Pei Peng

Toward High Carrier Mobility and Low Contact Resistance: Laser Cleaning of PMMA Residues on Graphene Surfaces

Poly(methyl methacrylate) (PMMA) is widely used for graphene transfer and device fabrication. However, it inevitably leaves a thin layer of polymer residues after acetone rinsing and leads to dramatic degradation of device performance. How to eliminate contamination and restore clean surfaces of graphene is still highly demanded. In this paper, we present a reliable and position-controllable method to remove the polymer residues on graphene films by laser exposure. Under proper laser conditions, PMMA residues can be substantially reduced without introducing defects to the underlying graphene. Furthermore, by applying this laser cleaning technique to the channel and contacts of graphene field-effect transistors (GFETs), higher carrier mobility as well as lower contact resistance can be realized. This work opens a way for probing intrinsic properties of contaminant-free graphene and fabricating high-performance GFETs with both clean channel and intimate graphene/metal contact.Graphical Abstract

Low insertion loss of 200 μm-long graphite coplanar waveguide

The graphene coplanar waveguide (CPW) has recently been found to have large insertion loss (typically larger than 50 dB/100 μm), which mainly results from the large resistance of graphene. The poor radio-frequency transmission property of graphene hampers its application in interconnect, a low loss material is thus required. In this paper, low-resistance graphite CPWs with effective graphite length up to 200 μm were fabricated. A record low insertion loss of graphite CPW (2.76 dB/100 μm) is demonstrated, and the average insertion loss of our graphite CPWs is only ∼1/5 of that of our monolayer graphene CPWs. Moreover, we find the insertion loss of graphite CPW may be even smaller at higher frequencies. Our investigation shows that graphite is a possible candidate for interconnect and may even be more applicable at ultra-high frequencies.

Synthesis of highly uniform monolayer graphene by etching the multilayer spots for electronic devices

We demonstrate a facile method to grow highly uniform monolayer graphene films on copper foils by atmospheric pressure chemical vapor deposition (APCVD). The technique in this method includes lowering flow ratio of methane/hydrogen and extending exposure time to hydrogen. All the multilayer islands will be etched away by hydrogen during this growth process, resulting in obtaining highly uniform monolayer graphene. A mechanism for the suppression of mutilayer spots based on the etching effect of hydrogen is proposed. The electron and hole room-temperature mobilities for the back-gated graphene transistors are up to about 3800 cm2V−1s−1 and 3300 cm2V−1s−1, respectively.

Enhancement of photocurrent in suspended monolayer graphene

The photocurrent in graphene has drawn much attention in recent years. The mechanisms of its production vary in different situations, such as at the interfaces of monolayer-bilayer junction or p-n junction. Here we demonstrate photocurrent generation in graphene-based field-effect transistors (GFETs) with a partially suspended area. Both Raman and photocurrent mapping were performed after a laser-induced doping under vacuum at room temperature. The resulting photocurrent in the suspended area is an order of magnitude larger than that in the supported area. The difference in photocurrent may be attributed to the thermoelectric effect.

Series resistance of signal line in graphene coplanar waveguide

In this paper, we focus on the influence of series resistance of signal line (comoprised of the sheet resistance of graphene and the contact resistance between graphene and metal) on the radio-frequency transmission property of graphene coplanar waveguide (CPW) and put forward ways to improve it. According to our research, the large series resistance (7.2 kΩ) of signal line is the main reason for the large insertion loss (|S21|) of graphene CPW. The insertion loss of graphene CPW is found to be reduced by either enlarging the area of graphene-metal contact or increasing the number of graphene layers.

Contact resistance between graphene and metal

In this paper, a 3D metal-graphene contact with different contact structures (i.e., embedded contact and edge contact) is studied using Kirchhoff model. The influences of some major factors, including metal resistivity, metal-graphene contact resistivity and sheet resistances of graphene both in the channel and under metal, on the total contact resistance are investigated. The sheet resistance of graphene under metal has much more effect on the total contact resistance than other factors. Methods for creating low resistance contact are also proposed.

Observation of sub-20nm line-defects in graphene by friction force microscopy

Graphene films, produced by chemical vapor deposition (CVD) on copper foils, generally have a lot of line-defects, such as grain boundaries and nanogaps between adjacent graphene domains. We demonstrated to identify such 1-D defects with size less than 20 nanometers using friction force microscopy (FFM). These line defects can be clearly observed in friction force images, while no height variation in simultaneously collected topographic ones. The possible reasons for line defects imaging under FFM mode is also studied.

Graphene Nucleation at Front Ends of Copper Twin Crystal

The graphene is easily grown on polycrystalline copper foils using metal-catalyzed chemical vapor deposition (CVD). Annealing twin readily occurred during this CVD process. However, there are only a few reports on the role of annealing twin on the graphene growth. In this paper, we present the investigations of the influence of copper twin crystal onthe nucleation of graphene. We found that there is no selective formation of graphene on these annealing twin crystal in general conditions of graphene growth.Interestingly, graphene preferentially nucleates at the front ends of twin crystals. We propose a micro-mechanism to explain the preferential graphene nucleation site based on spiral growth phenomena of copper twin crystals.This work may enable new possibilities for the controllable nucleation and wafer-scale growth of single-crystal graphene.

Influence of Polymer Residues on the Double Resonance Process of Bilayer Graphene

PMMA is commonly used for graphene transfer and device processing. However, it leaves a thin layer of polymer residues after standard acetone cleaning and causes electrical and thermal performance degradation of graphene devices. In this paper, we present a research of the impact of polymer residues on surface morphologies and electronic properties of bilayer graphene using Raman spectroscopy in combination with atomic force microscopy (AFM). The electronic structure of bilayer graphene is well captured in its Raman spectrum, of which the 2D band reveals four double resonance Raman scattering processes. The Raman analyses show universal blueshifts of the G band and the four 2D sub-bands P11, P12, P21, P22, as well as reduced intensity ratios of the sub-bands to the G band I(Pij)/I(G) after surface contamination by polymer residues, implying an electronic structure modulation in bilayer graphene. The effects are mainly attributed to p-type doping and extrinsic scattering induced by residual impurities and defects.

Impacts of dimensions, number of layers and contact structures on the loss of graphene-based coplanar waveguides through simulations

We present modeling and simulations of graphene coplanar waveguide (GCPW) under the frequency up to 50 GHz. Our simulation results show that the dimensions of GCPW greatly influence its insertion loss. In addition, different graphene layer numbers and structures of graphene-metal contact were also considered. We show that the usage of few-layer graphene and end-contact structure is able to decrease the insertion loss, which helps improve the radio-frequency performance of graphene coplanar waveguide.