Self-organized domain formation in low-dislocation-density GaN

The growth of high-quality GaN layers on a wafer size appropriate for device applications is based on heteroepitaxy on foreign substrates. Heteroepitaxial GaN layers with low densities (below 10⁶ cm⁻²) of extended structural defects can be achieved by lateral overgrowth of mask-patterned templates or by the growth of extremely thick GaN layers as a route towards free-standing GaN-pseudosubstrates. We present the microscopic analysis of such low-dislocation-density GaN layers by means of scanning cathodoluminescence microscopy (CL). Several state-of-the-art concepts of lateral overgrowth are compared, including two-step epitaxial lateral overgrowth of stripe masks (ELO), multi-stack ELO comprising several mask layers as well as an alternative approach involving in situ SiN nano-masks. The self-organized formation of typical microscopic growth domains with characteristic optical properties is evidenced by CL for all lateral overgrowth techniques. This behavior directly fingerprints the different growth rates and the specific impurity incorporation on non-equivalent GaN facets, e.g. (0001), or , always present during lateral overgrowth and closely related to the mask geometry. Accordingly, characteristic CL line shapes found in ELO on periodic, micrometer scale mask patterns are also detected for GaN on in situ SiN nano-masks and clearly reveal the individual facet structure during overgrowth. For thick GaN layers, CL is used to detect the spontaneous appearance of inclined facets inside inverted pyramidal defects. Optimized, thick GaN layers exclusively formed by (0001)-growth are proven to be laterally homogeneous despite periodically varying residual stress and dislocation density of the underlying ELO template.     

Studies of molecular ions. IX. Detection of low-lying ‘non-Koopmans’ states in polyacetylene molecular cations

Comparison of the photoelectron spectra of the title compounds with the electronic absorption spectra of the corresponding radical cations led to the detection of low lying ‘Non-Koopmans’-states in the ionic species.     

Resonance Raman studies on [FeCl4] and [FeBr4]

The resonance Raman spectra of [Et₄N] [FeCl₄] in nitromethane solution and of [Bu₄N] [FeBr₄] as a KBr disc at ≈ 80 K have been record     

The Rotational-Torsional Spectrum of the g'Gg Conformer of Ethylene Glycol: Elucidation of an Unusual Tunneling Path

The microwave spectrum of the energetically unfavored g'Gg conformer of ethylene glycol (CH(2)OH&bond;CH(2)OH) is reported. This spectrum is dominated by an interconversion geared-type large-amplitude motion during which each OH group in turn forms the intramolecular hydrogen bond. The microwave spectrum has been analyzed with the help of a Watson-type Hamiltonian plus a 1.4-GHz tunneling splitting. The rotational dependence of this tunneling splitting has been examined using an IAM approach and this yielded qualitative information on the tunneling path the molecule uses to interconvert between its two most stable conformers. Unexpectedly, but in agreement with ab initio calculations, when tunneling occurs between the energetically equivalent g'Gg and gGg' conformers, the OH groups are rotated stepwise through 240 degrees in the sense of a flip-flop rather than a concerted rotation and the molecule goes through the more stable g'Ga and aGg' forms. The electronic reasons for preferring a long rather than a short rotational path via a gGg form are discussed using calculated adiabatic vibrational modes. Copyright 2001 Academic Press.     

Magnetic field saturation in the Riga dynamo experiment

After the dynamo experiment in November 1999 [A. Gailitis et al., Phys. Rev. Lett. 84, 4365 (2000)] had shown magnetic field self-excitation in a spiraling liquid metal flow, in a second series of experiments emphasis was placed on the magnetic field saturation regime as the next principal step in the dynamo process. The dependence of the strength of the magnetic field on the rotation rate is studied. Various features of the saturated magnetic field are outlined and possible saturation mechanisms are discussed.     

Precision velocity measurements of pulsed supersonic jets

We introduce a straightforward experimental approach for determining the mean flow velocity of a supersonic jet with very high precision. While time measurements easily can achieve accuracies of Δt/t ≤ 10(-4), typically the absolute flight distances are much less well-defined. This causes significantly increased errors in calculations of the mean flow velocity and mean kinetic energy. The basic concept to improve on this situation is changing the flight distance in vacuo by precisely defined increments employing a linear translation stage. We demonstrate the performance of this method with a flight path that can be varied by approximately 15% with a tolerance of setting of 50 μm. In doing so, an unprecedented accurate value for the mean flow velocity of Δv/ < 3 × 10(-4) has been obtained without prior knowledge of the total distance. This very high precision in source pressure, temperature, and particle speed facilitates an improved energetic analysis of condensation processes in supersonic jet expansions. The technique is also of broad interest to other fields employing the strong adiabatic cooling of supersonic beams, in particular, molecular spectroscopy. In the presented case study, a thorough analysis of arrival time spectra of neutral helium implies cluster formation even at elevated temperatures.     

Vibrational spectroscopy and crystal structure analysis of two polymorphs of the di‐amino acid peptide cyclo(L‐Glu‐L‐Glu)

Cyclo(L ‐Glu‐L ‐Glu) has been crystallised in two different polymorphic forms. Both polymorphs are monoclinic, but form 1 is in space group P2₁ and form 2 is in space group C2. Raman scattering and FT‐IR spectroscopic studies have been conducted for the N,O‐protonated and deuterated derivatives. Raman spectra of orientated single crystals, solid‐state and aqueous solution samples have also been recorded. The different hydrogen‐bonding patterns for the two polymorphs have the greatest effect on vibrational modes with N H and CO stretching character. DFT (B3‐LYP/cc‐pVDZ) calculations of the isolated cyclo(L ‐Glu‐L ‐Glu) molecule predict that the minimum energy structure, assuming C₂ symmetry, has a boat conformation for the diketopiperazine ring with the two L ‐Glu side chains being folded above the ring. The calculated geometry is in good agreement with the X‐ray crystallographic structures for both polymorphs. Normal coordinate analysis has facilitated the band assignments for the experimental vibrational spectra. Copyright © 2009 John Wiley & Sons, Ltd.