Y. S. Huang

Characterization of RuO2 thin films by Raman spectroscopy

Abstract Raman scattering has been used as a technique for the characterization of RuO 2 thin films deposited on different substrates by the metal-organic chemical vapor deposition method and reactive sputtering under various conditions. Red shift and broadening of the linewidth of the Raman peaks are analyzed by the spatial correlation model. The intrinsic linewidth for the RuO 2 films deposited at high and low temperatures has to be adjusted to different values to achieve a good fit for the features. The lineshape and position of the Raman features vary for films deposited on different substrates under the same condition. These differences indicate the existence of stress induced by lattice mismatch and the differential thermal expansion coefficients between RuO 2 and the substrates. After annealing at 650°C in an oxygen atmosphere for 3 h, the linewidth decreases significantly for the RuO 2 thin films deposited at lower temperatures. The results show the improvement of the crystallinity of the films during the annealing process.

Characterization of RuO2 thin films deposited on Si by metal-organic chemical vapor deposition

We report characterization of RuO2 thin films, deposited on Si substrates by metal-organic chemical vapor deposition (MOCVD), by scanning electron microscopy, X-ray diffraction, electrical conductivity, spectrophotometry, ellipsometry and Raman scattering measurements. As-deposited RuO2 films are specular, crack free, and well adherent on the substrate. The Auger electron spectroscopy depth profile shows good compositional uniformity across the thickness of the films. As confirmed by X-ray investigations, the films crystallize with the correct rutile structure. The results of the electrical and optical studies of the MOCVD RuO2 films show a metallic character of these films. The results of Raman investigation indicate that a nearly strain free and high quality RuO2 thin film could be deposited on a Si substrate by MOCVD.

Characterization of IrO2 thin films by Raman spectroscopy

Abstract Raman scattering has been used as a technique for characterization of the sputtered IrO 2 thin films (SIROF) deposited on different substrates under various conditions. Compared with the spectrum of single crystal IrO 2 , red shift and broadening of the linewidth of the Raman peaks of SIROF are observed. X-ray diffraction measurements of SIROF were also carried out to assist the identification of the factor that influences the linewidth broadening of the Raman features. The results indicate that amorphous–crystalline transition for IrO 2 phase can be achieved at substrate temperature of 200–300 °C. The line-shape and position of the Raman features vary for films deposited on different substrates under the same conditions. These differences can be due to the existence of stress between IrO 2 and the substrates.

Temperature dependence of energies and broadening parameters of the band-edge excitons of single crystals

We have measured the temperature dependence of the spectral features in the vicinity of direct band-edge excitonic transitions of single crystals from 25 to 300 K using piezoreflectance (PzR). From a detailed lineshape fit of the PzR spectra, the energies and broadening parameters of the A and B excitons have been determined accurately. The origin of these excitonic transitions is discussed. The transition energies and their splittings vary smoothly with the tungsten composition x, indicating that the natures of the direct band edges are similar for the compounds. In addition, the parameters that describe the temperature variation of the energies and broadening function of the excitonic transitions are evaluated and discussed.

Raman spectrum of IrO2

We report the first Raman scattering measurement of the rutile structure compound IrO2. The four allowed first-order Raman active phonon modes have been observed and classified. The spectrum exhibits strong lines of A1g and Eg symmetries, a slightly broadened line of B2g symmetry, and a very weak sharp low-frequency B1g mode.

Optical absorption of ReS2 and ReSe2 single crystals

The optical absorption of synthetic ReS2 and ReSe2 single crystals is reported over a temperature range from 25 to 300 K. Analysis reveals that the absorption edges of ReS2 and ReSe2 are indirect allowed transitions. The indirect band gaps at various temperatures are determined and their temperature dependence is analyzed by the Varshni equation [Physica 34, 149 (1967)] and an empirical expression proposed by O’Donnel and Chen [Appl. Phys. Lett. 58, 2924 (1991)]. The parameters that describe the temperature dependence of energy gaps of these two materials are evaluated and discussed.

Characterization of sputtered iridium dioxide thin films

Iridium dioxide (IrO 2 ) thin films, deposited on Si substrates by reactive rf sputtering method under various conditions, were characterized by atomic force microscopy (AFM), x-ray diffraction (XRD), electrical-conductivity, spectrophotometry, ellipsometry and Raman scattering measurements. The average grain sizes of the films were estimated by AFM. A grain boundary scattering model was used to fit the relation between the average grain size and electrical resistivity. The optical and dielectric constants were determined by the ellipsometry measurements. The results of the electrical and optical studies show a metallic character of the films deposited at higher temperatures. The results of XRD and Raman scattering indicate that the IrO 2 films deposited at temperatures higher than 300 °C show the presence of (200) texture.

Nonlinear behaviors of valence-band splitting and deformation potential in dilute GaNxAs1−x alloys

Photoreflectance and piezoreflectance investigations have been performed on a series of GaNAs layers grown by low-pressure metal-organic chemical vapor deposition on Si-doped GaAs substrate. In addition to the observation of the fundamental band-gap and spin-orbit splitting, the valence-band splitting in thin GaNxAs1−x epilayers strained coherently by the GaAs substrate is observed in these modulation spectra. Comparing photoreflectance with piezoreflectance spectra, we clearly establish the transitions involving the heavy-hole and light-hole bands. We find that the valence-band splitting increases with increasing nitrogen composition, and it decreases with increasing temperature. We point out that the underlying origin of our observation can be attributed to the effect of lattice mismatch between GaNAs film and GaAs substrate. We also find that the deformation potential of GaNxAs1−x does not follow the linear interpolation of those for GaAs and GaN. It shows a strong composition dependence with a bowing ...

Temperature dependence of quantized states in an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure

Piezoreflectance (PzR) and contactless electroreflectance (CER) measurements of an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure as a function of temperature in the range of 20–300 K have been carried out. A careful analysis of the PzR and CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of EmnH(L) are evaluated. A detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of 11H exciton is evaluated and compared with that of the bulk material.

Temperature-dependent photoconductivity in β-In2S3 single crystals

The solar-energy absorption material β-In2S3 was characterized using temperature-dependent photoconductivity (PC) measurements in the temperature range between 30 and 340 K in this study, and thermoreflectance (TR) and photoluminescence (PL) measurements were carried out to identify near-band-edge transitions in the β-In2S3 tetragonal crystal. The experimental analyses of PL, PC, and TR confirmed that β-In2S3 is a direct semiconductor with a band gap of 2.073 eV at 30 K and 1.935 eV at 300 K. The PL and PC spectra manifest some defect-related features in the β-In2S3 single crystal. Two defect emissions and two band-edge luminescences were simultaneously detected in the PL spectrum at 30 K, and the temperature-dependent PC spectra of β-In2S3 from 160 to 300 K reveal an additional defectlike or band-to-band feature with an energy located above the conduction band edge (EC). The temperature dependences of the PC transition features in the β-In2S3 defect crystal were analyzed. The origin and mechanism of all ...