Z.G. Herro

Orientation-dependent phonon observation in single-crystalline aluminum nitride

In this study, we present a microspectroscopic investigation performed on different facets of a self-nucleated aluminum nitride (AlN) single crystal. We show that, apart from evaluating crystalline quality, Raman and Fourier transform infrared spectroscopy can provide means to detect the orientation of any AlN facet. Such local, nondestructive technique is very useful for selecting and evaluating samples of single crystalline AlN.

Plasmonic mode interferences and Fano resonances in Metal-Insulator-Metal nanostructured interface.

Metal-insulator-metal systems exhibit a rich underlying physics leading to a high degree of tunability of their spectral properties. We performed a systematic study on a metal-insulator-nanostructured metal system with a thin 6 nm dielectric spacer and showed how the nanoparticle sizes and excitation conditions lead to the tunability and coupling/decoupling of localized and delocalized plasmonic modes. We also experimentally evidenced a tunable Fano resonance in a broad spectral window 600 to 800 nm resulting from the interference of gap modes with white light broad band transmitted waves at the interface playing the role of the continuum. By varying the incident illumination angle shifts in the resonances give the possibility to couple or decouple the localized and delocalized modes and to induce a strong change of the asymmetric Fano profile. All these results were confirmed with a crossed comparison between experimental and theoretical measurements, confirming the nature of different modes. The high degree of control and tunability of this plasmonically rich system paves the way for designing and engineering of similar systems with numerous applications. In particular, sensing measurements were performed and a figure of merit of 3.8 was recorded ranking this sensor among the highest sensitive in this wavelength range.

Investigation of mass transport during PVT growth of SiC by 13C labeling of source material

Abstract We have investigated experimentally mass transport during physical vapor transport growth of silicon carbide (SiC). 13 C was used as trace in order to determine mass transport paths inside the SiC source material as well as inside the gas room. The 13 C concentration inside the SiC crystal, SiC source material and parts of the graphite growth cell was measured using micro-Raman spectroscopy and secondary ion mass spectroscopy. The SiC growth process was monitored using digital X-ray imaging. The presented method for the first time provides experimental access to mass transport which is of particular interest (i) for fundamental investigations, and (ii) for the verification of numerical modeling tools which are currently widely in use for the improvement of the SiC growth process.

Lattice-plane curvature and small-angle grain boundaries in SiC bulk crystals

SiC crystals grown by the physical vapour transport process along the [001] direction show a curvature of the crystal growth front in correspondence with the shape of the isotherms. A large radius for the curvature of the isotherms enhances the formation of an extended facet. Under the facet, the lattice planes are flat with a high crystal quality as expressed by rocking-curve half widths of 0.022°. In the non-faceted region, the lattice planes become bent, following the shape of the isotherms with a radius of typically 0.5 to 0.8 m and an increased rocking-curve half width of 0.3°. A reduction of the growth rate from 300 μm h -1 to 70 μm h -1 does not affect this behaviour significantly. The lattice-plane curvature and the development of the facet are predominantly affected by the shape of the isotherms. For crystals grown in the [015] direction, the lattice planes adjust only in a one-dimensional manner to the isotherms. In all cases, the lattice-plane curvature results from the formation of a high density of small-angle grain boundaries. They are generated by the condensation of dislocations with Burgers vectors in the ab plane.

Flux Growth of SiC Crystals from Eutectic Melt SiC-B4C

Well known obstacles in solution growth of SiC are very small growth rate limited by low solubility of carbon in silicon-based melts used up to now in flux growth and poor growth stability. The emphasis of our work was the search for alternative high-temperature solvents for SiC taking the example of SiC-B4C eutectic. The main problem of using this eutectic system is the choice of a practical crucible material. To solve this problem we have prepared polycrystalline SiC crucibles of high-purity and of theoretical density using the approach of PVT growth on graphite mandrels. Crystal growth experiments have been conducted in SiC crucibles at temperature of 23002350°C. Both self-nucleated SiC platelets up to 5 mm in diameter and epitaxial layers on PVT grown 6H-SiC substrates have been grown.

Analysis of Graphitization during Physical Vapor Transport Growth of Silicon Carbide

We have analyzed the graphitization process of the source material during physical vapor transport growth of SiC by comparison of experimental monitoring (digital x-ray imaging, and 13 C-labeling) and 2D numerical modeling of the sublimation and recrystallization process. Growth runs under different conditions (temperature and inert gas pressure) were used for verification of the calculated source evolution. Effects like formation of a condensed disk on top of the source material, consumption of SiC powder close to the hot graphite walls, mass transport through the core part and along the side walls could be confirmed. The rate of the sublimation and recrystallization effect, however, was overestimated by the model in the range of the experimental parameters in this study. Regardless of the latter, the crystal growth rate was described very well (modeling: 280µm/h, experiment: 310µm/h and 300µm/h).

Strategies for self-organization of Au nanoparticles assisted by copolymer templates

We discuss here different strategies for making arrays of Au nanoparticles using copolymer templates. Top-down and bottom-up routes are considered and the optical properties of as-prepared Au nanoparticles are discussed and compared to numerical simulations. Potential for applications such as biosensors or strain sensors is also assessed.

Natural Crystal Habit and Preferential Growth Directions during PVT of Silicon Carbide

We have investigated preferential growth directions and natural crystal habit during PVT growth of SiC. It is shown that the (-1100), (-1101) and (-1102) planes do not develop stable atomically flat facets, but interfaces consisting of a mixture of different faces. Contrary, the (01-15) facet is the most stable after the conventional (0001) facet. Hollow micropipe defects propagating along the [0001] axis of the seed are vanished in the grown crystal during growth performed on (01-15) plane. Polytype information is transmitted along the c-axis. Hollow cores generated at latter stages of growth are however aligned along growth direction. Growth along [01-15] is shown to be very promising for preparation of micropipe-free SiC. Introduction The faceting effect manifests itself very clearly in the natural platelet habit of free nucleated SiC crystals grown by the original Lely method in close-to-equilibrium conditions [1]. Modified Lely (PVT) method makes use of sufficiently higher thermal gradient, which leads to more complicated interface structures comprising faceted and nonfaceted areas. Closely packed (0001) faces are known to be stable under these conditions producing an atomically flat facet, whose size and position on the crystal interface depends on seed orientation in respect to growth direction. Faceted growth along the slowest [0001] direction is naturally stable, consequently large crystals are almost exclusively grown using (0001) seeds. Screw dislocations and dislocation-based micropipes (MP) are also aligned along the crystallographic c-axis. Because of this, crystal defects present in the seed substrate always propagate into the crystal and their density remains the same during the growth process (or even increases in improper growth conditions). Crystal growth experiments performed along [1-100] and [11-20] directions resulted in micropipe free crystals but grown boules suffered from stacking faults and high density of basal edge dislocations [2]. Recently epilayers grown on nonconventional (03-38) oriented substrates resulted in high quality growth as MP were stopped in the epilayer [3]. The target of our study was to find reasonable growth directions for PVT of SiC other than [0001] on the basis of detailed evaluation of natural crystal habit. ∗ Corresponding author: z.herro@ww.uni-erlangen.de Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 111-114 doi:10.4028/www.scientific.net/MSF.457-460.111

Development of a KOH Defect Etching Furnace with Absolute In-Situ Temperature Measurement Capability

Etching temperature and time are important parameters in the etching of SiC single crystals in molten KOH for defect studies. However, comparison of results of different research groups is difficult because of the way temperature measurements are being carried out. Until now the temperature of the melt has been measured indirectly with a temperature sensor placed outside the melt on the outer walls of the crucible of the etching furnace, resulting in varying etching conditions for varying setup designs. In this paper we developed an etching furnace with the capability of measuring the absolute temperature in-situ directly in the KOH melt. A new thermoelement, resistant to hot molten KOH was developed. Temperature profile measurements of the molten KOH were carried out and a calibration curve of the furnace was obtained. Based on our temperature measurements, we found that etching at 530 °C for 5 minutes was optimal for defect characterisation, both for defect statistics and for distinguishing between the etch pit morphologies. At 550 °C the etch pits become too large, overlap each other and the etching is no longer defect selective.

Growth of Large AlN Single Crystals Along the [0001] Direction

We have demonstrated growth of large AlN single crystals using (0001)-oriented AlN seeds. Boules with a diameter of 15 mm and length up to 12 mm were obtained from 5 mm seeds. Step flow growth was observed on both Al and N-polar surfaces. N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation.