Multiphoton Absorption Simulation of Sapphire Substrate under the Action of Femtosecond Laser for Larger Density of Pattern-Related Process Windows.

Xintian Cai,Zhiqiang Tian,Changkai Li,Chaoyue Ji,Sheng Liu,Xuan Wang,Cheng Lei

doi:10.3390/mi12121571

Xintian Cai, Zhiqiang Tian + Show 5 more

Open Access

https://doi.org/10.3390/mi12121571

Copy DOI

Abstract

It is essential to develop pattern-related process windows on substrate surface for reducing the dislocation density of wide bandgap semiconductor film growth. For extremely high instantaneous intensity and excellent photon absorption rate, femtosecond lasers are currently being increasingly adopted. However, the mechanism of the femtosecond laser developing pattern-related process windows on the substrate remains to be further revealed. In this paper, a model is established based on the Fokker–Planck equation and the two-temperature model (TTM) equation to simulate the ablation of a sapphire substrate under the action of a femtosecond laser. The transient nonlinear evolutions such as free electron density, absorption coefficient, and electron–lattice temperature are obtained. This paper focuses on simulating the multiphoton absorption of sapphire under femtosecond lasers of different wavelengths. The results show that within the range of 400 to 1030 nm, when the wavelength is large, the number of multiphoton required for ionization is larger, and wider and shallower ablation pits can be obtained. When the wavelength is smaller, the number of multiphoton is smaller, narrower and deeper ablation pits can be obtained. Under the simulation conditions presented in this paper, the minimum ablation pit depth can reach 0.11 μm and the minimum radius can reach 0.6 μm. In the range of 400 to 1030 nm, selecting a laser with a shorter wavelength can achieve pattern-related process windows with a smaller diameter, which is beneficial to increase the density of pattern-related process windows on the substrate surface. The simulation is consistent with existing theories and experimental results, and further reveals the transient nonlinear mechanism of the femtosecond laser developing the pattern-related process windows on the sapphire substrate.

Highlights

The development of microwave and optoelectronic devices has led to stricter requirements for semiconductor materials
When the femtosecond laser acts on the crystal surface, since the electroacoustic coupling time is usually on the order of picoseconds, the photon energy is first transferred to the electrons
The results show that within the action time of hundreds of femtoseconds, the free electron density rose to 4.2 × 1026 m−3, the laser intensity rose to 4 × 107 W·m−2 and dropped to nearly zero, and the absorption coefficient increased to 1.3 × 106 m−1

Summary

Introduction

The development of microwave and optoelectronic devices has led to stricter requirements for semiconductor materials. The third-generation wide-bandgap semiconductors represented by GaN, SiC, and diamond can meet the requirements of harsh environments, and they have received increased attention [1,2,3]. The dislocation density of wide-bandgap semiconductors can be as high as 108 –1010 cm−2 with a significant impact on electronic mobility and other properties, which seriously hinders their application and development [4,5]. It is important to reduce the dislocation density to improve the growth quality of the wide bandgap semiconductor film. Taking GaN as an example, numerous studies have shown that sapphire is one of the suitable substrates for film growing, and lateral epitaxy is the most effective method to reduce its dislocation density. When utilizing lateral epitaxial growth, it is necessary to

Methods

Results

Conclusion