R.J. Koval

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300°C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous growth regime to the mixed-phase (amorphous+microcrystalline) growth regime versus accumulated film thickness [the a→(a+μc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic layers by RF plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using the maximum possible flow ratio of H2 to SiH4 that can be sustained while avoiding the a→(a+μc) transition. More recent studies have suggested that a similar strategy is appropriate for optimization of p-type Si:H thin films. The simple phase diagrams can be extended to include in addition the thickness at which a roughening transition is detected in the amorphous film growth regime. It is proposed that optimization of a-Si:H in higher rate RF PECVD processes further requires the maximum possible thickness onset for this roughening transition.

Dependence of open-circuit voltage in hydrogenated protocrystalline silicon solar cells on carrier recombination in p/i interface and bulk regions

Contribution of carrier recombination from the p/i interface regions and the bulk to the dark current–voltage (JD–V) and short-circuit current–open-circuit voltage (Jsc–Voc) characteristics of hydrogenated amorphous-silicon (a-Si:H) p–i–n and n–i–p solar cells have been separated, identified, and quantified. Results are presented and discussed here which show that a maximum 1 sun Voc for a given bulk material can be validly extrapolated from bulk dominated Jsc–Voc characteristics at low illumination intensities.

Performance and stability of Si:H p–i–n solar cells with i layers prepared at the thickness-dependent amorphous-to-microcrystalline phase boundary

Systematic studies have been carried out on the transition from the amorphous to the microcrystalline phase in intrinsic Si:H as a function of the accumulated film thickness and the effect of this transition on p–i–n solar cell performance [J. Koh, Y. Lee, H. Fujiwara, C. R. Wronski, and R. W. Collins, Appl. Phys. Lett. 73, 1526 (1998)]. Guided by a deposition phase diagram obtained from real-time spectroscopic ellipsometry, cell structures having i layers deposited with different H2-dilution levels and thicknesses were investigated. For these structures, the fill factors are controlled by the bulk i layers. From the systematic changes in the fill factors, specifically their initial and degraded steady-state values and their degradation kinetics, the effects of the transition from the amorphous to the microcrystalline phase within the Si:H layers are identified, and insights are obtained into the properties of these structurally graded materials.

Extended phase diagrams for guiding plasma-enhanced chemical vapor deposition of silicon thin films for photovoltaics applications

Real time spectroscopic ellipsometry has been applied to develop extended phase diagrams that can guide the deposition of hydrogenated silicon (Si:H) thin films for highest performance solar cells. Previous such studies have shown that optimization of amorphous Si:H intrinsic layers by rf plasma-enhanced chemical vapor deposition (PECVD) is achieved using the maximum possible H2 dilution of SiH4 while avoiding a transition to the mixed-phase (amorphous+microcrystalline) growth regime. In this study, we propose that optimization of amorphous Si:H in higher rate rf PECVD processes further requires the largest possible thickness onset for a surface roughening transition detected in the amorphous film growth regime.

Light Induced Defect Creation Kinetics in Thin Film Protocrystalline Silicon Materials and Their Solar Cells

Using real time spectroscopic ellipsometry to characterize the microstructure and evolutionary growth of Si:H materials deposited with and without hydrogen dilution, phase diagrams were developed which clearly defined and established growth in the protocrystalline regime. Guided by these phase diagrams thin films and intrinsic layers in p-i-n cell structures were grown which consist solely of the protocrystalline phase so that the bulk uniform properties of the material could be characterized with confidence. Studies were carried out on the light induced changes in these films and cell structures that include the annealing out of defects as well as their creation under 1 sun illumination at temperatures from 25°C to 100°C that include the attainment of a degraded steady states (DSS). Defect states were characterized in films with electron mobility lifetimes (μτ), and subgap absorption at 1.2eV (α(1.2)); and in the i material of the p-i-n cells by the bulk limited fill factor (FF). The contributions of the different gap states to SWE are identified and characterized. The absence of direct correlations between α(1.2) with μτ and FF present in undiluted and diluted materials also found in protocrystalline Si:H. Similarities, on the other hand, are found between the μτ products and the FFs including the striking changes in the kinetics that occur at ∼40°C. Direct correlations between the changes in μτ and FF at different temperatures are presented. The reason for this correlation and lack of it for α(1.2) are briefly discussed with direct correlation of the α(1.2) to cell characteristics being presented, be it not with the FF.

Maximization of the open circuit voltage for hydrogenated amorphous silicon n-i-p solar cells by incorporation of protocrystalline silicon p-type layers

In studies of hydrogenated amorphous silicon (a-Si:H) n–i–p solar cells fabricated by rf plasma-enhanced chemical vapor deposition (PECVD), we have found that the maximum open circuit voltage (Voc) is obtained by incorporating p-type doped Si:H layers that are protocrystalline in nature. Specifically, these optimum p layers are prepared by PECVD in the a-Si:H growth regime using the maximum hydrogen-to-silane flow ratio possible without crossing the thickness-dependent transition into the mixed-phase (amorphous+microcrystalline) growth regime for the ∼200 A p-layer thickness. The strong dependence of the p-layer phase and solar cell Voc on the underlying i-layer phase also confirms the protocrystalline nature of the optimum Si:H p layer.

Real time analysis of amorphous and microcrystalline silicon film growth by multichannel ellipsometry

Real time spectroscopic ellipsometry (SE) has been applied to obtain insights into the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) thin films by plasma-enhanced chemical vapor deposition as a function of the H 2 -dilution gas flow ratio R = [H 2 ]/[SiH 4 ], the accumulated film thickness d b , and the substrate material. For depositions with 15 ≤ R ≤ 80 on clean amorphous semiconductor surfaces, for example, initial film growth occurs in a predominantly amorphous phase, as deduced from analyses of the real time SE data. However, after an accumulated thickness ranging from 3000 A for R = 15 to 30 A for R = 80, a roughening transition is observed in the SE analysis results as the Si film begins to develop a predominantly microcrystalline structure. We have identified this roughening transition as an amorphous-to-microcrystalline phase boundary in the deposition parameter space of d b and R. The thickness at which this boundary occurs decreases continuously with increasing R, and the position of the boundary is strongly substrate dependent. Based on these real time SE studies and detailed device analyses, we have found that the highest performance p-i-n solar cells are obtained in i-layer deposition processes maintained at the highest possible R value versus thickness without crossing the deposition phase boundary into the microcrystalline regime.

Real time spectroscopic ellipsometry studies of the nucleation and growth of p-type microcrystalline silicon films on amorphous silicon using B2H6, B(CH3)3 and BF3 dopant source gases

Real time spectroscopic ellipsometry (RTSE) has been applied to study the nucleation, coalescence, and growth processes for ∼100–200 A thick microcrystalline silicon (μc-Si:H) p-layers prepared by radio frequency (rf) plasma-enhanced chemical vapor deposition at 200 °C on amorphous silicon (a-Si:H) i-layers in the substrate/(n-i-p) device configuration. Analysis of the RTSE data provides the bulk p-layer dielectric function (2.5–4.3 eV), whose amplitude and shape characterize the void and crystalline Si contents in the p-layer. Among the parameters varied in this study of the deposition processes include the underlying a-Si:H i-layer surface treatment, the p-layer H2-dilution flow ratio, the p-layer dopant source gas and flow ratio, and the p-layer rf plasma power flux. Here we emphasize the differences among p-layer deposition processes using diborane, B2H6, trimethyl boron, B(CH3)3, and boron trifluoride, BF3, dopant source gases. We find that it is easiest to nucleate μc-Si:H p-layers immediately on th...

Absence of carrier recombination associated with the defect pool model in intrinsic amorphous silicon layers: Evidence from current–voltage characteristics on p–i–n and n–i–p solar cells

Forward bias current‐voltage characteristics ( JD ‐V) were studied for both p ‐i ‐n ~superstrate! and n ‐i ‐p ~substrate! (a-SiC:H p)/(a-Si:H i) solar-cell structures having different p/i interface layers and different thickness i-layers. Contributions of the p/i interfaces to the J D ‐V characteristics were separated, and the dependence on the thickness of the i-layers was established. Equivalence was observed in a comparison of the characteristics of p ‐i ‐n and n ‐i ‐p cells. The various JD ‐V characteristics are found to be consistent with uniform densities of defects in the i-layers, and thus inconsistent with the spatially varying large densities of defects predicted for solar-cell structures by the defect pool model.

Evolutionary phase diagrams for the deposition of silicon films from hydrogen-diluted silane

Abstract Phase diagrams that describe plasma-enhanced chemical vapor deposition (PECVD) of Si films at low substrate temperature (200°C) have been established using real time spectroscopic ellipsometry (RTSE) as a probe of film microstructural evolution and optical properties. These deposition phase diagrams describe the regimes over which predominantly amorphous and microcrystalline Si phases are obtained as a function of the accumulated film thickness and the hydrogen-to-silane gas flow ratio, R=[H2]/[SiH4]. The diagrams for different substrate materials demonstrate how general principles can be formulated and verified for design of optimized multistep i-layer components of amorphous silicon solar cells.