Thesis (Ph. D.)--Georgia State University, 2008. / Title from file title page. Nikolaus Dietz, committee chair, Brian Thoms, A. G. Unil Perera, Xiaochun He, committee members. Electronic text (215 p. : col. ill.) : digital, PDF file. Description based on contents viewed on July 14, 2008. Includes bibliographical references (p. 209-215).

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Calculations of the electronic properties of AIN, GaN, InN and their alloys are presented. Initial calculations are performed using the first principles pseudopotenial method to obtain accurate lattice constants. Further calculations then investigate bonding in the nitrides through population analysis and density of state calculations, the empirical pseudopotential method is also used in this work. Pseudopotentials 'or each of the nitrides are constructed using a functional form that allows strained material and alloys to be studied. The conventional k,p valence band parameters for both zincblende and wurtzite are obtained from the empirical band structure using two different methods. A Monte-Carlo fitting of the k.p band structure to the pseudopotential data (or an effective mass method for the zincblende structure) is used to produce one set. Another set is obtained directly from the momentum matrix elements and energy eigenvalues at the centre of the Brillouin zone. Both methods of calculating k.p parameters produce band structure in excellent agreement with the original empirical band calculations near the centre of the Brillouin zone. The advantage of the direct method is that it produces consistent sets of parameters, and can be used in studies involving a series of alloy compositions. Further empirical pseudopotential method calculations are then performed for alloys of the nitrides. In particular, the variation of the band gap with alloy composition is investigated, and good agreement with theory and experiment is found. The direct method is used to obtain k.p parameters for the alloys, and is contrasted with the fitting approach. The behaviour of the nitrides under strain is also studied. In particular, valence band offsets for nitride heterojunctions are calculated, and a strong forward-backward asymmetry in the band offset is found, in good agreement with other results in the literature.

We investigate the electronic band structure of device relevant GaInNAs/GaAs multiple quantum wells (MQWs) and veitical-cavity surface-emitting laser (VCSEL) structures. We report photo-modulated reflectance (PR) studies under applied pressure and variable temperature that probe the influence of N-related states on the electronic structure of dilute nitrogen (N) III-V MQWs. The pressure and temperature dependence of the intersubband transitions within the MQWs is reduced by addition of N. By matching our experimental results with a theoretical model important predictions for the ground-state electron effective mass and conduction band offset as a function of N and pressure are made. We present results of angle- and temperature-dependent electro-reflectance (ER) measurements on a dilute-N GaInNAs VCSEL and show that these explain how the corresponding VCSEL device can operate over a such a wide range of temperatures. We argue that intrinsic properties of dilute-N QWs provide novel ways to design laser devices, especially in the crucial telecommunication range of wavelengths. We show how non-destructive ER and PR measurements can be used, in order to estimate the QW transition energy when it is coupled with the cavity mode (CM). The energy of the main exciton is determined by monitoring the amplitude and the phase of the PR spectra. The ER measurements are presented on the GaInNAs VCSEL described in the previous paragraph. Furthermore we present a growth characterisation study on a representative InGaAs/GaAs/AlAs/AlGaAs as-grown VCSEL structure, using PR spectroscopy as a function of position on a non-uniform wafer. We also show how temperature dependent PR and the appropriate lineshape model can be used to obtain a full picture of the relative movements between the gain and the CM over the full range of temperature. This information allows calculating the material gain in the temperature range of interest, independent from the effect of the CM and also provides an alternative method for characterising the growth, which can be applied to uniform wafers. PR and non-destructive ER can be used to identify regions suitable for fabrication into devices. For this reason modulation spectroscopy can be very useful for industry to reject wafers where good alignment between the CM and the QW does not occur and can thus save on the time consuming and expensive fabrication procedures.

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