M. Schirra

Bright semipolar GaInN∕GaN blue light emitting diode on side facets of selectively grown GaN stripes

The authors demonstrate the fabrication and evaluation of bright semipolar GaInN∕GaN blue light emitting diodes (LEDs). The structures are realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes with triangular shape running along the ⟨112¯0⟩ direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140μm. Continuous wave on-wafer optical output powers as high as 700μW and 3mW could be achieved under dc conditions for 20 and 110mA, respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5nm for currents between 1 and 50mA. This is less than half the value measured on c-plane LEDs and confirms the reduced piezoelectric field in our LED structures.

Au-catalyzed growth processes and luminescence properties of ZnO nanopillars on Si

ZnO nanopillars are often grown on various substrates by catalytic growth processes through a vapor-liquid-solid (VLS) mechanism. However, on silicon substrates, even with the catalyzed growth processes, it is still very difficult to obtain highly oriented ZnO nanopillar arrays. In this work, it was found that in most cases the actual growth process of ZnO on Si catalyzed by Au was not of real VLS character. In the initial growth stage, the substrate surface is partially melted and then oxidized into a very thin layer of SiO2. Zn-rich alloys instead of ZnO are first deposited on the SiO2∕Si substrates and form polycrystalline hillocks in an atmosphere with low O2 partial pressure. The difficulty for ZnO to nucleate on SiO2∕Si is another reason preventing ZnO nanopillars from growing epitaxially on the substrates. Defects, steps, and∕or stress on the substrate surfaces may support the nucleation process and thus may influence the initial growth stage and the control of the growth orientation of the pillars...

Cathodoluminescence, photoluminescence, and reflectance of an aluminum nitride layer grown on silicon carbide substrate

Aluminum nitride (AlN) has an ultrawide direct band gap of approximately 6.1 eV at low temperature and is fully miscible with gallium nitride. This makes AlN a promising material for ultraviolet optoelectronic applications. Here, we apply cathodoluminescence, photoluminescence, and reflectance spectroscopies to the same AlN layer grown by metalorganic vapor phase epitaxy on silicon carbide. In cathodoluminescence and photoluminescence, we observe strong near band edge emission at ≈6 eV. The contribution appearing at an energetic position of 5.983 eV could be identified as A free exciton recombination, strongly redshifted due to strain effects. The spectra obtained by reflectance measurements show features at 5.985 eV and ≈6.2 eV which we assign to the A exciton—in accordance to our luminescence measurements—and a combination of the B and C free excitons, respectively.

Controlled catalytic growth and characterization of zinc oxide nanopillars on a-plane sapphire

Using the vapor-liquid-solid (VLS) technique, we have grown well-aligned nanopillars on [112¯0]-sapphire (a-plane) substrates at atmospheric pressure in a horizontal tube furnace employing gold catalyst seeds of different sizes and densities. It was the aim of the present work to find experimental conditions (source and template temperatures, temperature gradients, carrier gas flow, gold cluster size and density) under which controlled catalytic growth of nanopillars takes place. The VLS process is expected to result in a correlation of the zinc oxide (ZnO) pillar diameters with the gold catalyst cluster size. This is indeed found to hold true except for very small gold clusters. A minimum value of the pillar diameter of about 20 nm on a-plane sapphire is obtained in our experiments, which apparently represents a general limit. Structure characterization relies on high-resolution x-ray diffraction, atomic force microscopy, and high-resolution scanning electron microscopy. Electronic characterization is do...

Cathodoluminescence study of single zinc oxide nanopillars with high spatial and spectral resolution

Single zinc oxide (ZnO) nanopillars grown by the vapor-liquid-solid process on a-plane sapphire are investigated by spatially resolved cathodoluminescence on the nanometer scale in a scanning electron microscope. Spectra were recorded by exciting luminescence along single pillars and across the underlying wetting layer. Luminescence from the excited pillar spots exhibited sharp bound exciton lines and intrinsic free-exciton recombination, whereas the wetting layer showed only very weak luminescence. Along the pillars, virtually no shift of the lines was observed. This is interpreted as evidence for strain-free growth of the pillars. In the bottom region of the pillars aluminum incorporation from the substrate material was found. There are indications for an incorporation of gallium along the pillars due to unintentional doping introduced by the source material. The spot-resolved results are compared to integral photoluminescence measurements with large-area excitation carried out on the same sample region.

Growth of zinc oxide nanopillars on an iridium/yttria-stabilized zirconia/silicon substrate

Zinc oxide nanopillars were grown by a self-catalyzed growth process on an epitaxial Ir/yttria-stabilized zirconia/Si(111) multilayer structure in an optically heated tube furnace. The pillars obtained stand upright parallel to each other with their c-axis perpendicular to the sample surface. Problems due to alloying of Zn with Si are completely avoided, and no irregularities of the pillars in the initial growth phase are found. Cathodoluminescence measurements show narrow linewidths below 700μeV due to the excellent crystal quality. Termination of ZnO pillars with a flat metallic iridium layer is an attractive issue towards an optical cavity for laser action.

Incorporation of Ga in ZnO∕GaN epitaxial films

Growth of zinc oxide (ZnO) layers on gallium nitride (GaN) substrates benefits from the small lattice mismatch of these two materials. We report on spatially resolved cathodoluminescence studies of ZnO layers grown by a modified chemical vapor deposition process on GaN templates deposited on sapphire substrates. Line scans across the ZnO∕GaN interface reveal the incorporation of gallium from the template into the ZnO layer. Transmission electron microscopy and micro-Raman measurements both indicate that strain relaxation occurs within a distance of a few nanometers from the ZnO∕GaN interface. The diffusion coefficient of gallium in ZnO is determined.

Optimization of semipolar GaInN/GaN blue/green light emitting diode structures on {1-101} GaN side facets

Bluish-green semipolar GaInN/GaN light emitting diodes (LEDs) were investigated as possible candidates for high-brightness devices even in the long wavelength visible regime. To combine the high material quality known from c-GaN and the advantages of a reduced piezoelectric field, the LED structures were realized on the {1¯101} side facets of selectively grown GaN stripes with triangular cross section. Structural investigations using transmission electron microscopy, scanning electron microscopy, high resolution x-ray diffraction, and atomic force microscopy have been performed and could be related to the luminescence properties in photoluminescence and cathodoluminescence. The defect-related luminescence peaks at 3.3 eV and 3.42 eV typically observed in planar non- and semipolar GaN structures as fingerprints of prismatic and basal plane stacking faults, respectively, could be eliminated in our facet LED structures by optimized growth conditions. Furthermore, an indium incorporation efficiency for these {1¯101} facets is found to be about 50% higher as compared to c-plane growth, what helps significantly to achieve longer wavelength emission in spite of the reduced quantum confined Stark effect in such non- and semipolar materials. Combining these findings, we could realize a bluish-green semipolar light emitting diode on the side facets of our GaN stripes. Continuous wave on-wafer optical output powers as high as 240 µW@20mA could be achieved for about 500nm emission wavelength in electroluminescence measurements. The external efficiency was nearly constant for the investigated current range. Furthermore, the relatively small wavelength shift of about 3 nm for currents between 10mA and 100mA confirmed the reduced piezoelectric field in our LED structures.

ZnO Nanostructures: Optical Resonators and Lasing

We review briefly reports on lasing in different kinds of ZnO nanostructures ranging from clusters to pillars, tetrapods, and belts. Then detailed studies on individually in-situ accessible, well-faceted upright pillars grown on a metal interlayer on silicon substrate are presented. We observe in cathodoluminescence directly ultraviolet light standing wave resonator modes, and under pulsed excitation lasing activity based on (e,h) plasma recombination.