Hole Band Structure Research Articles

Enhanced visible light photocatalytic activity of SDBS/C/Nd/TiO2 synthesized by using anionic surfactant SDBS as template agent

<p>The catalyst SDBS/C/Nd/TiO2 was synthesized by sol-gel method using TiO2 co-doped with C and Nd and anionic surfactant SDBS as template agent. The prepared photocatalysts were characterized by X-ray diffraction (XRD), scanning electron (SEM), X-ray photoelectron spectrometer (XPS) and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and so on. Experimental results show that the crystal structure of the catalytic material is anatase type. Since doped elements change the energy band structure and electronic structure of TiO2, photogenerated electrons and holes can transition through the intermediate energy level, resulting in a decrease in the required excitation energy, thereby increasing the distance of its absorption edge moving toward the long wave direction, and meanwhile improving the utilization rate of visible light. The bandgap energy of the catalyst SDBS/C/Nd/TiO2 was 2.78 eV. Compared to pure TiO2 (3.1 eV), the band gap is reduced by 0.32eV. The photocatalytic experiments show that the degradation rate of the catalyst SDBS/C/Nd/TiO2 for methyl orange solution can reach 67%, and the catalytic performance is much improved than that of pure TiO2.</p>

Ytterbium doping reduces the dark current of UV photoelectric detector based on TiO2

Improved performance of TiO2-based UV photodetectors needs functional improvements in materials. Doping can modulate the electron and hole concentration and band structure of materials. This work shows that doping Yb into TiO2 improves the performance of TiO2-based UV photodetectors. It is found experimentally that doping Yb can cause Yb ions to replace Ti ions in lattice of TiO2, which is conducive to adjusting the physical and chemical properties from the perspective of energy band engineering. The dark current of the detector was less than 1 nA at bias of 5 V. Under UV radiation, the device achieved a maximum switching rate of 3.64 ⅹ 103 when the doping concentration of Yb was 5%, and the recovery time dropped from 1.4 s to 0.65 s.

Research Progress Over Cu2O/n-type Semiconductor Composites in Photocatalysis

Photocatalysis is a feasible technology to solve energy shortage and environmental pollution by using solar energy. Semiconductor photocatalysts with low cost, high stability and environmental friendliness are demonstrated advantages for the production of solar fuel, CO<sub>2</sub> reduction, and degradation of pollutants. Among them, Cu<sub>2</sub>O presents numerous potential for photocatalysis because of its narrow band gap and high activity under visible light. However, the rapid recombination of photoinduced electron-hole pairs and the instability of Cu<sub>2</sub>O under light irradiation limit its photocatalytic performance. In order to solve the above issues, researchers prefer to incorporate Cu<sub>2</sub>O with n-type semiconductors to design p-n heterojunction composites, thus regulating the band structure, promoting the separation and transfer of electrons and holes, and accelerating the redox reaction onto the surface. In this manuscript, the preparation methods of Cu<sub>2</sub>O/n-type semiconductor composites such as hydrothermal method, electrodeposition method, and in situ method are concluded, the photocatalytic applications including CO<sub>2</sub> reduction, hydrogen production, and degradation are presented, and the catalytic mechanism like Z-scheme, p-n heterojunction, etc. are discussed, respectively. This review also proposes that there are still challenges in broadening the photocatalytic application of Cu<sub>2</sub>O/n-type semiconductor composites.

The overlap of electron and hole wavefunctions in the InxGa1-xN/GaN graded quantum well LED is much superior to the symmetrically staggered: Even to that of a trapezoidal quantum well

The strong polarization fields in the InxGa1-xN/GaN quantum well (QW) LEDs, separates the electrons and holes spatially, which decreases the optical performance. Various non-square indium profiles of such QWs are studied to improve the overlap of electron and hole wavefunctions. We have investigated the overlap of electron and hole wave functions, band structures, field distributions and hole concentration of the symmetrically staggered, trapezoidal and graded QW LEDs, in the emission of the blue spectral region. The striking new results are presented with suitable depictions and discussions. In the trapezoidal and graded QW structures, the overlap of electron and hole wavefunctions is significantly increased as compared to the symmetrically staggered QW. The overlap of electron and hole wavefunctions of the graded structure is even higher than the trapezoidal QW. The best results are obtained for the graded structure, where the overlap function and hole concentration are significantly increased as compared to other QW structures. An important feature emerges from the graded structure that the change of the peak emission energy with increase in the current density is minimized.

The electron–hole overlap in the parabolic quantum well light emitting diode is much superior to the rectangular: even to that of a staggered quantum well

The strong piezoelectric field in the InxGa1−xN/GaN quantum well (QW) LEDs, separates the electrons and holes spatially, which decreases the luminescence. Various shapes and compositions of such QWs are studied to improve the performance. We have studied the transition energy (TE), overlap of electron and hole wave functions, band structures and field distributions of the parabolic QWs (PQW) through the self consistent solutions of Schrodinger and Poisson equations. The shape of the PQW is varied along with the compositions and dopings. The square of the overlap of electron and hole wave functions i.e. the transition probability (TP) is strikingly increased, compared to the rectangular QW and it is even higher than the symmetrically staggered QW. At a particular current density, for the same TE, the TP of the PQW increases more than two times that of the rectangular QWs. An important feature, desirable for the QW LEDs emerge. The change of the TE with increase in the current density is minimized. A brief theory, computational procedures and the results will be presented in details with suitable discussions.

Novel TiO2 prepared from titanyl sulphate by using pressurized water processing and its photocatalytic activity evaluation

TiO2 anatase was prepared by a novel preparation approach, combining utilization of titanyl sulphate as a precursor and pressurized water processing. In addition, two different chemical methods for preparation were used – thermal hydrolysis and induced hydrolysis. The photoactivity of developed TiO2 was examined in two environmentally-prospective reactions: azo-dye AO7 photodiscoloration (oxidation) and N2O photodecomposition (reduction). It was revealed that during the processing the temperature has a crucial effect on TiO2 crystallization contrary to pressure. TiO2 anatase of smaller crystallite-size showed significantly higher photoactivity in AO7 photodiscoloration than TiO2 anatase of large crystallite-size. However, in N2O photodecomposition all synthesized materials showed comparable activity. While the TiO2 anatase crystallite-size and surface area are the determining properties in azo-dye AO7 photodiscoloration, the electronic band structure and recombination rate of electron and holes are determining in N2O photodecomposition. The correlation enabling the estimation of TiO2 anatase crystallite-size from Raman FWHMs was found as well.

Optical Functions, Band Structure and Effective Masses of Electrons and Holes in InGaTe<sub>2</sub>

This paper has presented results of calculations of optical functions for e||c and e^c polarizations in 0 - 12 eV energy interval, band structure and effective masses of electrons and holes of ternary compound InGaTe2. Genesis of valence band was investigated by using group-theoretical analyses. The main features of spectra of optical functions, the parameters of transition and their theoretical nature were found out. Identified interband transitions are responsible for the main peaks in the optical functions. Calculated results are in good agreement with the known experimental data.

Investigation of surface plasmon coupling with the quantum well for reducing efficiency droop in GaN-based light emitting diodes

The spontaneous emission rate into Surface Plasmon Polariton (SPP) mode for the InGaN/GaN quantum well (QW) with SP coupling is presented taking into account the electron and hole band structures, the photon density of states, and evanescent fields of SPP. The optical properties of SP-enhanced InGaN QW structure with different QW layer number are investigated in detail by using the formula. It is observed that the energy of electron-hole pairs in the InGaN QW can be efficiently transferred into the SPP modes which will induce the significantly enhancement of the internal quantum efficiency (IQE) of SP-enhanced light emitting diodes (LEDs), especially for the original IQE in the range of 6%–25%. Furthermore, the distribution of electron and hole densities in each well layer can evidently affect the Purcell enhancement factor due to the distance dependence of the intensity of SP-QW coupling. The numerical results also indicate that the SP-enhanced LED can suppress the efficiency droop effect as long as the extraction efficiency of SPP mode is enough large.

In-plane mapping of buried InGaAs quantum rings and hybridization effects on the electronic structure

This work reports the investigation on the structural differences between InAs quantum rings and their precursor quantum dots species as well as on the presence of piezoelectric fields and asymmetries in these nanostructures. The experimental results show significant reduction in the ring dimensions when the sizes of capped and uncapped ring and dot samples are compared. The iso-lattice parameter mapped by grazing-incidence x-ray diffraction has revealed the lateral extent of strained regions in the buried rings. A comparison between strain and composition of dot and ring structures allows inferring on how the ring formation and its final configuration may affect optical response parameters. Based on the experimental observations, a discussion has been introduced on the effective potential profile to emulate theoretically the ring-shape confinement. The effects of confinement and strain field modulation on electron and hole band structures are simulated by a multiband k.p calculation.

Hole subbands in freestanding nanowires: six-band versus eight-band k⋅p modelling

The electronic structure of GaAs, InAs and InSb nanowires is studied using the six-band and the eight-band k⋅p models. The effect of the different Luttinger-like parameters (in the eight-band model) on the hole band structure is investigated. Although GaAs nanostructures are often treated within a six-band model because of the large bandgap, it is shown that an eight-band model is necessary for a correct description of its hole spectrum. The camel-back structure usually found in the six-band model is not always present in the eight-band model. This camel-back structure depends on the interaction between light and heavy holes, especially the ones with opposite spin. The latter effect is less pronounced in an eight-band model, but could be very sensitive to the Kane inter-band energy (EP) value.

On-State Performance Enhancement and Channel-Direction-Dependent Performance of a Biaxial Compressive Strained $ \hbox{Si}_{0.5}\hbox{Ge}_{0.5}$ Quantum-Well pMOSFET Along $ \langle \hbox{110} \rangle$ and $\langle \hbox{100} \rangle$ Channel Directions

pMOSFET performance of high Ge content (~50%) biaxial compressive strained SiGe heterostructure channel pMOSFETs is characterized, and performance between 〈110 〉 and 〈100 〉 channel orientations on a (001) substrate is compared for physical channel lengths down to ~80 nm. Temperature-dependent mobility and velocity are characterized for both channel directions. First, it is shown that high Ge content SiGe-based channels can deliver drive current enhancement over unstrained Si below sub-100-nm channel lengths. Second, it is found that, with a higher Ge content SiGe channel under biaxial compressive strain, there is a difference of drive current between 〈110 〉 and 〈100 〉 channel directions, and the difference increases when temperature is lowered and/or when channel length is scaled down. An external series resistance difference is detected between two channel directions, although it appears to be insufficient to explain all the direction-dependent drive current difference. Channel transport behavior in different channel orientations can be clearly observed with low external source/drain (S/D) series resistance achieved with a millisecond S/D dopant activation anneal process while controlling the thermal budget. Two possibilities have been investigated to understand channel-direction-dependent performance: possible differences in effects of device processing impact between two channel directions and anisotropic transport effects from an anisotropic hole band structure, particularly under biaxial compressive strain in a SiGe channel pseudomorphically grown on a Si substrate.

Spin injection and circular polarized electroluminescence from InAs-based spin-light emitting diode structures

We have investigated circularly polarized electroluminescence (EL) from hybrid II-Mn-VI/III–V light emitting diodes (LED’s) at low temperatures in magnetic fields upto 10 T. Both magnetic (the Brillouin paramagnet Cd1−xMnxSe) and nonmagnetic (CdSe) injectors were studied. Electrons, spin unpolarized (n-CdSe) or spin-polarized (n-CdMnSe), were injected into wide InAs quantum wells, where they recombined with unpolarized holes injected from p-type InAs/AlAsSb layers. Detailed measurements and modeling of the circular polarization of the resulting midinfrared EL were carried out to explore and quantify the additional complexities of this materials system compared with the extensively studied GaAs-based spin-LED structures. We show that optical and spin polarization in narrow gap semiconductors such as InAs are not simply related to each other. To analyze the complex relationship, we have developed and used a detailed rate equation model, which incorporates the band-structure of electrons and holes in a magnetic field, a finite ratio of recombination and spin-flip times, and the spin polarization of the CdMnSe spin-aligner as a function of injection current. The latter was determined in situ by circular polarized photoluminescence measurements on the injector material. Experimentally, the circular polarization degrees of magnetic and nonmagnetic structures are observed to be very similar, when the magnetic samples have low effective Mn incorporation. This results from a combination of the consequently low spin polarization of the aligner and comparable spin and recombination life times in InAs.

BAND STRUCTURE AND ELECTRON VELOCITY MEASUREMENT IN CARBON NANOTUBES AND GRAPHENE

We discuss recent measurements of the interband spectrocopy of carbon nanotubes and studies of cyclotron resonance in graphene, using these to examine the possible dependence of the band structure of graphene on the number of layers present and the role of Coulomb interactions. Cyclotron resonances gives a value for the electron velocity at the Dirac point of 1.093×106 ms-1, which is ~ 20% larger than would be expected from deductions of the band structure of carbon nanotubes. In addition, a significant asymmetry exists between band structure for electrons and holes, which gives rise to a 5% difference between the velocities at energies of 125 meV away from the Dirac point.

Ultrafast Spin Dynamics Including Spin-Orbit Interaction in Semiconductors

This Letter presents a theoretical investigation of ultrafast spin-dependent carrier dynamics in semiconductors due to strong spin-orbit coupling using holes in bulk GaAs as a model system. By computing the microscopic carrier dynamics in the anisotropic hole-band structure including spin-orbit coupling, we obtain spin-relaxation times in quantitative agreement with measured hole-spin relaxation times [Phys. Rev. Lett. 89, 146601 (2002)10.1103/PhysRevLett.89.146601]. We show that different optical techniques for the measurement of hole-spin dynamics yield different results, in contrast to the case of electron-spin dynamics.

Significant Enhancement of Hole Mobility in [110] Silicon Nanowires Compared to Electrons and Bulk Silicon

Utilizing sp3d5s* tight-binding band structure and wave functions for electrons and holes we show that acoustic phonon limited hole mobility in [110] grown silicon nanowires (SiNWs) is greater than electron mobility. The room temperature acoustically limited hole mobility for the SiNWs considered can be as high as 2500 cm2/V s, which is nearly three times larger than the bulk acoustically limited silicon hole mobility. It is also shown that the electron and hole mobility for [110] grown SiNWs exceed those of similar diameter [100] SiNWs, with nearly 2 orders of magnitude difference for hole mobility. Since small diameter SiNWs have been seen to grow primarily along the [110] direction, results strongly suggest that these SiNWs may be useful in future electronics. Our results are also relevant to recent experiments measuring SiNW mobility.

Cyclotron resonance of electrons and holes in graphene monolayers

We report studies of cyclotron resonance in monolayer graphene. Cyclotron resonances are detected by observing changes in the photoconductive response of the sample. An electron velocity at the Dirac point of 1.093 x 10(6) m s(-1) is obtained, which is the fastest velocity recorded for all known carbon materials. In addition, a significant asymmetry exists between band structure for electrons and holes, which gives rise to a 5% difference between the velocities at energies of 125 meV away from the Dirac point.

A comparison: 2D electron- and hole systems in the fractional quantum Hall regime

We investigated two-dimensional electron- (2DES) and hole systems (2DHS) in the fractional quantum Hall regime for filling factors ν=1/3 and ν=2/3. Due to a metallic top gate we are able to vary the electron-/hole density of the samples over a wide range. Measuring activated transport on these systems with perpendicular magnetic fields up to 18 T and temperatures down to about 30 mK allows to get an insight into the excitation spectra. Although the bandstructure of holes is complicated compared to those of electrons, the theoretical description of the fractional quantum Hall effect in the composite Fermion picture should hold for both types of particles. We analyzed and compared the results for the different filling factors and systems.

A Semianalytical Description of the Hole Band Structure in Inversion Layers for the Physically Based Modeling of pMOS Transistors

This paper presents a new semianalytical model for the energy dispersion of the holes in the inversion layer of pMOS transistors. The wave vector dependence of the energy inside the 2-D subbands is described with an analytical, nonparabolic, and anisotropic expression. The procedure to extract the parameters of the model is transparent and simple, and we have used the band structure obtained with the k ldr p method to calibrate the model for silicon MOSFETs with different crystal orientations. The model is validated by calculating several transport-related quantities in the inversion layer of a heavily doped pMOSFET and by systematically comparing the results to the corresponding k ldr p calculations. Finally, we have used the newly developed band-structure model to calculate the effective mobility of pMOS transistors and compare the results with the experimental data. The overall computational complexity of our model is dramatically smaller compared to a fully numerical treatment (such as the k ldr p method); hence, our approach opens new possibilities for the physically based modeling of pMOS transistors.

Electronic band structure and effective masses of electrons and holes in the α and β phases of silicon nitride

The electronic band structure of the two main crystallographic modifications of silicon nitride, namely, the α-Si3N4 and β-Si3N4 phases, is calculated from the first principles. The estimates obtained for the effective charges of silicon and nitrogen atoms and those for the effective masses of electrons and holes in the α-Si3N4 phase are in good agreement with the available experimental data for amorphous silicon nitride a-Si3N4. The calculations performed demonstrate that the effective mass tensor determined for the β-Si3N4 phase differs substantially from the effective mass tensor obtained for the α-Si3N4 phase.

Band-edge electroluminescence of crystalline silicon heterostructure solar cells

Monocrystalline Si solar cells exhibit above 50 K intrinsic electroluminescence which increases with temperature to external efficiencies beyond 0.03%. Spectra derived from the generalized Planck equation suggest that interface recombination is responsible for this temperature dependence. However, these modeled spectra predict too large emission above the gap. Spectra derived from the bandstructure and statistics of electrons, holes and phonons show that even at room temperature light is emitted only by creation of phonons while phonon annihilation does not contribute. The analysis suggests that detailed balance of absorption and emission rates is not achieved for emission at indirect gaps.