ZrTe5 and HfTe5 have attracted increasingly attention recently since the theoretical prediction of being topological insulators (TIs). However, subsequent works show many contradictions about their topolog-ical nature. Three possible phases, i.e. strong TI, weak TI, and Dirac semi-metal, have been observed in different experiments until now. Essentially whether ZrTe5 or HfTe5 has a band gap or not is still a question. Here, we present detailed first-principles calculations on the electronic and topological prop-erties of ZrTe5 and HfTe5 on variant volumes and clearly demonstrate the topological phase transition from a strong TI, going through an intermediate Dirac semi-metal state, then to a weak TI when the crystal expands. Our work might give a unified explain about the divergent experimental results and propose the crucial clue to further experiments to elucidate the topological nature of these materials.

Be doped GaN films grown by reactive molecular beam epitaxy (MBE) are investigated. The room temperature photoluminescence spectrum of the films that were studied shows features in the 390–420 nm range, similar to those observed in Mg doped GaN films, and indicates that Be can form acceptor states about 250 meV above the valence band of GaN. This is in contrast to previous works on GaN:Be films where the only luminescence feature seen was a broad peak centered at 2.16 eV (560 nm). Hot probe measurement indicates p-type conduction for GaN:Be doped films without any postgrowth annealing. Current–voltage measurements of fabricated mesas of MBE grown layers, consisting of a GaN:Be doped film grown over a Si doped GaN layer, show p–n diodelike rectification. A weak electroluminescence at 380–390 nm is observed when the device is driven with a pulsed current of 600 mA.

Novel ohmic contacts to n-ZnSe are demonstrated using single crystal Al films deposited on epitaxially grown ZnSe (100) by molecular beam epitaxy. Electron backscatter diffraction confirmed the single crystalline structure of the Al films. The (110)-oriented Al layer was rotated 45° relative to the substrate to match the ZnSe (100) lattice constant. The as-grown Al-ZnSe contact exhibited nearly ideal ohmic electrical characteristics over a large doping range of n-ZnSe without any additional treatment. The contact resistances are in a range of 10−3Ωcm2 for even lightly doped ZnSe (∼1017cm−3). Leaky Schottky behavior in lightly doped ZnSe samples suggested that Al-ZnSe formed a low-barrier height, Schottky limit contact. In situ grown Al could act as a simple metal contact to n-ZnSe regardless of carrier concentration with lower resistance compared to other reported contacts in literary studies. The reported novel metallization method could greatly simplify the ZnSe-based device fabrication complexity and lower the cost.

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