| Popis: |
This thesis deals with the electronic and optical properties of excitons bound to impurities in semiconductors. The first three chapters are concerned with the electronic structure of bound exciton states. The fundamental process in which a bound exciton is formed on a neutral acceptor in silicon is studied. Three lines are observed in the absorption spectra of the acceptors Al, Ga, and In in silicon. Of the various schemes used to explain these lines, it is determined from the relative oscillator strengths that the correct scheme is one in which two j=3/2 holes are coupled to form states with total angular momentum J=0 and J=2. The cubic environment of the crystal in the neighborhood of the impurity leads to a splitting of the J=2 state. In the bound exciton spectra of Si:Tl, four lines are observed in contrast to the three lines observed for the acceptors Al, Ga, and In. The oscillator strength of the thallium bound exciton is measured and continues the trend towards large oscillator strengths for the deep impurities. The excited state structure of the donor bound excitons in GaP is also presented. It is established that excitons bound at these impurities should have a structure which consists of a D- core to which a hole is loosely bound. Two different types of excited states have been identified for the donor bound excitons. Some of these states are excitations of the hole about the D- core of the bound exciton. Other states have been identified which are due to excitations of the D- electron core. These excitations have different symmetry than the ground state. The observation of a low lying excited state of the neutral indium acceptor is reported. This state is interesting because current theories of the electronic states of the acceptor do not predict it. The state may exist as the result of the Jahn-Teller effect. The systematics of the spectroscopy of multiple exciton complexes bound to donors in silicon are reported. It is found that the spectroscopy of these complexes changes very little between different impurities. This suggests that carrier-carrier interactions are more important than the impurity potential in determining the binding of excitons to the complexes. Any model of the structure of the complexes must take this into account. Measurements of the capture cross-section of a free exciton on a neutral In acceptor in silicon are reported. In lightly doped silicon the decay of the luminescence associated with In bound excitons is determined by the rate at which excitons are captured on the indium site. By measuring the decay time of the luminescence as a function of indium concentration and temperature, it is possible to obtain the capture cross-section of a free exciton on a site. The capture cross-section is strongly temperature dependent changing by two orders of magnitude between 10°K and 30°K. |
