Alternative interpretations of some of the observed effects discussed above, such as the appearance of E+ transition and the pressure dependence of the E− and E+ transitions, were also proposed [49–56]. For instance, it has been argued that the observed changes in the conduction band structure are a result of interactions between states originating from the extended states of the Γ, L, and/or X conduction-band minima. It has been argued that incorporation of N breaks the crystal symmetry and splits the degenerate L and X minima into the a1 and t2 states. The a1 states strongly interact with the states at the Γ minimum, leading to a downward shift of the conduction-band edge. The close proximity of the L
minimum energy at EV+1.705 eV to the energy of the localized N-state EN≈EV+1.65 eV was invoked in the argument that interaction with either of these states could be responsible for the E+ and E− transitions [44]. It was also proposed that the impuritylike band of interacting nitrogen pairs and cluster states is responsible for the downward shift of the conduction-band edge in GaNAs alloys [55,56]. Several groups have studied the N-induced effects on the L conduction-band edges by measuring the E1 (Λ4v,5v−Λ6c) transition near the L points of the Brillouin zone in GaNxAs1−x using different experimental methods [33,35,57,58]. Figure 2.11 shows PR spectra associated with the E1 transitions from the L4,5 valence-band edge to the L6
conduction-band minima in several GaNxAs1−x samples. A small increase of the E1 transition energy with increasing x has been observed relative to the E1 transition at 2.925 eV in GaAs.
is, within the experimental uncertainties, the same as downward shift of the E− transition, which represents the fundamental band-gap reduction in the GaNAs samples. This salient feature is a typical characteristic of two-level anticrossing interaction, in which the upward shift of the upper state and the downward shift of the lower state are exactly the same in magnitude. The much slower upward shift of the E1 compared with the E+ transition energy rules out the possibility that the E+ can be associated with N-induced Γv−Lc transition. The slow, monotonic increase of the E1 transition energy with N concentration and the lack of a splitting of the L-band edge are also in disagreement with the theoretical calculations [51] attributing the E+ spectral feature to transitions from Γv to configuration-weighted average of nitrogenlike a1(N) and L-like a1(Lc) states.
To further elucidate the role of the higher energy minima, the effects of N on the band structure of Ga1−yAlyAs alloys were investigated [58]. In these alloys, the Γ band-edge minimum shifts from about 0.5 eV below in GaAs to slightly over 0.5 eV above the X conduction-band minima in AlAs. The large relative energy shift is expected to strongly affect the strength ofthe interaction between those two minima. Ga1−yAlyNxAs1−x alloys used in the study were synthesized by implanting nitrogen ions into MOVPE-grown AlyGa1−yAs epitaxial films on GaAs substrates followed by postimplantation thermal annealing. Energy positions of the experimentally observed E+ and E− transitions for a GaN0.0085As0.9915 and fou Ga1−yAlyNxAs1−x samples are shown in Figure 2.13. The dependencies of the energies of the Γ, L, and X conduction-band minima on the Al content in AlyGa1−yAs are shown inthe figure. The band-gap E0 measured in the as-grown wafers was used to determine the Al concentration of the Ga1−yAlyAs epitaxial films. The inset in Figure 2.13 shows a comparison of the PR spectra between an as-grown Al0.35Ga0.65As sample and an N+- implanted Al0.35Ga0.65NxAs1−x sample.
ROSSANA HERNADEZ
electrnica del estado solido
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