Pressure-induced delocalization-to-localization transition of excitons in AlN
Pressure-induced delocalization-to-localization transition of excitons in AlN
复制标题
DOI：
10.1103/physrevb.75.201202
复制
发表时间：
2007-05
影响因子：
3.7
通讯作者：
R. Laskowski;N. Christensen
中科院分区：
文献类型：
--
作者：
R. Laskowski;N. Christensen
The properties of excitons in the rocksalt high-pressure phase of AlN are investigated theoretically by means of a first-principles approach based on solution of the Bethe-Salpeter equation. The excitons, in that phase, change nature when the applied pressure is varied, from being very extended in space at low pressures to being significantly more localized at higher pressures. The transition is associated with a sudden increase in the exciton binding energy. The change of the character of the excitons is related to the pressure-induced rearrangement of the energy bands. A commonly accepted classification of excitons in solids distinguishes between two limiting cases depending on the strength of the electron-hole e-h Coulomb attraction, or more specifically, on the degree of screening of the e-h interaction. For the large attraction limit, tightly bound excitons are described by the classical Frenkel model. 1 In the opposite limit, where the e-h attraction is weak, the Wannier model 2 applies. The strength of the e-h attraction makes these two limiting cases distinct in terms of localization and delocalization in reciprocal and real spaces. Frenkel excitons, which are mostly associated with atoms in excited states, are well localized in real space, therefore delocalized in reciprocal space. AWannier exciton has the opposite properties; it is very well localized in reciprocal space, but extends over a large volume in real space. Usually real-space-localized excitons are observed in molecular solids BN nanotubes 3 and layered compounds h-BN 4 . The spatially delocalized excitons are found in sp 3 covalently bonded semiconductors like II-VI and III-V compounds. Apparently, no material is so far known where the localization degree can be strongly varied just by changing external conditions. One might expect that since the dielectric constant of the tetrahedrally bonded semiconductors decrease with pressure, this reduced screening could stabilize real-space-localized excitons. We have not seen this, but in this work we report a theoretically predicted pressure-induced transition between delocalized and localized excitons in the rocksalt phase of AlN. The occurrence of the transition in that case is related to rearrangements of the bands under pressure due to differences in the deformation potentials of various direct gaps. At ambient conditions AlN is a direct-gap semiconductor crystallizing in the wurtzite structure. It is characterized by a large band gap, more than 6 eV, large bulk modulus, and high thermal conductivity. Rocksalt AlN is a high-pressure phase of AlN. The experimental transition pressure is reported in the range 14‐16 GPa see references in Ref. 5. The pressure-induced transformations of III nitrides were studied theoretically 5,6 with the linear muffin-tin-orbital LMTO method and the local density approximation LDA. The transition pressure for AlN has been estimated at 16.6 GPa. Our present calculations performed with the fullpotential linear augmented plane-wave method 7 and generalized gradient approximation 8 GGA result in a slightly lower value of 12.8 GPa. The calculated volume of rocksalt AlN at the transition pressure is VB180% of the zeropressure volume of wurtzite-type AlN. The scheme for calculating the optical response, with inclusion of electron-hole interactions, employed in this work is based on a solution of the equation of motion of the twoparticle Green’s function, known as the Bethe-Salpeter equation BSE. 9‐11 The computational procedure solves this equation in an approximate manner represented in the form of an effective eigenvalue problem with the BSE Hamiltonian: 12,13
