Atomic layer deposition for graphene device integration

Graphene is a two dimensional material with extraordinary properties, which make it an interesting material for many optical and electronic devices. The integration of graphene in these devices often requires the deposition of thin dielectric layers on top of graphene. Atomic layer deposition (ALD) is the method of choice to deposit these layers due to its ability to deposit ultra-thin, high quality films with sub-monolayer thickness control. ALD on graphene however, is a challenge due to the lack of reactive surface sites on graphene. This leads to the selective growth on grain boundaries, wrinkles and defect sites present in the graphene. In this review an overview of the different methods to achieve uniform deposition of ALD on graphene is presented. The advantages and disadvantages of each method are discussed from the perspective of ALD together with the opportunities for further research. Special emphasis is given to the recent advancements in the understanding of the ALD process conditions and their influence on the deposition uniformity on graphene. Particularly, improving the quality of the dielectric layers deposited by ALD while maintaining the pristine properties of graphene, will prove vital for the device integration of graphene.\u3cbr/\u3

Continuous and ultrathin platinum films on graphene using atomic layer deposition:a combined computational and experimental study

Integrating metals and metal oxides with graphene is key in utilizing its extraordinary material properties that are ideal for nanoelectronic and catalyst applications. Atomic layer deposition (ALD) has become a key technique for depositing ultrathin, conformal metal(oxide) films. ALD of metal(oxide) films on graphene, however, remains a genuine challenge due to the chemical inertness of graphene. In this study we address this issue by combining first-principles density functional theory (DFT) simulations with ALD experiments. The focus is on the Pt ALD on graphene, as this hybrid system is very promising for solar and fuel cells, hydrogen technologies, microreactors, and sensors. Here we elucidate the surface reactions underpinning the nucleation stage of Pt ALD on pristine, defective and functionalized graphenes. The employed reaction mechanism clearly depends on (a) the available surface groups on graphene, and (b) the ligands accompanying the metal centre in the precursor. DFT calculations also indicate that graphene oxide (GO) can afford a stronger adsorption of MeCpPtMe3, unlike Pt(acac)2, as compared to bare (non-functionalized) graphene, suggesting that GO monolayers are effective Pt ALD seed layers. Confirming the latter, we evince that wafer-scale, continuous Pt films can indeed be grown on GO monolayers using a thermal ALD process with MeCpPtMe3 and O2 gas. Besides, the current in-depth atomistic insights are of practical use for understanding similar ALD processes of other metals and metal oxides on graphene

Uniform atomic layer deposition of Al2O3 on graphene by reversible hydrogen plasma functionalization

A novel method to form ultrathin, uniform Al2O3 layers on graphene using reversible hydrogen plasma functionalization followed by atomic layer deposition (ALD) is presented. ALD on pristine graphene is known to be a challenge due to the absence of dangling bonds, leading to nonuniform film coverage. We show that hydrogen plasma functionalization of graphene leads to uniform ALD of closed Al2O3 films down to 8 nm in thickness. Hall measurements and Raman spectroscopy reveal that the hydrogen plasma functionalization is reversible upon Al2O3 ALD and subsequent annealing at 400 °C and in this way does not deteriorate the graphene’s charge carrier mobility. This is in contrast with oxygen plasma functionalization, which can lead to a uniform 5 nm thick closed film, but which is not reversible and leads to a reduction of the charge carrier mobility. Density functional theory (DFT) calculations attribute the uniform growth on both H2 and O2 plasma functionalized graphene to the enhanced adsorption of trimethylaluminum (TMA) on these surfaces. A DFT analysis of the possible reaction pathways for TMA precursor adsorption on hydrogenated graphene predicts a binding mechanism that cleans off the hydrogen functionalities from the surface, which explains the observed reversibility of the hydrogen plasma functionalization upon Al2O3 ALD

Area-selective atomic layer deposition of platinum using photosensitive polyimide

Area-selective atomic layer deposition (AS-ALD) of platinum (Pt) was studied using photosensitive polyimide as a masking layer. The polyimide films were prepared by spin-coating and patterned using photolithography. AS-ALD of Pt using poly(methyl-methacrylate) (PMMA) masking layers was used as a reference. The results show that polyimide has excellent selectivity towards the Pt deposition, after 1000 ALD cycles less than a monolayer of Pt is deposited on the polyimide surface. The polyimide film could easily be removed after ALD using a hydrogen plasma, due to a combination of weakening of the polyimide resist during Pt ALD and the catalytic activity of Pt traces on the polyimide surface. Compared to PMMA for AS-ALD of Pt, polyimide has better temperature stability. This resulted in an improved uniformity of the Pt deposits and superior definition of the Pt patterns. In addition, due to the absence of reflow contamination using polyimide the nucleation phase during Pt ALD is drastically shortened. Pt patterns down to 3.5 μm were created with polyimide, a factor of ten smaller than what is possible using PMMA, at the typical Pt ALD processing temperature of 300 °C. Initial experiments indicate that after further optimization of the polyimide process Pt features down to 100 nm should be possible, which makes AS-ALD of Pt using photosensitive polyimide a promising candidate for patterning at the nanoscale

The effect of residual gas scattering on Ga ion beam patterning of graphene

The patterning of graphene by a 30 kV Ga+focused ion beam(FIB) is studied by in-situ and ex-situRaman spectroscopy. It is found that the graphene surrounding the patterned target area can be damaged at remarkably large distances of more than 10 μm. We show that scattering of the Ga ions in the residual gas of the vacuum system is the main cause of the large range of lateral damage, as the size and shape of the tail of the ion beam were strongly dependent on the system background pressure. The range of the damage was therefore greatly reduced by working at low pressures and limiting the total amount of ions used. This makes FIB patterning a feasible alternative to electron beam lithography as long as residual gas scattering is taken into accoun

Diffraction enhanced transparency in a hybrid gold-graphene THz metasurface

\u3cp\u3eDiffraction enhanced transparency (DET) is a phenomenon based on interference in periodic lattices of resonators, leading to sharp transmission peaks where the associated anomalous dispersion can be used to delay THz radiation traversing the lattice. In this contribution we use an atomically thin graphene layer to strongly suppress DET altering the THz propagation. The response of this system is investigated both in the far-and near-field, where the suppression of DET can be measured both in the spectral response of the sample and in the changing near-field distributions surrounding the resonators. Changing the properties of the graphene layer allows for control over the transparency window, which is relevant for active THz devices.\u3c/p\u3

Graphene devices with bottom-up contacts by area-selective atomic layer deposition

\u3cp\u3eGraphene field-effect transistor devices were fabricated using a bottom-up and resist-free method, avoiding common compatibility issues such as contamination by resist residues. Large-area CVD graphene sheets were structured into device channels by patterning with a focused ion beam. Platinum contacts were then deposited by direct-write atomic layer deposition (ALD), which is a combination between electron beam induced deposition (EBID) and bottom-up area-selective ALD. This is a unique approach that enables nucleation of Pt ALD on graphene, and therefore these devices are the first reported graphene devices with contacts deposited by ALD. Electrical characterization of the devices confirms ambipolar transistor behaviour with typical field-effect mobilities in the range of 1000-1800 cm2 V-1 s-1. We observe clear signs of strong Pt-graphene coupling and contact induced hole doping, implying good contact properties in contrast to the conventionally weak bonding between Pt and graphene. We attribute these observations to the reduced amount of resist residue under the contacts, the improved wettability of the Pt due to the use of ALD, and the formation of a graphitic interlayer that bonds the Pt more strongly to the graphene. We conclude that direct-write ALD is a very suitable technique for metallization of graphene devices and to study the intrinsic properties of metal-graphene contacts in more detail. In addition, it offers unique opportunities to control the metal-graphene coupling strength.\u3c/p\u3

Bottom-up meets top-down:Tailored raspberry-like Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e-Pt nanocrystal superlattices

\u3cp\u3eSupported catalysts are widely used in industry and can be optimized by tuning the composition, chemical structure, and interface of the nanoparticle catalyst and oxide support. Here we firstly combine a bottom up colloidal synthesis method with a top down atomic layer deposition (ALD) process to achieve a raspberry-like Pt-decorated Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e (Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e-Pt) nanoparticle superlattices. This nanocomposite ensures the precision of the catalyst/support interface, improving the catalytic efficiency of the Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e-Pt nanocomposite system. The morphology of the hybrid nanocomposites resulting from different cycles of ALD was monitored by scanning transmission electron microscopy, giving insight into the nucleation and growth mechanism of the ALD process. X-ray photoelectron spectroscopy studies confirm the anticipated electron transfer from Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e to Pt through the nanocomposite interface. Photocurrent measurement further suggests that Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e superlattices with controlled decoration of Pt have substantial promise for energy-efficient photoelectrocatalytic oxygen evolution reaction. This work opens a new avenue for designing supported catalyst architectures via precisely controlled decoration of single component superlattices with noble metals.\u3c/p\u3

In-situ Raman spectroscopy to elucidate the influence of adsorption in graphene electrochemistry

\u3cp\u3eElectrochemistry on graphene is of particular interest due to graphene's high surface area, high electrical conductivity and low interfacial capacitance. Because the graphene Fermi level can be probed by its strong Raman signal, information on the graphene doping can be obtained which in turn can provide information on adsorbed atoms or molecules. For this paper, the adsorption analysis was successfully performed using three electroactive substances with different electrode interaction mechanisms: hexaammineruthenium(III) chloride (RuHex), ferrocenemethanol (FcMeOH) and potassium ferricyanide/potassium ferrocyanide (Fe(CN)\u3csub\u3e6\u3c/sub\u3e). The adsorption state was probed by analysing the G-peak position in the measured in-situ Raman spectrum during electrochemical experiments. We conclude that electrochemical Raman spectroscopy on graphene is a valuable tool to obtain in-situ information on adsorbed species on graphene, isolated from the rest of the electrochemical behaviour.\u3c/p\u3