Quantum wells optical transitions

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

A phenomenon which is related to the PPC effects is the optical metastability in GaN. Optical metastability has been observed in GaN epilayers and InGaN/GaN multiple quantum wells [24-26]. Optical metastability in bulk GaN was manifested by a photoinduced decrease in the output intensity of the bandedge transition at 365 nm followed by an increase in the output intensity of a new emission band at 378 nm [25], The recovery time associated with the observed optical metastability is very long (weeks). The cause for such an effect was attributed to the presence of traps in bulk GaN [25], In... 

Interband optical transitions in quantum wells are governed by selection rules determined by the symmetry of the wavefimctions. In an ideal one-dimensional potential well, only transitions between levels of identical quantum number would be allowed. In a real quasi-two-dimensional quantum well, band mixing for finite wave vectors within the layer plane leads to a weakening of the selection rule, so that transitions with An 0 may show up in optical spectra. 

The optical properties of GalnN/GaN quantum wells differ somewhat from the well-known behaviour of other III-V-based strained quantum well structures, partly due to the rather strong composition and well width fluctuations, possibly induced by a partial phase separation of InN and GaN. The even more dominant effect seems to be the piezoelectric field characteristic for strained wurtzite quantum wells, which strongly modifies the transition energies and the oscillator strengths. However, the relative influence of localisation and piezoelectric field effect is still subject to considerable controversy. 

For thick quantum wells (no significant quantisation) the shape of the optical gain spectra can be well described by a conventional band-to-band transition model [10], At least qualitatively, the same is true for thinner single and multiple quantum wells. However, the shape of the gain spectra alone cannot be used to decide on the transition mechanism. 

Shakeup represents a fundamental many-body effect that takes place in optical transitions in many-electron systems. In such systems, an absorption or emission of light is accompanied by electronic excitations in the final state of the transition. The most notable shakeup effect is the Anderson orthogonality catastrophe [5] in the electron gas when the initial and final states of the transition have very small overlap due to the readjustment of the Fermi sea electrons in order to screen the Coulomb potential of pho-toexcited core hole. Shakeup is especially efficient when the optical hole is immobilized, and therefore it was widely studied in conjunction with the Fermi edge singularity (FES) in metals [6-8] and doped semiconductor quantum wells [9-15]. Comprehensive reviews of FES and related issues can be found in Refs. [16,17]. 

In the following, we consider in some detail the transition from discrete to continuum spectra for the case of luminescence from highly excited semiconductor nanostructures. We wiU restrict ourselves to undoped semiconductors so that all carriers in conduction and valence band are optically excited. The luminescence is preceded by a fast carrier relaxation [76], so the recombination takes place when the electron and hole gases are in their respective ground states. In quantum wells, luminescence from high-density optically created electron-hole gases was studied in Refs. [77-79]. In confined structures, such as quantum dots, electrons and holes fill size-quantization energy states up to their respective Fermi... 

For nc-Si/SiO2 structures of type 1 the PL band maximum shifts from 1.3 to 1.7 eV when d decreases from 4.5 to 1.5 nm the intrinsic PL of nc-Si is commonly explained by the radiative recombination of excitons confined in nc-Si, while the size dependent spectral shift is attributed to the quantum confinement effect [21]. A considerable width of the PL band can be explained by nc-Si size distribution [21] as well as by phonon-assisted electron-hole recombination [22]. The external quantum yield of the exciton PL was found to reach -1 % for the samples with d = 3 - 4 nm at room temperature [18]. The lower quantum yield of the nc-Si/SiO2 structure in comparison with that observed for single Si quantum dots [22] and for III-V and II-VI compounds [22] can be explained by lower probability of the optical transitions, which are still indirect in nc-Si [21], as well as by the exciton energy migration in the assembly of closely packed nc-Si [18]. 

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