MICROSTRUCTURAL STUDIES OF NONCRYSTALLINE SOLID SILICON FILMS BY TO-LIKE MODES OF RAMAN-SPECTRA

Several microstructural parameters can be obtained from the HWHM (half width at half maximum) GAMMA of the TO-like mode and the peak positions in the Raman spectra of non-crystalline silicon materials (including multilayer and single thick films). These are the rms bond angle variation DELTA-theta = (GAMMA - 7.5)/3 (deg) [1], the average distortion energy per bond (u) of the interface, and the stress near the interface (sigma), in a-Si:H layers. The grain size of the mu-c-Si films can be also obtained from DELTA-d approximately 2-pi(B/DELTA-omega(TO))1/2[2]. The distortion energy is given by U = 3K(r-DELTA-theta-BAR)2, where K and r are the force constant of Si (9.2 x 10(3) dyn/cm2) and the bond length of Si (2.4 x 10(-8) cm) [3], respectively. Sigma is equal to n x DELTA-U, n is the total number of distorted Si-Si bonds per unit area near the interface, and DELTA-U is the average excess distortion energy. The microstructural parameters of thick single mu-c-Si:B films and different composition-modulated superlattices are compared when the following quantities are changed: (1) d(s) (thickness of the a-Si:H layer), (2) R (the NH3/SiH4 ratio). The microstructures can well be understood after proper theoretical models are adopted, i.e., using the Peierls-Nabarro model, it can be explained that the stress near the interface is inversely proportional to the thickness of the a-Si:H layer (d(s)) within the range of a few monolayers. The Pantelides model can be used to explain the overcoordination of nitrogen in the a-Si:H layer near the interface.