Showing papers on "Band gap published in 2002"
TL;DR: At pH less than 5, the absorption and emission spectra of individual nanotubes show evidence of band gap–selective protonation of the side walls of the tube, which is readily reversed by treatment with base or ultraviolet light.
Abstract: Fluorescence has been observed directly across the band gap of semiconducting carbon nanotubes. We obtained individual nanotubes, each encased in a cylindrical micelle, by ultrasonically agitating an aqueous dispersion of raw single-walled carbon nanotubes in sodium dodecyl sulfate and then centrifuging to remove tube bundles, ropes, and residual catalyst. Aggregation of nanotubes into bundles otherwise quenches the fluorescence through interactions with metallic tubes and substantially broadens the absorption spectra. At pH less than 5, the absorption and emission spectra of individual nanotubes show evidence of band gap-selective protonation of the side walls of the tube. This protonation is readily reversed by treatment with base or ultraviolet light.
3,635 citations
TL;DR: The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques as discussed by the authors.
Abstract: The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. These three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free-electron concentration of the sample. The peak energy exhibits very weak hydrostatic pressure dependence, and a small, anomalous blueshift with increasing temperature.
1,378 citations
TL;DR: In this paper, the authors used ab initio band calculations to find that mixing of the S 3p states with the valence band can contribute to the band gap narrowing, based on the theoretical analyses.
Abstract: Titanium dioxide (TiO2) doped with sulfur (S) was synthesized by oxidation annealing of titanium disulfide (TiS2). According to the x-ray diffraction patterns, TiS2 turned into anatase TiO2 when annealed at 600 °C. The residual S atoms occupied O-atom sites in TiO2 to form Ti–S bonds. The S doping caused the absorption edge of TiO2 to be shifted into the lower-energy region. Based on the theoretical analyses using ab initio band calculations, mixing of the S 3p states with the valence band was found to contribute to the band gap narrowing.
1,322 citations
TL;DR: In this article, the authors present the results of a joint study with the Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia and the Belarus Academy of Sciences, Brovki 17, 220072 Minsk, Belarus.
Abstract: (a) Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia (b) Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany (c) Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan (d) Institute of Solid State and Semiconductor Physics, Belarus Academy of Sciences, Brovki 17, 220072 Minsk, Belarus (e) LfI, University of Hannover, Schneiderberg 32, D-30167 Hannover, Germany
942 citations
TL;DR: In this article, a Ti-based oxysulfide, Sm(2,Ti(2)S(2),O(5), was studied as a visible light-driven photocatalyst.
Abstract: A Ti-based oxysulfide, Sm(2)Ti(2)S(2)O(5), was studied as a visible light-driven photocatalyst. Under visible light (440 nm < or = lambda < or = 650 nm) irradiation, Sm(2)Ti(2)S(2)O(5) with a band gap of approximately 2 eV evolved H(2) or O(2) from aqueous solutions containing a sacrificial electron donor (Na(2)S-Na(2)SO(3) or methanol) or acceptor (Ag(+)) without any noticeable degradation. This oxysulfide is, therefore, a stable photocatalyst with strong reduction and oxidation abilities under visible-light irradiation. The electronic band structure of Sm(2)Ti(2)S(2)O(5) was calculated using the plane-wave-based density functional theory (DFT) program. It was elucidated that the S3p orbitals constitute the upper part of the valence band and these orbitals make an essential contribution to the small band gap energy. The conduction and valence bands' positions of Sm(2)Ti(2)S(2)O(5) were also determined by electrochemical measurements. It indicated that conduction and valence bands were found to have satisfactory potentials for the reduction of H(+) to H(2) and the oxidation of H(2)O to O(2) at pH = 8. This is consistent with the results of the photocatalytic reactions.
830 citations
TL;DR: Wurtzite InN films were grown on a thick GaN layer by metalorganic vapor phase epitaxy as discussed by the authors, and growth of a (0001)-oriented single crystalline layer was confirmed by Raman scattering, x-ray diffraction, and reflection high energy electron diffraction.
Abstract: Wurtzite InN films were grown on a thick GaN layer by metalorganic vapor phase epitaxy. Growth of a (0001)-oriented single crystalline layer was confirmed by Raman scattering, x-ray diffraction, and reflection high energy electron diffraction. We observed at room temperature strong photoluminescence (PL) at 0.76 eV as well as a clear absorption edge at 0.7–1.0 eV. In contrast, no PL was observed, even by high power excitation, at ∼1.9 eV, which had been reported as the band gap in absorption experiments on polycrystalline films. Careful inspection strongly suggests that a wurtzite InN single crystal has a true bandgap of 0.7–1.0 eV, and the discrepancy could be attributed to the difference in crystallinity.
692 citations
TL;DR: These samples provide an opportunity for the study of the evolution of epitaxial strain as the shape of the core varies from nearly spherical to nearly cylindrical, in addition to their robust optical properties.
Abstract: We report the preparation and structural characterization of core/shell CdSe/CdS/ZnS nanorods. A graded shell of larger band gap is grown around CdSe rods using trioctylphosphine oxide as a surfactant. Interfacial segregation is used to preferentially deposit CdS near the core, providing relaxation of the strain at the core/shell interface. The reported synthesis allows for variation of the shell thickness between one and six monolayers, on core nanorods ranging from aspect ratios of 2:1 to 10:1. After an irreversible photochemical annealing process, the core/shell nanorods have increased quantum efficiencies and are stable in air under visible or UV excitation. In addition to their robust optical properties, these samples provide an opportunity for the study of the evolution of epitaxial strain as the shape of the core varies from nearly spherical to nearly cylindrical.
627 citations
TL;DR: It is proposed that these 3D metallic photonic crystals can be used to integrate various photonic transport phenomena, allowing applications in thermophotovoltaics and blackbody emission.
Abstract: Three-dimensional (3D) metallic crystals are promising photonic bandgap structures: they can possess a large bandgap, new electromagnetic phenomena can be explored, and high-temperature (above 1,000 degrees C) applications may be possible. However, investigation of their photonic bandgap properties is challenging, especially in the infrared and visible spectrum, as metals are dispersive and absorbing in these regions. Studies of metallic photonic crystals have therefore mainly concentrated on microwave and millimetre wavelengths. Difficulties in fabricating 3D metallic crystals present another challenge, although emerging techniques such as self-assembly may help to resolve these problems. Here we report measurements and simulations of a 3D tungsten crystal that has a large photonic bandgap at infrared wavelengths (from about 8 to 20 microm). A very strong attenuation exists in the bandgap, approximately 30 dB per unit cell at 12 microm. These structures also possess other interesting optical properties; a sharp absorption peak is present at the photonic band edge, and a surprisingly large transmission is observed in the allowed band, below 6 microm. We propose that these 3D metallic photonic crystals can be used to integrate various photonic transport phenomena, allowing applications in thermophotovoltaics and blackbody emission.
608 citations
TL;DR: In this paper, the fundamental band gap for InN is shown to be near 0.8 eV and the band gap increases with increasing Ga content, which is well fit with a bowing parameter of ∼1.4 eV.
Abstract: High-quality wurtzite-structured In-rich In1−xGaxN films (0⩽x⩽0.5) have been grown on sapphire substrates by molecular beam epitaxy. Their optical properties were characterized by optical absorption and photoluminescence spectroscopy. The investigation reveals that the narrow fundamental band gap for InN is near 0.8 eV and that the band gap increases with increasing Ga content. Combined with previously reported results on the Ga-rich side, the band gap versus composition plot for In1−xGaxN alloys is well fit with a bowing parameter of ∼1.4 eV. The direct band gap of the In1−xGaxN system covers a very broad spectral region ranging from near-infrared to near-ultraviolet.
590 citations
TL;DR: In this paper, the authors reported on the realization of wide band gap (5-6 eV), single-phase, metastable, and epitaxial MgxZn1−xO thin-film alloys grown on sapphire by pulsed laser deposition.
Abstract: We report on the realization of wide band gap (5–6 eV), single-phase, metastable, and epitaxial MgxZn1−xO thin-film alloys grown on sapphire by pulsed laser deposition We found that the composition, structure, and band gaps of the MgxZn1−xO thin-film alloys depend critically on the growth temperature The structural transition from hexagonal to cubic phase has been observed for (Mg content greater than 50 at %) (1⩾x⩾05) which can be achieved by growing the film alloys in the temperature range of 750 °C to room temperature Interestingly, the increase of Mg content in the film has been found to be beneficial for the epitaxial growth at relatively low growth temperature in spite of a large lattice mismatch between sapphire and cubic MgZnO alloys
547 citations
TL;DR: In this paper, a Kramers-Kronig consistent analytical expression was developed to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (e1,e2) versus photon energy E for the alloys.
Abstract: We have developed a Kramers–Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (e1,e2) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon–germanium (a-Si1−xGex:H) and silicon–carbon (a-Si1−xCx:H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (e1,e2) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where α increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (e1,e2) can be simulated assuming a single ...
TL;DR: The results suggest an unexpected radiation-induced, electronic-state-transition in the GaAs/AlGaAs 2DES, which exhibits vanishing diagonal resistance without Hall resistance quantization at low temperatures and low magnetic fields when the specimen is subjected to electromagnetic wave excitation.
Abstract: The observation of vanishing electrical resistance in condensed matter has led to the discovery of new phenomena such as, for example, superconductivity, where a zero-resistance state can be detected in a metal below a transition temperature Tc (ref. 1). More recently, quantum Hall effects were discovered from investigations of zero-resistance states at low temperatures and high magnetic fields in two-dimensional electron systems (2DESs)2,3,4. In quantum Hall systems and superconductors, zero-resistance states often coincide with the appearance of a gap in the energy spectrum1,2,4. Here we report the observation of zero-resistance states and energy gaps in a surprising setting5: ultrahigh-mobility GaAs/AlGaAs heterostructures that contain a 2DES exhibit vanishing diagonal resistance without Hall resistance quantization at low temperatures and low magnetic fields when the specimen is subjected to electromagnetic wave excitation. Zero-resistance-states occur about magnetic fields B = 4/5 Bf and B = 4/9 Bf, where Bf = 2πfm*/e,m* is the electron mass, e is the electron charge, and f is the electromagnetic-wave frequency. Activated transport measurements on the resistance minima also indicate an energy gap at the Fermi level6. The results suggest an unexpected radiation-induced, electronic-state-transition in the GaAs/AlGaAs 2DES.
TL;DR: A new organic molecular conductor, based on a spiro-biphenalenyl neutral radical, simultaneously exhibits bistability in three physical channels: electrical, optical, and magnetic.
Abstract: A new organic molecular conductor, based on a spiro-biphenalenyl neutral radical, simultaneously exhibits bistability in three physical channels: electrical, optical, and magnetic. In the paramagnetic state, the unpaired electrons are located in the exterior phenalenyl units of the dimer, whereas in the diamagnetic state the electrons migrate to the interior phenalenyl units and spin pair as a π-dimer. Against all expectations, the conductivity increases by two orders of magnitude in the diamagnetic state, and the band gap decreases. This type of multifunctional material has the potential to be used as the basis for new types of electronic devices, where multiple physical channels are used for writing, reading, and transferring information.
TL;DR: In this article, the authors propose a model to rationalise the vast range of experimental data available in literature at any excitation wavelength by a simple model, which considers the main factors influencing the Raman spectra.
TL;DR: In this paper, the first principles electronic structure calculations for the metallic rutile and the insulating monoclinic phase of vanadium dioxide were presented, and the density functional calculations allowed for a consistent understanding of all three phases.
Abstract: The results of first principles electronic structure calculations for the metallic rutile and the insulating monoclinic phase of vanadium dioxide are presented. In addition, the insulating phase is investigated for the first time. The density functional calculations allow for a consistent understanding of all three phases. In the rutile phase metallic conductivity is carried by metal orbitals, which fall into the one-dimensional band, and the isotropically dispersing bands. Hybridization of both types of bands is weak. In the phase splitting of the band due to metal-metal dimerization and upshift of the bands due to increased p-d overlap lead to an effective separation of both types of bands. Despite incomplete opening of the optical band gap due to the shortcomings of the local density approximation, the metal-insulator transition can be understood as a Peierls-like instability of the band in an embedding background of electrons. In the phase, the metal-insulator transition arises as a combined embedded Peierls-like and antiferromagnetic instability. The results for VO2 fit into the general scenario of an instability of the rutile-type transition-metal dioxides at the beginning of the d series towards dimerization or antiferromagnetic ordering within the characteristic metal chains. This scenario was successfully applied before to MoO2 and NbO2. In the compounds, the and bands can be completely separated, which leads to the observed metal-insulator transitions.
TL;DR: In this paper, the authors investigated the photocatalytic properties of perovskite-type materials AgMO3 (M: Ta and Nb) consisting of Ag+ and d0 ions and found that the band gaps of AgTaO3 and AgNbO3 were 3.4 and 2.8 eV, respectively.
Abstract: Photophysical and photocatalytic properties of perovskite-type materials AgMO3 (M: Ta and Nb) consisting of Ag+ and d0 ions were investigated. The band gaps of AgTaO3 and AgNbO3 were 3.4 and 2.8 eV, respectively, being 0.6 eV smaller than the band gaps of NaTaO3 (4.0 eV) and NaNbO3, even if the crystal structures of AgMO3 were similar to those of NaMO3. It was found from the electronic band structure study, using the plane-wave-based density functional method, that a hybrid orbital of Ag 4d and O 2p formed a valence band at a more negative level than O 2p orbitals. AgTaO3 showed photocatalytic activity for pure water splitting into H2 and O2 under UV-light irradiation. AgNbO3 has arisen as a new visible-light-driven photocatalyst possessing the ability to evolve H2 or O2 from water in the presence of sacrificial reagents.
TL;DR: Using model Hamiltonian and ab initio band structure methods it is demonstrated that even large band gap nonmagnetic materials as simple as CaO with a small concentration of Ca vacancies can exhibit extraordinary properties.
Abstract: We introduce a path to a possibly new class of magnetic materials whose properties are determined entirely by the presence of a low concentration of specific point defects. Using model Hamiltonian and ab initio band structure methods we demonstrate that even large band gap nonmagnetic materials as simple as CaO with a small concentration of Ca vacancies can exhibit extraordinary properties. We show that such defects will initially bind the introduced charge carriers at neighboring sites and depending on the internal symmetry of the clusters so formed, will exhibit "local" magnetic moments which for concentrations as low as 3% transform this nonmagnetic insulator into a half-metallic ferromagnet.
TL;DR: In this article, infrared reflection experiments were performed on wurtzite InN films with a range of free-electron concentrations grown by molecular-beam epitaxy, and the results showed a pronounced increase in the electron effective mass with increasing electron concentration, indicating a nonparabolic conduction band in InN.
Abstract: Infrared reflection experiments were performed on wurtzite InN films with a range of free-electron concentrations grown by molecular-beam epitaxy. Measurements of the plasma edge frequencies were used to determine electron effective masses. The results show a pronounced increase in the electron effective mass with increasing electron concentration, indicating a nonparabolic conduction band in InN. We have also found a large Burstein-Moss shift of the fundamental band gap. The observed effects are quantitatively described by the kip interaction within the two-band Kane model of narrow-gap semiconductors.
TL;DR: Optical measurements of solid hydrogen up to a pressure of 320 GPa at 100 K predict that metal hydrogen should be observed at about 450’GPa when the direct gap closes, at odds with the most recent theoretical calculations that predict much larger direct transition energies and the closure of an indirect gap.
Abstract: The quest for metallic hydrogen at high pressures represents a longstanding problem in condensed matter physics1,2. Recent calculations3,4,5,6 have predicted that solid hydrogen should become a molecular metal at pressures above 300 GPa, before transforming into an alkali metal; but the strong quantum nature of the problem makes the predictions difficult. Over a decade ago, an optical study7 of hydrogen was made using a diamond anvil cell to reach 250 GPa. However, despite many subsequent efforts, quantitative studies8,9,10,11 at higher pressures have proved difficult and their conclusions controversial. Here we report optical measurements of solid hydrogen up to a pressure of 320 GPa at 100 K. The vibron signature of the H2 molecule persists to at least 316 GPa; no structural changes are detected above 160 GPa, and solid hydrogen is observed to turn completely opaque at 320 GPa. We measure the absorption edge of hydrogen above 300 GPa, observing features characteristic of a direct electronic bandgap. This is at odds with the most recent theoretical calculations that predict much larger direct transition energies and the closure of an indirect gap3,4,5,6. We predict that metal hydrogen should be observed at about 450 GPa when the direct gap closes.
TL;DR: In this paper, the curvature of the nanotube wall is found not only to reduce the band gap of the tubes by hybridization, but also to alter the energies of the electronic states responsible for transitions in the visible energy range.
Abstract: We study the electronic dispersion in chiral and achiral isolated nanotubes as well as in carbon nanotube bundles. The curvature of the nanotube wall is found not only to reduce the band gap of the tubes by hybridization, but also to alter the energies of the electronic states responsible for transitions in the visible energy range. Even for nanotubes with larger diameters (1--1.5 nm) a shift of the energy levels of $\ensuremath{\approx}100 \mathrm{meV}$ is obtained in our ab initio calculations. Bundling of the tubes to ropes results in a further decrease of the energy gap in semiconducting nanotubes; the bundle of (10,0) nanotubes is even found to be metallic. The intratube dispersion, which is on the order of 100 meV, is expected to significantly broaden the density of states and the optical absorption bands in bundled tubes. We compare our results to scanning tunneling microscopy and Raman experiments, and discuss the limits of the tight-binding model including only $\ensuremath{\pi}$ orbitals of graphene.
TL;DR: In this article, the authors developed an effective medium description of a two-dimensional photonic band-gap medium composed of dielectric cylinders of large Dielectric constant, and derived an effective permittivity and permeability for the composite.
Abstract: We develop an effective medium description of a two-dimensional photonic band-gap medium composed of dielectric cylinders of large dielectric constant. Using the transfer matrix method we have calculated reflection coefficients for a slab of the composite and plane-wave incidence, as well as the (complex) wavevector for the infinite system. From these quantities we derive an effective permittivity and permeability for the composite. In the case of p-polarized incidence the composite displays a negative magnetic permeability at microwave frequencies due to single-scatterer resonances in the medium.
TL;DR: In this paper, a far ultraviolet (UV) spectroscopic ellipsometer system working up to 9 eV has been developed, and applied to characterize high-K-dielectric materials.
Abstract: A far ultraviolet (UV) spectroscopic ellipsometer system working up to 9 eV has been developed, and applied to characterize high-K-dielectric materials. These materials have been gaining greater attention as possible substitutes for SiO2 as gate dielectrics in aggressively scaled silicon devices. The optical properties of four representative high-K bulk crystalline dielectrics, LaAlO3, Y2O3-stabilized HfO2 (Y2O3)0.15–(HfO2)0.85, GdScO3, and SmScO3, were investigated with far UV spectroscopic ellipsometry and visible-near UV optical transmission measurements. Optical dielectric functions and optical band gap energies for these materials are obtained from these studies. The spectroscopic data have been interpreted in terms of a universal electronic structure energy scheme developed form ab initio quantum chemical calculations. The spectroscopic data and results provide information that is needed to select viable alternative dielectric candidate materials with adequate band gaps, and conduction and valence b...
TL;DR: In this paper, the band anticrossing model is extended over the entire Brillouin zone to explain the pressure behaviour of the lowest conduction band minimum in GaP1−xNx.
Abstract: In this paper we review the basic theoretical aspects as well as some important experimental results of the band anticrossing effects in highly electronegativity-mismatched semiconductor alloys, such as GaAs1−xNx and InyGa1−yAs1−xNx. The many-impurity Anderson model treated in the coherent potential approximation is applied to these semiconductor alloys, in which metallic anion atoms are partially substituted by a highly electronegative element at low concentrations. Analytical solutions of the Green's function provide dispersion relations and state broadenings for the restructured conduction bands. The solutions also lead to the physically intuitive and widely used two-level band anticrossing model. Significant experimental observations, including large bandgap reduction, great electron effective mass enhancement and unusual pressure behaviour of the bandgap, are compared with the predictions of the band anticrossing model. The band anticrossing model is extended over the entire Brillouin zone to explain the pressure behaviour of the lowest conduction band minimum in GaP1−xNx. Finally, we show that the band anticrossing can also account for the large bandgap bowing parameters observed in GaAsxSb1−x, InAsySb1−y and GaPxSb1−x alloys.
TL;DR: AgInZn7S9 with a 2.3 eV band gap, which was a solid solution of AgInS2 and ZnS semiconductors, showed photocatalytic activity for H2 evolution from an aqueous solution containing sacrificial reagents SO3(2-) and S2- under visible light irradiation even without a Pt cocatalyst.
TL;DR: It is shown that both properties are results of a large disparity between the fundamental gap and the apparent optical gap, a finding that could lead to a breakthrough in the study of bipolarly dopable wide-gap semiconductor oxides.
Abstract: Doping wide-gap materials p type is highly desirable but often difficult. This makes the recent discovery of p-type delafossite oxides, CuM(III)O2, very attractive. The CuM(III)O2 also show unique and unexplained physical properties: Increasing band gap from M(III) = Al,Ga, to In, not seen in conventional semiconductors. The largest gap CuInO2 can be mysteriously doped both n and p type but not the smaller gaps CuAlO2 and CuGaO2. Here, we show that both properties are results of a large disparity between the fundamental gap and the apparent optical gap, a finding that could lead to a breakthrough in the study of bipolarly dopable wide-gap semiconductor oxides.
TL;DR: In this paper, high-resolution x-ray photoelectron spectroscopy (XPS) was applied to characterize the electronic structures for a series of high-k materials (HfO2)x(Al2O3)1−x grown on (100) Si substrate with different HfO 2 mole fraction x.
Abstract: High-resolution x-ray photoelectron spectroscopy (XPS) was applied to characterize the electronic structures for a series of high-k materials (HfO2)x(Al2O3)1−x grown on (100) Si substrate with different HfO2 mole fraction x. Al 2p, Hf 4f, O 1s core levels spectra, valence band spectra, and O 1s energy loss all show continuous changes with x in (HfO2)x(Al2O3)1−x. These data are used to estimate the energy gap (Eg) for (HfO2)x(Al2O3)1−x, the valence band offset (ΔEν) and the conduction band offset (ΔEc) between (HfO2)x(Al2O3)1−x and the (100) Si substrate. Our XPS results demonstrate that the values of Eg, ΔEν, and ΔEc for (HfO2)x(Al2O3)1−x change linearly with x.
TL;DR: In this paper, a Cadmium-free high efficiency Cu(In, Ga)Se2 (CIGS) thin-film solar cells have been fabricated using chemical bath deposition (CBD)-ZnS buffer layers.
Abstract: Cadmium-free high efficiency Cu(In, Ga)Se2 (CIGS) thin-film solar cells have been fabricated using chemical bath deposition (CBD)-ZnS buffer layers. The use of CBD-ZnS, which is a wider band gap material than CBD-CdS, improved the quantum efficiency at short wavelengths, resulting in an increase in the short circuit current of fabricated solar cells. The best cell at present yielded an active area efficiency of 18.1%, which is the highest value reported previously for CIGS thin film solar cells with alternative buffer layers to CdS.
TL;DR: In this article, a survey of optical absorption, photoluminescence and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented.
Abstract: A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 × 10 18 and 4 × 10 19 cm -3 . It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich In x Ga 1-x N alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b ∼ 2.5 eV allows one to reconcile our results and the literature data for the band gap of In x Ga 1-x N alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples. At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN-In 2 O 3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.
TL;DR: In this paper, the optical properties of the samples were measured by spectroscopic ellipsometry at room temperature in the 1.5-5 eV photon energy region and the optical band gap absorption edge (E0) of the alloys showed a redshift from that of pure ZnO, reaching 350 meV for x = 0.22.
Abstract: Zn1−xCoxO (x⩽0.22) films were prepared on (0001)-oriented Al2O3 substrates by rf magnetron sputtering. The alloys show wurtzite crystal structure with the c-axis lattice constant increasing with increasing x. The optical properties of the samples were measured by spectroscopic ellipsometry at room temperature in the 1.5–5 eV photon energy region. As x increases, the optical band gap absorption edge (E0) of the alloys shows a redshift from that of pure ZnO, reaching 350 meV for x=0.22. The excitonic character of the E0 edge is gradually reduced as x increases and is replaced by the three-dimensional critical-point shape. Optical absorption structures are also observed below the E0 edge near 2 eV and interpreted as due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ ions substituting Zn2+ ions.