Xue-Song Hu

Electrical, spectral and optical performance of yellow?green and amber micro-pixelated InGaN light-emitting diodes

Micro-pixelated InGaN LED arrays operating at 560 and 600 nm, respectively, are demonstrated for what the authors believe to be the first time. Such devices offer applications in areas including bioinstrumentation, visible light communications and optoelectronic tweezers. The devices reported are based on new epitaxial structures, retaining conventional (0 0 0 1) orientation, but incorporating electron reservoir layers which enhance the efficiency of radiative combination in the active regions. A measured output optical power density up to 8 W cm−2 (4.4 W cm−2) has been achieved from a representative pixel of the yellow–green (amber) LED array, substantially higher than that from conventional broad-area reference LEDs fabricated from the same wafer material. Furthermore, these micro-LEDs can sustain a high current density, up to 4.5 kA cm−2, before thermal rollover. A significant blueshift of the emission wavelength with increasing injection current is observed, however. This blueshift saturates at 45 nm (50 nm) for the yellow–green (amber) LED array, and numerical simulations have been used to gain insight into the responsible mechanisms in this microstructured format of device. In the relatively low-current-density regime (<3.5 kA cm−2) the blueshift is attributable to both the screening of the piezoelectric field by the injected carriers and the band-filling effect, whereas in the high-current regime, it is mainly due to band-filling. Further development of the epitaxial wafer material is expected to improve the current-dependent spectral stability.

An improved multi-layer stopper in a GaN-based laser diode

In pursuit of a more efficient GaN-based laser diode, a multi-layer structure has recently been proposed to replace the traditional single AlGaN layer as a stopper layer in order to control electron overflow. Based upon the multi-layer idea, this paper combines the Poisson–Schrodinger self-consistent method and the transfer matrix method to investigate the transport properties of the stopper layer under polarization both for the electron and the hole. Two new structures proposed by references are compared with the original one. The reason for a better structure is analyzed and the conclusion is drawn that the multi-layer structure can improve the hole transport. Also, a varied Al content multi-layer structure is theoretically calculated. The Al content for AlGaN layers is gradually increased from the active region to give a better performance both for the purpose of electron blocking and hole tunneling.

Whole Genome Sequencing Identifies a Missense Mutation in HES7 Associated with Short Tails in Asian Domestic Cats.

Domestic cats exhibit abundant variations in tail morphology and serve as an excellent model to study the development and evolution of vertebrate tails. Cats with shortened and kinked tails were first recorded in the Malayan archipelago by Charles Darwin in 1868 and remain quite common today in Southeast and East Asia. To elucidate the genetic basis of short tails in Asian cats, we built a pedigree of 13 cats segregating at the trait with a founder from southern China and performed linkage mapping based on whole genome sequencing data from the pedigree. The short-tailed trait was mapped to a 5.6 Mb region of Chr E1, within which the substitution c. 5T > C in the somite segmentation-related gene HES7 was identified as the causal mutation resulting in a missense change (p.V2A). Validation in 245 unrelated cats confirmed the correlation between HES7-c. 5T > C and Chinese short-tailed feral cats as well as the Japanese Bobtail breed, indicating a common genetic basis of the two. In addition, some of our sampled kinked-tailed cats could not be explained by either HES7 or the Manx-related T-box, suggesting at least three independent events in the evolution of domestic cats giving rise to short-tailed traits.

Correlation of contact resistance with metal diffusion for oxidized Au/Ni/p-GaN contacts studied using Rutherford backscattering spectroscopy

The temperature- and time-dependent diffusion behaviours of oxidized Au/Ni/p-GaN ohmic contacts were studied with Rutherford backscattering spectroscopy/channelling (RBS/C). It was found that, after annealing at 450 °C, the Au diffused into the electrodes and contacted to the p-GaN and the O diffused into the electrodes to form NiO. Both of these processes are believed to be responsible for the sharp decrease in the specific contact resistance (ρc) at 450 °C. At 500 °C, the O diffused deeper into the interface. As a result, the ρc reached the lowest value at this temperature. The influence of the annealing time was also investigated. It was found that, at 500 °C, 1 min annealing caused obvious in-diffusion of Au and O and out-diffusion of Ni. The Au has even diffused into the p-GaN/metal interface. These diffusion behaviours are believed to cause the sharp decrease of ρc. Annealing for 5 min at 500 °C caused Au and Ni to in-diffuse and out-diffuse further, respectively. A longer annealing time, e.g. 10 min, only caused further in-diffusion of O. As for Au and Ni, no further in-diffusion and out-diffusion has been found for the samples annealed for 10 min at 500 °C. Therefore, the further decrease of ρc (5 × 10−4 Ω cm2) after 10 min annealing is attributed to the formation of more NiO at the interface. However, when the annealing temperature reached 600 °C, the ρc increases greatly. The reason for the increase of the ρc is also discussed.

Influence of the Pt barrier on Ti/Al/Pt/Au ohmic contacts to n-GaN studied by Rutherford backscattering spectrometry and x-ray diffraction

The diffusion barrier effect of Pt has been investigated in Ti/Al/Pt/Au ohmic contacts to n-GaN. Severe indiffusion, outdiffusion of Pt and outdiffusion of Ga have been found in 450 ?C annealed samples according to the Rutherford backscattering spectrometry (RBS) results. At the same time, the reaction between Ga and Pt has also been identified by glancing angle synchrotron irradiation x-ray diffraction (GASIXRD) in the samples annealed at 450 ?C. A higher annealing temperature causes Pt to indiffuse into GaN and react with GaN further. As a result, electrical degradation of the Ti/Al/Pt/Au metallization system was found for samples aged at 600 ?C. In contrast, no obvious degradation was found for the aged Ti/Al/Ni/Au samples. According to the RBS and GASIXRD results, it is suggested that the formation of Ga3Pt5 may cause degradation in the electrical properties of the Ti/Al/Pt/Au metallization system.