Influence of graphene oxide on metal-insulator-semiconductor tunneling diodes
© Lin et al. 2012
Received: 30 November 2011
Accepted: 26 June 2012
Published: 1 August 2012
In recent years, graphene studies have increased rapidly. Graphene oxide, which is an intermediate product to form graphene, is insulating, and it should be thermally reduced to be electrically conductive. We herein describe an attempt to make use of the insulating properties of graphene oxide. The graphene oxide layers are deposited onto Si substrates, and a metal-insulator-semiconductor tunneling structure is formed and its optoelectronic properties are studied. The accumulation dark current and inversion photocurrent of the graphene oxide device are superior to the control device. The introduction of graphene oxide improves the rectifying characteristic of the diode and enhances its responsivity as a photodetector. At 2 V, the photo-to-dark current ratio of the graphene oxide device is 24, larger than the value of 15 measured in the control device.
Metal-oxide-semiconductor field-effect transistors (MOSFETs) are frequently used in daily life. Strictly speaking, it is more precise to name such structures as metal-insulator-semiconductor field-effect transistors (MISFETs) since oxides represent only one class of the various insulators used today. To maximize the packing density and increase the operation speed of integrated circuits, transistor size must be reduced. The insulator thickness decreases as the dimension of a transistor becomes smaller, resulting in a significant gate tunneling current . Such a tunneling current in the vertical direction (from metal to semiconductor or from semiconductor to metal) of a metal-insulator-semiconductor (MIS) structure has been employed in a number of applications, such as microwave devices , flash memory devices , solar cells , and photodetectors . For photodetectors, the thickness of the insulator layer in the MIS tunneling diode is critical. For example, if the insulator layer is too thick, only limited tunneling can occur, leading to a small responsivity. On the contrary, if the insulator layer is too thin, Fermi level pinning may degrade the device current-voltage (IV) characteristics from rectifying behavior to ohmic behavior (or vice versa) [6, 7]. In addition to the thickness dependence, insulator composition also affects the IV characteristics of MIS devices. In this letter, we describe the effect with graphene oxide introduced in the insulator layer in MIS tunneling diodes. Graphene studies increased rapidly following the report of Novoselov et al. in 2004 describing graphene obtained from repeated peeling of graphite . Many methods to prepare graphene have since been reported [9–11]. One method involves chemical oxidation of graphite followed by exfoliation of graphene oxide (GO). These GO layers are insulating and could be thermally reduced to be electrically conductive . Nevertheless, the insulating property of GO may be utilized to certain applications. Each GO layer consists of the pure two-dimensional honeycomb lattice bearing functional groups, and the two-dimensional structure is not present in commonly employed insulators such as amorphous SiO2. Therefore, the influence of the GO layer in the MIS structure will be very interesting. In this study, we demonstrate MIS tunneling diodes with GO in the insulator layer. We deposited the GO layers onto the Si substrates and evaluated these structures based on the IV characteristics. We highlight the simple fabrication process employed to achieve high-performance GO-based MIS devices.
Graphite oxidation (modified Hummers method)
Graphite oxide was prepared using the modified Hummers method [13, 14]. First, H2SO4 (120 mL, 98%) was placed in an ice bath at < 5°C. Then, graphite powder (5 g) and KMnO4 (15 g) was slowly added and stirred in H2SO4 for 2 h. The temperature was controlled so as not to exceed 35°C. Deionized (DI) water (250 mL) was then slowly added to dilute the solution, which was then stirred for 2 h. DI water (700 mL) and H2O2 (20 mL) were then added and stirred for 12 h. The solution was filtered and the slurry on the filter paper was washed with HCl (3%) to remove inorganic impurities. The slurry was then washed with DI water (1 L) to remove the residual acid. Finally, the slurry was dried to yield graphite oxide.
Graphite oxide powder (30 mg) was added to DI water (20 mL). After stirring for 10 min, ultrasonication was performed for 2 h to exfoliate the GO sheets from the multilayer flakes. The solution was then centrifuged at 5,000 rpm for 15 min, and the supernatant was collected. Due to the polar oxygen-containing functional groups, GO sheets were well suspended in the supernatant , which was subsequently used for dip-coating.
Treatment on Si and GO dip-coating
In this study, 1- to 10-Ω cm p-type Si with native oxide was used as the substrates. The substrate was first cleaned and made hydrophilic by treatment with SC1 solution (NH4OH/H2O2/H2O = 1:2:8) for 15 min. The SC1 treatment rendered the surface hydrophilic by attaching polar hydroxyl groups as described in . The GO sheets were then deposited on substrates by dip-coating. The GO deposition was driven by the van der Waals interaction between the hydrophilic substrate and the polar oxygen-containing groups of the GO. Following dip-coating for 40 min, the dried substrate was covered with multilayers of GO sheets. Each single GO sheet might be approximately 1 nm thick as described in . However, the method of chemical exfoliation resulted in multiple-sheet deposition. From the profile of the atomic force microscopy (AFM) image, the thickness of the GO film in the GO device was estimated to be approximately 10 nm.
A 100-nm-thick Al gate with a circular area of 5 × 10−4 cm2 defined by a shadow mask was sputtered onto the GO-deposited side of the Si substrate. Al was also large-scale sputtered onto the back side of the Si substrate to form an ohmic contact. In order to identify the influence of the GO layer on the device characteristics, we also prepared and evaluated a control device without a GO film. White-light LEDs operating at a power density of 0.5 mW/cm2 were the illumination source used for photocurrent measurements.
Results and discussion
At 2 V, the photo-to-dark current ratio of the GO device is 24 compared to the value of 15 obtained for the control device. The photocurrent of the GO device is much larger than that of the control device due to the introduction of GO. Meanwhile, the dark current of the GO device is only slightly increased. Overall, the GO device can achieve a better photo-to-dark current ratio.
In , the photocurrent density of an MIS tunneling diode is approximately 0.03 mA/cm2 under 0.71-mW/cm2 illumination. The photocurrent density of our GO MIS tunneling diode is approximately 3 mA/cm2 under 0.5-mW/cm2 illumination; therefore, a much higher responsivity could be achieved via GO incorporation. Although the smaller dark current of the diode reported in  would result in a better photo-to-dark current ratio, the photo-to-dark current ratio of our GO device could increase if the GO is deposited on a high-quality thermal oxide as opposed to the native oxide.
Although thick GO flakes may prevent the tunneling of carriers, carriers can still flow through the native oxide that it is not covered with thick GO flakes for the GO sample. In other words, the GO sample is only partially covered with GO, and carriers can tunnel via the uncovered regions.
Graphene oxide was deposited on Si to form an Al/graphene oxide/native oxide/Si MIS tunneling diode. With GO insertion, the accumulation dark current and inversion photocurrent are greater than those measured in the control device. The photocurrent of the GO MIS tunneling diode is 1.34 × 10−6 A, while the photocurrent of the control diode is 1.94 × 10−7 A; thus, the GO-based device is a promising candidate for detector applications. Future work can involve optimization of GO coverage in MIS tunneling diodes.
CHL is an assistant professor in the Department of Opto-Electronic Engineering, National Dong Hwa University. WTY and CHC are currently working toward their master degree in the Department of Opto-Electronic Engineering, National Dong Hwa University. CCL is an assistant professor in the Department of Electrical Engineering in the same school.
The authors are grateful to the National Nano Device Laboratories of the Republic of China for the facility access and the National Center for High-Performance Computing of the Republic of China for the computer time and facility access. This work was supported by the National Science Council of the Republic of China under contract no. 98-2221-E-259-002-MY3.
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