Showing papers by "Robert M. Wallace published in 2016"

Two-dimensional gallium nitride realized via graphene encapsulation

Abstract: A method to synthesize 2D layers of gallium nitride on SiC is reported. Epitaxial graphene preliminarily grown on SiC allows intercalation of gallium atoms on the SiC substrate and stabilizes the 2D gallium nitride islands formed by ammonolysis. The spectrum of two-dimensional (2D) and layered materials ‘beyond graphene’ offers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (∼5.0–6.0 eV), has clearly established that 2D nitrides are key to advancing 2D devices1. A gap, however, remains between the theoretical prediction of 2D nitrides ‘beyond hBN’2,3 and experimental realization of such structures. Here we demonstrate the synthesis of 2D gallium nitride (GaN) via a migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene. We theoretically predict and experimentally validate that the atomic structure of 2D GaN grown via MEEG is notably different from reported theory2,3,4. Moreover, we establish that graphene plays a critical role in stabilizing the direct-bandgap (nearly 5.0 eV), 2D buckled structure. Our results provide a foundation for discovery and stabilization of 2D nitrides that are difficult to prepare via traditional synthesis.

Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2 Plasma Exposure.

Abstract: Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such stra...

Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides.

Abstract: Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%. Here, we perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2D materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. We then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Our results suggest that sulfur-based 2D materials a...

Systematic study of electronic structure and band alignment of monolayer transition metal dichalcogenides in Van der Waals heterostructures

Abstract: Two-dimensional transition metal dichalcogenides (TMDs) are promising low-dimensional materials which can produce diverse electronic properties and band alignment in van der Waals heterostructures. Systematic density functional theory (DFT) calculations are performed for 24 different TMD monolayers and their bilayer heterostacks. DFT calculations show that monolayer TMDs can behave as semiconducting, metallic or semimetallic depending on their structures; we also calculated the band alignment of the TMDs to predict their alignment in van der Waals heterostacks. We have applied the charge equilibration model (CEM) to obtain a quantitative formula predicting the highest occupied state of any type of bilayer TMD heterostacks (552 pairs for 24 TMDs). The CEM predicted values agree quite well with the selected DFT simulation results. The quantitative prediction of the band alignment in the TMD heterostructures can provide an insightful guidance to the development of TMD-based devices.

Remote Plasma Oxidation and Atomic Layer Etching of MoS2

Abstract: Exfoliated molybdenum disulfide (MoS2) is shown to chemically oxidize in a layered manner upon exposure to a remote O2 plasma. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and atomic force microscopy (AFM) are employed to characterize the surface chemistry, structure, and topography of the oxidation process and indicate that the oxidation mainly occurs on the topmost layer without altering the chemical composition of underlying layer. The formation of S–O bonds upon short, remote plasma exposure pins the surface Fermi level to the conduction band edge, while the MoOx formation at high temperature modulates the Fermi level toward the valence band through band alignment. A uniform coverage of monolayer amorphous MoO3 is obtained after 5 min or longer remote O2 plasma exposure at 200 °C, and the MoO3 can be completely removed by annealing at 500 °C, leaving a clean ordered MoS2 lattice structure as verified by XPS, LEED, AFM, and scanning tunneling microscopy. This work sho...

Charge Mediated Reversible Metal-Insulator Transition in Monolayer MoTe2 and WxMo1-xTe2 Alloy.

Abstract: Metal–insulator transitions in low-dimensional materials under ambient conditions are rare and worth pursuing due to their intriguing physics and rich device applications. Monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semimetallic distorted octahedral structure (T′) with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T′ phase is observed in the formation energy diagram of monolayer WxMo1–xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T′) of monolayer WxMo1–xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T′ phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T′ phase (semimetallic) in monolayer WxMo1–xTe2 alloy...

Contact Metal–MoS2 Interfacial Reactions and Potential Implications on MoS2-Based Device Performance

Abstract: Thin films of contact metals, specifically Au, Ir, Cr, and Sc, are deposited on exfoliated, bulk MoS2 using electron beam deposition under two different reactor base pressures to determine the contact metal–MoS2 interface chemistry and its dependence on the reactor ambient. The high work function metal Au does not react with MoS2 regardless of reactor ambient. In contrast, interfacial reactions between MoS2 and another high work function metal, Ir, are observed when it is deposited under both high vacuum (HV, ∼ 1 × 10–6 mbar) and ultrahigh vacuum (UHV, ∼ 1 × 10–9 mbar). Interfacial reactions occur between metals with low work functions (Cr, Sc) near the electron affinity of MoS2 when the contact metal is deposited under UHV conditions. In addition, Sc is rapidly oxidized on the MoS2 surface, whereas Cr is only partially oxidized when deposited under HV conditions. This indicates that deposition chamber ambient can affect the contact metal chemistry in addition to the chemistry present at the contact metal...

MoS2–Titanium Contact Interface Reactions

Abstract: The formation of the Ti–MoS2 interface, which is heavily utilized in nanoelectronic device research, is studied by X-ray photoelectron spectroscopy. It is found that, if deposition under high vacuum (∼1 × 10–6 mbar) as opposed to ultrahigh vacuum (∼1 × 10–9 mbar) conditions are used, TiO2 forms at the interface rather than Ti. The high vacuum deposition results in an interface free of any detectable reaction between the semiconductor and the deposited contact. In contrast, when metallic titanium is successfully deposited by carrying out depositions in ultrahigh vacuum, the titanium reacts with MoS2 forming TixSy and metallic Mo at the interface. These results have far reaching implications as many prior studies assuming Ti contacts may have actually used TiO2 due to the nature of the deposition tools used.

Controllable growth of layered selenide and telluride heterostructures and superlattices using molecular beam epitaxy

Abstract: Layered materials are an actively pursued area of research for realizing highly scaled technologies involving both traditional device structures as well as new physics. Lately, non-equilibrium growth of 2D materials using molecular beam epitaxy (MBE) is gathering traction in the scientific community and here we aim to highlight one of its strengths, growth of abrupt heterostructures, and superlattices (SLs). In this work we present several of the firsts: first growth of MoTe2 by MBE, MoSe2 on Bi2Se3 SLs, transition metal dichalcogenide (TMD) SLs, and lateral junction between a quintuple atomic layer of Bi2Te3 and a triple atomic layer of MoTe2. Reflected high electron energy diffraction oscillations presented during the growth of TMD SLs strengthen our claim that ultrathin heterostructures with monolayer layer control is within reach.

Partially Fluorinated Graphene: Structural and Electrical Characterization.

Abstract: Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF4 as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF4 plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices. Additionally, this ionic C–F component (ionic bonding characteristic charge sharing) is found to be present only at low fluorine content; continuous fluorination led to a comp...

Surface Analysis of WSe2 Crystals: Spatial and Electronic Variability

Abstract: Layered semiconductor compounds represent alternative electronic materials beyond graphene. WSe2 is one of the two-dimensional materials with wide potential in opto- and nanoelectronics and is often used to construct novel three-dimensional architectures with new functionalities. Here, we report the topography and the electronic property of the WSe2 characterized by means of scanning tunneling microscopy and spectroscopy (STM and STS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry. The STM images reveal the presence of atomic-size imperfections and a variation in the electronic structure caused by the presence of defects and impurities below the detection limit of XPS. Both STS and photoemission reveal a spatial variation in the Fermi level position. The analysis of the core levels indicates the presence of different doping levels. The presence of a large concentration of defects and impurities has a strong impact on the electronic properties of the WSe2 surface....

First principles kinetic Monte Carlo study on the growth patterns of WSe2 monolayer

Abstract: The control of domain morphology and defect level of synthesized transition metal dichalcogenides (TMDs) is of crucial importance for their device applications. However, current TMDs synthesis by chemical vapor deposition and molecular beam epitaxy is in an early stage of development, where much of the understanding of the process-property relationships is highly empirical. In this work, we use a kinetic Monte Carlo coupled with first principles calculations to study one specific case of the deposition of monolayer WSe2 on graphene, which can be expanded to the entire TMD family. Monolayer WSe2 domains are investigated as a function of incident flux, temperature and precursor ratio. The quality of the grown WSe2 domains is analyzed by the stoichiometry and defect density. A phase diagram of domain morphology is developed in the space of flux and the precursor stoichiometry, in which the triangular compact, fractal and dendritic domains are identified. The phase diagram has inspired a new synthesis strategy for large TMD domains with improved quality.

Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts.

Abstract: In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

Superacid Passivation of Crystalline Silicon Surfaces.

Abstract: The reduction of parasitic recombination processes commonly occurring within the silicon crystal and at its surfaces is of primary importance in crystalline silicon devices, particularly in photovoltaics. Here we explore a simple, room temperature treatment, involving a nonaqueous solution of the superacid bis(trifluoromethane)sulfonimide, to temporarily deactivate recombination centers at the surface. We show that this treatment leads to a significant enhancement in optoelectronic properties of the silicon wafer, attaining a level of surface passivation in line with state-of-the-art dielectric passivation films. Finally, we demonstrate its advantage as a bulk lifetime and process cleanliness monitor, establishing its compatibility with large area photoluminescence imaging in the process.

Reduction of Fermi level pinning at Au–MoS2 interfaces by atomic passivation on Au surface

Abstract: Monolayer molybdenum disulfide (MoS2), which is a semiconducting material with direct band gap of ~1.8 eV, has drawn much attention for application in field effect transistors (FETs). In this connection, it is very important to understand the Fermi level pinning (FLP) which occurs at metal–semiconductor interfaces. It is known that MoS2 has an n-type contact with Au, which is a high work function metal, representing the strong FLP at Au–MoS2 interfaces. However, such FLP can obstruct the attainment of high performance of field effect devices. In this study, we investigate the reduction of FLP at Au–MoS2 interfaces by atomic passivation on Au(111) using first-principles calculations. To reduce the FLP at Au–MoS2 interfaces, we consider sulfur, oxygen, nitrogen, fluorine, and hydrogen atoms that can passivate the surface of Au(111). Calculations show that passivating atoms prevent the direct contact between Au(111) and MoS2, and thus FLP at Au–MoS2 interfaces is reduced by weak interaction between atom-passivated Au(111) and MoS2. Especially, FLP is greatly reduced at sulfur-passivated Au–MoS2 interfaces with the smallest binding energy. Furthermore, fluorine-passivated Au(111) can form ohmic contact with MoS2, representing almost zero Schottky barrier height (SBH). We suggest that SBH can be controlled depending on the passivating atoms on Au(111).

Atomic and Electronic Structures of WTe2 Probed by High Resolution Electron Microscopy and ab Initio Calculations

Abstract: Transition metal dichalcogenides (TMDs) are a class of two-dimensional (2D) materials that have attracted growing interest because of their unique electronic and optical properties. Under ambient conditions, most TMDs generally exhibit 2H or 1T structures. Unlike other group VIb TMDs, bulk crystals and powders of WTe2 exist in a distorted 1T structure (Td) at room temperature and have semimetallic properties. There is so far a lack of direct atom-by-atom visualization, limiting our understanding of this distorted 2D layered material system. We present herein atomic resolution images of Td structured WTe2. The Td structure can be distinguished in the three major orientations along the [100], [010], and [001] zone axes. Subtle structural distortions are detected by atomic resolution imaging, which match well with the optimized structure relaxed by ab initio calculations. The calculations also showed that both crystal field splitting and charge density wave (CDW) interactions contribute to the stabilization ...

Origin of Indium Diffusion in High-k Oxide HfO2

Abstract: Indium (In) out-diffusion through high-k oxides severely undermines the thermal reliability of the next generation device of III-V/high-k based metal oxide semiconductor (MOS). To date, the microscopic mechanism of In diffusion is not yet fully understood. Here, we utilize angle resolved X-ray photoelectron spectroscopy (ARXPS) and density functional theory (DFT) to explore In diffusion in high-k oxide HfO2. Our ARXPS results confirm the In diffusion through as-prepared and annealed HfO2 grown on InP substrate. The theoretical results show that the In diffusion barrier is reduced to ∼0.88 eV in the presence of oxygen vacancies (VO), whereas this barrier is as high as ∼4.78 eV in pristine HfO2. Fundamentally, we found that the high feasibility of In diffusion is owing to In nonbonding with its neighboring atoms. These findings can be extended to understand the In diffusion in other materials in addition to HfO2.

Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures

Abstract: Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2–graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2–graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EGFH) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe2 and graphene and employ a charge transfer model to determine the WSe2 carrier density in both cases. The results indicate that WSe2–EGFH displays ohmic behavior at small biases due to a large hole density in the WSe2, whereas WSe2–EGPH forms a Schottky barrier junction.

Toward Atomic-Scale Patterned Atomic Layer Deposition: Reactions of Al2O3 Precursors on a Si(001) Surface with Mixed Functionalizations

Abstract: In this paper, we use density functional theory (DFT) calculations to investigate the initial surface reactions involved in the atomic layer deposition (ALD) of Al2O3 from H2O and Al(CH3)3 (trimethylaluminum, TMA) molecular precursors on the Si(001)-(2×1) reconstructed surface with different chemical terminations. Our results for the kinetic barriers and adsorption energies of both Al and oxygen precursors along different reaction pathways show the dependence of the ALD nucleation rate on the surface defects (Si dangling bonds or dimer trench) and how it can be modified with suitable p-doping. Finally, our ab initio thermodynamics study clearly determines the relation between typical ALD working conditions and the different chemical functionalizations of the Si(001) surface with the growth properties of Al2O3 nanofilms.

Band alignments between SmTiO3, GdTiO3, and SrTiO3

Abstract: The generation of a two-dimensional electron gas (2DEG) with unprecedented high density at the interface between two complex oxides has spurred interest in the growth and characterization of these materials. Interfaces between SrTiO3 and the rare-earth titanates SmTiO3 and GdTiO3 exhibit 2DEG densities of 3 × 1014 cm−2. Band alignments are key descriptors of these interfaces, and the authors report a joint experimental/computational investigation. Photoemission spectroscopy was used to measure the band alignments at the SmTiO3/GdTiO3 (110)o interface. In parallel, hybrid density functional calculations were performed. The measured and calculated band alignments for both the top of the O 2p band and the Ti 3d lower Hubbard band agree to within 0.13 eV. Our results also shed light on the position of the lower Hubbard band with respect to the O 2p valence band.

(Invited) Evaluation of Few-Layer MoS2 Transistors with a Top Gate and HfO2 Dielectric

Abstract: Top-gated, few-layer MoS2 transistors with a HfO2 gate dielectric are fabricated and subsequently characterized. Both transistor performance and gate-stack interface quality were characterized with current – voltage (I-V) and capacitance – voltage (C-V) measurements. The interface state density (Dit) was extracted and analyzed. The importance of annealing before high-k deposition to reduce the leakage current of the HfO2 gate oxide was shown. Finally, we show preliminary data of HfO2 on MoSe2 to show the possibility of extending this top-gate approach to other TMDs.

Formation of a ZnO/ZnS interface passivation layer on (NH4)2S treated In0.53Ga0.47As: Electrical and in-situ X-ray photoelectron spectroscopy characterization

Abstract: Atomic layer deposition is used to convert an (NH4)2S cleaned p-In0.53Ga0.47As with diethylzinc (DEZ) and water, resulting in the formation of a ZnO/ZnS interfacial passivation layer (IPL). The process is studied using in-situ X-ray photoelectron spectroscopy. DEZ reacts with sulfur and oxygen present on the surface, chemically reducing arsenic 3+ and gallium 3+ to lower oxidation states. The sulfur concentration remains constant during the deposition process while the oxygen concentration on the surface remains small, confirming that the IPL is composed of both ZnO and ZnS. Measurements of metal–oxide–semiconductor capacitors with HfO2 for the dielectric show that the ZnO/ZnS IPL can nearly eliminate frequency dispersion (<1% per frequency decade) in accumulation and results in small hysteresis (<60 mV) with a D it in the 1011 eV−1 cm−2 range in the midgap. Frequency dispersion is observed in the depletion region and is attributed to minority carrier generation from the ZnO present in the IPL.

Theoretical Demonstration of the Ionic Barristor.

Abstract: In this Letter, we use first-principles simulations to demonstrate the absence of Fermi-level pinning when graphene is in contact with transition metal dichalcogenides (TMDs). We find that formation of either an ohmic or Schottky contact is possible. Then we show that, due to the shallow density of states around its Fermi level, the work function of graphene can be tuned by ion adsorption. Finally we combine work function tuning of graphene and an ideal contact between graphene and TMDs to propose an ionic barristor design that can tune the work function of graphene with a much wider margin than current barristor designs, achieving a dynamic switching among p-type ohmic contact, Schottky contact, and n-type ohmic contact in one device.

In situ surface and interface study of crystalline (3×1)-O on InAs

Abstract: The oxidation behavior of de-capped InAs (100) exposed to O2 gas at different temperatures is investigated in situ with high resolution of monochromatic x-ray photoelectron spectroscopy and low energy electron diffraction. The oxide chemical states and structure change dramatically with the substrate temperature. A (3 × 1) crystalline oxide layer on InAs is generated in a temperature range of 290–330 °C with a coexistence of In2O and As2O3. The stability of the crystalline oxide upon the atomic layer deposition (ALD) of HfO2 is studied as well. It is found that the generated (3 × 1) crystalline oxide is stable upon ALD HfO2 growth at 100 °C.

Top-gated MoS2 capacitors and transistors with high-k dielectrics for interface study

Abstract: Top-gated MOS capacitors on bulk MoS2 and transistors of few-layer MoS2 were designed and fabricated. They can be potentially utilized on various TMD and high-k materials for fast and robust electrical characterization. The 3-terminal transistor test structure shows advantages of significant reduction of parasitic effects. C-V and I-V measurements were successfully conducted to characterize few-layer MoS2 transistors with sub-10 nm HfO2 dielectric.

Method of forming crystalline oxides on III-V materials

Abstract: A metal oxide semiconductor field effect transistor (MOSFET) includes a substrate having a source region, a drain region, and a channel region between the source region and the drain region, the substrate having an epitaxial III-V material that includes three elements thereon, a source electrode over the source region, a drain electrode over the drain region, and a crystalline oxide layer including an oxide formed on the epitaxial III-V material in the channel region, the epitaxial III-V material including three elements.