Antonova I V*
Rzhanov Institute of Semiconductor Physics SB RAS, Novosibirsk, Russia
Received: 29 September, 2016; Accepted: 12 October, 2016; Published: 13 October, 2016
I V Antonova, Rzhanov Institute of Semiconductor Physics SB RAS, Novosibirsk, Russia, E-mail:
Antonova IV (2016) Non-Organic Dielectric Layers for Graphene and Flexible Electronics. Int J Nanomater Nanotechnol Nanomed 2(1): 018-024. DOI: 10.17352/2455-3492.000010
© 2016 Antonova IV. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Graphene; Flexible electronics; Traditional gate dielectrics; Fluorinated grapheme; Oxide grapheme
Future electronics technology is expected to develop from rigid to flexible devices, which requires breakthroughs in materials’ properties, especially flexibility, in combination with desirable electrical insulating, semiconducting and metallic properties. Recently emerging 2D materials such as graphene are promising for an active conductive layer in a wide spectrum of flexible electronic devices. Developing optimized dielectrics for the graphene active layer is critical for graphene applications. The advances and limitations of qualitatively different traditional dielectric metal oxide layers (high-k dielectrics Al2O3, HfO2, and ZrO2) used as a gate in graphene field effect transistors on flexible substrates are considered in the first part of the present review. Its second part analyzes properties of novel dielectric materials (h-BN, Y2O3, graphene oxide, fluorinated graphene, composite dielectrics, ion gels) used for graphene transistors. Dielectric layers fabricated from fluorinated graphene or in combination with graphene oxide are the most promising graphene based flexible and transparent electronics.
Future electronics technology will evolve from rigid devices to bendable, rollable, foldable, stretchable or transparent ones that are wearable like clothes or accessories [1-3]. The first breakthrough in this direction occurred in the form of flexible electronics for a wide spectrum of applications (bio- and medical items, sensors and gadget displays on the textile or clothing electronics and so on). These flexible devices are expected to excel the rigid ones in durability, weight, and comfort. However their development necessitates breakthroughs in materials since in combination with desirable electrical insulating, semiconducting and metallic properties they need flexibility. Recently emerged 2D materials such as graphene, graphene derivates (graphene oxide, fluorinated graphene), hexagonal boron nitride (h-BN), and transition metal dichalcogenides are attractive because of their outstanding electrical and optical properties. Mechanical properties of these materials are different, and only part of them meets the requirements of flexible or stretchable electronics.
Graphene based printed electronics is also a recently emerged and a fast grown field that has attracted large scientific and technological interest for the past few years. Graphene presents great promise as an active layer in wide spectrum of devices of flexible electronics and, first of all, in field effect transistors. Recent reports demonstrate successful realization of graphene field effect transistors (FETs) on flexible or even on stretchable substrates [4-8]. To achieve such applications the development of optimized dielectrics for the graphene active layer is critical (gate and interlayer dielectrics or/and substrate for graphene). The carrier transport in graphene films takes place at the interfaces with the dielectric or the semiconductor; therefore, the quality of such interface and the interaction with nearby dielectric layers (charge carrier scattering) determine the device performance. Nevertheless, the development of dielectric materials that can achieve high-performance device operation, good mechanical properties, and low-temperature fabrication is not well established because the graphene thin film is very sensitive to surface conditions of dielectric layers .
In present review, the main materials applied nowadays as dielectric films for graphene based devices fabricated on the flexible substrate using traditional or printed technologies are discussed. The further opportunities for utilizing the graphene derivates such as graphene oxide and fluorinated graphene are also demonstrated.
Traditional materials for gate insulators in graphene FET on flexible substrates
Presently, an indium tin oxide (ITO) is widely used as a transparent conductor for optoelectronic devices. However, ITO has poor mechanical properties; it tends to crack easily or shows defects when strained . For these reasons, the use of graphene has been widely investigated in recent years as a transparent conductor for optoelectronic and photonic applications because of its combination of electrical, mechanical, and optical properties. The conductive graphene films or reduced oxide graphene layers are generally considered as a material for transistor channels or electrodes .
Traditional gate insulators SiO2, Al2O3, and HfO2 have several limitations for use in graphene transistors on flexible substrates, including low-facture strains less than 1%, poor mechanical strength, high growth temperatures, and poor interface between graphene and dielectric layers [3,11,12]. Nevertheless, oxides based high-k dielectric materials, such as Al2O3, HfO2, and ZrO2, are the most widely used in graphene FETs [13,14]. For example, Lu et al. , have demonstrated high-mobility and low-voltage graphene FETs, fabricated on a polyethylene terephthalate (PET) substrate with a high-capacitance natural aluminum oxide as the gate dielectric (evaporation of AL with its oxidation) in a self-aligned device configuration. The high capacitance of the native aluminum oxide and the self-alignment, which minimizes access resistance, yield a high current on/off ratio and an operation voltage below 3 V, along with high electron and hole mobility of 230 and 300 cm2/Vs, respectively. Moreover, the native aluminum oxide is resistant to mechanical bending and exhibits self-healing upon electrical breakdown.
The use of atomic layer deposition (ALD) with special precursors for high-k dielectrics allows decreasing the growth temperature and partially overcoming the limitation mentioned above. Petrone et al. , fabricate FETs on polyethylene naphthalate (PEN) substrate from graphene, grown by chemical vapor deposition (CVD) with a 6-nm gate dielectric of HfO2, conformally grown by ALD at 150 °C yielding a dielectric constant of k ≈ 13. Figure 1 demonstrates graphene FET schema and characteristics with and without strain. The source-togate current, Isg, is measured to remain below 0.5 pA over the entire strain range during device characterization, indicating negligible leakage current through the dielectric even at high strain; carrier mobility μ for these flexible FETs is ~1500 cm2/Vs and does not practically change with strain up to 1.75%.