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We critically examine how two processing steps commonly applied in the preparation of cadmium telluride (CdTe)-based solar cells, i.e., the cadmium chloride treatment and the subsequent stepwise bromine/methanol wet etching process, impact the structural and optical properties of polycrystalline CdTe thin-films. In particular, drawing upon a conjuncture of photothermal deflection spectroscopy and spectroscopic ellipsometry experimental results, we determine the spectral dependence of the optical absorption coefficient, α(E), over the photon energy range from 1.1 to 2.0 eV for samples of rf sputtered (RFS) and close space sublimation (CSS) CdTe. The impact of these processing steps on shaping the grazing incidence x-ray diffraction pattern is also examined. We extend the analysis to devices through interpretation of the spectral dependence of the external quantum efficiency associated with two cadmium chloride treated CdTe-based solar cells. The cells are comparably prepared with the exception of the absorber, one by RFS and the other by CSS. Through the use of our results for the thin-film CdTe optical functions and a model for the solar cell multilayer structure, we simulate the resultant external quantum efficiency spectrum. Through a critical contrast with the corresponding solar cell acquired experimental results, we glean insights into the carrier trapping and recombination processes that occur within the two types of CdTe absorbers. [ABSTRACT FROM AUTHOR]

Both the origins of the high open circuit voltages (VOC) in amorphous silicon solar cells having p layers prepared with very high hydrogen dilution and the physical structure of these optimum p layers remain poorly understood topics, with several studies offering conflicting views. This work attempts to overcome the limitations of previous studies by combining insights available from electronic measurements, real time spectroscopic ellipsometry, atomic force microscopy, and both high-resolution transmission electron microscopy (TEM) and dark field TEM of cross sections of entire solar cells. It is found that solar cells fabricated with p layers having a low volume fraction of nanocrystals embedded in a protocrystalline Si:H matrix possess lower recombination at the i/p interface than standard cells and deliver a higher VOC. The growth of the p layers follows a thickness evolution in which pure protocrystalline character is observed at the interface to the i layer. However, a low density of nanocrystallites nucleates with increasing thickness. The advantages offered by the protocrystalline character associated with the amorphous phase of the mixed-phase (amorphous+nanocrystalline) p layers prepared with excess H2 dilution account for the improved VOC of the optimum p layers. In this model, the appearance of a low volume fraction of nanocrystals near the top transparent conductor interface is proposed to be incidental to the high VOC. [ABSTRACT FROM AUTHOR]

We have developed a Kramers-Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (∈[sub 1], ∈[sub 2]) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon-germanium (a-Si[sub 1-χ]Ge[sub χ] :H) and silicon-carbon (a-Si[sub 1-χ]C[sub χ] :H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (∈1, ∈[sub 2]) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where a increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (∈[sub 1], ∈[sub 2]) can be simulated assuming a single Lorentz oscillator. The expression developed here provides an improved description of ∈[sub 2] (or α) in the below-band gap and near-band gap regions compared with previous approaches. Although the expression is more complicated analytically, it has numerous applications in the analysis and simulation of thin film a-Si:H based p-i-n and n-i-p multilayer photovoltaic devices. First, we describe an approach whereby, from a single accessible measure of the optical band gap, the optical functions can be generated over the full solar spectrum for a sample set consisting of the highest quality intrinsic a-Si:H based alloys prepared by plasma-enhanced chemical vapor deposition using the principle of maximal H[sub 2] dilution. Second, we describe quantitatively how such an approach can be modified for sample sets consisting of lower quality... [ABSTRACT FROM AUTHOR]

Presents information on a study which measured the unnormalized Stokes vector by using a rotating-compensator multichannel ellipsometer. Polarization state of light reflected from specular and textured SnO2: F films; Experimental details; Results and discussions.

Presents a study which reported the results of ellipsometry measurements performed during the growth of hydrogenated amorphous silicon thin films on crystalline silicon substrates. Formation of low-density microstructure in the initial stages of growth; Effect of sputtering on the glow-discharge decomposition of gases; Discussion on the pseudodielectric function trajectories.

Presents information on a study which extended in situ ellipsometry characterization of the nucleation and microstrucutral evolution of amorphous silicon (a-Si:H) to the effect of deposition parameters on these processes. Details about the structural evolution of a-Si:H; Important aspects of the ellipsometer/glow-discharge reactor system; General features of in situ trajectories.

Presents a study which examined the effect of substrate microstructure on the growth habit of high-quality hydrogenated amorphous silicon. Experimental details; Results and discussion; Conclusion.

Presents information on a study that examined the growth and microstructure of α-Si:H/α-SiN [subx] :H heterojunctions. Experimental details; Results and discussion on the study; Conclusions.

Presents information on a study which analyzed the in situ ellipsometry measurements taken during the deposition of hydrogenated amorphous silicon on crystalline silicon substrates in a convention rf glow-discharge apparatus. Nucleation behavior of hydrogenated amorphous silicon; Experimental details; Results and discussion; Conclusions.

Examines the optical properties of ion-beam-sputtered silicon and germanium using spectroscopic ellipsometry. Discrepancies between the structure of the p-silicon deduced from Raman and ellipsometry spectra; Significance of thin-film polycrystalline silicon; Psuedodielectric functions for ion-beam-deposited silicon prepared at different substrate temperatures.

Analyzes the structure of a hydrogenated amorphous silicon based n-1 two-layer system using spectroscopic ellipsometry measurements. Details on the experiment; Data analysis; Results of the study.

Investigates the growth rate and optical properties of thin nanocrystalline diamond films prepared over a range of the carbon monoxide(CO)/H[sub2] ratio, using real time spectroellipsometry (RTSE). Comparison between diamond growth and properties from CO/H[sub2] and CH[sub4]/H[sub2]/O[sub2] at similar phase-diagram points; Advantage of RTSE over Raman spectroscopy for characterizing and optimizing nanocrystalline diamond film; Significance of methyl radical for diamond film growth.

Discusses a study which applied a real time spectroellipsometry data analysis procedure to characterize depth profiles in the optical gap for semiconductor alloy thin films prepared by plasma enhanced chemical vapor deposition. Theoretical background; Description of the experimental setup; Findings.

Presents information on a study which investigated the stress in diamond films prepared by microwave plasma chemical vapor deposition as a function of methane concentration and deposition temperature. Calculation of thermal stress; Dependence of the total and intrinsic stress on methane concentration; Stress dependence on deposition temperature; Origin of tensile intrinsic stress.

Probes the evolution of surface roughness over a wide range of film thickness during the growth of glow discharge hydrogenated amorphous silicon (a-Si:H) using in situ ellipsometry. Differences in the resulting film properties; Factor to which the columnar morphology observed for SiH[sub4]:argon films is attributed; Description of the pseudodielectric function.

Reports on the application of real time spectroscopic ellipsometry to characterize the nucleation of diamond on c-silicon by tungsten filament-assisted chemical vapor deposition (CVD). Stages of diamond growth by filament-assisted CVD; Preparation of the diamond; Information on the structural model and parameters deduced in a linear regression analysis of the final spectra.

In situ and ex situ characterization methods have been applied to investigate the properties of (Ag,Cu)InSe2 (ACIS) thin films. Data acquired from real time spectroscopic ellipsometry (RTSE) experiments were analyzed to extract the evolution of the nucleating, bulk, and surface roughness layer thicknesses. The evolution of these layer thicknesses suggests a transition from Volmer-Weber to Stranski-Krastanov type behavior when Cu is replaced by Ag. The complex dielectric functions of ACIS at both deposition and room temperature as a function of film composition were also extracted from the RTSE data, enabling parameterization of the alloy optical properties. [ABSTRACT FROM AUTHOR]

Silver nanoparticle films were deposited by sputtering at room temperature and were annealed while monitoring by real time spectroscopic ellipsometry (SE). The nanoparticle dielectric functions (0.75 eV-6.5 eV) obtained by SE were modeled using Lorentz and generalized oscillators for the nanoparticle plasmon polariton (NPP) and interband transitions, respectively. The nanoparticle melting point could be identified from variations in the oscillator parameters during annealing, and this identification was further confirmed after cooling through significant, irreversible changes in these parameters relative to the as-deposited film. The variation in melting point with physical thickness, and thus average nanoparticle diameter, as measured by SE enables calculation of the surface energy density. [ABSTRACT FROM AUTHOR]

Engineered thin films consisting of periodic arrays of silicon microcrystallites in a hydrogenated amorphous silicon host matrix have been prepared by plasma-enhanced chemical vapor deposition where the hydrogen dilution of silane is modulated in multiple cycles. These types of films have been guided by a phase evolution diagram, depicting the deposition conditions and film thickness at which the material exhibits amorphous, microcrystalline, or mixed-phase (amorphous+microcrystalline) characteristics, developed for intrinsic Si:H prepared with varying H2 dilution on unhydrogenated a-Si:H. Real time spectroscopic ellipsometry (RTSE) has been used in situ to noninvasively determine the phase evolution of the resulting hydrogenated mixed-phase (amorphous+microcrystalline) silicon thin films and corroborated with dark-field transmission electron microscopy. Such tailored microstructures are of growing interest as components of thin film photovoltaic devices, and RTSE is shown to be a key technique for structure verification. [ABSTRACT FROM AUTHOR]

A multichannel ellipsometer in the dual rotating-compensator configuration has been developed for applications in real time Mueller matrix spectroscopy of anisotropic surfaces and thin films. The sequence of optical elements for this instrument configuration can be denoted PC[sub 1r](5ω)SC[sub 2r](3ω)A, where P, S, and A represent the polarizer, sample, and analyzer. In this sequence, C[sub 1r] and C[sub 2r] represent the first and second compensators, rotating with angular frequencies of 5ω and 3ω, respectively, where ω=π/T[sub C] is the base angular frequency (corresponding to 2 Hz) and T[sub C]=0.25 s is the fundamental optical period. The instrument can provide 170 point spectra over the wavelength range from 235 nm (5.3 eV) to 735 nm (1.7 eV) in all 16 elements of the unnormalized Mueller matrix with minimum acquisition and repetition times of T[sub C]=0.25 s. In this study, instrumentation calibration procedures are demonstrated in the transmission geometry without a sample, including (i) alignment of the two MgF[sub 2] zero-order biplate compensators, (ii) determination of the retardance and phase spectra for the compensators, (iii) determination of the offset angles for the optical elements, and (iv) characterization of the spectral response function of the ellipsometer. Calibration procedure (iv) allows the (1,1) element of the transmission Mueller matrix to be determined; thus the unnormalized Mueller matrix can be obtained. The dual rotating-compensator multichannel ellipsometer is assessed with respect to its performance in transmission without a sample, and is then applied in the transmission geometry to study anisotropy, depolarization, and light scattering effects for a MgF[sub 2] helicoidally sculptured thin film on a glass substrate. Numerous future applications of this instrument are anticipated for real time analysis of complex surfaces and thin films in the reflection geometry as well. [ABSTRACT FROM AUTHOR]

A unique rapid-scanning ellipsometer employing a rotating polarizer optical configuration and a multichannel detector for a 1.5–4.5 eV spectral range, has been developed recently for real time studies of film growth and surfaces. This is a new application of the photodiode array-based optical multichannel detector that entails waveform analysis of the incident irradiance at each photon energy. For accurate ellipsometric spectra {ψ(hν),Δ(hν)}, the raw data in the form of photon counts, integrated over four or more equal sectors of polarizer rotation, must be corrected for systematic errors originating from the detection system. Simple procedures are described to characterize and correct for the most significant errors including detection system nonlinearity, image persistence, and scattered stray light in the spectrograph/detector enclosure. [ABSTRACT FROM AUTHOR]

In a typical reflection ellipsometry experiment, one characterizes the polarization state change that a polarized light beam undergoes upon reflection from a specular surface. This measurement provides ρ≡rp/rs, where rp and rs are the complex amplitude reflection coefficients of the surface for p- and s-polarized waves. Nearly 15 years have passed since the development of automatic ellipsometers along with the detailed calibration, error analysis, and data reduction procedures to be used with them. More specifically, these powerful instruments permit (1) determination of bulk dielectric functions and nondestructive depth profiling of static multilayered materials through measurements as a function of photon energy and (2) characterization of dynamic surfaces in adverse environments through measurements as a function of time at fixed photon energy. In the 15 intervening years, the major research thrusts in ellipsometry have been the exploitation of these instruments in materials and process characterization in diverse areas of materials science and electrochemistry. One of the simplest and most popular automatic ellipsometers is a photometric instrument in which one or more of the optical elements (polarizer, compensator, and/or analyzer) is designed to rotate continuously. In this article, the major developments in rotating element ellipsometry are reviewed since the first report of an automatic rotating analyzer instrument by Cahan and Spanier in 1969. The topics covered include rotating element configurations, calibration, characterization of errors and imperfections, absolute accuracy, data reduction, precision, and a review of the real-time applications accessible to these instruments. The sections on calibration, errors, and data reduction include new material and are written from a practical standpoint to assist in instrumentation setup and troubleshooting. The development and improvement of next-generation rotating element instruments that... [ABSTRACT FROM AUTHOR]

Develops a multichannel spectroscopic ellipsometer based on the rotation-compensator principle. Application of thin film growth for real time Stoke vector spectroscopy; Advantage of the ellipsometer design over the rotating-polarizer design; Characterization of the time evolution of light scattering.

An expansion of the capabilities of high-speed, multichanneI spectroscopic ellipsometry (SE) is described that involves simultaneous measurement of the reflectance spectrum along with the two spectra in the ellipsometric angles (ψ, Δ). previously, a novel rotating-polarizer spectroscopic ellipsometer has been perfected that employs a photodiode array detector for high-speed acquisition of (ψ, Δ) spectra, designed for real-time studies of thin-film growth. For a polarizer angular rotation frequency of ω[sub 0], the (ψ, Δ) values at a given photon energy are deduced from the 2ω[sub 0] Fourier components of the detector irradiance, normalized by the dc component. A third parameter, the weighted reflectance R[sub A], can be obtained from the dc component and from a calibration based on the known optical properties of the substrate measured prior to film growth, With (ω[sub 0]/2π) = 12.5 Hz, three-parameter data sets, [ψ(hυ), Δ (h υ), R[sub A](h &upsilonl); 1.5 ≤ h υ ≤ 4.5 eV], can be acquired with a time ' resolution as short as 40 ms. Although R[sub A] provides complementary information to (ψ, Δ), it has yet to be exploited in conjunction with real-time SE until this study. A resilient analysis approach, based on mathematical inversion and least-squares fitting of the real-time three-parameter data sets, is designed to yield the film dielectric functions and thicknesses independently at each time during the early stages of thin-film growth. The three-parameter approach has been. applied successfully in studies of amorphous silicon (a-Si:H) thin films PrePared by plasma-enhanced chemical vapor deposition, and aluminum and silver films prepared by physical vapor deposition. For the a-Si:H films, R[sub A](h ν) exhibits deviations as large as ∼3% from that predicted in modeling (ψ, Δ) alone. The deviations result from light scattering by plasma... [ABSTRACT FROM AUTHOR]

Submonolayer sensitivity to thin-film nucleation and growth in real time on the millisecond scale has been achieved with a unique rotating polarizer multichannel ellipsometer. Continuous spectra in the ellipsometry angles {ψ(hv),Δ(hv)} consisting of ∼50 points from hv=1.5 to 4.3 eV have been obtained with acquisition and repetition times as short as 16 and 32 ms, respectively. As an example of the instrument capabilities, we present results for hydrogenated amorphous silicon (a-Si:H) growth on c-Si by plasma-enhanced chemical vapor deposition at a rate of 400 Å/min. In this example, the acquisition and repetition times are both 64 ms, and at this speed a precision in (ψ,Δ) of ∼0.02–0.03° is obtained under optimum conditions. We observe a-Si:H nucleation in the first 2 s of deposition, and detect relaxation of nucleation-induced surface roughness with submonolayer sensitivity. [ABSTRACT FROM AUTHOR]

In situ spectroscopic ellipsometry was applied to measure the dielectric functions &eh; of polycrystalline CdTe thin films prepared by magnetron sputtering. The observable critical point (CP) energies determined for the films in best fits to d2&eh;/dE2 are shifted relative to those for single crystal CdTe. The energy shift of the critical point designated as En is attributed to in-plane stress X with linear stress coefficient CX(En). By analyzing these shifts, it was found that stress (1) builds with thickness in as-deposited films, (2) relaxes with increasing substrate temperature at fixed thickness, and (3) is relaxed uniformly in CdCl2 treated films. [ABSTRACT FROM AUTHOR]

The dielectric function of Ag nanoparticle films, deduced from an analysis of in situ real-time spectroscopic ellipsometry (RTSE) measurements, is found to evolve with time during deposition in close consistency with the film structure, deduced in the same RTSE analysis. In the nucleation regime, the intraband dielectric function component is absent and plasmon polariton behavior dominates. Only at nuclei contact, does the intraband amplitude appear, increasing above zero. Both intraband and plasmon amplitudes coexist during surface smoothening associated with coalescence. The intraband relaxation time increases rapidly after surface smoothening is complete, also in consistency with the thin film structural evolution. [ABSTRACT FROM AUTHOR]

The dielectric functions [variant_greek_epsilon] of polycrystalline CdS and CdTe thin films sputter deposited onto Si wafers were measured from 0.75 to 6.5 eV by in situ spectroscopic ellipsometry. Differences in [variant_greek_epsilon] due to processing variations are well understood using an excited carrier scattering model. For each sample, a carrier mean free path λ is defined that is found to be inversely proportional to the broadening of each of the band structure critical points (CPs) deduced from [variant_greek_epsilon]. The rate at which broadening occurs with λ-1 is different for each CP, enabling a carrier group speed υg to be identified for the CP. With the database for υg, [variant_greek_epsilon] can be analyzed to evaluate the quality of materials used in CdS/CdTe photovoltaic heterojunctions. [ABSTRACT FROM AUTHOR]

Real-time spectroscopic ellipsometry (RTSE) is shown to be an effective contactless probe of radio frequency magnetron sputtered molybdenum thin films used as the back electrode in chalcopyrite [Cu(In,Ga)Se2] solar cells. A series of Mo thin films was sputtered onto soda-lime glass substrates at Ar pressures ranging from 4 to 20 mTorr. RTSE measurements reveal how Ar pressure affects the nucleation and growth mechanisms that influence the films’ ultimate grain structure and properties. Determinations of the free electron relaxation times at optical frequencies reveal that higher pressures lead to a smaller average grain size and increased void volume fraction. [ABSTRACT FROM AUTHOR]

BiFeO3 thin films have been deposited on (001) SrTiO3 substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO3 films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with ω rocking curve full width at half maximum values as narrow as 29 arc sec (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74 eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO3 films. [ABSTRACT FROM AUTHOR]