Session S20: Focus Session: Engineering Interfaces for New Materials III: Heterogeneous Interfaces

Epitaxial Growth of Iron and Iron Nitrides on Wurtzite Gallium Nitride (0001)

Magnetic transition metal-containing layers on GaN have potential spintronic applications. We explore the epitaxial growth of iron and iron nitride films on wurtzite ($w)$-GaN(0001). First, we investigate the growth of $\sim $ 1:1 iron nitride on $w$-GaN(0001) using rf N$_{2}$-plasma molecular beam epitaxy (MBE) and monitor growth with \textit{in-situ} reflection high energy electron diffraction (RHEED). We find that FeN grows epitaxially with zinc-blende structure and [111]-orientation on $w$-GaN(0001). To achieve high Fe content, such as Fe$_{4}$N, and Fe$_{16}$N$_{2}$, current efforts are aimed at reducing N content in the source gas. In the case of pure Fe deposition, x-ray diffraction and RHEED suggest the epitaxial relationship to be [110]$_{Fe}\vert \vert $ [0001]$_{GaN}$ with Fe in bcc structure. The assignment is based on lattice spacing measurements as well as angular dependence of the RHEED pattern. In this presentation, most recent results for Fe and Fe$_{x}$N$_{y}$ films grown on $w$-GaN(0001) will be presented.   

Observation of Standing Waves on the GaN(0001) Pseudo (1x1) Surface by Scanning Tunneling Microscopy at Room Temperature

The metallic pseudo-1x1 surface, consists of 2 to 2.5 ML Ga on top of the Ga-terminated GaN(0001), provides an ideally confined 2D electron gas (2DEG), which gives rise to complex standing wave patterns. Even at room temperature, these patterns can be observed by scanning tunneling microscopy (STM) and spectroscopy (STS). The analysis of the modulation of the local density of states within various confinement geometries as a function of the bias voltage shows that nearly free-electron like energy dispersive surface states are being probed.   

Surface Evolution During Sub-Monolayer Manganese Deposition onto Wurtzite Gallium Nitride (000-1) Surface.

While transition metal (TM)-doped gallium nitride (GaN) films have been explored as potential spintronic materials, the structural and magnetic effects of various TM adatoms on GaN surfaces are not well understood. In this work, we investigate the deposition of sub-monolayer quantities of Mn onto the N-polar GaN(000-1) 1$\times $1 surface. First, the GaN surface is prepared by molecular beam epitaxy. The smooth surface is then annealed to remove excess Ga adatoms. Next, the surface is exposed to a dose [approximately 0.05-0.1 monolayer (ML)] of Mn at substrate temperature of 200 $^{o}$C. Using \textit{in-situ} reflection high energy electron diffraction (RHEED), we observe the onset of clear 3$\times $ periodicity along [1-100] but only 1$\times $ along [11-20]. Additional 0.05-0.1 ML Mn doses lead to increasing intensity of the 2/3-order RHEED streaks, while 1/3-order and 1$^{st}$-order streaks weaken. For Mn doses up to about 1/3$^{rd}$ ML, the surface appears quite smooth, with the RHEED pattern stable upon heating the surface to 600 $^{\circ}$C. The results suggest a surface evolution process leading to a well-ordered Mn-containing structure at the GaN(000-1) surface.   

Van der Pauw and Hall Measurements on Ultra Thin Silicon-on-Insulator

Ultra-thin silicon-on-insulator (UTSOI) provides opportunities to study the role of the surface in electrical transport in Si. Because the Si layers can be as thin as 10 nm, surface states, surface induced band bending, and gap states at the oxide-Si interface dominate the carrier density. Transport measurements provide a sensitive probe of the carriers. Previous measurements of thin Si structures have shown that Si/SiO$_{2 }$interface traps deplete Si of mobile carriers, and sheet resistances reach 10$^{11}$ ohm/sq for a 20 nm thick sample [1]. Thus, any perturbation to the surface that induces even modest carrier densities can be detected in transport. We perform van der Pauw and Hall measurements on UTSOI structures with a variety of surface modifications, including hydrogen termination and epichlorohydrin surface attachment. UTSOI that was extremely resistive with oxide on both sides undergoes a drop in resistance of more than 3 orders of magnitude after surface modification. Hall and van der Pauw measurements, reveal the density and the sign of the carriers. We discuss the mechanisms for this increased conductivity. [1] Zhang P. et al. \textit{Nature} \textbf{439} 703 (2006)   

Segregation of O defects in Si:HfO$_{2}$ heterojunctions: A first principles investigation.

Driven by a need for device miniaturization in the microelectronic industry, Hf-based high-permittivity materials, such as HfO$_{2}$, have gained interest for their potential application as gate dielectrics. However, undesirable interfacial phases such as Hf silicides and SiO$_{x}$ are known to form and degrade the performance of devices. It has been postulated that these interfacial phases are related to the segregation of O defects (vacancy or interstitial) to the interface. In this work, we examine the thermodynamic and kinetic driving forces for the segregation of isolated and clustered O defects (vacancies and interstitials) to the Si:HfO$_{2}$ interface. Using first principles density functional theory calculations, we have determined the formation and migration energies per O defect within bulk HfO$_{2}$ and at the Si:HfO$_{2}$ interface. Our results indicate that isolated as well as a distribution of point defects display large driving forces for interface segregation, allowing for the formation of silicides and silicates. Thus, while an abrupt Si:HfO$_{2}$ interface may be stable in the absence of O defects, such an interface is unstable to the formation of other phases in the presence of O defects.   

Sub-Angstrom Distortions of an Epitaxial Oxide on Silicon (001)

As metal oxide semiconductor field effect transistor (MOSFET) devices are reduced to the nanometer length scale, atomistic control of the silicon-oxide interface is needed in order to fabricate optimally functioning devices. In this work, we present synchrotron x-ray diffraction measurements of a model system, barium oxide grown epitaxially on Si (001), with an interface phase of submonolayer strontium on silicon. Diffraction results show that the 2x1 surface phase that promotes epitaxy transforms into an interface phase between the oxide and silicon, which also has a 2x1 symmetry on the Si (001) surface. Quantitative analysis of the diffraction is consistent with three classes of models; these involve a 2x1 arrangement of alkaline earth metal in the interface phase, sub-angstrom distortions of the oxide film, or a combination of both. These measurements demonstrate how this reconstruction is a true interface phase that can be used to test our current understanding of silicon-oxide interface physics.   

Theoretical investigation of the interface structure of $\theta $-Al$_{2}$O$_{3}$/NiAl(001).

The atomic structure of $\theta $c-Al$_{2}$O$_{3}$/NiAl(001) interfaces has been investigated by using \textit{ab initio} pseudopotential method based on the density functional theory. Knowledge on physical origin of adhesion on oxide and metal interface is essential for the development of various industrial applications. However, the atomic structure of the interface has not been fully clarified yet. In this study, surface energies of the Al-terminate and Ni-terminate NiAl(001) are calculated. Geometry configurations and bond adhesion strength between$\theta $c-Al$_{2}$O$_{3}$ and NiAl(001) are determined..   

Calculated atomic arrangement and impurity bonding at a $\kappa $-Alumina -- Al(771) interface.

First principles optimization of a $\kappa $-Alumina -- Al(771) superlattice shows that the incompliant oxide causes substantial disorder in the adjacent, soft metal layers. An H ``probe atom'' is found to bind best in the disrupted metal region, suggesting that this is the locus of initial failure of a protective oxide layer.   

Local dielectric constants in metal-oxide and oxide-oxide interfaces: an ab initio approach to address interfacial effects

Recent experiments indicate that the dielectric constants of thin film oxides are strongly affected by interfaces formed between the oxide and metal or between different oxides. Such interfacial effects will be crucial to high-k dielectric stacks employing oxide materials with nanometer thickness. Therefore, systematic studies on interfacial effects are very important for materials selection and process design of gate stacks. In this presentation, we study on local dielectric constants in metal-oxide and oxide-oxide interfaces within the first-principles framework. Firstly, we introduce an efficient method to calculate local dielectric constants by employing slab models exposed to the vacuum. The static as well as optical dielectric constants are obtained from the change in electrostatic potentials upon the application of external electric fields. Our method can be easily adopted using conventional codes without any modification of the program. We apply this method to investigate interfacial dielectric constants in Au/MgO, Ni/ZrO2, Pt/HfO2, Ni/HfO2, Al/HfO2, SiO2/HfO2, and Al2O3/HfO2 interfaces. Our results show the presence of interfacial region with dielectric constants significantly different from that of the bulk. Microscopic explanations will be provided based on the dynamic charges and hardening/softening of phonons.   

The role of metal/transition metal oxide/organic interface

In this paper, we report a study with UPS and XPS data of metal/transition-metal-oxide/organic interfaces. Transition metal oxides are widely used in organic light- emitting (OLEDs) in recently years, such as Wo$_{3}$, ReO$_{3}$, MoO$_{3}$, and V$_{2}$O$_{5}$. These metal oxides have been proven to be good hole injection layers in OLEDs, interlayers in tandem OLEDs, and nanocomposite electrodes. Although a large number of studies have been made, little is known about the mechanism of metal/transition-metal-oxide/organic interfaces. UPS and XPS data performed by synchrotron radiation research show that these oxides would catch electrons from organic and results in p-type doping in organic material. In addition, there is a significant structure transition from insulating metal oxide to metallic metal oxide. As a result of high work function metallic metal oxides in anode structures and p-type doping organic hole transport layers (HTLs), holes can easily be injected from anode to HTLs. Current-voltage characteristics (I-V) and quantum-efficiency ($\eta $-J) measurements also show the improvement of device performance with insertion of thin transition metal oxides between anodes HTLs.   

Stability, structure, and electronic properties of chemisorbed oxygen and thin surface oxides on Ir(111)

Iridium-based catalysts are widely used in several important chemical reactions. Despite this, very little is known about the surface structure of the catalyst and the atomic and molecular processes involved. As a first step towards a microscopic understanding, we use density-functional theory, coupled with \textit{ab initio} atomistic thermodynamics [1], to investigate chemisorption of oxygen on Ir(111), and the stability of surface oxides. We find for on-surface adsorption, oxygen prefers the fcc-hollow site for all coverages considered, where with increasing coverage, the adsorption energy decreases substantially. Subsurface adsorption is found to be highly unfavourable. The most favourable surface-oxide-like structure has a tri-layer-like (O-Ir-O) configuration, which however, the $(p,T)$ phase diagram predicts is only metastable. For practically all conditions, except ultra-high vacuum, the bulk oxide is thermodynamically the most stable, and the only other stable phase predicted is the on-surface (2x2)-O structure for coverage 0.25 ML [2]. These studies point to the possible importance of oxidized iridium for heterogeneous oxidation reactions. [1] C. Stampfl, Catal. Today 105, 17 (2005). [2] H. Zhang, A. Soon, B. Delley and C. Stampfl, submitted to Phys. Rev. B.