Stimulated Emission in ZnO

E 11 c, respectively. For the Mg Ztii- O alloy system, linear dependence of the Cauchy parameters on x was assumed  

Even though n- and later on p-type doping have been reported in ZnO thin films [ 137], there is no demonstration of electrically pumped lasing in ZnO-based structures. However, optically pumped stimulated emission (SE) has been observed by many researchers from ZnO epitaxial layers grown by a variety of methods [138-141]. There have also been earlier demonstrations, dating back to 1966, of SE and lasing in vapor-grown ZnO platelets, which were cleaved parallel to the oaxis and pumped by an electron beam [142-144]. 

In the intermediate excitation density regime emissions due to biexcitonic, exciton-exciton, and exdton-carrier interactions may be observed. The inelastic collision between the exdtons results in an exciton exdted into a higher state and a photon with energy [145]  

These reports were followed by demonstrations of optically pumped lasing in ZnO [147, 148]. Using an Xe laser, Johnston [147] reported SE at 275 K from ZnO platelets but was unable to explain the data by exdton-LO-phonon theories. Later on, Reynolds et al. [ 149] showed optically pumped lasing in as-grown ZnO platelets using an HeCd laser with very low pump power (4 Wcm ) at 2 K. Extremely well-formed lasing modes were observed from which a changeover from absorption to emission could be clearly detected. 

As better quality ZnO layers and epitaxial cavities became available, more detailed investigations of optically pumped SE appeared in the literature. SE and lasing, which could survive even at temperatures as high as 550 K, have been observed by Bagnall et al. [139,150] in ZnO thin films grown by plasma-enhanced 

---

Stimulated Emission in ZnO 195 Table 3.9 Cauchy model parameters for the Mg ni /3 alloy system. 

ZnO microcrystaUite thin films possess ultraviolet (UV) laser emission at room temperature. The ZnO microcrystallites are hexagonal and they are self-assembled on sapphire substrates with parallel array. Fabry—Perot lasing cavities are formed by facets of the hexagons. At room temperature optical gain for UV-stimulated emission is excitonic in nature and in comparison with ZnO crystals the magnitude of a peak value is higher, which can be the initial first step towards the development of nanometer photoelectronics (Mershin et al., 2012 Sortino, 2012 Yin et al., 2004 Yu et al., 2003 Tang et al., 1998). 

At lower energies from 3.34 to 3.31 eV, two-electron sateDite (TES) recombination lines of the neutral donor bound excitons are observed. In some samples, a donor-acceptor pair (DAP) transition at 3.22eV that is again followed by phonon replicas is found, even though the chemical identity of the acceptor is unknown. In this chapter, a review of all these transitions is given. In addition, the refractive index of ZnO and MgZnO and stimulated emission and carrier dynamics in ZnO are discussed. 

---