We have fabricated a diode with a multiple stacked Si-QDs structure and a single layer graphene top electrode, and studied their electron field emission properties. Electron emission current from the diode was observed by the application of forward biases of 6.0 V and over, which is ~1.0 V lower than that from a diode with a Au top electrode. In addition, we found that electron emission efficiency was increased by a factor of ~ 100 by replacing the top electrode from Au to graphene. This result is explained by the reduction of electron scattering at the top electrode. The results lead to the development of high efficiency planar-type electron emission devices.

Introduction Electron field emission from Si-based nanocrystals has been stimulated considerable interest because electron emission devices are expected to be used for various applications [1, 2]. In our previous work, we have demonstrated electron field emission from multiple-stacked Si-quantum-dots (Si-QDs) structures with ultrathin Au top electrodes under biases of 6V and over, in which electric field concentrates on the upper dot layers [3]. To reduce electron scattering in top electrodes, we fabricated the multiple stacked Si-QDs structures with single layer graphene top electrodes and evaluated their electron emission properties. Experimental After conventional wet chemical cleaning steps, n-type Si(100) wafers were oxidized at 1000 °C in dry O2 to form ~3.0 nm thick SiO2. After that, Si-QDs with an areal dot density as high as ~4.7×1011 cm−2 and an average dot size of ~4.5 nm were formed on SiO2 by low pressure chemical vapor deposition (LPCVD) using pure SiH4, followed by thermal oxidation to cover the surface of the dots with ~1.5 nm-thick SiO2. By repeating such a process sequence, 11-fold stacked Si-QDs layers were formed. Single layer graphene was formed on a Cu foil by solid-source LPCVD using Poly-styrene. After spin coating poly methyl methacrylate (PMMA) on the graphene, the Cu film and residual Fe were removed by dipping in the Fe(NO3)3·9H2O solution and nitric acid solution (HNO3: H2O = 1:5), respectively. After that, PMMA/graphene was transferred on the multiple stacked Si-QDs layers. Subsequently, the PMMA was removed by dipping in acetone and IPA. We also fabricated ~10 nm-thick Au films as top electrodes on the 11-fold stacked Si-QDs by electron beam (EB) evaporation as a reference. Finally, Al films were formed as bottom electrodes by thermal evaporation. Results and Discussion When forward bias (|V s|) exceeds ~6.0 V, electron emission current (I e) from the multiple stacked Si-QDs with the graphene top electrode appears. In addition, with an increase in the |V s|, the I e was exponentially increased as in the case of the Au top-electrodes. It is interesting to note that the I e for the sample with the graphene top electrode was larger than that for the sample with the Au top electrode although the sample current (I s) was suppressed by using graphene as the top electrode (Fig.1). This result implies reduction of electron tunneling current, which can be interpreted in terms of density-of-state difference between the graphene and Au. In addition, an effective contact area of the graphene might be smaller than that of a designed value. We also summarized emission efficiencies as a function of |V s|, as shown Fig. 2. Obviously, in both cases, with an increase in the |V s|, emission efficiencies were increased exponentially. Notice that, by using the graphene top electrode, the emission efficiency was increased by a factor of ~100 compared with that of the sample with the Au top electrode under the same |V s|. This result indicates that, by using a single layer graphene as a top electrode, electron emission efficiency can be enhanced by the reduction of electron scattering within the top electrode. Conclusion The graphene top electrode plays a role in high efficiency electron emission from the stacked structures of Si-QDs due to the reduction of electron scattering within the top electrode. Acknowledgment This work was supported in part by Grant-in-Aid for Scientific Research (A) No. 19H00762 of MEXT Japan. The formation of the graphene top electrode was supported by associate professor Karita Gorap at Nagoya Institute of Technology. References [1] N. Koshida et al., Jpn. J. Appl. Phys., 34 (1995) L705. [2] N. Mori et al., Appl. Phys. Lett., 98 (2011) 062104. [3] D. Takeuchi et al., Thin Solid Films, 602 (2016) 68. [4] K. Golap et al., Appl. Phys. Lett., 111 (2017) 013504. Figure 1