InGaN quantum dots (QDs) have attracted many research interests in recent years for their potentials to realize long wavelength visible emission from green to red, which can pave a way to fabricate the phosphor-free white light emitting diodes (LEDs). In this paper, we reported our recent progresses on InGaN QD LEDs, the discussions were dedicated to the basic physics model of the strain relaxation in self-assembled InGaN QDs, the growth of InGaN QDs with a growth interruption method by metal organic vapor phase epitaxy, the optimization of GaN barrier growth in multilayer InGaN QDs, the demonstration of green, yellow-green and red InGaN QD LEDs, and future challenges.

pH sensor based on a gateless AlGaN/GaN high electron mobility transistor (HEMT) structure is reported in this paper. The sensing characteristics of the devices have been measured in hydrochloric acid solutions in pH=2 to pH=6 respectively. The device with gate sensing area of 250×400μm2 shows sensitivity of 0.0944mA/mm-pH in the linear region (Vds=0.5 V), while the one with gate sensing area of 15×400μm2 shows sensitivity of 2.19 mA/mm-pH in the saturation region (Vds=13 V). The results show that the carrier sheet density of the two dimensional electron gas (2DEG) could be effectively tuned by the solutions with different H+ concentration at AlGaN gate region, and higher sensitivity could be obtained in the saturation region than in the linear region. It is analyzed that the device with shorter gate length could reach saturation at lower voltage. Furthermore, sensing in the saturation region helps to overcome the measurement error caused by voltage fluctuations.

When the thickness of InGaN film grown on (0 0 0 1) GaN is beyond a critical value, the compressive strain will be relaxed through dislocation generation or three-dimensional growth. We calculate the InGaN critical thicknesses for dislocation generation and three-dimensional growth depending on the indium composition, respectively. The results show that both the critical thicknesses decrease with increase of the indium composition. When the indium composition is lower than 27%, the latter is smaller. This means three-dimensional growth will be induced more easily than the dislocation generation. But when the indium composition is higher than 27%, dislocations become to generate more easily. The extension of dislocations from GaN into InGaN can relax a part of strain and increases the critical thickness.Highlights► Dislocation generation and three-dimensional growth are distinguished. ► The different critical thicknesses of the two mechanisms are calculated. ► If In composition is lower than 27%, three-dimensional growth is induced first. ► If In composition is higher than 27%, dislocations can be generated more easily. ► Extension of dislocations from GaN into InGaN can increase the critical thicknesses.

•Critical thicknesses of h-BN on hexagonal crystals are investigated theoretically.•Stress of h-BN will be released preferentially through generating dislocations.•Large area of h-BN thin film can be obtained on frequently-used hexagonal crystals.•It is conducive to growing thick h-BN with large area on AlN at high temperature.•Dislocation-free large-scale graphene with a few monolayers can be grown on h-BN.Hexagonal crystals are suitable underlayer candidates for hexagonal boron nitride (h-BN) heteroepitaxy due to their similar in-plane atomic arrangement. When the thickness of h-BN is beyond a critical value, its accumulated stress resulting from the lattice mismatch can be relaxed by generating dislocation or changing into three-dimensional growth. Here we calculate the evolution of h-BN critical thickness with the growth temperature when it is grown on various frequently-used hexagonal crystals for both cases. The results show that in order to minimize the lattice mismatch, a low growth temperature is preferred when grown on GaN or Si(1 1 1) while on the contrary when grown on 6H-SiC or α-Al2O3. Besides, AlN is the most unique underlayer as its lattice mismatch with h-BN is relatively small (<0.7%) and they can even fully match around 1150 K, which means it can be used as a buffer layer for thick h-BN (>100 nm) growth. Moreover, large area of two-dimensional thin h-BN (5–15 nm) layer can be obtained on GaN, 6H-SiC, Si(1 1 1) or α-Al2O3 except for graphene. On the other hand, calculation indicates that large area of graphene can be grown on h-BN.