No abstract

We measured photoluminescence (PL) and Raman spectra for films deposited by hot-wire chemical vapor deposition using various hydrogen to silane ratios. We observed: (a) a PL peak energy increase from 1.25 to 1.4 eV as the material approaches the a- to μc-Si transition region; (b) a dual-PL peak at 1.3 and 1.0 eV for the film with a H dilution ratio of 3; and (c) as the H ratio increases, the 1.3 eV PL fades away and the low energy PL dominates. Meanwhile, a redshift of the peak position, a decrease of the intensity, and a narrower bandwidth for the low energy PL are also observed. The low energy PL is explained by band-tail radiative transitions from two types of grain boundaries.

The structure changes of thin films of amorphous ͑a͒ to microcrystalline (c) silicon are studied by Raman scattering in terms of three deposition parameters: the silane flow rate, the hydrogen flow rate, and the total gas pressure in hot-wire chemical vapor deposition. The Raman transverse optical ͑TO͒ mode is deconvoluted into two Gaussian functions for a-Si:H and intermediate components and one Lorenzian function for the c-Si component. We found that ͑a͒ in general, the change in structure is a function of the ratio of hydrogen to silane gas flow, R, but also depends on the SiH 4 flow rate and total gas pressure; ͑b͒ there is a narrow structural transition region in which the short-range order of the a-Si:H network improves, i.e., the variation in bond angle of the a-Si network decreases from ϳ10°to ϳ8°once the c-Si grains start to grow; and ͑c͒ when the films were deposited using a high SiH 4 flow rate of 22 sccm, the narrow TO mode with low peak frequency could be related to the column-like structures.

Films were prepared by hot wire chemical vapor deposition at ∼240 °C with varied hydrogen dilution ratios R=H2:SiH4 from 1 to 20. The optical and electronic properties as a function of microcrystallinity were studied. We found: (a) At low H dilution R⩽2, there is no measurable crystallinity by Raman spectroscopy and x-ray diffraction in the a-Si:H matrix, but an optical absorption peak at ∼1.25 eV appears; when R=2, the film shows the lowest subgap absorption, the highest photosensitivity, and the largest optical gap. (b) When R⩾3, the c-Si phase is measurable by Raman and a low-energy photoluminescence (PL) band (0.84–1.0 eV) appears in addition to the high-energy band (1.3–1.4 eV). Meanwhile, all the absorption spectra show a featureless line shape. (c) An energy redshift is observed for both PL peaks as the film grows thicker. Finally, (d) the conductivity activation energy first decreases from 0.68 to 0.12 eV, then increases with increasing microcrystallinity. A mode of two sets of energy bands of electronic states for these two-phase materials is suggested.

No abstract