Influence of Substrate Microstructure on the Transport Properties of

Subscriber access provided by KUNGL TEKNISKA HOGSKOLAN

Article

Influence of substrate micro-structure on the transport properties of CVD-graphene Andrey Babichev, Sergey Rykov, Maria Tchernycheva, Alexander Smirnov, Valery Davydov, Yurii Kumzerov, and Vladimir Butko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08479 • Publication Date (Web): 13 Dec 2015 Downloaded from http://pubs.acs.org on December 14, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

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

ACS Applied Materials & Interfaces

Influence of substrate micro-structure on the transport properties of CVD-graphene Andrey V. Babichev1,2,3*, Sergey A. Rykov4,5, Maria Tchernycheva1, Alexander N. Smirnov2,Valery Yu. Davydov2, Yurii A. Kumzerov2, and Vladimir Y. Butko2 1

Institut d’Electronique Fondamentale, UMR 8622 CNRS, University Paris Sud XI, Orsay

91405, France 2

Ioffe Institute, St. Petersburg 194021, Russia

3

ITMO University, St. Petersburg 197101, Russia

4

Brookhaven National Laboratory, Upton, NY 11973, USA

5

Peter the Great St. Petersburg Polytechnic University, St. Petersburg 195251, Russia

KEYWORDS: graphene, interface, CVD technique, micro-pyramid LED, TLM, contact resistance

ABSTRACT: We report the study of electrical transport in few-layered CVD-graphene located on nanostructured surfaces in view of its potential application as a transparent contact to optoelectronic devices. Two specific surfaces with a different characteristic feature scale are

ACS Paragon Plus Environment

1


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 2 of 29

analyzed: semiconductor micro-pyramids covered with SiO2 layer and opal structures composed of SiO2 nanospheres. Scanning Tunneling (STM) and Electron Microscopies (SEM) as well as Raman spectroscopy have been used to determine graphene/substrate surface profile. The graphene transfer on the opal face centered cubic arrangement of spheres with a diameter of 230 nm leads to graphene corrugation (graphene partially reproduces the opal surface profile). This structure results in a reduction by more than 3 times of the graphene sheet conductivity compared to the conductivity of reference graphene located on a planar SiO2 surface but does not affect the contact resistance to graphene. The graphene transfer onto an organized array of micro-pyramids results in a graphene suspension. Unlike opal case, the graphene suspension on pyramids leads to a reduction of both the contact resistance and the sheet resistance of graphene compared to resistance of the reference graphene/flat SiO2 sample. The sample annealing is favorable to improve the contact resistance to CVD-graphene, however it leads to the increase of its sheet resistance.

INTRODUCTION There is a strong economically-driven demand for alternative inexpensive transparent contacts to optoelectronic devices. Due to its high transparency, flexibility, low cost and availability of large area production1, graphene obtained by Chemical Vapor Deposition (CVD) is considered to be a serious candidate to replace indium tin oxide (ITO)2-7. Thanks to the possibility to transfer graphene on structured surfaces5,8 and to its high conductivity (a typical sheet resistance on a millimeter scale is 0.15 – 0.45 kΩ/sq9,10). CVD-graphene has recently been applied as a transparent contact to three-dimensional nanostructures based on nanowire arrays5,8,11, nanopillars12,13, micro-pyramids14,15, and single nanostructures16-19. In particular, graphene has been employed in nanostructured light-emitting diodes (LEDs) based on GaN micro-pyramids in


2

Page 3 of 29

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


view of solid state lightning applications14,15. Moreover, the mechanical flexibility of graphene also enables its use as a contact for flexible light sources15,20,21. For optimization of graphene-based transparent contacts to micro-pyramids LEDs and other nanostructured devices, the impact of the graphene/substrate interface on the graphene sheet conductivity and contact resistance to graphene should be investigated. A number of studies of deformed graphene layers have been performed, including Raman measurements of graphene on nanopillars22,23 and nanovoids arrays24 and STM band spectrum study of graphene nanobubbles, ridge, etc.25-30. However, despite the already reported applications of graphene as a contact layer, there have been almost no reports on the graphene sheet conductivity and metal contact resistance to graphene placed on nanostructured substrates used for optoelectronic applications (e.g. micro-pyramid LEDs). In this paper the contact and transport properties of few-layered graphene grown by CVD technique and transferred on submicron structured surfaces as well as the morphology of the graphene/substrate interface are investigated. In particular, the case of micro-pyramids having the same morphology as the nanostructured LEDs is analyzed in order to assess the key parameters of graphene-based contacts to this kind of devices. The results are compared with the properties of graphene located on an opal sphere array, which presents a nanostructuration with a smaller characteristic feature size than the pyramid case. We show that the sheet resistance and the contact resistance of the graphene layer transferred to micro-pyramid arrays decreases compared to resistance of the reference graphene/flat SiO2 sample, which is attributed to the graphene partial suspension. On the contrary, the graphene transferred to the opal sphere array exhibits an increase of the sheet resistance, which is attributed to the graphene short-scale corrugation.


3


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 4 of 29

EXPERIMENTAL SECTION Sample Structure. Few-layered graphene has been grown by CVD technique on a silicon substrate with an oxide layer and a nickel layer as described in Ref. 31. The thicknesses of the SiO2 layer obtained by dry thermal oxidation and the nickel layer deposited at 110 ºC by electron-beam evaporation system in these substrates were approximately 500 nm and 100 nm, respectively. In this growth method, the substrates were annealed in a quartz tube at 1000 ºC for 50 min in argon, for 15 min in Ar/hydrogen mixture, and for a few minutes in Ar/ hydrogen/CH4 mixture (the argon, hydrogen, and CH4 flow rates were ~ 900 sccm, ~ 300 sccm, and ~ 100 sccm, correspondingly). CH4 was used as the carbon source for CVD graphene growth. A rapid cooling of the graphene was done in argon at the flow rate of 2000 sccm. The number of layers has been characterized by Raman spectroscopy and by separate atomic force measurements (AFM) on a reference sample containing graphene transferred to a planar SiO2 surface. The average thickness is found to be equal to 4 monolayers. The opal structure used for graphene transfer consists of face centered cubic (fcc) arrangement of equally sized SiO2 spheres (diameter equals 230 nm) packed in 4 layers (cf. Fig. 1(а)), which was deposited on silicon substrate by Langmuir-trough-based technique, as described in Ref. 32. GaN micro-pyramids with an LED InGaN/GaN p-n internal structure organized into hexagonal lattice with a period of 1.0 µm and a height of 1.1 µm were fabricated by metal organic vapor phase epitaxy technique using selective area growth, similarly to Ref. 33. The pyramids were covered with 300 nm thick SiO2 layer deposited by Plasma Enhanced CVD technique at 300 ºC (cf. Fig. 1(с)). Device Fabrication. To transfer the graphene on opal spheres and on the SiO2 covered micropyramid array, wet transfer process has been used without poly(methyl methacrylate) protection


4

Page 5 of 29

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


layer5,8. The nickel layer was wet-etched by FeCl3 saturated aqueous solution, the floating graphene sheet was transferred to a clean deionized water and then deposited on top of the micro-pyramids ensemble or of the opal layer. Transfer tests using a different transfer medium (isopropyl alcohol instead of deionized water) have also been performed, however no significant difference in graphene adhesion and in final morphology has been observed. The graphene transfer preserves the sheet integrity over a large (cm2) surface as controlled by SEM observations. Figure 1 illustrates the morphology of graphene deposited on opal spheres (panels a and b) and on micro-pyramid array (panels c and d). Contrary to the metal deposition on the graphene surface after transfer, bottom contact configuration10 allows to avoid the contamination by the resist residues between the metal and graphene, thus leading to lower contact resistance and preserving graphene integrity on highly structured surfaces. Before graphene transfer, metal pads separated by a distance from 2 µm up to 200 µm were formed on the receptor substrates. The large length of metal pads (1 mm) was chosen, so that the current leakage outside the metal pads after graphene transfer can be neglected. These planar TLM structures with 100 µm-wide contact stripes have been fabricated by optical lithography using AZ 1505 resists and PMGI (from MicroChem Corp.) bottom layer. Contacts were formed by thermal metal evaporation and a lift-off process. The contacts consist of a bottom chromium adhesion layer (10 nm) and a top gold layer (250 nm). Gold was chosen to avoid any possible oxidation of the contacts. Prior to graphene transfer, samples with formed metal pads were cleaned in acetone, isopropyl alcohol and rinsed in deionized water.


5


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 6 of 29

Figure 1. Tilted SEM images of (a) Graphene deposited on the face centered cubic (fcc) arrangement of opal structure; (c) Graphene deposited on the organized array of micro-pyramids with SiO2 dielectric layer; (b) and (d) High magnification images of the samples from panels (a) and (c), respectively. White arrows in panels (a) and (d) indicate the positions of grain boundaries. Green arrows in panel (b) highlight the graphene sagging. Characterization techniques. The Raman spectra were measured by using a Horiba - Jobin Yvon Raman spectrometer (LabRam HR 800) equipped with a confocal microscope. A frequency doubled Nd:YAG laser operating at a wavelength of 532 nm was used as the excitation source. The scattered light was collected in the backscattering geometry and the spectral resolution was 1 cm-1. А power on the sample kept in the range from 0.02 to 1.0 mW to


6

Page 7 of 29

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


avoid laser induced heating. The laser beam was focused by a 100×microscope objective lens (0.9 NA) into the spot sizes of 1 µm onto the graphene sample. The morphology of the graphene transferred on opal structure and on micro-pyramid array was characterized by Scanning Electron Microscopy (SEM) Hitachi SU-8000, using a low acceleration voltage of 5 kV after electrical characterization to avoid any possible modification of the electrical properties due to electron beam exposure34. Scanning Tunneling Measurements (STM) were conducted at 298 K in ambient conditions under constant current with tunneling current of 0.1 nA and -0.5 V applied to the STM tip. To probe the graphene transport properties, current-voltage measurements were performed under vacuum (residual pressure about (1.5 – 3.7) × 10-6 Torr) and in ambient conditions using a Janis cryogenic probe station coupled to a Keithley 2636 Source-meter. RESULTS AND DISCUSSION Characterization by Raman Spectroscopy The structural quality and the number of graphene layers have been characterized by Raman technique for graphene samples located on a flat Si/SiO2 substrate (in the following referred to as a reference sample). Although for exfoliated graphene the number of layers is estimated by the intensity and the shape of the 2D Raman peak, this approach cannot be applied to few-layered CVD-graphene due to the mis-orientation of stacked layers. Indeed, the ordered AB stacking and therefore the corresponding electronic coupling between graphene layers does not occur in all regions, instead a turbostatic random stacking occurs35. Instead, to estimate the number of layers, we use I2D-line/IG-line intensity ratio as previously reported in Ref. 35, 36. Typical Raman spectra for the reference sample are shown in Fig. 2(a). The D peak intensity is relatively weak, which indicates the good structural quality of the graphene at least at the micrometer scale probed by


7


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 8 of 29

the focused laser beam. The I2D-line/IG-line ratio depends on the excitation spot position as seen from Fig. 2(a), which displays Raman spectra at different positions. The average full width at half-maximum (FWHM) of these lines are (22 ± 1) and (76 ± 7) cm-1, respectively. The average I2D-line/IG-line ratio is (1.8 ± 0.4), which corresponds to 4 monolayer graphene thickness33 (confirmed by separate AFM measurements).

Figure 2. (a) - Raman spectra of CVD-graphene located on a flat Si/SiO2 surface measured for three different points. The spectra are scaled to have similar height of the G peak. (b) Raman spectra of CVD-graphene placed on pyramid array measured for three different points. The spectra are scaled to have similar height of the 2D peak. The inset shows the slight downshift (by


8

Page 9 of 29

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


~4 cm-1) of the Raman 2D-mode and reduction of the FWHM of the G and of the 2D lines for graphene suspended on the pyramids with respect to the reference sample). After determining the number of layers and the structural quality of the few-layered CVDgraphene on planar Si/SiO2 reference substrate, we have recorded Raman spectra of the graphene located on pyramid array in order to probe the impact of the graphene partial suspension. A typical Raman spectrum is shown in Fig. 2(b). The difference in I2D-line/IG-line ratio between partially suspended graphene on the pyramids (cf. Fig. 2(b)) and the reference sample of supported graphene (cf. Fig. 2(a)) is in agreement with the literature data for suspended graphene. Indeed, a lower concentration of charged impurities in suspended graphene leads to a higher I2D-line/IG-line ratio compared to the supported graphene case36,37. The observed FWHM values of the G ((16 ± 2) cm-1) and of the 2D ((33 ± 1) cm-1) lines are smaller than the broadening of these lines in the reference sample similarly to the results reported for exfoliated37 and CVD20 graphene. A slight downshift (by ~4 cm-1) of the Raman 2D-mode for graphene suspended on the pyramids with respect to the reference sample is observed (cf. inset in Fig. 2(b)). This downshift is similar to the observations for graphene suspended on open trenches (exfoliated graphene case)36 and on nanopillars (CVD graphene case)22. Structural Characterization by SEM and STM technique. The graphene morphology on opal spheres is shown in Fig. 1(a) and (b). The graphene appears as an undulating transparent layer on top of the SiO2 spheres. SEM image demonstrates that the graphene closely follows the surface profile. Arrows in panels (a) and (d) indicate the positions of graphene grain boundaries, while in panel (b) the arrows highlight the graphene sagging. The maximal graphene sagging is estimated to be around 60 nm.


9


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 10 of 29

On the contrary, the graphene on micro-pyramid array does not follow the surface profile, but remains suspended between the pyramid apexes separated by 1 µm distance as shown in Fig. 1(c) and (d). SEM microscopy has not allowed to evaluate the degree of sagging of the graphene located on the micro-pyramid array, instead this point has been addressed by using STM measurements. Different areas with identical morphology were chosen for the STM scanning areas and for the Raman spectroscopy in order to avoid the graphene modification after STM scanning. Fig. 3 shows an example of a representative STM map, which confirms the graphene suspension between the pyramids. The graphene covers the pyramid tips but does not descend to the pyramid base. The typical sagging deduced from STM measurements is about 200-300 nm. For the pyramid array and a sample area of 2×2 µm2 the surface increase factor (i.e. the ratio between the corrugated graphene surface and the projected surface) is 1.229 calculated based on the STM measurements of the graphene morphology.

Figure 3. (a) STM map of the topography of graphene located on micro-pyramid array. (b) and (c) – characteristic examples of tunneling I-V curves close to the pyramid top (b) and between the pyramids (c). Panel (b) presents a linear behavior showing that the density of states does not change (characteristic behavior for tunneling between two metals). Panel (c) presents an S-shape


10

Page 11 of 29

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


tunneling I-V, which is due to the attraction of the sagged graphene to the STM tip during the measurement. Scanning tunneling spectroscopy (STS) measurements were attempted to assess the modification of the band spectrum in graphene suspended on a sub-micrometer scale, following the approach previously applied to single graphene nanobubbles, ridges, etc.25-30. The majority of current-voltage (I-V) characteristics for STM tip position between the pyramids exhibit an Sshape with a maximal current saturating at 100 nA for both polarities (cf. Fig. 3(c)). This shape indicates that during the I-V measurement the STM tip-to-graphene distance was changing. The STM tip position was fixed during the acquisition of local scanning tunneling spectra. However the suspended areas of graphene have a possibility to move relative to the STM tip in the electric field created by bias voltage. The distance between tip and the graphene layer changes not too much – by ~1 nm estimated from tunneling current amplitude. This effect does not impact the measurement of the graphene profile and corrugation (cf. Fig. 3(a)). However, since the tunneling current is extremely sensitive to the tip-to-graphene distance, the band spectrum results are not reliable in these conditions. In some cases (close to the pyramid top where the graphene is not sagging and is therefore stable) linear I-V curves are observed characteristic for a metal-tometal tunnel current (cf. Fig. 3(b)), in other positions we observed previously discussed nonlinear characteristics (Fig. 3(c)). These non-linear behaviors were also observed for STS analyses of graphene located on opal spheres. Due to these experimental challenges coming from the instability of the corrugated graphene, we were unable to extract the modification of the band spectrum using STS. We note that in previous studied on single graphene nanobubbles, ridge, etc., where tunneling I-Vs were successfully measured, the characteristic scales were below 15 nm, so that the graphene attraction to the STM tip was negligible in comparison to our case.


11


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 12 of 29

Graphene Transport Properties. Four and nine sets of TLM structures were tested for graphene located on opal spheres and on micro-pyramid array, respectively. For graphene on fcc opal spheres the measured I-V curves exhibit a linear behavior as a function of the inter-contact distances from 2 up to 200 µm (cf. inset to Fig. 4(a)). This is different from the reference sample containing graphene on a planar SiO2 surface, which shows a deviation of I-V curves from linearity as reported in our previous work10. Since the typical values of the carrier free mean path (approx. 30–70 nm in studied CVD few-layered graphene10) are small compared to the SiO2 sphere diameter and to the curvature radius, the transport at the scale of several hundreds of nanometers is diffusive. Therefore, the transfer length method (TLM) to study the contact resistance and the in-plane resistance of graphene is valid. Fig. 4(a) shows the dependence of the resistance on the inter-contact distance at ambient conditions (T=298 K). The reported distance has been recalculated to account for the graphene corrugation on the opal surface using a corrective geometrical factor due to graphene sagging of 1.085 extracted from SEM images (cf. Fig. 1(b)). Table I summarizes the contact parameters extracted from the sets of TLM measurements. The value of the transfer length, LT, is much smaller than the length of the contact pads, which indicates the presence of current crowding effects near metal contacts. Therefore, the contact resistance Rc is a more relevant parameter to estimate the contact quality than the specific contact resistivity ρc. Rc value of graphene located on the opal spheres is equal to (9.5 ± 1.4) kΩ·µm, which is closed to the value that we have obtained for the graphene on the reference planar SiO2 sample (Rc = (10.1±1.4) kΩ·µm)10. Contrary to the contact resistance, which is similar to the planar graphene case, the sheet resistance Rsh of graphene located on opal spheres increases by


12

Page 13 of 29

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


more than 3 times with respect to the planar SiO2 reference and becomes (1.18 ± 0.31) kΩ/sq at room temperature.

Figure 4. (a) Typical resistance of the graphene on opal structure as a function of inter-contact distance. Inset shows the I-V curves for different inter-contact distances measured at T=298 K. (b) Typical resistance of the graphene on micro-pyramid array as a function of inter-contact distance. Inset shows the I-V curves for different inter-contact distances measured at T=298 K. The modification of the resistance of graphene located on SiO2 opal spheres can be due to different mechanisms such as (i) the carrier scattering on fixed charges and (ii) the modification of the band spectrum of the suspended corrugated graphene. The scattering on charged impurities is a dominant mechanism for graphene located on SiO239,40. However, it is unlikely that the replacement of the flat (reference) SiO2 surface by SiO2 opal spheres may lead to the significant increase of the number of charge traps at the interface and be responsible for the increased charge scattering. We note that no argon plasma activation or chemical oxidation in e.g. RCA1 has been used, which could have induced additional charge traps. A more plausible explanation is the modification of the band spectrum of graphene (including the bandgap formation) induced by the mechanical deformation in the graphene located on a nanostructured surface due to


13


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 14 of 29

pseudomagnetic fields. In Ref. 41 the authors numerically predict a bandgap formation for triangular symmetry of strains created by depositing graphene on profiled surfaces. The fcc arrangement of the opal spheres studied in this work qualitatively resemble to this theoreticallyconsidered case. Experimental confirmations of the presence of pseudomagnetic fields in single graphene nanobibbles, ridge, etc with characteristic scales around 15 nm have also previously been reported25-30. Therefore we tentatively explain the observed increase of the graphene sheet resistance for a sub-micron scale corrugation by the modification of the band spectrum of graphene induced by its deformation. We note that this effect is undesirable for contact applications, but can find some alternative applications. For example, this effect may be used to control the graphene resistance using a stretchable PDMS layer covered with SiO2 spheres42. Even small substrate deformations in this case would lead to sphere displacement and to the graphene conductivity variation, which opens interesting perspectives for graphene strain engineering. Table I. Contact properties of few-layered CVD-graphene transferred to different surfaces (Rc – contact resistance, Rsh – sheet resistance, LT – transfer length, ρc – specific contact resistivity). Rc, kΩ·µm

Rsh, kΩ/sq

LT, µm

ρc, Ω×cm2

9.5 ± 1.4

1.18 ± 0.31

8.2 ± 2.8

(0.7 ± 0.2)×10-3

as transferred

6.9 ± 0.7

0.18 ± 0.03

39.2 ± 10.3

(2.7 ± 0.9)×10-3

in vacuum, 48 hours

6.9 ± 0.8

0.30 ± 0.07

24.8 ± 7.5

(1.7 ± 0.7)×10-3

after annealing, ambient conditions

0.8 ± 0.2

0.30 ± 0.03

2.6 ± 0.5

(2.1 ± 0.8)×10-5

10.1 ± 1.4

0.37 ± 0.08

25.7 ± 8.9

(3.9 ± 2.1)×10−3

Measurement conditions Graphene on opal spheres as transferred Graphene on pyramids

Graphene on planar Si/SiO2* as transferred

* The results for the reference graphene on planar Si/SiO2 sample have been previously reported by our group in Ref. 10.


14

Page 15 of 29

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


The graphene located on the micro-pyramid array also presents linear I-V curves in ambient conditions (cf. inset of Fig. 4(b). With respect to the graphene on planar SiO2 reference sample, the contact resistance is reduced down to (6.9 ± 0.7) kΩ·µm and the sheet resistance is reduced to (0.18 ± 0.03) kΩ/sq. The considerable reduction of the sheet resistance is attributed to the partial graphene suspension. We note that with respect to the opal case, the graphene deformation takes place on a much larger scale (micrometer scale instead of a hundred nanometer scale) and the area of suspended regions is significantly larger, which may explain the difference in the sheet resistance behavior. The analyses of the same sample under vacuum (residual pressure (1.5 – 3.7) × 10-6 Torr) showed that the contact resistance remained almost unchanged whereas the sheet resistance increased to (0.30 ± 0.07) kΩ/sq due to the reduction of residual doping of suspended graphene. Sample annealing (under nitrogen flux of 500 sccm for 1 hour at 400 C°) leads to a reduction of the contact resistance (down to 0.8 ± 0.2 kΩ·µm) and to an increase of the sheet resistance (up to 0.30 ± 0.03 kΩ/sq). This sheet resistance increase is attributed to the impurity desorption, which reduces the residual graphene doping. Therefore, thermal annealing is favorable to improve the contact resistance to CVD-graphene. However, to compensate the increase of the sheet resistance additional graphene doping (e.g. using AuCl33,9) is desirable. The achieved small values of the metal to graphene contact resistance (i.e. (0.8 ± 0.2 kΩ·µm) for graphene suspended on GaN micro-pyramids to be compared with (0.9 ± 0.1 kΩ·µm) for graphene on planar SiO2 recently reported by Graphenea Inc.43) show the high promise of CVD graphene to be used as a transparent contact to GaN micro-pyramid LEDs. For further details on this application please refer to Supporting Information.


15


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 16 of 29

It is worth noting that our noninvasive contacting method10, for which the graphene is transferred onto the predefined electrical contacts, allows to avoid the carrier scattering by resist residues, which typically contributes to the contact resistance in the conventional contacting process, for which the metal is deposited on top of the graphene layer using lithography and liftoff procedure. Moreover, our contacting method allows to avoid graphene tearing even in the case of highly structured surfaces as the GaN micro-pyramids LEDs, which may occur for standard contacting procedure14. The future optimization of graphene transparent contacts for optoelectronic devices based on micro-pyramids may include additional doping to decrease the planar resistance of graphene. CONCLUSION In this work we have analyzed the morphology and the contact and transport properties of fewlayered CVD-graphene located on submicron structured surfaces. For graphene located on an opal structure more than three-fold increase of the sheet resistance is observed in comparison to a reference graphene/flat SiO2 surface. This effect is attributed to the graphene corrugation on submicron scale. We show that for graphene deposited on micro-pyramids with a typical micronic inter-pyramid pitch there is no increase of the graphene contact and sheet resistances, which may have appeared because of the graphene corrugation. Sample annealing leads to a reduction of the contact resistance down to (0.8 ± 0.2) kΩ·µm), while increasing the in-plane resistance. Lower contact resistance between the suspended CVD-graphene and metal has been achieved in comparison to the values reported for planar geometry43. This shows high promise of the graphene based transparent contacts to GaN micro-pyramid LEDs.

ASSOCIATED CONTENT


16

Page 17 of 29

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


Supporting Information. Application of Graphene as a Transparent Contact to Micropyramid LEDs. AUTHOR INFORMATION Corresponding Author *Tel.: +33 1 69 15 40 51. E-mail: [email protected] (A.V.B.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the RFBR (project no. 14-02-31485, 15-02-08282, 14-02-01212), by Scholarship of the President of the Russian Federation (grant no. SP-4716.2015.1). A. Babichev acknowledges support from the Russian Science Foundation (project no. 15-12-00027) for electrical measurements support. The authors thank O. Kryliuk for providing micro-pyramid samples, A. Fokin for fruitful discussions and H. Zhang for assistance with micro-pyramid LED characterization.

REFERENCES (1) Kobayashi, T.; Bando, M.; Kimura, N.; Shimizu, K.; Kadono, K.; Umezu, N.; Miyahara, K.; Hayazaki, S.; Nagai, S.; Mizuguchi, Y.; Murakami, Y.; Hobara, D. Production of a 100-MLong High-Quality Graphene Transparent Conductive Film by Roll-to-Roll Chemical Vapor Deposition and Transfer Process. Appl. Phys. Lett. 2013, 102 (2), 023112.


17


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 18 of 29

(2) Weber, C. M.; Eisele, D. M.; Rabe, J. P.; Liang, Y.; Feng, X.; Zhi, L.; Lyon, J. L.; Williams, R.; Vanden Bout, D. A.; Stevenson, K. J. Graphene‐Based Optically Transparent Electrodes for Spectroelectrochemistry in the UV–Vis Region. Small 2010, 6 (2), 184–189. (3) Choe, M.; Cho, C. Y.; Shim, J. P.; Park, W.; Lim, S. K.; Hong, W. K.; Lee, B. H.; Lee, D.S.; Park, S.-J.; Lee, T. Au Nanoparticle-Decorated Graphene Electrodes for GaN-Based Optoelectronic Devices. Appl. Phys. Lett. 2012, 101 (3), 031115. (4) Huang, H. M.; Chang, C. Y.; Hsu, Y. S.; Lu, T. C.; Lan, Y. P.; Lai, W. C.; Enhanced Internal Quantum Efficiency in Graphene/InGaN Multiple-Quantum-Well Hybrid Structures. Appl. Phys. Lett. 2012, 101 (6), 061905. (5) Babichev, A. V.; Zhang, H.; Lavenus, P.; Julien, F. H.; Egorov, A. Y.; Lin, Y. T.; Tu, L. W.; Tchernycheva, M. GaN Nanowire Ultraviolet Photodetector with a Graphene Transparent Contact. Appl. Phys. Lett. 2013, 103 (20), 201103. (6) Xu, Q.; Yang, M.; Cheng, Q.; Zhong, J.; Wu, S.; Zhang, Z.; Cai, W.; Zhang, F.; Wu, Z. GaN Light-Emitting Diodes with an Al-Coated Graphene Layer as a Transparent Electrode. Appl. Phys. Express 2014, 7 (8), 082103. (7) Babichev, A. V.; Butko, V. Y.; Sobolev, M. S.; Nikitina, E. V.; Kryzhanovskaya, N. V.; Egorov, A. Y. Electroluminescence of GaPxNyAs1−x−y Nanoheterostructures Through a Transparent Electrode Made of CVD Graphene. Semiconductors 2012, 46 (6), 796–800. (8) Zhang, H.; Babichev, A. V.; Jacopin, G.; Lavenus, P.; Julien, F. H.; Egorov, A. Y.; Zhang, J.; Pauporté, T.; Tchernycheva, M. Characterization and Modeling of a ZnO Nanowire


18

Page 19 of 29

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


Ultraviolet Photodetector with Graphene Transparent Contact. J. Appl. Phys. 2013, 114 (23), 234505. (9) Kim, K. K.; Reina, A.; Shi, Y.; Park, H.; Li, L. J.; Lee, Y. H.; Kong, J. Enhancing the Conductivity of Transparent Graphene Films via Doping. Nanotechnology 2010, 21 (28), 285205. (10) Babichev, A. V.; Gasumyants, V. E.; Egorov, A. Y.; Vitusevich, S.; Tchernycheva, M. Contact Properties to CVD-Graphene on GaAs Substrates for Optoelectronic Applications. Nanotechnology 2014, 25 (33), 335707. (11) Fan, G.; Zhu, H.; Wang, K.; Wei, J.; Li, X.; Shu, Q.; Guo, N.; Wu, D. Graphene/Silicon Nanowire Schottky Junction for Enhanced Light Harvesting. ACS Appl. Mater. Interfaces 2011, 3 (3), 721–725. (12) Lee, C.; Kim, B. J.; Ren, F.; Pearton, S. J.; Kim, J. Large-Area Suspended Graphene on GaN Nanopillars. J. Vac. Sci. Technol. B 2011, 29 (6), 060601. (13) Feng, T.; Xie, D.; Lin, Y.; Zang, Y.; Ren, T.; Song, R.; Zhao, H.; Tian, H.; Li, X.; Zhu, H.; Liu, L.; Graphene based Schottky Junction Solar Cells on Patterned Silicon-Pillar-Array Substrate. Appl. Phys. Lett. 2011, 99 (23), 233505. (14) Kang, J.; Li, Z.; Li, H.; Liu, Z.; Li, X.; Yi, X.; Ma, P.; Zhu, H.; Wang, G. Pyramid Array InGaN/GaN Core–Shell Light Emitting Diodes with Homogeneous Multilayer Graphene Electrodes. Appl. Phys. Express 2013, 6 (7), 072102.


19


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 20 of 29

(15) Wang, L.; Ma, J.; Liu, Z.; Yi, X.; Zhu, H.; Wang, G. In Situ Fabrication of Bendable Microscale Hexagonal Pyramids Array Vertical Light Emitting Diodes with Graphene as Stretchable Electrical Interconnects. ACS Photonics 2014, 1 (5), 421–429. (16) Fu, X. W.; Liao, Z. M.; Zhou, Y. B.; Wu, H. C.; Bie, Y. Q.; Xu, J. Yu, D. P. Graphene/ZnO Nanowire/Graphene Vertical Structure Based Fast-Response Ultraviolet Photodetector. Appl. Phys. Lett. 2012, 100 (22), 223114. (17) Kim, S.; Janes, D. B.; Choi, S. Y. Ju, S. Nanoscale Contacts Between Semiconducting Nanowires and Metallic Graphenes. Appl. Phys. Lett. 2012, 101 (6), 063122. (18) Tchernycheva, M.; Lavenus, P.; Zhang, H.; Babichev, A. V.; Jacopin, G.; Shahmohammadi, M.; Julien, F. H.; Ciechonski, R.; Vescovi, G.; Kryliouk, O. InGaN/GaN Core–Shell Single Nanowire Light Emitting Diodes with Graphene-Based P-Contact. Nano Lett. 2014, 14 (5), 2456–2465; (19) Tchernycheva, M.; Neplokh, V.; Zhang, H.; Lavenus, P.; Rigutti, L.; Bayle, F.; Julien, F. H.; Babichev, A.; Jacopin, G.; Largeau, L.; Ciechonski, R.; Vescovi, G.; Kryliouk, O. Core– Shell InGaN/GaN Nanowire Light Emitting Diodes Analyzed by Electron Beam Induced Current Microscopy and Cathodoluminescence Mapping, Nanoscale 2015, 7 (27), 11692–11701. (20) Choi, J. H.; Cho, E. H.; Lee, Y. S.; Shim, M.-B.; Ahn, H. Y.; Baik, C.-W.; Lee, E. H.; Kim, K.; Kim, T.-H.; Kim, S.; Cho, K.-S.; Yoon, J.; Kim, M.; Hwang, S. Fully Flexible GaN Light‐Emitting Diodes through Nanovoid‐Mediated Transfer. Adv. Opt. Mater. 2014, 2 (3), 267– 274.


20

Page 21 of 29

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


(21) Dai, X.; Messanvi, A.; Zhang, H.; Durand, C.; Eymery, J.; Bougerol, C.; Julien, F. H.; Tchernycheva, M. Flexible Light Emitting Diodes Based on Vertical Nitride Nanowires. Nano Lett. 2015, 15 (10), 6958–6964. (22) Reserbat-Plantey, A.; Kalita, D.; Han, Z.; Ferlazzo, L.; Autier-Laurent, S.; Komatsu, K.; Li, C.; Weil, R.; Ralko, A.; Marty, L.; Guéron, S.; Bendiab, N.; Bouchiat, H.; Bouchiat, V. Strain Superlattices and Macroscale Suspension of Graphene Induced by Corrugated Substrates. Nano Lett. 2014, 14 (9), 5044–5051. (23) Tomori, H.; Kanda, A.; Goto, H.; Ootuka, Y.; Tsukagoshi, K.; Moriyama, S.; Watanabe, E.; Tsuya, D. Introducing Nonuniform Strain to Graphene Using Dielectric Nanopillars. Appl. Phys. Express 2011, 4 (7), 075102. (24) Zhu, X.; Shi, L.; Schmidt, M. S.; Boisen, A.; Hansen, O.; Zi, J.; Xiao, S.; Mortensen, N. A. Enhanced Light–Matter Interactions in Graphene-Covered Gold Nanovoid Arrays. Nano Lett. 2013, 13 (10), 4690–4696. (25) Levy, N.; Burke, S. A.; Meaker, K. L.; Panlasigui, M.; Zettl, A.; Guinea, F.; Castro Neto, A. H.; Crommie, M. F. Strain-Induced Pseudo–Magnetic Fields Greater than 300 Tesla in Graphene Nanobubbles. Science 2010, 329 (5991), 544–547. (26) Yan, H.; Sun, Y.; He, L.; Nie, J. C.; Chan, M. H. Observation of Landau-Level-Like Quantization at 77 K along a Strained-Induced Graphene Ridge. Phys. Rev. B 2012, 85 (3), 035422.


21


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 22 of 29

(27) Guo, D.; Kondo, T.; Machida, T.; Iwatake, K.; Okada, S.; Nakamura, J. Observation of Landau Levels in Potassium-Intercalated Graphite under a Zero Magnetic Field. Nat. Commun. 2012, 3, 1068. (28) Yeh, N. C.; Teague, M. L.; Yeom, S.; Standley, B. L.; Wu, R. P.; Boyd, D. A.; Bockrath, M. W. Strain-Induced Pseudo-Magnetic Fields and Charging Effects on CVD-Grown Graphene. Surf. Sci. 2011, 605 (17), 1649–1656. (29) Lu, J.; Neto, A. C.; Loh, K. P. Transforming Moiré Blisters into Geometric Graphene Nano-Bubbles. Nat. Commun. 2012, 3, 823. (30) Yan, W.; He, W. Y.; Chu, Z. D.; Liu, M.; Meng, L.; Dou, R. F.; Zhang, Y.; Liu, Z.; Nie, J.-C.; He, L. Strain and Curvature Induced Evolution of Electronic Band Structures in Twisted Graphene Bilayer. Nat. Commun. 2013, 4, 2159. (31) Babichev, A. V.; Gasumyants, V. E.; Butko, V. Y. Resistivity and Thermopower of Graphene Made by Chemical Vapor Deposition Technique. J. Appl. Phys. 2013, 113 (7), 076101. (32) Romanov, S. G.; Vogel, N.; Bley, K.; Landfester, K.; Weiss, C. K.; Orlov, S.; Korovin, A. V.; Chuiko, G. P.; Regensburger, A.; Romanova, A. S.; Kriesch, A.; Peschel, U. Probing Guided Modes in a Monolayer Colloidal Crystal on a Flat Metal Film. Phys. Rev. B 2012, 86 (19), 195145. (33) Stankevic, T.; Mickevicius, S.; Schou Nielsen, M.; Kryliouk, O.; Ciechonski, R.; Vescovi, G.; Bi, Z.; Mikkelsen, A.; Samuelson, L.; Gundlach, C.; Feidenhans'l, R. Measurement of Strain in InGaA/GaN Nanowires and Nanopyramids. J. Appl. Crystallogr. 2015, 48 (2), 344–349.


22

Page 23 of 29

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


(34) Dorgan, V. E.; Behnam, A.; Conley, H. J.; Bolotin, K. I.; Pop, E. High-Field Electrical and Thermal Transport in Suspended Graphene. Nano Lett. 2013, 13 (10), 4581–4586. (35) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Lett. 2008, 9 (1), 30–35. (36) O'Brien, M.; Nichols, B. CVD Synthesis and Characterization of Graphene Thin Films; U.S. Army Research Laboratory; ARL-TR-5047: Adelphi, MD, 2010. http://www.dtic.mil/cgibin/GetTRDoc?AD=ADA513793. (37) Berciaud, S.; Ryu, S.; Brus, L. E.; Heinz, T. F. Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers. Nano Lett. 2008, 9 (1), 346–352. (38) Ni, Z. H.; Yu, T.; Luo, Z. Q.; Wang, Y. Y.; Liu, L.; Wong, C. P.; Miao, J.; Huang, W.; Shen, Z. X. Probing Charged Impurities in Suspended Graphene Using Raman Spectroscopy. ACS Nano 2009, 3 (3), 569–574. (39) Lafkioti, M.; Krauss, B.; Lohmann, T.; Zschieschang, U.; Klauk, H.; Klitzing, K. V.; Smet, J. H. Graphene on a Hydrophobic Substrate: Doping Reduction and Hysteresis Suppression Under Ambient Conditions. Nano Lett. 2010, 10 (4), 1149–1153. (40) Wu, Y.; Lin, Y. M.; Bol, A. A.; Jenkins, K. A.; Xia, F.; Farmer, D. B.; Zhu, Y.; Avouris, P. High-Frequency, Scaled Graphene Transistors on Diamond-Like Carbon. Nature 2011, 472 (7341), 74–78.


23


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 24 of 29

(41) Guinea, F.; Katsnelson, M. I.; Geim, A. K. Energy Gaps and a Zero-Field Quantum Hall Effect in Graphene by Strain Engineering. Nat. Phys. 2010, 6 (1), 30–33. (42) Ge, J.; Yin, Y.. Responsive Photonic Crystals. Angew. Chem. Int. Ed. 2011, 50 (7), 1492– 1522. (43) Asadi, K.; Timmering, E. C.; Geuns, T. C.; Pesquera, A.; Centeno, A.; Zurutuza, A.; Klootwijk, J. H.; Blom, P. W. M.; de Leeuw, D. M. Up-Scaling Graphene Electronics by Reproducible Metal-Graphene Contacts. ACS Appl. Mater. Interfaces 2015, 7 (18), 9429–9435.

Table of content figure description: Left: STM map of the topography of graphene located on micro-pyramid array. Right: Typical resistance of the graphene on micro-pyramid array as a function of inter-contact distance. Inset: Tilted SEM image of graphene deposited on the organized array of micro-pyramids covered with SiO2 dielectric layer.


24

Page 25 of 29

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


Figure 1. Tilted SEM images of (a) Graphene deposited on the face centered cubic (fcc) arrangement of opal structure; (c) Graphene deposited on the organized array of micro-pyramids with SiO2 dielectric layer; (b) and (d) High magnification images of the samples from panels (a) and (c), respectively. White arrows in panels (a) and (d) indicate the positions of grain boundaries. Green arrows in panel (b) highlight the graphene sagging. 177x128mm (300 x 300 DPI)



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

Figure 2. (a) - Raman spectra of CVD-graphene located on a flat Si/SiO2 surface measured for three different points. The spectra are scaled to have similar height of the G peak. (b) Raman spectra of CVDgraphene placed on pyramid array measured for three different points. The spectra are scaled to have similar height of the 2D peak. The inset shows the slight downshift (by ~4 cm-1) of the Raman 2D-mode and reduction of the FWHM of the G and of the 2D lines for graphene suspended on the pyramids with respect to the reference sample). 85x118mm (300 x 300 DPI)


Page 26 of 29

Page 27 of 29

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


Figure 3. (a) STM map of the topography of graphene located on micro-pyramid array. (b) and (c) – characteristic examples of tunneling I-V curves close to the pyramid top (b) and between the pyramids (c). Panel (b) presents a linear behavior showing that the density of states does not change (characteristic behavior for tunneling between two metals). Panel (c) presents an S-shape tunneling I-V, which is due to the attraction of the sagged graphene to the STM tip during the measurement. 177x51mm (300 x 300 DPI)



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

Figure 4. (a) Typical resistance of the graphene on opal structure as a function of inter-contact distance. Inset shows the I-V curves for different inter-contact distances measured at T=298 K. (b) Typical resistance of the graphene on micro-pyramid array as a function of inter-contact distance. Inset shows the I-V curves for different inter-contact distances measured at T=298 K. 177x74mm (300 x 300 DPI)


Page 28 of 29

Page 29 of 29

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


Table of content figure description: Left: STM map of the topography of graphene located on micro-pyramid array. Right: Typical resistance of the graphene on micro-pyramid array as a function of inter-contact distance. Inset: Tilted SEM image of graphene deposited on the organized array of micro-pyramids with SiO2 dielectric layer. 83x35mm (300 x 300 DPI)


Influence of Substrate Microstructure on the Transport Properties of

Recommend Documents