We demonstrate that the phonon frequencies that are reported in the literature for indium nitride (InN), films are all consistently correlated to the strain state of the material. Raman spectroscopy measurements and X-ray investigations of large size InN quantum dots grown on c-plane GaN are combined, which show these frequencies to experience a blue shift with increasing compression. The InN dots are weakly strained, most probably due to the formation of dislocations at the InN/GaN interface. We report the observation of a broad absorption in the 1.25 eV region that is typical of thin InN films. Such a feature we attribute to light absorption at the energy of the fundamental direct band gap of InN, while we attribute the low energy 650-800 meV photoluminescence to an extrinsic recombination process analogous to the processes that produce the blue band in AlN and the yellow band in GaN. (C) 2004 Elsevier B.V. All rights reserved.

The behavior of the E-2 and A(1)(LO) optical phonons of hexagonal indium nitride under hydrostatic pressure was studied using Raman spectroscopy. Linear pressure coefficients and the corresponding Gruneisen parameters for both modes were determined for the wurtzite structure up to 11.6 GPa, close to the starting pressure of the hexagonal to rocksalt phase transition of InN. Raman spectra acquired within the 11.6 to 13.2 GPa pressure range suggest that wurtzite InN undergoes a gradual phase transition, and the reverse transformation exhibits a strong hysteresis effect during the downstroke.

Using measurements of phonons frequencies in large size InN quantum dots deposited by Metal-organic vapor phase epitaxy, we found these frequencies to experience a blue shift with increasing compression. Next we show that all the phonon frequencies reported in the literature are correlated to the strain state of InN and are, within the experimental uncertainty, consistent with each other. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.

The composition dependence of the band-gap reduction of GaAs1-xNx grown by molecular beam epitaxy and metal organic vapor phase epitaxy was investigated using transmission, reflection, and low-temperature photoluminescence (PL) spectroscopy for N incorporations ranging from doping concentrations up to x=5 × 10-2. We identified four different regimes of N incorporation with distinctly different band-gap scaling. N-doped GaAs shows sharp PL lines due to N cluster states, but no significant change in the band gap. In the ultradilute region (10-5≤x≤ 1.5 × 10-3) a strong band-gap reduction was observed which scales according to x, irrespective of the local distribution of N atoms in the As sublattice. The same band-gap scaling was observed for ultradilute InGaAsN after corrections for strain and In alloying. In an intermediate compositional region (1.5 × 10-3≤x≤2.5×10-2) ΔEg scales according to x2/3. At higher concentrations (x>2.5 × 10-2) ΔEg weakens due to effects connected with N oversaturation of the As sublattice.

Piezoelectric effects on the optical properties of GaN/AlN quantum dots have been investigated by both continuous-wave and time-resolved photoluminescence (TRPL) measurements. The increase in excitation density in CW-PL leads to a blue shift of the transition energy. The TRPL measurements reveal a very large blue-shift (0.6 eV) of the PL peak energy, which reduces with time delay in a complex way, due to the strong transition-energy dependence of the carrier recombination time. The results are discussed in terms of respective roles played by the population of excited levels and by the screening of internal electric fields by accumulation of electron-hole dipoles in the quantum boxes. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Bragg mirrors are highly interesting structures for a large set of applications including vertical cavity lasers and the upcoming range of devices based on microcavities. Although the nitride semiconductors are performing fairly well in optoelectronic applications, it is not straightforward to realize Bragg mirrors based on this material system, due to the low optical index differences between GaN and AlN. Moreover, the lattice parameter difference between these materials will generate crystal defects, which prevent the stacking of a large number of periods, adding to the difficulty. In this work, we have grown high reflectivity Bragg mirrors, with a band centered in the visible blue range. The structures were first modelled, then grown by low pressure MOCVD, and were optimised using an in-situ reflectivity system.;This in-situ reflectivity measurement was compared to a calculated profile, to enable real-time control of the structures. The samples were characterized by transmission electron microscopy and reflectivity. It was possible to realize samples with 90% reflectivity near 400 nm.