An analysis of photoluminescence (PL) spectra for a range of single and multiple InGaN/GaN quantum wells, as a function of temperature, is presented. The well-known anomalous ‘S-shape’ behaviour is observed for samples emitting over a very wide range of energies. We present an analysis of this range of data, supplemented by other published data, in terms of different types of recombination sites within the wells. A quantitative model, based on previous work, to fit the temperature dependence of the emission peak energy is developed and gives good fits over an extended temperature range. The fitting parameters obtained are discussed in the light of values obtained from the literature and the intense piezoelectric fields present in the samples. Furthermore, the linewidth dependence of the PL emission peak energy in the region of the ‘S-shape’ is also analysed.

In this paper, we describe the growth and characterization of InGaN single quantum wells with emission peaks in the blue, green, amber and red spectral regions, grown by metal-organic vapour phase epitaxy. Starting from the growth of a blue-emitting (peak ~430 nm) InGaN quantum well at 860°C the InGaN growth temperature was progressively reduced. The photoluminescence peak wavelength, measured at low temperature, shifts through the green and orange spectral regions and reaches 670 nm for an InGaN growth temperature of 760°C. This corresponds to an energy lower than the currently accepted band-gap of the binary compound, InN. Spectral characteristics of the luminescence peaks will be discussed, including an analysis of the phonon-assisted contribution. Low energy secondary ion mass spectrometry analysis provides information on the indium content and thickness of the `blue' and `red' quantum wells. The results are combined to discuss the origin of the `sub-band-gap' luminescence in terms of the combined influence of InN-GaN segregation and the effect of intense piezoelectric fields.

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An analysis of the fraction of strongly localized excitons (FSLE) for a number of InGaN/GaN quantum wells emitting from violet to green, as a function of temperature, and a number of wells grown under the same conditions is reported. The analysis is based on the Huang-Rhys parameters, obtained from different phonon replica intensities and measured using photoluminescence spectroscopy (PL). It is suggested that the FSLE is related to separately localized excitons rather than tightly localized excitons due to potential fluctuations, concluding that the overall exciton localization is a result of a competition between these two types of localization centres.

This chapter presents an introductory review on quantum cascade lasers (QCLs). An overview is prefaced, including a brief description of their beginnings and operating basics. Materials used, as well as growth methods, are also described. The possibility of developing GaN-based QCLs is also shown. Summarizing, the applications of these structures cover a broad range, including spectroscopy, free-space communication, as well as applications to near-space radar and chemical/biological detection. Furthermore, a number of state-of-the-art applications are described in different fields, and finally a brief assessment of the possibilities of volume production and the overall state of the art in QCLs research are elaborated.