Jiejun Wu

Ultrafast optical response of the Au–BaO thin film stimulated by femtosecond pulse laser

The pump-probe method was applied to study the dependence of the transient optical transmissivity upon time delay for the Au–BaO composite thin film stimulated by a femtosecond pulsed laser. It was observed that the light absorption of the thin film increased rapidly and then resumed its initial value in several picoseconds. Optical relaxation is a process in which nonequilibrium electrons, excited by laser pulses and originating from Au ultrafine particles, return to the equilibrium state. In this article, the value of the electron-phonon coupling constant g of gold ultrafine particles in the composite thin film was calculated theoretically.

Large-roll growth of 25-inch hexagonal BN monolayer film for self-release buffer layer of free-standing GaN wafer

Hexagonal boron nitride (h-BN) is known as promising 2D material with a wide band-gap (~6 eV). However, the growth size of h-BN film is strongly limited by the size of reaction chamber. Here, we demonstrate the large-roll synthesis of monolayer and controllable sub-monolayer h-BN film on wound Cu foil by low pressure chemical vapor deposition (LPCVD) method. By winding the Cu foil substrate into mainspring shape supported by a multi-prong quartz fork, the reactor size limit could be overcome by extending the substrate area to a continuous 2D curl of plane inward. An extremely large-size monolayer h-BN film has been achieved over 25 inches in a 1.2” tube. The optical band gap of h-BN monolayer was determined to be 6.0 eV. The h-BN film was uniformly transferred onto 2” GaN or 4” Si wafer surfaces as a release buffer layer. By HVPE method, overgrowth of thick GaN wafer over 200 μm has been achieved free of residual strain, which could provide high quality homo-epitaxial substrate.

In situ self-release of thick GaN wafer from sapphire substrate via graded strain field engineering

We present a scheme for in situ self-release of thick GaN 2-in. wafer from sapphire substrate by engineering the gradient of misfit strains. Release energies of a-, m-, and c-planes of wurtzite GaN are systematically calculated under different biaxial strains by using first-principles method. The results reveal that the c-plane separation will take place under graded strains around −2.8%, where a drastic transition interface of release energy may strongly reduce the strength of c-plane bonding. Based on this mechanism, uniform thick GaN epilayer (>450 μm) is grown on (0001) sapphire substrate by hydride vapor phase epitaxy and subjected to a graded compressive strain field by bowing, fulfilling the c-plane separation condition. As a result, high quality free-standing GaN wafer (350 μm) can be achieved by self-release simply during the cooling process.

Angular distribution of polarized spontaneous emissions and its effect on light extraction behavior in InGaN-based light emitting diodes

Angular intensity distributions of differently polarized light sources in multiple quantum wells (MQWs) and their effects on extraction behavior of spontaneous emission from light emitting diode (LED) chips have been studied. Theoretical calculation based on k·p approximation, ray tracing simulation and angular electroluminescence measurement were applied in this work. It is found that the electron-hole recombination in the InGaN MQWs produces a spherical distribution of an s-polarized source and a dumbbell-shaped p-polarized source. Light rays from different polarized sources experience different extraction processes, determining the polarization degree of electro-luminescence and extraction efficiency of LEDs.

Inclined Dislocation Generation in Compressive-Strain-Enhanced Mg-Doped GaN/Al0.15Ga0.85N Superlattice with AlN Interlayer

The a- and a+c-type threading dislocation (TD) inclinations were observed in Mg-doped tensile-strained GaN/Al0.15Ga0.85N superlattice (SL) with a high-temperature (HT) AlN interlayer (IL). Most of the inclined dislocations are a-type, toward the equivalent [100] line direction, following dislocation climb model, while a+c-type dislocations gliding on (101) second slip plane inclined toward the [110] line direction. The analysis by comparing samples gave no evidence of Mg dopants to affect the TD inclination. The stress evolution, specifically, the stress transformation of the tensile-stress-relieved AlGaN barrier to the compressive-strain-enhanced GaN well caused by the HT-AlN IL, dominantly affected the a- and a+c-type TD inclinations.

Changing oblique angles of pyramid facets fabricated by wet etching of N polar GaN

Wet etching was performed on N polar GaN, which was fabricated by laser lift-off from a sapphire substrate. Dodecagonal pyramids appeared on the N-polar GaN surface after immersion into hot H3PO4 solution even if it had been etched previously with hot KOH solution. According to the symmetry of the space group of C6v4-P6mc, the oblique angle and crystallographic plane indices of the pyramid facets were obtained. It was observed that the oblique angles of the etched facets decreased from the tip to the base of the pyramids. The etching rate was fast when the etching temperature was above 130 °C, and the oblique angle at the base was reduced. The enhancement of light output with increasing etching temperature has been confirmed. The polarization charges on the different facets were assigned to a kinetics-limited process by the special behavior of the hot H3PO4 etching.