Study On The Effect Of Hydrogen Content On The Thermo-optic Effect Of Amouphous Silicom Films

The combination of low absorption in the near-infrared and large thermo-optic effects make amorphous silicon uniquely suited for the optoelectronic devices, also the advantages of low production cost and ease of integration make amorphous silicon thin film widely applied in the field of semiconductor. Since some optoelectronic devices’ design is based on the thermo-optic effect of amorphous silicon, and the thermo-optical coefficient of the material is the key factor affecting their optical properties. Parameters like band gap, polarization can characterize the thermo-optic coefficient, and they may vary with the changes of the micro-structure in the material. While the hydrogen content in the film and hydrogen bonding are the primary factors affecting the microstructure of thin films. Therefore, the changes in the hydrogen content of the film affecting on the thermo-optical effect was studied from both the theoretical simulation and process in this paper.Firstly, based on the study of the amorphous structure and basic physical principles of thermo-optical effect, the theory that the changes of hydrogen content of amorphous silicon thin film affects thermo-optical effect was summarized. The expression of amorphous silicon’s thermal-optical coefficient was derived, and the relationship of hydrogen with thermal coefficient was established from the amorphous silicon single-oscillator model. The thermo-optical coefficient under different hydrogen content was simulated and analyzed. The simulation results provide a theoretical basis for the experimental design.Then, in order to test the thermal coefficient of amorphous silicon thin film, a measurement platform based on FILMeasure-20 was designed and built. Some researches and analysis about hydrogen content affecting the thermo-optical effect at 1550 nm were did using the platform, film samples including the thermal annealing amorphous silicon thin film and N-doped amorphous silicon thin film.The amorphous silicon thin film sample was deposited by PECVD equipment and was annealing at different temperatures for a short time. Combined with the microstructure and the bonding methods of the sample it can be concluded that the hydrogen content represented by FTIR absorption intensity peak of the film gradually increased from 100℃ to 300℃, and then decreased from 500℃ to 700℃ compared with the film sample that has not been annealed. While the thermo-optic coefficient of the film also increased first and then decreased.The mixed gas with SiH₄ and NH₃ were passed into PECVD to prepare N-doped amorphous silicon film. Also the effect of changes in the hydrogen content and Si-H bonding methods of the sample on the thermo-optical effect was studied. It was found that when the nitrogen content of the film was low, FTIR absorption peak intensity of the film was high, and when the hydrogen content was larger, the characterization like refractive index and thermo-optical effect of N-doped amorphous silicon thin film would be closer to amorphous silicon. When the film mixed with higher nitrogen content, the Si-N absorption peak intensity would increase and affected the Si-H bonding mode, the refractive index and thermal optical properties of the N-doped amorphous silicon thin film would decreased, which would be more close to the silicon nitride.Through theoretical analysis and experimental results, it can be concluded that the changes of process parameters can effectively change the hydrogen bonded way and hydrogen content of the film, so as to the thermo-optical coefficient, which is of great reference value to the actual production practices in optoelectronic devices.