Tiefeng Wei

Hillocks and hexagonal pits in a thick film grown by HVPE

A GaN film with a thickness of 250@mm was grown on a GaN/sapphire template in a vertical hydride vapor phase epitaxy (HVPE) reactor. The full-width at half-maximum (FWHM) values of the film were 141 and 498arcsec for the (002) and (102) reflections, respectively. A sharp band-edge emission with a FWHM of 20meV at 50K was observed, which corresponded to good crystalline quality of the film. Some almost circular-shaped hillocks located in the spiral growth center were found on the film surface with dimensions of 100@mm, whose origin was related to screw dislocations and micropipes. Meanwhile, large hexagonal pits also appeared on the film surface, which had six triangular {101@?1} facets. The strong emission in the pits was dominated by an impurity-related emission at 377nm, which could have been a high-concentration oxygen impurity.

Hydride Vapor Phase Epitaxy Growth of Semipolar (1013)GaN on Patterned m-Plane Sapphire

We have investigated the hydride vapor-phase epitaxy growth of (1013)-oriented GaN thick films on patterned sapphire substrates (PSSs) ( 1010). From characterization by atomic force microscopy, scanning electron microscopy, double-crystal X-ray diffraction, and photoluminescence (PL), it is determined that the crystalline and optical qualities of (1013) GaN epilayers grown on the cylindrical PSS are better than those on the flat sapphire. However, two main crystalline orientations ( 1013) and (1122) dominate the GaN epilayers grown on the pyramidal PSS, demonstrating poor quality. After etching in the mixed acids, these ( 1013) GaN films are dotted with oblique pyramids, concurrently lining along the (3032) direction, indicative of a typical N-polarity characteristic. Defect-related optical transitions of the (1013) GaN epilayers are identified and detailedly discussed in virtue of the temperature-dependent PL. In particular, an anomalous blueshift-redshift transition appears with an increase in temperature for the broad blue luminescence due to the thermal activation of the shallow level.

Tunable bandgap in hybrid perovskite CH3NH3Pb(Br3−yXy) single crystals and photodetector applications

We report the synthesis of CH3NH3Pb(Br3−yXy) (X=Cl and I) single crystals via a stepwise temperature control approach. High-quality CH3NH3Pb(Br3−yXy) crystals with a tunable bandgap from 1.92eV to 2.53eV have been prepared successfully in this way. And further experiments revealed the influence of halogen content and preparation temperature on the structural and optical properties of these crystals. It is observed that chlorine can lower the critical nucleation energy, which results in crystallizing at lower temperature with the chlorine content increasing, while the nucleation energy increases slowly with increasing iodine content. Moreover, in contrast to Frank–van der Merwe growth with low heating rate, high heating rate leads to a mass of small size single crystals and Stranski-Krastanov growth. The single crystals with tunable band gap and impressive characteristics enable us to fabricate high performance photodetectors for different wavelengths.

Nearly full-dense and fine-grained AZO:Y ceramics sintered from the corresponding nanoparticles

Aluminum-doped zinc oxide ceramics with yttria doping (AZO:Y) ranging from 0 to 0.2 wt.% were fabricated by pressureless sintering yttria-modified nanoparticles in air at 1,300°C. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, a physical property measurement system, and a densimeter were employed to characterize the precursor nanoparticles and the sintered AZO ceramics. It was shown that a small amount of yttria doping can remarkably retard the growth of the as-received precursor nanoparticles, further improve the microstructure, refine the grain size, and enhance the density for the sintered ceramic. Increasing the yttria doping to 0.2 wt.%, the AZO:Y nanoparticles synthetized by a coprecipitation process have a nearly sphere-shaped morphology and a mean particle diameter of 15.1 nm. Using the same amount of yttria, a fully dense AZO ceramic (99.98% of theoretical density) with a grain size of 2.2 μm and a bulk resistivity of 4.6 × 10−3 Ω·cm can be achieved. This kind of AZO:Y ceramic has a potential to be used as a high-quality sputtering target to deposit ZnO-based transparent conductive films with better optical and electrical properties.

Preparation of AZO Nanoparticles, Ceramic Targets and Thin Films by a Co-precipitaition Method

We comprehensively study the co-precipitation preparation of aluminum doped zinc oxide (AZO) nanoparticles, ceramic target and thin film deposition. The When the calcination temperature exceeded 700 °C, ZnAl2O4 phase appeared. The resistivity and relative density of the AZO target pressed from nanoparticles were 3×10-3 Ω∙cm and 99.1%, respectively. The minimum resistivity of AZO thin films prepared by DC sputtering of the ceramic target reached 4.1×10–4 Ω∙cm with the mobility of 33 cm2/v∙s and the carrier concentration of 4.5 ×1020 cm-3. The average optical transmittance of the AZO thin films in the visible wavelength range (400-800 nm) was more than 80%.