Enhanced End-Contacts by Helium Ion Bombardment to ... - MDPI

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Enhanced End-Contacts by Helium Ion Bombardment to Improve Graphene-Metal Contacts Kunpeng Jia 1 , Yajuan Su 1, *, Jun Zhan 1 , Kashif Shahzad 1 , Huilong Zhu 1 , Chao Zhao 1,2 and Jun Luo 1,2, * 1

2

*

Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics of Chinese Academy of Sciences, 3 Bei-Tu-Cheng West Road, Chaoyang District, Beijing 100029, China; [email protected] (K.J.); [email protected] (J.Z.); [email protected] (K.S.); [email protected] (H.Z.); [email protected] (C.Z.) University of Chinese Academy of Sciences, Beijing 100049, China Correspondence: [email protected] (Y.S.); [email protected] (J.L.); Tel.: +86-10-8299-5920 (Y.S.); +86-10-8299-5876 (J.L.)

Academic Editors: Ho Won Jang and Soo Young Kim Received: 23 June 2016; Accepted: 29 July 2016; Published: 26 August 2016

Abstract: Low contact resistance between graphene and metals is of paramount importance to fabricate high performance graphene-based devices. In this paper, the impact of both defects induced by helium ion (He+ ) bombardment and annealing on the contact resistance between graphene and various metals (Ag, Pd, and Pt) were systematically explored. It is found that the contact resistances between all metals and graphene are remarkably reduced after annealing, indicating that not only chemically adsorbed metal (Pd) but also physically adsorbed metals (Ag and Pt) readily form end-contacts at intrinsic defect locations in graphene. In order to further improve the contact properties between Ag, Pd, and Pt metals and graphene, a novel method in which self-aligned He+ bombardment to induce exotic defects in graphene and subsequent thermal annealing to form end-contacts was proposed. By using this method, the contact resistance is reduced significantly by 15.1% and 40.1% for Ag/graphene and Pd/graphene contacts with He+ bombardment compared to their counterparts without He+ bombardment. For the Pt/graphene contact, the contact resistance is, however, not reduced as anticipated with He+ bombardment and this might be ascribed to either inappropriate He+ bombardment dose, or inapplicable method of He+ bombardment in reducing contact resistance for Pt/graphene contact. The joint efforts of as-formed end-contacts and excess created defects in graphene are discussed as the cause responsible for the reduction of contact resistance. Keywords: graphene; contact resistance; helium ion bombardment; defects; end-contact

1. Introduction In the past decade, graphene has attracted worldwide attention as a miracle material with unique electrical and physical properties [1–3]. Graphene holds great promise in a range of device applications, such as field effect transistors, photo-detectors, radio frequency (RF) devices, and spintronics [4–8]. For all these application scenarios, the large specific contact resistance (Rc ) between graphene and metals is one of grand challenges which hinders the progressive improvement on device performance [9–18]. In order to access the intrinsic excellent electrical properties of graphene, the Rc between graphene and metals as low as possible is desired. There has been plentiful studies on the properties of graphene-metal (G-M) contact [11,16,19–21] and methods to reduce G-M contact resistance. Choi et al. [22] and Robinson et al. [23] proposed a plasma treatment to make the graphene surface hydrophilic, thus enhancing the bonding between graphene and metals.

Nanomaterials 2016, 6, 158; doi:10.3390/nano6090158

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Nanomaterials 2016, 6, 158

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By using this method Robinson got as low contact resistance as 10−7 Ω·cm2 . Similar to this plasma treatment, ultraviolet/ozone (UVO) was used to clean the graphene surface after lithography which Nanomaterials 6, 158 2 of 13 due reduced contact2016, resistance to 7 × 10−7 Ω·cm2 and 568 Ω·µm respectively [24,25]. Nevertheless, to the difficulty in tuning the processing parameters properly when applying these methods, the Robinson got as low contact resistance as 10−7 Ω·cm2. Similar to this plasma treatment, excellent uniformity in reduced R over a wide range cannot be easily realized, or evenreduced the Rc will c to clean the graphene ultraviolet/ozone (UVO) was used surface after lithography which be degraded under some the graphene/metals end-contacts, contact resistance to 7 circumstances. × 10−7 Ω·cm2 andAlternatively, 568 Ω·μm respectively [24,25]. Nevertheless, due to initially the disclosed in a theoretical work [26], is claimed to possess great potential in improving R dramatically. difficulty in tuning the processing parameters properly when applying these methods, thec excellent The utilization ofreduced end-contacts reducing Rc is also confirmed [20,27,28]. uniformity in Rc overin a wide range cannot be easily realized,by orexperimental even the Rc willefforts be degraded under somenotably circumstances. the graphene/metals end-contacts, in 2a) [20], In those works, reducedAlternatively, Rc were demonstrated by introducing holes initially (~2.2 × disclosed 10−9 Ω·cm theoretical work (100 [26], Ω is·µm) claimed great in improving Rc dramatically. The to etched zigzag edges [27],toorpossess line cuts (125potential Ω·µm) [28] in the contact area of graphene utilization of end-contacts in reducing R c is also confirmed by experimental efforts [20,27,28]. In those form end-contacts with metals. It is worth noting that although Rc can be reduced using end-contacts, works, notably reduced Rc were demonstrated by introducing holes (~2.2 × 10−9 Ω·cm2) [20], etched the approach to introducing various patterns in the contact area of graphene is too complicated zigzag edges (100 Ω·μm) [27], or line cuts (125 Ω·μm) [28] in the contact area of graphene to form to be implemented in practical applications, since in these cases the lithography and subsequent end-contacts with metals. It is worth noting that although Rc can be reduced using end-contacts, the dry etching needed in order toinshape the holes, edges, line cuts [20,27,28]. approachistousually introducing various patterns the contact area ofzigzag graphene is tooorcomplicated to be Moreover, this additional lithography for introducing patterns in the contact area of graphene implemented in practical applications, since in these cases the lithography and subsequent dry may incuretching the problem of polymer graphene, degrade Rc .Moreover, In this regard, is usually needed inresidues order to on shape the holes,which zigzagwould edges,possibly or line cuts [20,27,28]. this while additional lithography introducing patterns in theconcept contact to area of graphene incur the a simple efficient methodfor featuring the end-contacts reduce Rc is ofmay great importance problem ofInpolymer residues on graphene, would possibly degrade Rc. In this regard, and a simple and interest. this work, a novel methodwhich to form end-contacts between graphene various while efficient method featuring the end-contacts concept to reduce R c is of great importance and metals (Ag, Pd, and Pt) is explored. In this method, defects are introduced to graphene by light He+ interest. Inand thisthereafter work, a novel method toare form end-contacts graphene and various metals bombardment end-contacts formed betweenbetween graphene and metals. Since the thermal (Ag, Pd, and Pt) is explored. In this method, defects are introduced to graphene by light He+ annealing treatment is a common yet effective way to reduce Rc [29,30], the effect of annealing on Rc bombardment and thereafter end-contacts are formed between graphene and metals. Since the with and without He+ bombardment is also investigated in this work. thermal annealing treatment is a common yet effective way to reduce Rc [29,30], the effect of annealing on Rc with and without He+ bombardment is also investigated in this work.

2. Experiment

2. Experiment Thermal chemical vapor deposition (CVD) grown graphene [31,32] was transferred onto a heavily p-type doped silicon wafer vapor with 100 nm thermal oxide [33]. The optical microscopy (OM) and Thermal chemical deposition (CVD) grown graphene [31,32] was transferred ontoatomic a force heavily microscopy inspect the morphology of transferred graphene, shown p-type(AFM) dopedwere siliconemployed wafer withto100 nm thermal oxide [33]. The optical microscopy (OM)as and atomic force microscopy (AFM) that werethe employed to inspect morphology of transferred graphene, in Figure 1a,b. The results display graphene film isthe continuous with a large single layer ratio. as shown in Figure 1a,b. The results display that the graphene film is continuous with a large single To further characterize the quality of the graphene, Raman scattering spectroscopy was implemented layer ratio. To further characterize the quality of the graphene, Raman scattering spectroscopy was in and the Raman spectrum is shown in Figure 1c. The Raman scattering measurement was performed implemented and the Raman spectrum is shown in Figure 1c. The Raman scattering measurement air using a 50× objective and the excitation laser energy was 2.41 eV (514 nm). Low laser power was was performed in air using a 50× objective and the excitation laser energy was 2.41 eV (514 nm). Low used to avoid the sample damage caused by heating. The G peak and 2D peak appearing at ~1585 and laser−power was used to avoid the sample damage caused by heating. The G peak and 2D peak ~2700appearing cm 1 respectively are~2700 observed and the ratio 2D/G peaks is higher indicating that the at ~1585 and cm−1 respectively areofobserved and the ratio ofthan 2D/G2peaks is higher transferred graphene is single layer. than 2 indicating that the transferred graphene is single layer.

(a)

(b)

(c) 2D G

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Figure 1. (a) Optical and (b) Atomic force microscopy (AFM) images of chemical vapor deposition

Figure 1. (a) Optical and (b) Atomic force microscopy (AFM) images of chemical vapor deposition (CVD)-grown graphene that is transferred onto SiO2/Si substrate; (c) showing the Raman spectrum of (CVD)-grown graphene that is transferred onto SiO2 /Si substrate; (c) showing the Raman spectrum of the as-transferred graphene. The color scale of AFM image is −20–20 nm. the as-transferred graphene. The color scale of AFM image is −20–20 nm.

Transferring line method (TLM) is used to extract the contact resistance between graphene and metals. The fabrication of TLM test is structures schematically shownresistance in Figure between 2a–f. Notegraphene that Transferring line method (TLM) used toisextract the contact conventionally in the fabrication of graphene-based devices, photoresist (PR) is spin-coated directly and metals. The fabrication of TLM test structures is schematically shown in Figure 2a–f. Note on graphene and in thereafter the contactof window is opened bydevices, lithography. In such a case, that conventionally the fabrication graphene-based photoresist (PR)PR is residues spin-coated usually occur which cannot be removed effectively. PR residues not only influence the properties of directly on graphene and thereafter the contact window is opened by lithography. In such a case, graphene but also impede the good contact between graphene and metals. In order to eliminate the


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PR residues usually occur which cannot be removed effectively. PR residues not only influence the properties of graphene but also impede the good contact between graphene and metals. In order to eliminate the adverse effect of PR residues on the contact properties, a ~12-nm-thick aluminum oxide (AlOx ) isolation layer is deposited between PR and graphene as a hardmask by Atomic Layer Nanomaterials 2016,[34] 6, 158(cf. Figure 2a,b). The graphene diffusion strips were protected by 3 of 13 Deposition (ALD) patterned positive PR followed by Ar plasma etching to remove un-protected AlOx and underneath graphene adverse effect of PR residues on the contact properties, a ~12-nm-thick aluminum oxide (AlOx) (cf. Figure 2c). layer Afteristhe definition of aPR graphene diffusion strip, another lithography using negative isolation deposited between and graphene as a hardmask by Atomic Layer Deposition PR was used[34] to define the2a,b). contact followedstrips by wet using dilute solution (ALD) (cf. Figure Thewindows graphene diffusion wereetching protected by patterned positive of PRH3 PO4 (1:3) atfollowed 40 ◦ C toby remove the unwanted AlOx layer in theseAlO contact (cf. Figure 2d). Afterwards, Ar plasma etching to remove un-protected x and windows underneath graphene (cf. Figure + bombardment, 2c). After the definition of a into graphene using negative PR was metals the samples were categorized two diffusion sets. Forstrip, the another first setlithography without He used to define the contact windows followed by wet etching using dilute solution H3PO4 (1:3) at (X/Au = 40/10 nm, X = Ag, Pd or Pt) were deposited directly on graphene byofe-beam evaporation 40 °C to remove the unwanted AlOx layer in these contact windows (cf. Figure 2d). Afterwards, the + (cf. Figure 2e). For another set, the contact windows were shot by energetic He and then the same samples were categorized into two sets. For the first set without He+ bombardment, metals (X/Au = metals were deposited by e-beam evaporation (cf. Figure 2d,e). The fabrication of TLM test structures 40/10 nm, X = Ag, Pd or Pt) were deposited directly on graphene by e-beam evaporation (cf. Figure was finished by a lift-off to remove PR and metals Figure 2f).were For both 2e). For another set, the process contact windows wereunwanted shot by energetic He+ and then(cf. the same metals sets, the TLM test structures were electrically characterized before and after a thermal annealing. deposited by e-beam evaporation (cf. Figure 2d,e). The fabrication of TLM test structures was finished The thermal annealing process implemented in a tube furnace min under by a lift-off process to removewas unwanted PR and metals (cf. Figure 2f). at For420 both◦ C/30 sets, the TLM test a low structures were electrically characterized beforeprocess, and after50 a thermal annealing. annealing pressure of 40 Pa. During the whole annealing sccm argon (Ar)The gasthermal was steadily pumped process was implemented inelectron a tube furnace at 420 °C/30 min under of a low pressure of TLM 40 Pa.test During into the tube furnace. Scanning microscope (SEM) images as-fabricated structures the whole annealing process, 50 sccm argon (Ar) gas was steadily pumped into the tube furnace. are shown in Figure 2g,h. The width of defined graphene diffusion strips is 15 µm and the gap between Scanning electron microscope (SEM) images of as-fabricated TLM test structures are shown in Figure two G-M contacts varies from 3 to 40 µm. The characterizations of specific contact resistance for 2g,h. The width of defined graphene diffusion strips is 15 μm and the gap between two G-M contacts different G-M contacts were using Keithley 4200 semiconductor analyzer varies from 3 to 40 μm. Theperformed characterizations of aspecific contact resistance for differentparameter G-M contacts (IMECAS, China). were Beijing, performed using a Keithley 4200 semiconductor parameter analyzer (IMECAS, Beijing, China).

(c) (a)

Graphene

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SiO2

SiO2

Si

Si

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PR1

AlOx

Metal

SiO2 Si

He+

(d) PR2

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SiO2

SiO2

SiO2

Si

Si

Si

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Figure 2. Schematics of the process showing the fabrication flow of transferring line method (TLM)

Figuretest 2. Schematics of the process showing the fabrication flow of transferring line method (TLM) test structures. (a) CVD-grown graphene is transferred onto SiO2/Si substrate; (b) The AlOx isolation structures. CVD-grown graphene transferred onto SiO2 /Si substrate; The AlO isolation layer layer (a) is deposited on graphene byisAtomic Layer Deposition (ALD); (c) The (b) definition of xgraphene is deposited onstrip graphene by Atomic Layer (ALD); (c) opening The definition of windows graphene diffusion by the first lithography andDeposition plasma etching; (d) The of contact bydiffusion a second lithography and wetand etching (Afterwards, optional He+opening bombardment); (e) Metals are deposited strip by the first lithography plasma etching; (d) The of contact windows by a second by e-beam Lift-off process to remove unwanted photoresist (PR) and metals; lithography and evaporation; wet etching(f) (Afterwards, optional He+ bombardment); (e) Metals are deposited by showing the electron microscope (SEM) images of as-fabricated TLMmetals; test structures. e-beam(g,h) evaporation; (f)scanning Lift-off process to remove unwanted photoresist (PR) and (g,h) showing The gap between two G-M contacts varies from 3 to 40 μm. the scanning electron microscope (SEM) images of as-fabricated TLM test structures. The gap between two G-M contacts varies from 3 to 40 µm.


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3. Results 3.1. Effect of Annealing on the G-M Contact Properties For the extraction of G-M contact resistance, four probe configuration illustrated in Figure 3a is employed in TLM test structures. The current flows from probe 1 to probe 4 and the voltage drop is measured between probe 2 and probe 3 at the same two landing pads. The ideal linear relationship between voltage drop and input current shown in Figure 3b demonstrates good ohmic contacts between graphene and metals. It is well known that, for the TLM test structures, the measured total resistance (RT ) consists of graphene sheet resistance (RS ), G-M contact resistance (RGM ), metal wire resistance (RW ), and probe-pad contact resistance (RPP ) as seen in Equation (1). RT = RS (lG ) + 2RGM + RW (lW ) + 2RPP

(1)

Among them, RS relates to the length of graphene diffusion strip (lG ) between two G-M contacts and the value of RW depends on the length of metal wires between pads and G-M contact windows (lW ). Benefitting from the four point probe configuration in TLM method, RPP can be ignored. The slope in Figure 3b yields RT which is the sum of RS , 2RGM , and RW . Normally, Rw is small which can be omitted in Equation (1) if metal wires are thick and/or short. However, since in this work the deposited metal wires are thin i.e., 50-nm-thick and long, Rw cannot be omitted which is also a disturbing factor for the linear plotting of RT vs. lG . In order to get a perfect linear plotting, it is a must to subtract Rw from RT . The sheet resistances of metal wires are extracted to be 0.96, 3.72 and 4 Ω/ for Ag/Au, Pd/Au and Pt/Au, respectively. As a result, the values of Rw can be calculated according to lW and the width of metal wires and be subtracted from RT . After the Rw subtraction, perfect linear relationships between RT and lG for all G-M contacts are accomplished as shown in Figure 4. For the contacts between Ag, Pd, Pt, and graphene without He+ bombardment, the linear plots of RT vs. lG before annealing (a–c) and after annealing (d–f) are shown in Figure 4. For each lG , 10 data points are used and the linear fitting is performed using concatenate fit method. The perfect linear fittings for all G-M contacts are evident demonstrating the validity of employed method to extract Rc in this work. The interception of the fitted red line with RT (Y-axis) yields the value of 2RGM . Since the width of the graphene diffusion strip is 15 µm, Rc is therefore calculated by RGM × 15 µm which is also offered in each corresponding plot. For all three G-M contacts, it is clearly observed that Rc values are reduced significantly after annealing which confirms the effectiveness of the annealing in improving G-M contacts [30]. Except the plot of Rc with different metal work functions [35,36] in Figure 5a for three G-M contacts before annealing, all extracted Rc data in Figure 4 are summarized in Figure 5b for the sake of easy comparison. In Figure 5a, filled black circles represent the extracted Rc values for different metals (Ag, Pd, and Pt) and filled red squares depict the mean Rc values with error bar. As seen, albeit the metal work function alters from ~4.26 to ~5.65 eV for different metals, the Rc value remains nearly invariable, manifesting that Rc is barely—or even not—relevant to metal work functions before annealing. This observation is also in accordance with previous finds of E. Watanabe’s et al. [37] in which different G-M contacts were investigated. It seems that the work function of the graphene under different metals is pinned to a particular value regardless of the work function of used metals as indicated in [38].


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Nanomaterials 2016, 2016, 6, 6, 158 158 Nanomaterials

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Probe 44 Probe

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Probe 33 Probe Pad B B Pad

V

llGG Width Width

Current Current bias bias

W llW

Pad A A Pad Probe 11 Probe

Probe 22 Probe

Figure 3. 3. (a) (a) Four probe probe configurations configurations and and (b) (b) measured measured current-potential current-potential drop drop characteristics characteristics of of Figure (a) Four Four configurations and current-potential characteristics typical G-M contact with different l G in TLM test structures. in TLM TLM test test structures. structures. typical G-M contact with different lG G in

(a)

(b)

(c)

(d)

(e)

(f)

Figure 4. The The linear plots plots of of R RT vs. llGG for for the contacts contacts between Ag, Ag, Pd, Pt Pt and graphene graphene without He He++ Figure Figure 4. 4. The linear linear plots of RTT vs. vs. lG for the the contacts between between Ag, Pd, Pd, Pt and and graphene without without He+ bombardment before annealing annealing (a–c) and and after annealing annealing (d–f). For For each llGG,, 10 10 data points points are used used bombardment bombardment before before annealing (a–c) (a–c) and after after annealing (d–f). (d–f). For each each lG , 10 data data points are are used and the liner fitting is performed using concatenate fit method. Extracted R c value is shown in each and is performed performed using using concatenate concatenate fit method. Extracted and the the liner liner fitting fitting is fit method. Extracted R Rcc value value is is shown shown in in each each corresponding plot. corresponding corresponding plot. plot.

Nanomaterials 2016, 6, 158 Nanomaterials 2016, 6, 158

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Figure 5. resistance as aasfunction of metal workwork functions (Ag, Pd, Filled Figure 5. (a) (a)Specific Specificcontact contact resistance a function of metal functions (Ag,and Pd,Pt). and Pt). black circles (●) represent extracted R c values for different metals (Ag, Pd, and Pt). Filled red squares Filled black circles ( ) represent extracted Rc values for different metals (Ag, Pd, and Pt). Filled red (■) depict meanthe Rcmean values error bar; (b) bar; Extracted averageaverage Rc values with error forbar all squares (the ) depict Rcwith values with error (b) Extracted Rc values withbar error G-M (Ag, Pd andPd Pt/graphene) before before and after for allcontacts G-M contacts (Ag, and Pt/graphene) andannealing. after annealing.

By comparing Rc values for all G-M contact before and after annealing shown in Figure 5b, it is By comparing Rc values for all G-M contact before and after annealing shown in Figure 5b, obvious that thermal annealing is helpful in reducing G-M contact resistance. In [30], it is argued that it is obvious that thermal annealing is helpful in reducing G-M contact resistance. In [30], it is the mechanism behind improved G-M contact property after thermal annealing is, however, not argued that the mechanism behind improved G-M contact property after thermal annealing is, attributed to the removal of resist residues, instead to numerous end-contacts between metals and however, not attributed to the removal of resist residues, instead to numerous end-contacts between dangling bonds in graphene formed by the reaction of dissolved carbon atoms from graphene lattice metals and dangling bonds in graphene formed by the reaction of dissolved carbon atoms from sites with metals [30]. It is not unexpected for Ni used in [30] that the Rc value is reduced significantly graphene lattice sites with metals [30]. It is not unexpected for Ni used in [30] that the Rc value is after annealing since it is a chemically adsorbed metal. The intriguing point in this work is why reduced significantly after annealing since it is a chemically adsorbed metal. The intriguing point physically adsorbed metals like Pt and Ag also show remarkably reduced Rc values after annealing, in this work is why physically adsorbed metals like Pt and Ag also show remarkably reduced Rc i.e., from 247.43 and 285.07 to 180.75 and 227.81 Ω·μm for Pt/graphene and Ag/graphene contacts, values after annealing, i.e., from 247.43 and 285.07 to 180.75 and 227.81 Ω·µm for Pt/graphene and respectively. In order to clarify this point, elaborate characterizations of graphene in G-M contact Ag/graphene contacts, respectively. In order to clarify this point, elaborate characterizations of windows before and after annealing are implemented by Raman scattering spectrum. The contact graphene in G-M contact windows before and after annealing are implemented by Raman scattering windows are opened using aqua regia (HCl:HNO3 = 3:1) to remove metals thus exposing the spectrum. The contact windows are opened using aqua regia (HCl:HNO3 = 3:1) to remove metals graphene under metals [30]. The Raman spectra of as-exposed graphene (before and after annealing) thus exposing the graphene under metals [30]. The Raman spectra of as-exposed graphene (before are displayed in Figure 6. As obviously seen, for all G-M contacts (Ag, Pd, and Pt), a distinct D peak and after annealing) are displayed in Figure 6. As obviously seen, for all G-M contacts (Ag, Pd, appears at ~1350 cm−1 for the graphene after −annealing in comparison to the graphene before and Pt), a distinct D peak appears at ~1350 cm 1 for the graphene after annealing in comparison annealing. Note that the Raman spectra of the graphene before annealing resemble those of asto the graphene before annealing. Note that the Raman spectra of the graphene before annealing transferred graphene on SiO2 (cf. Figure 1c), indicating that the D peak does not originate from the resemble those of as-transferred graphene on SiO2 (cf. Figure 1c), indicating that the D peak does metal deposition and aqua regia attack and this is also in consistent with Ref. [30]. Consequently, the not originate from the metal deposition and aqua regia attack and this is also in consistent with appearance of D peak can be solely attributed to the structural defects in the graphene after annealing. Ref. [30]. Consequently, the appearance of D peak can be solely attributed to the structural defects in As aforementioned, these structural defects result from the formation of numerous end-contacts the graphene after annealing. As aforementioned, these structural defects result from the formation of between graphene and metals after annealing and this enhances G-M contact property prominently. numerous end-contacts between graphene and metals after annealing and this enhances G-M contact Along with this guideline, the presence of end-contacts between graphene and metals is critical to property prominently. Along with this guideline, the presence of end-contacts between graphene and achieve extremely low specific contact resistance for G-M contacts. The approach to introduce defects metals is critical to achieve extremely low specific contact resistance for G-M contacts. The approach to thus forming end-contacts is, therefore, naturally brought up which is investigated in the following introduce defects thus forming end-contacts+ is, therefore, naturally brought up which is investigated part. In the following part, a self-aligned He bombardment method to induce defects in the graphene in the following part. In the following part, a self-aligned He+ bombardment method to induce defects within the contact windows and therefore to form end-contacts after annealing is explicitly illustrated. in the graphene within the contact windows and therefore to form end-contacts after annealing is explicitly illustrated.


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The Raman spectra of the graphene in the G-M contact windows before and after annealing. FigureFigure 6. The6.Raman spectra of the graphene in the G-M contact windows before and after annealing.

3.2. Effect of He+ Bombardment on the G-M Contact Properties

3.2. Effect of He+ Bombardment on the G-M Contact Properties

The graphene in G-M contact windows was bombarded by He+ to intentionally induce defects

+ to−2intentionally 13 cm The graphene inRaman G-M contact bombarded He induce defects within it. The energy andspectra dose ofwindows impinging He is 35 keV and windows 2by  10 . After He+ annealing. bombardment, Figure 6. The the graphenewas in +the G-M contact before and after + 13 − 2 + withinthe it.graphene The energy and dose of impinging He is 35 andscattering 2 × 10 spectrum. cm . After bombardment, in the contact windows is inspected by keV Raman TheHe Raman spectra + Bombardment on the G-M Contact Properties 3.2. Effect of He of the bombarded and un-bombarded graphene by as reference are shownspectrum. in Figure 7a. Qualitatively, the graphene in the contact windows is inspected Raman scattering The Raman spectra D peak forand the bombarded graphene is much more pronounced compared that for the + to intentionally of thethe bombarded graphene reference are in Figureto 7a. Qualitatively, The graphene inun-bombarded G-M contact windows was as bombarded by Heshown induce defects + bombardment. un-bombarded one, illustrating that the graphene is indeed impaired by the He + 13 −2 + the Dwithin peakit.for bombarded more compared to that for the Thethe energy and dose ofgraphene impingingis Hemuch is 35 keV andpronounced 2  10 cm . After He bombardment, To reckon quantitatively the damage created by He+ bombardment in the graphene, a parameter ID/IG + bombardment. the grapheneone, in the contact windows is inspected by Raman scattering spectrum. TheHe Raman spectra un-bombarded illustrating that the graphene is indeed impaired by the is given in Equation (2) [39]: + bombardment of thequantitatively bombarded and un-bombarded graphene reference are shown Figure 7a.aQualitatively, To reckon the damage created by Heas in theingraphene, parameter ID /IG 2 2 2 2 2) the D peak for the bombarded graphene is much more pronounced compared I r − r πr π(r −r πrS2 to that for the A S S A S is given in EquationD (2) [39]: =C [exp (− ) − exp (− )] + C [1 − exp⁡(− )] (2)

A 2 S un-bombardedIGone, illustrating by the He rA − 2rS2 that the L2Dgraphene is indeed L2D impaired L2D+ bombardment. " ! !# + bombardment in the graphene, a parameter ID/IG To reckon quantitatively the damage created by He 2 2 2 2 2 2 π rA where − rS the D band scattering πr r − r πrdetermine whereIDrS and rA are length scales that the region takes S S place. CA 2A (2)S[39]: exp − − exp − + C [ 1 − exp (− )] (2) is given in=Equation S IG rS determines the disorderedL2D area and rA is the radius of rA −radius 2r2S of the structurally L2D L2D the area 2 2 2 2 ID pointrdefects πrS2the D band π(r πraS2 measure of the surrounding the scattering A − rS in which A −rS ) takes place. CA is = CA 2 )] + CS [1 − exp⁡(− 2 )] (2) 2 [exp (− 2 ) − exp (− 2 IG length rA −ofscales 2rthe Ldetermine Lratio value G ratio SDis the value of the ID D/IG in the highlytakes wheremaximum rS and rpossible that theCLregion where band scattering D in graphene. D S ID/I A are disordered [39,40]. LDscales is the mean defect distance and the defect proportional toof 1/L D2. area place.where rS determines the radius of that the determine structurally area and rAisscattering is the radius the rS andlimit rA are length thedisordered region where thedensity D band takes place. In accordance with the empirical data inthe [39], A = (4.2 ± 0.1), CS = (0.87 ± 0.05), rA = (3.00 ± 0.03) nm, surrounding the point defects which D Cband scattering takesrAplace. measure rS determines the radius ofinthe structurally disordered area and is the Cradius the areaof the A is a of and rS = (1.00 ± 0.04) nm, the ID/IG as a function of LD for the graphene with and without He+ surrounding thevalue pointofdefects D band scattering place. CAID is/I a Gmeasure maximum possible the IDin /IGwhich ratio the in graphene. CS is thetakes value of the ratio inof thethe highly bombardment is plotted in Figure 7b. It can be seen that as the ID/IG increases from 0.032 for the unmaximum possible value of the I D /I G ratio in graphene. C S is the value of the I D /I G ratio in the highly disordered limit [39,40]. LD is the mean defect distance and the defect density is proportional to 1/LD 2 . bombarded graphene to 0.143 for the bombarded one, the LD increases dramatically from 58.05 nm disorderedwith limit the [39,40]. LD is thedata meanindefect and±the defect density is to 1/LD±2. 0.03) In accordance empirical [39], distance Cin (4.2 0.1), CS = (0.87approximately ±proportional 0.05), rA = 4.4 (3.00 A = to accordance 27.56 nm. This means that the defect density the graphene is increased times In with the empirical data in [39], C A = (4.2 ± 0.1), CS = (0.87 ± 0.05), rA = (3.00 ± 0.03) nm, nm, and rS He = (1.00 ± 0.04) nm, the ID /IG as a function of LD for the graphene with and without He+ + bombardment. using and rS = (1.00 ± 0.04) nm, the ID/IG as a function of LD for the graphene with and without He+

bombardment is plotted in Figure 7b. It can be seen that as the ID /IG increases from 0.032 for the bombardment is plotted in Figure 7b. It can be seen that as the ID/IG increases from 0.032 for the unun-bombarded graphene to 0.143 for the bombarded(b) one, the LD increases dramatically from 58.05 nm (a) bombarded graphene to 0.143 for the bombarded one, the LD increases dramatically from 58.05 nm to 27.56 nm. This means that the defect density in the grapheneisisincreased increased approximately 4.4 times to 27.56 nm. This means that the defect density in the graphene approximately 4.4 times + usingusing He He bombardment. + bombardment.

(a)

(b)

Figure 7. (a) Raman spectra of the graphene in G-M contact windows with/without He+ bombardment before metal depositions; (b) The calculated relationship between ID/IG and mean defect distance (LD).

Figure 7. (a) Raman spectra of the graphene in G-M contact windows with/without He+ bombardment

Figure 7. (a) Raman spectra of the graphene in G-M contact windows with/without He+ bombardment before metal depositions; (b) The calculated relationship between ID/IG and mean defect distance (LD). before metal depositions; (b) The calculated relationship between ID /IG and mean defect distance (LD ).


8 of 13 8 of 13

For Ag, Ag, Pd, and Pt/graphene Pt/graphene contacts contacts with He++ bombardment, bombardment, the linear plots of before For of R RTT vs. lGG before annealing (a–c) (a–c) and and after after annealing annealing (d–f) (d–f) are are shown shown in in Figure Figure 8. 8. Similarly Similarly to to the theG-M G-Mcontacts contacts without without annealing He++bombardment, bombardment,10 10data datapoints pointsare areused usedand and the the linear linear fitting fitting is is performed performed using using concatenate concatenate fit fit He method. As thethe effectiveness of annealing in improving the G-M contacts withcontacts He+ bombardment method. Asseen, seen, effectiveness of annealing in improving the G-M with He+ is also evident.isHowever, how However, the He+ bombardment the specific contact is still bombardment also evident. how the He+impacts bombardment impacts the resistance specific contact not clear. In order to clear. provide a direct for the readers, the for the G-M contacts resistance is still not In order to comparison provide a direct comparison forRthe readers, R c values for c values + bombardment with/without He+with/without bombardmentHe are summarized inare Figure 9. In Figure 9a, the comparison made the G-M contacts summarized in Figure 9. In Figure is9a, the for three G-M and without He+with bombardment before annealing. It isbefore obvious that except comparison is contacts made forwith three G-M contacts and without He+ bombardment annealing. It + + Pd, He bombardment leads an increase in leads Rc value forincrease the Ag/graphene Pt/graphene contacts. is obvious that except Pd, Hetobombardment to an in Rc valueand for the Ag/graphene and + bombardment The defects induced Hedefects be blamed forshould the increase of Rcfor value and this Pt/graphene contacts.by The induced byshould He+ bombardment be blamed the increase of Rc value and observation also agrees withformed the G-Mbycontacts formed byon metal sputtering observation alsothis agrees well with the G-M well contacts metal sputtering graphene [41]. on [41]. The defects grapheneresult will probably result in the scattering of thereof carriers the andcarriers’ thereof Thegraphene defects in graphene willinprobably in the scattering of carriers and the carriers’ meanisfree path is shortened, in turn, gives rise increase to the increase Rc [11]. In Figure mean free path shortened, which in which turn, gives rise to the of Rc of [11]. In Figure 9b, + + 9b, the comparison is made for G-M threecontacts G-M contacts and He without He bombardment after the comparison is made for three with andwith without bombardment after annealing. + bombardment, annealing. AsAg/graphene seen, for Ag/graphene and Pd/graphene contacts with He+ bombardment, R c values As seen, for and Pd/graphene contacts with He the Rthe values are c + are reduced dramatically by and 15.1% and 40.1% compared to their counterparts without He+ reduced dramatically by 15.1% 40.1% compared to their counterparts without He bombardment. bombardment. Note thatRthe obtained Rc values in this work G-M for allcontacts, three G-M 193.33, Note that the obtained in this work for all three i.e.,contacts, 193.33, i.e., 118.28 and c values 118.28 189.15 Ω·µ m Ag, Pd, Pt/graphene respectively, arethe among thespecific lowest specific 189.15 and Ω·µm for Ag, Pd,for Pt/graphene contacts contacts respectively, are among lowest contact contact resistances [11,29,37]. Nevertheless, for Pt/graphene contact He+ bombardment after resistances [11,29,37]. Nevertheless, for Pt/graphene contact with He+ with bombardment after annealing, + annealing, theisRincreased c value is increased little bit compared to its counterpart without He+ bombardment the Rc value a little bit acompared to its counterpart without He bombardment and this and this be tentatively interpreted as what follows. could becould tentatively interpreted as what follows.

(a)

(b)

(c)

(d)

(e)

(f)

Figure Figure 8. 8. The The linear linear plots plots of of R RTT vs. vs. llGGfor forthe theAg, Ag,Pd, Pd, and and Pt/graphene Pt/graphenecontacts contactswith with He He++bombardment bombardment before before annealing annealing (a–c) (a–c) and and after annealing (d–f). For For each each llGG, ,10 10data datapoints pointsare areused usedand andthe the liner liner fitting fitting is is is shown in in each corresponding plot. is performed performedusing usingconcatenate concatenatefitfitmethod. method.Extracted ExtractedRcRvalue shown each corresponding plot. c value


(a)

9 of 13 9 of 13

(b)

(c)

Figure 9. of R Rcc for without He He++ bombardment Figure 9. The The comparison comparison of for three three G-M G-M contacts contacts with with and and without bombardment (a) (a) before before annealing and (b) after annealing; (c) Detailed R c values in (a,b) are summarized. B.A.: Before Annealing; annealing and (b) after annealing; (c) Detailed Rc values in (a,b) are summarized. B.A.: Before A.A.: After A.A.: Annealing. Annealing; After Annealing.

In Figure 10, an attempt is made to schematically illustrate why the Rc value is lower for In Figure 10, an attempt is made to schematically illustrate why the Rc value is lower for Ag/graphene and Pd/graphene contacts whereas is higher for Pt/graphene with He++ bombardment Ag/graphene and Pd/graphene contacts whereas is higher for Pt/graphene with He bombardment after annealing. We think the discussion about the higher or lower of contact resistance after after annealing. We think the discussion about the higher or lower of contact resistance after annealing annealing should take both the capability of metals forming end-contacts with defects and carrier should take both the capability of metals forming end-contacts with defects and carrier scattering scattering induced by defects within graphene into account [41–44]. It is assumed that for each G-M induced by defects within graphene into account+[41–44]. It is assumed that for each G-M contact contact there is an optimum dose (Doptimum) of He bombardment where the lowest Rc value (Rcmin) there is an optimum dose (Doptimum ) of He+ bombardment where the lowest R value (Rcmin ) takes + dose isc Doptimum, the takes place. Four cases happen to the dose of+ He+ bombardment. (1) If He created + place. Four cases happen to the dose of He bombardment. (1) If He dose is Doptimum , the created defects in graphene can be completely consumed by the formation of end-contacts between metals defects in graphene can be completely consumed by the formation of end-contacts between metals and graphene. As a result, Rcmin can be obtained; (2) If He+ dose is less than Doptimum, Rc value is reduced and graphene. As a result, Rcmin can be obtained; (2) If He+ dose is less than Doptimum , Rc +value is but not to Rcmin in that as-formed end-contacts are not saturated; (3) On the other hand, if He dose is reduced but not to Rcmin in that as-formed end-contacts are not saturated; (3) On the other hand, more than Doptimum but less than Dmax, though the formation of saturated end-contacts leads to the if He+ dose is more than Doptimum but less than Dmax , though the formation of saturated end-contacts reduction of Rc, excess created defects enhance the carriers scattering which unfortunately degrades leads to the reduction of Rc , excess created defects enhance the carriers scattering which unfortunately Rc concurrently. The joint efforts of as-formed end-contacts and excess created defects render the Rc degrades Rc concurrently. The joint efforts of as-formed end-contacts and excess created defects value not the lowest Rcmin; (4) If He+ dose is more than Dmax, the effort of excess created defects prevails render the Rc value not the lowest Rcmin ; (4) If He+ dose is more than Dmax , the effort of excess over that of saturated end-contacts and this leads to drastically increased Rc value that is even larger created defects prevails over that of saturated end-contacts and this leads to drastically increased Rc than Rc0 i.e., the Rc value for G-M contacts without He+ bombardment after annealing. Specifically, value that is even larger than Rc0 i.e., the Rc value for G-M contacts without He+ bombardment after for the Ag/graphene and Pd/graphene contacts, they belong to cases (2) and (3) where their Rc values annealing. Specifically, for the Ag/graphene and Pd/graphene contacts, they belong to cases (2) and are reduced but possibly not to the lowest values Rcmin. It is worth noting that for Pd/graphene contact, (3) where their Rc values are reduced but possibly not to the lowest values Rcmin . It is worth noting the employed dose 2  1013 cm−2 is even closer to its Doptimum since its reduction percentage 40.1% is that for Pd/graphene contact, the employed dose 2 × 1013 cm−2 is even closer to its Doptimum since its much bigger than 15.1% for Ag/graphene contact. For Pt/graphene contact, it belongs to case (4) reduction percentage 40.1% is much bigger than 15.1% for Ag/graphene contact. For Pt/graphene where the effort of excess created defects prevails over that of as-formed saturated end-contacts, contact, it belongs to case (4) where the effort of excess created defects prevails over that of as-formed resulting in even larger Rc value compared to its counterpart without He+ bombardment after saturated end-contacts, resulting in even larger Rc value compared to its counterpart without He+ annealing. The specific contact resistance as a function of ion bombardment dose is still under bombardment after annealing. The specific contact resistance as a function of ion bombardment dose investigation and will be revealed as a continuation of this work in the near future. is still under investigation and will be revealed as a continuation of this work in the near future.

Nanomaterials 2016, 6, 158 Nanomaterials 2016, 6, 158 Nanomaterials 2016, 6, 158

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RC RC Rc0 Rc0

Rcmin Rcmin

Doptimum Dmax D Dmax optimum He++ Bombardment Dose He Bombardment Dose

Figure 10. Schematic representation of the specific contact resistance (Rc) after annealing vs. He++ Figure 10. Schematic representation of the specific contact resistance (Rc ) after annealing vs. He + Figure 10. Schematic representation of the specific contact resistance (Rc) after annealing vs. He bombardment bombardment dose. dose. bombardment dose.

Similarly to the discussions in previous section, the elaborate characterizations of graphene in Similarly to the discussions in previous the elaborate characterizations of graphene in discussions previous section, section, theand elaborate characterizations G-M contact windows with He++ bombardment before after annealing are also performed by + G-M contact windows with He bombardment before and after annealing are also performed by Raman scattering spectrum. He The bombardment Raman spectrabefore of as-exposed graphene with He++ bombardment + Raman scattering spectrum. Raman spectra of graphene with He bombardment spectrum.are The of as-exposed as-exposed graphene withPd, Heand bombardment (before and after annealing) depicted in Figure 11. As can be seen, for all Ag, Pt/graphene (before and after annealing) are depicted in Figure 11. As can be seen, for all Ag, Pd, and Pt/graphene (before and after annealing) are depicted in Figure 11. As can be seen, for all Ag, Pd, and Pt/graphene contacts, a tiny D peak appears at ~1350 cm−1 for the graphene before annealing in comparison to as−1 for the graphene before annealing in comparison −1 contacts, a tiny D peak appears at ~1350 cm a tiny D peak appears at ~1350 cm for the graphene before annealing in comparison to as+ transferred graphene in Figure 1c and this is ought to be the result of He bombardment. However, + bombardment. + bombardment. to as-transferred graphene in Figure 1c and this is ought to be the result of He transferred graphene in Figure 1c and this is ought to be the result of He However, + for the graphene with He bombardment after annealing, the D peak becomes startlingly + However, for Ifthe graphene with He+ bombardment annealing, Dpeak peak becomes startlingly for the graphene with He annealing, the the D conspicuous. scrutinized, the bombardment intensities of D after peak after for the graphene with He++ becomes bombardment in all + conspicuous. If scrutinized, the intensities of D peak for the graphene with He bombardment in all peak for the graphene with He + G-M contact windows are higher than those for the graphene without He +bombardment in Figure 6. + bombardment G-M observation contact windows areevidence higher than for the graphene without is Hefavorable bombardment in Figure Figureof 6. in 6. This is good thatthose the formation of end-contacts in the presence This observation is good evidence that the formation of end-contacts is favorable in the presence of He This observation is good evidence of end-contacts is favorable in resistance the presence of+ + bombardment. He We believe thethat keythe to formation achieve extremely low specific contact is to bombardment. Weinbelieve thethus key to achieve extremely low specific contact resistance is to introduce He+ bombardment. We believe the key toplenty achieve extremely low specific contact is to introduce defects graphene forming of end-contacts after annealing likeresistance other methods defects in graphene thus forming plenty of end-contacts after annealing like other methods to form introduce defects in graphene thus forming plenty of end-contacts after annealing like other methods to form end-contacts [20,27,28]. The approaches to introduce defects into graphene can be diverse, end-contacts [20,27,28]. The approaches tobyintroduce defects into graphene can be diverse, not only by to form [20,27,28]. Thealso approaches to introduce intospecies. graphene be diverse, + bombardment, not only end-contacts by He but the implantation ofdefects other ion It iscan a remarkable + + He bombardment, but alsotoby the of other contact ion species. It is species. a may remarkable that the not that only by optimum He bombardment, butimplantation also the implantation of other ion It isdepending afact remarkable fact the dose achieve theby lowest specific resistance vary on optimum dose to achieve the lowest specific contact resistance may vary depending on the ion species fact that the optimum dose to achieve the lowest specific contact resistance may vary depending on the ion species used or on the implantation energy. used or on the implantation energy. the ion species used or on the implantation energy.

Figure 11. The Raman spectra of the graphene in the G-M contact windows with He+ bombardment Figureand 11. The The Raman spectra of of the the graphene graphene in in the the G-M G-Mcontact contactwindows windowswith withHe He++ bombardment bombardment before afterRaman annealing. Figure 11. spectra before and and after after annealing. annealing. before

4. Conclusions 4. Conclusions To summarize, the specific contact resistance for three G-M contacts (Ag, Pd, and Pt/graphene) Toreduced summarize, specific from contact resistance forand three G-M Ω·µ contacts Pd,197.33, and Pt/graphene) are all afterthe annealing 285.07, 267.60, 247.43 m to (Ag, 227.81, and 180.75 are all reduced after annealing from 285.07, 267.60, and 247.43 Ω·µ m to 227.81, 197.33, and 180.75


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4. Conclusions To summarize, the specific contact resistance for three G-M contacts (Ag, Pd, and Pt/graphene) are all reduced after annealing from 285.07, 267.60, and 247.43 Ω·µm to 227.81, 197.33, and 180.75 Ω·µm, respectively. This indicates that not only chemically adsorbed metal (Pd) but also physically adsorbed metals (Ag and Pt) readily form end-contacts at intrinsic defect locations in graphene. Along with this guideline, a novel method, in which self-aligned He+ bombardment to induce exotic defects in graphene and subsequent thermal annealing to form end-contacts, was proposed in order to further reduce the specific contact resistance. Achieved results show that the specific contact resistances are reduced significantly by 15.1% and 40.1% for Ag/graphene and Pd/graphene contacts with He+ bombardment compared to their counterparts without He+ bombardment, respectively. For the Pt/graphene contact, the contact resistance is, however, not reduced as anticipated with He+ bombardment and this might be ascribed to either inappropriate He+ bombardment dose, or inapplicable method of He+ bombardment in reducing contact resistance for Pt/graphene contact. The effort of as-formed end-contacts prevailing over that of excess created defects is attributed to the reduction in Rc values for G-M contacts with He+ bombardment after annealing. By manipulating the He+ bombardment dose in conjunction with Ag, Pd, and Pt metals, the lowest Rc value for each G-M contact could be possibly accomplished. It is worth noting that the proposed He+ bombardment and metal depositions share the same lithography mask and this processing simplicity demonstrates that our proposed method is very efficient in improving the contact properties for graphene-based devices in the future. Acknowledgments: This work has been supported by the Opening Project of Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics od the Chinese Academy of Sciences, National Sciences and Technology Major Project (Grant No. 2011ZX02707-3), the National Natural Science Foundation of China (Grant No. 61306124 and No. 61204123) and by the Youth Innovation Promotion Association of CAS under Grant No. 2015097. Author Contributions: K.J., J.L., Y.S., H.Z. and C.Z. conceived and designed the experiments; K.J. and K.S. performed the experiments; K.J. analyzed the data; K.J. and J.Z. contributed reagents/materials/analysis tools; K.J., J.L. and Y.S. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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