Artificial Graphene on Si Substrates: Fabrication and Transport Characteristics

Abstract

Artificial graphene (AG) based on a honeycomb lattice of semiconductor quantum dots (QDs) has been of great interest for exploration and applications of massless Dirac Fermions in semiconductors thanks to the tunable interplay between the carrier interactions and the honeycomb topology.Here, an innovative strategy to realize AG on Si substrates is developed by fabricating a honeycomb lattice of Au nanodisks on a Si/GeSi quantum well. The lateral potential modulation induced by the nanoscale Au/Si Schottky junction results in the formation of quantum dots arranged in a honeycomb lattice to form AG. Nonlinear current−voltage curves of the AG reveal conductance phase transitions with switch on/off voltages, a large electric hysteresis loop, and a strong sharp current peak accompanied by a group of differential-conductance peaks and negative differential conductance around the switch-on voltage, which can be modulated by temperature and light. These features are interpreted by a model based on the Coulomb blockade effect, the collective resonant tunneling, and the coupling of holes in the AG. Our results not only demonstrate an approach to the formation but also will greatly stimulate the characterizations and the applications of innovative semiconductor-based AG.

Carrier Dynamics for the Collective Resonant Tunneling in Quantum Dot-Based Artificial Graphene

“…It reveals more details about the CRT associated with the strong coupling of QDs and network topology in a hexagonal lattice. The group of d I /d V peaks results from the progressive alignment of energy levels of minibands in QDs of AG with sweeping-up voltage . In addition, the different amplitudes of the d I /d V peaks demonstrate that the densities of states composing the miniband of AG are not uniform.…”

“…The progressive alignment of minibands in neighboring QDs with sweeping voltages leads to the resonant tunneling of holes between QDs, which results in the dramatic increase in current at voltage V on and a current peak at voltage V on + Δ V . The miniband width is proportional to Δ V , and the average number q r of holes in the QD of AG for the CRT at voltage V on + Δ V can be estimated by q normalr ( V ) ≈ true C V P L e , .25em C = 4 π ε normals normali R , .25em V = V normalo normaln + normalΔ V where C is the self-capacitance of the QD, ε si is the dielectric constant of Si, R is the radius of the QD, P is the effective period of QDs, L is the distance between two electrodes, and e is the elementary charge. Obviously, the voltage V on + Δ V for the current peak increases with the sweeping rate.…”

“…These features have been explained in terms of the Coulomb blockade effect, the CRT, and the coupling of holes in QD-based AG. 28 For simplicity, the intradot interaction and interdot coupling of multiple holes in QDs of AG give rise to the minibands composed of a few states. The details of the minibands in the momentum space are in need of further studies.…”

“…The group of dI/dV peaks results from the progressive alignment of energy levels of minibands in QDs of AG with sweeping-up voltage. 28 In addition, the different amplitudes of the dI/dV peaks demonstrate that the densities of states composing the miniband of AG are not uniform. More interestingly, the voltage gap ΔV, corresponding to the group of dI/dV peaks, increases with the voltage sweeping rate.…”

“…Given the nanoscale metal/semiconductor Schottky contact, that is, Au nanodisk/Si contact, a nanoscale depletion region with modified energy bands appears in the Si/Ge 0.38 Si 0.62 /Si heterostructure underneath the Au nanodisk. It gives rise to the formation of a QD in the GeSi QW, 28 as denoted by the dark-blue region in the light-blue QW in Figure 1a. The Au nanodisks in a honeycomb lattice naturally result in QD-based AG.…”