Optical spectroscopy of the band formation and band structure of metastable semiconductor systems

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The aim of the project is to study the electronic, vibronic, and magnetic properties and the correlations between them of various metastable semiconductor alloys and their heterostructures. In these systems band gap engineering can be achieved by perturbing strongly a host semiconductor by incorporating a few percent of an isovalent impurity. At the beginning of the project we will focus on the most prominent members of this new class of semiconductor alloys: Ga(N,As) and (Ga,In)(N,As). Later on we will also investigate the less studied alloys Ga(N,P) and (Ga,In)(N,P). The quaternary (Ga,In)(N,As) is already employed as active material for lasers in telecommunications and for high efficiency solar cells. Understanding and controlling the unique properties of this alloy will considerably improve these devices. We will study the band formation and the interaction between Ninduced and host-like bands with increasing anion substitution as well as the alteration of bands during the phase transitions of the metastable systems. Optical spectroscopy under high pressures and in magnetic fields will be used to study electron and phonon states in these alloys. The experimental results will yield stringent tests of the recently proposed microscopic models of the band formation in these semiconductors. In addition, we will determine important band structure parameters for a k.p-like description of the band structure. Such k.p descriptions form the basis of gain modelling in devices containing these alloys. Furthermore, the band offsets shall be determined for the heterosystems under consideration Ga(N,As)/GaAs, (Ga,In)(N,As)/GaAs, Ga(N,P)/GaP and (Ga,In) (N,P)/GaP. Micro-spectroscopy shall be used to obtain information about the spatial extension of the areas with different band gaps (but the same mean alloy composition) in the quaternary compounds. The different band gaps seem to be correlated to the different nearest-neighbour environments of the isovalent impurity.