ALBERT

Notes: Triisopropyl indium (TIPIn) has been investigated as an alternative to trimethyl indium for use in chemical-beam epitaxy (CBE). In previous CBE studies of GaAs/AlGaAs growth, the replacement of methyl-containing precursors with ethyl- and isopropyl-containing precursors has been shown both to widen the substrate temperature window available for growth, and also to reduce unintentional carbon incorporation in the grown layers. In the present study of (100)InxGa1−xAs (0≤x≤0.1) growth using the new TIPIn source, in situ modulated-beam mass spectrometry studies have demonstrated a similar, and technologically very important, widening of the substrate temperature window. Furthermore, use of the new precursor combination, TIPIn and triisopropyl gallium, is also shown to generate state-of-the-art InGaAs material with electrical and optical properties directly comparable to corresponding material grown using molecular-beam epitaxy. © 1996 American Institute of Physics.

Notes: The photoluminescence (PL) at λ≤850 nm of boron-doped porous Si films anodically etched, passively etched, annealed, and reactive ion etched (RIE) under systematically varied conditions is described and discussed. As previously observed, the PL yield η of films etched from 7 to 20 Ω cm wafers in HF/H2O/C2H5OH solutions rapidly degraded during 40 mW illumination in air at 488 nm. In addition, it was totally quenched by O2 annealing or RIE but not by H2 RIE. However, the yield of films etched in HF/H2O only decreased by less than 10% following similar illumination for 7 h, and O2 annealing or RIE reduced it by 0%–50% only. The instability under illumination, the effects of oxygen annealing and RIE, Auger line shape measurements, and surface charge buildup during Auger analysis are consistent with a significantly higher density of weak Si—Si bonds in films etched in ethanol-containing solutions. These bonds are apparently broken by photoinduced oxidation, oxygen annealing, or O2 RIE, but the oxygen does not efficiently passivate the newly generated dangling bonds. While the peak emission wavelength λmax of the films etched from these wafers shifted with annealing and etching conditions, it invariably peaked in the ∼7600–8600 A(ring) range; η reached ∼1.7%. H2 RIE had no effect on η, but λmax was blueshifted. This behavior is attributed to similar breaking of weak Si—Si bonds followed by efficient passivation of the newly generated dangling bonds.The blueshift results from the higher H/Si ratio; the invariant yield is believed to be limited by the density of dangling bonds at the SiOxHy/Si interface which is apparently unaffected by the treatments. It is therefore believed that the PL is at least partially due to SiHx or SiOxHy complexes on the crystallite and column surfaces, which are similar to those found at the surface of some H-rich a-Si:H. Size effects, however, cannot be ruled out. In contrast to films etched from 7 to 20 Ω cm wafers, λmax of those prepared from 1 to 2 Ω cm substrates was consistently ∼7000 A(ring) after a brief exposure to air. While η exhibited a complex dependence on the O2 RIE power, λmax was unchanged by either O2 or H2 plasmas. The emission from heavily doped ∼0.02 Ω cm films was usually undetectable. However, H2 RIE resulted in observable PL. © 1995 American Institute of Physics.

Notes: Abstract A glasshouse study was undertaken to determine the physiological and morphological changes in cocksfoot (Dactylis glomerata L.) during regrowth after defoliation. Individual plants were arranged in a mini-sward in a randomized complete block design. Treatments involved harvesting each time one new leaf had expanded (one-leaf stage), up to the six-leaf stage, with the plants separated into leaf, stubble (tiller bases) and roots. Stubble and root water-soluble carbohydrate (WSC), stubble and leaf dry matter (DM), tiller number per plant and leaf quality (crude protein (CP), estimated metabolizable energy (ME) and mineral content) were measured to develop optimal defoliation management of cocksfoot-based pastures. WSC concentration in stubble and roots was highest at the five- and six-leaf stages. Mean WSC concentration (g kg−1 DM) was greater in stubble than roots (32·7 ± 5·9 vs. 9·4 ± 1·5 respectively). There was a strong positive linear relationship between plant WSC concentration and leaf DM, root DM and tillers per plant after defoliation (Adj R2 = 0·72, 0·88 and 0·95 respectively). Root DM plant−1 and tiller DM tiller−1 decreased immediately following defoliation and remained low until the three-leaf stage, then increased from the four-leaf stage. Tillers per plant remained stable until the four-leaf stage, after which they increased (from 9·9 ± 0·5 to 15·7 ± 1·0 tillers plant−1). Estimated metabolizable energy concentration (MJ kg−1 DM) was significantly lower at the six-leaf stage (11·01 ± 0·06) than at any previous leaf regrowth stage, whereas CP concentration (g kg−1 DM) decreased with regrowth to the six-leaf stage. Both the levels of ME and CP concentrations were indicative of a high quality forage throughout regrowth (11·37 ± 0·04 and 279 ± 8·0 for ME and CP respectively). Results from this study give a basis for determining appropriate criteria for grazing cocksfoot-based pastures. The optimal defoliation interval for cocksfoot appears to be between the four- and five-leaf stages of regrowth. Delaying defoliation to the four-leaf stage allows time for replenishment of WSC reserves, resumption of root growth and an increase in tillering, and is before herbage is lost and quality falls due to onset of leaf senescence.

Notes: We report electroabsorption studies of poly(2-methoxy-5-ethyl(2′-hexyloxy) para-phenylene vinylene) light-emitting diodes. An electric field develops during operation which opposes the field of the applied bias. The counter field builds up within 5 s of turning on the device, increases in magnitude with the operating voltage, and decays exponentially with a time constant between 15 and 32 s. We attribute the counter field to bulk carrier traps and discuss its relevance to the increase of the turn-on voltage as organic light-emitting diodes degrade. © 1999 American Institute of Physics.

Notes: We have studied the device degradation of single layer poly(2,5-dialkoxy-p-phenylenevinylene) light emitting diodes by electroabsorption spectroscopy. The applied direct current (dc) bias generates an opposing internal field. This internal field rises as the applied dc bias is increased. The development of the internal field is less pronounced in vacuum than in an ambient atmosphere and is no longer apparent for devices that were prepared and tested under an inert atmosphere in a glovebox. For the devices that were tested in air and under dynamic vacuum conditions we have also observed a change in the flat band voltage of the devices due to an aging effect on the electrodes. The combination of these two processes leads to an increase in the device turn-on voltage with increasing operating time. © 1999 American Institute of Physics.

Notes: The local environment of CAs acceptors in InxGa1−xAs has been determined from the localized vibrational modes (LVMs) of both isolated CAs impurities and H–CAs pairs using infrared (IR) absorption and Raman scattering techniques. In as-grown layers, a single LVM due to CAs was observed which broadened and shifted to lower energies with increasing x. The introduction of hydrogen led to the formation of H–CAs pairs and a single antisymmetric A1− mode (stretch) and a single symmetric A+1 mode (XH) were observed for all samples. All the LVMs were identified with carbon in CAsGa4 cluster configurations implying that less than 5% of the detectable carbon atoms are present in clusters incorporating one or more CAs–In bonds. © 1995 American Institute of Physics.

Notes: The first reported use of tri-isopropyl gallium (TiPGa) in chemical beam epitaxy (CBE) is described. Hall measurements performed on the resulting undoped GaAs epitaxial layers indicate an order of magnitude reduction in unintentional carbon impurity levels compared to structures grown under comparable conditions using the standard CBE precursor, triethyl gallium. 2 K photoluminescence spectra match those recorded elsewhere from state-of-the-art high purity GaAs material grown by molecular beam epitaxy, and 77 K Hall measurements on intentionally n-type doped GaAs layers confirm residual acceptor levels in the low 1014 cm−3 range. The early data obtained already provide a clear indication of the important potential of TiPGa as an improved precursor for the CBE growth of Ga-containing III–V materials.