InxAl1-xN/Al0.53Ga0.47N multiple quantum wells on Al0.5Ga0.5N buffer with variable in-plane lattice parameter

The structural and luminescent properties of InxAl1-xN/Al0.53Ga0.47N multiple quantum wells (MQW) grown on an Al0.5Ga0.5N buffer partially relaxed with respect to an underlying AlN-template are reported. A significant redshift and improvement of ultraviolet (UV) photoluminescence (PL) intensity is found for InAlN MQWs grown on AlGaN buffers with higher relaxation degree. This is attributed to a higher QW indium incorporation as confirmed also by X-ray diffraction (XRD). The nature of room temperature time resolved PL is studied and discussed from the point of view of the possibility of a type I-type II band lineup transition in the InAlN-AlGaN system

Ultraviolet stimulated emission in AlGaN layers grown on sapphire substrates using ammonia and plasma-assisted molecular beam epitaxy

Ammonia and plasma‐assisted (PA) molecular beam epitaxy modes are used to grow AlN and AlGaN epitaxial layers on sapphire substrates. It is determined that the increase of thickness of AlN buffer layer grown by ammonia‐MBE from 0.32 μm to 1.25 μm results in the narrowing of 101 X‐Ray rocking curves whereas no clear effect on 002 X‐Ray rocking curve width is observed. It is shown that strong GaN decomposition during growth by ammonia‐MBE causes AlGaN surface roughening and compositional inhomogeneity, which leads to deterioration of its lasing properties. AlGaN layers grown by ammonia‐MBE at optimized temperature demonstrate stimulated emission (SE) peaked at λ = 330 nm, 323 nm, 303 nm and 297 nm with the SE threshold values of 0.7 MW cm−2, 1.1 MW cm−2, 1.4 MW cm−2 and 1.4 MW cm−2, respectively. In comparison to these, AlGaN layer grown using PA‐MBE pulsed modes (migration‐enhanced epitaxy, metal‐modulated epitaxy, and droplet elimination by thermal annealing) shows a SE with a relatively low threshold (0.8 MW cm−2) at the considerably shorter wavelength of λ = 267 nm

AlGaN/GaN high electron mobility transistor heterostructures grown by ammonia and combined plasma-assisted ammonia molecular beam epitaxy

The structural properties and surface morphology of AlN epitaxial layers grown by ammonia (NH3) and plasma-assisted (PA) molecular beam epitaxy (MBE) at different growth conditions on (0001) sapphire were investigated. The lowest RMS roughness of ~0.7 nm was achieved for the sample grown by NH3 MBE at a substrate temperature of 1085 °C and NH3 flow of 100 standard cm3 min−1. Atomic force microscopy measurements demonstrated a terrace-monolayer step-like surface morphology. Furthermore, the optimal substrate temperature for growth of GaN and AlGaN layers was determined from analysis of the GaN thermal decomposition rate. Using the optimized growth conditions, high electron mobility transistor heterostructures were grown by NH3 MBE on different types of AlN nucleation layer deposited by NH3 MBE or PA MBE. The grown heterostructures demonstrated comparable two-dimensional electron gas (2DEG) properties. The maximum 2DEG mobility of ~2000 cm2 V–1 s–1) at a 2DEG density of ~1.17 × 1013 cm−2 was achieved for the heterostructure with a PA MBE-grown AlN nucleation layer. The obtained results demonstrate the possibility of successful combination of different epitaxial approaches within a single growth process, which will contribute to the development of a new type of hybrid epitaxy that exploits the advantages of several technologies

A neutral DNA sequence-selective vector for interaction studies: Fluorescence binding experiments directed towards a carbohydrate-DNA carrier

The distamycin-type γ-linked covalent dimer -Py-γ-Py-Ind has been shown to be a neutral selective vector capable of transporting recognition elements to the minor groove of DNA for further structural studies. Comparison of fluorescence binding constants of the complexes formed by the vectors (R-Py-γ-Py-Ind 1 and 3) with ct-DNA and poly(dA-dT) showed that -Py-γ-Py-Ind is a neutral (–ATAT–)-selective vector. We also provide experimental data that show that the vector can be used as a sugar-carrier to the DNA. Thus, modifying the vector at the C terminus with sugars of different configurations (4–7 D vs. 8 L), and with both α- and β-linkages (5 and 4, respectively) to the oligoamide fragment provides efficient DNA binders (Ka = 1.1 104 to 3.2 105 M–1). Moreover, the sugar residue is able to modulate the binding to the different DNA polymers studied and, even more relevantly,the sugar contributes to the selectivity of binding: β-D-Gal- Py-γ-Py-Ind (6) is the most selective –ATAT–sugar–oligoamide ligand {ΔΔG° 6 [poly(dA-dT) – ct-DNA] = –4.0 kcal mol–1}. We have used fluorescence quantum yield values to ensure the presence of similar free state conformation for ligands 1–8 and we can thus correlate the differences in the measured binding energies with the changes in the shape of the structural elements. Finally, we have demonstrated that the sequence-selective sugar carrier makes a 1:1 complex with the Drew–Dickerson oligonucleotide dodecamer, thus opening the road to more detailed structural and thermodynamic studies of sugar-oligoamide DNA short oligonucleotide complexes in solution.Financial support for this work was provided by the Ministerio de Educación y Ciencia (MEC) (BQU2003-03550-C03-02 and CTQ2006-10874-C02-02) and a European Research Training Network (RTN) project (HPRN-CT-2002-00190). P. P. is grateful to the Consejo superior de Investigaciones Científicas for an I3P predoctoral fellowship. S. A. thanks RTN and MEC for a Juan de la Cierva postdoctoral contract.Peer reviewe