Band Alignment In Heterostructures Research Articles

Discovery of Stable Surfaces with Extreme Work Functions by High‐Throughput Density Functional Theory and Machine Learning

AbstractThe work function is the key surface property that determines the energy required to extract an electron from the surface of a material. This property is crucial for thermionic energy conversion, band alignment in heterostructures, and electron emission devices. This work presents a high‐throughput workflow using density functional theory (DFT) to calculate the work function and cleavage energy of 33,631 slabs (58,332 work functions) that are created from 3,716 bulk materials. The number of calculated surface properties surpasses the previously largest database by a factor of ≈27. Several surfaces with an ultra‐low (<2 eV) and ultra‐high (>7 eV) work function are identified. Specifically, the (100)‐Ba‐O surface of BaMoO3 and the (001)‐F surface of Ag2F have record‐low (1.25 eV) and record‐high (9.06 eV) steady‐state work functions. Based on this database a physics‐based approach to featurize surfaces is utilized to predict the work function. The random forest model achieves a test mean absolute error (MAE) of 0.09 eV, comparable to the accuracy of DFT. This surrogate model enables rapid predictions of the work function (≈ 105 faster than DFT) across a vast chemical space and facilitates the discovery of material surfaces with extreme work functions for energy conversion and electronic device applications.

Exploration of underlap induced high-k spacer with gate stack on strain channel cylindrical nanowire FET for enriched performance

This research explores a comprehensive examination of gate underlap incorporated strained channel Cylindrical Gate All Around Nanowire FET having enriched performances above the requirement of the 2 nm technology node of IRDS 2025. The device installs a combination of strain engineering based quantum well barrier system in the channel region with high-k spacers sandwiching the device underlaps and stack high-k gate-oxide. The underlaps are prone to parasitic resistance and various short channel effects (SCEs) hence, are sandwiched by HfO2 based high-k. This SCE degradations and a strong electric field in the drain-channel region is rendered controlling the leakages. The strain based Nanosystem engineering is incorporated with Type-II heterostructure band alignment inducing quantum well barrier mechanism in the ultra-thin cylindrical channel region creating an electrostatic charge centroid leading to energy band bending and splitting among the two-fold and four-fold valleys of the strained Silicon layer. This provides stupendous electron mobility instigating high current density and electron velocity in the channel. Thereby, the device is susceptible to on-current enhancement via ballistic transport of carriers and carrier confinement via succumbing of quantum charge carriers. The device transconductance, Ion, Ioff, Ion/Ioff ratio are measured and the output performance (ID-VDS) characteristics is determined providing emphatic enrichments in contrast to the existing gate all-around FETs as well as the 2 nm technology node data of IRDS 2025. Hence, the strained channel Nanowire FET device developed here is presented here as the device of the future for various digital applications, RF applications and faster switching speed.

Gate controllable band alignment transition in 2D black-arsenic/WSe2 heterostructure

Controlling the manner of band alignment of heterostructures increases design freedom with novel physical properties, enables the design of new functional devices, and improves device performance, but the lattice matching limits the diversity of traditional heterostructures. Van der Waals heterostructures (vdWHs) fabricated by rationally mechanical restacking different two-dimensional (2D) layered materials or sequential synthesis can overcome this limitation. However, it is difficult to achieve full control over the band alignment for a specific vdWHs by means of an applied vertical electric field. Here, we take advantage of the band structure alignment properties of narrow-bandgap black-arsenic (b-As) and large-bandgap WSe2 to realize b-As/WSe2 vdWHs with type-I band alignment. The band alignment can be tuned from type I to type II by gate electric field, which greatly improves the photoresponsivity over 103. An ultra-fast photoresponse of about 570 ns is obtained, which is much better than that of vdWHs with the same structure. The b-As/WSe2 vdWHs also can achieve high-performance rectifier phototransistor with an ultra-high rectification ratio exceeding 106, a small conductance slope of about 86 mV/dec, and a low curvature coefficient of about 46 V−1. Our work paves the way for the exploitation of b-As heterojunction for ultra-fast and low-power optoelectronic applications.

Strain and interfacial defect tailored electronic structures of two-dimensional WSe2/h-BN van der Waals heterostructure

In this work, strain and interfacial defect tailored electronic structures of h-BN/WSe2 heterostructure are investigated systematically. The results show that the WSe2/h-BN heterostructure is a direct bandgap semiconductor (1.211[Formula: see text]eV) with type-I band alignment compared with the isolated h-BN and WSe2 monolayers. Applying the in-plane strain can well adjust the electronic structure of heterostructure, resulting in a transition from indirect to direct bandgap at the strain of −2% for the h-BN/WSe2 heterostructures. The bandgap of h-BN/WSe2 heterostructure monotonically increases at the compressive strains from −6% to −2%, whereas decreases at the tensile strains from 0% to 8%. In addition, introducing of vacancy defects and n- or p-type doping can effectively alter the band alignment of heterostructure. When the N and B vacancies or C doping are introduced in the h-BN layer, a significant transform from type-I to type-II band alignment is observed. These results suggest the h-BN/WSe2 heterostructure becomes a good candidate for the application of optoelectronics and nanoelectronics devices.

Band alignment study of 2D-2D heterointerface of MoSe2 with Ti3C2Tx (transport layer) for flexible broadband photodetection

This work signifies the band alignment study of solution processed MoSe2 with MXene (Ti3C2Tx) using Ultraviolet Photoelectron Spectroscopy and utilizing the same for broadband photodetection. Based on the work function values calculated from ultraviolet photoelectron spectroscopy (UPS), MXene functions as an excellent transport layer for the photogenerated excitons generated in MoSe2. Due to the wideband absorbance of MoSe2, the photodetection experiments were carried out from the visible to NIR range (554 nm–780 nm). To examine the heterointerface's band alignment and carrier migration mechanism between MoSe2 and MXene, work function values were extracted from UPS and used to understand the actual energy band diagram of the MoSe2/MXene interface. The fabricated MoSe2/MXene photodetector exhibits Responsivity of 6.58 mA/W and 9.82 mA/W for visible and NIR light illumination supporting that the fabricated device has a high response to the NIR spectrum due to its high absorption at NIR light incidence. The durability of the MoSe2/MXene photodetector was tested four times by examining the device response, wherein change in the Responsivity values was observed to be negligible. Understanding the band alignment of heterostructure through proper extraction of the work function values from UPS is a step ahead in choosing perfect materials for photodetection and optoelectronic applications.

Doping Engineering in the MoS2 /SnSe2 Heterostructure toward High-Rejection-Ratio Solar-Blind UV Photodetection.

The intentionally designed band alignment of heterostructures and doping engineering are keys to implement device structure design and device performance optimization. According to the theoretical prediction of several typical materials among the transition metal dichalcogenides (TMDs) and group-IV metal chalcogenides, MoS2 and SnSe2 present the largest staggered band offset. The large band offset is conducive to the separation of photogenerated carriers, thus MoS2 /SnSe2 is a theoretically ideal candidate for fabricating photodetector, which is also verified in the experiment. Furthermore, in order to extend the photoresponse spectrum to solar-blind ultraviolet (SBUV), doping engineering is adopted to form an additional electron state, which provides an extra carrier transition channel. In this work, pure MoS2 /SnSe2 and doped MoS2 /SnSe2 heterostructures are both fabricated. In terms of the photoelectric performance evaluation, the rejection ratio R254 /R532 of the photodetector based on doped MoS2 /SnSe2 is five orders of magnitude higher than that of pure MoS2 /SnSe2 , while the response time is obviously optimized by 3 orders. The results demonstrate that the combination of band alignment and doping engineering provides a new pathway for constructing SBUV photodetectors.

The WS2/AlN heterostructure band alignment by semi-local functional and its optical properties by the Bethe-Salpeter approximation on top of GW calculations

In this study, the electronic properties and band alignment of the heterostructures of WS2/AlN at three different separation distances of d1 = 2.77, d2 = 4.34, and d3 = 6.37 Å, were investigated by GGA approximation. The calculation of energy changes in terms of volume alternations revealed that the heterostructure at d2 = 4.34 Å is the most energetically favored configuration. The WS2/AlN showed the band gap of 1.68, 1.52, and 1.69 eV by GGA at separation distances of d1, d2, and d3, respectively showing the band alignment of type II. Since the accuracy of the optical properties and detecting the excitonic states relies on the accuracy of the band structure, the quasi-particle calculation based on the G0W0 approximation was performed on the most energetically favored WS2/AlN. The G0W0 calculations revealed that the heterostructure of WS2/AlN at d2 having a bandgap of 3 eV. The Bethe-Salpeter equation on top of G0W0 calculations showed that the exciton state having the binding energy of 0.64 eV.

Heterostructure Silicon Solar Cells with Enhanced Power Conversion Efficiency Based on Si x /Ni3+ Self-Doped NiO x Passivating Contact.

Developing efficient crystalline silicon/wide-band gap metal-oxide thin-film heterostructure junction-based crystalline silicon (c-Si) solar cells has been an attractive alternative to the silicon thin film-based counterparts. Herein, nickel oxide thin films are introduced as the hole-selective layer for c-Si solar cells and prepared using the reactive sputtering technique with the target of metallic nickel. An optimal Ni3+ self-doped NiOx film is obtained by tuning the reactive oxygen atmosphere to construct the optimized c-Si/NiOx heterostructure band alignment. A thin SiOx interlayer was further introduced to reduce the defect of the c-Si/NiOx interface with the UV–ozone (UVO) treatment. The constructed p-type c-Si/SiOx/NiOx/Ag solar cell exhibits an increase in the open voltage from 586 to 611 mV and achieves a 19.2% conversion efficiency.

Band alignment engineering of a Ruddlesden–Popper perovskite-based heterostructure constructed using Cs2SnI2Cl2 and α-In2Se3: The effects of ferroelectric polarization switching and electric fields

Ferroelectricity is an important source of the fascinating optoelectronic properties of heterostructures. Interfaces formed between ferroelectrics and oxides or transition metal dichalcogenides have undergone rapid development. However, the integration of ferroelectrics and two-dimensional Ruddlesden–Popper (RP) perovskites has rarely been studied so far. Herein, we use first-principle calculations to investigate ferroelectric polarization and electric field control of band alignments within the RP perovskite-based heterostructure α-In2Se3/Cs2SnI2Cl2. Our calculations demonstrate that the band alignment of heterostructure can be changed from type-II to type-III by switching the ferroelectric polarization direction in the α-In2Se3 layer. Furthermore, application of an external electric field can modulate the band structure of the α-In2Se3/Cs2SnI2Cl2 heterostructure and induce the band alignment transition. These findings highlight the importance of ferroelectric polarization switching in band alignment engineering and suggest the possibility of electric field-tunable multi-band alignment in Ruddlesden–Popper perovskite-based heterostructures.

Direct Synthesis and Enhanced Rectification of Alloy-to-Alloy 2D Type-II MoS2(1- x ) Se2 x /SnS2(1- y ) Se2 y Heterostructures.

The interfacial tunable band alignment of heterostructures is coveted in device design and optimization of device performance. As an intentional approach, alloying allows band engineering and continuous band-edge tunability for low-dimensional semiconductors. Thus, combining the tunability of alloying with the band structure of a heterostructure is highly desirable for the improvement of device characteristics. In this work, the single-step growth of alloy-to-alloy (MoS2(1- x ) Se2 x /SnS2(1- y ) Se2 y ) 2D vertical heterostructures is demonstrated. Electron diffraction reveals the well-aligned heteroepitaxial relationship for the heterostructure, and a near-atomically sharp and defect-free boundary along the interface is observed. The nearly intrinsic van der Waals (vdW) interface enables measurement of the intrinsic behaviors of the heterostructures. The optimized type-II band alignment for the MoS2(1- x ) Se2 x /SnS2(1- y ) Se2 y heterostructure, along with the large band offset and effective charge transfer, is confirmed through quenched PL spectroscopy combined with density functional theory calculations. Devices based on completely stacked heterostructures show one or two orders enhanced electronmobility and rectification ratio than those of the constituent materials. The realization of device-quality alloy-to-alloy heterostructures provides a new material platform for precisely tuning band alignment and optimizing device applications.

A nonlocal approach for semianalytical modeling of a heterojunction vertical surrounding-gate tunnel FET

A semianalytical model based on a nonlocal approach is proposed for an undoped tunnel field-effect transistor (TFET) with a vertical surrounding-gate structure. The heterostructure band alignment is computed by applying the affinity rule on suitable potential expressions obtained from the two-dimensional (2-D) electrostatic solution for all device regions. The fringing field, doping-induced degeneracy, ambipolarity, and dual modulation effects are included with the aim of obtaining a large domain of validity. The core model is completed with expressions for the capacitance of the terminals and validated against numerical simulations obtained using ATLAS-2D software. An investigation of the types of band alignment and the impact of doping on the device performance is also conducted. The developed models could be implemented into commercial simulators to investigate circuits based on such multigate field-effect transistors (FETs).

Structural and Optical Properties of Multilayer Heterostructure of CdTe/CdSe Thin Films

Abstract CdTe/CdSe multilayer heterostructures thin films were prepared by thermal evaporation method. The CdTe/CdSe layers deposited on glass substrate and effects of annealing temperature on optical and structural properties of thin films were investigated. The XRD analysis revealed that CdTe and CdSe were in cubic (111) and hexagonal (100) forms. By increasing the annealing temperature, intensity of XRD peaks increased for multilayer heterostructures. Band alignment in heterostructures of CdTe/CdSe was of type II. Energy band gaps values for CdTe/CdSe multilayer heterostructures increased with respect to bulk compounds. During emission, red and blue shifts are observed in visible region in photoluminescence spectrum of CdTe/CdSe samples. Due to better crystallinity of multilayer thin film, 1LO, 2LO and 3LO phonon modes were observed in Raman spectrum.

Ultrafast Exciton Dynamics and Light-Driven H2 Evolution in Colloidal Semiconductor Nanorods and Pt-Tipped Nanorods

Colloidal quantum confined one-dimensional (1D) semiconductor nanorods (NRs) and related semiconductor-metal heterostructures are promising new materials for efficient solar-to-fuel conversion because of their unique physical and chemical properties. NRs can simultaneously exhibit quantum confinement effects in the radial direction and bulk like carrier transport in the axial direction. The former implies that concepts well-established in zero-dimensional quantum dots, such as size-tunable energetics and wave function engineering through band alignment in heterostructures, can also be applied to NRs; while the latter endows NRs with fast carrier transport to achieve long distance charge separation. Selective growth of catalytic metallic nanoparticles, such as Pt, at the tips of NRs provides convenient routes to multicomponent heterostructures with photocatalytic capabilities and controllable charge separation distances. The design and optimization of such materials for efficient solar-to-fuel conversion require the understanding of exciton and charge carrier dynamics. In this Account, we summarize our recent studies of ultrafast charge separation and recombination kinetics and their effects on steady-state photocatalytic efficiencies of colloidal CdS and CdSe/CdS NRs and related NR-Pt heterostructures. After a brief introduction of their electronic structure, we discuss exciton dynamics of CdS NRs. By transient absorption and time-resolved photoluminescence decay, it is shown that although the conduction band electrons are long-lived, photogenerated holes in CdS NRs are trapped on an ultrafast time scale (∼0.7 ps), which forms localized excitons due to strong Coulomb interaction in 1D NRs. In quasi-type II CdSe/CdS dot-in-rod NRs, a large valence band offset drives the ultrafast localization of holes to the CdSe core, and the competition between this process and ultrafast hole trapping on a CdS rod leads to three types of exciton species with distinct spatial distributions. The effect of the exciton dynamics on photoreduction reactions is illustrated using methyl viologen (MV(2+)) as a model electron acceptor. The steady-state MV(2+) photoreduction quantum yield of CdSe/CdS dot-in-rod NRs approaches unity under rod excitation, much larger than CdSe QDs and CdSe/CdS core/shell QDs. Detailed time-resolved studies show that in quasi-type II CdSe/CdS NRs and type II ZnSe/CdS NRs strong quantum confinement in the radial direction facilitates fast electron transfer and hole removal, whereas the fast carrier mobility along the axial direction enables long distance charge separation and slow charge recombination, which is essential for efficient MV(2+) photoreduction. The NR/MV(2+) relay system can be coupled to Pt nanoparticles in solution for light-driven H2 generation. Alternatively, Pt-tipped CdS and CdSe/CdS NRs provide fully integrated all inorganic systems for light-driven H2 generation. In CdS-Pt and CdSe/CdS-Pt hetero-NRs, ultrafast hole trapping on the CdS rod surface or in CdSe core enables efficient electron transfer from NRs to Pt tips by suppressing hole and energy transfer. It is shown that the quantum yields of photodriven H2 generation using these heterostructures correlate well with measured hole transfer rates from NRs to sacrificial donors, revealing that hole removal is the key efficiency-limiting step. These findings provide important insights for designing more efficient quantum confined NR and NR-Pt based systems for solar-to-fuel conversion.

Gate-tunable resonant tunneling in double bilayer graphene heterostructures.

We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

Effects of Interface States on Photoexcited Carriers in ZnO/Zn2SnO4 Type-II Radial Heterostructure Nanowires

Type-II band alignment of heterostructure contributes to spatially separate electrons and holes leading to an increase in minority carrier lifetime, which has much more advantages in photocatalytic activities and photovoltaic device applications. Here, Zn2SnO4-sheathed ZnO radial heterostructure nanowires were constructed to investigate systematically interfacial charge separation. The lattice mismatch between ZnO and Zn2SnO4 induces interface states to exist at their heterointerface. At low pump fluence, photoexcited charges are localized within the ZnO core rather than separated due to the large interface barrier. Correspondingly, only ZnO-related bandedge ultraviolet (UV) and green emissions are dominated in photoluminescence spectra. At high pump fluence, however, impurities are ionized and electrons trapped in interface states are excited, resulting in a decrease in interface barrier, which makes photogenerated charges efficiently separated at their heterointerface by direct tunneling, and, consequently, an additional blue-violet emission, attributed to the heterointerface recombination of electrons in Zn2SnO4 conduction band (CB) and holes in ZnO valence band. Additionally, the heterointerface can separate effectively photoexcited carriers and form a photovoltaic effect. Our results provide the localization/separation condition of photogenerated charges for the type-II band alignment of core/shell heterostructure, which should be very useful for the realization of underpinned mechanism of the developed optoelectronic devices.

Coexistence of type-I and type-II band alignment in Ga(Sb, P)/GaP heterostructures with pseudomorphic self-assembled quantum dots

Band alignment of heterostructures with pseudomorphic GaSb1 − xPx/GaP self-assembled quantum dots (SAQDs) lying on a wetting layer was studied. Coexistence of type-I and type-II band alignment was found within the same heterostructure. Wetting layer has band alignment of type-I with the lowest electronic state belonging to the XXY valley of GaSb1 − xPx conduction band, in contrast to SAQDs, which have band alignment of type-II, independently of the ternary alloy composition x. It is shown that type-I-type-II transition is a result of GaP matrix deformation around the SAQD.

Chemical and electronic properties of Fe/MgO/Ge heterostructures for spin electronics

We report on the chemical and electronic properties of epitaxial Fe/MgO/Ge(001) heterostructures probed by X-ray Photoemission Spectroscopy. At variance with the Fe/MgO/Fe system, annealing at 570 K produces a sizable interdiffusion at the upper Fe/MgO interface, while at 470 K this process is inhibited. The XPS analysis of band alignment in heterostructures annealed at 470 K grown onto an intrinsic Ge substrate indicates that the Fermi level is placed at the center of the MgO gap and that the Schottky barrier height is 0.35±0.1 eV, thus indicating a partial depinning of the Fermi level.

Charge transport and trapping in InN nanowires investigated by scanning probe microscopy

Charge transport and trapping in InN nanowires (NWs) and their networks have been investigated using scanning current voltage microscopy (SIVM) and scanning gate microscopy (SGM). SIVM maps indicate highly conducting NWs and nanojunctions as well as significant variation in surface barrier height along the NWs, which are strongly affected by deformations. SIVM measurements were used to determine the electrical conductivity and carrier mobility of individual NWs exploiting the unusually large probe current under reverse bias, arising out of possible type II heterostructure band alignment. Strong correlation between surface barrier change and electrical conductivity of the NW was observed, which can be explained by considering a high density of electron accumulation at the NW surface. SGM measurements performed on NW field effect transistors reveal large scale trapping of carriers under reverse bias, while cyclic drain current-probe voltage measurements indicate both trapping and detrapping rates to be strongly dependent on the magnitude of the reverse bias. Application of a negative probe bias pulse resulted in a drain current recovery transient with time constant of tens of seconds indicating large activation energy for the traps whose density is estimated to be in excess of 2×1013 cm−2.

Modification of heterojunction band offsets at III–V/IV/III–V interfaces

III–V/IV semiconductor heterojunctions (with IV=Si, Ge, and III–V=GaAs, AlAs) as well as III–V/IV/III–V single and multiple quantum well structures have been synthesized by molecular beam epitaxy and characterized by a variety of in situ and ex situ characterization techniques. The goal was to investigate the composition profile achievable at the interface, the band discontinuities, the charged versus neutral character of the interfaces, and the dependence of the band discontinuities on the local interface environment. Relatively abrupt composition profiles can be achieved through appropriate growth protocols. Large deviations from the commutativity and transitivity rules of heterojunction band offsets in these systems are related to the establishment of inequivalent local interface environments. Such deviations can in principle be exploited to tune the band alignment in heterostructures.

GexSi1−x/Si Heterostructures: Physics and Device Applications

ABSTRACTWork on GexSi1−x strained layer epitaxy is reviewed including: The limits of single layer and superlattice growth, and comparison with theory; Physical properties including bandgap and heterostructure band alignment; Work on non-random alloys; Device applications including MODFET's, PIN and APD photodetectors.