In the present paper, studies on the state of strain in single and ensembles of nanocolumns investigated by photoluminescence spectroscopy will be presented. The GaN nanocolumns were either grown in a bottom-up approach or prepared in a top-down approach by etching compact GaN layers grown on Si(111) and sapphire (0001) substrates. Experimental evidence for strain relaxation of the nanocolumns was found. The difference and development of the strain value for different nanocolumns could be verified by spatially resolved micro-photoluminescence on single nanocolumns separated from their substrate. A common D0X spectral position at 3.473 eV was found for all separated single GaN nanocolumns independent of the substrate or processing technique used, as expected for a relaxed system.

Spin-orbit scattering in a polarization-doped Al 0.30 Ga 0.70 N / GaN two-dimensional electron gas with one occupied subband is studied at low temperatures. At low magnetic fields weak antilocalization is observed, which proves that spin-orbit scattering occurs in the two-dimensional electron gas. From measurements at various temperatures the elastic scattering time tr , the dephasing time , and the spin-orbit scattering time so are extracted. Measurements in tilted magnetic fields were performed, in order to separate spin and orbital effects. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2162871͔ GaN-based layer systems are very interesting candidates regarding the realization of future spin-based electronics ͑spintronics͒. The interest in this material for spintronics was ignited by the theoretical prediction that diluted magnetic semiconductor based on GaN should show Curie temperatures above room temperature. 1 GaN-based diluted magnetic semiconductors are promising materials for spin injection because of their expected good matching to AlGaN / GaN heterostructures. However, for a successful realization of spintronic devices an essential prerequisite is that the spin orientation can be controlled externally. A prominent example is the spin transistor proposed by Datta and Das. 2 Here, the spin orientation in a two-dimensional electron gas ͑2DEG͒ is controlled by a gate electrode via the Rashba spin-orbit coupling originating from a macroscopic electric field in an asymmetric quantum well. 3 For semiconductor quantum wells with a large band gap channel layer, i.e., GaN, it is not obvious that the Rashba effect is suffciently strong, because the Rashba coupling parameter ␣ decreases with increasing band gap. However, recently it was shown theoretically that owing to the polarization field at the AlGaN / GaN heterointerface and owing to the large carrier concentration a relatively large Rashba spinsplitting energy can be expected. 4 Experimentally information on the Rashba effect can be obtained from the characteristic beating pattern in the Shubnikov-de Haas oscillations 5,6 or by analyzing weak antilocalization. 7,8 For some AlGaN / GaN heterostructures a characteristic beating pattern was observed in the magnetoresistance, which was interpreted as the presence of the Rashba effect. [9][10][11][12] In addition to the Rashba effect, the lack of inversion symmetry in zincblende-type or wurtzite-type semiconductors can also alter the spin orientation during the electron propagation ͑Dresselhaus term͒. 13 The question to which extent spin-orbit coupling affects the electron transport in an AlGaN / GaN 2DEG is still not solved completely. One reason is, that the observation of beating effects in the Shubnikov-de Haas oscillations is not an unambiguous signature for the presence of spin-orbit coupling. 11,14 In contrast, we focused on analyzing weak antilocalization at very low magnetic fields in a polarizationdoped AlGaN / GaN heterostructure which is a unambiguous indication of spin-orbit effects. Randomi...

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The magnetotransport in a set of identical parallel Al x Ga 1−x N / GaN quantum wire structures is investigated. The width of the wires ranges between 1110 and 340 nm. For all sets of wires, clear Shubnikov-de Haas oscillations are observed. We find that the electron concentration and mobility are approximately the same for all wires, confirming that the electron gas in the Al x Ga 1−x N / GaN heterostructure is not deteriorated by the fabrication procedure of the wire structures. For the wider quantum wires, the weak antilocalization effect is clearly observed, indicating the presence of spin-orbit coupling. For narrow quantum wires with an effective electrical width below 250 nm, the weak antilocalization effect is suppressed. By comparing the experimental data to a theoretical model for quasi-one-dimensional structures, we come to the conclusion that the spin-orbit scattering length is enhanced in narrow wires.

Metalorganic vapor phase epitaxy (MOVPE) of GaN was performed using hydrogen (H 2 ), nitrogen (N 2 ) and H 2 /N 2 mixtures thereof as the carrier gas in the high temperature buffer growth range. The effect of carrier gas on the structural and morphological characteristics of the epilayers was systematically studied using interference and atomic force microscopy (AFM), photoluminescence (PL) measurements at 2 K, Raman spectroscopy and X-ray diffraction (XRD). The higher the N 2 content in the carrier gas, the more pinholes are observed, the lower compressive strain and the higher dislocation density in the layers. A carrier gas composition range was defined at which GaN layers with acceptable structural and morphological characteristics are achieved.