Influence of different oxidants on the band alignment of HfO_2 films deposited by atomic layer deposition

Based on X-ray photoelectron spectroscopy （XPS）, influences of different oxidants on band alignment of HfO2 films deposited by atomic layer deposition （ALD） are investigated in this paper. The measured valence band offset （VBO） value for H2O-based HfO2 increases from 3.17 eV to 3.32 eV after annealing, whereas the VBO value for O3-based HfO2 decreases from 3.57 eV to 3.46 eV. The research results indicate that the silicate layer changes in different ways for H2O-based and O3-based HfO2 films after the annealing process, which plays a key role in generating the internal electric field formed by the dipoles. The variations of the dipoles at the interface between the HfO2 and SiO2 after annealing may lewd the VBO values of H2O-based and O3-based HfO2 to vary in different ways, which fits with the variation of fiat band （VFB） voltage.

Effect of Sb composition on the band alignment of InAs/GaAsSb quantum dots

Aiming to achieve InAs quantum dots(QDs) with a long carrier lifetime,the effects of Sb component in cap layers on the band alignment of the InAs/GaAsSb QDs have been studied.InAs QDs with high density and uniformity have been grown by molecular beam epitaxy.With increasing Sb composition,the InAs/GaAsSb QDs exhibit a significant redshift and broadening photoluminescence(PL).With a high Sb component of 22%,the longest wavelength emission of the InAs/GaAs_(0.78)Sb_(0.22) QDs occurs at 1.5 μm at room temperature.The power-dependence PL measurements indicate that with a low Sb component of 14%,the InAs/GaAs_(0.86)Sb_(0.14) QDs have a type-Ⅰ and a type-Ⅱ carrier recombination processes,respectively.With a high Sb component of 22%,the InAs/GaAs_(0.78)Sb_(0.22) QDs have a pure type-Ⅱ band alignment,with three type-Ⅱ carrier recombination processes.Extracted from time-resolved PL decay traces,the carrier lifetime of the InAs/GaAs_(0.78)Sb_(0.22) QDs reaches 16.86 ns,which is much longer than that of the InAs/GaAs_(0.86)Sb_(0.14) QDs(2.07 ns).These results obtained here are meaningful to realize high conversion efficiency intermediate-band QD solar cells and other opto-electronic device.

Double interface modification promotes efficient Sb2Se3 solar cell by tailoring band alignment and light harvest

The band alignment at the front interfaces is crucial for the performance of Sb_(2)Se_(3) solar cell with superstrate configuration.Herein,a Sn O_(2)/Ti O_(2) thin film,demonstrated beneficial for carrier transport in Sb_(2)Se_(3) device by the first-principle calculation and experiment,is proposed to reduce the parasitic absorption caused by CdS and optimize the band alignment of Sb_(2)Se_(3) solar cell.Thanks to the desirable transmittance of SnO_(2)/TiO_(2) layer,the Sb_(2)Se_(3) solar cell with SnO_(2)/TiO_(2)/(CdS-38 nm) electron transport layer performances better than (CdS-70 nm)/Sb_(2)Se_(3) solar cell.The optimized band alignment,the reduced interface defects and the decreased current leakage of Sb_(2)Se_(3) solar cell enable the short-circuit current density,fill factor,open-circuit voltage and efficiency of the Sb_(2)Se_(3) solar cell increase by 26.7%,112%,33.1%and 250%respectively when comparing with TiO_(2)/Sb_(2)Se_(3) solar cell without modification.Finally,an easily prepared Sn O_(2)/Ti O_(2)/CdS ETL is successfully applied on Sb_(2)Se_(3) solar cell by the first time and contributes to the best efficiency of 7.0%in this work,which is remarkable for Sb_(2)Se_(3) solar cells free of hole transporting materials and toxic CdCl_(2) treatment.This work is expected to provide a valuable reference for future ETL design and band alignment for Sb_(2)Se_(3) solar cell and other optoelectronic devices.

Energy band alignment at ferroelectric/electrode interface determined by photoelectron spectroscopy

The most important interface-related quantities determined by band alignment are the barrier heights for charge trans- port, given by the Fermi level position at the interface. Taking Pb（Zr, Ti）O3 （PZT） as a typical ferroelectric material and applying X-ray photoelectron spectroscopy （XPS）, we briefly review the interface formation and barrier heights at the inter- faces between PZT and electrodes made of various metals or conductive oxides. Polarization dependence of the Schottky barrier height at a ferroelectric/electrode interface is also directly observed using XPS.

Analysis of flatband voltage shift of metal/high-k/SiO_2/Si stack based on energy band alignment of entire gate stack

A theoretical model of flatband voltage （VFB） of metal/high-k/Si02/Si stack is proposed based on band alignment of entire gate stack, i.e., the VFB is obtained by simultaneously considering band alignments of metal/high-k, high-k/SiO2 and SiO2/Si interfaces, and their interactions. Then the VFB of TiN/HfO2/SiO2/Si stack is experimentally obtained and theoretically investigated by this model. The theoretical calculations are in good agreement with the experimental results. Furthermore, both positive VFB shift of TiN/HfO2/SiO2/Si stack and Fermi level pinning are successfully interpreted and attributed to the dielectric contact induced gap states at TiN/HfO2 and HfO2/SiO2 interfaces.

A theoretical investigation of the band alignment of type-I direct band gap dilute nitride phosphide alloy of GaN_xAs_yP_(1-x-y)/GaP quantum wells on GaP substrates

The GaP-based dilute nitride direct band gap material Ga（NAsP） is gaining importance due to the monolithic integra- tion of laser diodes on Si microprocessors. The major advantage of this newly proposed laser material system is the small lattice mismatch between GaP and Si. However, the large threshold current density of these promising laser diodes on Si substrates shows that the carrier leakage plays an important role in Ga（NAsP）/GaP QW lasers. Therefore, it is necessary to investigate the band alignment in this laser material system. In this paper, we present a theoretical investigation to optimize the band alignment of type-I direct band gap GaNxAsyP1-x-y/GaP QWs on GaP substrates. We examine the effect of nitrogen （N） concentration on the band offset ratios and band offset energies. We also provide a comparison of the band alignment of type-I direct band gap GaNxAsyP1-x-y/GaP QWs with that of the GaNxAsyP1-x-y/Al2Ga1-2P QWs on GaP substrates. Our theoretical calculations indicate that the incorporations of N into the well and AI into the barrier improve the band alignment compared to that of the GaAsP/GaP QW laser heterostructures.

Band alignment between NiO_(x) and nonpolar/semipolar GaN planes for selective-area-doped termination structure

Band alignment between NiO_(x) and nonpolar GaN plane and between NiO_(x) and semipolar GaN plane are measured by x-ray photoelectron spectroscopy. They demonstrate that the maximum value of the valence band in the unintentional-doped a-plane, m-plane, and r-plane GaN are comparable to each other, which means that all the substrates are of n-type with similar background carrier concentrations. However, the band offset at the NiO_(x)/GaN interface presents obvious crystalline plane dependency although they are coated with the same NiO_(x) films. By fitting the Ga 3 d spectrum obtained from the NiO_(x)/GaN interface, we find that relatively high Ga–O content at the interface corresponds to a small band offset. On the one hand, the high Ga–O content on the GaN surface will change the growth mode of NiO_(x). On the other hand, the affinity difference between Ga and O forms a dipole which will introduce an extra energy band bending.

Determination of band alignment between GaO_(x)and boron doped diamond for a selective-area-doped termination structure

An n-GaO_(x)thin film is deposited on a single-crystal boron-doped diamond by RF magnetron sputtering to form the pn heterojunction.The n-Ga Ox thin film presents a small surface roughness and a large optical band gap of 4.85 e V.In addition,the band alignment is measured using x-ray photoelectron spectroscopy to evaluate the heterojunction properties.The GaO_(x)/diamond heterojunction shows a type-Ⅱstaggered band configuration,where the valence and conduction band offsets are 1.28 e V and 1.93 e V,respectively.These results confirm the feasibility of the use of n-GaO_(x)as a termination structure for diamond power devices.

Band alignment of p-type oxide/ε-Ga2O3 heterojunctions investigated by x-ray photoelectron spectroscopy

Theε-Ga2O3 p-n heterojunctions(HJ)have been demonstrated using typical p-type oxide semiconductors(NiO or SnO).Theε-Ga2O3 thin film was heteroepitaxial grown by metal organic chemical vapor deposition(MOCVD)with three-step growth method.The polycrystalline SnO and NiO thin films were deposited on theε-Ga2O3 thin film by electron-beam evaporation and thermal oxidation,respectively.The valence band offsets(VBO)were determined by x-ray photoelectron spectroscopy(XPS)to be 2.17 eV at SnO/ε-Ga2O3 and 1.7 eV at NiO/ε-Ga2O3.Considering the bandgaps determined by ultraviolet-visible spectroscopy,the conduction band offsets(CBO)of 0.11 eV at SnO/ε-Ga2O3 and 0.44 eV at NiO/ε-Ga2O3 were obtained.The type-Ⅱband diagrams have been drawn for both p-n HJs.The results are useful to understand the electronic structures at theε-Ga2O3 p-n HJ interface,and design optoelectronic devices based onε-Ga2O3 with novel functionality and improved performance.

Band alignment of Ga_2O_3/6H-SiC heterojunction

A high-quality Ga2O3 thin film is deposited on an SiC substrate to form a heterojunction structure. The band alignment of the Ga2O3/6H-SiC heterojunction is studied by using synchrotron radiation photoelectron spectroscopy, The energy band diagram of the Ga2O3/6H-SiC heterojunction is obtained by analysing the binding energies of Ga 3d and Si 2p at the surface and the interface of the heterojunction. The valence band offset is experimentally determined to be 2.8 eV and the conduction band offset is calculated to be 0.89 eV, which indicate a type-II band alignment. This provides useful guidance for the application of Ga2O3/6H-SiC electronic devices.

Energy band alignment of 2D/3D MoS_(2)/4H-SiC heterostructure modulated by multiple interfacial interactions

The interfacial properties of MoS_(2)/4H-SiC heterostructures were studied by combining first-principles calculations and X-ray photoelectron spectroscopy.Experimental(theoretical)valence band offsets(VBOs)increase from 1.49(1.46)to 2.19(2.36)eV with increasing MoS_(2) monolayer(1L)up to 4 layers(4L).A strong interlayer interaction was revealed at 1L MoS_(2)/SiC interface.Fermi level pinning and totally surface passivation were realized for 4H-SiC(0001)surface.About 0.96e per unit cell transferring forms an electric field from SiC to MoS_(2).Then,1L MoS_(2)/SiC interface exhibits type I band alignment with the asymmetric conduction band offset(CBO)and VBO.For 2L and 4L MoS_(2)/SiC,Fermi level was just pinning at the lower MoS_(2)1L.The interaction keeps weak vdW interaction between upper and lower MoS_(2) layers.They exhibit the type II band alignments and the enlarged CBOs and VBOs,which is attributed to weak vdW interaction and strong interlayer orbital coupling in the multilayer MoS_(2).High efficiency of charge separation will emerge due to the asymmetric band alignment and built-in electric field for all the MoS_(2)/SiC interfaces.The multiple interfacial interactions provide a new modulated perspective for the next-generation electronics and optoelectronics based on the 2D/3D semiconductors heterojunctions.

Improving the band alignment at PtSe_(2)grain boundaries with selective adsorption of TCNQ

Grain boundaries in two-dimensional(2D)semiconductors generally induce distorted band alignment and interfacial charge,which impair their electronic properties for device applications.Here,we report the improvement of band alignment at the grain boundaries of PtSe_(2),a 2D semiconductor,with selective adsorption of a presentative organic acceptor,tetracyanoquinodimethane(TCNQ).TCNQ molecules show selective adsorption at the PtSe_(2)grain boundary with strong interfacial charge.The adsorption of TCNQ distinctly improves the band alignment at the PtSe_(2)grain boundaries.With the charge transfer between the grain boundary and TCNQ,the local charge is inhibited,and the band bending at the grain boundary is suppressed,as revealed by the scanning tunneling microscopy and spectroscopy(STM/S)results.Our finding provides an effective method for the advancement of the band alignment at the grain boundary by functional molecules,improving the electronic properties of 2D semiconductors for their future applications.

Two dimensional GeO_(2)/MoSi_(2)N_(4)van der Waals heterostructures with robust type-Ⅱ band alignment

Constructing two-dimensional(2D)van der Waals heterostructures(vdWHs)can expand the electronic and optoelectronic applications of 2D semiconductors.However,the work on the 2D vdWHs with robust band alignment is still scarce.Here,we employ a global structure search approach to construct the vdWHs with monolayer MoSi_(2)N_(4)and widebandgap GeO_(2).The studies show that the GeO_(2)/MoSi_(2)N_(4)vdWHs have the characteristics of direct structures with the band gap of 0.946 eV and typeII band alignment with GeO_(2)and MoSi_(2)N_(4)layers as the conduction band minimum(CBM)and valence band maximum(VBM),respectively.Also,the direct-to-indirect band gap transition can be achieved by applying biaxial strain.In particular,the 2D GeO_(2)/MoSi_(2)N_(4)vdWHs show a robust type-II band alignment under the effects of biaxial strain,interlayer distance and external electric field.The results provide a route to realize the robust type-II band alignment vdWHs,which is helpful for the implementation of optoelectronic nanodevices with stable characteristics.

Band alignment and polarization engineering inκ-Ga_(2)O_(3)/GaN ferroelectric heterojunction

Ferroelectric-semiconductor heterostructures offer an alternative strategy to manipulate polarization towards advanced devices with engineered functionality and improved performance.In this work,we report on the heteroepitaxial construction,band structure alignment and polarization engineering of the single-phasedκ-Ga_(2)O_(3)/GaN ferroelectric/polar heterojunction.A type-II band alignment is determined at theκ-Ga_(2)O_(3)/GaN polar hetero-interface,with a valence band offset of(1.74±0.1)eV and a conduction band offset of(0.29■0.1)eV.Besides the band edge discontinuity,charge dipoles induced by spontaneous polarization lead to the observed band bending with built-in potentials of 0.9 and 0.33 eV,respectively,at theκ-Ga_(2)O_(3)surface andκ-Ga_(2)O_(3)/GaN interface.The polarization switching properties of ferroelectricκ-Ga_(2)O_(3)are identified with a remanent polarization of approximately 2.7μC/cm^(2)via the direct hysteresis remanent polarization/voltage(P-V)loop measurement.These findings allow the rational design ofκ-Ga_(2)O_(3)ferroelectric/polar heterojunction for the application of power electronic devices,advanced memories and even ultra-low loss negative capacitance transistors.

Energy band alignment of HfO2 on p-type(100)InP

The band alignment of HfO2 film on p-type （100） InP substrate grown by magnetron sputtering was investigated.The chemical states and bonding characteristics of the system were characterized by X-ray photoelectron spectroscopy （XPS）.The results show that there is no existence of Hf-P or Hf-In and there are interfacial In2O3 and InPO4 at the interface.Ultraviolet spectrophotometer （UVS） was employed to obtain the band gap value of HfO2.In 3d and Hf 4f core-level spectra and valence spectra were employed to obtain the valence band offset of HfO2/InP.Experimental results show that the （5.88 ± 0.05） eV band gap of HfO2 is aligned to the band gap of InP with a conduction band offset （△Ec） of （2.74 ± 0.05） eV and a valence band offset （△Ev） of （1.80 ± 0.05） eV.Compared with HfO2 on Si,HfO2 on InP exhibits a much larger conduction band offset （1.35 eV larger）,which is beneficial to suppress the tunneling leakage current.

Strain drived band aligment transition of the ferromagnetic VS_(2)/C_(3)N van der Waals heterostructure

Exploring two-dimensional(2D)magnetic heterostructures is essential for future spintronic and optoelectronic devices.Herein,using first-principle calculations,stable ferromagnetic ordering and colorful electronic properties are established by constructing the VS_(2)/C_(3)N van der Waals(vdW)heterostructure.Unlike the semiconductive properties with indirect band gaps in both the VS2 and C3N monolayers,our results indicate that a direct band gap with type-Ⅱband alignment and p-doping characters are realized in the spin-up channel of the VS_(2)/C_(3)N heterostructure,and a typical type-Ⅲband alignment with a broken-gap in the spin-down channel.Furthermore,the band alignments in the two spin channels can be effectively tuned by applying tensile strain.An interchangement between the type-Ⅱand type-Ⅲband alignments occurs in the two spin channels,as the tensile strain increases to 4%.The attractive magnetic properties and the unique band alignments could be useful for prospective applications in the next-generation tunneling devices and spintronic devices.

Band offsets and electronic properties of the Ga_(2)O_(3)/FTO heterojunction via transfer of free-standing Ga_(2)O_(3) onto FTO/glass

The determination of band offsets is crucial in the optimization of Ga_(2)O_(3)-based devices,since the band alignment types could determine the operations of devices due to the restriction of carrier transport across the heterogeneous interfaces.In this work,the band offsets of the Ga_(2)O_(3)/FTO heterojunction are studied using x-ray photoelectron spectroscopy(XPS)based on Kraut’s method,which suggests a staggered type-II alignment with a conduction band offset(DEC)of 1.66 eV and a valence band offset(DEV)of2.41 eV.Furthermore,the electronic properties of the Ga_(2)O_(3)/FTO heterostructure are also measured,both in the dark and under ultraviolet(UV)illuminated conditions(254 nm UV light).Overall,this work can provide meaningful guidance for the design and construction of oxide hetero-structured devices based on wide-bandgap semiconducting Ga_(2)O_(3).

Anomalous photoluminescence enhancement and resonance charge transfer in type-II 2D lateral heterostructures

Type-Ⅱband alignment can realize the efficient charge transfer and separation at the semiconductor heterointerface,which results in photoluminescence(PL)quenching.Recently,several researches demonstrated great enhancement of localized PL at the interface of type-Ⅱtwo-dimensional(2D)heterostructure.However,the dominant physical mechanism of this enhanced PL emission has not been well understood.In this work,we symmetrically study the exciton dynamics of type-Ⅱlateral heterostructures of monolayer MoS_(2) and WS_(2) at room temperatures.The strong PL enhancement along the one-dimensional(1D)heterointerface is associated with the trion emission of the WS_(2) shell,while a dramatic PL quenching of neutral exciton is observed on the MoS_(2) core.The enhanced quantum yield of WS2trion emission can be explained by charge-transfer-enhanced photoexcited carrier dynamics,which is facilitated by resonance hole transfer from MoS_(2) side to WS_(2) side.This work sheds light on the 1D exciton photophysics in lateral heterostructures,which has the potential to lead to new concepts and applications of optoelectronic device.

Visualizing interface states in In_(2)Se_(3)–WSe_(2)monolayer lateral heterostructures

Recent findings of two-dimensional(2D)ferroelectric(FE)materials provide more possibilities for the development of 2D FE heterostructure electronic devices based on van der Waals materials and the application of FE devices under the limit of atomic layer thickness.In this paper,we report the in-situ fabrication and probing of electronic structures of In_(2)Se_(3)–WSe_(2) lateral heterostructures,compared with most vertical FE heterostructures at present.Through molecular beam epitaxy,we fabricated lateral heterostructures with monolayer WSe_2(three atomic layers)and monolayer In_(2)Se_(3)(five atomic layers).Type-Ⅱband alignment was found to exist in either the lateral heterostructure composed of anti-FEβ′-In_(2)Se_(3) and WSe_(2) or the lateral heterostructure composed of FEβ*-In_(2)Se_(3)and WSe_2,and the band offsets could be modulated by ferroelectric polarization.More interestingly,interface states in both lateral heterostructures acted as narrow gap quantum wires,and the band gap of the interface state in theβ*-In_(2)Se_(3)–WSe_(2)heterostructure was smaller than that in theβ′-In_(2)Se_(3)heterostructure.The fabrication of 2D FE heterostructure and the modulation of interface state provide a new platform for the development of FE devices.

Insights into the efficient charge separation over Nb_(2)O_(5)/2D-C_(3)N_(4) heterostructure for exceptional visible-light driven H_(2) evolution

Two-dimensional carbon nitride(2 D-C_(3) N_(4))nanosheets are promising materials in photocatalytic water splitting,but still suffer from easy agglomeration and fast photogene rated electron-hole pairs recombination.To tackle this issue,herein,a hierarchical Nb_(2) O_(5)/2 D-C_(3) N_(4) heterostructure is precisely constructed and the built-in electric field between Nb_(2)O_(5) and 2 D-C_(3) N_(4) can provide the driving force to separate/transfer the charge carriers efficiently.Moreover,the strongly Lewis acidic Nb_(2)O_(5) can adsorb TEOA molecules on its surface at locally high concentrations to facilitate the oxidation reaction kinetics under irradiation,resulting in efficient photogene rated electrons-holes separation and exceptional photocatalytic hydrogen evolution.As expected,the champion Nb_(2)O_(5)/2 D-C_(3)N_(4) heterostructure achieves an exceptional H2 evolution rate of 31.6 mmol g^(-1) h^(-1),which is 213.6 times and 4.3 times higher than that of pristine Nb_(2)O_(5) and2 D-C_(3)N_(4),respectively.Moreover,the champion heterostructure possesses a high apparent quantum efficiency(AQE)of 45.08%atλ=405 nm and superior cycling stability.Furthermore,a possible photocatalytic mechanism of the energy band alignment at the hetero-interface is proposed based on the systematical characterizations accompanied by density functional theory(DFT)calculations.This work paves the way for the precise construction of a high-quality heterostructured photocatalyst with efficient charge separation to boost hydrogen production.