An interesting implication of the band-structure modification due to N in Ga(N,As) and (Ga,In (N,As) is the strong nonparabolicity of the conduction band predicted by theor.In comparison with GaAs, the electron effective mass is expected to increase by about 50% at k=0 for x≈1%, with a further mass increase for k≠0. There are, to date, several experimental studies of this interesting effect. Skierbiszewski et al. [51] observed a strong increase of the electron effective mass at the Fermi level in (Ga,In)(N,As):Se with increasing free-electron concentration up to a value of 0.4 m0 for n=6×1019 cm−3. Hai et al. [52,53] showed by cyclotron resonance on 7-nm GaNxAs1−x/GaAs QWs that the electron effective mass at the conduction band edge in GaNxAs1−x increases with x, whereas the hole effective masses are similar to those of GaAs. They reported values of 0.12 m0 and 0.19 m0 for x=1.2% and 2.0%, respectively. Wu et al. [54] pointed out that the confined states of Ga(N,As)/GaAs QWs can only be correctly described when the Ninduced changes of the electron effective mass are taken into account. Baldassari Höger von Högersthal et al. [55] reported a value of 0.15 m0 for x=1.6% determined by magnetophotoluminescence. A sharp increase of the electron effective mass was observed for N contents below 0.5% [56]. Similar experiments were performed by Wang et al. [58]. The electron effective mass is usually reported to strongly increase with increasing N content, apart from a report of transport experiments where the opposite trend was claimed [57]. The experimental data scatter considerably. This is partly a result of different assumptions underlying the models used for extracting the actual effectivemass values from the experimental data obtained by different experimental techniques. In addition, as discussed below in detail, effective-mass results obtained on bulk samples and quantum wells
of the same x are not necessarily comparable. The effective-mass issue is discussed from a different perspective.
A further complication arises because the conduction band of GaNxAs1−x is strongly nonparabolic. This can also be understood qualitatively in the framework of the levelrepulsion model. The closer the conduction-band states of the host are to the N level, the stronger is the level repulsion. Consequently, this means for QWs that the effective masses of the electron subbands in the conduction band must depend on N content, well width, and confinement energy. This manifests itself in the hydrostatic-pressure dependence of the interband transitions enhhn of Ga(N,As)/GaAs QWs, as can be seen in Figure 5.7. The figure shows series of PR spectra obtained under hydrostatic pressure at 300 K for two GaN0.018As0.982/GaAs QWs of width 8 nm and 4 nm. In the first series of spectra in Figure 5.7a, three signals can be clearly detected at all pressures; a fourth one can be discerned in the spectra for pressures exceeding 0.7 GPa. The signal at the highest energy originates from the GaAs barrier.
ROSSANA HERNANDEZ
Electronica del estado solido
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