Showing papers by "Robert M. Wallace published in 2009"

Detection of Ga suboxides and their impact on III-V passivation and Fermi-level pinning

Abstract: The passivation of interface states remains an important problem for III-V based semiconductor devices. The role of the most stable bound native oxides GaOx (0.5≤x≤1.5) is of particular interest. Using monochromatic x-ray photoelectron spectroscopy in conjunction with controlled GaAs(100) and InGaAs(100) surfaces, a stable suboxide (Ga2O) bond is detected at the interface but does not appear to be detrimental to device characteristics. In contrast, the removal of the Ga 3+ oxidation state (Ga2O3) is shown to result in the reduction of frequency dispersion in capacitors and greatly improved performance in III-V based devices.

Atomic Layer Deposition of Dielectrics on Ge and III–V Materials for Ultrahigh Performance Transistors

Abstract: The prospect of utilizing alternative transistor channel materials for ultrahigh performance transistors will require suitable gate dielectrics for surface-channel field-effect devices. With the utilization of deposited gate dielectrics in large-scale production for Si-based integrated circuits by atomic layer deposition, extending this technology to channel materials that exhibit high bulk mobility behavior is of interest. A review of the current status for atomic layer deposited high-κ dielectrics on Ge and III–V channel materials is presented.

Copper-metal deposition on self assembled monolayer for making top contacts in molecular electronic devices.

Abstract: Molecular electronics is an attractive option for low-cost devices because it involves highly uniform self-assembly of molecules with a variety of possible functional groups. However, the potential of molecular electronics can only be turned into practical applications if reliable contacts can be established without damaging the organic layer or contaminating its interfaces. Here, a method is described to prepare tightly packed carboxyl-terminated alkyl self-assembled monolayers (SAMs) that are covalently attached to silicon surfaces and to deposit thin metallic copper top contact electrodes without damage to this layer. This method is based on a two-step procedure for SAM preparation and the implementation of atomic layer deposition (ALD) using copper di-sec-butylacetamidinate [Cu(sBu-amd)]2. In situ and ex situ infrared spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrical measurements are used to characterize the chemical modification of the Si/SAM in...

In situ studies of Al2O3 and HfO2 dielectrics on graphite

Abstract: Deposition of Al2O3 and HfO2 dielectrics on graphite is studied as a route to the formation of a high-κ dielectric on graphene. Electron beam evaporation of metal Al and Hf is followed by a separate oxidation step. Reactive e-beam deposition of HfO2 by introduction of O2 to the deposition chamber is also demonstrated as an alternative to the two-step metal deposition and oxidation approach. We employ in situ x-ray photoelectron spectroscopy to study reactions between the substrate and deposited film and ex situ atomic force microscopy to examine the dielectric film morphology.

The significance of core-level electron binding energies on the proper analysis of InGaAs interfacial bonding

Abstract: The detection and removal of interfacial oxides on InGaAs semiconductors is of critical importance for their implementation as high-mobility channels for improved complementary metal oxide semiconductor device performance. X-ray photoelectron spectroscopy is a powerful tool to determine the chemical bonding at these interfaces. To correctly analyze these spectra, one must consider the binding energies and escape depths of the core-level electrons being detected, as monolayer level interfacial oxides (As–O and Ga–O) are detectable only in certain surface sensitive spectral regions. Also, inherent asymmetries associated with the In spectra must be taken into account for analysis of In-oxide bonding.

Near band edge Schottky barrier height modulation using high-κ dielectric dipole tuning mechanism

Abstract: Schottky barrier height tuning using high-κ/SiO2 interfacial dipoles is reported. Schottky barrier heights of 1.0 and 0.2 eV are observed in a TaN/p-Si diode by insertion of thin layers of high-κ (LaOx,AlOx) and SiO2 at the metal-semiconductor interface. The dipole tunes the effective work function of TaN/p-Si by more than 0.8 eV to achieve effective Schottky barrier heights near conduction and valence band edge. LaOx (n-type) and AlOx (p-type) have a dipole potential offsets estimated to be 0.3 and 0.5 V, respectively. Applications to lowering contact resistivity are discussed, as well as a comparison of other dipole offsets.

Report on the 47th IUVSTA Workshop ‘Angle-Resolved XPS: the current status and future prospects for angle-resolved XPS of nano and subnano films’

Abstract: Report on the 47th IUVSTA Workshop ‘Angle-Resolved XPS: the current status and future prospects for angle-resolved XPS of nano and subnano films’ A. Herrera-Gomez,a,b∗ J. T. Grant,c P. J. Cumpson,d† M. Jenko,e F. S. Aguirre-Tostado,b C. R. Brundle,f T. Conard,g G. Conti,h C. S. Fadley,i J. Fulghum,j K. Kobayashi,k L. Kover,l H. Nohira,m R. L. Opila,n S. Oswald,o R. W. Paynter,p R. M. Wallace,b W. S. M. Wernerq and J. Wolstenholmer

Characterization of the ''clean-up'' of the oxidized Ge(100) surface by atomic layer deposition

Abstract: While the “clean-up” effect on III-V substrates has recently been well documented interfacial reactions during atomic layer deposition (ALD) on Ge substrates are not fully explored. The “clean-up” of Ge oxides is studied by interrupting the ALD process following individual precursor pulses for in situ monochromatic x-ray photoelectron spectroscopy analysis. Germanium oxides are found to be reduced by TMA and water, while an interfacial GeON layer is only affected by the initial TMA pulse. Oxide free germanium surfaces behave analogously to a surface with initial native oxides since they are oxidized measurably prior to the first TMA pulse due to residual oxidants in a commercial ALD chamber.

Half-Cycle Atomic Layer Deposition Reaction Study Using O3 and H2O Oxidation of Al2O3 on In0.53Ga0.47As

Abstract: The effect of water and ozone as the oxidant in the atomic layer deposition (ALD) of aluminum oxide on the ammonium-sulfide-passivated In 0.53 Ga 0.47 As surface is compared using X-ray photoelectron spectroscopy (XPS) after each "half-cycle" of the ALD process. While the first half-cycle of the aluminum precursor tri-methyl aluminum (TMA) reduces the residual native oxides to within detection limits of XPS, the ozone oxidation process causes significant reoxidation of the substrate in comparison to the water-based process. Subsequent TMA pulses fail to remove the excess interfacial oxides caused by ozone oxidation, resulting in the formation of an oxide interlayer.

New insight into Fermi-level unpinning on GaAs: Impact of different surface orientations

Abstract: We have systematically studied NMOSFETs, MOSCAPs, and the interfacial chemistry on GaAs (100), (110), (111)A and (111)B-four different crystalline surfaces with direct ALD Al 2 O 3 . We found that a much higher drain current on GaAs(111)A NMOSFET can be achieved compared to that obtained on the other 3 surfaces. Also, the results of MOS-CAPs and the interfacial chemistry obtained on the (111)A surface are very different from those others. These experimental results conclusively demonstrate that Fermi-level on the GaAs (111)A surface is indeed unpinned and Fermi-level pinning is not an intrinsic property of GaAs, but is orientation dependent thus related to surface chemistry.

Surface Studies of III-V Materials: Oxidation Control and Device Implications

Abstract: Despite the long history of research on III-V semiconductor materials, the determination and control of the individual oxidation states of these substrates is still poorly understood. In this study, multiple surface treatments have been employed to determine the presence and concentration of the individual oxidation states of both As and Ga. The removal of these surface oxides is demonstrated through particular techniques. It is established that not all of these oxidation states are of poor electrical quality, while eliminating the presence of certain oxidation states results in a marked improvement in MOS device performance.

Structural and Electrical Properties of HfO2/n-InxGa1-xAs structures (x: 0, 0.15, 0.3 and 0.53)

Abstract: In this work results are presented of an investigation into the structural and electrical properties of HfO2 films on GaAs and InxGa1-xAs substrates for x: 0.15, 0.30, and 0.53. The capacitancevoltage responses of the GaAs and InxGa1-xAs (x: 0.15 and 0.30) are dominated by an interface defect response. Analysis of these samples at 77K indicates that the defect density is > 2.5x1013 cm-2. For the HfO2/In0.53Ga0.47As system, 77K capacitance-voltage responses indicate surface accumulation is achieved. The results are consistent with a high defect density, with an energy level {greater than or equal to}0.75 eV above the valence band in the HfO2/InxGa1-xAs system, where the defect energy with respect to the valence band, does not change with the composition of the InxGa1-xAs. The HfO2/In0.53Ga0.47As interface exhibits two defects at 0.3eV (1.7x1013cm-2eV) and 0.61eV (1.5x1013cm-2eV) above the valance band edge. The defect at 0.61eV is removed by forming gas annealing at 325oC.

Atomic-Layer-Deposited Al2O3 as Gate Dielectrics for Graphene-Based Devices

Abstract: We present a facile route to deposit a uniform Al2O3 layer on a highly oriented pyrolytic graphite (HOPG) surface using atomic layer deposition (ALD). Al2O3 layers deposited from TMA (trimethylaluminum)/ H2O chemistry showed selective deposition only on step edges. However, TMA/O3 chemistry resulted in the deposition of Al2O3 layers on basal planes of HOPG, which has a chemically inert surface. An O3-pretreatement followed by Al2O3 deposition using TMA/O3 chemistry produced conformal and uniform Al2O3 dielectric layers with a small RMS roughness of ~ 0.2 nm. This suggests that O3-pretreatement prior to ALD deposition makes the chemically inert HOPG surface reactive toward ALD precursors, which leads to the desired two-dimensional growth mode. High-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy revealed that this O3 process used for Al2O3 deposition does not introduce a significant defect concentration to the top graphene layer. The dielectric constant of the deposited Al2O3 film on top of the HOPG surface was found to be ~ 9 from C-V measurements.

Thermal stability of lanthanum in hafnium-based gate stacks

Abstract: The controlled addition of La to HfxSi1−xO2/SiO2/Si dielectric stacks has been shown to enable the engineering of the work function to appropriate levels when TaN or TiN is employed as the capping metal gate. Work function tuning has been suggested to be controlled by La diffusion into the Hf-based dielectric as a result of further thermal treatments. In this paper, we performed high resolution angle resolved x-ray photoelectron spectroscopy (ARXPS) studies to investigate the chemical depth profile distribution of TaN/La2O3/HfO2/SiO2/Si dielectric stacks exposed to a nitridation treatment by NH3 at 700 °C. The stoichiometry and distribution of the HfO2 and SiO2 layers was examined using a self-consistent ARXPS analysis. This study shows that La diffuses to the SiO2/HfO2 interface, and that subsequent rapid thermal annealing at 1000 °C for 5 s does not significantly change the La distribution.

In-situ Studies of High-κ Dielectrics for Graphene-Based Device

Abstract: The discovery and experimental isolation of graphene, a single layer of carbon atoms in a hexagonal lattice, has spurred a tremendous amount of research in recent years due to its unique transport and physical properties (1). These novel characteristics suggest that graphene is a viable candidate material for nanoelectronic switches beyond the end of the Si-CMOS roadmap. All such proposed devices will require the integration of a scalable high-κ dielectric. Engineering of chemical interactions and bonding at the dielectric-graphene interface will be crucial to the performance and reliability of any device. However, the inert nature of the basal plane of graphene presents challenges to the deposition of thin dielectrics. We present in-situ studies of high-κ dielectric formation by electron beam evaporation of metals and subsequent controlled oxidation. The work was performed using an in-situ UHV cluster system with analytical and deposition modules, including monochromatic x-ray photoelectron spectroscopy (XPS), electron beam evaporation, oxidation and annealing as described elsewhere (2). In an effort to overcome the inert nature of the graphene surface, metals were deposited by e-beam evaporation on the surface of natural graphite and subsequently oxidized to form a thin dielectric. Two metals, Al and Hf, were studied in this work as a model system to establish the interfaces associated with Al2O3 and HfO2 on graphene. The metals were deposited on exfoliated and annealed graphite surfaces. The metals were oxidized using high-purity O2, UV O3 and RFgenerated O3 at several temperatures ranging from 25°C to 300°C. We observed that the Al films were fully oxidized for the exfoliated graphite surfaces, but not for the annealed graphite surfaces as determined from the metallic states in the Al 2p XPS spectrum shown in Fig. 1. The metallic state of the Al 2p XPS peak is clearly detected for the cleaned surface but not in the unannealed case. A calculation of thickness based upon C 1s attenuation indicates that the thickness of the deposited metal layer is 8.3Ǻ, assuming the overlayer is uniform. In order to better understand the interface chemistry at the graphite/metal(oxide) interface we also evaluated the interface between graphite and Hf, which has a larger electronegativity difference with C than Al. The formation of HfC bonds was observed immediately after deposition of Hf at a substrate temperature of 25°C. Oxidation in an O3 environment at 200°C converted the HfC to HfO2. Furthermore, a significant difference is observed in the C 1s peak of oxidized Al and oxidized Hf. The high binding energy side of the C1s for the HfO2 shows a larger asymmetry than that from the original graphite surface in comparison to Al2O3 formation (not shown). This data suggests that C-C bonds are broken at the graphene/Hf interface during the oxidation process. A companion sample with singleand few-layer graphene flakes on SiO2 processed simultaneously with the graphite sample shows complete consumption of the graphene layer as a result of HfC to HfO2 conversion. The consumption of graphene was confirmed by optical microscope images and Raman spectroscopy before and after in-situ processing, supporting the fact that graphene is damaged during deposition and oxidation of Hf on C. This also supports utilizing large area graphite as a reasonable surface with which to investigate graphene interfacial chemistry. The effect of the chamber background pressure during metal deposition was also investigated. Deposition at 4×10 mbar pressure led to reduced HfC formation, below the detection limit of XPS as shown in Fig. 2 The effect of carbide bond formation and damage to graphene layers during oxidation will have a major effect on the selection of metals and processes for both metal contact deposition and gate dielectric integration.

Contact resistance studies of metal on HOPG and graphene stacks

Abstract: Contact resistance is one of the major factors limiting the performance of future nanoelectronic devices Although there has been significant progress in graphene based devices since 2004[1, 11, 13, 19, 20], there have been few studies of factors such as metal type, metal workfunction and number of layers in the graphene stack on metal/graphene contact resistance In this work, contact resistance measurements of various metals (Cr, Ni, Pd, Pt) on Highly Oriented Pyrolytic Graphite (HOPG) and graphene (single layered and few layered) were performed The total resistance is independent of distance indicating a contact resistance dominated system The contact resistance is observed to be similar for a wide variety of metals although the metal workfunction varies from 43 eV to 56 eV Similar measurements were performed for metal on single‐ and multi‐layer graphene The electrical results are compared with XPS measurements of thin metal on HOPG Issues regarding the characterization and interpretation of contact resistance for metal‐semimetal systems are discussed

Physical and optoelectronic characterization of reactively sputtered molybdenum-silicon-nitride alloy metal gate electrodes

Abstract: With continued transistor scaling, work function tuning of metal gates has become important for advanced complementary-metal-oxide-silicon applications. The work function tuning of reactively sputtered MoxSiyNz (also referred to as MoSiN) gates has been studied through the incorporation of nitrogen. The nitrogen concentration in the MoSiN films was altered by controlling the gas flow ratio, RN=N2/(N2+Ar), during gate deposition. The sheet resistance (Rs) of blanket MoSiN films, measured using four-point resistance method, was found to increase as the gas flow ratio was varied from 10% to 40%. Current-voltage measurements confirmed excellent electrical stability of MoSiN/SiO2/p-Si gate stack for applied electric fields ranging up to 6 MV/cm. High frequency capacitance-voltage measurements were used to extract the MoSiN work function (Φm) using the relationship between the flatband voltage (VFB) and the oxide thickness (tox). The extracted MoSiN/SiO2 interfacial barrier heights, obtained through the interna...

Materials Science of Graphene for Novel Device Applications

Abstract: Realization of graphene based devices will require a controlled integration of graphene into a device structure with multiple material components of metals and insulators. To investigate the fundamental materials problems of graphene devices, a complimentary team of researchers at UTD is applying experimental and theoretical methods to the graphene/metal and graphene/dielectric interface problems. To assess the large area graphene synthesis and the corresponding material properties, graphene oxides and grain boundaries in graphene are also examined by experiments in close connection with modeling study. Through a strong collaborative research in synthesis, characterization, and modeling, we have developed a fundamental understanding of graphene material properties which can facilitate diverse graphene based devices applications.

High rate gas dosing for tip based nanofabrication processes

Abstract: Tip based nanofabrication (TBN) processes promise unprecedented degrees of control and precision for the manufacture of nanostructured materials and devices. These processes use atomic force microscope or scanning tunneling microscope tips to create localized electric fields, electron beams, and other catalyzing conditions to control and detect the position, size, dimension, and orientation of nanostructures. Tip based approaches have deposited metals, oxides, and organic molecules to name a few. Often, a gas phase precursor is required to provide the material for the deposit. The TBN conditions for gas dosing are unique compared to other fabrication processes, e.g., chemical vapor deposition. The manufacture of precision nanostructures requires a contamination-free environment, and hence ultrahigh vacuum conditions must be maintained in the chamber. This can cause a gas jet from a doser to spread into a wide fan resulting in a small precursor flux with a broad distribution. This makes it difficult to mee...

Impact of surface preparations on the transport characteristics of In x Ga 1−x As metal-oxide-semiconductor field effect transistors (MOSFETs)

Abstract: Although III–V semiconductors have intrinsically higher electron mobility compared to Si, a high-quality gate stack with low D it is still required to realize III–V surface channel MOSFETs Recently, significant effort has been focused on improving high-k/III–V interfaces using different interfacial passivation layers (IPL) and surface passivation techniques These include MBE deposition of Ga 2 O 3 (Gd 2 O 3 ) [1], deposition of amorphous Si (a-Si) IPL [2], SiH 4 -NH 3 based passivation [3], PH 3 plasma based passivation [4] and deposition of dielectrics in direct contact with the substrate [2] In this work In x Ga 1−x As (x=053, x=065) MOSFET transport characteristics have been studied for three different ex-situ surface passivation techniques with atomic layer deposited (ALD) HfO 2 gate dielectric

First-principles study of HfO2: GaAs interface passivation by Si and Ge

Abstract: We investigated the HfO2:GaAs interface electronic structure and interface passivation by first principles calculations. The HfO2:GaAs interface of HfO2 terminated with four O atoms and GaAs terminated two Ga atoms is found to be the most energetically favorable. It is found that the interface states mainly arise from the interfacial charge mismatch, more specifically from the electron loss of interfacial As. Si or Ge as an interfacial passivating layer helps to maintain the charge of interfacial As and hence reduce the interface states.

Interfacial engineering of InGaAs/high-k metal-oxide-semiconductor field effect transistors (MOSFETs)

Abstract: The electrical properties of In x Ga 1-x As (x=0.53, x=0.65) MOSFETs have been studied for three different ex-situ surface passivation techniques (HF clean, (NH 4 ) 2 S clean and PECVD a-Si interlayer) with HfO 2 gate dielectric (4 nm and 8 nm) and Ni/Au gate metal. In 0.65 Ga 0.35 As devices demonstrate much higher drive current than In 0.53 Ga 0.47 As devices. Devices with an a-Si IPL exhibit the highest drive current and mobility, whereas (NH 4 ) 2 S surface cleaned devices show higher drive current, trans-conductance and mobility compared to HF cleaned devices. Also devices with a-Si IPL and (NH 4 ) 2 S clean shows the highest and the lowest sub-threshold swing, respectively. Furthermore charge pumping measurements show a similar average mid-gap D it indicating that mid-gap D it is not related to the improvement in transport characteristics.