| Summary: | 碩士 === 國立臺灣大學 === 物理研究所 === 96 === This thesis concerns with the studies on the optical and electrical properties of Ⅲ-Nitride semiconductors. Photoreflectance (PR), piezoreflectance (PZR), photoluminescence (PL), photoconductivity (PC), resonant PL, and optically enhanced PC, and polarization resolved PL from a cleaved sample edge (edge photoluminescence) are carried out to study the physical properties of Ⅲ-Nitride materials, including dilute GaNxAs1-x alloys and GaNAs/GaAs single quantum wells. Many peculiar phenomena have been observed, which are very useful for the understanding as well as application of Ⅲ-Nitride materials. These results are presented as follows.
(1) Nonlinear behaviors of valence-band splitting and deformation potential in dilute GaNxAs1-x alloys
Photoreflectance and piezoreflectance investigations have been performed on a series of GaNAs layers grown by low pressure metalorganic chemical vapor deposition on Si-doped GaAs substrate. In addition to the observation of the fundamental band gap and spin-orbit splitting, the valence-band splitting in thin GaNxAs1-x epilayers strained coherently by the GaAs substrate is observed in these modulation spectra. Comparing photoreflectance with piezoreflectance spectra, we clearly establish the transitions involving the heavy-hole and light-hole bands. We find that the valence-band splitting increases with increasing nitrogen composition, and it decreases with increasing temperature. We point out that the underlying origin of our observation can be attributed to the effect of lattice mismatch between GaNAs film and GaAs substrate. We also find that the deformation potential of GaNxAs1-x does not follow the linear interpolation of those for GaAs and GaN. It shows a strong composition dependence with a bowing in the small composition
(2) Upside-down tuning of light- and heavy-hole states in GaNAs/GaAs single quantum wells by thermal expansion and quantum confinement
Polarization resolved photoluminescence from a cleaved sample edge (edge photoluminescence) was used to investigate the valence-band splitting in GaNAs/GaAs single quantum wells. The spectra resulting from the heavy- and light-hole transitions shows an interesting phenomenon, in which the light-hole state is above the heavy-hole state at low temperature, they degenerate at about 195 K, and then it is below the heavy-hole state at higher temperature. We point out that the underlying origin of our observation can be attributed to the competitive effect between the tensile strain induced by lattice mismatch and quantum confinement. It is known that the large density of heavy hole states is beneficial for laser diodes. Our result is therefore very useful for the application of GaNAs/GaAs quantum wells in optical devices.
(3) Nature of 0.97 eV defect emission in GaNAs epifilms
GaNAs epifilms with different nitrogen contents grown on n+-(001) GaAs substrates by molecular beam epitaxy have been characterized by using different optical techniques, including photoluminescence (PL), photoconductivity (PC), resonant PL, and optically enhanced PC measurements. The PL spectra of GaNAs epifilms show two peaks, which contain a dominant emission near the GaNAs alloy band gap and a relatively weak deep band around 0.97 eV. In PC spectra, the responsivity of GaNAs epifilms around the low energy feature was found to increase when the epifilm was illuminated with a background Ar+ ion laser. It is a useful result to determine that the location of the defect level is above the top of the valence band about 0.38 eV. In the resonant PL spectrum, the 0.97 eV peak splits into four fine structures. From all our results, we identify that the origin of the 0.97 eV defect emission is related to AsGa antisite defects.
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