Ding Xun-Min

Positive and Negative Pulse Etching Method of Porous Silicon Fabrication

We present a new method in which both positive and negative pulses are used to etch silicon for fabrication of porous silicon (PS) monolayer. The optical thickness and morphology of PS monolayer fabricated with different negative pulse voltages are investigated by means of reflectance spectra, scanning electron microscopy and photoluminescence spectra. It is found that with this method the PS monolayer is thicker and more uniform. The micropores also appear to be more regular than those made by common positive pulse etching. This phenomenon is attributed to the vertical etching effect of the PS monolayer being strengthened while lateral etching process is restrained. The explanation we propose is that negative pulse can help the hydrogen cations (H+) in the electrolyte move into the micropores of PS monolayer. These H+ ions combine with the Si atoms on the wall of new-formed micropores, leading to formation of Si–H bonds. The formation of Si–H bonds results in a hole depletion layer near the micropore wall surface, which decreases hole density on the surface, preventing the micropore wall from being eroded laterally by F− anions. Therefore during the positive pulse period the etching reaction occurs exclusively only at the bottom of the micropores where lots of holes are provided by the anode.

Temperature: a critical parameter affecting the optical properties of porous silicon

The optical properties of porous silicon (PS) samples fabricated by pulse etching in a temperature range from −40 to 50°C have been investigated using reflectance spectroscopy, photoluminescence spectroscopy, and scanning electron microscopy (SEM). The dependence of the optical parameters, such as the refractive index n and the optical thickness (nd) of PS samples, on the etching temperature has been analyzed in detail. As the etching temperature decreases, n decreases, indicating a higher porosity, and the physical thickness of PS samples also decreases. Meanwhile, the reflectance spectra exhibit a more intense interference band and the interfaces are smoother. In addition, the intensity of the PL emission spectra is dramatically increased.

Studies on the microstructure,optical and electrical properties of organic microcavity devices based on a porous silicon reflector

A novel type of microcavity organic light-emitting diode based on a porous silicon distributed Bragg reflector (PS-DBR) has first been achieved and its microstructure, optical, and electrical properties have also been investigated in detail. The microcavity is made up of the central active organic multilayer sandwiched between a top silver film and a bottom PS-DBR, formed by electrochemical etching of p++-Si substrate. The fieldemission scanning electron microscopy cross-section images show the nanometer-scale layered structure and flat interfaces inside the microcavity. The reflectivity (relative to an Al mirror) of the PS-DBR is up to 99%, and the stopband is about 160 nm wide. Resonant cavity mode appears as a tip in the reflectivity spectrum of the Si-based organic multilayer films, which is a symbol that the Si-based organic multilayer structure is indeed a microcavity. The peak widths of the electroluminescence (EL) spectra from the cavities emitting green and red light are greatly reduced from 85 nm and 70 nm to 8 nm and 12 nm, respectively, as compared with those measured from non-cavity structures. Note that the EL emission from the cavity devices is single-mode, and the off-resonant optical modes are highly suppressed. Moreover, increases of a factor of about 6 and 4 of the resonant peak intensity from the cavities emitting green and red light are also observed, respectively. In addition, the current-brightness-voltage characteristics and effect parameters on the lifetime of the cavity devices are also discussed. The present technique for obtaining enhanced EL emission from Si-based organic microcavity may also be another novel effective method for realizing Si-based optoelectronics device integration.

An angle resolved photoemission study of InP(1̄1̄1̄) clean surface

The energy dispersive relation of surface dangling bond state on InP() clean surface has been measured by the angle resolved photomission. The theoretical calculation based on a slab model and extended Huckel theory agreed well with the experimental result if a surface relaxed atomic structure model for InP() (1×1) is postulated.

The effect of sodium adlayers on the adsorption of oxygen on Ag(100)

The application of HREELS to the study of the Ag(100)/Na+O2 system reveals the presence of molecularly adsorbed oxygen species at room temperature. This may help explain why alkali metals can enhance the selectivity of an Ag catalyst towards the formation of C2H4O.