Junction investigation of graphene/silicon Schottky diodes
© Mohammed et al.; licensee Springer. 2012
Received: 16 December 2011
Accepted: 20 April 2012
Published: 11 June 2012
Here we present a facile technique for the large-scale production of few-layer graphene flakes. The as-sonicated, supernatant, and sediment of the graphene product were respectively sprayed onto different types of silicon wafers. It was found that all devices exhibited current rectification properties, and the supernatant graphene devices have the best performance. Schottky junctions formed between graphene flakes and silicon n-type substrates exhibit good photovoltaic conversion efficiency while graphene/p-Si devices have poor light harvesting capability.
Since the first demonstration of graphene nanosheets in 2004, it has been considered as a chemically stable and mechanically strong new material. Tremendous work has been devoted to this material because it has exhibited outstanding properties particularly in the optoelectronic field. The two dimensional honeycomb lattice of the graphene leads to a hybridization of sp2 which, in turn, leads to extraordinary electrical properties with ultrahigh carrier mobility (approximately 100,000 cm2/Vs). A monolayer of graphene has a thickness of 0.34 nm and absorbs 2.3 % of white light; even graphene layers of 1,000 nm thick still have a transparency of approximately 70 %, which makes it possible to use graphene as transparent electrodes. Graphene has the advantages over carbon nanotubes of being naturally compatible with thin film processing, enabling large device areas and hence, high operating powers. Also, graphene is more readily scalable and has a lower contact resistance. The combination of these enticing electrical and optical properties of the graphene motivated the researchers to experience it in the field of optoelectronic devices.
Previous use of graphene in organic solar cell applications was mainly confined as flexible transparent electrodes to replace transparent indium tin oxide or fluorine-doped tin oxide for collecting charge carriers. Recently, graphene-on-silicon configurations were made into solar cells by using membrane transfer technique. However, the fabrication process based on membrane transfer is expensive and difficult to scale up. The previous studies have explored electron transport in graphene; the Schottky barriers between graphene and silicon have not been studied thoroughly. The Schottky barriers have been observed at bulk, highly ordered pyrolytic graphite/silicon interfaces, but no photocurrents could be measured; a comparison of n- and p-type substrates was not given in this prior work. Moreover, the local effect of light absorption on the J-V characteristics of graphene/silicon interfaces has not been studied.
In this work we present a facile technique for the large-scale production of few-layer graphene flakes. Next, as-sonicated (So), supernatant (Su) and sediment (Se) of the graphene product were respectively sprayed onto n-/p-type silicon wafers. The current rectification properties of the formed Schottky junctions between different graphene sources and silicon substrates were compared.
The graphene flakes were synthesized through chemical vapor deposition of acetylene on a MgO-supported Fe-Co bimetallic catalyst (Fe-Co/MgO with a stoichiometric composition of 2.5:2.5:95 wt%). The catalyst was prepared by using the impregnation technique. Initially, the weighted amounts of Fe(NO3)3·9H2O and Co(NO3)2·6H2O were dissolved in ethanol under agitation. Subsequently, the MgO powder with a surface area of 130 m2/g was mixed the solution and followed by drying at 60°C overnight. The catalyst was obtained by calcinating the resulting mixture in air at 500°C for 2 h. Graphene sheets can grow on the catalyst system from pyrolysis of acetylene at 1,000°C with the argon flow as carrier gas. The mixture product can be collected after 30 min of reaction and cooled under argon flow for about 10 min. Impurities like catalystsupport MgO and Fe-Co metal particles can be removed by washing the mixture product with hydrochloric acid under sonication. The purified graphene sheets can be obtained after filtration and washing.
To characterize the morphological properties of graphene nanosheets, several techniques such as microscopy and X-ray diffraction (XRD) were utilized. Atomic force microscopy (AFM) images were obtained on Veeco Dimension 3100 AFM system (Veeco Instruments, Inc., NY, USA). Scanning electron microscopy (SEM) images were obtained using a JEOL 7000 F high-resolution scanning electron microscope (JEOL Ltd., Tokyo, Japan). This microscope has a resolution of 1.2 nm at an accelerating voltage of 15 kV and a working distance of 10 mm. The final products were mounted on aluminum pins with double-sided carbon tape and their corresponding SEM images were obtained. Elemental analysis was performed with Genesis energy dispersive spectrometer system (EDAX Inc., Mahwah, NJ, USA). The X-ray powder diffraction profiles of graphene sheets were recorded in θ-2θ mode on Bruker D8 Discovery diffraction system (Bruker AXS Corporation, Madison, WI, USA). The monochromatic Cu Kα radiation line and general area detector diffraction system were used as an excitation source and detector, respectively. The experiments were carried out in Bragg-Brentano geometry.
To understand how a different graphene source affect the G/n-Si Schottky junction properties, current density-voltage (J-V) characteristics were investigated in the dark and under illumination using a solar simulator at air mass coefficient 1.5 (approximately 100 mW/cm2) inside a glove box in a nitrogen environment. The illumination was on the graphene flake side. The devices were irradiated in an area of 1 × 1 cm2 and data were recorded using a Keithley 2400 source meter (Keithley Instruments Inc., Cleveland, OH, USA).
Results and discussions
The crystallinity and number of layers in the graphene nanosheets can be analyzed by XRD technique. The XRD profile of the graphene sheets grown by radio frequency catalytic chemical vapor deposition method was shown in Figure2c. The typical features for graphite at 25.3° (002) and 49.1° (004) are identified in this graphene XRD pattern. The 44.7° (100) and 74.7° (110) diffraction peaks originate from the two-dimensional in-plane symmetry along the graphene sheets. The layer-to-layer distance (d-spacing) between two subsequent graphene sheets can be calculated from the (002) diffraction peak position. The width of the diffraction peak can be used to evaluate the crystallite size by using the Scherrer equation (thickness = 0.9 λ/(B cos θ), where λ is the x-ray wavelength, B is the full width at half maximum of the diffraction peak, and θ is the Bragg angle). Based on the values of the d-spacing and the size of crystallite, the graphene sheets in this work were estimated to have in average of about four layers.
Usually graphene flakes, generally known to exhibit weak p-type conductivity, and different Schottky junction solar cells composed of the graphene and n-/p-type Si substrates were evaluated (Figures5a,b). By virtue of the formation of Schottky barrier, excellent rectification characteristics are observed (Figure5a). Moreover, these devices exhibit pronounced photovoltaic effects upon white light illumination. As shown in Figure5b, the device made of p-Si wafer shows the worst performance, with a power conversion efficiency of less than 0.005 %. In contrast, substantial increase of to 0.02 % is observed when graphene flakes are deposited onto n-type silicon wafers. Further increase in the conversion efficiency was observed for devices made of Se and Su graphene flakes. It might be due to the fact that Su graphene has much less impurities which could cause exciton quenching.
Although the photovoltaic conversion efficiency of the G/Si solar cells is relatively low, the photovoltaic performance could be improved by doping graphene with nitrogen in G/p-Si configuration, which could be attributed to the enhanced n-type conductivity of the graphene film at higher N doping concentration, or treating graphene with a strong acid such as SOCl2 or nitric acid in G/n-Si devices. Additionally, increasing the graphene flake size would be helpful for the improvement of photovoltaic conversion of both devices.
We developed chemical vapor deposition approach to the synthesis of graphene at large scale and low cost, which makes the wide applications of graphene possible. The graphene coating on n-Si wafer forms Schottky junction with rectification behavior. The fabrication of Schottky junctions has the merits of low cost and simplicity. By comparing different graphene sources, it was found that the supernatant graphene is the most desirable material for the fabrication of G/n-Si junction with excellent rectifying capability and good photovoltaic conversion efficiency. Although G/p-Si also exhibit rectification behavior, they demonstrate poor photovoltaics conversion efficiency due to the weak p-type conductivity of our graphene flakes.
MM is Ph.D. candidate connected with the Department of Physics and Astronomy, University of Arkansas at Little Rock. ZL is a visiting professor at the Department of Physics and Astronomy, University of Arkansas at Little Rock, JC and TC are the professors at Department of Physics and Astronomy, University of Arkansas at Little Rock.
This work was supported by NSF EPSCOR ASSET II project (EPS-1003970). Thanks are also given to Nanotechnology Center at University of Arkansas at Little Rock for using their facilities. MM gratefully acknowledges the scholarship support from Iraqi government.
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