Optical Spectroscopic Ellipsometry Research Articles

Studying the structural, optical spectroscopic ellipsometry and electrical properties of variable-CdS thickness/CdTe for solar cell applications

"The structural and optical properties of CdS window layer at different thicknesses in the range of (100-300 nm) deposited on pre-cleaned glass substrates (CdS/glasses) via the thermal evaporation process are studied in the current work. The structural analysis is done using Rietveld refinement and atomic pressure microscope techniques. The films of CdS/glass showcase a wurtzite behavior. XRD and AFM tests are confirmed that the structural parameters improve as the thickness of the CdS-layer increases. The optical constants (the refractive index n, the extinction coefficient, k and the bandgap energy values are estimated from spectroscopic ellipsometry (SE) via the construction of an optical model. The refractive index of the CdS/glass films increases with the increase of CdS-layer thickness. This, in turn, is due to the rise of the size of the crystal in the thin layers. It is also found that as the thickness of the CdS-layer rises, so does the overall behavior of the extinction coefficient. Additionally, the bandgap energy of the direct transition decreases from 2.45 eV (d=100 nm) to 2.25 eV (d=300 nm). The dark and illumination I-V photovoltaic characteristics of fabricated devices are explored by depositing a p-CdTe thin layer (500 nm) over varying thicknesses of CdS thin films (100- 300 nm) prepared on glass substrates (2 mm) and thus, the Ni/n-CdS/i-AgSe/p-CdTe/Pt heterojunction has been successfully fabricated with an AgSe buffer layer deposited directly on the p-CdTe absorber laryer. The n-CdS window layer is deposited on i-AgSe buffer layer. In dark conditions, based on the dependence of the forward and reverse current-voltage, the essential behavior related to the fabricated diode has been determined. As well, in the illumination case, the open-circuit voltage, the short-circuit current, the fill factor, the power conversion efficiency, (PCE), photoresponsivity, quantum efficiency, dependence of generated photocurrent on the light intensity, dependence of the generated photocurrent on wavelength (λ)for the studied solar cell are computed and discussed."

Observation of the surface layer of lithium metal using in situ spectroscopy

We have investigated the surface of lithium metal using x-ray photoemission spectroscopy and optical spectroscopic ellipsometry. Even if we prepare the surface of lithium metal rigorously by chemical cleaning and mechanical polishing inside a glovebox, both spectroscopic investigations show the existence of a few tens of nanometer-thick surface layers, consisting of lithium oxides and lithium carbonates. When lithium metal is exposed to room air (∼50% moisture), in situ real-time monitoring of optical spectra indicates that the surface layer grows at a rate of approximately 24 nm/min, presumably driven by an interface-controlled process. Our results hint that surface-layer-free lithium metals are formidable to achieve by a simple cleaning/polishing method, suggesting that the initial interface between lithium metal electrodes and solid-state electrolytes in fabricated lithium metal batteries can differ from an ideal lithium/electrolyte contact.

Spectroscopic ellipsometry and morphological characterizations of nanocrystalline Hg1-xMnxO oxide diluted magnetic semiconductor thin films

The structural, optical spectroscopic ellipsometry and morphological properties of nanocrystalline Hg1-xMnxO (0 ≤ x ≤ 0.2) oxide diluted magnetic semiconductor thin films synthesized by electron beam deposition technique have been investigated by means of X-ray diffraction (XRD), spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) techniques. All the deposited films crystallize in the form of a hexagonal crystal structure. The integration between X-ray and atomic force microscopy data shows a very good agreement for the nanosize nature of the deposited films. For the surface roughness, good matching between the results of SE and AFM measurements was observed. In the spectral range 200–1200 nm, the real part (ε1) and imaginary part (ε2) of the dielectric constant of nanocrystalline Hg1-xMnxO films have been extracted from SE measurements. At fixed energy value, the ε1, consequently the refractive index is found to decrease with increasing Mn2+ dopant. In contrary, the energy gap (Eg) of the deposited films determined from the ε2 is found to increase as the Mn2+ concentration increases. The variation of both the ε1 and Eg as a function of Mn2+ concentration is understood based on Lorentz-Lorenz relation and sp-d exchange interaction, respectively. The results reported here show that Mn-doped HgO nanocrystalline films could be employed in the fabrication of nanoscale optical and magneto-optical devices.

Band gap narrowing in ferroelectric KNbO3-Bi(Yb,Me)O3 (Me=Fe or Mn) ceramics

The direct optical bandgap in ferroelectric KNbO3-Bi(Yb,Me)O3 (Me = Fe or Mn) ceramics fabricated by the solid state reaction method varies from 3.2 eV for KNbO3 down to 2.2 eV for 0.95KNbO3-0.05BiYbO3, as revealed by optical spectroscopic ellipsometry. This narrowing of bandgap is accompanied by an apparent increase of the room-temperature relative permittivity from 320 for KNbO3 to 900 for 0.95KNbO3-0.05BiYbO3. All compositions studied exhibit dielectric anomalies associated with structural phase transitions, and their ferroelectric nature is corroborated by the presence of a sharp mixed mode (at ∼190 cm−1) and by a Fano-type resonant dip in their Raman spectra.

Spectral Ellipsometry of Cobalt-Ions Implanted Silicon Surface

Monocrystalline silicon wafers implanted by cobalt ions with energy of 40 keV at a fluence range from 6.6×1012 to 2.5×1017 Co+-ion/cm2 were investigated by optical spectroscopic ellipsometry. By comparison of experimental data with modeling it is shown that the ellipsometric measurements are accurate and reliable method for monitoring of a low-dose ion implantation process.

Temperature dependent spectroscopic ellipsometry of Ag2Se and Ag2S with phase transitions from ionic to superionic conductivity state

AbstractTemperature dependent infrared (IR) and optical spectroscopic ellipsometry methods are used to investigate peculiarities of the transitions from the ionic conductivity to the superionic conductivity states in Ag2S and Ag2Se compounds. The structural phase transitions (SPTs), known as β→α transition in these semiconductors, are investigated also with spectroscopic ellipsometry methods. The electronic band structure of Ag2S at the SPT changes sharply from the monoclinic to the bcc phase, which occurs at T = 453 K in the super ionic conductivity state, where all the interband transitions in optic region, corresponding to the monoclinic structure of β‐phase, disappear. The α‐phase of Ag2S is characterised with new lines in the dielectric functions ε1 and ε2, which arise at 5‐6.5 eV. The interband transitions do not change in the SPT from the orthorhombic to the bcc in Ag2Se, which occurs at T > 406 K. In IR ellipsometry for the degenerated Ag2Se, the free carriers' plasma energy is displaced abruptly from ħω=0.075 eV at T=300 K to ħω=0.125 eV at T=338 K. The same effect is seen in the IR ellipsometry for Ag2S, where negative ε1 appears at T=434‐440 K. These facts indicate a drastic increase of the free carriers concentrations in both of the semiconductors at temperatures before the β→α SPT. In Ag2S the second abrupt shift of the free carriers plasma energy from 0.035eV to 0.1 eV occurs at T=453 K, corresponding to β→ α SPT. The second shift is due to the electronic band structure change of Ag2S. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pulsed laser deposition with simultaneous in situ real-time monitoring of optical spectroscopic ellipsometry and reflection high-energy electron diffraction

We present a pulsed laser deposition system that can monitor growth by simultaneously using in situ optical spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction (RHEED). The RHEED precisely monitors the number of thin-film layers and surface structure during the deposition, and the SE measures the optical spectra of the samples simultaneously. The thin-film thickness information obtained from RHEED facilitates the SE modeling process, which allows extracting the in situ optical spectra, i.e., the dielectric functions of thin-films during growth. The in situ dielectric functions contain indispensable information about the electronic structure of thin-films. We demonstrate the performance of this system by growing LaMnO3+δ (LMO) thin-films on SrTiO3 (001) substrates. By using in situ SE and RHEED simultaneously, we show that real-time thickness and dielectric functions of the LMO thin-films can be effectively extracted. The simultaneous monitoring of both optical SE and RHEED offers important clues to understand the growth mechanism of atomic-scale thin-films.

Optical Characterization of Thiolate Self-Assembled Monolayers on Au(111)

We have investigated the optical response of thiolate self-assembled monolayers (SAMs) deposited from the liquid phase on well characterized, (111)-textured gold films based on the use of in situ and ex situ optical spectroscopic ellipsometry. We considered SAMs formed by several molecules with thiol functionality, focusing on the octadecanethiol (C18) SAM model system. We were able to show the tiny spectroscopic variations induced by the monolayer thick films with great reproducibility and high signal-to-noise ratio. We identified spectral features related to the alkyl chain and to the S−Au interface, providing a reliable spectral “fingerprint” of the formation of densely packed thiolate layers. By comparing data with simulations based on several effective models developed within the framework of Fresnel approach, we identified the main optical features related to the thiolate interface and in particular an absorption band whose spectral weight increases regularly from 500 nm toward the IR limit. We also...

Double CeO 2/YSZ buffer layer for the epitaxial growth of YBa 2Cu 3O 7 − δ films on Si (001) substrates

Both single YSZ and double CeO 2/YSZ buffer layers were considered for the epitaxial growth of YBaCuO films on silicon (001) substrates. The paper reports on the influence of YSZ and CeO 2 layer thicknesses on in-plane epitaxial relationships and superconducting properties of YBaCuO films. All the films were grown by pulsed laser ablation. The thickness and interface quality was deduced from grazing X-ray reflectivity and optical spectroscopic ellipsometry measurements. The structural properties have been characterized by X-ray diffraction analysis (XRD). θ-2θ scan XRD spectra showed that, in appropriate conditions, both YSZ and CeO 2 films have (001) orientation on Si (001) and YSZ/Si (001) substrates respectively. C-axis textured YBaCuO films were obtained on each buffer layer. A single YSZ buffer layer was considered first. Even when critical temperatures T c (zero resistance) of up to 88 K were obtained on YSZ/Si, ϕ-scan XRD spectra indicated that two different in-plane orientations exist in the YBaCuO films. With the introduction of an intermediate 10 nm thick CeO 2 layer on a 70 nm thick YSZ layer, the total YBaCuO in-plane alignment was obtained. YSZ and CeO 2 films were cube on cube aligned, whereas YBaCuO films were totally 45° rotated, around the c-axis, with respect to the Si (001) substrate. Critical temperatures T c ( R = 0) of 88.5 K and critical current densities above 10 6 A/cm 2 at 77 K confirm the high quality of the 40 nm thick YBaCuO films. The films are stable upon aging and thermal cycling as proved by their successful use for the realization of sensitive suspended YBaCuO microbolometers.