Polaron-exciton binding energy

We emphasize that the small value of the polaron-exciton binding energy results from a cancellation (indirect) of the electron-electron and electron-lattice contributions. This cancellation demonstrates the need for correlation effects to be taken into account when describing the excited state wavefunctions. 

The combination of a hole polaron and an electron polaron, with binding energies Ep+ and Ep-, respectively, results in the formation of an exciton. Their difference corresponds to Et and is also referred to as the single particle energy gap Egsp -... 

One can say that the obtained by us experimental results upon 2D exciton localization (taking place due to the growth of the crystal dielectric permeability anisotropy parameter) with o are very close to [27] where the behaviour of polaron excitons in parabolic quantum dots were considered and shown that the dot size decrease results in increasing the exciton binding energy. 

If the exciton binding energy relative to the separate charge pair is larger than that of the polarons relative to their free particle bands, the polaronic optical transitions will be hidden under the excitonic ones, and a one-electron approximation of the polaron certainly fails. This is yet another reason why optical signatures of the polaron may give dubious results (see Section IV.C.4). Such a situation can have practical consequences, to which we return in the discussion of electroluminescence in Chapter 12, Section V.C. 

These experiments yield an exciton binding energy of 0.5 eV relative to separate electron and hole. This is a large value. An important consequence, although one that is never mentioned, is that in such a case, the polaron binding energy u>p (see Fig. 7 of Chapter 11) must be larger than 0.25 eV for the absorption between polaron levels in the gap to occur below that of the exciton in practice, the polaron absorption, if it exists, will remain hidden under the excitonic absorption. 

In thick ( 300 pm) crystals of GaN electronic excitons of shallow dopants have been observed in far infrared absorption at 215 cm 1 [44], Interpreted as the ls-2p transition of a residual shallow donor, its binding energy was calculated to be (35.5 0.5) meV. Further modes at 149 and 242 cm 1 have been observed in mixed phase GaN/GaAs in Raman scattering and have been associated with electronic excitations of shallow donors in cubic and sphalerite GaN, respectively [45] see also [46], Far infared absorption at 23.2 cm 1 in magnetic fields has been used to determine the effective electron mass in GaN, m = 0.20 0.005 m, (corrected for polaron effects) in cyclotron resonance [47]. 

---