Fowler Nordheim Research Articles

Radiation tolerance, charge trapping, and defect dynamics studies of ALD-grown Al/HfO2/Si nMOSCAPs

The radiation response, long-term performance, and reliability of HfO2-based gate dielectric materials play a critical role in metal oxide semiconductor (MOS) technology for space device applications. Al/HfO2/Si atomic layer-deposited devices were irradiated by gamma and swift heavy ions. An increase in the leakage current and charge trapping has been observed as the gamma irradiation dose varied from 25 to 100 krad. The density of oxide traps is found to increase with an increase in the gamma dose while the interface trap density is found to decrease. Another set of samples were irradiated by 120 MeV Au ions to study the SHI-induced defect annealing/creation of defects and intermixing effects in HfO2/Si-based devices. The formation of an interfacial layer of HfSiO at a fluence of at 5 × 1013 cm−2 is revealed by X-ray reflectivity analysis. The densities of interface- and oxide-trapped charges are found to decrease up to a critical fluence of 1 × 1012 cm−2 and then increase with further increase in fluence to 5 × 1013 cm−2. The presence of the interlayer, due to the swift heavy ion-induced intermixing, has been confirmed by X-ray photoelectron spectroscopy measurements. Various current conduction mechanisms in both substrate and gate injection cases were used to understand the basic mechanisms of direct, Fowler–Nordheim, and Poole–Frenkel tunneling, as well as Schottky emission in these devices. These studies elucidated the radiation tolerance and charge-trapping behavior of Al/HfO2/Si nMOS capacitors.

Fowler–Nordheim Law Correlated with Improved Field Emission in Self‐Assembled NiCr2O4 Nanosheets

Electric field emission (FE) properties are measured on self‐assembled NiCr2O4 nanosheets in a planner “diode” arrangement at a base pressure of ≈1.0 × 10−8 mbar. The turn‐on field at FE current density 1 μA cm−2 and the threshold field at FE current density 10 μA cm−2 are observed as 4.10 and 4.94 V μm−1, respectively. The local work function (Φ) is calculated as 4.358 eV, using density functional theory (DFT) in Quantum Espresso code and field enhancement factor (β) intensified up to 2074 from the sharp edges of the NiCr2O4 nanosheet arrays’ emitter surface. An exemplary FE current stability is observed from the current–time plot over a period of 4.5 h. The field enhancement factor (β), inferior turn‐on field, and superior stability compared with any other pristine nanosheet compound recommend its potential application in flat panel display and vacuum micro‐/nanoelectronics.

A First Evaluation of Thick Oxide 3C-SiC MOS Capacitors Reliability

Despite the recent advances in 3C-SiC technology, there is a lack of statistical analysis on the reliability of SiO2 layers on 3C-SiC, which is crucial in power MOS device developments. This article presents a comprehensive study of the medium- and long-term time-dependent dielectric breakdowns (TDDBs) of 65-nm-thick SiO2 layers thermally grown on a state-of-the-art 3C-SiC/Si wafer. Fowler–Nordheim (F-N) tunneling is observed above 7 MV/cm and an effective barrier height of 3.7 eV is obtained, which is the highest known for native SiO2 layers grown on the semiconductor substrate. The observed dependence of the oxide reliability on the gate active area suggests that the oxide quality has not reached the intrinsic level. Three failure mechanisms were identified and confirmed by both medium- and long-term results. Although two of them are likely due to extrinsic defects from material quality and fabrication steps, the one dominating the high field (>8.5 MV/cm) should be attributed to the electron impact ionization within SiO2. At room temperature, the field acceleration factor is found to be $\approx 0.906$ dec/(MV/cm) for high fields, and the projected lifetime exceeds 10 years at 4.5 MV/cm.

Квантовая теория эмиссии электронов из структуры "металл-диэлектрик" в сильных электрических полях

A generalized formula is derived for the electron emission current in relation to the temperature, the electric field, and the electronic work function for a “metal–dielectric” system. The formula takes into account the quantum nature of the image forces. In deriving it, the Fermi–Dirac distribution and the quantum image potential obtained in terms of the electron–polaron theory are used. In the limit of the classical potential of image forces, the well-known Richardson–Schottky and Fowler–Nordheim formulas are obtained for thermionic emission and field emission, respectively. It is shown that at high temperatures and electric fields E ≥ 10 MV/cm, the polaron contribution to the electron emission current increases with increasing field and decreases with increasing temperature. The decrease in current is related to an increase in effective electronic work function due to the electron-polaron effect. Extrapolation formulas convenient to obtain theoretical estimates are derived for the thermionic and the field emission current.

Influence of the distribution of local field enhancement factors on the shape of the current-voltage characteristics of carbon-nanotube-based large-area emitters

Experimental current-voltage characteristics (IVCs) of carbon nanotube (CNT)-based large-area field emitters (LAFEs) were measured and analyzed using a “two-class model”, in which some emitters had a set of local field enhancement factors (FEFs) significantly larger than those of the other emitters. An online measurement of the experimental IVCs was carried out and, interestingly, showed the presence of strong nonlinearity in the form of “upward bends” in the corresponding orthodox Fowler-Nordheim (FN) plots. Our studies were conducted over a wide range of currents, and excellent agreement between our experimental results and the proposed theory confirmed the general validity of the two-class FEF model of orthodox field emission. The results presented here were also supported by a bimodal distribution of the brightness level observed by an analysis of the glow patterns, since the brightness was expected to depend on the site's emitted current. We believe this is an important step for LAFE characterization when strong nonlinearites are present in the FN plots, since an FEF similar to the classically defined FEF was recently proven to be applicable to CNTs in the context of full quantum ab initio calculations.

A detailed study on frequency dependent electrical characteristics of MOS capacitors with dysprosium oxide gate dielectrics

Frequency dependent electrical characteristics of Al/Dy2O3/p-Si/Ag MOS capacitors have been systematically investigated incorporating interfacial chemical structures of the capacitors to specify where interface state signals arise from. The results show that capacitance (C) increases with decreasing in the applied voltage frequency due to the response time dependency of the interface states. Conductance (G) curves have a single peak at the relatively high frequencies as expected. Interestingly, the G curves exhibit two distinct peaks at 400 kHz and lower frequencies. These G peaks are associated with acceptor-like interface (Dit,ac) and donor-like interface (Dit,dn) states. Electrochemical analysis has also depicted that non-stoichiometric DySiOx and SiOx bonds are present at the interface between Dy2O3/Si gate stack. Owing to interactions between mobile charge carriers and possible defect sites present at the DySiOx and SiOx, the Dit,ac and Dit,dn signals have been observed at G curves. In addition, the C hysteresis loop widths decrease with decreasing in the voltage frequencies due to opposite polarity and response time dependencies of the Dit,ac and Dit,dn states. The oxide, interface and border trap densities of capacitors exhibit slight frequency variations and their orders are comparable with device used in the state of the art microelectronics technologies. On the other hand, barrier potential varies from 0.97 eV to 1.136 eV depending on the voltage frequencies. Leakage current density was found to be 1.61 × 10−7 Acm−2 at −1 V and Fowler–Nordheim tunneling mechanism dominates the leakage current conduction. Considering to these results, it can be concluded that the Dy2O3 shows promising dielectric and insulating behavior for the application in future microelectronics technology.

Analysis of damage curing in a MOSFET with joule heat generated by forward junction current at the source and drain

This study demonstrates a method for curing the gate dielectric of a MOSFET using Joule heat (JH) generated by forward bias current in the PN-junction in the drain-to-body (D-B) and source-to-body (SB). To accurately quantify the curing effect by the D-B JH and the SB JH, the interface trap density (Nit) of the gate dielectric was laterally profiled using a charge pumping characterization method. The curing method was applied to repair damage in the gate dielectric caused by Fowler-Nordheim (FN) and hot-carrier injection (HCI) stress. When FN stress was applied to the device, there is no difference in curing the damage by the D-B JH and the SB JH. However, when HCI stress, which asymmetrically causes more damage to the drain, was applied, the D-B JH showed better recovery than the SB JH.

Micro Flowers of SrS/Bi2S3 Nanocomposite and Its Field Emission Properties

The three-dimensional hierarchical SrS/Bi2S3 heterostructures were synthesized by a template-free single-step hydrothermal method. The structural and morphological studies revealed the formation of a single crystalline orthorhombic heterostructure with rod-like morphologies possessing a high aspect ratio. The field emission properties of SrS/Bi2S3 nanorods were investigated. J–E and the Fowler–Nordheim (F–N) plot, as well as long-term field emission (FE) stability, were studied. SrS/Bi2S3 nanoflowers have enhanced the FE properties more than the virgin Bi2S3. The observed values of the re-producible turn-on field for SrS/Bi2S3 defined to draw an emission current density of ca. 1 µA/cm2 were found to be ca. 2.50 V/µm, and of the threshold field to draw a current density of ca. 10 µA/cm2 were found to be ca. 3.00 V/µm (without visible light illumination). A maximum emission current density of ca. 527 μA/cm2 was drawn without light and a current density of ca. 1078 μA/cm2 with light, which is higher than that of pristine Bi2S3.

The Murphy-Good plot: a better method of analysing field emission data.

Measured field electron emission (FE) current–voltage Im(Vm) data are traditionally analysed via Fowler–Nordheim (FN) plots, as versus 1/Vm. These have been used since 1929, because in 1928 FN predicted they would be linear. In the 1950s, a mistake in FN's thinking was found. Corrected theory by Murphy and Good (MG) made theoretical FN plots slightly curved. This causes difficulties when attempting to extract precise values of emission characterization parameters from straight lines fitted to experimental FN plots. Improved mathematical understanding, from 2006 onwards, has now enabled a new FE data-plot form, the ‘Murphy–Good plot'. This plot has the form versus 1/Vm, where η ≅ 9.836239 (eV/ϕ)1/2 and ϕ is the local work function. Modern (twenty-first century) MG theory predicts that a theoretical MG plot should be ‘almost exactly' straight. This makes precise extraction of well-defined characterization parameters from ideal Im(Vm) data much easier. This article gives the theory needed to extract characterization parameters from MG plots, setting it within the framework of wider difficulties in interpreting FE Im(Vm) data (among them, use of ‘smooth planar emitter methodology'). Careful use of MG plots could also help remedy other problems in FE technological literature. It is suggested that MG plots should replace FN plots.

Laminate Al2O3/Ta2O5 Metal/Insulator/Insulator/Metal (MIIM) Devices for High-Voltage Applications

We evaluate for high-voltage (HV) applications a series of metal/insulator/insulator/metal (MIIM) devices with relatively thick asymmetric Al2O3/Ta2O5 bilayer insulator barriers in which the Al2O3:Ta2O5 layer thickness ratio is varied from 1:1 to 1:9, with the thickness of the Al2O3 layer fixed at 30 nm. The impact of Ta2O5 to Al2O3 stack order and layer thickness ratio on current versus voltage asymmetry, reverse leakage, breakdown, and programable resistance ratio is investigated. Regions of operation are distinguished, and the responsible underlying dominant conduction mechanisms, including Schottky emission over the various barriers, ohmic conduction and Fowler–Nordheim tunneling through Al2O3, and defect-based Frenkel–Poole emission through Ta2O5 are identified. It was found that under positive bias, the low-resistance state in one-time programable usage is controlled by the thickness and properties of the Ta2O5 layer while the high-resistance state is determined by the thickness and properties of the Al2O3 layer. These results demonstrate that atomic layer deposition (ALD) bilayers are an effective way to engineer the electrical properties of HV metal-insulator-metal (MIM) devices.

Microporous networks of NiMn2O4 as a potent cathode material for electric field emission

The electric field-induced sterling electron emission of NiMn2O4 microporous networks synthesized via the sol-gel auto combustion route was investigated. Some primary characterization techniques such as x-ray diffraction, Fourier-transform infrared spectroscopy, and Raman spectroscopy were performed to confirm the pure crystallinity and metal oxide (Ni–O and Cr–O) stretching vibrations and also to provide a molecular fingerprint of the NiMn2O4 porous network. The distinct field emission (FE) properties of the NiMn2O4 microporous network was observed which was correlated with an electric field induced electron tunneling F–N (Fowler–Nordheim) model from a nearly planner conducting emitter surface with triangular potential-energy barrier approximation. A low turn-on field of 4.15 V µm−1 and threshold field of 5.25 V µm−1 were detected to draw emission current densities of 1 µA cm−2 and 10 µA cm−2 respectively. The local work function (Φ) of 5.509 eV for the NiMn2O4 porous network was computed using density functional theory (DFT) and it exhibits an impressive field enhancement factor (β) of 3381 with good FE current stability. These results demonstrate the potential application of this material for future vacuum micro/nanoelectronics and FE panel display applications.

NH3and NO + NH3Annealing of 4H-SiC Trench MOSFETs: Device Performance and Reliability

As the gate oxide (GOX)-SiC interface of SiC MOSFETs is crucial for device performance, it requires special attention. To improve interface quality, device performance, and reliability, commonly, a postoxidation anneal (POA) is applied. Different gas compositions can be used to passivate the SiC interface and bulk oxide. We study the effects of ammonia (NH3) and a new combined NO + NH3 POA on 4H-SiC trench MOSFET test structures and compare them to the common nitric oxide (NO) POA. Our studies are not only limited to device performance represented by channel mobility but also contain reliability studies in the form of bias temperature instability (BTI), sweep hysteresis, Fowler–Nordheim (FN) tunneling and GOX breakdown. We find that both, NH3 and NO + NH3 result in higher mobility than NO annealed test structures. On the other hand, ammonia containing POAs deteriorate device reliability, e.g., leading to larger BTI and higher tunneling currents. As both can be detrimental in certain applications, they must be optimized. We show that NO + NH3 anneals may offer an acceptable compromise between the characteristics of NO and NH3 with regard to tunneling and device performance. However, it shows more positive BTI (PBTI) drift than NH3 and NO. We will show that a full characterization of the GOX is essential to evaluate, optimize, and benchmark new annealing strategies.

Field Emission From Nanostructured Tendril Bundles

Field emission from tungsten surfaces with nano-tendril bundles formed after exposure to helium plasma with nitrogen or neon impurities was measured using a vacuum diode device. Field enhancement factor and effective emission area were calculated using the Fowler–Nordheim formula and compared for different types of nanostructures. The field enhancement factor for these structures can reach several thousands and the effective emission area can be more than 1000 nm2. Anode luminescence was detected using long-exposure photography and allowed for estimation of current density near the anode. Unipolar arc tracks were observed on the samples with the highest field emission intensity. We show that field emission from nano-tendril bundles is much higher than that from tungsten fuzz or pure tungsten, and fusion-relevant plasma conditions may lead to arc initiation.

Leakage Current Conduction Mechanism of Au-Pt-Ti/ HfO2-Al2O3/n-InAlAs Metal-Oxide-Semiconductor Capacitor under Reverse-Biased Condition

Au-Pt-Ti/high-k/n-InAlAs metal-oxide-semiconductor (MOS) capacitors with HfO2-Al2O3 laminated dielectric were fabricated. We found that a Schottky emission leakage mechanism dominates the low bias conditions and Fowler–Nordheim tunneling became the main leakage mechanism at high fields with reverse biased condition. The sample with HfO2 (4 m)/Al2O3 (8 nm) laminated dielectric shows a high barrier height ϕB of 1.66 eV at 30 °C which was extracted from the Schottky emission mechanism, and this can be explained by fewer In–O and As–O states on the interface, as detected by the X-ray photoelectron spectroscopy test. These effects result in HfO2 (4 m)/Al2O3 (8 nm)/n-InAlAs MOS-capacitors presenting a low leakage current density of below 1.8 × 10−7 A/cm2 from −3 to 0 V at 30 °C. It is demonstrated that the HfO2/Al2O3 laminated dielectric with a thicker Al2O3 film of 8 nm is an optimized design to be the high-k dielectric used in Au-Pt-Ti/HfO2-Al2O3/InAlAs MOS capacitor applications.

A radiation-hardened Sense-Switch pFLASH cell for FPGA

In this paper, a radiation-hardened Sense-Switch pFLASH cell is presented. It employs a programming/erasing transistor (T1) as well as a signal transmission transistor (T2). They share the common floating and controlling gates. The proposed cell with the total ionizing dose (TID) tolerance of more than 50 Krad(Si) is prepared successfully based on 0.13 μm eFlash technology. Firstly, the “ON/OFF” functions are realized by both the band-to-band tunneling induce hot electron (BBHE) and the Fowler-Nordheim (FN) tunneling mechanisms. Secondly, the “ON/OFF” states are characterized, and the endurance and the TID effects are also studied. Finally, the experimental results show that the mean value of the T1/T2 threshold window and uniformity of them are approximately 10 V and 3.3%, respectively. Under the working voltage of −1.5 V, the leakage current of T2 at OFF state is as low as 1 pA/μm, and the average driving current of T2 at ON state is around 126.8 μA/μm, and the uniformity of driving current can reach 6.31%. Meanwhile, the number of program and erase cycles can be up to 10,000 times. Therefore, the Sense-Switch pFLASH cell, which has low leakage, high endurance, better TID tolerance and other excellent characteristics, can meet the basic switch core requirements of radiation-hardened programmable logic devices.

Interfacial‐Tunneling‐Effect‐Enhanced CsPbBr3 Photodetectors Featuring High Detectivity and Stability

AbstractInorganic halide perovskite (HP)‐based photodetectors (PDs) have exhibited fast response speed and high responsivity, but with low detectivity due to the high dark current of devices. Additionally, the intrinsic instability of HPs and interface deterioration originating from ion migration inhibit their practical applications severely. A tunneling organic layer is introduced to solve both these problems. Light‐induced charge carriers can flow across the interfacial (poly(methyl methacrylate), PMMA) layer with appropriate thickness via the Fowler–Nordheim tunneling effect. Due to the effective control of dark current, the photo‐/dark‐current ratio reaches a giant value of 2.13 × 108, and the peak detectivity is as high as 1.24 × 1013 Jones. With such superiority, light signal as weak as 244 pW is accurately imaged by a PD array. Additionally, the hydrophobic organic layer inhibits the destruction of HPs caused by moisture and ion migration induced interface reaction, and negligible response attenuation is observed during continuous work in a humid environment for 48 h. This heterojunction structure design provides a new strategy to enhance the performance and stability of perovskite‐based photoelectric and photovoltaic devices.

Hunting down the ohmic contact of organic field-effect transistor**Project supported by the Slovak Research and Development Agency (Grant Nos. APVV-17-0501 and APVV-17-0522) and the Slovak Grant Agency for Science (Grants No. 1/0776/15).

We report properties of contact resistances observed on pentacene organic field-effect transistors (OFET) with four different source/drain electrodes, namely, copper (Cu), gold (Au), silver (Ag), and germanium (Ge). The metals were selected to provide a wide range of energy barriers for charge injection, from blocking contact to smooth injection. All OFETs exhibited strong voltage dependence of the contact resistance, even for devices with smooth injection, which is in strong disagreement with the definition of ohmic contacts. A comparison with current crowding, resistive network, Fowler–Nordheim tunneling, and electric field enhanced thermionic injection (Schottky emission) pointed to importance of local electric fields and/or electrostatic field charges.

Ten Approaches to Define the Field Emission Area

The emission area is one of the main parameters of field emission systems. A variety of formal determination techniques for the emission area is studied. Using the model hemisphere-on-cylindrical-post constructed with the COMSOL Multiphysics software package, the relation between well-known estimates of the emission area is shown. The corresponding fractions of the emission currents are computed. It is demonstrated that the formal emission area (the ratio of the total current to the current density at the vertex) ensures approximately 75% of the emission current. The effect of abnormal behavior on the effective emission area is obtained (cutoff from the trend line of the current–voltage characteristics) for varying voltage level in standard Fowler–Nordheim coordinates. An experimental assessment of the area with application of modified Fowler–Nordheim coordinates ln(I/U2–η/6) vs 1/U is proposed. The method is applied for analysis of field emission data of the multipoint nanocomposite emitter with carbon nanotubes.

Residual Gas Adsorption and Desorption in the Field Emission of Titanium-Coated Carbon Nanotubes.

Titanium (Ti)-coated multiwall carbon nanotubes (CNTs) emitters based on the magnetron sputtering process are demonstrated, and the influences of modification to CNTs on the residual gas adsorption, gas desorption, and their field emission characteristic are discussed. Experimental results show that Ti nanoparticles are easily adsorbed on the surface of CNTs due to the “defects” produced by Ar+ irradiation pretreatment. X-ray photoelectron spectroscopy (XPS) characterization showed that Ti nanoparticles contribute to the adsorption of ambient molecules by changing the chemical bonding between C, Ti, and O. Field emission of CNTs coated with Ti nanoparticles agree well with the Fowler–Nordheim theory. The deviation of emission current under constant voltage is 6.3% and 8.6% for Ti-CNTs and pristine CNTs, respectively. The mass spectrometry analysis illustrated that Ti-coated CNTs have a better adsorption capacity at room temperature, as well as a lower outgassing effect than pristine CNTs after degassing in the process of field emission.

Influence of reactive-ion-etching depth on interface properties in Al2O3/n-GaN MOS diodes

Influence of etching depth on the interface properties in Al2O3/n-GaN MOS diodes has been investigated. The n-GaN surface was etched by inductively coupled reactive-ion-etching (RIE) with shallow (20 nm) and deep etching (1200 nm) depth. The resulting Al2O3/n-GaN interface properties were then evaluated by the capacitance/conductance–voltage (C/G–V) and current–voltage (I–V) characteristics measurements. It was found that: (i) the interface state density of the shallow-etched MOS diode is almost the same as that of the un-etched reference sample, (ii) the deep-etched MOS diode showed around five times higher interface state density than the shallow-etched MOS diode, (iii) the increased interface state density with deep etching was not recovered by Tetra-Methyl-Ammonium Hydroxide (TMAH) treatment nor by post annealing, and (iv) the deep-etched MOS diode indicated about five orders of magnitude higher leakage current, which could be attributed to the lowering of the effective barrier height in the Fowler–Nordheim type conduction. The results obtained in this study indicate that improved RIE process has to be developed for the fabrication of GaN MOS FETs that require deep etching such as vertical trench-type FETs.