Metal Insulator Semiconductor Structures Research Articles

Theory and Simulation of Metal–Insulator–Semiconductor (MIS) Photoelectrodes

A metal-insulator-semiconductor (MIS) structure is an attractive photoelectrode-catalyst architecture for promoting photoelectrochemical reactions, such as the formation of H2 by proton reduction. The metal catalyzes the generation of H2 using electrons generated by photon absorption and charge separation in the semiconductor. The insulator layer between the metal and the semiconductor protects the latter element from photo-corrosion and, also, significantly impacts the photovoltage at the metal surface. Understanding how the insulator layer determines the photovoltage and what properties lead to high photovoltages is critical to the development of MIS structures for solar-to-chemical energy conversion. Herein, we present a continuum model for charge-carrier transport from the semiconductor to the metal with an emphasis on mechanisms of charge transport across the insulator. The polarization curves and photovoltages predicted by this model for a Pt/HfO2/p-Si MIS structure at different HfO2 thicknesses agree well with experimentally measured data. The simulations reveal how insulator properties (i.e., thickness and band structure) affect band bending near the semiconductor/insulator interface and how tuning them can lead to operation closer to the maximally attainable photovoltage, the flat-band potential. This phenomenon is understood by considering the change in tunneling resistance with insulator properties. The model shows that the best MIS performance is attained with highly symmetric semiconductor/insulator band offsets (e.g., BeO, MgO, SiO2, HfO2, or ZrO2 deposited on Si) and a low to moderate insulator thickness (e.g., between 0.8 and 1.5 nm). Beyond 1.5 nm, the density of filled interfacial trap sites is high and significantly limits the photovoltage and the solar-to-chemical conversion rate. These conclusions are true for photocathodes and photoanodes. This understanding provides critical insight into the phenomena enhancing and limiting photoelectrode performance and how this phenomenon is influenced by insulator properties. The study gives guidance toward the development of next-generation insulators for MIS structures that achieve high performance.

Trap‐Assisted Memristive Switching in HfO2‐Based Devices Studied by In Situ Soft and Hard X‐Ray Photoelectron Spectroscopy

AbstractMemristive devices are under intense development as non‐volatile memory elements for extending the computing capabilities of traditional silicon technology by enabling novel computing primitives. In this respect, interface‐based memristive devices are promising candidates to emulate synaptic functionalities in neuromorphic circuits aiming to replicate the information processing of nervous systems. A device composed of Nb/NbOx/Al2O3/HfO2/Au that shows promising features like analog switching, no electro‐forming, and high current‐voltage non‐linearity is reported. Synchrotron‐based X‐ray photoelectron spectroscopy and depth‐dependent hard X‐ray photoelectron spectroscopy are used to probe in situ different resistance states and thus the origin of memristive switching. Spectroscopic evidence for memristive switching based on the charge state of electron traps within HfO2 is found. Electron energy loss spectroscopy and transmission electron microscopy support the analysis. A device model is proposed that considers a two‐terminal metal–insulator–semiconductor structure in which traps within the insulator (HfO2/Al2O3) modulate the space charge region within the semiconductor (NbOx) and, thereby, the overall resistance. The experimental findings are in line with impedance spectroscopy data reported in the companion paper (Marquardt et al). Both works complement one another to derive a detailed device model, which helps to engineer device performance and integrate devices into silicon technology.

Pulsed laser deposited Dy and Ta doped hafnium- zirconium oxide thin films for the high-k applications

Hf0.6Zr0.2Dy0.1Ta0.1O2 thin films were deposited on Si (100) at 600°C using pulsed laser deposition for gate oxide applications because of the compatibility of the elements with the CMOS process. Special emphasis was placed on the properties of Hf0.6Zr0.2Dy0.1Ta0.1O2 thin films within metal-insulator-semiconductor (MIS) structure (Pt/Hf0.6Zr0.2Dy0.1Ta0.1O2/Si) for high-k application in logic devices. The capacitance-voltage and current-voltage measurements are used to determine the electrical characteristics of MIS capacitor structures. The minimum equivalent oxide thickness and maximum dielectric constant of the thin films obtained were as low as 1.2 nm and as high as 35.7 at 10 kHz, respectively. Results on the gate stacks obtained based on the MIS structures appear to be promising as the high-k material for logic devices.

Sub-bandgap near-infrared photovoltaic response in Au/Al2O3/n-Si metal–insulator–semiconductor structure by plasmon-enhanced internal photoemission

Silicon sub-bandgap near-infrared (NIR) (λ > 1100 nm) photovoltaic (PV) response by plasmon-enhanced internal photoemission was investigated. The Si sub-bandgap NIR PV response, which remains unexploited in Schottky junction-like solar cell device, was examined using nanometer sized Au/Al2O3/n-Si junction arrays. This kind of metal–insulator–semiconductor structure was similar in functionality to Schottky junction in NIR absorption, photo-induced charge separation and collection. It showed that NIR absorption increased steadily with increasing volume of Au nanoparticles (NPs) till a saturation was reached. Simulation results indicated the formation of localized surface plasmon on the surfaces of Au NPs, which was correlated well with the observed NIR absorption. On the other hand, the NIR PV response was found sensitive to the amount and size of Au NPs and thickness of Al2O3. Chemical and field-effect passivation of n-Si by using Al2O3 and SiO2 were used to optimize the NIR PV response. In the current configuration, the best PV conversion efficiency was 0.034% at λ = 1319 nm under illumination power of 0.1 W/cm2.

Ga2O3 metal–insulator-semiconductor solar-blind photodiodes with plasmon-enhanced responsivity and suppressed internal photoemission

Metal-semiconductor-metal (MSM) architectures are popular for achieving high-responsivity Ga2O3 solar-blind photodetectors (SBPDs), however, the hot-electron-induced internal photoemission (IPE) effect restricts their detecting performance. Herein, we demonstrate the rational design of an Al/Al2O3/Ga2O3 metal–insulator-semiconductor (MIS) SBPD that has merits of enhanced responsivity, suppressed sub-gap response and ultralow dark current based on the simulation results obtained using Lumerical software. For the cylindrical patterned detectors with Al/Al2O3/Ga2O3 MIS structures, the optimized dimensions of Al electrodes with a conformed ultra-thin (2 nm) Al2O3 layer support the surface plasmon polariton resonances at 250 nm, thus improving the photoresponsivity to 74 mA W−1. Furthermore, the sandwiched Al2O3 layer lifts the barrier for hot electrons in electrodes, which significantly suppresses the IPE-induced sub-gap photoresponse by more than 105 in magnitude with respect to the Al/Ga2O3 MSM counterpart. Optical and electrical field distributions are overlapped in cylindrically patterned MIS detectors, simultaneously improving the excitation and collection efficiencies of excess carriers and resulting in the 103-boosted rejection ratio.

A numerical modeling of the frequency dependence of the capacitance–voltage and conductance–voltage characteristics of GaN MIS structures

The capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics of GaN metal–insulator–semiconductor (MIS) structures have a frequency dependence due to the capture and emission of electrons by the high density of the interface states. However, the details of how an interface state affects C–V and G–V characteristics is still not well understood. In this paper, we report a numerical modeling method that can simulate the frequency dependent C–V and G–V characteristics of GaN MIS structures.

Evidence of reduced interface states in Al2O3/AlGaN MIS structures via insertion of ex situ regrown AlGaN layer

We report on the impact of the 3 nm thick ex situ AlGaN regrown layer prior to insulator deposition on the interfacial properties of Al2O3/AlGaN/GaN metal–insulator–semiconductor (MIS) structures. MIS-capacitors (MIScaps) with regrown AlGaN layer exhibited anomalously excessive threshold voltage shift compared to reference sample without regrown AlGaN, suggesting highly reduced interface states density (D it). Moreover, MIScaps with regrown AlGaN layer exhibited “spill-over” in the capacitance–voltage profiles, further evidencing the improved Al2O3/AlGaN interfaces. Fabricated three-terminal MIS-HEMTs with regrown AlGaN showed less hysteresis in transfer curves, enhanced maximum drain current, and increased linearity over the reference device.

Analysis of recombination centers near an interface of a metal–SiO2–Si structure by double carrier pulse deep-level transient spectroscopy

This paper proposes a new double carrier pulse deep-level transient spectroscopy (DC-DLTS) method that is applicable for evaluating metal–insulator–semiconductor (MIS) structures and the recombination centers in carrier-selective contact solar cells. Specifically, this study evaluated recombination characteristics of defects induced in bulk Si near SiO2/Si interfaces by reactive plasma deposition (RPD). In this method, a pulse voltage was first applied to inject majority carriers. Subsequently, a second pulse voltage was applied, which allowed minority carriers to be injected into the MIS structure. With these two types of carrier injections, carriers were recombined in recombination-active defects, and the DC-DLTS spectrum changed. During the injection of minority carriers, some majority carriers were thermally emitted from the defects, resulting in a decrease in the signal intensity. The recombination activity was analyzed by considering the effect of thermal emission on the change in signal intensity. The number of induced defect types and defect properties were estimated using Bayesian optimization. According to the results, three types of electron traps were generated using the RPD process. Based on the DC-DLTS results, defects with energy level 0.57 eV below the conduction band and capture cross section of ∼10−15 cm2 act as recombination centers.

High-performance vertical GaN field-effect transistor with an integrated self-adapted channel diode for reverse conduction

A vertical GaN field-effect transistor with an integrated self-adapted channel diode (CD-FET) is proposed to improve the reverse conduction performance. It features a channel diode (CD) formed between a trench source on the insulator and a P-type barrier layer (PBL), together with a P-shield layer under the trench gate. At forward conduction, the CD is pinched off due to depletion effects caused by both the PBL and the metal–insulator–semiconductor structure from the trench source, without influencing the on-state characteristic of the CD-FET. At reverse conduction, the depletion region narrows and thus the CD turns on to achieve a very low turn-on voltage (V F), preventing the inherent body diode from turning on. Meanwhile, the PBL and P-shield layer can modulate the electric field distribution to improve the off-state breakdown voltage (BV). Moreover, the P-shield not only shields the gate from a high electric field but also transforms part of C GD to C GS so as to significantly reduce the gate charge (Q GD), leading to a low switching loss (E switch). Consequently, the proposed CD-FET achieves a low V F of 1.65 V and a high BV of 1446 V, and V F, Q GD and E switch of the CD-FET are decreased by 49%, 55% and 80%, respectively, compared with those of a conventional metal–oxide–semiconductor field-effect transistor (MOSFET).

Study of MIS structures based on CdHgTe and HfO2 applied by PEALD

We investigate the HfO2/Hg0.78Cd0.22Te interface fabricated by plasma-enhanced atomic layer deposition (PEALD) at 120 °C During the deposition of HfO2, no donor-like defects are introduced into mercury cadmium telluride. X-ray photoelectron spectroscopy and ellipsometry were used to establish the optimal process regime at 120 °C and to demonstrate how HfO2 layer composition and growth rate per cycle depend on post-plasma purge time; the optimum is achieved at 6 s. Increasing the post-plasma purge time decreases the carbon and nitrogen impurity concentration in the HfO2 layer. Measurements of the admittance of metal-insulator-semiconductor (MIS) structures over the surface of a sample show that the electro-physical properties are uniform. We discuss the method of measuring the admittance of MIS structures that allows us to minimize the contribution of slow states with trapped charge on shape and shift of the C–V curve. The results demonstrate that the densities of fixed charge, slow states, and fast interfacial traps at the HfO2/MCT interface are greater than that for Al2O3/MCT (also formed by PEALD). The interface trap density is estimated from a normalized parallel conductance map, and the HfO2 film adheres well.

Evaluation of the border traps in LPCVD Si3N4/GaN/AlGaN/GaN MIS structure with long time constant using quasi-static capacitance voltage method

We extract the electric properties of border traps with long time constant in low-pressure chemical vapor deposition (LPCVD) Si3N4/GaN/AlGaN/GaN metal–insulator–semiconductor (MIS) structure using quasi-static capacitance voltage method. The energy and depth distribution of the border traps is calculated based on the analysis of energy band diagram and charging dynamic of border traps in the MIS structures. With this method, it is found that LPCVD Si3N4/GaN/AlGaN/GaN MIS structure have a high density of border traps in the order up to 1021 cm−3 eV−1 located at energy level between E C,GaN − 0.04 eV and E C,GaN − 0.66 eV with distance of 1.0–4.2 nm from the Si3N4/GaN interface. Microstructure analysis suggests that the high density of border traps is possibly correlated to the oxygen content at the Si3N4/GaN interface. Meanwhile, the proposed method is also suitable for MIS or metal-oxide-semiconductor structure on other semiconductors, providing another powerful tool to analysis the physical properties of border traps.

Effects of carbon impurity in ALD-Al2O3 film on HAXPES spectrum and electrical properties of Al2O3/AlGaN/GaN MIS structure

In this study, the effects of carbon impurity in the atomic layer deposited-Al2O3 film on the hard X-ray photoelectron spectroscopy (HAXPES) spectra and the electrical properties of metal–insulator–semiconductor (MIS) structure were measured. The carbon concentration in the Al2O3 film was adjusted by varying the deposition conditions (precursor: trimethylaluminum, Al(CH3)3, or dimethylaluminum hydride, Al(CH3)2H, oxidant, and deposition temperature) in the atomic layer deposition process. The HAXPES measurements revealed the correlation between full width at half maximum (FWHM) of Al 1 s and O 1 s spectra and the carbon concentration in the Al2O3 film. Furthermore, the negative charges in the Al2O3 film could change the FWHM, attributed to the carbon impurity. The correlation between the carbon concentration and the electrical characteristics of the Al2O3/AlGaN/GaN MIS structure was analyzed. The interface state density and the effective charge density were dependent on the carbon concentration in the Al2O3 film.

Role of Surface Coverage and Film Quality of the TiO2 Electron Selective Layer for Optimal Hole-Blocking Properties.

Titanium dioxide (TiO2) is a commonly used electron selective layer in thin-film solar cells. The energy levels of TiO2 align well with those of most light-absorbing materials and facilitate extracting electrons while blocking the extraction of holes. In a device, this separates charge carriers and reduces recombination. In this study, we have evaluated the hole-blocking behavior of TiO2 compact layers using charge extraction by linearly increasing voltage in a metal–insulator–semiconductor structure (MIS-CELIV). This hole-blocking property was characterized as surface recombination velocity (SR) for holes at the interface between a semiconducting polymer and TiO2 layer. TiO2 layers of different thicknesses were prepared by sol–gel dip coating on two transparent conductive oxide substrates with different roughnesses. Surface coverage and film quality on both substrates were characterized using X-ray photoelectron spectroscopy and atomic force microscopy, along with its conductive imaging mode. Thicker TiO2 coatings provided better surface coverage, leading to reduced SR, unless the layers were otherwise defective. We found SR to be a more sensitive indicator of the overall film quality, as varying SR values were still observed among the films that looked similar in their characteristics via other methods.

A Poisson–Nernst–Planck Model of Ion Transport and Interface Segregation in Metal–Insulator–Semiconductor Structures and Solar Cells

A numerical model that describes the transport of mobile ionic species in metal–insulator–semiconductor (MIS) and photovoltaic (PV) devices subject to temperature and voltage stress is presented. The finite element method (FEM) is used to solve the Nernst–Planck equation while imposing Poisson's equation self‐consistently as a restriction for the electrostatic potential. This allows the contribution of the ionic species to the potential to be taken into account. Using a variational formulation eases the implementation of diverse boundary conditions, including the incorporation of segregation kinetics at the device interfaces. Segregation across the dielectric–semiconductor interface is relevant to modeling the electronic device degradation in systems where contamination reaches the semiconductor. The model in closed systems with no‐flux boundary conditions is validated first. In the limiting case of low contamination levels with respect to the gate bias, the FEM solution matches analytically derived approximations. Then, the implementation is broadened to include an open boundary at the dielectric–semiconductor interface to account for leakage of ions. The predicted time dependence of the flatband voltage in Na‐contaminated MIS test structures agrees well with measurements. The model successfully captures the role of long‐range ion transport at concentrations of relevance to electronic and PV device instability and neuromorphic computing.

Investigation of the Effects of Rapid Thermal Annealing on the Electron Transport Mechanism in Nitrogen-Doped ZnO Thin Films Grown by RF Magnetron Sputtering.

Nitrogen-doped ZnO (ZnO:N) thin films, deposited on Si(100) substrates by RF magnetron sputtering in a gas mixture of argon, oxygen, and nitrogen at different ratios followed by Rapid Thermal Annealing (RTA) at 400 °C and 550 °C, were studied in the present work. Raman and photoluminescence spectroscopic analyses showed that introduction of N into the ZnO matrix generated defects related to oxygen and zinc vacancies and interstitials. These defects were deep levels which contributed to the electron transport properties of the ZnO:N films, studied by analyzing the current–voltage characteristics of metal–insulator–semiconductor structures with ZnO:N films, measured at 298 and 77 K. At the appliedtechnological conditions of deposition and subsequent RTA at 400 °C n-type ZnO:N films were formed, while RTA at 550 °C transformed the n-ZnO:N films to p-ZnO:N ones. The charge transport in both types of ZnO:N films was carried out via deep levels in the ZnO energy gap. The density of the deep levels was in the order of 1019 cm−3. In the temperature range of 77–298 K, the electron transport mechanism in the ZnO:N films was predominantly intertrap tunneling, but thermally activated hopping also took place.

On the impact of a metal-insulator-semiconductor structured n-electrode for AlGaN-based DUV LEDs.

In this work, a 280 nm AlGaN-based deep ultraviolet light-emitting diode (DUV LED) with a metal-insulator-semiconductor (MIS) structured n-electrode is fabricated and studied. The SiO2 insulator layer is adopted to form the MIS structure by using an atomic layer deposition system. After adopting the MIS-structured n-electrode, the SiO2 intermediate layer enables electron affinity for the contact metal to be higher than the conduction band of the n-AlGaN layer, which favors the electrons to be injected into the n-AlGaN layer by intraband tunneling rather than thermionic emission. Moreover, the thin SiO2 insulator can share the applied bias, which makes the n-AlGaN layer surface less depleted and thus further facilitates the electron injection. The improved electron injection capability at the metal-semiconductor interface helps reduce the contact resistance and increase electron concentration in the active region, which then improves external quantum efficiency and wall-plug efficiency for the proposed DUV LED.

Highly Tunable Enhancement and Switching of Nonlinear Emission from All-Inorganic Lead Halide Perovskites via Electric Field.

Herein, we demonstrate a highly tunable enhancement and switching of nonlinear emission from all-inorganic metal halide perovskites based on an asymmetrically biased metal-insulator-semiconductor (MIS) structure. We achieve 2 orders of magnitude enhancement of the two-photon-pumped photoluminescence (TPL) from CsPbBr3 microplates with the MIS structure, due to comprehensive effects including localized field effect, trap-filling effect, and collection enhancement. In particular, taking advantage of electric-field-induced passivation/activation of Br vacancies, we realize highly tunable TPL enhancement, ranging from ∼61.2-fold to ∼370.3-fold. Moreover, we demonstrate an efficient modulation of the two-photon-pumped lasing from the MIS structure, which exhibits electric field induced switching with a high on/off ratio of 67:1. This work has opened new avenues for steering carrier transport and nonlinear emission in lead halide perovskites, which shows great promise for realizing high-efficiency and tunable nonlinear nanophotonic devices.

(Invited) Fabrication and Characterization of Ge-Based MIS Structures

Germanium (Ge) is a very attractive channel material for future high-performance metal-insulator-semiconductor (MIS) devices because it exhibits much higher low-field electron (two times higher) and hole (four times higher) mobilities than Si. In addition, the smaller band gap of Ge than that of Si should allow the further reduction of power consumption in advanced integrated circuits and the lower melting point of Ge than that of Si also enables Ge-based device integration at low processing temperatures. However, until the early years of the 2000s, the reported electrical properties of high-k/Ge interfaces had been so poor that one of the critical issues in establishing Ge-based MIS technology was the effective passivation of Ge surfaces by dielectrics with superior interface properties. Although the quality of the Ge-MIS structure has been remarkably improved in the past decade, the relationship between the processing methods and the quality has not been discussed precisely.In this paper, we report on the fabrication of Ge MIS structures with various high-k dielectric materials and interlayer materials through various methods. We also introduce the convenient method to characterize the interface trap densities (Dit's) of the MIS capacitors by spectroscopic measurement of the interface trap density in an insulator/Ge interface at room temperature [1]. The effects of the insertion of interlayers into the interface of high-k insulator/Ge and the application of heat treatment to the Ge MIS structures will be also presented.For instance, the Dit of Ta2O5/GeNx/Ge fabricated by electron-cyclotron-resonance (ECR) plasma nitridation and sputtering was estimated to be 4×1011 cm-2 · eV-1 [2], that of Si3N4/GeNx/Ge by ECR plasma nitridation to be 1×1011 cm-2 · eV-1 [3-6], and that of Al2O3/GeO2/Ge by ECR plasma oxidation and sputtering to be 4.5×1010 cm-2 · eV-1 [7-9]. These results have summarized that the adequate surface preparations of the interfaces and films by the ECR plasma process with low energy plasma irradiation have been effective to realize the low Dit of MIS structures with GeO2 and/or GeNx.These results have expanded to another MIS structures fabricated by more conventional film formation technique such as the atomic layer deposition (ALD) and the radio frequency (RF) magnetron sputtering. The Dit of Al2O3/Al germanate/Ge by ALD with trimethylaluminum (TMA) and microwave-generated atomic oxygen was estimated to be 2×1011cm-2 · eV-1 [10], and that of HfO2/Ge by RF magnetron sputtering to be 5.9×1011 cm-2 · eV-1. These results have summarized that the heat treatment after the fabrication of MIS strucures have affected interface properties of germanate/Ge and oxide/Ge.Although the mechanism by which the process conditions act on the interface state is now under estimation, it has been speculated that the fabrication method, the insertion of the interlayers, and the application of heat treatment have a great influence on the interface characteristics. The structures and the fabrication method of various MIS will be discussed.Part of this work was carried out under the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University.

Graphene-Silicon Device for Visible and Infrared Photodetection.

The fabrication of a graphene–silicon (Gr-Si) junction involves the formation of a parallel metal–insulator–semiconductor (MIS) structure, which is often disregarded but plays an important role in the optoelectronic properties of the device. In this work, the transfer of graphene onto a patterned n-type Si substrate, covered by Si3N4, produces a Gr-Si device, in which the parallel MIS consists of a Gr-Si3N4-Si structure surrounding the Gr-Si junction. The Gr-Si device exhibits rectifying behavior with a rectification ratio up to 104. The investigation of its temperature behavior is necessary to accurately estimate the Schottky barrier height (SBH) at zero bias, φb0 = 0.24 eV, the effective Richardson’s constant, A* = 7 × 10–10 AK–2 cm–2, and the diode ideality factor n = 2.66 of the Gr-Si junction. The device is operated as a photodetector in both photocurrent and photovoltage mode in the visible and infrared (IR) spectral regions. A responsivity of up to 350 mA/W and an external quantum efficiency (EQE) of up to 75% are achieved in the 500–1200 nm wavelength range. Decreases in responsivity to 0.4 mA/W and EQE to 0.03% are observed above 1200 nm, which is in the IR region beyond the silicon optical band gap, in which photoexcitation is driven by graphene. Finally, a model based on two parallel and opposite diodes, one for the Gr-Si junction and the other for the Gr-Si3N4-Si MIS structure, is proposed to explain the electrical behavior of the Gr-Si device.

Local increase in compressive strain (GaN) in gate recessed AlGaN/GaN MISHFET structures induced by an amorphous AlN dielectric layer

We fabricated and characterized metal insulator semiconductor (MIS) structures by applying amorphous AlN thin layers as a dielectric in gate recessed AlGaN/GaN heterostructure field effect transistors (HFETs). Micro photoluminescence measurements performed on MISHFET devices reveal a local non-uniform distribution of strain in the source—gate recess—drain region. Furthermore, a reduction of compressive stress up to 0.3 GPa in GaN after gate recessing was experimentally determined. The local stress increases by ∼0.1 GPa and ∼0.2 GPa after the deposition of 4 and 6 nm thin AlN layers in the gate recessed structures, respectively. Additionally, an increase in sheet charge density in the devices under investigation from ∼ to ∼ was evaluated by capacitance–voltage measurements. Therefore, strain engineering by applying amorphous AlN layers in gate recessed MISHFETs can significantly improve their device characteristics.