PHASE DIAGRAMS FOR GUIDING SILICON THIN FILM DEPOSITION IN PHOTOVOLTAICS APPLICATIONS AS DERIVED BY REAL TIME SPECTROSCOPIC ELLIPSOMETRY

Open Access
Author:
Ferlauto, Andre Santarosa
Graduate Program:
Materials
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 15, 2001
Committee Members:
  • Robert W Collins, Committee Chair
  • Christopher Roman Wronski, Committee Chair
  • Russell Messier, Committee Member
  • Thomas Nelson Jackson, Committee Member
Keywords:
  • microcrystalline silicon
  • solar cells
  • thin film
  • microstructure
  • ellipsometry
  • amorphous silicon
  • surface roughness
  • optical properties
Abstract:
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.&nbsp; 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.&nbsp; In the deposition phase diagrams, transitions lines are drawn that identify the bulk layer thicknesses (d<sub>b</sub>) separating different film growth regimes as a function of one key deposition variable.&nbsp; 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+&micro;c)]; and (iii) an onset of surface smoothening associated with the coalescence of the microcrystals, leading to a single-phase microcrystalline Si:H (&micro;c-Si:H) growth regime [(a+&micro;c) --> &micro;c]. <br><br>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.&nbsp; Using such insights, the deposition phase diagrams were applied in studies of the effect of H<sub>2</sub>-dilution and substrate on Si:H layer deposition.&nbsp; 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 H<sub>2</sub>-dilution level R=[H<sub>2</sub>]/[SiH<sub>4</sub>] while avoiding the amorphous-to-(mixed-phase microcrystalline) transition [a --> (a+&micro;c)].&nbsp; 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 &Aring;). <br><br>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.&nbsp; 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+&micro;c) transition to lower R.&nbsp; In contrast, increases in the total gas pressure lead to a shift of the a --> (a+&micro;c) transition to much larger R values.&nbsp; As a result, a large window opens in R, whereby the films are amorphous and exhibit smooth, stable surfaces up to relatively large d<sub>b</sub> values (d<sub>b</sub>>2000 &Aring;).&nbsp; These results suggest that the total gas pressure, together with the H<sub>2</sub>-dilution can be used in the optimization of Si:H PECVD processes at higher rates. <br><br>A database for the optical properties of the different materials used in multijunction a-Si:H-based solar cells was also established.&nbsp; 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.&nbsp; In particular, new analytical expressions have been developed for the optical functions of amorphous semiconductor absorber layers and doped microcrystalline layers.&nbsp; 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.