Competition of Radiation Processes in 6H-SiC Observed by Luminescence

We report on the results of photoluminescence and thermoluminescence measurements of various 6H-SiC crystals. At low temperature in all n-type samples two bands with maxima at 2.7 eV (blue) and 1.8 eV (orange) were detected. In the p-type material only blue band was observed. The measurements performed at a broad range of temperatures showed totally different behaviour of photoluminescence intensity of both bands. The presented results could be explained in the model assuming well established donor-acceptor pair recombination for the blue band emission and the conduction band - deep defect transition for the orange band. The proposed model was confirmed by thermoluminescence measurements of the orange band which showed peaks at 30 K, 80 K, 100 K, 150 K attributed to ionization of subsequent shallow donor levels

Nature of Donors in SiC

6H-SiC samples were examined by ESR technique in temperature range from 5 K up to 300 K. Two kinds of ESR lines were observed: a single line at g = 2.0054 ± 0.0007, called X-line, and a triplet corresponding to isolated nitrogen defect. Ionization energy of X defect was determined as about 60 meV and the ionization energy of isolated nitrogen was determined as about 200 meV below SiC conduction band

On GaN Crystallization by Ammonothermal Method

GaN crystals are grown using ammonothermal method at pressures below 5 kbar and temperatures below 550°C. In this method, GaN is synthesised from high purity metallic gallium. The main role in the low temperature GaN crystallization is played by the chemically active and dense ammonia and dissolved mineralizer. Morphology of the obtained crystals as well as solubility experiments prove that gallium nitride is dissolved and crystallised from solution. Physical properties of GaN crystals obtained using ammonothermal method depend on the growth conditions and the type of mineralizer. All GaN samples reveal very intensive photoluminescence, also at room temperature. The spectra of crystals grown with lithium compound mineralizer are shifted towards higher energies in comparison to crystals grown with potassium based mineralizer. At helium temperatures, phosphorescence is also observed

Optical and Electrical Studies of FR1 and FR2 Defects in GaAs

The systematic EPR, optical absorption, photoluminescence and thermally stimulated current studies of acceptor defects in bulk GaAs were performed. For the first time, parallel EPR and optical absorption experiments allowed to find the absorption spectrum due to the photoionization of FR1 defect with the threshold at 0.19 eV. Photoluminescence studies showed two families of bands in the energy range of about 1.25 to 1.35 eV. We tentatively ascribed them to FR1 and FR2 complexes with shallow donors. Thermally stimulated current measurements showed two peaks at 90 K and 110 K assigned to FR1 and FR2 respectively

Structural dynamics of GaN microcrystals in evolutionary selection selective area growth probed by X-ray microdiffraction

A method to grow high quality, single crystalline semiconductor material irrespective of the substrate would allow a cost-effective improvement to functionality and performance of optoelectronic devices. Recently, a novel type of substrate-insensitive growth process called Evolutionary Selection Selective Area Growth (ES-SAG) has been proposed. Here we report the use of X-ray microdiffraction to study the structural properties of GaN microcrystals grown by ES-SAG. Utilizing high resolution in both direct and reciprocal spaces, we have unraveled structural dynamics of GaN microcrystals in growth structures of different dimensions. It has been found that the geometric proportions of the growth constrictions play an important role: 2.6â.Î 1/4m and 4.5â.Î 1/4m wide growth tunnels favor the evolutionary selection mechanism, contrary to the case of 8.6â.Î 1/4m growth tunnels. It was also found that GaN microcrystal ensembles are dominated by slight tensile strain irrespective of growth tunnel shape

Some Optical and Epr Properties of Strain-Free GaN Crystals Obtained by Ammono Method

AMMONO GaN is grown spontaneously from ammonia solution in form of regular, well shaped, few micrometer crystals. Photoluminescence spectra of these crystals are characterized by fixed positions of very narrow exciton lines (FWHM down to 1 meV), where free excitons A, B, C, resolved two donor bound excitons and acceptor bound exciton are visible. Fixed position of exciton lines is in contrast to small changes of line energies which have been always observed for epitaxial GaN layers because of strain present in them. Free electron concentration of AMMONO GaN is less than few times 10$\text{}^{15}$ cm$\text{}^{-3}$, as estimated from EPR signal of shallow donor. The above-mentioned facts qualified these crystals as state of the art strain-free, model material for basic parameter measurements of GaN. In this work, results of PL and EPR measurements performed on AMMONO GaN crystals are presented and discussed

GaN Synthesis by Ammonothermal Method

It is shown that ammonothermal method can be successfully used to synthesize GaN powder of good crystallographic quality from ammonia solution at high pressure and a moderate temperature. The size of obtained GaN powder grains was of a few micrometers. The improvement of the powder crystalline quality (examined by X-ray rocking curve, scanning electron microscopy and luminescence measurements) with increasing molar proportion of mineralizer was observed. It was therefore possible to conclude that high molar proportion of mineralizer in ammonia solution plays a crucial role in the polycrystal growth process. Visible luminescence of high efficiency from the GaN powder was found