Use of AlInN layers in optical monitoring of growth of GaN-based structures on free-standing GaN substrates

When lattice matched to GaN, the AlInN ternary alloy has a refractive index ~7% lower than that of GaN. This characteristic can be exploited to perform in situ reflectometry during epitaxial growth of GaN-based multilayer structures on free-standing GaN substrates, by insertion of a suitable Al0.82In0.18N layer. The real-time information on growth rates and cumulative layer thicknesses thus obtainable is particularly valuable in the growth of optical resonant cavity structures. We illustrate this capability with reference to the growth of InGaN/GaN multiple quantum-well structures, including a doubly periodic structure with relatively thick GaN spacer layers between groups of wells. Al0.82In0.18N insertion layers can also assist in the fabrication of resonant cavity structures in postgrowth processing, for example, acting as sacrificial layers in a lift-off process exploiting etch selectivity between Al0.82In0.18N and GaN

Photoluminescence of near-lattice-matched GaN/AlInN quantum wells grown on free-standing GaN and on sapphire substrates

Near-lattice-matched GaN/Al1−xInxN single quantum wells, grown using both free-standing GaN and conventional GaN-on-sapphire substrates, are studied by photoluminescence (PL) and PL excitation spectroscopies. PL spectra distinguish luminescence originating in the wells, barriers, and underlying GaN buffer layers. The spectra also reveal significant differences between structures grown simultaneously on the different substrates. The quantum well transition energy decreases as the well width increases due to the intense in-built electric fields, estimated to be 3.0±0.5 MeV/cm, that persist in strain free GaN/Al1−xInxN. Screening of these fields is studied using the excitation power dependence of the P

Photoluminescence and phonon satellites of single InGaN/GaN quantum wells with varying GaN cap thickness

Variations in thickness of the GaN caps above single InGaN quantum wells have been studied using photoluminescence spectroscopy. Data are presented from two series of samples designed to promote energy transfer to luminescent species on the surface. Improvements in the optical properties as the GaN cap thickness increases from 2.5 to 15 nm are accompanied by clear changes in the intensity of the LO-phonon satellites. Analysis of the strength of successive phonon satellites and the associated Huang-Rhys factors indicates that the amount of localization of the excitons is increased for the thinner cap samples. Surface depletion fields are also considered

Raman-scattering study of the InGaN alloy over the whole composition range

We present Raman-scattering measurements on InxGa1−xN over the entire composition range of the alloy. The frequencies of the A1(LO) and E2 modes are reported and show a good agreement with the one-mode behavior dispersion predicted by the modified random-element isodisplacement model. The A1(LO) mode displays a high intensity relative to the E2 mode due to resonant enhancement. For above band-gap excitation, the A1(LO) peak displays frequency shifts as a function of the excitation energy due to selective excitation of regions with different In contents, and strong multiphonon scattering up to 3LO is observed in outgoing resonance conditions

Quantum dot emission from site-controlled ngan/gan micropyramid arrays

InxGa1−xN quantum dots have been fabricated by the selective growth of GaN micropyramid arrays topped with InGaN/GaN quantum wells. The spatially, spectrally, and time-resolved emission properties of these structures were measured using cathodoluminescence hyperspectral imaging and low-temperature microphotoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit decay lifetimes of approximately 0.5 ns, with linewidths down to 650 me

Optical spectroscopy of gan microcavities with thicknesses controlled using a plasma etch-back

The effect of an etch-back step to control the cavity length within GaN-based microcavities formed between two dielectric Bragg mirrors was investigated using photoluminescence and reflectivity. The structures are fabricated using a combination of a laser lift-off technique to separate epitaxial III-N layers from their sapphire substrates and electron-beam evaporation to deposit silica/zirconia multilayer mirrors. The photoluminescence measurements reveal cavity modes from both etched and nonetched microcavities. Similar cavity finesses are measured for 2.0 and 0.8 mm GaN cavities fabricated from the same wafer, indicating that the etchback has had little effect on the microcavity quality. For InGaN quantum well samples the etchback is shown to allow controllable reduction of the cavity length. Two etch steps of 100 nm are demonstrated with an accuracy of approximately 5%. The etchback, achieved using inductively coupled plasma and wet chemical etching, allows removal of the low-quality GaN nucleation layer, control of the cavity length, and modification of the surface resulting from lift-off

(In,Ga)N/GaN microcavities with double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial layer

Comparable microcavities with 3/2 (~240 nm) active regions containing distributed (In,Ga)N quantum wells, grown on GaN substrates and bounded by two dielectric mirrors, have been fabricated by two different routes: one using laser lift-off to process structures grown on GaN-on-sapphire templates and the second using freestanding GaN substrates, which are initially processed by mechanical thinning. Both exploit the properties of an Al0.83In0.17N layer, lattice matched to the GaN substrate and spacer layers. In both cases cavity quality factors >400 are demonstrated by measurements of the cavity-filtered room-temperature excitonic emission near 410 nm

Structural and optical properties of MOCVD AllnN epilayers

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GDP, share prices and share returns: Australian and New Zealand evidence

With the aim of predicting share market returns, many empirical studies have delved into how financial and macroeconomic variables can be used to forecast return variability. The aim of this paper is to examine whether the ratio of aggregate share price to GDP can capture the variation of future returns on the aggregate share market within Australia and New Zealand. Using quarterly and semi-annual data for the period 1991-2003 for New Zealand and 1982-2006 for Australia, this study finds that the ratio of share price to GDP indeed captures a significant amount of the variation of returns on the New Zealand share market as well as the Australian share market; however results for Australian data do vary, depending on the sample period. Results in this paper generally provide support for the theory behind previous papers, specifically that of Rangvid (2006)

Virulence of Russian Wheat Aphid, Diuraphis Noxia (Kurdjumov) (Homoptera: Aphididae) Populations in Kenya

The Russian wheat aphid (RWA) Diuraphis noxia (Kurdjumov) is a serious pest of wheat in Kenya. Development and use of RWA resistant wheat (Triticum aestivum L.) varieties, has been constrained by RWA populations evolving with differential virulence to given resistant host plants. To fully exploit host plant resistance (HPR) in management of RWA, local populations of RWA have to be evaluated for differential virulence and biotypes in order to develop and deploy cultivars that exhibit cross biotype resistance. A study was conducted at KARI-Njoro to characterize virulence of RWA populations from the endemic areas (Eldoret, Mau Narok, Njoro and Egerton) in Kenya. A factorial experiment in randomized complete block design replicated three times was set up to evaluate seedling resistance to RWA with variety and aphid collection source as main factors in the screen house. Five adult RWA aphids from each of the four collection locations were used to infest four host genotypes; PI624933-1  containing Dn4 gene, 2414-11-2  containing Dn7 gene, KRWA9 which contains an unknown Dn gene and a susceptible check, K.KWALE, for 28 days to determine virulence of the RWA aphids  to seedlings of the four wheat genotypes in the greenhouse. Data was recorded on damage scores, plant height, plant height reduction, shoot biomass and biomass reduction of test plants 28 days after infestation. Results of an analysis of variance of these plant parameters show that Egerton population was more virulent than populations selected from other areas as it caused more damage on resistant lines. Keywords: Diuraphis noxia, biotype, virulence, RWA, Wheat.