Phase diagrams for guiding silicon thin film deposition in photovoltaics applications as derived by real time spectroscopic ellipsometry

Real time spectroscopic ellipsometry measurements of the evolution of the microstructural and optical properties of hydrogenated silicon (Si:H) films during growth have been applied to develop deposition phase diagrams. These diagrams provide guidance for the optimization of rf plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon (a-Si:H) films for applications in high performance, high stability solar cells. In the deposition phase diagrams, transitions lines are drawn that identify the bulk layer thicknesses (db) separating different film growth regimes as a function of one key deposition variable. The identified transitions include (i) an onset of surface roughening from a stable-surface growth regime such that the Si:H film is amorphous on both sides of the onset [a → a]; (ii) an onset of surface roughening associated with the nucleation of Si microcrystals, leading to a mixed-phase growth regime [a → (a + muc)]; and (iii) an onset of surface smoothening associated with the coalescence of the microcrystals, leading to a single-phase microcrystalline Si:H (muc-Si:H) growth regime [(a + muc) → muc].; RTSE measurements of several Si:H film depositions, complemented by atomic force microscopy (AFM) images were employed in investigations of the physical mechanisms underlying such transitions. Using such insights, the deposition phase diagrams were applied in studies of the effect of H2-dilution and substrate on Si:H layer deposition. Comparisons of the phase diagrams and solar cell performance results have indicated that optimum rf PECVD of a-Si:H intrinsic layers (i-layers) is performed in the amorphous growth regime with the maximum possible H2-dilution level R = [H2]/[SiH 4] while avoiding the amorphous-to-(mixed-phase microcrystalline) transition [a → (a + muc)]. Furthermore, optimization requires the largest possible thickness onset for the roughening transition detected in the amorphous regime [a → a], thus ensuring film growth with a smooth, stable surface throughout deposition of a relatively thick layer (>1000 A).; The phase diagrams were also applied in investigations of the effects of the rf PECVD parameters on Si:H film growth in order to obtain insights into i-layer deposition processes at high rates. The phase diagram results indicate that increases in rf plasma power lead to detrimental effects on film growth, and that a moderate increase in substrate temperature exerts only a weak reversal of the effects of high power due in part to a shift of the a → (a + muc) transition to lower R. In contrast, increases in the total gas pressure lead to a shift of the a → (a + muc) transition to much larger R values. As a result, a large window opens in R, whereby the films are amorphous and exhibit smooth, stable surfaces up to relatively large db values (db > 2000 A). These results suggest that the total gas pressure, together with the H2-dilution can be used in the optimization of Si:H PECVD processes at higher rates.; A database for the optical properties of the different materials used in multijunction a-Si:H-based solar cells was also established. In most cases, the optical functions of the different materials were described in terms of simple analytical expressions based on a few physically-relevant, wavelength-independent parameters. In particular, new analytical expressions have been developed for the optical functions of amorphous semiconductor absorber layers and doped microcrystalline layers. It was shown that for a set of high electronic quality thin films, including intrinsic a-Si:H and its alloys with Ge and C, the optical properties throughout the visible range can be described in terms of a single parameter, the optical band-gap.