Introduction

ZnO with wide band gap of 3.3 eV and large exciton binding energy of 60 meV has been widely studied and used in many technological applications. The transparent conducting films, Al-doped ZnO (AZO), with high electrical conductivity and high transparency in the visible light region have been extensively applied in electrical and optical applications such as solar cells, flat panel displays, anode contact for organic light-emitting diodes, and electro-optical devices. From other reports, it was found that the deposition condition plays the crucial factor to influence the microstructure, optical and electrical properties of the films. There are many methods used for the preparation of AZO films such as RF or DC [1–3] magnetron sputtering [4–6], pulsed-laser deposition (PLD) [7, 8], sol–gel process [9], and spray pyrolysis [10].

The ion beam sputtering deposition (IBSD) used in this work is a relatively unique method in which a high packing-density film can be fabricated by the IBSD with atom-by-atom or molecule-by-molecule transport growth mechanism at low substrate temperature [11, 12]. The high quality AZO films with an optimum aluminum component were easily fabricated as both Zn and Al targets were co-sputtered by an energetic ion beam as reported previously [13, 14]. The influence of oxygen partial pressure during deposition process on the physical properties of AZO films is important since the oxygen vacancies were attributed to the photoluminescence and played as electrical donors in the ZnO films [15–21]. In most of sputtering deposition methods, the oxide ceramic targets are usually used to deposit the films to prevent losing the oxygen atoms in the films which results in the worse properties of AZO films. Therefore, in order to investigate a particular role of oxygen in the AZO films, various oxygen partial pressures were used when both Zn and Al metallic targets were sputtered by argon ions during the film fabrication.

It is known that the resistivity of the AZO film can be adjusted by the deposition conditions (for example, [22]); however, it is still debatable how the resistivity actually correlated to the structure and defects in the film. In this study, structural, electrical, and optical properties of the AZO films prepared under various partial oxygen pressures were investigated to understand how property modifications due to the structure changes caused by various oxygen partial pressures. Different optical band gaps observed in the films prepared under various oxygen partial pressures are closely related to the carrier concentrations in the films. Furthermore, the conductivity of the film was dependent on the crystallinity, grain size, carrier concentration, oxygen vacancies, and zinc interstitials.

Experiments

The AZO films were deposited on the B270 glass substrate with the size of 1.3 cm × 1.3 cm by ion beam co-sputtering process at room temperature. Both metallic targets Zn and Al with purity of 99.99 % mounted side by side on a water-cooling copper block were sputtered by the high energetic argon ion beam during the deposition. The glass substrates were cleaned by ethanol in an ultrasonic cleaner and blown dry by nitrogen gas prior to the deposition. A base pressure of 4.0 × 10−6 Torr was achieved using a cryopump. Then, the argon working gas and the oxygen ambient gas were fed into the ion source and the chamber, respectively. The ion beam voltage was kept at 1000 V with the ion beam current of 20 mA. Four different oxygen partial pressures of 1.0 × 10−4, 1.3 × 10−4, 1.7 × 10−4, and 2.0 × 10−4 Torr were used and the total pressure of the chamber was kept at 2.4 × 10−4 Torr during the deposition. The detailed experimental set-up can be found in our previous publication [13]. The aluminum concentrations in the films were maintained at 1.5 atomic % and determined by Energy-dispersive X-ray spectroscopy. The transmission spectra of AZO films fabricated under variation of oxygen partial pressures were measured by UV–Visible-Near IR spectrometer (Cary 5E). An X-ray diffraction (XRD) apparatus (Rigaku Multiflex) with a Cu Kα line (1.54055 Å) was used to characterize the structure of AZO films. The resistivity, carrier concentration and carrier mobility were measured at room temperature by Hall measurement with the Van der Pauw method. The chemical compositions and states of the elements in the film were characterized at room temperature by X-ray photoelectron spectroscopy (XPS) apparatus (Perkin-Elmer PHI1600).

Results and discussion

The structure of AZO films characterized by XRD with respect to the different oxygen partial pressures is shown in Fig. 1. The (001) and (002) ZnO diffraction peaks were detected in the films, and the most intense (002) peak was obtained in the film prepared under the oxygen partial pressure of 1.3 × 10−4 Torr. The obvious (002) peaks shown in the XRD patterns indicate that AZO films have a high texture of the crystalline (002) orientation along the c-axis perpendicular to the substrate surface. Since the peak intensity is closely related to the crystallinity of the film, the higher the oxygen partial pressure, the lower the crystallinity of the film in general. Apparently, a film with better crystallinity resulting in lowest resistivity, as reported previous [13], can be prepared under the oxygen pressure of 1.3 × 10−4 Torr.

Fig. 1
figure 1

The XRD patterns of AZO films prepared at different oxygen partial pressure

Full size image

The grain size and atomic d-spacing of AZO films calculated by Scherrer’s formula and Bragg’s diffraction condition, respectively, as functions of oxygen partial pressures are plotted in Fig. 2. As the oxygen partial pressure increasing, the worse structure in the film is produced in general due to the poor crystallinity with the smaller grains while the d-spacing remains about the same first and then increases little when the pressure is equal or higher than 1.7 × 10−4 Torr. It means that the number of Al3+ ions replacing the Zn2+ ions in AZO structure is reduced and the compressive stress in the films, which can be calculated from the difference between the lattice constant of ZnO and d-spacing [14], increases little as the oxygen partial pressure increases. The bigger d-spacing in AZO film indicates that less Al atoms replace the Zn atoms in the lattice due to the smaller ionic radius of Al3+ (53–67.5 pm) than that of Zn2+ (74–104 pm) such that the resistivity increases [13, 23]. The extrinsic defects in the ZnO films due to the Al impurity play as donors in the films since each Al3+ ion donates an extra electron compared to Zn2+ ion in ZnO structure. Therefore, the donor concentration reduces little as oxygen pressure increases. Moreover, from our previous study [13], the smaller the grain size in the film, the higher the scattering at the boundary, and hence the higher the resistivity.

Fig. 2
figure 2

The grain size and atomic d-spacing of AZO films were plotted as functions of oxygen partial pressures

Full size image

In ZnO film intrinsic defects, oxygen vacancies and zinc interstitials, act as donors to produce carrier electrons, influence the electrical and optical properties as the carrier concentration is modified. [20, 21, 24–26]. In order to understand the relationship between oxygen pressure and electrical property of the film, the resistivity, carrier concentration and carrier mobility were measured by Hall measurement with the Van der Pauw method, and the results are shown in Fig. 3. All the films were shown n-type semiconductors after Hall measurements. It is well known that the resistivity is proportional to the reciprocal product of carrier concentration and mobility. In Fig. 3, it is found that the carrier concentration increases first and then drops to a value of 8.58 × 1020/cm3 as the oxygen partial pressure is increased. Moreover, the carrier mobility decreases initially and then increases as the oxygen gas increases further. It was observed that the trend of carrier concentration was opposite to that of mobility in the pattern when the oxygen partial pressure increased. The low carrier concentration and the high carrier mobility seen as the oxygen partial pressure increasing can be attributed to the defects in the film. The charge carriers, electrons, generated by the oxygen vacancy/zinc interstitial donors decreased as the oxygen partial pressure increased which was seen in XPS spectra shown below. The lowest resistivity, 7.8 × 10−4 Ω cm, due to the overall effect of both concentration and mobility was obtained in the film prepared under 1.3 × 10−4 Torr. Combined to the intense (002) peak in Fig. 1 and the resistivity results the optimum oxygen partial pressure to deposit the AZO film with lowest resistivity and highest crystallinity is 1.3 × 10−4 Torr in this study.

Fig. 3
figure 3

The electrical properties of AZO films was measured with respect to oxygen partial pressure

Full size image

The optical spectra of AZO films prepared under different oxygen partial pressures measured by spectrometer were plotted in Fig. 4. In the visible light range, the transmittance of AZO films is above 80 % in average regardless of the oxygen partial pressure. The transmittance falls sharply due to the band-to-band absorption in the UV wavelength region. It shows that the different oxygen partial pressures have no significant influence on the transmittance in the range of visible light but the absorption edge of AZO films does shift toward the longer wavelength when the oxygen pressure is higher than 1.3 × 10−4 Torr. The shift is caused by Burstein-Moss effect influenced by the carrier concentrations in the film.

Fig. 4
figure 4

The optical spectra of AZO films with different oxygen partial pressure were observed

Full size image

Using the relation between (αhν)2 and hν for the AZO films prepared under different oxygen partial pressures, the optical band gap can be determined and the results are shown in Fig. 5, where α is the absorption coefficient. The optical band gap was obtained using the extrapolation from the plots of (αhν)2 versus hν and the numerical values of the optical band gap for the AZO films with the increase of oxygen partial pressure are 3.76, 3.81, 3.68, and 3.60 eV. According to the Burstein-Moss effect, the optical band gap widening is directly proportional to the 2/3 power of the carrier concentration of the film [14]. Therefore, the absorption edge shifts to the longer wavelength with increasing oxygen partial pressure as a result of the decrease of the carrier concentration in the film.

Fig. 5
figure 5

Plot of (αhν)2 as a function of hν of AZO films to determine the optical energy band gap of the film

Full size image

From the results of optical, structural, and electrical properties of the films it was confirmed again that the low resistivity and high transparency of AZO films prepared at room temperature by the IBSD method can be obtained with an optimum oxygen partial pressure.

To study the film’s surface compositions and chemical bonding environment XPS is applied. Since both the Al concentrations, 1.5 at.%, and the atomic sensitivity factor of XPS Al2p signal, 0.11[27], were so low that no clear Al signal was detected by XPS. The XPS O1s and Zn2p3/2 spectra were shown in Figs. 6 and 7, respectively. The Zn2p3/2 signal can be fitted by two components centered at 1021.5 and 1022.3 eV. The dominant low binding energy (BE) component (ZnI) can be assigned to either the Zn atoms in the Zn–O bonds. [19, 28] or the Zn with 2 + oxidation state. The high BE component (ZnII) can be assigned to either the Zn in the Zn–O bonds surrounded by oxygen vacancies or the Zn with higher than 2 + oxidation state. Because the ZnII are Zn atoms not exactly occupied in the lattice site of regular Zn ions, they may be related to the Zn interstitials. The ZnI first decreased little in 1.3 × 10−4 Torr film and then increased as the oxygen partial pressure increased. The ZnII decreased as the oxygen partial pressure increased.

Fig. 6
figure 6

The original and fitted XPS spectra of Zn2p3/2, two components centered at 1021.5 and 1022.3 eV were used to fit the spectra

Full size image
Fig. 7
figure 7

The original and fitted XPS spectra of O1s, three components were applied, 530.12 ± 0.02, 531.47 ± 0.03 and 532.45 ± 0.02 eV to fit the spectra

Full size image

The XPS O1s signal can be fitted by three components, located at 530.12 ± 0.02, 531.47 ± 0.03, and 532.45 ± 0.02 eV, respectively as shown in Fig. 7. The high binding energy component (HBC, OIII) located at 532.45 ± 0.02 eV can be attributed to the chemisorbed oxygen, H2O, or –CO3 on the surface of the film [29–33]. The median binding energy component (MBC, OII) is assigned to the oxygen in the oxygen deficient region [34–36], i.e., oxygen in O-Zn bonds but surrounded by the oxygen vacancies. The MBC was also assigned to OH on the surface in the literature [37, 38]. The dominant low binding energy component (LBC, OI) is originated from the oxygen in the O-Zn bonds [39–42]. The XPS O1s spectra with three components were previously observed in AZO and ZnO films by other researchers [28, 43–45]. The XPS fitting parameters, peak binding energy positions and full width at half maxima (FWHMs) of O1s and Zn2p3/2 signals were listed in Table 1. Core-level photoelectrons originated from the bonding elements such as LBC O1s and ZnI have less fluctuation in the position and small FWHMs as shown in Table 1. The relative concentrations of various O1s components and Zn2p3/2 components as functions of oxygen partial pressures were plotted in Fig. 8 in which both the OII and ZnII decreased as the oxygen partial pressure increasing. The general trends on the ZnI, OIII, and OI were ZnI and OI increased and OIII decreased as the oxygen partial pressure increasing. Kun Ho Kim et al. pointed out previously that the resistivity of the AZO film was mainly dependent on the carrier concentration [46]. Since the OII is related to the oxygen vacancies and ZnII to Zn interstitials, both decreases indicated the donors were reduced; thus, the low carrier concentration and, therefore, the high resistivity as also detected by Hall measurement shown in Fig. 3.

Table 1 The XPS fitting parameters, peak binding energy positions and full width at half maxima (FWHMs) of O1s and Zn2p3/2 signals
Full size table
Fig. 8
figure 8

The relative concentrations of HBC (OIII), MBC (OII), and LBC (OI) components in XPS O1s signal and ZnI and ZnII components in Zn2p3/2 signal as functions of oxygen partial pressures were plotted

Full size image

Conclusion

The AZO films contained 1.5 at.% Al with the low resistivity and high transparency can be obtained by the ion beam co-sputtering deposition without heating substrate during the deposition under 1.3 × 10−4 Torr oxygen partial pressure. The crystallinity is getting worse and the grain sizes become smaller as the various oxygen partial pressures increases. The increase of the resistivity was caused by the decrease of the carrier concentration as oxygen partial pressure increases. Under the optimum oxygen partial pressure of 1.3 × 10−4 Torr the lowest resistivity and the intense (002) peak was observed simultaneously in AZO film. Regardless of the oxygen partial pressure, the average transmittance of AZO films was above 80 % in the visible region; however, the “Red Shift” phenomenon was observed in the UV region as oxygen partial pressure increased due to the Burstein-Moss effect on the optical energy band gap. Finally, the compositions and chemical bonding environment in the films were examined by XPS in which three oxygen components, LBC, MBC, and HBC and two zinc components, ZnI, and ZnII were applied to fit the spectra. Both MBC O1s, related to the oxygen vacancies, and ZnII, related to the Zn interstitials, decreased as the oxygen partial press increased which indicated that the carrier concentration reduced accordingly. Combined to the worse crystallinity, smaller grain size, and lower carrier concentration, as the pressure increasing, it can be concluded that the resistivity of the film enhances as the oxygen partial pressure increases.