Surface State Density Research Articles

Surface Dispersion Suppression in High-Frequency GaN Devices

GaN-based high electron mobility transistors (HEMTs) are shown to have excellent properties, showing themselves to perform well among the throng of solid-state power amplifiers. They are particularly promising candidates for next-generation mobile communication applications due to their high power density, frequency, and efficiency. However, the radio-frequency (RF) dispersion aroused by a high surface-state density inherent in nitrides causes the degradation of GaN devices’ performance and reliability. Although various dispersion suppression strategies have been proposed successively—including surface treatment, passivation, field plate, cap layer, and Si surface doping—outcomes were not satisfactory for devices with higher frequencies until the emergence of a novel N-polar deep recess (NPDR) structure broke this deadlock. This paper summarizes the generation of dispersion, several widespread dispersion containment approaches, and their bottlenecks under high frequencies. Subsequently, we highlight the NPDR structure as a potential substitute, evaluate its technical benefits, and review the continuous exertions in recent years.

PEI N-doped graphene quantum dots/p-type silicon Schottky diode

Graphene Quantum Dots (GQDs) are graphene nanoparticles with exceptional properties and a remarkable material for new technologies. Especially, GQDs/semiconductor junctions (GQDs/S), which have a high potential to open doors to new fields of study such as sensing, detection, and communication have started to attract great attention in the scientific community in the last few years. Functionalization of GQDs with several functional groups to develop their properties is one of the hot topics of research. In this context, we have investigated the properties of functionalized GQDs/p-type Si diode in this study. Polyethylenimine (PEI) functionalized nitrogen-doped GQDs (PEI/N/GQDs) have been successfully produced by the hydrothermal method. Afterward, we fabricated the Al/PEI/N/GQDs/p-Si diode. The parameters of GQDs-based Schottky diode have been investigated from current-voltage (I-V) measurements at 300 K. The barrier heights (Φb) of the diode obtained from Thermionic Emission (TE) theory, Norde’s, and Cheung’s approaches are 0.76 eV, 0.82 eV, and 0.66 eV, respectively. The ratio of rectification (RR) of the diode has been calculated to be approximately 2.8 × 104 at ± 5 V. In addition, the surface state density (Nss) versus (Ess-Ev) profile of the diode has been also obtained, by taking into account the voltage dependence of the ideality factor (n) and Φb. The average value of them has been found as 1.90 × 1013 eV−1cm−2, which is very suitable for this structure. The obtained results show that the prepared Al/PEI/N/GQDs/p-Si has Schottky diode behavior.

Metal‐Gallium Arsenide Based Tribovoltaic Nanogenerators and its Application for High‐Precision Self‐Powered Displacement Sensors

AbstractTriboelectric nanogenerators (TENGs) based on metal and semiconductors have attracted great interest due to their direct‐current (DC) output characteristics and better integration with an integrated circuit. In order to increase the DC output performance, a new type of metal‐gallium arsenide tribovoltaic nanogenerator (MG‐TVNG) is developed. The DC output characteristics of gallium arsenide‐based and silicon‐based devices are compared by a conductive atomic force microscope (CAFM), and the output of gallium arsenide‐based devices is dozens of times higher than that of silicon‐based devices, which verifies the influence of physical factors such as carrier mobility and surface state density on the tribovoltaic effect. The current density of 28.3 A m−2 can be obtained when using a single metal probe to slide on p‐type gallium arsenide, outperforming previously reported silicon TVNGs. The effects of operating parameters such as sliding velocity and load pressure on the output of the MG‐TVNG are further characterized. An MG‐TVNG‐based micro/nano grid displacement sensor is also successfully demonstrated and shows good potential in microscale displacement detecting.

Electrophysical characteristics of power MOSFETs additionally implanted with nitrogen ions

The electrical characteristics of power MOSFETs additionally implanted with nitrogen ions have been studied. Ion implantation of nitrogen was carried out through a protective oxide of 23 nm thickness with energies of 20 and 40 keV and doses of 1 ⋅ 1013‒5 ⋅ 1014 cm–2. Rapid thermal annealing was carried out at temperature of 900 or 1000 °C for 15 s. It has been established that nitridisation of the gate dielectric makes it possible to reduce the noise of the gate leakage currents and their dispersion. In the direct order of heat treatment (first rapid thermal annealing, and then the removal of the protective oxide), for samples prepared with an additional operation of nitrogen ion implantation, there is an increase in the threshold voltage compared to control samples. The capacitance of the gate dielectric in the case of implantation of nitrogen ions in the direct order of heat treatment is less than for control samples. It has been established that in the direct order of rapid thermal annealing, the doses of nitrogen ion implantation do not cause significant changes in the maximum value of the current-voltage slope. At the same time, in all studied cases, there is a shift in the maximum value of the current-voltage slope towards higher gate voltages. In the reverse order of heat treatment (first the removal of the protective oxide, and then rapid thermal annealing), there are no significant differences in the value of the threshold voltage for the samples created with additional nitrogen implantation and the control ones. The maximum value of the current-voltage slope also does not experience significant changes. It is shown that in the voltage range from – 0.15 to 0 V, the drain current of nitrogen-implanted samples manufactured using the direct order of heat treatment is higher than for control samples, and the drain current of nitrogen-implanted samples obtained with the reverse order of heat treatment it is lower compared to control samples. Results are explained by a decrease in the density of surface states at the Si – SiO2 interface in MOS-structures created using an additional operation of nitrogen ion implantation in the direct order of heat treatment.

Passivation of Mid-Gap Electronic States at Calcium Aluminosilicate Glass Surfaces upon Water Exposure: An Ab Initio Study.

When a clean glass surface is exposed to humid air, a thin water layer forms on the hydrophilic surface. Using ab initio molecular dynamics, we simulate the changes in the electronic structure of a CaO-Al2O3-SiO2 glass model upon vacuum fracture and subsequent exposure to H2O. When the glass is fractured, dangling bonds form, which lower the band gap of the surface by ∼1.8 eV compared to the bulk value due to mid-gap surface states. When H2O adsorbs onto the vacuum-fractured surface, the band gap increases to a value closer to that of the bulk band gap. Using two different hydroxylation methods, we find that the calculated band gap of the glass surface depends on the hydroxylation state. Surfaces with ∼4.5 OH/nm2 have smaller band gaps due to unfilled surface states, and surfaces with ∼2.5 OH/nm2 have larger band gaps with no apparent unfilled surface states. The resulting changes in the electronic structure, quantified by electron affinity and work function values, are hypothesized to play an important role in the electrostatic charge transfer based on the principles of surface state theory, which posit that the density of electronic surface states determines the amount of electronic charge transfer to or from material surfaces.

Improved Surface Passivation of Rear SiNx/AlOx Stacks in p‐Type Boron‐Doped Czochralski Silicon Wafers During Dark Annealing at Elevated Temperatures

Herein, the surface passivation quality of p‐type boron‐doped Czochralski silicon (Cz‐Si) wafers with passivated emitter and rear cell (PERC) structure during 200 °C dark annealing is investigated. During such a treatment, a reduction in the saturation current density (J 0) along with the degradation and regeneration of minority carrier lifetime can only be observed in PERC‐structured wafers that fast fired at high temperatures instead of in unfired wafers, which is possibly correlated with hydrogen injected into the silicon bulk during fast firing. Besides, no notable variations in J 0 can be observed in fast‐fired SiN x /n + /p‐Si/n + /SiN x symmetrical structure wafers during the same treatment, although their minority carrier lifetime undergoes degradation and regeneration cycles, demonstrating that the improved surface passivation comes from the rear surface of PERC structure. The capacitance–voltage measurement on the fabricated rear surface samples with metal‐insulator‐semiconductor (MIS) structure further demonstrates that the improved surface passivation is not related to field‐effect passivation. By comparing the surface state density of rear surface during dark annealing, it can be concluded that the improved rear surface passivation originates from the hydrogenation of surface defects such as dangling bonds, possibly by the outdiffused hydrogen from silicon bulk during such a treatment.

Multiple roles for LaFeO3 in enhancing the Photoelectrochemical performance of WO3

For photoelectrochemical (PEC) water splitting, constructing heterojunctions and loading co-catalysts are effective means to realizing sufficient light absorption, effective photogenerated carrier separation and fast charge transport. However, during implementation, the PEC performance of the catalyst is affected by both parasitic light absorption and reflection and the change in energy band structure due to the creation of new interfaces. Herein, in order to minimize the effect of recombination of photogenerated electron-hole pairs on the catalyst PEC performance due to the nascent interface arising from the co-catalyst compounding, WO3 and Ni/Co co-doped LaFeO3 (LFO) are constructed as heterojunctions, in which NiCo-LFO acts both as a part of the heterojunction to enhance photogenerated carrier separation and a co-catalyst to enhance the conductivity and modulate the surface state density at the catalyst-electrolyte interface. The current density of NiCo-LFO/WO3 reaches 3.92 mA cm−2, which is more than 7 times that of LFO/WO3. This work provides a reference for the efficient water splitting of B-site doped, especially the co-doped perovskite oxide as multifunctional roles integrated with conventional photoelectrodes.

Nanometric Moiré Stripes on the Surface of Bi2Se3 Topological Insulator.

Mismatch between adjacent atomic layers in low-dimensional materials, generating moiré patterns, has recently emerged as a suitable method to tune electronic properties by inducing strong electron correlations and generating novel phenomena. Beyond graphene, van der Waals structures such as three-dimensional (3D) topological insulators (TIs) appear as ideal candidates for the study of these phenomena due to the weak coupling between layers. Here we discover and investigate the origin of 1D moiré stripes on the surface of Bi2Se3 TI thin films and nanobelts. Scanning tunneling microscopy and high-resolution transmission electron microscopy reveal a unidirectional strained top layer, in the range 14–25%, with respect to the relaxed bulk structure, which cannot be ascribed to the mismatch with the substrate lattice but rather to strain induced by a specific growth mechanism. The 1D stripes are characterized by a spatial modulation of the local density of states, which is strongly enhanced compared to the bulk system. Density functional theory calculations confirm the experimental findings, showing that the TI surface Dirac cone is preserved in the 1D moiré stripes, as expected from the topology, though with a heavily renormalized Fermi velocity that also changes between the top and valley of the stripes. The strongly enhanced density of surface states in the TI 1D moiré superstructure can be instrumental in promoting strong correlations in the topological surface states, which can be responsible for surface magnetism and topological superconductivity.

Ti doped α-Fe2O3 electrodes for water oxidation

Development of low-cost electrocatalysts is a challenging task and has been the focus in the field of advanced water oxidation in comparison to ruthenium and iridium oxide-based electrodes. The current research focused on Ti doped α-Fe2O3 electrodes for water oxidation. The water oxidation rate initially enhanced and then decreased as Ti doping levels increase. The enhanced activity can be ascribed to the increased surface area and charge transfer rate constant kct. The increase in kct results from a greater potential drop on the Helmholtz layer, which is mainly attributed to an increased surface state density. The decreased performance for the electrodes with high Ti levels is due to the formation of a TiO2-Fe2O3 mixture where TiO2 shows poor activity. Moreover, the mechanism of charge transfer is discussed. This study may provide an alternative hematite semiconductor anode for water dark oxidation, environmental pollution control, and may even be helpful for better understanding the photoelectrochemical oxidation of water under the weak light illumination and high bias potential.

Sliding Schottky diode triboelectric nanogenerators with current output of 109 A/m2 by molecular engineering of Si(211) surfaces

Triboelectric nanogenerators (TENGs) are an autonomous and sustainable power-generation technology, seeking to harvest small vibrations into electricity. Here, by achieving molecular control of oxide-free Si crystals and using conductive atomic force microscopy, we address key open questions and use this knowledge to demonstrate zero-applied-bias current densities as high as 109 A/m2. Key to achieve this output, is to use a proton-exchangeable organic monolayer that simultaneously introduces a sufficiently high density of surface states (assessed as changes to carrier recombination velocities) coupled to a strong surface dipole in the form of a surface alkoxide anion (Si–monolayer–O−). We also demonstrate that the DC output of a Schottky diode TENG does not track the energy released as friction. This removes the complexity of controlling an unavoidable stick–slip motion, bypassing the requirement of aligning sliding motion and substrate topographical features. We reveal that there is no apparent correlation between the current of a static (biased) junction and the tribocurrent of the same junction when under motion and unbiased.

Highly response and humidity-resistant gas sensor based on polyaniline-functionalized Bi2MoO6 with UV activation

The bismuth molybdate (Bi2MoO6) has shown great application potential in the field of gas sensing owing to its abundant Lewis acid sites, suitable energy band and unique layered structure. In this work, a room temperature, highly response, selectivity and humidity-resistant NH3 gas sensor based on the Bi2MoO6/Polyaniline (BMO/PANI) nanocomposite was successfully developed by an in-situ chemical oxidative polymerization process. The obtained samples were characterized by X-ray diffractometer, UV–vis diffuse reflectance spectra, Transmission electron microscopy, X-ray photoelectron spectroscopy, etc. A series of gas-sensing properties studies were conducted with the assistance of UV light, which indicate that UV light illumination could significantly enhance the response of the prepared BMO/PANI sensor and short recovery time (71s). Additionally, the sensor also demonstrated excellent selectivity, repeatability, long-term stability, and moisture resistance. These outstanding characteristics of the sensor are attributed to the dramatic increase of surface state density upon activation by the UV light and the distinctive heterogeneous interface strengthens the interfacial charge transfer.

Effective Passivation of InGaAs Nanowires for Telecommunication Wavelength Optoelectronics

AbstractCatalyst‐free InGaAs nanowires are promising building blocks for optoelectronic devices operating at telecommunication wavelengths. Despite progress, the applications of InGaAs nanowires remain limited due to their high density of surface states that degrade their optical properties. Here, InGaAs nanowires with superior optical properties are achieved by effectively suppressing their surface states with an InP passivation shell. Optimal InP shell growth conditions and thickness to maximize the minority carrier lifetime are identified. The photoluminescence intensity of these passivated InGaAs nanowires is up to three orders of magnitude higher than that of their bare counterparts. Moreover, a long minority carrier lifetime of up to ≈13 ns is measured with these passivated nanowires at room temperature. Optimal passivation of InGaAs nanowires with an emission wavelength of 1530 nm results in an ultra‐low surface recombination velocity of ≈280 cm s−1. In addition to the shell, the crystal structure of these nanowires plays an important role in the luminescence intensity. Combined cathodoluminescence mapping and high‐resolution transmission electron microscopy along the nanowires reveal significantly lower emission intensities in wurtzite predominant sections of the nanowires than zinc blende predominant ones.These insights on the optimal passivation of InGaAs provide directions for engineering high‐performance nanoscale‐devices in the telecommunication wavelength.

Ultra-High Responsivity and Enhanced Trap Assisted Charge Transfer by utilizing Ti3C2TX(MXene) as a Transport Layer for ReS2 based Flexible Broadband Photodetector: A better Alternative to Graphene

Even though there are many reports on low-cost flexible broadband photodetector the main issue is the low value of the responsivity due to non-uniformity of the substrate. The reason is the lack of suitable transport material. Graphene has proven to be the best till date, but, on unconventional substrates such as biodegradable paper, responsivity values still remain an unresolved issue. This report demonstrates for the first time the use of Ti3C2TX(MXene) as transport layer, utilizing 2D ReS2 as active material on paper substrate as broadband photodetector and further compared its performance with graphene. Responsivity values calculated for ReS2/MXene and ReS2/graphene under visible and NIR illumination are 40.8 AW−1, 26.9 AW−1, and 26.5 AW−1, 17.5 AW−1, respectively, which suggests that MXene as transport layer largely enhances performance. To understand the phenomena, detailed mechanisms studies are presented in terms of density of surface states and energy band diagram. Surface states density was found to be larger in MXene compared to graphene structure, suggests that the lifetime of the carrier is high with MXene as transport layer compared to Graphene due to trap assisted charge transfer. This ReS2/MXene heterostructure based environment friendly photodetector can find potential applications in future flexible optoelectronics.

Role of Oxygen and Fluorine in Passivation of the GaSb(111) Surface Depending on Its Termination

The mechanism of the chemical bonding of oxygen and fluorine on the GaSb(111) surface depending on its termination is studied by the projector augmented-waves method within density functional theory. It is shown that on an unreconstructed (111) surface with a cation termination, the adsorption of fluorine leads to the removal of surface states from the band gap. The binding energy of fluorine on the cation-terminated surface in the most preferable Ga-T position is lower by ~0.4 eV than that of oxygen, but it is significantly lower (by ~0.8 eV) on the anion-terminated surface. We demonstrate that the mechanism of chemical bonding of electronegative adsorbates with the surface has an ionic–covalent character. The covalence of the O–Sb bond is higher than the F–Sb one, and it is higher than both O–Ga and F–Ga bonds. Trends in the change in the electronic structure of the GaSb(111) surface upon adsorption of fluorine and oxygen are discussed. It is found that an increase in the oxygen concentration on the Sb-terminated GaSb(111) surface promotes a decrease in the density of surface states in the band gap.

Mitigation of Edge and Surface States Effects in Two-Dimensional WS2 for Photocatalytic H2 Generation.

Large scale development of the 2D transition metal di-chalcogenides (TMDC) relies on landmark improvement in performance, which could emerge from nanostructuration. Using p-WS2 nanoflakes with different degrees of exfoliation and fracturing, perspectives were provided to develop high-surface-area 2D p-WS2 films for the photocatalytic hydrogen generation. The critical role of inter-nanoflakes contacts within high-surface-area 2D films was demonstrated, highlighting the benefit of plane/plane versus edge/plane contacts. Evidence of the high density of surface states displayed by these 2D films was provided through electrochemical measurements. In addition to operating as recombination centers, the surface states were shown to give rise to deleterious Fermi-level pinning (FLP), which dramatically decreased the efficiency of charge carrier separation. Lastly, promising strategies yielding FLP suppression via surface states modification were proposed. In particular, use of a multifunctional ultrathin film displaying healing, catalytic, and n-type semiconduction properties was shown to greatly enhance charge carrier separation and transport to the photo-electrode/electrolyte interface. When the 2D photoelectrodes were fabricated with the above prerequisites (i. e., a high proportion of plane/plane contacts and a successful surface states chemical modification), a photocurrent up to 4.5 mA cm-2 was achieved for the first time on 2D p-WS2 photocathodes for hydrogen generation.

Density of Surface States: Another Key Contributing Factor in Triboelectric Charge Generation.

The triboelectric nanogenerator (TENG) has been invented as a technology for harvesting mechanical energy, as well as for allocating quantized charge for scientific instruments. The charge generation of the TENG is mainly related to the triboelectric effect or contact electrification (CE) as usually described by the potential-well-electron-cloud model, while the triboelectric charge transfer is related to the difference in the occupied energy levels of electrons. However, in our experiment, we observed an abnormal triboelectric charge generation phenomena between ternary materials, which cannot be explained by the occupied energy level difference only. To address this issue, we proposed the model based on the density of surface states (DOSS) as another key contributing factor to the triboelectric charge generation. To demonstrate our model, we introduced an approach to measure the DOSS through applying external electric field between two triboelectric surfaces. Our experiments confirmed the contribution of the DOSS to the triboelectric charge generation, with the derived charge density consistent with the measured results, which verified our model. We also predicted that the FEP has the potential to achieve a high charge density of ∼5.6 × 10-4 C/m2, which is close to the reported maximum values. This study provides another key contributing factor to the triboelectric charge generation, which may provide a more complete model for guiding the material selection and modification to tailor the surface charge generated by the CE.

Signatures of nontopological patches on the surface of topological insulators

The non-trivial topology in the layered $\text{FeTe}_{0.55}\text{Se}_{0.45}$ (FTS) superconductor has been suggested by both theory and experiment to be strongly dependent on the Te concentration. Motivated by this together with the Te fluctuations expected from alloy disorder, we develop a simple layered model for a strong topological insulator that allows us to describe a scenario where topologically trivial domains permeate the sample. We refer to such a phase as topological domain disordered and study the local density (LDOS) of the topological surface states that can be measured using scanning tunneling spectroscopy (STS) in this phase. We find that topologically trivial domains on the surface, where one would expect the topological surface state to be absent, appear as regions of suppressed LDOS surrounded by domain walls with enhanced LDOS. Furthermore, we show that studying the energy dependence of the STS should allow us to distinguish the topologically trivial parts of the surface from other forms of disorder. Finally, we discuss implications of such local disappearance of the topological surface states for the observation of Majorana modes in vortices.

Study of interfacial design for direct-current tribovoltaic generators

As a newly discovered physical phenomenon, tribovoltaic effect has potential applications in various fields. Herein, we systematically study the relationship between interfacial properties of the tribovoltaic generator and its output performance using a conductor-semiconductor (C-S) structure. Based on several similar physical models, we derive an equation to quantify the effects of contact area, load force and the density of surface states of the semiconductor on performance of the tribovoltaic generator. The experimental results are in good agreements with the theoretical analysis. Moreover, the contribution of interfacial electric field and interfacial magnetic field to the tribovoltaic effect are discussed, respectively. By using epitaxial silicon (Si) film, the built-in electric field near the C-S interface can be regulated, which leads to suppression of the recombination effect of the tribovoltaic charges, and the short-circuit current of the tribovoltaic generator can be increased by 66.83%. This work paves the way for electrode design and better output performance of flexible DC tribovoltaic generator in the future.

Effect of Hydrogen Implantation Dose on the Relaxation of Electrophysical Characteristics of Silicon-on-Insulator Structures after Exposure to X-rays

Experimental studies of the relaxation of electrophysical parameters of SOI structures with different doses of hydrogen implantation after exposure to stationary X-ray radiation are presented. Investigation of high-frequency CV characteristics and pseudo-MOS transistors made it possible to obtain information about the accumulated charge, the density of surface states, mobility of carriers. The impurity composition and depth profile of hydrogen concentration were determined by the SIMS method. Structural perfection of the layers and interfaces was evaluated using the XRR and XRD methods. A different nature of the relaxation dependence and the recovery time of the electrophysical parameters for SOI structures with different doses of hydrogen implantation are recorded. The values of mobility and charge density are higher for the structure with a lower hydrogen implantation dose. Keywords: Ion implantation, silicon on insulator, radiation resistance, X-ray diffraction, pseudo-MOS transistor, residual hydrogen, SIMS.

Development of methods for studying intergranular surface features in semiconductor heterogeneous polycrystals of bismuth-antimony tellurides with the imposition of electric and deformation fields

The article discusses the technique and tools for studying the role of the effective density of electronic surface states innanocrystalline semiconductor films when cyclic deformation is applied directly from the analysis of experimental data. The surface electron states play the role of recombination and trapping centers depending on the number of carriers, the electron capture section and hole, the concentration of surface states of their type and energy position. To determine the effective density of surface states, we found both a change in the Fermi level and a change in the density of the effective surface charge. The effective density of electronic surface states was determined from the measured variations in the active resistance and capacity of nanocrystalline films of bismuth-antimony tellurides upon the application of irreversible deformation, and its strain dependence was found. From data analysis, one can judge the irreversibility of heterogeneous structures during deformation, that is, the electronic structure of a nanocrystalline semiconductor film changes greatly when cyclic deformation is applied.