Photoluminescence polarization degree

We report on polarization sensitive microphotoluminescence studies of wurtzite type GaN/AlN quantum dots grown on (0001) sapphire substrates by molecular beam epitaxy. Typical linearly polarization degree of photoluminescence is about 0.06. Photoluminescence intensity of some particular samples decreases over time under continuous-wave excitation on a timescale of minutes. Emission of such samples is linearly polarized with polarization degree of about 0.15. Possible reasons of the observed behavior are discussed. 

The PL spectra of the sample 417 at orthogonal directions of polarization are shown on Fig. 2. It is seen, that its photoluminescence is linearly polarized with the polarization degree P = 0.15, which was defined as... 

Analogous g-values may be defined for the degree of circular polarization in emission [or circularly polarized photoluminescence (CPPL)] and circularly polarized electroluminescence (CPEL), eg. gCppL = 2(JL - 1R)/(1L + 1r), where IL and IR denote the intensity of left- and right-handed circularly polarized emission, respectively. CPPL should not be confused with fluorescence-detected CD. 

Even though the degree of the anisotropy reached for this double layer system is lower than that found for the single polymeric layers, it shows photoluminescence and electroluminescence emission with sensitive polarization depencences. 

The in-plane polarization anisotropy can be enhanced by anisotropic strain. This can be achieved by choosing a nonpolar orientation with an appropriate substrate. In the extreme case of M-plane GaN on liAl02, the degree of linear polarization can be increased to its maximum value of one for all three transitions between the three uppermost valence bands (VBs) and the conduction band (CB), corresponding to complete linear polarization for all three transitions. This optical anisotropy can be observed in transmission (absorption) and reflection, as well as photoluminescence (PL) and photoreflectance (PR) spectroscopy. It can therefore be used for polarization filtering, polarization-sensitive photodetectors (PSPDs), and polarized light emitters. For anisotropically strained C-plane GaN films on (1120) sapphire, the in-plane polarization properties have been previously reported in Refs. 1-3. 

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