Dead Layer Research Articles

Enhanced UV–Vis Rejection Ratio in Metal/BaTiO3/β‐Ga2O3 Solar‐Blind Photodetectors

AbstractThe fabrication and characterization of metal/BaTiO3/β‐Ga2O3 solar‐blind photodetectors are reported. β‐Ga2O3 is a promising material for solar‐blind photodetectors due to its large bandgap and the availability of low defect‐density melt‐grown substrates. In this work, structures are introduced that employ high‐permittivity dielectric/semiconductor heterojunctions to enhance the performance of a Schottky photodetector. It is shown that integrating the high‐k dielectric BaTiO3 reduces the dark current by ≈104, all but eliminates illumination induced Schottky barrier lowering, and increases the UV–vis rejection ratio by a factor greater than 9 × 103 compared to a Schottky photodetector. It is hypothesized that the high permittivity of the dielectric overcomes the influence of self‐trapped holes in Ga2O3 to reduce the peak electric field at the dielectric/metal interface, thereby eliminating the effects of Schottky barrier lowering on illuminated β‐Ga2O3 photodetectors. Additionally, it is hypothesized that the increase in the UV–vis rejection ratio is caused by the “dead layer” that forms at the BaTiO3/Pt interface.

Impact of interface structure on electronic states in 4H-SiC inversion layer

Abstract We investigate impact of interface structures on the electronic states in the 4H-SiC(0001) inversion layers, considering the full band structure of bulk 4H-SiC by using the empirical pseudopotential method (EPM).
Our results reveal that a certain interface structure has a dead layer which repels electrons from the interface by about 0.3 nm.
In addition, it is found that the contribution of an upper conduction band valley of bulk 4H-SiC leads to a strong localization of the electronic state of the 1st subband under high confinement electric fields.
We also perform calculations based on the effective mass approximation (EMA) and compare the subband energies between results of the EPM and the EMA.
This comparison reveals that the impact of the full band structure considered in the EPM calculation is comparable to doubling the effective mass in the EMA calculation.

Elimination of dead layer in silicon particle detectors via induced electric field based charge collection

The front surface of semiconductor particle detectors typically contains undepleted recombination active regions that the impinging particles pass through before reaching the sensitive area of the device. These so-called dead layers pose a fundamental limitation for achievable energy resolutions and are unavoidable in externally doped pn-junction detectors. Here, we fabricate a silicon particle detector using an alternative method for charge collection that extends the sensitive region to the front surface of the device and minimizes the dead layer. The junction is realized by inducing an electric field at the surface of the detector using a charged thin film. Such an approach has previously been implemented in photodiodes, which have demonstrated effective collection of charge carriers from the very surface of the devices. Our detector displays low leakage currents and recombination, which allow efficient charge collection throughout the device, as demonstrated by excellent internal quantum efficiency of the device. The detector is further characterized using detection of alpha particles as a case example. We achieve 20 keV energy resolutions that are, already without extensive device optimization, on the same level with commercial externally doped silicon particle detectors. Notably, the design shows promise for detection of shallow penetrating charged particles, which is very sensitive to dead layers.

A novel 4H–SiC thermal neutron detector based on a metal-oxide-semiconductor structure

Energy resolution and leakage current are two key parameters for semiconductor neutron detectors. Metal-Oxide-Semiconductor (MOS) detectors introduce an additional oxide layer compared to Schottky Barrier Diodes (SBDs) detectors in order to reduce leakage current. However, this also leads to a degradation in energy resolution due to the introduction of an additional dead layer. Therefore, optimizing the thickness of the oxide layer is important for MOS detectors. In this study, a 4H–SiC-based MOS thermal neutron detector was designed with comprehensive consideration of the oxide layer. Subsequently, a prototype of the detector was fabricated and tested. The prototype of the MOS detector achieved an nA-level leakage current under a reverse bias of −200 V. Additionally, it demonstrated good energy resolution performance in a moderated D-T neutron source test. The MOS detector was able to clearly distinguish between the two reaction channels of 10B converter events, which is consistent with the theoretical branching ratio. This work highlights the potential application of 4H–SiC-based MOS detectors for thermal neutron detection.

Tunnel oxide passivating contact enabled by polysilicon on ultra-thin SiO2 for advanced silicon radiation detectors

Conventional silicon junction detectors encounter significant carrier recombination within the heavily doped p+ and n+ layers, as well as beneath the metal contact regions, creating the so-called “dead layers”, especially on the detector side. In this study, we present the tunnel oxide passivating contact with doped polysilicon on oxide, which demonstrates exceptional surface passivation and carrier selectivity. The key innovation lies in an ultra-thin (~ 1.5 nm) interfacial oxide layer that facilitates efficient majority carrier transportation via tunneling while effectively block minority carriers. Remarkably low saturation current densities, ranging from 5 to 10 fA/cm2 even with the metal contact, underscore the superiority of both n-type and p-type tunnel oxide passivating contacts. In contrast, conventional p–n junction or high-low junction exhibit saturation current densities ranging from 10 to 90 fA/cm2 in the studied p+ and n+ layers with surface passivation schemes due to Auger recombination and surface recombination, and 1000–6000 fA/cm2 with metal contacts due to intense metal-induced recombination at the interface. These findings indicate the potential and superiority of implementing n-type tunnel oxide passivating contact on the detector side and p-type contact on the back side for advanced silicon radiation detectors. This approach would enable thorough collection of generated charge carriers along the track of incident ionizing radiation particles, leading to improved energy resolution and reduced noise levels.

Origin of the Surface Magnetic Dead Layer in Rare‐Earth Titanates

Origin of the Surface Magnetic Dead Layer in Rare‐Earth Titanates

Characterization of predominant genotype (QX) of avian infectious bronchitis virus isolated from layer chickens in Bangladesh

Avian infectious bronchitis (AIB) is a highly transmissible infection that affects the poultry industry globally. This study aims to isolate and characterize emerging strains of infectious bronchitis virus (IBV) from field samples of layer chickens in Bangladesh. A total of 108 samples (trachea, lung, and kidney) were taken from dead and sick layer chickens from 18 farms in 4 areas detecting outbreaks in Bangladesh. The samples were processed and inoculated in embryonated chicken eggs (ECEs) and finally screened by the trypsin-induced hemagglutination (THA) test. Using various techniques such as hemagglutination inhibition (HI), agar gel immuno-diffusion (AGID), virus neutralization test (VNT), reverse transcription-polymerase chain reaction (RT-PCR), and nucleotide sequencing, we were able to identify and confirm the isolated IBV viruses. The study also determined the hemagglutination (HA) pattern of isolated virus using avian and mammalian red blood cells. The pathogenicity of the isolated IBV was determined using embryonated chicken eggs and day-old chicks. The study found that 8 samples were positive for IBV using ECEs, and 4 were positive by the THA test. These isolates were confirmed using HI, AGID, and VN tests. S1 gene-based RT-PCR confirmed all four isolates as IBV, with the recent isolates belonging to the genotype-QX and being similar to IBV isolates from Thailand, Saudi Arabia, and India. The HA pattern of the recent isolates showed that the isolated IBV was virulent. The pathogenicity test also revealed that the four isolates were highly pathogenic. The study indicated that the prevalent genotype (QX) of the IBV strain is present in the layer chicken population of Bangladesh.

Calibration and performance evaluation of an N-type HPGe detector for a collimated and focused PGNAA system by experimental and Monte Carlo methods

The MCNP code was utilized to assess the operational status of an N-type high-purity germanium (HPGe) detector and to evaluate its potential for a collimated and focused Prompt Gamma Neutron Activation Analysis (PGNAA) detection system. In order to calibrate the detector, a narrow beam γ collimator was designed to scan the detector in both axial and radial directions. The curvature of the crystal front edge and the non-uniform description of the detector dead layer (DL) were taken into account when adjusting the effective crystal size. A collimated 137Cs γ source was used to assess the detection efficiency in the scanning experiments. Monte Carlo studies were performed using 0.662 MeV γ rays, and separately, a uniform γ-ray energy distribution from 0 to 1.6 MeV. Lastly, the effects of a collimation aperture with different sizes and different incident directions were considered to discuss the relationship between the spatial resolution and suitable measurement scenarios. Therefore, a detailed calibration method for non-uniform dead layers was introduced through a comparison between experiments and Monte Carlo simulations. This work reevaluates the performance of HPGe detectors and provides valuable data support for a detailed system design and γ spectroscopy analysis in future.

Origin of giant enhancement of phase contrast in electron holography of modulation-doped [formula omitted]-type GaN

The electron optical phase contrast probed by electron holography at n-n+ GaN doping steps is found to exhibit a giant enhancement, in sharp contrast to the always smaller than expected phase contrast reported for p-n junctions. We unravel the physical origin of the giant enhancement by combining off-axis electron holography data with self-consistent electrostatic potential calculations. The predominant contribution to the phase contrast is shown to arise from the doping dependent screening length of the surface Fermi-level pinning, which is induced by FIB-implanted carbon point defects below the outer amorphous shell. The contribution of the built-in potential is negligible for modulation doping and only relevant for large built-in potentials at e.g. p-n junctions. This work provides a quantitative approach to so-called dead layers at TEM lamellas.

Modeling of N and P-Type of Coaxial Ge Detectors Using MCNPX and the Effect of Dead Layer Variation on Its Response Function

Abstract This study assessed the response function of a p-type and n-type coaxial high-purity germanium (HPGe) detector via Monte Carlo N-Particle eXtended (MCNPX). MCNPX was employed to model the Coaxial Ge detectors, and for a precise simulation, the dimensions of the dead layer of germanium crystals were added. The dead layer was separated into front and lateral surfaces, and the thickness of each dead layer was modeled. In this work, the simulated detectors have been performed at different energy lines using a radioactive source Eu-152 to study the response function of each with dead layer variations for the front dead layer and study the range of relative deviation of the Monte Carlo simulation outputs from the manufactured declared data. The results proved that the n-type coaxial HPGe detector is more sensitive to the dead layer change than the p-type with a thick change of 0.01 mm. This research has significant effects on the efficiencies of the radiation detection systems in the energy range ∼ (120–1410) keV.

Scintillation characteristics of the EJ-299-02H scintillator.

A study of the dead layer thickness and quenching factor of a plastic scintillator for use in ultracold neutron (UCN) experiments is described. Alpha spectroscopy was used to determine the thickness of a thin surface dead layer to be 630 ± 110nm. The relative light outputs from the decay of 241Am and Compton scattering of electrons were used to extract Birks' law coefficient, yielding a kB value of 0.087 ± 0.003 mm/MeV, consistent with some previous reports for other polystyrene-based scintillators. The results from these measurements are incorporated into the simulation to show that an energy threshold of (∼9keV) can be achieved for the UCNProBe experiment. This low threshold enables high beta particle detection efficiency and the indirect measurement of UCN. The ability to make the scintillator deuterated, accompanied by its relatively thin dead layer, gives rise to unique applications in a wide range of UCN experiments, where it can be used to trap UCN and detect charged particles in situ.

Emergent quasi-two-dimensional ferromagnetic state with perpendicular magnetic anisotropy at La0.7Sr0.3MnO3/SrCuO2 interface

Due to the strong interlayer coupling between multiple degrees of freedom, oxide heterostructures usually produce distinct interfacial phases with unexpected functionalities. Here, we report on the realization of quasi-two-dimensional ferromagnetic state in ultrathin La0.7Sr0.3MnO3 (LSMO) layer down to two unit cells (u.c.), being sandwiched by the planar infinite-layer structured SrCuO2 layers (P-SCO). We find the LSMO/P-SCO interface coupling has greatly suppressed the magnetic dead layer of LSMO, resulting in an emergent interfacial ferromagnetic phase. Thus, robust ferromagnetic order can be maintained in the 2 u.c.-thick LSMO layer (∼7.7 Å), showing a Curie temperature of ∼260 K and remarkable perpendicular magnetic anisotropy. X-ray absorption spectra reveal notable charge transfer from Mn to Cu at the interface, and thus, resulted preferential d3z2−r2 orbital occupation for interfacial Mn ions plays an important role in the inducing of perpendicular magnetic anisotropy in quasi-two-dimensional LSMO layer. Our work demonstrates a unique approach for tuning the properties of oxides via an interface engineering of oxygen coordination in perovskite/infinite-layer heterostructures.

Enhancement of magnetization and optical properties of CuFe2O4/ZnFe2O4 core/shell nanostructure

In this work, core/shell of CuFe2O4/ZnFe2O4 nanostructure composite was prepared by hydrothermal method. X-ray diffraction (XRD) analysis, transmission electron microscope imaging, energy dispersive X-ray (EDX), and Fourier transform infrared techniques were used to prove the phase formation, morphology, elemental analysis, and cation distribution of core/shell structure, respectively. Furthermore, measurement of the optical properties proved the decrease of photoluminescence (PL) efficiency. The magnetic measurements showed an enhancement of the magnetization by about 63% relative to pure Cu ferrite, and the magnetization curve exhibited superparamagnetic behavior. These results were explained in terms of the depression of the magnetic dead layer thickness in the core/shell structure. The results unleash the promising applications of the prepared samples as transformer cores in the high frequency range and as a photocatalytic agent for water purification and hydrogen production.

Unraveling the effect of annealing on the structural and microstructural evolution of NiFe2O4@SiO2 core-shell type nanocomposites

The comparison between as-received and multistage annealed NiFe2O4@SiO2 (NFO@SiO2) core-shell nanoparticles is presented. The studied ferrites were synthesized by co-precipitation followed by microemulsion. The presence of cubic inverse spinel ferrite structure with tetrahedral and octahedral iron occupancy is confirmed in all analyzed samples. The structural characterization revealed the gradual crystallization of NFO cores up to about 12 nm crystallite size at the last annealing stage. Simultaneously, a partial crystallization of the silica matrix is evidenced. The superparamagnetic (SPM) behaviour is demonstrated based on complex magnetic performance and confirmed by Mössbauer spectrometry. The spin-canting phenomenon on the surface dead magnetic layer (MDL) is revealed based on the Yafet-Kittel model. A slight change in MDL thickness versus heat treatment is proved. The X-ray photoemission and absorption studies confirmed the complex nature of the Fe-based states. The Si–O–Ni/Fe intermixing on the NFO – SiO2 nanoparticles interface is revealed by the core-level lines analysis, proving the existence of a disordered layer on the NFO surface.

Impact of HfO2 Dielectric Layer Placement in Hf0.5Zr0.5O2‐Based Ferroelectric Tunnel Junctions for Neuromorphic Applications

AbstractThe use of Hf0.5Zr0.5O2 (HZO) films within hafnia‐based ferroelectric tunnel junctions (FTJ) presents a promising avenue for next‐generation non‐volatile memory devices. HZO exhibits excellent ferroelectric properties, ultra‐thinness, low power consumption, nondestructive readout, and compatibility with silicon devices. In this study, Mo/HZO/n+ Si devices are investigated, incorporating a 1 nm HfO2 dielectric layer at the top and bottom of the HZO ferroelectric layer. Comparing the FTJ device configurations, it is observed that the metal‐ferroelectric‐dielectric‐semiconductor (MFIS) outperforms the metal‐dielectric‐ferroelectric‐semiconductor (MIFS) in terms of ferroelectricity, displaying a high 2Pr value of ≈69 µC cm−2. Additionally, MFIS exhibits lower leakage current, higher tunneling electro‐resistance ratio, and a thin dead layer during short pulse switching, as confirmed through DC double sweeping of I−V characteristics. The modified half‐bias scheme demonstrates a maximum array size of 191 for MFIS, showcasing its superior performance over MIFS. Synaptic characteristics, including potentiation, depression, paired‐pulse facilitation, spike‐rate‐dependent plasticity, and excitatory postsynaptic current, are measured using MFIS, highlighting its outstanding ferroelectric properties. As a physical reservoir, the FTJ device implements 16 states of 4 bits in reservoir computing. Finally, pattern recognition using a deep learning neural network achieves high accuracy with using the Modified National Institute of Standards and Technology dataset.

Optimization of magnetic properties of MnFe2O4 by modulating molarity of NaOH as precipitating agent

Abstract MnFe2O4 nanoparticles were synthesized using the co-precipitation method with a wide range of molar concentrations of sodium hydroxide 0.76 M−3.0 M. X-ray diffraction, field effect scanning electron microscopy, transmission electron microscopy, and vibrating sample magne-tometry were employed to characterise the structural, morphological, and magnetic characteristics of nanoparticles. Field effect scanning electron microscopy and transmission electron microscopy images show that the particles were spherical in shape for all the samples except for sample prepared at a molar concentration of 1.3 M. Particle shape was found to depend on the molar concentration of NaOH. The hysteresis loops of the samples possessed a very small area and low coercivity. The crystallite size (cs), saturation magnetisation, coercivity, retentivity, squareness ratio and anisotropy constant were found to be dependent on the molar concentration on NaOH. M S was noted to be at a maximum of 64.4 emu g−1 at a molar concentration of 1.3 M. The ratio t/cs (where t is the thickness of the dead layer) was calculated to account for the variation in M S. H C was found to be maximum of ∼52 Oe at molar concentrations between 1.0 M and 2.0 M. M r and M r/M S were found to be a maximum of 8.95 emu g−1 and 0.15, respectively, for the molar concentration of 2.0 M.

Atomic‐Scale Study of Dead Layers in Epitaxial Perovskite Dielectric Thin Films with Oxide and Metal Top Electrodes

AbstractPerovskite‐oxide‐based capacitors, which exhibit high charge storage capacity, have attracted considerable attention as a potential candidate for overcoming the limitations of nanoscale integration. Unfortunately, a dead layer forms in these capacitors at the interface between the electrode and the dielectric, which degrades the charge storage capacity; thus, this layer has been extensively investigated. The dead layer in perovskite‐oxide‐based capacitors exhibits different characteristics depending on the electrode materials; however, a method for minimizing this layer is lacking. In this study, the charge storage capacity of a perovskite‐oxide‐based capacitor is evaluated considering the effect of the Ru and SrRuO3 top electrodes on the SrRuO3/Ba0.5Sr0.5TiO3 stack. Dead layers at the interface between each top electrode material and the dielectric are studied on the atomic scale. The results indicate that the Ru metal electrode causes oxygen to diffuse from the dielectric to the electrode, forming elongated perovskite oxide at the interface, which acts as a dead layer. However, minimizing the dead layer at the top interface increases the dielectric permittivity from 667 to 953. Consequently, the phenomenon and mechanism of the dead layer are intuitively identified. This study proposes a method to overcome the limitations of next‐generation dynamic random access memory (DRAM).

Magnetism of MoS2 bilayers with intercalated and surface adsorbed Fe

Magnetic thin films composed of two-dimensional van der Waals materials will play an important role in future spintronics devices. Recently, Tuli et al. performed in-situ MOKE measurements of Fe deposited on an MoS2 substrate and found that a magnetic dead layer (MDL) exists up to approximately 2 Å (Tuli et al. (2021)). To understand the origin of the MDL, we investigate MoS2 bilayers with intercalated and surface adsorbed Fe using first-principles calculations. The results show that the Fe-intercalated bilayer MoS2 is more stable than the bilayer MoS2 with surface adsorbed Fe. The Fe-intercalated bilayer MoS2 is non-magnetic and behaves as the MDL. The systematic investigation of variation in the inter-layer distance between the intercalated Fe layer and surface of MoS2 layer reveals that the strong hybridization between Fe and S is the origin of the MDL. We also show that further intercalation of Fe makes the film magnetic. The results provide a deep understanding of the MDL in two-dimensional van der Waals magnetic materials.

Effect of the nature of the solid substrate on spatially heterogeneous activated dynamics in glass forming supported films.

We extend the force-level elastically collective nonlinear Langevin equation theory to treat the spatial gradients of the alpha relaxation time and glass transition temperature, and the corresponding film-averaged quantities, to the geometrically asymmetric case of finite thickness supported films with variable fluid-substrate coupling. The latter typically nonuniversally slows down motion near the solid-liquid interface as modeled via modification of the surface dynamic free energy caging constraints that are spatially transferred into the film and which compete with the accelerated relaxation gradient induced by the vapor interface. Quantitative applications to the foundational hard sphere fluid and a polymer melt are presented. The strength of the effective fluid-substrate coupling has very large consequences for the dynamical gradients and film-averaged quantities in a film thickness and thermodynamic state dependent manner. The interference of the dynamical gradients of opposite nature emanating from the vapor and solid interfaces is determined, including the conditions for the disappearance of a bulk-like region in the film center. The relative importance of surface-induced modification of local caging vs the generic truncation of the long range collective elastic component of the activation barrier is studied. The conditions for the accuracy and failure of a simple superposition approximation for dynamical gradients in thin films are also determined. The emergence of near substrate dead layers, large gradient effects on film-averaged response functions, and a weak non-monotonic evolution of dynamic gradients in thick and cold films are briefly discussed. The connection of our theoretical results to simulations and experiments is briefly discussed, as is the extension to treat more complex glass-forming systems under nanoconfinement.

Enhanced Ferromagnetism in Atomically Thin Oxides Achieved by Interfacial Reconstruction

AbstractDiscoveries of ferromagnetic materials with ultrathin thickness are of great importance for both fundamental science and technological applications. Transition metal oxides (TMOs) provide promising candidates in the context of next‐generation spintronics, despite the severe decay of ferromagnetism as the thickness reduces to the nanometer regime. Here, an efficient strategy to eliminate the magnetic dead layer in atomically thin oxides is presented, by using the epitaxial interface of 3d and 5d oxide monolayers that reconciles both strong exchange interaction and large uniaxial magnetic anisotropy. Combining multiple experimental methods, a ferromagnetic transition in an ultrathin oxide heterostructure comprised of only one La0.2Sr0.8MnO3 monolayer sandwiched by SrIrO3 monolayer (total thickness of three unit‐cells) is unambiguously demonstrated. Remarkably, a largely enhanced saturation magnetization (2 µB Mn−1) and Curie temperature (80 K) are observed for the single manganite monolayer, as compared to previously reported ferromagnetic monolayer oxides. The results demonstrate a general strategy for creating robust ferromagnetism in ultrathin TMOs, potentially enabling novel oxide spin‐orbitronic devices.