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InGaN/GaN quantum wells grown by Metal Organic Chemical Vapor Phase Epitaxy (MOVPE) were initially studied by optical measurements and X-ray diffraction measurements. The comparison of these two techniques indicated that indium is not distributed homogeneously, which was confirmed by transmission electron microscopy in the nanometer scale. The experimental results of Secondary Ion Mass Spectrometry (SIMS) measurements showed that this analytic method can provide specific information on In spatial distributions not accessible by other methods. SIMS data revealed that In fluctuations occur only in the lower part of 2 nm thick InGaN quantum wells, whereas the QW composition is quite uniform in the upper parts. From the experimental data, one may estimate a SIMS depth resolution of about 0.2 nm and of about 1 μm in lateral directions.

Bovine serum albumin (BSA) is often employed as a proteinaceous component for synthesis of luminescent protein-stabilized gold nanoclusters (AuNC): intriguing systems with many potential applications. Typically, the formation of BSA-AuNC conjugate occurs under strongly alkaline conditions. Due to the sheer complexity of intertwined chemical and structural transitions taking place upon BSA-AuNC formation, the state of albumin enveloping AuNCs remains poorly characterized. Here, we study the conformational properties of BSA bound to AuNCs using an array of biophysical tools including vibrational spectroscopy, circular dichroism, fluorescence spectroscopy and trypsin digestion. The alkaline conditions of BSA-AuNC self-assembly appear to be primary responsible for the profound irreversible disruption of tertiary contacts, partial unfolding of native α-helices, hydrolysis of disulfide bonds and the protein becoming vulnerable to trypsin digestion. Further unfolding of BSA-AuNC by guanidinium hydrochloride (GdnHCl) is fully reversible equally in terms of albumin's secondary structure and conjugate's luminescent properties. This suggests that binding to AuNCs traps the albumin molecule in a state that is both partly disordered and refractory to irreversible misfolding. Indeed, when BSA-AuNC is subjected to conditions favoring self-association of BSA into amyloid-like fibrils, the buildup of non-native β-sheet conformation is less pronounced than in a control experiment with unmodified BSA. Unexpectedly, BSA-AuNC reveals a tendency to self-assemble into giant twisted superstructures of micrometer lengths detectable with transmission electron microscopy (TEM), a property absent in unmodified BSA. The process is accompanied by ordering of bound AuNCs into elongated streaks and simultaneous decrease in fluorescence intensity. The newly discovered self-association pathway appears to be specifically accessible to protein molecules with a certain restriction on structural dynamics which in the case of BSA-AuNC arises from binding to metal nanoclusters. Our results have been discussed in the context of mechanisms of protein misfolding and applications of BSA-AuNC.

This paper describes two types of MOCVD-grown n-type GaN layers (Samples A and B) with similar carrier concentration but behaved differently under galvanic photo-etching. In order to understand this behavior, Transmission Electron Microscopy (TEM) for cross-section and plan-view samples, Secondary Ion Mass Spectroscopy (SIMS) and photoluminescence (PL) techniques were applied. SIMS studies showed that Si, C and O are approximately at the same concentration in both samples, but Sample B also contained Fe and Mg. Both GaN samples were grown on sapphire substrate with Ga growth polarity, which was confirmed by Convergent Beam Electron Diffraction (CBED). Despite a smaller layer thickness in Sample B, the density of edge dislocations is almost one order of magnitude lower than in Sample A. In addition, planar defects formed in this sample in the transition area between the undoped buffer and Si doped layers resulted in a substantial decrease in the density of screw dislocations at the sample surface. These planar defects most probably gave rise to the PL lines observed at 3.42 eV and 3.32 eV. The new PL lines that only appeared in Sample B might be related to Mg impurities found in this sample. There were no detectable gettering of these impurities at dislocations using different diffraction conditions. However, Fe rich platelets were found only in Sample B due to the presence of Fe as well as hexagonal features, similar to defects reported earlier in highly Mg-doped GaN. These structural and chemical non-uniformities between the two GaN samples can explain their different etching behaviors. This paper demonstrates that samples with similar carrier concentrations do not necessarily ensure similar structural and optical properties and that additional material characterization are needed to ensure that devices built on such samples have similar performance.

There are two physical phenomena governing the light emission in InGaN quantum structures: the internal electric fields and the In composition fluctuations. Both these effects manifest through the blue shift of the wavelength emission with the excitation intensity and both of them have the pronounced influence on the light emitting properties of these structures. In order to discriminate between these two effects, we fabricated two identical structures: one with the quantum barriers doped with silicon (method for internal electric field screening) and the other with an undoped active region. Under the optical excitation the emission peak shifts by almost 35 nm (Si doped) and 50nm (without Si). Additionally, we studied temperature dependence of the emission peak position. In case of low temperatures and at RT and high pumping energy, emission energy position is almost the same for both samples. Our observations lead us to the conclusion that at low temperatures and at high pumping regime the Quantum Confined Stark Effect (QCSE) is totally suppressed. While this is understandable that at high carrier injection QCSE is screened, the origin of the low temperature effect is much less clear. We can speculate that at the lowest temperature the carriers are localized eliminating the spatial separation of holes and electrons wavefunctions. Measured cathodoluminescence (CL) maps show the same level of the indium fluctuations for both samples. At higher excitation the fluctuations starts to be less visible suggesting band filling of states. Finally we compare recombination times by means of time resolved photoluminescence.

In this work we investigate InGaN/GaN short period superlattices (SPSLs) grown on (0001) GaN and on relaxed In0.2Ga0.8N buffers with varying degree of plastic relaxation. The SPSLs were fabricated by plasma assisted molecular beam epitaxy (PAMBE). Despite the nominal SPSL structure consisted of InN monolayers (MLs) embedded in GaN barriers, we observed single monolayer InGaN layers instead of InN. We study the photoluminescence (PL) of such SPSLs as a function of the substrates lattice constant. Increase of the substrate lattice constant from a = 3.183 A (for GaN grown on sapphire) to a = 3.216 A resulted in a peak emission shift from 379 nm to 419 nm. We attribute the PL red-shift to the increased In incorporation in the ML-thick InGaN on relaxed buffers as a consequence of the reduced lattice mismatch between the substrate and the film. Our experiment confirms that coherent growth of InN is prohibited on GaN and points out the important role of substrate lattice constant engineering for realization of InN/GaN SPSLs.

Studies of ambient-pressure and high-pressure behavior of photoluminescence (PL) for series of InxAl1-xN layers are presented. The measured evolution of PL energy (E-PL) with x is characterized by a clear decrease of E-PL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band-gap energy changes E-G with x. However, values of E-PL are clearly lower than E-G, for 0

GaN c-plane substrates were patterned to obtain 30-70 mm wide differently angled regions. The patterning technique was based on multilevel pattern photolithography and ion etching and was similar to the one used for the diffraction optics elements fabrication. The region angles were between 0.2 and 3.4 degrees with respect to the c-plane. It is shown that photoluminescence and cathodoluminescence wavelengths of InGaN/GaN quantum wells grown by metalorganic vapor phase epitaxy depend on each region's angle. Laser diodes grown on freestanding patterned GaN are also demonstrated. The lasing wavelength of chips grown in differently angled substrate regions are different. We attribute those differences to indium content differences in each of the angled regions.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

A patterned GaN/sapphire template with separate regions angled between 0.4 and 2° to the wurtzite c-plane was used to grow a 50 nm In0.1Ga0.9N layer. The photoluminescence wavelength varied between 403 and 389 nm according to the increased region's angle. The indium content measured using X-rays was reduced in regions with a higher miscut angle. Patterned freestanding GaN with separate regions angled by 0.35 and 0.85° to the c-plane was used to fabricate ridge-waveguide diode lasers. Each laser stripe was placed inside one of the angled regions. Lasing wavelengths of 405.8±0.2 and 401.0±1 nm were obtained for devices grown in those regions.