Yuzo Shinozuka

Self-Trapping in Mixed Crystal –Clustering, Dimensionality, Percolation–

Self-trapping of a Frenkel exciton in binary mixed crystal A c B 1- c , with excitation energy difference Δ = e B - e A and exciton-phonon coupling constant g , is studied. Impurity-assisted self-trapping is possible through cooperation of Δ and g even when neither of them alone is enough for localization. In this situation, one expects amalgamated absorption spectrum representing unrelaxed extended exciton and Stokes-shifted broad emission band representing relaxed localized exciton. We study whether relaxed exciton is localized (S-state) at a particular site of, or extended (B-state) throughout, the A -cltuster of various sizes, and how such cluster state changes, as c increases, into free (F) state extended throughout the crystal. The results are compared with recent experiments on TlBr c Cl 1- c . Sudden appearance of edge emission (due to F) at c ∼0.3, at the cost of broad band (due to S and B), is ascribed to the combined effect of size-dependent discontinuity of S-B transition in Br-clusters and th...

Stability of an Electron in Deformable Lattice –Force Range, Dimensionality and Potential Barrier–

Local and global stabilities of an electron in deformable lattice with dimensionality δ and force range index λ are studied within the adiabatic approximation. Depending on the sign of the stability index: σ=δ-2λ-2, one finds the free state to be locally stable (σ>0) ith discontinuous transition to globally stable self-trapped state beyond a critical value of coupling constant g , to be marginal (σ=0) with local (as well as global) stability being governed by g , or to be locally instabilized (σ <0) towards the optimum lattice distortion which grows continuously from zero with increasing g . A phase diagram is presented for an electron interacting simultaneously with two types of phonon fields with \(\sigma \lessgtr 0\). The potential barrier for self-trapping of a Wannier exciton is studied with particular attention to the effect of its internal motion, and is compared with the observation.

A Wannier Exciton in a Quantum Well: Subband Dependence

Variational calculations are performed to investigate the subband dependence of the ground (1 s type) and a few excited (2 s and 2 p type) states of a Wannier exciton in a quantum well. It is shown that (i) binding energies and oscillator strengths of excitons depend strongly not only on the width of the well but also on (electron and hole) subbands of the well with which excitons are associated and (ii) as the well changes from a thin well to a thick one, binding energies of most excitons for higher subbands do not change monotonically between two- and three-dimensional results.

Electron-Lattice Interaction in Nonmetallic Materials : Configuration Coordinate Diagram and Lattice Relaxation

Electron-lattice interactions in nonmetallic materials are reexamined in the many-electron scheme. The difference in the stable atomic configuration between two electronic states is the origin of the electron-lattice interaction. We show the relationship among the adiabatic potentials, one electron (hole) energy and the lattice elastic energy, paying attention to the electron-hole symmetry. Correct configuration coordinate diagrams for deep-level defects in semiconductors are presented which can be used even when the number of carriers changes due to creation and recombination. Radiative and nonradiative carrier capture and recombination processes at deep-level defects are described consistently with particular attention to the charge of a defect, the thermal and the optical depths of a bound carrier, the correlation between successive electron and hole captures, and the energy dissipation to the lattice through the interaction mode.

Electronic Structures of Quantum Well of A1-xBx Alloy Semiconductor in Coherent Potential Approximation

The electronic structures of a quantum well constructed from the binary alloy semiconductor A1-xBx are studied by coherent potential approximation (CPA). A tight binding model is used for a single particle (electron, hole, or Frenkel exciton) in a well composed of a rectangular array of Nx×Ny×Nz sites. The effects of diagonal randomness are included as the coherent potential Σ(E), which is assumed to be the same for all sites, and is self-consistently determined with the average Greens function. For slab (∞,∞,Nz) and wire (∞,Ny,Nz) structures, the energy density of states ρ(E) is composed of Nz (or Ny×Nz) subbands with the remains of a two (one)-dimensional van-Hove singularity. When x (or 1-x) is small, a B (A) impurity band always appears on the lower (higher) energy side of the lowest (highest) host subband. The metamorphosis of ρ(E) and the absorption spectrum I(E) due to the creation of a Frenkel exciton by changing the well size and the dimensionality is extensively discussed.

Mechanisms of capture- and recombination-enhanced defect reactions in semiconductors

Proposed mechanisms on defect reactions in semiconductors (defect creation, annihilation, multiplication, reconstruction, impurity diffusion, etc.) are reexamined with particular attention to the instability of the lattice and the transient lattice vibration induced by successive carrier captures. (1) Thermal activation process to overcome the potential barrier U n : it depends on the electronic state n and the reaction rate is given by p c exp(-U n /k B T). (2) Instability mechanism: the lattice relaxation after an electronic transition at a defect promptly induces the reaction coordinate QR. (3) Phonon kick mechanism (single capture): if the relaxation mode Q 1 partially includes Q R , an electronic transition to the state n enhances the defect reaction during the lattice relaxation time τ 2π/ △ ω where △ω is the width of the frequency distribution of related phonons. (4) Phonon kick mechanism (recombination): if N pairs of electron and hole are captured within a short period τ 2π/ △ ω and the central frequency ω R of QR is not so different from ω 0 of Q 1 , the band gap energy Eg is transformed by a series of coherent carrier captures into the lattice vibration energy. The defect reaction rate is given by (ω 0 /2π)exp(-E act i /k B T) because only the first capture (i = e, h) is to be activated. On the other hand, if ω R is much different from ω 0 , the rate is (ω 0 2π)exp(-U* 0 /k B T) with U 0 -(E act i + E tb i because the N phonon-kicks are out of phase.

Self-trapping in mixed crystal. II. Concentration dependence

We studied self-trapping of a Frenkel exciton in a binary mixed crystal A x B 1- x with randomness in the exciton atomic (e i ) and the transfer ( t i j ) energies, and the short range exciton-lattice interaction D i . The concentration dependence of the energies of relaxed excitons and the emission energies are calculated for self-trapped ( S i ) states at i =A or B site and free (F) states using the coherent potential approximation. Extrinsic self-trapping and the F-S transition are discussed with phase diagrams obtained for various parameters Δ=e B -e A , D i , the exciton band width T i ( i =A, B) and the concentration x . The results are compared with recent experiments on mixed ionic crystals and II-VI compound semiconductors.

Self-shrinking of electronic states below the mobility edge in disordered semiconductors

Abstract The stability of an electron in the static random potential field and the phonon field is studied with a continuum model and a variational method. It is shown that a dicotomy in electronic states below the mobility edge takes place when electron-lattice interaction is strong enough. The electronic relaxation process is discussed with the phase diagram of the relaxed states.

Interacting quasi-band model for electronic states in alloy semiconductors: Relation to average t-matrix approximation and band anticrossing model

A new variational theory is proposed for electronic states in alloy semiconductors with arbitrary diagonal and off-diagonal randomness and any concentration. In AcABcB substitutional alloys, the theory derives the mutual interaction between two quasi-A and -B electronic bands whose effective band widths are proportional to cA and cB, respectively, i.e., an interacting quasi-band model. The model provides satisfactory results near-band-edge states, especially for band bowing. For diagonal randomness, the theory corresponds to the average t-matrix approximation, and in the dilute limit, a formula similar to the band anticrossing model, which has been frequently applied to GaN-related alloys, is obtained.

Transient lattice vibration induced by coherent carrier captures at a deep-level defect and the effect on defect reactions

Abstract We study theoretically the dynamics of the transient lattice vibrations induced by successive carrier captures by a deep-level defect. After each carrier capture, the interaction mode Q 1 ( t ) coupled to the defect level shows a damping oscillation in a period ∼2π/Δ ω , where Δ ω is the width of the phonon frequency distribution. The induced vibration in turn enhances the next carrier capture. The induced lattice vibration energy induced by a nonradiative recombination of an electron–hole pair is larger for shorter time interval between two captures. The possibility of defect reaction is discussed in connection with the coherence in successive captures and the induced vibration, which depend on the carrier densities, n e(h) , the capture cross sections, σ e(h) , the activation energies, E act e(h) and Δ ω .