Hydrostatic pressure experiments on dilute nitride alloys

GaN_xAs_1-x and Ga_1-yIn_yN _xAs_1-x are the most prominent members of a novel class of non-amalgamation type semiconductor alloys where a fraction x of the anions of the host (e.g., GaAs or Ga_1-yI_yAs) is replaced by N isovalent impurity atoms. The localized N-states in GaN_xAs _1-x and Ga_1-yIn_yN_xAs_1-x form a series of discrete energy levels (e.g., isolated N-state, N-pairs and higher order N-cluster states) resonant with the conduction band of the host. The effect of the alloying with nitrogen on the bandstructure of GaN _xAs_1-x and Ga_1-yIn_yN _xAs_1-x can be well parameterized using a band-anticrossing (BAC) model, namely, assuming a level repulsion between an effective N-state and the conduction band-edge state. The dependence of several physical properties on nitrogen incorporation can be predicted qualitatively in the framework of this model, e.g., a tremendous increase of the electron effective mass in Gan_xAs_x with increasing x, a huge cross section for scattering of electrons by N impurities in electronic transport, etc. Most of these predictions can be tested and verified by performing hydrostatic pressure experiments which, within the picture of the BAC model, allow one to tune continuously the energy difference between the host-like conduction band edge and the effective N-level within one and the same specimen. Several examples of this kind will be discussed. Furthermore, we will demonstrate the limitations of the BAC model in the case of GaN_xP_1-x and also of GaN_xAs_1-x.,. In particular, we will show several examples where the description by a single effective N-state fails and that the multiplicity of the N-states needs to be taken into account. Again hydrostatic pressure experiments prove to be a useful and suitable tool for revealing the effects due to N-cluster states.