Excitonic Crystal and Nanotechnology

Abstract

The role and application of bound excitons in nanoscience and technology are discussed in this chapter. Bound excitons are well studied in semiconductors, especially in gallium phosphide doped by nitrogen (GaP:N). Doping of GaP with N leads to isoelectronic substitution of the host P atoms by N in its crystal lattice and to creation of the electron trap with a giant capture cross-section. Therefore, any non-equilibrium electron in the vicinity of the trap will be captured by N atom, attracting a nonequilibrium hole by Coulomb interaction and creating the bound exciton -short-lived nanoparticle with the dimension of the order of 10nm (it is the Bohr diameter of bound exciton in GaP:N). Note, that none of nanotechnology methods are used in creation or selection of dimensions of these nanoparticles -only natural forces of electron-hole interaction and electron capture by the traps are necessary for creation of these nanoparticles. As the result we get something like neutral short-lived atom analoguea particle consisting of heavy negatively charged nucleus (N atom with captured electron) and hole. So called "zero vibrations" do not destroy possible solid phase of bound excitons having these heavy nuclei that gives an opportunity to reach their crystal state -short-lived excitonic crystal.Thus using bound excitons as short-lived analogues of atoms and sticking to some specific rules, including the necessity to build in the GaP:N single crystal the excitonic super lattice with the identity period equal to the bound exciton Bohr dimension, we get a unique opportunity to create a new solid state media -consisting from short-lived nanoparticles excitonic crystal, obviously, with very useful and interesting properties for application in optoelectronics, nanoscience and technology. The following will discuss methods of preparation and possible application of GaP excitonic crystals and nanocrystals in optoelectronics. Preparation and Properties of GaP with Ordered Position of N ImpuritiesCrystals that are grown under conventional laboratory conditions naturally contain a varied assortment of defects such as displaced host and impurity atoms, vacancies, dislocations, and impurity clusters. These defects result from relatively rapid growth conditions and inevitably lead to the deterioration of optical and mechanical properties of the crystal. For instance, defects in freshly-prepared GaP:N single crystals completely suppress their luminescence at room temperature, which is very bright in the same, but aged perfect crystal.With the lapse of time driving forces such as impurity diffusion, strain relaxation, and thermodynamic minimization of the free energy associated with properly directed chemical bonds can result in an ordered distribution of impurity and host atoms. Evaluation of the characteristic time of such reordering, based on the known Ising model, suggests that the time for the substitution reaction associated with N diffusion along P sites in GaP:N is about 15 years at room temperature [1]. Accordingly, observations ...

Properties of GaP Studied over 50 Years

“…All of the results presented here and included in summary reviews [25,26,[32][33][34] may sufficiently change the approach to the selection of materials necessary for electronics, and to make cheaper and simpler technology for the preparation of the selected materials and device structures on which they are based. This study of long-term convergence of bulk-and nanocrystal properties [15,17] brings a novel perspective to improving the quality of semiconductor crystals.…”

“…The quality of GaP nanoparticles was improved using mild aqueous synthesis and different colloidal reactions of Ga and P sources in toluene [26][27][28][29][30][31][32][33][34][35][36][37]. Ultrasonication and ultracentrifugation were applied during the synthesis and selection of nanoparticles to increase their quality and to control the size of the nanoparticles.…”

“…At the relevant concentrations of N, the anion sublattice can be represented as a row of anions where N substitutes P atoms with the period equal to the Bohr diameter of the bound exciton in GaP (approximately 10 nm) (Figure 7a). At some level of excitation, all the N sites will be filled by excitons, thereby creating an excitonic crystal (Figure 7b), which is a new phenomenon in solid-state physics and a very interesting medium for application in optoelectronics and nonlinear optics [4,15,17,25,26,[32][33][34].…”

“…Jointly with Refs. [25,26,[31][32][33][34], this review provides a generalization of the results on these long-term observations of luminescence, absorption, Raman light scattering, and microhardness of the bulk single crystals in comparison with the same properties of the highquality GaP nanocrystals. It is shown that the combination of these characterization techniques elucidates the evolution of these crystals over the course of many decades.…”

New Prospect for Optoelectronics

“…Main results, as they occurred and as they were understood and interpreted for more than half a century, are presented in many works. The most important ones are covered in [1][2][3][4][5][6][7][8][9][10][11]. Since pure and doped samples of Gallium Phosphide were originally prepared in the 1960s [1,2], followed by the introduction of the excitonic crystal concept in the 1970s [3], the author discovered stimulated emission with indirect optical transitions [4] and interesting non-linear optical…”