Metal-induced Gap States Model Research Articles

Barrier heights and strong fermi-level pinning at epitaxially grown ferromagnet/ZnO/metal Schottky Interfaces for opto-spintronics applications

Schottky contacts at the ferromagnet/ZnO interface are good candidates for the realization and control of several semiconductor emerging magnetic phenomena such spin injection and spin-controlled photonics. In this work, we demonstrate the epitaxial growth of single-phase and wurtzite-ZnO thin films on fcc Pt/Co0.30Pt0.70 (111) electrodes by molecular beam epitaxy technique. While the magnetic properties of the Pt/Co0.30Pt0.70 buffer remain unchanged after the ZnO growth, the electric measurements of back-to-back Schottky diodes reveal Schottky barrier heights at the metal/ZnO interfaces in the range of 590–690 meV using Cu, Pt and Co0.30Pt0.70 contacts. A pinning factor S and a charge neutrality level (CNL) ΦCNL of 0.08 and 4.94 eV, respectively, are obtained indicating a strong Fermi-level pining with a CNL level that lies 0.64 eV bellow the conductance band of ZnO semiconductor. These experimental findings indicate that Co0.30Pt0.70/ZnO interface follows the metal-induced gap states model and can open a pathway for the realization of opto-spintronics applications such spin-LEDs.

Schottky barrier heights of defect-free metal/ZnO, CdO, MgO, and SrO interfaces

The Schottky barrier heights (SBHs) of defect-free interfaces of ZnO, CdO, MgO, and SrO with various metals and different terminations are investigated by density functional supercell calculations. The oxide bands are corrected for their density functional bandgap error by applying a U-type term to their metal-d and O-p states where necessary. The p-type SBHs are found to decrease linearly with increasing metal work function. The pinning factor S of the non-polar and polar interfaces is similar for each oxide. S is found to be 0.26, 0.56, 0.74, and 0.96 for CdO, ZnO, MgO, and SrO, respectively, with S increasing with increasing oxide ionicity. The calculated pinning factors are generally consistent with the metal-induced gap state model in terms of variation in ionicity and dielectric constant. A significant shift of SBHs from the non-polar to the polar interfaces of 0.4, 1, and 0.5 eV for ZnO, MgO, and SrO, respectively, is found, which can be explained by an interfacial dipole. Our results are also useful to describe Co,Fe|MgO interfaces in magnetic tunnel junctions.

Analytical study of Metal-Insulator-Semiconductor contacts for both p- and n-InGaN

This paper has studied the metal–insulator-semiconductor (MIS) contact for both p- and n-InGaN. We found that the insulator layer thickness has a remarkable effect on the Fermi level pinning and barrier height reduction in MIS contacts. Schottky's formation with doped InGaN is explained by extending the MIS contacts' using the metal-induced gap states (MIGS) model. The J-V characteristics clarify the current transport mechanism through the MIS contact with InGaN semiconductor for different temperatures. We observed less control on the barrier height reduction in the case of n-InGaN through changing the thickness of the interfacial layer. The calculated contact resistivities of MIS contact with p- and n-InGaN show better results than the metal–semiconductor contact counterparts. These findings are highly significant to design and fabricate the InGaN based switching devices for converter applications.

Extending the metal-induced gap state model to include silicides

The weaker Fermi-level pinning and face dependence seen in silicides are explained with the addition of defect gap states, leading to a more complete metal-induced gap state model.

Extending the metal-induced gap state model of Schottky barriers

Fermi level pinning at Schottky barriers strongly limits the minimization of contact resistances in devices and thereby limits the scaling of modern Si electronic devices, so it is useful to understand the full range of behaviors of Schottky barriers. The authors find that some semiconductor interfaces with compound metals like silicides have apparently weaker Fermi level pinning. This occurs as these metals have an underlying covalent skeleton, whose interfaces with semiconductors lead to miscoordinated defect sites that create additional localized interface states that go beyond the standard metal-induced gap states (MIGSs) model of Schottky barriers. This causes a stronger dependence of Schottky barrier height on the metal and on interface orientation. These states are argued to be an additional component needed to extend the MIGS model.

Termination-dependence of Fermi level pinning at rare-earth arsenide/GaAs interfaces

The properties of metal/semiconductor interfaces are generally described by the metal-induced gap states (MIGS) model. However, rare-earth (RE) arsenide interfaces are found not to follow the MIGS model in having very different Schottky barrier heights (SBHs) for the Ga- or As-terminations of polar (100) or (111) RE-As/GaAs interfaces. Density function supercell calculations find this effect is due to localized defect interface states located on the mis-coordinated atoms of these interfaces that pin their SBHs at very different energies for each termination as determined by the anion sublattice bonding. Band offsets of semiconducting ScN/GaN interfaces also depend on their termination as determined by the same defect interface states. This pinning mechanism dominates any MIGS mechanism when it arises. Nonpolar (110) interfaces have little change in bonding, so they have no defect interface states, and we find their SBH is pinned by MIGS at the charge neutrality level. Hence, traditional MIGS models should be extended to include such interface states in a more general description.

Schottky barrier height at metal/ZnO interface: A first-principles study

The Schottky barrier heights (SBHs) of various metals on ZnO are investigated by first-principles calculation. The SBHs decrease linearly with increasing metal work function, which follows the prediction of the metal-induced gap states (MIGS) model. The pinning factor S is calculated to be 0.56 which indicates moderate pinning effect. A closer look at the interfacial electronic structure shows the dominant rule of oxygen in forming the MIGS. To extend the concept of MIGS model to the band alignment between semiconductors, a calculation is performed on Si/ZnO interface. Si is found to have a type-II band alignment with ZnO, the conduction band offset (CBO) and valence band offset (VBO) are calculated to be 0.5 eV and 2.5 eV respectively. The results agree with the experimental values and the predicted values based on the charge neutrality level (CNL) method.

Face Dependence of Schottky Barriers Heights of Silicides and Germanides on Si and Ge

Density functional supercell calculations of the Schottky barrier heights (SBH) of metal germanides and silicides on Si or Ge find that these vary with the facet, unlike those of elemental metals. In addition, silicides and germanides show a stronger dependence of their SBHs on the work function than those of elemental metals, as seen experimentally. Both effects are beyond the standard metal induced gap states model. NiSi2 is found to have a much lower SBH on n-Si(100) than on n-Si(111), as seen experimentally. It is shown how such results can be used to design lower SBH contacts for n-Ge, which are needed technologically. The SBHs of the better behaved Si/silicide interfaces can be used to benchmark the behavior of the less well behaved Ge-germanide interfaces for this purpose. The dependence of the SBH of epitaxial Pb-Si(111) on its reconstruction is also covered.

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

Effect of Nano Al Interlayer on the Schottky Contact Property Between Metal and Hg3In2Te6 Wafer.

In this paper, the fabrication and characterization of Schottky diodes based on Hg3In2Te6 (short for MIT) wafer are presented. The Al interlayer with 1 nm thickness was grown on the MIT wafer by Vacuum thermal evaporation method. Diodes characterization of metal/Al/MIT contact were performed by I-V measurement. Then the Schottky barrier height and ideality factor were calculated from I-V test results by means of the thermionic emission model. The results showed that the Schottky barrier heights of Ni/Hg3In2Te6, Au/Hg3In2Te6 were reduced from 0.86, 0.85 eV to 0.74, 0.81 eV respectively, and ideality factors decreased from 1.88 and 1.89 to 1.67 and 1.58 accordingly when the Al thin film about 1 nm in thickness was used for the interlayer of the MIT Schottky diode. It was found that the Al interlayer can significantly alleviates the Fermi-level pinning, and the interface defect mechanisms for the depinning were discussed based on interface states model and the metal-induced gap states model.

Influence of interface structure on electrical properties of NiGe/Ge contacts

We have investigated the electrical properties of NiGe/n-type Ge contacts with various crystalline structures. We found that the Schottky barrier height (SBH) of the NiGe/Ge contact could be modulated by controlling the crystalline structures of the NiGe layer. An epitaxial NiGe(100)/n-type Ge(110) contact shows an SBH 0.15 eV lower than that of the polycrystalline NiGe/n-type Ge(110) contact, whereas the SBH of the epitaxial NiGe(111)/n-type Ge(001) contact is not so low compared with that of the polycrystalline NiGe/n-type Ge(001) contact with Fermi level pinning, even though an epitaxial NiGe layer with good crystalline quality is formed. This distinctive modulation of SBH could not be explained by considering only the metal-induced gap states model. We consider that this modulation of SBHs is a consequence of the variation in the density of the interface states induced by the extrinsic factors, such as the dangling bonds and/or defects. Our results give us a hint that the modulation of the SBH by controlling the crystalline structures is possible.

Effect of surface structure on workfunction and Schottky-barrier height in SrRuO3/SrTiO3 (001) heterojunctions

We present an ab-initio theoretical study of work functions and surface energies of SrRuO3 (001) surfaces and Schottky-barrier heights (SBHs) at various interfaces in SrRuO3/SrTiO3 (001) heterostructure within the framework of the density-functional theory. The SrRuO3 workfunctions are found to exhibit strong dependence on surface terminations. The workfunction of two defect-free SrRuO3 (001) surface terminations, viz., SrO and RuO2, differ by as much as 2.37 eV. The p-type SBH at the RuO2/SrO/TiO2 interface is calculated to be 1.27 eV. The substitution of interfacial SrO layer by isoelectronic BaO layer induces small change in the p-SBH (∼0.06 eV). However, the p-SBH is reduced significantly (∼0.5 eV) as the RuO2 layer is substituted by MnO2 layer due to large change in the interfacial dipole. The p-SBH at different interfaces in SrRuO3/SrTiO3 structures are also estimated using semi-empirical metal-induced-gap-states (MIGS) model. The estimated values are found to be larger by ∼2 eV than those obtained using ab-initio method, rendering the validity of MIGS model questionable in the prediction of SBH in all-oxide metal/dielectric heterojunctions. The modification of SBH by interfacial doping offers the possibility of contact resistance control in SrRuO3/SrTiO3 heterostructures and related devices.

Mechanism of Schottky barrier height modulation by thin dielectric insertion on n-type germanium

Although high channel electron mobility has been reported after some passivation techniques, the performance of n-channel Ge metal-oxide-semiconductor field-effect transistor is still limited by the high Schottky barrier height at the metal/n-Ge contact interface, which comes from the Fermi level pinning effect. Recent experiments demonstrated that the Schottky barrier height can be reduced by inserting a thin dielectric layer between metal and Ge. However, the mechanism has not been well clarified. In this paper, the metal induced gap state model, the dipole layer model, and the fixed charge model are verified by varying contact metals, dielectric thicknesses, as well as the annealing temperatures. The pinning factor is improved slightly by dielectric insertion but its value is independent of the dielectric thickness and is still much lower than the ideal value of the non-pinning case. This pinning effect is consistent with the Fermi level pinning at the metal/TiO2 interface. After thermal process, no interfacial layer forms at the TiO2/Ge interface and the TiO2 crystallizes gradually after annealing but the Schottky barrier height increases. Since the amount of fixed charges in the thin dielectric layer estimated from a metal-insulator-semiconductor structure is about 2 × 1011 cm−2 and is insufficient to produce the observed 0.5 eV Schottky barrier height reduction, it is thus recommended that the main mechanism comes from the change of interface dipoles and the annealing effect is attributed to the short-range ordering of the TiO2 layer. Furthermore, dielectric with low conduction band offset which has good thermal stability should be explored.

Electrical characterization of Schottky contacts to N-polar GaN

The Schottky barrier heights of several metals (Cu, Au, Pd, Ni, and Pt) to N-polar GaN were extracted using current–voltage and capacitance–voltage measurements. The dependence of barrier height on metal was found to vary linearly with the electronegativity of the metal as predicted by the metal-induced gap states (MIGS)-and-electronegativity model. However, the magnitude of the barrier heights are lower than those predicted by the MIGS model for GaN and lower than the experimentally measured barrier heights for Ga-polar GaN. It is likely that the polarization-induced charge at the N-polar GaN surface is responsible for the reduced barrier height.

Evaluating the use of electronegativity in band alignment models through the experimental slope parameter of lanthanum aluminate heterostructures

In this work, photoelectron spectroscopy is used to characterize the band alignment of lanthanum aluminate heterostructures which possess a wide range of potential applications. It is found that our experimental slope parameter agrees with theory using the metal-induced gap states model while the interface induced gap states (IFIGS) model yields unsatisfactory results. We show that this discrepancy can be attributed to the correlation between the dielectric work function and the electronegativity in the IFIGS model. It is found that the original trend, as established largely by metals, may not be accurate for larger band gap materials. By using a new correlation, our experimental data shows good agreement of the slope parameter using the IFIGS model. This correlation, therefore, plays a crucial role in heterostructures involving wider bandgap materials for accurate band alignment prediction using the IFIGS model.

Schottky barrier height and conduction mechanisms in ferroelectric bismuth titanate

Band structure of ferroelectric bismuth titanate is calculated by first-principles computations under the framework of density functional theory. Using the metal induced gap state model, the Schottky barrier height on Pt electrode is estimated to be as high as 1.26 eV, which indicates that the Schottky effect may not be the dominant conduction mechanism in bismuth titanate. By further comparisons with the experimental data, we conclude that the leakage current behavior of bismuth titanate films is dominated by bulk limited conduction mechanisms and can be reduced by better processing conditions or doping.

Ultrathin K∕p-Si(001) Schottky diodes as detectors of chemically generated hot charge carriers

Ultrathin potassium layers were deposited on hydrogen passivated Si(001)−1×1 surfaces at a temperature of 120K in the thickness range from submonolayers up to 3nm. They were investigated with Auger spectroscopy, work function, and current-voltage measurements. After the formation of a wetting layer, potassium deposition leads to island growth. The surface hydrogen atoms are removed by the adsorption process. By attaching an electrical contact to the thin film, the current-voltage characteristics of the Schottky diodes could be determined. The analysis yields a homogeneous Schottky barrier height of 0.55eV for K∕p-Si(001) diodes in agreement with the metal induced gap state model. Exposing the diodes to molecular oxygen generates a strong chemicurrent signal which first increases with exposure, passes a maximum, and drops exponentially. The chemicurrent transient is attributed to a nucleation and growth formation of oxide islands and gives strong evidence for the existence of precurser states prior to oxidation.

Tuning of Fermi level position at HfNx/SiO2 interface

We demonstrate that the vacuum work function of HfNx, as measured in situ by Kelvin probe, can be tailored on the entire silicon band gap by changing the nitrogen content of the HfNx alloy. We observed an increase in the effective work function of HfNx on SiO2, extracted from capacitance-voltage measurements, only for x<1, and saturation is reached at a value of 4.6 eV for x>1. The measured value of the effective work function for pure Hf (4.1 eV) can be explained by the metal induced gap states model, while the HfNx behavior is explained by oxidation at the HfNx/SiO2 interface.

Evidence for strong Fermi-level pinning due to metal-induced gap states at metal/germanium interface

The purpose of this paper is to understand metal/germanium (Ge) junction characteristics. Electrode metals with a wide work function range were deposited on Ge. All metal/p-Ge and metal/n-Ge junctions have shown Ohmic and Schottky characteristics, respectively, with the strong Fermi-level pinning. The charge neutrality level (CNL) at metal/Ge interface is close to the branch point calculated for the bulk Ge. Moreover, the pinning level is hardly modulated by annealing in forming gas, forming metal-germanide/Ge interfaces or changing the substrate orientation. These results suggest that Fermi level at metal/Ge interface is intrinsically pinned at the CNL characterized by the metal-induced gap states model.

Band gap and Schottky barrier heights of multiferroic BiFeO3

Bi Fe O 3 is an interesting multiferroic oxide and a potentially important Pb-free ferroelectric. However, its applications can be limited by large leakage currents. Its band gap is calculated by the density-functional based screened exchange method to be 2.8eV, similar to experiment. The Schottky barrier height on Pt or SrRuO3 is calculated in the metal induced gap state model to be over 0.9eV. Thus, its leakage is not intrinsic.