# Optimizing Laser-Induced Voltage Signal in SnO2 Thin Films by Changing Oxygen Pressure

Authors: Xiaohong Zhou; Xiaofang Zhou; Hui Zhang
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Laser-induced voltage (LIV) effect of the thin film can be applied for the laser energy measurement and millimeter-wave detection. The amplitude and corresponding velocity of LIV signals depended on the film material and preparation process. In the study, the effect of oxygen pressure on the LIV signals of SnO2 thin films was studied and got the surprised result that the maximum LIV peak reach about 4.02 V and response time with only 98 ns when oxygen pressure was 30 Pa. The crystal quality of the films was analyzed by X-ray diffraction (XRD) and LIV measurements, and a linear relationship between LIV peak voltage and laser energy was also disclosed. Those findings indicate that the laser energy/power meters designed with SnO2 thin film with optimized oxygen pressure would have the merits high precision, high sensitivity, and fast response.


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## Body

## 1. Introduction

The LIV effect had been observed in the thin films of high-temperature superconductor cuprates YBa2Cu3O7 −δ and colossal magnetoresistance manganese La1 −xCaxMnO3 since 1990 [1, 2]. It has attracted great attention because of its potential applications in detecting laser radiations for a wide spectral range [3–5] and can be applied to reveal information about the optical and thermal properties of shale [6]. The LIV effects could be explained by atomic layer thermopile (ALT) mode with Seebeck effects. The structure of anisotropy leads to different electronegativity, electron mobility, and thermal conductivity along the ab-axis and c-axis [3]. Lots of novel film materials were synthesized to enlarge the difference of the Seebeck coefficient ΔS = Sab − Sc and to get a fast response and high sensitivity in light detectors such as La0.9Sr0.1NiO3, MxCoO2 (M = Na, Ca, or Sr), SrTiO3 −δ/Si, and BaNb0.3Ti0.7O3/Si [7–9]. Those multicomplex oxide films more or less have the problem of selective sputtering under the ultraviolet laser, which is a factor that hinders the development of detection elements. Simple oxides, such as SnO2, only contain one or two cations and could avoid the laser sputtering by selecting the type of cation, which could get a high LIV peak and fast response. SnO2 is an n-type semiconductor with a broad bandwidth ranging from 2.7 to 4.6 eV[10], and it has been widely used for gas sensors, oxidation catalysts, and optoelectronic devices [10–13]. However, there are very few studies on SnO2 thin films applied for laser energy meters using the LIV effects principle. The LIV single depends on the structure of SnO2 films, which was determined by the preparation methods and the manufacturing process. SnO2 thin films have been prepared by various techniques such as chemical vapor deposition (CVD), pulsed laser deposition (PLD), sputtering, and sol-gel coating [14–17]. Compared to these preparation methods, PLD is a very attractive technique for growing qualified SnO2 thin films due to its merits in maintaining uniform stoichiometry, avoiding chemical contamination, and epitaxial growth [18]. The whole PLD process can be divided into two stages: the film growth step and in situ annealing step, including the process parameters such as oxygen pressure, temperature, time, and so on, which can affect the structure of film greatly. Preiss et al. [13] studied the structure of SnO2 films with the oxygen pressure varying from 0.01 to 0.2 m bar during the film growing stage, and the X-ray diffraction results showed the mainly amorphous structure of the as-deposited films except the film deposited at 0.05 mbar. The oxygen pressure in the annealing step would also be a key parameter of the crystal structure of SnO2 films, which may achieve the purpose of adjusting the anisotropic Seebeck coefficient of the film and can be optimized to get appropriate LIV signals for laser energy measurement application.In this study, the LIV effects signal was investigated in simple oxide SnO2 thin films grown on tilted Al2O3 single-crystal substrate, which was cut with 10° tilting angles along the (0001) direction. The SnO2 thin films were annealed under different oxygen pressure in situ after PLD. The effects of the annealing oxygen pressure on structure and LIV effects were analyzed through XRD and LIV measurements.


## 2. Experimental

SnO2 target was synthesized by the solid-state reaction method. The high purity powder SnO2 (99.9%) was calcined in the furnace at 1050°C for 24 hours, cooled, and taken out to be ground sufficiently. Ground powder was molded and sintered at 1380°C for 8 hours.The sintered samples were analyzed by a BDX3200 X-ray diffractometer (Cu, Kα), and the results are shown in Figure 1. The XRD spectrum showed that the SnO2 polycrystalline target synthesized under the above conditions had a tetragonal structure, and the lattice constant a = b = 0.4738 nm, c = 0.3187 nm, and no impurity phase was formed.Figure 1
XRD spectra of the SnO2 target.SnO2 thin films were prepared on tilted Al2O3 single-crystal substrate (99.99% purity), which was cut with 10°tilting angles along the (0001) direction by PLD, using a KrF excimer laser with the wavelength of 248 nm. Before the deposition of films, the substrates were cleaned in an ultrasonic bath using acetone and methanol for 8 min. Cleaned substrates were placed in the PLD chamber. Sintered SnO2 ceramic target (20 mm diameter and 5 mm thickness) was used for the deposition of films. The distance between the target and the substrate was kept at 45 mm. The laser parameters used were set as follows: single pulse laser energy was 300 mJ (millijoule), repetition frequency was 5 Hz, deposition was performed for 15 min, and the film thickness was about 250 nm. During the deposition, the deposition oxygen pressure was kept at 0.5 Pa, and substrate temperature was maintained at 750°C. After deposition, the thin films were in situ annealed at 750°C with the oxygen pressure at 0.5 Pa, 5 Pa, 30 Pa, and 100 Pa for 30 min immediately and respectively.The LIV measurement system used is the same as that reported in the literature [19–21]. The pulsed laser source is an LPX300 type I excimer pulsed laser (λ = 248 nm, pulse width: 28 nm), and the laser spot area is 1 cm × 3 cm. The photosensitive surface of the film sample area is 2 mm × 0.8 mm. The LIV signal of the film sample was acquired using a Tektronix TDS210 oscilloscope with an oscilloscope sampling frequency of 1 GHz.


## 3. Results and Discussion

The crystal state and orientation of the film have a lot of influence on the LIV signal [4, 22]. Excessive defects in the film markedly reduce the sensitivity of the film, which is manifested by a sharp decrease in signal peak value and a corresponding increase in response time [23].Figure2 shows the XRD (mono-2 box) of SnO2/Al2O3 films prepared at different annealing oxygen pressures. Curve from bottom to top, respectively, corresponds to 100 Pa, 5 Pa, 30 Pa, and 0.5 Pa annealing oxygen pressure to prepare SnO2 thin film. It can be seen from the figure that there was a tetragonal rutile structure in the vicinity of 2θ = 37.95° and 2θ = 81.14° except for the corresponding substrate peaks A12O3 (0006) and A12O3 (00012) in the vicinity of 2θ of 41.70° and 90.65°, respectively. Diffraction peaks at 2θ = 37.95° and 2θ = 81.14° correspond to tetragonal rutile structure found in nearby SnO2 (200) and SnO2 (400). In addition, no additional diffraction peaks were observed. This result indicates that the prepared film has a tetragonal rutile structure of pure SnO2 and a definite orientation along the a-axis.Figure 2
XRD curves for the SnO2 (200) diffraction peak of the film grown at 750°C, 0.5 Pa oxygen pressure, and different annealing oxygen pressure at 0.5 Pa, 5 Pa, 30 Pa, and 100 Pa.As the annealing oxygen pressure increases from 0.5 Pa to 100 Pa, the diffraction peak position of the SnO2 film changes, especially in the large-angle region. Take SnO2 (200) as an example: the detailed information is shown in Figure 3. With the annealing oxygen pressure increasing from 0.5 Pa to 100 Pa, the diffraction peaks vary from 38.02° to 37.82°, which indicate the lattice parameters were slightly changed under different oxygen pressures. There was a tendency that the higher the oxygen pressure, the less 2θ of diffraction peak, which could be explained by oxygen vacancy defects. There were many oxygen vacancy defects because of oxygen lack, and the c-axis lattice parameters of SnO2 were shortened.Figure 3
Calculated parameters depending on the peaks of XRD.Furthermore, it was found that the full width at half maximum (FWHM) of SnO2 (200) reached the minimum value, and the intensity value was extraordinary under 30 Pa oxygen pressure in Figure 3. It indicated that the SnO2 thin film had good crystallinity under 30 Pa oxygen pressure during the annealing stage. The oxygen atmosphere could keep the suitable proportion of oxygen in the film and help to improve the crystalline quality of the film not only during the laser deposition process [12,13] but also during the annealing stage. Low oxygen pressure would lead to oxygen defects, and too high oxygen pressure would also damage the structure because of oversaturated oxygen content, shown in Figure 3; the FWHM values under 0.5 Pa and 100 Pa oxygen pressure were wider than that under 30 Pa.Figure4 shows LIV signal of SnO2/Al2O3 thin films under a different annealed oxygen pressure of the laser energy density is 100 mJ/cm2. It can be observed that the LIV signals change significantly, including peak voltage and response time. As the annealing oxygen pressure increase, the peak voltage increases first and then decreases. The peak voltage of LIV was about 2.8 V under 0.5 Pa annealing oxygen pressure and increased to 3.4 V with oxygen pressure at 5 Pa. The peak voltage reached the value of 4.02 V when annealing oxygen pressure was 30 Pa and decreased to 3.6 V with oxygen pressure at 100 Pa. The peak voltage of the SnO2 thin film is stronger than the reported peak voltages obtained in YBCO [1], LCMO [8], and La0.9Sr0.1NiO3 [7]. In addition, the response time of the LIV signal is 124 ns, 86 ns, 98 ns, and 117 ns for the annealing oxygen pressure at 0.5 Pa, 5 Pa, 30 Pa, and 100 Pa, respectively. According to the peak voltage and the response time of the LIV signal, the optimal annealing oxygen pressure is nearly 30 Pa for the excellent sensitivity of the LIV signal. It could be made clear that the oxygen content was greatly improved, and the crystal structure became complete after annealing under oxygen pressure. The anneal step can transfer the depositing layers into a polycrystalline structure [13]. Liu et al. [9] also found the structure of the film was uniform all over the substrate’s surface, and most of the grains were separated with clear grain boundaries at various annealing oxygen pressures. However, the conductivity would also be affected if the crystal structure was too perfect. The mechanism of conductivity is closely related to the oxygen voids, and there was not any conclusion that the better the crystal crystallization quality, the better the conductivity. High-temperature annealing could change the crystallization quality, which leads to a change in the optical and electrical properties of the film if under different annealing temperatures [24] and different oxygen pressure [25]. The value of the oxygen content determines the concentration of carriers, affecting the value of the anisotropic Seebeck coefficient [24]. There was an optimal oxygen content value; if the oxygen content was too high, there would be only a few oxygen voids as the carriers. Bansal et al. [16] analyzed the defect of SnO2 films grown under different oxygen partial pressure by photoluminescence (PL) emission spectra and found that the 14 SCCM oxygen flow rate yields the highest carrier density comparing oxygen flow rate at 13 SCCM, 15 SCCM, and 16 SCCM. On the other hand, if the oxygen content was too low, there would be too many defects in the films causing crystallization problems and also affect the anisotropy Seebeck coefficient. The morphology of the films was also affected by the oxygen pressure. Preiss et al. [13] took SEM images showing the higher oxygen pressure delivers smaller grains and a more porous microstructure, and Liu et al. [9] took AFM images of films at various annealing oxygen pressure (900 Pa, 3 kPa, and 10 kPa), which revealed the surface roughness (Ra) value was minimized at 3 kPa annealing oxygen pressure, where achieved high LIV value. The combination function of crystallization, grain boundary, and roughness would take roles in the electronic conductivity and thermal conductivity in the ab-plane, which led to considerable anisotropy of the conductivity and the Seebeck coefficient.Figure 4
LIV signals of SnO2/Al2O3 thin films as a function of annealing oxygen pressure; the inset is the linear relation between the peak values of the laser incident energy density.A linear relationship exists between the peak voltage and the laser incident energy density regardless of the annealing oxygen pressure, as shown in the inset picture of Figure4. The linear relationship could be explained with an atomic layer thermopile model and could be formalized with an equation, which shows the peak voltage varies linearly with laser energy while other parameters are invariable [4]. Besides, the linear relationship in SnO2 thin film is similar to the reported La1 −xSrxCoO3 [26] and La0.9Sr0.1NiO3 [7] thin film. The peak voltages were about three times later when the laser energy density increased from 24 mJ/cm2 to 100 mJ/cm2, which shows a comparable broad energy density range in La0.9Sr0.1NiO3, and the laser energy/power meters made by SnO2 thin films show a broad detection range. Combined with the high peak voltage and the low response time described above, laser energy/power meters designed with SnO2 thin film would have the merits high precision, high sensitivity, and fast response. It indicated that SnO2 thin film would be a potential candidate for laser energy/power detectors.


## 4. Conclusions

A single a-axis oriented single-crystal SnO2 film was successfully grown on a 10°tilted Al2O3 substrate by the PLD method, and the LIV effects were found in the film. The results show that the crystal quality and the peak value of the LIV signal of the film depended on the annealing oxygen pressure, which was mainly reflected in the effect of the crystal quality on the film’s conductivity and carrier concentration, resulting in the change of the film’s electrical transport properties. The optimized annealing oxygen pressure was 30 Pa, in which the thin SnO2 film had good crystallinity and could get the maximum LIV peak of 4.02 V and minimize response time by 98 ns. In addition, there was a good and broad relationship between LIV peak voltage and laser energy in SnO2 thin film, which indicates the laser energy/power meters designed with SnO2 thin film would have the merits high precision, high sensitivity, and fast response.



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*Source: 3303595-2022-07-15.xml*