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摘要
Objective Pulsed laser deposition (PLD) is a technique, for removing material from the surface of a target that uses laser energy pulses. It has several advantages over other depositions methods, including high particle energy to form film, fast deposition rate, and no restriction on the target materials. PLD technology has advanced rapidly in recent years, and it is now widely used in the production of metals, ceramics, transparent electrodes, and high-temperature superconducting films. For traditional PLD technology, nanosecond single-pulse lasers are commonly used as excitation sources. One disadvantage of using nanosecond pulsed laser is the possibility of selective ablation, which could result in a lack of stoichiometry during the process. This is a critical challenge that nanosecond PLD (ns-PLD) technologies for scientific research and industrial applications. With the increasing availability of commercial ultrashort laser sources, in recent years, and its distinct advantage of efficient laser ablation, the ultrashort pulse PLD is gaining popularity as a method for producing thin films. Ultrashort pulse PLD demonstrates its potential capacity to control the emission of droplets due to the diverse ablation mechanisms, even though it may not be the ultimate solution in smooth film deposition. Furthermore, the pulse sequence presented in this article has the potential to change the laser-matter interaction, which can be used to improve the deposit' s surface quality and optical properties. Methods A method for pulsed deposition of picosecond laser based on different pulse burst modes is presented, consisting of four main components: seed oscillator, pulse selector, laser amplifier, and power controller (Fig. 1). The laser burst mode is set from 1 to 4 (Fig. 2) with a 532 nm output wavelength, 100 kHz laser frequency, 33.3 ns intrapulse interval, and 10 ;is interpulse string interval. Zinc oxide (ZnO) transparent conductive thin films are deposited on glass substrates and single-crystal silicon substrates via the proposed method. The effect of different pulse burst modes on the crystal structure, surface morphology, and optical properties of the ZnO film is studied thoroughly using spectroscopic ellipsometry, atomic force microscopy, X-ray diffractometry, ultraviolet-visible spectrophotometry, and scanning electron microscopy. Results and Discussions We obtain film thickness (Fig. 3), refractive index, and extinction coefficient data (Table 1) created for various burst modes (from 1 to 4) and discover that the deposition rate decreases and are accompanied by an increase in refractive index as the number of burst modes increases. To begin, the intrapulse period in multipulse mode is set to 33.3 ns and the average plasma velocity is around 104 cm/s. Therefore, using a burst mode of 4, the plasma in the multipulse mode is not completely disengaged from the target when the last pulse is incident, resulting in laser-plasma contact and partial absorption of the pulsed laser energy by the plasma, and limiting target material extraction. Furthermore, the energy distribution of laser pulses is related to the fact that the first pulse energy gradually decreases as the multipulse burst mode increases in size, reducing the first energy interaction between the laser and the target. The roughness diminishes as the multipulse burst mode is increased (Fig. 4). As the laser-plasma interaction is strengthen, the plasma's kinetic energy increases, resulting in longer plasma lifetimes, longer diffusion durations on the substrate, and eventually favoring the orderly formation of thin films. In the single-pulse mode of laser deposition, large particles and droplets are present; however, the laser-plasma interaction in the multipulse mode can further heat up and break down the large particles in the plasma, resulting in fewer large particles on the film surface and smoother, denser films (Figs. 5 and 6). The crystal structure of the prepared films is examined using XRD (Fig. 7) and the crystal structure data for different pulse burst modes (from 1 to 4) are compared, as shown in Table 1, and it is discovered that the different pulse burst mode does not affect the crystal structure. However, when the pulse burst mode is 4, it has some subtle effects on the crystal size, diffraction peak angle, and film intensity, with larger crystal size and better film quality. The transmittance curves of the films deposited in various pulse burst modes are determined (Fig. 8), In the visible range (380-800 nm), the average transmittances of the films are 90. 31%, 92. 72%, 93. 98%, and 94. 81%, respectively. The optical band gaps (Fig.9) are 3.317, 3.343, 3.362, and 3.427 eV, which are comparable to the normal ZnO bandgap (3.3 eV), which corresponds to the tendency of the central wavelength of the absorption edge in the transmittance curve to move in the direction of short wave. Finally, we calculate resistivity curves for the deposited films under various pulse burst modes (Fig. 10) and discover that the film resistivity is lowest when the pulse burst mode is 4. Conclusions The high-quality ZnO films are deposited on glass and silicon substrates using a laser deposition process in different burst modes, with the number of subpulses in each burst increasing from 1 to 4. The effects of different burst modes of picosecond lasers on the film thickness, roughness, surface topography, crystal structure, optical properties, and electrical properties of ZnO films are investigated. When the pulse burst mode is set to 4, the film surface has less roughness, smaller particle size, higher transmittance, better crystalline quality, and lower resistivity when compared with other burst modes. This is extremely important for relevant optical applications to the production of ZnO thin film.
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