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Register a new userElectron Transport across Vertical Silicon / MoS${_2}$ / Graphene Heterostructures: Towards Efficient Emitter Diodes for Graphene-Base Hot Electron Transistors
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Heterostructures comprising of silicon (Si), molybdenum disulfide (MoS${_2}$) and graphene are investigated with respect to the vertical current conduction mechanism. The measured current-voltage (I-V) characteristics exhibit temperature dependent asymmetric current, indicating thermally activated charge carrier transport. The data is compared and fitted to a current transport model that confirms thermionic emission as the responsible transport mechanism across the devices. Theoretical calculations in combination with the experimental data suggest that the heterojunction barrier from Si to MoS${_2}$ is linearly temperature dependent for T = 200 to 300 K with a positive temperature coefficient. The temperature dependence may be attributed to a change in band gap difference between Si and MoS${_2}$, strain at the Si/MoS${_2}$ interface or different electron effective masses in Si and MoS${_2}$, leading to a possible entropy change stemming from variation in density of states as electrons move from Si to MoS${_2}$. The low barrier formed between Si and MoS${_2}$ and the resultant thermionic emission demonstrated here makes the present devices potential candidates as the emitter diode of graphene-base hot electron transistors for future high-speed electronics.
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Graphene/silicon heterostructures have attracted tremendous interest as a new platform for diverse electronic and photonic devices such as barristors, solar cells, optical modulators, and chemical sensors. The studies to date largely focus on junctions between graphene and lightly-doped silicon, where a Schottky barrier is believed to dominate the carrier transport process. Here we report a systematic investigation of carrier transport across the heterojunctions formed between graphene and highly-doped silicon. By varying the silicon doping level and the measurement temperature, we show that the carrier transport across the graphene/p++-Si heterojunction is dominated by tunneling effect through the native oxide. We further demonstrate that the tunneling current can be effectively modulated by the external gate electrical field, resulting in a vertical tunneling transistor. Benefited from the large density of states of highly doped silicon, our tunneling transistors can deliver a current density over 20 A/cm2, about two orders of magnitude higher than previous graphene/insulator/graphene tunneling transistor at the same on/off ratio.
We propose and analyze the concept of the vertical hot-electron terahertz (THz) graphene-layer detectors (GLDs) based on the double-GL and multiple-GL structures with the barrier layers made of materials with a moderate conduction band off-set (such as tungsten disulfide and related materials). The operation of these detectors is enabled by the thermionic emissions from the GLs enhanced by the electrons heated by incoming THz radiation. The electron heating is primarily associated with the intraband absorption (the Drude absorption). We calculate the responsivity and detectivity as functions of the photon energy, GL doping, and the applied voltage for the GL detectors (GLDs) with different number of GLs. The detectors based on the cascade multiple-GL structures can exhibit a substantial photoelectric gain resulting in the elevated responsivity and detectivity. The advantages of the THz detectors under consideration are associated with their high sensitivity to the normal incident radiation and efficient operation at room temperature at the low end of the THz frequency range. Such GLDs with a metal grating, supporting the excitation of plasma oscillations in the GL-structures by the incident THz radiation, can exhibit a strong resonant response at the frequencies of several THz (in the range, where the operation of the conventional detectors based on A$_3$B$_5$ materials, in particular THz quantum-well detectors, is hindered due to a strong optical phonon radiation absorption in such materials).
We report on room temperature THz detection by means of antenna-coupled field effect transistors fabricated by using epitaxial graphene grown on silicon carbide substrate. Two independent detection mechanisms are found: plasma wave assisted-detection and thermoelectric effect, which is ascribed to the presence of junctions along the FET channel. The superposition of the calculated functional dependence of both the plasmonic and thermoelectric photovoltages on the gate bias qualitatively well reproduces the measured photovoltages. Additionally, the sign reversal of the measured photovoltage demonstrates the stronger contribution of the plasmonic detection compared to the thermoelectric mechanism. Although responsivity improvement is necessary, these results demonstrate that plasmonic detectors fabricated by epitaxial graphene on silicon carbide are potential candidates for fast large area imaging of macroscopic samples.
Palladium diselenide (PdSe2) is a recently isolated layered material that has attracted a lot of interest for the pentagonal structure, the air stability and the electrical properties largely tunable by the number of layers. In this work, PdSe2 is used in the form of multilayer as the channel of back-gate field-effect transistors, which are studied under repeated electron irradiations. Source-drain Pd leads enable contacts with resistance below 350 kOhm um. The transistors exhibit a prevailing n-type conduction in high vacuum, which reversibly turns into ambipolar electric transport at atmospheric pressure. Irradiation by 10 keV electrons suppresses the channel conductance and promptly transforms the device from n-type to p-type. An electron fluence as low as 160 e-/nm2 dramatically change the transistor behavior demonstrating a high sensitivity of PdSe2 to electron irradiation. The sensitivity is lost after few exposures, that is a saturation condition is reached for fluence higher than 4000 e-/nm2. The damage induced by high electron fluence is irreversible as the device persist in the radiation-modified state for several hours, if kept in vacuum and at room temperature. With the support of numerical simulation, we explain such a behavior by electron-induced Se atom vacancy formation and charge trapping in slow trap states at the Si/SiO_2 interface.
We theoretically study the inelastic scattering rate and the carrier mean free path for energetic hot electrons in graphene, including both electron-electron and electron-phonon interactions. Taking account of optical phonon emission and electron-electron scattering, we find that the inelastic scattering time $tau sim 10^{-2}-10^{-1} mathrm{ps}$ and the mean free path $l sim 10-10^2 mathrm{nm}$ for electron densities $n = 10^{12}-10^{13} mathrm{cm}^{-2}$. In particular, we find that the mean free path exhibits a finite jump at the phonon energy $200 mathrm{meV}$ due to electron-phonon interaction. Our results are directly applicable to device structures where ballistic transport is relevant with inelastic scattering dominating over elastic scattering.
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