Quantum wells (QWs) of the ternary nitride alloy indium gallium nitride (InGaN) form the active region in standard mass-market LEDs emitting in the violet to green spectral range. The Institute’s MOCVD growth team has extensive experience in growing such structures emitting at wavelengths between 375 and ~700 nm, and much of our publication output has concerned collaborative studies on these. In recent work, we have focussed particularly on incorporating InGaN QWs emitting at 390-410 nm into two types of structure:

1. Microcavities containing distributed QWs, and ultimately intended to further physical studies of the strong light-matter coupling regime.
   
2. Structures containing a single near-surface QW emitting, intended to explore non-radiative energy transfer to overlayers of light-emitting polymers.

Work on the microcavity structures is pursued jointly with colleagues from Strathclyde Physics Department, and other external collaborators with whom we work in two EU projects. Our own microcavity research has focussed strongly on growth on free-standing GaN substrates which have recently become available, and also involves exploiting lattice-matched AlInN interlayers (as discussed in more detail below) to fulfil roles in growth monitoring and post-growth processing. The work on energy transfer is a collaboration with Imperial College, forming part of a Research Councils UK Basic Technology project. The non-radiative energy transfer process is analogous to the Förster interaction in purely molecular systems, and is of interest because of its fast timescale, and correspondingly high efficiency. The studies with our custom-grown structures gave the first demonstration of this form of energy transfer between an inorganic QW and an organic material of any type, and were published in a highly-cited interdisciplinary journal [Advanced Materials, 18, 334 (2006)].

Selective area growth (SAG) is a specialised form of epitaxy, in which a single-crystal seed layer is covered by a patterned mask of a material which does not support crystal growth. In the case of GaN seed layers, silicon oxide is generally used as the mask material. Our own interests in SAG of GaN concern the formation of micropyramid arrays, in which the form of the individual pyramids reflects the hexagonal crystal symmetry of the GaN seed layer.

AlInN is the least studied of the three ternary alloys [AlInN, InGaN and AlGaN] possible in the group- III-metal-nitrogen system. However, interest in this material has developed rapidly since mid-2003, and derives from the fact that AlInN with an InN fraction of around 17% is lattice-matched to GaN. At the same time, the lattice-matched AlInN alloy has a large refractive index contrast to GaN, a direct bandgap around 0.9 eV higher than that of GaN, and dissimilar chemical properties to GaN. Our own work on AlInN growth has concentrated particularly on the use of lattice-matched AlInN interlayers in resonant-cavity structures grown on free-standing GaN substrates. Here, an important function of the AlInN layer is to allow in situ growth monitoring by optical techniques routinely applied in heteroepitaxial growth [see Applied Physics Letters, 87, 151901 (2005)]. We also plan to develop various other applications of AlInN-GaN heterostructures, including GaN QWs with AlInN barriers for emission in the near-ultraviolet, unipolar transistors, and microelectromechanical structures fabricated by selective wet etching. The relative immaturity of the AlInN alloy means that there is still uncertainty over such basic materials properties as the variation of lattice constant with composition, vibrational mode behaviour, and Stokes shift, which we are actively investigating with colleagues from Strathclyde Physics Department and overseas laboratories.

The group has strong links on materials characterisation with colleagues from Strathclyde Physics Department, and several international collaborators, including: University of Aveiro, Portugal; Institute of Nuclear Technology, Portugal; CNRS-CRHEA, France; University of Montpellier II, France; Paul Drude Institute, Germany; Institut Jaume Almera, Spain; and SUNY Buffalo, USA.

Industrial collaborators include: Epichem Metalorganics, MATS (UK), and Sharp Laboratories of Europe.