Defect-induced Gap States Research Articles

Thermally stable Bi2Te3/WSe2 Van Der Waals contacts for pMOSFETs application

Novel van der Waals (vdW) contacts formed by layered Bi2Te3 are found effective in improving the performance of WSe2 pMOSFETs. As compared with conventional transition metal-based Ni/Au S/D contacts, over 103 times on-state current improvement is achieved. vdW interface formation between Bi2Te3 and WSe2 is confirmed by X-ray diffraction analysis and scanning transmission electron microscope observation. An atomically flat Bi2Te3/WSe2 vdW interface, where the number of defects could be reduced as small as possible, contributes to the suppression of Fermi-level pinning caused by defect-induced gap states. Moreover, the semimetal-like characteristics of Bi2Te3 are also effective in minimizing the impact of metal-induced gap states. These features offer WSe2 pMOSFETs with exceptional S/D junction characteristics, including suppressed off-state leakage and a higher on–off ratio. In addition, it is found that WSe2 pMOSFETs with Bi2Te3 S/D contacts have excellent thermal stability, maintaining device performance even after 400 °C annealing, which is very promising for CMOS back-end-of-line application. The layered tellurides, reconciling low contact resistance and high thermal stability, are promising, particularly from the perspective of their application in the manufacturing process.

Native point defects in 2D transition metal dichalcogenides: A perspective bridging intrinsic physical properties and device applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) hold immense promise as ultrathin-body semiconductors for cutting-edge electronics and optoelectronics. In particular, their sustained charge mobility even at atomic-level thickness as well as their absence of surface dangling bonds, versatile band structures, and silicon-compatibility integration make them a prime candidate for device applications in both academic and industrial domains. Despite such high expectations, group-VI TMDs reportedly exhibit a range of enigmatic properties, such as substantial contact resistance, Fermi level pinning, and limited unipolar charge transport, which are all rooted in their inherent defects. In other words, intrinsic physical properties resulting from their native defects extend their influence beyond the material level. Bridging point-defect-induced material properties and their behavior at the device level, this Perspective sheds light on the significance of crystalline defects beyond a rather simple defect–property relationship. As a distinctive approach, we briefly review the well-established defect model of conventional III–V semiconductors and further apply it to the emergent defect behaviors of 2D TMDs such as their defect-induced gap states. Within the main discussion, we survey a range of behaviors caused by the most prevalent intrinsic defect, namely, vacancies, within 2D TMDs, and their implications for electronic and optoelectronic properties when employed at the device level. This review presents an in-depth summary of complexities in material properties as well as device characteristics arising from intrinsic point defects and provides a solid foundation for the cross-links among native defects and material/device properties.

Role of Chalcogen Defect Introducing Metal-Induced Gap States and Its Implications for Metal-TMDs' Interface Chemistry.

The contact resistance of the transition metal dichalcogenide (TMD) devices is not comparable to that of their silicon counterparts, probably due to a lack of clarity in their interface chemistry. Looking beyond the conventional Schottky-Mott rule, the metal chalcogen orbital overlaps, tunnel barrier, and metal-induced gap states (MIGSs) are crucial factors determining different metals' contact properties with TMDs. Exploring their properties helps TMDs' contact resistance engineering, driven mainly by their orbital overlaps and perturbing parameters. This work presents the interface chemistry of TMDs (MoS2, MoSe2, WS2, and WSe2) with different metals (Au, Cr, Ni, and Pd) in detail using density functional theory computations. Additionally, the work discusses the role of the chalcogen vacancy and interstitial defects in the metal-TMD interactions and corresponding MIGS features. The investigations reveal that Au does not show any significant MIGS due to its weak interactions with all the TMDs. However, other investigated metals have a strong affinity with TMDs, making significant MIGS contributions. All the metals offer n-type doping characteristics to TMDs due to valence charge transfer from the metals toward TMDs. The chalcogen vacancy boosts the orbital overlaps of the TMDs with all the metals. The vacancy reduces metal-TMD interfacial distance, which can be a promising technique to reduce the tunnel barrier and contact resistance. The MIGS and defect-induced gap states (DIGSs) reflect the possibility of Fermi-level pinning in the TMDs' contacts with Cr, Ni, and Pd. Besides, the work discloses that the chalcogen vacancy converts an n-type Pd-TMD interface into p-type due to reverse charge transfer after the vacancy. Chalcogen interstitial impurity also helps with contact resistance engineering for some metal-TMD systems by reducing the bond distance of the metal TMDs. Our study highlights the possibility of defect-assisted and MIGS-based contact engineering at the metal-TMD interfaces.

Optical study of Ga2-xSnxO3 (0 ≤ x ≤ 0.7) thin films using spectroscopic ellipsometry and cathodoluminescence

Diagnosis of the semiconductor defect levels is essential for their device applications. The optical and electrical properties of Ga2-xSnxO3 (GTO, 0 ≤ x ≤ 0.7) films were investigated to look into the defect levels. Amorphous GTO thin films were grown on Si substrates with various Sn contents via radio frequency magnetron sputtering deposition. Thermal annealing at 900 °C for two hours was performed after the deposition of films at 500 °C to achieve the polycrystalline phase. We examined the physical properties of amorphous β-Ga2O3 thin films grown using various O2 gas flows during growth, and of the post-annealed GTO films. Using spectroscopic ellipsometry, the optical constants in the spectral range between 1.0 and 6.0 eV were determined. We estimated the optical gap energy of the GTO layers using the Tauc method. The optical gap energy was approximately 4.9 eV for amorphous β-Ga2O3 films. Optical structures owing to defects were found below optical gap energy in the spectra of the complex refractive index. Using cathodoluminescence (CL) spectroscopy, there were several peaks discovered in the range between 1.5 and 3.0 eV. The CL peaks correspond to transitions between valence (conduction band) and defect-induced gap states. We identified the 1.5 − 1.6 eV peak as the transition between defect states and the conduction band. The extinction coefficient (k) peak became sharper at 5.3 eV suggesting a strong excitonic behavior at the fundamental gap for amorphous Ga2O3 films.

Improving performance of monolayer arsenene tunnel field-effect transistors by defects.

We systematically investigate the transport properties of monolayer arsenene tunneling field-effect transistors (TFETs) along the armchair and zigzag directions using first-principles calculations based on density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) approach. We introduce five types of defects at the source-channel interface and study their influences on the device performance. The pristine arsenene TFETs along the armchair direction have large ON-state currents due to the small effective masses, but still cannot meet the International Technology Roadmaps of Semiconductor 2022 (ITRS 2022) requirements for high performance (HP) devices. The adsorption of one and two H atoms can significantly enhance the ON-state currents to above 1412 μA μm−1 and reduce subthreshold swing (SS) to below 60 mV decade−1 for both n- and p-type devices, satisfying the ITRS 2022 requirements for HP devices. Besides, the p-type As and the n-type Li adatoms can improve the performance of p-type and n-type devices, respectively. The pristine arsenene TFETs along the zigzag direction with low ON-state currents already meet the ITRS 2022 requirements for low-power (LP) devices. The performance of the p-type TFETs as LP devices can be improved by p-type SV and the As adatom by increasing the ON-state currents and/or reducing the SS. On the other hand, the adsorption of one H adatom can remarkably increase the ON-state current of the p-type TFET to 1563 μA μm−1 and reduce SS to 34 mV decade−1, allowing the device to work as an HP device. We further confirm that the enhancement of the ON-state currents is due to the shortening of the band-to-band tunneling path through the defect induced gap states. Our calculations provide a theoretical guide to improve the performance of TFETs based on arsenene or other monolayer materials by suitable defects.

A New Opportunity for 2D van der Waals Heterostructures: Making Steep-Slope Transistors.

The use of a foreign metallic cold source (CS) has recently been proposed as a promising approach toward the steep-slope field-effect-transistor (FET). In addition to the selection of source material with desired density of states-energy relation (D(E)), engineering the source:channel interface for gate-tunable channel-barrier is crucial to CS-FETs. However, conventional metal:semiconductor (MS) interfaces generally suffer from strong Fermi-level pinning due to the inevitable chemical disorder and defect-induced gap states, precluding the gate tunability of the barriers. By comprehensive materials and device modeling at the atomic scale, it is reported that 2D van der Waals (vdW) MS interfaces, with their atomic sharpness and cleanness, can be considered as general ingredients for CS-FETs. As test cases, InSe-based n-type FETs are studied. It is found that graphene can be spontaneously p-type doped along with slightly opened bandgap around the Dirac-point by interfacing with InSe, resulting in superexponentially decaying hot carrier density with increasing n-type channel-barrier. Moreover, the D(E) relations suggest that 2D transition-metal dichalcogenides and 2D transition-metal carbides are a rich library of CS materials. Graphene, Cd3 C2 , T-VTe2 , H-VTe2 , and H-TaTe2 CSs lead to subthreshold swing below 60 mV dec-1 . This work broadens the application potentials of 2D vdW MS heterostructures and serves as a springboard for more studies on low-power electronics based on 2D materials.

First Principles Calculations on the Stoichiometric and Defective (101) Anatase Surface and Upon Hydrogen and H2Pc Adsorption: The Influence of Electronic Exchange and Correlation and of Basis Set Approximations.

Anatase TiO2 provides photoactivity with high chemical stability at a reasonable cost. Different methods have been used to enhance its photocatalytic activity by creating band gap states through the introduction of oxygen vacancies, hydrogen impurities, or the adorption of phthalocyanines, which are usually employed as organic dyes in dye-sensitized solar cells. Predicting how these interactions affect the electronic structure of anatase requires an efficient and robust theory. In order to document the efficiency and accuracy of commonly used approaches we have considered two widely used implementations of density functional theory (DFT), namely the all-electron linear combination of atomic orbitals (AE–LCAO) and the pseudo-potential plane waves (PP–PW) approaches, to calculate the properties of the stoichiometric and defective anatase TiO2 (101) surface. Hybrid functionals, and in particular HSE, lead to a computed band gap in agreement with that measured by using UV adsorption spectroscopy. When using PBE+U, the gap is underestimated by 20 % but the computed position of defect induced gap states relative to the conduction band minimum (CBM) are found to be in good agreement with those calculated using hybrid functionals. These results allow us to conclude that hybrid functionals based on the use of AE–LCAO provide an efficient and robust approach for predicting trends in the band gap and the position of gap states in large model systems. We extend this analysis to surface adsorption and use the AE–LCAO approach with the hybrid functional HSED3 to study the adsorption of the phthalocyanine H2Pc on anatase (101). Our results suggest that H2Pc prefers to be adsorbed on the surface Ti5c rows of anatase (101), in agreement with that seen in recent STM experiments on rutile (110).

Defect induced gap states in monolayer MoS2 control the Schottky barriers of Pt-mMoS2 interfaces

Vacancies can significantly affect the performance of monolayer MoS2 (mMoS2) nanodevices because defect induced gap states can introduce large Schottky barriers at Pt-mMoS2 interfaces. Effects of adsorbed gases at S-vacancies on the defect induced gap states and Schottky barriers of Pt-mMoS2 interfaces have been studied by first-principles calculations. The defect induced gap states are occupied (unoccupied) ones when electron-rich (electron-poor) gases adsorb at S-vacancies. The occupied gap states in mMoS2 result in n-type Schottky barriers, whereas unoccupied gap states cause p-type Schottky barriers. Moreover, both the n-type and p-type Schottky barriers of Pt-mMoS2 interfaces decrease when the gap states are closer to the valence bands of mMoS2 because the gap states determine the direction and the amount of charge transfer at interfaces. The n-type and p-type Schottky barriers of Pt-mMoS2 interfaces are reduced to 0.36 and 0 eV by adsorbing high concentrations Cl2 and CO, respectively. Furthermore, adsorbing electron-poor gases (CO and NO) at S-vacancies change the n-type Pt-mMoS2 interfaces to p-type ones. These findings provide guidance to develop approaches to design high performance metal-mMoS2 interfaces with low Schottky barriers.

Two-dimensional mono-elemental semiconductor with electronically inactive defects: the case of phosphorus.

The deep gap states created by defects in semiconductors typically deteriorate the performance of (opto)electronic devices. This has limited the applications of two-dimensional (2D) metal dichalcogenides (MX2) and underscored the need for a new 2D semiconductor without defect-induced deep gap states. In this work, we demonstrate that a 2D mono-elemental semiconductor is a promising candidate. This is exemplified by first-principles study of 2D phosphorus (P), a recently fabricated high-mobility semiconductor. Most of the defects, including intrinsic point defects and grain boundaries, are electronically inactive, thanks to the homoelemental bonding, which is not preferred in heteroelemental system such as MX2. Unlike MX2, the edges of which create deep gap states and cannot be eliminated by passivation, the edge states of 2D P can be removed from the band gap by hydrogen termination. We further find that both the type and the concentration of charge carriers in 2D P can be tuned by doping with foreign atoms. Our work sheds light on the role of defects in the electronic structure of materials.

Point defect-induced transport bandgap widening in the downscaled armchair graphene nanoribbon device

We report on ab initio study of the electronic states and transport characteristics for armchair graphene nanoribbon devices with point defects. Geometric optimization of the point defect channel revealed the self-organizing property of the graphene. Density of state (DOS) calculation shows the point defect-induced gap states. However, the quantum transport calculations revealed that these defect-induced gap states are not contributing to the carrier transport across the channel. This is clarified further from the wavefunction analysis, which shows the spatial localization of the wavefunction around the defect regions. Most importantly, the widening of the transport bandgap with the increase in number of point defects was found even though the DOS bandgap remains unchanged. The impact of these point defects on the device characteristics varies depending on their type and location in the channel. The orientation of the defects plays a vital role in the carrier scattering, which is a crucial factor to be considered in downscaled devices.

Mechanism of the Fermi level pinning at organic donor–acceptor heterojunction interfaces

We investigate the energy level alignment and the Fermi level pinning mechanism at organic donor–acceptor heterojunctions interfaces by using the model organic–organic heterojunctions (OOHs) with well-defined molecular orientation of the standing copper (II) phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) films on the standing copper-hexadecafluoro-phthalocyanine (F16CuPc) thin films on SiO2. We identify two distinct regions for the energy level alignment by in situ ultraviolet photoelectron spectroscopy investigation. In region (I) where the work function (WF) of the underlying substrate is larger than the ionization potential (IP) of the top organic layers, the substrate Fermi level is pinned at the leading edge of the HOMO peak accompanied by a decreasing of the WF; in region (II) where the WF is smaller than the IP of the top organic layers, a downward shift of both the HOMO and vacuum level is observed. In connection with the defect induced gap states, we provide a detailed explanation for this thickness dependent energy level alignment and Fermi level pinning mechanism at the organic donor–acceptor OOH interface.

Probing nanoscale potential modulation by defect-induced gap states on GaAs(110) with light-modulated scanning tunneling spectroscopy

We investigated charged defects on an n-GaAs(110) surface using light-modulated scanning tunneling spectroscopy. Tunneling via a single defect-induced gap state under photoillumination was observed for the isolated atomic defects. Screened Coulomb potentials induced around a charged Ga vacancy and a step edge were visualized, for the first time, with a nanometer spatial resolution. Furthermore, the charge states of the individual defects were determined on the atomic level.

Study of the Effect of Metal/Semiconductor Interface Properties on a Resistance Switching Device

AbstractN-doped ZnO thin films were deposited by pulsed laser deposition on SiO2/Si substrates. X-ray diffraction analysis revealed that the films had the wurtzite structure, and were highly oriented along the c-axis direction. Au and Al electrical contacts were deposited by sputtering on the top surface of the samples, forming a two-terminal structure in each case. The current-voltage characteristics of the two terminal structure, and the temperature dependence of the resistance switching effect, were studied in the 125-300 K temperature range. The results of these measurements are presented and discussed in terms of the different Schottky barrier heights, as well as in terms of interfacial defect-induced gap states.

Generation of white light from optically pumped gallium nitride epilayers

We describe results of optical pumping experiments on gallium nitride (GaN) epilayers grown on sapphire and capped with a layer of aluminum gallium nitride (AlGaN). Our samples show the well-known yellow luminescence (in response to exposure with ultraviolet radiation) that derives from transitions to and from defect-induced gap states. We show that it is possible to enhance this downconversion luminescence by subjecting samples to mechanical stress through ultrasonic wave propagation. When double pumped with both ultraviolet and visible blue radiations, the samples generate broadband visible radiation that appears white to the human eye.

The Band Offsets of Isomeric Boron Carbide Overlayers

ABSTRACTSemiconducting boron carbide overlayers, formed from the decomposition of orthocarborane and metacarborane have been studied by angle resolved photoemission. The incurrence of surface photovoltage and the photovoltaic process, from the photoemission experiment, reveal band offsets in the orthocarborane multilayer configurations that are invereted relative to single layer configurations. Defect induced gap states which trap charge at the heterostructure interface is used as one explanation of these results. The role of defects is also used to help illuminate why opposite semiconducting type materials are formed from the decomposition of isomer carborane molecules.

Spin injection at Heusler/semiconductor interfaces: First-principles determination of potential discontinuity and half-metallicity

Technologically relevant properties, such as potential discontinuity and half-metallic behavior, are determined by means of an accurate first-principles approach for Co2MnGe/GaAs and Co2MnGe/Ge interfaces. Interface states appear in both sides of the junctions, so that half-metallicity, typical of bulk Co2MnGe, is locally lost. As for the potential discontinuity, the character of the contact is dramatically affected by the semiconductor side: In Co2MnGe/GaAs, irrespective of the atomic termination, the Fermi level is pinned within the energy band gap, whereas it lies close to the valence-band maximum in the case of Co2MnGe/Ge. This gives rise to a rectifying contact in the first case and ohmic for holes in the second case, both potentially useful for spin–injection purposes. Finally, we investigate the effects of possible Co–Mn antisites in bulk Co2MnGe and find that half-metallicity is only locally destroyed, since defect-induced gap states are shown to be screened in a couple of nearest-neighbor atomic shells.

Defect-induced band gap states and the contact charging effect in wide band gap insulators

Color centers, created by electron bombardment, have been studied as model defects on the (100) surfaces of the wide band gap insulators NaCl and KCl with electron energy loss spectroscopy (EELS) and UV photoelectron spectroscopy (UPS). Both salts were grown as thin epitaxial films on the same Ge(100) surface so that they could be directly compared in situ. At substrate temperatures below 200 K, characteristic losses of F and M centers could be identified on both materials. Close to room temperature, however, high electron exposures resulted in additional losses in the band gap due to surface and bulk plasmons of Na and K clusters. Only the defects turn out to be chemically reactive. Color centers dissociate water, but leave salicylic acid (SA) intact as a molecule, which, however, is more strongly bound compared with the undistorted surfaces. Both the color centers and the adsorbed molecules provide unoccupied electronic states in the band gap to which thermal electronic excitation is possible. We propose that these states are essential for charge exchange during contact charging and discuss this phenomenon in context with our experiments.

Defect-induced gap states in quasi-one-dimensional systems

A simple scheme for estimating the single-particle energies of defect-induced states in quasi-one-dimensional systems from the band structures of the defect-free systems is proposed. It is based on a transfer-matrix formulation of the Schrodinger equation and is shown to be most valid for extended defects. A number of applications are presented, including solitons, polarons, twistons, and ring-torsion defects in conjugated polymers, solitons in hydrogen-bonded systems, structural and substitutional defects in Se helices, and interfaces between crystalline materials. The scheme provides thereby a unifying description of a large number of different defects.