Origin of Monochromatic Electron Emission From Planar-Type Graphene/h-BN/n-Si Devices

We previously reported highly monochromatic electron emission from the planar-type electron emission devices based on a graphene/hexagonal boron nitride (h-BN) heterostructure. In this paper, the electron energy distribution (EED) of these devices is examined to clarify the mechanism of monochromatic electron emission. We find that the monochromaticity of the electron beam depends significantly on the electronic structure of the substrate material; for the devices with an n-type silicon substrate, the narrowest FWHM of the electron beam is 0.18 eV, whereas that of devices with a metallic (Nb) substrate is 0.33 eV. At the same time, simulations considering the electron scattering by phonons acceptably reproduced the shape of each EED spectrum considering the small energy loss due to out-of-plane acoustic phonon modes in h-BN. Thus, the monochromatic electron emission from the $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$ device is ascribed to a combination of the narrow energy distribution of electrons at the conduction band of the n-$\mathrm{Si}$ substrate and small phonon energy of the h-BN insulating layer. These features also realize the excellent emission properties in addition to the monochromaticity of the beam, such as a high emission current density of $9.3{\mathrm{A}/\mathrm{cm}}^2$, insensitivity to environmental pressure up to 10 Pa, and long lifetime of more than 7 days with little decay.

Figure 1

Schematic diagram of the measurement system for the electron emission properties of graphene/h-BN electron emission devices.

Figure 2

Simulation model of planar-type electron emission devices with a structure of $\mathrm{graphene}/h$-$\mathrm{BN}/\mathrm{Nb}$ or $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$.

Figure 3

(a) Electron emission characteristics and (b) FN plots of anode current for the $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$ device. (c) Electron emission characteristics and (d) FN plots of anode current for the $\mathrm{graphene}/h$-$\mathrm{BN}/\mathrm{Nb}$ device. Blue dashed, red solid, and green dashed lines indicate cathode current, anode current, and emission efficiency, respectively.

Figure 4

Time drift of anode current for the $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$ emission device during dc operation (gate voltage: 30 V).

Figure 5

Anode current density for the $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$ device at different pressures.

Figure 6

Comparison of experimental (dots), simulated (solid line) electron energy distribution (EED), electron distribution in the substrate (initial EED, dashed lines) spectra for graphene/h-BN devices with (a) n-$\mathrm{Si}$ and (b) Nb substrates. Fitting parameters ħω and λ_i are 0.04 eV and 1.0 nm, respectively. (c) Comparison of experimental and simulated EED spectra for a $\mathrm{graphene}/{\mathrm{Si}\mathrm{O}}_2/n$-$\mathrm{Si}$ structure. Fitting parameters are ħω  = 0.15 [eV] and λ_i = 0.9 [nm].

Figure 7

Energy spectrum of emitted electrons from a $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$ emission device at a gate voltage of 35 V.