Showing papers on "Band offset published in 2019"

A dielectric-defined lateral heterojunction in a monolayer semiconductor

Abstract: Owing to their low dimensionality, two-dimensional semiconductors, such as monolayer molybdenum disulfide, have a range of properties that make them valuable in the development of nanoelectronics. For example, the electronic bandgap of these semiconductors is not an intrinsic physical parameter and can be engineered by manipulating the dielectric environment around the monolayer. Here we show that this dielectric-dependent electronic bandgap can be used to engineer a lateral heterojunction within a homogeneous MoS2 monolayer. We visualize the heterostructure with Kelvin probe force microscopy and examine its influence on electrical transport experimentally and theoretically. We observe a lateral heterojunction with an approximately 90 meV band offset due to the differing degrees of bandgap renormalization of monolayer MoS2 when it is placed on a substrate in which one segment is made from an amorphous fluoropolymer (Cytop) and another segment is made of hexagonal boron nitride. This heterostructure leads to a diode-like electrical transport with a strong asymmetric behaviour. A lateral heterojunction with diode-like electrical transport can be created in a homogeneous MoS2 monolayer by using a substrate in which one segment is made from an amorphous fluoropolymer and another segment from hexagonal boron nitride.

A two-dimensional GeSe/SnSe heterostructure for high performance thin-film solar cells

Abstract: Based on the first-principles calculations, we demonstrated that a GeSe/SnSe heterostructure has type-II band alignment and a direct band gap, which can effectively prevent the recombination of photogenerated electron–hole pairs. Moreover, the GeSe/SnSe heterostructure also exhibits strong optical absorption intensity, which can reach the order of 105 cm−1. Our predicted photoelectric conversion efficiency (PCE) for the GeSe/SnSe heterostructure reaches 21.47%. We also found that the hole carrier mobility of the GeSe/SnSe heterostructure along the x direction has been significantly improved to 6.42 × 104 cm2 V−1 s−1, which is higher than that of black phosphorus (1 × 104 cm2 V−1 s−1). By applying a vertical external electric field, we found that the band gap and band offset of the GeSe/SnSe heterojunction can be effectively tuned. The revealed type-II band alignment, strong optical absorption, superior PCE and superior hole carrier mobility of the GeSe/SnSe heterostructure imply that this new proposed material has broad application prospects in solar cells.

Deep-Ultraviolet Photodetection Using Single-Crystalline β-Ga2O3/NiO Heterojunctions

Abstract: In recent years, β-Ga2O3/NiO heterojunction diodes have been studied, but reports in the literature lack an investigation of an epitaxial growth process of high-quality single-crystalline β-Ga2O3/NiO thin films via electron microscopy analysis and the fabrication and characterization of an optoelectronic device based on the resulting heterojunction stack. This work investigates the thin-film growth of a heterostructure stack comprising n-type β-Ga2O3 and p-type cubic NiO layers grown consecutively on c-plane sapphire using pulsed laser deposition, as well as the fabrication of solar-blind ultraviolet-C photodetectors based on the resulting p-n junction heterodiodes. Several characterization techniques were employed to investigate the heterostructure, including X-ray crystallography, ion beam analysis, and high-resolution electron microscopy imaging. X-ray diffraction analysis confirmed the single-crystalline nature of the grown monoclinic and cubic (201) β-Ga2O3 and (111) NiO films, respectively, whereas electron microscopy analysis confirmed the sharp layer transitions and high interface qualities in the NiO/β-Ga2O3/sapphire double-heterostructure stack. The photodetectors exhibited a peak spectral responsivity of 415 mA/W at 7 V reverse-bias voltage for a 260 nm incident-light wavelength and 46.5 pW/μm2 illuminating power density. Furthermore, we also determined the band offset parameters at the thermodynamically stable heterointerface between NiO and β-Ga2O3 using high-resolution X-ray photoelectron spectroscopy. The valence and conduction band offsets values were found to be 1.15 ± 0.10 and 0.19 ± 0.10 eV, respectively, with a type-I energy band alignment.

Tailoring the film morphology and interface band offset of caesium bismuth iodide-based Pb-free perovskite solar cells

Abstract: Bismuth-based halide perovskites (Bi-HaP) are low toxicity and air-stable materials with promising photo-absorber properties. In this study, we fabricated Bi-HaP (Cs3Bi2I9 and CsBi3I10) films by a solution process followed by solvent annealing and investigated the crystal growth and optoelectronic properties of these materials. A compact and large grain morphology of the Bi-HaP films was realized by annealing under ambient solvent vapor conditions. Collective analysis of XRD patterns, and Raman spectra, absorption and PL spectra of the fabricated films corroborates that the Cs3Bi2I9 film (Eg ∼ 2.08 eV) with a hexagonal crystal phase is more stable under annealing conditions in a wide temperature range and ambient solvent vapor annealing conditions as compared to the other CsBi3I10 thin film, having the narrower Eg ∼ 1.8 eV, of the Bi-HaP family. We have achieved the best power conversion efficiency as high as ∼1.26% with the open circuit voltage of 0.74 V for the device fabricated with Cs3Bi2I9. The analysis of material properties and device characteristics indicates that morphology tailoring, surface chemistry control, and interface band offset engineering are important for the further improvement of Bi-HaP-based devices.

NumericalSimulation of Planar Heterojunction PerovskiteSolar Cells Based on SnO 2 Electron Transport Layer

Abstract: The perovskite solar cells attracted great attention owing to their low cost and high performance. SnO2 as electron transport layer has been mostly used in the perovskite solar cells due to its exc...

Tailoring excitonic states of van der Waals bilayers through stacking configuration, band alignment, and valley spin.

Abstract: Excitons in monolayer semiconductors have a large optical transition dipole for strong coupling with light. Interlayer excitons in heterobilayers feature a large electric dipole that enables strong coupling with an electric field and exciton-exciton interaction at the cost of a small optical dipole. We demonstrate the ability to create a new class of excitons in hetero- and homobilayers that combines advantages of monolayer and interlayer excitons, i.e., featuring both large optical and electric dipoles. These excitons consist of an electron confined in an individual layer, and a hole extended in both layers, where the carrier-species-dependent layer hybridization can be controlled through rotational, translational, band offset, and valley-spin degrees of freedom. We observe different species of layer-hybridized valley excitons, which can be used for realizing strongly interacting polaritonic gases and optical quantum controls of bidirectional interlayer carrier transfer.

Thickness-dependent bandgap and electrical properties of GeP nanosheets

Abstract: Recently there have been extensive efforts to develop novel two-dimensional (2D) layered structures, owing to their fascinating thickness-dependent optical/electrical properties. Herein, we synthesized thin GeP nanosheets that had a band gap (Eg) of 2.3 eV, which is a dramatic increase from the value in the bulk (0.9 eV) upon exfoliation. This Eg value is close to that of the GeP monolayer predicted by first-principles calculations (HSE06 functional). The calculations also indicate a strong dependence of Eg on the number of layers (2.306, 1.660, 1.470, and 1.397 eV for mono-, bi-, tri-, and tetralayers, respectively), and that the band edge positions are suitable for water splitting reactions. Field-effect transistor devices were fabricated using the p-type GeP nanosheets of various thicknesses, and the devices demonstrated a significant decrease in the hole mobility but an increased on–off ratio as the layer number decreased. The larger on–off ratio (104) for the thinner ones is promising for use in novel 2D (photo)electronic nanodevices. Further, liquid-exfoliated GeP nanosheets (thickness = 1–2 nm) deposited on Si nanowire arrays can function as a promising photoanode for solar-driven water-splitting photoelectrochemical (PEC) cells. Based on the calculated band offset with respect to the Fermi levels for the two half-reactions in the water splitting reaction, the performance of the PEC cell can be explained by the formation of an effective p-GeP/n-Si heterojunction.

Polymer Doping Enables a Two-Dimensional Electron Gas for High-Performance Homojunction Oxide Thin-Film Transistors.

Abstract: High-performance solution-processed metal oxide (MO) thin-film transistors (TFTs) are realized by fabricating a homojunction of indium oxide (In2 O3 ) and polyethylenimine (PEI)-doped In2 O3 (In2 O3 :x% PEI, x = 0.5-4.0 wt%) as the channel layer. A two-dimensional electron gas (2DEG) is thereby achieved by creating a band offset between the In2 O3 and PEI-In2 O3 via work function tuning of the In2 O3 :x% PEI, from 4.00 to 3.62 eV as the PEI content is increased from 0.0 (pristine In2 O3 ) to 4.0 wt%, respectively. The resulting devices achieve electron mobilities greater than 10 cm2 V-1 s-1 on a 300 nm SiO2 gate dielectric. Importantly, these metrics exceed those of the devices composed of the pristine In2 O3 materials, which achieve a maximum mobility of ≈4 cm2 V-1 s-1 . Furthermore, a mobility as high as 30 cm2 V-1 s-1 is achieved on a high-k ZrO2 dielectric in the homojunction devices. This is the first demonstration of 2DEG-based homojunction oxide TFTs via band offset achieved by simple polymer doping of the same MO material.

Strong interlayer hybridization in the aligned SnS2/WSe2 hetero-bilayer structure

Abstract: The combination of monolayers of different two-dimensional (2D) materials into van der Waals hetero-bilayer structures creates unprecedented physical phenomena, acting as a powerful tool for future devices. Understanding and exploiting these phenomena hinge on knowing the electronic structure and the hybridization of hetero-bilayer structures. Here, we show strong hybridization effects arising between the constitutive single layers of a SnS2/WSe2 hetero-bilayer structure grown by chemical vapor deposition. Surprisingly, the valence band maximum position of WSe2 is moved from the K point for the single layer WSe2 to the Γ point for the aligned SnS2/WSe2 hetero-bilayer. Additionally, a significant photoluminescence quenching is observed for the SnS2/WSe2 hetero-bilayer structure with respect to the WSe2 monolayer. Using photoluminescence spectroscopy and nano-angle-resolved photoemission spectroscopy techniques, we demonstrate that the SnS2/WSe2 heterostructure present a type-II band alignment. These findings directly answer many outstanding questions about the electronic band structure and the band offset of SnS2/WSe2 hetero-bilayers for envisaging their applications in nanoelectronics.

Strain and electric field modulated electronic structure of two-dimensional SiP(SiAs)/GeS van der Waals heterostructures

Abstract: The van der Waals (vdW) heterostructures that combine different two-dimensional (2D) materials can effectively improve the electronic and optical properties. Recently, the group-IV monochalcogenide GeS, as a potential candidate material for vdW heterostructures, has attracted much attention due to its puckered structure and unique electronic properties. However, the indirect band gap of GeS limits its applications in electronic devices. Here, the electronic structure of 2D SiP(SiAs)/GeS heterostructures is investigated systematically by first-principles calculations. The typical type-II band alignment appears in the SiP(SiAs)/GeS heterostructures, which can effectively facilitate the separation of photogenerated electron and hole pairs. Especially, the SiAs/GeS vdW heterostructure shows a direct band gap of 0.953 eV, which makes it have potential applications in optoelectronic devices. Additionally, by increasing or decreasing the interlayer distance, the binding energy of the heterostructure increases, where the charge transfer can be modulated. Furthermore, the charge transfer, band gap and band offset of the heterostructures are sensitive to the in-plane strain. Moreover, the large band offset at a positive electric field can enhance the photogenerated charge separation when the SiP(SiAs)/GeS heterostructures are irradiated with light. These calculated results indicate that the SiP(SiAs)/GeS vdW heterostructures are good candidates for low-dimensional optoelectronic devices.

Constructing the Band Alignment of Graphitic Carbon Nitride (g-C3N4)/Copper(I) Oxide (Cu2O) Composites by Adjusting the Contact Facet for Superior Photocatalytic Activity

Abstract: g-C3N4/Cu2O p–n junction composites with different crystal facets were prepared by a simple one-step route, which showed strong facet-dependent photoactivity, where the degradation rate value k for 20% C3N4–Cu2O truncated cubes composites with {100} facets heterojunction was ∼2.5 times higher than that of pure Cu2O truncated cubes. Meanwhile, k value for 20% C3N4–Cu2O octahedra with {111} facets heterojunction exceeds that of pure Cu2O octahedra by a factor of ∼1.9. p–n heterojunctions with type II energy alignment are determined by XPS analysis. Larger band energy offset (0.96 eV for ΔECBO, 1.73 eV for ΔEVBO) was observed in 20% C3N4–Cu2O truncated cubes composites compared with that of 20% C3N4–Cu2O octahedra composites (0.85 eV for ΔECBO, 1.64 eV for ΔEVBO). The bigger band offset means stronger driving force of the electron transfer between Cu2O truncated cubes with {100} facets and C3N4, indicating band alignment of the heterojunction was facet-dependent, the properly larger band offsets between Cu2O...

Band gap and band alignment prediction of nitride-based semiconductors using machine learning

Abstract: Nitride has been drawing much attention owing to its wide range of applications in optoelectronics and there remains plenty of room for materials design and discovery. Here, a large set of nitrides has been designed, with their band gap and alignment being studied by first-principles calculations combined with machine learning. The band gap and band offset against wurtzite GaN accurately calculated by the combination of the screened hybrid functionals of HSE and DFT-PBE were used to train and test machine learning models. After comparison among different machine learning techniques, when elemental properties are taken as features, support vector regression (SVR) with radial kernel performs best for predicting both the band gap and band offset with a prediction root mean square error (RMSE) of 0.298 eV and 0.183 eV, respectively. The former is within the HSE calculation uncertainty and the latter is small enough to provide reliable predictions. Additionally, when the band gap calculated by DFT-PBE was added into the feature space, the band gap prediction RMSE decreased to 0.099 eV. Through a feature engineering algorithm, the elemental feature space-based band gap prediction RMSE further drops by around 0.005 eV and the relative importance of elemental properties for band gap prediction was revealed. Finally, the band gap and band offset of all designed nitrides were predicted and two trends were noticed: as the number of cation types increases, the band gap tends to narrow while the band offset tends to increase. The predicted results will provide useful guidance for precise investigation of nitride engineering.

Energy level alignment of Cu (In ,Ga ) (S ,Se ) 2 absorber compounds with In 2 S 3 , NaIn 5 S 8 , and CuIn 5 S 8 Cd-free buffer materials

Abstract: Motivated by environmental reasons, In2S3 is a promising candidate for a Cd-free buffer layer in Cu(In, Ga)(S, Se)(2) (CIGSSe)-based thin-film solar cells. For an impactful optimization of the In-2 S-3 alternative buffer layer, however, a comprehensive knowledge of its electronic properties across the absorber-buffer interface is of foremost importance. In this respect, finding a favorable band offset between the absorber and the buffer layers can effectively reduce the carrier recombination at the interface and improve open-circuit voltage and fill factor, leading to higher conversion efficiencies. In this study, we investigate the band alignment between the most common CIGSSe-based absorber compounds and In2S3. Furthermore, we consider two chemically modified indium sulfide layers, NaIn(5)S(8 )and CuIn5S8, and we discuss how the formation of these secondary phases influences band discontinuity across the interface. Our analysis is based on density functional theory calculations using hybrid functionals. The results suggest that Ga-based absorbers form a destructive clifflike conduction-band offset (CBO) with both pure and chemically modified buffer systems. For In-based absorbers, however, if the absorber layer is Cu-poor at the surface, a modest favorable spikelike CBO arises with NaIn5S8 and CuIn5S8.

Revisit of the band gaps of rutile SnO 2 and TiO 2 : a first-principles study

Abstract: From the recent experimentally observed conduction band offset and previously reported band gaps, one may deduce that the valence band offset between rutile SnO2 and TiO2 is around 1 eV, with TiO2 having a higher valence band maximum. This implication sharply contradicts the fact that the two compounds have the same rutile structure and the Γ3+ VBM state is mostly an oxygen p state with a small amount of cation d character, thus one would expect that SnO2 and TiO2 should have small valence band offset. If the valence band offset between SnO2 and TiO2 is indeed small, one may question the correctness of the previously reported band gaps of SnO2 and TiO2. In this paper, using first-principles calculations with different levels of computational methods and functionals within the density functional theory, we reinvestigate the long-standing band gap problem for SnO2. Our analysis suggests that the fundamental band gap of SnO2 should be similar to that of TiO2, i.e., around 3.0 eV. This value is significantly smaller than the previously reported value of about 3.6 eV, which can be attributed as the optical band gap of this material. Similar to what has been found in In2O3, the discrepancy between the fundamental and optical gaps of SnO2 can be ascribed to the inversion symmetry of its crystal structure and the resultant dipole-forbidden transitions between its band edges. Our results are consistent with most of the optical and electrical measurements of the band gaps and band offset between SnO2 and TiO2, thus provide new understanding of the band structure and optical properties of SnO2. Experimental tests of our predictions are called for.

Band-Offset Degradation in van der Waals Heterojunctions

Abstract: van der Waals heterojunctions based on two-dimensional materials are attractive for tunnel field-effect transistors, but their atomically clean and electronically sharp junction interfaces fail to realize satisfactory tunneling behavior. This work reveals the phenomenon of band offset degradation in these heterojunctions, induced by interlayer charge transfer, which can impede the band-alignment configuration that is key to turning on such devices. These discoveries help to explain the calculated deviations in heterojunction band alignment, and should be highlighted in tunnel-device exploration.

Strain effect on band structure of InAlAs digital alloy

Abstract: Recently, InAlAs digital alloys have been shown to exhibit unique electronic dispersion properties, which can be used to make low-noise avalanche photodiodes. In this paper, the strain effect is analyzed for its impact on the band structure of the InAlAs digital alloy. Simulation using a tight binding model that includes the strain effect yields bandgap energies that are consistent with experimental results. The bandgap would be larger without strain. In addition, a positive relationship has been found between minigaps of the InAlAs digital alloy and the band offset between bulk InAs and AlAs at the same position in k-space.

Achieving a direct band gap and high power conversion efficiency in an SbI3/BiI3 type-II vdW heterostructure via interlayer compression and electric field application

Abstract: Type-II van der Waals (vdW) heterostructures are considered as a class of competitive candidates of high-efficiency photovoltaic materials, due to their spontaneous electron-hole separation. However, most of the vdW heterostructures possess an indirect gap and a large band offset, which would lead to low photon-to-electron conversion efficiency. Taking an SbI3/BiI3 vdW heterostructure as an illustrative example, we propose interlayer compression and vertical electric field application as two effective strategies to modulate the electronic and photovoltaic properties of type-II vdW heterostructures. Our results reveal that a lattice-matched SbI3/BiI3 vdW heterostructure has an indirect band gap of 1.34 eV with the conduction band minimum (CBM) at the Γ point and the valence band maximum (VBM) between the Γ and M points. The power conversion efficiency (PCE) of an SbI3/BiI3-based excitonic solar cell (XSC) is predicted to be about 14.42%. When compressing the heterostructure along the vdW gap direction, the highest valence band state at the Γ point is lifted significantly and the VBM gradually approaches the Γ point, implying an indirect-direct gap transition. This interesting evolution can be attributed to the increasing k-dependent electronic hybridization of the pz orbitals of interlayer adjacent I atoms with a reduced interlayer distance. Moreover, the interlayer compression also enhances the PCE of the system monotonically. When applying a vertical electric field, the band alignment of the heterostructure undergoes a transition from type-II to type-I and then returns to type-II between 0.1 and 0.6 V A-1. Meanwhile, the PCE of the SbI3/BiI3 XSC could be enhanced up to 21.63%. This work provides guidance for improving the electronic and photovoltaic properties of type-II vdW heterostructures.

The electronic properties and band-gap discontinuities at the cubic boron nitride/diamond hetero-interface

Abstract: Clarifying the electronic states and structures of the c-BN/diamond interface is of extreme importance for bundling these two different wide-band gap materials in order to synthesize hybrid structures with new functional properties. In this work, the structural optimization and property determinations were carried out on (100) and (111) c-BN/diamond hetero-interface by using first principles total energy calculations. A 12-layers c-BN above the diamond was found to be energetically reasonable for the calculations of the properties of the hetero-interface. Based on the calculation of the chemical potentials for the c-BN/diamond interface, the hetero-interface with the C–B configuration is the most energetically favorable structure under the (111) and (100) surfaces of diamond, respectively. The calculations of band structure and density of states for C–N bond configuration indicate that the main contribution to the density of the interface states near the EF is from the N 2s 2p, B 2p and C 2p orbitals while that for C–B bond configuration is mainly from the B 2p, N 2p and C 2p orbitals. The electron density difference, binding energy and band offset were also calculated, demonstrating that the C–B bond was found to be remarkably stronger than other adjacent bonds. Furthermore, a band offset of 0.587 eV for the (111) c-BN/diamond hetero-interface with the C–N bond configuration has been obtained, which is in good agreement with the previous experimental result (0.8 eV), suggestting that the C–N bond may exist in synthesized c-BN/diamond epitaxy using different growth methods. This should allow the design of a hybrid structure of c-BN/diamond thereby opening a new pathway towards high temperature electronics, UV photonics and (bio-) sensor applications.

Direct observation of an electrically degenerate interface layer in a GaN/sapphire heterostructure.

Abstract: An electrically degenerate layer deteriorates the optoelectric performance of a wide band gap semiconductor grown on an insulator substrate. This detrimental effect can be passively avoided by using a buffer layer to harbor various lattice defects. However, the longstanding scientific questions regarding the microscopic origin of the degenerate interface layer and the effect of local changes in the atomic structure and chemical environment at an interface on the functionality of a desired film have remained unanswered. Moreover, this is key information for the development of ultrathin optoelectronic devices. In this study, we discuss the direct observation of a degenerate interface phase at the GaN/sapphire interface on an atomic scale. By combining high-resolution transmission electron microscopy and electron energy loss spectroscopy, we detect the presence of an ultrathin (∼6.5 A) α-Ga2O3-x layer near the GaN/sapphire interface, which is subjected to ∼4.5% biaxial compressive strain and contains many oxygen vacancies. Density functional theory calculations show that the presence of a defective α-Ga2O3-x thin layer in the GaN and sapphire heterostructure remarkably reduces the band offset between the α-Ga2O3-x conduction band and the GaN valence band, thereby exerting a significant influence on the conductivity enhancement of the interface. Our results provide an unprecedented integrated picture of the degenerate interface phenomenon on an atomic scale, which would evolve the fundamental understanding about a wide band gap semiconductor heterostructure system.

Band offset models of three-dimensionally bonded semiconductors and insulators.

Abstract: The band offsets of heterojunctions of three-dimensionally (3D) bonded semiconductors lie between two limits, the electron affinity rule (unpinned limit) and the matching of the charge neutrality l...

Charge-Transfer-Induced Photoluminescence Properties of WSe2 Monolayer–Bilayer Homojunction

Abstract: The charge-transfer process in transition-metal dichalcogenides (TMDCs) lateral homojunction affects the electron-hole recombination process of in optoelectronic devices. However, the optical properties of the homojunction reflecting the charge-transfer process has not been observed and studied. In this work, we investigated the charge-transfer-induced emission properties based on monolayer (1L)-bilayer (2L) WSe2 lateral homojunction with dozens of nanometer monolayer region. On the one hand, the photoluminescence (PL) emission of bilayer WSe2 from the homojunction area blue shifts ∼23 and ∼31 meV for direct and indirect bandgap emission, respectively, compared with the bare WSe2 bilayer region. The blue shift of the emission spectrum in the bilayer WSe2 is ascribed to the decrease in binding energy induced by charge transfer from monolayer to bilayer. On the other hand, the energy shift shows a tendency to increase as the temperature decreases. The energy blue shift is ∼57 meV for direct bandgap emission at 80 K, which is larger than that (∼23 meV) at room temperature. The larger-energy blue shift at low temperature is derived from the larger driving force under larger band offset. Our observations of the unique optical properties induced by efficient charge transfer are very helpful for exploring novel TMDC-based optoelectronic devices.