Band-edge Optical Transitions Research Articles

Very high oscillator strength in the band-edge light absorption of zincblende, chalcopyrite, kesterite, and hybrid perovskite solar cell materials

In semiconducting solar-cell absorbers, high absorption coefficient (alpha) near the band-edge region is critical to maximize the photocurrent generation and collection. Nevertheless, despite the importance of the band-edge absorption characteristics, the quantitative analysis of the band-edge optical transitions has not been performed. In this study, we have implemented systematic density functional theory (DFT) calculation, focusing on the band-edge oscillator strength of seven practical solar cell absorbers (GaAs, InP, CdTe, CuInSe2, CuGaSe2, Cu2ZnSnSe4, and Cu2ZnSnS4) with zincblende, chalcopyrite and kesterite structures. We find that all these crystals exhibit the giant oscillator strength near the band gap region, revealing the fact that alpha in the band gap region is enhanced significantly by the anomalous high oscillator strength. In high-energy optical transitions, however, the oscillator strength reduces sharply and the absorption properties are determined primarily by the joint density-of-state contribution. Based on DFT results, we show that the giant oscillator strength in the band edge region originates from a unique tetrahedral-bonding structure, with a negligible effect of constituent atoms.

Band-edge optical transitions in a nonpolar-plane GaN substrate: exciton–phonon coupling and temperature effects

We present a detailed investigation of the band-edge optical transitions involving the interacting exciton–phonon system, especially first-order longitudinal optical (LO) phonon-assisted luminescence of bound and free excitons in m- and c-plane GaN substrates in a low temperature range from 4 K to 40 K. The main luminescence features of all of the three kinds of excitons can be well described by the theoretical models that take exciton-LO-phonon coupling into account. The effective Bohr radii of the excitons play a key role in determining the Huang–Rhys factor characterizing the exciton-LO-phonon coupling strength in GaN. An interesting oscillatory structure is found to appear in the low-temperature luminescence spectra of the nonpolar-plane GaN substrate, which needs to be clarified by further investigations.

Linearly Polarized Excitons in Single- and Few-Layer ReS2 Crystals

Rhenium disulfide (ReS2), a layered group VII transition metal dichalcogenide, has been studied by optical spectroscopy. We demonstrate that the reduced crystal symmetry, as compared to the molybdenum and tungsten dichalcogenides, leads to anisotropic optical properties that persist from the bulk down to the monolayer limit. We find that the direct optical gap blueshifts from 1.47 eV in the bulk to 1.61 eV in the monolayer limit. In the ultrathin limit, we observe polarization-dependent absorption and polarized emission from the band-edge optical transitions. We thus establish ultrathin ReS2 as a birefringent material with strongly polarized direct optical transitions that vary in energy and orientation with sample thickness.

Energy band structure ofCuInS2and optical spectra ofCuInS2nanocrystals

Using first principles calculations we describe the energy band structure of bulk ${\mathrm{CuInS}}_{2}$. The energy band parameters for the multiband effective mass approximation that describes the band edges of this semiconductor are obtained by fitting them to the first principles spectra. Within the multiband effective mass approximation we develop a theoretical description for the structure of band-edge levels and optical properties of the ${\mathrm{CuInS}}_{2}$ nanocrystals. For the nanocrystals of spherical shape, the optical transitions are weakly allowed between the electron and hole ground states due to the tetragonal symmetry of the crystal lattice, resulting in a large Stokes shift of photoluminescence up to 300 meV in the smallest nanocrystals. This theory of the band-edge optical transitions in ${\mathrm{CuInS}}_{2}$ NCs can be applied to spherical NCs made of other chalcopyrite compounds.

Absorption-edge anisotropy of Cu 2ZnSiQ 4 (Q = S, Se) quaternary compound semiconductors

Polarization-dependent absorption characterization of Cu 2ZnSiQ 4 (Q = S, Se) quaternary single crystal compound semiconductors were carried out in the temperature range of 10–300 K. The absorption measurements were performed on the as grown basal plane with the normal along [2 1 0] and the axis c parallel to the long edge of the crystal platelet. A significant shift towards lower energy was observed in the absorption spectra of E⊥ c polarization with respect to those corresponding to E∥ c polarization. A comprehensive analysis of the absorption spectra revealed that the absorption edges of the studied crystals are indirect allowed transitions. A schematic representation of the plausible assignments for the observed near band edge optical transitions for Cu 2ZnSiQ 4 was proposed. The variation of the indirect transition energies with temperature were analyzed by Varshni and Bose–Einstein expressions. The parameters that describe the temperature dependence of the indirect transition energies with different polarizations were evaluated and discussed.

Electronic and optical properties of nonpolara-plane GaN quantum wells

In this paper we present a detailed study of the electronic band structure of a series of nonpolar $a$-plane GaN/AlGaN multiple quantum wells (QWs) of varying well width using complementary results from x-ray diffraction, polarization-dependent photoluminescence excitation spectroscopy, and $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory. When excited with unpolarized light, excitonic transitions involving different electron subbands are resolved in the excitation spectra. For linearly polarized $(E\ensuremath{\perp}c,E\ensuremath{\parallel}c)$ excitation, these are shown to consist of overlapping transitions involving different hole subbands. These results are then analyzed in detail using strain data determined by the x-ray diffraction measurements in combination with the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory to calculate the bulk band structure and the relative oscillator strength of an $a$-plane GaN film under strain. The results are compared with those of an unstrained $c$-plane film. This analysis reveals that the experimentally observed polarization anisotropy can be attributed to anisotropic strain in the $c$ plane. Based on the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian, we apply an effective mass approximation, taking into account strain and nonparabolicity effects, to calculate the single-particle states and energies for the different quantum wells. The possible influence of the weak spin-orbit coupling on the results is studied in detail. Starting from the single-particle energies and including excitonic binding energies, the band edge optical transitions are calculated and successfully compared to the experimental data. Our analysis gives an estimate for the conduction- to valence-band offset ratio of 45:55 for nonpolar GaN/AlGaN QW structures. Additionally, our study also allows us to investigate the magnitude of the crystal-field splitting and spin-orbit coupling in GaN systems.

Ultrafast Electron Transfer Dynamics in CdSe/CdTe Donor−Acceptor Nanorods

Photoinduced charge transfer dynamics in type-II CdSe/CdTe donor−acceptor nanorods were studied in real-time using femtosecond broadband pump−probe spectroscopy. Following photoexcitation of CdTe (λex = 620 nm), spectral bleaching from the state-filling signals of the band-edge optical transitions was measured using a femtosecond white light pulse (λprobe = 350−750 nm). Excitation of the lowest energy CdTe transition shows an ultrafast change in the bleaching signals of both the CdTe and CdSe lowest energy 1S transitions as well as the characteristic carrier transfer band. Our results indicate that interfacial interconduction band electron transfer CdTe to CdSe occurs on the 500 fs time scale in these heteromaterials.

Mechanisms for Photogeneration and Recombination of Multiexcitons in Semiconductor Nanocrystals: Implications for Lasing and Solar Energy Conversion

One consequence of strong spatial confinement of electronic wave functions in ultrasmall semiconductor nanocrystals is a great enhancement of carrier-carrier interactions, which has a dramatic effect on the spectral and dynamical properties of both single and multiexciton states. Strong carrier-carrier interactions open new nanocrystal-specific energy relaxation and recombination channels associated, e.g., with electron-hole energy transfer and ultrafast nonradiative Auger recombination. Further, they lead to extremely efficient direct photogeneration of multiple electron-hole pairs (excitons) by single photons known as carrier (or exciton) multiplication. This review focuses on the effect of Coulomb interactions on carrier recombination and photogeneration mechanisms in nanocrystals based on II-VI (e.g., CdSe) and IV-VI (e.g., PbSe) compounds. The specific topics discussed here include the fine structure of the band-edge optical transitions and its effect on temperature-dependent single-exciton recombination dynamics, Auger recombination of multiexcitons in size- and shape-controlled nanocrystals with a specific emphasis on optical-gain properties of nanocrystalline materials (including quantum rods and multicomponent core-shell heterostructures), and the direct generation of multiple excitons via carrier multiplication and its implications in photovoltaic technologies.

Effects of the orientational superlattice on the electronic and vibrational properties of CuPt ordered GaInP alloys

AbstractA brief review is given of studies on a new semiconductor superlattice—an orientational superlattice—observed recently in spontaneously ordered III–V alloys GaInP. New experimental data, including band edge optical transitions and Raman transitions, are presented to illustrate the unique and intrinsic properties of orientational superlattices. Copyright © 2001 John Wiley & Sons, Ltd.

The conduction band and selection rules for interband optical transitions in strained and heterostructures

The conduction-band alignments of and heterostructures grown on (111) and (001) crystallographic planes, respectively, are analysed. We have obtained the selection rules for interband dipole optical transitions in the heterostructures, and discussed the possibilities for specifying the types of the lowest conduction-band minima. We show that this can be done by, for example, exploring the polarization of different phonon-assisted band-edge optical transitions. The conduction-band minima may be at different L or points of the Brillouin zone, depending on the structure parameters. Although bulk Ge, Si, and their alloy are indirect-gap semiconductors, the heterostructures and can have a direct band gap. We found the parameter regions where type-I and type-II band alignment were realized, and those where the band gap was direct in quantum well (QW) structures. It is shown that in direct-gap QW structures grown on (001) planes, direct interband optical transitions between the nearest electron and hole subbands are allowed, but the same transitions are forbidden for direct-gap structures grown on (111) planes.

Optical transitions in GaAs/AlAs superlattices with different miniband widths

Band edge optical transitions and their external electric field effects are systematically studied in GaAs/AlAs superlattices with different miniband widths (2Δ) by low temperature photocurrent spectroscopy. Distinct Π excitons related with saddle-type M 1 critical points at the miniband top are clearly observed both for heavy-hole and light-hole excitons. Under the applied field strength F, discrete transitions due to Wannier-Stark localization are observed and it is found that their intensities of nth order Stark ladder transitions with negative indices show F −2 n and Δ 2 dependences in agreement with the tight binding theory.

Direct observation of miniband-edge singularities in the optical spectra of GaAs/AlAs superlattices

Band-edge optical transitions in GaAs/AlAs superlattices with different miniband widths are investigated by low-temperature-photocurrent, photoluminescence-excitation, and reflectance spectroscopy. Direct spectroscopic evidence is given for excitonic effects at saddle-type critical points which are related to ${\mathit{M}}_{1}$ Van Hove singularities at the upper miniband edges, for both heavy-hole and light-hole excitons. Excellent agreement is obtained between the observed transition energies and the miniband widths deduced from Kronig-Penney model calculations.

Phonon-Assisted Recombination of Free Excitons in Compound Semiconductors

In this paper we study the band-edge optical transitions in direct semiconductors involving the interacting exciton-phonon system, especially the longitudinal-optical (LO)-phonon-assisted recombination of free excitons. By introducing an extended Green's-function approach to include propagation in and between the various bands in the exciton spectrum, we obtain rather directly the transition rate to lowest order in the exciton-photon coupling which contains the effects of the interactions between the exciton and lattice vibrations to all orders, and thus includes line broadening, shift, asymmetry, and all renormalizations. Under the appropriate conditions (sufficiently separated bands), this result is equivalent to an expression previously derived by Toyozawa, but has the virtue that all terms appearing in it (in particular, the asymmetry term) are given in precise and general expressions. The relation of the general result to that obtained by conventional perturbation theory is discussed. The radiative decay of free excitons by the one- LO- and two-LO-phonon-assisted processes are formulated in the framework of perturbation theory. The limitation of this approach and its relationship to that in which the exciton-photon interaction is treated more accurately are briefly discussed. It is also shown that the second-order perturbation-theory result for the one-phonon-assisted processes (both for absorption and emission), including the contributions from the full intermediate state (hydrogenic) spectrum, can be evaluated exactly in closed form. The one- and two-LO-phonon-assisted emission spectra for CdS are calculated for several temperatures up to 77\ifmmode^\circ\else\textdegree\fi{}K using only experimentally determined parameters and taking the anisotropy of the valence band into account. Except for the one-LO peak at $T=77\ifmmode^\circ\else\textdegree\fi{}$K, where the polariton effects are important, the calculated line shapes and widths, as well as the intensity ratio of peaks, are found to be in good accord with the observed spectra. The corresponding calculations for ZnO at 77\ifmmode^\circ\else\textdegree\fi{}K are in good agreement with experimental spectrum. The zero-LO exciton peak due to the one-acoustic-phonon-assisted process is calculated in weak exciton-photon coupling approximations for both the deformation potential and piezoelectric couplings. The widths of the calculated lines are orders of magnitude smaller than the observed widths, and we conclude that the difficulty lies in the use of the weak exciton-photon coupling approach.