Silicene Field Effect Transistor Research Articles

2D Honeycomb Silicon: A Review on Theoretical Advances for Silicene Field-Effect Transistors

Catalysed by the success of mechanical exfoliated free-standing graphene, two dimensional (2D) semiconductor materials are successively an active area of research. Silicene is a monolayer of silicon (Si) atoms with a low-buckled honeycomb lattice possessing a Dirac cone and massless fermions in the band structure. Another advantage of silicene is its compatibility with the Silicon wafer fabrication technology. To effectively apply this 2D material in the semiconductor industry, it is important to carry out theoretical studies before proceeding to the next step. In this paper, an overview of silicene and silicene nanoribbons (SiNRs) is described. After that, the theoretical studies to engineer the bandgap of silicene are reviewed. Recent theoretical advancement on the applications of silicene for various field-effect transistor (FET) structures is also discussed. Theoretical studies of silicene have shown promising results for their application as FETs and the efforts to study the performance of bandgap-engineered silicene FET should continue to improve the device performance.

Silicene, silicene derivatives, and their device applications.

Silicene, the ultimate scaling of a silicon atomic sheet in a buckled honeycomb lattice, represents a monoelemental class of two-dimensional (2D) materials similar to graphene but with unique potential for a host of exotic electronic properties. Nonetheless, there is a lack of experimental studies largely due to the interplay between material degradation and process portability issues. This review highlights the state-of-the-art experimental progress and future opportunities in the synthesis, characterization, stabilization, processing and experimental device examples of monolayer silicene and its derivatives. The electrostatic characteristics of the Ag-removal silicene field-effect transistor exhibit ambipolar charge transport, corroborating with theoretical predictions on Dirac fermions and Dirac cone in the band structure. The electronic structure of silicene is expected to be sensitive to substrate interaction, surface chemistry, and spin-orbit coupling, holding great promise for a variety of novel applications, such as topological bits, quantum sensing, and energy devices. Moreover, the unique allotropic affinity of silicene with single-crystalline bulk silicon suggests a more direct path for the integration with or revolution to ubiquitous semiconductor technology. Both the materials and process aspects of silicene research also provide transferable knowledge to other Xenes like stanene, germanene, phosphorene, and so forth.

Current saturation and kink effect in zero-bandgap double-gate silicene field-effect transistors

Double gate silicene field effect transistor is investigated using Density Functional Theory (DFT) and Non-Equilibrium Green's Function (NEGF) formalism. The results suggest that with an increase in gate bias, bandgap is introduced in silicene which results in reduction in device current. The increase in silicene bandgap is also related to the reduction in channel length. It is observed that drain to source current (IDS) saturates on increasing drain to source voltage (VDS). On increasing VDS beyond saturation region, at some value of VDS kink effect is seen which is due to switching in the type of carriers at the drain end due to ambipolar channel. Transconductance (gm) is seen to reduce with reduction in channel length, however, gm improves with reduced oxide thickness due to better gate controllability. The output characteristics do not change much with oxide thickness.

Spin-valley caloritronics in silicene near room temperature

Two-dimensional silicene, with an observable intrinsic spin-orbit coupling, has a great potential to perform fascinating physics and new types of applications in spintronics and valleytronics. By introducing an electromotive force from a temperature difference in ferromagnetic silicene, we discover that a longitudinal spin Seebeck effect can be driven even near room temperature, with spin-up and spin-down currents flowing in opposite directions, originating from the asymmetric electron-hole spin band structures. We further propose a silicene field-effect transistor constructed of two ferromagnetic electrodes and a central dual-gated region, and find that a valley Seebeck effect appears, with currents from two different valleys flowing in opposite directions. The forbidden transport channels are determined by either spin-valley dependent band gaps or spin mismatch. By tuning the electric field in the central region, the transport gaps depending on spin and valley vary correspondingly, and a transition from valley Seebeck effect to spin Seebeck effect is observed. These spin-valley caloritronic results near room temperature are robust against many real perturbations, and thus suggest silicene to be an excellent candidate for future energy-saving technologies and bidirectional information processing in solid-state circuits.

Completely independent electrical control of spin and valley in a silicene field effect transistor

One-atom-thick silicene is a silicon-based hexagonal-lattice material with buckled structure, where an electron fuses multiple degrees of freedom including spin, sublattice pseudospin and valley. We here demonstrate that a valley-selective spin filter (VSSF) that supports single-valley and single-spin transport can be realized in a silicene field effect transistor constructed of an npn junction, where an antiferromagnetic exchange field and a perpendicular electric field are applied in the p-doped region. The nontrivial VSSF property benefits from an electrically controllable state of spin-polarized single-valley Dirac cone. By reversing the electric field direction, the device can operate as a spin-reversed but valley-unreversed filter due to the dependence of band gap on spin and valley. Further, we find that all the possible spin-valley configurations of VSSF can be achieved just by tuning the electric field. Our findings pave the way to the realization of completely independent electrical control of spin and valley in silicene circuits.

Electrically tunable spin polarization in silicene: A multi-terminal spin density matrix approach

Recent realized silicene field-effect transistor yields promising electronic applications. Using a multi-terminal spin density matrix approach, this paper presents an analysis of the spin polarizations in a silicene structure of the spin field-effect transistor by considering the intertwined intrinsic and Rashba spin–orbit couplings, gate voltage, Zeeman splitting, as well as disorder. Coexistence of the stagger potential and intrinsic spin–orbit coupling results in spin precession, making any in-plane polarization directions reachable by the gate voltage; specifically, the intrinsic coupling allows one to electrically adjust the in-plane components of the polarizations, while the Rashba coupling to adjust the out-of-plan polarizations. Larger electrically tunable ranges of in-plan polarizations are found in oppositely gated silicene than in the uniformly gated silicene. Polarizations in different phases behave distinguishably in weak disorder regime, while independent of the phases, stronger disorder leads to a saturation value.

Silicene field effect transistor with high on/off current ratio and good current saturation

We investigate theoretically the possibility of exploiting the electrically tunable band gap property of silicene to achieve field effect transistor with improved characteristics. We find that the silicene field effect transistor where a band gap is introduced through a perpendicular electric field shows a subthreshold swing smaller than 60 mV/decade and a switching effect with high on/off current ratio exceeding $$10^{5}$$105. We find also that the device output characteristic displays a very good saturation due to improved pinch-off of the channel, stemming from the electrically induced band gap.

Silicene field-effect transistors operating at room temperature.

Free-standing silicene, a silicon analogue of graphene, has a buckled honeycomb lattice and, because of its Dirac bandstructure combined with its sensitive surface, offers the potential for a widely tunable two-dimensional monolayer, where external fields and interface interactions can be exploited to influence fundamental properties such as bandgap and band character for future nanoelectronic devices. The quantum spin Hall effect, chiral superconductivity, giant magnetoresistance and various exotic field-dependent states have been predicted in monolayer silicene. Despite recent progress regarding the epitaxial synthesis of silicene and investigation of its electronic properties, to date there has been no report of experimental silicene devices because of its air stability issue. Here, we report a silicene field-effect transistor, corroborating theoretical expectations regarding its ambipolar Dirac charge transport, with a measured room-temperature mobility of ∼100 cm(2) V(-1) s(-1) attributed to acoustic phonon-limited transport and grain boundary scattering. These results are enabled by a growth-transfer-fabrication process that we have devised--silicene encapsulated delamination with native electrodes. This approach addresses a major challenge for material preservation of silicene during transfer and device fabrication and is applicable to other air-sensitive two-dimensional materials such as germanene and phosphorene. Silicene's allotropic affinity with bulk silicon and its low-temperature synthesis compared with graphene or alternative two-dimensional semiconductors suggest a more direct integration with ubiquitous semiconductor technology.

Can a silicene transistor be realized?

In the past decade a new exciting class of materials has been developed, which is not three-dimensional, but only two-dimensional in nature. Graphene is by far the most famous example of this new class of materials. Graphene exhibits a wealth of exotic and intriguing properties, which has resulted in a myriad of scientific breakthroughs. However, graphene also suffers from a severe drawback: it is gapless, implying that a graphene based field-effect transistor is not within reach. Silicene, the silicon analog of graphene, is in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the silicene lattice is slightly buckled and composed of two vertically displaced sub-lattices. By breaking the sub-lattice symmetry a band gap can be opened, which would in principle allow the realization of a silicene field-effect transistor. This piece introduces the various band engineering options for silicene as well as additional hurdles that will need to be overcome before the door can be opened to a silicene transistor

Tunable Bandgap in Silicene and Germanene

By using ab initio calculations, we predict that a vertical electric field is able to open a band gap in semimetallic single-layer buckled silicene and germanene. The sizes of the band gap in both silicene and germanene increase linearly with the electric field strength. Ab initio quantum transport simulation of a dual-gated silicene field effect transistor confirms that the vertical electric field opens a transport gap, and a significant switching effect by an applied gate voltage is also observed. Therefore, biased single-layer silicene and germanene can work effectively at room temperature as field effect transistors.