Yeeheng Lee

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.

OPTIMIZATION OF HYDROGENATED AMORPHOUS SILICON P-I-N SOLAR CELLS WITH TWO-STEP I LAYERS GUIDED BY REAL-TIME SPECTROSCOPIC ELLIPSOMETRY

Hydrogenated amorphous silicon (a-Si:H) p–i–n solar cell performance has been optimized using a two-step i-layer growth process. This effort has been guided by real-time spectroscopic ellipsometry (RTSE) studies of the nucleation and growth of a-Si:H films by plasma-enhanced chemical vapor deposition at 200 °C using a variable H2-dilution gas flow ratio R=[H2]/[SiH4]. RTSE studies during film growth with R>15 reveal a transition from the amorphous to microcrystalline (a→μc) phase at a critical thickness that decreases with increasing R. From such results, the optimum two-step process was designed such that the initial stage of the i layer (∼200 A) is deposited at much higher R than the bulk to ensure that the film remains within the amorphous side of the a→μc phase boundary, yet as close as possible to this boundary at low i-layer thicknesses.

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.

Initial, rapid light‐induced changes in hydrogenated amorphous silicon materials and solar cell structures: The effects of charged defects

Large, rapid light induced changes are reported for photoconductivities, electron mobility‐lifetime products, and forward bias currents in hydrogenated amorphous silicon (a‐Si:H) films and Schottky barrier cell structures. The absence of concurrent changes in subgap absorption and quantum efficiencies are clearly inconsistent with the widely held view that the kinetics of degradation in a‐Si:H materials and cells can be quantified solely in terms of neutral dangling bond defects. The self‐consistent analysis of all the results was carried out for films and Schottky barrier structures by including charged defects and using a three‐Gaussian distribution of donorlike and acceptorlike defect states. Such self‐consistent development of ‘‘operational’’ parameters for these gap states offers a method for reliable and quantitative correlations between solar cell performance and stability with the properties of their bulk materials.

Selfconsistent Analysis of Mobility-Lifetime Products and Subgap Absorption on Different PECVD A-SI:H Films

The photoconductivity and subband gap absorption measurements over a wide range of generation rate(G) have been carried out on diluted and undiluted a-Si:H. It is found that in these high quality films there are significant differences in the functional dependence of mobility-lifetime ({micro}{tau}) products on G. In addition to the different values of subgap absorption ({alpha}) there are also distinct differences in the dependence of {alpha} on photon energy (E) as well as G. It is difficult to self consistently analyze the results on the undiluted film with the previously used three gaussian distribution, particularly at high generation rates. Self consistent analysis is obtained when the (+/0) transitions of negative charged defects and the (0/{minus}) transitions of positive charged defects are introduced respectively closer to the valence and conduction bands. This new gap state distribution is a better representation for the defect pool model and potential fluctuation model.

Real time spectroscopic ellipsometry characterization of structural and thermal equilibration of amorphous silicon–carbon alloy p layers in p-i-n solar cell fabrication

Real time spectroscopic ellipsometry (RTSE) has been applied to investigate the near-surface optical changes that occur during p/i interface processing for hydrogenated amorphous silicon carbon alloy (a-Si1−xCx:H, x≈0.05) p layers prepared at ∼200 °C by plasma-enhanced chemical vapor deposition in the p-i-n solar cell configuration. Trimethylboron [B(CH3)3] was used as the p-type dopant source gas in order to avoid p-layer surface contamination that occurs when using diborane (B2H6). We have analyzed the changes in the RTSE data detected after extinguishing the plasma for a-Si1−xCx:H p-layer deposition while maintaining the p layer near its growth temperature. We have attributed these changes to: (i) structural equilibration characterized by the emission of bonded hydrogen (∼2 at. %) from the p layer into the vacuum, and (ii) thermal equilibration characterized by near-surface temperature variations (∼7 °C) due to gas composition and pressure variations within the reactor. From the RTSE data, the kinetics...

Microstructural evolution of a-Si:H prepared using hydrogen dilution of silane studied by real time spectroellipsometry

We have applied real time spectroellipsometry to measure the nucleation and growth of hydrogenated amorphous silicon (a-Si:H) films prepared by plasma-enhanced chemical vapor deposition from H2-diluted SiH4 on crystalline Si (c-Si) substrates at 200°C. For a H2-dilution ratio R=[H2]/[SiH4] of 10, optimum microstructural evolution is observed during the growth of 0.3 μm a-Si:H film, namely, smoothening during coalescence followed by long-term surface stability. At lower and higher R values, surface roughening in the thick film regime (d>20 nm) is larger, particularly for R≥20 due to crystallite development. Although dilution levels of 20≤R≤30 lead to microcrystallinity in thick films (0.3 μm), the structural evolution and optical properties in thin films (<20 nm) are characteristic of high quality a-Si:H. Thus, real time spectroellipsometry suggests that in the preparation of i-layers for solar cells, R∼10 may be optimum for the bulk i-layer whereas a 10 to 20 nm layer with much larger R may be beneficial at the p/i-interface. These suggestions have been verified in p–i–n cells deposited on specular SnO2-coated glass substrates.

Light-induced changes in hydrogen-diluted a-Si : H materials and solar cells: A new perspective on self-consistent analysis

Abstract We report a study on a-Si: H materials and p(a-SiC : H)/i(a-Si : H)/n(μc-Si) solar cells prepared without and with hydrogen dilution (10 : 1) at substrate temperatures between 240°C and 130°C. In contrast to previously reported studies, the cell characteristics in the annealed state of these ∼4000 A thick cells could be directly correlated with the properties of their corresponding i-layer materials. Also, despite the importance of the p/i interface regions, very similar kinetics of light-induced changes are observed in the cells and the corresponding films. In particular, both cells and films fabricated with hydrogen dilution reach a degraded steady state in less than 100 h of AM 1 illumination, which offers a well-defined “marker” for the direct correlation of their respective light-induced changes. Advantage is also taken of the differences in degradation kinetics between diluted and undiluted materials in fabricating custom-designed cells in which these well-characterized intrinsic materials are incorporated into either the bulk or the p/i interface regions.

Stability of a-Si:H solar cells and corresponding intrinsic materials fabricated using hydrogen diluted silane

We report on a study in which properties of p(a-SiC:H)/i(a-Si:H)/n(/spl mu/c-Si) a-Si:H solar cells and their i-materials prepared with hydrogen dilution are investigated and compared with films and cells prepared without hydrogen dilution. The cells and the corresponding intrinsic films were fabricated in a multi-chamber PECVD system with pure silane (SiH/sub 4/) and silane diluted with hydrogen in the ratio [H/sub 2/]/[SiH/sub 4/]=10. The initial performance of both types of cells (/spl sim/4000 /spl Aring/ thick) fabricated without optical enhancement are quite similar but the diluted cells are significantly more stable. Despite the reported importance of the interface regions in determining their solar cell characteristics, a direct correlation between the degradation of the diluted solar cells and their intrinsic films is observed in this study. Both diluted cells and films reach a steady state of degradation under AM1 illumination within 100 hours. Distinctly different kinetics from the undiluted materials and cells and the ability to reach steady state degradation in less than 100 hours offer a new probe for improving our understanding of the mechanisms limiting cell performance.

Real Time Spectroscopic Ellipsometry Studies of the Solid Phase Crystallization of Amorphous Silicon

We have introduced real time spectroscopic ellipsometry (RTSE) for characterization of the solid phase crystallization (SPC) of intrinsic and n-type amorphous silicon (a-Si:H) thin films. RTSE has several advantages in the study and design of SPC processes for thin film transistor and solar cell fabrication. These include the capability of obtaining (i) calibration data that yield the near surface temperature of the film during processing, (ii) the volume fraction of the crystalline Si component of the film continuously versus time during SPC, and (iii) a measurement of the grain size and quality of the final polycrystalline Si film. For the thin layers studied here (∼150-1000 A), we demonstrate excellent fitting of the SPC dynamics to the Avrami-Johnson-Mehl theory for random nucleation and two-dimensional crystallite growth. For a-Si:H n-layers, the crystallization time over the range from 565 to 645°C appears to be weakly activated with an energy of 0.6 eV.