Band Gap Of ZnSe Research Articles

Extended photoresponse and multi-band luminescence of ZnO/ZnSe core/shell nanorods

Aligned ZnO/ZnSe core/shell nanorods (NRs) with type-II energy band alignment were fabricated by pulsed laser deposition of ZnSe on the surfaces of hydrothermally grown ZnO NRs. The obtained ZnO/ZnSe core/shell NRs are composed of wurtzite ZnO cores and zinc blende ZnSe shells. The bare ZnO NRs are capable of emitting strong ultraviolet (UV) near band edge (NBE) emission at 325-nm light excitation, while the ZnSe shells greatly suppress the emission from the ZnO cores. High-temperature processing results in an improvement in the structures of the ZnO cores and the ZnSe shells and significant changes in the optical properties of ZnO/ZnSe core/shell NRs. The fabricated ZnO/ZnSe core/shell NRs show optical properties corresponding to the two excitonic band gaps of wurtzite ZnO and zinc blende ZnSe and the effective band gap between the conduction band minimum of ZnO and the valence band maximum ZnSe. An extended photoresponse much wider than those of the constituting ZnO and ZnSe and a multi-band photoluminescence including the UV NBE emission of ZnO and the blue NBE emission of ZnSe are observed.

Optical Properties of Pure ZnSe Nanocrystals Synthesized by Laser Ablation in Organic Liquids

Synthesis of the zinc selenide (ZnSe) nanocrystals (NCs) by pulsed laser ablation approach is reported in two distinct liquid media (ethanol and acetone) by means of the 1st harmonic of high frequency Nd:YAG laser. Based on the X-ray diffraction (XRD) pattern, ZnSe NCs have both wurtzite and zinc blende structures with some overlapping peaks. Transmission electron microscopy (TEM) reveals that as-synthesized NCs are relatively monodispersed and spherical in shape. UV–Vis spectra indicate that the band gap of ZnSe NCs in acetone and ethanol is blue shifted comparing to the band gap of bulk ZnSe which is due to quantum confinement effect. According to the XRD results, TEM observations and UV–Vis spectroscopy, as-synthesized ZnSe NCs in ethanol are larger than the ones in acetone. Two kinds of band edge and deep level emissions are identified by means of the room temperature photoluminescence spectroscopy.

Facile and large-scale synthesis of water-soluble Fe:ZnSe semiconductor nanocrystals

A method for the large-scale synthesis of water-soluble Fe:ZnSe nanocrystals was developed, which manipulates the optical properties by controlling the reaction temperature. The Fe:ZnSe nanocrystals crystallize well, and possess cubic zinc blende structure, and the obvious broadening of the diffraction peaks is attributed to their small size nature. The formation of mercaptoacetic acid capped Fe:ZnSe nanocrystals was confirmed by FT-IR analysis. The emission peaks of the resulting nanocrystals can be tuned from 419nm to 453nm, and the absorption edges of the obtained nanocrystals are blue-shifted compared with the corresponding band gap of bulk ZnSe due to the quantum confinement effect. Finally, the Fe:ZnSe nanocrystals yield in a single reaction was determined to be 2.88g.

Synthesis and optoelectronic properties of p-type nitrogen doped ZnSe nanobelts

Single-crystal nitrogen (N) doped p-type ZnSe nanobelts (NBs) with zinc blende structure were synthesized in ammonia atmosphere via a thermal evaporation method. The p-type conductivity of ZnSe:N NBs was confirmed by field-effect transistors (FETs) based on individual NBs. High-performance photodetectors were constructed based on ZnSe:N NBs, which show high sensitivity and relatively fast response speed to the incident light with a sharp cut-off at 460nm, corresponding to the band-gap of ZnSe. The high photosensitivity and relatively fast response speed are attributable to the high crystal quality of the ZnTe nanowires. These results reveal that such single-crystalline ZnSe NBs are excellent candidates for optoelectronic applications.

Strain-assisted bandgap modulation in Zn based II-VI semiconductors

The electronic structure of bulk ZnX (X = O, S, Se, and Te) under uniaxial strain along the [0001] direction or equibiaxial strain along the (0001) plane is investigated using hybrid density functional theory calculations and many-body perturbation theory. It is shown that compressive uniaxial (or tensile equibiaxial) strains lead to a structural phase transition in all the ZnX systems. This is accompanied by large reductions in the bandgap of ZnSe and ZnTe, spanning the entire visible spectrum.

Photomodulation reflectance study of temperature dependence of the band gap of ZnSe1‐xOx

AbstractWe investigated the band gap of ZnSe1‐xOx alloys (x =2.7% and 5.3%) by photomodulation reflectance spectroscopy from 10 K to 300 K. The temperature dependence of band gap of ZnSe1‐xOx exhibited slight deviation from the expectation of the band anticrossing model. The deviation was explained by the decrease of the anticrossing interaction between the oxygen states and conduction band at low temperature. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characteristics of Zn–Se–S thin layers by dip method

Zn–Se–S composite thin films of mixed cubic and hexagonal phases of ZnSe and ZnS are synthesized by dip process at room temperature. Polycrystalline nature of the films was observed from XRD. Optical band gap of Zn–Se–S thin film was found to be in between individual band gaps of ZnSe and ZnS. The electrical conductivity was found to be in the range of 10 −5–10 −2 (Ω cm) −1. Due to above properties, these films find applications as a buffer layer in solar cells.

Preparation of ultrawide ZnSe nanoribbons with the function of lasing cavity

Ultrawide ZnSe nanoribbons are synthesized by the simple thermal evaporation. The microstructure of ZnSe nanoribbons is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), photoluminescence (PL) and Raman spectrum. It is found that the strong emission near the band gap of ZnSe centered at 460 nm is obtained in these nanoribbons. More importantly, ZnSe nanoribbons can act as lasing emitting optical cavities. Raman studies indicate that the longitudinal optic (LO) and transverse optic (TO) phonon confinements of the ZnSe nanoribbons shift to lower frequency.

Green synthesis of ZnSe quantum dots using in-situ synthesised sodium selenide

Single-step green synthesis of zinc blende ZnSe quantum dots (QDs) is performed using sodium selenide and zinc stearate. The present method is non-toxic and is considered reasonably green route where stearate ions released during the reaction act as surfactant. The size quantisation effect is revealed by blue shifted absorption bands with a band-gap energy of ~2.9 eV with respect to bulk ZnSe band-gap of 2.7 eV. Photoluminescence measurement with FWHM <80 nm revealed trap state emission. Broadened XRD pattern confirmed cubic (zinc blende) crystal structure of ZnSe. The particle size was estimated to be about 5 nm from the XRD line broadening, which matched well with TEM analysis.

Cascade processes in inelastic light scattering in ZnSe nanowire structures

The resonance Raman scattering of light in MBE-grown structures with ZnSe nanowires (10–20 nm in diameter) with an Au film deposited on the substrate used as a catalyst was investigated. The thicknesses of the Au layers were 2, 10, and 100 A. The photon energy of the He-Cd pump laser (λ = 441.6 nm) was in excess of the band gap of bulk ZnSe, and the measurements were conducted at room temperature. Under these conditions, the Raman spectra are defined by a cascade process in which the electron interacting with a longitudinal optical phonon transfers between real band states with a certain probability of radiative recombination at each step. The blue shift of the luminescence maximum associated with the quantum confinement of carriers in the nanowire has been observed. The average nanowire diameter derived from the magnitude of this shift agrees well with electron microscopy measurements.

High Photocatalytic Activity of Heterojunction of Zinc Selenide Grown on Nanoscaled Titanium Oxide

High-quality nanoscaled anatase phase titanium oxide can be obtained from the conversion of ammonium oxotrifluorotitanate by the thermal treatment in oxygen. The photocatalytic activity of nanoscaled titanium oxide obtained at the thermal treatment temperature of shows the highest photocatalytic activity and is times of commercial P-25. The energy bandgap of ZnSe is near the maximum intensity of solar spectrum and acts as a sensitizer. The heterojunction of zinc selenide/titanium oxide prepared by metallorganic vapor-phase epitaxy shows 2.0 times the photocatalytic activities of commercial P-25.

Multiphoton photoconductivity and optical nonlinearities in ZnSe and CdSe direct band gap crystals

The multiphoton photoconductivity properties of ZnSe and CdSe direct band gap crystals have been studied using frequency-doubled Nd:YAG laser excitation. Two-photon absorption in both crystals has been observed. The photoconductivity measurements and nonlinearity in Z-scan traces confirms the existence of a lower valence band, from where one-photon transitions are forbidden but two-photon transitions are allowed. The value of the two-photon absorption cross section γ2 in both crystals has been calculated by using second-order time-dependent perturbation theory.

Conductivity and photoconductivity in undoped ZnSe nanowire array

Arrays of free-standing ZnSe nanowires with the length of 8–10μm and diameters of 80–150nm were fabricated by Au-catalyzed vapor-liquid-solid growth. Current-voltage characteristics of the arrays over the temperature interval of 90–400K showed a superlinear character. The differential conductance varied between two saturating regimes at low and high biases, respectively. This behavior was explained using a model of nonuniform wires with concentration fluctuations along them. The nanowire photoconductivity had a spectral edge corresponding to the ZnSe band gap and a strong frequency dispersion, presumably due to carrier capture by deep centers.

Local electronic structures of ZnSe∕Si nanotapes and their luminescence properties

ZnSe∕Si nanotapes have been fabricated via a co-assisted vapor-phase transfer mechanism. Local electronic structure investigation along the nanotape radial direction suggests that the Si surface can be terminated by the ZnSe, leading to the one-dimensional (1D) anisotropic growth of Si, and further results in direct interface between ZnSe and Si in the nanotapes. The co-growth mechanism not only results in nanowire heterostructures, but also effectively incorporates Si into ZnSe and thus modifies its luminescence properties, leading to luminescence peak both above and below the original band gap of pure phase ZnSe.

Electric field effects in photoreflectance spectra of ZnSe epilayers grown on GaAs by molecular beam epitaxy

Photoreflectance (PR) is used to investigate the internal electric field of a series of ZnSe/GaAs heterostructures having different layer thicknesses. The room and low temperature PR spectra exhibited Franz–Keldysh oscillations at energies above the ZnSe band gap. From the period of these oscillations, the electric field at the ZnSe/GaAs interface was determined and behaviour decreasing with temperature was observed. The calculated values for the electric field (10–50 kV cm−1) are in agreement with the typical values in semiconductors. The dependence of the electric field on the layer thickness is attributed to the presence of additional states introduced by the misfit dislocations that are formed to relieve the epilayer strain.

Quasiparticle band structure of ZnS and ZnSe

We calculate the quasiparticle band structure of ZnS and ZnSe using the plane-wave pseudopotential method and the GW approximation for the self-energy. The Zn semicore $3d$ states are treated as valence states. A systematic study of the role of various approximations is presented, including the local density approximation (LDA) and generalized gradient approximation (GGA) for the ground-state properties, the role of self-consistency both in the dielectric function and the self-energy $\ensuremath{\Sigma},$ the effect of off-diagonal matrix elements of $(\ensuremath{\Sigma}\ensuremath{-}{V}_{\mathrm{xc}})$ in the Kohn-Sham orbital basis, and the plasmon-pole approximation. This study demonstrates that the LDA and GGA give similar results, and that self-consistency in updating the quasiparticle energies improves the accuracy of the band gap as well as the energies of the semicore $3d$ states. The calculated quasiparticle band gaps of ZnS and ZnSe agree well with experimental values when the GW approximation is used. There is some discrepancy for the calculated quasiparticle energies of the semicore states and some possible reasons are discussed.

Characterization of ZnSe films grown on GaAs substrates with In xGa l− xAs and Al xGa 1− xAs buffer layers

In this work we present a study of ZnSe films grown on buffer layers of the ternary compounds In x Ga 1− x As and Al 1− x Ga x As by means of photoreflectance (PR), and photoluminescence (PL) spectroscopies and transmission electron microscopy (TEM). The buffer layers of the ternary compounds were grown by molecular beam epitaxy (MBE) employing concentrations with 0.01< x<0.3 and thickness of 5000 Å. The films of ZnSe were grown by MBE in a separate II–VI system with a thickness of 6000 Å. The measurements of PR allowed us to determine the energy value of the ZnSe band gap, the magnitude of the internal electric fields, and to evaluate the quality of the buffer layers and the ZnSe film. The intensity and width of the excitonic related peaks in the PL spectra and the presence of lines related with defects allowed us to assess the crystalline quality of the films. TEM results have a good correlation with the optical spectroscopies showing that the better samples are those grown on buffer layers of Al 0.01Ga 0.99As and In 0.01Ga 0.99As.

Temperature and Isotopic Mass Dependence of the Direct Band Gap in Semiconductors: LCAO Calculations

Using perturbation theory, within a tight-binding (LCAO) approach, we have calculated the effects of electron–phonon interaction on the lowest direct band gap of Ge, GaAs and ZnSe. The agreement of the calculated temperature coefficient with experimental data is satisfactory. Following our approach, we have also calculated the dependence of the direct band gap on isotopic mass. The results are in close agreement with the experimental data.

Temporal evolution of resonant Raman-scattering in ZnCdSe quantum dots

We investigated ZnCdSe/ZnSe quantum-dot structures which include planar and coherently strained three-dimensional islands with different sizes. Optical excitation of these islands well below the ZnSe band gap leads to a resonant enhancement of the Zn0.7Cd0.3Se longitudinal-optical (LO) phonon-scattering efficiency and makes the 2LO and 3LO multiphonon emission observable. Resonant excitation with a power density of about 1.3 MW/cm2 using a micro-Raman setup results in an exponential decrease of the 1LO, 2LO, and 3LO intensity with irradiation time. This decay behavior is not observed for pure ZnSe crystals and can be avoided for the ZnCdSe/ZnSe structures using much lower excitation densities. The decrease in intensity is accompanied by a shift of the LO mode to higher frequencies resulting from a lower cadmium concentration in the alloy. From these experimental findings, we conclude that resonant excitation at a certain power density leads to cadmium out-diffusion from the planar quantum dots, which shifts the resonance away from the excitation energy.

Band gaps and quasiparticle energy calculations on ZnO, ZnS, and ZnSe in the zinc-blende structure by theGWapproximation

We have calculated the quasiparticle band gaps of ZnO, ZnS, and ZnSe in zinc-blende structure within the GW approximation using a full random-phase approximation dielectric matrix. The linear muffin-tin orbital basis was used for this calculation and the $3d$ orbitals of the Zn atom were treated as valence band in every case. The calculated band gaps are 3.59, 3.97, and 3.10 eV for ZnO, ZnS, and ZnSe, respectively. The gaps of ZnS and ZnSe are in good agreement with the experimental values and so is the gap of ZnO if we compare it with the experimental optical gap of wurtzite ZnO.