ICMCTF2012 Session TSP: TS Poster Session

Epitaxial graphene on SiC shows n-type behavior due to interaction with the SiC substrate. However, for metal-oxide-semiconductor applications, it is important to induce p-type doping in graphene. Recent work has shown that an Au layer deposited on a graphene monolayer (GML) develops into an intercalation layer between the GML and the Si-terminated SiC substrate when the system is annealed at 800ºC for 5 min [1]. Within the GML, a p-doping effect is observed. Furthermore, it has been reported that by controlling the Au coverage level GMLs ranging from strongly n-doped to weakly p-doped can be formed [2]. However, it is still a problem to achieve a strongly p-doped GML by intercalation of an Au layer.

Using first principles calculations, we study the effect of Au intercalation on the electronic properties of epitaxial graphene grown on SiC substrates [3]. A GML on SiC restores the shape of the pristine graphene dispersion, where doping levels between strongly n-doped and weakly p-doped can be achieved by altering the Au coverage. In addition, we predict that Au intercala-tion between the two C layers of bilayer graphene grown on SiC makes it possible to achieve a strongly p-doped graphene state, where the p-doping is controlled the Au coverage.

References:

[1] B. Premlal, M. Cranney, F. Vonau, D. Aubel, D. Casterman, M. M. De Souza, and L. Simon, Appl. Phys. Lett. 94, 263115 (2009).

[2] I. Gierz, T. Suzuki, R. T. Weitz, D. S. Lee, B. Krauss, C. Riedl, U. Starke, H. Hochst, J. H. Smet, C. R. Ast, and K. Kern, Phys. Rev. B 81, 235408 (2010).

[3] Y. C. Cheng and U. Schwingenschlögl, Appl. Phys. Lett. 97, 193304 (2010).

In this study, the SiGeO_x film was taken as the resistive switching layer in Pt/SiGeO_x/TiN memory cells because germanium and silicon are extremely compatible with the prevalent complementary metal oxide semiconductor (CMOS) process . To enhance memory switching parameters, a compatible SiGeON layer between SiGeO_x and TiN is proposed to control the disruption length of filaments. Compared with Pt/SiGeO_x/TiN memory cells, the proposed Pt/SiGeO_x/SiGeON/TiN cells is effective improvement the characteristics of memory switching parameters including excellent characteristic with good endurance of more than 10⁷ times, long retention time of 10⁴ s in 125℃ and more stable in resistance switching state. Because the nitrogen introduced can effective minimize the dispersions of oxygen. It is a simple method to enhance the resistance switching parameters which introduce only the gas of ammonia in the manufacturing process. The most merit of this method is that the bi-layer structure is composed of the unitary material.

Recently, nanoscale materials have attracted considerable attention in the past few years due to their features and potential applications in various areas. ZnO nanoparticles are of great interest because of their three-dimensional quantum confinement, which strongly enhances the excitation radiative recombination. Nanoscale or submicronsized ZnO materials have also been synthesized through various methods, such as sol–gel coating, sputtering technique, atomic layer deposition etc. In this study, we using a cosputtering technique to deposit ZnO-SiO₂ nanocomposite layer, the structure of ZnO nanoclusters embedded in the nanocomposite matrix can be fabricated. The sizes of the ZnO nanoclusters distributed from 2 to 7 nm in the ZnO-SiO₂ nanocomposite layer were examined using a high resolution transmission electron microscope (HRTEM). The mechanism of the electroluminescence emission peak at 376 and 427 nm from the Ga:ZnO/i-ZnO-SiO₂ nanocomposite/p-GaN n-i-p heterostructured light-emitting devices (LEDs) were attributed to the radiative recombination occurred from the ZnO clusters and the Mg acceptor levels in the p-GaN layer.

X-ray Photoelectron Spectroscopy (XPS) is a widely used technique for the chemical characterization of surfaces. In this work we would like to present evidence that XPS characterization of the Argon gas trapped within an oxide film deposited by magnetron sputtering can be used to determine the presence or absence of crystalline structure within the film.

It is known that, during sputter deposition, a certain percentage of noble gas can be trapped within the growing film. Although these embedded gases are not expected to interact chemically with their environment, their electronic structure has been shown to react to the characteristics of their host. The shifts in binding energy and their correlation to material characteristics can be successfully determined using XPS, particularly in metallic and semiconducting materials [1]. In the case of insulating materials, however, the interpretation of the energy shifts in core levels has not been so thoroughly investigated.

Aluminium oxide films were deposited by RF magnetron sputtering, driven by 13.56 MHz and 71 MHz. The films were deposited under different biasing and temperature conditions to ensure varying degrees of crystallinity, and characterized by X-ray diffraction. The Ar2p core level of the embedded Argon atoms was investigated by XPS.

In totally amorphous samples, the Ar2p peak was fitted using a single component at ~242 eV. However, in the cases of films with γcrystalline phases, the Ar2p peak needed to be fitted with two different components at ~241 eV and ~242 eV, respectively. In order to confirm the association of the lower binding energy component to the crystalline phase, the samples were bombarded in-situ with Neon ions to destroy the crystallinity of the film without introducing additional Argon. The in-situ sputtering with Neon of crystalline samples resulted in the disappearance of the lower BE component in the Ar2p signal. This indicates that the embedded noble gas has the potential to provide a fingerprint for crystallinity that can be used during standard XPS characterization of the films.

The work is funded by DFG within SFB-TR 87.

[1] A. Rastgoo Lahrood et al. Thin Solid Films 520 (2011) 1625-1630

Transition metal dichalcogenide (TMD) crystals are characterized by their distinct layered atomic structures, with strong covalent bonds comprising each layer, but weak van der Waals forces holding the layers together. The relationship between chemical bonding in a material and its thermal conductivity (k) is well-known, however the thermal properties of TMD thin films with such highly anisotropic chemical bonds have only recently been investigated with remarkable results, such as ultra-low k_z. Materials with very low thermal conductivity in the z-axis, but higher k_x and k_y have potential as next-generation thermal barrier or heat spreading materials. Molecular dynamics (MD) simulations predicted k_x=k_y=4k_z for perfect TMD crystals (MoS₂ in this case). Experiments to determine k_x,_y and k_z were conducted by developing processes to grow crystalline TMD thin film materials with strong (002) (basal planes parallel to surface) or (100) (perpendicular basal planes) preferred orientation. Initially, no correlation between structure and thermal conductivity was apparent, as water intercalation and reactivity to ambient air resulted in a thermal “short-circuit” across basal planes, such that the time between deposition and k measurement had a stronger impact on thermal conductivity than film orientation. Experiments to measure intrinsic thermal conductivity of MoS₂ revealed values approximately one order of magnitude lower than those predicted using MD simulations, however, measurement of k_x=k_y=4k_z was consistent with simulation results. Simulations to evaluate the dependence of thermal conductivity on grain size was evaluated, which correlated well to measured values. Comparison of measured k values for strongly (002) oriented MoS₂, WS₂, WSe₂ and other materials with analogous crystal structures are discussed in the context of the Slack Law, which accounts for intrinsic physical properties of the crystal, but not film microstructure.

The control of texture in fcc nitride coatings by varying the film thickness was demonstrated on polycrystalline TiAlN coatings grown by pulsed DC magnetron sputtering. Development of off-axis texture with film thickness was observed. For this purpose the evolution of texture versus thickness was studied by a set of analytical x-ray diffraction (XRD) methods like θ–2θ and pole figures, while scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the microstructure and changes in texture with thickness. The stresses along the (111) and (002) orientation were obtained by sin²ψ method. Based on the results obtained, the texture formation mechanism is divided in three different stages of film growth. Films at low thickness lead to the development of (002) orientation due to the surface energy minimization. Meanwhile, the competitive growth promotes the growth of (111) planes parallel to film surface at higher thickness. However, contrary to the prediction of growth models, the (002) grains are not completely overlapped by (111) grains at higher thickness. Rather the (002) grains still constitute the surface, but are tilted away from the substrate normal showing substantial in-plane alignment to allow the (111) planes remain parallel to film surface. Intrinsic stress along (111) and (002) shows a strong dependence with preferred orientation. The stress level in (002) grains which was compressive at low thickness changes to tensile at higher thickness. This change in the nature of stress allows the (002) planes to tilt away in order to promote the growth of <111> parallel to film normal and to minimize the overall energy of system due to high compressive stress stored in the (111) grains. The change in surface morphology with thickness was observed using SEM. An increase in surface roughness with film thickness was observed which indicates the development of (111) texture parallel to film surface. TEM observations support the XRD results regarding texture change. Film hardness was measured by nanoindentation and a correlation between (111) texture, stress and hardness is obtained. The results indicate that texture development is a complex interplay between thermodynamic and kinetic forces. An attempt is made to understand this phenomenon of off-axis accommodation of (002) at higher thicknesses, which is a new result not reported previously.