Bound excitons in silicon carbide

Sammanfattning: The investigation of silicon carbide (SiC) is strongly motivated by the potential of this material for various demanding electronic applications. The main virtue of SiC is its toughness compared to other more common semiconductors. SiC is hard, can withstand high temperatures, high electric fields, chemically hostile environments, high levels of radiation and is able to accommodate high frequencies. An important aspect of any semiconductor crystal is the defects it contains. In fact, semiconductors owe their usefulness to the controlled incorporation of defects, but at the same time their usefulness is often limited by the presence of unwanted defects. This thesis is concerned with the study of defects in SiC by means of photoluminescence (PL) spectroscopy. Many defects can bind excitons at low temperatures. It is the radiative recombination of such bound excitons (BE) that has been investigated in this thesis. The main contribution of the work is to clarify the behaviour and electronic structure of BE's associated with a number of common defects in SiC. The work is presented in six papers.In paper I, the properties of a PL spectrum appearing after room temperature electron irradiation of 4H-SiC are reported. The spectrum is very complicated, consisting of a myriad of sharp no-phonon PL lines as well as broader phonon assisted transitions. We show in the paper how the use of PL excitation (PLE) spectroscopy, a technique that has hitherto been largely neglected in the study of SiC, greatly facilitates the investigation of this spectrum. From our results we suggest that the PL spectrum is due to BE recombination at several different isoelectronic defects.In papers II and III we report a comprehensive study of the so-called D1-BE in SiC. Paper II contains several novel experimental results pertaining to the D1 -BE in the 4H-polytype of SiC. Paper III extends the investigation to two other polytypes, namely 3C- and 6H-SiC, and offers an explanation to all the experimental findings in the form of a detailed theoretical model. The D1-BE isshown to be bound at an isoelectronic defect. The exciton consists of a tightly bound hole and a weakly bound electron. A series of effective-mass-like excited states associated with the electron are observed with PLE spectroscopy.Papers IV and V deal with well known hydrogen related defects in 6HSiC. Paper IV reports the excitation spectra of the BE's residing at these defects. Several excited states of the BE's are detected. Paper V is concerned with the quenching of the hydrogen related PL under prolonged laser excitation. By studying the temporal changes of the PL intensity as a function of excitation energy, it is shown that the quenching is recombination enhanced. Also, we supply evidence for the existence of another configuration of the defect, that cannot be detected directly in the PL spectrum.In paper VI, the excitation spectra of BE's associated with nitrogen donors in 4H-, and 6H-SiC are reported. Several high energy excited states are observed. The electronic structure of the BE's is discussed in terms of a shell model.