The interplay between extended and localized electronic states in semiconductor alloys can significantly influence their properties at low temperatures. In the case of donors or acceptors, localized states determine whether a semiconductor appears as an insulator or as a metal at low temperatures. It is well known that the material can undergo an insulator to metal transition when their concentration is raised above some threshold. This phenomenon has been studied extensively during the past decades, but nevertheless many unanswered questions persist.
In this context, the material system (B,Ga,In)As turns out to be a very interesting and complex system from a fundamental point of view with manifold interactions of localized and extended states. Isovalent boron is found to generate highly localized states resonant with the conduction band. These states are very close to the conduction band edge, which makes them accessible by applying hydrostatic pressure.
The first part of this work addresses the influence of those isovalent localized states on the electronic properties of (B,Ga,In)As. Despite the difficulties brought about by the complexity of the system, its properties can be well explained by the model proposed in this work. Most valuable were the measurements under hydrostatic pressure that revealed a pressure induced metal-insulator transition. One of the main ideas in this context is the trapping of carriers in localized B-related cluster states that appear in the bandgap at high pressure. The key conclusion that can be drawn from the experimental results is that boron atoms seem to have the character of isovalent electron traps, rendering boron as the first known isovalent trap induced by cationic substitution.
In the second part, thermoelectric properties of (B,Ga,In)As and (Ga,In)(N,As) are studied.It was found that although the electric-field driven electronic transport in n-type (Ga,In)(N,As) and (B,Ga,In)As differs considerably from that of n-type GaAs, the temperature-gradient driven electronic transport is very similar for the three semiconductors, despite distinct differences in the conduction band structure of (Ga,In)(N,As) and (B,Ga,In)As compared to GaAs. This similar behavior in case of temperature-gradient driven electric transport is caused by the similarity of the dispersions of the extended phonon states of the three semiconductor materials in conjunction with the dominance of the phonon drag effect.
The third part addresses the influence of magnetic interactions on the transport properties near the metal-insulator transition (MIT). Here, two scenarios are considered: Firstly the focus is set on ZnMnSe:Cl, a representative of so called dilute magnetic semiconductors (DMS). In this material Mn(2+) ions provide a large magnetic moment due to their half filled inner 3d-shell. It is well known that the resulting interaction between these localized magnetic moments and the electron spins leads to a spin splitting of the band states. However, little is known about the modifications of the impurity band transport due to magnetic interactions. It is shown that magnetic interactions in conjunction with disorder effects are responsible for the unusual magnetotransport behavior found in this and other II-Mn-VI semiconductor alloys.In the second scenario, a different magnetic compound, namely InSb:Mn, is of interest. It is a representative of the III-Mn-V DMS, where the magnetic impurity Mn serves both as the source of a large localized magnetic moment and as the source of a loosely bound hole due to its acceptor character. Up to now, little is known about the influence of magnetic donors or acceptors on the metal-insulator transition. However, as it will be shown, there exists an extremely interesting doping regime close to the metal-insulator transition where localized states of magnetic impurities can dramatically alter the transport properties. This work tries to shed some light on this topic by comparing magnetic InSb:Mn and non-magnetic InSb:Ge which reveal distinct differences in their electric resistivity near the metal-insulator transition and by presenting a model that is able to explain the unusual experimental findings.
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