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Carrier Polarity Control in #-MoTe2 Schottky Junctions Based on Weak Fermi Level Pinning Shu Nakaharai, Mahito Yamamoto, Keiji Ueno, and Kazuhito Tsukagoshi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02036 • Publication Date (Web): 20 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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ACS Applied Materials & Interfaces

Carrier Polarity Control in α-MoTe2 Schottky Junctions Based on Weak Fermi Level Pinning Shu Nakaharai,†* Mahito Yamamoto,† Keiji Ueno,‡ Kazuhito Tsukagoshi†* †

WPI Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials

Science (NIMS), Tsukuba 305-0044, Japan, ‡Department of Chemistry, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan. KEYWORDS: Transition Metal Dichalcogenide, Schottky Junction, Fermi Level Pinning, Field Effect Transistor, Carrier Injection

ABSTRACT

The polarity of the charge carriers injected through Schottky junctions of alpha-phase molybdenum ditelluride (α-MoTe2) and various metals has been characterized. We found that the Fermi level pinning in the metal/α-MoTe2 Schottky junction is so weak that the polarity of the carriers (electron or hole) injected from the junction can be controlled by the work function of the metals, in contrast to other transition metal dichalcogenides such as MoS2. From the estimation of the Schottky barrier heights, we obtained p-type carrier (hole) injection from a Pt/α-MoTe2 junction with a Schottky barrier height of 40 meV at the valence band edge. n-Type carrier (electron) injection from Ti/α-MoTe2 and Ni/α-MoTe2 junctions was also observed with

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Schottky barrier heights of 50 and 100 meV, respectively, at the conduction band edge. In addition, enhanced ambipolarity was demonstrated in a Pt–Ti hybrid contact with a unique structure specially designed for polarity-reversible transistors, in which Pt and Ti electrodes were placed in parallel for injecting both electrons and holes.

INTRODUCTION Reduction of the total power consumption in large-scale integrated circuits (LSIs) is required because the amount of power consumed worldwide for information processing has increased exponentially. The low-voltage operation of current on/off switching in metal-oxidesemiconductor (MOS) field-effect transistors (FETs) is considered an essential solution to address this problem. For this reason, ultrathin body channels such as silicon-on-insulator, germanium-on-insulator or III–V-on-insulator structures have been developed for low-power consumption LSI technology based on better gate control over the channel, enhanced carrier mobility, and immunity to short channel effects.1 The ultimate style of the ultra-thin body channels is two-dimensional (2D) materials such as transition metal dichalcogenide (TMDC) semiconductors2–7 including MoS2 or WSe2, because these 2D materials possess only an atomicscale thickness and potentially atomically flat surfaces without dangling bonds. Because of these advantages, transistors on TMDCs are actively being developed for future low-power consumption LSIs.8–20


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One of the serious issues in TMDC semiconductors, however, is the Schottky barriers to the carrier injection from the metal contacts.21–31 In particular, difficulty in controlling the polarity of the carriers (electrons or holes) injected from the metal contacts is a critical issue for the application in complementary-MOS (CMOS)-like circuits. Although a high electron mobility of 700 cm2 V−1s−1 was reported in few-layer molybdenum disulfide (MoS2) with nearly ohmic Schottky junctions of the Sc contacts,24 carrier injection from metal contacts is almost limited to electrons when the Schottky barriers are electrostatically modulated by a solid gate. The origin of this problem is generally known as Fermi level pinning, which is commonly found in semiconductors. Because of this effect, the Fermi levels of the metals at the metal/MoS2 interfaces come close to the conduction band edge of the semiconducting MoS2, and even in a junction with a large work function metal such as Pt, the metal injects electrons rather than holes.24 To overcome this problem, techniques using an oxide contact,25 ionic liquid gates with electric double-layer gating,32–35 or substitutional doping36 are being investigated. Tungsten diselenide (WSe2) is also a TMDC semiconductor, and it can exhibit ambipolar carrier conduction. In this case, the Fermi levels of the metals at metal/WSe2 junctions are pinned close to the midgap of the semiconductor, and their carrier injection relies mainly on tunneling through the Schottky barrier rather than the thermionic emission over high barriers (more than hundreds of millielectron volts) of both electrons and holes.23 An enhancement in ambipolarity was also realized by thinning the Schottky barriers for both electrons and holes by applying a large gate electric field from the liquid gate.37–40 For the application of WSe2 to the CMOS-like architecture with conventional solid gates, the control of the polarity of the injected carriers can be achieved by choosing appropriate contact metals.16, 17, 23 However, additional treatments are commonly required to adjust the carrier injection properties in either or both p-FET and n-FET on a WSe2


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sheet. For example, surface doping of potassium for degenerately doped access regions (the region between the contact and top gate) in n-FETs,18 channel doping with tetrafluorotetracyanoquinodimethane (F4TCNQ) for p-FETs,19 or electrostatic doping for both p- and nFETs20 have been applied for realizing a CMOS-like inverter. The ideal method for controlling the carrier polarity should be based on simple tuning of the Schottky barrier height without additional doping for electrons or holes; however, such simple carrier polarity control of purely or nearly ohmic Schottky junctions has not yet been realized in TMDCs with solid gates except for the above-mentioned complicated elaborations. Recently, outstanding ambipolar carrier transport has been reported41 in alpha-phase molybdenum ditelluride (α-MoTe2), which is also a TMDC semiconductor with a band gap smaller than other TMDC semiconductors.42–60 This material sometimes exhibits p-type carrier conduction,42,43,46 and it was also used for the p-FET part of an inverter circuit in combination with n-FET on MoS2.59 However, detailed investigations of Fermi level pinning in α-MoTe2 have not yet been fully accomplished. In this work, we have systematically studied the carrier injection characteristics in α-MoTe2 Schottky junctions with various metals from the point of view of the Fermi level pinning phenomena. We found that the Fermi level pinning in α-MoTe2 Schottky junctions is so weak that the Fermi levels at the junction can be tuned depending on the work function of the contact metals. The experimental results show that a junction with a small work function metal such as Ti injected electrons, whereas a junction with a large work function such as Pt injected holes. The Schottky barrier heights for both electrons and holes have been precisely evaluated by a procedure based on the thermionic emission scheme under the flat band condition.24 Based on the ability to control the carrier polarity in α-MoTe2 Schottky junctions, we demonstrate an enhanced ambipolar transistor operation in a Pt–Ti hybrid contact device that is designed for the


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injection of both electrons and holes to an intrinsic semiconductor of α-MoTe2. This unique contact structure is expected to enlarge the drive current in polarity-reversible transistors in which the transistor polarity can be electrostatically controlled by an electrostatic signal.60

RESULTS AND DISCUSSION Electric characteristics were taken in devices of the structure shown in Fig. 1(a), which has a pair of gated Schottky diodes and a back-gated FET in between. The back-gate bias, VBG, modulation of drain currents, ID, in a Pt contact device at different drain biases, VD, is shown in Fig. 1(b). Here, the ID values are normalized with the junction width and divided by VD, and the curves were aligned with the back-gate bias value of Vmin at which the minimum ID was given. The obtained data exhibited a p-type transistor behavior with large on currents when VBG−Vmin < 0 and off currents when VBG−Vmin > 0. Conversely, Ti and Ni contact devices behaved as n-type transistors (Fig. 1(c) and (d)). In every case, a weak VD dependence of ID/VD in the on state implies a nearly ohmic transport with a very low Schottky barrier, whereas a strong VD dependence of ID/VD in the off state should be understood in terms of a gate bias modulation of the Schottky barrier thickness for carrier tunneling. This feature enables us to distinguish the carrier injection nature between the tunneling and the thermionic emission. We have fabricated multiple devices with these contact metals, and found that all of the devices reproduced the carrier polarity determined by the contact metals, except for some failed devices with a high contact resistance. We obtained the maximum hole mobility of 12 cm2V−1s−1 in the Pt contact device, and electron mobilities of 8 and 5 cm2V−1s−1 in the Ti and Ni devices, respectively (see Supporting Information).


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The Schottky barrier heights in the three contacts were evaluated from the temperature dependence of ID. The ID−VBG characteristics of the Pt contact device at different temperatures at VD = 100 mV are shown in Fig. 2(a). Here, the curves are also aligned with the Vmin value. The on state current decreased as the temperature decreased, and the degree of its down shift was found to be much weaker than that of the off state current. This behavior also suggests that the carrier transport of the on state is very close to the ohmic regime with a very small barrier height, and, conversely, the off state current is supposed to be based on the thermally assisted tunneling through the Schottky barrier rather than thermionic emission. In the present work, however, we focus on the transport of the on state rather than the off state. To confirm the above predictions about the carrier injection characteristics quantitatively, we estimated the barrier height in the three devices with different contact metals by following the method by Das et al.24 based on the thermionic emission scheme. The current flowing through a Schottky barrier is expressed as

= ∗ exp − 1 − exp − ,

(1)

where q is the elementary charge (q > 0 for electrons), T is the temperature, A* is Richardson’s constant, kB is Boltzmann’s constant, and ΦB is the injection barrier potential measured from the Fermi level. Note that exp(−qVD/kBT) is much smaller than unity in the present experiment, and the injection barrier energy, qΦB, is always positive for injection of both electrons and holes. We extracted qΦB values from the temperature dependence of ID by evaluating the slope in the semilogarithmic plot of ID/T2 versus 1000/T in Fig. 2(b) according to equation (1). The extracted qΦB values were plotted as a function of VBG−Vmim in Fig. 3(a). Here, the carrier injection was divided into two regimes, and their border corresponds to the flat band condition at which the


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true value of the Schottky barrier height is obtained as follows. When |VBG−Vmim| is small, the injection barrier for holes is formed at the drain contact by the bending of the semiconductor’s band by the back-gate bias, as shown in Fig. 3(b). Here, the holes are injected by thermionic emission. In this case, as Das et al. pointed out,24 the injection barrier qΦB exhibits a linear relation to |VBG−Vmim|, which is easily seen in the region labeled (I) in Fig. 3(a). As |VBG−Vmim| increases and reaches |VFB|, the injection barrier is lowered and it comes to the flat band regime (labeled (II) in Fig. 3(a)) at which the semiconducting band at the junction becomes flat, as shown in Fig. 3(c). Under this condition, the injection barrier qΦB coincides with the Schottky barrier qΦSB. In the present case of a Pt contact device, it was found to be = 40 meV, as

indicated by a broken horizontal line in Fig. 3(a). When |VBG−Vmim| exceeds |VFB|, the semiconducting band is raised up further by the gate bias and more holes are accumulated in the channel (Fig. 3(d), and labeled (III) in Fig. 3(a)). With a sufficiently large gate bias, the Schottky barrier becomes thin enough for the tunneling of holes, while the barrier height is unchanged, and the tunneling component starts to contribute to the total on current (Fig. 3(d)). Because of this tunneling component, the nominal value of the extracted barrier height becomes an underestimated one and hence it deviates from equation (1). In the case of Ti and Ni contact devices, which were found to be n-type as in Fig. 1(c) and (d), we also estimated the Schottky ! barrier heights for electrons using the same procedure, obtaining = 50 meV and =

100 meV, as shown in Figs. S3 and S4, respectively. In the evaluation process of presented in Figs. 2 and 3, the measurements were done at

VD = 100 meV, which is much smaller than the band gap. At VD = 2 V, which is larger than the band gap, we also obtained a quite similar value of the Schottky barrier height in the same device, as summarized in the Supporting Information (Fig. S2). Also in the case of Ni and Ti, the


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obtained barrier heights did not exhibit any significant difference between the measurements at VD = 100 mV and 2 V, as summarized in Figs. S3 and S4. In the case of Au and Pd contacts, the current levels of electrons and holes were comparable to each other in the ID−VBG characteristics (Fig. S5), and these current levels were between the on and off currents of n-type and p-type contacts. Based on these experimental results, the Fermi levels of metals at the Schottky junctions can be considered to be around the midgap of the semiconductor.23 In these junctions, tunneling is the dominant process of carrier injection of both electrons and holes, and it is difficult to estimate the precise value of the Schottky barrier height by the same method as that used for the on currents of Pt, Ti and Ni contacts. The Fermi levels of the metals in the Schottky junction are summarized in Fig. 4. Depending on the work function of the metals, qφM, the polarity of the Schottky junction was determined; a small work function metal such as Ti (qφTi = 4.33 eV) made an n-type junction, and a large work function metal such as Pt (qφPt = 5.65 eV) made a p-type junction. In the case of Au and Pd with medium work functions (qφAu = 5.1 eV, qφPd = 5.12 eV), the Fermi levels were around the midgap of the semiconductor. Only Ni was an exception, because the work function of 5.15 eV suggests its Fermi level is around the midgap but the Schottky junction clearly showed n-type behavior. One possible reason for this discrepancy could be that Ni is likely to react with the αMoTe2 surface to form interface states, which pin the Fermi level of the metal below the conduction band edge, whereas a less reactive metal such as Pt can make a p-type junction without forming the interface states even at a high evaporation temperature. A similar behavior of Schottky barrier formation in Ni was also reported in WSe2 whereby the Ni contact behaved as an n-type junction and the Pd contact behaved as a p-type junction.23 Interestingly, in the case of heavily Nb-doped MoS2, it was reported that Ni contact formed a p-type junction36 in contrast


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to our case of n-type junction of Ni-MoTe2. This difference suggests that we can expect a variety of method for carrier type control by substitutional doping in TMDCs. In the case of Ti contact, it is difficult to distinguish whether Ti reacted with MoTe2 or not, because the extracted Fermi level of Ti at the Schottky junction is supposed to be close to the original energy level of Ti. The degree of Fermi level pinning effect can be quantified using the slope of the ΦSB−φM curves, " ⁄"#$ , which is also referred to as the pinning factor. In our experiment, a pinning factor of 0.7 was calculated from the Pt and Ti data. This value is much larger than silicon (~0.3) and other TMDC semiconductors. For example, in MoS2, the reported pinning effect is so strong, with a pinning factor as small as 0.1, that most of the metals including Pt only form n-type Schottky junctions.24 As has been reported, chalcogen vacancies in a sheet of TMDC materials at the junction are mainly responsible for the Fermi level pinning.26–28 In this regard, the formation energies of the relevant vacancies in tellurides including MoTe2 and WTe2 have been reported to be larger than MoS2 or WSe2, suggesting a smaller density of vacancies and accordingly an easier control of Fermi levels against the band gap in these telluride semiconductors.27 These reported trends of the pinning effect are consistent with our present work. As far as we can infer from the above discussion, the weakness of pinning of Pt, Au and Pd could be explained by the lower density of vacancies than the case of MoS2, while the reaction of Ni, and presumably Ti, with α-MoTe2 could be the reason for pinning of their Fermi levels. For more decisive conclusion for the mechanism of pinning, of course, requires more intensive analysis based on detailed investigation of interfaces. To demonstrate the ability to control the carrier polarity in α-MoTe2 Schottky junctions without any additional treatment, we fabricated a Pt–Ti hybrid contact with a unique structure in which two contacts of Pt and Ti are placed in parallel so that electrons and holes are injected


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from Ti and Pt contacts, respectively. Figure 5(a) shows an image and a schematic illustration of the device. Here, 5-nm-thick Pt contacts are partially covered by a stack of Ti/Au (5/30 nm). The illustration in Fig. 5(b) shows that the semiconducting band bends upward in the Pt/α-MoTe2 junction and bends downward in the Ti/α-MoTe2 junction. Since these two contact metals are electrically connected, they share the same Fermi level. Consequently, the band alignment at the Pt–Ti hybrid contact junction forms the structure shown in Fig. 5(b). By applying a negative back-gate bias to the device, the semiconducting band is lifted up, and the valence band edge of the semiconductor comes close to the Fermi level. As a result, the barrier for holes is lowered, and holes are injected from the Pt/α-MoTe2 junction (Fig. 5(c)). Similarly, when a positive backgate bias is applied, the semiconducting band is pushed down and electrons are injected from the Ti/α-MoTe2 junction (Fig. 5(d)). Therefore, both electrons and holes can be injected from the hybrid contact to an intrinsic semiconductor according to the polarity of the gate bias. Figure 5(e) shows the ID–VBG characteristics of the Pt–Ti hybrid contact device, along with data for different devices of Pt contact and Ti contact for comparison. The drain current of the hybrid contact device follows the on state part of the ID–VBG curve of the Pt contact device in the negative VBG−Vmin region and that of the Ti contact device in the positive VBG−Vmin region. This confirms that the hybrid contact works as expected in our model depicted in Fig. 5(b–d) with appropriately working p- and n-type contacts on a sheet. The unique concept of a hybrid contact is necessary for polarity-reversible transistors in which both electrons and holes must be injected to an intrinsic semiconductor channel.60–64 In our previous publication,60 we demonstrated the fabrication and operation of a dual-gated transistor on α-MoTe2 in which the transistor polarity can be controlled by a gate bias of one of the top gates. In this device, only Ti was used for the contacts, and it was found that the drive current in


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the p-FET mode was limited compared with the n-FET mode. This problem was proposed to be caused by the poor hole injection property from the Ti/α-MoTe2 junction, and so it should be solved by the proposed Pt–Ti hybrid contacts. It should also be noted that, with a positive drain bias and a grounded source electrode, electrons are injected from the source contact and holes are injected from the drain contact, depending on the polarity of the back-gate bias. Consequently, the polarity-reversible transistors can be optimized by using Ti for the drain contact and Pt for the source contact. This is basically true, because the flow of electrons from a semiconductor to a Pt contact corresponds to the forward direction of a Schottky diode, and the flow of holes from the semiconductor to the Ti contact is also in the forward direction. However, in this structure, the direction of the current flowing is fixed one-way, which limits the flexibility in the circuit design. Also, the barrier at the junction where the carriers are ejected should remain high, resulting in a high parasitic resistance particularly in the case of a low-operating voltage of lowpower consumption circuits. The proposed concept of hybrid contacts also contributes to reduce the series resistance at the contacts.

CONCLUSIONS The Fermi level pinning effect in Schottky junctions of metals and α-MoTe2 is much weaker than other TMDC semiconductors, and accordingly the carrier polarity (electrons or holes) of the injected carriers through the junctions can be controlled by tuning the work function of the contact metals. In our experiments, Schottky barrier heights were estimated under flat band condition, obtaining a barrier height of 40 meV for holes in Pt contact, and 50 and 100 meV barrier heights for electrons in Ti and Ni contacts, respectively. These results indicate that α-


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MoTe2 is the most suitable material for CMOS applications among the TMDC semiconductors. Based on these results, we demonstrated ambipolar carrier injection through a Pt–Ti hybrid contact in which Pt and Ti contacts were placed in parallel so that both electrons and holes were injected from Ti and Pt, respectively, without any additional treatment for adjusting the carrier injection properties. The concept of a hybrid contact is expected to enhance the symmetry of the currents and reduce the series resistance in polarity-reversible transistors.

EXPERIMENTAL SECTION Throughout the fabrication process, we took special care for the samples not to be oxidized by air exposure. We stored the α-MoTe2 crystal and exfoliated flakes in vacuum cell as long as possible. The α-MoTe2 flakes were exfoliated by tape from a bulk single crystal grown by chemical vapor transport,64 and deposited on a high-doped silicon wafer with a 285-nm-thick surface thermal oxide layer. Thin flakes of α-MoTe2 were found using an optical microscope. Air exposure time in the process of exfoliating and locating α-MoTe2 flakes was less than about half an hour. The number of α-MoTe2 layers of each flake was estimated to be three to six layers, which was confirmed by Raman spectroscopy51 after the measurements. Electron beam lithography with PMMA (poly(methyl methacrylate)) resist was used for defining the contacts and pads. Metals of Ti, Ni, Au, Pd and Pt were deposited by thermal evaporation with a tungsten boat by resistance heating with an applied current of around 80 A for Pt, or from 40 to 60 A for other metals (Ti, Au, Ni, Pd). The chamber base pressure was typically 4×10-4 Pa for Au, Ni, Pd and Pt, and in the case of Ti, it became slightly low down to 1×10-4 Pa by evaporated Ti. The evaporation rate was typically from 0.05 to 0.1 nm/s. In the cases of Pd, Au and Pt, 5-nm-thick


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contact layers were first fabricated, and then we repeated the same fabrication process for additional Ti/Au (5/30 nm) layers over the contact layer. The Ni contact devices had a 15-nmthick Ni contact layer that was covered with a 20-nm-thick Au layer, and the Ti contact devices had a simple stack of Ti/Au = 5/30 nm. The total air exposure time in the full process of fabrication and measurement was about half an hour. According to the literature,66 the work functions of Ti, Ni, Pd, Au and Pt are 4.33, 5.15, 5.12, 5.1 and 5.65 eV, respectively. The distance between the source and drain was fixed as 1 µm, and the junction width ranged between 1 and 2.5 µm depending on the flake size. The measurements were performed in a probe station under vacuum and at a controlled temperature. The source electrodes were always grounded, and a bias was applied from the drain electrodes for two-terminal measurements using source/measure units. The silicon substrates were biased as the back-gate electrodes. Due to our careful handling of the samples, the influence of surface oxide MoOx is supposed to be not significant. As inferred from the field mobility values in Fig. S1, the contact resistances should be comparable with other TMDC cases. The influence of channel oxidation was also ignorable because, in our previous experiment, we confirmed that a device with a mono-layer αMoTe2 flake and Ti contacts worked as an ambipolar (but slightly asymmetric) semiconductor just like a few-layered α-MoTe2 device, while the current level was lower than the few-layered case. Also, the mono-layer flake in this device exhibited a Raman spectrum which is the same as that shown in our previous work with a mono-layer one.51 Therefore, the oxidation of α-MoTe2 in our experiment was suppressed sufficiently and its influence was ignorable.

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Numbers 15K06006, 25107004.


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FIGURE CAPTIONS Figure 1. (a) Optical image of one of the fabricated devices. A pair of Pt contacts indicated by blue broken lines were attached on the surface of a few-layer α-MoTe2 flake indicated by red broken lines, and the Pt contacts were covered with Ti/Au layers. Note that the Ti layer does not touch the α-MoTe2 channel directly. The substrate was a silicon wafer with a 285-nm-thick surface oxide layer. (b) Back-gate modulation of the drain current at room temperature in the Pt contact device shown in (a) at different drain biases of 0.1, 0.2, 0.5, 1 and 2 V. Here, the drain currents were divided by the drain biases and the junction width. Curves were aligned with the Vmin values at which the minimum ID/VD was given. This device worked as a p-type FET. With the same measurement conditions, an n-type FET operation was observed in Ti contact (c) and Ni contact (d) devices. Figure 2. (a) Temperature dependence of the drain current in the Pt contact device. The temperatures were from 125 K to 275 K with 25 K steps, and the drain bias was 100 mV. (b) ID/T2 is plotted against 1000/T at different values of VBG−Vmin. The solid lines are a guide for the eye. Figure 3. (a) The gate bias dependence of the carrier injection barrier, qΦB, in the Pt contact device shown in Fig. 2(a) and (b). The Schottky barrier height of 40 meV was extracted by the qΦB value at which the qΦB value deviated from the linear relation to the gate bias (red solid line). The VBG−Vmin value of this condition is referred to as the flat band voltage, VFB. (b) The band configuration of the Schottky junction at the drain contact when the back-gate bias is lower than VFB, which is labeled (I) in (a). In this case, holes around the Fermi level were injected into


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the channel by thermionic emission with a barrier height of qΦB. (c) The band configuration at the flat band condition, which is labeled (II) in (a). At this configuration, qΦB corresponds to the Schottky barrier, . (d) The band configuration when the back-gate bias exceeds VFB, which

is labeled (III) in (a). In this case, holes are injected into the channel by both thermionic emission and tunneling. Because of this tunneling component, the nominal value of qΦB becomes lower than the true value of .

Figure 4. Schematic illustration of the band alignment in a Schottky junction of the metal/αMoTe2 interface. The Fermi levels of the metals at the junction are in line with the work function difference except for Ni. Here, the difference in the work function between Ti and Pt was weakly compressed at the junction, which is evidence of the weak Fermi level pinning effect in the metal/α-MoTe2 Schottky junction. Figure 5. (a) Optical micrograph of the fabricated Pt–Ti hybrid contact device. Two Pt contacts are covered with Ti/Au layers, which also work as Ti contacts. This structure is also illustrated schematically in the lower picture. (b) Band diagram of the Schottky junction of the source contact of the Pt–Ti hybrid device. The direction of bending of the semiconductor’s band at the junction of the Pt and Ti parts is opposite to each other. (c) With a negative gate bias, holes can be injected from the Pt part of the contact. In this case, the device behaves as a p-FET. (d) With a positive gate bias, electrons can be injected from the Ti part of the contact. In this case, the device behaves as an n-FET. (e) Back-gate modulation of drain currents in the Pt contact (ptype), Ti contact (n-type) and Pt–Ti hybrid contact (ambipolar) devices at VD = 100 mV and at room temperature. The currents in the Pt–Ti hybrid contact followed the on currents of the Pt contact (VBG−Vmin < 0) and Ti contact (VBG−Vmin > 0), confirming the carrier injection model explained in (b)–(d).


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications Web site at DOI: Field-effect mobility and Schottky barrier height evaluations in three devices with Pt, Ti and Ni contacts, and gate-bias modulation of conductance in Au and Pd contact devices.

Corresponding Authors *E-mail: (S.N.) [email protected]; (K.T.) [email protected]. Present Address (M.Y.) Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.

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25


(a)

(b) 10

Pt (p-type)

1

ID/VD (μS/μm)

Ti/Au

α-MoTe

Pt

2

10−1

VD = 2 V

10−2 10−3

100 mV

10−4

2μm

10−5 −60 −40 −20

0

20

40

60

VBG − Vmin (V) (d)

10 1 10−1

10

Ti (n-type)

ID/VD (μS/μm)

(c)

ID/VD (μS/μm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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VD = 2 V 10−2 10−3 10−4

100 mV

10−5 −45 −30 −15

1

Ni (n-type)

10−1

VD = 2 V 10−2 10−3 10−4

100 mV

0

15

30

45

60

10−5 −30 −15

VBG − Vmin (V)

0

15

30

45

60

75

VBG − Vmin (V)

Figure 1. (a) Optical image of one of the fabricated devices. A pair of Pt contacts indicated by blue broken lines were attached on the surface of a few-layer α-MoTe2 flake indicated by red broken lines, and the Pt contacts were covered with Ti/Au layers. Note that the Ti layer does not touch the α-MoTe2 channel directly. The substrate was a silicon wafer with a 285-nmthick surface oxide layer. (b) Back-gate modulation of the drain current at room temperature in the Pt contact device shown in (a) at different drain biases of 0.1, 0.2, 0.5, 1 and 2 V. Here, the drain currents were divided by the drain biases and the junction width. Curves were aligned with the Vmin values at which the minimum ID/VD was given. This device worked as a p-type FET. With the same measurement conditions, an n-type FET operation was observed in Ti contact (c) and Ni contact (d) devices.


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(b)

(a) −6

T = 275 K

10−8

−20 V

125 K −10

10

−12

10

10−14 −60

VBG −Vmin = −25 V

10−12

Pt ID/T2 (A/K2)

10

ID (A/μm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


−15 V

10−14

−10 V

10−16 −5 V

VD = 100mV

−40

−20

0

20

40

10−18 4

VBG −Vmin (V)

5

6

7

8

1000/T (K−1)

Figure 2. (a) Temperature dependence of the drain current in the Pt contact device. The temperatures were from 125 K to 275 K with 25 K steps, and the drain bias was 100 mV. (b) ID/T2 is plotted against 1000/T at different values of VBG−Vmin. The solid lines are a guide for the eye.


2


(d)

(c)

(III) |VBG−Vmin | > |VFB|

(b)

(II) |VBG−Vmin | = |VFB|

(I) |VBG−Vmin| < |VFB|

Flat Band

qΦB

Tunneling EF

EC

qΦB = qΦSB

h

Thermionic

Pt

EF

h Thermionic

qΦB

EV

Pt

EF

h

Eg

Pt

Thermionic

(III)

(I)

(II)

100

(a)

80

qΦB (meV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

Pt

60 40 = ~40 mV

20 0 −30

VD = 100 mV

−25

−20

−15

−10

−5

VBG −Vmin (V)

Figure 3. (a) The gate bias dependence of the carrier injection barrier, qΦB, in the Pt contact device shown in Fig. 2(a) and (b). The Schottky barrier height of 40 meV was extracted by the qΦB value at which the qΦB value deviated from the linear relation to the gate bias (red solid line). The VBG−Vmin value of this condition is referred to as the flat band voltage, VFB. (b) The band configuration of the Schottky junction at the drain contact when the back-gate bias is lower than VFB, which is labeled (I) in (a). In this case, holes around the Fermi level were injected into the channel by thermionic emission with a barrier height of qΦB. (c) The band configuration at the flat band condition, which is labeled (II) in (a). At this configuration, qΦB corresponds to the Schottky barrier, qΦSBPt. (d) The band configuration when the back-gate bias exceeds VFB, which is labeled (III) in (a). In this case, holes are injected into the channel by both thermionic emission and tunneling. Because of this tunneling component, the nominal value of qΦB becomes lower than the true value of qΦSBPt.


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Vacuum Level 50 meV

100 meV

EC

Ti (4.33 eV) Midgap

Au,Pd (5.1, 5.12 eV)

Eg

Ni (5.15 eV)

EV 40 meV

Pt (5.65 eV)

Figure 4. Schematic illustration of the band alignment in a Schottky junction of the metal/αMoTe2 interface. The Fermi levels of the metals at the junction are in line with the work function difference except for Ni. Here, the difference in the work function between Ti and Pt was weakly compressed at the junction, which is evidence of the weak Fermi level pinning effect in the metal/α-MoTe2 Schottky junction.


4


Pt

(a)

3 μm

(c)

Pt

(b) h

VBG < 0

α-MoTe2

Pt

EF

EC EF

Ti/Au

α-MoTe2

h

e

(d)

EV Ti

E

Pt

VBG > 0

y x

y

EF

e Ti

Ti x

(e) Pt-Ti Hybrid (ambipolar)

10−7 10−8

ID (A/μm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10−9

Pt (p-type)

10−10 10−11 10−12

−40

Ti (n-type) VD = 100 mV

−20

0

20

40

VBG − Vmin (V)

Figure 5. (a) Optical micrograph of the fabricated Pt–Ti hybrid contact device. Two Pt contacts are covered with Ti/Au layers, which also work as Ti contacts. This structure is also illustrated schematically in the lower picture. (b) Band diagram of the Schottky junction of the source contact of the Pt–Ti hybrid device. The direction of bending of the semiconductor’s band at the junction of the Pt and Ti parts is opposite to each other. (c) With a negative gate bias, holes can be injected from the Pt part of the contact. In this case, the device behaves as a p-FET. (d) With a positive gate bias, electrons can be injected from the Ti part of the contact. In this case, the device behaves as an n-FET. (e) Back-gate modulation of drain currents in the Pt contact (p-type), Ti contact (n-type) and Pt–Ti hybrid contact (ambipolar) devices at VD = 100 mV and at room temperature. The currents in the Pt–Ti hybrid contact followed the on currents of the Pt contact (VBG−Vmin < 0) and Ti contact (VBG−Vmin > 0), confirming the carrier injection model explained in (b)–(d).


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Table of Contents Graphic

Pt h

Pt-Ti Hybrid (ambipolar)

10−7 EF 10−8

α-MoTe2 Pt

EC

VBG < 0

EF EV

VBG > 0

ID (A/μm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10−9

Pt (p-type)

10−10 10−11

Ti EF

e Ti

Ti (n-type) 10−12 −40

VD = 100 mV −20

0

20

40

VBG − Vmin (V)


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Carrier Polarity Control in Î±-MoTe2 Schottky ... - ACS Publications

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