A specific question about the Himalayas is whether the orogeny grew by distributed extrusion or discrete thrusting. To place firm constraints on tectonic models for the orogeny, kinematic, thermobarometric and geochronological investigations have been undertaken across the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, south‐central Tibet. The GHC in this section is divided into the lower, upper and uppermost GHC, with kinematically top‐to‐the‐south, alternating with top‐to‐the‐north shear senses. A new thrust named the Nyalam thrust is recognized between the lower and upper GHC, with a 3 kbar pressure reversion, top‐to‐the‐south thrust sense, and was active after the exhumation of the GHC. Peak temperature reached ∼749 °C in the cordierite zone, and decreased southwards to 633–667 °C in the kyanite and sillimanite‐muscovite zones, and northwards to greenschist facies at the top of the South Tibetan Detachment System (STDS). Pressure at peak temperature reached a maximum value in the kyanite zone of 9.0–12.6 kbar and decreased northwards to ∼4.1 kbar in the cordierite zone. Zircon U‐Pb ages of a sillimanite migmatite and an undeformed leucogranite dyke cutting the mylonitized rocks in the STDS reveal a long‐lived partial melting of the GHC, which initiated at 39.7–34 Ma and ceased at 14.1 Ma. Synthesizing the obtained and collected results, a revised channel flow model is proposed by considering the effect of heat advection and convection by melt and magma migration. Our new model suggests that distributed processes like channel flow dominated during the growth of the Himalayan orogen, while discrete thrusting occurred in a later period as a secondary process.

Epitaxial growth of AlN films on c-sapphire using a multilayer structure has been investigated by metalorganic chemical vapor deposition adopting multiple alternation cycles of low-and high-temperature (LT-HT) growth. It is found that the surface morphology and crystal quality can be greatly improved using three alternation cycles with X-ray diffraction ω-scan full width at half maximum values of 311 and 548 arcsec for the (0002) and (10−12) peaks, respectively, which are induced by the alternation of the three-dimensional (3D) and two-dimensional (2D) growth modes caused by the LT-HT process. The first 3D-2D cycle is found to play a major role in threading dislocation reduction, while the second and third cycles mainly account for tensile stress relaxation. CrystEngCommThis journal is

Developing efficient active region structures with low sensitivity to threading dislocation density (TDD) is of great technical significance for AlGaN-based deep ultraviolet (DUV) light-emitting devices. Here, we propose an active region strategy by introducing bunching effect to form self-assembled sidewall quantum wells (SQWs) with much stronger carrier confinement, resulting in a significant enhancement of internal quantum efficiency (from 46% to 59%) compared to the commonly adopted multiple quantum wells (MQWs) due to the lower sensitivity to TDD. As a demo, an AlGaN-based DUV light-emitting diode (LED) with the proposed active region involving both SQWs and MQWs presents dual-band emission and a consequent 68% enhancement in light output power compared to the DUV-LED with only MQWs as the active region, suggesting that the proposed architecture is fully suitable for the development of high performance DUV light-emitting devices even based on poor or medium quality materials.

Improving the contact characteristics of Ti/Al/Ni/Au on plasma etched n-AlGaN has been attempted by an active pretreatment, which can provide Si and N atoms to occupy the possible metal and N vacancies. It is found that the contacts on both the as-grown and plasma-etched + pretreatment n-Al0.5Ga0.5N present truly Ohmic in nature, whereas the contacts on plasma-etched samples without pretreatment still remain rectifying. Surface atomic concentration analysis indicates that the plasma etching induced N or metal vacancies mostly act as acceptor-like states, leading to a severe compensation. Fortunately, these states can be effectively removed by the presently proposed pretreatment, and thus the Fermi level is raised up toward the conduction band edge, ensuring the formation of Ohmic contacts.

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