Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...

Infrared dielectric functions and phonon modes of high-quality ZnO films

Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different b...

Femtosecond laser-induced periodic surface structures revisited: A comparative study on ZnO

A new Schottky junction ultraviolet photodetector (UVPD) is fabricated by coating a free-standing ZnO nanorod (ZnONR) array with a layer of transparent monolayer graphene (MLG) fi lm. The single-crystalline [0001]-oriented ZnONR array has a length of about 8‐11 μ m, and a diameter of 100 ∼ 600 nm. Finite element method (FEM) simulation results show that this novel nanostructure array/MLG heterojunction can trap UV photons effectively within the ZnONRs. By studying the I‐V characteristics in the temperature range of 80‐300 K, the barrier heights of the MLG fi lm/ZnONR array Schottky barrier are estimated at different temperatures. Interestingly, the heterojunction diode with typical rectifying characteristics exhibits a high sensitivity to UV light illumination and a quick response of millisecond rise time/fall times with excellent reproducibility, whereas it is weakly sensitive to visible light irradiation. It is also observed that this UV photodetector (PD) is capable of monitoring a fast switching light with a frequency as high as 2250 Hz. The generality of the above results suggest that this MLG fi lm/ZnONR array Schottky junction UVPD will have potential application in future optoelectronic devices.

Monolayer Graphene Film on ZnO Nanorod Array for High‐Performance Schottky Junction Ultraviolet Photodetectors

Electronic band structure and optical properties of zinc monochalcogenides with zinc-blende- and wurtzite-type structures were studied using the ab initio density functional method within the local-density approximation (LDA), generalized-gradient approximation, and $\mathrm{LDA}+U$ approaches. Calculations of the optical spectra have been performed for the energy range $0--20\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, with and without including spin-orbit coupling. Reflectivity, absorption and extinction coefficients, and refractive index have been computed from the imaginary part of the dielectric function using the Kramers-Kronig transformations. A rigid shift of the calculated optical spectra is found to provide a good first approximation to reproduce experimental observations for almost all the zinc monochalcogenide phases considered. By inspection of the calculated and experimentally determined band-gap values for the zinc monochalcogenide series, the band gap of ZnO with zinc-blende structure has been estimated.

Electronic structure and optical properties of ZnX ( X=O, S, Se, Te): A density functional study

The optical dielectric functions for polarization perpendicular and parallel to the c-axis (optical axis) of pulsed-laser-deposition grown wurtzite MgxZn1−xO (0⩽x⩽0.29) thin films have been determined at room temperature using ellipsometry for photon energies from 1 to 5 eV. The dielectric functions reveal strong excitonic contributions for all Mg concentrations x. The band gap energies (E0A=3.369 eV for ZnO to 4.101 eV for x=0.29) show a remarkable blueshift. The exciton binding energy (61 meV for ZnO) decreases to approximately 50 meV for x≈0.17 and increases to approximately 58 meV for x=0.29. In contrast to ZnO, the MgxZn1−xO alloys are found uniaxial negative below the band gap energy, opposite to previously reported results.

Dielectric functions (1 to 5 eV) of wurtzite MgXZn1 -XO (x≤0.29) thin films

Optical properties of epitaxial ZnO layers have been studied in the spectral region from 1.5 to 5.4 eV using four-zone null spectroscopic ellipsometry. An existing model dielectric function based on excitonic structure near direct band gap has been improved by including a high-energy absorption term. Surface layer, corresponding to the surface roughness, was found to be essential to fit the spectroellipsometric data obtained. Two kinds of samples have been studied: ZnO layers prepared on (0001) and (1120)-oriented sapphire substrates. The surfaces of the first ones were found to be more rough.

C.-H. Ino

Papers

Spectroscopic ellipsometry of epitaxial ZnO layer on sapphire substrate