| Recently, since combined unique quantum confinement effect of quantum dots (QDs) with ultrahigh carrier mobility of graphene, the QDs/graphene hybrid structures demonstrate excellent photoresponse properties in the field of photodetectivity and become research focus.In this paper, Ge QDs were successfully deposited on graphene/SiO2/Si by using ion beam sputtering deposition (IBSD), as well as the Ge QDs/graphene hybrid structure photodetection was prepared. The element information of Ge atoms on graphene surface was indentified by X-ray photoelectron spectroscopy (XPS). On the other hand, the effect of growth parameters on surface morphologies of samples and the interaction between Ge QDs and graphene were characterized by atomic force microscope (AFM) and Raman spectra. The formation of Ge islands and the adsorption of Ge atoms on graphene surface were investigated by the so-called Zinsmeiser theory and the first principles, respectively. Last, the influence of light power and gate voltage on the responsivity of Ge QDs/graphene field effect transistor (FET) photodection was measured by optoelectrnic tests. The work principle of the device was also discussed.The results show that there is a large impact of growth parameters on morphology of Ge QDs grown on graphene. At room temperature, when the sputtering time is relatively short, the strength of interaction between Ge and graphene is strong since the doping effect turns to be dominant, while more defects were induced in graphene. With increasing sputtering time of Ge layer, the sizes of Ge QDs on graphene shrink and the dots density increase, but the strength of interaction between Ge and graphene is lowered. Compared with that in samples grown at room temperature, the sizes and density of Ge QDs on graphene both decrease, the morphological uniformity and crystalline quality of Ge QDs are improved when growth temperature is relatively high. Moreover, it is found that there is a bond formation between Ge and C atoms in the sample grown at 500℃. This sample appears the typical photoluminescence (PL) peak located at 1500-1850nm range that iss belonged to the information of Ge QDs, indicating the obvious quantum effect of Ge dots.The time dependence of Ge islands morphology was qualitatively explained by using the so-called Zinsmeister theory. Under the assumptions that the whole sputtering process is not influnced by the temperature, atomic collision and energy, it shows that with the increase of sputtering time, the sizes and density of Ge QDs shrink and enhance, respectively. The first principles calculated results demonstrate the 3D islands growth of Ge on graphene. By comparing different coverage cases, it is found that the interaction between Ge and graphene is stronger and the structural change of graphene is larger when the coverage of Ge is smaller. Charge density difference results reveal the nature of the interaction further, i.e., the charges transfer between Ge and graphene is the main reason for the formation of the interaction of Ge QDs/graphene.Based on the Ge QDs/graphene hybrid structures grown by using EBSD, we fabricated the FET photodetectors. The output, transfer characteristics and responsivity were characterized. The device demonstrates the responsivity of 4.3AW"1 at 808nm infrared light irradiation, and relatively large value of β in absolute,0.92. By using traditional FET theory, the effect of light irradiation on responsivity was built, and an ultrahigh responsivity of 1012 AW-1 is predicted at ultra-low irradiation cases. The optoelectronic features indicate that the device based on Ge QDs/graphene hybrid structure fabricated by IBSD can improve the efficiency of carrier transfer largely, and overcome the limit of ligands barrier at the interface of Ge/graphene. Our results can supply a prototype for fabricating novel optoelectronic devices based on the QDs/graphene hybrid nanostructures and provide ways for improvement in responsivity of the novel optoelectronics. |
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