Binglei Fu

Analysis of Photoluminescence Thermal Quenching: Guidance for the Design of Highly Effective p-type Doping of Nitrides.

A contact-free diagnostic technique for examining position of the impurity energy level of p-type dopants in nitride semiconductors was proposed based on photoluminescence thermal quenching. The Mg ionization energy was extracted by the phenomenological rate-equation model we developed. The diagnostic technique and analysis model reported here are priorities for the design of highly effective p-doping of nitrides and could also be used to explain the abnormal and seldom analyzed low characteristic temperature T0 (about 100 K) of thermal quenching in p-type nitrides systems. An In-Mg co-doped GaN system is given as an example to prove the validity of our methods. Furthermore, a hole concentration as high as 1.94 × 1018 cm−3 was achieved through In-Mg co-doping, which is nearly one order of magnitude higher than typically obtained in our lab.

Anomalous luminescence efficiency enhancement of short-term aged GaN-based blue light-emitting diodes

The origin of anomalous luminescence efficiency enhancement of short-term aged GaN-based blue light-emitting diodes was studied. We found that the intensity of the electroluminescence and photoluminescence spectra were both increased in the very beginning period of aging. With the help of a rate-equation model, we concluded that this kind of luminescence efficiency enhancement is a joint effect of the defect reduction in active layers and the changes out of active layers, for example the Mg acceptor annealing.

Co-doping of magnesium with indium in nitrides: first principle calculation and experiment

In this work, an effective strategy for achieving efficient p-type doping in wide bandgap nitride semiconductors was proposed to overcome the fundamental issue of high activation energy. We demonstrated that a hole concentration as high as 1.4 × 1018 cm−3 could been achieved through In–Mg co-doping. The electronic structure of the system and the formation energy of impurity were analyzed via first principle calculation to clarify the underlying physics and the ambiguity in understanding of the origin of the high hole concentration. Our results indicated that the original valence band maximum of the host materials could be modified, thus improving the p-type dopability. We showed that the calculated ionization energy e(−/0) of acceptor is only about 135 meV, which is much smaller than that of the isolated Mg acceptor.

Electronic structure of SrSn 2 As 2 near the topological critical point

Topological materials with exotic quantum properties are promising candidates for quantum spin electronics. Different classes of topological materials, including Weyl semimetal, topological superconductor, topological insulator and Axion insulator, etc., can be connected to each other via quantum phase transition. For example, it is believed that a trivial band insulator can be twisted into topological phase by increasing spin-orbital coupling or changing the parameters of crystal lattice. With the results of LDA calculation and measurement by angle-resolved photoemission spectroscopy (ARPES), we demonstrate in this work that the electronic structure of SrSn2As2 single crystal has the texture of band inversion near the critical point. The results indicate the possibility of realizing topological quantum phase transition in SrSn2As2 single crystal and obtaining different exotic quantum states.

The Effect of Growth Pressure and Growth Rate on the Properties of Mg-Doped GaN

In this work, the effects of growth pressure and growth rate on electrical and structural properties of Mg-doped GaN were investigated. It has been shown that enhanced growth rates induced by higher growth pressures may lead to decreased structural and electrical properties of p-type GaN layers. If the growth rate is kept unchanged, higher growth pressures will be beneficial for the quality of Mg-doped GaN due to the enhanced NH3 overpressure.

Phosphor-free InGaN micro-pyramid white light emitting diodes with multilayer graphene electrode

We reported the combination of micro-pyramid active layers and graphene electrode to realize the phosphor-free InGaN based white light emitting diodes (LEDs). SEM and TEM measurements were used to characterize the structural qualities of the micro pyramid arrays. According to CL and EDX analyses, the two emitting peaks originated from the indium segregation during the MOCVD process. Multilayer graphene was used to simplify the fabrication process for the electrical connection of the micro-pyramid arrays. The wavelength shift of two emitting peaks were both small with the increase of injection currents, indicating the weak QCSE. What was more, the reverse current leakage was also quite low for the phosphor-free InGaN micro-pyramid white light emitting diodes arrays connected by graphene.

Enhanced performance of InGaN-based light emitting diodes through a special etch and regrown process in n-GaN layer

We reported that the peak efficiency together with the efficiency droop in InGaN-based light emitting diodes could be effectively modified through a simple and low-cost etch-regrown process in n-GaN layer. The etched n-GaN template contained pyramid arrays with inclined side planes. The following lateral overgrowth process from the etched n-GaN template substantially reduced the edge dislocation density and residential compressive strain in epilayers. The efficiency droop of LED samples thus could be modified due to the reduced polarization field, resulting from the strain relaxation in epilayers. What is more, the peak efficiency and reverse current leakage were also modified due to the reduction of dislocations.

Elimination of defects in In–Mg codoped GaN layers probed by strain analysis

The effect of In codoping effect in GaN:Mg layers were investigated through strain analysis. A hydrostatic lattice expansion is induced by In doping which cannot be simply explained by the size effect of In and Mg dopants. Together with the photoluminescence spectrums, this lattice expansion is a strong evidence for the elimination of nitrogen vacancies. The biaxial strain and edge dislocations are reduced with In doping. A theoretic model of interaction between In atoms and edge dislocations are established to explain this phenomenon.

Selective Area Growth and Characterization of GaN Nanorods Fabricated by Adjusting the Hydrogen Flow Rate and Growth Temperature with Metal Organic Chemical Vapor Deposition

GaN nanorods are successfully fabricated by adjusting the flow rate ratio of hydrogen (H2)/nitrogen (N2) and growth temperature of the selective area growth (SAG) method with metal organic chemical vapor deposition (MOCVD). The SAG template is obtained by nanospherical-lens photolithography. It is found that increasing the flow rate of H2 will change the GaN crystal shape from pyramid to vertical rod, while increasing the growth temperature will reduce the diameters of GaN rods to nanometer scale. Finally the GaN nanorods with smooth lateral surface and relatively good quality are obtained under the condition that the H2:N2 ratio is 1:1 and the growth temperature is 1030°C. The good crystal quality and orientation of GaN nanorods are confirmed by high resolution transmission electron microscopy. The cathodoluminescence spectrum suggests that the crystal and optical quality is also improved with increasing the temperature.

The Effects of Mg Back Diffusion Capping Layers on the Performance Enhancement of Blue Light Emitting Diodes With a p-InGaN Last Barrier

The effects of thin Mg back diffusion capping layers on the performance enhancement of InGaN/GaN light emitting diodes (LEDs) with p-InGaN last barrier are investigated experimentally and numerically. By inserting a thin capping layer before the p-InGaN last barrier, the Mg back diffusion effect could be effectively suppressed and the optical performance of LEDs could be enhanced due to the improved hole injection and reduced electron leakage. What is more, the p-InGaN LED with GaN capping layer showed further reduced efficiency droop compared with that with InGaN capping layer as a result of the more uniform hole distribution in active layers.