Interfacial Gap States Research Articles

Spin-selective contact type and strong Fermi level pinning at a CrI3/metal interface

An interface in heterostructures formed by stacking two-dimensional magnets with metals could offer unprecedented electronic and magnetic properties. Here, in a heterostructure with monolayer CrI3 adsorbed on metals (Pb, Ag, Hf, and Au), we find that the nature of the CrI3-metal interface contact is spin-selective, i.e. Ohmic and Schottky contacts for the majority and minority spin channels, respectively. This spin-selective phenomenon is attributed to n-doping, and thus half metallicity of CrI3 in the heterostructure. For the Schottky contact, the slope parameter of a Schottky barrier height is found around 0.15, indicating a strong Fermi level pinning at the CrI3/metal interface. This strong Fermi level pinning is a result of two interfacial behaviors: interface gap states induced by a strong interaction between CrI3 and metals, and a interface dipole generated by charge transfer. These findings could pave the way for the free design of an electrical contact type by manipulating the spin channel in a heterostructure, and enable two-dimensionalmagnets-based versatile spintronics.

Modulated interfacial band alignment of β-Ga2O3/GaN heterojunction by the polarization charge transfer

Designing β-Ga2O3/GaN heterojunction has been regarded as a promising approach to optimize the application of β-Ga2O3. Here, band characters of β-Ga2O3/GaN heterojunction are deeply understood by the polarization effect of GaN, which is vital to design high-performance β-Ga2O3/GaN heterojunction. Due to the inconsistent band bending and orbital distribution induced by GaN polarization effect, just N-terminated β-Ga2O3/GaN heterojunction theoretically shows the type-II band alignment with two-dimensional electron gases (2DEGs). Such 2DEGs are just actually observed for N-terminated β-Ga2O3/GaN heterojunctions with weak interfacial interaction, but obvious interfacial energy barriers are found for heterojunctions with strong interfacial interactions which can weak the polarization charge transfer. The interfacial energy barriers can be turned into energy grooves upon introducing O-dopant with low concentration into the GaN region of heterojunction. In addition, all Ga-terminated β-Ga2O3/GaN heterojunctions possess 2DEG band alignments, but abundant interfacial gap states are observed due to the lost electron of Ga atom from β-Ga2O3 region and accumulated polarization electron of N atom from GaN region. In addition, band bending characters of β-Ga2O3/GaN heterojunctions are negligibly dependent of the atomic arrangements of β-Ga2O3 and GaN surfaces. These works deeply understand the interfacial performances of β-Ga2O3/GaN heterojunctions and provide useful guidance for realizing high-performance β-Ga2O3/GaN heterojunctions.

Impacts of the Electron Transport Layer Surface Reconstruction on the Buried Interface in Perovskite Optoelectronic Devices.

Using density functional theory combined with ab initio molecular dynamics, we comprehensively investigated the performance enhancement mechanism of the device after surface reconstruction by passivating different halogen groups (i.e., F or Cl) at the ETL/perovskite interface. We demonstrated that the halogen group at the ETL layer could stabilize the geometric structure of the perovskite surface by balancing the interfacial interaction, ionic migration, and lead iodide framework. Even though halogen passivation decreased and increased the interface charge transfer at the O- and SnO-terminated MAPbI3/SnO2 interfaces, respectively, halogen passivation optimized surface reconstruction and could theoretically relieve the interface carrier recombination according to the changes in conduction band offsets generated by halogen passivation. Furthermore, the interfacial carrier recombination of the MAPbI3/SnO2 interface was also connected to the interfacial gap states, which were smaller for O-terminated MAPbI3/SnO2 interfaces with halogen passivation-induced surface reconstruction but larger for the SnO-terminated cases. Hence, our findings have implications for the design of buried interface optimization in perovskite optoelectronic devices.

The formation and role of the SiO2 oxidation layer in the 4H-SiC/β-Ga2O3 interface

Although 4H-SiC has been regarded as an excellent substrate for the epitaxial growth of β-Ga2O3, an additional oxidation layer could be formed at the substrate during the high temperature growth process. First-principles calculations were employed to investigate the formation and role of the oxidation layer, which was vital for fabricating high quality β-Ga2O3. Through inserting O atoms and emitting CO molecules, the 4H-SiC substrate was spontaneously oxidized, and the oxidation site and process were dependent of the substrate termination. Such inconsistent characters disappeared for the 4H-SiC/β-Ga2O3 interface, and the SiO2 tetrahedron was preferably formed at all the 4H-SiC/β-Ga2O3 interfaces. Moreover, the 4H-SiC/β-Ga2O3 interfaces with Si-terminations and OGa1-termiantions were easier oxidized due to the lower migration energies of O atoms, in particular the O1 atoms. In addition, the oxidization layer accumulated abundant electrons at the SiO2/β-Ga2O3 contact region and turned the type-I band alignment for 4H-SiC/β-Ga2O3 interface into the type-II band alignment, which might be useful to the charge transport of electronic device. However, many interfacial gap states were introduced into the band gaps of 4H-SiC/β-Ga2O3 interfaces, which were harmful to the optoelectronic device.

Photoelectronic Properties of End-bonded InAsSb Nanowire Array Detector under Weak Light

A simple fabrication of end-bonded contacts InAsSb NW (nanowire) array detector to weak light is demonstrated in this study. The detector is fabricated using InAsSb NW array grown by molecular beam epitaxy on GaAs substrate. The metal-induced gap states are induced by the end-bonded contact which suppresses the dark current at various temperatures. The existence of the interface dipole due to the interfacial gap states enhances the light excitation around the local field and thus upgrades the photoresponsivity and photodetectivity to the weak light. The light intensity of the infrared light source in this report is 14 nW/cm2 which is about 3 to 4 orders of magnitude less than the laser source. The responsivity of the detector has reached 28.57 A/W at room temperature with the light (945 nm) radiation, while the detectivity is 4.81 × 1011 cm·Hz1/2 W−1. Anomalous temperature-dependent performance emerges at the variable temperature experiments, and we discussed the detailed mechanism behind the nonlinear relationship between the photoresponse of the device and temperatures. Besides, the optoelectronic characteristics of the detector clarified that the light-trapping effect and photogating effect of the NWs can enhance the photoresponse to the weak light across ultraviolet to near-infrared. These results highlight the feasibility of the InAsSb NW array detector to the infrared weak light without a cooling system.

Oxidized-monolayer tunneling barrier for strong Fermi-level depinning in layered InSe transistors

In two-dimensional (2D)-semiconductor-based field-effect transistors and optoelectronic devices, metal–semiconductor junctions are one of the crucial factors determining device performance. The Fermi-level (FL) pinning effect, which commonly caused by interfacial gap states, severely limits the tunability of junction characteristics, including barrier height and contact resistance. A tunneling contact scheme has been suggested to address the FL pinning issue in metal–2D-semiconductor junctions, whereas the experimental realization is still elusive. Here, we show that an oxidized-monolayer-enabled tunneling barrier can realize a pronounced FL depinning in indium selenide (InSe) transistors, exhibiting a large pinning factor of 0.5 and a highly modulated Schottky barrier height. The FL depinning can be attributed to the suppression of metal- and disorder-induced gap states as a result of the high-quality tunneling contacts. Structural characterizations indicate uniform and atomically thin-surface oxidation layer inherent from nature of van der Waals materials and atomically sharp oxide–2D-semiconductor interfaces. Moreover, by effectively lowering the Schottky barrier height, we achieve an electron mobility of 2160 cm2/Vs and a contact barrier of 65 meV in two-terminal InSe transistors. The realization of strong FL depinning in high-mobility InSe transistors with the oxidized-monolayer presents a viable strategy to exploit layered semiconductors in contact engineering for advanced electronics and optoelectronics.

A first-principles study of the structural and electronic properties of the epitaxial Ge(1 1 1)/La2O3(0 0 1) heterostructure

The effects of the atomic configuration of the epitaxial Ge(1 1 1)/La2O3(0 0 1) interface on the electrical properties of the structure were studied using first-principles calculations. The interface stability of this heterostructure is susceptible to the atomic configuration of the interface. The Ge–O-bonded interface without interfacial gap states is generally more stable than the Ge–La-bonded interface, which involves interfacial gap states. The band alignment is affected by the charge transfer depending on the interface atomic configuration, and the band bending across the La2O3 region was observed due to the electronic dipole inside the La2O3.

Geometric structure and electronic properties of wurtzite GaN/HfO2 interface: A first-principles study

III-V interface with high-k oxide, especially HfO2, is crucial to the development of high mobility microelectronic devices. In this work, we systematically investigated the wurtzite GaN/cubic HfO2 interface based on the first-principles calculations with density functional theory in terms of the geometric structure and electronic properties. In order to mimic the high-k growth conditions, the interfacial oxygen contents in the slab interface models varied to study the interface stability and the relevant electronic structures. It is found that the oxygen rich interface, i.e., oxygen content of 83.3% (O5), shows the most stability in a large range of the oxygen chemical potential from 0 eV to −4.34 eV. Through the calculations of local density of states and Bader charge analysis, we noticed that increasing the interfacial oxygen content leads to the increase of the valence band offset (VBO) and the decrease of the conduction band offset (CBO), respectively. More importantly, interface O5 displays a promising VBO (0.86 eV) and CBO (1.34 eV), which meets the industrial requirements to confine the carrier in the III-V channel materials. Furthermore, no interfacial gap states are observed in interface O5, indicating that O5 is free of Fermi level pinning. This theoretical exploration suggests that varying oxygen content at the interface could result in the optimal interface for the applications of high mobility electronic devices.

Understanding dipole formation at dielectric/dielectric hetero-interface

Band alignment and dipole formation at the hetero-interface still remain fascinating and, hence, are being intensively investigated. In this study, we experimentally investigate the dipole formation by employing a dielectric/dielectric (Al2O3/GeO2) interface. We investigate the dipole dependence on various post-deposition annealing (PDA) ambiences from the viewpoints of electrical extraction and the X-ray photoelectron spectroscopy measurement. The core level shift at the Al2O3/GeO2 interface is consistent with the dipole changes in various PDA ambiences. We discover that the dipole formation can be well explained by the interface gap state and charge neutrality level theory. These results further confirm the feasibility of gap state theory in explaining the band alignment at hetero-junctions. This study can be a booster to enhance the comprehension of dipole origin at hetero-junction interfaces.

Chemical interaction dictated energy level alignment at the N,N′-dipentyl-3,4,9,10-perylenedicarboximide/CH3NH3PbI3 interface

Here, we report the electronic structures at the N, N′-dipentyl-3,4,9,10-perylenedicarboximide (PTCDI-C5)/CH3NH3PbI3 interface identified in-situ by X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. Strong chemical reactions are found to occur upon the deposition of PTCDI-C5 molecules on CH3NH3PbI3. Electron donation from PTCDI-C5 molecules to CH3NH3PbI3 leads to the filling of surface states and the emergence of an interfacial gap state with its onset tailed to the Fermi level. As a consequence, the downward surface band bending resulting from surface states acting as donor states at the pristine perovskite surface is reduced by 0.2 eV. After the energy level alignment at the interface is established, the perovskite conduction band minimum is found to be in line with the lowest unoccupied molecular orbital favoring the electron extraction with a moderate valence band maximum-highest occupied molecular orbital offset of ∼0.7 eV. The present results demonstrate that interfacial chemical reactions can dictate energetics at organic/perovskite interfaces. Understanding the chemical interaction and resultant electronic structures at those interfaces is crucial for efficient and long-term stable perovskite-based devices when passivation of chemical active sites and matched energy level could be readily reached.

Research on c-HfO2 (0 0 1)/α-Al2O3 (1 -1 0 2) interface in CTM devices based on first principle theory

With the growing application of high-k dielectrics, the interface between HfO2 and Al2O3 play a crucial role in CTM devices. To clearly understand the interaction of the HfO-AlO interface at the atomic and electronic scale, the bonding feature, electronic properties and charge localized character of c- HfO2 (0 0 1)/α-Al2O3 (1 -1 0 2) interface has been investigated by first principle calculations. The c- HfO2 (0 0 1)/α-Al2O3 (1 -1 0 2) interface has adhesive energy about -1.754 J/m2, suggesting that this interface can exist stably. Through analysis of Bader charge and charge density difference, the intrinsic interfacial gap states are mainly originated from the OII and OIII types oxygen atoms at the interface, and only OIII type oxygen atoms can localized electrons effectively and are provided with good reliability during P/E cycles, which theoretically validate the experimental results that HfO2/Al2O3 multi-layered charge trapping layer can generate more effective traps in memory device. Furthermore, the influence of interfacial gap states during P/E cycles in the defective interface system have also been studied, and the results imply that defective system displays the degradation on the reliability during P/E cycles, while, the charge localized ability of interfacial states is stronger than intrinsic oxygen vacancy in the trapping layer. Besides, these charge localized characters are further explained by the analysis of the density of states correspondingly. In sum, our results compare well with similar experimental observations in other literatures, and the study of the interfacial gap states in this work would facilitate further development of interface passivation.

Reconsideration of Metal Work Function at Metal/Semiconductor Interface

It is known that vacuum work function (WF) of metal is composed of bulk and surface terms [1]. The former is characterized by bulk properties of many-body effect and kinetic energy of electrons. The latter is by potential drop through the surface dipole induced by wave function tailing of electron from metal to vacuum through the potential barrier from Fermi-level to vacuum level. This fact implicates that the surface term of WF is sensitive to a counterpart of interface. Namely, the surface term at semiconductor or insulator interface should be different from that at vacuum surface, though vacuum WF is commonly used to discuss band alignment at metal/semiconductor (M/S) or metal/insulator/semiconductor interfaces. On the other hand, Metal induced gap states (MIGS) is the intrinsic Fermi level pinning (FLP) mechanism at M/S interface, and it is also described as a dipole induced by wave function tail into the band gap of semiconductor [2]. We note that the MIGS and surface term of WF at M/S interface are described by common physics by considering that both of hole and electron tailing from metal as shown in Fig. 1. In other words, MIGS seems just corresponded to surface term at M/S interface. Schottky barrier height (SBH) for conduction band of semiconductor at M/S interface is commonly described as shown in Eq. (1). However, considered with above discussion, the FLP caused by MIGS should rather be described as a modulation of WF. Therefore, if the FLP in Eq. (1) is caused by MIGS, the Eq. (1) should be rewritten with a definition of interface WF as shown in Eq. (2), and then from Eqs. (1) and (2), the interface WF can be written as shown in Eq. (3). This indicates that the S parameter of MIGS is a weight average parameter. Next, we would like to discuss how to suppress the WF modulation and MIGS. We think there are two approaches. The one is passive approach from semiconductor side. As shown in Fig. 1, the modulation of wave function tailing is enhanced by narrowing the gap of semiconductor, which is consistent with general trend discussed in MIGS model [2]. Therefore, for example, effective gap widening by inserting ultra-thin dielectrics between metal and semiconductor is effective. The other is active approach from metal side. Since surface term is only modulated in WF, WF modulation should be suppressed by reducing a weight of surface term in WF. Considering the result of first principle calculation that the surface term is drastically decreased by reducing free electron density n in metal [1], we noticed low n metal seems effective. It is also consistent with Heine’s original idea [3]. Here, we emphasize that MIGS can be controlled from both semiconductor and metal sides independently, and that it is different from common other FLP models based on interface gap states only in semiconductor side. We think that metal/germanium (Ge) interface is well understandable based on this view, which means that MIGS is a dominant origin of strong FLP at metal/Ge interface in Eq.(1) [4]. It is well known that the passive approach is effective to increase S parameter [5]. Recently we also demonstrated deviation from the strong FLP trend and S parameter increasing at low n metal/Ge interface by the active approach [6,7]. In that work, interface structure dependence on SBH was also observed at low n metal/Ge interface [6,7]. It might be due to an appearance of surface structure dependence on WF and various local dipoles by suppression of strong WF modulation to align Fermi-level with FLP energy level characterized by bulk Ge. We discussed M/S interface WF in terms of the wave function tailing from metal. We proposed that interface WF is different from vacuum WF in principle and MIGS is corresponded to surface term of interface WF. Furthermore, the strong FLP at metal/Ge interface is reasonably understandable based on the view. Acknowledgement This work was supported by JST CREST Grant Number JPMJCR14F2, Japan, and partly by JSPS KAKENHI Grant Number JP17K06374. Reference [1] N. D. Lang, and W. Kohn, Phys. Rev. B 3, 1215 (1971). [2] W. Mönch, Appl. Surf. Sci. 92, 367 (1996). [3] V. Heine, Phys. Rev. 138, A1689 (1965). [4] T. Nishimura, et al., Appl. Phys. Lett. 91, 123123 (2007). [5] e.g. T. Nishimura, et al., Appl. Phys. Express 1, 051406 (2008). [6] T. Nishimura, et al., Appl. Phys. Express 9, 081201 (2016). [7] T. Nishimura, et al., Ext. Abs. IEEE Semiconductor Interface Specialist Conference 2016. Figure 1

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.

Indium passivation impact on HfO2/GaAs interface: A first-principles study

The interface quality between III–V semiconductor and high-k material is critical for improving the performance of the metal-oxide-semiconductor field effect transistors. Utilizing first-principles calculations, we explore the electronic properties of the GaAs/HfO2 interface when indium interfacial atom defects are introduced. The stability of the interfaces will increase with the increasing of the number of indium interfacial atoms. Meanwhile, the band offsets also depend on the number of indium atom substitutions and interstitials. Furthermore, the substitutions suppress the interfacial gap states near the VBM and have the partially passivation, but the effect of interstitial passivation on the interfacial states are limited.

Effect of metal oxide additions to quality on Ge/GeO2 interfaces

Alloying amorphous GeO2 with Y2O3 or related group IIIA oxides is known experimentally to improve its properties as a gate dielectric in field effect transistors. The mechanism of this is studied here by density functional calculations. The metal site coordination is found to be 6–7, by increasing the oxygen coordination to 3 or higher. The alloying is found to increase the bulk modulus. Alloying also increases the diffusion energy of the oxygen vacancies in GeO2 next to the metal and also increases the vacancy formation energy of oxygens that are second neighbors of the metal sites. In this way, a relatively small metal concentration can reduce the O vacancy diffusion rate and thereby the GeO evolution rate. Oxygen vacancies at the Ge/GeO2 interface next to a metal site are found to divide into two types, those which rebond across the vacancy (La, Hf) and those without rebonding (Y, Sc, Al), the latter being preferable as they do not give rise to interfacial gap states.

Atomic and electronic structures of a-ZnSnO3/a-SiO2interface byab initiomolecular dynamics simulations

The interface between amorphous ZnSnO3 and amorphous SiO2 was investigated by ab initio molecular dynamic simulations. The radial distribution function at the interfacial region shows the significant reduction of the coordination numbers of Zn and Sn and slight decrease in the bond lengths of Zn–O and Sn–O, while keeping those of Si. These phenomena were explained in terms of the differences in both the coordination states of oxygen polyhedra and the connectivity of oxygen polyhedra between amorphous ZnSnO3 and amorphous SiO2. The interfacial energy was calculated to be 0.73 J cm−2 by the annealing at 300 K, while it was 0.29 J cm−2 by the annealing at 900 K, respectively. The reduction of the interfacial energy at higher annealing temperature was considered to be attributed to the increase of the number of M–O–Si bonds (M=Zn and Sn). The interfacial states in the density of states observed at the interface region was induced by the 2p states of oxygen having the decreased coordination number, and the interfacial gap states were not removed even by the 900 K annealing.

Overcoming Electrode‐Induced Losses in Organic Solar Cells by Tailoring a Quasi‐Ohmic Contact to Fullerenes via Solution‐Processed Alkali Hydroxide Layers

It is shown that the performance of inverted organic solar cells can be significantly improved by facilitating the formation of a quasi‐ohmic contact via solution‐processed alkali hydroxide (AOH) interlayers on top of n‐type metal oxide (aluminum zinc oxide, AZO, and zinc oxide, ZnO) layers. AOHs significantly reduce the work function of metal oxides, and are further proven to effectively passivate defect states in these metal oxides. The interfacial energetics of these electron collecting contacts with a prototypical electron acceptor (C60) are investigated to reveal the presence of a large interface dipole and a new interface state between the Fermi energy and the C60 highest occupied molecular orbital for AOH‐modified AZO contacts. These novel interfacial gap states are a result of ground‐state electron transfer from the metal hydroxide‐functionalized AZO contact to the adsorbed molecules, which are hypothesized to be electronically hybridized with the contact. These interface states tail all the way to the Fermi energy, providing for a highly n‐doped (metal‐like) interfacial molecular layer. Furthermore, the strong “light‐soaking” effect is no longer observed in devices with a AOH interface.

Materials Design on the Origin of Gap States in a High-κ/GaAs Interface

Given the demand for constantly scaling microelectronic devices to ever smaller dimensions, a SiO2 gate dielectric was substituted with a higher dielectric-constant material, Hf(Zr)O2, in order to minimize current leakage through dielectric thin film. However, upon interfacing with high dielectric constant (high-κ) dielectrics, the electron mobility in the conventional Si channel degrades due to Coulomb scattering, surface-roughness scattering, remote-phonon scattering, and dielectric-charge trapping. III-V and Ge are two promising candidates with superior mobility over Si. Nevertheless, Hf(Zr)O2/III-V(Ge) has much more complicated interface bonding than Si-based interfaces. Successful fabrication of a high-quality device critically depends on understanding and engineering the bonding configurations at Hf(Zr)O2/III-V(Ge) interfaces for the optimal design of device interfaces. Thus, an accurate atomic insight into the interface bonding and mechanism of interface gap states formation becomes essential. Here, we utilize first-principle calculations to investigate the interface between HfO2 and GaAs. Our study shows that As−As dimer bonding, Ga partial oxidation (between 3+ and 1+) and Ga− dangling bonds constitute the major contributions to gap states. These findings provide insightful guidance for optimum interface passivation.

Electronic properties of InP (001)/HfO2 (001) interface: Band offsets and oxygen dependence

Using ab-initio methods, atomic structures and electronic properties of InP (001)/HfO2 (001) interface are studied within the framework of density functional theory. We examine the InP/HfO2 model interface electronic structures under varying oxidation conditions. The effects of indium and phosphorous concentrations on interfacial bonding, defect states, band offsets, and the thermodynamic stability at the interface are also investigated. The origin of interfacial gap states in InP (001)/HfO2 (001) interface are proposed, mainly from the P-rich oxides, which is validated by our experimental work. This highlights the importance of surface passivation prior to high-κ deposition based on the in situ spectroscopic results of atomic layer deposition of HfO2 on InP.

Chemical trends and passivation of defects at Al2O3:GaAs/InAs/InP/GaSb interfaces

Graphical abstractDisplay Omitted Energy levels of interface defects for GaAs/Al2O3.Comparison with InAs, InP and GaSb.Calculational methods for defects. The III-V compound semiconductor: high-K oxides interfaces have been calculated by first principles density functional theory. The chemical trend of the energy levels of native defects including the Ga dangling bond, As dangling bond, and As-As anti bonding state has been compared among different semiconductors. The defect calculation shows that these defects in InAs, InP, and GaSb will not contribute to the Fermi level pinning. Nitrogen doping at the interface could also eliminate the As-As dimer at the interface and could passivate the interface without interfacial gap states.