Novel Conductive Filament MetalâInterlayerâSemiconductor Contact

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Functional Inorganic Materials and Devices

Novel Conductive Filament Metal-Interlayer-Semiconductor Contact Structure for Ultra-Low Contact Resistance Achievement Seung-Hwan Kim, Gwang-Sik Kim, June Park, Changmin Lee, Hyoungsub Kim, Jiyoung Kim, Joon Hyung Shim, and Hyun-Yong Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07066 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Applied Materials & Interfaces

Novel Conductive Filament Metal-InterlayerSemiconductor Contact Structure for Ultra-Low Contact Resistance Achievement Seung-Hwan Kim,1 Gwang-Sik Kim, 1 June Park,2 Changmin Lee,3 Hyoungsub Kim,3 Jiyoung Kim,4 Joon Hyung Shim,5 and Hyun-Yong Yu1,2,* 1

School of Electrical Engineering, Korea University, Seoul 02841, Korea

2

Department of Semiconductor Systems Engineering, Korea University, Seoul 02841, Korea

3

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon

16419, Korea 4

Department of Materials Science and Engineering, University of Texas at Dallas, Texas 75080,

United States 5

School of Mechanical Engineering, Korea University, Seoul 02841, Korea

*corresponding author: [email protected]

KEYWORDS: conductive filament, metal-induced gap state, fermi-level pinning, metalinterlayer-semiconductor structure, source/drain contact, III-V semiconductor.

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ABSTRACT: In the post-Moore era, it is well known that contact resistance has been a critical issue in determining the performance of complementary metal–oxide–semiconductor (CMOS) reaching physical limits. Conventional Ohmic contact techniques, however, have hindered rather than helped the development of CMOS technology reaching its limits of scaling. Here, a novel conductive filament metal–interlayer–semiconductor (CF-MIS) contact—which achieves ultralow contact resistance by generating CFs and lowering Schottky barrier height (SBH)—is investigated for potential applications in various nanodevices in lieu of conventional Ohmic contacts. This universal and innovative technique, CF-MIS contact, forming the CFs to provide a quantity of electron paths as well as tuning SBH of semiconductor, is firstly introduced. The proposed CF-MIS contact achieves ultra-low specific contact resistivity, exhibiting up to ~×700,000 reduction compared to that of the conventional metal-semiconductor (MS) contact. This study proves the viability of CF-MIS contacts for future Ohmic contact schemes, and that they can easily be extended to mainstream electronic nanodevices that suffer from significant contact resistance problems.


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1. INTRODUCTION Since the past few decades, the growth and advancement of Si-based complementary metal–oxide–semiconductor (CMOS) technology has been highly dependent upon high device performance, and large-scale integration.1 However, as the CMOS technology is reaching its physical limits in the sub-10 nm technology regime, III-V compound semiconductors, and Ge are becoming sound candidates for replacing Si, owing to their excellent electron mobilities.1,2 Moreover, new device structures have been hailed for future nanodevice technologies.3—6 Although these candidates have the advantageous properties for use in next-generation CMOS technologies, they all have serious issues pertaining to their contacts that should be overcome to achieve high performance.7—9 The source/drain (S/D) Ohmic contact issue cannot be perfectly solved by alternating channel materials or changing device structures, because the root cause of that is scaled S/D region. Therefore, the S/D contact resistance—which is a major factor in the total resistance of highly scaled electronic devices—becomes a key issue that hinders device performance.9 Many types of contacts for electronic nanodevices have been reported that reduce the thickness of the depletion region at semiconductor surface via dopant segregation and form alloy metal which has low resistivity (e.g., silicide and germanide for Si and Ge, respectively, and alloy contacts for III-V compound semiconductors).9—13 However, currently, there has been little progress for the sub-10 nm regime. The physical limitations of the contacts have deeply challenged further scaling of the electronic nanodevices.14,15 Therefore, a new, and better concepts, techniques, and approaches for contact Ohmic formation should be studied to improve the performance of these devices and continue development in electronic technologies. To solve these problems, new S/D contact engineering, and lowering the Schottky barrier height (SBH) of the semiconductor contacts, metal–interlayer–semiconductor (MIS) contacts, by


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inserting ultra-thin wide-bandgap material between the metal and the semiconductor, has been studied.16—29 This wide-bandgap interlayer can unpin the Fermi level through alleviating the metal-induced gap states (MIGS), resulting in SBH lowering. Furthermore, the type of interlayer materials, which have a low conduction band offset (CBO) to semiconductor, is important to reduce tunneling resistance between the metal and the semiconductor. Also, since this MIS contact structure does not consume semiconductor in the S/D region, it is more suitable than the conventional contacts in terms of scaling down. Although the interlayer can reduce contact resistance by unpinning the Fermi level, ultra-low contact resistance cannot be achieved as demanded by the sub-10 nm regime due to low film conductivity of the wide-bandgap interlayer. Therefore, a novel technique, conductive filaments (CFs) formation in the interlayer, that brings a significant improvement in electrical conduction through the interlayer by eliminating undesirable tunneling resistance while maintaining its SBH lowering effect should be developed to attain ultra-low contact resistance. In our quest to achieve both high electrical conduction in the contact, and low SBH, we developed a novel contact technique, which is the formation of CFs inside the interlayer. The proposed CFs-formed MIS (CF-MIS) contacts present excellent electrical properties via an electroforming step in the interlayer.


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2. EXPERIMENTAL DETAILS 2.1. MIS Contact Fabrication. The Si-doped GaAs (Nd = 1 × 1018 cm–3) was cleaned sequentially with acetone, 2-propanol, and deionized water to remove organics, followed by 20% of HCl solution (1 min) wet cleaning to remove the native oxide, and (NH4)2S solution (1 min) for passivation to prevent native oxide formation after wet cleaning. The cleaned substrates were immediately loaded in atomic layer deposition (ALD). The TiO2 film was formed by repeating cycles of a titanium tetraisopropoxide (TTIP) precursor with an H2O gas reactant, followed by N2 purging at 250 °C. The ZnO film was formed by repeating cycles of a diethyl zinc (DEZ) precursor (0.5 sec) with an H2O gas reactant (1 sec), and N2 purging (15 sec) following each step, at 150 °C. The Al2O3 film was formed by repeating cycles of a trimethyl aluminum (TMA) precursor (0.5 sec) with an H2O gas reactant (1 sec), and N2 purging (15 sec) after each step, in 250 °C chamber temperature. After the ultra-thin interlayers were deposited, the Ti and Au contacts were deposited by electron-beam evaporation, with TLM pattern using a mask aligner. 2.2. CFs Formation. The gold probe tips were attached to the top electrode, and the back side of the GaAs substrate, whose details are shown in Figure S1. The voltage was swept in a range from 0 to −5 V to form the CFs in the interlayers, and sequentially swept in a range from −5 to 0 V to compare the current difference between before and after the electroforming step, with a compliance current of 0.1 A. Most interlayers were electrically broken at ~−4.5 V, and the currents of the MIS and CF-MIS contacts were measured by electrical measurement setup (Keithley 4200-SCS). 2.3. Electrical Characterization. The reverse and forward current densities of the MS, the MIS, and the CF-MIS contacts were measured by Keithley 4200-SCS, whose applied voltage ranged


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from −1 to 1 V. The I–V characteristics of TLM patterned back-to-back diode were measured to extract specific contact resistivity, whose details are shown in Figure S2. 2.4. Characterization of CFs. The conductive atomic force microscopy (CAFM) analysis in contact mode was performed using an AFM setup (XE-100, Park systems) at room temperature. Current mapping was obtained using a Cr/Pt-Ir coated tip scanning across of 5 × 5 µm2, with a contact force of 10.04 nN under applied voltage of 1 V. A focused ion beam (FIB) milling process (Nova 200, FEI) were performed to prepare the transmission electron microscopy (TEM) specimen. The CF-MIS structure were scanned by a field emission TEM (JEM-2100F, JEOL LTD.) at 200 kV. The X-ray photoelectron spectroscopy (XPS) analysis to confirm the oxygen vacancies was performed using an XPS system (X-tool, Ulvac PHI), which has an aluminum Kα line X-ray source of 1486.6 eV photon energy, 24.1 W, and 15 kV. A fitting method based on a Lorentzian-Gaussian combination, a Shirley background type, and a carbon peak (C 1s) calibration were used to analyze various peaks of interlayers.


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3. RESULTS AND DISCUSSION Figure 1a shows a schematic overview of the CF-MIS contact, and the zoom-in illustrates the CFs between the metal contact (Au/Ti), and the GaAs substrate. The details of the device fabrication process are shown via cross-sectional schematics, and a process flow chart in Figure S3. Figure 1b shows the resistive switching behavior of the Ti/TiO2 (2 nm)/n-GaAs contact with a compliance current of 0.1 A (Figure S4 shows the same for other interlayer thicknesses). The negative electroforming step—a voltage sweep from 0 to −5 V on the MIS contacts—is used to properly form the CFs, because the electrical breakdown behavior cannot be observed in forward bias due to the rectifying property of the contact. When the positive voltage is applied to the metal contact, electrical breakdown isn’t observed up to maximum compliance current because current level of the MIS contacts is already high. Before electroforming step, the current in the Ti/TiO2/n-GaAs contact stays in high resistance state (HRS) with reverse bias, but suddenly increases at −4.7 V. As the voltage is swept back from −5 to 0 V after the electroforming step, the current is significantly increased, which means that it has switched from HRS to low resistance state (LRS) as the CFs are formed in the TiO2 interlayer. The details of the CF formation process are shown in Figure S1. Figure 1c shows the J–V characteristics of the Ti/CFs-formed TiO2/n-GaAs contact, and the Ti/n-GaAs contact for different TiO2 interlayer thicknesses. A weakly rectifying contact is already formed at the Ti/n-GaAs contact because the GaAs substrate is moderately doped by Si dopant (Nd: 1 × 1018 cm–3). To unpin the Fermi level at the GaAs surface, and lower the SBH, ultra-thin TiO2 is inserted between the Ti and the GaAs; and to form the CFs, a negative electroforming step is performed. The behavior transition of the contact from Schottky-like to Ohmic-like is observed for all the CF-MIS contacts. The CF-MIS contact with 2 nm-thick TiO2 exhibits the highest current density: ~×25,000 higher than that of


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the Ti/n-GaAs contact at −1 V. Contacts with other TiO2 thicknesses exhibit similar or slightly low current densities compared to that with 2 nm thickness of TiO2, because the Fermi-level is fully unpinned and tunneling resistance is slightly high due to its thickness for the CF-MIS contact with 3 and 5 nm thicknesses of the TiO2. For CF-MIS contact with 1-nm-thick TiO2, the tunneling resistance is low but the Fermi-level isn’t fully unpinned yet. The J–V characteristics of the Ti/TiO2/n-GaAs contact, and the Ti/n-GaAs contact before electroforming step are presented in Figure S5a. The Ohmic I–V characteristics induced by CF-MIS contacts can be explained by band diagrams, as illustrated in Figure 2. The strong Fermi level pinning causes large SBH at metal– semiconductor (MS) contacts for most semiconductor materials because the charge neutrality level (CNL), which determines the pinning point, is commonly located near the edge of the valence band of the semiconductor, as shown in Figure 2a.16—18 The rectifying I–V characteristics result from the large SBH at contacts. In contrast, the CF-MIS structure makes the contact exhibit Ohmic I–V characteristics in two mechanisms: (i) lowered SBH induced by the Fermi level unpinning effect of interlayer, and (ii) formation of highly conductive current paths provided by conductive filaments, as shown in Figure 2b. The Fermi level can effectively be unpinned by the insertion of an ultra-thin widebandgap material between the metal and the semiconductor.20 To increase the probability of electron tunneling between the metal and the semiconductor, studies using TiO2 and ZnO interlayers, which have low CBO to semiconductors, have been conducted.26—29 The TiO2, ZnO, and Al2O3 interlayers, whose electron affinity values are 4.33, 4.35, and 2.58 eV and CBOs to GaAs are −0.26, −0.28, and +1.49 eV, respectively, are used in this study to verify effect of the CBO differences on their electrical characteristics. When the interlayer, which has positive CBO


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value to semiconductor such as Al2O3, is used for the MIS contact, the tunneling probability is very low to pass through the interlayer, but when the interlayer, which has near zero or negative CBO value to semiconductor such as TiO2 and ZnO, is used for the MIS contact, the tunneling probability is significantly increased compared to high CBO materials. After the CFs are formed in the interlayer via electroforming, major electron current paths through the interlayer can be generated;30—34 the zoom-in images of Figure 2b show the possible electron current paths from the metal to the semiconductor before and after the electroforming step. Electron paths are still possible before the electroforming step where the interlayer is thin enough to tunnel easily, i.e., (i) direct tunneling can occur. When it comes to using an interlayer of low CBO to the semiconductor, electrons can pass through the interlayer by following mechanisms: (ii) Thermionic emission, which is thermally activated flow of electrons over the interlayer barrier; and (iii) Fowler-Nordheim (FN) tunneling, which is electron tunneling from the metal to the conduction band of the interlayer at the triangular barrier as shown in upper zoom-in image in Figure 2b. These current paths are strongly dependent on the interlayer thickness, and its CBO to the semiconductor. However, after the electroforming step, major electron current paths are created by the formation of CFs in the interlayer due to (iv) the generation of electron flow via donations from free oxygen ions; and due to (v) trap-to-trap tunneling, which is the hopping of electrons in the localized traps, such as oxygen vacancies as shown in lower zoom-in image in Figure 2b. The details of conduction mechanisms from the cathode to the anode through the oxide has been comprehensively reviewed by Yu et al.34 These major new electron current paths provide the highest conductivity between the metal and the semiconductor, regardless of the kind of interlayer material, and the interlayer thickness. Furthermore, these oxygen vacancies also indicate that the interlayers are slightly doped.


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Therefore, band bending can be occurred at the interlayer because of interlayer doping, which means its tunneling thickness is reduced. It can also pull down the surface potential of the semiconductor, causing slight SBH lowering.27,35 The interfacial dipole can also be induced by oxygen vacancies in the interlayer. Since the oxygen densities of the CFs-formed interlayers are relatively lower than those of the as-deposited interlayers, relatively positive potential is induced at the semiconductor surface by oxygen ion diffusion.35 For these reasons, the CF-MIS structures can improve its electrical conductivity by forming CFs in the interlayer, as well as inducing the SBH lowering effect at the semiconductor surface as shown in lower zoom-in image in Figure 2b. Then, ZnO, and Al2O3 are also used as interlayers to examine whether this proposed CFMIS structure is affected by the CBO of the interlayers to semiconductor. Figure 3a and c show the resistive switching behaviors of the Ti/ZnO (2 nm), and Al2O3 (2 nm)/n-GaAs contacts with a compliance current of 0.1 A (Figure S6 and S7 show the same for other interlayer thicknesses). Like the CF-MIS contact with TiO2 interlayer, the negative electroforming step is performed to form the CFs in the ZnO and Al2O3 interlayers. The reverse current of Ti/ZnO and Al2O3/n-GaAs contacts then suddenly increases somewhere between −4.5 and −5 V. Figure 3b shows the J–V characteristics of the MS contact, and the CF-MIS contact for different ZnO interlayer thicknesses. The ZnO, which has low CBO to GaAs, is as suitable for the MIS structure as TiO2. The J–V characteristics of the Ti/CFs-formed ZnO/n-GaAs contacts are shown in Figure 3b. They exhibit forward-reverse current symmetry, meaning that Ohmic contacts have formed, and the reverse current densities of all the CF-MIS contacts have been greatly improved from that of the MS contact. The optimal thickness of the ZnO for the highest electrical conductivity is observed to be 2 nm; other thicknesses of ZnO exhibit similar or


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slightly low conductivities for the same reason of the CF-MIS contacts with TiO2 interlayer. The current density of Ti/ZnO (2 nm)/n-GaAs contact is ~×24,000 higher than that of Ti/n-GaAs contact at −1 V. Figure S5b shows the J–V characteristics of both the Ti/ZnO/n-GaAs contacts, and the Ti/n-GaAs contacts. All the MIS contacts show higher current densities than that of the Ti/n-GaAs contact due to low CBO between ZnO and GaAs. Figure 3d shows the J–V characteristics of the MS contact, and the Ti/Al2O3/n-GaAs contact for different interlayer thicknesses. Like TiO2, and ZnO, the reverse current densities of the CF-MIS contacts with Al2O3 are greatly improved from that of the MS contact, even for thick Al2O3 interlayers, because the conductivity of the interlayer is increased by the formation of CFs and the effective SBH is lowered by alleviating the effect of the Fermi-level pinning. The optimal current density is exhibited at the CF-MIS contact with 2-nm-thick Al2O3 interlayer, which is ~×10,500 higher than that of the Ti/n-GaAs contact at −1 V. The reverse current densities of the Ti/Al2O3/n-GaAs contacts with 1-nm and 2-nm-thick Al2O3 interlayers are slightly higher than that of the MS contact, but the optimal interlayer thickness is 1 nm, contrary to both the Ti/TiO2/n-GaAs contact and the Ti/ZnO/n-GaAs contact, as shown in Figure S5c. Since the Al2O3 has wider band gap than that of TiO2 and ZnO, it can unpin the Fermi-level with thinner layer.20 Furthermore, because the Al2O3 has high CBO to the GaAs, the current densities of the Ti/Al2O3/n-GaAs contacts with interlayers thicker than optimal thickness are significantly degraded. By comparing the reverse current densities of the different MIS contacts, a trade-off between the Fermi level unpinning effect, and the increase in tunneling resistance is observed due to its wide-band gap and high CBO to GaAs. To verify existence of the CFs in the interlayers after the electroforming step, CAFM analysis was carried out. Figure 4a, b, and c show the CAFM images of the TiO2, ZnO, and


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Al2O3 interlayers in the LRS; and the inset images show the same in the HRS. Area of 5 × 5 µm2 are scanned at 1 V, with a compliance current of 10 nA. While all the as-deposited samples— non-electroformed interlayers—exhibit insulating properties, the CFs-formed interlayers show high electrical conductivity. The root-mean-square (RMS) currents of the TiO2, ZnO, and Al2O3 films measured in HRS are ~283, ~285, and ~320 pA, respectively. However, the interlayers in the LRS present partially high conductive regions, which are expected with the formation of CFs. The CAFM experiments prove that the CFs certainly exist inside the interlayers after the electroforming process, and that they are randomly distributed over whole interlayer area. Additionally, high-resolution TEM (HR-TEM) is used to ascertain the ultra-thin interlayers, and the Ti/TiO2/n-GaAs, the Ti/ZnO/n-GaAs, and the Ti/Al2O3/n-GaAs structures, as shown in Figure 4d, e, and f, respectively. It is confirmed that the interlayers are smoothly, and uniformly deposited on the GaAs surface by atomic layer deposition (ALD). Figure 5 shows the XPS spectra results of Ti 2p, Zn 2p, Al 2p, and O 1s to verify the generation of oxygen vacancies after electroforming. A negative shift towards the lower binding energy intends that oxygen vacancies have increased in the case of the Ti 2p peak.18,36 Figure 5a shows the Ti 2p peaks for the as-deposited, and the electroformed TiO2 films at ~458.63 eV, and ~458.33 eV, respectively. Contrary to the Ti 2p peaks, as shown in Figure 5b, the Zn 2p peak shifted from ~1021.78 eV to ~1022.4 eV, which indicates the change in the amount of Zn2+ ions, corresponding to the oxygen vacancies that are generated in the ZnO interlayer after electroforming.26,37 The Al 2p peaks of the Al2O3 interlayers before and after electroforming step are shown in Figure 5c. The Al 2p peak for the as-deposited Al2O3 film is at ~74.3 eV, shifting to ~74.4 eV after electroforming. A positive shift to the higher binding energy may be attributed to the increase in oxygen vacancies.38


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The O 1s peaks of the TiO2, ZnO, and Al2O3 films after electroforming step are shown in Figure 5d, e, and f, respectively. Insets of Figure 5d, e, and f exhibit the O 1s peaks of each asdeposited interlayer. All the electroformed interlayers show larger amounts of the oxygen vacancies compared to those of as-deposited interlayers, which verifies that the electroforming step effectively generates oxygen vacancies inside the interlayers. Note that the CFs are generally identified as oxygen vacancies, or ionized metals.32,33,39,40 The electroformed TiO2 film exhibits the largest amount of the oxygen vacancies, as shown in Figure 5d, e, and f, which is in accordance with the experimental electrical data. Therefore, these dielectric materials, which can both effectively unpin the Fermi level, and generate large amounts of the oxygen vacancies, are suitable as interlayers in CF-MIS contacts to improve electrical characteristics. Figure 6a shows the reverse current densities of the Ti/CF-formed TiO2, ZnO, and Al2O3/n-GaAs, at a bias voltage of −1 V for different interlayer thicknesses. After the interlayer conductivities are improved by the formation of CFs after electroforming, their current densities are significantly increased. The reverse current densities of all CF-MIS contacts with the TiO2, ZnO, and Al2O3 are almost constant regardless of their thicknesses because of the effects of SBH lowering, and the extremely high interlayer conductivity due to CF formation. The Ti/CF-formed TiO2/n-GaAs contact exhibits the highest current density, whose value is ~×25,000 higher than that of Ti/n-GaAs contact. The comparison of reverse current densities between the MIS contact, and the CF-MIS contact is shown in Figure S8. Consequently, no matter what the interlayer material is, or how the thickness of interlayer is, CF-MIS contacts exhibit better electrical contact characteristics after the electroforming step. The advantages of this innovative technique allow the CF-MIS contact to be a universal Ohmic contact for next-generation electronic nanodevices. To examine the stability of the CF-MIS contacts, it is confirmed that current levels at −1 and 1 V


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are maintained for > 107 s, and it is estimated that they can be stable up to 10 years, as shown in Figure S9. Also, the CF-MIS contacts exhibit excellent cyclic endurance, more than 104. Figure 6b shows the specific contact resistivity (ρc) of the CF-MIS contacts, obtained by the transmission line model (TLM) method. The details of ρc extraction are demonstrated in Figure S2, and Note S1. Compared to the MS contact (~1 Ω cm2), the CF-MIS contacts with the 2-nm-thick TiO2, ZnO, or Al2O3 show significantly ultra-low ρc values ((1.5 ± 0.6) × 10–6 Ω cm2, (2.3 ± 0.5) × 10–6 Ω cm2, and (5.1 ± 0.6) × 10–6 Ω cm2, respectively). The lowest ρc value exhibited was a ~×700,000 reduction compared to that of the MS contact. These results indicate that an Ohmic contact of ultra-low contact resistance can be achieved via CF formation, regardless of the interlayer materials, and the interlayer thickness. Figure 6c summarizes the ρc values of the primary Ohmic contact examples in GaAs, and our works.41—46 Most contacts—both alloyed and non-alloyed Ohmic contacts—exhibit ~10–5 Ω cm2, or ~10–6 Ω cm2 for moderately doped, or heavily doped GaAs, respectively. However, the novel CF-MIS contact proposed in this work exhibits one of lowest ρc values ((1.5 ± 0.6) × 10–6 Ω cm2) ever obtained, even though the doping concentration of the GaAs substrate was moderate (1 × 1018 cm–3). It is also expected that ultra-low ρc can be achieved with increase in substrate doping concentration.

4. CONCLUSIONS We have developed a novel Ohmic contact technique by introducing the CF-MIS structures, resulting in achieving remarkable improvements in the electrical properties of contacts. We reported ultra-low ρc value ((1.5 ± 0.6) × 10–6 Ω cm2) between the metal and GaAs, while achieving both CF formation in the interlayer, and SBH lowering at the GaAs. Although a


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GaAs substrate, a fundamental material of the III-V compound semiconductor, has been used to develop the proposed technique, we expect that this novel CF-MIS contact scheme can be extended to other semiconductors. Therefore, we believe that the proposed CF-MIS contact technique can be a mainstream Ohmic contact technique for next generation electronic devices.


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ASSOCIATED CONTENT Supporting Information Schematic illustration for the CFs formation and the electrical measurement method, Total resistance vs. electrode spacing in TLM measurements, Schematic illustration and fabrication process flow of the MIS contact, Resistive switching behaviors of the Ti/TiO2/n-GaAs, Electrical properties of the MIS contacts, Resistive switching behaviors of the Ti/ZnO/n-GaAs, Resistive switching behaviors of the Ti/Al2O3/n-GaAs, Comparison electrical properties between the MIS and the CF-MIS contacts, Retention characteristics of the CF-MIS contacts, Details of ρc extraction method.

AUTHOR INFORMATION Corresponding Author Hyun-Yong Yu School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, 02841, Korea (E-mail: [email protected], Tel: +82-2-3290-4830, Fax: +82-2-921-0544). ACKNOWLEDGMENTS This work was supported in part by the Technology Innovation Program within the Ministry of Trade, Industry and Energy, Korea, under Grant 10052804, in part by the Nano·Material Technology Development Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4910426), in part by


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the Basic Science Research Program within the Ministry of Science, ICT, and Future Planning through the NRF of Korea under Grant 2017R1A2B4006460, and in part by the research project of Samsung Electronics.


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Figure 1. Illustration and electrical characteristics of the CF-MIS contact. a) Schematic overview and zoom image of the CF-MIS contact. b) I–V characteristics of the Ti/TiO2 (2 nm)/n-GaAs contacts before and after negative electroforming step to confirm the formation of the CFs, measured under voltage sweep from 0 to −5 V, and back from −5 to 0 V. c) J–V characteristics of the Ti/n-GaAs, and the Ti/CFs-formed TiO2/n-GaAs contact for different interlayer thicknesses.


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Figure 2. Band structures of the CF-MIS contact. Band structures of a) the Ti/n-GaAs, and b) the Ti/CFs-formed interlayer/n-GaAs contacts, where upper and lower zoom-in images on the right side of b) show possible electron paths from the metal to the semiconductor before, and after electroforming, respectively.


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Figure 3. Electrical characteristics transition between the MIS, and the CF-MIS contacts. I–V characteristics of a) Ti/ZnO/n-GaAs, and c) Ti/Al2O3/n-GaAs contacts before and after negative electroforming step to confirm the formation of CFs, measured under voltage sweep from 0 to −5 V, and back from −5 to 0 V. b, d) J–V characteristics of the Ti/n-GaAs, and the Ti/CFs-formed ZnO and Al2O3/n-GaAs contacts for different interlayer thicknesses.


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Figure 4. Verification of the formation of CFs. CAFM images for the CFs-formed a) TiO2, b) ZnO, and c) Al2O3 interlayers to verify existence of CFs in the interlayers. The small images show the respective insulating properties of the as-deposited interlayers. The cross-sectional HRTEM images of the d) Ti/TiO2/n-GaAs, e) Ti/ZnO/n-GaAs, and f) Ti/Al2O3/n-GaAs contacts.


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Figure 5. Analysis of the oxygen vacancies. XPS spectra results for a) Ti 2p, b) Zn 2p, c) Al 2p, and d-f) O 1s. Shift in each metal peak represents the oxygen vacancies formed at each electroforming step. The O 1s peaks show the amount of oxygen vacancies in the TiO2, ZnO, and Al2O3 after electroforming. The insets in d-f) show the amount of oxygen vacancies in the TiO2, ZnO, and Al2O3 before electroforming step, respectively.


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Figure 6. Summary of the electrical characteristics of the CF-MIS contacts. a) JR values of the Ti/n-GaAs contact, and Ti/CFs-formed TiO2, ZnO, and Al2O3/n-GaAs contacts for different interlayers thicknesses at a bias voltage of −1 V, and b) ρc of the Ti/n-GaAs contact and the Ti/CFs-formed TiO2, ZnO, and Al2O3/n-GaAs contacts for different interlayers thicknesses. c) Summary of Ohmic contact data in GaAs of primary examples and our works. The “*” sign indicates that the ρc value is obtained by simulation based on their results.


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Novel Conductive Filament MetalâInterlayerâSemiconductor Contact

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