Fabrication and characteristics of flexible normally-off AlGaN/GaN HEMTs

In this paper, we present a method for removing a high electron mobility transistor (HEMT) silicon substrate using mechanical grinding and deep silicon etching technology and successfully transferred the epitaxial wafer to a PET substrate to achieve the flexible normally-off HEMT. By testing the output characteristics and transfer characteristics of the Si-substrate HEMT and PET-substrate HEMT, we have demonstrated that the PET-substrate HEMT has excellent performance and successfully achieved the mechanical flexibility. Furthermore, we analyzed the physical mechanisms of the change in PET-substrate and Si-substrate HEMT characteristics, as well as flexible HEMT performance under bent and flattened states. The flexible HEMT array demonstrates significant potential in integration with other flexible devices, such as GaN-based micro-LED arrays

Characteristics of underwater lighting based on white LEDs

This work systematically analyzed the application of LEDs in underwater white lighting. The characteristics of a RGB-LED and a phosphor-converted LED (PC-LED) have been compared in different water types. The green light shows the smallest attenuation, while the red light has the greatest attenuation coefficient. Such different attenuation effects at different wavelengths lead to the different spectra, CIE coordinates and color rendering index (CRI) of the two kinds of LEDs. With increasing distance, the illuminance of the PC-LED decreases much more rapidly than the RGB-LED due to the stronger attenuation of the wide phosphor-based yellow spectrum. However, the CIE coordinates calculated from the spectra of the PC-LED vary little due to the wider yellow spectrum. On the contrary, the CIE coordinates shift is very clear for the RGB-LED. What's more, the CRI of the RGB-LED increases at distances from 0 to 1.5m, and then decreases at longer distances, but the CRI of the PC-LED keeps decreasing at all distances, which can be explained by the variation of the R values of the color samples due to wavelength dependent light attenuation. This work will benefit the applications and designs of LED-based underwater lighting

Microstructural properties of over-doped GaN-based diluted magnetic semiconductors grown by MOCVD

We have grown transition metal (Fe, Mn) doped GaN thin films on c-oriented sapphire by metal-organic chemical vapor deposition. By varying the flow of the metal precursor, a series of samples with different ion concentrations are synthesized. Microstructural properties are characterized by using a high-resolution transmission electron microscope. For Fe over-doped GaN samples, hexagonal Fe 3N clusters are observed with Fe 3N(0002) parallel to GaN (0002) while for Mn over-doped GaN, hexagonal Mn 6N 2.58 phases are observed with Mn 6N 2.58(0002) parallel to GaN(0002). In addition, with higher concentration ions doping into the lattice matrix, the partial lattice orientation is distorted, leading to the tilt of GaN(0002) planes. The magnetization of the Fe over-doped GaN sample is increased, which is ascribed to the participation of ferromagnetic iron and Fe 3N. The Mn over-doped sample displays very weak ferromagnetic behavior, which probably originates from the Mn 6N 2.58

The Optical Luminosity Function of Gamma-Ray Bursts Deduced from ROTSE-III Observations

We present the optical luminosity function (LF) of gamma-ray bursts (GRBs) estimated from a uniform sample of 58 GRBs from observations with the Robotic Optical Transient Search Experiment III (ROTSE-III). Our GRB sample is divided into two sub-samples: detected afterglows (18 GRBs) and those with upper limits (40 GRBs). We derive R-band fluxes for these two sub-samples 100 s after the onset of the burst. The optical LFs at 100 s are fitted by assuming that the co-moving GRB rate traces the star formation rate. While fitting the optical LFs using Monte Carlo simulations, we take into account the detection function of ROTSE-III. We find that the cumulative distribution of optical emission at 100 s is well described by an exponential rise and power-law decay, a broken power law,and Schechter LFs. A single power-law (SPL) LF, on the other hand, is ruled out with high confidence