Femtosecond laser improves the quality of bismuth film and enhances ultra-wideband photoelectric detection

Researchers from the Changchun Institute of Optics, Precision Mechanics and Physics of the Chinese Academy of Sciences and Peking University have developed a new technology for improving the quality of bismuth films as topological insulators for ultra wideband photodetectors. Relevant research achievements were published in Optics Express under the title of "Femtosecond laser upgrading the quality of bismuth films to enhance ultra broadband photodetection".

Topological insulator bismuth (Bi) has attracted extensive attention in the preparation of room temperature, ultra-wideband and high-performance photodetectors because of its gapless edge state and insulating body state characteristics. These detectors can span the range from ultraviolet to far infrared and even terahertz. However, the surface roughness and grain boundary of bismuth film greatly affect its photoelectric conversion and carrier transmission, and limit its photoelectric performance.

Femtosecond laser can provide non-contact high-precision manufacturing on various materials due to its ultra-high peak power and ultra-short duration. In this paper, researchers have demonstrated the method of femtosecond laser treatment to change the surface morphology and physical and chemical properties of bismuth film and improve the quality of bismuth film. After adjusting the laser parameters such as pulse energy and scanning speed, the average surface roughness measurement value is reduced from Ra=44 nm to 6.9 nm, and the grain boundary is obviously eliminated. The photoelectric conversion and carrier transmission of bismuth film are improved, and its photoelectric performance is enhanced. Therefore, the optical responsivity of bismuth film has increased by about 2 times in the ultra-wide spectrum range from visible light to middle infrared. The research shows that femtosecond laser processing is helpful to improve the performance of topological insulator ultra-wideband photodetectors.

Upgrading bismuth thin film by femtosecond laser

In the research, silicon dioxide (SiO2) - based bismuth film (about 300 nm thick) was prepared by DC magnetron sputtering for further femtosecond laser processing. As shown in the schematic diagram in Figure 1, the commercial titanium sapphire femtosecond laser amplifier (Spitfire Ace, Spectra Physics) is used as the light source to send the coil polarized infrared (800 nm, 1 kHz, 40 femtosecond) pulse sequence. After the beam is expanded, a cylindrical lens with a focal length of f=50 mm is focused on the bismuth film fixed on the high-precision three-dimensional translation platform to form a linear beam spot with a diameter of about 30 mm. The sample was scanned at a speed of 1 mm/s and a large area of surface treatment was completed. Surprisingly, after femtosecond laser irradiation, the external surface and internal grain boundaries can be manipulated simultaneously, as shown in the left and right pictures in Figure 1.

Figure 1 shows the schematic diagram of femtosecond laser processing of bismuth film.

Change of surface morphology and grain boundary
In order to better understand the effect of laser parameters on the surface roughness and grain boundary of bismuth thin films, researchers have experimentally studied the variation of the surface after laser treatment with the incident energy flux per pulse. Figure 2 (a) - (f) shows a set of atomic force microscope (AFM) images, showing the changes of surface morphology under the influence of different lasers. The average surface roughness (Ra) and root-mean-square average surface roughness (Rq) calculated in Fig. 2 (g) are functions of laser flux F per pulse. In addition, the high-resolution SEM images of bismuth film before and after femtosecond laser processing can reveal the changes of crystal grain boundaries, as shown in Fig. 3 (a) - (e). From Fig. 3 (a) - (b), it can be seen that the grain boundary of the film gradually disappears with the increase of f. From Fig. 3 (a) - (b), it can be seen that the original film is composed of many grains, and the grain size distribution is very wide, ranging from tens of nanometers to hundreds of nanometers. Especially, there are some large protruding particles on the surface, which has a significant impact on the smoothness. The cross section scanning electron microscope images show that the size of these protruding particles is even larger than the thickness of the original film. (i) Raman spectra of the original and laser-treated bismuth films were measured with different laser fluxes.

Figure 2 shows the evolution of the surface morphology of bismuth thin films treated by femtosecond laser with the pulse energy flux.

Fig. 3 shows the SEM and Raman spectrum analysis of bismuth film.

Photoelectric Properties of Modified Bismuth Thin Films
The photoelectric properties of bismuth thin films after laser treatment were evaluated by using broadband laser excitation. The optical image and illustration shown in Fig. 4 (a) compare the morphology of two parts of the sample surface before (left) and after (right) laser treatment. Obviously, even for centimeter-level laser processing, the surface roughness of the sample can be greatly reduced due to the disappearance of large protruding particles. The researchers also used atomic force microscopy (AFM) to carry out three-dimensional (3D) characterization of the morphological changes of the film surface, as shown in Fig. 4 (b) and c. The prominent particles on the surface of bismuth film were effectively removed, and the surface morphology was obviously smooth. By comparing the AFM measurement data before and after laser processing, it can be found that their average height difference is about 200 nm, and the maximum value is about 300 nm.

Figure 4 shows the comparison of surface roughness of bismuth film before and after laser treatment.

Researchers have proved that femtosecond laser processing can be used as an effective method to modify bismuth films, including surface roughness and grain boundary. By adjusting the laser parameters, not only the surface roughness of the film can be continuously changed, but also the grain boundary can be eliminated at the same time, thus significantly improving the carrier transmission. Compared with the original sample, even at room temperature, the laser-treated bismuth film shows twice the optical response in the broadband range from visible light to middle infrared (MIR). In general, this method can be further used to improve the quality of other topological insulators, and will certainly promote the application of topological insulator materials in the field of ultra-wideband photoelectric detection. The authors also hope that their research can inspire more research on femtosecond laser processing of modulated topological quantum materials.

Source: OFweek