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Hexagonal boron nitride encapsulation is the method of choice for protecting graphene from environmental doping and impurity scattering. It was previously demonstrated that metal–organic vapor phase epitaxy (MOVPE) grows epitaxially ordered, uniform BN layers on epigraphene (graphene grown on SiC). Due to graphene’s nonwetting properties, h-BN growth starts preferentially from the graphene ledges. We use this fact here to selectively promote the growth of high-quality flat h-BN on epigraphene by patterning epigraphene microstructures prior to BN growth. Thin h-BN films (down to 6 nm) grown by MOVPE show a smooth and pleated surface morphology on epigraphene, whereas crumpled BN is observed on the SiC. Cross-sectional high-resolution transmission electron microscopy images and fluorescence imaging confirm the higher BN quality grown on the epigraphene. Transport measurements reveal p-doping, as expected from hydrogen intercalation of epigraphene and regions of high and low mobility. This method can be used to produce structurally uniform high-quality h-BN/epigraphene micro/nanoscale heterostructures.

Hexagonal boron nitride (h-BN) can be used as a p-doped material in wide-bandgap optoelectronic heterostructures or as a release layer to allow lift-off of grown three-dimensional (3D) GaN-based devices. To date, there have been no studies of factors that lead to or prevent lift-off and/or spontaneous delamination of layers. Here, we report a unique approach of controlling the adhesion of this layered material, which can result in both desired lift-off layered h-BN and mechanically inseparable robust h-BN layers. This is accomplished by controlling the diffusion of Al atoms into h-BN from AlN buffers grown on h-BN/sapphire. We present evidence of Al diffusion into h-BN for AlN buffers grown at high temperatures compared to conventional-temperature AlN buffers. Further evidence that the Al content in BN controls lift-off is provided by comparison of two alloys, Al0.03B0.97N/sapphire and Al0.17B0.83N/sapphire. Moreover, we tested that management of Al diffusion controls the mechanical adhesion of high-electron-mobility transistor (HEMT) devices grown on AlN/h-BN/sapphire. The results extend the control of two-dimensional (2D)/3D hetero-epitaxy and bring h-BN closer to industrial application in optoelectronics.

The electro-optic (EO), thermo-optic, and dielectric coefficients of pure as well as Nd-doped YCa4O(BO3)3 crystals are determined. The frequency and wavelength dependence of the EO coefficients were determined in the 0-150 MHz and 0.5-1.3 µm ranges, respectively. The analysis of the frequency dependences of both EO and dielectric properties provides the unclamped and clamped values of the whole set of coefficients. It appears that the acoustic contributions to the EO and dielectric properties are very low. Small differences in the properties found for the two crystals are attributed to the Nd doping ions. It is to be noticed that, surprisingly, the birefringence (n_X - n_Y) was found to be independent of the temperature in the range 20-40°C.

The electro-optic properties and their frequency dispersion of the 2-amino-5-nitropyridium monohydrogen L-tartrate (2A5NPLT) are investigated at room temperature at a wavelength of 632.8 nm. The results show a large increase (more than three times higher) of the electro-optic coefficient values in the 2A5NPLT crystal compared with the 2A5NPDP crystal, which is one of the first crystals engineered with the 2-amino-5-nitropyridine highly polarizable chromophore.

The electro-optic properties and their frequency dispersion of the 2-amino-5-nitropyridium monohydrogen L-tartrate (2A5NPLT) are investigated at room temperature at a wavelength of 632.8 nm. The results show a large increase (more than three times higher) of the electro-optic coefficient values in the 2A5NPLT crystal compared with the 2A5NPDP crystal, which is one of the first crystals engineered with the 2-amino-5-nitropyridine highly polarizable chromophore.

We show that a Z-transform-based time-response analysis of the electro-optical response of a crystal to a step voltage with a short rise time allows one to obtain the dispersion of the electro-optical coefficients over a wide frequency range. We describe the method employed and present the results obtained for the main electro-optic coefficients (r_22, r_61, and r_c) of a standard LiNbO_3 crystal. We also show that this method is able to provide even small values of the electro-optic coefficients as well as the dispersion within a wide frequency range, which is limited only by the rise time of the step voltage.

The effect of indium (In) in the barrier of InGaN/GaN multiple quantum well (MQW) has been studied for MQWs with and without semi‐bulk InGaN buffer. From simulation, the optimum In content in the barrier with 3–5 nm width is 5–7% to get the optimal material quality and internal quantum efficiency (IQE) of ∼65% for 450–480 nm emission range. Simulation shows a reduction of the potential barrier due to band flattening, a more homogeneous distribution of electrons and holes in the active region and subsequently, a more radiative recombination rate with InGaN as barrier layer. Both cathodoluminescence (CL) and photoluminescence (PL) experimental results show a blue‐shift of emission wavelength along with an enhancement in the emission intensity when GaN barrier is replaced with InGaN barrier, for a MQW structure both with and without the semi‐bulk InGaN buffer. We attribute this blue shift to the reduced polarization mismatch and increased effective bandgap. This InGaN barrier‐related improvement in IQE and efficiency droop could be useful for the realization of longer wavelength “green‐gap” range LEDs where poor IQE and efficiency droop are more prominent due to high indium (In) in the active region.

We report on the optimization of p‐GaN layers for high indium (In) content InGaN applications by optimizing temperature, precursor CP2Mg flow rate and III/V ratio and report also on the optimization of p‐contact performance. Using MOVPE, a 150 nm thick p‐type GaN with moderate Mg doping and a 50 nm contact layer with high Mg doping concentration were grown on standard GaN templates at growth temperatures in the range of 850–1000 °C. Hall measurement yields hole concentration of 4.8 × 1017cm−3for the optimized sample. SIMS shows Mg concentration of 1.7 × 1020cm−3on average in the heavily doped and 4 × 1019cm−3on average in the moderately doped p‐GaN layer for the optimized p‐GaN. A multilayer Pd/Ag/Ni/Au metal contact has been deposited on this p‐GaN and studied using the CTLM technique. Optimization of both p‐GaN layers and p‐contact processing led to a low resistance contact with specific contact resistivity of 6 × 10−4Ω cm2. We believe, the very high Mg concentration of the surface layer in intimate contact with the contact metal reduces the Schottky barrier height and band bending. This optimization of p‐GaN is an important step towards high efficiency green LEDs and solar cells.