We have investigated the transverse mode pattern and the optical field confinement factor of gallium nitride (GaN) laser diodes (LDs) theoretically.For the particular LD structure,composed of approximate 4 μm thick n-GaN substrate layer,the maximum optical confinement factor was found to be corresponding to the 5 th order transverse mode,the so-called lasing mode.Moreover,the value of the maximum confinement factor varies periodically when increasing the n-side GaN layer thickness,which simultaneously changes and increases the oscillation mode order of the GaN LD caused by the effects of mode coupling.The effects of the thickness and the average composition of Al in the AlGaN/GaN superlattice on the optical confinement factor are also presented.Finally,the mode coupling and optimization of the layers in the GaN-based LD are discussed.
The microwave photonic filters (MPFs) based on serially coupled silicon microring resonators (MRRs) are theoretically analyzed for the application of 60-GHz millimeter wave wireless personal area networks. This is achieved by calculating the improvement of bit error ratio (BER). According to the simulation results, the requirement of signal-to-noise ratio (SNR) of the received data can be reduced by 14 dB for the same BER with and without MPFs. The performance of the MPF with five serially coupled microring structures is better than that of the MPF with a single microring, owing to the improvement of the shape factor.
The formation process of silicon-nanocrystals (Si-NCs) in the amorphous silicon/silicon dioxide (a-Si/SiO2) multilayer structure during thermal annealing is theoretically studied with a modified model based on the Gibbs free energy variation. In this model, the concept of average effective interfacial free energy variation is introduced and the whole formation process consisting of nucleation and subsequent growth is considered. The calculating results indicate that there is a lower limit of the silicon layer thickness for forming Si-NCs in a-Si/SiO2 multilayer, and the oxide interfaces cannot constrain their lateral growth. Furthermore, by comparing the results for a-Si/SiO2 and a-Si/SiNx multilayers, it is found that the constraint on the crystal growth from the dielectric interfaces depends on the difference between interfacial free energies.
Two strain-state samples of GaN,labelled the strain-relief sample and the quality-improved sample,were grown by hydride vapour phase epitaxy (HVPE),and then characterized by high-resolution X-ray diffraction,photoluminescence and optical microscopy.Two strain states of GaN in HVPE,like 3D and 2D growth modes in metal-organic chemical vapour deposition (MOCVD),provide an effective way to solve the heteroepitaxial problems of both strain relief and quality improvement.The gradual variation method (GVM),developed based on the two strain states,is characterized by growth parameters' gradual variation alternating between the strain-relief growth conditions and the quality-improved growth conditions.In GVM,the introduction of the strain-relief amplitude,which is defined by the range from the quality-improved growth conditions to the strain-relief growth conditions,makes the strain-relief control concise and effective.The 300-μm thick bright and crack-free GaN film grown on a two-inch sapphire proves the effectiveness of GVM.
We report on the growth and fabrication of nonpolar a-plane light emitting diodes with an in-situ SiN x interlayer grown between the undoped a-plane GaN buffer and Si-doped GaN layer.X-ray diffraction shows that the crystalline quality of the GaN buffer layer is greatly improved with the introduction of the SiN x interlayer.The electrical properties are also improved.For example,electron mobility and sheet resistance are reduced from high resistance to 31.6 cm 2 /(V · s) and 460 /respectively.Owing to the significant effect of the SiN x interlayer,a-plane LEDs are realized.Electroluminescence of a nonpolar a-plane light-emitting diode with a wavelength of 488nm is demonstrated.The emission peak remains constant when the injection current increases to over 20 mA.
Non-polar a-plane (110) GaN films have been grown on r-plane (102) sapphire substrates by metal organic chemical vapour deposition. The influences of Ⅴ/Ⅲ ratio on the species diffusion anisotropy of a-plane GaN films were investigated by scanning electron microscopy, cathodoluminescence and high-resolution x-ray diffraction measurements. The anisotropy of a-plane GaN films may result from the different migration length of adatoms along two in-plane directions. Ⅴ/Ⅲ ratio has an effect on the growth rates of different facets and crystal quality. The stripe feature morphology was obviously observed in the film with a high V/III ratio because of the slow growth rate along the [100] direction. When the Ⅴ/Ⅲ ratio increased from 1000 to 6000, the in-plane crystal quality anisotropy was decreased due to the weakened predominance in migration length of gallium adatoms.
