B/N co-doped graphene oxide gel with extremely-high mobility and I_ON/I_OFF for large-area field effect transistors

Abstract

Thin films of large-area graphene and graphene-based materials are highly desired for electrical applications. However, the current state-of-art synthesis methods produce large-area graphene films with multiple grain boundaries (GBs) that highly hinder their charge carrier mobilities and I_on/I_off ratios. Here, we demonstrate a femtosecond laser ablation process to produce B and N co-doped graphene oxide (GO) gels with controllable total doping percentage (between 0.8 at%-2.3 at%) and effectively reduced GBs concentration. The charge carrier mobilities and I_on/I_off ratios of the produced large-area gel (∼100 × 2400 μm²) field effect transistors (FETs) revealed extremely-high values of up to 9000 ± 3000 cm²/V and 9.7E+5, respectively, comparable to the state-of-art values of a single monolayer graphene nanoflake. The increased total doping percentage also proved to improve the chemical reactivity of the gels. This femtosecond laser ablation approach could prove effective for large-area FETs with controllable mobility, I_on/I_off ratio, and chemical reactivity.

Introduction

Graphene is a two-dimensional carbon allotrope with an extremely high theoretical charge carriers mobility [1]. These theoretical properties of graphene made it a potential candidate for next-generation metal oxide semiconductor field-effect transistors (MOSFETs) and complementary metal oxide semiconductor (CMOS) logic, due to their scaling challenges [2]. While the theoretical mobility of monolayer graphene is remarkable, there have been many challenges fabricating a device with ultra-high mobilities, since many factors decrease the mobility of graphene. Mobility drastically decreases with increased number of graphene layers [3]. Grain boundaries (GBs) in graphene exhibit high scattering mechanisms of charge carriers, resulting in diminished mobility between graphene nanoflakes [4]. Structural defects that reduce the crystallinity of graphene also act as scattering centers and limit the achievable mobility [5]. It was additionally found that the substrate itself has a great effect over the mobility in graphene, with hexagonal boron nitride (BN) being a promising dielectric to replace silicon dioxide [6,7]. While much progress has been achieved in addressing many of these factors, a large-area and volume fabrication of graphene for electronic applications still remains a challenge, mostly due to the presence of GBs and the inability to produce wafer-scale high mobility graphene [[8], [9], [10]].

Besides mobility, there is one more important parameter that limits the use of graphene for field effect transistor (FET) applications. This parameter is the I_ON/I_OFF ratio, which is the ratio between the current in the “ON” state when a bias is applied and the “OFF” state when no bias is applied on the FET device [11]. Graphene has an extremely low I_ON/I_OFF ratio due to its zero bandgap [12]. When a bandgap is produced, usually this leads to an increase in the I_ON/I_OFF ratio, but not to the same scale of the current state-of-art MOSFETs (I_ON/I_OFF ∼10⁷) [13].

Graphene oxide (GO) gel has been studied in the past few years as an approach to effectively remove some of the GBs and improve the electrical performance of GO [14,15]. Here in, we present a novel boron nitride (B/N) co-doped GO gel (B/N-GO gel) synthesis via femtosecond laser ablation of a solution precursor. The ability to co-dope GO with B/N via femtosecond laser has been discussed in our previous work [16]. The motivation for the B/N co-doping is three-fold. First, doping allows control over the charge carrier concentration and control over the electrical properties. Second, B/N co-doping was demonstrated to increase the bandgap in graphene-based materials [16], so it should potentially allow control over the I_ON/I_OFF ratio [12]. Lastly, forming the GO gel reduces the chemical reactivity of the gel by removing some of the GBs [17] and lowers its appeal in multiple applications (e.g. sensors). Therefore, the B/N are introduced to restore some of the chemical reactivity [18].

The electron and hole mobilities of the fabricated B/N-GO gels were measured in large-area (∼100 × 2400 μm²) back-gated FETs, with average values up to 9000 ± 3000 cm²/V and 5000 ± 2600 cm²/V, respectively. The fabricated gel FETs showed an I_ON/I_OFF ratio of up to 9.7E+5, demonstrating a great potential for future FET applications.