Different technologies have been evaluated in order to create vertical and smooth facets for GaN-based laser diodes on nonand semipolar substrates. Laser assisted scribing and cleaving proved to be a suitable method to create c-, a-and m-plane facets for devices grown on nonpolar, polar and semipolar GaN planes. The RMS roughness measured by atomic force microscopy was found to be less than 1 nm. Semipolar facets showed tilted or multifaceted cleavage planes. Dry etching using inductively coupled plasma etching (ICP) resulted in vertical facets for devices grown on polar c-plane GaN, while the facets of semipolar ð1122Þ GaN on sapphire exhibited a slight tilt depending on the crystal orientation. Wet chemical postprocessing of semipolar facets can improve the tilt angle but suffers from a roughening of the facets due to selective dislocation etching. By using a focussed ion beam (FIB) it is possible to create vertical and smooth facets independent of the crystal orientation.
We studied InGaAs quantum dots (QD) formation, the evolutions of QD density, and relaxation as function of indium content and layer thickness on GaAs(001). The results as well as literature show that deposition of InAs after QD formation increases linearly the QD density. This indicates a single constant energy is released per QD. The strain energy at the onset of QD formation and relaxation is constant, i.e., independent of the indium content. Thus relaxation and QD formation can be described better by using a constant critical strain energy of 4.0 eV/nm2 compared to theories based on balancing the energy/force of dislocations with the strain. QD were only found above 40–50% of indium, while for lower indium contents a dislocation network forms. The reason is a surface transition from c(4×4) to (2×4)-like which promotes nucleation and hence QD formation.
The authors report the first ever in situ scanning tunnelling microscopy measurements in metal-organic vapor phase epitaxy (MOVPE) using an active cooling shield for operation up to 650°C. Apart from fundamental considerations and constraints, some MOVPE specific problems are discussed. The authors present images representing the current status and discuss the effect of tip interaction with the MOVPE process.
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