ALBERT

Notes: The surface of shallow Ga[Al]As heterostructures is locally oxidized with an atomic force microscope. The electron gas underneath the oxide is depleted. We demonstrate experimentally that these depleted regions of the two-dimensional electron gas can be made highly resistive at liquid nitrogen temperatures. Thus, local anodic oxidation of high electron mobility transistors with an atomic force microscope provides a novel method to define nanostructures and in-plane gates. Two examples, namely antidots and quantum point contacts as in-plane gate transistors have been fabricated and their performance at low temperatures is discussed. © 1998 American Institute of Physics.

Notes: [Auszug] Quinn et al. report a strong association between myopia in children and their exposure to night-time lighting during their first two years. We have been unable to confirm this surprising result, but we find that myopic parents are more likely to employ night-time lighting aids for their ...

Notes: Wheat (Triticum aestivum L.) cultivars showed differential tolerance to chlorsulfuron. Cultivar Kotare showed no injury symptoms following foliar applications of chlorsulfuron at 15 or 60 g a.i. ha−1, while cultivars Rongotea and Lancer showed early damage in pot and field experiments at both rates of chlorsulfuron. Cultivars Abele and Jasper were intermediate in their response. The number of spikelets per ear was the only yield component affected by chlorsulfuron and was reduced in Lancer and Rongotea. Retention, uptake and transiocation of chlorsulfuron were not different between Kotare and Ron-gotea. Within 48 h of application, Kotare metabolized 92–2% of [14C]chlorsulfuron, while Lancer and Rongotea metabolized only 43–5% and 63% of the herbicide, respectively. The concentration of chlorsulfuron in young tissues of Kotare, Lancer and Rongotea, 48 h after application was calculated as 1.2, 31.9 and 15.6 ng g−1 dry weight, respectively. It is concluded that differential rates of metabolism are the main reason for differences in sensitivity to chlorsulfuron between the wheat cultivars tested. Le mécanisme des différences de tolérance au chlorsulfuron entre variétés de blé Des variétés de blé (Triticum aestivum L.) ont montré des différences de tolérance au chlorsul-furon. On n'a aucun symptôme de phytotoxicité sur la variété‘Kotare’ après une application foliaire de chlorsulfuron à 15 ou 60 g m.a. ha−1, alors que les variétés ‘Rongotea’ et ‘Lancer’ présentaient des symptômes précoces aux deux doses, dans des expériences en pot et au champ. La réponse des variété‘Abele’ et ‘Jasper’était intermédiaire. Le nombre de grains par épillet était la seule composante du rendement affectée parle chlorsulfuron et était réduit chez Lancer et Rongotea. La rétention, la pénétration et la migration de chlorsulfuron n'était pas différentes entre Kotare et Rongotea. Quarante huit heures après le traitement, Kotare avait metabolise 92,2% du [14C]chlorsulfuron, alors que Lancer et Rongotea ne métabolisaient que respectivement 43,5 et 63% de l'herbicide. La concentration de chlorsulfuron dans les tissus jeunes de Kotare, Lancer et Rongotea 48 h après la traitement a étéévaluée à respectivement 1,2,31,9 et 15,6 ng g−1 de matière sèche. Il est conclu que des différences de vitesse de métabolisation sont la raison principale des différences de sensibilité au chlorsulfuron observées chez les variétés de blé testées. Mechanismen der unterschiedlichen Reaktion von Weizensorten auf Chlorsulfuron-Behandlun-gen Es wurde eine unterschiedliche Toleranz bei Weizensorten (Triticum aestivum L.) gegenüber Chlorsulfuron-Behandlungen beobachtet. Bei der Sorte ‘Kotare’ traten nach Behandlungen mit 15 oder 60 g AS ha keine Schadsymtome auf, während die Sorten ‘Rongotea’ und ‘Lancer’ sowohl in Topfals auch in Freilandversuchen bei beiden Dosen frühe Schädigungen erlitten; die Sorten ‘Abel’ und ‘Jasper’ reagierten mittelstark. Unter den Ertragskomponenten war nur die Zahl der Ährchen pro Ähre betroffen, sie war bei ‘Lancer’ und ‘Rongotea’ reduziert. Aufnahme und Translokation des Wirkstoffs waren bei ‘Kotare’ und ‘Rongotea’ gleich. Bei ‘Kotare’ war [14C]Chlorsulfuron 48 h nach der Applikation zu 92,2 % metabolisiert, bei ‘Rongotea’ zu 63 % und bei ‘Lancer’ zu 43,5 %. In jungen Geweben dieser Sorten lagen die Chlor-sulfuron-Konzentrationen 48 h nach der Anwendung bei 1,2, 15,6 und 31,9 ng g−1 TM. Die unterschiedlichen Metabolisierungsraten bei den untersuchten Weizensorten wurden für den Hauptgrund für die Toleranzunterschiede gegenüber Chlorsulfuron gehalten.

Notes: GaN(0001¯) films were grown by molecular beam epitaxy using ammonia and elemental Ga. The surface reactivity and growth kinetics of GaN(0001¯) were investigated as a function of growth parameters using desorption mass spectroscopy. Growth proceeds either by island nucleation or by step flow, depending on the steady state surface coverage of Ga. Three Ga adsorption states were found on the surface, one chemisorption and two weak states. One of the weak states corresponds to Ga adsorbed on a gallided surface, while the other corresponded to an intrinsic physisorption state on a hydrogen-passivated, nitrided surface. An abrupt growth mode transition between excess Ga and excess nitrogen was found as a function of growth parameters. The transition was modeled by rate equations based on growth at step edges and the three types of adsorption states. © 2000 American Institute of Physics.

Notes: Homoepitaxial, GaN films on both c-plane surfaces of bulk GaN crystals were examined using reflection high-energy electron diffraction (RHEED). Differences in the RHEED pattern, time development of the RHEED intensity, and surface reconstructions were observed. The substrate surfaces were prepared either by mechanical polishing [GaN(0001)A] or by chemo-mechanically polishing [GaN(0001¯)B]. Then films were grown by molecular beam epitaxy; Ga was provide by a Knudsen cell and nitrogen from NH3. On the B surface, the Ga rich reconstructions reported by Smith and co-workers [Phys. Rev. Lett. 79, 3934 (1997)] were observed. On the A surface, a (2×2) reconstruction was observed. Both reconstructions were much sharper than those seen on GaN films grown on sapphire. RHEED measurements of the specular intensity vs time showed that two different surface terminations could be maintained on the B surface, one of which is a stable, gallided surface, while the other is a nitrided surface, which is unstable in vacuum. If the nitrided surface is heated in vacuum it changes to the gallided surface in several minutes at 800 °C. Only one termination was detected on the A surface. The results are complemented by desorption mass spectroscopy measurements, and the resulting surfaces were then investigated using atomic force microscopy and scanning tunneling microscopy. We were able to distinguish the two surface terminations on the B surface, and a unique annealing process under NH3 will be documented. Preliminary investigation of the A surface revealed decorated step edges. The results were compared to films grown on sapphire with different nucleation layers, which can be grown to yield either polarity. © 1999 American Institute of Physics.

Notes: We fabricated quantum wires of different geometries in Ga[Al]As heterostructures by local oxidation of the semiconductor surface with an atomic force microscope. By magnetotransport measurements at low temperatures on these wires the electronic width is determined and compared to the geometrical width. An extremely small lateral depletion length of the order of 15 nm and a high specularity of the scattering at the confining walls is found. Furthermore, we demonstrate experimentally that these quantum wires can be tuned by a combination of in-plane gates and top gates. © 1999 American Institute of Physics.

Notes: A single-electron transistor has been realized in a Ga[Al]As heterostructure by oxidizing lines in the GaAs cap layer with an atomic force microscope. The oxide lines define the boundaries of the quantum dot, the in-plane gate electrodes, and the contacts of the dot to source and drain. Both the number of electrons in the dot as well as its coupling to the leads can be tuned with an additional, homogeneous top gate electrode. Pronounced Coulomb blockade oscillations are observed as a function of voltages applied to different gates. We find that, for positive top-gate voltages, the lithographic pattern is transferred with high accuracy to the electron gas. Furthermore, the dot shape does not change significantly when in-plane voltages are tuned. © 1999 American Institute of Physics.