRD). A four-point probe (Model RT-70, NAPSON, Japan) was employed to measure the sheet resistivity for all films. The carrier density and mobility of the films were measured using the Hall effect in Van der Pauw method. All film thickness values and electrical characteristics were the averages calculated from measurements at different positions on the samples. Optical transmittance and absorbance were measured by an ultraviolet/visible/ near-infrared spectrophotometer (UV/VIS/NIR spectrophotometer, Hitachi U4001) within a wavelength range between 200-1100 nm. The transmission percentages were then calculated from the percentage transmission data of the corresponding bare substrates. Figure 1 shows the film resistivity against annealing temperature plotted for as-deposited and annealed SCO films. Resistivities of all samples were measured by a standard four-point probe technique. The resistivity increased first as a film was annealed at 373 and 473 K, and then started to decrease as the annealing temperature reached 623 K. The changes in carrier mobility of as-deposited and annealed films are shown in Fig. 2 . The mobility of a film decreased with annealing temperature at 373 and 473 K, and then started to increase at 623 K. This trend matched the changes occurring in resistivity between 373-623 K. Figure 3 depicts the carrier density vs. annealing temperature. It is clear that the carrier density increased with annealing temperature. According to previous research work, the increase in the number of carriers would decrease the carrier mobility due to the scattering of carriers.
Results and Discussions

Electrical and structural properties
14) It is agreed widely that the film resistivity increases as the carrier mobility decreases in TCO films. As discussed above, the major difficulty in developing a p-type TCO is to increase the mobility of carriers in the film, so one distinctive factor that affects film resistivity is the variations in the mobility of carriers. However, it was observed that when the annealing temperature reached 623 K there were larger SrCu 2 O 2 crystals formed in the film being annealed. Figure 4 displays the GIAXRD patterns (2) as a function of annealing temperature. There were no identical peaks found among the patterns of the films that had been annealed at 373 and 473 K. However, the patterns of a SCO film annealed at 623 K revealed a peak of (211) plane (JCPDS relative card #38-1178) and indicated that larger conductive SrCu 2 O 2 crystals had been created. Crystals of larger sizes lead to less grain boundaries and reduce the scattering of grain boundaries. Therefore, the carrier mobility and film resistivity were improved, similar to what had been reported from Lee et al.
16) The resistivity of the SCO film annealed at 623 K in an atmosphere of pure oxygen was hence further improved. It was also found that the peak of SrCu 2 O 2 had disappeared after annealing at 723 K. When the conductive SrCu 2 O 2 phase in the film vanished due to the annealing process, it resulted in an increase of film resistivity. Similar results were found in previous research. 13) Due to the high resistivity which typically leads to less practicality, films annealed at 723 K will be excluded in discussions on optical properties. Figure 5 shows the transmission spectra of films annealed at different temperatures with oxygen. The transmission spectra of the films were measured for wavelengths between 200-1100 nm. The transmittance of an annealed film was improved as the annealing temperature increased. The transmittance in the visible range at 550 nm was slightly higher from 52.6% to 58.2%. The defects formed during the sputtering deposition process could have hindered the transmission of light. The annealing process could provide sufficient energy for atoms to migrate and by which less defects could be obtained. Thus the annealing process could help reduce defect density and improve transmittance. [17] [18] [19] It is a general rule that the absorption coefficient, , in relation to the photon energy, h, is given by
Optical properties
Where A is a constant and E g is the optical energy gap. Figure 6 is a plot of ðhÞ 2 vs. photon energy (h) for asdeposited and annealed SCO films. The optical band gap (E g ) was obtained from the linear portion where h was zero.
18)
The E g evaluated slightly shifted from 3.6 to 3.65 eV as the annealing temperature increased. This increase could have been caused by the increase of carrier density in annealed SCO films. During the annealing process, excess oxygen could have been introduced into SCO films and provided additional carriers. 20) This shift in the band gap was as expected.
Hall measurements
To identify the major conducting type of the annealed films, Hall measurements were taken at 298 K. The measured Hall coefficients of these films were all positive values. The Hall coefficients of annealed SCO films fell in the range of 9:28 Â 10 À2 to +5.46. The conducting type of the films was affirmatively p-type.
Conclusion
Annealed SCO films exhibited that the resistivity of a SCO film could be improved with a proper annealing temperature. The demerits of low carrier mobility strongly affect the resistivity of a SCO film but which can be reduced by a common annealing technique. Analysis of GIAXRD results showed that larger crystals of SrCu 2 O 2 were generated on the SCO film when annealed at 623 K, and lowest resistivity of 1.63 m was observed due to the improved carrier mobility. However, an annealing temperature above 723 K could damage the conductive SrCu 2 O 2 phase and the conductivity of a SCO film. The carrier density and optical transmittance were both improved as the annealing temperature increased. Hall measurements verified all the SCO films to be p-type. 

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