Two-Dimensional Gallium Selenide (GaSe) Material for Nanoelectronics Application

Alloying plays a central role in tailoring the material properties of 2D transition-metal dichalcogenides (TMDs). However, despite widespread reports, the details of the alloying mechanism in 2D TMDs have remained largely unknown and are yet to be further explored. Here, we combine a set of systematic experiments with ab initio density functional theory (DFT) calculations to unravel a defect-mediated mechanism for the alloying of monolayer TMD crystals. In our alloying approach, a monolayer MoSe2 film serves as a host crystal in which exchanging selenium (Se) atoms with sulfur (S) atoms yields a MoS2 xSe2(1- x) alloy. Our study reveals that the driving force required for the alloying of CVD-grown films with abundant vacancy-type defects is significantly lower than that required for the alloying of exfoliated films with fewer vacancies. Indeed, we show that pre-existing Se vacancies in the host MoSe2 lattice mediate the replacement of chalcogen atoms and facilitate the synthesis of MoS2 xSe2(1- x) alloys. Our DFT calculations suggest that S atoms can bind to Se vacancies and then diffuse throughout the host MoSe2 lattice via exchanging the position with Se vacancies, further supporting our proposed defect-mediated alloying mechanism. Beside native vacancy defects, we show that the existence of large-scale defects in CVD-grown MoSe2 films causes the fracture of alloys under the alloying-induced strain, while no such effect is observed in exfoliated MoSe2 films. Our study provides a deep insight into the details of the alloying mechanism and enables the synthesis of 2D alloys with tunable properties.

The variational Hartree-Fock-Roothaan (HF) method with correlation corrections introduced through Many Body Perturbation Theory (MBPT) and the variational Density Functional Theory (DFT) have been investigated for atomic systems to provide insights into the strengths and weaknesses of each variational approach to solving the multicenter many-electron Hamiltonian. The HF+MBPT method, having been found to be more reliable and physically relevant from the atomic investigations, is used to investigate the electronic structures and associated properties of the BF3*NH3 molecular complex, and through cluster methods, the most likely locations of the transitional metal impurities V2+, Cr+, Mn2+ and Mn0 in Silicon.

The band lineup, or alignment, of semiconductors is investigated via first-principles calculations based on density functional theory (DFT) and many-body perturbation theory (MBPT). Twenty-one semiconductors including C, Si, and Ge in the diamond structure, BN, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe in the zinc-blende structure, and GaN and ZnO in the wurtzite structure are considered in view of their fundamental and technological importance. Band alignments are determined using the valence and conduction band offsets from heterointerface calculations, the ionization potential (IP) and electron affinity (EA) from surface calculations, and the valence band maximum and conduction band minimum relative to the branch point energy, or charge neutrality level, from bulk calculations. The performance of various approximations to DFT and MBPT, namely the Perdew-Burke-Ernzerhof (PBE) semilocal functional, the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, and the $GW$ approximation with and without vertex corrections in the screened Coulomb interaction, is assessed using the $GW{\ensuremath{\Gamma}}^{1}$ approximation as a reference, where first-order vertex corrections are included in the self-energy. The experimental IPs, EAs, and band offsets are well reproduced by $GW{\ensuremath{\Gamma}}^{1}$ for most of the semiconductor surfaces and heterointerfaces considered in this study. The PBE and HSE functionals show sizable errors in the IPs and EAs, in particular for group II-VI semiconductors with wide band gaps, but are much better in the prediction of relative band positions or band offsets due to error cancellation. The performance of the $GW$ approximation is almost on par with $GW{\ensuremath{\Gamma}}^{1}$ as far as relative band positions are concerned. The band alignments based on average interfacial band offsets for all pairs of 17 semiconductors and branch point energies agree with explicitly calculated interfacial band offsets with small mean absolute errors of both $\ensuremath{\sim}0.1\text{eV}$, indicating a good overall transitivity of the band offsets. The alignment based on IPs from selected nonpolar surfaces performs comparably well in the prediction of band offsets at most of the considered interfaces. The maximum errors are, however, as large as 0.3, 0.4, and 0.7 eV for the alignments based on the average band offsets, branch point energies, and IPs, respectively. This margin of error should be taken into account when performing materials screening using these alignments.

Two-dimensional (2D) materials, viz. transition metal dichalcogenides (TMD) and transition metal oxides (TMO), offer a platform that allows the creation of heterostructures with a variety of properties. The optoelectronic industry has observed an upheaval in the research arena of MoS2 based van der Waals (vdW) heterostructures (HTSs) and Janus structures. Therefore, interest towards these structures is backed by the ability to select their electronic and optical properties. The present study investigates the photocatalytic abilites of bilayer, MoS2 and Janus (MoSSe) based vdW HTSs, viz. MoS2/TMO, MoS2/TMD, MoSSe/TMO and MoSSe/TMD, by a first-principles based approach under the framework of (hybrid) density functional theory (DFT) and many body perturbation theory (GW approximation). We have considered HfS2, ZrS2, TiS2 and WS2, and HfO2, T-SnO2 and T-PtO2 from the families of TMDs and TMOs, respectively. The photocatalytic properties of these vdW HTSs are thoroughly investigated and compared with their respective individual monolayers by visualizing their band edge alignments, electron–hole recombination and optical properties. Strikingly, we observe that, despite most of the individual monolayers not performing optimally as photocatalysts, type II band edge alignment is noticed in vdW HTSs and they appear to be efficient photocatalysts via the Z-scheme. Moreover, these vdW HTSs have also shown promising optical responses in the visible region. Finally, electron–hole recombination, H2O adsorption and hydrogen evolution reaction (HER) results establish that MoSSe/HfS2, MoSSe/TiS2, MoS2/T-SnO2 and MoSSe/ZrS2 are probable highly efficient Z-scheme photocatalysts.

Currently, transition metal dichalcogenides (TMDs) are under intense investigation due to their potential uses. [1] One reason for the interest is because TMDs exhibit a bandgap. This provides a key aspect that can enable their use in future transistors as alternative channel materials with high mobility values while also providing robust on/off ratios. Because of the need to overcome the short channel effects and lower the power consumption of field effect transistors (FETs) as their dimensions scaled down drastically, extensive research has been done on layer-by-layer TMDs over the past few years. [2-4] Furthermore, due to the variety of TMDs, which results in different bandgaps and overall offsets [5], there are theoretical predictions suggesting that TMDs are best introduced in “broken gap” tunneling field effect transistors (TFETs). Some of these device designs include dielectrics in contact with TMD material. As we wait for advances in TMD materials growth, researchers are investigating relevant TMDs with most transistor structures being bottom gate devices for the ease of integration with an emphasis on the TMD material. However, there have been very few studies to investigate high-k dielectrics on these TMD materials. [6-8] Furthermore, even fewer studies are done on the interface characterization between dielectric and TMD, with even fewer still on the evaluation of this interface electrically [9, 10] and the implications to device performance and reliability. An obstacle of integrating high-k dielectrics on these two-dimensional (2-D) materials is the lack of bonds available at the surface that enables thin film deposition. On the other hand, the research on metal/TMD contacts is also important to improve the transistor performance. To reduce the contact resistance, the metal selection [2], doping techniques [11] and the understanding of carrier transport near the metal/TMD interface [12] still require scientific investigation. Therefore, in this work, we will demonstrate top-gated few-layer MoS2 transistors with HfO2 dielectric and the subsequent electrical characterization. Few-layer MoS2 flakes were exfoliated from commercial natural MoS2 crystals and transferred onto the SiO2 /Si substrate. Using photolithography with a lift-off process, the source and drain of the transistor were formed with Au / Ti. Thereafter, a 15-minutes in-situ UV-O3 surface functionalization [6] was performed, and HfO2 was deposited at 200˚C using atomic layer deposition (ALD) immediately after the treatment. The final step of fabrication was deposition and patterning of Au / Cr metal gate. Electrical measurements were performed using a Keithley 4200 SCS and an Agilent E4980A LCR meter. The interface between MoS2 and HfO2 was studied by capacitance-voltage (C-V) measurements. Interface trap density (Dit) was extracted using the conventional high-low frequency method. From the characterization results of a MoS2 transistor, the IDS-VGS exhibited an Ion/Ioff ratio of 106, and an ultra-low leakage current (~10-14 A) indicating a continuous dielectric thin film. In C-V measurements, a “hump” was observed in the depletion region, which indicates a large number of interface traps in the gate stack. It explains the SS degradation in the I-V measurements. The IDS-VDS curves show a non-linear region at low VDS, which are caused by non-optimized contacts. Moreover, we extracted the Dit, where a peak value of 1.2×1013 cm-2eV-1 was extracted. More physical characterization and analysis are needed to understand the origin of these interface traps. With this transistor test structure; however, the ability to study other TMDs beyond MoS2, HfO2, and Au/Ti contacts are possible. This work is partially funded by National Science Foundation (NSF) award 1407765.

The isolation of graphene has sparked an unprecedented world wide research activity on the topic of two-dimensional (2D) materials. However, even though graphene has a high potential for various applications, its metallic character is a significant limitation for a large variety of electron devices, starting with the conventional field effect transistor. This limitation could be overcome in the near future by the use of layered van der Waals crystals such as black phosphorus (i.e., phosphorene, which is one to few layers of black phosphorus) and also by other semiconducting 2D materials such as transition metal dichalcogenides (TMDCs) of general formula MX2, where M is the metal and X the chalcogen (S, Se and Te). For instance, phosphorene exhibits a direct band gap varying between ~ 0.3 eV for bulk crystals to ~ 2 eV for a monolayer. In layered TMDC structures, each layer typically comprises 3 atomic planes (corresponding to a thickness below 8 Å), and consists of an hexagonally-organized atomic layer of metal atoms sandwiched between two planes of chalcogen atoms, also in hexagonal configurations. M–X bonds inside each layer are covalent; however, the sandwiching chalcogen layers are linked by weak van der Waals bonds, so that a bulk crystal is easy to cleave along the chalcogen planes. As with phosphorene, most semiconducting TMDC materials also see their band gap increasing as thickness decreases and they switch from indirect to direct band gap for a monolayer. To date, the most studied semiconductor TMDCs are MoS2 and WS2. Because they are atomically thin, 2D materials are particularly well suited for flexible electronics applications. After a brief description of the major features of phosphorene and TMDCs (crystal structures, relevant electronic properties as a function of thickness …), the talk will highlight recent results concerning field effect transistor characteristics. In particular, carrier mobility values as high as 5000 cm2/Vs have been measured in phosphorene films. Since local substitutional-type doping cannot be used in 2D materials, there is a general contacting problem (Schottky barrier formation), which is of paramount importance for device applications. In particular, two types of contacts can be formed: the top contact and the edge contact, which behave differently. Generally speaking, the semiconductor thickness is much smaller than the depletion length at the contact, which is an intriguing and novel situation. In the second part of the talk, various synthesis methods will be presented and discussed, particularly in view of their possible use in the field of large area electronics. For most devices fabricated so far, the 2D layers were obtained by exfoliation of bulk crystals. However, recent progress in synthesis include molecular beam epitaxy, controlled chemical vapour deposition and vapour phase transport. Those various approaches will be discussed, as well as the newly-introduced atomic layer deposition (ALD) process. As most 2D materials are grown on a separate substrate, the transfer problem will be highlighted, particularly concerning phosphorene which is highly hygroscopic, and sensitive to the presence of light. The presentation will be concluded by some perspectives concerning the industrialization of 2D materials.

Monolayer (ML) transition metal dichalcogenides (TMDs) have emerged as promising channel materials for future transistors due to their atomically thin geometry, enabling aggressive channel length scaling. Their short electrostatic screening length, sizable bandgap, and high carrier mobility make ML TMDs strong candidates for advanced technology nodes. This thesis presents several key advancements that demonstrate the potential of ML TMD transistors based on statistical performance evaluation. First, we fabricated double-gated monolayer WS₂ field-effect transistors (FETs) with a 30 nm channel length, achieving excellent performance, a drive current of 400 µA/µm. Technology Computer-Aided Design (TCAD) simulations were employed to optimize the device structure by introducing an air-gap spacer between the source and top gate, which effectively suppressed off-state degradation and mitigated negative threshold voltage shifts. Second, we significantly reduced contact resistance in ML MoS₂ FETs by using conventional metal, Nickle, bringing it down to approximately 500 Ω·µm through contact interface engineering. Lastly, we developed a contact strategy using metallic Nb_0.3W_0.7Se₂ as a contact for monolayer and bilayer WSe₂ transistors with channel lengths down to 100 nm, the resulting 2D/2D contacted FETs exhibit high on-current densities of up to 375 µA/µm and 1.1 mA/µm on monolayer and bilayer WSe₂ channels, respectively.The pristine, dangling-bond-free nature of monolayer (ML) transition metal dichalcogenides (TMDs) poses a significant challenge for top dielectric formation, which is a key element in enhancing electrical performance. Two different methods have been introduced in this thesis for forming interfacial layers to enable the successful growth of top dielectrics. The first approach involves the formation of a physical metal oxide layer via the natural oxidation of pre-deposited metal atoms. By utilizing a pre-deposited Aluminum seeding layer, an ML WS₂ transistor exhibiting an ideal subthreshold slope of approximately 63 mV/dec has been demonstrated. The second method integrates amorphous boron nitride (aBN), grown at 250^◦C, achieving a seeding-layer-free process for fabricating double-gated field-effect transistors (FETs). This integration results in a low interface trap density of less than 10¹² cm⁻² and an ideal subthreshold slope of around 60 mV/dec over a two-order current range (10^-4 to 10^-6 µA/µm). Furthermore, we investigated the interface trapped density landscape of ML MoS₂, a temperature dependent properties of ML MoS₂. Due to its inherent two-valley degeneracy and reduced phonon scattering, the mobility versus carrier density exhibits a gentler slope near −1, compared to −1.5 in multilayer MoS₂. Notably, ML MoS₂ shows a dielectric-constant-independent mobility behavior, with an extracted mobility in the range of 45 to 55 cm²/V·s.We also demonstrated the first on-chip P-bit core by integrating a P-bit with ML MoS₂ transistors. The low processing temperature of ML MoS₂ FETs enables their fabrication alongside probabilistic magnetic tunnel junctions (MTJs) without degrading MTJ performance. The high on-current of ML MoS₂ allows for effective resistance matching between the MTJs and input resistance, which is critical for the performance of the P-bit core.

Halide perovskites (HPs) are widely viewed as promising photovoltaic and light-emitting materials for their suitable band gaps in the visible spectrum. Density functional theory (DFT) calculations employing (semi)local exchange-correlation functionals usually underestimate the band gaps for these systems. Accurate descriptions of the electronic structures of HPs often demand higher-order levels of theory such as the Heyd-Scuseria-Ernzerhof (HSE) hybrid density functional and GW approximations that are much more computationally expensive than standard DFT. Here, we investigate three representative types of HPs, ABX3 halide perovskites, vacancy-ordered double perovskites (VODPs), and bond disproportionated halide perovskites (BDHPs), using DFT+U+V with onsite U and intersite V Hubbard parameters computed self-consistently without a priori assumption. The inclusion of Hubbard corrections improves the band gap prediction accuracy for all three types of HPs to a similar level of advanced methods. Moreover, the self-consistent Hubbard U is a meaningful indicator of the true local charge state of multivalence metal atoms in HPs. The inclusion of the intersite Hubbard V is crucial to properly capture the hybridization between valence electrons on neighboring atoms in BDHPs that have breathing-mode distortions of halide octahedra. In particular, the simultaneous convergence of both Hubbard parameters and crystal geometry enables a band gap prediction accuracy superior to HSE for BDHPs but at a fraction of the cost. Our work highlights the importance of using self-consistent Huabbard parameters when dealing with HPs that often possess intricate competitions between onsite localization and intersite hybridization.

Many-body Green's function perturbation theories, such as the GW and Bethe-Salpeter formalisms, are starting to be routinely applied to study charged and neutral electronic excitations in molecular organic systems relevant to applications in photovoltaics, photochemistry or biology. In parallel, density functional theory and its time-dependent extensions significantly progressed along the line of range-separated hybrid functionals within the generalized Kohn-Sham formalism designed to provide correct excitation energies. We give an overview and compare these approaches with examples drawn from the study of gas phase organic systems such as fullerenes, porphyrins, bacteriochlorophylls or nucleobases molecules. The perspectives and challenges that many-body perturbation theory is facing, such as the role of self-consistency, the calculation of forces and potential energy surfaces in the excited states, or the development of embedding techniques specific to the GW and Bethe-Salpeter equation formalisms, are outlined.

The isotope shifts (IS) of the resonance lines 4s2S1/2−4p2PJin CaII (seven isotopes) have been measured by fast ion beam collinear laser spectroscopy (FIBCLS) with non-optical detection of the resonant laser-ion interaction. The electronic field shift factorF was derived from the experimental IS via a King plot procedure using the mean square nuclear charge radii from muon spectroscopy andelectron scattering. TheJ-independent resultF=−283(6) Mhz/fm2 was obtained. TheF-value resulting from extensive many body perturbation theory (MBPT) calculations, recently performed by Martensson-Pendrill et al., is in excellent agreement with the experimental value, confirming the reliability of the MBPT procedure.

A simplified strong orthogonality projection has been introduced for the use in the many body perturbation theory (MBPT) or coupled cluster method (CCM) calculations with explicitly correlated geminals. This approximate projection efficiently eliminates the undesired strong orthogonality violating components from the perturbative or CCM pair functions. Moreover it permits dropping the exact strong orthogonality projectors from the pair equations. This results in a dramatic simplification of atomic and molecular calculations with explicitly correlated geminals.

Semiconducting transition metal dichalcogenides (TMDC) have been growing interest for both scientific interest and practical applications because of their various material properties. Potential applications and fundamental understanding in new classes of semiconductor materials motivates the fabrication of field-effect transistors (FETs) that are indispensable for probing of carrier transport properties, accumulation of charge and interfacial phenomena. This paper reports a fabrication and characterization of TMDC FETs with heavily-doped silicon-on-insulator substrate. The silicon processing is sophisticated technology that can be extended to the reliable operation of TMDC FETs with high reproducibility. In this presented approach, the heavily-doped SOI can work as a gate electrode in FETs, while the carrier injection into TMDC layer is possible by preparing overlap structure between gate electrode and source/drain contacts. The effectiveness of our approach was demonstrated through the fabrication and characterization of FETs.P-type SOI(88 nm)/BOX(145 nm) wafer was cleaned with SPM and HF solutions. For heavily-doped n-type SOI (n+ SOI), phosphorus-doped spin-on-glass (SOG) was deposited with spin coating and baked on a hot plate at 180 oC. The substrate was annealed at 1000 oC in N2 for 30 min to perform solid phase diffusion. Then, SOG was removed by 1 % HF for 30min. For heavily-doped p-type SOI (p+ SOI), B+ ions were implanted into SOI layer. Activation was performed at 1000 oC in Ar for 30 min. SOI layer was patterned by chemical dry etching with CF4/O2. Thermal SiO2 was grown by dry oxidation at 900 oC for 10 min. Next, AlOx was deposited by RF sputtering. After contact opening with 1 % HF, Al (20 nm)/TiN (20 nm) were deposited by RF sputtering and lift-off for electrical contact to SOI. The substrates were subjected to annealing in forming gas at 420 oC for 30 min to improve the quality of gate dielectrics and electrical contact of gate pad. Al (10 nm)/Au (40nm) was deposited by thermal evaporation and lift-off for contact electrodes. Subsequently, the substrate was exposed to oxygen plasma to form hydroxyl groups on the surface of AlOx. Then, the substrate was immersed into 2-propanol containing 5 mM n-octadecylphosphonic acid (ODPA) for 6 hours at room temperature. Annealing was conducted at 100 oC in N2 for 30 min to stabilize ODPA. The gate dielectrics consists of hybrid ODPA/AlOx/SiO2. Mechanically exfoliated WS2 was transferred to the substrate with the PDMS elastomer. Finally, devices were annealed in N2 at 150 oC for 30 min to improve source/drain contact.The FET operation was observed for both n+ and p+ SOI gate. The threshold voltage (Vth) was evaluated by linear extrapolation method with the drain current measured as a function of gate voltage at a low drain voltage of 0.05 V. In the case of n+ SOI gate electrode, the Vth of -0.71 V, On/Off ratio of 103～104 and subthreshold slope (SS) of 150 mV/dec were evaluated. On the other hand, the Vth of -0.13 V, On/Off ratio of 103～104 and subthreshold slope (SS) of 107 mV/dec were obtained for p+ SOI gate electrode. The difference in Vth between n+ and p+ SOI gate electrodes is derived from the difference in the Fermi level of SOI. The proposed method is suited for research in electrical characteristics of various TMDC semiconductors.

Contact engineering on monolayer layer (ML) semiconducting transition metal dichalcogenides (TMDs) is considered the most challenging problem toward using these materials as a transistor channel in future advanced technology nodes. The typically observed strong Fermi-level pinning induced in part by the reaction of the source/drain contact metal and the ML TMD frequently results in a large Schottky barrier height, which limits the electrical performance of ML TMD field-effect transistors (FETs). However, at a microscopic level, little is known about how interface defects or reaction sites impact the electrical performance of ML TMD FETs. In this work, we have performed statistically meaningful electrical measurements on at least 120 FETs combined with careful surface analysis to unveil contact resistance dependence on interface chemistry. In particular, we achieved a low contact resistance for ML MoS2 FETs with ultrahigh-vacuum (UHV, 3 × 10-11 mbar) deposited Ni contacts, ∼500 Ω·μm, which is 5 times lower than the contact resistance achieved when deposited under high-vacuum (HV, 3 × 10-6 mbar) conditions. These electrical results strongly correlate with our surface analysis observations. X-ray photoelectron spectroscopy (XPS) revealed significant bonding species between Ni and MoS2 under UHV conditions compared to that under HV. We also studied the Bi/MoS2 interface under UHV and HV deposition conditions. Different from the case of Ni, we do not observe a difference in contact resistance or interface chemistry between contacts deposited under UHV and HV. Finally, this article also explores the thermal stability and reliability of the two contact metals employed here.

In many cases, density-functional theory (DFT) with current standard approximate functionals offers a relatively accurate and computationally cheap description of the short-range dynamic electron correlation effects. However, in general, standard DFT does not treat the dispersion interaction effects adequately which, on the other hand, can be described by many-body perturbation theory (MBPT). It is therefore of interest to develop a hybrid model which combines the best of both the MBPT and DFT approaches. This can be achieved by splitting the two-electron interaction into long-range and short-range parts; the long-range part is then treated by MBPT and the short-range part by DFT. This work deals with the formulation of a general MBPT-DFT model (i.e., valid for any type of zeroth-order Hamiltonian) based on such a range separation. Applying the Rayleigh-Schr\"odinger formalism in this context, one finds that the generalized Bloch equation becomes self-consistent at each order of perturbation. This complication arises because the short-range part of the energy is a functional of the exact electron density, which is expanded in a perturbation series. In order to address this ``self-consistency problem'' and provide computable orbital-based expressions for any order of perturbation, a general one-electron reduced-density-matrix-based formalism is proposed. Two applications of our general formalism are presented: The derivation of a hybrid second-order M\o{}ller-Plesset-DFT model and the formulation of a range-separated optimized effective potential method based on a hybrid G\"orling-Levy-type MBPT-DFT model.

In Appendix A, a detailed description of the electronic structure calculation techniques based on the so-called density functional theory (DFT) was presented. As mentioned and illustrated in that section, DFT is widely used to investigate the electronic properties of materials, their defects, interfaces, etc. Unfortunately, the semi-local approximations of DFT, such as the local density approximation (LDA) and gradient generalized approximation (GGA), suffer from a well-known substantial underestimation of the band gap. This may be interpreted as a result of the fact that DFT does not properly describe excited states of a system. This failure of DFT may also induce a wrong estimation of the position of the electronic defect/dopant levels in the band gap.