Schottky Barrier Height Reduction Research Articles

P-type Schottky-barrier-free contact to MoS2 via layer-number-assisted interface engineering

Bilayer and few-layer MoS2 show intrinsically higher electronic quality and substantially improved device performance than monolayer, and high-quality MoS2 wafers with controlled layer number have been grown. In addition, MoS2 has different polymorphs such as the semiconducting H phase and semimetallic distorted T (dT) phase, and vertically stacked dT-/H-MoS2 metal-semiconductor junctions (MSJs) have been synthesized. However, the contact mechanism of dT-/H (few layer)-MoS2 MSJs remains to be elucidated, and p-type ohmic contact to MoS2 is difficult to realize due to the high ionization energy of MoS2. In the current work, we reveal a mechanism of two-dimensional (2D) semiconductors (2DSCs) layer-number-assisted metal-semiconductor (SC) interface engineering for Schottky barrier height (SBH) reduction and p-type ohmic contact is achieved based on this mechanism; 2DSCs here mean H (few layer)-MoS2 and metal-SC interfaces are the dT-/H-MoS2 interfaces with increasing 2DSC layers. Specifically speaking, the mechanism is as follows: (1) two competing effects coexist, namely, the interface dipole (ΔV) at the metal-SC interface and the quasibonding (QB) between all adjacent layers, with ΔV (QB) increasing (decreasing) SBH; (2) the effect of QB beats ΔV and hence the overall effect is decreasing SBH at the metal-SC interface; and (3) the SBH reduction effect increases with increasing 2DSC layers. The mechanism of 2DSC layer-number-assisted metal-SC interface engineering should apply also for other 2DSCs with a suitable metal and hence our current work paves an avenue for ohmic contact to few-layer 2DSCs by the accumulated interlayer QBs that widely present in 2DSCs. Published by the American Physical Society 2024

Rich Sulfur Vacancies and Reduced Schottky Barrier Height Synergistically Enable Au/ZnIn2S4 with Enhanced Photocatalytic CO2 Reduction into CO.

Constructing the plasmonic metal/semiconductor heterostructure with a suitable Schottky barrier height (SBH) and the sufficiently reliable active sites is of importance to achieve highly efficient and selective photocatalytic CO2 reduction into hydrocarbon fuels. Herein, we report Au/sulfur vacancy-rich ZnIn2S4 (Au/VSR-ZIS) hierarchical photocatalysts, fabricated via in situ photodepositing Au nanoparticles (NPs) onto the nanosheet self-assembled ZnIn2S4 (ZIS) micrometer flowers (MFs) with rich sulfur vacancies (VS). Density functional theory (DFT) calculations confirm that for the Au/VSR-ZIS system, the Au NPs serve as the reaction sites for H2O oxidation, and the VSR-ZIS MFs serve as those for CO2 reduction. The rich VS in the Au/VSR-ZIS hybrid can reduce its SBH so as to boost more hot electrons in the Au NPs across its Schottky barrier and then inject into the conduction band (CB) of the VSR-ZIS MFs. In addition, VS can also act as the electron sink to trap the photogenerated electrons, retarding the recombination of photogenerated carriers. The two merits effectively enhance the photogenerated electron density in the surface of VSR-ZIS MFs, availing CO2 photoreduction. In addition, the introduction of rich VS in the Au/VSR-ZIS hybrid can offer more active sites, benefiting the CO2 adsorption and accelerating the desorption of CO* from the surface of the photocatalyst. Therefore, under visible light illumination with no sacrificial reagent, the optimum photocatalyst (Au/VSR-ZIS-0.4) presents the enhanced and selective CO2 photoreduction into CO (8.15 μmol g-1h-1 and near 100%), which are superior to those of most of ZIS-based and plasmon-based photocatalysts. The photocatalytic activity is about 40.0-fold as high as that of the Vs-poor-ZIS (VSP-ZIS) MFs. This work contributes a viable strategy for designing highly efficient plasmonic photocatalysts by using the synergism of the anion vacancies and the optimized SBH induced by them.

Lowering the Schottky Barrier Height by Quasi-van der Waals Contacts for High-Performance p-Type MoTe2 Field-Effect Transistors.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) offer advantages over traditional silicon in future electronics but are hampered by the prominent high contact resistance of metal-TMD interfaces, especially for p-type TMDs. Here, we present high-performance p-type MoTe2 field-effect transistors via a nondestructive van der Waals (vdW) transfer process, establishing low contact resistance between the 2D MoTe2 semiconductor and the PtTe2 semimetal. The integration of PtTe2 as contacts in MoTe2 field-effect transistors leads to significantly improved electrical characteristics compared to conventional metal contacts, evidenced by a mobility increase to 80 cm2 V-1 s-1, an on-state current rise to 5.0 μA/μm, and a reduction in Schottky barrier height (SBH) to 48 meV. Such a low SBH in quasi-van der Waals contacts can be assigned to the low electrical resistivity of PtTe2 and the high efficiency of carrier injection at the 2D semimetal/2D semiconductor interfaces. Imaging via transmission electron microscopy reveals that the 2D semimetal/two-dimensional semiconductor interfaces are atomically flat and exceptionally clean. This interface engineering strategy could enable low-resistance contacts based on vdW architectures in a facile manner, providing opportunities for 2D materials for next-generation optoelectronics and electronics.

First Principle Study on Schottky Barrier at Ni/Graphene/4H-SiC Interface

High stability 4H-SiC ohmic contact is currently a key technical challenge that silicon carbide devices urgently need to overcome. It is important to reduce the Schottky barrier height (SBH) at the Ni/4H-SiC interface to optimize ohmic contact. In this paper, the mechanisms of graphene layer changing Ni/4H-SiC interface Schottky barrier height (SBH) are studied based on the first-principles method within the local density approximation. Theoretical studies have shown that graphene intercalation can reduce the SBH of Ni and 4H-SiC interfaces. The reason of SBH reduction may be that the graphene C atoms saturate the dangling bonds on the 4H-SiC surface and the influence of the metal-induced energy gap state at the interface is reduced. In addition, the new phase formed at the interface of graphene and silicon carbide has a lower work function. Furthermore, an interfacial electric dipole layer may be formed at the 4H-SiC/graphene interface which may also reduce the SBH. These results make them to be promising candidates for future radiation resistant electronics.

Fermi-level depinning in Mo/Si junctions by insertion of amorphous Si-rich Mo silicide film formed via gas-phase reactions of MoF6 with SiH4

We demonstrate the reduction of the electron Schottky barrier height (SBH) to 0.48 eV at Mo/n-type Si junctions through the insertion of a semimetal Si-rich Mo silicide (MoSi n , n = 7.9) film. Raman scattering measurements elucidated the persistence of the same amorphous structure within the MoSi n film even when subjected to temperatures as high as 900 °C. This excellent thermal stability yielded a notable result: preservation of the SBH modulation effect even following annealing at 700 °C. Moreover, we investigated the capacitance−voltage characteristics of MOS capacitors, revealing that MoSi n film has a remarkably low effective work function, measuring 4.1 eV when deposited onto SiO2. The deposition of MoSi n film was accomplished with an excellent coverage by using MoF6 and SiH4 gases. Thus, MoSi n film is a promising contact material in advanced CMOS transistors.

Benchmarking the yttrium content in the low temperature/low humidity electrical properties of yttrium-doped barium cerate

Yttrium-doped barium cerate (BCY) is one of the best proton-conducting ceramics to be used in fuel cells/electrolyzers at intermediate temperatures, due to its high hydration enthalpy and high conductivity. Nonetheless, it has commonly been discarded for these applications due to its instability in the presence of carbonaceous and humidified atmospheres. A recent work has addressed this issue, by suggesting the use of this material in very low water vapor partial pressures (pH2O ∼ 10−4 atm) [Electrochimica Acta 334 (2020) 135625] and at low temperatures (T ≤ 400 °C). Under these conditions, BCY can still offer pure protonic conductivity while also offering suitable stability. In the current work, we provide a comprehensive analysis of the effects of the acceptor dopant concentration (Y2O3) in the structural, microstructural, and electrical properties in a wide compositional series of BaCe1-xYxO3-δ (x = 0.05, 0.10, 0.20 and 0.30) at low levels of humidity in the low temperature range (100–400 °C). The results show that BaCe0.9Y0.1O3-δ (BCY10) offers the highest bulk conductivity in these conditions, while further increase in dopant concentration leads to impairment due to “proton trapping” effects. In contrast, the specific grain boundary conductivity increases with increasing dopant concentration due to a reduction of Schottky barrier height at the space-charge layer. We demonstrate that, by operating at very low temperatures (e.g., below 400 °C), the peak performing, BCY10 material can still operate at very low levels of water vapor partial pressure (pH2O ∼ 10−4 atm), where its stability can be maintained. This work paves the way for the optimization of this material, with a focus on utilizing low humidity levels and low temperatures, where it can offer a competitive advantage over that of the majority of other proton-conducting perovskites for potential applications operating under these conditions.

Reduction of Schottky barrier height and contact resistivity in metal contacts to n-type silicon by insertion of interfacial arsenic monolayers

Interfacial arsenic monolayers are used to reduce the Schottky barrier at metal contacts to n-type silicon and consequently reduce the resistance of such contacts. Building on earlier work on group V elemental monolayers and III-V atomic bilayers on (111) silicon, we confirm for the first time that interfacial engineering may achieve very significant reduction of contact resistivity, by a factor of about 100 for aluminum contacts to n-type silicon doped 1 × 1019 cm−3. The effect of an arsenic monolayer is explained theoretically based on the formation of an additional electronic dipole between the metal and the silicon which acts to counter the pinned Schottky barrier between the two. Arsenic monolayers are formed by MBE, RPCVD and MOVPE and characterized through a combination of RBS and SIMS analysis.

All-Transfer Electrode Interface Engineering Toward Harsh-Environment-Resistant MoS2 Field-Effect Transistors.

Nanoscale electronic devices that can work in harsh environments are in high demand for wearable, automotive, and aerospace electronics. Clean and defect-free interfaces are of vital importance for building nanoscale harsh-environment-resistant devices. However, current nanoscale devices are subject to failure in these environments, especially at defective electrode-channel interfaces. Here, harsh-environment-resistant MoS2 transistors are developed by engineering electrode-channel interfaces with an all-transfer of van der Waals electrodes. The delivered defect-free, graphene-buffered electrodes keep the electrode-channel interfaces intact and robust. As a result, the as-fabricated MoS2 devices have reduced Schottky barrier heights, leading to a very large on-state current and high carrier mobility. More importantly, the defect-free, hydrophobic graphene buffer layer prevents metal diffusion from the electrodes to MoS2 and the intercalation of water molecules at the electrode-MoS2 interfaces. This enables high resistances of MoS2 devices with all-transfer electrodes to various harsh environments, including humid, oxidizing, and high-temperature environments, surpassing the devices with other kinds of electrodes. The work deepens the understanding of the roles of electrode-channel interfaces in nanoscale devices and provides a promising interface engineering strategy to build nanoscale harsh-environment-resistant devices.

Effective Schottky barrier height and interface trap density reduction engineering using 2-dimensional reduced graphene oxide interlayer for metal-interlayer-semiconductor contact structure

The contact resistance at the metal-semiconductor (MS) interface has become the foremost challenge to ensuring high performance in nanoelectronics as devices are scaled down. A metal-interlayer-semiconductor (MIS) contact structure is a promising technique for Fermi-level unpinning to overcome the issue of fundamental contact resistance. Herein, an MIS contact structure with a 2-dimensional reduced graphene oxide (rGO) interlayer is proposed as an effective source/drain contact structure. The demonstration of a metal-rGO-semiconductor MIS (rGO MIS) contact structure with different interlayer thicknesses indicated that the rGO interlayer with a thickness of a few nanometers significantly enhanced the reverse current density (JR) by ∼2.0 × 103 times compared to that of the MS contact. In addition, the effective Schottky barrier height was reduced from 0.576 to 0.205 eV. This drastic improvement is attributed to the unique properties of rGO, such as its wide band gap, electrical conductivity, and Van der Waals interface leading to a metal-induced gap state reduction, tunneling resistance lowering, and passivation effect when it is adopted as an interlayer of the MIS contact. This result demonstrates the promising potential of using rGO as an MIS interlayer, which will pave the way toward high-performance emerging nanoelectronic devices.

Resistance degradation in sputtered sodium potassium niobate thin films and its relationship to point defects

The reliability of piezoelectric thin films is crucial for piezoelectric micro-electromechanical system applications. The understanding of resistance degradation in piezoelectric thin films requires knowledge about point defects. Here, we show the resistance degradation mechanism in the lead-free alternative sodium potassium niobate (KNN) thin films and the relationship to point defects in both field-up and field-down polarities. The conduction mechanism of KNN thin films is found to be Schottky-limited. Furthermore, a reduction in Schottky barrier height accompanies the resistance degradation resulting from interfacial accumulation of additional charged defects. We use thermally stimulated depolarization current measurements and charge-based deep level transient spectroscopy to characterize the defects in KNN thin films. Our results show that oxygen vacancies accumulate at the interface in field-up polarity, and multiple defects accumulate in field-down polarity, potentially oxygen vacancies and holes trapped by potassium vacancies. We use wafer deposition variation to create samples with different film properties. Measurement results from these samples correlate resistance degradation with the defect concentration. We find the natural logarithm of leakage current to be linearly proportional to the defect concentration to the power of 0.25. The demonstrated analysis provides a precise and meaningful process assessment for optimizing KNN thin films.

The Variation of Schottky Barrier Height Induced by the Phase Separation of InAlAs Layers on InP HEMT Devices

We investigated the effect of phase separation on the Schottky barrier height (SBH) of InAlAs layers grown by metal–organic chemical vapor deposition. The phase separation into the In-rich InAlAs column and Al-rich InAlAs column of In0.52Al0.48As layers was observed when we grew them at a relatively low temperature of below 600 °C. From the photoluminescence spectrum investigation, we found that the band-gap energy decreased from 1.48 eV for a homogeneous In0.52Al0.48As sample to 1.19 eV for a phase-separated InxAl1−xAs sample due to the band-gap lowering effect by In-rich InxAl1−xAs (x > 0.7) region. From the current density–voltage analysis of the InAlAs Schottky diode, it was confirmed that the phase-separated InAlAs layers showed a lower SBH value of about 240 meV than for the normal InAlAs layers. The reduction in SBH arising from the phase separation of InAlAs layers resulted in the larger leakage current in InAlAs Schottky diodes.

Phase Separation Induced Schottky Barrier Height Change in InAlAs/InP Heterostructure-based HEMT Devices

The effect of phase separation phenomenon in InAlAs layers grown by metal-organic chemical vapor deposition on the Schottky barrier height (SBH) is investigated. The phase separation into In-rich and Al-rich InxAlyAs columns of InAlAs (x = 0.52, y = 0.48) layers was observed when we grow them at a relatively low temperature below 600 °C. From the photoluminescence spectrum investigation, we found that the band-gap energy decreased from 1.48 eV for a homogeneous In0.52Al0.48As sample to 1.19 eV for a phase-separated InxAl1-xAs sample due to the band-gap lowering effect by In-rich InxAl1-xAs (x > 0.7) region. From the current density-voltage analysis of the InAlAs Schottky diode, it was also confirmed that the phase-separated InAlAs layers showed a lower SBH value of about 250 meV than it for the normal InAlAs layers. The reduction of SBH arising from the phase separation of InAlAs layers resulted in the larger leakage current in InAlAs Schottky diodes.

Reduced Schottky barrier height at metal/CVD-grown MoTe2 interface

We demonstrated that Schottky barrier height (SBH) at the metal/CVD-grown MoTe2 interface can be significantly reduced with tunnel contact by inserting a thin Al2O3 layer regardless of the metal work function. The existence of strong Fermi level pinning (FLP) at the metal/MoTe2 interface was verified, while depinning cannot be achieved with Al2O3 insertion. Thus, the fixed charges inside the Al2O3 were proposed to be responsible for the effective SBH reduction in virtue of the eliminated SBH reduction after the post-annealing treatment. This work provides a feasible way to solve the contact issue and favors for the fabrication of high performance MoTe2-based electronic devices.

Quasi-bonding-induced gap states in metal/two-dimensional semiconductor junctions: Route for Schottky barrier height reduction

The van der Waals interface and interfacial reconstruction between two-dimensional (2D) transition-metal dichalcogenides (TMDCs) and metal electrodes significantly influence their device performance. During the growth of $\mathrm{Mo}{\mathrm{S}}_{2}$ and $\mathrm{W}{\mathrm{S}}_{2}$ on metals, the participation of sulfur atoms leads to the Au(100) surface forming a ${\mathrm{Au}}_{4}{\mathrm{S}}_{4}@\mathrm{Au}(100)\text{\ensuremath{-}}(2\sqrt{2}\ifmmode\times\else\texttimes\fi{}2\sqrt{2})R{45}^{\ensuremath{\circ}}$ reconstructed phase [Luo et al., Nat. Commun. 11, 1011 (2020)]. To reveal the nature of the reconstructed interface interactions, here we perform a comparative study for Au/TMDCs junctions with and without Au(100) surface reconstruction by density-functional theory calculations. Metal-induced gap states are apparent in the unreconstructed junction, while with reconstruction, significant quasi-bonding-induced gap states (QBIGSs) appear above the valence band maximum of TMDCs, which are antibonding states from the quasi-bonding interaction at the ${\mathrm{Au}}_{4}{\mathrm{S}}_{4}$/TMDCs interface. The QBIGS favors the formation of $p$-type contacts and significantly reduce the $p$-type Schottky barrier height (SBH). It is anticipated that QBIGS commonly exist at the chalcogenide-reconstructed metal/TMDCs junctions. This study opens a different route for $p$-type SBH reduction in metal/2D TMDCs junctions.

Analysis of Schottky barrier heights and reduced Fermi-level pinning in monolayer CVD-grown MoS2 field-effect-transistors

Chemical vapor deposition (CVD)-grown monolayer (ML) molybdenum disulfide (MoS2) is a promising material for next-generation integrated electronic systems due to its capability of high-throughput synthesis and compatibility with wafer-scale fabrication. Several studies have described the importance of Schottky barriers in analyzing the transport properties and electrical characteristics of MoS2 field-effect-transistors (FETs) with metal contacts. However, the analysis is typically limited to single devices constructed from exfoliated flakes and should be verified for large-area fabrication methods. In this paper, CVD-grown ML MoS2 was utilized to fabricate large-area (1 cm × 1 cm) FET arrays. Two different types of metal contacts (i.e. Cr/Au and Ti/Au) were used to analyze the temperature-dependent electrical characteristics of ML MoS2 FETs and their corresponding Schottky barrier characteristics. Statistical analysis provides new insight about the properties of metal contacts on CVD-grown MoS2 compared to exfoliated samples. Reduced Schottky barrier heights (SBH) are obtained compared to exfoliated flakes, attributed to a defect-induced enhancement in metallization of CVD-grown samples. Moreover, the dependence of SBH on metal work function indicates a reduction in Fermi level pinning compared to exfoliated flakes, moving towards the Schottky–Mott limit. Optical characterization reveals higher defect concentrations in CVD-grown samples supporting a defect-induced metallization enhancement effect consistent with the electrical SBH experiments.

NO2 capture on graphene/WSe2 heterostructure with high sensitivity and selectivity modulated by external electric field

The adsorption performance of ten gas molecules (H2, N2, O2, CO, CO2, H2O, NO, NO2, SO2 and CH4) on graphene/WSe2 heterostructure is studied by means of first-principles calculations. The results show that gas molecules are physisorbed on the heterostructure surface owing to the weak interface interaction between gas molecules and graphene/WSe2. Meanwhile, NO2 has the strongest interactions with the graphene/WSe2 heterostructure among these gas molecules, suggesting the heterostructure has the highest selectivity towards NO2. Moreover, NO2 behaves as an electron acceptor and attracts electrons from graphene/WSe2 heterostructure, resulting in the reduction of Schottky barrier height of the heterostructure. Further, the controlled external electric field regulates the adsorption properties of the gas-graphene/WSe2 system in terms of the adsorption energies, band structures, and charge transfer. It is found that the sensitivity of NO2 on graphene/WSe2 heterostructure can be enhanced under large positive electric field (E = +0.5 V/Å), which induced by the charge redistribution between gas and the heterostructure. Our work will provide a preference to design the WSe2-based sensors with high sensitivity and selectivity.

Enrichment of high-purity large-diameter semiconducting single-walled carbon nanotubes.

Semiconducting single-walled carbon nanotubes SWCNTs (s-SWCNTs) are considered one of the most promising alternatives to traditional silicon-based semiconductors. In particular, large-diameter s-SWCNTs (>1.2 nm) exhibit more advantages over small-diameter ones in high-performance electronic applications because of their higher charge carrier mobility and reduced Schottky barrier height. Great efforts have been made to enriching large-diameter s-SWCNTs from mass-produced raw CNTs that contain both metallic SWCNTs and s-SWCNTs. Among separation technologies, the effective and scalable ones are conjugated polymer wrapping (CPW), gel permeation chromatography (GC), aqueous two-phase extraction (ATPE), and density gradient ultracentrifugation (DGU). In this review, we survey recent progress on enriching large-diameter s-SWCNTs using those methods and outline the strategies and challenges in the separation according to the electronic type and chirality of SWCNTs. Finally, we highlight some applications of the enriched large-diameter s-SWCNTs and outlook for the future of SWCNT-based electronic devices.

First principle study on modulating of Schottky barrier at metal/4H-SiC interface by graphene intercalation

In the production of SiC electronic devices, one of the main challenges is the fabrication of good Ohmic contacts due to the difficulty in finding the metals with low Schottky barriers of wide band gap SiC. Therefore, reducing the Schottky barrier height (SBH) at the metal/SiC interface is of great importance. In this paper, the effects of graphene intercalation on the SBH in different metals (Ag, Ti, Cu, Pd, Ni, Pt)/4H-SiC interfaces are studied by combining the average electrostatic potential and local density of states calculation methods based on first-principles plane wave pseudopotential density functional theory. The calculation results show that single-layer graphene intercalation can reduce the SBH of metal/4H-SiC contact. When the two layers of graphene are inserted, the SBH are further reduced. Especially, the contact between Ni and Ti exhibits negative SBH values, inferring that good Ohmic contacts are formed. When layers of graphene continue to increase, the SBH no longer changes obviously. By analyzing the differential charge density and the local density of states of the interface, the mechanism of SBH reduction may be that the dangling bonds on the SiC surface are saturated by the graphene C atoms and the influence of the metal-induced energy gap state at the interface is reduced, thereby reducing the interface state density. In addition, graphene and the corresponding new phases at the interface have low work functions. Moreover, an interfacial electric dipole layer may be formed at the SiC/graphene interface which also contributes to barrier reduction.

Influence of the interface structure and strain on the rectification performance of lateral MoS2/graphene heterostructure devices.

We systematically study the influence of interface configuration and strain on the electronic and transport properties of lateral MoS2/graphene heterostructures by first-principles calculations and quantum transport simulations. We first identify the favorable heterostructure configurations with C-S and/or C-Mo bonds at the interfaces. Strain can be applied to graphene or MoS2 and would not change the relative stabilities of different heterostructures. Band alignment calculations show that all the lateral heterostructures have n-type Schottky contacts. The current-voltage characteristics of the lateral MoS2/graphene heterostructure diodes exhibit good rectification performance. Too strong and too weak interface interactions do not benefit electronic transport. The MoS2/graphene heterostructures with moderate C-S bonds at the interface have larger currents through the junctions than those with C-Mo bonds at the interface. The maximal rectification ratio of the lateral diode with strain applied to MoS2 can reach up to 105. With strain applied to graphene, the currents through the heterostructures can increase by 1-2 orders of magnitude due to the reduced Schottky barrier heights at the interface, but the rectification ratio is reduced with a maximal value of 104. Our calculations can serve as a theoretical guide to design rectifier and diode devices based on two-dimensional lateral heterostructures.

Atomic layer deposited Al2O3 passivation layer for few-layer WS2 field effect transistors

We have investigated the effect of an Al2O3 passivation layer on the performance of few-layer WS2 FETs. While the performance of WS2 FETs is often limited by a substantial decrease in carrier mobility owing to charged impurities and a Schottky barrier between the WS2 and metal electrodes, the introduction of an Al2O3 overlayer by atomic layer deposition (ALD) suppressed the influence of charged impurities by high-κ dielectric screening effect and reduced the effective Schottky barrier height. We argue that n-doping of WS2, induced by positive fixed charges formed at Al2O3/WS2 interface during the ALD process, is responsible for the reduction of the effective Schottky barrier height in the devices. In addition, the Al2O3 passivation layer protected the device from oxidation, and maintained stable electrical performance of the WS2 FETs over 57 d. Thus, the ALD of Al2O3 overlayer provides a facile method to enhance the performance of WS2 FETs and to ensure ambient stability.