Rongchuan Fang

Femtosecond laser-induced periodic surface structure on diamond film

We report the laser-induced periodic surface structure (LIPSS) with periodicity about a quarter of the laser wavelength on unpolished diamond film treated by a P-polarized femtosecond laser. The short period LIPSS is parallel to the laser polarization and independent on the incidence angle. The LIPSS perpendicular to the laser polarization with periodicity shorter than a third of the laser wavelength slightly dependent on the incidence angle is also observed as well as the LIPSS perpendicular to the laser polarization with periodicity dependent on the incidence angle. The results are explained by interference of the incident laser and surface scattered wave related to the excited electrons during laser interactions with diamond, being in excellent agreement with a previously developed theory.

Optical properties of ZnO/GaN heterostructure and its near-ultraviolet light-emitting diode

We report on photoluminescence in a ZnO/GaN heterostructure, which showed a donor–acceptor pair emission band at 3.270 eV and the longitudinal optical phonon replicas at 12 K. In comparison with p-type GaN, the positions of the peaks are redshifted. This may be associated with the variation of the residual strain in the GaN layer of the heterostructure. Using this heterostructure, near-ultraviolet light-emitting diodes were fabricated and their electroluminescence properties were characterized.

Characterization of the diamond growth process using optical emission spectroscopy

In situ optical emission spectroscopy was applied to an electron assisted hot filament diamond growth process. Emission lines from the Balmer series of atomic hydrogen, molecular hydrogen, CH, CH+, and Ar were observed in the visible range when CH4, Ar, and H2 were used as the input gases. The relative concentration of atomic hydrogen was estimated by using the emission line of Ar at 750.4 nm as an actinometer. The effects of deposition conditions, including filament temperature, substrate temperature, bias voltage, and methane concentration in the source gas, on the species distribution, electron temperature, and diamond film growth were investigated. The vertical spatially resolved measurements show that the concentration of atomic hydrogen decreases as the detection point moves far from the filament and suddenly drops near the substrate. The horizontal spatially resolved measurements show that the homogeneous region of reaction species and the electron temperature near the substrate define the homogene...

Evidence of the role of positive bias in diamond growth by hot filament chemical vapor deposition

Diamond films have been deposited on a positively biased silicon substrate by hot filament chemical vapor deposition. It is found that the size distribution of the diamond particle is uniform under bias conditions. The effects of the bias on reactive gas composition were investigated by in situ infrared absorption and in situ optical emission with Ar actinometry. These techniques indicate that the bias does not significantly influence the gas composition. Diamond growth under bias conditions for a small region masked by metal Mo is similar to that without bias. These results confirm that the influence of bias on diamond growth is caused by electron and/or negative ion bombardment on the surface of the substrate and the growing crystallites rather than by the change in gaseous environments.

Analysis of optical emission spectroscopy in diamond chemical vapor deposition

Abstract We used optical emission spectroscopy (OES) to study the gas phase chemistry in hot filament chemical vapor deposition (HFCVD) diamond processes. The results show that the methane concentration strongly influenced the intensity ratios of CH, CH+ and Hγ to Hβ, and the effects of the pressure and filament temperature on the relative concentrations of the species were also analyzed. Spatially resolved OES implied that a relative high concentration of atomic H existed near the substrate surface, which is favorable for diamond film growth. Although the relatively high concentrations of CH and CH+ to atomic H are beneficial to increasing diamond nucleation density, they are very harmful to the growth of diamond films.

Photoluminescence and its decay of the dye/porous‐silicon composite system

Photoluminescence (PL) and its decay in the composite system of dye/porous silicon, consisting of oxidized porous silicon (Psi) and impregnated dyes, have been studied. We find that the PL spectra of the dyes are similar to that of dyes in ethanol solution while being quite different from those of dye films absorbed on the surface of c‐Si. The line shape of the spectra is more symmetric for impregnated dyes than that in solution. The PL decay of the impregnated dyes has been measured. The lifetime is about 100 ps for rhodamine B impregnated in as‐prepared Psi and it becomes much shorter for that in the annealed Psi layers. The influence of absorbed dyes on the PL decay of Psi has also been measured. It shows that impregnated dyes provide new nonradiative processes for the deactivation of the excited Psi.

Two-step growth of high quality diamond films

Diamond films with high quality were successfully deposited using a novel growth method-two-step growth technique-in hot filament chemical vapor deposition (CVD) chamber. The characterization of diamond films were investigated by scanning electron microscopy and Raman scattering, which show noticeable improvement in quality of diamond films. The high surface resistivity of diamond films was also investigated. It suggests that the decreasing of activated carbon concentration near the substrate surface and the increasing of secondary nucleation density of diamond are responsible for the well-behaved diamond film growth. In addition, the thermal stability of the diamond layer surface is also beneficial to the growth of the diamond film.

Raman investigation of amorphous carbon in diamond film treated by laser

Micro-Raman spectroscopy was employed to investigate the structural changes of diamond films prepared by hot filament chemical vapor deposition and treated by femtosecond (fs) laser and nanosecond (ns) lasers. Breit–Wigner–Fano and Lorentzian line shape simulations were used to fit the spectra. For 266 nm ns laser treated samples, increasing laser power density results in the transformation of amorphous carbons in diamond films into nanocarbon clusters whose size increases and saturates rapidly at around 5.1 nm. At the same time, the Raman G peak position considerably shifts upwardly with increasing laser power density. The different change behavior of the nanocarbons and G peak is interpreted in light of the charge transfer from the graphite π bands to the localized edge states. As the 266 nm laser power density is high enough, a Raman peak in the range of 1150–1200 cm−1 appears, which is attributed to the presence of amorphous diamond. In the case of fs laser treated samples, much more power density (>1...

Study of diamond film growth mechanism on porous silicon during hot-filament chemical vapor deposition

Abstract The nucleation and growth process of diamond film on porous silicon in a hot-filament chemical vapor deposition system were investigated. The nucleation density of 3.6×107 cm−2 was obtained. We find that almost all of the nuclei occur at the edge of the etched pores, and the continuous diamond films are successfully deposited without seeding diamond particles. The characters of diamond films were determined by scanning electronic microscopy, Raman spectra and X-ray photoelectron spectroscopy. No strains are found in diamond films, and no intermediates are found between the films and porous silicon substrates.

Growth and characterization of diamond films deposited by d.c. discharge assisted hot filament chemical vapor deposition

Diamond films have been deposited by d.c. discharge assisted hot filament chemical vapor deposition. The diamond nucleation density was significantly enhanced more than five orders of magnitude by this d.c. discharge assisted process. The effects of deposition parameters including deposition time, temperature, and discharge current on the diamond growth and gas phase composition were studied by Raman scattering, scanning electron microscopy, and optical emission spectroscopy. The mechanisms of the discharge enhanced nucleation of diamond are discussed.