Low Surface State Density Research Articles

Delocalization of topological surface states by diagonal disorder in nodal loop semimetals

The effect of Anderson diagonal disorder on the topological surface (``drumhead'') states of a Weyl nodal loop semimetal is addressed. Since diagonal disorder breaks chiral symmetry, a winding number cannot be defined. Seen as a perturbation, the weak random potential mixes the clean exponentially localized drumhead states of the semimetal, thereby producing two effects: (i) the algebraic decay of the surface states into the bulk; (ii) a broadening of the low energy density of surface states of the open system due to degeneracy lifting. This behavior persists with increasing disorder, up to the bulk semimetal-to-metal transition at the critical disorder $W_{c}$. Above $W_{c}$, the surface states hybridize with bulk states and become extended into the bulk.

Protection of isolated and active regions in AlGaN/GaN HEMTs using selective laser annealing* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51577169 and 51777187), the National Key Research and Development Program of China (Grant No. 2017YFB0402804), and the “Science and Technology Innovation 2025” Major Program of Ningbo (Grant No. 2018B10098).

AlGaN/GaN high-electron-mobility transistors with Au-free ohmic contacts are fabricated by selective laser annealing and conventional rapid thermal annealing. The current transport mechanism of ohmic contacts is investigated. High-temperature annealing can be avoided in the isolated region and the active region by selective laser annealing. The implanted isolation leakage current is maintained 10−6 mA/mm even at 1000 V after selective laser annealing. On the contrary, high-temperature annealing will cause obvious degradation of the isolation. The morphology of AlGaN surface is measured by atomic force microscope. No noticeable change of the AlGaN surface morphology after selective laser annealing, while the root-mean-square roughness value markedly increases after rapid thermal annealing. The smaller frequency dispersion of capacitance–voltage characteristics confirms the lower density of surface states after selective laser annealing. Thus, dynamic on-resistance is effectively suppressed.

External Control of GaN Band Bending Using Phosphonate Self-Assembled Monolayers.

We report on the optoelectronic properties of GaN(0001) and (11̅00) surfaces after their functionalization with phosphonic acid derivatives. To analyze the possible correlation between the acid's electronegativity and the GaN surface band bending, two types of phosphonic acids, n-octylphosphonic acid (OPA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA), are grafted on oxidized GaN(0001) and GaN(11̅00) layers as well as on GaN nanowires. The resulting hybrid inorganic/organic heterostructures are investigated by X-ray photoemission and photoluminescence spectroscopy. The GaN work function is changed significantly by the grafting of phosphonic acids, evidencing the formation of dense self-assembled monolayers. Regardless of the GaN surface orientation, both types of phosphonic acids significantly impact the GaN surface band bending. A dependence on the acids' electronegativity is, however, only observed for the oxidized GaN(11̅00) surface, indicating a relatively low density of surface states and a favorable band alignment between the surface oxide and acids' electronic states. Regarding the optical properties, the covalent bonding of PFOPA and OPA on oxidized GaN layers and nanowires significantly affects their internal quantum efficiency, especially in the nanowire case due to the large surface-to-volume ratio. The variation in the internal quantum efficiency is related to the modification of both the internal electric fields and surface states. These results demonstrate the potential of phosphonate chemistry for the surface functionalization of GaN, which could be exploited for selective sensing applications.

Effect of carrier drift-diffusion transport process on thermal quenching of photoluminescence in GaN

Thermal quenching of the defect-related photoluminescence (PL) has been widely used to determine the fundamental properties of point defects in GaN such as the activation energy (E t ) and capture cross section. However, in the present work, we have shown using drift-diffusion modeling and experimental analysis that the thermal quenching of defect-related PL from GaN does not only depend of the defect parameters but also is strongly governed by the carrier drift and diffusion in the depletion region. In particular, we have found that these processes significantly influence the slope of PL thermal quenching and temperature T 0 at which the quenching begins. As a result, E t obtained from the Arrhenius plot of the defect-related PL intensity in GaN provides the correct values of the defect activation energy only in the case of low surface state density ( cm−2eV−1) corresponding to the weak surface band bending. However, in the case of high D 0 giving rise to the significant surface band bending, the E t value should be cautiously interpreted because it may not correspond entirely to the real defect activation energy due to the drift-diffusion process. Finally, we have shown that our finding can explain in a coherent manner various anomalous behaviors of PL thermal quenching reported in the literature, such as tunable, abrupt or sample dependent thermal quenching. We believe that our findings can be very interesting for nitride growth technology community and optoelectronic device applications.

The Features of the Lateral Photovoltaic Effect in the Fe<sub>3</sub>O<sub>4</sub>/SiO<sub>2</sub>/n-Si Structure Depending on Silicon Substrate Orientation

We report on the results of a study of the lateral photovoltaic effect in theFe3O4/SiO2/n-Si structure grown on Si(001) and Si(111) substrates. It was found that in theFe3O4/SiO2/Si(001) structure the LPE sensitivity is a half times as much, and the photoresponseparameters are about 3 times less than those in the Fe3O4/SiO2/Si(111) structure. It is supposed thata higher sensitivity and faster photoresponse in the Fe3O4/SiO2/Si(001) structure, compared with theFe3O4/SiO2/Si(111) structure, are caused by a lower density of surface states at the SiO2/Si(001)interface than at the SiO2/Si(111) interface.

Mechanisms of GaAs surface passivation by a one-step dry process using low-frequency plasma enhanced chemical deposition of silicon nitride

III-V materials are known for their high surface states densities which degrade the performances of photonic and electronic devices. Plasma-enhanced chemical vapor deposition of SixNy is commonly used to passivate surface states and to encapsulate III-V devices and circuits. The plasma is generated with an RF excitation source usually set at a high frequency of 13.56 MHz. To obtain a high quality III-V/SixNy interface, many chemical surface treatments are required before deposition, as well as post-deposition thermal annealing. In this work, we demonstrate that a low-frequency plasma allows for an efficient passivation process by a simple deposition of SixNy, without any wet chemical pretreatment nor post-deposition annealing. This dry process is more compatible with the passivation and encapsulation of completed III-V devices and circuits. Metal-insulator-semiconductor capacitors were fabricated by depositing SixNy on GaAs at a frequency of 90 kHz. CV measurements indicated the unpinning of the Fermi level and a low density of surface states in the mid 1011 cm−2 eV−1 range. X-ray photoelectron spectra have shown the complete suppression of native oxides and an important reduction of elemental arsenic from the GaAs surface. The mechanisms of surface passivation by ionic hydrogen bombardment inherent to the low-frequency plasma deposition of SixNy are discussed.

Ambipolar Charge Transport in Two-Dimensional WS2 Metal-Insulator-Semiconductor and Metal-Insulator-Semiconductor Field-Effect Transistors.

Two-dimensional (2D) materials with ambipolar transport characteristics have attracted considerable attention as post-complementary metal-oxide semiconductor (CMOS) materials. These materials allow for electron- or hole-dominant conduction to be achieved in a single channel of the field-effect transistors (FETs) without an extrinsic doping. In this study, all-2D metal-insulator-semiconductor (MIS)-based devices, which were composed of all-2D graphene, hexagonal boron nitride, and WS2, exhibited ambipolar and symmetrical transport characteristics with a low surface state density (Dit,min ≈ 7 × 1011 cm-2·eV-1). Hole- or electron-dominant inversion under the influence of electrostatic doping was obtained in a WS2-based 2D capacitor up to a frequency range of 1 MHz. n- and p-channel conductions with enhancement-mode operations were selectively realized in a single MISFET, which presented a current on/off ratio of >106 and high field-effect mobility (μe = 58-67 cm2/V·s and μh = 19-30 cm2/V·s). Furthermore, a monolithic CMOS-like logic inverter, which employed a single WS2 flake, exhibited a high gain of 78. These results can be used to reduce the footprints of the device architectures and simplify the device fabrication processes of next-generation CMOS integrated circuits.

Surface states on (001) oriented β -Ga2O3 epilayers, their origin, and their effect on the electrical properties of Schottky barrier diodes

Surface states on (001) oriented halide vapor phase epitaxy (HVPE) grown β-Ga2O3 epilayers were explored through the determination of the Schottky barrier height (SBH) as a function of the metal work function using Cr, Cu, Ni, and Au Schottky barrier diodes. SBH is found to be nearly pinned between 1.2 and 1.35 eV in the HVPE grown epilayers. The position of the Fermi level pinning is closely matched with the energy level of the oxygen vacancy [VO(III)] state (EV + 3.57 eV) in the energy bandgap of β-Ga2O3, indicating that Fermi level pinning is due to oxygen vacancy type surface states on (001) oriented β-Ga2O3 epitaxial layers. The Fermi level is found to be relatively unpinned on the bulk β-Ga2O3 (001) substrate, suggesting the presence of lower density of oxygen vacancy states on its surface. Hence, the HVPE growth process was found to be responsible for the presence of oxygen vacancy states [VO(III)] in the epilayer. Moreover, this work highlights the role of these surface states in determining the SBH on β-Ga2O3 (001) epilayers and also explains the reason behind the scattered data of SBH values reported in the literature. In addition to these results, we also showed an increment in the built-in potential and the reduction of reverse leakage current for the epilayer with lower surface state density, which gives a direct evidence of the effect of surface states on the properties of β-Ga2O3 (001) Schottky barrier diodes.

Negative Fermi-Level Pinning Effect of Metal/n-GaAs(001) Junction Induced by a Graphene Interlayer.

It is demonstrated that the electric dipole layer due to the overlapping of electron wave functions at the metal/graphene contact results in a negative Fermi-level pinning effect on the region of the GaAs surface with low interface-trap density in the metal/graphene/n-GaAs(001) junction. The graphene interlayer plays the role of a diffusion barrier, preventing the atomic intermixing at the interface and preserving the low interface-trap density region. The negative Fermi-level pinning effect is supported by the decrease of the Schottky barrier with the increase of the metal work function. Our work shows that the graphene interlayer can invert the effective work function of the metal between high and low, making it possible to form both Schottky and Ohmic-like contacts with identical (particularly high work function) metal electrodes on a semiconductor substrate possessing low surface-state density.

Strain-induced changes in AlGaN/GaN two-dimensional electron gas structures with low surface state densities

We report on the effects of applied external strain on AlGaN/GaN two dimensional electron gas structures exhibiting a negative strain dependence of the sheet carrier density (ns). Flexible AlGaN/GaN heterojunctions, grown on two-dimensional boron nitride (BN)-on-sapphire templates, were released from the substrate via strain-induced separation at the weak BN van der Waals interface and then transferred to flexible substrates. By releasing the AlGaN/GaN layer from the substrate, residual strain was removed which allowed for isolation and study of the effects of the externally applied strain. By bending samples, uniaxial strain up to 0.15% was applied as measured by the shifts in the GaN E2H Raman mode. Hall effect measurements revealed a 2.5% decrease in ns with 0.11% applied tensile strain, which is contrary to the increase expected from the piezoelectric effect. The observed decrease in ns is attributed to a relatively large increase in the AlGaN surface barrier height. This effect, which is rarely reported, is observable due to a low surface state density (2.2 × 1012 cm−2 eV–1) in the samples. Illumination was found to dramatically alter the ns-strain dependence, an effect potentially related to detrapping of electrons in the GaN buffer.

Behavioral Features of MIS Memristors with a Si3N4 Nanolayer Fabricated on a Conductive Si Substrate

The effect of the material of the metal plates (Au, Ta, W) and exposure to a high-power blue laser on the memristive characteristics of metal–insulator–semiconductor (MIS) capacitors with a Si3N4 film 6 nm thick fabricated on the basis of n+-Si is studied. It is shown that bipolar switching by the current appears only in capacitors with Au. The causes of the absence of bipolar switching in capacitors with Ta and W are explained. The switching of capacitors with Ta by the current and light and the photomemory effect are detected. It is shown that, despite the high doping level of the semiconductor substrate, it decreases the MIS memristor response rate due to a high density of surface states localized at the Si3N4/n+-Si interface. However, illumination allows a significant increase in the response rate due to a decrease in the semiconductor resistivity. The surface state densities are determined. To improve the frequency characteristics of MIS memristors, it is necessary to achieve a low surface state density.

Effect of Post Nitridation Annealing on Thermally Grown Silicon Oxynitride Characteristics

The ability to block the direct current is one of the main requisite for the gate dielectric in a metal-oxide-field-effect-transistor. Since thickness of oxide is inversely related with oxide capacitance, therefore oxide capacitance can be increased as an alternative to oxide thickness to meet the scaling requirements. Oxynitrides and high-k materials as gate dielectric have been considered as promising candidates to overcome the above problems. At greater physical thickness, these materials have the similar capacitance value which avoids tunneling effect. SiON has been used as a substitute for SiO2 because of its high dielectric permittivity and low density of surface states. In this paper, the effect of post nitridation annealing on thermally grown silicon oxynitride characteristics have been investigated by analyzing high-frequency capacitance-voltage (C-V) and current- voltage characteristics (I-V) of n-Si/SiON/Ti-Pt MOS capacitor. Post nitridation annealing was carried out at 800°C for 15 min in Nitrogen (N2) ambient. Ellipsometery, FESEM, Atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) was used to study the effect of post nitridation annealing on the physical characteristics of deposited silicon oxynitride film. Cheung’s approach has been used for evaluating barrier height (ϕbo ) and ideality factor (η). We present a comparative study between the as-deposited and annealed film and it was found that the physical and electrical properties (k, η, ϕbo etc) improved by post nitridation annealing is attributed to the improvement in the interface properties of Si/SiON. Moreover, the value of positive fixed charge density (Q f ~1.8×1012 cm−2) is effective for field-effect passivation of n-type Si surfaces. For both as-deposited and annealed SiON film, the maximum current through the Si/SiON/Ti-Pt device never exceeded a value of 10–8A/cm2 which indicates that the film were very good insulators. These results indicate that the obtained SiON film is an appropriate gate insulator for advanced nano electronic devices.

Investigation of charge carrier depletion in freestanding nanowires by a multi-probe scanning tunneling microscope

Profiling of the electrical properties of nanowires (NWs) and NW heterocontacts with high spatial resolution is a challenge for any application and advanced NW device development. For appropriate NW analysis, we have established a four-point prober, which is combined in vacuo with a state-of-the-art vapor-liquid-solid preparation, enabling contamination-free NW characterization with high spatial resolution. With this ultrahigh-vacuum-based multi-tip scanning tunneling microscopy (MT-STM), we obtained the resistance and doping profiles of freestanding NWs, along with surface-sensitive information. Our in-system 4-probe STM approach decreased the detection limit for low dopant concentrations to the depleted case in upright standing NWs, while increasing the spatial resolution and considering radial depletion regions, which may originate from surface changes. Accordingly, the surface potential of oxide-free GaAs NW {112} facets has been estimated to be lower than 20 mV, indicating a NW surface with very low surface state density.

Особенности поведения МДП мемристоров с нанослоем Si-=SUB=-3-=/SUB=-N-=SUB=-4-=/SUB=-, изготовленных на основе проводящей подложки Si

AbstractThe effect of the material of the metal plates (Au, Ta, W) and exposure to a high-power blue laser on the memristive characteristics of metal–insulator–semiconductor (MIS) capacitors with a Si_3N_4 film 6 nm thick fabricated on the basis of n ^+-Si is studied. It is shown that bipolar switching by the current appears only in capacitors with Au. The causes of the absence of bipolar switching in capacitors with Ta and W are explained. The switching of capacitors with Ta by the current and light and the photomemory effect are detected. It is shown that, despite the high doping level of the semiconductor substrate, it decreases the MIS memristor response rate due to a high density of surface states localized at the Si_3N_4/ n ^+-Si interface. However, illumination allows a significant increase in the response rate due to a decrease in the semiconductor resistivity. The surface state densities are determined. To improve the frequency characteristics of MIS memristors, it is necessary to achieve a low surface state density.

Surface State Density Determines the Energy Level Alignment at Hybrid Perovskite/Electron Acceptors Interfaces

Substantial variations in the electronic structure and thus possibly conflicting energetics at interfaces between hybrid perovskites and charge transport layers in solar cells have been reported by the research community. In an attempt to unravel the origin of these variations and enable reliable device design, we demonstrate that donor-like surface states stemming from reduced lead (Pb0) directly impact the energy level alignment at perovskite (CH3NH3PbI3-xClx) and molecular electron acceptor layer interfaces using photoelectron spectroscopy. When forming the interfaces, it is found that electron transfer from surface states to acceptor molecules occurs, leading to a strong decrease in the density of ionized surface states. As a consequence, for perovskite samples with low surface state density, the initial band bending at the pristine perovskite surface can be flattened upon interface formation. In contrast, for perovskites with a high surface state density, the Fermi level is strongly pinned at the conduction band edge, and only minor changes in surface band bending are observed upon acceptor deposition. Consequently, depending on the initial perovskite surface state density, very different interface energy level alignment situations (variations over 0.5 eV) are demonstrated and rationalized. Our findings help explain the rather dissimilar reported energy levels at interfaces with perovskites, refining our understanding of the operating principles in devices comprising this material.

New mechanism of semiconductor polarization at the interface with an organic insulator

A semiconductor—organic-insulator system with spatially distributed charge is created with a uniquely low density of fast surface states (N ss ) at the interface. A system with N ss ≈ 5 × 1010 cm–2 is obtained for the example of n-Ge and the physical characteristics of the interface are measured for this system with liquid and metal field electrodes. For a system with an organic insulator, the range of variation of the surface potential from enrichment of the space-charge region of the semiconductor to the inversion state is first obtained without changing the mechanism of interaction between the adsorbed layer and the semiconductor surface. The effect of enhanced polarization of the space-charge region of the semiconductor occurs due to a change in the spatial structure of mobile charge in the organic dielectric layer. The system developed in the study opens up technological opportunities for the formation of a new generation of electronic devices based on organic film structures and for experimental modeling of the electronic properties of biological membranes.

Physical and photoelectrochemical characterizations of the pyrochlore La1.9Ba0.1Sn2O7: Application to chromate reduction under solar light

Abstract The substituted pyrochlore La 1.9 Ba 0.1 Sn 2 O 7 was prepared by nitrate route and the physical and electrochemical characterizations were investigated. The optical gap is found to be 3.01 eV and the conductivity is characteristic of a semiconducting behavior with activation energy of 0.42 eV. The conduction band of La 1.9 Ba 0.1 Sn 2 O 7 (−2.64 V SCE ), predicted from electro negativities of the constituent atoms, is close to that determined experimentally (−2.99 V SCE ). The capacitance measurement (C −2 –V) exhibits a linear behavior, characteristic of p type conductivity, from which a flat band potential of −0.05 V SCE and hole density N D of 8.42 × 10 14 cm −3 were determined, in agreement with the small electrical conductivity (σ 300K ∼3 × 10 −6 Ω −1 cm −1 ). The Nyquist plot showed a semicircle which is a contribution of capacitance and resistance in parallel. A bulk resistance contribution (R b = 71 kΩ cm 2 ) and a low density of surface states are observed. The center of the arc is localized below the real axis with a depletion angle of 14° ascribed to a constant phase element (CPE). As application, chromate was successfully converted to Cr 3+ on the pyrochlore under sunlight. ∼1 h was necessary to reach the adsorption equilibrium for an initial chromate concentration of 10 −4 M. The conduction band is more cathodic than the potential of HCrO 4 − which is spontaneously reduced under solar light. The best performance is obtained at pH ∼ 3 with a catalyst dose of 1 mg/mL. 40% of chromate disappears with a quantum yield of 0.53% in less than 2 h. The process obeys to a first order kinetic with an apparent rate constant of 2.3 × 10 −3 min −1 .

Photoelectrochemical characterization of chalcopyrite Cu(In0.6Ga0.4)3Se5

The compound Cu(In0.6Ga0.4)3Se5 prepared by fusion in sealed tube is a wide band gap semiconductor; it crystallizes in the P-chalcopyrite structure with a direct transition of 1.42 eV. Its transport properties exhibit a semi-conducting behavior, which seem to be not intrinsic but rather attributed to selenium vacancies, and the conductivity is well described by small-polaron hopping: σ = σ0exp{−29/kT}(Ohm cm)−1 with an effective mass of ∼2m 0. The thermo-power is negative and changes little with temperature, suggesting that the conduction mechanism is mostly due to electron hopping. The analyzed material shows a chemical stability over a broad pH range; the semi-logarithmic plot in KOH solution displays an exchange current density of 27 μA cm−2 and a corrosion potential of −0.204 VSCE. The capacitance measurement (C −2–V) exhibits a linear behavior, characteristic of n type conductivity, from which a flat band potential of −0.530 VSCE and an electron density of 3.49 × 1020 cm−3 are determined. The Nyquist plot shows a semicircle due to the predominance of the bulk contribution (=127 Ohm cm2) and a low density of surface states. The centre is localized below the real axis with a depression angle of 12° ascribed to a constant phase element (CPE). The straight line in the low frequencies region is due to the Warburg diffusion.

Mechanism of charge transfer in injection photodetectors based on the M(In)-n-CdS-p-Si-M(In) structure

The mechanism of charge transfer in a new-type selective (with a tunable spectrum) injection photodetector based on the M(In)-n-CdS-p-Si-M(In) structure with the internal amplification has been analyzed. It has been shown that, in this structure, there is a mutual compensation of the drift and diffusion fluxes of charge carriers. The counter drift and diffusion fluxes of nonequilibrium carriers at reverse current densities I ∼ 10−8–10−7 A/cm2 lead to the appearance of sign-reversal points of the photosensitivity in the short-wavelength and long-wavelength regions of the spectrum. The mutual compensation of the counter drift and diffusion current fluxes at current densities of the order of ∼10−6 A/cm2 leads to the appearance of a sublinear section in the reverse current-voltage characteristic over a wide range of bias voltages. It has been found that the n-SdS-p-Si heterojunction has a low density of surface states at the interface. This makes it possible to develop an injection photodetector based on the considered structure with a high spectral sensitivity Sλ = 5.0 × 104 A/W) and a high integrated sensitivity Sint = 2.8 × 104 A/lm or 4.5 × 106 A/W in the for-ward direction of the current.

Physical and photoelectrochemical properties of Sb-doped SnO2 thin films deposited by chemical vapor deposition: application to chromate reduction under solar light

Sb-doped SnO2 thin films (Sb-SnO2) are prepared by chemical vapor deposition. The X-ray diffraction indicates a rutile phase, and the SEM analysis shows pyramidal grains whose size extends up to 200 nm. The variation of the film thickness shows that the elaboration technique needs to be optimized to give reproducible layers. The films are transparent over the visible region. The dispersion of the optical indices is evaluated by fitting the diffuse reflectance data with the Drude–Lorentz model. The refractive index (n) and absorption coefficient (k) depend on both the conditions of preparation and of the doping concentration and vary between 1.4 and 2.0 and 0.2 and 0.01, respectively. Tin oxide is nominally non-stoichiometric, and the conduction is dominated by thermally electrons jump with an electron mobility of 12 cm2 V−1 s−1 for Sb-SnO2 (1 %). The (C2–V) characteristic in aqueous electrolyte exhibits a linear behavior from which an electrons density of 4.15 × 1018 cm−3 and a flat-band potential of −0.83 VSCE are determined. The electrochemical impedance spectroscopy shows a semicircle attributed to a capacitive behavior with a low density of surface states. The center lies below the real axis with a depletion angle (12°), due to a constant phase element, i.e., a deviation from a pure capacitive behavior, presumably attributed to the roughness and porosity of the film. The straight line at low frequencies is attributed to the Warburg diffusion. The energy diagram reveals the photocatalytic feasibility of Sb-SnO2. As application, 90 % of the chromate concentration (20 mg L−1, pH ~3) disappears after 6 h of exposure to solar light.