S.B. Thapa

Photoluminescence of highly excited AlN: Biexcitons and exciton-exciton scattering

Low-temperature photoluminescence spectra of nominally undoped high quality AlN layers on SiC and Al2O3 substrates are reported. Under high excitation conditions, we observe several bands that increase superlinearly with the excitation density. Based on temperature and excitation level dependences recorded on different samples, we identify a band 36 meV below the free A-exciton transition as due to exciton-exciton scattering (P2 band) and a second band down-shifted from the A-exciton transition by 27 meV as due to biexciton recombination. The combined data yield an exciton binding energy of 48 meV.

Highly conductive modulation doped composition graded p-AlGaN/(AlN)/GaN multiheterostructures grown by metalorganic vapor phase epitaxy

In this study, we present theoretical and experimental results regarding highly conductive modulation doped composition graded p-AlGaN/(AlN)/GaN multiheterostructures. Based on simulation results, several multiheterostructures were grown by metalorganic vapor phase epitaxy. Using high resolution x-ray diffraction and x-ray reflectometry, the abruptness of the AlGaN/AlN/GaN interfaces could be determined. Using electron holography, the energetic profile of the valence band could be measured, yielding important information about the vertical carrier transport in such multiheterostructures. The electrical properties of the samples were investigated by measuring the lateral (σL) and vertical (σV) conductivity, respectively. The free hole concentration of a sample optimized in terms of lateral conductivity was measured to be 1.2×1019 cm−3 (295 K) with a mobility of 7 cm2/V s, yielding a record σL of 13.7 (Ω cm)−1. Low temperature Hall measurements (77 K) proved the existence of a two-dimensional hole gas at th...

Simulation supported analysis of the effect of SiNx interlayers in AlGaN on the dislocation density reduction

SiNx interlayers can act as anti-surfactants and drastically reduce the dislocation density in pure GaN layers. In our work we could observe a very efficient dislocation annihilation of the a-type threading dislocations (TD) at a fractional SiNx monolayer even in AlxGa1?xN layers with relatively high Al content of x=0.2, grown on c-plane sapphire by MOVPE. The investigations were focused on the effect of the SiNx interlayer on the dislocation density reduction of the a-type TDs. Weak-beam dark-field (WBDF) and high-resolution (HR) TEM analyses directly at the SiNx interface indicate that the most frequently occurring effect for the reduction of the a-type TDs is the conversion of an a-type TD into an a-type basal dislocation due to lateral overgrowth of SiNx by AlGaN. To confirm this effect, an appropriate dislocation model was developed for the a-type TD in AlGaN and its bending due to the SiNx nano-mask. Corresponding image calculations were performed and compared with the experiments.

Epitaxial growth of coaxial GaInN-GaN hetero-nanotubes

We report about the successful realization of a coaxial hetero structure grown by MOVPE around ZnO nanocolumns. At higher overgrowth temperatures, the ZnO cores completely dissolved leaving GaN nanotube structures with excellent properties. Such tubes could be sheathed by a GaInN-GaN single quantum well structure, as confirmed by photoluminescence and transmission electron microscopy.

Quantitative Dislocation Analysis of 2H AlN:Si grown on (0001) Sapphire

In the last few years aluminium nitride (AlN) has attracted much attention due to its extremely large direct band gap of approximately 6.0 eV and its impressive chemical and thermal stability. Thus AlN and AlxGa1−xN ternary alloys are promising materials for high-power high temperature electronic applications and optoelectronic devices in UV range. For group-III nitride wafers are still not available in sufficient amount and quality, AlN has to be grown on foreign substrates such as Al2O3 (Sapphire). Due to the lattice mismatch between the AlN/Al2O3 interface of about −11.7%, compressive stress is induced in the crystal system. The strain energy is reduced by the formation of threading dislocations, decreasing the crystal quality [1, 2]. Thus it is still a big challenge to grow AlN directly on foreign substrates with small dislocation density. To make the material suitable for semiconductor devices an efficient doping is necessary to achieve sufficient conductivity. Unfortunately Si doped AlN is still highly resistive mainly due to its large activation energy of several 100meV for the Si dopants. Thus high doping densities of up to 1020cm−3 are necessary, however affecting crystal quality.