Indirect Band Gap Transition Research Articles

Strain Effects on Properties of Phosphorene and Phosphorene Nanoribbons: a DFT and Tight Binding Study**Supported by the National Natural Science Foundation of China under Grant Nos 51572219 and 11447030, the Natural Science Foundation of Shaanxi Province of China under Grant Nos 2014JM2-1008 and 2015JM1018, and the State Key Laboratory of Transient Optics and Photonics Technology 2015 Annual Open Fund under

We perform comprehensive density functional theory calculations of strain effect on electronic structure of black phosphorus (BP) and on BP nanoribbons. Both uniaxial and biaxial strain are applied, and the dramatic change of BP’s band structure is observed. Under 0–8% uniaxial strain, the band gap can be modulated in the range of 0.55–1.06 eV, and a direct–indirect band gap transition causes strain over 4% in the y direction. Under 0–8% biaxial strain, the band gap can be modulated in the range of 0.35–1.09 eV, and the band gap maintains directly. Applying strain to BP nanoribbon, the band gap value reduces or enlarges markedly either zigzag nanoribbon or armchair nanoribbon. Analyzing the orbital composition and using a tight-binding model we ascribe this band gap behavior to the competition between effects of different bond lengths on band gap. These results would enhance our understanding on strain effects on properties of BP and phosphorene nanoribbon.

First-Principles Investigation of Phase Stability, Electronic Structure and Optical Properties of MgZnO Monolayer.

MgZnO bulk has attracted much attention as candidates for application in optoelectronic devices in the blue and ultraviolet region. However, there has been no reported study regarding two-dimensional MgZnO monolayer in spite of its unique properties due to quantum confinement effect. Here, using density functional theory calculations, we investigated the phase stability, electronic structure and optical properties of MgxZn1−xO monolayer with Mg concentration x range from 0 to 1. Our calculations show that MgZnO monolayer remains the graphene-like structure with various Mg concentrations. The phase segregation occurring in bulk systems has not been observed in the monolayer due to size effect, which is advantageous for application. Moreover, MgZnO monolayer exhibits interesting tuning of electronic structure and optical properties with Mg concentration. The band gap increases with increasing Mg concentration. More interestingly, a direct to indirect band gap transition is observed for MgZnO monolayer when Mg concentration is higher than 75 at %. We also predict that Mg doping leads to a blue shift of the optical absorption peaks. Our results may provide guidance for designing the growth process and potential application of MgZnO monolayer.

Structural and Optical Characteristics of GeSn Quantum Wells for Silicon-Based Mid-Infrared Optoelectronic Applications

This paper reports the study of Ge0.95Sn0.05/Ge0.91Sn0.09/Ge0.95Sn0.05 single quantum well (SQW) and double quantum wells (DQWs). The quantum well (QW) structures were grown on Ge buffered Si substrates using an industrial standard reduced-pressure chemical vapor deposition system. Pseudomorphically grown structures were observed using x-ray diffraction measurements. Defect-free interfaces between each layer were revealed using cross-sectional transmission electron microscopy. Atomic-scale high-resolution transmission electron microscopy and Fourier transform patterns exhibited the high crystalline quality of QWs. Temperature-dependent photoluminescence (PL) was performed, and the emission peaks attributed to the QW region were identified. The dominant optical transition changed from direct bandgap transition at 300 K to indirect bandgap transition at 10 K. Theoretical calculation showed the type-I band alignment for the QWs. Moreover, the Γ and L valley electron distributions and non-radiative lifetimes were evaluated, which further explained the PL characteristics of the QW samples.

Effects of oxygen contamination on monolayer GeSe: A computational study

Natural oxidation is a common degradation mechanism of both mechanical and electronic properties for most of the new two-dimensional materials. From another perspective, controlled oxidation is an option to tune material properties, expanding possibilities for real-world applications. Understanding the electronic structure modifications induced by oxidation is highly desirable for new materials like monolayer GeSe, which is a new candidate for near-infrared photodetectors. By means of first-principles calculations, we study the influence of oxygen defects on the structure and electronic properties of the single layer GeSe. Our calculations show that the oxidation is an exothermic process, and it is nucleated in the germanium sites. The oxidation can cause severe local deformations on the monolayer GeSe structure and introduces a deep state in the bandgap or a shallow state near the conduction band edge. Furthermore, the oxidation increases the bandgap by up to 23 %, and may induce direct to indirect bandgap transitions. These results suggest that the natural or intentionally induced monolayer GeSe oxidation can be a source of new optoelectronic properties, adding another important building block to the two-dimensional layered materials.

Exploring Direct to Indirect Bandgap Transition in Silicon Nanowires: Size Effect

We have investigated the electronic band structure of [110] silicon nanowires (SiNWs) using first-principles calculations. We find that, in the ultrathin diameter regime, SiNWs have a direct bandgap, but the energy difference between the indirect and direct fundamental bandgaps decreases as the nanowire diameter increases. This indicates that larger [110] SiNWs could have an indirect bandgap. Fundamentally, a series of quantitative direct–indirect bandgap transitional diameters are obtained for different cross-sectional geometries, with the largest values for SiNWs with triangular cross section.

Systematic study of Ge1−xSnx absorption coefficient and refractive index for the device applications of Si-based optoelectronics

The absorption coefficient and refractive index of Ge1−xSnx alloys (x from 0% to 10%) were characterized for the wavelength range from 1500 to 2500 nm via spectroscopic ellipsometry at room temperature. By applying physical models to fit the obtained data, two empirical formulae with extracted constants and coefficients were developed: (1) Absorption coefficient. The absorption regarding Urbach tail, indirect and direct bandgap transitions were comprehensively taken into account; (2) refractive index. The Sellmeier coefficients associated with dispersion relationship were extracted. In these formulae, the Sn composition and strain percentage were the input parameters, by inputting which the spectral absorption coefficient and spectral refractive index can be obtained. Since the absorption coefficient is key information to determine the performance of the photodetectors including operation wavelength range, responsivity, and specific detectivity, and the refractive index is very useful for the design of the anti-reflection coating for photodetectors and the layer structure for waveguides, the developed formulae could simplify the optoelectronic device design process due to their parameter-based expressions.

Novel optical properties of MoS2 on monolayer zinc tellurium substrate

The electronic structure and optical properties of the MoS2/ZnTe monolayer have been investigated through first-principles calculations. According to the different atoms’ parallelism, we have discussed five interface growth configurations: Te–Mo, Te–S, Hollow, Zn–Mo, and Zn–S systems. It has been shown that the monolayer MoS2/ZnTe exhibits semiconducting behavior and the band gap of MoS2 can be effectively tuned to 1.60 or 1.28 eV in MoS2 and ZnTe heterobilayer. An unexpected direct–indirect band gap transition is also observed, which is dependent on the stacking pattern and interlayer spacing on MoS2. The optical properties of MoS2/ZnTe heterobilayer reflect the phenomenon of red shift. The absorption edge of pure monolayer molybdenum disulfide is 0.8 eV, while the absorption edges of Te–Mo, Te–S, Hollow, Zn–Mo, and Zn–S systems become 0 eV. As a potential material, these provide a possible way to design effective FETs out of MoS2 on a ZnTe monolayer.

SiyGe1−x−ySnx films grown on Si using a cold-wall ultrahigh-vacuum chemical vapor deposition system

Silicon germanium tin alloys were grown directly on Si substrates using a cold-wall ultrahigh-vacuum chemical vapor deposition system at 300 °C, where commercially available precursors of silane, germane, and stannic chloride were used to grow the epitaxial layers. The crystallinity and growth quality of the SiyGe1−x−ySnx films were investigated through material characterization methods including x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. Rutherford backscattering measurements show that 2%–5% of the Sn and 3%–5% of the Si were successfully incorporated. Investigation of the material growth parameters shows that a flow rate of stannic chloride higher than 1 sccm results in etching of the film, while an increase in the silane flow rate results in amorphous film growth. The photoluminescence study shows clear emission peaks ascribed to direct and indirect bandgap transitions, which are in agreement with theoretical calculations.

Flux-mediated syntheses, structural characterization and low-temperature polymorphism of the p-type semiconductor Cu2Ta4O11

A new low-temperature polymorph of the copper(I)-tantalate, α-Cu2Ta4O11, has been synthesized in a molten CuCl-flux reaction at 665°C for 1h and characterized by powder X-ray diffraction Rietveld refinements (space group Cc (#9), a=10.734(1)Å, b=6.2506(3)Å, c=12.887(1)Å, β=106.070(4)°). The α-Cu2Ta4O11 phase is a lower-symmetry monoclinic polymorph of the rhombohedral Cu2Ta4O11 structure (i.e., β-Cu2Ta4O11 space group R3̅c (#167), a=6.2190(2)Å, c=37.107(1)Å), and related crystallographically by ahex=amono/√3, bhex=bmono, and chex=3cmonosinβmono. Its structure is similar to the rhombohedral β-Cu2Ta4O11 and is composed of single layers of highly-distorted and edge-shared TaO7 and TaO6 polyhedra alternating with layers of nearly linearly-coordinated Cu(I) cations and isolated TaO6 octahedra. Temperature dependent powder X-ray diffraction data show the α-Cu2Ta4O11 phase is relatively stable under vacuum at 223K and 298K, but reversibly transforms to β-Cu2Ta4O11 by at least 523K and higher temperatures. The symmetry-lowering distortions from β-Cu2Ta4O11 to α-Cu2Ta4O11 arise from the out-of-center displacements of the Ta 5d0 cations in the TaO7 pentagonal bipyramids. The UV–vis diffuse reflectance spectrum of the monoclinic α-Cu2Ta4O11 shows an indirect bandgap transition of ∼2.6eV, with the higher-energy direct transitions starting at ∼2.7eV. Photoelectrochemical measurements on polycrystalline films of α-Cu2Ta4O11 show strong cathodic photocurrents of ∼1.5mA/cm2 under AM 1.5G solar irradiation.

Strain-induced direct–indirect bandgap transition and phonon modulation in monolayer WS2

In situ strain photoluminescence (PL) and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition (CVD)-grown monolayer tungsten disulphide (WS2) under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition (K-K) and indirect bandgap transition (Γ-K, ≥2.5%), exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E’ mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide (TMD) materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

Solution phase synthesis of indium gallium phosphide alloy nanowires.

The tunable physical and electronic structure of III-V semiconductor alloys renders them uniquely useful for a variety of applications, including biological imaging, transistors, and solar energy conversion. However, their fabrication typically requires complex gas phase instrumentation or growth from high-temperature melts, which consequently limits their prospects for widespread implementation. Furthermore, the need for lattice matched growth substrates in many cases confines the composition of the materials to a narrow range that can be epitaxially grown. In this work, we present a solution phase synthesis for indium gallium phosphide (In(x)Ga(1-x)P) alloy nanowires, whose indium/gallium ratio, and consequently, physical and electronic structure, can be tuned across the entire x = 0 to x = 1 composition range. We demonstrate the evolution of structural and optical properties of the nanowires, notably the direct to indirect band gap transition, as the composition is varied from InP to GaP. Our scalable, low-temperature synthesis affords compositional, structural, and electronic tunability and can provide a route for realization of broader In(x)Ga(1-x)P applications.

Semiconductor to metal transition in bilayer phosphorene under normal compressive strain

Phosphorene, a two-dimensional analog of black phosphorous, has been a subject of immense interest recently, due to its high carrier mobilities and a tunable bandgap. So far, tunability has been predicted to be obtained with very high compressive/tensile in-plane strains, and vertical electric field, which are difficult to achieve experimentally. Here, we show using density functional theory based calculations the possibility of tuning electronic properties by applying normal compressive strain in bilayer phosphorene. A complete and fully reversible semiconductor to metal transition has been observed at strain, which can be easily realized experimentally. Furthermore, a direct to indirect bandgap transition has also been observed at strain, which is a signature of unique band-gap modulation pattern in this material. The absence of negative frequencies in phonon spectra as a function of strain demonstrates the structural integrity of the sheets at relatively higher strain range. The carrier mobilities and effective masses also do not change significantly as a function of strain, keeping the transport properties nearly unchanged. This inherent ease of tunability of electronic properties without affecting the excellent transport properties of phosphorene sheets is expected to pave way for further fundamental research leading to phosphorene-based multi-physics devices.

Probing local strain at MX(2)-metal boundaries with surface plasmon-enhanced Raman scattering.

Interactions between metal and atomically thin two-dimensional (2D) materials can exhibit interesting physical behaviors that are of both fundamental interests and technological importance. In addition to forming a metal–semiconductor Schottky junction that is critical for electrical transport, metal deposited on 2D layered materials can also generate a local mechanical strain. We investigate the local strain at the boundaries between metal (Ag, Au) nanoparticles and MX2 (M = Mo, W; X = S) layers by exploiting the strong local field enhancement at the boundary in surface plasmon-enhanced Raman scattering (SERS). We show that the local mechanical strain splits both the in-plane vibration mode E2g(1) and the out-of-plane vibration mode A1g in monolayer MoS2, and activates the in-plane mode E1g that is normally forbidden in backscattering Raman process. In comparison, the effects of mechanical strain in thicker MoS2 layers are significantly weaker. We also observe that photoluminescence from the indirect bandgap transition (when the number of layers is ≥2) is quenched with the metal deposition, while a softened and broadened shoulder peak emerges close to the original direct-bandgap transition because of the mechanical strain. The strain at metal–MX2 boundaries, which locally modifies the electronic and phonon structures of MX2, can have important effects on electrical transport through the metal–MX2 contact.

Growth and Characterization of Epitaxial Ge1-XSnx Alloys and Heterostructures Using a Commercial CVD System

Group IV alloys incorporating Sn have several possible applications ranging from optoelectronics to stress engineering in FinFET based CMOS designs. From an optoelectronics perspective the benefits of incorporating Sn into Ge is realized in the reduction of the difference between the direct and indirect band gap transitions resulting in more efficient luminescence/ absorption. Furthermore, alloys of Ge1-xSnx are theoretically expected to become a direct band gap material for Sn contents of approximately 10 at. % [1]. With regards to logic applications of Sn containing alloys the possibilities include the passive function as a common strain relieved buffer (SRB) for both NMOS and PMOS applications as well as PMOS S/D stressor for an active GeSn channel FinFET devices [2]. Given these potential application one of the key issues lies in the lack of viable growth process on a commercial deposition system.The Ge1-xSnx alloys and Ge buffer layers presented here were grown in an ASM Epsilon® 2000 Plus chemical vapor deposition (CVD) system This system is ideally suited for the low deposition temperatures required to form these metastable alloys. A specialized growth approach has been adopted to grow the Ge1-xSnx layers at temperatures of less than 450°C.Material and optical characterization of these alloys indicate that the materials are of high crystal quality. Figure 1 shows a series symmetrical (004) 2θ-ω XRD scans for Ge1-xSnx alloys with Sn contents ranging from 0.5 to 10 at. % in which the perpendicular lattice constant increases with Sn content. Rutherford Backscattering Spectroscopy (RBS) in combination with ion channeling was used to verify the thickness and composition of the alloys as well as to determine substitutionality of the Sn within the Ge lattice (Fig.1). The thickness and composition of these alloys was in close agreement with those obtained from SIMS. Further structural characterization of the Ge1-xSnx alloys was obtained by cross-sectional Transmission Electron Microscopy (XTEM) analysis. Figure 3 shows a bright field XTEM image of a Ge0.93Sn0.07/Ge/Si heterostructure in which no threading defects are observed propagating through the GeSn epilayer. Room temperature photoluminescence studies of the as-deposited alloys show a strong well-defined peak which shifts to lower energy with increasing Sn contents. Growth of heterostructures with different alloy compositions have also been demonstrated in which abrupt compositional transitions are observed. Optical devices based on these alloys have been fabricated.

Competition of optical transitions between direct and indirect bandgaps in Ge1−xSnx

Temperature-dependent photoluminescence (PL) study has been conducted in Ge1−xSnx films with Sn compositions of 0.9%, 3.2%, and 6.0% grown on Si. The competing between the direct and indirect bandgap transitions was clearly observed. The relative peak intensity of direct transition with respect to the indirect transition increases with an increase in temperature, indicating the direct transition dominates the PL at high temperature. Furthermore, as Sn composition increases, a progressive enhancement of direct transition was observed due to the reduction of direct-indirect valley separation, which experimentally confirms that the Ge1−xSnx could become the group IV-based direct bandgap material grown on Si by increasing the Sn content.

Strain and orientation modulated bandgaps and effective masses of phosphorene nanoribbons.

Passivated phosphorene nanoribbons, armchair (a-PNR), diagonal (d-PNR), and zigzag (z-PNR), were investigated using density functional theory. Z-PNRs demonstrate the greatest quantum size effect, tuning the bandgap from 1.4 to 2.6 eV when the width is reduced from 26 to 6 Å. Strain effectively tunes charge carrier transport, leading to a sudden increase in electron effective mass at +8% strain for a-PNRs or hole effective mass at +3% strain for z-PNRs, differentiating the (mh*/me*) ratio by an order of magnitude in each case. Straining of d-PNRs results in a direct to indirect band gap transition at either -7% or +5% strain and therein creates degenerate energy valleys with potential applications for valleytronics and/or photocatalysis.

Phonon-assisted transient electroluminescence in Si

The phonon-replica infrared emission is observed at room temperature from indirect band gap Si light-emitting diode under forward bias. With increasing injection current density, the broadened electroluminescence spectrum and band gap reduction are observed due to joule heating. The spectral-resolved temporal response of electroluminescence reveals the competitiveness between single (TO) and dual (TO + TA) phonon-assisted indirect band gap transitions. As compared to infrared emission with TO phonon-replica, the retarder of radiative recombination at long wavelength region (∼1.2 μm) indicates lower transition probability of dual phonon-replica before thermal equivalent.

Dependence of the Absorption and Optical Surface Plasmon Scattering of MoS2 Nanoparticles on Aspect Ratio, Size, and Media

The optical and electronic properties of suspensions of inorganic fullerene-like nanoparticles of MoS2 are studied through light absorption and zeta-potential measurements and compared to those of the corresponding microscopic platelets. The total extinction measurements show that, in addition to excitonic peaks and the indirect band gap transition, a new peak is observed at 700-800 nm. This spectral peak has not been reported previously for MoS2. Comparison of the total extinction and decoupled absorption spectrum indicates that this peak largely originates from scattering. Furthermore, the dependence of this peak on nanoparticle size, shape, and surface charge, as well as solvent refractive index, suggests that this transition arises from a plasmon resonance.

New Visible Light Absorbing Materials for Solar Fuels, Ga(Sbx)N1-X

In this work large band gap bowing of dilute antimonide alloys of gallium nitride, Ga(Sbx)N1-x has been investigated. Our computational calculations using first principles density functional theory2 revealed that a small amount of Sb incorporation is sufficient to achieve a significant band gap reduction in GaN from 3.4eV to 2eV. Theoretical calculations predicted that Ga(Sb)xN1-x alloys with 2 eV band gap straddle the electrochemical potentials of the hydrogen and oxygen evolution reactions. Theoretical computations with Sb composition beyond 7% change the electronic band gap from direct to indirect.Synthesis of crystalline GaSbxN1-x alloys were carried out using metal organic chemical vapor deposition using trimethyl gallium (TMGa) and Trimethyl Antimony (TMSb) and ammonia at x values ranging from 0-8%. Synthesis was carried out on different planar substrates and GaN nanowires. X-ray diffraction measurements showed a monotonic increase in the lattice with increase in antimonide composition which corroborates with theoretical calculations. Optical measurements like UV-Vis spectroscopy and photocurrent spectroscopy suggested a rapid decrease in band gap from 3.4 to 2 eV with small concentration of antimonide incorporation. Experimental data from optical measurements indicated direct band gap transition for alloys less than 7at% and an indirect band gap transition for alloys beyond 7% as shown in fig. 1. In addition Mott Schottky measurements showed that Ga(Sb)xN1-x alloys ranging from 0-8% straddle the water oxidation and reduction potentials in agreement to computational calculations. Moreover, the photo-electrochemical data on activity and stability suggest that these alloys are highly suitable for solar water splitting under visible light irradiation. Fig. 1 Tauc plots for direct transition and indirect transition for 2 % Sb (a,b) and 8 % Sb (c,d). Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).

Sensitive detection of surface- and size-dependent direct and indirect band gap transitions in ferritin

Ferritin is a protein nano-cage that encapsulates minerals inside an 8 nm cavity. Previous band gap measurements on the native mineral, ferrihydrite, have reported gaps as low as 1.0 eV and as high as 2.5–3.5 eV. To resolve this discrepancy we have used optical absorption spectroscopy, a well-established technique for measuring both direct and indirect band gaps. Our studies included controls on the protein nano-cage, ferritin with the native ferrihydrite mineral, and ferritin with reconstituted ferrihydrite cores of different sizes. We report measurements of an indirect band gap for native ferritin of 2.140 ± 0.015 eV (579.7 nm), with a direct transition appearing at 3.053 ± 0.005 eV (406.1 nm). We also see evidence of a defect-related state having a binding energy of 0.220 ± 0.010 eV . Reconstituted ferrihydrite minerals of different sizes were also studied and showed band gap energies which increased with decreasing size due to quantum confinement effects. Molecules that interact with the surface of the mineral core also demonstrated a small influence following trends in ligand field theory, altering the native mineral’s band gap up to 0.035 eV.