P. I. Rovira

Rotating-compensator multichannel ellipsometry: Applications for real time Stokes vector spectroscopy of thin film growth

A multichannel spectroscopic ellipsometer based on the rotating-compensator principle was developed and applied to measure the time evolution of spectra (1.5–4.0 eV) in the normalized Stokes vector of the light beam reflected from the surface of a growing film. With this instrument, a time resolution of 32 ms for full spectra is possible. Several advantages of the rotating-compensator multichannel ellipsometer design over the simpler rotating-polarizer design are demonstrated here. These include the ability to: (i) determine the sign of the p-s wave phase-shift difference Δ, (ii) obtain accurate Δ values for low ellipticity polarization states, and (iii) deduce spectra in the degree of polarization of the light beam reflected from the sample. We have demonstrated the use of the latter spectra to characterize instrument errors such as stray light inside the spectrograph attached to the multichannel detector. The degree of polarization of the reflected beam has also been applied to characterize the time evo...

Evolutionary phase diagrams for plasma-enhanced chemical vapor deposition of silicon thin films from hydrogen-diluted silane

Real-time optical studies have been applied to develop phase diagrams that characterize plasma-enhanced chemical vapor deposition (PECVD) of silicon thin films at low temperature (200 °C). The deposition phase diagrams describe regimes over which predominantly amorphous and microcrystalline Si phases are obtained as a function of the accumulated thickness and the hydrogen-to-silane gas flow ratio R=[H2]/[SiH4] in the PECVD process. The diagrams for different substrates provide insights into optimization of amorphous Si materials and solar cells.

Transmission ellipsometry of a thin-film helicoidal bianisotropic medium

Experimental evidence is introduced for circularly birefringent and dichroic behavior in a MgF2 thin-film helicoidal bianisotropic medium (TFHBM), which is a rotationally inhomogeneous and anisotropic material. Optical rotation (OR) of an incident linearly polarized monochromatic plane wave is observed upon transmission through the MgF2/glass TFHBM/substrate system, and it is accompanied by an ellipticity in the polarization of the transmitted plane wave. Both OR and ellipticity spectra have remarkable features within a narrow wavelength zone not unlike the Cotton effect in isotropic chiral media. The features in the experimental spectra are in accord with theoretical predictions.

Alignment and calibration of the MgF 2 biplate compensator for applications in rotating-compensator multichannel ellipsometry

Biplate compensators made from MgF2 are being used increasingly in rotating-element single-channel and multichannel ellipsometers. For the measurement of accurate ellipsometric spectra, the compensator must be carefully (i) aligned internally to ensure that the fast axes of the two plates are perpendicular and (ii) calibrated to determine the phase retardance delta versus photon energy E. We present alignment and calibration procedures for multichannel ellipsometer configurations with special attention directed to the precision, accuracy, and reproducibility in the determination of delta (E). Run-to-run variations in external compensator alignment, i.e., alignment with respect to the incident beam, can lead to irreproducibilities in delta of approximately 0.2 degrees . Errors in the ellipsometric measurement of a sample can be minimized by calibrating with an external compensator alignment that matches as closely as possible that used in the measurement.

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.

Rotating-compensator multichannel transmission ellipsometry of a thin-film helicoidal bianisotropic medium

Abstract We have employed a novel multichannel transmission ellipsometer based on the rotating-compensator principle to characterize a MgF 2 thin film helicoidal bianisotropic medium (HBM) deposited on a glass substrate. A HBM is a rotationally inhomogeneous anisotropic material obtained in thin film form by deposition at a glancing angle onto a rotating substrate to yield helical columns or helices with (i) an alignment perpendicular to the substrate surface, (ii) an in-plane scale of the order of nanometres, and (iii) a helix pitch of the order of the wavelength of visible light. The rotating-compensator multichannel ellipsometer provides a complete description of the optical response of this material to an incident linearly polarized plane wave. Evidence is introduced for circularly birefringent and dichroic behavior based on the observed rotation and ellipticity imparted to a linearly polarized monochromatic plane wave when it is transmitted at normal incidence through the ambient/HBM/substrate system. The spectra in both the optical rotation and ellipticity exhibit strong features in a narrow wavelength zone (430–450 nm) not unlike the Cotton effect in isotropic chiral media. The features in the experimental spectra are found to be in accord with theoretical predictions, using values for the pitch of the helices and their packing density estimated from scanning electron microscopy and gravimetry, respectively.

Rotating-compensator multichannel ellipsometry for characterization of the evolution of nonuniformities in diamond thin-film growth

A multichannel spectroscopic ellipsometer based on the rotating-compensator principle has been applied to obtain the evolution of spectra (1.5–4.0 eV) in the normalized Stokes vector of the light beam reflected from the surface of a nanocrystalline diamond film during growth. Spectra in the ellipsometry angles (ψ, Δ) provide the time evolution of the microstructure and optical properties of the film in thin layers, whereas the spectra in the degree of polarization provide the time evolution of nonuniformities in the growth process attributed to light scattering by diamond nuclei in the initial stage of growth and to thickness gradients over the probed area in thicker layers.

Optics of textured amorphous silicon surfaces

We have performed real time Stokes vector spectroscopy (also called polarimetry) during the preparation of hydrogenated amorphous silicon (a-Si:H) p‐i‐n solar cells on textured tin-oxide (SnO2) surfaces. With this spectroscopy (1.5‐4.0 eV), the irradiance, Ir, and polarization parameters,fOQ; vU; pg, for the specularly reflected beam are obtained with 0.8 s resolution versus time during solar cell preparation. Here Q and v are the tilt and ellipticity angles of the polarization state, respectively, and p is the degree of polarization. From an analysis of these spectra, we establish a model for textured a-Si:H intrinsic-layer growth that includes both microscopic and macroscopic structural components. Using such a model, the evolution of structural and optical properties of the solar cell components can be determined, and the results provide realistic inputs for optical modeling of solar cells. ” 2000 Elsevier Science B.V. All rights reserved.

Real time optics of p-type silicon deposition on specular and textured ZnO surfaces

Using insights from real time spectroscopic ellipsometry (RTSE) studies, we have developed an evolutionary phase diagram that describes rf plasma-enhanced chemical vapor deposition (PECVD) of p-type silicon (Si:H) films on specular zinc-oxide (ZnO)-coated glass substrates at 200/spl deg/C. Using this diagram, optimized conditions have been identified for the deposition of 200-/spl Aring/-thick, single-phase, p-type microcrystalline Si:H (/spl mu/c-Si:H) on glass/(specular ZnO). These conditions have also been applied for p-layer incorporation into an amorphous Si:H (a-Si:H)-based p-i-n solar cell fabricated on a glass/(textured SnO/sub 2/)/ZnO bilayer substrate. We find that the microstructural evolution, phase, and optical properties are similar for the optimized 200-/spl Aring/-thick /spl mu/c-Si:H p-layers prepared on the specular and textured ZnO. These results demonstrate that the presence of macroscopic substrate texturing does not necessitate process reoptimization for /spl mu/c-Si:H film growth on ZnO.