The Optimum Fabrication Condition of p-Type Antimony Tin Oxide Thin Films Prepared by DC Magnetron Sputtering

Transparent Sb-doped tin oxide (ATO) thin films were fabricated on quartz glass substrates via a mixed (SnO 2 + Sb 2 O 3 ) ceramic target using direct current (DC) magnetron sputtering in ambient Ar gas at a working pressure of 2 × 10 torr. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Hall-effect, and UV-vis spectra measurements were performed to characterize the deposited films. The substrate temperature of the films was investigated in two ways: (1) films were annealed in Ar ambient gas after being deposited at room temperature or (2) they were deposited directly at different temperatures. The first process for fabricating the ATO films was found to be easier than the second process. The deposited films showed p-type electrical properties, a polycrystalline tetragonal rutile structure, and their average transmittance was greater than 80% in the visible light range at the optimum annealing temperature of 500C.The best electrical properties of the film were obtained on a 10wt% Sb 2 O 3 -doped SnO 2 target with a resistivity, hole concentration, and Hall mobility of 0.55Ω⋅cm, 1.2 × 10 cm, and 0.54 cmVs, respectively.

Recently, the p-type doping of SnO 2 thin films was studied using metal elements in group III.For example, Al [20,21] and Ga [22] have been incorporated into the structure as the acceptor impurities.Later, Ji et al. [23] and Ni et al. [24] also obtained p-type SnO 2 thin films doping In and Sb.Besides, Ni et al. [25] found that the incorporation of group IIA atoms (Zn) was also suitable for doping SnO 2 , and Mao et al. [26] reported an In-Ga codoping method to realize p-type SnO 2 thin films with a high mobility.
The previous research has been focused on thin films synthesized by RF magnetron sputtering [20,22,[24][25][26][27][28][29][30], the sol-gel [31,32], spay-pyrolysis [33], or PLD [34].Among these deposition methods, RF magnetron sputtering method is widely used because of high deposition rate, good adhesion, and easy control of the electrical properties of the films by controlling processing parameters such as sputtering power, pressure, and temperature, particularly postannealing temperatures.However, no report explained clearly how to achieve the p-type by annealing and why thin films were not directly prepared with the various temperatures as well as why the thin films were deposited at 200 ∘ C after postannealing.The results of the previous research have not yet solved this problem, and, furthermore, a DC magnetron sputtering process has not yet been studied.Because the ionic radius of the Sb 3+ ion (0.76 Å) is similar to that of Sn 4+ (0.71 Å), we studied optical, electrical properties and the crystal structure of Sb-doped SnO 2 films deposited on quartz substrates by DC magnetron sputtering from ceramic targets made of a mixture of SnO 2 and Sb 2 O 3 .The parameters investigated were the substrate temperature during fabrication, the annealing temperature, and the percentage of the Sb 2 O 3 dopant.

Experimental Techniques
SnO 2 :Sb (ATO) thin films were deposited on quartz substrates from a ceramic target composed of a mixture of (SnO 2 + Sb 2 O 3 ) by DC magnetron sputtering in ambient Ar gas at a working pressure of 2 × 10 −3 torr with the Sb 2 O 3 (wt%) varying from 0% to 15%, on a Univex 450 system.The quartz glass substrates were ultrasonically cleaned in 10% NaOH/acetone/deionized water solution for 35 min.Before deposition, the target was presputtered for 15 min to remove any contaminants on the target surface.The sputtering power and the target to sample distance were 15 W and 7 cm, respectively.
The substrate temperature of the films was investigated in two ways: (1) the films were deposited directly at different temperatures from room temperature to 500 ∘ C and heating and cooling rate 20 ∘ C/min, respectively, and (2) they were annealed in ambient Ar gas after being deposited at room temperature with the same heating and cooling rate as part 1.
The thicknesses of the thin films were measured using the SCOUT program by fitting the modeled spectrum to the experimental spectrum.By the controlling the deposition time, the thickness of SnO 2 and ATO films were about 400 nm.The structure of the ATO films was studied using an X-ray diffractometer (D8 ADVANCE) operated at 40 kV and 40 mA with Cu-K radiation.The optical transmittance was measured in the wavelength range 200-1100 nm using a UV-vis Jasco V-530.The film resistivity was measured at room temperature using the four-point probe method.A Hall measurement system (HMS 3000), which operated at room temperature, was used to measure the carrier concentration and the mobility.X-ray photoelectron spectroscopy (XPS) was measured at room temperature on an ESCALAB and the I-V characteristics were measured using a semiconductor testing system (Keithley 2400).particular, the temperature was applied directly to the films during the fabrication process; however, S6 and A6 films were deposited at 500 ∘ C followed by annealing at 600 ∘ C because the evaporation rate from substrates is greater than the condensation rate on substrates at temperatures above 500 ∘ C. Figure 1 shows that all films made at room temperature had an amorphous structure; the films started to crystallize at 200 ∘ C with a SnO 2 tetragonal rutile structure (JCPDS Number 41-14445).The films had better crystalline quality at approximately 400 ∘ C marked by the appearance of the (110) peak, and the intensity of the peak sharply increased for the films deposited at 500 ∘ C and annealed at 600 ∘ C. Figure 2 shows that ATO films started to crystallize at 200 ∘ C with a SnO 2 tetragonal rutile structure and no peaks from antimony or antimony oxide phases appear in the XRD patterns, indicating that antimony could be incorporated into the tin oxide lattice.Several authors also showed all SnO 2 films doping atoms such as Ta [35], In-Ga [26], Sb [24,34], Ga [22], Zn [28], and In [31] are in rutile phase without any secondary phases of dopant atoms or their oxides, indicating the incorporation of dopants into host lattice.Figure 2 also shows that there is the appearance of Sn 2 O 3 phase (101) for films deposited at 500 ∘ C and the films deposited at 500 ∘ C followed by annealing at 600 ∘ C had one more Sn 2 O 3 phase (230).In particular, the SnO 2 phase (110) disappeared and phase (101) appeared for ATO 500 ∘ C followed by annealing 600 ∘ C while it contrasted to SnO 2 films.This could also be the substitution of Sn 4+ by Sb 3+ .In fact, the report [36,37] had also the same results that the (110) surface transformed to (101) surface which is rich in Sn 2+ atoms (i.e., rich oxygen vacancy) as Sn 4+ were substituted by Sn 2+ .

Results and Discussion
The above results indicate that the ATO films can be made p-type through the annealing step after deposition at room temperature.Therefore, to avoid evaporating the material on the substrate, which occurs when the film is deposited at a higher temperature, the film can be deposited at room temperature and then annealed at various temperatures.
The XRD patterns of the as-deposited SnO 2 thin films and those annealed below 550 ∘ C are shown in Figure 3.The films become polycrystalline and are in the rutile phase with (110), (101), and (211) orientations.When the SnO 2 films were annealed at 550 ∘ C and 600 ∘ C, the Sn 2 O 3 phase appeared and can contribute to the positive carriers, which result in the ptype films.It shows clearly in electrical properties (Table 2).
Figure 4 shows that the peak intensity of the ATO films is lower than those of the SnO 2 films under the same conditions, indicating that the dopant Sb 3+ ions could replace the Sn 4+ ions sites.When annealed above 500 ∘ C, the Sb 3+ are activated to substitute Sn 4+ significantly and there is appearance of Sn 2 O 3 phase which is not recorded at SnO 2 films of the same fabricated conditions.The energy released from the replacement of Sn 4+ by Sb 3+ also contributes to the oxygenation of Sn 2 O 3 to SnO 2 when annealed at 600 ∘ C.This oxygenation does not occur at SnO 2 films of the same fabricated conditions.The ATO films have lower resistivities (Table 2) with the presence of the Sn 2 O 3 phase at an annealing temperature of 550 ∘ C.However, to confirm whether the positive contribution of Sn 3+ is significant, the ATO films were annealed for 2 hours.
Figure 5 shows that the reflection intensity for ( 110  phase to SnO 2 phase and vice versa could be the attribution to films energy released from the substitution of Sn 4+ by Sb 3+ as mentioned in the discussion of Figure 4. Next, we investigated the effect of the doping percentage on the structure of the ATO films.Figure 6 illustrates the XRD patterns of the as-deposited samples annealed for 2 h at 500 ∘ C with various Sb 2 O 3 -doping percentages.The 5% and 10% ATO films do not show the presence of a Sn 2 O 3 phase as 15% ATO films.Thus, the number of Sb 3+ ions substituting for Sn 4+ ions was enough to supply the energy required for the oxidation process of Sn 2 O 3 transforming to SnO 2 at minimum doping percentages of 5 wt% Sb 2 O 3 .1, all of the SnO 2 films deposited at various temperatures including sample S6 exhibit n-type conductivity due to oxygen vacancies and tin interstitials.This result has been also given in [38,39] at a certain deposition temperature.On the other hand, the samples A5 and A6 of 15% ATO films showed p-type conductivity.Clearly, antimony atoms substituted for tin sites at 500 ∘ C temperature and this substitution was recorded at the deposition temperatures of 600 and 700 ∘ C as mentioned in [34].In particular, the carrier concentration increased from 10 15 to 10 16 , and the carrier mobility decreased from 19.3 to 3.49 after annealing at 600 ∘ C following the direct deposition at 500 ∘ C. The decrease in the carrier mobility is attributed to the increase in ionized acceptors as mentioned in [24].Therefore, it is obvious that the annealing method plays an important role in the electrical properties of the ATO films.

Electrical Properties of ATO. As seen in Table
As seen in Table 2, the SnO 2 and ATO films were nonconductive at room temperature and were n-type at 200 ∘ C; however, the ATO films had much lower resistivities compared to the SnO 2 films.This effect might also be explained as follows: creating donors such as oxygen vacancies, tin interstitials as mentioned in [23,25,28,31,32,38,40], and unintentional hydrogen dopants as in agreement with previous theoretical and experimental studies [41][42][43][44] compensate by the Sn 2+ acceptors [29,32] at room temperature, leading to nonconductive SnO 2 and ATO films, while Sb appear as a result of the dissociation of Sb 2 O 3 at 200 ∘ C, which supplies more negative carriers for the ATO films leading to a resistivity of the ATO films lower than that of the SnO 2 film.This dissociation of Sb 2 O 3 shows clearly at optical properties (Figure 10).The SnO 2 films annealed at a temperature in the range of 300-500 ∘ C are nonconductive because Sn +2 is oxidized to Sn 4+ [45], the oxygen vacancies are filled with the oxygen present in the ambient air, and unintentional hydrogen donors are driven out of films [43,46].The 15% ATO films annealed at 300 ∘ C are n-type, whereas, in the range of 400-600 ∘ C, the 15% ATO films show p-type conductivity because the Sb 3+ substitutes for Sn 4+ .Several authors have also observed this phenomenon that the conduction type of In-Ga co- [26], Sb- [24], Ga- [27], Zn- [25,28], or Indoped SnO 2 [23] changed from n-type to p-type at a certain annealing temperature.The SnO 2 films annealed at 550 ∘ C are p-type, which is attributed to the dissociation from SnO 2 to Sn 2 O 3 being in agreement with results of structure analyzing.However, the resistivity of SnO 2 film is much higher than that of the 15% ATO film annealed at 550 ∘ C. It was found that hole carriers were formed by Sb 3+ replacing Sn 4+ rather than by Sn 3+ replacing Sn 4+ .Indeed, the hole concentration of the SnO 2 films decreased at 550 ∘ C because the Sn 2 O 3 phases were oxidized and this also increased the mobility.Contrarily, the hole concentration of the 15% ATO films first increased with increases in the annealing temperature, and the highest hole concentration was obtained after annealing at 550 ∘ C for 1 h.As the annealing temperature was further increasing, the hole concentration value started to decrease because of disappearing of acceptor Sn 3+ and native donors being able to appear.The 15% ATO films annealed at 550 ∘ C for 1 h had low resistivities (0.83 Ω⋅cm) and high hole concentrations (4.4 × 10 18 cm −3 ).However, in the following section, we increased the annealing time of the ATO films to confirm the effect of the Sn 3+ on the electrical properties.As we see in Table 3, the resistivity increased slightly due to decreasing hole mobility as samples were annealed at 550 ∘ C for 2 h compared to 1 h regardless existence of Sn 2 O 3 phase in films.Contrarily, the resistivity decreases as samples were annealed at 500 ∘ C for 2 h compared to 1 h without existing Sn 2 O 3 phase in films.In conclusion, the contribution of the hole concentration is primarily from the substitution of Sn 4+ by Sb 3+ , while the contribution from Sn 3+ is minor.The other issue, we determined the effect of the doping percentage on the electrical properties.
As seen in Table 4, the resistance first decreases with increasing Sb concentration, and the lowest resistance is obtained at a doping concentration of 10 wt%.The 10% ATO films had the lowest resistivity (0.55 Ω⋅cm) and the highest hole concentration (1.2 × 10 19 cm −3 ).As the wt% Sb 2 O 3 was further increased, the resistance value started to increase.Our present observations suggest that beyond a certain doping concentration, the doping atoms do not occupy the lattice sites but instead produce some types of defects.

Optical Properties.
At room temperature, both SnO 2 and ATO films have absorption edge in visible light at 431 nm (2.87 eV) for SnO 2 films and 350 nm for ATO films; this indicates existence of defect level Sn 2+ [47] which served as an acceptor; electron at acceptor energy level Sn 2+ absorbed light energy so that it transited to conduction band (Figure 7); this transited energy differ from exciton transited energy.At 200 ∘ C, the absorption edge significantly shifts to the shorter wavelength side; at above 200 ∘ C, this shifts was slight.This is due to acceptor Sn 2+ transform to Sn 4+ , particular at 500 ∘ C band gap energy is exciton absorbed energy (Figures 7 and 8).
Figure 9 presents the transmittance spectra of SnO 2 films with different annealing temperatures.At room temperature, the transmittance spectra of SnO 2 films were taken from Figure 7. Above 200 ∘ C, acceptor Sn 2+ were supplied energy to transform to Sn 4+ so that the average transmittance in the visible region increased gradually above 80%; furthermore, the absorption edge gradually shifts to the shorter wavelength side.Meanwhile, Figure 10 shows that the ATO films have lower average transmittances compared to the SnO 2 films at temperatures below 400 ∘ C, which indicates the decomposition of Sb 2 O 3 to form Sb which has discrete distribution in films, and this causes a decrease in the transmittance due to their absorption.It does not occur at ATO films deposited at 200 ∘ C. The transmittance in the visible region increased to 80% at 500 ∘ C.This effect occurs because, with increasing temperature, the doping Sb enters the film from interstitial sites and substitutes into other positions.
Figure 11 presents the spectral transmittance of 5%, 10%, and 15% ATO films with the same thickness annealed at 500 ∘ C for 2 h.The average transmissions of the 5%, 10%, and 15% ATO films in the visible region (300-800 nm) were  approximately 80%.All these results are promising for diverse applications in the field of transparent electronics.12 and 13 show that the results of the XPS spectra of the O 1s and the Sn core-level of the ATO films with different Sb concentrations were annealed at 500 ∘ C for 2 h after being deposited at room temperature and SnO 2 films were annealed at 550 ∘ C for 1 h after being deposited at room temperature.All peak positions were corrected for surface charge by the C 1s peak at 284.5 eV.The O 1s spectrum can be deconvoluted into three peaks for the SnO 2 (530, 531.5, and 532.5 eV) using XPSEAK41 software, and the profile of each peak was taken as a Lorentzian-Gaussian mixed function, respectively.For SnO 2 films, deconvolutions contained the 3 peaks of O 1s at 530, 531, and 532.5 eV.Two  peaks at 530 and 531.5 eV can be attributed to O 2− ions in the rutile structure of the Sn 4+ ion array [48] and the O 2− ions in oxygen-deficient regions [49].The peak at 532.5 eV is assigned to the adsorbed oxygen species [50].The above deconvolutions of all ATO films contained the 3 peaks of O 1s and 1 peak at 529.5 eV due to the contribution of the Sb 3+ species [50].This peak was attributed to the substitution of Sn 4+ by Sb 3+ .It showed clearly that the peak for the Sn 3d 3/2 of ATO films shifted to a lower binding energy compared to that of SnO 2 films.The atomic percentages and compositional ratios of all the samples were calculated by the ratio between the integrated areas of O 531.5 and O 530 peaks and the integrated areas of the 2Sn and 1.5Sb peaks, and the results are listed in  from 0.221-0.866,which indicates the existence of numerous oxygen deficiencies (the ratio was 1 for stoichiometric ATO films).The oxygen deficiencies prove substitution of Sn 4+ by Sb 3+ .Moreover, the O 1s peak of 10 wt% Sb 2 O 3 was asymmetric proving the highest substitution of Sn 4+ by Sb 3+ , resulting in the highest concentration of holes (relative to the lowest ratio).

Properties of p-Type ATO/n-Type Si Heterojunction.
To confirm the fabrication of p-type conductive ATO films, a ptype 10% ATO was selected as a p-type layer due to its high hole concentration of 1.2 × 10 19 , and they were fabricated on an n-type Si substrate.The current-voltage characteristics of the p-type ATO/n-type Si structure are plotted in Figure 14.Indium electrodes were placed both on the p-type and the ntype layers, as shown in the upper left inset.The lower right inset shows the I-V curves for a pair of In contacts on the ptype and n-type material layers.The linear I-V characteristics indicate good ohmic contact behavior.It is found that the I-V curve of the heterojunction p-type ATO/n-type Si is nonlinear.This characteristic clearly indicates the formation of p-n junction at the interface.Figure 14 also shows a forward turn-on voltage of 1.09 V.

Conclusions
In summary, p-type transparent conducting ATO films were prepared by DC magnetron sputtering.It was found that an annealing temperature of 500 ∘ C for 2 h was necessary for high p-type conductivity.The best electrical properties of the film were obtained from 10 wt% Sb 2 O 3 -doped SnO 2 target with a resistivity, hole concentration, and Hall mobility of 0.55 Ω⋅cm, 1.2 × 10 19 cm −3 , and 0.54 cm 2 V −1 s −1 , respectively.The XPS analysis indicated that Sb exists in the form of Sb 3+ in the film and acts as an acceptor.The ATO films show polycrystalline tetragonal rutile structure and their average transmittance is above 80% in the visible light range.The p-n heterojunction diode composed of p-ATO/n-Si exhibits good I-V characteristics, thereby, demonstrating that a p-type ATO film and its fabrication technology might have a promising future in practical applications.

Figure 1 :
Figure 1: The X-ray diffraction patterns of SnO 2 films deposited at different temperatures.

Figure 2 :
Figure 2: The X-ray diffraction patterns of ATO films deposited at different temperatures.

Figure 3 :
Figure 3: The X-ray diffraction patterns of SnO 2 films annealed at different temperatures for 1 h after being deposited at room temperature.

Figure 4 :
Figure 4: The X-ray diffraction patterns of 15% ATO films annealed at different temperatures for 1 h after being deposited at room temperature.

Figure 5 :Figure 6 :
Figure 5: The X-ray diffraction patterns of 15% ATO films annealed at different temperatures for 2 h after being deposited at room temperature.

Figure 7 :Figure 8 :
Figure 7: The UV-visible spectra of SnO 2 films prepared at different annealing temperatures.

Figure 9 :Figure 10 :
Figure 9: The UV-visible spectra of SnO 2 films prepared at different annealing temperatures after being deposited at room temperature.

Figure 14 :
Figure 14: The I-V characteristics of the p-type ATO/n-Si heterojunction.The insets show a schematic diagram of the p-n heterojunction (upper left) and the I-V characteristics of In contacts on the p-type and n-type (lower right).

Table 1 :
The results of the Hall measurements of SnO 2 and 15% ATO films that were directly deposited at different temperatures.

Table 2 :
The results of the Hall measurements of SnO 2 and ATO films annealed at different temperatures after being deposited at room temperature.

Table 3 :
The results of the Hall measurements of 15% ATO films annealed at different temperatures for 1 h and 2 h after being deposited at room temperature.

Table 4 :
The results of the Hall measurements of (a) 5% ATO, (b) 10% ATO, and (c) 15% ATO films annealed at 500 ∘ C after being deposited at room temperature.

Table 5 :
XPS analyses of the surface of the SnO 2 and ATO films.
5 ]/(2[Sn] + 1.5[Sb]) ratio must have been 1, but the results in Table5give 1.427, indicating that valence state of Sn was not 4 perfectly and including lower valence state such as Sn 3+ .These results were in agreement with the existence of acceptor Sn 3+ discussed in structure and electrical properties (3.1 and 3.2).For ATO, this ratio varied