Showing papers by "Robert M. Wallace published in 2010"

First-principles study of metal–graphene interfaces

Abstract: Metal-graphene contact is a key interface in graphene-based device applications, and it is known that two types of interfaces are formed between metal and graphene. In this paper, we apply first-principles calculations to twelve metal-graphene interfaces and investigate the detailed interface atomic and electronic structures of physisorption and chemisorption interfaces. For physisorption interfaces (Ag, Al, Cu, Cd, Ir, Pt, and Au), Fermi level pinning and Pauli-exclusion-induced energy-level shifts are shown to be two primary factors determining graphene’s doping types and densities. For chemisorption interfaces (Ni, Co, Ru, Pd, and Ti), the combination of Pauli-exclusion-induced energy-level shifts and hybridized states’ repulsive interactions lead to a band gap opening with metallic gap states. For practical applications, we show that external electric field can be used to modulate graphene’s energy-levels and the corresponding control of doping or energy range of hybridization.

Characteristics of high-k Al2O3 dielectric using ozone-based atomic layer deposition for dual-gated graphene devices

Abstract: We present characteristics of dual-gated graphene devices with an Al2O3 gate dielectric formed by an O3-based atomic-layer-deposition process. Raman spectra reveal that a O3 process at 25°C on single-layered graphene introduces the least amount defects, while a substantial number of defects appear at 200 °C. This graphene device with O3-based Al2O3 dielectric demonstrates a heterojunction within the graphene sheet when applying VTG and VBG and possesses good dielectric properties with minimal chemical doping, including a high dielectric constant ∼8, low hysteresis width ∼0.2 V, and low leakage current and a carrier mobility of 5000 cm2/V s 25°C in ambient.

Comparison of the sputter rates of oxide films relative to the sputter rate of SiO2

Abstract: There is a growing interest in knowing the sputter rates for a wide variety of oxides because of their increasing technological importance in many different applications. To support the needs of users of the Environmental Molecular Sciences Laboratory, a national scientific user facility, as well as our research programs, the authors made a series of measurements of the sputter rates from oxide films that have been grown by oxygen plasma-assisted molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, electrochemical oxidation, or sputter deposition. The sputter rates for these oxide films were determined in comparison with those from thermally grown SiO2, a common reference material for sputter rate determination. The film thicknesses and densities for most of these oxide films were measured using x-ray reflectivity. These oxide films were mounted in an x-ray photoelectron or Auger electron spectrometer for sputter rate measurements using argon ion sputtering. Although the primary objec...

Fermi level unpinning of GaSb (100) using plasma enhanced atomic layer deposition of Al2O3

Abstract: N-type and p-type GaSb metal-oxide-semiconductor capacitors (MOSCAPs) with atomic-layer-deposited (ALD) and plasma-enhanced-ALD (PEALD) Al2O3 dielectrics are studied to identify the optimum surface preparation and oxide deposition conditions for a high quality oxide-semiconductor interface. The ALD Al2O3/GaSb MOSCAPs exhibit strongly pinned C-V characteristics with high interface state density (Dit) whereas the PEALD Al2O3/GaSb MOSCAPs show unpinned C-V characteristics (low Dit). The reduction in Sb2O3 to metallic Sb is suppressed for the PEALD samples due to lower process temperature, identified by x-ray photoelectron spectroscopy analysis. Absence of elemental Sb is attributed to unpinning of Fermi level at the PEALD Al2O3/GaSb interface.

Impact of Interfacial Oxygen Content on Bonding, Stability, Band Offsets, and Interface States of GaAs:HfO2 Interfaces

Abstract: A theoretical study of the structural and electronic properties of GaAs:HfO2 interface is performed using the density functional theory method. For the interfaces with Ga−O bonds (formed between Ga-terminated GaAs surface and O-terminated HfO2 surface), we found that the interfaces with 10% and 20% interfacial oxygen removed are thermodynamically stable over a wide range of O chemical potential. By gradually decreasing the interfacial O content from 100% to 30% (by changing O chemical potential corresponding to varying the growth condition), the valence band offset increases from 1.06 to 3.34 eV. The analysis of the electronic structures indicates that for interfaces with high interfacial oxygen content, the interface gap states are induced by interfacial Ga dangling bonds and As−As dimer pairs, which are formed by interface atomic structure reconstruction. The decreasing interfacial oxygen content causes the decrease of the charge states of interfacial Hf and Ga leading to metallic interface states. For ...

First-principles study of GaAs(001)-β2(2×4) surface oxidation and passivation with H, Cl, S, F, and GaO

Abstract: The interactions of oxygen atoms on the GaAs(001)-β2(2×4) surface and the passivation of oxidized GaAs(001)-β2(2×4) surface were studied by density functional theory. The results indicate that oxygen atoms adsorbed at back-bond sites satisfy the bond saturation conditions and do not induce surface gap states. However, due to the oxygen replacement of an As dimer atom at a trough site or row site, the As–As bond is broken, and gap states are produced leading to the Fermi level pinning because of unsaturated As dangling bonds. Atomic H, Cl, S, F, and the molecular species GaO were examined to passivate the unsaturated As dangling bond. The results show that H, Cl, F, and GaO can remove such gap states. It is also found that the interaction of S with the unsaturated As dangling bond does not remove the gap states, and new gap states are generated upon single S adsorption. A higher S coverage forms S–S dimer pairs which passivate two unsaturated As atoms, and removes the As-induced gap states.

An in situ examination of atomic layer deposited alumina/InAs"100… interfaces

Abstract: Undoped InAs(100) wafers were either passivated with sulfur from a (NH4)2Sx solution or etched with NH4OH and then characterized with monochromatic x-ray photoelectron spectroscopy (XPS) before and after in situ deposition of Al2O3 by atomic layer deposition. Sulfur passivation minimized oxidation. Trimethyl aluminum (TMA) exposure reduced trivalent indium and arsenic oxidation states. The In1+ chemical state persisted while elemental arsenic remained at the Al2O3/InAs interface prior to TMA exposure and possibly a mixture of As–As and As–Al bonds were present afterwards. The In 3d5/2 peak line shape from bulk InAs differed from previous XPS experiments on epitaxial InxGa1−xAs.

The effect of graphite surface condition on the composition of Al2O3 by atomic layer deposition

Abstract: We present a study of the nucleation of atomic layer deposition of Al2O3 on highly oriented pyrolytic graphite (HOPG) using trimethlyaluminum (TMA) with ozone as the oxidant (TMA/O3). In situ x-ray photoelectron spectroscopy (XPS) is used to study TMA/O3 depositions on HOPG. We examine the dependence of TMA/O3 nucleation on deposition temperature and characterize the morphology and uniformity of deposited films by ex situ atomic force microscopy. The impact of several predeposition surface treatments of the graphite surface condition is discussed, particularly with regard to the presence of adsorbed atmospheric contamination.

Synthesizing graphene from metal-carbon solutions using ion implantation

Abstract: A method and semiconductor device for synthesizing graphene using ion implantation of carbon. Carbon is implanted in a metal using ion implantation. After the carbon is distributed in the metal, the metal is annealed and cooled in order to precipitate the carbon from the metal to form a layer of graphene on the surface of the metal. The metal/graphene surface is then transferred to a dielectric layer in such a manner that the graphene layer is placed on top of the dielectric layer. The metal layer is then removed. Alternatively, recessed regions are patterned and etched in a dielectric layer located on a substrate. Metal is later formed in these recessed regions. Carbon is then implanted into the metal using ion implantation. The metal may then be annealed and cooled in order to precipitate the carbon from the metal to form a layer of graphene on the metal's surface.

Effects of O3 and H2O oxidants on C and N-related impurities in atomic-layer-deposited La2O3 films observed by in situ x-ray photoelectron spectroscopy

Abstract: The effect of H2O and O3 oxidants on the behavior of residual C and N-related impurities as well as Si out-diffusion and interfacial layer formation in atomic-layer-deposited La2O3 films grown at 250 °C were examined using in situ x-ray photoelectron spectroscopy. The silicate formation was suppressed in a La2O3 film grown using O3 compared to that deposited using H2O, but interfacial layer growth was enhanced. The accumulation of C and N-related residues with low binding energy, which originated from incomplete reactions, was suppressed in La2O3 films grown using O3. However, the use of O3 resulted in La-carbonate phase in film.

Structure of Ultra-Thin Diamond-Like Carbon Films Grown with Filtered Cathodic Arc on Si(001)

Abstract: The structure of 3 nm and 15 nm diamond-like carbon films, grown on Si(001) by filtered cathodic arc, was studied by angle-resolved X-ray photoelectron spectroscopy (ARXPS) and transmission electron microscopy (TEM). The ARXPS data was deconvolved by employing simultaneous-fitting, which allowed for a clear deconvolution of the Si 2p and C 1s spectra into their different chemical contributions. An analysis of the take-off angle dependence of the peak intensities allowed for an independent identification of the physical origin of the chemical species. It was shown that the C 1s peak at 283.3 eV and the Si 2p peak at 99.6 eV correspond to SiC, and that the C/Si interface of the 3 nm film consists of a stoichiometric approximately 1 nm SiC layer. To quantify the sp(3)-sp(2) ratio it was necessary to take into account not only their associated C 1s XPS-peak intensities, but also their take-off angle dependence. The thickness of the films obtained through ARXPS closely agrees with cross-sectional TEM images.

Reduced Metal Contamination in Atomic-Layer-Deposited HfO2 Films Grown on Si Using O3 Oxidant Generated Without N2 Assistance

Abstract: The physicochemical and electrical properties of atomic-layer-deposited HfO 2 films grown using 0 3 generated with and without N 2 assistance were examined. Compared to the films grown using conventionally generated 0 3 with N 2 assistance, the HfO 2 film grown using O 3 generated without N 2 assistance had less metal impurities (mainly Cr) possibly due to the etching of stainless ozone delivery line. This induced a higher physical density of the film and a lower Si concentration in the film, which resulted in enhanced dielectric properties and reliability such as dielectric breakdown and resistance to constant voltage stress.

Fermi level unpinning of GaSb(100) using Plasma Enhanced ALD Al 2 O 3 dielectric

Abstract: Antimonide based compound semiconductors have gained considerable interest in recent years due to their superior electron and hole transport properties [1–3]. Among the various high mobility material systems (Fig. 1), arsenic-antimonide based MOS-HEMTs have great potential to enable complementary logic operation at low supply voltage. Integrating a high quality dielectric is key to demonstrating a scalable arsenic-antimonide MOS-HEMT architecture for 15 nm logic technology node and beyond. It is hypothesized that an ultra-thin GaSb surface layer is more favorable toward high-к integration than In 0.2 Al 0.8 Sb barrier as it avoids Al at the interface and the associated surface oxidation. Here, we study the effects of various surface passivation approaches on the capacitance-voltage characteristics (C-V) and the surface chemistry of n-type and p-type GaSb(100) MOS capacitors made with ALD and Plasma Enhanced ALD (PEALD) Al 2 O 3 dielectric. We demonstrate for the first time, unpinned Fermi level in GaSb MOS system with high-к PEALD Al 2 O 3 dielectric using admittance spectroscopy and XPS analysis.

First-Principles and Quantum Transport Studies of Metal-Graphene End Contacts

Abstract: Metal-graphene contact is of critical significance in graphene-based nanoelectronics. There are two possible metal-graphene contact geometries: side-contact and end-contact. In this paper, we apply first-principles calculations to study metal-graphene end-contact for these three commonly used electrode metals (Ni, Pd and Ti) and find that they have distinctive stable end-contact geometries with graphene. Transport properties of these metal-graphene-metal (M-G-M) end-contact structures are investigated by density functional theory non-equilibrium Green’s function (DFT-NEGF) algorithm. The Transmission as a function of chemical potential (E-EF) shows asymmetric curves relative to the Fermi level. Transmission curves of Ni-G-Ni and Ti-G-Ti contact structures indicate that bulk graphene sheet is n-doped by Ni and Ti electrodes, but that of Pd-G-Pd shows p-doping of graphene by Pd electrode. The contact behaviors of these electrodes are consistent with experimental observations.

Contact resistance reduction to FinFET source/drain using dielectric dipole mitigated Schottky barrier height tuning

Abstract: We demonstrate for the first time contact resistance reduction using dielectric dipole mitigated Schottky barrier height (SBH) tuning on a FinFET source/drain. Different techniques for forming a SiO 2 /AlO x dipole layer are investigated using diodes. FinFETs, with contacts containing a SBH tuning dipole layer, are also presented. Reduction of the SBH by 100meV from the AlO x /SiO 2 dipole results in a 10Ω-µm2 reduction in specific contact resistivity (ρ CO ) and a 100Ω-µm reduction in FinFET source/drain resistance (R S/D ). Larger reductions of ρ CO should be possible if chemically formed or atomic-layer deposited SiO 2 is used in the dipole layer instead of interfacial SiO 2 due to the larger SBH reduction (ΔSBH ∼300meV) obtained from these oxidation methods. Contact formation without the need for silicide makes this technique very promising for emerging devices, alternative channel materials, and sub-22nm CMOSFETS.

Interfacial Chemistry of Oxides on III-V Compound Semiconductors

Abstract: This chapter reviews the interfacial chemistry observed for III-V semiconductors, with particular attention to native and deposited oxide gate dielectrics. The chemical states observed at the III-V surface as studied by x-ray photoelectron spectroscopy are presented, and the suppression of oxide formation with deposition techniques, particularly atomic layer deposition, is discussed. The resultant electrical properties of these interfaces are then reviewed, with attention to the interpretation of capacitance-voltage and related transistor measurements.

Band Offset Control by Interfacial Oxygen Content at GaAs:HfO2 interfaces

Abstract: The impact of interfacial oxygen content on the band offsets of GaAs:HfO2 interfaces was investigated using the density functional theory (DFT) method. Reference potential method was used to determine the band offsets. Moreover, GW correction was utilized to find more accurate value of the valence band edge of HfO2 and hence obtain more accurate band offsets. With gradually decreasing the interfacial O content from 100% to 30% (by changing O chemical potential corresponding to varying the growth condition), the valence band offset increases from 1.06 to 3.34 eV. It is found that this increase of the valence band offsets is inversely proportional to the charge loss of interfacial Ga atoms. Specifically, less charge loss of interfacial Ga induces less charge transfer from GaAs to HfO2 side. Consequently, the less charge loss of interfacial Ga essentially leads to an increase of the valence band offsets.