Chengming Li

The ultrafast excitation processes in femtosecond laser-induced damage in dielectric omnidirectional reflectors

A pump and probe system is developed, where the probe pulse duration τ is less than 60fs while the pump pulse is stretched up to 150–670fs. The time-resolved excitation processes and damage mechanisms in the omnidirectional reflectors SiO2∕TiO2 and ZnS∕MgF2 are studied. It is found that as the pump pulse energy is higher than the threshold value, the reflectivity of the probe pulse decreases rapidly during the former half, rather than around the peak of the pump pulse. A coupled dynamic model based on the avalanche ionization (AI) theory is used to study the excitation processes in the sample and its inverse influences on the pump pulse. The results indicate that as pulse duration is longer than 150fs, photoionization (PI) and AI both play important roles in the generation of conduction band electrons (CBEs); the CBE density generated via AI is higher than that via PI by a factor of 102–104. The theory explains well the experimental results about the ultrafast excitation processes and the threshold fluences.

Investigation of cracks in GaN films grown by combined hydride and metal organic vapor-phase epitaxial method

Cracks appeared in GaN epitaxial layers which were grown by a novel method combining metal organic vapor-phase epitaxy (MOCVD) and hydride vapor-phase epitaxy (HVPE) in one chamber. The origin of cracks in a 22-μm thick GaN film was fully investigated by high-resolution X-ray diffraction (XRD), micro-Raman spectra, and scanning electron microscopy (SEM). Many cracks under the surface were first observed by SEM after etching for 10 min. By investigating the cross section of the sample with high-resolution micro-Raman spectra, the distribution of the stress along the depth was determined. From the interface of the film/substrate to the top surface of the film, several turnings were found. A large compressive stress existed at the interface. The stress went down as the detecting area was moved up from the interface to the overlayer, and it was maintained at a large value for a long depth area. Then it went down again, and it finally increased near the top surface. The cross-section of the film was observed after cleaving and etching for 2 min. It was found that the crystal quality of the healed part was nearly the same as the uncracked region. This indicated that cracking occurred in the growth, when the tensile stress accumulated and reached the critical value. Moreover, the cracks would heal because of high lateral growth rate.

A Theoretical Calculation of the Impact of GaN Cap and

The electron mobility in a 2-D electron gas in a GaN/Al_xGa_1-xN/GaN heterostructure limited by GaN cap-thickness-fluctuation (CTF) and Al_xGa_1-xN barrier thickness-fluctuation (BTF) scattering is calculated considering the strong spontaneous and piezoelectric polarization. The calculated results reveal that the electron mobility limited by CTF and BTF scattering is lower than that limited by interface roughness scattering if the Al_xGa_1-xN barrier layer is thin enough (several nanometers).

\hbox{Al}_{x}\hbox{Ga}_{1-x}\hbox{N}

The electron mobility limited by spacer layer thickness fluctuation (SLTF) scattering on the two-dimensional electron gas in AlGaAs/GaAs modulation-doped heterostructure is investigated. Although the SLTF scattering and the interface roughness scattering are both induced by the roughness of the AlGaAs/GaAs interface, they are two different scattering mechanisms. The interface roughness will lead to the fluctuation of the distance from the electrons to the ideal interface and the fluctuation of the spacer layer thickness. The former induces the fluctuation of the electron potential, which works as the interface roughness scattering potential. The latter induces the fluctuation of the sheet carrier density in the channel, which causes a fluctuation in the quantization energy level. The quantization energy level fluctuation works as the SLTF scattering potential. Compared with the interface roughness scattering, the results reveal that the SLTF scattering becomes the dominant scattering mechanism when the do...

Barrier Thickness Fluctuations on Two-Dimensional Electron Gas in a

We theoretically study the influence of spacer layer thickness fluctuation (SLTF) on the mobility of a two-dimensional electron gas (2DEG) in the modulation-doped AlxGa1−xAs/GaAs/AlxGa1−xAs quantum well. The dependence of the mobility limited by SLTF scattering on spacer layer thickness and donor density are obtained. The results show that SLTF scattering is an important scattering mechanism for the quantum well structure with a thick well layer.