Metal-Free CVD Graphene Synthesis on 200 mm Ge/Si(001) Substrates

Chemical Vapor Deposition Growth of Graphene on 200 mm Ge(110)/Si Wafers and Ab Initio Analysis of Differences in Growth Mechanisms on Ge(110) and Ge(001)

For the fabrication of modern graphene devices, uniform growth of high-quality monolayer graphene on wafer scale is important. This work reports on the growth of large-scale graphene on semiconducting 8 inch Ge(110)/Si wafers by chemical vapor deposition and a DFT analysis of the growth process. Good graphene quality is indicated by the small FWHM (32 cm^-1) of the Raman 2D band, low intensity ratio of the Raman D and G bands (0.06), and homogeneous SEM images and is confirmed by Hall measurements: high mobility (2700 cm²/Vs) and low sheet resistance (800 Ω/sq). In contrast to Ge(001), Ge(110) does not undergo faceting during the growth. We argue that Ge(001) roughens as a result of vacancy accumulation at pinned steps, easy motion of bonded graphene edges across (107) facets, and low energy cost to expand Ge area by surface vicinals, but on Ge(110), these mechanisms do not work due to different surface geometries and complex reconstruction.

Toward Nonvolatile Spin-Orbit Devices: Deposition of Ferroelectric Hafnia on Monolayer Graphene/Co/HM Stacks

While technologically challenging, the integration of ferroelectric thin films with graphene spintronics potentially allows the realization of highly efficient, electrically tunable, nonvolatile memories through control of the interfacial spin-orbit driven interaction occurring at graphene/Co interfaces deposited on heavy metal supports. Here, the integration of ferroelectric Hf_0.5Zr_0.5O₂ on graphene/Co/heavy metal epitaxial stacks is investigated via the implementation of several nucleation methods in atomic layer deposition. By employing in situ Al₂O₃ as a nucleation layer sandwiched between Hf_0.5Zr_0.5O₂ and graphene, the Hf_0.5Zr_0.5O₂ demonstrates a remanent polarization (2Pr) of 19.2 μC/cm². Using an ex situ, naturally oxidized sputtered Ta layer for nucleation, we could control 2Pr via the interlayer thickness, reaching maximum values of 28 μC/cm² with low coercive fields. Magnetic hysteresis measurements taken before and after atomic layer deposition show strong perpendicular magnetic anisotropy, with minimal deviations in the magnetization reversal pathways due to the Hf_0.5Zr_0.5O₂ deposition process, thus pointing to a good preservation of the magnetic stack including single-layer graphene. X-ray diffraction measurements further confirm that the high-quality interfaces demonstrated in the stack remain unperturbed by the ferroelectric deposition and anneal. The proposed graphene-based ferroelectric/magnetic structures offer the strong advantages of ferroelectricity and ferromagnetism at room temperature, enabling the development of novel magneto-electric and nonvolatile in-memory spin-orbit logic architectures with low power switching.

Reliable metal-graphene contact formation process flows in a CMOS-compatible environment

The possibility of exploiting the enormous potential of graphene for microelectronics and photonics must go through the optimization of the graphene-metal contact. Achieving low contact resistance is essential for the consideration of graphene as a candidate material for electronic and photonic devices. This work has been carried out in an 8'' wafer pilot-line for the integration of graphene into a CMOS environment. The main focus is to study the impact of the patterning of graphene and passivation on metal-graphene contact resistance. The latter is measured by means of transmission line measurement (TLM) with several contact designs. The presented approaches enable reproducible formation of contact resistivity as low as 660 Ω μm with a sheet resistance of 1.8 kΩ/□ by proper graphene patterning, passivation of the channel and a post-processing treatment such as annealing.

Towards the Growth of Hexagonal Boron Nitride on Ge(001)/Si Substrates by Chemical Vapor Deposition

The growth of hexagonal boron nitride (hBN) on epitaxial Ge(001)/Si substrates via high-vacuum chemical vapor deposition from borazine is investigated for the first time in a systematic manner. The influences of the process pressure and growth temperature in the range of 10^-7-10^-3 mbar and 900-980 °C, respectively, are evaluated with respect to morphology, growth rate, and crystalline quality of the hBN films. At 900 °C, nanocrystalline hBN films with a lateral crystallite size of ~2-3 nm are obtained and confirmed by high-resolution transmission electron microscopy images. X-ray photoelectron spectroscopy confirms an atomic N:B ratio of 1 ± 0.1. A three-dimensional growth mode is observed by atomic force microscopy. Increasing the process pressure in the reactor mainly affects the growth rate, with only slight effects on crystalline quality and none on the principle growth mode. Growth of hBN at 980 °C increases the average crystallite size and leads to the formation of 3-10 well-oriented, vertically stacked layers of hBN on the Ge surface. Exploratory ab initio density functional theory simulations indicate that hBN edges are saturated by hydrogen, and it is proposed that partial de-saturation by H radicals produced on hot parts of the set-up is responsible for the growth.

Advanced wearable biosensors for the detection of body fluids and exhaled breath by graphene

Given the huge economic burden caused by chronic and acute diseases on human beings, it is an urgent requirement of a cost-effective diagnosis and monitoring process to treat and cure the disease in their preliminary stage to avoid severe complications. Wearable biosensors have been developed by using numerous materials for non-invasive, wireless, and consistent human health monitoring. Graphene, a 2D nanomaterial, has received considerable attention for the development of wearable biosensors due to its outstanding physical, chemical, and structural properties. Moreover, the extremely flexible, foldable, and biocompatible nature of graphene provide a wide scope for developing wearable biosensor devices. Therefore, graphene and its derivatives could be trending materials to fabricate wearable biosensor devices for remote human health management in the near future. Various biofluids and exhaled breath contain many relevant biomarkers which can be exploited by wearable biosensors non-invasively to identify diseases. In this article, we have discussed various methodologies and strategies for synthesizing and pattering graphene. Furthermore, general sensing mechanism of biosensors, and graphene-based biosensing devices for tear, sweat, interstitial fluid (ISF), saliva, and exhaled breath have also been explored and discussed thoroughly. Finally, current challenges and future prospective of graphene-based wearable biosensors have been evaluated with conclusion. Graphene is a promising 2D material for the development of wearable sensors. Various biofluids (sweat, tears, saliva and ISF) and exhaled breath contains many relevant biomarkers which facilitate in identify diseases. Biosensor is made up of biological recognition element such as enzyme, antibody, nucleic acid, hormone, organelle, or complete cell and physical (transducer, amplifier), provide fast response without causing organ harm.

Influence of plasma treatment on SiO2/Si and Si3N4/Si substrates for large-scale transfer of graphene

One of the limiting factors of graphene integration into electronic, photonic, or sensing devices is the unavailability of large-scale graphene directly grown on the isolators. Therefore, it is necessary to transfer graphene from the donor growth wafers onto the isolating target wafers. In the present research, graphene was transferred from the chemical vapor deposited 200 mm Germanium/Silicon (Ge/Si) wafers onto isolating (SiO₂/Si and Si₃N₄/Si) wafers by electrochemical delamination procedure, employing poly(methylmethacrylate) as an intermediate support layer. In order to influence the adhesion properties of graphene, the wettability properties of the target substrates were investigated in this study. To increase the adhesion of the graphene on the isolating surfaces, they were pre-treated with oxygen plasma prior the transfer process of graphene. The wetting contact angle measurements revealed the increase of the hydrophilicity after surface interaction with oxygen plasma, leading to improved adhesion of the graphene on 200 mm target wafers and possible proof-of-concept development of graphene-based devices in standard Si technologies.

Wafer-Scale Integration of Graphene-Based Photonic Devices

Graphene and related materials can lead to disruptive advances in next-generation photonics and optoelectronics. The challenge is to devise growth, transfer and fabrication protocols providing high (≥5000 cm2 V-1 s-1) mobility devices with reliable performance at the wafer scale. Here, we present a flow for the integration of graphene in photonics circuits. This relies on chemical vapor deposition (CVD) of single layer graphene (SLG) matrices comprising up to ∼12000 individual single crystals, grown to match the geometrical configuration of the devices in the photonic circuit. This is followed by a transfer approach which guarantees coverage over ∼80% of the device area, and integrity for up to 150 mm wafers, with room temperature mobility ∼5000 cm2 V-1 s-1. We use this process flow to demonstrate double SLG electro-absorption modulators with modulation efficiency ∼0.25, 0.45, 0.75, 1 dB V-1 for device lengths ∼30, 60, 90, 120 μm. The data rate is up to 20 Gbps. Encapsulation with single-layer hexagonal boron nitride (hBN) is used to protect SLG during plasma-enhanced CVD of Si3N4, ensuring reproducible device performance. The processes are compatible with full automation. This paves the way for large scale production of graphene-based photonic devices.

Robust atomic-structure of the 6 × 2 reconstruction surface of Ge(110) protected by the electronically transparent graphene monolayer

Wafer-scale growth of the unidirectional graphene monolayer on Ge surfaces has rejuvenated the intense study of the surfaces and interfaces of semiconductors underneath graphene. Recently, it was reported that the Ge atoms in the Ge(110) surface beneath a graphene monolayer underwent a rearrangement and formed an ordered (6 × 2) reconstruction. However, a plausible atomic model related to this (6 × 2) reconstruction is still lacking. Here, by using scanning tunnelling microscopy/spectroscopy (STM/S) and density functional theory (DFT) calculations, we deeply investigated the structural and electronic properties of the Ge(110) (6 × 2) surface encapsulated by a graphene monolayer. The (6 × 2) surface reconstruction was confirmed for the post-annealing-graphene-covered Ge(110) surface via STM, and was found to be quite air-stable, owing to the protection of the graphene monolayer against surface oxidation. Our study disclosed that the topographic features of the topmost graphene monolayer and the Ge(110) surface could be selectively imaged by utilizing suitable scanning biases. According to the STM results and DFT calculations, a rational ball-and-stick model of the (6 × 2) reconstruction was successfully provided, in which an elemental building block comprising two Ge triangles and two isolated Ge atoms adsorbed on the unreconstructed ideal Ge(110) surface. Local density of states of the graphene/Ge surface was explored via scanning tunneling spectroscopy (STS), presenting four well-defined differential conductance (dI/dV) peaks, protruding at energies of 0.2, 0.4, 0.6 and 0.8 eV, respectively. The four peaks predominantly originated from the surface states of the reconstructing adatoms and were well reproduced by our theoretical simulation. This result means that the Ge surface is very robust after being encapsulated by the epitaxial graphene, which could be advantageous for directly fabricating graphene/Ge-hybrid high-speed electronics and optoelectronics based on conventional microelectronics technology.

High-Mobility Epitaxial Graphene on Ge/Si(100) Substrates

Graphene was shown to reveal intriguing properties of its relativistic two-dimensional electron gas; however, its implementation to microelectronic applications is missing to date. In this work, we present a comprehensive study of epitaxial graphene on technologically relevant and in a standard CMOS process achievable Ge(100) epilayers grown on Si(100) substrates. Crystalline graphene monolayer structures were grown by means of chemical vapor deposition (CVD). Using angle-resolved photoemission spectroscopy and in situ surface transport measurements, we demonstrate their metallic character both in momentum and real space. Despite numerous crystalline imperfections, e.g., grain boundaries and strong corrugation, as compared to epitaxial graphene on SiC(0001), charge carrier mobilities of 1 × 10⁴ cm²/Vs were obtained at room temperature, which is a result of the quasi-charge neutrality within the graphene monolayers on germanium and not dependent on the presence of an interface oxide. The interface roughness due to the facet structure of the Ge(100) epilayer, formed during the CVD growth of graphene, can be reduced via subsequent in situ annealing up to 850 °C coming along with an increase in the mobility by 30%. The formation of a Ge(100)-(2 × 1) structure demonstrates the weak interaction and effective delamination of graphene from the Ge/Si(100) substrate.

Direct growth of graphene on Ge(100) and Ge(110) via thermal and plasma enhanced CVD

The integration of graphene into CMOS compatible Ge technology is in particular attractive for optoelectronic devices in the infrared spectral range. Since graphene transfer from metal substrates has detrimental effects on the electrical properties of the graphene film and moreover, leads to severe contamination issues, direct growth of graphene on Ge is highly desirable. In this work, we present recipes for a direct growth of graphene on Ge via thermal chemical vapor deposition (TCVD) and plasma-enhanced chemical vapor deposition (PECVD). We demonstrate that the growth temperature can be reduced by about 200 °C in PECVD with respect to TCVD, where usually growth occurs close to the melting point of Ge. For both, TCVD and PECVD, hexagonal and elongated morphology is observed on Ge(100) and Ge(110), respectively, indicating the dominant role of substrate orientation on the shape of graphene grains. Interestingly, Raman data indicate a compressive strain of ca. − 0.4% of the graphene film fabricated by TCVD, whereas a tensile strain of up to + 1.2% is determined for graphene synthesized via PECVD, regardless the substrate orientation. Supported by Kelvin probe force measurements, we suggest a mechanism that is responsible for graphene formation on Ge and the resulting strain in TCVD and PECVD.

Epitaxial graphene/Ge interfaces: a minireview

The recent discovery of the ability to perform direct epitaxial growth of graphene layers on semiconductor Ge surfaces led to a huge interest in this topic. One of the reasons for this interest is the chance to overcome several present-day drawbacks on the method of graphene integration in modern semiconductor technology. The other one is connected with the fundamental studies of the new graphene-semiconductor interfaces that might help with the deeper understanding of mechanisms, which governs graphene growth on different substrates as well as shedding light on the interaction of graphene with these substrates, whose range is now spread from metals to insulators. The present minireview gives a timely overview of the state-of-the-art field of studies of the graphene-Ge epitaxial interfaces and draws some conclusions in this research area.

Contact resistance and mobility in back-gate graphene transistors

The metal-graphene contact resistance is one of the major limiting factors toward the technological exploitation of graphene in electronic devices and sensors. High contact resistance can be detrimental to device performance and spoil the intrinsic great properties of graphene. In this paper, we fabricate back-gate graphene field-effect transistors with different geometries to study the contact and channel resistance as well as the carrier mobility as a function of gate voltage and temperature. We apply the transfer length method and the y-function method showing that the two approaches can complement each other to evaluate the contact resistance and prevent artifacts in the estimation of carrier mobility dependence on the gate-voltage. We find that the gate voltage modulates both the contact and the channel resistance in a similar way but does not change the carrier mobility. We also show that raising the temperature lowers the carrier mobility, has a negligible effect on the contact resistance, and can induce a transition from a semiconducting to a metallic behavior of the graphene sheet resistance, depending on the applied gate voltage. Finally, we show that eliminating the detrimental effects of the contact resistance on the transistor channel current almost doubles the carrier field-effect mobility and that a competitive contact resistance as low as 700 Ω·μm can be achieved by the zig-zag shaping of the Ni contact.

Investigation of the Oxidation Behavior of Graphene/Ge(001) Versus Graphene/Ge(110) Systems

The oxidation behavior of Ge(001) and Ge(110) surfaces underneath the chemical vapor deposition (CVD)-grown graphene films has been investigated experimentally and interpreted on the basis of ab initio calculations. Freshly grown samples were exposed to air for more than 7 months and periodically monitored by X-ray photoelectron spectroscopy, scanning electron microscopy, and Raman spectroscopy. The oxidation of Ge(110) started with incubation time of several days, during which the oxidation rate was supposedly exponential. After an ultrathin oxide grew, the oxidation continued with a slow but constant rate. No incubation was detected for Ge(001). The oxide thickness was initially proportional to the square root of time. After 2 weeks, the rate saturated at a value fivefold higher than that for Ge(110). We argue that after the initial phase, the oxidation is limited by the diffusion of oxidizing species through atomic-size openings at graphene domain boundaries and is influenced by the areal density and by the structural quality of the boundaries, whereby the latter determines the initial behavior. Prolonged exposure affected the surface topography and reduced the strain in graphene. In the last step, both the air-exposed samples were annealed in vacuum at 850 °C. This removed oxygen from the substrate and restored the samples to their initial state. These findings might constitute an important step toward further optimization of graphene grown on Ge.

Graphene-based wearable sensors

The human body is a "delicate machine" full of sensors such as the fingers, nose, and mouth. In addition, numerous physiological signals are being created every moment, which can reflect the condition of the body. The quality and the quantity of the physiological signals are important for diagnoses and the execution of therapies. Due to the incompact interface between the sensors and the skin, the signals obtained by commercial rigid sensors do not bond well with the body; this decreases the quality of the signal. To increase the quantity of the data, it is important to detect physiological signals in real time during daily life. In recent years, there has been an obvious trend of applying graphene devices with excellent performance (flexibility, biocompatibility, and electronic characters) in wearable systems. In this review, we will first provide an introduction about the different methods of synthesis of graphene, and then techniques for graphene patterning will be outlined. Moreover, wearable graphene sensors to detect mechanical, electrophysiological, fluid, and gas signals will be introduced. Finally, the challenges and prospects of wearable graphene devices will be discussed. Wearable graphene sensors can improve the quality and quantity of the physiological signals and have great potential for health-care and telemedicine in the future.

Submicron Size Schottky Junctions on As-Grown Monolayer Epitaxial Graphene on Ge(100): A Low-Invasive Scanned-Probe-Based Study

We report on the investigation of the Schottky barrier (SB) formed at the junction between a metal-free graphene monolayer and Ge semiconductor substrate in the as-grown epitaxial graphene/Ge(100) system. In order to preserve the heterojunction properties, we defined submicron size graphene/Ge junctions using the scanning probe microscopy lithography in the local oxidation configuration, a low-invasive processing approach capable of inducing spatially controlled electrical separations among tiny graphene regions. Characteristic junction parameters were estimated from I-V curves obtained using conductive-atomic force microscopy. The current-voltage characteristics showed a p-type Schottky contact behavior, ascribed to the n-type to p-type conversion of the entire Ge substrate due to the formation of a large density of acceptor defects during the graphene growth process. We estimated, for the first time, the energy barrier height in the as-grown graphene/Ge Schottky junction (φ_B ≈ 0.45 eV) indicating an n-type doping of the graphene layer with a Fermi level ≈ 0.15 eV above the Dirac point. The SB devices showed ideality factor values around 1.5 pointing to the high quality of the heterojunctions.

Graphene on Group-IV Elementary Semiconductors: The Direct Growth Approach and Its Applications

Since the first development of large-area graphene synthesis by the chemical vapor deposition (CVD) method in 2009, CVD-graphene has been considered to be a key material in the future electronics, energy, and display industries, which require transparent, flexible, and stretchable characteristics. Although many graphene-based prototype applications have been demonstrated, several important issues must be addressed in order for them to be compatible with current complementary metal-oxide-semiconductor (CMOS)-based manufacturing processes. In particular, metal contamination and mechanical damage, caused by the metal catalyst for graphene growth, are known to cause severe and irreversible deterioration in the performance of devices. The most effective way to solve the problems is to grow the graphene directly on the semiconductor substrate. Herein, recent advances in the direct growth of graphene on group-IV semiconductors are reviewed, focusing mainly on the growth mechanism and initial growth behavior when graphene is synthesized on Si and Ge. Furthermore, recent progress in the device applications of graphene with Si and Ge are presented. Finally, perspectives for future research in graphene with a semiconductor are discussed.

Direct CVD Growth of Graphene on Technologically Important Dielectric and Semiconducting Substrates

To fabricate graphene based electronic and optoelectronic devices, it is highly desirable to develop a variety of metal-catalyst free chemical vapor deposition (CVD) techniques for direct synthesis of graphene on dielectric and semiconducting substrates. This will help to avoid metallic impurities, high costs, time consuming processes, and defect-inducing graphene transfer processes. Direct CVD growth of graphene on dielectric substrates is usually difficult to accomplish due to their low surface energy. However, a low-temperature plasma enhanced CVD technique could help to solve this problem. Here, the recent progress of metal-catalyst free direct CVD growth of graphene on technologically important dielectric (SiO2, ZrO2, HfO2, h-BN, Al2O3, Si3N4, quartz, MgO, SrTiO3, TiO2, etc.) and semiconducting (Si, Ge, GaN, and SiC) substrates is reviewed. High and low temperature direct CVD growth of graphene on these substrates including growth mechanism and morphology is discussed. Detailed discussions are also presented for Si and Ge substrates, which are necessary for next generation graphene/Si/Ge based hybrid electronic devices. Finally, the technology development of the metal-catalyst free direct CVD growth of graphene on these substrates is concluded, with future outlooks.

Fast, Noncontact, Wafer-Scale, Atomic Layer Resolved Imaging of Two-Dimensional Materials by Ellipsometric Contrast Micrography

Adequate characterization and quality control of atomically thin layered materials (2DM) has become a serious challenge particularly given the rapid advancements in their large area manufacturing and numerous emerging industrial applications with different substrate requirements. Here, we focus on ellipsometric contrast micrography (ECM), a fast intensity mode within spectroscopic imaging ellipsometry, and show that it can be effectively used for noncontact, large area characterization of 2DM to map coverage, layer number, defects and contamination. We demonstrate atomic layer resolved, quantitative mapping of chemical vapor deposited graphene layers on Si/SiO₂-wafers, but also on rough Cu catalyst foils, highlighting that ECM is applicable to all application relevant substrates. We discuss the optimization of ECM parameters for high throughput characterization. While the lateral resolution can be less than 1 μm, we particularly explore fast scanning and demonstrate imaging of a 4″ graphene wafer in 47 min at 10 μm lateral resolution, i.e., an imaging speed of 1.7 cm²/min. Furthermore, we show ECM of monolayer hexagonal BN (h-BN) and of h-BN/graphene bilayers, highlighting that ECM is applicable to a wide range of 2D layered structures that have previously been very challenging to characterize and thereby fills an important gap in 2DM metrology.

The graphene/n-Ge(110) interface: structure, doping, and electronic properties

The implementation of graphene in semiconducting technology requires precise knowledge about the graphene-semiconductor interface. In our work the structure and electronic properties of the graphene/n-Ge(110) interface are investigated on the local (nm) and macro (from μm to mm) scales via a combination of different microscopic and spectroscopic surface science techniques accompanied by density functional theory calculations. The electronic structure of freestanding graphene remains almost completely intact in this system, with only a moderate n-doping indicating weak interaction between graphene and the Ge substrate. With regard to the optimisation of graphene growth it is found that the substrate temperature is a crucial factor, which determines the graphene layer alignment on the Ge(110) substrate during its growth from the atomic carbon source. Moreover, our results demonstrate that the preparation route for graphene on the doped semiconducting material (n-Ge) leads to the effective segregation of dopants at the interface between graphene and Ge(110). Furthermore, it is shown that these dopant atoms might form regular structures at the graphene/Ge interface and induce the doping of graphene. Our findings help to understand the interface properties of the graphene-semiconductor interfaces and the effect of dopants on the electronic structure of graphene in such systems.