Exciton lifetime temperature dependence

FIGURE 4 Temperature dependence of the recombination lifetimes of the excitonic transitions in Ill-nitride epilayers and MQWs. 

The FEs produced at low temperature by illumination with photons in the vicinity or above Eg have finite lifetimes that depend on temperature (see [34] for silicon), their binding energies, and on the band structure of the semiconductor (the lifetime is larger in semiconductors with indirect gap than direct gap). During their lifetime, they can diffuse in the crystal and be trapped by impurities and defect to become bound excitons (BEs) with energies slightly different from that of the FE. 

This dependence of the dressed exciton dispersion ( k) for angle 9 = 0 when the transition dipole moment is perpendicular to chain is displayed in Fig. 4.4. For another orientation of the exciton transition dipole moment the dependence of k) can be very different. For excitons with small transition dipole moment the renormalization of the exciton dispersion due to account of retardation is usually small and can be important only at low temperature of order of 1-2 K or less because of the smallness of the parameter A/Efx. In the same situation the radiative width of exciton states with small wavevectors determined by the same parameter A/EM can be a hundred-fold larger than the radiative width of a molecule in solution. Very interesting is the problem of the temperature dependence of the radiative lifetime and we come back to the discussion of this problem later. 

CdS clusters of narrow size distribution were studied by Eychmuller et al. [63], In this case a rather narrow luminescence band can be observed near the absorption band. The decay kinetics of this excitonic luminescence is multiexponential with a typical lifetime on the order of nanoseconds, much longer than the expected exciton lifetime. The temperature dependence of the excitonic luminescence shows complex behavior. Again, the authors use the three-level thermal equilibrium model to explain the data. The excitonic luminescence is identified as delayed luminescence occurring by detrapping of trapped electrons. Furthermore, they invoke the concept... 

The dependence of lifetime on temperature in the range above RT shows an activation energy in the order of 10-25 meV [Bu3, Ool]. This was proposed to be a consequence of the exchange splitting of the exciton between the singlet and the triplet state. While at RT both states are populated, only the lower triplet state is populated at temperatures below 20 K. However, it has been shown that even for crystallites of low symmetry the calculated values of the exchange splitting are too low compared with experimental observations [De3]. Calculations of the radiative lifetime of the triplet exciton that take into account spin-orbit interactions are reported to be consistent with experimental results [Nal]. 

Carrier and exciton dynamics in InGaN/GaN MQWs have also been studied at a high optical pumping power [34], At 7 K, a radiative decay lifetime of 250 ps was observed for the dominant transition at a generated carrier density of 1012/cm2. The time-resolved measurement showed that the decay of PL has a bimolecular recombination characteristic. At room temperature, the carrier recombination was found to be dominated by non-radiative processes with a measured lifetime of 130 ps. Well width dependence of carrier and exciton dynamics in InGaN/GaN MQWs has also been measured [35]. The dominant radiative recombination at room temperature was attributed to the band-to-band transition. Combined with an absolute internal quantum efficiency measurement, a lower limit of 4 x 10 9 cm3/s on the bimolecular radiative recombination coefficient B was obtained. At low temperatures, the carrier... 

Table 25.1 Material dependent lifetime of triplet excitons and the highest TOF mobility obtained at low temperature.

Even when the intramolecular vibrations do not mix with the phonons, they are coupled by the intermolecular potential, through its dependence on the internal molecular coordinates. This coupling determines the band structure of the vibrational excitons or vibrons in molecular crystals, which can be studied by inelastic neutron scattering (see Fig. 12). In the case of TCB crystals the vibron band structure has been observed recently by laser phosphorescence [59, 104]. This is a rather special achievement since, normally, optical techniques probe only the g = 0 excitations. It is related to the long lifetime of triplet electronic excitons in TCB, which are scattered into different q states. By phosphorescent de-excitation q 0 triplet excitons to the electronic ground state it is possible to detect the g 0 vibrons, with intensities controlled by the (very low) temperature and by the phosphorescence delay time. 

Figs. 12-12a.b show the PIA spectrum, and its dependence on temperature and laser chopping frequency, of a CN-derivative of P(PV) the frequency dependence gives an indication of the lifetime of the excitons. 

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