Band-edge Transition Energies Research Articles

Giant spin-selective bandgap renormalization in CsPbBr3 colloidal nanocrystals

The spin-dependence of the strongly correlated phenomena and many-body interactions play an important role in quantum information science. In particular, it is quite interesting but unclear how the spin degree of freedom ramifies the bandgap renormalization, one of the fundamental many-body phenomena. We report the first room-temperature observation of giant spin-selective bandgap renormalization (SS-BGR) in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_{3}$ colloidal nanocrystals using time-resolved circularly polarized femtosecond pump-probe spectroscopy. The SS-BGR results from many-body interactions among carriers with the same spin that renormalize their joint density of states by $57\ifmmode\pm\else\textpm\fi{}$ 1 meV, visualized here as photoinduced absorption (PIA) below band-edge transition energy when the pump and probe are co-polarized. The hallmark result of spectrally resolved SS-BGR is in stark contrast to the usually submerged signal in II-VI and III-V semiconductors and is 3 orders of magnitude larger than that observed in Ge/SiGe quantum wells, highlighting the unique and beneficial band structure of the metal-halide semiconductors. We propose that the PIA, due to SS-BGR, can be used to describe the spin-polarization and spin-relaxation dynamics. The experimental and theoretical findings open up new possibilities for optical manipulation of spin degrees of freedom and their many-body interactions in metal-halide perovskite nanocrystals for potential room-temperature applications.

Temperature Dependent Excitonic Transition Energy and Enhanced Electron-Phonon Coupling in Layered Ternary SnS2-xSex Semiconductors with Fully Tunable Stoichiometry.

In this study, a series of SnS2-xSex (0 ≤ x ≤ 2) layered semiconductors were grown by the chemical–vapor transport method. The crystal structural and material phase of SnS2-xSex layered van der Waals crystals was characterized by X-ray diffraction measurements and Raman spectroscopy. The temperature dependence of the spectral features in the vicinity of the direct band edge excitonic transitions of the layered SnS2-xSex compounds was measured in the temperature range of 20–300 K using the piezoreflectance (PzR) technique. The near band-edge excitonic transition energies of SnS2-xSex were determined from a detailed line-shape fit of the PzR spectra. The PzR characterization has shown that the excitonic transitions were continuously tunable with the ratio of S and Se. The parameters that describe the temperature variation of the energies of the excitonic transitions are evaluated and discussed.

Symmetry Breaking Induced Activation of Nanocrystal Optical Transitions

ABSTRACTWe have analysed the effect of symmetry breaking on the optical properties of semiconductor nanocrystals due to doping by charged impurities. Using doped CdSe nanocrystals as an example, we show the effects of a Coulomb center on the exciton fine-structure and optical selection rules using symmetry theory and then quantify the effect of symmetry breaking on the exciton fine structure, modelling the charged center using a multipole expansion. The model shows that the presence of a Coulomb center breaks the nanocrystal symmetry and affects its optical properties through mixing and shifting of the hole spin and parity sublevels. This symmetry breaking, particularly for positively charged centers, shortens the radiative lifetime of CdSe nanocrystals even at room temperature, in qualitative agreement with the increase in PL efficiency observed in CdSe nanocrystals doped with positive Ag charge centers [A. Sahu et.al., Nano Lett. 12, 2587, (2012)]. The effect of the charged center on the photoluminescence and the absorption spectra is shown, with and without the presence of compensating charges on the nanocrystal surface. While spectra of individual nanocrystals are expected to shift and broaden with the introduction of a charged center, configuration averaging and inhomogeneous broadening are shown to wash out these effects. The presence of compensating charges at the NC surface also serves to stabilize the band edge transition energies relative to NCs with no charge centers.

Study of the near band-edge excitonic transition energies of the mixed layered compounds of Mo1-xWx Se2 single crystals

The temperature dependence of the spectral features in the vicinity of the direct band edge of Mo1-xWxSe2 single crystals was measured in the temperature range of 25–300 K using piezoreflectance (PzR). The near band-edge excitonic transition energies of the mixed layered compounds of Mo1-xWxSe2 single crystals determined accurately from a detailed line-shape fit of the PzR spectra. From a detailed line-shape fit of the PzR spectra, the temperature dependence of the energies of the band-edge excitons are determined accurately.

Double surface effect causes a peak in band-edge photocurrent spectra: a quantitative model

Band-edge photocurrent spectra are typically observed in either of two shapes: a peak or a step. In this study, we show that the photocurrent band-edge response of a GaN layer forms a peak, while the same response in GaN nanowires takes the form of a step, and both are red-shifted to the actual band-edge energy. This apparent inconsistency is not limited to GaN. The physics of this phenomenon has been unclear. To understand the physics behind these observations, we propose a model that explains the apparent discrepancy as resulting from a structure-dependent surface effect. To test the model, we experiment with a GaAs layer, showing that we can deliberately switch between a step and a peak. We use GaAs because it is available at a semi-insulating doping level. We demonstrate that using this quantitative model one may obtain the exact band-edge transition energy, regardless of the red-shift variance, as well as the density of the surface state charges that cause the red shift. The model thus adds quantitative features to photocurrent spectroscopy.

Thickness fluctuation relations in carrier dynamics of CdTe/ZnTe quantum dots

We investigated the influence of thickness fluctuations on the carrier dynamics of CdTe/ZnTe quantum dots (QDs). The temperature dependence of both the red-shift in the band-edge transition energy and broadening of the emission line were evaluated using different models. We showed that the quantum confinement effect and thermal escape of the QDs can be extended to significantly higher temperatures. These results were confirmed by using the discrete recombination model to investigate localized and delocalized states. Taking place into the reducing fluctuations of QDs that the thermally activated transition energies and the carrier scattering via phonons are enhanced.

Growth and optical studies of tungsten disulphide single crystals doped with gold

Tungsten disulphide single crystals doped with gold (WS2:Au) have been grown by the chemical vapor transport method using iodine as a transporting agent. The structure of the materials was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), and Raman measurements. The doping effects of the materials are characterized by surface photovoltage (SPV) and piezoreflectance (PzR) measurements. The SPV spectrum reveals an impurity level located below the A exciton. The direct band-edge excitonic transition energies for WS2:Au show redshifts and the broadening parameters of the excitonic transition features increase due to impurity scattering.

Piezoreflectance study of near band edge excitonic-transitions of mixed-layered crystal Mo(SxSe1-x)2 solid solutions

The temperature dependence of the spectral features in the vicinity of the direct band edge of mixed-crystals Mo(SxSe1-x)2 solid solutions is measured in the temperature range of 25–295 K by using piezoreflectance (PzR). The near band-edge excitonic transition energies of Mo(SxSe1-x)2 solid solutions were determined accurately from a detailed line-shape fit of the PzR spectra. The near band-edge excitonic transition energies were found to vary smoothly with the increase of S content x, indicating that the natures of the direct band edges of Mo(SxSe1-x)2 solid solutions are similar. The temperature dependences of near band edge transition energies were analyzed using Bose-Einstein expressions in the temperature range from 25 to 295 K. The parameters that described the temperature variation of the energies and broadening function of the excitonic transitions were evaluated and discussed.

Dielectric function of Cu(In, Ga)Se2-based polycrystalline materials

The dielectric functions of Cu(In, Ga)Se2(CIGS)-based polycrystalline layers with different Ga and Cu compositions have been determined by applying spectroscopic ellipsometry (SE) in a wide energy range of 0.7–6.5 eV. To suppress SE analysis errors induced by rough surface and compositional fluctuation, quite thin CIGS layers (<60 nm) with high uniformity toward the growth direction have been characterized using a self-consistent SE analysis method. We find that the optical model used in many previous studies is oversimplified particularly for the roughness/overlayer contribution, and all the artifacts arising from the simplified analysis have been removed almost completely in our approach. The CIGS dielectric functions with the variation of the Ga composition [x = Ga/(In + Ga)] revealed that (i) the whole CIGS dielectric function shifts toward higher energies with x, (ii) the band gap increases linearly with x without the band-gap bowing effect, and (iii) the overall absorption coefficients are significantly smaller than those reported earlier. Furthermore, the reduction of the Cu composition [y = Cu/(In + Ga)] leads to (i) the linear increase in the band-edge transition energy and (ii) the decrease in the absorption coefficient, due to the smaller interaction of the Cu 3d orbitals near the valence band maximum in the Cu-deficient layers. When y > 1, on the other hand, the free-carrier absorption increases drastically due to the formation of a semi-metallic CuxSe phase with a constant band gap in the CIGS component. In this study, by using a standard critical-point line-shape analysis, the critical point energies of the CIGS-based layers with different Ga and Cu compositions have been determined. Based on these results, we will discuss the optical transitions in CIGS-based polycrystalline materials.

Optical study of GaAs 1− xSb x layers grown on GaAs substrates by gas-source molecular beam epitaxy

The optical properties of GaAs 1− x Sb x (5.9% ≤ x ≤ 9.7%) layers grown on GaAs substrates by gas-source molecular beam epitaxy were characterized by photoreflectance (PR) and piezoreflectance (PzR). Identification of conduction to heavy-hole (HH) band and conduction to light-hole (LH) band transitions originated from the strained induced valence band splitting have been achieved by comparing the relative intensities of PR and PzR spectra. The results indicate increases of the valence band splitting with increasing of Sb content. The temperature dependences of near band edge transition energies were analyzed using Varshni and Bose–Einstein expressions in the temperature range from 15 to 300 K. The parameters that describe the temperature variations of the near band edge transition energies and broadening function were evaluated and discussed.

Temperature-dependent study of the band-edge excitonic transitions of Cu 2ZnSiS 4 single crystals by polarization-dependent piezoreflectance

The temperature dependence of the band-edge excitonic transitions of Cu 2ZnSiS 4 single crystals were characterized by using polarization-dependent piezoreflectance (PzR) in the temperature range of 10–300 K. The PzR measurements were carried out on the as-grown basal plane with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The PzR spectra revealed polarization-dependent E ⊥ ex and E | | ex features for E⊥ c and Е||c polarization, respectively. Both E ⊥ ex and E | | ex features are associated with the interband excitonic transitions at Γ point and can be explained by crystal-field splitting of valence band. From a detailed lineshape fit to the PzR spectra, the temperature dependence of the transition energies and broadening parameters of the band-edge excitons were determined accurately. The temperature dependence of near band-edge excitonic transition energies were analyzed using Varshni and Bose–Einstein expressions. The temperature dependence of the broadening parameter of excitonic features also has been studied in terms of a Bose–Einstein equation that contains the electron (exciton)–longitudinal optical phonon-coupling constant. The parameters that describe the temperature variation of the excitonic transition energies and broadening parameters were evaluated and discussed.

Mapping the Optical Properties of CdSe/CdS Heterostructure Nanocrystals: The Effects of Core Size and Shell Thickness

Here we present the first comprehensive report on CdSe/CdS heterostructure nanocrystals. The effects of core size and shell thickness on the optical properties of CdSe/CdS heterostructure nanocrystals are investigated. We report a reliable synthetic method to grow thick CdS shells on CdSe cores with sizes ranging from 2.5-4.7 nm. We provide a calibration curve, which enables determination of CdS shell thickness (+/-0.1 nm) over a wide range of core sizes, circumventing the need for time-consuming HRTEM analyses. Epitaxial growth of the shells was verified by HRTEM, XRD, and SAED. In-situ reaction measurements revealed the average per particle (p) deposition rates for cadmium and sulfur to be k(Cd) = 5.38 x 10(-25) mol s(-1) p(-1) and k(S) = 4.83 x 10(-24) mol s(-1) p(-1). Faster sulfur deposition rates are attributed to the absence of strong sulfur binding ligands in the growth medium. Through the rigorous use of high resolution transmission electron microscopy, a direct link between the dimensions of the heterostructures and their band-edge transition energies, quantum yields, and excited state lifetimes is established. The experiments show that the band-edge transition energies of the core samples, which initially span approximately 431 meV, condense to span only 163 meV after the growth of a 6 monolayer-thick CdS shell. Furthermore, shifts in the band-edge transition energies were found to be extremely sensitive to core size. The QY of the as-prepared core/shells ranged from 25 to 60%. The QYs and band-edge lifetimes of the core/shells were found to depend upon the ligands adsorbed to the particle surface. These data prove that one or both of the charge carriers still has access to the particle surface despite the presence of a 2.2 nm thick CdS shell.

Temperature dependent band gap shrinkage in GaN: Role of electron–phonon interaction

We present an experimental investigation of temperature dependent band-gap shrinkage in GaN using photoluminescence spectroscopy. The near-band-edge transition energy shifts to lower energy with increasing temperature. The parameters that describe the temperature-dependent red-shift of the band-edge transition energy are evaluated using different models. It has been found that the semi-empirical relation based on phonon-dispersion related spectral function leads to an excellent fit to the experimental data.

Piezoreflectance and photoreflectance study of annealing effects on GaAs0.916Sb0.084 and GaAs0.906Sb0.075N0.019 films on GaAs grown by gas‐source molecular beam epitaxy

AbstractThermal annealing effects of GaAs0.916Sb0.084 and GaAs0.906Sb0.075N0.019 films grown on GaAs substrates by gas‐source molecular beam epitaxy have been characterized by piezoreflectance (PzR) and photoreflectance (PR). By a comparison of relative intensity of PzR and PR spectra, the identification of conduction to heavy‐hole (HH) band and conduction to light‐hole (LH) band transitions originated from the strained induced valence band splitting have been achieved. The near band edge transition energies are blue‐shifted, and the splitting of HH and LH bands is reduced after thermal annealing treatment. The annealing effects of GaAs0.906Sb0.075N0.019 are found to be more pronounced than that of GaAs0.916Sb0.084. The temperature dependences of near band edge transition energies are analyzed using Varshni and Bose–Einstein expressions in the temperature range from 15 K to 300 K. The parameters that describe the temperature variations of the near band edge transition energies are evaluated and discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependent electromodulation characterization of Zn1−x−yBexMgySe mixed crystals

We report a detailed investigation of the temperature dependence of the band-edge excitonic transitions of three Zn1−x−yBexMgySe mixed crystals using contactless electroreflectance (CER) and photoreflectance (PR) in the temperature range of 15–400 K. The samples were grown by the modified high pressure Bridgman methods. The fundamental transition energy E0 and broadening parameters are determined via a lineshape function fit to the CER and PR spectra. The parameters that describe the temperature dependence of the band-edge excitonic transition energy and broadening parameters are evaluated and discussed.

Photoluminescence and photoreflectance study of annealing effects on GaAs0.909Sb0.07N0.021 layer grown by gas-source molecular beam epitaxy

Thermal annealing effects of a GaAs0.909Sb0.07N0.021 film grown on GaAs substrate via gas-source molecular beam epitaxy have been characterized by photoluminescence (PL) and photoreflectance (PR) techniques. PL measurements show the evolution of luminescence feature with the thermal annealing treatment. The conduction to heavy-hole (HH) band and conduction to light-hole (LH) band transitions originated from the strained induced valence band splitting in GaAs0.909Sb0.07N0.021 layer have been observed by the PR measurements. The near band edge transition energies are slightly blueshifted, and the splitting of HH and LH bands is reduced with rising annealing temperature. The temperature dependences of near band edge transition energies are analyzed using Varshni and Bose–Einstein expressions in the temperature range from 15to300K. The parameters that describe the temperature variations of the near band edge transition energies are evaluated and discussed.

Effect of threading edge dislocations on the photoluminescence spectra forn-type wurtzite GaN

A computational approach to study the effect of threading edge dislocation density on photoluminescence in $n$-type wurtzite GaN is presented and the calculated results are compared with experimental data. A calculation domain with a dipole of edge dislocations in three-dimensional real space is analyzed and its size is varied to study the effect of dislocation density. A $6\ifmmode\times\else\texttimes\fi{}6$ $\mathbf{k}∙\mathbf{p}$ Hamiltonian is solved to determine the energy levels and wave functions for valence electrons using the finite element method; the conduction band is calculated separately by taking advantage of the wide band gap of $3.44\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The spontaneous emission spectrum is then evaluated. The electrostatic potential of the edge dislocation, due to its electron-acceptor nature, and the deformation potential associated with the strain field of the edge dislocation reduce the intensity and broaden the peak of the band edge emission. Besides the band edge peak, three distinct peaks are observed below the band edge transition energy. These peaks are from transitions to dislocation-localized hole states with energies within the band gap created by the electrostatic potential of the edge dislocations. Compared to the effect of the electrostatic potential, the strain field effect is negligible in generating emission peaks below the band edge transition energy, but it is still significant enough to contribute to broadening the band edge peak. The calculated band edge peak intensities closely agree with experimentally measured band edge photoluminescence intensities. The calculated and experimental data both show a significant reduction of band edge peak intensity for a dislocation density higher than ${10}^{7}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ as the dislocation density increases. The intensity of band edge peaks reduces significantly with high dislocation densities and low carrier concentrations, which indicates that GaN becomes dislocation sensitive like other III-V materials if insufficient carriers are present.

Band structure and absorption properties ofGaAs1−xNx∕InAs1−yNyshort period superlattices strained toInP(001)

The $\mathbit{k}∙\mathbit{p}$ approximation and the band anticrossing model modified for the strain are used to describe the electronic states of the strained bulk ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ and ${\mathrm{InAs}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}$ ternaries in the vicinity of the center of the Brillouin zone ($\ensuremath{\Gamma}$ point) and their respective band offsets have been evaluated, before implementing them into the superlattice structure. By minimizing the total mechanical energy of the stack of the alternating layers of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ and ${\mathrm{InAs}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}$ in the superlattice, the ratio of the thicknesses of the epilayers is determined to make the structure lattice matching on $\mathrm{InP}(001)$. Energy miniband structure of the superlattice is then investigated using the transfer-matrix formalism, predicting the evolution of the band-edge transition energies for different nitrogen concentrations and thickness combinations. The results show the potential to significantly reduce the band gap compared to quaternary alloys of similar average concentration and to obtain photon absorption and emission energies in the range of $0.65--0.35\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for a typical nitrogen concentration of $\ensuremath{\le}5%$. Finally, the optical-absorption coefficient of such a superlattice, as a function of the nitrogen concentration, the change in electron effective masses, and the temperature are estimated under the anisotropic medium approximation.

Photoluminescence studies in sol–gel derived Zno films

The photoluminescence (PL) properties of high quality ZnO thin films grown on Si (1 0 0) substrates using spin coating technique are investigated as a function of temperature in the range 10–300 K. The PL spectra shows dominant donor bound excitonic emission along with free exciton related emission in the UV region. The corresponding activation energy of thermal quenching is found to be ∼ 59.7 meV . The parameters that describe the temperature dependent red shift of the band-edge transition energy are evaluated using different models. The broadening of the PL peak due to increase in temperature is mainly attributed to the exciton–LO phonon coupling.

The temperature dependence of the band gap shrinkage due to the electron–phonon interaction inAlxGa1−xAs

The photoluminescence spectrum of band edge transitions inAlxGa1−xAs is studied as a function of temperature and electron concentration. The parameters thatdescribe the temperature dependence redshift of the band edge transition energy areevaluated using different models. We find that a semi-empirical relation based on a phonondispersion related spectral function leads to an excellent fit to the experimentaldata.