Interface Band Bending Research Articles

Nanostructured p-TiO2/n-GaN heterostructure as a potential photoelectrode for efficient charge separation

This work proposes p-TiO2/n-GaN as a new semiconductor heterostructure, which holds great promise as a photoelectrode material. To fabricate p-TiO2/n-GaN heterostructures, wurtzite GaN is grown in two different morphologies by molecular beam epitaxy, and a TiO2 overlayer is formed by atomic layer deposition. The XRD and Raman experiments confirm the anatase crystal structure of TiO2 and SEM shows the conformal coating of TiO2 on GaN. The system with GaN nanowall network morphology (TiO2/GaN NWN, calls as TGN) showed better photoelectrochemical response with a photocurrent density of ∼0.65 mA cm−2 and an IPCE of 17% compared to 0.24 mA cm−2 and 6% of the planar heterostructure (TiO2/GaN epilayer, called TGE), at an applied bias of 1.24 V and at an incident power of only 13 mW cm−2. Chronoamperometric analysis show that electrodes are highly stable. The p–n diagram derived by using the data from cathodoluminescence (CL) and valence band (VB) spectra showed higher barrier height for the TGN structure due to interfacial band bending, and thus, favoring photogenerated charge separation. The evolution of hydrogen in the form of bubbles is visually evident at an applied bias of −0.56V for TGN. The enhanced photocurrent density, cathodic shift in the onset potential and the stability show the superiority of TGN to TGE in charge separation as well as in the photoelectrochemical performance.

Spin accumulation in topological insulator thin films—influence of bulk and topological surface states

Current-induced spin torques in topological insulators (TIs) include both field-like torques as well as damping-like torques. In many experimental spin torque measurements, the Fermi energies are such that both of the topological surface states (TSSs) and the bulk states are occupied. In this work, we calculate the electric field-induced spin accumulation that leads to these torques in TI thin film systems, and account for the contributions of both of the TSSs as well as bulk states in the presence of interfacial band bending and disorder scattering. We analyze how the band structure of a TI thin film is affected by the film thickness, magnetization coupling, and band bending. The variation of the band structure due to these factors in turn affects the spin accumulation along both the in-plane and out-of-plane directions, and hence the induced spin torques. We show that the in-plane spin accumulation is influenced more by the TSS, whereas the bulk bands have a significant effect on the out-of-plane spin accumulation. The predicted variation of spin accumulation with sample thickness, and doping, substrate, and ferromagnetic material choices, mirrors the widely varying spin torque measurements in experiments.

Large process-dependent variations in band alignment and interface band gaps of Cu2ZnSnS4/CdS solar cells

Electron–hole recombination at the Cu2ZnSnS4/CdS interface is believed to play a major role in limiting the efficiency of Cu2ZnSnS4 solar cells. In this work, we experimentally determine detailed Cu2ZnSnS4/CdS interface band diagrams as a function of process conditions, and correlate them to chemical processes occurring during interface formation and subsequent post-annealing. The newly devised experimental method involves a combination of photoemission spectroscopy and spectroscopic ellipsometry. Our measurements reveal that, under most process conditions, the band gaps of both Cu2ZnSnS4 and CdS decrease by several hundred meV near the interface. Furthermore, interface band bending and conduction band offsets are highly process-dependent and roughly correlated to the amount of chemical interdiffusion. The interface electronic properties are found to be unfavorable under all process conditions studied in this work, either due to a cliff-like conduction band offset, or to substantial band gap narrowing in Cu2ZnSnS4, or to both effects. According to the present study, the least harmful process conditions for the interface electronic properties are a low CdS deposition temperature without post-annealing. Even in such a case, a minimum open circuit voltage loss of 230 mV is expected due to interface- or near-interface recombination.

The Role of Misorientation in Direct Wafer Bonded III-V Materials

Wafer bonding represents an important method to study the effects of wafer misorientation, both tilt and twist, on the properties of the bonded interface since both forms of misorientation can be controllably induced. Of particular interest in this study, the impact on electronic transport across the interface can be systematically studied for III-V materials systems. Both twist and tilt between bonded wafers is found to induce additional interfacial energy band bending that impedes electronic transport across the interface. The effect of rotational misalignment is found to exhibit a cyclic behavior, contrary to some studies reported in literature. The findings presented here suggest that crystallographic planar symmetry plays a role in influencing the bonded interface electrical properties.

Energy level alignment and hole injection property of poly(9-vinylcarbazole)/indium tin oxide interface

The energy level alignment at the interface between an organic layer and an electrode plays a critical role in the performance of organic electronic devices. To improve the charge injection efficiency, the energy barrier between the charge transport level of an organic layer and the Fermi level of an electrode should be reduced. Especially, poly(9-vinylcarbazole) (PVK) is a polymeric semiconductor that is widely used as a hole transport and electron blocking layer in various optoelectronic devices. Thus, an understanding of the energy level alignment between PVK and an electrode is of great importance. In this study, the energy level alignment of PVK and indium tin oxide (ITO) was determined with X-ray and ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy measurements. The effect of UV-ozone (UVO) treatment on the formation of a hole injection barrier (Φh) in ITO was also investigated. In both cases of bare ITO and UVO ITO, only small interface dipole and band bending were observed, which indicates charge transfer between PVK and ITO is miniscule. The UVO treatment significantly increases the work function of ITO from 4.05 eV to 4.40 eV, which results in the reduction of Φh from 1.70 eV to 1.50 eV. This reduced Φh value dramatically improves the current density-voltage characteristics of PVK-based hole-only devices.

Boosting interfacial charge transfer for efficient water-splitting photoelectrodes: progress in bismuth vanadate photoanodes using various strategies

Abstract

Investigation of the Nano-Heterojuntion Electrochemistry Effect By Using in-Situ Spectrum and Electrical Measurement System

In this research, the broadband light (from 365-940 nm) and room temperature gas (NO, CO) sensing can be achieved by using nano-heterojunction (Ge and SnO2 nanowire) device; the detail analysis can be measured by in-situ spectrum and electrical measurement system. The in-situ spectrum and electrical measurement can provide the response immediately to investigate the interface defect, band bending and Joule-heating effect through the response of the devices. For broadband light detection experiment, the visible light (532 nm) beam was illuminated at different parts of the nano-heterjunction device (p-type semiconductor, n-type semiconductor, and interface). The broadband light responses only happen at interface part, not at both nanowires. For gas detection experiment, the Raman spectrum and electrical signal can be used for studying gas molecule and nano-heterjunction dynamics. Base on the abovementioned experiment, we can find the parameters to create and design new nano-device in smart way.

Facilitating the charge transfer of ZnMoS4/CuS p–n heterojunctions through ZnO intercalation for efficient photocatalytic hydrogen generation

A thin intercalation in a p–n heterojunction is utilized to induce interfacial band bending, thus generating a favorable carrier flow for photocatalytic reactions.

Tuning the Schottky rectification in graphene-hexagonal boron nitride-molybdenum disulfide heterostructure

It was still a great challenge to design high performance of rectification characteristic for the rectifier diode. Lately, a new approach was proposed experimentally to tune the Schottky barrier height (SBH) by inserting an ultrathin insulated tunneling layer to form metal–insulator-semiconductor (MIS) heterostructures. However, the electronic properties touching off the high performance of these heterostructures and the possibility of designing more efficient applications for the rectifier diode were not presently clear. In this paper, the structural, electronic and interfacial properties of the novel MIS diode with the graphene/hexagonal boron nitride/monolayer molybdenum disulfide (GBM) heterostructure had been investigated by first-principle calculations. The calculated results showed that the intrinsic properties of graphene and MoS2 were preserved due to the weak van der Waals contact. The height of interfacial Schottky barrier can be tuned by the different thickness of hBN layers. In addition, the GBM Schottky diode showed more excellent rectification characteristic than that of GM Schottky diode due to the interfacial band bending caused by the epitaxial electric field. Based on the electronic band structure, we analyzed the relationship between the electronic structure and the nature of the Schottky rectifier, and revealed the potential of utilizing GBM Schottky diode for the higher rectification characteristic devices.

Study on interface engineering of layer-by-layer structure for applications in organic photodetector

The interfacial electronic structure of layer-by-layer 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/fullerene C60 was investigated using ultra-violet photoemission spectroscopy (UPS). Photoemission data of period of DCJTB/C60/DCJTB films suggested the formation of surface dipole and interfacial band bending across the interfaces, which greatly facilitates the charge transfer from DCJTB to C60 and from C60 to DCJTB layer as well. When applied this layer-by-layer structure to a near-infrared photodetector, a maximum of photocurrent was achieved by the device with 3 periods of DCJTB/C60 thin films. Finally, the detailed work mechanism of this detector was discussed.

Critical Interface States Controlling Rectification of Ultrathin NiO-ZnO p-n Heterojunctions.

Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/oxide' heterojunctions are of interest for UV optical sensing, gas sensing, photocatalysis, charge confinement layers, piezoelectric nanogenerators, and flash memory devices. These heterojunctions can also be used as current rectifiers and potentially as recombination layers in tandem photovoltaic stacks by making the two oxide layers ultrathin. In the ultrathin geometry, understanding and control of interface electronic structure and chemical reactions at the oxide/oxide' interface are critical to functionality, as oxygen atoms are shared at the interface of the dissimilar materials. In the studies presented here the extent of chemical reactions and interface band bending is monitored using X-ray and ultraviolet photoelectron spectroscopies. Interface reactivity is controlled by varying the near surface composition of nickel oxide, nickel hydroxide, and nickel oxyhydroxide using standard surface-treatment procedures. A direct correlation between relative percentage of interface hydroxyl chemistry (and hence surface Lewis basicity) and the local band edge alignment for ultrathin p-n junctions (6 nm NiO/30 nm ZnO) is observed. We propose an acid-base formulism to explain these results: the stronger the acid-base reaction, the greater the fraction of interfacial electronic states which lower the band offset between the ZnO conduction band and the NiO valence band. Increased interfacial gap states result in larger reverse bias current of the p-n junction and lower rectification ratios. The acid-base formulism could serve as a future design principle for oxide/oxide' and other heterojunctions based on dissimilar materials.

Switching charge transfer of C3N4/W18O49 from type-II to Z-scheme by interfacial band bending for highly efficient photocatalytic hydrogen evolution

Z-scheme composite represents an ideal system for photocatalytic hydrogen evolution, but the charge transfer mechanism is still ambiguous, and how to design and construct such system is a big challenge. Herein, we demonstrate that C3N4-W18O49, the type-II composite, can be switched to direct Z-scheme via modulating the interfacial band bending. Experiment and DFT computation results reveal that the adsorption of triethanolamine (TEOA) on C3N4 surface significantly uplifts its Femi level, inverses the continuous interfacial band bending to interrupted one, and thus switches the composite from type-II to Z-scheme, without the assistance of any electron shuttles. Importantly, this Z-scheme C3N4/W18O49 composites exhibit much better photocatalytic H2 activity compared with pure C3N4, and obtain H2 evolution rate of 8597μmolh−1g−1 (AQY of 39.1% at 420nm) with Pt as cocatalyst and TEOA as hole scavenger. Also, using this hypothesis we successfully explain why C3N4/WO3 is inherent Z-scheme composite but the performance is not as good as C3N4/W18O49 and why TEOA is the best hole scavenger for C3N4. This work is expected to give deep insights into understanding the charge transfer in semiconductor composites and rationally designing and constructing Z-scheme photocatalyst for hydrogen evolution.

Relation between Interfacial Band‐Bending and Electronic Properties in Organic Semiconductor Pentacene

AbstractThere is still a challenge in organic electronics to effectively tailor interfacial electronic structures for achieving desired electronic properties for optoelectronic devices. This study realizes the tuning of interfacial electronic structures in organic semiconductor pentacene by employing substrates with different work functions. Pentacene layers show downward band‐bending with a low work function, flat band with a middle work function, and upward band‐bending with a high work function. The formation of band‐bending is explained by the electrostatic potential according to the Fermi level position. Different interfacial electronic structures can influence the carrier transport across the interface. Their diodes show different rectifying behaviors due to band‐bending, and ohmic transport is observed for the flat band structure. This work is a promising exploration to realize desired functions of semiconductor devices by tailoring interfacial electronic structures.

Photovoltage Behavior in Perovskite Solar Cells under Light-Soaking Showing Photoinduced Interfacial Changes

The photovoltage of perovskite solar cells (PSCs) was studied over a wide range of light intensities, showing changes from pristine to light-soaking (LS) conditions, explained using a specific model of spatial charge distribution. Migration of ions and vacancies under photovoltage conditions results in localized charge redistribution manifested as positive charge accumulation at the TiO2 or TiO2–MgO interlayer–perovskite interface, signifying photoinduced interfacial upward band bending. Consequentially, generation of an electrostatic potential (Velec) and an increase in interfacial recombination rate are confirmed. The magnitude and effect of Velec and interfacial recombination on the photovoltage depend on the illumination intensity and on the LS duration. PSCs with mesoporous Al2O3 showed similar changes, validating the role of the compact TiO2. Faster generation and a gradual increase of Velec are apparent under LS, which expresses the constant migration of ions and vacancies toward the interface. The...

Rectifying Characteristics and Semiconductor-Metal Transition Induced by Interfacial Potential in the Mn3CuN/n-Si Intermetallic Heterojunction.

The Mn3CuN/n-Si heterojunction device is first designed in the antiperovskite compound, and excellent rectifying characteristics are obtained. The rectification ratio (RR) reaches as large as 38.9 at 10 V, and the open-circuit voltage Voc of 1.13 V is observed under temperature of 410 K. The rectifying behaviors can be well described by the Shockley equation, indicating the existence of a Schottky diode. Simultaneously, a particular semiconductor-metal transition (SMT) behavior at 250 K is also observed in the Mn3CuN/n-Si heterojunction. The interfacial band bending induced inversion layer, which is clarified by the interfacial schematic band diagrams, is believed to be responsible for the SMT and rectifying effects. This study can develop a new class of materials for heterojunction, rectifying devices, and SMT behaviors.

Surface States in Ternary CdSSe Quantum Dot Solar Cells.

Ternary CdSSe quantum dot-sensitized solar cells (QDSCs) have demonstrated advantages such as wide absorption ranges and tunable band structures. However, the oxygen additives absorbed on such multicomponent quantum dot (QD) surfaces induce band bending at the TiO₂/CdSSe interface and prevent charge transport in QDSCs, as determined via X-ray photoelectron spectroscopy (XPS) and synchrotron-based X-ray Absorption Near-Edge Structure (XANES) analysis. Annealing of TiO₂/CdSSe QDs photoanodes was conducted at different temperatures under Ar atmospheres to eliminate oxygen additives and interfacial band bending. The short-circuit current (J(sc))of the annealed ternary CdSSe QDSCs is obviously improved, whereas the TiCl4 treatment and MgO coating of the TiO₂ nanocrystals are assisted by the annealing to compensate for the loss of opencircuit voltage (V(oc)) and fill factor (FF). Ternary CdSSe QDSCs with efficiencies of 4.72% have been achieved using the optimized annealing conditions.

Spatially Resolved Imaging on Photocarrier Generations and Band Alignments at Perovskite/PbI2 Heterointerfaces of Perovskite Solar Cells by Light-Modulated Scanning Tunneling Microscopy.

The presence of the PbI2 passivation layers at perovskite crystal grains has been found to considerably affect the charge carrier transport behaviors and device performance of perovskite solar cells. This work demonstrates the application of a novel light-modulated scanning tunneling microscopy (LM-STM) technique to reveal the interfacial electronic structures at the heterointerfaces between CH3NH3PbI3 perovskite crystals and PbI2 passivation layers of individual perovskite grains under light illumination. Most importantly, this technique enabled the first observation of spatially resolved mapping images of photoinduced interfacial band bending of valence bands and conduction bands and the photogenerated electron and hole carriers at the heterointerfaces of perovskite crystal grains. By systematically exploring the interfacial electronic structures of individual perovskite grains, enhanced charge separation and reduced back recombination were observed when an optimal design of interfacial PbI2 passivation layers consisting of a thickness less than 20 nm at perovskite crystal grains was applied.

Engineering of interface band bending and defects elimination via a Ag-graded active layer for efficient (Cu,Ag)2ZnSn(S,Se)4 solar cells

We reveal a new approach for forming a Ag-graded absorber to overcome the large open-circuit voltage deficit in (Cu,Ag)2ZnSn(S,Se)4 solar cells.

Tuning contact transport mechanisms in bilayer MoSe2 transistors up to Fowler–Nordheim regime

Atomically thin molybdenum diselenide (MoSe2) is an emerging two-dimensional (2D) semiconductor with significant potential for electronic, optoelectronic, spintronic applications and a common platform for their possible integration. Tuning interface charge transport between such new 2D materials and metallic electrodes is a key issue in 2D device physics and engineering. Here, we report tunable interface charge transport in bilayer MoSe2 field effect transistors with Ti/Au contacts showing high on/off ratio up to 107 at room temperature. Our experiments reveal a detailed map of transport mechanisms obtained by controlling the interface band bending profile via temperature, gate and source-drain bias voltages. This comprehensive investigation leads to demarcating regimes and tuning in transport mechanisms while controlling the interface barrier profile. The careful analysis allows us to identify thermally activated regime at low carrier density, and Schottky barrier driven mechanisms at higher carrier density demonstrating the transition from low-field direct tunneling/ thermionic emission to high-field Fowler–Nordheim tunneling. Furthermore, we show that the transition voltage Vtrans to Fowler–Nordheim correlates directly to the difference between the chemical potential of the metal electrode and the conduction band minimum in the 2D semiconductor, which opens up opportunities for new theoretical and experimental investigations. Our approach being generic can be extended to other 2D materials, and the possibility of tuning contact transport regimes is promising for designing MoSe2 device applications.

The Roles of Band Bending, Surface Misorientation, and Passivation on Electrical Transport Across III-V Bonded Structures

With results from bonding of several different materials combinations, surface orientations, and passivation treatments, as well as an assessment of barrier heights reported in the literature for transport in polycrystalline III-V materials as well as for other III-V bonded materials combinations, a general model has emerged. Lattice mismatch does not play a major role in limiting conductivity across an interface; however, misalignment (through twist or tilt) produces interfaces with increased resistance. Passivation techniques are most important for these systems which possess significant interface band-bending. The electrical characteristics of a grain boundary, whether in polcrystalline material or at a bonded interface, are determined by the presence of defect states and interfacial charge which create band bending and Fermi level pinning. The barrier heights and widths from zero bias conductance vs bias measurements over a wide range of temperatures and for different materials can be matched to band structure simulations of bonded semiconductor heterojunctions with resultant I-V curves showing good agreement between experiment and theory. Based on this study, which combines experimental results and transport modelling, it is expected that narrow bandgap III-V semiconductors should show lower barrier heights; but more importantly, modelling indicates that high doping at mismatched interfaces helps to significantly reduce the interface barrier height.