Change In Band Alignment Research Articles

Composition Modulation-Mediated Band Alignment Engineering from Type I to Type III in 2D vdW Heterostructures.

Band alignment engineering is crucial for facilitating charge separation and transfer in optoelectronic devices, which ultimately dictates the behavior of Van der Waals heterostructures (vdWH)-based photodetectors and light emitting diode (LEDs). However, the impact of the band offset in vdWHs on important figures of merit in optoelectronic devices has not yet been systematically analyzed. Herein, the regulation of band alignment in WSe2/Bi2Te3- xSex vdWHs (0 ≤ x ≤ 3) is demonstrated through the implementation of chemical vapor deposition (CVD). A combination of experimental and theoretical results proved that the synthesized vdWHs can be gradually tuned from Type I (WSe2/Bi2Te3) to Type III (WSe2/Bi2Se3). As the band alignment changes from Type I to Type III, a remarkable responsivity of 58.12 AW-1 and detectivity of 2.91×1012 Jones (in Type I) decrease in the vdWHs-based photodetector, and the ultrafast photoresponse time is 3.2 µs (in Type III). Additionally, Type III vdWH-based LEDs exhibit the highest luminance and electroluminescence (EL) external quantum efficiencies (EQE) among p-n diodes based on Transition Metal Dichalcogenides (TMDs) at room temperature, which is attributed to band alignment-induced distinct interfacial charge injection. This work serves as a valuable reference for the application and expansion of fundamental band alignment principles in the design and fabrication of future optoelectronic devices.

Comparative study of mechanical stress-induced flat-band voltage change in MOS capacitor and threshold voltage change in MOSFET fabricated on 4H-SiC (0001)

The impact of mechanical uniaxial stress on electrical characteristics of 4H-SiC (0001) n-type MOSFET (n-MOSFET) was systematically investigated by a mechanical 4-point bending method. Expected variation of field-effect mobility with stress was observed and for the first time, a direct relationship between uniaxial stress and significant change in threshold voltage (V th) on lateral SiC MOSFET was investigated systematically. The observed change of V th was as large as 40 mV with a stress of 170 MPa. By comparing with flat-band voltage (V fb), it was concluded that a change in V th mainly consists of change in V fb on n-MOS capacitor with mechanical stress. Even though the possible origins of such V fb change with stress are not clarified yet, they were suggested to be either the change in band alignment or the change in fixed charge density induced by electronic structure change.

Momentum matching and band-alignment type in van der Waals heterostructures: Interfacial effects and materials screening

Momentum-matched type II van der Waals heterostructures (vdWHs) have been designed by assembling layered two-dimensional semiconductors (2DSs) with special band-structure combinations----that is, the valence band edge at the \ensuremath{\Gamma} point (the Brillouin-zone center) for one 2DS and the conduction-band edge at the \ensuremath{\Gamma} point for the other [Ubrig et al., Nat. Mater. 19, 299 (2020)]. However, the band offset sizes, band-alignment types, and whether momentum matched or not, all are affected by the interfacial effects between the component 2DSs, such as the quasichemical-bonding (QB) interaction between layers and the electrical dipole moment formed around the vdW interface. Here, based on density-functional theory calculations, first we probe the interfacial effects (including different QBs for valence and conduction bands, interface dipole, and the synergistic effects of these two aspects) on band-edge evolution in energy and valley (location in the Brillouin zone) and the resulting changes in band alignment and momentum matching for a typical vdWH of monolayer InSe and bilayer $\mathrm{W}{\mathrm{S}}_{2}$, in which the band edges of subsystems satisfy the special band-structure combination for a momentum-matched type II vdWH. Then, based on the conclusions of the studied interfacial effects, we propose a practical screening method for robust momentum-matched type II vdWHs. This practical screening method can also be applied to other band-alignment types. Our current study opens a way for practical screening and designing of vdWHs with robust momentum matching and band-alignment type.

First-principles study for the electric field influence on electronic and optical properties of AlN/g-C3N4 heterostructure

Constructing a heterostructure and applying an external electric field are effective methods for enhancing photocatalytic efficiency. In this study, we systematically investigate the electronic and optical properties of an AlN/g-C3N4 heterostructure under varying electric fields using first principles. Our findings indicate that the AlN/g-C3N4 heterostructure demonstrates high stability, and van der Waals (vdW) interactions exist between the interfaces. Compared with monolayers AlN and g-C3N4, the reduced bandgap of AlN/g-C3N4 enhances electron escape ability. Furthermore, we observe that the bandgap of AlN/g-C3N4 reduces when an electric field is applied and the band alignment changes. Importantly, −0.4 V/Å AlN/g-C3N4 is not only a type-II heterostructure, forming a built-in electric field, but it also has a band alignment that spans the redox reaction of water. Consequently, the compound rate of electron–hole pairs substantially reduces, enhancing the possibility of AlN/g-C3N4 photocatalytic water splitting. Our findings provide a theoretical foundation for related experimental preparations.

(Invited) Combinatorial Synthesis and Interface Analysis for Development of High Dielectric Constant Thin Films

Compound semiconductor-based power devices such as SiC and GaN require high-temperature operational passive devices such as a capacitor. SiC based active devices are capable of operating over 250°C. For module and system level advancement, the high-temperature operational passive should be developed urgently. In the case of capacitors, current available capacitors can efficiently function up to 175°C only. Therefore, a thin film capacitor that can be operated at high-temperatures and integrated monolithically in the proximity of active device should be developed. In this talk, I will introduce our achievements of development of high temperature operational dielectric film materials and its interface control of metal/oxide stacking structure mainly.By employing combinatorial method and the thermal diffusion theory, we found some candidates for the dielectric materials based on ferroelectric materials. The BaTiO3 based relaxor ferroelectrics with a high dielectric constant (> 200) and free of hazardous elements are promising candidates. Among the BaTiO3 based relaxor ferroelectrics, we have chosen x[BaTiO3]-(1-x)[Bi(Mg2/3Nb1/3)O3] (BT-BMN). The permittivity of 400 and the stability <8% from room temperature to 400°C has been obtained, which are promising as a high-temperature dielectric medium. For the practical device application, a thermal stable electrode on the dielectrics is also required. To control the interface stability of the electrode/BT-BMN interfaces, several electrodes were investigated by photoelectron spectroscopy (PES). From the viewpoints of the oxidization energy, cost, and Bi thermal diffusion energy, RuO2, TiC, and TaC were selected as the electrode candidates to compare with the Pt electrode. To probe the interface structure and the band alignment non-destructively, we employed a combination of conventional x-ray PES using Al Kα light source (SXPES: hν= 1486.6 eV) and hard x-ray PES (HAXPES, hν= 5.95 keV). HAXPES is a powerful tool for investigating heterostructure interfaces due to its longer inelastic mean free path (IMFP) than that of SXPES. The IMFP of Pt 4f7/2, which was the heaviest element with the shortest IMFP in our measurement, was 4.68 nm, and HAXPES probed approximately 15 nm (probing depth: 3×IMFP) below the surface. For the Pt electrode, which is usually used as an electrode for Bi-contained oxides, the Bi diffusion into the electrode layer and the change of band alignment were clearly observed after annealing at 400°C. In contrast, the TiC electrode inhibited the Bi diffusion and did not show any change of the band alignment after annealing, meaning that the TiC electrode is a potential candidate for the electrode of Bi-contained oxide for high temperature operational devices.In the presentation, we also introduce the development of new high-k gate materials for GaN and Ge channels by combining the combinatorial method and HAXPES analysis. For a Ge channel, which has been attracting a lot of attention as a replacement for the Si channel used in current Si-based metal-oxide-semiconductor (CMOS) devices. This is because the Ge channel has high electron and hole mobility, which lead to a higher drive current, and Ge has a narrower band gap than Si thus allowing supply voltage scaling. However, Ge has the same issue as Si, namely an unintentionally oxidized layer with a low dielectric constant (~5.6) can form at an oxide/Ge interface. Furthermore, in contrast with SiO2, which is a good insulating layer for MOS devices, GeOx is thermodynamically unstable and water soluble. These properties cause high defect densities at the interface between high-k and Ge and a large hysteresis in the capacitance-voltage characteristic. To overcome these issues, we have proposed the direct growth of (110) rutile TiO2 or non-oxide materials on (100) Ge substrates.

Field-Dependent Band Structure Measurements in Two-Dimensional Heterostructures.

In electronic and optoelectronic devices made from van der Waals heterostructures, electric fields can induce substantial band structure changes which are crucial to device operation but cannot usually be directly measured. Here, we use spatially resolved angle-resolved photoemission spectroscopy to monitor changes in band alignment of the component layers, corresponding to band structure changes of the composite heterostructure system, that are produced by electrostatic gating. Our devices comprise graphene on a monolayer semiconductor, WSe2 or MoSe2, atop a boron nitride dielectric and a graphite gate. Applying a gate voltage creates an electric field that shifts the semiconductor bands relative to those in the graphene by up to 0.2 eV. The results can be understood in simple terms by assuming that the materials do not hybridize.

(Invited) Study on Impact of MOS Interface Passivation Processes on Band Alignment and Flat-Band Voltage of 4H-SiC Gate Stacks

A sufficiently large threshold voltage (Vth) is generally demanded for high-voltage power MOSFETs with 4H-SiC, however, the increase in channel doping concentration to tune Vth inevitably induces a significant reduction of carrier mobility in the inversion channel, due to Coulomb scattering at SiO2/4H-SiC interface [1]. Another serious problem is that negative Vth shifts are often observed by employing SiC surface nitridation process, usually done by a post-oxidation annealing (POA) in NO ambient at elevated temperature. Such processes introduce nitrogen atoms selectively to the surface of SiC by substituting the topmost carbon atoms [2]. Currently such nitrogen passivation process is inevitable to fabricate 4H-SiC MOS gate stacks with sufficiently low interface state density and good device reliability. Therefore the development of technique to design Vth of 4H-SiC MOSFETs is strongly demanded based on the understanding of the factors to cause Vth shift.First we systematically studied the flat-band voltage (Vfb) of SiO2/4H-SiC (0001) n-type MOS capacitors with various oxide thickness, fabricated by conventional dry oxidation followed by NO-POA. From the oxide thickness dependence of Vfb we could deduce the expected Vfb value when the effects of fixed charges are removed. As a result we found an anomalous negative shift of Vfb in several hundreds of mV irrespective of oxide thickness, which was enlarged by extending NO annealing duration [3]. This phenomenon is quite unexpected but would be explainable by assuming a dipole layer formation at the interface by SiC surface nitridation. Considering the polar Si-N bond formation with high density ~1014 cm2 at the interface, the SiC surface with Si-coordinated N atoms could introduce a dipole layer.SiO2/4H-SiC band alignment was investigated by two kinds of methods: the valence-band spectra analysis using x-ray photoelectron spectroscopy and the energy barrier height characterization of Fowler-Nordheim (F-N) current analysis. A clear trend was observed that the valence-band offset between 4H-SiC (0001) and SiO2 changes monotonically by the NO-POA durations to result in a increase of conduction-band offset at SiO2/SiC interface, which naturally explains the unexpected negative shift of Vfb observed in MOS capacitors. To our surprise, this trend appeared to the opposite direction for the gate stacks on 4H-SiC (000-1) or (1-100) fabricated with similar processes. For the MOS capacitors on (0001), we could clearly see a decreasing trend of F-N tunneling current by NO annealing duration which was attributable to the increase of conduction-band offset, while the opposite trend was observed for the capacitors on (000-1) and (1-100). Even though the defect-assisted conduction mechanisms such as Pool-Frenkel model appears to dominantly determine the temperature-dependence and electric-field-dependence of the leakage current at room temperatures or higher, obviously higher leakage current level for a given electric field was observed on (000-1) and (1-100) stacks than (0001), which should be attributable to the significant difference of the band alignment in several hundreds of mV. These results clarify that there are significant intrinsic difference of the gate leakage characteristics among SiC gate stacks on different crystal orientations, as well as the value of Vfb, explainable by the difference of band alignment.Next we examined a way to tune the Vfb of SiC gate stacks by the introduction of another dipole layer, by depositing of just a few nm-thick Al2O3 layer on top of SiO2 after NO-POA, based on the concept of oxide interface dipole layers reported for various high-k/SiO2 interfaces [4]. As a result we successfully demonstrated ~1V Vfb shift without a deterioration of interface state density only by Al2O3 deposition and post-deposition annealing. This technique is expected as a method not only to overcome the unexpected change of Vfb by nitrogen passivation process, but to achieve larger Vth without increasing the channel doping to avoid the deterioration of channel mobility.In conclusion, we found that nitrogen passivation of SiO2/4H-SiC gate stacks resulted in a significant change of band alignment to increase the conduction band offset on (0001) stack but to decrease the offset on (000-1) or (1-100) stacks, which results in a difference of gate leakage characteristics as well as Vfb. Control of the formation of dipole layers at SiO2/SiC interface and/or that formed in the multilayer dielectrics would have important roles to tune the band alignment of SiC gate stacks.[1] M. Noguchi et al., Jpn. J. Appl. Phys. 58, 031004 (2019).[2] J. Rozen et al., J. Appl. Phys. 105, 124506 (2009).[3] T. H. Kil and K. Kita, Appl. Phys. Lett. 116, 122103 (2020).[4] K. Kita and A. Toriumi, Appl. Phys. Lett. 94, 132902 (2009).

Tuning band alignment at a semiconductor-crystalline oxide heterojunction via electrostatic modulation of the interfacial dipole

We demonstrate that the interfacial dipole associated with bonding across the SrTiO3/Si heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into SrTiO3. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type-II to type-III. The transferred charge, resulting in built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Functionalizing the interface dipole to enable electrostatic altering of band alignment opens new pathways to realize novel behavior in semiconducting heterojunctions.

(Invited) Study on Impact of MOS Interface Passivation Processes on Band Alignment and Flat-Band Voltage of 4H-SiC Gate Stacks

Change of band alignment at SiO2/4H-SiC was observed by extending the duration of interface nitridation process to passivate the interface defects. The conduction-band offset on (0001) stack increases but that on (000-1) and (1-100) stacks decrease, which does not result only in a difference of flatband voltage (Vfb) but also in a difference of gate leakage characteristics among those stacks on different crystal faces. An introduction of an additional dipole layer in the gate stack was demonstrated by depositing a few-nm-thick Al2O3 on SiO2 to cause a positive shift of Vfb without degrading the quality of SiO2/SiC MOS interface. Control of those factors to cause Vfb shift is crucially important to reduce the mobility degradation in SiC MOSFET inversion channel by increasing the channel doping concentration to tune the threshold voltage.

Immediate and Temporal Enhancement of Power Conversion Efficiency in Surface-Passivated Perovskite Solar Cells.

This work reports strategies for improving the power conversion efficiency (PCE) by capitalizing on temporal changes through the storage effect and immediate improvements by interface passivation. It is demonstrated that both strategies can be combined as shown by PCE improvement in passivated perovskite solar cells (PSCs) upon ambient storage because of trap density reduction. By analyzing the dominant charge recombination process, we find that lead-related traps in perovskite bulk, rather than at the surface, are the recombination centers in both as-fabricated and ambient-stored passivated PSCs. This emphasizes the necessity to reduce intrinsic defects in the perovskite bulk. Furthermore, storage causes temporal changes in band alignment even in passivated PSCs, contributing to PCE improvement. Building on these findings, composition engineering was employed to produce further immediate PCE improvements because of defect reduction in the bulk, achieving a PCE of 22.2%. These results show that understanding the dominant recombination mechanisms within a PSC is important to inform strategies for producing immediate and temporal PCE enhancements either by interface passivation, storage, composition engineering, or a combination of them all to fabricate highly efficient PSCs.

Geometric Calibration Updates to Landsat 7 ETM+ Instrument for Landsat Collection 2 Products

The Landsat 7 (L7) spacecraft and its instrument, the enhanced thematic mapper plus (ETM+), have been consistently characterized and calibrated since its launch in April of 1999. These performance metrics and calibration updates are determined through the U. S. Geological Survey (USGS) Landsat image assessment system (IAS), which has been performing this function since launch. Starting in 2016, the USGS adopted a tiered collection management structure for its Landsat data products that ensures a consistent method of processing for the Landsat archive within a given collection while allowing a set of calibration updates to be performed between any two given collections. The time frame between 2016 and the end of 2020 was part of the Landsat data Collection 1, in the middle of 2020 was the start of the Landsat Collection 2 data products. The start of a given collection initiates the reprocessing of the Landsat archive, which may involve one or more of a set of updated calibration parameters, improvements in the support data needed for product generation, and improved algorithms used in both the processing flow of products along with the characterization and calibration of the Landsat instruments and spacecraft. This paper discusses only the ETM+ geometric spacecraft and instrument calibration improvements for Collection 2. Three ETM+ calibration updates were made for the ETM+; updates to the thermal band odd-to-even detector alignment, sensor to attitude control system (ACS) alignment, and a cold-to-warm focal plane alignment adjustment. The sensor alignment updates impact only the accuracy of the systematic terrain products (L1GT), which are the products generated before applying any corrections based on the ground control used in registration. The band alignment changes impacted only bands 5, 6, and 7 within the focal plane. Other geometric calibration updates, such as scan mirror alignment, are done on a routine basis and are not part of the Collection 2 updates due to their more dynamic characteristics.

Promising pyro-photo-electric catalysis in NaNbO3 via integrating solar and cold-hot alternation energy in pyroelectric-assisted photoelectrochemical system

Herein, a novel catalytic process named “pyro-photo-electric catalysis” is firstly realized by combining pyroelectric catalysis (PREC) and photoelectrochemical (PEC) catalysis based on NaNbO 3 nanocubes. The films show enhanced photocurrent density of 0.37 mA/cm 2 at 1.23 V vs. a reversible hydrogen electrode (RHE) under 20–50 °C heating-cooling cycles and illumination, which is 3.1 and 1.6 times higher than that of PREC (0.09 mA/cm 2 ) and PEC catalysis (0.14 mA/cm 2 ). It is found that the higher catalysis efficiency can be the result of increased carrier concentration, including photogenerated carriers and pyro-generated carriers, and accelerated charge separation and transfer due to the change in band alignment at NaNbO 3 /electrolyte caused by the imbalance of polarization charges and the induced pyroelectric potential. This study confirms the feasibility of pyroelectric-assisted PEC performance and realizes the energy harvesting of cold-hot alternation thermal energy and solar energy. It also provides insights into the rational design for designing new and ecofriendly photoelectrodes with efficient PEC water splitting. • The pyro-photo-electric catalysis combining pyroelectric and PEC catalysis based on NaNbO 3 . • The PEC performance of NaNbO 3 is significantly improved upon the assist of pyroelectric effect. • The pyroelectric effect generates pyro-generated carriers under cold-hot cycle. • The induced pyroelectric potential accelerating charge separation and transfer. • The pyroelectric-assisted PEC activity realizes the energy harvesting of thermal energy and solar energy.

Band Alignment at the Al2O3/β-Ga2O3 Interface with CHF3 Treatment**Supported by the Guangdong Province Key Technologies Research and Development Program (Grant No. 2019B010128001), the National Natural Science Foundation of China (Grant No. 61774041), and the Shanghai Science and Technology Innovation Program (Grant No. 19520711500).

The energy band alignment at the atomic layer deposited Al2O3/β-Ga2O3 interface with CHF3 treatment was characterized by x-ray photoelectron spectroscopy and secondary ion mass spectrometry (SIMS). With additional CHF3 plasma treatment, the conduction band offset increases from 1.95±0.1 eV to 2.32±0.1 eV; and the valence band offset decreases from 0.21±0.1 eV to −0.16±0.1 eV. As a result, the energy band alignment changes from type I to type II. This energy band alignment transition could be attributed to the downshift of the core-level of Ga 3d, resulting from the Ga–F bond formation in the F-rich interfacial layer, which is confirmed by the SIMS results.

Changes in band alignment during annealing at 600 °C of ALD Al2O3 on (InxGa1 − x)2O3 for x = 0.25–0.74

Changes in valence band offsets (VBOs) as a result of annealing of heterostructures of atomic layer deposited Al2O3 on (InxGa1 − x)2O3 (where x = 0.25–0.75), grown by pulsed laser deposition, are reported. The heterostructures have been annealed at 600 °C to simulate the expected thermal budget during device fabrication. The VBOs decrease significantly as a result of annealing, with the change being larger at higher indium concentrations. The decrease in VBO ranges from −0.38 eV for (In0.25Ga0.75)2O3 to −1.28 eV for (In0.74Ga0.26)2O3 and is likely due to increased interfacial disorder at the heterointerface as well as phase differences between gallium-rich samples and indium-rich samples. After annealing, the band alignment remains type I (nested gap) for x = 0.25, 42, and 60 but becomes type II for the (In0.74Ga0.26)2O3 sample.

Efficient band structure engineering and visible-light response in ZrS2/GaS heterobilayer by electrical field or external strains

Via first-principle methods, the electronic structures and optical properties of 2D ZrS2/GaS van der Waals heterostructure (vdWH) are studied. It is found that the band alignment changes from type-II to type-I under negative electrical field, and compressive strains. The transition points are -0.2 V/Å and -1%, respectively. The band gap changes efficiently under positive electrical field and compressive strains. The tensile strains increase the optical adsorption coefficients in ultraviolet regions, while the compressive strains increase the optical adsorption coefficients in visible region significantly.

Tunable SnSe2 /WSe2 Heterostructure Tunneling Field Effect Transistor.

The burgeoning 2D semiconductors can maintain excellent device electrostatics with an ultranarrow channel length and can realize tunneling by electrostatic gating to avoid deprivation of band-edge sharpness resulting from chemical doping, which make them perfect candidates for tunneling field effect transistors. Here this study presents SnSe2 /WSe2 van der Waals heterostructures with SnSe2 as the p-layer and WSe2 as the n-layer. The energy band alignment changes from a staggered gap band offset (type-II) to a broken gap (type-III) when changing the negative back-gate voltage to positive, resulting in the device operating as a rectifier diode (rectification ratio ~104 ) or an n-type tunneling field effect transistor, respectively. A steep average subthreshold swing of 80 mV dec-1 for exceeding two decades of drain current with a minimum of 37 mV dec-1 at room temperature is observed, and an evident trend toward negative differential resistance is also accomplished for the tunneling field effect transistor due to the high gate efficiency of 0.36 for single gate devices. The ION /IOFF ratio of the transfer characteristics is >106 , accompanying a high ON current >10-5 A. This work presents original phenomena of multilayer 2D van der Waals heterostructures which can be applied to low-power consumption devices.

GaAsSb spacer effect in quasi-type-II InAs coupled-QDs for intraband absorption enhancement

We designed an InAs coupled quantum-dot (QD) structure with a GaAsSb spacer to form an intermediate band (IB). The electron and hole states are calculated using the k ⋅ p method. The numerical results revealed the band alignment changes to be quasi-type-II with 16% Sb. The 1 nm AlAs layers around the QD and 1 nm GaAs layer help in broadening the intraband absorption spectrum from the far infrared region to infrared range. The coupling QD structure with an 8.5 nm GaAsSb spacer and 16% Sb concentration exhibits better photoelectric efficiency for intermediate band solar cell in the simulation, with a 3.3% enhancement of the same PIN structure with GaAs as the intrinsic region. Introducing a GaAsSb layer in the coupling QD structure will also release the maximum shear stress in QD, exhibiting a 3% release with 16% a GaAsSb spacer.

Interfacial Defect Engineering on Electronic States of Two-Dimensional AlN/MoS2 Heterostructure

The effects of vacancies and doping defects on the electronic and magnetic states of two–dimensional (2D) AlN/MoS2 heterostructure are investigated by first–principle calculation. Because of charge transfer from the AlN layer to the interstitial region between layers, the energy band structure of the AlN/MoS2 heterostructure is not a simple superposition of those of AlN and MoS2. The energy band alignment can be further tuned by introducing vacancies and doping at the interface. When the AlN layer is decorated by N vacancies or n–type doped, noticeable charge transfer to the conduction band of the MoS2 layer is observed and the band alignment maintains type–I. However, in the case of p–type doping, for instance, C substituting for N (CN) in the AlN sublayer, the band alignment changes to type–II. Moreover, Al vacancies and BeAl/CN doping produce asymmetrical spin–up and spin–down states, which leads to magnetization of the AlN/MoS2 heterostructure. The results demonstrate the significant effects of interf...

Chemical sensing by band modulation of a black phosphorus/molybdenum diselenide van der Waals hetero-structure

We report on a new chemical sensor based on black phosphorus/molybdenum diselenide van der Waals hetero-junctions. Due to the atomically thin nature of two-dimensional (2D) materials, surface adsorption of gas molecules can effectively modulate the band alignment at the junction interface, making the device a highly sensitive detector for chemical adsorptions. Compared to sensors made of homogeneous nanomaterials, the hetero-junction demonstrates considerably lower detection limit and higher sensitivity toward nitrogen dioxide. Kelvin probe force microscopy and finite element simulations have provided experimental and theoretical explanations for the enhanced performance, proving that chemical adsorption can induce significant changes in band alignment and carrier transport behaviors. The study demonstrates the potential of van der Waals hetero-junction as a new platform for sensing applications, and provides more insights into the interaction between gaseous molecules and 2D hetero-structures.

Electronic structure of transparent conducting Mo-doped indium oxide films grown by polymer assisted solution process

Transparent conducting properties of molybdenum doped indium oxide thin films by polymer-assisted solution (PAS) were investigated as a function of Mo doping concentration. Indium oxide films with molybdenum of ~1.5% (MIO) showed lowest electrical resistivity with 7.89×10−4 Ω cm, compared to those of indium oxide films with 4.39×10−3 Ω cm. The enhancement of the transparent conducting properties by molybdenum doping were explained by the electronic structure in terms of chemical bonding states, band edge states below the conduction band, and band alignment. Molybdenum doping into indium oxide films resulted in the evolution of the conduction band and the change of band alignment.