Intrinsic Optical Transitions Research Articles

Enhancement of intrinsic optical transitions in silicon nanocrystals by state localization

Silicon photoluminescence and lasing have been critical issues to breakthrough bottlenecks in the understanding of luminescence mechanisms. Unfortunately, long-standing disputes about the exciton recombination mechanism and fluorescence lifetime remain unresolved, especially about whether silicon nanocrystals (Si NCs) can realize fast direct-bandgap-like optical transitions. Here, using ground-state and excited-state density functional theory (DFT), we obtained intrinsic phonon-free optical transitions at sizes from Si22 to Si705, showing that very small Si NCs can realize a strong direct optical transition. Orbital labeling results show that this rapid transition does not come from the {\Gamma}-{\Gamma} -like transition, contrary to the conclusions from the effective mass approximation (EMA) and that {\Gamma}-X mixing leads to a quasi-direct bandgap. This anomalous transition is particularly intense with decreasing size (or enhancement of quantum confinement). By investigating electron and hole distributions generated in the optical transition, localized state-induced enhanced emission (LIEE) in Si NCs was proposed. Quantum confinement (QC) distorts the excited-state electron spatial distribution by localizing Bloch waves into the NC core, resulting in increased hole and electron overlap, thus inducing a fast optical process. This work resolves important debates and proposes LIEE to explain the anomalous luminescence--a phase transition from weak (or none) luminescent state to strong optical transition, which will aid attempts at realizing high-radiative-rate NCs materials and application-level Si lasers.

Selective Sensors of Nitrogen Dioxide Based on Thin Tungsten Oxide Films under Optical Irradiation

It is shown that NO2 present in air, beginning at a concentration of 1 ppm, can be selectively detected by sensors based on Au/WO3:Au thin films activated by laser diode radiation with maximum intensity at 400 nm instead of constant heating. The radiation-activated photodesorption reduces the time of sensor response to NO2. A high humidity of air under conditions of room-temperature irradiation additionally increases the device sensitivity to NO2 due to the appearance of additional adsorption sites. The absence of a sensor response to reducing gases and varying oxygen concentration in the atmosphere is caused by the photodesorption of chemisorbed $${\text{O}}_{2}^{ - }$$ species during their interaction with holes generated in intrinsic optical transitions in the near-surface region of WO3 film.

Anisotropic Photoluminescence from Isotropic Optical Transition Dipoles in Semiconductor Nanoplatelets.

Many important light-matter coupling and energy-transfer processes depend critically on the dimensionality and orientation of optical transition dipoles in emitters. We investigate individual quasi-two-dimensional nanoplatelets (NPLs) using higher-order laser scanning microscopy and find that absorption dipoles in NPLs are isotropic in three dimensions at the excitation wavelength. Correlated polarization studies of the NPLs reveal that their emission polarization is strongly dependent on the aspect ratio of the lateral dimensions. Our simulations reveal that this emission anisotropy can be readily explained by the electric field renormalization effect caused by the dielectric contrast between the NPLs and the surrounding medium, and we conclude that emission dipoles in NPLs are isotropic in the plane of the NPLs. Our study presents an approach for disentangling the effects of dipole degeneracy and electric field renormalization on emission anisotropy and can be adapted for studying the intrinsic optical transition dipoles of various nanostructures.

Study of Lateral Scaling Impact on the Frequency Performance of SiGe Heterojunction Bipolar Phototransistor

The influence of the lateral scaling such as emitter width and length on the frequency behavior of SiGe bipolar transistor is experimentally studied. Electrical transistors of different emitter sizes are designed and fabricated by using a commercial bipolar transistor technology. The effect of peripheral current and collector current spreading on electrical bipolar transistor performances are analyzed in regards to the state of the art. Furthermore, the lateral scaling effect on SiGe phototransistor electrical and opto-microwave frequency behavior is studied. The impact of the lateral flow of photo-generated carriers toward the optical opening in phototransistor structure is investigated. Moreover, the 2-D carrier flow effect on the opto-microwave frequency behavior of the phototransistor is characterized through opto-microwave scanning near-field optical microscopy measurements, in the course of which the intrinsic parameters, such as transit time and junction capacitances are extracted over the surface of the phototransistor. An intrinsic optical transition frequency of 6.5 GHz is measured for $10\times 10\,\,\mu \text {m}^{2}$ .

Alkane Encapsulation Induces Strain in Small-Diameter Single-Wall Carbon Nanotubes

Encapsulation of linear alkane molecules in the endohedral volumes of small-diameter single-wall carbon nanotubes (SWCNTs) is shown to induce diameter-dependent strain on the hexagonal lattice of carbon atoms composing the tubular structure. For the smallest diameter nanotubes, such as the (6,5), (9,1), (8,3), and (10,0), encapsulation leads to expansive radial strain. This effect is demonstrated through precision measurements of induced shifts in the energy of the intrinsic optical transitions of single-chirality nanotube populations. The effect on the optical transitions from strain is found to exceed that of the effective dielectric medium change when comparing the same SWCNT population filled with an alkane versus those filled with water. This differs from encapsulation of alkanes into larger-diameter nanotubes, for which dielectric effects dominate because of the relative sizes of the guest molecules and the SWCNT cavity. For the SWCNT species examined in this work, the interior cavity diameters are ...

4f fine-structure levels as the dominant error in the electronic structures of binary lanthanide oxides.

The ground-state 4f fine-structure levels in the intrinsic optical transition gaps between the 2p and 5d orbitals of lanthanide sesquioxides (Ln2 O3 , Ln = La…Lu) were calculated by a two-way crossover search for the U parameters for DFT + U calculations. The original 4f-shell potential perturbation in the linear response method were reformulated within the constraint volume of the given solids. The band structures were also calculated. This method yields nearly constant optical transition gaps between Ln-5d and O-2p orbitals, with magnitudes of 5.3 to 5.5 eV. This result verifies that the error in the band structure calculations for Ln2 O3 is dominated by the inaccuracies in the predicted 4f levels in the 2p-5d transition gaps, which strongly and non-linearly depend on the on-site Hubbard U. The relationship between the 4f occupancies and Hubbard U is non-monotonic and is entirely different from that for materials with 3d or 4d orbitals, such as transition metal oxides. This new linear response DFT + U method can provide a simpler understanding of the electronic structure of Ln2 O3 and enables a quick examination of the electronic structures of lanthanide solids before hybrid functional or GW calculations.

First-principle study of optical properties of Cu-doped CdS

Based on the full-potential linearized augmented plane wave (FLAPW) method, we study the electronic structure and optical properties in the Cu-doped CdS semiconductor. We find that the d orbitals of Cu are strongly hybridized with the p orbitals of S near the Fermi level, resulting in p and d charge transfer between Cu and S atoms. These lead to transit from a semi-metal to metal with increase of Cu dopant concentrations, enhancing the conductivity of CdS significantly. As a consequence, the optical transition between valence band and unoccupied d states of Cu enhances greatly, and is stronger than the intrinsic optical transition of CdS semiconductor. Along with the increase of Cu concentrations, the absorption spectra gradually shift to higher energy (blue shifts), and optical absorption of Cd1−xCuxS increases in the visible range.

Fabrication and Photoluminescence Properties of Graphite Fiber/ZnO Nanorod Core–Shell Structures

Graphite fiber/ZnO nanorod core-shell structures were synthesized by thermal evaporation process. The core-shell hybrid architectures were comprised of ZnO nanorods grown on the surface of graphite fiber. In addition, Hollow ZnO hierarchical structure can be obtained by oxidizing the graphite fiber. Room temperature photoluminescence (PL) of the as-made graphite fiber/ZnO nanorod structures shows two UV peaks at around 3.274 eV and 3.181 eV. The temperature-dependent photoluminescence spectra demonstrate the two UV emissions are attributed to the intrinsic optical transitions and extrinsic defect-related emissions in ZnO. These hybrid structures may be used as the building block for fabrication of nanodevices.

Microstructural, Optical, and Electrical Properties of SnO Thin Films Prepared on Quartz via a Two-Step Method

A simple, cost-effective, two-step method was proposed for preparing single-phase SnO polycrystalline thin films on quartz. X-ray diffraction (XRD) analysis demonstrated that the annealed films were consisted of polycrystalline alpha-SnO phase without preferred orientation, and chemical composition analysis of the single phase in nature was analyzed using X-ray photoelectron spectroscopy (XPS). Transmittance spectra in UV-vis-IR range indicated that the average transmittance of both the as-deposited and the annealed SnO thin films was up to 70%. The optical band gap decreased from 3.20 to 2.77 eV after the annealing process, which was attributed to the crystalline size related quantum size effect. Photoluminescence (PL) spectrum of the annealed film showed only a weak peak at 585 nm, and no intrinsic optical transition emission was observed. Moreover, the p-type conductivity of SnO film was confirmed through Hall effect measurement, with Hall mobility of 1.4 cm(2) V(-1) s(-1) and hole concentration of 2.8 x 10(16) cm(-3).

Optical selection rule for the lower Davydov excitons in co-oligomer single crystals

Intrinsic optical transitions of lower Davydov excitons in thiophene/phenylene co-oligomer (TPCO) crystals were investigated. Lower Davydov excitons are either optically allowed or forbidden depending on the number of thiophene rings constituting the TPCOs. The TPCO molecules are orientated nearly parallel to each other like $H$ aggregates and are aligned in herringbone fashion within a crystalline layer. The transition selection rules are dominated by the components of the molecular transition dipole moments lying in the herringbone planes of the respective crystals. The selection rules at the exciton band bottoms of the TPCOs differ from those of oligothiophenes in spite of these two compounds having similar molecular structures. The selection rules vary drastically with the molecular alignment because of the $H$ aggregation. TPCO and oligothiophene crystals are treated as $H$ aggregates as a first approximation. This treatment is very useful for understanding the optical transitions of conjugated materials.

Evidence for large configuration-induced band-gap fluctuations inGaAs1−xNxalloys

We have measured the near-band-gap absorption of ${\text{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ thin films with $xl0.012$. The spectra were analyzed with a model which allows a precise determination of the band gap and of the width of the optical transitions; the latter is found to increase, for $xg0.002$, well beyond what is expected in a III-V alloy. Ab initio calculations were performed within the generalized-gradient approximation with the exchange-correlation functional of Engel and Vosko [Phys. Rev. B 47, 13164 (1993)]. They reveal that the band gap depends markedly on nitrogen atomic configuration. The anomalous broadening of the intrinsic optical transitions thus gives strong evidence for configuration-induced band-gap fluctuations.

The effects of concomitant In and N incorporation on the photoluminescence of GaInNAs

We investigated the effects of concomitant In- and N-incorporation on the photoluminescence (PL) of GaInNAs grown by molecular beam epitaxy. In comparison with the N-free GaInAs epilayer, the PL spectra of the GaInNAs epilayer exhibit an anomalous S-shape temperature dependence of dominant luminescence peak. Through further careful inspection, two PL peaks are clearly discerned and are associated with the interband excitonic recombinations and excitons bound to N-induced isoelectronic impurity states, respectively. By comparing the PL spectra of GaInNAs/GaAs quantum wells (QWs) with those of In-free GaNAs/GaAs QWs grown under similar conditions, it is found that the concomitant In- and N-incorporation reduces the density of impurities and has an effect to improve the intrinsic optical transition of GaInNAs, but also enhance the N-induced clustering effects. At last, we found that rapid thermal annealing can significantly reduce the density of N-induced impurities.

Room temperature intrinsic optical transition in GaN epilayers: The band-to-band versus excitonic transitions

The mechanism of room-temperature (RT) intrinsic optical transition in high quality and purity GaN epilayer grown by metalorganic chemical vapor deposition (MOCVD) has been investigated. Our results show that the band-to-band instead of excitonic transition is the dominant transition in MOCVD grown GaN epilayer at RT. This conclusion is supported by the observation of the excitation intensity dependence of the photoluminescence emission peak position and by a model calculation. The band-to-band transition energy at RT at the limit of low carrier concentration has been determined to be 3.429 eV. Since the band-to-band transition is the dominant optical transition at RT, it thus suggests that the electron-hole plasma is most likely responsible for gain in GaN blue lasers similar to the case in other III–V semiconductor lasers.

Synthesis of β-FeSi 2 for optical applications by Fe triple-energy ion implantation into Si(100) and Si(111) substrates

Ion-beam synthesis of β-FeSi 2 layers was performed by Fe triple-energy ion implantation into Si(100) and Si(111) substrates and subsequent two-step annealing. By keeping the first-step annealing temperature constant at 600°C, the physical properties were characterized as a function of the second-step annealing temperature ( T a). X-ray diffraction measurements revealed that only the β phase was formed on Si(100) substrates for T a below 930°C, whereas the α phase coexisted with the β phase for all T a values even on as-implanted Si(111) substrates. The electrical resistivities of β-FeSi 2 formed on Si(111) substrates with T a = 875 and 915°C were found to be 0.1–0.3 Ω cm. It was shown by optical absorption measurements that the band gap decreased from 0.83 to 0.78 eV with increasing T a. Samples with both Si(100) and Si(111) substrates with T a = 875°C exhibited a defect-related optical absorption band at around 0.7 eV. In the 2 K photoluminescence spectra of these two samples with T a = 875°C, only a very broad emission band peaking at 0.8 eV was observed. However, the Si(111) sample with T a = 915°C showed a sharp emission line at 0.837 eV, which was assigned to the intrinsic optical transition of β-FeSi 2.

Optical spectroscopy of ordered GaAs(001) surfaces obtained by sulfide passivation

Reflectance anisotropy (RA) spectroscopy has been used to study at 300 K the intrinsic optical transitions on (Na 2·9H 2O) and (NH 4) 2S-treated GaAs(001) surfaces and for unpassivated surfaces. As a function of annealing temperature, we observe arsenic and sulfur dimers oriented along the [11̄0] direction as well as gallium diners oriented along [110]. The differences as a function of surface treatments are related to the chemistry of the passivated surfaces and to the amount of residual impurities. We obtain surface structures with a reduced band bending, stable down to 120 K and consistent with known arsenic-rich and gallium-rich reconstructions.

Optical study of surface dimers on sulfur-passivated (001)GaAs

Reflectance anisotropy (RA) spectrosocpy has been used to study at 300 K the intrinsic optical transitions on sulfur-passivated surfaces. The spectra allow to follow the modification of surface dimers after annealing at a temperature Ta. For Ta≊440 °C, the S-treated surface is covered by arsenic and sulfur dimers oriented along the [11̄0] direction. Upon subsequent heating, sulfur is desorbed and there appear gallium dimers oriented along [110]. Sulfur passivation has allowed to obtain stable surface structures, with a strongly reduced band bending, which are consistent with the known (2×1) and (4×6) reconstructions.

Band-to-band optical absorption in narrow-gap Hg1−xCdxTe semiconductors

The infrared absorption spectra have been measured at temperatures from 4.2 to 300 K for thin bulk samples of Hg1−xCdxTe with compositions, x, from 0.170 to 0.443 with thicknesses smaller than 10 μm. The spectral range of the investigations includes the exponential absorption edges and intrinsic absorption spectra determined by electron transitions from the valence band to the conduction band. From the measured absorption spectra, the energy-band gap was determined by the photon energy at the absorption transition point between the Urbach exponential region and the Kane region. The data were fitted by a calculation of the intrinsic optical transition using Kane’s theory. The dependence of the energy-band gap on composition and temperature was determined in the alloy composition range from x=0 to 0.443 and for x=1, for temperatures from T=4.2 to 300 K. The absorption coefficient, αg, at the band-gap energy versus composition and temperature, and the exponential rule satisfied by the absorption edge are also obtained.