Semiconductor Layers Research Articles

Advanced Characterization of 1 eV GaInAs Inverted Metamorphic Solar Cells

In this work, 1 eV Ga0.7In0.3As inverted metamorphic (IMM) solar cells were analyzed to achieve a deeper understanding of the mechanism limiting their improvement. For this purpose, high-resolution X-ray diffraction (HRXRD), transmission electron microscopy (TEM), high-resolution cross-sectional cathodoluminescence (CL), and transient in situ surface reflectance were carried out. Additionally, the photovoltaic responses of the complete devices were measured using the external quantum efficiency (EQE) and numerically simulated through Silvaco TCAD ATLAS. The combination of structural characterization of the semiconductor layers and measurements of the solar cell photovoltaic behavior, together with device modeling, allows us to conclude that the lifetime of the bulk minority carriers is the limiting factor influencing the PV response since the recombination at the interfaces (GaInP window–GaInAs emitter and GaInAs base–GaInP back surface field (BSF)) does not impact the carrier recombination due to the favorable alignment between the conduction and the valance bands. The advanced characterization using cross-sectional cathodoluminescence, together with transient in situ surface reflectance, allowed the rejection of the formation of traps related to the GaInAs growth conditions as being responsible for the decrement in the minority-carrier lifetime. Conversely, the TEM and HRXRD revealed that the presence of misfit dislocations in the GaInAs layer linked to strain relaxation, which were probably formed due to an excessive tensile strain in the virtual substrate or an incorrect combination of alloy compositions in the topmost layers, was the dominant factor influencing the GaInAs layer’s quality. These results allow an understanding of the contributions of each characterization technique in the analysis of multi-junction solar cells.

High resistive buffer layers by Fermi level engineering

An efficient carrier compensation mechanism in semiconductor layers by Fermi-level engineering is demonstrated using the modulation-doping of a deep acceptor and a shallow donor. The punch-through of the depletion region across the whole stack of modulation-doped layers shifts the Fermi level closer toward the midgap position, resulting in the compensation of residual background free carriers. The method represents an alternative to achieve semi-insulating properties in semiconductor materials where a suitable deep acceptor or donor state at the midgap position is not available. We demonstrate the applicability of the concept with a commercially important GaN case study using carbon (deep acceptor) and Si (shallow donor) doping. A strong enhancement of breakdown field strength and reduced charge pileup effects are observed due to the efficient pinning of the Fermi level.

Effect of Annealing and Etching Times on Anatase TiO2 Hollow Sphere

The development of high-efficiency photocatalysts plays an important role in the application of solar energy conversion. Titanium dioxide (TiO2) with an anatase crystalline phase is well-known as semiconductor thin layers for solar cells. This work has constructed a novel TiO2 hollow sphere (HST) in a 2-step synthesis. The first step is coating the SiO2 template with TiO2 to build a core-shell of SiO2@TiO2 (CSST). The second step is etching via sonication to elute the SiO2 from CSST and construct the HST. The annealing of CSST for 1 to 6 hours and etching for 1 to 7 hours has resulted in the HST with different crystallite sizes and microstrains. The HST of ~90 nm has been fabricated with crystallite size of 9.53 to 20.54 nm and microstrain from 0.34 to 3.42. It was found that the optimum crystallite size and microstrain of HST obtained via annealing the CSST for 2 hours and etching for 5 hours has the best photooxidation of I- under UV irradiation. The optimum crystallite size and microstrain of HST via annealing and etching times can be recommended for the future of solar cell fabrication and applications.

Observation of periodic structures and pseudogaps in pristine compound α-TiNCl by STM/STS and break junction methods

Scanning tunneling microscopy and spectroscopy (STM/STS) and break-junction tunnel spectroscopy (B.TTS) measurements are performed on the pristine layered α-TiNCl semiconductor (pri-TiNCl), being the precursor of superconducting nitride chlorides. The STM topography of pri-TiNCl shows basic crystal structures with the lattice periods of a0≈ 0.38 nm and b0≈ 0.31 nm, ensuring that a clean a–b surface of micro-single crystals is obtained. From the STS measurements, the averaged conductance versus voltage dependence dI/dV(V) reveals kink structures at relatively high bias voltages of |F| ∼ −50 mV and −100 mV. The spatial (r-) conductance distributions dI/dV (V, r) (dI/dV maps) proportional to those of quasiparticle local densities of states (LDOSes) exhibit the bias-independent streak structures with the period of (4.8 ± 0.2) b0, which are predominantly observed within the range of |V| < 95 mV. The temperature dependence of the dI/dV B.TTS spectra shows the gap structure with the energy scale of 4Σ = 180 meV vanishing at Ts ∼ 120 K. Thus, the ratio of the gap Σ to the transition temperature Ts is 2Σ/kBTs ∼ 10. Here, kB is the Boltzmann constant. Such a ratio is typical of the pseudogap features in cuprate superconductors and dielectric gap characteristics in layered chalcogenides with charge-density waves.

Next level epitaxy (NLE) – a novel approach for the mass production of semiconductor layers

Next level epitaxy (NLE) – a novel approach for the mass production of semiconductor layers

Effect of magnetic particle size in semi-conductive layer on charge accumulation within insulation layer of HVDC cable

The key problem limiting the development of high-voltage direct current (HVDC) cable is the space charge accumulation within the insulation layer. A method is proposed to reduce the accumulation of space charge of cable insulation by a semi-conductive layer modified by magnetic particles. The physical essence is to change the charge migration path by the action of the Lorentz force in the semi-conductive layer. SrFe12O19 particles with different particle sizes from 80 nm to 1000 nm are prepared by the sol-gel self-combustion method. The magnetic properties and resistivity of the semi-conductive layer composites with various particle sizes are investigated. In addition, the effect of the semi-conductive layer electrode for charge accumulation in the insulation layer is studied. Finally, the relationship among SrFe12O19 particle size, magnetization intensity of the semi-conductive layer, and charge inhibition effect of the insulation layer is analyzed. The results of the experiments demonstrate that residual magnetization of magnetic particles has a significant impact on the space charge suppression. When SrFe12O19 particle size is 200 nm, the charge inhibition effect is the best. This work can provide a new view for the charge suppression of insulation layer.

Defect Profiling of Oxide‐Semiconductor Interfaces Using Low‐Energy Muons

AbstractMuon spin rotation with low‐energy muons (LE‐µSR) is a powerful nuclear method where electrical and magnetic properties of surface‐near regions and thin films can be studied on a length scale of ≈200 nm. This study shows the potential of utilizing low‐energy muons for a depth‐resolved characterization of oxide‐semiconductor interfaces, i.e., for silicon (Si) and silicon carbide (4H‐SiC). The performance of semiconductor devices relies heavily on the quality of the oxide‐semiconductor interface; thus, investigation of defects present in this region is crucial to improve the technology. Silicon dioxide (SiO2) deposited by plasma‐enhanced chemical vapor deposition (PECVD) and grown by thermal oxidation of the SiO2‐semiconductor interface are compared with respect to interface and defect formation. The nanometer depth resolution of LE‐µSR allows for a clear distinction between the oxide and semiconductor layers, while also quantifying the extension of structural changes caused by the oxidation of both Si and SiC. The results demonstrate that LE‐µSR can reveal unprecedented details on the structural and electronic properties of the thermally oxidized SiO2‐semiconductor interface.

Momentum matching and band-alignment type in van der Waals heterostructures: Interfacial effects and materials screening

Momentum-matched type II van der Waals heterostructures (vdWHs) have been designed by assembling layered two-dimensional semiconductors (2DSs) with special band-structure combinations----that is, the valence band edge at the \ensuremath{\Gamma} point (the Brillouin-zone center) for one 2DS and the conduction-band edge at the \ensuremath{\Gamma} point for the other [Ubrig et al., Nat. Mater. 19, 299 (2020)]. However, the band offset sizes, band-alignment types, and whether momentum matched or not, all are affected by the interfacial effects between the component 2DSs, such as the quasichemical-bonding (QB) interaction between layers and the electrical dipole moment formed around the vdW interface. Here, based on density-functional theory calculations, first we probe the interfacial effects (including different QBs for valence and conduction bands, interface dipole, and the synergistic effects of these two aspects) on band-edge evolution in energy and valley (location in the Brillouin zone) and the resulting changes in band alignment and momentum matching for a typical vdWH of monolayer InSe and bilayer $\mathrm{W}{\mathrm{S}}_{2}$, in which the band edges of subsystems satisfy the special band-structure combination for a momentum-matched type II vdWH. Then, based on the conclusions of the studied interfacial effects, we propose a practical screening method for robust momentum-matched type II vdWHs. This practical screening method can also be applied to other band-alignment types. Our current study opens a way for practical screening and designing of vdWHs with robust momentum matching and band-alignment type.

Stable 2R van der Waals heterostructures of NbS2 and MSe2 for M=Mo and W

In this letter, we investigate the stable and commensurate van der Waals heterostructures of metallic and semiconducting $1H$ transition-metal dichalcogenides, NbS$_2$ and MoSe$_2$ (WSe$_2$), which possess almost the same lattice constant of the pristine honeycomb structure. In the most stable structure, the metallic and semiconducting layers are stacked in a similar manner to $3R$ stacking but the period is a pair of a metallic layer and a semiconducting layer. The heterostructure aligns the spin-polarization in each valley among all layers and induces spin-selective charge transfer between the metallic and semiconducting layers. Especially in hetero-trilayers, the electronic spin is conserved due to mirror symmetry along the out-of-plane axis in contrast to the $3R$ stacking structure. A drastic enhancement of spin Hall effect is numerically shown as an example of electronic spin transport phenomena in the hetero-trilayers.

Depth-distribution of resistivity within ion-irradiated semiconductor layers revealed by low-kV scanning electron microscopy

Low-kV scanning electron microscopy imaging was used to visualize the 2D profiles of internal resistivity distribution in 600 keV He2+ ion-irradiated epitaxial GaAs and Al(0.55)Ga(0.45)As. The influence of the dopant concentration on DIVA (damage-induced voltage alteration) contrast formation has been studied in this paper. The threshold irradiation fluencies (the fluencies below which no damage-related contrast is observed) were defined for each studied material. The results show that the same level of damage in the material caused by ion irradiation becomes visible at lower threshold fluence in the case of lower-doped sample of the same composition. The aluminum content in the composition of materials exposed to ion irradiation and subsequent DIVA contrast formation mechanism was considered as well. The carrier concentration in irradiated layers has been studied by Raman spectroscopy and photoluminescence measurements, which confirmed that the increase of the resistivity of the material caused by ion-irradiation damage generation is resulting from the formation of deep states in the bandgap trapping free carriers.

Fullerene adducts from malonic esters of triethylene glycol and C60: electronic and electrochemical properties

In this work, dimeric and monomeric fullerene-C60 derivatives with triethylene glycol units were synthesised through the Bingel–Hirsch reaction and characterised with thermogravimetric analysis (TGA), Nuclear Magnetic Resonance (NMR) spectroscopy and Mass Spectrometry (MS) techniques. Also, the target compounds were studied from a physical chemistry perspective to investigate their photophysical and electrochemical properties using UV–Vis and fluorescence spectroscopy, Cyclic (CV), and Square Wave Voltammetry (SWV). All previous emphasised the aim to evaluate the features to be an electron transporting layer on perovskite solar cells (PSC), on this regard, both compounds exhibited good thermal stability and alignment of LUMO energy levels with MAPbBr3 as the perovskite semiconductor layer.

Research Progress of Organic Field-Effect Transistor Based Chemical Sensors.

Due to their high sensitivity and selectivity, chemical sensors have gained significant attention in various fields, including drug security, environmental testing, food safety, and biological medicine. Among them, organic field-effect transistor (OFET) based chemical sensors have emerged as a promising alternative to traditional sensors, exhibiting several advantages such as multi-parameter detection, room temperature operation, miniaturization, flexibility, and portability. This review paper presents recent research progress on OFET-based chemical sensors, highlighting the enhancement of sensor performance, including sensitivity, selectivity, stability, etc. The main improvement programs are improving the internal and external structures of the device, as well as the organic semiconductor layer and dielectric structure. Finally, an outlook on the prospects and challenges of OFET-based chemical sensors is presented.

A biomaterial-silicon junction for photodetection.

Bio-integrated optoelectronics can be interfaced with biological tissues, thereby offering opportunities for clinical diagnosis and therapy. However, finding a suitable biomaterial-based semiconductor to interface with electronics is still challenging. In this study, a semiconducting layer is assembled comprising a silk protein hydrogel and melanin nanoparticles (NPs). The silk protein hydrogel provides a water-rich environment for the melanin NPs that maximizes their ionic conductivity and bio-friendliness. An efficient photodetector is produced by forming a junction between melanin NP-silk and a p-type Si (p-Si) semiconductor. The observed charge accumulation/transport behavior at the melanin NP-silk/p-Si junction is associated with the ionic conductive state of the melanin NP-silk composite. The melanin NP-silk semiconducting layer is printed as an array on an Si substrate. The photodetector array exhibits uniform photo-response to illumination at various wavelengths, thus providing broadband photodetection. Efficient charge transfer between melanin NP-silk and Si provides fast photo-switching with rise and decay constants of 0.44​s and 0.19​s, respectively. The photodetector with a biotic interface comprising an Ag nanowire-incorporated silk layer as the top contact can operate when underneath biological tissue. The photo-responsive biomaterial-Si semiconductor junction using light as a stimulus offers a bio-friendly and versatile platform for artificial electronic skin/tissue.

Effect of Organic / Inorganic Gate Materials on the Organic Field-Effect Transistors Performance

The choice of gate dielectric materials is fundamental for organic field effect transistors (OFET), integrated circuits, and several electronic applications. The operation of the OFET depends on two essential parameters: the insulation between the semiconductor layer and the gate electrode and the capacitance of the insulator. In this work, the electrical behavior of a pentacene-based OFET with a top contact / bottom gate was studied. Organic polyvinyl alcohol (PVA) and inorganic hafnium oxide (HfO2) were chosen as gate dielectric materials to lower the operation voltage to achieve the next generation of electronic applications. In this study, the performance of the OFET was studied using monolayer and bilayer gate insulators. To model and analyze a device's electrical properties, MATLAB was used. Two main parameters were studied: switching ratio (Ion/Ioff) and subthreshold swing (SS), as well as the effect of dielectric capacitance on the gate dielectric materials. The PVA/HfO2 bilayer gate dielectric gave the best results in Ion/Ioff ratio, SS and transconductance of 9.05´10-7, -1.52, and -4.99 x10-5A/V respectively, which is because the dielectric capacitance has increased.

Plasma etching for fabrication of complex nanophotonic lasers from bonded InP semiconductor layers

Integrating optically active III-V materials on silicon/insulator platforms is one potential path towards improving the energy efficiency and performance of modern computing. Here we demonstrate the applicability of direct wafer bonding combined with plasma etching to the fabrication of complex nanophotonic systems out of InP layers. We explore and optimise the plasma etching of InP, validating existing processes and developing improved ones. We explore the use of microdisk lasing as a way to evaluate fabrication fidelity, and demonstrate that we can create complex lasing systems of interest to us: coupled disk cavities and random network lasers.

Analysis of Near-Field Thermophotovoltaic Devices Using Graphene–Germanium Schottky Cell

In this work, we investigate the performance of graphene-based Schottky junction thermophotovoltaic (TPV) devices in near-field conditions. Despite the low cost and excellent photoelectric properties of graphene, earlier studies have focused primarily on the contribution of the graphene layer to the photocurrent, assuming an internal quantum efficiency (IQE) of 100%. Our numerical model of a graphene/germanium Schottky junction TPV device reveals that the semiconductor layer predominates in the generation photocurrent, with an IQE of graphene less than 40%. We also evaluate the photocurrent densities generated by the semiconductor and graphene at an emitter temperature of 1000 K and a vacuum gap of 100 nm. Results show that using an indium tin oxide (ITO)-covered tungsten (W) emitter can increase photocurrents by a factor of around 10 and 11 for the semiconductor and graphene, respectively. Additionally, using a hyperbolic metamaterial (HMM) emitter can enhance photocurrents by around 4.7 and 5.2 times for the semiconductor and graphene, respectively. However, this comes at the cost of higher heat flux from the HMM emitter. Our findings will provide valuable insights for the design and optimization of TPV devices to improve their photocurrent and efficiency.

Oxygen vacancy region formation in BaTiO3 adjacent to the interface between the internal electrode and the dielectric layer in Ni–Sn internal electrode multilayer ceramic capacitor exhibiting leakage current suppression

This study underlines the ceramic BaTiO3 dielectric layer adjacent to the electrode of long-term reliability-improved Ni–Sn alloy internal electrode BaTiO3-based multilayer ceramic capacitor to clarify the cause of electric barrier formation. Electron energy loss spectroscopy measurements of the Ti L 3,2 near the edges and the O K near the edge structure changes to characterize the existence of an oxygen vacancy region, approximately 60 nm in width, and generated in BaTiO3 adjacent to the interface. Accordingly, the n-type semiconductor layer of BaTiO3 that originated from the oxygen vacancies, led to the formation of a rigid Schottky barrier at the interface.

Performance of MOS Capacitor with Different Dielectric Material Simulated Using Silvaco TCAD Tools

Metal oxide semiconductor (MOS) capacitor is a trilayer device that comprises of metal, dielectric, and semiconductor layer. The advancement of MOS technology has greatly give huge improvement to MOS devices which lead to scaling down the MOS devices. The reduction of dielectric thickness of conventional dielectric material has coming to an end, therefore as alternative new material with high mobility carrier is suggested to overcome the problem. The objectives of this work are to study the performance of MOS capacitor. Two parameters were varied, first the semiconductor material which is silicon (Si), germanium (Ge) and silicon germanium (SiGe) and second is the dielectric material that is silicon dioxide (SiO2) and silicon nitride (Si3N4). The performance of the MOS capacitor is evaluated based on the capacitance-voltage (C-V) and current-voltage (I-V) characteristics. Silvaco TCAD tool were use as as simulation tool for the method of investigation. Result shown that the performance of the MOS capacitor increased when Ge and SiGe were used as semiconductor material and Si3N4 as dielectric layer. It can see that with VT of 4.15 V for MOS with Ge and 4.28 V for MOS with SiGe. For the C-V properties Cmax value for both devices are F and F, respectively. The results show that there is around 100% increment in capacitance value when Ge is used as semiconductor layer but there is no increment or decrement in capacitance value when SiGe is used. Based on the obtained results, Ge is chosen as the best semiconductor material.

Density-tuned effective metal-insulator transitions in two-dimensional semiconductor layers: Anderson localization or Wigner crystallization

Electrons (or holes) confined in 2D semiconductor layers have served as model systems for studying disorder and interaction effects for almost 50 years. In particular, strong disorder drives the metallic 2D carriers into a strongly localized Anderson insulator (AI) at low densities whereas pristine 2D electrons in the presence of no (or little) disorder should solidify into a Wigner crystal at low carrier densities. Since the disorder in 2D semiconductors is mostly Coulomb disorder arising from random charged impurities, the applicable physics is complex as the carriers interact with each other as well as with the random charged impurities through the same long-range Coulomb coupling. By critically theoretically analyzing the experimental transport data in depth using a realistic transport theory to calculate the low-temperature 2D resistivity as a function of carrier density in 11 different experimental samples covering 9 different materials, we establish, utilizing the Ioffe-Regel-Mott criterion for strong localization, a direct connection between the critical localization density for the 2D metal-insulator transition (MIT) and the sample mobility deep into the metallic state, which for clean samples could lead to a localization density low enough to make the transition appear to be a Wigner crystallization. We believe that the insulating phase is always an effective Coulomb disorder-induced localized AI, which may have short-range WC-like correlations at low carrier densities. Our theoretically calculated disorder-driven critical MIT density agrees with experimental findings in all 2D samples, even for the ultra-clean samples. In particular, the extrapolated critical density for the 2D MIT seems to vanish when the high-density mobility goes to infinity, indicating that transport probes a disorder-localized insulating ground state independent of how low the carrier density might be.

Transmission pulse photoacoustic response of thin semiconductor plate

In modern high-speed semiconductor electronics, there is a high-rate heating of components that is affected by very fast relaxation processes. The relaxation times of these processes should be known for thermal management of these devices. For almost half a century, photoacoustic techniques have been successfully developed and used to investigate physical semiconductor properties. In order to enable observation of fast relaxation processes, the model of pulse photoacoustic signal is proposed that includes influence of thermal relaxations. It is shown that these processes can be observed in a thin semiconductor layer by choosing the frequency of short pulse train and their duty cycle.