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We report on optically pumped lasing at 500 nm on InGaN laser structures grown by plasma assisted molecular beam epitaxy. The InGaN laser structures were grown under group III-rich conditions on bulk (0001) GaN substrates. The influence of the nitrogen flux and growth temperature on the indium content of InGaN layers was studied. We demonstrate that at elevated growth temperatures, where appreciable dissociation rate for In-N bonds is observed, the indium content of InGaN layers increases with increasing nitrogen flux. We show that growth of InGaN at higher temperatures improves optical quality of InGaN quantum wells, which is crucial for green emitters. The influence of piezoelectric fields on the lasing wavelength is also discussed. In particular, the controversial issue of partial versus complete screening of built-in electric field at lasing conditions is examined, supporting the former case. [ABSTRACT FROM AUTHOR]

Systematic studies of InxGa1-xN layers (0≤x<0.13) doped with Mg were performed. Samples were grown by metal organic vapor phase epitaxy. Intermediate Mg doping in the range of 2×1019 cm-3 was chosen to achieve a maximum hole carrier concentration, pH (as measured by Hall effect) of 4×1018 cm-3 in samples with high x. We confirmed reports on decreasing resistivity in InxGa1-xN:Mg epitaxial layers observed with increasing x. This finding is very important for applications. In the performed research we attempted to separate contributions to pH increase resulting from increase in In-content and an associated decrease in growth temperature, Tgr (necessary to obtain high x). For this purpose In-content increase was achieved by means of either (i) lowering the growth temperature (from 1020 to 830 °C) or by (ii) varying an intended GaN substrate miscut. We demonstrated that the increase in pH in InxGa1-xN:Mg is caused by higher In concentration while a drop in Tgr plays a secondary role. Studies of photoluminescence in the InGaN:Mg layers exhibit band-to-band radiative recombination which has created much controversy. The most important feature of samples grown at temperatures 860 °C and below, is a green band observed in InGaN:Mg layers (not in GaN:Mg obtained at the same Tgr) dominating the whole spectrum at room temperature. Its maximum shifts from 2.5 to 2.2 eV with increasing x up to 0.13. Presence of this band was previously reported for InGaN:Mg. We present arguments that it originates from deep donor level. [ABSTRACT FROM AUTHOR]

Light emission of nitride single quantum well light-emitting diodes operating in the range from 395 to 440 nm, grown by metal-organic vapor phase epitaxy, was investigated by means of photoluminescence (PL) and electroluminescence (EL) spectroscopies as functions of temperature and current injection level. The indium content of the active layers was varied via the growth temperature, which ranged from 770 to 810 °C. It was found that samples with higher indium contents (grown at lower temperatures) exhibit larger luminescence full widths at half maximum (FWHMs) and higher EL intensities. The larger FWHM points to a larger amplitude of the potential profile fluctuations, which suggests that these fluctuations may be useful for the increase in the device output power. In agreement with this result, the performed examination by PL measurements shows large thermal stability of the luminescence intensity for the lowest growth temperature sample. On the other hand, a nontrivial dependence of thermal stability on growth temperature suggests the additional involvement of different nonradiative recombination center concentrations in the individual samples, for example, because of dissimilar substrate qualities. [ABSTRACT FROM AUTHOR]

Nanostructured GaN surfaces sputtered with Au provide very promising and reproducible platforms for surface enhanced Raman scattering (SERS). The enhancement factor (EF) in SERS is expected to depend strongly on the local metal (Au) surface structure (size, distribution and morphology). Herein we show how temperature-induced coalescence followed by recrystallization of Au on GaN nano-columns occurs well below the melting point of gold. This process is reflected in SEM, TEM images and SERS spectra of Au/GaN – bound p ‐mercaptobenzoic acid (4-MBA), a model Raman scatterer. SERS signals of 4-MBA bound to Au/GaN reach minimum for platforms exposed to t = 350 °C before regaining the intensity when annealed in the range of 450–900 °C. The results have been discussed in the light of the nature of SERS active sites – so called “hot spots” and structure of Au nanoparticles. [ABSTRACT FROM AUTHOR]

We describe surface patterning as a method to obtain discrete regions of different local miscut angles, δ, in semipolar ( 20 2 ¯ 1 ) GaN substrates. During patterning the region angle δ was varied between 0° and ±2° with respect to the growth plane. We chose two nonequivalent miscut directions: a 〈 1 ¯ 2 1 ¯ 0 〉 and c ׳ 〈 1 ¯ 01 4 ¯ 〉. On such patterned substrates In x Ga 1− x N/GaN quantum wells were grown by a Metalorganic Vapor Phase Epitaxy method. Angled regions were not planarized during growth and their initial miscut angles remained. We studied structural, morphological and optical properties of In x Ga 1− x N/GaN quantum wells as a function of δ . The intended In concentration, x , was about 13% on exact oriented ( 20 2 ¯ 1 ) planes. For miscut towards the a -direction, indium content decreases and luminescence energy increases with δ . For miscut towards c ׳, we observe quite different behaviors, indium content and quantum well width are constant and luminescence energy increases with δ . We propose the explanation of these effects on the basis of sample morphology, In-content and built-in electric field. The obtained results are important for understanding the role of miscut (intentional and unintentional) in basic properties of semipolar InGaN/GaN quantum well structures. It is also worth to note that, the spectral width of luminescence was the smallest for regions miscut 2° towards c ׳ direction, which can be important for semipolar optoelectronic devices. [ABSTRACT FROM AUTHOR]

We demonstrate lasing emission of optically excited InGaN LD structures without intentional resonant cavity formation. We observe the equal mode-spacing character of this effect in the back-scattering geometry after exceeding the threshold excitation intensity. The homogeneity of the effect and stable mode spacing exclude participation of defects or wafer edges in lasing. We propose a lasing mechanism based on optically excited 2D electron-hole plasma oscillations, which act as a dynamical grating and resonantly couple the lasing modes separated by the plasma frequency, similar to the case of DFB lasers. The observed anomalous mode spacing is determined by the eigenfrequency of the plasma oscillations. [ABSTRACT FROM AUTHOR]

It is shown that in polar InGaN QWs emitting in the blue-green spectral region a Stokes shift between spontaneous emission (SE) and optical transition observed in contactless electroreflectance (CER) spectrum (absorption-like technique) can be observed even at room temperature, despite the fact that the SE is not associated with localized states. Time resolved photoluminescence measurements clearly confirm that the SE is strongly localized at low temperatures whereas at room temperature the carrier localization disappears and the SE can be attributed to the fundamental transition in this QW. The Stokes shift is observed in this QW system because of the large built-in electric field, i.e., the CER transition is a superposition of all optical transitions with non-zero electron-hole overlap integrals and, therefore, the energy of this transition does not correspond to the fundamental transition of InGaN QW. Lasing from this QW has been observed at the wavelength of 475 nm, whereas the SE was observed at 500 nm. The 25 nm shift between the lasing and SE is observed because of a screening of the built-in electric field by photogenerated carriers. However, our analysis shows that the built-in electric field inside the InGaN QW region is not fully screened under the lasing conditions. [ABSTRACT FROM AUTHOR]

Abstract: A recently developed photoetching system for n-type GaN, a KOH solution containing the strong oxidizing agent potassium peroxydisulphate (K2S2O8), was studied in detail. By careful selection of the etching parameters, such as the ratio of components and the hydrodynamics, two distinct modes were defined: defect-selective etching (denoted by KSO-D) and polishing (KSO-P). Both photoetching methods can be used under open-circuit (electroless) conditions. Well-defined dislocation-related etch whiskers are formed during KSO-D etching. All types of dislocations are revealed, and this was confirmed by cross-sectional TEM examination of the etched samples. Extended electrically active defects are also clearly revealed. The known relationship between etch rate and carrier concentration for photoetching of GaN in KOH solutions was confirmed for KSO-D etch using Raman measurements. It is shown that during KSO-P etching diffusion is the rate-limiting step, i.e. this etch is suitable for polishing of GaN. Some constraints of the KSO etching system for GaN are discussed and peculiar etch features, so far not understood, are described. [ABSTRACT FROM AUTHOR]