Size And Temperature Dependence Of The Band Gap Of Semiconductor Materials

In recent years,with miniaturization of semiconductor materials down to nanoscale, a large number of fascination phenomena of nanomaterials have been found by intensive experimental studies. However, the classical approaches such as continuum medium mechanics and the quantum mechanics approach have encountered severe difficulties because of the boundary effect. Therefore, a model of overcoming the limitations uncounted in both approaches and understanding their physical origin and predicting general trend of material properties with size reduction is highly demanded.Nanomaterials are different from the corresponding bulk and single atom due to low coordination surface atoms and their interaction. The recently advanced bond-order-length-strength (BOLS) correlation mechanism suggests that the size dependent material property is mainly attributed to the interaction between the under-coordinated atoms in the surface skins. The coordination number imperfection in the surface skin leads spontaneous bond length contraction and bond energy strengthening. This causes densification and localization of charge, energy, mass at surface region, and hence modifies atomic coherency, Hamiltonian, etc. By extending the BOLS correlation mechanism to temperature domain, an approach of local bond average (LBA) has been developed, which states that: (i) the entire specimen or a specific location of a specimen can be represented by a representative bond; (ii) the detectable quantities of a specimen can be obtained once the relationship between the detectable quantity and the bond identities (bond order, nature, length, and strength) of the representative bond and the response of these bond identities to the stimulus is established.The thesis focuses on the size and temperature of band gap of semiconductor materials IV (Si and Ge), III-V (GaN and AlN), II-VI (ZnSe, ZnS, and ZnO). Major progress is summarized as follows: (i) the size induced trend is determined by the surface-to-volume ratio, (ii) broken-bond-induced local strain and quantum trapping at the surface of skin depth dictates the EG expansion, (iii) only the outermost three atomic layers contribute to the band gap broaden yet bonds in the core interior remain as they are in the bulk, (iv) the thermally induced change of band gap arises from both bond length expansion and bond strength weakening with increasing temperature.The exceedingly good agreement between predictions and measurements confirmed that if one bond breaks, the remainder between the under-coordinated atoms will become shorter and stronger. Then, local strain and quantum trapping are formed immediately at the sites surrounding the under-coordinated atoms. Consequently, binding energy density and cohesive energy are modified, which dominates the size dependence of mechanical and thermal properties for nanomaterials, respectively. Therefore, the BOLS correlation mechanism and LBA approximation could pave a path to bridge the gap between classical approach in macroscopic system and quantum confinement approach in atomic level.