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Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society

AlGaAs several models have been proposed including distinctively different trapping times for electrons and holes and DX-type behavior of impurities [7-9] and spatial charge separation [6]. In general orthogonalization of the dopant states and the host states induces both classes of defect states: Hydrogenic levels dominated by the wave function of the band edges due to Coulombic interaction and strongly localized states controlled by the atomic bond strength directly. The latter are represented by an average of the entire Brillouin zone. Within the vicinity of the bandgap hydrogenic levels in most cases dominate but a charge transfer into a strongly localized level may occur whenever it coincides with the Fermi level. Carriers can therefore become trapped in such strongly localized neutral charge states (DO) [10]. In addition a structural relaxation of the donor impurity can occur in the vicinity of the electron transfer condition and may lead to an activation barrier between hydrogenic and strongly localized relaxed state. Proposed by Chadi and Chang [1] this widely accepted model of a structural relaxation explains the metastability and the activation barrier between the different states of these DX-centers. Due to the similarities of the properties in AlGaAs and the observation in AIGaN it must be feared that the n-type doping limitations in AIGaN might not be overcome by selection of any type of donor dopant. This would significantly affect the applicability of A1GaN alloys and AIN in a wide range of devices. At present the results in AIGaN might be limited by growth technology and we therefore choose a model for alloying GaN with AIN which has proven suitable also in the case of AIGaAs alloys. In this analogy alloying with AIN should be equivalent to application of large hydrostatic pressure to the respective binary compound, i.e., GaN. In GaN dopant impurities Si [11] and 0 [12,13] are found to be effective donors. In addition, the vacancy of nitrogen VN has frequently been referred to as a possible origin of high unintentional n-type conductivity. By applying large pressures we investigate the electronic band and impurity structure of GaN in search for possible carrier trap mechanisms. GaInN at variable InN fraction pseudomorphically grown on GaN acts as a suitable model for biaxial stress conditions in GaN, while hydrostatic pressure is applied to GaN as a model for AIGaN alloys. In GaN both major classes of donor dopants namely Si representing group-IV element on group-Ill site and 0 representing group-VI on group-V site are studied. We employ a contactless and purely optical method for monitoring the free carrier concentration by Raman spectroscopy within a diamond anvil cell. In the result we identify the major donor dopant in as-grown highly n-type GaN from high pressure synthesis and propose a promising donor species for high Al-content alloys. EXPERIMENTAL For the biaxial pressure experiments single GaljxlnxN/GaN heterostructures