Xuekun Lu

Si1−xGex/Si asymmetric 2×2 electro‐optical switch of total internal reflection type

Based on plasma dispersion of Si1−xGex, we have fabricated asymmetric 2×2 switches of total internal reflection type, in which Si1−xGex was grown by molecular beam epitaxy. The optimum intersecting angle is 4°, and the crosstalk is less than −10.6 dB at 76 mA injection current. The insertion loss is 2.8 dB, and the switch time is 0.6 μs.

Crystalline quality of Si epilayers influenced by Sb doping

Reflection high energy electron diffraction and Raman scattering show that the crystalline quality of Sb-doped epilayers is strongly dependent on the doping conditions. Surface roughening, which is induced by surface segregation of the dopants during growth, degrades the quality of the film grown and develops with growth. Surface roughening could be explained by a two-state-exchange model of Sb segregation.

Molecular beam epitaxy growth of Ge on Si(111) substrates covered by a SiO2 mask

Abstract The chemical reaction of Ge beam with thermally grown SiO 2 in ultra-high vacuum (UHV) producing volatile Ge and Si oxides was described. Using this experimental result, a Ge film grown in the windows of the SiO 2 mask at the Si(111) substrate without leaving any Ge residuals at the SiO 2 was realized. The Ge film grown in the mask windows was formed by islands. The transition from coheren Stranski-Krastanov (SK) mode to true SK mode for Ge growing at the Si substrate was discussed.

Observation of boron doping induced surface roughening in silicon molecular beam epitaxy

Boron doping induced surface roughening was observed in conventional silicon molecular beam epitaxy. Reflection high energy electron diffraction and cross‐sectional transmission electron microscopy revealed that as growth continued, the growth surface remained no longer planar but developed {113} facets. The facets evolved along with growth, and finally resulted in a severely roughened surface. The evolution of the roughening was found to remain the same in the boron doping concentration range of 1×1017–2×1020 cm−3 and the growth temperature range of 500–650 °C. This surface roughening effect is attributed to boron segregation behavior.

Application of HREELS to Al/GaAs and Al/GaP interfaces

Abstract By measuring the threshold of electronic transitions in the electron energy loss spectrum of a semiconductor surface, the band gap of the substrate could be determined in the case that surface states in the gap have been removed. For the systems of Al/GaAs(100) and Al/GaP( 1 1 1 ), such kinds of measurements reveal that Al-Ga exchange reactions take place in the vicinity of the interface even at room temperature. Annealing of the two systems to 400–450°C leads to different results: ternary alloy AlGaAs is formed in the former case while for the latter, the species of the reaction products remain unchanged.

Photoluminescence from trapped excitons in quantum well structures

Photoluminescence spectra from quantum well structures grown at high temperatures are investigated. The luminescence properties are found to be very different from those of free excitons. To describe correctly the spectral lineshape, the radiative recombination from excitons trapped on the local potential fluctuations in quantum wells must be considered. At low sample temperatures, the luminescence is mainly from the trapped excitons. With increasing temperature, trapped excitons are thermally activated into free excitons and luminescence peaks shift to higher energies. By comparing measured spectra with the calculated spectra, the trap density and the trap energy are derived. The origin of the trap and its relation with the crystal growth are discussed.

Ge Beam Treatment of Si Substrate for Molecular Beam Epitaxy

A new surface cleaning method for Si MBE is described in which the Si surface is exposed to Ge beams while the substrate is kept at certain temperature. It has been proved that the thin passivation layer of SiO 2 on the Si substrate will react with Ge at a relatively low temperature (620°C), and the products are volatile. The residual Ge on Si substrate can be reduced to less than 0.1 monolayer (ML). Ge beam treatment turns out to be an effective low temperature technique for preparing Si substrate, especially for the heteroepitaxial growth of Ge x Si 1-x /Si.

Co-evaporative Sb doping and crystalline quality in Si molecular beam epitaxy

The co-evaporative Sb doping in Si molecular beam epitaxy was studied with the emphasis on the crystalline quality of the films. By using reflective high energy electron diffraction, cross-sectional transmission electron microscopy, spread resistance profiling and secondary ion mass spectroscopy, it was found that doping growths at high substrate temperatures (≥ 500°C) would degrade the crystalline quality of the films. This degradation was attributed to Sb segregation. In contrast, the films would retain good quality in doping growths at low substrate temperatures (< 500°C) due to suppression of the segregation. Growth at low substrate temperature (350–400°C) plus post-annealing is recommended for growing Sb-doped Si films.