Real Time Spectroellipsometry Research Articles

Optical depth profiling of band gap engineered interfaces in amorphous silicon solar cells at monolayer resolution

Real time spectroellipsometry and a two-layer optical analysis have been applied to obtain alloy composition (x) and optical gap (Eg) depth profiles with ∼3 Å resolution and sensitivities better than ±0.01 in x and ±0.02 eV in Eg for graded amorphous semiconductor alloy thin films prepared by plasma-enhanced chemical vapor deposition. Graded amorphous silicon–carbon alloy (a-Si1−xCx:H) layers incorporated at the i–p interfaces of a-Si:H n-i-p solar cells have been studied using these methods, and the layer characteristics have been related to improvements in solar cell performance.

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.

Analysis of the growth processes of plasma-enhanced chemical vapor deposited diamond films from CO/H2 and CH4/H2 mixtures using real-time spectroellipsometry

Real-time spectroscopic ellipsometry (RTSE) has been applied to study the growth of nanocrystalline diamond thin films by microwave plasma-enhanced chemical vapor deposition on silicon substrates. The goal of this research is to characterize the diamond film growth process as a function of the gas source and substrate temperature, comparing the results obtained using various mixtures of CO and H2 with those obtained from a standard mixture of CH4 highly diluted in H2. The capabilities of RTSE have been exploited to establish the true near-surface substrate temperature under the specific diamond film growth conditions, as well as the deposition rates for a succession of films prepared on the same substrate under different conditions. The latter capability allows large regions of parameter space to be scanned expeditiously. As a result of this study, a low-temperature growth process has been identified that yields high deposition rates (up to 2.5 μm/h) at relatively low microwave plasma powers (0.5 kW). In contrast to the commonly-used H2-rich mixtures of CH4 or CO and H2 that exhibit monotonic reductions in the growth rate with decreasing substrate temperature from 800 to 400 °C, CO-rich mixtures of CO and H2 exhibit an increase and a well-defined maximum as the temperature is reduced over this range. At a CO/H2 gas flow ratio of 18, for example, the growth rate peaks near 450 °C and is a factor of ∼20 higher than that obtained with the standard H2-rich mixtures of CH4/H2 and CO/H2. These observations suggest a different diamond growth mechanism from the CO-rich mixtures of CO/H2 with potentially important applications for low-temperature substrate materials.

Real time spectroellipsometry for optimization of diamond film growth by microwave plasma-enhanced chemical vapor deposition from CO/H2 mixtures

Real time spectroellipsometry has been applied to determine the deposition rate and thickness evolution of the nondiamond (sp2-bonded) carbon volume fraction in very thin (&amp;lt;1000 Å), but fully coalesced, nanocrystalline diamond films prepared on Si substrates by microwave plasma-enhanced chemical vapor deposition from gas mixtures of CO and H2. At a substrate temperature of ∼800 °C, high quality diamond films can be obtained over two orders of magnitude in the CO/H2 gas flow ratio, from 0.04, the lowest value explored, to ∼5. A well-defined minimum in the sp2 C volume fraction (0.03 in a 600 Å film) is observed for a CO/H2 ratio of 0.2, corresponding to the C–H–O diamond-growth phase-diagram coordinate XH/Σ=[H]/{[H]+[C]} of 0.9. Under these conditions, the deposition rate increases with increasing temperature over the range of ∼400–800 °C with an activation energy of 8 kcal/mol, behavior identical to that observed for diamond film growth from a CH4/H2 ratio of 0.01. This observation shows that the dominant film precursors in the diamond growth process from CO/H2=0.2 are hydrocarbons whose flux at the growing film surface is controlled through the reaction of excited CO with H or H2 in the plasma. A broad subsidiary minimum in the sp2 C content is observed, centered near a CO/H2 ratio of 2, corresponding to an XH/Σ value of ∼0.5. Under these gas flow conditions, the deposition rate is a complicated function of temperature, exhibiting a peak near 550 °C. This peak shifts to lower temperature with further increases in the CO/H2 ratio above 2, suggesting a nonhydrocarbon precursor and a different growth mechanism for diamond prepared at high CO/H2 ratio and low temperature.

Correlation of real time spectroellipsometry and atomic force microscopy measurements of surface roughness on amorphous semiconductor thin films

We have correlated the results of real time spectroellipsometry (SE) and ex situ atomic force microscopy (AFM) measurements of surface roughness on amorphous semiconductor thin films. Roughness layer thicknesses deduced from real time SE, using a conventional approach based on the Bruggeman effective medium theory, closely obey a relationship of the form: ds(SE)≊1.5 drms(AFM)+4 Å, for 10≤dS(SE)&amp;lt;100 Å, where drms(AFM) is the root-mean-square roughness from AFM. The slope and intercept of this relationship provide insights into the origin and interpretation of the optically deduced roughness layer thicknesses.

Real time spectroellipsometry characterization of optical gap profiles in compositionally-graded semiconductor structures: Applications to bandgap engineering in amorphous silicon-carbon alloy solar cells

We have applied a real time spectroellipsometry data analysis procedure developed previously [S. Kim and R. W. Collins, Appl. Phys. Lett. 67, 3010 (1995)] to characterize depth profiles in the optical gap for compositionally-graded semiconductor alloy thin films prepared by plasma enhanced chemical vapor deposition. The analysis procedure employs a two-layer (four-medium) optical model consisting of the ambient, a thin surface roughness layer and outer-layer (5–15 Å) whose properties are to be determined, and a pseudo-substrate that contains the past history of the graded-layer deposition. The ellipsometric spectra (2.3–4.0 eV) are analyzed to provide, not only the depth-profile of the optical gap and alloy composition for the graded layer, but also the instantaneous deposition rate and the surface roughness layer thickness versus time or accumulated layer thickness. To apply the previous analysis approach, it was necessary to (i) parameterize the dielectric function of the alloys as a continuous function of composition over the desired alloy range and (ii) express the optical gap as an accurate function of alloy composition. As an example, we have applied the extended analysis to obtain the depth-profile of the optical gap and alloy composition with &amp;lt;15 Å resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si1−xCx:H) film prepared by continuously varying the gas flow ratio z=[CH4]/{[CH4]+[SiH4]}. In order to demonstrate the technological importance of such structures, the graded layer has been incorporated at the p/i interface of widegap a-Si1−xCx:H p-i-n solar cells, and improvements in open-circuit voltage have been observed.

Nucleation and Growth of μc-Si:H n- and p-Type Layers in a-Si:H p-i-n AND n-i-p Solar Cells: Real Time Spectroellpsometry Studies

AbstractWe have applied real time spectroellipsometry (RTSE) to study the growth of microcrystalline silicon n- and p-layers [μc-Si:H:(P,B)] incorporated into amorphous silicon (a-Si:H) p-i-n and n-i-p solar cells, respectively. In previous research, we have applied RTSE to characterize a-Si:H solar cells having only amorphous component layers. The μc-Si:H(P,B) component layers, however, pose a more difficult RTSE analysis problem for two reasons. First, the near-surface of the underlying i-layer is modified in the μc-Si:H:(P,B) growth process, and second, the microstructural evolution near the i/(n,p) interfaces is very complicated. From RTSE spectra (1.5 &lt; hv &lt; 4 eV) collected every ∼4–15 s during growth, we have extracted the time evolution of the μc-Si:H:(P,B) layer microstructure, thicknesses, and optical properties along with the modifications that the near-surface i-layer properties undergo in the formation of the i/(n,p) interfaces. We suggest that the beneficial optical properties of the microcrystalline layers may be due to size effects in the crystallites that make up the films.

Advances in the Characterization of Compositionally-graded Layers in Amorphous Semiconductor Solar Cells by Real Time Spectroellipsometry

AbstractWe have developed a real time spectroellipsometry data analysis procedure that allows us to characterize compositionally- graded amorphous semiconductor alloy thin films prepared by plasma-enhanced chemical vapor deposition (PECVD). As an example, we have applied the analysis to obtain the depth-profile of the optical gap and alloy composition with ≤15 Å resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si1−xCx:H) film prepared by continuously varying the gas flow ratio z=[CH4]/{[CH4]+[SiH4]} in the PECVD process. The graded layer has been incorporated at the p/i interface of widegap a-Si1−xCx:H (x∼0.05) p-i-n solar cells, and consistent improvements in open-circuit voltage have been demonstrated. The importance of the graded-layer characterization is the ability to relate improvements in device performance directly to the physical properties of the interface layer, rather to the deposition parameters with which they were prepared.

Structural Evolution of Top-Junction a-Si:C:H:B and Mixed-Phase (Microcrystalline Si)-(a-Sil-xCx:H) p-Layers in a-Si:H n-i-p Solar Cells

AbstractWe have applied real time spectroellipsometry (RTSE) to study hydrogenated amorphous silicon (a-Si:H) solar cells fabricated in the Cr/n-i-p configuration using plasma-enhanced chemical vapor deposition (PECVD) in a single-chamber system. The microstructural evolution of the n-, i-, and p-layers of the devices has been determined, including the thicknesses of the bulk, interface, and surface roughness layers versus time. The optical properties of the individual layers, including the dielectric functions and optical gaps, have also been obtained in the same analysis. In this study, we have focused on i/p interface formation and, in particular, on the nucleation process for differently-prepared a-Si:C:H and mixed-phase μc-Si:H/a-Si1-xCx:H p-layers on the a-Si:H i-layer. From the thickness dependence of the p-layer void volume fraction, we can obtain an estimate of the thickness at which nuclei make contact to form a continuous film. For the mixed-phase p-layers, the nuclei contact thickness can be reduced by exposing the i-layer to a H2-plasma prior to p-layer deposition. We have found that for similarly-prepared p-layers this reduction in contact thickness leads to an increase in open-circuit voltage of the solar cell

Real time spectroellipsometry study of the evolution of bonding in diamond thin films during nucleation and growth.

Real time spectroellipsometry (RTSE) has been performed during the nucleation and growth of nanocrystalline thins films prepared by plasma-enhanced chemical vapor deposition onto Si substrates. RTSE shows that a significant volume fraction of nondiamond (or ${\mathrm{sp}}^{2}$-bonded) carbon forms during thin film coalescence and is trapped near the substrate interface between $\ensuremath{\sim}300\AA{}$ diamond nuclei. Although this behavior is observed over a range of C${\mathrm{H}}_{4}$/${\mathrm{H}}_{2}$/${\mathrm{O}}_{2}$ gas compositions, the interfacial ${\mathrm{sp}}^{2}$-bonded carbon volume can be minimized by operating just within the growth-etch boundary of the diamond growth phase diagram.

Optical properties of ultrathin crystalline and amorphous silicon films.

The complex dielectric functions $\left(2&lt;h\ensuremath{\nu}&lt;4\mathrm{eV}\right)$ of ultrathin crystalline silicon $\left(c\ensuremath{-}\mathrm{Si}\right)$ and amorphous silicon $\left(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}\right)$ films consisting of isolated clusters have been measured at 250 \ifmmode^\circ\else\textdegree\fi{}C by real time spectroellipsometry. For $c\ensuremath{-}\mathrm{Si}$ cluster films $\ensuremath{\sim}12\AA{}$ thick, a well-defined absorption onset near 3 eV is observed that blueshifts with decreasing thickness, consistent with quantum confinement of electrons. A much broader absorption onset is observed for $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ cluster films of similar thickness and is attributed to an electron mean free path less than the cluster size, which limits the observation of confinement effects.

Preparation of ultrathin microcrystalline silicon layers by atomic hydrogen etching of amorphous silicon and end-point detection by real time spectroellipsometry

The etching of hydrogenated amorphous silicon (a-Si:H) in thermally generated atomic hydrogen has been investigated in detail, utilizing real time spectroellipsometry for characterization and end-point detection. When properly controlled, etching can yield ultrathin microcrystalline Si (μc-Si:H) films of relatively high density on virtually any substrate material. These films are unique in that their microstructure is established by the crystallization of the near-surface a-Si:H, rather than by the nucleation of crystallites on the substrate, as occurs for plasma-enhanced chemical vapor-deposited μc-Si:H films.

Real time spectroellipsometry study of the interaction of hydrogen with ZnO during ZnO/a-Si1−xCxH interface formation

Using real time spectroellipsometry (SE), we have studied the interfacial interactions that occur when i- and p-type hydrogenated amorphous silicon-carbon alloys (a-Si1−xCx:H) are deposited from hydride-containing plasmas onto transparent, conducting films of ZnO. The SE spectra collected during the nucleation of a-Si1−xCx:H onto ZnO reveal a widening of the near-interface optical gap of ZnO by ∼0.1 eV, an effect attributed to the penetration of atomic H from the plasma. The SE data, along with ex situ secondary ion mass spectrometry, reveal that the H diffuses into ZnO to depths ≳200 Å. The defects that result from H incorporation in ZnO (e.g., O vacancies) lead to a shift in the near-interface Fermi level higher into the ZnO conduction band and to an estimated enhancement in the electron concentration by ∼1020 cm−3.

Real Time Ellipsometry Study of the Interfaces Formation of Microcrystalline Silicon

ABSTRACTA detailed study by real time spectroellipsometry of the early stage of the growth of microcrystalline silicon (μc-Si) on various substrates is presented. In practical applications μc-Si films are generally deposited on hydrogenated amorphous silicon (a-Si:H) or on transparent conducting oxides (TCO). In the case of a-Si:H, the beginning of the deposition is described by an uniform growth of a 60 Å thick porous μc-Si. However a nucleation-coalescence mechanism is observed during the growth of the first 60–70 Å thick μc-Si on smooth crystalline silicon (c-Si) substrate. In contrast a strong chemical reduction of the TCO substrates is observed during the first 10–20s of the exposure to the hydrogen diluted plasma. Then the ellipsometric measurements reveal an induction period of 30–60 s for the nucleation of microcrystallites. The influence of the preparation conditions of μc-Si on the TCO / μc-Si interfaces formation is emphasized.