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Ultrathin and Atomically Flat Transition-Metal Oxide: Promising Building Blocks for Metal-Insulator Electronics Qingsong Cui, Maryam Sakhdari, Bhim Prasad Chamlagain, Hsun-Jen Chuang, Yi Liu, Mark Ming-Cheng Cheng, Zhixian Zhou, and Pai-Yen Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11302 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016

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ACS Applied Materials & Interfaces

Ultrathin and Atomically Flat Transition-Metal Oxide: Promising Building Blocks for MetalInsulator Electronics Qingsong Cui1, Maryam Sakhdari1, Bhim Chamlagain2, Hsun-Jen Chuang2, Yi Liu3, Mark MingCheng Chen1, Zhixian Zhou2*, and Pai-Yen Chen1* 1

Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI

48202 2

Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201

3

Department of Chemistry, Wayne State University, Detroit, MI 48202

*) Author to whom correspondence should be addressed, electronic mail: [email protected], [email protected] KEYWORDS: Insulating nanomaterials, 2D materials, transition-metal oxides, metal-insulatormetal diodes, metal-insulator electronics, tunneling devices

ACS Paragon Plus Environment

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Abstract We present a new and viable template-assisted thermal synthesis method for preparing amorphous ultra-thin transition-metal oxides (TMOs) such as TiO2 and Ta2O5, which are converted from crystalline two-dimensional (2D) transition-metal dichalcogenides (TMDs) down to a few atomic layers. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM) were used to characterize the chemical composition and bonding, surface morphology, and atomic structure of these ultrathin amorphous materials to validate the effectiveness of our synthesis approach. Furthermore, we have fabricated metal-insulator-metal (MIM) diodes using the TiO2 and TaO2 as ultrathin insulating layers with low potential barrier heights. Our MIM diodes show a clear transition from direct tunneling to Fowler-Nordheim tunneling, which was not observed in previously reported MIM diodes with TiO2 or Ta2O5 as the insulating layer. We attribute the improved performance of our MIM diodes to the excellent flatness and low pinhole/defect densities in our TMO insulting layers converted from 2D TMDs, which enable the low-threshold and controllable electron tunneling transport. We envision that it is possible to use the ultrathin TMOs converted from 2D TMDs as the insulating layer of a wide variety of metal-insulator and field-effect electronic devices for various applications ranging from microwave mixing, parametric conversion, infrared photodetection, emissive energy harvesting, to ultrafast electronic switching.


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INTRODUCTION In recent years, there has been a growing interest in metal-insulator-metal (MIM) heterostructures that serve as the fundamental building blocks of nanoscale metal-insulator electronics.1-14 Historically, diode-like characteristics of MIM heterostructures have found versatile applications in mixing, detection, and parametric conversion of electromagnetic radiation, potentially being capable of ultrahigh-speed and room-temperature operations.4-10 Since a MIM diode operates based on the femtosecond-fast transport mechanism of quantum tunneling, the electron transport can instantaneously respond to the impressed ac voltage, thereby allowing for the effective rectification of microwave and even infrared into dc power.4-10 Increased attention has been recently devoted to the environmental energy harvesting using nanoscale MIM devices.13-16 Particularly, infrared nano-antennas integrated with MIM diodes (so-called nano-rectennas), although still in its infancy, hold great promise in harvesting plentiful thermal radiation (~1017 W13) that Earth continuously emits into outer space. MIM devices also present interesting possibilities for hot-electron transistors,17 non-volatile switching memories,18 macroelectronics,19 and flat-panel displays,20 thanks to their ultrahigh operating speed and excellent scalability potential (vertical dimension less than 10 nm). In the optoelectronics domain, MIM nanostructures enable control and manipulation of light-matter interactions at the nanoscale, creating a new paradigm for designing on-chip nanoantennas21-24 nano-cavities, waveguides,25 and newly discovered hot-electron photodetectors and photovoltaic cells.26,27 Insulating barrier layer plays a crucial role in the performance of MIM devices. Although a TMO film can be grown by conventional thin-film deposition techniques28-30, its quality is controllable only when the film is sufficiently thick, which, however, prohibits tunneling


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transports of electrons. In fact, how to effectively control quantum mechanical tunneling through a thin dielectric layer represents a fundamental material challenge in the pursuit of highperformance MIM devices. The effective modulation of quantum tunneling inherently requires an ultrathin insulator layer (typically < 10 nm) that is nearly pinhole-free and has low interfacial roughness and uniform stoichiometry. Although the electronic band structure of an insulating layer is a critical parameter affecting the performance of a MIM device, it does not uniquely control the device figure of merits. Other parameters such as surface properties and defects in the insulating layer also significantly impact the metal-insulator interface and thus the device characteristics.31 To date, nanometer-scale MIM insulators have been prepared by various fabrication methods, including anodization,32 thermal oxidation,8,33 and plasma oxidation.6,34,35 These approaches, however, exhibit poor yield and nondeterministic performance. Although surface oxidation can realize a native oxide film at the atomic scale (e.g. Al-Al2O3, W-WO236,37), it shows very poor repeatability and integrability, as well as limited choice of insulator and metal types in engineering the electronic structure of MIM junctions. It is, however, desirable to pair up insulators having a wide range of bandgaps and electron affinities with arbitrary metals, allowing the electronic structure to be flexibly tailored for different targeted applications. Rough surfaces, pinholes and poor conformity, regardless of synthesis methods, have been commonly observed in conventional MIM devices, leading to thermally activated hopping and diffusive transport, as well as undesired interface effects due to surface traps, defects, and interfacial dipoles.38 These effects are generally detrimental to the overall device performance39 and make the control of quantum mechanical tunneling problematic, compromising the reliability, stability, and predictive utility of the devices. With the rapid advent of high-κ dielectrics, ultrathin and atomically smooth amorphous insulating layers have been successfully prepared by the atomic-


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layer deposition (ALD). Although the ALD technique may allow high comformality, precise thickness control, and device uniformity, it still fails to prepare large electron affinity oxides (e.g. TiO2, Ta2O5, and Nb2O5) with satisfactorily low defects and oxygen vacancy traps2,3. Insulating layers with large electron affinities (low tunnel barrier heights) are desired to achieve a low voltage operation in many practical applications. In fact, the dominant transport mechanism in ALD transition-metal oxides (TMOs) of large electron affinities was found to be dominated by thermally-sensitive emissions, such as trap-assisted tunneling and Frankel-Poole emission2,3. Although certain TMOs with large electron affinities (e.g. TiO2, Ta2O5, and Nb2O5) seem promising for achieving low-threshold MIM devices, high-quality and ultrathin TMO insulating layers, necessary for controllable quantum tunneling, are still lacking. In this Letter, we propose a new class of amorphous TMO materials as potential candidates for high-quality, ultrathin insulators. Here, we demonstrate, for the first time, template-assisted synthesis (TAS) of ultrathin TMOs. Particularly, we focus on the synthesis of two types of TMOs, TiO2 and Ta2O5, which offer exceptional physical and electronic properties, such as large electron affinity, wide bandgap, excellent surface flatness and homogeneity, which are of essential importance for realizing high performance metal-insulator electronics. We will provide below the experimental and theoretical evidence that these ultrathin amorphous insulators, when used in MIM diodes, may allow controllable tunneling transport at moderate low bias voltages. These MIM devices exhibit clear evidence for a mechanistic transition from direct tunneling to Fowler-Nordheim (FN) tunneling (or field emission).

RESULTS AND DISCUSSION


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Figure 1. (a) Schematics for the template-assisted synthesis (TAS) of ultrathin TMOs, which is based on thermally activated oxygen annealing, with sulfur atoms in 2D TMDs being replaced by oxygen atoms. XPS spectra for (b) the synthesized ultrathin TiO2 and its 2D TiS2 template, and (c) the synthesized ultrathin Ta2O5 and its 2D TaS2 template. Figure 1(a) illustrates this template-assisted approach, where a metallic 2D-layered transition-metal dichalcogenide (TMD) serves as an atomic-scale template for the formation of ultrathin TMOs by thermal assisted oxygen annealing. While semiconducting TMDs such as MoS2, MoSe2 and WSe2 are stable in air, metallic TMDs such as TiS2, TaS2 and TaSe2 are prone to oxidation in air, especially at elevated temperatures. When a 2D TMD gains sufficient thermal


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energy at the ambient oxygen (O) concentration, its atomic structure can be reconfigured into amorphous TMO (e.g. TiS2 → TiO2; TaS2 → Ta2O5), while preserving the geometry and topography of the TMD template. In this study, few-layer 2D TiS2 and TaS2 were exfoliated from commercially available bulk crystals. They serve as ultrathin and atomically flat templates with very low defect and impurity levels for the template-confined synthesis of ultrathin TMOs. The 2D TMD templates were then heated at 300°C for 3 hours on a hot plate in ambient environment to accelerate the oxidation process. After cooling to room temperature, ultrathin TMO layers with essentially the same geometry and topography of their TMD templates are obtained. This is achieved because sulfide (S) or selenium (Se) atoms share analogous covalent bonding with oxygen species, as they lie in the same column (group) on the periodic table. It is worth noting that the proposed approach is not limited to TiO2 and Ta2O5, but can be straightforwardly applied to the synthesis of a variety of other TMOs (e.g. Nb2O5, ZrO2 and WO2). X-ray photoelectron spectroscopy (XPS) was conducted to analyze the composition, phase, and chemical states of synthesized ultrathin TMOs before and after oxidation. Figure 1(b) and 1(c) show Ti 2p and Ta 4f photoelectron spectra before (blue line) and after (red line) the templateconfined oxidation. As can be seen in Figure 1(b), the Ti 2p orbit of transferred TiS2 flakes has two peaks located at 455.6 and 461.7 eV, which are assigned to the doublet of Ti 2p3/2 and Ti 2p1/2. It is clearly evident that after oxidation the binding energies for Ti 2p3/2 and Ti 2p1/2 are shifted to 458.1 eV and 463.7 eV, consistent with the spectral profile of TiO2. Figure 1(c) shows the Ta 4f7/2 and 4f5/2 peaks before and after the oxidation process. The pristine TaS2 has two peaks at 22.7 eV (Ta 4f7/2) and 24.7 eV (Ta 4f5/2), while the oxidized sample has two peaks shifted to 25.8 eV and 27.7 eV, corresponding the doublet of Ta 4f7/2 and Ta 4f5/2 in Ta2O5.40-42


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From the XPS spectra, it is evident that the TAS process has successfully converted 2D TMDs into TMOs. The topography and surface roughness of the ultrathin samples before and after the thermal oxidation process were characterized by noncontact-mode atomic force microscopy (AFM). Figure 2(a) and 2(b) show AFM images of 2D TiS2 and TaS2, and their oxidized counterparts, TiO2 and Ta2O5. The surface RMS roughness is less than 0.3 nm for ultrathin TiO2 and Ta2O5, indicating that the atomically flat surfaces of 2D TiS2 and TaS2 are retained after the oxidation.

Figure 2. AFM image for (a) ultrathin TiO2 oxidized from pristine 2D TiS2 and (b) ultrathin Ta2O5 oxidized from pristine 2D TaS2. Scanning transmission electron microscopy (STEM) was conducted to further characterize the detailed phase and atomic structures of prepared ultrathin TMOs. TiO2 and Ta2O5 samples for STEM analysis were prepared on a Si/SiO2 substrate coated a thin Pt film, and passivated by a


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layer of Ti/Au thin film to top. Focused ion beam (FIB) was used to cut very thin slices of the cross-sectional areas of TiO2 and Ta2O5 samples for STEM analysis. A schematic illustration of a sample for STEM cross-sectional analysis is shown in Figure 3(a). The top panels of Figure 3(b) and 3(c) show the high-resolution STEM images of ultrathin TiO2 and Ta2O5 sandwiched between Pt and Ti/Au metal films. The cross-sectional STEM analysis demonstrates a sharp and clean interface between the pinhole-free insulator and metals. The corresponding energydispersive X-ray spectroscopy (EDS) mapping of these nanomaterials are also provided in bottom panels of Figs. 3(b) and 3(c). It is evident that sulfur atoms are nearly absent in the TMO layer, strongly indicating that TaS2 and TiS2 have been fully oxidized by substituting all the sulfur atoms with oxygen atoms. We found that the oxidized materials were amorphous, since no crystalline structures were observed in the high-resolution TEM images or selected-area diffraction patterns (not shown here).

Figure 3. (a) Optical microscope image of MIM nanodiodes constructed by Pt/ TiO2/Ti/Au (top) and Pt/Ta2O5/Ti/Au (bottom). (b) and (c) are TEM images for MIM nanodiodes in (a) and the corresponding EDS mapping results.


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In order to get more physical insights into the electrical quality of the TMOs as ultrathin insulating layers and the predominant transport mechanisms (i.e. tunneling and/or defect-related conduction), we have fabricated dissimilar MIM nanodiodes by sandwiching an ultrathin TMO (TiO2 or Ta2O5) between the top (Ti) and bottom (Pt) electrodes. Here 2D TiS2 and TaS2 flakes with thicknesses ranging from 2 to 15 nm were prepared from their bulk crystals by mechanical exfoliation. The dielectric constant (κ) of our ultrathin TiO2 is determined to be ~ 18 by capacitance-voltage (CV) measurement, consistent with its amorphous phase determined by TEM analysis. We note that a crystalline TiO2 has a significantly higher κ value of 80, while the κ value for amorphous TiO2 is typically between 15 and 25 without oxygen annealing treatment.43-47 In the following, we will demonstrate that a dissimilar MIM diode comprising an ultrathin TMO insulator can operate based on the direct tunneling (low voltage region) and FN tunneling (high voltage region), with a distinct mechanistic transition. This is very different from those MIM diodes comprising ALD TMOs, where the electron transport mostly relies on thermally-activated conductions mechanisms, such as Frankel-Poole emission and trap-assisted tunneling48,49 (thermally excited electrons emitted from traps into the conduction band of insulator).50 To this end, we have constructed and characterized current density-voltage (J-V) responses for several Pt/TiO2/Ti and Pt/Ta2O5/Ti diodes, as shown in Figs. 4(a) and 4(b).


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Figure 4. Current density versus voltage for MIM diodes with (a) ultrathin TiO2 tunnel barriers and (b) ultrathin Ta2O5 tunnel barriers. (c) Energy band diagram under equilibrium, forward bias, and reverse bias. (d) Corresponding FN plots for devices in (a) and (b). The room temperature J-V characterization was performed using the semiconductor parameter analyzer, of which the source-measure unit was used to apply the bias voltage and measure the current response of MIM devices shown in Figure 4(a). The left panel of Figure 4(c) shows the energy band diagram for Ti/TiO2/Pt and Ti/Ta2O5/Pt nanodiodes. Known from the literature, the work functions for metals are: ΦTi = 4.33 eV and Φ Pt = 6.35 eV ,51 creating a work function difference ∆Φ = 2.02 eV, and the electron affinities for insulators are: Φ TiO2 = 3.9 eV 52 and

Φ Ti2O5 = 3.35 eV. 53 Here a forward bias is defined when Pt, which has a higher work function, is


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positively biased with respect to Ti. In this scenario, the positive current occurs when electrons flow from Ti to Pt. Similarly, a reverse current occurs when Ti is positive with respect to Pt, as illustrated in Figure 4(c). Because of the asymmetry of work functions for two different metals, the forward bias current is expected to be much greater than the reverse bias current, particularly when operated in the FN tunneling (high voltage) regime. The J-V characteristics of MIM diodes are sensitive to the height, width, and shape of tunnel barrier, as well as the effective electron mass inside the barrier. As observed in Figure 4(a) and 4(b), the tunneling current density may increase dramatically as the insulator thickness decreases because the tunneling probability of electrons through a thin barrier increases exponentially with decreasing the barrier thickness. We also note that in an extremely thin oxide, the image force arising from images of tunneling electrons in both metal surfaces may appreciably lower the barrier height (Schottky barrier lowering effect), which further increases the tunneling current (see the supporting material for the importance of image force). Moreover, a TiO2 MIM diode is expected to have a lower turnon voltage than a Ta2O5 MIM diode of comparable insulator thickness (e.g. 10 nm TiO2 versus 8 nm Ta2O5) because TiO2 has a larger electron affinity and thus a smaller barrier height. In order to provide a quantitative understanding of our experimental observations, we have also performed numerical calculations using the Wentzel-Kramer-Brillouin (WKB) method,54 with realistic values for metal work functions and the oxide electron affinities/dielectric constants (details are shown in the supporting material). We found that the theoretical results, as shown by dashed lines in Figure 4, display good agreement with experimental results, verifying that the transport mechanism is due primarily to quantum mechanical tunneling. In our theoretical calculation, the electron effective mass in oxides, summarized in the supporting material, was extracted empirically. We note that the effective electron mass is thickness dependent, as the


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band structure and effective mass of an ultrathin film could differ significantly from its bulk crystal. For ultrathin barriers, as is the case for the results reported here, the electron effective mass tends to increase with decreasing TMO thickness, consistent with what was reported in the literature.55 As pointed out by Simmons,10,56 at low forward/reverse voltages, where the applied bias is less than the barrier height, the direct tunneling mechanism would dominate, as illustrated in subpanels 1 and 3 of Figure 4(c). In the direct tunneling regime, the J-V characteristic is linear such that the MIM junction acts like an ohmic resister. In the low-voltage regime, the

)

(

relationship between tunneling current and applied bias follows: J ∝ V exp −2∆s 2m* Φ / h , where m* is the electron effective mass, h is the reduced Planck constant, and Φ and ∆s are the effective barrier height and width, respectively. On the other hand, when the applied bias exceeds the barrier height, the FN tunneling becomes the dominant transport mechanism, as illustrated in subpanels 2 and 4 of Figure 4(c), and the current increase exponentially with the

 * 3 s m − 4 ∆ 2 Φ voltage as: J ∝ V exp   3hqV  2

(

  , where q is the electronic charge. It is common to plot  

)

ln J / V 2 against 1 / V (so-called FN plot) for elucidating the conduction mechanism. In the FN 3   −  4∆s 2m* Φ   J   1 tunneling regime, the FN plot yields a linear trend ln  2  ∝ with a 3hq V V 

negative slope that depends on the barrier height. In the zero and low-bias direct tunneling regime,

(

the

FN

plot,

however,

exhibits

a

logarithmic

growth

as

)

ln J / V 2 ∝ ln (1/ V ) − 2∆s 2m* Φ / h. In the transition regime, when the applied bias is close to


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the barrier height, the mechanism experiences a transition from logarithmic growth to the linear decay. The information extracted the FN plots provides crucial guidance in identifying whether or not the conduction is due to quantum mechanical tunneling. Figure 4(d) shows the corresponding FN plots for MIM devices in Figs. 4(a) and 4(b). It is clearly evident that all devices with thin insulators exhibit a logarithmic growth in the low-bias direct tunneling regime (observable only for sufficiently thin barrier), while a distinct transition from logarithmic growth to the linear decay is seen in the high-bias FN tunneling regime. As a result, one can concluded from FN plots and fitted theoretical results [Figs. 4(a) and (b)] that the transport mechanism is due primarily to the quantum mechanical tunneling, instead of the defect-related conduction observed in conventional TiO2 and Ta2O5 films,57 thanks to the extremely low defect density and surface roughness of our atomically flat ultrathinTiO2 and Ta2O5 converted from 2D TMD crystals.

Figure 5. Extracted (a) nonlinearity and (b) responsivity for MIM devices in Figure 4. In general, figure of merits, including nonlinearity ( η NL ) and responsivity ( η RES ), are used to evaluate the performance of MIM devices9,58. η NL is defined as the ratio of the differential


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conductance (dJ/dV) to the conductance (J/V), which is a measure of the deviation from a linear resistor. On the other hand, η RES , defined as the ratio of the second derivative of the J-V curve (d2J/dV2) to the differential conductance (dJ/dV), serves as a quantitative measure of the rectified dc current as a function of input radiation (ac) power. Figure 5(a) and 5(b) show the corresponding η NL and η RES for the MIM devices in Figure 4. The asymmetry polarity and maximum nonlinearity and responsivity of MIM diodes are determined primarily by the built-in field induced by the work function difference of two metal electrodes. Since the choice of metal electrodes is fixed, the fabricated MIM devices have similar values for η NL and ηRES . We found that η RES could approach 10 V-1 around zero bias (Ti-TiO2-Pt diodes). Such a value, although not optimized yet, is greater than most MIM diodes (typically7,35,59,60) designed for microwave and infrared detection applications. When the MIM devices are biased, a strong nonlinearityηNL ~ 6 can be achieved, which is quite high in comparison with previously reported tunneling structures. 7,35,59,60

We note that these ultrathin high quality insulators can be readily paired and integrated

with a variety of metals or semiconductors, for building high-performance nanodevices, including but not limited to MIM diodes and field-effect transistors.

CONCLUSIONS To sum up, we have successfully prepared a new class of amorphous ultrathin transitionmetal oxides, such as TiO2 and Ta2O5, which are transformed from corresponding 2D transitionmetal dichalcogenides via a simple and effective template-assisted synthesis method. TiO2 and Ta2O5 are known to possess a narrow bandgap, large electron affinity, and high dielectric constant, making them insulators for metal-insulator electronics. We have conducted detailed


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material characterizations of their atomic structures, compositions, and surface morphologies, and found that these ultrathin insulators are of ultrahigh quality: ultrathin and atomically flat, spatially uniform, and nearly defect-free. Moreover, we have fabricated MIM diodes and demonstrated that direct tunneling (at low voltages) and FN tunneling (at high voltages) are the main mechanism for electron transport through these ultrathin insulators, which is further verified by theoretical calculations. We envision that ultrathin TMOs with attractive structural and electronic properties may enable low-threshold, controllable quantum tunneling in nanoscale MIM devices, with potential uses in ultrafast switching components, microwave detection and mixing, infrared photodetection, and energy harvesting.

METHODS Device Fabricaton: First, we patterned the bottom electrodes consisting of 10 nm of Pt with 5 nm of Ti adhesion layer on Si substrates with 290 nm-thick thermal oxide using the electron beam lithography (EBL) followed by electron beam deposition and lift-off. Next, ultrathin 2D TiS2 or TaS2 flakes were transferred onto top of the Pt electrodes,61-64 and heated to 300 ᵒC for 3 hours in ambient air to convert the multilayer 2D TMD crystals to ultrathin amorphous TMOs. Finally, top electrodes were made by EBL patterning and deposition of 10 nm Ti and 20 nm Au.65 Characterization: XPS measurements were performed using a Ktratos Axis Ultra XPS system with a monochromatic Al Source. The photoelectrons were separated by pass energy of 20 eV and 0.1 eV scanning step. The C 1s reference line at 284.6 eV was used to correct the charging effect. Optical microscopy and Park-Systems XE-70 noncontact mode atomic microscopy (AFM)


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were used to identify and characterize the thickness and surface morphology of the samples. Field-emission high resolution STEM (JEOL 2010F, operated at 200 kV) equipped an energy dispersive spectrometer (EDS) was used to obtain high resolution microstructure images and chemical compositions. The TEM samples were prepared with a Dual Beam TESCAN GAIA3 focused ion beam (FIB) workstation. Electrical properties of the MIM devices were measured by a Keithley 4200 semiconductor parameter analyzer. Conflicts of interest: The authors declare no competing financial interest. Acknowledgement P.Y.C. would like to acknowledge the Air Force Research Laboratory Summer Faculty Fellowship Program (ARFL-SFFP) and fruitful discussions with Dr. Shin Mou at AFRL. H.C., B.C., and Z.Z. acknowledge partial support by NSF grant number DMR-1308436 and the WSU Presidential Research Enhancement Award.

ASSOCIATED CONTENT Supporting Information: Quasi-analytical formula for electric tunnel effects in metal-insulator-metal systems, numerical approaches for electric tunnel effects in metal-insulator-metal systems, comparison between theory and experiment. This material is available free of charge via the Internet at http://pubs.acs.org.”


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