Study of Si–Ge Interdiffusion in Laser Recrystallization of Ge Epitaxial Film on Si Substrate

Direct epitaxial growth of germanium (Ge) film on silicon (Si) substrate (GOSS) holds great potential in micro-electronics and optoelectronics. However, due to the 4.2% lattice mismatch between Si and Ge, it is difficult to directly obtain high quality Ge by epitaxy on Si substrate. Laser recrystallization technology provides a simple, efficient and low-cost way to improve the crystal quality of epitaxial Ge film grown on Si substrate. This technology is essentially a process of thermally induced phase transformation. By controlling the laser process parameters, epitaxial film of a certain thickness is melted, so that lattice rearrangement and recrystallization are achieved, and high-quality thin Ge/Si can be prepared. Laser recrystallization is a high temperature thermal process, and Si–Ge interdiffusion may detrimentally occur. In this paper, the mechanism of Si–Ge interdiffusion is discussed. Based on Fick's law of diffusion, a numerical model for Si–Ge interdiffusion of GOSS is established. On this basis, the process simulation of thermal annealing and laser recrystallization Si–Ge interdiffusion is carried out by Sentaurus Process simulation. The results show that compared with the traditional thermal annealing, the Si–Ge interdiffusion of Ge on Si almost does not occur in the process of laser recrystallization. By reasonably controlling the process parameters of laser recrystallization, the thin Ge film near the Si–Ge interface does not melt, which can not only improve the crystal quality of Ge epitaxial layer, but also effectively avoid the Si–Ge interdiffusion in the process of laser recrystallization. Through this research, we have aimed at predicting and control the Si–Ge interdiffusion, providing an important technical reference for the preparation of high quality GOSS by laser recrystallization technology.

DETERMINATION OF THE STRUCTURAL STATE AND STABILITY OF THE LASER CRYSTALLIZED Cd1-xМnxTe CRYSTAL SURFACE

The modified surface layers of the Cd1-xMnxTe crystals were obtained by the laser recrystallization of the crystal surface with the use of millisecond and nanosecond ruby ​​lasers. The determination and diagnostics of the layer structural state were performed by the study the electron channeling patterns in the SEM. The AFM studies showed that mechanically stable contact regions in the CdTe crystal – Cu film system can be formed, depending on the laser energy density and beam defocusing. On the base of the ellipsometric studies, it was found that while irradiating the Cd1-xMnxTe crystal surface, the refractive index of the oxide film on the modified surface changes depending on the laser beam energy density, which can be interpreted as the formation of the oxides of the different chemical composition.

Optimization of Laser Recrystallization Process for GeSn Films on Si Substrates Based on Finite Difference Time Domain and Finite Element Method

GeSn alloy on Si substrate has the advantages of high carrier mobility, high radiation recombination efficiency, compatibility with the Si process, and is widely used in the field of semiconductor optoelectronics. However, due to the high lattice mismatch between the GeSn epitaxial layer and the Si substrate, how to prepare a perfect GeSn film on the Si substrate is an issue. The 808 nm continuous wave laser recrystallization technology can significantly improve the quality of the GeSn alloy epitaxial layer by melting and recrystallization, which provide another technical way for solving this problem. Optimized laser recrystallization related process parameters is necessary when laser recrystallization technology is used to prepare high quality GeSn alloy on Si substrate. For this purpose, the absorption, reflection and transmission models of GeSn alloy epitaxial layer/Si substrate system irradiated by 808 nm continuous wave laser are established using finite difference time domain software FDTD Solutions. The thickness-related process parameters of GeSn alloy epitaxial layer and SiO2 capping layer are optimized. In addition, the temperature distribution model of 808 nm continuous wave laser irradiation of GeSn alloy epitaxial layer on Si substrate system is obtained by COMSOL Multiphysics simulation. The process parameters related to laser recrystallization temperature are optimized and listed, which can be used as important technical references for the growth of low defect density GeSn layer on Si substrate assisted by the laser recrystallization technology.

Thermophysics Simulation of Laser Recrystallization of High-Ge-Content SiGe on Si Substrate

The high-Ge-content SiGe material on the Si substrate can be applied not only to electronic devices but also to optical devices and is one of the focuses of research and development in the field. However, due to the 4.2% lattice mismatch between Si and Ge, the epitaxial growth of the high-Ge-content SiGe epitaxial layer directly on the Si substrate has a high defect density, which will seriously affect the subsequent device performance. Laser recrystallization technique is a fast and low-cost method to effectively reduce threading dislocation density (TDD) in epitaxial high-Ge-content SiGe films on Si. In this paper, by means of finite element numerical simulation, a 808 nm laser recrystallization thermal physics model of a high-Ge-content SiGe film (for example, Si0.2Ge0.8) on a Si substrate was established (temperature distribution physical model of Si0.2Ge0.8 epitaxial layer under different laser power, Si0.2Ge0.8 epitaxial layer thickness, and initial temperature). The results of this paper can provide important technical support for the preparation of high-quality high-Ge-content SiGe epilayers on Si substrates by laser recrystallization.

Deposition of Al-Doped Zinc Oxide by Direct Pulsed Laser Recrystallization at Room Temperature on Various Substrates for Solar Cell Applications

In this study, a method combining room temperature pulsed laser deposition (PLD) and direct pulsed laser recrystallization (DPLR) are introduced to deposit superior transparent conductive oxide (TCO) layer on low melting point flexible substrates. As an indispensable component of thin film solar cell, TCO layer with a higher quality will improve the overall performance of solar cells. Alumina-doped zinc oxide (AZO), as one of the most promising TCO candidates, has now been widely used in solar cells. However, to achieve optimal electrical and optical properties of AZO on low melting point flexible substrate is challenging. Recently developed direct pulsed laser recrystallization (DPLR) technique is a scalable, economic and fast process for point defects elimination and recrystallization at room temperature. It features selective processing by only heating up the TCO thin film and preserve the underlying substrate at low temperature. In this study, 250 nm AZO thin film is pre-deposited by pulsed laser deposition (PLD) on flexible and rigid substrates. Then DPLR is introduced to achieve a uniform TCO layer on low melting point flexible substrates, i.e. commercialized Kapton polyimide film and micron-thick Al-foil. Both finite element analysis (FEA) simulation and designed experiments are carried out to demonstrate that DPLR is promising in manufacturing high quality AZO layers without any damage to the underlying flexible substrates. Under appropriate experiment conditions, such as 248 nm in laser wavelength, 25 ns in laser pulse duration, 15 laser pulses at laser fluence of 25 mJ/cm2, desired temperature would result in the AZO thin film and activate the grain growth and recrystallization. Besides laser conditions, the thermal conductivity and crystallinity of the substrate serve as additional factors in the DPLR process. It is found that the substrate’s thermal conductivity correlates positively with the AZO crystal size; the substrate’s crystallinity correlates positively with the AZO film’s crystallinity. The thermal expansion of substrate would also contribute to the film tensile stress after processed by DPLR technique. The overall results indicate that DPLR technique is useful and scalable for flexible solar cell manufacturing.