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 μc-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.