The effect of the initial nitridation of the sapphire substrate on the GaN crystal quality as a function of substrate temperature was studied. GaN layers were grown by plasma-assisted molecular beam epitaxy (MBE) on sapphire substrates nitridated at different substrate temperatures. A strong improvement in the GaN crystal quality was observed at 100 °C nitridation temperature. Symmetric (0004) ...

Picosecond time-resolved photoluminescence ~PL! spectroscopy has been used to investigate the optical properties of GaN pyramids overgrown on hexagonal-patterned GaN~0001! epilayers on sapphire and silicon substrates with AlN buffer layers. We found that: ~i! the release of the biaxial compressive strain in GaN pyramids on GaN/AlN/sapphire substrate led to a 7 meV redshift of the spectral peak ...

We have investigated the use of tightly focused, temporally shaped femtosecond (fs)-laser pulses for producing nanostructures in two dielectric materials (sapphire and phosphate glass) with different characteristics in their response to pulsed laser radiation. For this purpose, laser pulses shaped by third-order dispersion (TOD) were used to generate temporally asymmetric excitation pulses, lea...

Understanding the freezing of salt solutions near solid surfaces is important in many scientific fields. Here we use sum frequency generation (SFG) spectroscopy to study the freezing of a NaCl solution next to a sapphire substrate. During cooling we observe two transitions. The first corresponds to segregation of concentrated brine next to the sapphire surface as we cool the system down to the ...

We describe epitaxial methods for two different semipolar GaN orientations on patterned sapphire substrates: With (101̄2) (r-plane) sapphire substrates we achieve planar (112̄2) GaN layers with smooth surfaces on a large scale. In the case of (112̄3) (n-plane) patterned wafers the growth of (101̄1) GaN is possible. We optimized the growth conditions for (112̄2) GaN (especially the growth temperature...

We report the remarkably improved crystal quality of semi-insulating GaN grown by metalorganic chemical vapor deposition on an AlN buffer layer, which is deposited on sapphire substrate. The electrical and structural properties are characterized by dark current–voltage transmission line model and X-ray diffraction measurements. It is found that the crystal quality of the GaN epilayer is strongl...

This study investigates and models the grinding process of single crystal sapphire. Five parameters: the wheel speed, the feed speed, the vertical feed, the ultrasonic assistance and the crystallographic direction were considered via a design of experiments (DoE) approach. The responses were multiple but can be divided in three groups: the process, the machine and the grinding quality. DoE resu...

We investigated the efficiency droop and polarization-induced internal electric field of InGaN blue light-emitting diodes (LEDs) grown on silicon(111) and c-plane sapphire substrates. The efficiency droop of the LED sample grown on silicon substrates was considerably lower than that of the identically fabricated LED sample grown on sapphire substrates. Consequently, the LED on silicon showed hi...

We present ab initio computations of the ionization of two-electron atoms by short pulses of intense linearly polarized Ti:sapphire laser radiation beyond the one-dimensional approximation. In the model the electron correlation is included in its full dimensionality, while the center-of-mass motion is restricted along the polarization axis. Our results exhibit a rich double ionization quantum d...

We demonstrate a significant improvement in signal-to-noise ratio in coherent anti-Stokes Raman scattering (CARS) spectroscopy/microscopy, using two highly synchronized picosecond Ti:sapphire lasers. A temporal jitter between the pulse trains from the two independent commercial lasers is reduced from a few picoseconds to ~21 fs , maintained over several hours. The tight synchronization brings t...