Investigation of hydrogen induced phase transition from a-Si:H to μc-Si:H using real time infrared spectroscopy

In this paper, we present the use of real time process monitors to study hydrogen induced phase transition from hydrogenated amorphous silicon (a-Si:H) to hydrogenated microcrystalline silicon (μc-Si:H) films. We determine the H bonding configurations and H concentration in the film using a real time, high sensitivity infrared reflectance spectroscopy. Spectroscopic ellipsometry is used to determine whether the film is amorphous or crystalline, and to measure the film thickness. The Si-H bonding and film microstructure are monitored in three sets of experiments: During deposition of μc-Si-H using DC reactive magnetron sputtering; exposure of unhydrogenated amorphous silicon (a-Si) to atomic hydrogen; exposure of a-Si to hydrogen plasma. In the first experiment, a transition layer of a-Si:H is deposited before μc-Si:H growth starts. Results show no evidence of enhanced etching during μc-Si:H growth compared to a-Si:H deposition under same processing conditions. In the second experiment, hydrogen coverage of the a-Si:H surface drops at higher temperatures due to thermal desorption. Lastly, when exposed to hydrogen plasma a-Si film is first hydrogenated, and then it transforms to microcrystalline phase with a concomitant decrease in H content. Based on these results, we conclude that the role of H is to assist in the nucleation of μc-Si phase and provide enough surface coverage to facilitate the diffusion of surface species. We also suggest that introduction of hydrogen treatment (using atomic hydrogen/hydrogen plasma) during the initial growth of γ-Si:H on glass or oxide substrates will be able to eliminate the a-Si:H transition layer. Role of the real time process monitors will be crucial in developing such processes.