| Amorphous silicon was widely used in optical systems due to the advantages of high thermo-optic coefficient, low cost and being integrated easily. In recent years, the design of low-loss and high-performance optical devices based on the thermo-optic effect of amorphous silicon has become the focus of research in the field of semiconductor optics, such as thermo-optic tunable thin film filter, thermo-optical switches, and infrared detectors. High thermo-optic coefficient of the thin film was required when it was used in thermo-optic effect based devices. However, when it was used in other situations, the changes of the refractive index of amorphous silicon with temperature may induce system instability and performance degradation of devices. Thus, relatively lower thermo-optic coefficient was required to avoid these kind thermal induced effects. The characteristics of amorphous silicon vary a lot with different deposition conditions, so it is of great practical significance to research the correlation between preparation conditions and the thermo-optic effect of amorphous silicon.Firstly, influence factors of thermo-optic properties of amorphous silicon were analyzed theoretically. Polarizability was concluded as the main factor and positive correlated with thermo-optic coefficient. The model was established on the basis of thin film components, which is called amorphous silicon H cluster model. Polarizability was calculated to research the influence of the amount of H and different silicon-hydrogen groups to thermo-optic coefficient at the wavelength 770 nm, 1330 nm and 1550 nm with changing the amount of silicon-hydrogen groups in a single cell. It was found that thermo-optic properties of amorphous silicon were very sensitive to H content. Increasing the H content appropriately by increasing the ratio of SiH2 and SiH3 clusters could enhance the thermo-optic effect of amorphous silicon. On the contrary, too much H would degrade the performance of thin film and weaken the thermo-optic effect.Secondly, to research the correlation between thermo-optic properties and deposition conditions, two technical issues were necessarily to be solved: the preparation of homogeneous thin film and the measurement of thermo-optic coefficient of thin film accurately. To solve the first problem, the reaction chamber was redesigned by optimizing its structure on the aspects of the improvements of airflow filed homogeneity, temperature filed homogeneity and electromagnetic filed homogeneity. It was observed that the homogeneity of amorphous silicon thin film improved a lot due to the results measured by 3D profiler. To solve the second problem, new method of the measurement of thermo-optic coefficient of thin film was created by building the test platform based on FILMeasure-20 fiber optical spectrometer after the summarization of domestic and foreign methods of measurement.At last, the test platform was used to research the thermo-optic coefficient at the wavelength 1330 nm of amorphous silicon thin film deposited in different conditions: different radio-frequency power, reaction pressure and annealing temperature. The test platform was proved to be reliable due to the high consistence between measured results and theoretical value. And the results were listed below: First, relatively higher radio-frequency power, proper reaction pressure and short time annealing at the temperature of 500℃ could increase the thermo-optic coefficient of amorphous silicon, particularly, the 500℃ short time annealing could make the increasement up to 63.5%. Second, relatively lower radio-frequency power, higher reaction pressure and annealing at temperatures above 800℃ could reduce the thermo-optic coefficient of amorphous silicon. What’s more, the 900℃ annealing could make the thermo-optic coefficient drop down to the magnitude of 10-5 K-1. In a real world application, appropriate parameters of PECVD should be selected for different use. |
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