P. H. Hao

Study of the Raman peak shift and the linewidth of light‐emitting porous silicon

The correlation between the Raman peak shift and the linewidth of porous silicon is studied. The experimental result does not fit with the relationship predicted by the phonon confinement model. By taking into account both the phonon confinement and the effect of strain, the calculated Raman line shape coincides fairly well with the measured spectrum. The built‐in strain of porous silicon varies with the porosity of the sample and is on the order of 10−3.

Large blue shift of light emitting porous silicon by boiling water treatment

A boiling water treatment of light emitting porous silicon can give rise to a large blue shift of its photoluminescence spectrum and meanwhile strengthen the skeleton of porous Si by filling up many pores with aqueous oxide. A stable blue‐green light emission at the peak wavelength down to 500 nm is achieved. FTIR measurements show that the formation of Si dihydride on the sidewall surfaces of the Si rods is not responsible to the visible luminescence for the very thin Si wires.

Electrochemical sulfur passivation of GaAs

An anodic sulfurized treatment of GaAs has been developed to passivate its surfaces preventing oxidation. The photoemission core level spectra show that the surface Ga and As atoms are bonded to S atoms to form a thick sulfurized layer. No oxygen uptakes on the sulfurized GaAs surface as illustrated by the high resolution electron energy loss spectroscopy. The results of photoluminescence spectra verify that the passivated surface has low surface recombination velocity and can protect the photoassisted oxidation under laser illumination.

Critical conditions for achieving blue light emission from porous silicon

By observing the luminescence micrographic images and measuring the decay behaviors of photoluminescence spectra, it is found that the blue light‐emitting porous silicon obtained by boiling water treatment behaves very similarly to the red light‐emitting sample. It is thus believed that the blue light emission is originated from the porous silicon skeleton rather than impurity contaminations. The achievement of blue light emission requires the proper control of the size of the Si nanostructures, effective passivation of the internal surfaces of porous silicon layer, and keeping a mechanically strong Si skeleton. A theoretical estimation and the experiments show that the simultaneous fulfillment of these conditions is quite critical, which explains the poor reproducibility of achieving blue emission experimentally.

Energy band lineup at the porous‐silicon/silicon heterointerface measured by electron spectroscopy

The energy band gap of light‐emitting porous silicon is determined by high‐resolution electron energy loss spectroscopy, and the valence band edge of porous silicon with respect to its Fermi level is measured by ultraviolet photoelectron spectroscopy. By combining the results with that measured from clean Si, a picture of band lineup at the porous‐silicon/p‐Si heterointerface is proposed, in which 70% of the total band gap discontinuity occurs at the valence band edge.

Improvement of electroluminescence properties of light-emitting porous silicon

By applying some post-pore-forming treatments, we have achieved solid state electroluminescence of an Au/PS/Si structure. Stable visible light emission is observed under forward bias with a threshold voltage and current density down to 6 V and 30 mA cm-2.

Optical Study of the Porous Silicon Layer

The porous-Si samples were prepared for optical studies by using the PL, Raman scattering, as well as the absolute reflectance and ellipsometry methods. The results show that the porous Si has low optic constants, and can trap the visible photons of more than g5%, but give no evidence of a strong direct interband transition existing in the visible region. The Lorentz oscillator and Bruggeman EMA models were used in data analyses. The calculations show that the layer dispersion effect may result in a red shift of the PL peak. The possible mechanism for the PL and Raman enhancement as well as the photon trap phenomenon was discussed, and was attributed mainly to randomly multiple micrereflections inside the porous-Si layer having extremely large internal micro-surfaces.