Graphene Hybrid Devices Fabricated by a

Subscriber access provided by UNIV OF CAMBRIDGE

Functional Nanostructured Materials (including low-D carbon)

Single GaAs Nanowire/Graphene Hybrid Devices Fabricated by a Position Controlled Micro-Transfer and Imprinting Technique for Embedded Structure ANJAN MUKHERJEE, Hoyeol Yun, Dong-Hoon Shin, Jungtae Nam, A. Mazid Munshi, Dasa Dheeraj, Bjørn-Ove Fimland, Helge Weman, Keun Soo Kim, Sang Wook Lee, and Dong-Chul Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20581 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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 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 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.

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 25 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

Single GaAs Nanowire/Graphene Hybrid Devices Fabricated by a Position-Controlled Micro-Transfer and Imprinting Technique for Embedded Structure

Anjan Mukherjee †, Hoyeol Yun, Dong Hoon Shin¶, Jungtae Nam ǁ, A. Mazid Munshi†, Dasa L. Dheeraj†, Bjørn-Ove Fimland†, Helge Weman†*, Keun Soo Kim ǁ, Sang Wook Lee¶*, and Dong-Chul Kim †, §*★

†

Department of Electronic Systems, Norwegian University of Science and Technology (NTNU), NO-7491, Trondheim, Norway

 School

¶ Department

§

ǁ

of Physics, Konkuk University, 05029, Seoul, Republic of Korea

of Physics, Ewha Womans University, 03760, Seoul, Republic of Korea

CrayoNano AS, Sluppenvegen 6, NO-7037, Trondheim, Norway

Department of Physics and Graphene Research Institute, Sejong University, 05006,

Seoul, Republic of Korea

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

*Authors to whom the correspondence should be addressed. [email protected], [email protected] ★Present

e-mail address: [email protected]

KEYWORDS: Single Nanowire Device, GaAs, Graphene, Schottky Contact, Embedded Nanowire

Abstract

We developed a new technique to fabricate single nanowire devices with reliable graphene/nanowire contacts by using a position controlled micro-transfer and an embedded nanowire structure in a planar junction configuration. A thorough study of electrical properties and fabrication challenges of single p-GaAs nanowire/graphene devices was carried out in two different device configurations: 1) a graphene bottom-contact device where the nanowiregraphene contact junction is formed by transferring a nanowire on top of graphene and 2) a graphene top-contact device where the nanowire-graphene contact junction is formed by transferring graphene on top of an embedded nanowire. For the graphene top-contact devices, graphene-nanowire-metal devices where graphene is used as one electrode and metal is the other electrode to a nanowire, and graphene-nanowire-graphene devices where both electrodes to a nanowire are graphene were investigated and compared with conventional metal/p-GaAs nanowire devices. Conventional metal/p-GaAs nanowire contact devices were further investigated in embedded and non-embedded nanowire device configurations. A significantly improved current in the embedded device configuration is explained with a ‘parallel resistors model’ where the high resistance parts with the metal-semiconductor Schottky contact and the low resistance parts with non-contacted facets of the hexagonal nanowires are taken into


Page 2 of 25

Page 3 of 25 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


consideration. Consistently, the non-embedded nanowire structure is found to be depleted much easier than the embedded nanowires from which an estimation for a fully depleted condition has also been established.

1. Introduction

Semiconductor nanowires (NWs)/graphene hybrid heterostructures have recently demonstrated their strong potential ability towards high efficiency and low-cost optoelectronic device applications1-5. It has also been reported that the semiconductor/graphene Schottky junction can be used as the main building block for many chemical and biological sensors6-7. Many superlative properties like the unique linear energy band dispersion, high electron mobility, ultra-high transparency over a broad range of wavelengths and high flexibility make graphene a preferable candidate as a transparent electrode to semiconducting NWs for many optoelectronic applications8-11. Besides that, in comparison with the present semiconductor thin film/graphene optoelectronic devices, these hybrid hetero-devices could also show additional interesting phenomena originated from the NW geometry, such as the light trapping and polarization dependent absorption etc.12-13. Though there have been several reports on NW array/graphene hybrid devices where graphene was transferred on top of the NW array and used as a transparent electrode3-4, single NW/graphene hybrid device cases are still rare. Until now, a few graphene-single NW-metal structures, where the contact asymmetry between the graphene-NW and metal-NW junctions was utilized to form rectifying Schottky devices, have been demonstrated using InAs, CdS, GaN and GaAs NWs5,

14-16.

In these types of devices, the metal-NW contacts have been



considered to be ohmic and thus the rectifying effect is exclusively ascribed to the grapheneNW interface, which clearly indicates the presence of a high Schottky barrier at the grapheneNW junction. This high energy barrier at the graphene-NW interface is thus impeding the formation of a non-rectifying or ohmic-like device by using only graphene electrodes (without any metal) to contact a single NW. It has also been demonstrated that the metal can diffuse inside the NW, especially during its annealing process, which can change its electrical and optical properties drastically17.On the other hand, graphene is an inert material that does not form any type of chemical bonding to the semiconducting NW18-19, and thus could serve as a much better alternative electrode to single NWs. A few reports also show that an intermediate ultra-thin metal layer was used in between the single NW and the graphene to reduce the contact resistance, which indicates the difficulty in achieving good contact between the NW and graphene14-15. Although the single NW contacting by itself is a very complicated process, the primary reason for the limited reports on these hybrid devices is due to the 3-dimensional morphology of a single NW, which makes it even more challenging to fabricate such hybrid devices. Especially, the hexagonal cross-section of the single NW and the large topological height difference between the support and the top of the dispersed NW pose a challenge for conformal coverage of the NW by a transferred atomically thin sheet of graphene, making the contacts very unstable and unreliable. This is in contrast with 2-dimensional films or NW arrays where the flat top surface aids a good graphene transfer and therefore a better quality of the contact. In this report, we present a new way to fabricate reliable single NW/graphene devices using a position-controlled micro-transfer and an imprinting technique for embedded NW configuration. This technique enables us to select and transfer a single NW (or graphene) on a target graphene (or a target single NW) at the desired location by using a micro-manipulator20. Applying this technique, both NW/graphene bottom- and top-contact devices, where bottom


Page 4 of 25

Page 5 of 25 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


(top)- contact devices means that NW (graphene) is transferred on top of graphene (NW) such that the NW-graphene contact junction is formed at the bottom (top) of the NW, were made and compared to understand their fabrication challenges and potential applications. For the fabrication of the first approach, NW/graphene bottom-contact devices, the weak adhesion between polymer resist and a hydrophobic surface was utilized. By dispersing NWs on a hydrophobic surface, single NWs could easily be detached with a polymer resist and selectively transferred on top of the target graphene, forming a NW/graphene contact at the bottom21. For the second approach, NW/graphene top-contact devices, a key issue is how to make single NWs embedded in a flat-surfaced structure, which is critical for the successful transfer of graphene on top of the NWs without any damage to the graphene. Using a curable photoresist layer, single NWs are transferred and imprinted to form an embedded NW structure. A monolayer of graphene was subsequently transferred on top of the NW to produce the NW/graphene top contact. In this work, Be-doped p-GaAs NWs were chosen to fabricate single NW/graphene devices, as GaAs NWs are one of the most important III-V semiconductor NW structures with a high potential for future high-efficiency solar cells22-25 and photodetectors26-28. These hybrid junction devices have been compared with conventional GaAs NW/metal junction devices for a better understanding of the benefit of using graphene as a replacement for metal electrodes. The metal-contacted p-GaAs NW devices were further compared both in their embedded and non-embedded NW device configuration. A significant improvement in I-V characteristics with higher electrical current has been observed with the embedded NW structures, which could qualitatively be explained by a ‘parallel resistors model’ where the total cross-sectional resistance of the hexagonal NW comes from a parallel combination of high resistance parts with the depletion layer formed by a Schottky contact and low resistance parts with non-contacted facets of the NW. Numerical estimation for a fully depleted condition shows



that the non-embedded NW device structure is more readily depleted, consistent with the observed higher resistance than for the embedded NW device structure.

2. Experimental Details Be-doped p-GaAs NWs were grown on Si (111) substrate29 in a solid source Varian Gen II molecular beam epitaxy (MBE) system by using the Ga-assisted vapor-liquid-solid technique. The nominal Be concentration from the temperature of the Be effusion cell is estimated to be ~4.4 × 1018/cm3. The average diameter and length of the NWs are 125 nm ± 3% and 4.5 µm ± 5%, respectively. For the fabrication of GaAs NW/graphene bottom-contact devices where the junction is formed by overlapping a NW on top of graphene, a graphene sheet was first prepared, which can be either exfoliated graphene from Kish graphite flakes or chemical vapor deposition (CVD) grown graphene. Micro-patterning was carried out with an e-beam resist, e.g. Poly (methyl methacrylate) (PMMA) coating and electron beam lithography (EBL). After O2 -plasma etching the micro-sized graphene/PMMA stack was then transferred onto a Si/SiO2 substrate by using a frame assisted transfer technique with the help of a micromanipulator, as illustrated in Figure 1(a) and (b)21. The PMMA layer on graphene was subsequently removed by acetone for contact to NW. GaAs NWs were dispersed on a hydrophobic substrate (tape) and then separated from the substrate by releasing off a spin-coated PMMA layer. As shown in Figure 1(c), a GaAs NW can be fixed underneath the PMMA membrane and this stacking structure helps select and transfer a single NW to any target substrate. Although the spin-coated PMMA layer covered the top surface of the GaAs NW, the bottom part of the NW is still open (see the inset in Figure 1. (c)) so that the oxide layer formed on the NW surface was removed by wet etching to make good contact to the graphene at the bottom29. The selected GaAs NW was then perpendicularly aligned to the length of the graphene sheet and the top PMMA layer was completely removed


Page 6 of 25

Page 7 of 25 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


afterward so that the metal electrodes could be connected to the GaAs NW and graphene layer separately (Figure 1(d)). After the oxide layer etching of the NW surface, contacts to the single GaAs NW and graphene layer were fabricated by EBL patterning and deposition of Pt (5 nm)/Ti (10 nm)/Pt (10 nm)/Au (200 nm)30-31 and Pd (20 nm)/Au (30 nm)32-33 metal layers, respectively (Figure 1(e)).

Another GaAs NW/graphene junction device was fabricated by transferring a graphene layer on top of the GaAs NW to form a graphene/NW junction at the top. This is an important structure as it allows to investigate the hybrid junction optically through the top transparent graphene. We found it very challenging to achieve reliable contacts between graphene and the GaAs NW for the top junction by just transferring a graphene sheet on top of a single NW. This type of device structure shows a very high instability with very poor contact between GaAs NW and graphene. Transfer of a graphene sheet directly on top of a NW with a hexagonal crosssection always creates air gaps, unavoidably formed between the bottom three facets of the NW and the transferred graphene, and also causes high local strains in the graphene sheet (Figure 1 (f)). Typical scanning electron microscope (SEM) images of this non-embedded NW with a transferred graphene sheet on top are shown in Figure S1 in Supporting Information, where the formation of air gaps along the NW length is clearly visible. This inevitably results in a very poor and unstable contact of graphene to the single NW.



Figure 1. Schematics of the fabrication process of single GaAs NW/graphene bottom contact device and an illustration of the air gap issue in graphene/single NW top contact device: (a) Frame-assisted micro-transfer of graphene/PMMA stack onto Si/SiO2 substrate. (b) Graphene/PMMA stack on the substrate after the transfer process. (c) Detachment of the NW/PMMA stack from its hydrophobic substrate (top) and transfer of the NW/PMMA stack on the target substrate with graphene (bottom). (d) Perpendicular alignment of a single GaAs NW on top of the graphene sheet. (e) Formation of metal contacts to the single GaAs NW and graphene. (f) The air gap (circular dashed area) formed between the bottom three facets of a NW placed on a flat surface and the transferred graphene on top.

In order to overcome the air gap issue, a new technique to fabricate single GaAs NW/graphene hybrid devices was developed using an imprinting method to embed single NWs in a curable photoresist layer. A diluted version of Shipley S1800 photoresist having a thickness in the order of the NW diameter was firstly spin-coated on a Si/SiO2 substrate (Figure 2(a)). Single NWs were subsequently transferred onto the photoresist surface with the help of a hydrophobic tape as shown in Figure 2 (b). After the successful transfer, NWs were imprinted into the resist through a precisely controlled mechanical pressing, and the top hydrophobic tape was removed from the substrate without disturbing the NW’s positions (Figure 2 (c)). In order to provide


Page 8 of 25

Page 9 of 25 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


strong stability to this embedded NW structure, the resist was then cured at high temperature (215 °C) for 15 minutes. This imprinting method was optimized to make a ~50 % embedded NW configuration in a flat-surfaced structure, which buries the bottom three facets of the NW inside the cured resist (Supporting Information, Figure S3). This method avoids the formation of air gaps and ensures that the transferred graphene forms a conformal coverage on the three top-facets of the embedded NW (Supporting Information, Figure S1(c)). A frame-assisted CVD graphene/PMMA stack was transferred on top of the NW after removal of the oxide layer from the NW surface by wet etching (Figure 2 (d)). The PMMA layer on top of the graphene was patterned by EBL and in this case, the patterned PMMA layer was used as a protection layer to the graphene sheet (Figure 2 (e)). After the development of the pattern, the exposed graphene region was etched out by O2 plasma. The patterned PMMA layer was then removed, which leaves a patterned graphene underneath (Figure 2 (f)). Patterned graphene and embedded NW were then connected to the outer pre-fabricated metal pads by EBL patterning and depositing Pd (20 nm)/Au (30 nm) and Pt (5 nm)/Ti (10 nm)/Pt (10 nm)/Au (40 nm), respectively (Figure 2 (g)). Using this embedded method, two different types of single p-GaAs NW/graphene devices were fabricated. The first is a metal-NW-graphene device and the second is a graphene-NWgraphene device. For the metal-NW-graphene device, a single embedded p-GaAs NW was contacted by a graphene electrode at one side and two metal electrodes (Pt/Ti/Pt/Au) on the other side (Figure 2 (g)). For the graphene-NW-graphene device, a single embedded p-GaAs NW was contacted by two square-shaped graphene electrodes at the two ends of the NW (Figure 2 (h)). In addition, in order to see the effect of this new imprinting method in the conventional metal/NW contact devices, two groups of p-GaAs/metal devices were further fabricated in an embedded and non-embedded NW configuration.



Figure 2. Schematics of the fabrication process of graphene/single GaAs NW top contact device: (a) Spin coating of curable photoresist on Si/SiO2 substrate. Yellow solid cubes at the corners represent pre-fabricated Au metal pads. (b) Transfer of a single GaAs NW on the resist-coated surface prior to its baking. (c) Mechanical pressing of a single GaAs NW into the photoresist layer forming an embedded NW structure and curing. (d) Transfer of graphene/PMMA stack on the target substrate with the embedded NW. (e) Patterning and O2 plasma etching of graphene. (f) Removal of PMMA to expose patterned graphene on top of the embedded NW. (g) Formation of metal contacts to the single GaAs NW and graphene for the metal-NW-graphene device. (h) Schematic representation of the grapheneNW-graphene device.

The electrical measurements on fabricated devices were performed with a Keithley 2636A Sourcemeter in a dark room at room temperature. The NW diameter, the embedded profile of the imprinted NW, and especially the channel length and contact width for embedded


Page 10 of 25

Page 11 of 25 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


and non-embedded metal-NW-metal devices were characterized by using atomic force microscopy (AFM). In addition, scanning photocurrent microscopy measurements were carried out on the graphene-NW-graphene top junction device to investigate the photocurrent response at the graphene/GaAs NW contact area through the graphene electrodes. A 532 nm green laser was used with a laser power of 10 µW and a raster scan step of 20 nm in x- and y-direction.

3. Results and Discussions

Electrical properties of the single p-GaAs NW/graphene bottom junction device were investigated by measuring the I-V characteristics for all possible drain (D)- source (S) electrode combinations as shown in Fig. 3 (a) and (c). In case of the metal contacts to p-GaAs NW measured between 1D and 2S, a symmetric I-V curve that is non-linear at the low-bias voltage region and becomes linear at high voltages was measured, indicating the formation of symmetric contact barriers at the metal/NW junctions (inset of Figure 3(c)). This is consistent with our previous observations of typical I-V characteristics of metal-contacted p-doped GaAs NW29. For the metal/p-doped GaAs NW/graphene contact of the device, a rectifying behavior is consistently observed in all drain-source combinations, where the metal electrode is at bias (D) and the graphene electrode is at ground (S). At negative voltages the current is less than ~ 480 nA at -2 V, which is ~35-70 times smaller than that observed in the metal/p-GaAs NW/metal junction counterpart which has even longer channel length than the metal-NWgraphene devices. At positive voltages, it shows high currents (5 - 10 µA @ 2 V) comparable to the current observed in the metal/p-GaAs NW/metal junction (~ 15 µA @ 2 V) as shown in the inset of Figure 3 (c). This can be qualitatively explained in terms of a back-to-back Schottky diode model. The Schottky barriers at the contacts are asymmetric and the barrier formed at the graphene/NW junction is dominant over the metal/NW junction barrier. The strong



suppression of the current at negative voltages appears from the reverse bias mode at the NW/graphene Schottky junction. At positive voltages, the NW/graphene Schottky junction is in the forward bias mode and the current is limited by the reverse bias mode at the NW/metal junction, as is observed in the metal/ NW/metal junction device. The metal/embedded p-GaAs NW/graphene top junction device and its I-V characteristics are presented in Figure 3 (b) and (d), respectively. Metal contacts of the embedded p-GaAs NW, measured between 1D and 2S, show a typical symmetric I-V curve (inset of Figure 3 (d)), similar to that observed in metal electrode pairs of the graphene/p-GaAs NW bottom junction device (inset of Figure 3 (c)). However, in contrast to what was observed in the graphene/pGaAs NW bottom contact device, the I-V characteristics of a metal/embedded pGaAs/graphene top junction device show symmetric (non-rectifying or ohmic-like) I-V curves, almost similar to each other in all possible drain-source measurement combinations (1D-3S, 1D-4S, 2D-3S & 2D-4S). This transition from a rectifying to a non-rectifying I-V curve for the metal-NW-graphene part in the top-contact device compared to a bottom-contact device indicates a dramatical reduction in the contact barrier at the NW/graphene interface in the embedded NW/graphene top junction configuration. There could be several reasons for this. One is that, in the case of the bottom contact device, to transfer a single NW at the desired location on the graphene sheet and to make a proper alignment are a bit time-consuming process, which increases the chance of an oxide layer to form at the bottom part of the NW during the transfer process34. Another is the wetting of the material transferred on top. GaAs NW is a hard material much less flexible than graphene. When transferred, graphene seems to get wetted better on the bottom surface (embedded NW surface and the cured photoresist) and form the electrical contact much better, compared to the bottom junction case where the hard GaAs NW is transferred on top of graphene for junction formation.


Page 12 of 25

Page 13 of 25 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


In addition, the I-V characteristic from the metal-embedded NW-metal contacts (1D-2S) of the same device (inset of Figure 3 (d)) shows a considerably lower turn on/off voltage and an improved linearity with a decrease of the non-linear region at low bias compared to that of the graphene/NW bottom contact device (inset of Figure 3 (c)). This behavior will be discussed in detail later with the results of the metal-NW-metal contact devices as a comparison study between embedded and non-embedded structure.

Figure 3. (a) Optical microscope image of p-GaAs NW/graphene bottom junction device. The blue dotted rectangle in the image shows the position of a rectangular-shaped graphene sheet below the NW. Metal contacts are marked as 1 & 2 and graphene contacts are marked as 3 & 4. (b) SEM image of graphene/embedded p-GaAs NW top junction device, where metal electrodes are marked as 1 & 2 and graphene electrodes are marked as 3 & 4. (c) Current-voltage characteristics of graphene/single p-GaAs NW bottom junction device measured with all possible drain-source combinations. The bias voltage is applied to the metal electrodes (drain) w.r.t the graphene electrodes (source). I-V characteristics of the metal contacts of the same single p-GaAs NW measured between 1D and 2S is



shown in the inset. (d) I-V characteristics of graphene/single p-GaAs NW top junction device measured with all possible drain-source combinations. I-V characteristics of the metal contacts of the same single p-GaAs NW measured between 1D and 2S is shown in the inset.

Graphene/embedded p-GaAs NW/graphene top contact devices have also been fabricated. The transferred graphene sheet was trimmed into two square-shaped electrodes by using EBL and O2 plasma etching to contact the two ends of an embedded p-GaAs NW (Figure 4 (a)). I-V characteristics of the graphene/embedded p-GaAs NW/graphene top contact device is shown in Figure 4 (c). All four combinations of source and drain show similar non-rectifying, symmetric I-V characteristics with similar current levels. This proves the consistency and reliability of this new imprinting method for an embedded NW structure with graphene contact. Electrical transport through the graphene electrodes shows linear, ohmic behavior with a 2-probe resistance of 1 - 2 k (Figure 4 (d)). The effect of the 2-probe graphene resistance on the electrical transport through the graphene/NW/graphene junction is negligible. It is worth to mention here that the conductance of graphene in this type of device structure is distinctly dependent on the smoothness of the photoresist surface after the imprinting of the NW. A rough photoresist surface would increase the chance for formations of wrinkles and defects in the transferred graphene sheet compared to a smooth photoresist surface. This is why, following a previous report on the fabrication of a single NW/metal Hall bar device in an embedded NW structure, using a (resist) spacer layer covering also the top of the NW in a planar structure and an etch down process to form an embedded NW configuration is not useful here since the etch down process by O2 plasma creates a very rough surface, especially adjacent to the NW edge on the spacer layer35. In order to compare with our developed imprinting technique, two metalembedded NW-metal devices have also been fabricated by following the previously reported etch down process35 (see Figure S2 in the Supporting Information), where a high surface


Page 14 of 25

Page 15 of 25 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


roughness adjacent to the NW edge is easily noticeable, which clearly indicates that this method is not suitable for NW/graphene contacts.

The height profile of the top part of the embedded NW that is outside the cured photoresist layer shows a gentle variation from 55 to 65 nm along the NW axis (Supporting Information, Figure S3). This means an approximately 52% embedment of the NW on an average. The height variation at the graphene electrode along the NW axis is less than ~ 2 nm, which should be good enough for stable conformal contact of graphene to the NW.



Figure 4. (a) SEM image of a graphene/embedded p-GaAs NW/graphene top contact device. Four metal electrodes on graphene are marked as 1, 2, 3 & 4. (b) Two-dimensional scanning photo-current microscopy (SPCM) image on the graphene/p-GaAs NW/graphene device measured at zero bias. The fitted semi-transparent SEM image of the device shows the positions of the photo-generated currents. Two bright photocurrent spots (a blue one at the drain side and a red one at the source side) were observed exactly at the graphene/NW junction positions. Solid red data points represent the photocurrent variation along the length of the NW. (c) I-V characteristics of the graphene/embedded p-GaAs NW/graphene top junctions measured with all possible drain-source combination. (d) I-V characteristics of the metal-graphene-metal contacts on the same device measured through the graphene electrode itself.

One of the unique merits with the graphene electrode on top of a NW is that graphene is transparent over a broad frequency range, allowing to investigate the underlying NW and its contact to graphene optically through the graphene electrode. Confocal photocurrent mapping of the graphene-NW-graphene top junction device manifests the position of the graphene/NW junctions (Figure 4 (b)). The positive (negative) current at the drain (source) graphene electrode at zero bias confirms the Schottky junction formed by the p-doped NW and graphene. The photocurrent spots at the graphene electrodes have similar size and current level, indicating similar Schottky junction barriers at the two different contact positions of the NW with the top graphene. This is consistent with the symmetric behavior in the I-V curve of the device.

As noted earlier, improved I-V characteristics were observed in the metal-embedded NW-metal contact device compared to a conventional, non-embedded NW with metal electrodes. To investigate this further, a group of metal contacted p-GaAs NW devices with embedded and non-embedded configuration was fabricated and measured (Supporting Information, Figure S4). The similar NW diameter, channel length, and contact widths have only been taken into consideration for comparison of these two groups of metal-NW-metal devices. The diameter of


Page 16 of 25

Page 17 of 25 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 NW was measured from AFM line scan across the NW at the mid-gap of the channel length of each device, whereas the contact width and the channel length were measured along the length of the device as shown in the Supporting information, Figure S5 & S6. The measured values of these parameters and even the (%) of embedment of the NW for embedded devices have been summarized in Supporting Information, Table S.Ι. Very consistent improvement with a relative increase of current and more linear I-V characteristics was observed in embedded NW devices as compared to the non-embedded NWs (Figure 5(a)). To support this result even further, a set of 4-probe devices were fabricated using metal electrodes of similar contact widths (~250 nm) in conventional non-embedded NW configuration (see Supporting Information, Figure S7). Contact resistance and resistivity of the NW has been summarized in Supporting Information, Table S.ΙΙ. from 4-probe metal-NW (nonembedded) measurements. It is worth to highlight that the contact resistance (~60-80 kΩ) from these non-embedded metal-NW-metal devices (Table S.ΙΙ in Supporting Information) were found to be much higher than that of the total 2-probe resistance (~20-30 kΩ) of the embedded metal-NW-metal devices (Figure 5 (c)). This clearly signifies that the contact resistance has been reduced dramatically in embedded NW-metal configuration, which justifies the importance of our developed imprinting method.

This may look counter-intuitive if one considers the contact areas between the NW and metal electrodes. As shown in Figure 5 (b), the non-embedded NW devices have more contact area than the embedded ones. One would anticipate that this should give less contact resistance and correspondingly higher current in non-embedded NW devices. However, this is only true if the NW is electronically homogeneous in its cross-sectional area. On the contrary, our semiconducting NWs have different electronic states at the interface with metal contacts. For the NW facets contacted to the metal electrode, a Schottky junction is formed with a depletion ACS Paragon Plus Environment


layer below the surface, which can be highly resistive at low bias. This does not happen when the NW facets are embedded in a cured photoresist (PR) or contacted to a bottom insulating substrate. A high conductance state, similar to the intact core, should be retained in this case3637.

A simple ‘parallel resistors model’ is introduced in consideration of the two different resistance states (Supporting information, Section 1). In the model, the cross-sectional resistance of a NW is from a combination of six resistors that can be either high-resistive RSC or low-resistive RNW. The parallel combination of the resistors makes the portion of RNW important in reducing the total cross-sectional area resistance. The basic assumptions of RSC > RNW and their parallel connections result in a higher resistance in the non-embedded NW device compared to that of the embedded NW device. It is worth to note that this is the case at low bias. At high bias, the depletion layer becomes close to zero and the differential resistance (dV/dI) will show the intrinsic resistance of the NW itself. In fact, it is observed that the differential resistances are comparable at high bias in embedded (V > 0.5 V) and non-embedded (V > 1.5 V) devices (Figure 5 (a)). In a few theoretical studies, it has been shown that the Schottky barrier formation is quite different in a cylindrical NW with all-around metal contact compared to a planar junction with metal contact and how that control the transport property of a thin NW38-39. A different electric field distribution in the cylindrical NW and a reduction of the image-force barrier-lowering effect make the Schottky barrier higher and wider for the cylindrical NW-metal junction than the conventional planar junction. This suggests that, if the non-embedded NW devices which have 5 out of 6 facets of the hexagonal NW contacted to metal is approximated to a cylindrical NW with all-around metal contacts, it should have a Schottky barrier higher and wider than the planar metal contact case which can approximate our embedded NW devices. It has also been reported how the size of a nanocontact to a free-standing NW can switch the electrical transport


Page 18 of 25

Page 19 of 25 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


from a Schottky to an Ohmic behavior, where a smaller contact size to NW diameter ratio has been found to be ohmic40. This is a clear evidence, similar to our observation, that the geometrical effect influences the contact depletion and therefore controls the carrier transport, especially for thin NWs. Under the approximation of the non-embedded NW devices and embedded NW devices to a cylindrical NW with all-around metal contact and a planar metal contact, respectively, a fully depleted condition for the NW can be established (Supporting information, Section 2). The estimation of a parameter, β, shows that β for the embedded NW is ~3.7 times larger than β for the non-embedded NW. This means that either higher barrier height or lower carrier concentration (or both) is required to deplete the embedded NW fully compared to the nonembedded NW. In other words, the non-embedded NW with conventional all-around metal contact is more readily depleted than the embedded NW with a metal contact on top facets.



Figure 5. (a) I-V characteristics of a group of embedded and non-embedded metal/single p-GaAs NW/metal contact devices. The red data points represent the I-V of non-embedded NW/metal devices and the blue data points represent the I-V of embedded NW/metal devices.

(b) Schematic representation of the five and three-facet

depletion regions in a non-embedded and embedded NW/metal device at zero bias. (c) Statistical summary of measured currents at 1.5 V from non-embedded & embedded NW/metal top contact devices, grapheneembedded NW-metal top-contact devices, and graphene-embedded NW-graphene top-contact devices. For graphene-NW-metal bottom contact devices, the data are the currents measured at a reverse bias of – 1.5 V, which corresponds to the reverse mode at the NW/graphene junction. The top horizontal axis represents the number of devices measured for each type of devices.

All measured data are briefly summarized in Figure 5 (c). The NW/graphene bottom contact devices show the highest resistance that was calculated at reverse bias for NW/graphene junction. Metal/embedded NW/metal devices show the lowest mean resistance, followed by the conventional non-embedded metal/NW/metal devices. To check the device-to-device variability due to the graphene transfer, the 2-probe resistance of the graphene was measured from the I-V characteristics of metal-graphene-metal structures of same channel length, as shown in Figure 4 (d). For comparison, only devices with similar 2-probe resistance have been considered. The resistances of graphene/embedded NW contact devices show higher values than for the metal/embedded NW/metal contact devices. This is not so unreasonable if we consider the nature of the graphene contact. Graphene is a two-dimensional material with no dangling bonds at the surface. Therefore, it does not form any chemical bonding to the NW, unlike the conventional contact by metal deposition. The van der Waals gap formed between graphene and NW could reduce the current flow further in addition to the conventional Schottky barrier. Still, it can also imply that there is room for improving the graphene contact to embedded NW further.


Page 20 of 25

Page 21 of 25 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


5. Conclusion In summary, we have developed a new technique to fabricate reliable single GaAs NW/graphene hybrid devices by using a position controlled micro-transfer and an imprinting technique for embedded NW structure in a planar junction configuration. Both bottom- and topjunction devices were made and compared. Highly rectifying I-V characteristic observed in NW/graphene bottom-contact devices is attributed to a high contact barrier at the NW/graphene junction due to possible oxidization of the NW at the junction interface or poor wetting of the NW on graphene. For NW/graphene top-contact devices, the presence of air gaps unavoidably formed between the bottom three-facets of the NW and the transferred graphene is considered as the main reason causing unstable and unreliable electrical contacts between the NW and top graphene. This issue was clearly solved by imprinting the NW in a curable photoresist layer which buries the bottom three-facets of the NW. I-V characteristics of embedded NW/graphene top-contact devices show a symmetric behavior with the current level comparable to the conventional metal/NW/metal contact devices, which signifies a large reduction of contact barrier at the NW/graphene interface. Two different types of embedded NW devices with graphene electrode, graphene/NW/metal contact devices, and graphene/NW/graphene contact devices, were investigated. Both show similar I-V characteristics, and hence prove the consistency and reliability of the newly developed technique. Photocurrent mapping shows that graphene may serve as a high-quality transparent and conductive electrode to single NWs in an embedded structure. Conventional, non-embedded NW/metal devices were further compared with the embedded NW/metal devices. The improvement in electrical transport in embedded NW/metal devices has been explained in terms of a ‘parallel resistors model’. In addition, a condition for full depletion of the embedded and non-embedded NWs with metal contact has been investigated theoretically.



Associated Content Supporting Information SEM image of graphene/GaAs top-contact with and without embedded NW structure, SEM image metal-NW-metal device made by top-down etching, AFM image and line scan profile of graphene-NW-graphene device, SEM image of metal-NW-metal device in non-embedded and embedded NW configuration, Characterization of NW diameter, channel length and contact width from AFM line scan, Contact resistance and NW resistivity measurement from 4-probe NW-metal devices, Parallel Resistors Model, Depletion Limited Model.

Acknowledgments We acknowledge the financial support from the Research Council of Norway NANO2021 (Grant #: 228758, Grant #: 239206) and for the support from the Norwegian Micro- and NanoFabrication Facility, NorFab (245963/F50). S. W. Lee acknowledges the Basic Science Research Program (NRF-2015R1A2A2A05050829), International Collaboration Program (NRF-2016K2A9A1A03905001) through the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP). This research was also financially supported by the Ministry of Trade, Industry, and Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the International Cooperative R&D program. K. S. Kim acknowledges the Priority Research Centers Program (Grant #: 2010-0020207) by the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology, and International Research Center Program through the National Science Foundation of Korea (Grant #: 2018K1A4A3A01064272) funded by the Ministry of Science, ICT and Future Planning.


Page 22 of 25

Page 23 of 25 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


References (1) Munshi, A. M.; Dheeraj, D. L.; Fauske, V. T.; Kim, D.-C.; van Helvoort, A. T.; Fimland, B.-O.; Weman, H., Vertically Aligned GaAs Nanowires on Graphite and Few-Layer Graphene: Generic Model and Epitaxial Growth. Nano Lett., 2012, 12 (9), 4570-4576. (2) Park, H.; Chang, S.; Jean, J.; Cheng, J. J.; Araujo, P. T.; Wang, M.; Bawendi, M. G.; Dresselhaus, M. S.; Bulović, V.; Kong, J., Graphene Cathode-Based ZnO Nanowire Hybrid Solar Cells. Nano Lett., 2012, 13 (1), 233-239. (3) Babichev, A.; Zhang, H.; Lavenus, P.; Julien, F.; Egorov, A. Y.; Lin, Y.; Tu, L.; Tchernycheva, M., GaN Nanowire Ultraviolet Photodetector with a Graphene Transparent Contact. Appl. Phys. Lett., 2013, 103 (20), 201103. (4) Wu, Y.; Yan, X.; Zhang, X.; Ren, X., A Monolayer Graphene/GaAs Nanowire Array Schottky Junction Self-Powered Photodetector. Appl. Phys. Lett., 2016, 109 (18), 183101. (5) Miao, J.; Hu, W.; Guo, N.; Lu, Z.; Liu, X.; Liao, L.; Chen, P.; Jiang, T.; Wu, S.; Ho, J. C., High‐Responsivity Graphene/InAs Nanowire Heterojunction Near‐Infrared Photodetectors with Distinct Photocurrent On/Off Ratios. Small., 2015, 11 (8), 936-942. (6) An, X.; Liu, F.; Jung, Y. J.; Kar, S., Tunable Graphene-Silicon Heterojunctions for Ultrasensitive Photodetection. Nano Lett., 2013, 13 (3), 909-916. (7) Kim, H.-Y.; Lee, K.; McEvoy, N.; Yim, C.; Duesberg, G. S., Chemically Modulated Graphene Diodes. Nano Lett., 2013, 13 (5), 2182-2188. (8) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films. Science., 2004, 306 (5696), 666-669. (9) Li, X.; Zhu, Y.; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Piner, R. D.; Colombo, L.; Ruoff, R. S., Transfer of Large-Area Graphene Films for High-Performance Transparent Conductive Electrodes. Nano Lett., 2009, 9 (12), 4359-4363. (10) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J., Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science., 2008, 321 (5887), 385-388. (11) Chen, C.-C.; Aykol, M.; Chang, C.-C.; Levi, A.; Cronin, S. B., Graphene-Silicon Schottky Diodes. Nano Lett., 2011, 11 (5), 1863-1867. (12) Garnett, E.; Yang, P., Light Trapping in Silicon Nanowire Solar Cells. Nano Lett., 2010, 10 (3), 1082-1087. (13) Yan, R.; Gargas, D.; Yang, P., Nanowire Photonics. Nat. Photon., 2009, 3 (10), 569-576 (14) Ye, Y.; Dai, Y.; Dai, L.; Shi, Z.; Liu, N.; Wang, F.; Fu, L.; Peng, R.; Wen, X.; Chen, Z., High-Performance Single CdS Nanowire (Nanobelt) Schottky Junction Solar Cells with Au/Graphene Schottky Electrodes. ACS Appl. Mater. Interfaces., 2010, 2 (12), 3406-3410. (15) Tchernycheva, M.; Lavenus, P.; Zhang, H.; Babichev, A.; Jacopin, G.; Shahmohammadi, M.; Julien, F.; 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. (16) Luo, Y.; Yan, X.; Zhang, J.; Li, B.; Wu, Y.; Lu, Q.; Jin, C.; Zhang, X.; Ren, X., A Graphene/Single GaAs Nanowire Schottky Junction Photovoltaic Device. Nanoscale., 2018, 10 (19), 9212-9217. (17) Fauske, V. T.; Huh, J.; Divitini, G.; Dheeraj, D. L.; Munshi, A. M.; Ducati, C.; Weman, H.; Fimland, B.-O.; van Helvoort, A. T., In Situ Heat-Induced Replacement of GaAs Nanowires by Au. Nano Lett., 2016, 16 (5), 3051-3057.



(18) Qi, J. S.; Huang, J. Y.; Feng, J.; Shi, D. N.; Li, J., The Possibility of Chemically Inert, Graphene-Based All-Carbon Electronic Devices with 0.8 eV Gap. ACS Nano., 2011, 5 (5), 3475-3482. (19) Skoda, M.; Dudek, I.; Jarosz, A.; Szukiewicz, D., Graphene: One Material, Many Possibilities-Application Difficulties in Biological Systems. J Nanomater., 2014, 2014, 190. (20) Kim, Y. D.; Kim, H.; Cho, Y.; Ryoo, J. H.; Park, C.-H.; Kim, P.; Kim, Y. S.; Lee, S.; Li, Y.; Park, S.-N.; Shim Yoo, Y.; Yoon, D.; Dorgan, V. E.; Pop, E.; Heinz, T. F.; Hone, J.; Chun, S.-H.; Cheong, H.; Lee, S. W.; Bae, M.-H.; Park, Y. D., Bright Visible Light Emission from Graphene. Nat. Nanotechnol., 2015, 10, 676-681 (21) Yun, H., Electrical Studies on Low-Dimensional Nanomaterial Based Stacking Structures Prepared by Micro-Transfer Methods 2015, PhD Thesis, Konkuk University. (22) Åberg, I.; Vescovi, G.; Asoli, D.; Naseem, U.; Gilboy, J. P.; Sundvall, C.; Dahlgren, A.; Svensson, K. E.; Anttu, N.; Björk, M. T., A GaAs Nanowire Array Solar Cell with 15.3% Efficiency at 1 Sun. IEEE J Photovolt., 2016, 6 (1), 185-190. (23) Krogstrup, P.; Jørgensen, H. I.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; i Morral, A. F., Single-Nanowire Solar Cells Beyond the Shockley-Queisser Limit. Nat. Photon., 2013, 7 (4), 306-310 (24) Yao, M.; Huang, N.; Cong, S.; Chi, C.-Y.; Seyedi, M. A.; Lin, Y.-T.; Cao, Y.; Povinelli, M. L.; Dapkus, P. D.; Zhou, C., GaAs Nanowire Array Solar Cells with Axial p-i-n Junctions. Nano Lett., 2014, 14 (6), 3293-3303. (25) LaPierre, R., Numerical Model of Current-Voltage Characteristics and Efficiency of GaAs Nanowire Solar Cells. J. Appl. Phys., 2011, 109 (3), 034311. (26) Dai, X.; Zhang, S.; Wang, Z.; Adamo, G.; Liu, H.; Huang, Y.; Couteau, C.; Soci, C., GaAs/AlGaAs Nanowire Photodetector. Nano Lett., 2014, 14 (5), 2688-2693. (27) Wang, H., High Gain Single GaAs Nanowire Photodetector. Appl. Phys. Lett., 2013, 103 (9), 093101. (28) Thunich, S.; Prechtel, L.; Spirkoska, D.; Abstreiter, G.; Fontcuberta i Morral, A.; Holleitner, A., Photocurrent and Photoconductance Properties of a GaAs Nanowire. Appl. Phys. Lett., 2009, 95 (8), 083111. (29) Dheeraj, D.; Munshi, A.; Christoffersen, O.; Kim, D.; Signorello, G.; Riel, H.; van Helvoort, A.; Weman, H.; Fimland, B., Comparison of Be-doped GaAs Nanowires Grown by Au-and Ga-Assisted Molecular Beam Epitaxy. J Cryst Growth., 2013, 378, 532-536. (30) Stichtenoth, D.; Wegener, K.; Gutsche, C.; Regolin, I.; Tegude, F.; Prost, W.; Seibt, M.; Ronning, C., P-Type Doping of GaAs Nanowires. Appl. Phys. Lett., 2008, 92 (16), 163107. (31) Okada, H.; Shikata, S.-i.; Hayashi, H., Electrical Characteristics and Reliability of Pt/Ti/Pt/Au Ohmic Contacts to p-Type GaAs. Jpn. J. Appl. Phys., 1991, 30 (4A), L558. (32) Kim, M.; Yoon, D.; Choi, S. K.; Cheong, H.; Yoon, H. A.; Lee, S. W., Polarization Dependence of the Photocurrent Due to an Anisotropic Electron-Photon Interaction in PdGraphene-Pd Devices. J. Korean Phys. Soc., 2013, 63 (5), 1019-1022. (33) Anzi, L.; Mansouri, A.; Pedrinazzi, P.; Guerriero, E.; Fiocco, M.; Pesquera, A.; Centeno, A.; Zurutuza, A.; Behnam, A.; Carrion, E. A., Ultra-Low Contact Resistance in Graphene Devices at the Dirac Point. 2D Mater., 2018, 5 (2), 025014. (34) Lee, R. S.; Kim, T. K.; Lee, S. W.; Cho, K. Y.; Choi, J. H.; Kim, M. Y.; Shin, J. C., Electrical Properties of Surface-Passivated GaAs Nanowires. Appl. Sci. Convergence Technol., 2018, 27 (6), 166-168. (35) Hultin, O.; Otnes, G.; Borgström, M. T.; Björk, M.; Samuelson, L.; Storm, K., Comparing Hall Effect and Field Effect Measurements on the Same Single Nanowire. Nano Lett., 2015, 16 (1), 205-211.


Page 24 of 25

Page 25 of 25 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


(36) Bercu, B.; Geng, W.; Simonetti, O.; Kostcheev, S.; Sartel, C.; Sallet, V.; Lérondel, G.; Molinari, M.; Giraudet, L.; Couteau, C., Characterizations of Ohmic and Schottky-Behaving Contacts of a Single ZnO Nanowire. Nanotechnol., 2013, 24 (41), 415202. (37) Wen, J.; Zhang, X.; Gao, H.; Wang, M., Current-Voltage Characteristics of the Semiconductor Nanowires Under the Metal-Semiconductor-Metal Structure. J. Appl. Phys., 2013, 114 (22), 223713. (38) Park, H.; Beresford, R.; Hong, S.; Xu, J., Geometry-and Size-Dependence of Electrical Properties of Metal Contacts on Semiconducting Nanowires. J. Appl. Phys., 2010, 108 (9), 094308. (39) Calahorra, Y.; Yalon, E.; Ritter, D., On the Diameter Dependence of Metal-Nanowire Schottky Barrier Height. J. Appl. Phys., 2015, 117 (3), 034308. (40) Lord, A. M.; Maffeis, T. G.; Kryvchenkova, O.; Cobley, R. J.; Kalna, K.; Kepaptsoglou, D. M.; Ramasse, Q. M.; Walton, A. S.; Ward, M. B.; Köble, J. r., Controlling the Electrical Transport Properties of Nanocontacts to Nanowires. Nano Lett., 2015, 15 (7), 4248-4254.

Table of Contents (TOC) graphic


Graphene Hybrid Devices Fabricated by a

Recommend Documents