eV Gap Research Articles

High Performance Group IV Photodiodes with Tunable Absorption Edges based on Ternary SiGeSn Alloys

We describe the fabrication of ternary SiGeSn photodiodes and compare the optical and electrical properties of heterostructure devices with the same target compositions (Si: 8-11 %, Sn: 1.7-2.3 %) grown directly on Si(100) and Ge(100) substrates via UHV-CVD reactions of SnD4, Si3H8 and Ge2H6. We show that the quantum efficiency and dark current levels of near lattice-matched alloys grown on Ge wafers are superior to those of comparable Ge devices. The corresponding optoelectronic properties of analogous SiGeSn devices grown on Si are significantly degraded due to the inferior microstructure resulting from the lattice mismatch between the substrate and epilayer. Finally, we also show that minor variations in composition produce significant shifts in the absorption edge of the devices grown on Ge. Collectively the SiGeSn diodes described here exhibit enhanced performance over any previous Sn based group-IV system, underscoring the potential of this 1 eV gap material as versatile candidates for next generations of Si-based optoelectronics, including multijunction photovoltaics.

Electronic and magnetic properties of Ti2O3, Cr2O3, and Fe2O3 calculated by the screened exchange hybrid density functional

The electronic and magnetic properties of the transition metal sesqui-oxides Cr2O3, Ti2O3, and Fe2O3 have been calculated using the screened exchange (sX) hybrid density functional. This functional is found to give a band structure, bandgap, and magnetic moment in better agreement with experiment than the local density approximation (LDA) or the LDA+U methods. Ti2O3 is found to be a spin-paired insulator with a bandgap of 0.22 eV in the Ti d orbitals. Cr2O3 in its anti-ferromagnetic phase is an intermediate charge transfer Mott–Hubbard insulator with an indirect bandgap of 3.31 eV. Fe2O3, with anti-ferromagnetic order, is found to be a wide bandgap charge transfer semiconductor with a 2.41 eV gap. Interestingly sX outperforms the HSE functional for the bandgaps of these oxides.

Palladium in the Gap: Cluster Assemblies with Band Edges Localized on Linkers

Cluster-assembled materials composed of [Pd2As14]4– polyvalent anions, where a palladium dimer links two As7 clusters, have been synthesized in zero- and two-dimensional architectures. Their dimensionality was controlled through the selection of the counterions, and their structures were characterized by single-crystal X-ray diffraction. Reflectance spectroscopy measurements indicate that the zero- and two-dimensional assemblies have band gaps of 1.35 and 1.15 eV, respectively, and are smaller than the 1.80 eV gap of the As73– cluster. Theoretical investigations reveal that the frontier orbitals of the cluster building blocks are located primarily on the Pd dimer. Further, these orbitals are embedded inside the intrinsic band gap of the As73– clusters, reducing the band gap energy with edge states that are localized on the Pd sites. It is suggested that such a periodic array of narrow localized states may lead to interesting transport and optical properties as the carriers may become localized at these sites.

Electronic structure and polaronic excitation in FeVO4

We investigated the electronic properties of FeVO4 films using optical, valence band x-ray photoelectron, and infrared spectroscopies. These studies show that FeVO4 is a direct bandgap system with a 2.7 eV gap with the Fermi level in the middle of the valence band maximum and conduction band minimum. A polaronic excitation is also observed in the middle infrared, indicating the importance of charge-lattice coupling in this multiferroic material. Fits to a model for the optical response of large polarons yield a binding energy of approximately 130 meV.

Zn incorporation and (CuIn) 1− xZn 2 xSe 2 thin film formation during the selenization of evaporated Cu and In precursors on Al:ZnO coated glass substrates

CuInSe 2 thin films with typical 1.0 eV gap energy and tetragonal chalcopyrite structure have been obtained on soda–lime glass substrates by the reaction of sequentially evaporated Cu and In layers with elemental selenium vapor, at 500 °C in flowing Ar. When analogous deposition and reaction processes were performed on Al:ZnO coated glasses, some increment in the band gap energy and diminution in the crystalline interplanar spacings have been detected for the resulting films with an extent that depends on the Cu/In atomic ratio of the evaporated precursor layers. This fact has been related to Zn incorporation into the selenized film, with quaternary (CuIn) 1− x Zn 2 x Se 2 compound formation that is influenced by the presence of copper selenide phases during the reaction process. Such deductions are supported by the optical, structural and compositional characterizations that have been performed comparatively on samples prepared by selenization of evaporated metallic precursors with two different Cu/In ratios (0.9 and 1.1) on bare and Al:ZnO coated glass substrates.

The Possibility of Chemically Inert, Graphene-Based All-Carbon Electronic Devices with 0.8 eV Gap

Graphene is an interesting electronic material. However, flat monolayer graphene does not have significant gap in the electronic density of states, required for a large on-off ratio in logic applications. We propose here a novel device architecture, composed of self-folded carbon nanotube-graphene hybrids, which have been recently observed experimentally in Joule-heated graphene. These experiments demonstrated the feasibility of cutting, folding, and welding few-layer graphene in situ to form all-carbon nanostructures with complex topologies. The electronic gap of self-folded nanotubes can be combined with the semimetallicity of graphene electrodes to form a "metal-semiconductor-metal" junction. By ab initio calculations we demonstrate large energy gaps in the transmission spectra of such junctions, which preserve the intrinsic transport characteristics of the semiconducting nanotubes despite topologically necessary disinclinations at the flat graphene-curved nanotube interface. These all-carbon devices are proposed to be constructed by contact probe cutting and high-temperature annealing and, if produced, would be chemically stable at room temperature under normal gas environments.

First Principles Calculations of Optical Properties of Cu3N and Cu4N

First-principles calculations were performed to study the electronic and optical properties of Cu3N and Cu4N. The calculated density of states show that Cu3N is a semiconductor with a 0.5ev gap while Cu4N is metallic. The dielectric functions, absorption coefficients, refractive coefficients and extinction coefficients of Cu3N and Cu4N were obtained and compared with some experiment results. It shows the calculated results agree well with the experimental values.

Synthesis of boron nitride multi-walled nanotubes by laser ablation technique

Carbon nanotubes are known as prospective material for the optoelectronics, vacuum electronics, non-linear optics and for the composite material synthesis. Boron nitride nanotubes (BNNTs) were obtained only recently. It is expected that these nanotubes possess the unique properties. BN is much more chemically inert than carbon. Its oxidation temperature is above 1000°C as the graphite is totally oxidized already at 650°C. BNNTs are wide-gap semiconductors with the 6 eV gap. In case of the introducing of the carbon atoms into the BN nanotube wall its bandgap may be varied in wide range. This property is important for the UV optics.

Comment on “Curvature effects on electronic properties of small radius nanotube” [Appl. Phys. Lett. 91, 033102 (2007)

carbon nanotubes using the density functional theory/ nonequilibrium Green’s function DFT/NEGF approach. The main conclusions were that due to curvature effects, the 2,2 nanotube is a semiconducting tube with an energy gap of 0.68 eV and the 3,3 tube exhibits a negative differential conductance at a bias of 1.8 V. The purpose of this comment is to demonstrate that a different conclusion about the properties of a 2,2 tube can be reached by choosing a different basis set for the simulations. Also, the length of the electrodes used can significantly affect the results. Further, the use of a polarized basis set for a 3,3 tube indicates an extended range over which there is a negative differential conductance. Figure 1 in Ref. 1 shows the simulated structures for the two aforementioned tubes in the central region of a twoprobe system. Each of the electrodes consists of just one period 0.2463 nm of each tube. An important assumption in the DFT/NEGF approach is that the electrodes are semiinfinite, meaning that the electrodes are assumed to be repeated infinitely in the transport direction and that they have bulklike properties. 2 The short electrodes described above may well allow an interaction between the central region and the repeated images of the electrodes. To verify this, we simulated the structures with four periods of each tube in the central region and the same short electrodes using the package ATOMISTIX. 3 In Ref. 1, the basis set used is not mentioned so we started with the single-zeta SZ basis set the simplest available basis set in ATOMISTIX within the local density approximation that was used in Ref. 1. The results for the transmission coefficient as a function of energy are shown in Fig. 1, and the 0.68 eV gap of the 2,2 tube is exactly reproduced. The simulations were repeated with the same basis set but the electrode length was increased to two periods 0.4926 nm. The results are also shown in Fig. 1 and the corresponding gap is only 0.39 eV. Fixing the length of the electrodes and using the double-zeta DZ basis set gives a very narrow gap of 0.1 eV. Finally, including polarization by using both the SZ polarized SZP and the DZ polarized DZP basis sets shows a metallic tube with a zero gap rather than a semiconducting tube with a sizable bandgap, as predicted using the input parameters in Ref. 1. In fact, including polarization in the basis set is essential to accurately describe high-curvature effects. 4 Taking into account all of the above, we suggest that inclusion of polarization in the basis set is necessary for accurate simulation of very small-radius tubes, such as the 2,2. We note that the DZ set is more complete than the SZP set, 3 and it predicts a small-bandgap tube while the SZP predicts a zero-gap metallic tube, as does the DZP set. This implies that the results of the polarized sets cannot be due to any overcompleteness of the DZP set but rather are a result of the polarization per se. Moreover, barring overcompleteness in the basis set, the total energy of the system Etot should decrease using a more complete basis set, i.e., Etot is variational with the number of basis functions. 5 Indeed, Etot was 1098.188 Ry for the SZ set, 1104.014 Ry for the SZP set, 1107.034 Ry for the DZ set, and 1109.838 Ry for the DZP set. This assures that the DZP set is not overcomplete. We also calculated Etot using the DZ doublepolarized DZDP basis set and found it to be 1109.143 Ry, i.e., it is larger than that predicted by the DZP basis set. This suggests that the DZDP basis set would be an overcomplete basis set for this system.

Calculated phonon band structure and density of states and interpretation of the Raman spectrum in rocksalt ScN

As one of the simplest transition-metal nitrides, ScN is quite interesting. After this material was thought to be a semimetal for a long time,1 it was finally concluded to be a semiconductor with an indirect gap of about 1 eV and smallest direct gap of about 2 eV.2–7 It was recently found to be a potentially interesting magnetic semiconductor host by adding Mn as dopant.8,9 Its crystal growth in thin-film form was improved considerably in recent years.4,10–12 It was also predicted that alloys or superlattices with IIIa column elements Ga, Al, or In might have interesting properties involving a hexagonal form of ScN.13–15 Yet, several properties of this material are only poorly known. Here we study the vibrational properties. While calculated phonon band structures were reported previously for the hypothetical hexagonal form,14 we are only aware of one previous study of the phonons in rocksalt ScN.16 That paper does not discuss the comparison with the Raman spectra in any detail and furthermore uses a different pseudopotential from the one used here, which leads to significant differences. In the rocksalt structure, first-order Raman scattering is forbidden by symmetry. Yet, a Raman spectrum was observed and reported by Travaglini et al.1 and by Gu et al.12 It was interpreted as a disorder-induced first-order Raman spectrum representing the density of phonon states. In infrared absorption, one expects to see the TO and LO modes at the zone center because these are optically allowed, but the presence of free carriers complicates this picture. Travaglini et al.1 extracted TO to be 45 meV or 362 cm −1 from an analysis of the infrared reflectivity, which exhibits a plasmon LO-phonon-coupled mode because of the large free-carrier concentration in their samples. Here, we present phonon band structures and density of states DOS and compare carefully with the Raman data by Travaglini et al.1 We find that the latter shows not a pure but weighted density of states, with a larger intensity for the LO L modes. Several features can be accounted for with Van Hove singularities at the zone boundary. The emphasis of the LO L modes is explained in terms of their dominant role in the breathing-induced motions around the cation which exhibit the strongest electron-phonon coupling.

Influence of passivating interlayer on Ge/HfO 2 and Ge/Al 2O 3 interface band diagrams

The energy band alignment between Ge, HfO 2 and Al 2O 3 was analyzed as influenced by passivating interlayers (ILs) of different composition (GeO 2, Ge 3N 4, Si/SiO x ). From internal photoemission and photoconductivity experiments we found no IL-sensitive dipoles at the Ge/HfO 2 interfaces, the latter being universally characterized by conduction and valence band offsets of 2.1 and 3.0 eV, respectively. However, in the case of HfO 2 growth using H 2O-based atomic layer deposition, the Ge oxide IL appears to have a narrower bandgap, 4.3 eV, than the 5.4–5.9 eV gap of bulk germania. Accordingly, formation of this IL yields significantly reduced barriers for hole and, particularly, electron injection from Ge into the insulator. Changing to a H-free process for HfO 2 and Al 2O 3 deposition suppresses the formation of the narrow-gap Ge oxide.

Scanning tunnelling spectroscopy and ab initio calculations of single-walled carbonnanotubes interfaced with highly doped hydrogen-passivated Si(100) substrates

The electronic properties of isolated single-walled carbon nanotubes (SWNTs) adsorbed onton- and p-doped hydrogen-passivated Si(100) surfaces are studied by ultrahigh vacuum scanningtunnelling spectroscopy and ab initio density-functional methods. SWNTs identified assemiconductors (s-SWNTs) have well-defined conduction and valence band edges separated by a≈1 eV gap, with the mid-gap Fermi level implying that the s-SWNTs areundoped. Relative s-SWNT/H-Si(100) band alignments inferred fromdI/dV plots are sensitive to the polarity of the substrate doping. Band structure calculations for a(12,4) s-SWNT corroborate experimental data: n-type (p-type) doping of the substrateleads to a shift of the surface bands lower (higher) in energy relative to those of thes-SWNT. The adsorption energy and charge transfer calculated for the (12,4) s-SWNTphysisorbed onto H-Si(100) are considerably less than values reported for the same tube onunpassivated Si(100) and are registration independent. The atomistic results presented herehave critical implications to hybrid electronic and photonic devices that rely upon a directinterface between a SWNT and a technologically relevant semiconductor such as Si orGaAs.

Photophysical Properties of Hydroxylated Amphiphilic Poly(p-phenylene)s

A homologous series of polyhydroxylated poly(p-phenylene)s with different alkoxy groups (C6PPPOH, C12PPPOH, and C18PPPOH) were synthesized with use of the Suzuki polycondensation reaction. Comparative studies of the structure correlation between their photophysical properties and film morphology is described. The absorption and emission spectra of polymers in solution and thin films showed similar features indicating that the electronic properties in solution were retained in the film state. Compared to the polymer with the short alkoxy chains (C6PPPOH), the polymers with long alkoxy groups (C12PPPOH and C18PPPOH) showed improved film forming properties with continuous and smooth film morphology. The absorption properties of the C12PPPOH showed an enhanced effective conjugation length and high quantum yield implying planarization of the backbone through alkoxy chain packing (C12H25O-) and potential hydrogen bonds. No overlap in the absorption and emission spectra was observed, which indicated minimized excimer formation or excitation energy transfer in the films. Time-resolved fluorescence measurements showed that the decay times increased from 43 ps (C6PPPOH) to 78 ps (C12PPPOH) and 99 ps (C18PPPOH). Electrochemical studies were performed for all polymers and the observed oxidation potential for C6PPPOH was higher than that of C12PPPOH and C18PPPOH. In addition, the C12PPPOH has the lowest band gap of DeltaE = 2.59 eV when compared to the 3.1 (C6PPPOH) and 2.61 eV (C18PPPOH) gaps. The optical band gaps estimated from the absorption onset of the polymers are significantly higher than those obtained from electrochemical data. C12PPPOH was chosen for investigating the charge carrier mobility by the time-of-flight (TOF) technique. The observed results also showed negative field dependent values of the drift mobility for the polymer C12PPPOH.

Nanometer-scale spatial inhomogeneities of the chemical and electronic properties of an ion implanted Mn–Ge alloy

Electron energy loss spectroscopy maps using a transmission electron microscope were used to investigate with nanometer spatial resolution the Mn distribution of a Mn x Ge 1− x ion implanted alloy ( x ≃ 4%). Mn is fully diluted in the Ge matrix in a subsurface implanted layer, showing concentration inhomogeneities at the nm scale. In the deep implanted layers the presence of Mn rich clusters—either amorphous or in the Mn 5Ge 3 phase—is directly evidenced. Scanning Tunneling Microscopy/Spectroscopy directly shows that the Mn 5Ge 3 clusters are metallic, while those smaller and amorphous are semiconducting with 0.45 ± 0.05 eV band gap. The Ge matrix with Mn dilution is semiconducting with 0.60 ± 0.05 eV gap. Electronic structure results are compared with ab-initio calculations.

Electronic resonant images of an ion implanted Si(111) substrate observed by wavelength tunable optical second harmonic microscopy

This paper demonstrates that the spatial distribution of electronic states of an arsenic ion implanted Si(111) substrate can be observed by using a wavelength tunable second harmonic (SH) microscope in a wide photon energy range from 2ℏω=1.96to5.19eV. The contrast in the SH intensity images between the As-doped area and the nondoped area depends greatly on the SH photon energy. For 2ℏω>3eV, optical second harmonic generation (SHG) from the nondoped area was stronger than from the doped area, and the contrast was reversed for 2ℏω⩽2.33eV. The contrast in the SH intensity images was considerably different from that in the linear optical reflection images, indicating that spectroscopic SH microscopy can provide different informations on electronic levels from that associated with the linear optical response. It is suggested that the larger SH intensity from the nondoped area for 2ℏω>3eV results from the resonant SHG enhancement effect associated with the bulk Si E1 (3.4eV) and E2 (4.3eV) gaps. In the case of the doped area, a small resonant enhancement of the SH intensity was observed around 2ℏω=2.33eV. This resonance may result from an energy level created by the ion implantation.

Chromatic Dispersion in Ge-Doped SiO<sub>2</sub>-Based Single Mode Fibres due to Temperature Dependence of the Ultraviolet Absorption: Numerical and Experimental Results

The main information transmission limitation in optical communications employing SiO2 optical fibres is imposed by the optical pulses temporal broadening, resulting from the dispersive properties of the transmission medium (chromatic dispersion). Usually, this dispersion is modelled assuming a -0.4 meV/°C dependence of the 11 eV gap energy. We have monitored the 4.3 eV absorption band, with temperature and obtained a thermal dependence of -0.026 meV/°C. With this value we can predict the first and second order chromatic dispersion for a wavelength of 1550 nm.

Single crystal growth and characterization of strontium tartrate

Conditions for the growth of large and perfect single crystals of strontium tartrate trihydrate by controlled ionic diffusion of reacting ions through silica hydrogel have been optimized. The TGA/DTA thermograms reveal specific decomposition reactions. The values of several structural and physical parameters have been determined. The anisotropy in Vicker's microhardness leads to a study of fracture toughness, brittleness index and yield strength. The measurement of DC conductivity reveals hopping and band-type conduction mechanism with specific activation energy values in different temperature domains. Optical absorption study shows the existence of indirect transition at 5.20 eV and direct gap at 5.49 eV. The phonons invoked have been identified as internal vibration of strontium ions at 643 cm −1.

Ab initio investigation of the magnetic states of Ca2MnO4 and Ca2MnO3.5

Abstract An analysis of the electronic and magnetic properties of Ca 2 MnO 4 and Ca 2 MnO 3.5 is carried out within local spin density functional theory using the augmented spherical wave method. From energy differences between the hypothetic magnetic configurations both systems are found to be insulating antiferromagnets in the ground state with a ∼1 eV gap. However we identify an intermediate half metallic ferromagnetic state with the Hund’s rule expected moments for Mn IV (3 μ B ) and Mn III (4 μ B , high spin HS configuration), respectively. The latter result of moment magnitude finds support in recent experimental evidence of Mn III bismuth oxide as a ferromagnet in its ground state. This is characterized by a small density of states (DOS) magnitude of itinerant states in spin (↑) channel pointing to a metallic-like behavior as it is experimentally evidenced. For both Ca 2 MnO 4 and Ca 2 MnO 3.5 the chemical bonding characteristics are resolved for the two spin channels. Relationship to colossal magnetoresistive compounds is proposed.

First principles search of hard materials within the SiCN ternary system

Starting from C 3N 4 and Si 3N 4 stoichiometries and from the pseudocubic model structure of the former, intermediate phases SiC 2N 4 and Si 2CN 4 are proposed and geometry optimised within density functional built pseudopotential method using both local density (LDA) and generalised gradient approximations (GGA). The ternary compounds are found to be less stable than the two binary systems but the trends in the calculated magnitudes of the bulk moduli B 0 from the fit of the E( V) curves with Birch equation of state: B 0 (SiC 2N 4)=334.5 GPa and B 0 (Si 2CN 4)=270.3 GPa can be interpolated from those of the two extreme compounds: B 0 (C 3N 4)=424.1 GPa and B 0 (Si 3N 4)=219.8 GPa. This translates the chemical role of the substituting element on one hand and allows validating Cohen's semiempirical law relating B 0 to the inverse powers of the average interatomic distances on the other hand. From a mismatch of the chemical bonding in Si(C)NC(Si) chain observed by the electron localisation function (ELF) plot we propose an interpretation for the instability of the intermediate ternary phases. The electronic structure (density of states and band structures) obtained from augmented spherical wave (ASW) calculations of the relaxed structures point to semiconducting behaviour with smaller band gaps for the intermediate phases (∼2 eV, compared with the ∼4 eV gap of binaries).

Electrical properties of short DNA oligomers characterized by conducting atomic force microscopy

Complementary, single-strands of DNA (ssDNA), one bound to a gold electrode and the other to a gold nanoparticle were hybridized on the surface to form a self-assembled, dsDNA bridge between the two gold contacts. The adsorption of a ssDNA monolayer at each gold interface eliminates non-specific interactions of the dsDNA with the surface, allowing bridge formation only upon hybridization. The technique used, in addition to providing a good electrical contact, offers topographical contrast between the gold nanoparticles and the non-hybridized surface and enables accurate location of the bridge for the electrical measurements. Reproducible AFM conductivity measurements have been performed and significant qualitative differences were detected between conductivity in single- and double-strand DNA. The ssDNA was found to be insulating over a 4 eV range between ±2 V under the studied conditions, while the dsDNA, bound to the gold nanoparticle, behaves like a wide band gap semiconductor and passes significant current outside of a 3 eV gap.