Poster abstract

We measured and analyzed, at the temperature up to 750 C, the electrical properties (resistivity) of gallium nitride crystals grown from Mg-rich Ga melt under high N2 pressure. The studied material is characterized by high background concentration of O donors (~5x10^19 cm-3), which compensate Mg acceptors. Generally, the resistivity of as-grown crystals is high, due to nearly perfect compensation of dopants. We have studied the thermally activated time-depending processes leading to activation of dopants (i.e. annealing, leading to irreversible increase of conductivity) as well as characteristic temperature dependencies of resistivity at different stage of these processes. We found that 1. The electrical properties can be described by two impurity/defect levels: For the adequate explanation of the resistivity behavior, (especially above 500 C), beside the shallow Mg acceptor level (Ev+0.1 eV), a second level lying at Ev+0.9 eV should be introduced. 2. There are two different thermoactivated processes which lead to the activation of the corresponding defect/impurity centers. The first process, which lead to appearance of shallow Mg acceptors, is observed at temperatures as low as ~300 C and terminates below 600 C (we find a rough estimate of activation energy characteristic for this process as equal to about 1.5 eV ). We find however that the number of Mg acceptors activated in this process does not exceed 1% of the total Mg concentration present in the material. In the second process, observed above 600 C, the considerable concentration of the deep centers of unknown nature is generated. The possible nature of the centers as well as the mechanisms of their activation is discussed. The low-temperature annealing process might be tentatively attributed to the dissociation of Mg-H complexes i.e. to the mechanism observed in MOCVD grown layers (the presence of hydrogen in our material is confirmed by SIMS), although in the case of MOCVD samples such process starts at more elevated temperature. The second process seems to be worthy of further studies for two reasons: - it results in generation of important concentration of defects (comparable to this of the main dopants) - it is observed in the temperature range usually employed to activation of Mg acceptors in MOCVD grown nitride layers. -