Direct-indirect Crossover Research Articles

Band Alignment at GaxIn1–xP/AlyIn1–yP Alloy Interfaces from Hybrid Density Functional Theory Calculations

The composition dependence of the natural band alignment at the GaxIn1–xP/AlyIn1–yP alloy interface is investigated via hybrid functional based density functional theory. The direct–indirect crossover for the GaxIn1–xP and AlyIn1–yP alloys is calculated to occur for x = 0.9 and y = 0.43. The calculated GaxIn1–xP/AlyIn1–yP interface band alignment shows a crossover from type‐I to type‐II with increasing Ga content x. The valence band offset is essentially positive irrespective of the alloy compositions, and amounts up to 0.56 eV. The conduction band offset varies between −0.85 and 1.16 eV.

Interfacial optoelectronic process in organic-inorganic van der Waals hetero-structure

The tragedy of creating inorganic van der Waals heterostructure has inspired worldwide efforts to integrate various organic materials and distinct inorganic two dimensional (2D) materials to construct organic-inorganic van der walls heterostructures, which hold the great potentials for future flexible electronic and optoelectronic applications. Inorganic 2D materials, e.g., graphene and transition metal dichalcogenides (TMDs), have been extensively investigated for next-generation flexible nanoelectronics, nanophotonics and optoelectronics applications. These materials are benefit in exceptional electronic, optical and optoelectronic properties, such as high tunable optical bandgaps, high carrier mobility, direct-indirect bandgap crossover, and strong spin-orbit coupling etc., but limited in narrow absorption band, high cost and relatively difficult fabrication of high-quality single-crystals. In contrast, organic materials have many advantages, such as low cost, transparency, flexibility, light weight and easy processing, which makes them great candidates for large-area displays, solid-state lighting, sensor and organic solar cell, however, they are shorted in low dielectric constant, low carrier mobility and poor thermo-stability. By marrying the fields of organics and 2D materials, its easy to expect outstanding optoelectronic properties that are not present in either material alone. Recently, tremendous efforts have been made to explore the possible combination of organic materials and inorganic 2D materials to create optoelectronic flexible devices of organic-inorganic van der Waals heterostructures with better properties and even new functionalities that are not accessible to us in other heterostructures. Over the past few years, many research groups all over the world have shown substantial progress and excellent results have been generated from such organic-2D material heterostructures. Among them, finding the possible combination of organics and 2D materials, investigating the properties of such heterostructures, and testing the functionality of the corresponding devices are the main targets currently in this field. The optimization of such devices with excellent performance is strongly relied on a fundamental understanding of the organic-2D material interface. For instance, the band alignment at the interface of organic and 2D TMD directly determine the basic physical properties of heterostructures. However, the interfacial features of such heterostructures, e.g., interfacial charge transfer, surface screening effect, molecular doping and so on, are barely known. Its therefore vital to study the underlying physical mechanism. Here, we review the latest progress of this field first: (1) The fabrication of the organic-2D material heterostructures and the devices; (2) the performances of such devices. Substantially, the characterization methods and the related techniques are fully reviewed and discussed based on the specific demands of organic-2D material heterostructures. Three parts were included: (1) The characterization of interfacial material structure and molecular conformation; (2) interfacial electronic structure and defects; (3) carrier dynamics. The progress in this field including the utilization of several key techniques including transmission electron microscopy, scanning probe microscopy and transient absorption spectroscopy et al., are comprehensively reviewed, and the potential characterization and measurement methods are discussed in detail. Besides, the main challenges of such heterostructures in future flexible electronic and optoelectronic applications are discussed as well. We believe this review will therefore shed light on the booming the development of this field by guiding the selection of the proper materials, the creation of the desired organic-2D material heterostructures and the optimization of the devices.

Zero-point motion and direct-indirect band-gap crossover in layered transition-metal dichalcogenides

Two-dimensional transition-metal dichalcogendes $MX_2$ (es. MoS$_2$, WS$_2$, MoSe$_2$, \ldots) are among the most promising materials for bandgap engineering. Widely studied in these compounds, by means of ab-initio techniques, is the possibility of tuning the direct-indirect gap character by means of in-plane strain. In such kind of calculations however the lattice degrees of freedom are assumed to be classical and frozen. In this paper we investigate in details the dependence of the bandgap character (direct vs. indirect) on the out-of-plane distance $h$ between the two chalcogen planes in each $MX_2$ unit. Using DFT calculations, we show that the bandgap character is indeed highly sensitive on the parameter $h$, in monolayer as well as in bilayer and bulk compounds, permitting for instance the switching from indirect to direct gap and from indirect to direct gap in monolayer systems. This scenario is furthermore analyzed in the presence of quantum lattice fluctuation induced by the zero-point motion. On the basis of a quantum analysis, we argue that the direct-indirect bandgap transitions induced by the out-of-plane strain as well by the in-plane strain can be regarded more as continuous crossovers rather than as real sharp transitions. The consequences on the physical observables are discussed.

Is Graphitic Silicon Carbide (Silagraphene) Stable?

Graphene is considered to be the most likely candidate for the postsilicon era; however, the problem with its zero band gap is challenging to overcome. A close relative of silicon, silicon carbide is expected to have a stable 2D polymorph which happens to be a wide-gap semiconductor. Unfortunately, the so-called silagraphene has proven to be elusive. To date, neither theoretical nor experimental studies have been conclusive. Here, we employ computational methods to determine the stable arrangements of silagraphene and establish their accurate band structure. We also experimentally validate our models by preparing and characterizing a number of graphitic features. Silagraphene exhibits a wide spectrum of optoelectronic properties (360–690 nm) as well as an unusual band structure with highly anisotropic transport properties, which originates from its nondispersive band near its K-point. This feature makes direct-indirect gap crossover extremely sensitive to ambient conditions, making silagraphene suitable f...

Exciton states in asymmetric GaInNAs/GaAs coupled quantum wells in an applied electric field

The electronic and optical properties of exciton states in GaInNAs/GaAs coupled quantum well (CQW) structure have been theoretically investigated by solving the Schrödinger equation in real space. The effect of well width on the exciton states has been also studied by varying the well width from 5 nm to 10 nm in asymmetric structures. The electron, hole and exciton states are calculated in the presence of an applied electric field. It is found that there are two direct (bright) exciton states with the largest oscillator strengths. Their energies weakly depend on the electric field due to the compensation between the blue shift and red shift of the electron–hole pair states. In addition, these two states are overlap in the case of symmetric CQWs and one of them is then shifted to higher energy in asymmetric CQWs. The ground state exciton has the binding energy of approximately 7.3 meV and decrease to around 3.0 meV showing the direct to indirect transition of the ground state. The direct–indirect crossover is observed at different electric field for different structure. It happens at the electric field when the e1–e2 electron anticrossing or h1–h2 hole anticrossings is observed, so that the crossover can be controlled by the well width of CQWs structure.

Correlation between the structural change and the electrical transport properties of indium nitride under high pressure.

We report on the intriguing structural and electrical transport properties of compressed InN. Pronounced anomalies of the resistivity, Hall coefficient, electron concentration, and mobility are observed at ∼11.5 GPa, accompanied by a wurtzite-rocksalt structural transition confirmed using high-pressure XRD measurements and first-principles calculations. The pressure-tuned electrical properties of wurtzite and rocksalt InN are also studied, respectively. Particularly, compression pressure significantly decreases the electron concentration of rocksalt InN by two orders of magnitude and increases the mobility by ten fold. The obvious variations in electrical parameters can be rationalized by our band structure simulations, which reveal a direct-indirect energy crossover at 10 GPa, followed by the rapidly increasing patterns of the energy gap with a pressure coefficient of 33 meV GPa-1. Moreover the electron effective mass and energy gap are found to well satisfy with the k·p model. Definite correlation between the structural change and the electrical transport properties should shed a new light on building InN-based applications in the future.

Magnetic field-induced direct–indirect crossover in AlxGa1−xAs

We induce a direct–indirect band gap crossover in a single AlxGa1−xAs sample by using high magnetic fields. A reduction in the photoluminescence (PL) peak energy and onset of an asymmetric peak lineshape at magnetic fields >38 T result from a competition between the radiative recombination lifetime and the carrier migration time as the alloy becomes indirect. By analyzing the progression of the PL lineshape as the magnetic field is increased we show that the crossover is an alloy disorder-broadened, first-order phase transition.

Design of Next Generation Full Spectrum Solar Cells Using Intermediate Band Semiconductors

The intermediate band solar cell (IBSC) is an attractive concept to achieve photovoltaic conversion efficiencies exceeding the Shockley-Quiessar efficiency limit for a single junction cell. In an IBSC, a partially occupied energy band located within the bandgap region of a semiconductor serves as a ‘stepping stone’ for the absorption of lower energy photons that are nominally not absorbed by the cell. In theory, the addition of this ‘intermediate band’ would provide a high photocurrent while maintaining a high output voltage.1 Recently, an IBSC device using the highly-mismatched semiconductor GaNAs alloy (HMA’s) has demonstrated optical activity from the three energy bands and is a proof of principle for the IBSC concept.[2] It has been proposed that the GaAsPN dilute nitride alloy system is another promising candidate material for IBSC’s.3 Calculations based on the band anticrossing model4 show that the intermediate band formed by the localized N level in GaNxAsyP1-x-y alloys with y<0.7 will be narrow and completely separated from the conduction band and hence more suitable as an intermediate band semiconductor for IBSC’s.3 The location of the IB (E-) and the conduction band (E+) of this material system can be tuned more effectively for optimal absorption of the solar spectrum by changing the As/P ratio (y).. Furthermore, GaAsyP1-y exhibits a direct-indirect gap crossover at approximately y=0.5, and for alloys with compositions y>0.5 the photon absorption occurs at the lower energy Γ band, yet electron transport may take place in the indirect X band. This could result in an IBSC absorber material with a high absorption coefficient that retains long carrier lifetimes.5 This work presents the design and characterization of a GaAsPN-based IBSC device. In order to fabricate a functioning IBSC device, first p-type, and n-type GaNAsP were grown and characterized in terms of their optical and electrical properties. We found that efficient p and n-type doping of GaNAsP can be achieved with Be and Si, respectively. Next, GaAsPN-based IBSC devices were grown by gas-source molecular beam epitaxy (MBE) on GaP substrate with a thick (~1.5µm) compositionally graded GaAsP buffer layers. Both a blocked intermediate band structure, where the intermediate band is electrically isolated from the valence and conduction bands, and an unblocked intermediate band structure (as a reference structure) were considered. The design of the structure was optimized to achieve efficient charge-carrier extraction, yet still demonstrate absorption from the three possible band-to-band transitions.The external quantum efficiency, both with and without a white-light bias, was used to evaluate the spectral response of the devices and the optical activity of the intermediate band. I-V measurements using both AM 1.5 and under 30x concentration were used to evaluate the PV device performance.[1] Luque, A., Marti, A. & Stanley, C. Understanding intermediate-band solar cells. Nature Photonics 6, 146–152 (2012).[2] López, N., Reichertz, L., Yu, K.M., Campman, K. & Walukiewicz, W. Engineering the Electronic Band Structure for Multiband Solar Cells. Phys. Rev. Lett. 106, 028701 (2011).[4] Shan, W. et al. Band anticrossing in GaInNAs alloys. Phys. Rev. Lett. 82, 1221–1224 (1999).[5] Yu, K. M. et al. Multiband GaNAsP quaternary alloys. Appl. Phys. Lett 88, 092110 (2006).[6] Y. J. Kuang, et al. GaNAsP: An intermediate band semiconductor grown by gas-source molecular beam epitaxy, Appl. Phys. Lett. 102, - (2013).

Fundamental band gap and direct-indirect crossover in Ge1-x-ySixSny alloys

The Ge-like indirect band gap of Ge1−x−ySixSny alloys has been determined over an extended y > x range using photoluminescence spectroscopy from films grown on Ge-buffered Si substrates. It is found that the compositional dependence of this transition can be fit with a bilinear expression of the form (in eV): Eind=(0.668±0.008)+(0.67±0.15)x−(1.77±0.16)y. These energies are significantly below the prediction from a simple linear interpolation between Si, Ge, and α-Sn, revealing a large negative bowing in the compositional dependence similar to that found earlier for direct transitions. The direct-indirect crossover boundary is found to lie along the compositional line y = (0.062 ± 0.014) + (0.76 ± 0.23)x, which in the limit x → 0 agrees with earlier results for the binary Ge1−ySny alloy.

Next generation of Ge1−ySny (y = 0.01-0.09) alloys grown on Si(100) via Ge3H8 and SnD4: Reaction kinetics and tunable emission

Film growth and reaction kinetics studies have shown that trigermane (Ge3H8) is a superior Ge source for the epitaxial synthesis of Ge1−ySny/Si(100) alloys using ultra-high vacuum chemical vapor deposition. The Ge3H8/SnD4 combination yields 3-4 times higher growth rates than the traditional Ge2H6/SnD4 approach, with film Sn/Ge ratios reflecting the corresponding gas-phase stoichiometries much more closely. These advances have led to optical quality Ge1−ySny layers with Sn concentrations up to at least 9% and thicknesses approaching 1 μm. These thick films are found to be crucial for the observation of a strong, tunable photoluminescence signal near the threshold of the predicted direct-indirect bandgap crossover.

Direct-indirect crossover in GaxIn1-xP alloys

The energy and composition of the direct to indirect bandgap crossover in GaxIn1-xP significantly influences its potential for optoelectronic devices, such as solar cells and light emitting diodes, however considerable discrepancies still remain in the literature with regard to the precise value of the crossover composition xc. We revisit this issue in GaxIn1-xP films epitaxially grown on GaAs substrates. Observation of concurrent yet distinct direct and indirect transitions in Ga0.719In0.281P at 2 K using time integrated and time resolved photoluminescence studies places the crossover very near the composition xC = 0.71.

Electronic and structural properties of zincblende Al xIn 1− xN

Numerical calculations based on first-principles are applied to study the electronic and structural properties of ternary zincblende AlInN alloy. The results indicate the lattice constant has a small deviation from the Vegard’s law. The direct and indirect bowing parameters of 4.731 ± 0.794 eV and 0.462 ± 0.285 eV are obtained, respectively, and there is a direct–indirect crossover near the aluminum composition of 0.817. The bulk modulus is monotonically increased with an increase of the aluminum composition, and the deviation parameter of bulk modulus of 10.34 ± 9.37 GPa is obtained. On the contrary, the pressure derivative of bulk modulus is monotonically decreased with an increase of the aluminum composition.

Electronic and structural properties of zincblende AlxGa1-xN

Numerical calculations based on first-principles are applied to study the electronic and structural properties of zincblende AlxGa1-xN. The results show that the lattice constant has a very small deviation from the linear Vegard’s law. The direct and indirect bowing parameters of 0.295±0.034 eV and -0.125±0.060 eV are obtained, respectively, and there is a direct-indirect crossover near x=0.692. Besides, the bulk moduli and their pressure derivatives are monotonically increased with an increase of the aluminum composition x. The deviation parameter of the bulk modulus of -5.32±1.60 GPa is obtained.

Band-Gap Bowing Parameters of the Zincblende Ternary III–Nitrides Derived from Theoretical Simulation

The band-gap bowing parameters of unstrained zincblende ternary III–nitride alloys are investigated numerically with the CASTEP simulation program. Direct and indirect band-gap bowing parameters of 1.379 eV and 1.672 eV for InxGa1-xN, 0.755 eV and 0.296 eV for AlxGa1-xN, and 2.729 eV and 3.624 eV for AlxIn1-xN are obtained. Simulation results show that the direct band-gap energy is always smaller than its indirect counterpart for InxGa1-xN, indicating that the zincblende InxGa1-xN is a direct band-gap semiconductor. There is a direct-indirect crossover near x=0.571 for AlxGa1-xN, and x=0.244 for AlxIn1-xN. The relationship between band-gap energy and lattice constant for zincblende InxGa1-xN, AlxGa1-xN, and AlxIn1-xN is also provided.

Electronic structure and radiative lifetimes of ideal Zn 1− xBe xSe alloys

The electronic band structures of Zn 1− x Be x Se alloys are computed employing the virtual crystal approximation and empirical pseudopotentials. Pseudopotential form factors for ZnSe are fitted to experimentally determined critical point energies, and those for BeSe to generalized density functional theory computed band structures. The direct–indirect crossover alloy composition ratio is predicted to be x=0.43. At x=0.45, when Zn 1− x Be x Se is lattice matched to Si, radiative lifetimes of ideal structures are computed to be 4–6 μs over the 250–400 K temperature range.

Electron transport across bulk (Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P barriers determined from the I-V characteristics of n-i-n diodes measured between 60 and 310 K

The electron transport characteristics of five n-i-n diodes with (Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P intrinsic barrier regions of various aluminum composition x were determined from the measured I-V characteristics between 60 and 310 K. From these measurements, three different transport regimes were identified. Fowler-Nordheim tunneling was observed at temperatures below 215, 260, 110, 150, and 120 K for aluminum compositions of x=0.4, 0.5, 0.6, 0.7, and 1.0, respectively, with applied electric fields in excess of 5 MV/m. The temperature dependence of the Fowler-Nordheim tunneling currents is shown in AlGaInP for the first time with direct bandgap AlGaInP exhibiting a strong linear decrease in apparent barrier height with increasing temperature. The measured barrier height using the thermionic emission model yields values close to the expected conduction band offset between the GaInP spacer layers and the AlGaInP intrinsic barriers, as measured using high-pressure photoluminescence, and provides a novel technique for measuring the direct-indirect crossover composition in AlGaInP. It is shown that the lowest lying conduction band in AlGaInP is the dominant barrier to electron transport. This has important implications for the design of AlGaInP laser diodes.

Schottky barrier heights of In 0.5(Al xGa 1− x) 0.5P (0≤ x≤1) lattice matched to GaAs

We report a systematic investigation of the Schottky characteristics for the entire aluminum composition of In 0.5(Al x Ga 1− x ) 0.5P for the first time, using current–voltage measurements. The n-type Schottky barrier height φ bn has a minimum value of 0.85 eV at x=0 and a maximum value of about 1.0 eV at the direct–indirect crossover point ( x=0.55). With further increase of Al composition, φ bn decreases to 0.94 eV at x=1.0. The improved Schottky barrier height provides In 0.5(Al x Ga 1− x ) 0.5P FETs, such as MESFETs and HEMTs, an additional advantage for power applications due to the reduced gate leakage current. We also found that both n-type and p-type Δ φ b= φ b(In 0.5(Al x Ga 1− x ) 0.5P)− φ b(In 0.5Ga 0.5P) agree very well with their Δ E C and Δ E V, respectively, for the entire Al composition range. This general correlation can be most appropriately explained by Tersoff's “intrinsic state pinning” model.

Photoluminescence of quasi-direct transitions in disordered In/sub 1-x/Ga/sub x/P/graded GaP alloys

We have examined the photoluminescence and photoluminescence kinetics of a series of In/sub 1-x/Ga/sub x/P alloys in an effort: 1) to elucidate the electronic structure of the conduction band versus alloy composition, especially near the direct-indirect crossover; 2) to determine precisely the composition of the direct-indirect crossover, and its temperature dependence; and 3) to understand the nonradiative decay mechanism and its temperature dependence. We find that the fundamental bandgap is only determined by the /spl Gamma//sub 1c/ and X/sub 1c/ states in samples with Ga-compositions ranging from 0.58 to 0.75, and that the 2-K direct-indirect crossover from /spl Gamma//sub 1c/, to X/sub 1c/ occurs at x=0.69 and is not strongly temperature-dependent. Further, we find, in agreement with our spectroscopic ellipsometry measurements at room temperature, that the mixing near crossover is rather complicated and leads to the previous observation of quasi-direct transitions. Our combined photoluminescence and spectroscopic ellipsometry measurements have therefore clearly resolved the controversy regarding the bandgap crossover. This has strong implications for the realization of InGaP-based efficient light-emitting devices with emission at higher energies.

Low pressure metalorganic vapor phase epitaxy and characterization of (Al,Ga)Sb/GaSb heterostructures

Ternary AlxGa1 − xSb layers and AlSb/GaSb multi-quantum-well structures were grown by low pressure metalorganic vapor phase epitaxy (LP-MOVPE). First, a comparative investigation on the GaSb and AlSb growth kinetics with the standard methyl and ethyl group-III precursors (TMGa, TMAl, TEGa, TEAl) was performed to establish which starting materials should be preferentially utilized. Our studies revealed that only the combination of the ethyl precursors TEGa, TEAl and TESb exhibit a well matched reactivity for the LP-MOVPE deposition of (Al,Ga)Sb/GaSb heterostructures. Ternary AlxGa1 − xSb layers (0 <x < 1) grown on GaSb substrates showed low temperature photoluminescence responses for Al contents smaller than the direct-indirect crossover concentration. Furthermore, GaSb/AlSb multi-quantum-well stacks with good structural and optical quality were deposited which could be verified by X-ray diffraction, Raman and photoluminescence measurements.

Intrawell and interwell magnetoexcitons in InxGa1-xAs/GaAs coupled double quantum wells.

We theoretically study the ground and many of the excited optically active s states of spatially direct (intrawell) and indirect (interwell) quasi-two-dimensional excitons in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs coupled double quantum wells (DQW's) in quantizing magnetic fields B\ensuremath{\gtrsim}2 T. The evolution of the eigenstates and oscillator strengths with the perpendicular magnetic B and electric E fields (when, in particular, the direct-indirect crossover and a number of anticrossings occur in the spectra) is studied. The results of the theory are in good agreement with the recently obtained experimental data [L. V. Butov, A. Zrenner, G. Abstreiter, A. V. Petinova, and K. Eberl, Phys. Rev. B 52, 12 153 (1995)]. The physical origin of the excitonic symmetric-antisymmetric splittings ${\mathrm{\ensuremath{\Delta}}}_{\mathit{X}}$ in DQW's is discussed. It is shown that, in the wide-barrier regime, ${\mathrm{\ensuremath{\Delta}}}_{\mathit{X}}$ are determined by the two-particle tunneling through the barrier and suppressed by the excitonic effects. \textcopyright{}1996 The American Physical Society.