In an article published in energiesThe researchers presented a novel approach to improve the optoelectric properties of silicon (Si), including optical conductivity and bandgap, by applying the Direct Ultrafast Pulsed Nanostructure Formation (DUPNF) technique.
The 3D nanolattice Si structures were created by varying the power and frequency of high-intensity picosecond laser pulses. The optical characteristics of the 3D nanolattice Si structures were investigated by optical spectroscopy in the near-infrared (NIR) and visible (VIS) regions.
Energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) imaging techniques analyzed the structural properties of Si structures. Varying the laser processing power and frequency led to a change in the structural properties and bandgap of the 3D nanolattice Si structures.
The development of the Si structures of the 3D nanolattice and the modification of their characteristics were visible as the frequency and power of the laser pulses were increased. Compared with bulk silicon, the critical decay field of Si structures was more significant.
Benefits of Nanostructures
Nanomaterials have gained popularity as a result of advances in technology and nanoscience. The material’s characteristics improve at the nanoscale and when scaled down, the size decreases, allowing the material to be used in compact devices.
Nanomaterials are essential as today’s global technology demands smaller devices without reduced functionality.
Silicon, one of the most widely available semiconductors, has several exciting applications in electronics, sensors, power generation, and biomedical systems. However, the use of silicon in many high-power applications is restricted by its relatively low indirect bandgap of 1.12 eV. Its low critical breakdown field, at 0.2 MV/cm, also acts as a limitation in its high power applications.
The switch to nanomaterials has allowed the alteration of the characteristics of the materials. Nanomaterials have been observed to have unique characteristics contrary to bulk materials. The synthesis of 3D nanonetwork Si structures using various methods, including spray pyrolysis, chemical vapor deposition, sol-gel technology, and laser deposition, has been extensively studied and published by various academics.
However, since some of these procedures involve a costly setup and preparation environment, research has shifted to newer innovative synthesis techniques. A unique approach for the economical and environmentally friendly synthesis of Si structures in ambient settings has been demonstrated.
The direct method of ultrafast pulsed nanostructure formation provided a simple one-step process to create Si structures with predetermined optical characteristics.
Changes in the characteristics of the 3D nanolattice Si structures synthesized through the direct formation of ultrafast pulsed nanostructures were reported as a function of laser parameters such as power and frequency.
The amount of energy used for the ablation and the power of the laser had a significant impact on the formation of the nanostructures. Finally, a plasma layer was formed due to the interaction of the laser with the material and the absorption of energy from the substrate.
The interaction between the laser and the material was controlled with ultrashort laser pulses, which also helped reduce damage to the material. The ultrashort pulses aided in the disintegration of the surface into fine particles deposited on the sample.
The power and frequency parameters were varied to determine the effects on the duration and energy of the laser pulse.
Synthesize the improved Si nanostructures
Twenty-micrometer-thick n-type silicon wafers (Si-100) were used for the experiment. To synthesize the nanostructures, silicon was irradiated with a pulsed fiber laser with a pulse duration of 150 ps and a scanning speed of 100 mm/s by applying the direct ultrafast pulsed nanostructure formation technique.
By varying the power and frequency of the laser (1200, 1000, 800 and 600 kHz), four samples were generated, which were named sets “S” and “A”. The “A” samples (constant power) had fluctuating frequency and constant power at 15 W.
The “S” samples, with constant pulse energy, had fluctuating frequency and variable power at 10, 13.3, 16.7, and 20 W with continuous energy (110 mJ).
The experimental setup for the direct ultrafast pulsed nanostructure formation technique consisted of a sample holder, a spectrometer, a light source, and optical cables. Optical tests were performed on the samples using light spectroscopy, and reflectance spectra were obtained and examined. SpectraSuite software was used to record and examine the reflectance curve and data.
Samples were first examined by SEMl; sample S4 contained more 3D nanonetwork structures. In the region where the direct ultrafast pulsed nanostructure formation approach was used, samples S1 to S3 had higher brightness.
A similar trend was also present in samples A. A1 to A4 showed that the colors of the samples became lighter and brighter. When examined with the unaided human eye, the excised area of samples S2 and S3 was comparable to sample A4.
In general, the samples showed an increase in their band gap after applying the direct technique of ultrafast pulsed nanostructure formation due to the formation of more nanofibers that improved the band gap of the material.
The number of overlaps between different energy levels in the material’s band structure decreased as the number of nanoparticles produced increased, increasing the material’s band gap.
The optical conductivity of the samples was also evaluated using the reflectance information collected by light spectroscopy. The relationship between the band gap of the material and the refractive index affects the optical conductivity. Consequently, the optical conductivity decreased as the band gap of the samples increased after applying the direct ultrafast pulsed nanostructure formation technique.
New approach improves the optoelectric characteristics of silicon
In this paper, a novel approach of employing the direct technique of ultrafast pulsed nanostructure formation was proposed to generate 3D nanonetwork Si structures with enhanced features.
Various 3D nanolattice silicon structures were formed by changing laser parameters, including power and frequency. Spectroscopic methods were used to determine the characteristics of the Si structures of the 3D nanolattice. Finally, the bandgap of the material was enhanced and the variation in pulse duration and pulse energy was used to measure the bandgap change.
The relationship between the bandgap and the refractive index is crucial in determining the other optical characteristics of the material. It was observed that as the band gap expanded, the refractive index decreased. Other optical parameters were also analyzed and it was found that as the dielectric constant of the samples increased, their optical conductivity decreased.
The critical field of decay was also calculated and a similar critical field for gallium arsenide (GaAs) was obtained for the nanostructure samples.
The findings of this work contribute to possible ways to improve the optoelectric characteristics of semiconductors using the simple, fast, direct and ultrafast pulsed nanostructure formation technique.
NS Jamwal, A. Kiani, 2022. Synthesis of 3D Nanonetwork Si Structures via the Direct Ultrafast Pulsed Nanostructure Formation Technique. energies. https://www.mdpi.com/1996-1073/15/16/6005