Electrical and optical properties of gallium phosphide films

The electrical and optical properties of gallium phosphide films, prepared by the explosive evaporation and three-temperature methods, are studied. It is shown that film properties are strongly dependent on method of preparation and substrate temperature. Gallium phosphide films prepared by explosive evaporation having perfect structure possess the best electrical and optical properties.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 54–58, June, 1971.     

Forbidden optical transitions between impurity levels in silicon and gallium phosphide

Experimental estimates are made of absorption cross sections for forbidden optical transitions from the ground state to long-lived excited states of P, As, Sb, In, and Ga impurities in silicon and Te impurities in gallium phosphide. The results can be used to predict the possibility of long-wavelength stimulated emission being excited as a result of the population inversion of long-lived impurity states in these materials.     

Ellipsometric analysis and optical absorption characterization of gallium phosphide nanoparticulate thin film

Gallium phosphide (GaP) nanoparticulate thin films were easily fabricated by colloidal suspension deposition via GaP nanoparticles dispersed in N,N-dimethylformamide. The microstructure of the film was performed by x-ray diffraction, high resolution transmission electron microscopy and field emission scanning electron microscopy. The film was further investigated by spectroscopic ellipsometry. After the model GaP+void|SiO₂ was built and an effective medium approximation was adopted, the values of the refractive index n and the extinction coefficient k were calculated for the energy range of 0.75 eV-4.0 eV using the dispersion formula in DeltaPsi2 software. The absorption coefficient of the film was calculated from its k and its energy gaps were further estimated according to the Tauc equation, which were further verified by its fluorescence spectrum measurement. The structure and optical absorption properties of the nanoparticulate films are promising for their potential applications in hybrid solar cells.     

Surface mode absorption and infrared optical properties of gallium phosphide nanoparticles

The surface optical (SO) mode of ellipsoidal gallium phosphide (GaP) nanoparticles is investigated by infrared transmission spectroscopy. The surface mode theory of diatomic cubic particles is generalized and applied to GaP nanoparticles in a systematic treatment. The Fröhlich mode of GaP nanoparticles has been observed in our experiment. As far as surface mode frequency is concerned, the result of the experiment agrees with that of the theoretical calculation. The characteristics of the SO mode peak, including frequency shift, broadening and line shape, are analyzed. The frequency shift is attributed to the surrounding medium effect, surface oxidation and the aggregation effect as well as intrinsic point defects; the broadening is mainly due to the non-spherical particle shape, aggregation and quantum confinement effect; and the line shape is related to the particle shape and the damping function.     

The optical properties of Be,Mg and Zn-diffused gallium phosphide

Interest in the Ga-site acceptors Be and Mg was stimulated by the possibility that they might produce efficient luminescence on association with O, analogous to the well-known red Zn-O luminescence in GaP but at higher transition energy. Attention was directed to diffusion doping by Be and Mg of GaP O-doped during growth because the reactivity of Be and Mg with O renders double doping during crystal growth very difficult. Structured green donor-acceptor pair spectra were observed at 1.6°K from many Be-diffused crystals, yielding an accurate measure of (E_A)_BE, 50 ± 1 meV. Moderately efficient orange-red luminescence was also observed below ∼ 100 °K from these crystals, but the intensity of this luminescence decreased rapidly to negligible levels by ∼ 200°K. This luminescence also contains sharp structure at 1.6°K, of a form characteristic of the decay of excitons bound to complex centres. Many sharp phonon replicas occur, involving local modes as well as characteristic GaP modes. One set of no-phonon lines, at least, near 2.19 eV, shows zero-field splitting, luminescence decay times and behaviour in magnetic and external strain fields characteristic of exciton decay at a centre with <100>; or <111>-type symmetry axes, containing no extra electronic particles. The exciton state is split by 2.4 meV by J-J coupling, and the axial field of the centre splits the hole states by ∼ 1.0 meV. These bound excitons are specifically characteristics of diffused GaP and appear analogous to bound excitons observed below 2.12 eV in Zn-diffused GaP. It is probable that the relevant centres contain diffusion components such as Be or Zn interstitials and improbable that O_P is involved. By contrast, weak orange bound exciton luminescence observed in Mg-diffused GaP does involve O, presumably as O_P. No analysis of the magneto-optical behaviour of this Mg-related bound exciton was possible in our crystals, so its symmetry axis was not established. It is possible that this is the Mg_Ga-O_P bound exciton. If so, the two-fold reductions in the exciton localisation energy from ∼ 0.32 eV to ∼ 0.15 eV and in the mass of the Ga-site substituent has produced dramatic changes in the form of the phonon cooperation between the Zn-O and “Mg-O” excitons. The “Mg-O” exciton luminescence is not dominant in our crystals, even at low temperature. The exciton state is again split by a local crystal field as well as by J-J coupling, but here the former splitting is predominant; 2_∈0 = 3.9 meV, Δ = 0.60 meV.     

Forbidden band width and effective charge in crystal lattice of ZnS

The optical and thermal widths of the forbidden band are determined by the absorption spectrum and temperature dependence of the conductivity respectively. The difference between them is determined also by the spectral sensitivity of different phosphors to optical stimulation. The effective charge of an ion in ZnS is determined in several ways by using the value of this difference, the refractive index, dielectric constant and frequencies in the Raman spectrum and the spectrum of infra-red absorption of ZnS. It was found to be equal to approximately half of the electron charge.     

Structural and optical properties of CdS thin films

CdS thin films are deposited onto glass substrates by vacuum evaporation at 373 K and the films are annealed at different temperatures. Rutherford backscattering spectrometry (RBS) and X-ray diffraction techniques are used to determine the thickness, composition, crystalline structure and grain size of the films. The films show a predominant hexagonal phase with small crystallites. The optical band gap of the films are estimated using the optical transmittance measurements. A decrease in the band gap is observed for the annealed films. The Raman peak position of the CdS A₁ (LO) mode did not change much whereas, the full width at half maximum (FWHM) is found to decrease with annealing.     

The optical properties of thin erbium films

The effective dielectric constant, ε′₂, of very thin films of erbium on sodium chloride substrates was determined from measurements of normal incidence reflectance and transmittance in the visible spectrum. ε′₂ showed a maximum which moved to longer wavelengths as the film thickness increased. Electron microscopy revealed that the film islands grew flatter and more irregular with thickness. The shape factor of the islands, F, was calculated by a modified Maxwell-Garnett method and became smaller as the film thickness increased.     

Multiphoton transitions at the direct and indirect band gaps of gallium phosphide

The non linear photoconductivity of GaP has been studied using three-photon and two-photon excitation by a neodymium and a ruby laser. The non linear cross sections for three- and two-photon transitions corresponding to the direct gap and for two photon-phonon assisted ones have been obtained. The results are compared with theoretical calculations.     

Electronic and optical characteristics of thin hexagonal gallium nitride films specified by surfaces

A computational procedure using density functional theory and generalized to the case of periodic finite-layered structures in the basis of valence orbitals with the inclusion of polarization functions is applied to calculation of the energy spectrum of electrons and curves for the optical absorption of thin hexagonal gallium-nitride films. It is shown that, to stabilize the atomically pure surface, it is necessary to place additional atoms onto it (those of gallium on one side of the film and those of nitrogen on the other side in the ratio: one additional atom to four surface ones, three of which form chemical bonds with the additional atom). In this case, the calculated curve for the optical absorption is close to the experimental one. It is also shown that the attachment of additional atoms to the atomically pure surface enhances its capability of emitting electrons.     

Structural and optical properties of sputtered ZnO thin films

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential 〈0 0 1〉 orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400-800 nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing.     

Structural characterization and optical properties of ZnSe thin films

Zinc selenide (ZnSe) thin films (d = 0.11-0.93 μm) were deposited onto glass substrates by the quasi-closed volume technique under vacuum. Their structural characteristics were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The experiments showed that the films are polycrystalline and have a zinc blende (cubic) structure. The film crystallites are preferentially oriented with the (1 1 1) planes parallel to the substrate surface. AFM images showed that the films have a grain like surface morphology. The average roughness, R_a = 3.3-6.4 nm, and the root mean square roughness, R_rms = 5.4-11.9 nm, were calculated and found to depend on the film thickness and post-deposition heat treatment.The spectral dependence of the absorption coefficient was determined from transmission spectra, in the range 300-1400 nm.The values of optical bandgap were calculated from the absorption spectra, E_g = 2.6-2.7 eV.The effect of the deposition conditions and post-deposition heat treatment on the structural and optical characteristics was investigated.     

Preparation and optical properties of barium titanate thin films

Barium titanate (BTO) films were prepared by sol-gel spin-coating technique. The crystal structure and optical properties of BTO films have been investigated. The results indicate that the BTO films are single perovskite phase having tetragonal symmetry. The band gap of the BTO films increases with the increasing of layer number and decreasing of solution concentration. The transmittance and band gap of the BTO films annealed at 900 °C is more than that of the BTO films annealed at 700 °C when wavelength is 200-1000 nm. When wavelength is 400-1000 nm, the absorption coefficient α obtained by experiment is higher than that obtained by calculation (close to zero).     

Structural and optical properties of thermally evaporated thin films

Chalcogenide glass Se₅₅Ge₃₀As₁₅ have amorphous structure in both as-deposited and annealed conditions. The optical properties of the as-deposited and annealed films were studied using spectrophotometric measurements of transmittance, T(λ), and reflectance, R(λ), at normal incidence of light in the wavelength range 200–2500 nm. Neither annealing temperature nor film thickness can influence spectral response on refractive index and absorption index of films. The type of electronic transition responsible for optical properties is indirectly allowed transition with energy gap of 1.94 eV and phonon energy of 40 meV. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–Didomenico (WD) model. The width of band tails of localized states into the gap (ΔE), the single oscillator energy (E_o), the dispersion energy (E_d), the optical dielectric constant (ε_∞), the lattice dielectric constant (ε_L), the plasma frequency (ω_p) and the free charge carrier concentration (N) were estimated.     

Electrical and luminescent properties of gallium phosphide with radiation defects

The energy spectrum of local levels and the effect of radiation damage on intensity and character of cathode luminescence lines in the visible region of the spectrum in gallium phosphide single crystals prepared from a solution-melt by the Chochralski method are examined. It is shown that as a result of irradiation by fast electrons with E_e=1 Mev and Co 60 -quanta, both donor and acceptor centers are introduced into the original crystals. New radiation centers producing lines in the spectral range studied (500–600 nm) were not observed after irradiation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 49–54, June, 1976.     

Effects of electron-beam irradiation on structural,electrical, and optical properties of amorphous indium gallium zinc oxide thin films

High-energy electron-beam irradiation of indium gallium zinc oxide (IGZO) films improved the short-range arrangement. The increase in band gap was used as an indication of such improvement. X-ray diffraction confirmed that the films treated with a DC voltage of 2–4.5 kV for duration of up to 35 min are in the amorphous state or nanocrystalline phase. Higher energy electron-beam irradiation led to increased conductivity, which mainly comes from the drastic increase in electron concentration. Electron-beam treatment could be a viable route to improve the contact resistance between the source/drain and channel layer in thin-film transistor devices.     

Measurement of optical and surface properties of PLZT thin films

Lanthanum-modified lead zirconate titanate (Pb_0.93La_0.07(Zr_0.3Ti_0.7)_0.93O₃, PLZT7/30/70) thin films with and without a seeding layer of PbTiO₃ (PT) were successfully deposited on indium-doped tin oxide (ITO) coated glass substrate via spin coating in conjunction with a sol–gel process, and a top transparent conducting thin film of SnO₂ was also prepared in the same way. The thicknesses of PLZT and PT layers are 0.5 μm and 24 nm, respectively. The retardance of PLZT film was measured by a new heterodyne interferometer and enhanced by application of a seeding layer of PT. The Pockels linear electro-optical coefficient of PLZT film with a PT layer was determined to be 3.17 × 10^?9 m/V when the refractive index is considered as 2.505, which is one order larger than 1.4 × 10^?10 m/V for PLZT12/40/60 doped with Dy reported in the literature. The root-mean-square (rms) roughness of PLZT thin film with a PT layer (R_rms = 6.867 nm) was larger than that of PLZT film (R_rms = 0.799 nm). From the comparisons, the average transmittance of PLZT film with a PT seeding layer was 77.01%, which was a little smaller than that of PLZT film (around 80.75%). Experimental results imply that the PT seeding layer plays a key role in the increase of retardance value, leading to a higher Pockels coefficient.     

Investigation of optical properties of thin CuGaTe2 films

Thin films of copper tellurogallate were obtained by the method of pulsed laser spraying. The structure of the films and the parameters of the unit cell were determined by the x-ray method. It is shown that the films possess p-type conduction and their resistivity decreases with an increase in the substrate temperature. From the transmission near the edge of the fundamental absorption band a determination was made of the energy of interband transitions, crystalline cleavage, and spin-orbital splitting. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 946–949, November–December, 1998.