Recent Advances in Graphene Homogeneous p–n Junction for Optoelectronics

Abstract

Graphene has been widely used as electrodes and active layers in optoelectronics due to its diverse excellent performances, such as high mobility, large thermal conductivity and high specific surface area etc. Methodology for constructing p-n junction has becoming an important consideration in improving the performance of optoelectronic devices and broadening of its application in related fields. Currently, graphene based p-n junction has been explored and different structures have also been investigated. This review paper summaries the recent progress on graphene homogeneous p-n junction, ranging from preparation of front-end materials (e.g. p-and n-type graphene) to building of planar and vertical p-n junctions. Furthermore, p-n junction via electrical modulation is described. The requirements for building the graphene homogeneous p-n junction, and the advantages and drawbacks of the different structures of the p-n junction are also discussed. Finally, a preferential technique to fabricate high performance p-and n-type graphene and building of the p-n junction is evaluated. This paper therefore provides an important indication on the future direction on the application of graphene in optoelectronics. 2 ranging applications, such as logical rectifier circuit [10,11] , field-effect optoelectronic transistor [12-14] , multimode non-volatile memory [15] , rectifier memory [16,17] , optoelectronic storage [18,19] , photovoltaic device [20] , photodetector [21,22] and gas sensor [23,24] etc. Consequently, it is obvious to study new material system for the p-n junction that will enhance the performance of current devices and also lead to the development of new devices. In general, p-n junction can be categorized into heterojunction and homojunction. If same materials are used, the p-n junction is known as homogeneous junction or homojunction. There are many advantages of homogeneous p-n junction, for example, there is no issue on lattice mismatch, which will result in surface defects, since p and n layers are of the same materials. Moreover, the use of the same materials in homogeneous p-n junction leads to uniform energy band gap across the device structure, as shown in Figure 1a. Depletion region is formed at the junction between p and n regions, and the alignment of Fermi level (E f) promotes band bending at the depletion region when the same semiconducting materials are used to construct the p-n junction, as represented in Figure 1b. Such homogeneous p-n junction is of great significance in the field of optoelectronics. Figure 1c illustrates the charge transport mechanism under illumination. Upon radiation of light with certain wavelength (hυ ≧ Eg), electrons jump to conduction band in p, n and depletion regions. These electrons and holes arriving at the depletion region are subsequently swept into n-type and p-type regions respectively via the built-in field. As a result, current is generated and a certain voltage is produced due to the accumulation of charges under open circuit conditions. Furthermore, this lead...

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