Fundamental Optical Transition Research Articles

Excitons in coupled quantum dots

Properties of excitons in vertically coupled GaAs/AlGaAs quantum dots were investigated using the variational method within the envelope function and effective mass approximations. It was found that when the thickness of the spacer layer becomes less than about one exciton Bohr radius, both the exciton binding energy and the fundamental optical transition energy are reduced compared to those in isolated quantum dots. This is a result of increased space extension of exciton due to the penetration of carrier wave functions into the spacer layer and corresponding reduction in confinement energy which dominates over the Coulomb interaction between the electron and the hole.

MeV energy Lithium ion irradiated crystalline GaAs: an optical study

An optical study of GaAs 〈1 0 0〉 , undoped, irradiated with 50 MeV Li-ion at room temperature with a fluence ranging from 1×10 11 to 1×10 14 ions/cm 2 using Pelletron ion beam facility of Nuclear Science Centre (NSC), New Delhi has been carried out in the spectral regions of the fundamental optical absorption and interband transition. The damage induced by the MeV ion in the surface and deep inside the semiconductor has been monitored by determining the ion fluence dependence of the optical constants: absorption coefficient ( α), reflection ( R), ion-irradiation induced defect density ( N s) and optical energy gap ( E g) in the spectral range 200– 2500 nm . The present study reveals that the ion irradiation does not induce significant changes in the surface of a semiconductor. The fundamental optical absorption spectra are, however, greatly modified by MeV energy ion–solid interaction.

One- and two-photon photocurrents from tunable organic microcavity photodiodes

We have constructed multilayer thin-film organic microcavity photodiodes with the photoactive layer comprised of a spin-coated conjugated polymer and an evaporated C60 layer. The electrodes are designed as semitransparent mirrors which form a resonant cavity structure. The photocurrent spectra show distinct maxima at the optical resonances of the cavities, which are located up to 200 nm below the fundamental optical transition of the polymer. The design allows a simple tuning of the spectral response by varying the layer thickness. Microcavity photodiodes are also shown to be highly sensitive two-photon detectors, which exhibit a factor 500 improvement in the two-photon response compared to devices without photonic confinement.

InAs δ-layer structures in GaAs grown by MOVPE and characterised by luminescence and photocurrent spectroscopy

Photocurrent, electroluminescence, and photoluminescence spectroscopy were used for the characterisation of laser structures containing ultrathin InAs δ-layers in GaAs matrix surrounded by AlGaAs waveguide and grown by low-pressure metal organic vapour phase epitaxy. Three types of δ-layer structures for laser active layers were investigated: single layers with different thickness ( W L), different numbers of identical layers ( N), and seven identical δ-layers separated by GaAs spacers of variable thickness ( S L). Measurement revealed two fundamental optical transition between electron and heavy and light hole states in the δ-layers. Both transitions are shifted to lower energies by hundreds of meV when W L and N increases or S L decreases. While the effect of W L can be explained by a quantum model accounting for the influence of stress and quantum state coupling, data obtained from multilayer structures exhibit significant deviation from theory.

Optical investigations of InGaAsN/GaAs single quantum well structures

The influence of nitrogen content on the optical properties of InGaAsN/GaAs single quantum wells (SQWs) with nitrogen concentration up to 5.2% and relatively high indium content have been investigated by photoreflectance (PR) and photoluminescence (PL) spectroscopy. The PR measurements have allowed us to observe, besides of the fundamental 11H optical transition, transitions between excited states in QWs. The identification of optical transitions present in PR spectra has been possible due to the theoretical calculations based on the envelope function model. We have found the relatively high red shift of all optical transitions with increasing nitrogen content. The temperature dependence of PL peak energy have been investigated, too. The thermal stability of PL peak energy is stronger for structures with higher N content. Fitting with Varshni empirical relation to PL data we have found that at low temperatures carriers are localised. For structures with high nitrogen content we observed S-shaped energy peak behaviour.

Coulomb effects on the fundamental optical transition in semiconducting single-walled carbon nanotubes: Divergent behavior in the small-diameter limit

We have experimentally investigated the diameter dependence of Coulomb effects on the optical transition in semiconducting single-walled carbon nanotubes. The absorption band due to the lowest band-to-band transition shifts to the higher-energy side compared to the transition energy calculated by a tight-binding model. The blueshift originating from the Coulomb effect increases with decreasing tube diameter d as d - 1 , 3 . We have found that this dependence is attributed to both the increase of the band gap and the enhancement of the exciton binding energy with a decrease of the diameter.

Optical transitions in non-stoichiometric LaMnO3 identifying the charge-transfer transitions

Using the Mott–Littleton approach we examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn4+ and O− in LaMnO3 lattice. The reasonable agreement with the experiment of our predicted energies, linewidths and oscillator strengths leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5eV is associated with O(2p)–Mn(3d) transition of charge-transfer character, whereas the band near 2.3eV is rather associated with the presence of Mn4+ and/or O− self-trapped holes in the as-grown non-stoichiometric LaMnO3 compound. We do not exclude other contributions to the latter band.

Lattice relaxation and charge-transfer optical transitions due to self-trapped holes in nonstoichiometric LaMnO3 crystal

We use the Mott-Littleton approach to evaluate polarisation energies in LaMnO$_3$ lattice associated with holes localized on both Mn$^{3+}$ cation and O$^{2-}$ anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O-site is estimated as 2.4 eV which is appreciably greater than that of 0.8 eV for a hole localized at the Mn-site, indicating on the strong electron-phonon interaction in the former case. Using a Born-Haber cycle we examine thermal and optical energies of the hole formation associated with electron ionization from Mn$^{3+}$, O$^{2-}$ and La$^{3+}$ ions in LaMnO$_3$ lattice. For these calculations we derive a phenomenological value for the second electron affinity of oxygen in LaMnO$_3$ lattice by matching the optical energies of La$^{4+}$ and O$^-$ hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn$^{4+}$ state in LaMnO$_3$ host lattice, but the energy of a hole in the O$^-$ state is only higher by a small amount, 0.75 eV, rather suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn$^{4+}$ and O$^-$ in LaMnO$_3$ lattice. The reasonable agreement with experiment of our predicted energies, linewidths and oscillator strengths leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with O(2p) - Mn(3d) transition of charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn$^{4+}$ and/or O$^-$ self-trapped holes in non-stoichiometric LaMnO$_3$ compound.

Nature of the fundamental band gap in GaNxP1−x alloys

The optical properties of GaNxP1−x alloys (0.007⩽x⩽0.031) grown by gas-source molecular-beam epitaxy have been studied. An absorption edge appears in GaNxP1−x at energy below the indirect ΓV–XC transition in GaP, and the absorption edge shifts to lower energy with increasing N concentration. Strong photomodulation signals associated with the absorption edges in GaNxP1−x indicate that a direct fundamental optical transition is taking place, revealing that the fundamental band gap has changed from indirect to direct. This N-induced transformation from indirect to direct band gap is explained in terms of an interaction between the highly localized nitrogen states and the extended states at the Γ conduction-band minimum.

Optical anisotropy of the (100) surfaces in AlxGa1−x As ternary compounds

The reflectance anisotropy spectra of the clean (100) surfaces of the AlxGa1−xAs ternary compounds at aluminum concentrations 0≤x≤0.5 have been measured and thoroughly studied. In the spectral range from 1.6 to 3.5 eV, the signal caused by the optical transitions in the arsenic dimers dominates in the spectra of the clean arsenic-terminated GaAs surfaces. For the ternary compounds, an increase in the aluminum concentration brings about the broadening of this signal and its shift toward the low-energy range. This is explained by the appearance of additional signals associated with the optical transitions in the nonequivalent arsenic dimers, in which a part of the Ga atoms in the bulklike bonds is replaced by the Al atoms. An increase in the number of the substituted gallium atoms leads to a decrease in the energy of optical transition in the dimer. The fundamental optical transition energies are determined for the nonequivalent dimers.

Lineshape analysis of photoreflectance spectra from InGaAs/GaAs quantum wells

The fundamental optical transitions in In0.15Ga0.85As/GaAs single symmetric quantum wells (QWs) are studied through photoreflectance (PR) measurements and their dependence on the well distance from the surface. A phase rotation of the lineshape of the PR signal is observed as was predicted in our previous works. PR spectra of several samples, measured at 77 K, are compared with results of PR lineshape calculations, and a fairly good agreement is found. The quantum-confined Stark effect is shown to be the dominant modulation mechanism in the QW. Pronounced interference effects make PR spectra from QWs sensitive to the cap layer thickness.

Exciton Effects of Optical Transitions in Single-Wall Carbon Nanotubes

Absorption spectra have been measured in the near-infrared region for a thin-film sample of single-wall carbon nanotubes of 1.2–1.5 nm diameter. Two absorption peaks are observed at 0.7 and 1.2 eV. The observed spectrum has been compared with the calculation based on the tight-binding model taking into account the distribution of diameter and the helical arrangement of tubes. We have found that a one-dimensional exciton effect plays an important role in the fundamental optical transition of semiconducting single-wall carbon nanotubes.

Photoelectrochemical studies with inorganic cage structures of metal dichalcogenides

Abstract Inorganic fullerene-like (IF) and nanotube nanoparticles of WS 2 and SnS 2 with an oxide core have been prepared by the reaction of the respective oxide nanoparticles with H 2 S in a reducing atmosphere. Ethanolic suspensions of these nanoparticles were prepared and they were subsequently deposited electrophoretically onto conductive substrates. To study their photoelectrochemical properties, the nanoparticles were immersed in a selenosulphate electrolyte and illuminated. The spectral response of the photoelectrodes was measured and the fundamental optical transitions were determined. The mechanism of charge excitation, conduction and transfer is discussed in view of the experimental results.

Effects of interface-layers composition and strain distribution on the optical transitions of InAs quantum dots on InP

In the present work we have investigated the optical properties of the self-assembled quantum dots (SAQD's) on an InP substrate. The dots are grown by gas source molecular-beam epitaxy (GSMBE) and characterized by photoluminescence (PL) and atomic force microscopy. The energy of the fundamental optical transitions measured by PL present a redshift compared to calculated values. Two hypotheses have been tested to explain this apparent difference: the existence of an intermediate ${\mathrm{InAs}}_{y}{\mathrm{P}}_{1\ensuremath{-}y}$ layer, with a composition depending on the experimental conditions, changes the value of the transition energy, and the strain induced in the InP confinement barrier by the dot as pointed out by Tersoff, has the same effect. The present study concludes with a discussion of the presence of a thin ${\mathrm{InAs}}_{y}{\mathrm{P}}_{1\ensuremath{-}y}$ interface layer originating from As/P exchange kinetic energy at the second interface on top of the dots.

The role of piezoelectric fields in GaN-based quantum wells

In this contribution, we focus on the consequences of the piezoelectric field, which is an inherent consequence of the commonly used wurtzite phase of GaN, on the optical properties of strained GaN-based quantum well structures. We demonstrate that both in GaN/AlGaN and in GaInN/GaN single quantum well structures, the piezoelectric field leads to a Stark-shift of the fundamental optical transitions, which can lead to luminescence emission far below the bulk bandgap. Due to the spatial separation of the electron and hole wavefunctions in such structures, the oscillator strength of these transitions may become extremely small, many orders of magnitude lower than in the field-free case. From specially designed structures, we can even determine the sign of the piezoelectric field and relate it to the polarity of the layers. Under high-excitation conditions, as found in a laser diode, the piezoelectric field is almost completely screened by the injected carriers. As a consequence, the stimulated emission is significantly blue-shifted compared to the photoluminescence, which has sometimes been confused with localization effects.

Preparation and optical studies on flash evaporated Sb2S3 thin films

Thin films of Sb2S3 were prepared by flash evaporation. As-deposited films were found to be amorphous and highly resistive (R>106Ω) in nature. An amorphous-crystalline phase transition occurs at 498 K on subsequent annealing in nitrogen atmosphere. The fundamental optical transition is direct in nature and the optical band gaps (Eg) lie in the region 1.70–2.07 and 1.42–1.65 eV for amorphous and crystalline films, respectively. A near-linear decrease in Eg with decreasing S/Sb ratio was observed suggesting the dependence of the band gap on film composition.

Optical and transport properties of B-doped GaSe semiconductor single crystal

The optical energy band-gap and the nature of the fundamental optical absorption transitions in GaSe:B single crystals have been determined by UV-VIS and photoacoustic spectroscopy. The thermal diffusivity, carrier diffusion coefficient, bulk recombination mean lifetime and surface recombination velocity have been measured using the photoacoustic detection technique.

Studies on chemically deposited CdS 1 − xSe x mixed thin films

A solution growth process is employed for the deposition of CdS 1 − x Se x thin film composites with 0 ≤ x≤ 1. Cadmium acetate, thiourea and sodium selenosulphate were used as the basic source materials. The samples were obtained at 55 °C in an alkaline medium and were characterised by optical and electrical characterisation techniques. The layer thickness is found to be decreased as x is varied from 0 to 1. The optical absorption studies showed a high coefficient of absorption ( α = 10 4 cm −1) with an allowed direct type of transition. The optical energy gap decreases continuously (typically from 2.42 eV to 1.74 eV) as x is increased from 0 to 1. For values of x between 0.5 and 0.8, two absorption edges have been observed; one at approximately 2.40 eV corresponding to the fundamental optical transitions in CdS and the other decreasing continuously, which corresponds to the fundamental transitions in CdS 1 − x Se x solid solution. The electrical conductivity is found to decrease up to x = 0.5 and to increase thereafter for further increase in x. The activation energy is more or less the same for all compositions and has no systematic dependence on the composition parameter x.

Fundamental optical transitions in GaN

A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time-resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.

Formula omitted] quantum boxes obtained by self-organized growth: Intrinsic electronic properties and applications

Spatially or spectrally selective photoluminescence (PL) studies, as well as time-resolved PL, highlight the superior intrinsic electronic properties of InAs GaAs quantum boxes (QBs) obtained by self-organized molecular beam epitaxy. A given QB displays a single PL line, with linewidth much smaller than the thermal energy kT. The emission of resonantly excited QBs is slightly Stokes shifted (0.7 meV) with respect to their fundamental optical transition. Ultrafast carrier capture and relaxation (≈20 ps) is observed in spite of the large energy between QB levels. Usually foreseen applications of QBs in optoelectronics are however still out of reach due to the inhomogeneous broadening induced by size fluctuations and, for the smaller QBs, thermoemission. Novel applications of such QBs in optoelectronics, such as efficient light emitting diodes on silicon, are proposed and validated experimentally.