Near-band-edge emission

After postdeposition Ar and Ar/S-annealing, the films were studied again using PL. S-anneals reduced the relative intensities of the PL1, PL4, and broad emission bands, whereas Ar-anneals increased the relative intensity of the PL1 band. This can be seen in Fig. 6.28. We can also see in this figure that S-anneals suppressed the broad near-band-edge emission from the trailing edge samples. When EDS measurements were performed on the films after S-anneals, an... 

The CL spectrum of the ZnO film consisted of intense, near-band-edge ultraviolet emission with a wavelength maximum at 387 nm and a full width at half maximum of 21 nm. This emission is of excitonic nature and is a result of the radiative annihilation of free and bound excitons. A broad defect-related green band with much lower intensity near 510 nm, typical for ZnO, was also observed (Fig. 1). The CL spectrum of the p-Alo. i2Gao 8sN(Mg) film consisted of a very weak near-band-edge emission with at 356 nm, and of a more intense broad band with a maximum at 410 nm. 

Figure 2.34 CL linewidth of the ZnO near-band-edge emission as a function of LT buffer layer thickness and growth temperature. The linewidth decreases with increasing growth temperature and layer thickness of LT ZnO buffer. (After Ref [153].)...

In Fig. 3, these compositions are compared with their respective CL near band-edge emission energies. Additionally, in Fig. 4, the CL emission peaks are compared with bandgap values obtained by scanning electron microscopy. Using a parabolic model, the following relationship describes the CL peak emission 2 hne emission) (Eq. (1)) as a function aluminum mole fraction for 0 < x < 0.96. 

The low temperature (4.2K) CL of undoped AlxGai.xN films containing up to 0.96 mole fraction of aluminum exhibited near band-edge emission which has been... 

Figure 3. The relationship between aluminum mole faction and 4.2K CL near band-edge emission from AlGaN thin films deposited directly on vicinal and on-axis 6H-SiC (0001) substrates.

Figure 4. Comparsion of 4.2K CL near band-edge emission and bandgap as determined by spectroscopic ellipsometry at room temperature.

Optical band edge excitation yields a strong blue emission. Near band edge excitation yields a green emission which has been ascribed to a WO3 group (Ref. 48, 50 compare Sect. 3.4.1). At low temperatures and under near band edge excitation two research groups 48,49) reported a zero-phonon line at about 370 nm (next to the... 

Photoluminescence emission spectra of ZnO/PVA nanocomposite fihns under an excitation at 325 nm showed an intense PL emission centered around 364 nm, and a weaker and broad emission around 397 nm. ZnO/ PVA nanocomposite films prepared with OA modified ZnO nanoparticles compared to films prepared with pristine ZnO. The PL emissions observed in ZnO nanorods at 468 and 563 nm decrease considerably in intensity and are almost quenched in the composite films. The green emission in ZnO originates mainly from the deep surface traps, which can almost be removed via surface passivation by the polymer. Figure 12.13 shows the PL spectra of PVA and ZnO/PVA nanocomposite thin films for three different concentrations, 1, 2 and 3 wt% of OA modified ZnO, which gives maximum PL intensity. The composite films show intense luminescence emission centered aroimd 364 nm in the UV region and intensity of this emission peak is foimd to increase with an increase of ZnO content in the composite. The PL intensity at 397 nm is found to be more prominent in this case. The surface modification of ZnO by the polymer matrix removes defect states within ZnO and facilitates sharp near-band-edge PL emission at 364 nm. 

In steady-state PL, the shape of the spectrum is determined by the level of excitation intensity as the defect-related PL often saturates at power densities on the order of to 10 Wcm, and the overall PL spectrum may be skewed in favor of the excitonic emission at higher excitation densities. Similarly, focusing the laser beam and using small monochromator slit widths would also skew the PL in favor of excitonic transitions. In such a case, the chromatic dispersion of the lenses used to collect the PL, as well as the different effective sizes of the emission spots for the ultraviolet (UV) and visible emission attributed in particular to photon recycling process [24], may lead to a noticeable artificial enhancement of the UV (near band edge) over the visible part in the PL spectrum (mainly defect related). Qualitative terms such as "very intense PL attesting to the high quality of the material are omnipresent in the literature on ZnO. In contrast to the wide use of PL measurements, relatively little effort has been made to estimate the absolute value of the PL intensity or its quantum efficiency (QE) for a quantitative analysis. 

In this section the electronic structure of conjugated polymers is discussed. They form a special class of materials with particular types of excitations (such as the solitons) and properties, introduced briefly in Chapter 11. These problems are discussed here essentially in relation to the spectroscopic properties. The related but distinct subject of electrical conductivity is treated in Section IV. To set the scene, we first present some typical results visible absorption and emission spectra and resonance Raman spectra. We consider the theoretical issues in Section III.B, then return to the meaning of the experimental results in Section III.C. The interesting nonlinear optical properties of CPs will be considered in Section III.D. These sections are concerned with electronic states within the gap or near the band edges the structure (i.e., the dispersion relations) of valence and conduction bands is also of theoretical interest and is considered in Section III.E. 

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