Optical properties of Cd1−xZnxS thin films for CuInGaSe2 solar cell application

The photovoltaic Cd_1−xZn_xS thin films, fabricated by chemical bath deposition, were successfully used as n-type buffer layer in CuInGaSe₂ (CIGS) solar cells. Comprehensive optical properties of the Cd_1−xZn_xS thin films were measured and modeled by spectroscopic ellipsometry (SE), which is proven to be an excellent and non-destructive technique to determine optical properties of thin films. The optical band gap of Cd_1−xZn_xS thin films can be tuned from 2.43 eV to 3.25 eV by controlling the Zn content (x) and deposition conditions. The wider-band-gap Cd_1−xZn_xS film was found to be favorable to improve the quantum efficiency in the wavelength range of 450-550 nm, resulting in an increase of short-circuits current for solar cells. From the characterization of quantum efficiency (QE) and current-voltage curve (J-V) of CIGS cells, the Cd_1−xZn_xS films (x = 0.32, 0.45) were demonstrated to significantly enhance the photovoltaic performance of CIGS solar cell. The highest efficiency (10.5%) of CIGS solar cell was obtained using a dense and homogenous Cd_0.68Zn_0.32S thin film as the buffer layer.     

Significance of Al on the morphological and optical properties of Ti1−xAlxN thin films

TiN and Ti_1−xAl_xN thin films with different aluminum concentrations (x = 0.35, 0.40, 0.55, 0.64 and 0.81) were synthesized by reactive magnetron co-sputtering technique. The structure, surface morphology and optical properties were examined using Grazing Incidence X-ray Diffraction (GIXRD), Atomic Force Microscopy (AFM), Raman spectroscopy and spectroscopic ellipsometry, respectively. The structure of the films were found to be of rocksalt type (NaCl) for x = 0.0–0.64 and X-ray amorphous for x = 0.81. AFM topographies show continuous mound like structure for the films of x between 0.0 and 0.64, whereas the film with x = 0.81 showed smooth surface with fine grains. Micro-Raman spectroscopic studies indicate structural phase separation of AlN from TiAlN matrix for x > 0.40. Ti_1−xAl_xN has the tendency for decomposition with the increase of Al concentration whereas c-TiN and hcp-AlN are stable mostly. The optical studies carried out by spectroscopic ellipsometry measurements showed a change from metallic to insulating behavior with the increase in x. These films are found to be an insulator beyond x = 0.81.     

Optical constants of electroplated Bi2Te3 films by Mueller matrix spectroscopic ellipsometry

We synthesized polycrystalline Bi_2 + xTe_3 − x (− 0.2< x <0.2) thin films by electrodeposition in acidic medium. Since Bi₂Te₃-like structure may be uniaxially anisotropic due to its rhombohedral crystallographic system, we investigated their optical behavior using ex and in situ Mueller matrix spectroscopic ellipsometry in the wavelength range of 470 to 830 nm (1.5-2.6 eV). We found that room-temperature electroplated polycrystalline appears optically isotropic and that no depolarization effect occurs from the first steps of growth until several micrometers thick films. Additional ex situ measurements permit to obtain their optical constants from far-ultraviolet to near-infrared (190-2100 nm).     

Analytical model for the optical functions of indium gallium nitride with application to thin film solar photovoltaic cells

This paper presents the preliminary results of optical characterization using spectroscopic ellipsometry of wurtzite indium gallium nitride (In_xGa_1−xN) thin films with medium indium content (0.38 < x < 0.68) that were deposited on silicon dioxide using plasma-enhanced evaporation. A Kramers-Kronig consistent parametric analytical model using Gaussian oscillators to describe the absorption spectra has been developed to extract the real and imaginary components of the dielectric function (?₁, ?₂) of In_xGa_1−xN films. Scanning electron microscope (SEM) images are presented to examine film microstructure and verify film thicknesses determined from ellipsometry modeling. This fitting procedure, model, and parameters can be employed in the future to extract physical parameters from ellipsometric data from other In_xGa_1−xN films.     

Compositional dependence on the optical properties of amorphous As2 − xS3 − xSbx thin films

In this paper, we report results of the optical properties of thermally deposited As_2 − xS_3 − xSb_x thin films with x = 0.02, 0.07, 0.1 and 0.15. We have characterized the deposited films by Fourier Transform Infrared, Raman and X-ray photoelectron spectroscopy (XPS). The relationship between the structural and optical properties and the compositional variation were investigated. It was found that the optical bandgap decreases with increase in Sb content. The XPS core level spectra show a decrease in As₂S₃ percentage with increase in Sb content. This is confirmed from the shifting of the Raman peak from AsS₃ vibrational mode towards SbS₃ vibrational mode.     

Study of GeYSi1 − Y:H films deposited by low frequency plasma

In this work, we report a study of the optical properties measured through spectral transmission and spectroscopic ellipsometry in Ge:H and Ge_YSi_1 − Y:H (Y ≈ 0.97) films deposited by low frequency (LF) PE CVD with hydrogen (H) dilution. The dilution was varied in the range of R = 20 to 80. It was observed that H-dilution influences in a different way on the interface and bulk optical properties, which also depend on incorporation of silicon. The films with low band tail characterized by its Urbach energy, E_U, and defect absorption, α_D, have been obtained in Ge:H films for R = 50 with E_U = 0.040 eV and α_D = 2 × 10³ cm^− 1 (hν ≈ 1.04 eV), and in Ge_YSi_1 − Y:H films for R=75 with E_U = 0.030 eV and α_D = 5 × 10² cm^− 1 (hν ≈ 1.04 eV).     

Structural and optical properties of rock-salt Cd1 − xZnxO thin films prepared by thermal decomposition of electrodeposited Cd1 − xZnxO2

Cd_1 − xZn_xO nanocrystalline thin films with rock-salt structure were obtained through thermal decomposition of Cd_1 − xZn_xO₂ (x = 0, 0.37, 0.57, 1) thin films which were electrodeposited from aqueous solution at room temperature. X-ray diffraction results showed that the Zn ions were incorporated into rock salt-structure of CdO and the crystal lattice parameters decreased with the increase of Zn contents. The bandgaps of the Cd_1 − xZn_xO thin films were obtained from optical transmission and were 2.40, 2.51, 2.63 and 3.25 eV, respectively.     

Effect of dopant concentration on the structural, electrical and optical properties of Mn-doped ZnO films

The Mn-doped ZnO (Zn_1 − xMn_xO) thin films with manganese compositions in the range of 0-8 at.% were deposited by radio-frequency (RF) magnetron sputtering on quartz glass substrates at room temperature (RT). The influence of Mn concentration on the structural, electrical and optical properties of Zn_1 − xMn_xO films has been investigated. X-ray diffraction (XRD) measurements reveal that all the films are single phase and have wurtzite structure with (002) c-axis orientation. The chemical states of Mn have been identified as the divalent state of Mn²⁺ ions in ZnO lattice. As the content of Mn increases, the c-lattice constant and the optical band gap of the films increase while the crystalline quality deteriorates gradually. Hall-effect measurements reveal that all the films are n-type and the conductivity of the films has a severe degradation with Mn content. It is also found that the intensity of RT photoluminescence spectra (PL) is suppressed and saturates with Mn doping.     

CuAl1 − xInxSe2 solid solutions: Dielectric function and inter-band optical transitions

The dielectric function of bulk CuAl_1 − xIn_xSe₂ with composition x varying from x = 0.07 to x = 0.6 were studied over the photon energy region 1.0-6.0 eV at room temperature by spectroscopic ellipsometry. Information on the inter-band optical transitions was obtained from the results of the standard critical point analysis of the obtained dielectric function. With increasing Indium content, all spectral features of the obtained dielectric functions were found to gradually shift towards lower energies. The details of this shift for each critical point retrieved from the obtained dielectric function were disclosed. A compositional dependence of the optical transitions in Γ point of the Brillouin zone was verified to be strong. Such dependence for N and T points turned out to be weak by comparison. The later fact was accounted for a small compositional shift of the conduction band states in N and T points as compared to Γ point.     

Optical properties of thin films of mixed Ni-W oxide made by reactive DC magnetron sputtering

Thin films of Ni_xW_1 − x oxides with x = 0.05, 0.19, 0.43 and 0.90 were studied. Films with thicknesses in the range 125-250 nm were deposited on silicon wafers at room temperature by reactive DC magnetron co-sputtering from targets of Ni and W. The films were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and spectroscopic ellipsometry (SE). XRD spectra and SEM micrographs showed that all films were amorphous and possessed a columnar structure. The ellipsometric angles Ψ and Δ of as-deposited films were measured by a rotating analyzer ellipsometer in the UV-visible-near infrared range (0.63-6.18 eV) and by an infrared Fourier transform rotating compensator ellipsometer in the 500-5200 cm⁻¹ wavenumber range. SE measurements were performed at angles of incidence of from 50 ° to 70 °. Parametric models were used to extract thicknesses of the thin films and overlayers of Ni_xW_1 − x oxide at different compositions, band gaps and optical constants. Features in the optical spectra of the Ni_xW_1 − x oxides were compared with previous data on tungsten oxide, nickel oxide and nickel tungstate.     

Structural, electrical and optical properties of transparent Zn1 − xMgxO nanocomposite thin films

Zn_1 − xMg_xO thin films of various Mg compositions were deposited on quartz substrates using inexpensive ultrasonic spray pyrolysis technique. The influence of varying Mg composition and substrate temperature on structural, electrical and optical properties of Zn_1 − xMg_xO films were systematically investigated. The structural transition from hexagonal to cubic phase has been observed for Mg content greater than 70 mol%. AFM images of the Zn_1 − xMg_xO films (x = 0.3) deposited at optimized substrate temperature clearly reveals the formation of nanorods of hexagonal Zn_1 − xMg_xO. The variation of the cation-anion bond length to Mg content shows that the lattice constant of the hexagonal Zn_1 − xMg_xO decreases with corresponding increase in Mg content, which result in structure gradually deviating from wurtzite structure. The tuning of the band gap was obtained from 3.58 to 6.16 eV with corresponding increase in Mg content. The photoluminescence results also revealed the shift in ultraviolet peak position towards the higher energy side.     

Study of flash evaporated CuIn1 − xGaxTe2 (x = 0, 0.5 and 1) thin films

CuIn_1 − xGa_xTe₂ thin films with x = 0, 0.5 and 1, have been prepared by flash evaporation technique. These semiconducting layers present a chalcopyrite structure. The optical measurements have been carried out in the wavelength range 200-3000 nm. The linear dependence of the lattice parameters as a function of Ga content obeying Vegard's law was observed. The films have high absorption coefficients (4 · 10⁴ cm^− 1) and optical band gaps ranging from 1.06 eV for CuInTe₂ to 1.21 eV for CuGaTe₂. The fundamental transition energies of the CuIn_1 − xGa_xTe₂ thin films can be fitted by a parabolic equation namely E_g1(x) = 1.06 + 0.237x − 0.082x². The second transition energies of the CuInTe₂ and CuGaTe₂ films were estimated to be: E_g2 = 1.21 eV and E_g2 = 1.39 eV respectively. This variation of the energy gap with x has allowed the achievement of absorber layers with large gaps.     

Optical properties and DC electrical conductivity of Ge28−xSe72Sbx thin films

Thin films of Ge_28−xSe₇₂Sb_x (x=0, 8, 16, 24 at%) with thickness of 200 nm are prepared by thermal evaporation onto glass substrates under vacuum of 5.3×10⁻⁵ mbar. Optical reflectance and transmittance of these films are measured at room temperature in the light wavelength region from 200 to 1100 nm. The estimated optical energy gap, E_g, is found to decrease from 2 eV (0 at% Sb) to 1.5 eV (24 at% Sb), whereas the band tail width, E_e, increases from 0.062 to 0.077 eV, respectively. The refractive index, n, and extinction coefficient, κ, are determined as functions of wavelength. The DC electrical conductivity, σ, of films is measured as a function of temperature in the range from 300 to 360 K. The extracted value of activation energy, ΔE, is found to decrease from 0.95 eV (0 at% Sb) to 0.74 eV (24 at% Sb). Optical and electrical behavior of films can be explained in terms of cohesive energy (CE) and Se-Se defect bonds.     

Effect of substrate temperature on the optical parameters of thermally evaporated Ge-Se-Te thin films

Thin films of Ge₁₀Se_90 − xTe_x (x = 0, 10, 20, 30, 40, 50) glassy alloys were deposited at three substrate temperatures (303 K, 363 K and 423 K) using conventional thermal evaporation technique at base pressure of ~ 10^− 4 Pa. X-ray diffraction results show that films deposited at 303 K are of amorphous nature while films deposited at 363 K and 423 K are of polycrystalline nature. The optical parameters, refractive index and optical gap have been derived from the transmission spectra (using UV-Vis-NIR spectrophotometer) of the thin films in the spectral region 400-1500 nm. This has been observed that refractive index values remain almost constant while the optical gap is found to decrease considerably with the increase of substrate temperature. The decrease in optical gap is explained on the basis of change in nature of films, from amorphous to polycrystalline state, with the increase of substrate temperature. The optical gap has also been observed to decrease with the increase of Te content.     

Spectroscopic ellipsometry study of CuCdTeO thin films grown by reactive co-sputtering

The complex dielectric function of CdTeO_x and CuCdTeO thin films was determined by spectroscopic ellipsometry in the photon energy range of 1.5 to 5 eV. The films were grown onto glass slides substrates by reactive rf co-sputtering using CdTe and Cu targets in an Ar + O₂ atmosphere. Films with different Cu concentrations were obtained by varying the power on the Cu target. The dielectric function of the films is represented by a generalized Lorentz harmonic oscillator expression. Three-dimensional type line-shapes for the critical points E₁ and E₁ + Δ₁ of CdTe were identified in CdTeO_x and CuCdTeO films even for Cu and O concentrations above 20 at.%. This latter result can be indicative of CdTe alloying with those elements widening the possibilities for new photovoltaic materials.     

Electrical and optical investigations on Pb1 − 3x/2LaxZr0.2Ti0.8O3 thin films obtained by radiofrequency assisted pulsed laser deposition

Thin films of ferroelectric relaxor Pb_1 − 3x/2La_xZr_0.2Ti_0.8O₃, x = 0.22 have been integrated in an oxidic heterostructure for electro-optical investigations. The quadratic electro-optic behavior and optical properties have been studied by means of variable angle spectroscopic ellipsometry method in reflection mode. Birefringence values up to δΔ = 0.17° have been obtained for quadratic compositions at λ = 540 nm and 65° angle of incidence. Structural, chemical and morphologic properties of Pb_1-3x/2La_xZr_0.2Ti_0.8O₃ (x = 0.22) thin films have been investigated by x-ray diffraction and atomic force microscopy techniques. The dielectric and ferroelectric behavior has been investigated using dielectric spectroscopy and a ferroelectric test system.     

Properties of transparent conducting oxides formed from CdO alloyed with In2O3

The structure, optical and electrical properties of transparent conducting oxide films depend greatly on the methods of preparation, heat treatment, type and level of dopant. Thin films of (CdO)_1−x(In₂O₃)_x have been grown by electron beam evaporation technique for different concentrations of In₂O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2). Increase of doping led to increased carrier concentration as derived from optical data and hence to increased electrical conductivity, which degraded the transparency of the films. An improvement of the electrical and optical properties of Cadmium indium oxide (CdIn₂O₄) has been achieved by post-deposition annealing. A resistivity value of 7 × 10^− 5 Ω cm and transmittance of 92% in the near infrared region and 82% in the visible region have been obtained after annealing at 300 °C for 90 min in air.     

Research on the novel GeSe2-In2Se3-KBr chalcohalide optic glasses

(1 − x)(0.75GeSe₂-0.25In₂Se₃) − x(1/3In₂Se₃-2/3KBr) (x = 0, 0.15, 0.3, 0.45, 0.525) chalcohalide glasses were prepared by traditional melt-quenching method and its glass-forming region was determined. The physical, thermal and optical properties of the GeSe₂-In₂Se₃-KBr ternary glass system are reported. The results show that the GeSe₂-In₂Se₃-KBr glass system has relatively high glass transition temperature (T_g = 286-335 °C) and good thermal stability. With increasing KBr content from 0 to 35 mol%, a blue-shift from 750 to 620 nm at the visible absorbing cutting-off edge is observed. The red-shifting of the transmission cutting-off edge at the long-wave IR band occurs linearly with increasing KBr. The allowed direct transition and indirect transition of samples were calculated according to the classical Tauc equation. The direct optical band gaps and indirect optical band gaps were in the range from 1.649 to 1.931 eV and 1.513 to 1.863 eV, respectively.     

The dielectric functions and optical band gaps of thin films of amorphous and cubic crystalline Mg~ 2NiH~ 4

Magnesium nickel hydride films have earlier been suggested for several optoelectronic applications, but the optical properties and band gap have not been firmly established. In this work, the dielectric functions and the optical band gaps of thin films of Mg₂NiH₄ have been determined experimentally from optical modeling using spectroscopic ellipsometry and spectrophotometry in the photon energy range between 0.7 and 4.2 eV. Samples were prepared by reactive sputtering, resulting in a single-layer geometry that could easily be studied by ellipsometry. Crystalline samples were prepared by annealing the as-deposited amorphous films ex-situ. The resulting films remained in the high temperature cubic Mg₂NiH₄ structure even after cooling to room temperature. Tauc analysis of the dielectric functions shows that Mg₂NiH₄ films exhibit a band gap of 1.6 eV for the amorphous structure and 2.1 eV for the cubic crystalline structure.     

Growth and characterization of electrodeposited Cu(In,Al)Se2 thin films

Thin films of CuIn_1 − xAl_xSe₂ were grown using a cathodic electrodeposition technique. The CuIn_1 − xAl_xSe₂ films were electrodeposited on SnO₂ coated glass from aqueous baths containing different Al contents using deposition potentials ranging from − 650 mV to − 850 mV versus a saturated calomel electrode. The electrodeposited films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis of X-rays, atomic force microscopy, and UV-VIS-NIR spectroscopy. The results show that single phase CuIn_1 − xAl_xSe₂ films with Al content x around 0.27-0.33 having good stoichiometry can be produced in the above potential range. XRD and SEM studies show that films deposited at − 650 mV and − 750 mV have good crystallinity while those grown at − 850 mV have comparatively poorer crystallinity. SEM studies show that the particle size of the films grown at − 650 and − 750 mV is in the micron range but is around 100 nm when grown at − 850 mV. Optical studies show that the optical band gap shifts with Al content from 1.21 eV for x = 0.27 to about 1.42 eV for x = 0.33. The as-grown as well as vacuum annealed films were n-type in conductivity with resistivity in the range 3-5 × 10⁻³ Ω cm.