Current transport in graphene/AlGaN/GaN vertical heterostructures probed at nanoscale

Quantum capacitance of coupled two-dimensional electron gases

Quantum capacitance effect is observed in nanostructured material stacks with quantum limited density of states. In contrast to conventional structures where two-dimensional electron gases (2DEG) with reduced density of states interact with a metal plate, here we explore the quantum capacitance effect in a unique structure formed by two 2DEG in a graphene sheet and AlGaN/GaN quantum well. The total capacitance of the structure depends non-linearly on the applied potential and the linear density of states in graphene leads to enhanced electric field leakage into the substrate causing a dramatic 50% drop in the overall capacitance at low bias potentials. We show theoretical projections of the quantum capacitance effect in the proposed device stack, fabricate the structure and provide experimental verification of the calculated values at various temperatures and applied potentials. The wide swing in the total capacitance is sensitive to the chemical potential of the graphene sheet and has multiple applications in molecular sensing, electro-optics, and fundamental investigations.

Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al_0.5Ga_0.5N/sapphire templates by propane (C₃H₈) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C₃H₈ flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp²/sp³ hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq^-1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.

InGaN islands and thin films grown on epitaxial graphene

In this work, the growth of InGaN on epitaxial graphene by molecular beam epitaxy is studied. The nucleation of the alloy follows a three-dimensional (3D) growth mode in the observed temperature range of 515 °C-765 °C, leading to the formation of dendrite-like islands. Careful Raman scattering experiments show that the graphene underneath is not degraded by the InGaN growth. Moreover, lateral displacement of the nuclei during an atomic force microscopy (AFM) scan demonstrates weak bonding interactions between the InGaN and the graphene. Finally, a longer growth time of the alloy gives rise to a compact thin film in a partial epitaxial relationship with the SiC underneath the graphene.

Conductive Atomic Force Microscopy of Semiconducting Transition Metal Dichalcogenides and Heterostructures

Semiconducting transition metal dichalcogenides (TMDs) are promising materials for future electronic and optoelectronic applications. However, their electronic properties are strongly affected by peculiar nanoscale defects/inhomogeneities (point or complex defects, thickness fluctuations, grain boundaries, etc.), which are intrinsic of these materials or introduced during device fabrication processes. This paper reviews recent applications of conductive atomic force microscopy (C-AFM) to the investigation of nanoscale transport properties in TMDs, discussing the implications of the local phenomena in the overall behavior of TMD-based devices. Nanoscale resolution current spectroscopy and mapping by C-AFM provided information on the Schottky barrier uniformity and shed light on the mechanisms responsible for the Fermi level pinning commonly observed at metal/TMD interfaces. Methods for nanoscale tailoring of the Schottky barrier in MoS₂ for the realization of ambipolar transistors are also illustrated. Experiments on local conductivity mapping in monolayer MoS₂ grown by chemical vapor deposition (CVD) on SiO₂ substrates are discussed, providing a direct evidence of the resistance associated to the grain boundaries (GBs) between MoS₂ domains. Finally, C-AFM provided an insight into the current transport phenomena in TMD-based heterostructures, including lateral heterojunctions observed within Mo_xW_1-xSe₂ alloys, and vertical heterostructures made by van der Waals stacking of different TMDs (e.g., MoS₂/WSe₂) or by CVD growth of TMDs on bulk semiconductors.

Frictional Drag Effect between Massless and Massive Fermions in Single-Layer/Bilayer Graphene Heterostructures

The emergence of two-dimensional layered materials offers an excellent opportunity for the fabrication of double-layer electron systems that are in close proximity but electronically isolated. In this work, heterostructures consisting of one single-layer graphene (SLG) and one bilayer graphene (BLG) separated by an ultrathin hBN layer are successfully fabricated, enabling the experimental investigation of the interlayer frictional drag effect between massless and massive fermions. With varying carrier densities, a giant positive peak of drag response emerges at the double neutrality point, around which nonmonotonic temperature dependent behaviors of drag resistance are further observed. These observations can be attributed to the anticorrelations in the distributions of e-h puddles between layers. More importantly, as the system shifts toward the strong coupling regime, the carrier density dependence of drag resistance R_drag shows a crossover from 1/n³ to 1/n² for the density matched cases, which is a unique characteristic predicted for massless-massive fermion systems. Consequently, a generalized carrier dependent expression (1/(|n_S| + |n_B|)²) is obtained for the strong coupling regime, where n_S and n_B are the carrier densities of SLG and BLG, respectively. Our study provides insight into the electronic frictional effects between massless and massive fermions and thus will promote the investigations of interlayer interactions in hybrid structures hosting different types of carriers.

Probing the uniformity of hydrogen intercalation in quasi-free-standing epitaxial graphene on SiC by micro-Raman mapping and conductive atomic force microscopy

In this paper, micro-Raman mapping and conductive atomic force microscopy (C-AFM) were jointly applied to investigate the structural and electrical homogeneity of quasi-free-standing monolayer graphene (QFMLG), obtained by high temperature decomposition of 4H-SiC(0001) followed by hydrogen intercalation at 900 °C. Strain and doping maps, obtained by Raman data, showed the presence of sub-micron patches with reduced hole density correlated to regions with higher compressive strain, probably associated with a locally reduced hydrogen intercalation. Nanoscale resolution electrical maps by C-AFM also revealed the presence of patches with enhanced current injection through the QFMLG/SiC interface, indicating a locally reduced Schottky barrier height (Φ_B). The Φ_B values evaluated from local I-V curves by the thermionic emission model were in good agreement with the values calculated for the QFMLG/SiC interface using the Schottky-Mott rule and the graphene holes density from Raman maps. The demonstrated approach revealed a useful and non-invasive method to probe the structural and electrical homogeneity of QFMLG for future nano-electronics applications.

Role of GO and r-GO in resistance switching behavior of bilayer TiO2 based RRAM

Graphene-based resistance random access memory devices (RRAMs) have shown promise as a suitable replacement for flash memories, owing to their fast switching speed, low programming voltage, better scalability and great reliability. Furthermore, recent research works have shown bi-layer RRAM devices exhibiting better performance along the same parameters, where titania is one of the most commonly used materials. In the present work, we have studied the resistance switching behavior in a bi-layer RRAM device structure of TiO₂ with graphene oxide (GO) and reduced graphene oxide (rGO). Switching mechanism in these devices has been investigated by detailed experimental characterization in conjunction with a finite element modeling (FEM) simulation. A dual conical conductive filament has been used in the present work, based on the modeling of the electroforming process carried out by FEM. It has been demonstrated that for the GO/TiO₂ based hybrid RRAM device structure, GO acts as an active filament formation layer, whereas in the rGO/TiO₂ bi-layer structure, rGO acts as a mere electrode.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.

High-Performance Schottky Diode Gas Sensor Based on the Heterojunction of Three-Dimensional Nanohybrids of Reduced Graphene Oxide-Vertical ZnO Nanorods on an AlGaN/GaN Layer

A Schottky diode based on a heterojunction of three-dimensional (3D) nanohybrid materials, formed by hybridizing reduced graphene oxide (RGO) with epitaxial vertical zinc oxide nanorods (ZnO NRs) and Al_0.27GaN_0.73(∼25 nm)/GaN is presented as a new class of high-performance chemical sensors. The RGO nanosheet layer coated on the ZnO NRs enables the formation of a direct Schottky contact with the AlGaN layer. The sensing results of the Schottky diode with respect to NO₂, SO₂, and HCHO gases exhibit high sensitivity (0.88-1.88 ppm^-1), fast response (∼2 min), and good reproducibility down to 120 ppb concentration levels at room temperature. The sensing mechanism of the Schottky diode can be explained by the effective modulation of the reverse saturation current due to the change in thermionic emission carrier transport caused by ultrasensitive changes in the Schottky barrier of a van der Waals heterostructure between RGO and AlGaN layers upon interaction with gas molecules. Advances in the design of a Schottky diode gas sensor based on the heterojunction of high-mobility two-dimensional electron gas channel and highly responsive 3D-engineered sensing nanomaterials have potential not only for the enhancement of sensitivity and selectivity but also for improving operation capability at room temperature.

Vertical Charge Transfer and Lateral Transport in Graphene/Germanium Heterostructures

Heterostructures consisting of two-dimensional (2D) materials and conventional semiconductors have attracted a lot of attention due to their application in novel device concepts. In this work, we investigated the lateral transport characteristics of graphene/germanium heterostructures and compared them with the transport properties of graphene on SiO₂. The heterostructures were fabricated by transferring a single layer of graphene (Gr) onto a lightly doped germanium (Ge) (100) substrate. The field-effect measurements revealed a shift in the Dirac voltage of Gr on the Ge substrates compared to that of the Gr on SiO₂. Transfer length model measurements show a significant difference in the sheet resistance of Gr on Ge compared to that of the Gr on SiO₂. The results from the electrical and structural characterization suggest that a charge transfer in the order of 10¹² cm^-2 occurs between Gr and Ge resulting in a doping effect in the graphene sheet. A compact electrostatic model extracted the key electronic properties of the Gr/Ge interface. This study provides valuable insights into the electronic properties of Gr on Ge, which are vital to the development of novel devices based on mixed 2D and 3D structures.

Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions

After a decade of intensive research on two-dimensional (2D) materials inspired by the discovery of graphene, the field of 2D electronics has reached a stage with booming materials and device architectures. However, the efficient integration of 2D functional layers with three-dimensional (3D) systems remains a significant challenge, limiting device performance and circuit design. In this review, we investigate the experimental efforts in interfacing 2D layers with 3D materials and analyze the properties of the heterojunctions formed between them. The contact resistivity of metal on graphene and related 2D materials deserves special attention, while the Schottky junctions formed between metal/2D semiconductor or graphene/3D semiconductor call for careful reconsideration of the physical models describing the junction behavior. The combination of 2D and 3D semiconductors presents a form of p-n junctions that have just marked their debut. For each type of the heterojunctions, the potential applications are reviewed briefly.

Fabrication of Straight Silicon Nanowires and Their Conductive Properties

Straight Si nanowires (Si NWs) with tens to hundreds of micrometers in length and 40-200 nm in diameter are achieved by annealing a Si substrate coated with metallic Fe. The influences of annealing gas and temperature on the formation of Si NWs are investigated. It is found that the annealing gas has significant impacts on the microstructure of the NWs. By increasing the hydrogen ratio in the forming gas, straight and crystal Si NWs with thin oxide shells are obtained. Both the conductive properties along and perpendicular to the NW are investigated by conductive atomic force microscopy (CAFM) on single NWs. The conductance perpendicular to the NW is too poor to be detected, while a weak conductance can be measured along the NW. The results indicate that the measured resistance mainly comes from the contact(s), and the Si NWs exhibit typical semiconductive conductance themselves, which should have potential applications in nanoelectronics.

Passivation effect of graphene on AlGaN/GaN Schottky diode

Monolayer graphene was used as a passivation layer on a AlGaN/GaN diode to reduce surface leakage current and increase flat-band voltage.