PL intensity

The resulting PL intensity depends on the absorption of the incident light and the mechanism of coupling between the initial excited states and the relaxed excited states that take part in emission. The spectrum is similar to an absorption spectrum and is useful because it includes higher excited levels that normally do not appear in the thermalized PL emission spectra. Some transitions are apparent in PLE spectra from thin layers that would only be seen in absorption data if the sample thickness were orders of magnitude greater. 

A useftil applicadon of time-dependent PL is the assessment of the quality of thin III-V semiconductor alloy layers and interfaces, such as those used in the fabri-cadon of diode lasers. For example, at room temperature, a diode laser made with high-quality materials may show a slow decay of the acdve region PL over several ns, whereas in low-quality materials nonradiative centers (e.g., oxygen) at die cladding interface can rapidly deplete the free-carder population, resulting in much shorter decay times. Measurements of lifetime are significandy less dependent on external condidons than is the PL intensity. 

Spatial information about a system can be obtained by analyzing the spatial distribution of PL intensity. Fluorescent tracers may be used to image chemical uptake in biological systems. Luminescence profiles have proven useftil in the semiconductor industry for mapping impurity distributions, dislocadons, or structural homogeneity in substrate wafers or epilayers. Similar spatial infbrmadon over small regions is obtained by cathodoluminescence imaging. 

Figure 4 Spatial variation of PL intensity of an InGaAsP epitaxial layer on a 2-in InP substrate shows results of nonoptimal growth conditions. (Data from a Waterloo Scientific SPM-200 PL mapper, courtesy of Bell Northern Research)...

Figure 7-32. PL decay of CW) DOO-PPV al several doping concentrations, (b) Change in PL intensity, correlated to the delect spacing. Inset PL spectra vs doping concentration.

Figure 7-3. (a) Zeeman splming of triplet exeiton sublevels as a Imietion ol the applied magnetic Held, (b) Change in PL intensity at magnetic resonance. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

PL spectra of Mn-doped ZnS nanoparticles optically annealai in air (a) and in vacuum (b) are shown in Fig. 2. For Mn-doped ZnS nanoparticles, the PL band is seen at around 585mn. When Mn-doped ZnS nanoparticles were annealed in air, PL intensity is increased more significantly with UV irradiation time compared with ones ann ed in vacuum. PL spectra of Pr-doped ZnS nanoparticles axe shown in Fig. 3. The broad emission at 430 nm corresponds to the emission of the undoped ZnS nanoparticles. The other peak is relaftrii to the Pr-related complexes. The effect of the optical aimealing in air is more notable than in vacuum on the enhancement of luminescent intensity. The incre e of PL intensity for Pr-doped ZnS nanoparticles in mr is more rapid than undoped or Mn-doped ZnS nanoparticles. 

Undoped, Mn, and Pr-doped ZnS namopartides synthesized by wet chemiral mdhod were optically annealed in air or vacuum. PL emission inoeas with annulling time. This increase is attributed to the photo-oxidation, enhancanent in the crystal quality, and diffiision of the luminescent ions. PL intensity of nanoparticles annealed in air increased more significantly due to the photo-oxidation compared with the nanoparticles annealed in vacuum. Mn and Pr-codoped ZnS nanoparticles emitted white light due to the effects of dopants. The optical annealing enhanced the emission intensity. 

Figure 21. (a) Evolution of PL intensity at 1540 nm in both pumping conditions as a function of the annealing temperature for the Er... 

Fig. 6.16 The normalized integrated absorbance of SiH and SiH2 species (filled symbols), the PL intensity (open circles) and the specific surface area (open diamonds) as a...

For micro PS a decrease in the specific resistivity by two or three orders of magnitude is observed if the dry material is exposed to humid air [Ma8] or vapors of polar solvents, e.g. methanol [Be6]. This sensitivity of PS to polar vapors can be used to design PS-based gas sensors, as discussed in Section 10.4. This change in resistivity with pore surface condition becomes dramatic if the pores are filled with an electrolyte. From the strong EL observed under low anodic as well as low cathodic bias in an electrolyte it can be concluded that micro PS shows a conductivity comparable to that of the bulk substrate under wet conditions [Ge8]. Diffusion doping has been found to reduce the PS resistivity by more than five orders of magnitude, without affecting the PL intensity [Ell]. 

Fig. 7.9 The PL spectra of various silicon-based materials (a) at RT and (b) at 77 K (note the logarithmic scale of the PL intensity) (a-Si amorphous Si, pc-Si microcrystalline Si). After [St8].

Microcrystallites of direct semiconductors usually show a simple exponential decay of the PL intensity P with time, with time constants r in the ps and ns range at RT. A similar simple exponential decay (r = 20ms at 2 K) is observed for PL from mesoporous silicon of high porosity, which shows a weak confinement effect... 

In addition to the red PL band, an IR PL band between about 0.8 and 1.3 eV is usually present in most PS samples [Fa6, Pi3, Mal7, Mo7, Ku4, Pe5], as shown in Fig. 7.9. At RT the intensity of the IR band is weak. At cryogenic temperatures it becomes much stronger and can even be more intense than the red band. The PLE spectra for the red and the IR bands are identical despite their large difference in PL peak energy, as shown in Fig. 7.11a. Furthermore a correlation between the peak position of the two bands has been observed, as shown in Fig. 7.13 [Ku6]. The PL decay, however, is found to be different For the IR band a if2 dependence of PL intensity on time, with little dependence on temperature, is observed, while a stretched exponential decay with strong temperature dependence is observed for the red band, as shown in Fig. 7.11b. 

The dependence of the PL intensity and peak position on oxidation temperature for three different PS samples is shown in Fig. 7.20. Oxidation at 600°C destroys the PL, while the initial PL intensity is restored or even increased after oxidation at 900°C. This effect can be understood as a quenching of PL because of a high density of defects generated during the desorption of hydrogen from the internal surface of PS. Electron spin resonance (ESR) investigations show a defect with an isotropic resonance (g= 2.0055) in densities close to 101 cm for oxidation at 600°C [Pel, Me9]. This corresponds to one defect per crystallite, if the crystallite diameter is assumed to be about 5 nm in diameter. 

The spectral distribution of the PL from OPS is found to be similar to that of as-prepared PS [Ta6]. In some cases a green band is found and has been ascribed to point defects in Si02 [Ka9]. The slow red-orange band is dominant, while the fast blue-green band contributes significantly to the PL intensity for highly oxidized samples [Kol], While the red band is correlated with the presence of small... 

Fig. 7.20 Luminescence intensity and peak position versus RTO processing temperature for PS samples grown on p-type silicon substrates (A 1 Q cm, B 1 Q cm, C 0.07 12 cm). Note the anti-correlation of the PL intensity and of the ESR signal (taken for sample series A). After [Pel],...

The best fit is obtained with B = 100 K and A = 2.4 X I0-4 eV K l. These values are closed to those obtained from the bulk phase (B = 180 K and A = 4 x I0-4 eV K l, respectively) (76). On decreasing temperature, the PL intensity increases and its maximum is shifted to higher energy. To determine the peak intensities and positions, the PL spectrum is simulated as before by assuming two Gaus-sians. The temperature dependence of the photoluminescence due to direct transition obeys an Arrhenius law given by (34) ... 

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