Fowler-Nordheim Tunneling Mechanism Research Articles

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.

Electron transport mechanism through ultrathin Al2O3 films grown at low temperatures using atomic–layer deposition

Alumina (Al2O3) films of different thicknesses have been grown at different low temperatures (100 °C–250 °C) by atomic–layer deposition on n–type Si substrate. The robustness of the Al2O3 film as a barrier has been investigated based on Al/Al2O3/Si metal–insulator–semiconductor structures. The electron transport through the Al2O3 layer was fitted well by the Fowler–Nordheim tunneling mechanism, from which the barrier heights (conduction band offset between Si and Al2O3) were deduced. It was discovered that the growth temperature and film thickness both influenced the carrier transport and barrier height. The Al/Al2O3/Si structure with an ultrathin 3 nm Al2O3 fabricated at 150 °C showed the largest barrier height, the lowest tunneling current density (4.9 × 10–8 A cm−2 at 5 MV cm−1), and the highest breakdown field strength of 18.3 MV cm−1. Using Au to replace Al as the electrode could suppress the tunneling current significantly. The Al2O3 films were also examined by x-ray photoelectron spectroscopy to determine their chemical constituents.

Dramatically Enhanced Broadband Photodetection by Dual Inversion Layers and Fowler-Nordheim Tunneling.

Silicon photonics is now widely accepted as a key technology in a variety of systems. But owing to material limitations, now it is challenging to greatly improve the performance after decades of development. Here, we show a high-performance broadband photodetector with significantly enhanced sensitivity and responsivity operating over a wide wavelength range of light from near-ultraviolet to near-infrared at low power consumption. The specially designed textured top ceiling electrode works effectively as an antireflection layer to greatly improve the absorption of near-infrared light, thereby overcoming the absorption limitation of near-infrared light. Instead of the conventional p-n junction and p-intrinsic-n junction, we introduce a ∼15 nm thick alumina insulator layer between a p-type Si substrate and n-type ZnO nanowire (NW) arrays, which significantly enhances the charge carrier separation and collection efficiency. The photosensing responsivity and sensitivity are found to be nearly 1 order of magnitude higher than that of a reference device of p-Si/n-ZnO NW arrays, significantly higher than the commercial silicon photodiodes as well. The light-induced charge carriers flow across the appropriate thickness of insulator layer via the quantum mechanical Fowler-Nordheim tunneling mechanism. By virtue of the piezo-phototronic effect, the charge density at the interfaces can be tuned to alter the energy bands and the potential barrier distance for tunneling. Additionally, along with the use of incident light of different wavelengths, the influence of the insulator layer on the transport of electrons and holes separately is further investigated. The demonstrated concepts and study would lead to sensitivity improvement, quality enhancement of data transfer, decrease of power consumption, and cost reduction of silicon photonics.

Design and Characterization of Semi-Floating-Gate Synaptic Transistor.

In this work, a study on a semi-floating-gate synaptic transistor (SFGST) is performed to verify its feasibility in the more energy-efficient hardware-driven neuromorphic system. To realize short- and long-term potentiation (STP/LTP) in the SFGST, a poly-Si semi-floating gate (SFG) and a SiN charge-trap layer are utilized, respectively. When an adequate number of holes are accumulated in the SFG, they are injected into the nitride charge-trap layer by the Fowler–Nordheim tunneling mechanism. Moreover, since the SFG is charged by an embedded tunneling field-effect transistor existing between the channel and the drain junction when the post-synaptic spike occurs after the pre-synaptic spike, and vice versa, the SFG is discharged by the diode when the post-synaptic spike takes place before the pre-synaptic spike. This indicates that the SFGST can attain STP/LTP and spike-timing-dependent plasticity behaviors. These characteristics of the SFGST in the highly miniaturized transistor structure can contribute to the neuromorphic chip such that the total system may operate as fast as the human brain with low power consumption and high integration density.

Half-volt operation of IGZO thin-film transistors enabled by ultrathin HfO2 gate dielectric

Amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) enabled by an ultrathin, 5 nm, HfO2 film grown by atomic-layer deposition were fabricated. An ultra-low operation voltage of 1 V was achieved by a very high gate capacitance of 1300 nF/cm2. The HfO2 layer showed excellent surface morphology with a low root-mean-square roughness of 0.20 nm and reliable dielectric properties, such as low leakage current and high breakdown electric field. As such, the a-IGZO TFTs exhibit desirable properties such as low power devices, including a small subthreshold swing of 75 mV/decade, a low threshold voltage of 0.3 V, and a high on/off current ratio of 8 × 106. Furthermore, even under an ultralow operation voltage of 0.5 V, the on/off ratio was also up to 1 × 106. The electron transport through the HfO2 layer has also been analyzed, indicating the Poole-Frenkel emission and Fowler-Nordheim tunneling mechanisms in different voltage ranges.

Electroluminescent Yb2O3:Er and Yb2Si2O7:Er nanolaminate films fabricated by atomic layer deposition on silicon

Atomic layer doped Yb2O3:Er and Yb2Si2O7:Er nanolaminate films are fabricated on silicon by atomic layer deposition, and ∼1530 nm electroluminescence (EL) is obtained from the metal-oxide-semiconductor light-emitting devices (MOSLEDs) based on these films. The Yb2O3 films transfer to Yb2Si2O7 phase after annealing above 1000 °C. Intense photoluminescence from Yb2Si2O7 film confirms high efficiency and energy transfer under optical excitation, but the limited electron conduction restricts the EL performance. EL from the Yb2O3:Er MOSLED outperforms, presenting an external quantum efficiency up to 8.5% and the power efficiency of 1 × 10−3. The EL is derived to result from the impact excitation of Er3+ ions by hot electrons, which stem from Fowler-Nordheim tunneling mechanism under sufficient bias voltage. The critical distance for the cross relaxation of doped Er3+ ions in nanolaminate Yb2O3 matrix is experimentally determined to be ∼3 nm. Such devices manifest the technological potential of Er-doped Yb-oxides for applications in silicon-based optoelectronics.

Temperature-dependent electronic charge transport characteristics at MoS2/p-type Ge heterojunctions

Significant effort has been devoted to constructing two-dimensional transition metal dichalcogenides-based hybrid heterojunctions with enhanced performance or unique functionalities for versatile device applications in emerging electronics and optoelectronics. In this study, we report the temperature-dependent electronic charge transport characteristics in MoS2/p-type Ge heterojunction diodes. From the current-voltage (I-V) characteristics of the heterojunction device, it is observed that different transport phenomena can occur depending upon the temperature and bias voltage. The charge transport is dominated by thermionic emission in the high-temperature regime above 300 K, whereas in low-temperature regime below 300 K, the charge transport mechanism transitions from the thermionic emission to the tunneling mechanism associated with trap sites. In particular, the I-V characteristics in the low-temperature regime show a transition from the direct tunneling at a low bias to the Fowler-Nordheim tunneling mechanism at a high bias. This could be well described by the electrical analyses on temperature-dependent I-V behavior and corresponding energy band diagrams.

Temperature dependent negative differential resistance behavior in multiferroic metal organic framework (CH3)2NH2 Mn (HCOO)3 crystals

Abstract We present the temperature dependent negative differential resistance behavior of (CH3)2NH2Mn (HCOO)3 crystals prepared by solvothermal synthesis. The paramagnetic to antiferromagnetic transition temperature, TN and paraelectric to ferroelectric transition temperature, Tc were at 8 K and 184 K respectively. Above 100 K, negative differential resistance peaks in the low bias voltage region (between −4 V and +4 V) of the current-voltage characteristic were observed till 224 K. Fowler Nordheim tunneling mechanism was the dominant current conduction mode in the temperature independent high bias region (±5 V to ±10 V). The highest peak to valley ratio of 10 was recorded at 176 K with a peak current, I peak of 2.7 × 10−11 A. The peak Vpeak and valley Vvalley were recorded at 1.5 V and 2.7 V respectively. The dielectric transition was found to be accompanied by an electronic transition of the metal to insulator type around 204 K.

Low voltage operation of IGZO thin film transistors enabled by ultrathin Al2O3 gate dielectric

An ultrathin, 5 nm, Al2O3 film grown by atomic-layer deposition was used as a gate dielectric for amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The Al2O3 layer showed a low surface roughness of 0.15 nm, a low leakage current, and a high breakdown voltage of 6 V. In particular, a very high gate capacitance of 720 nF/cm2 was achieved, making it possible for the a-IGZO TFTs to not only operate at a low voltage of 1 V but also exhibit desirable properties including a low threshold voltage of 0.3 V, a small subthreshold swing of 100 mV/decade, and a high on/off current ratio of 1.2 × 107. Furthermore, even under an ultralow operation voltage of 0.6 V, well-behaved transistor characteristics were still observed with an on/off ratio as high as 3 × 106. The electron transport through the Al2O3 layer has also been analyzed, indicating the Fowler–Nordheim tunneling mechanism.

Effects of Nd3+-substitution for Bi-site on the leakage current, ferroelectric and dielectric properties of Na0.5Bi0.5TiO3 thin films

Perovskite Na0.5(Bi1−xNdx)0.5TiO3 (x = 0, 0.01, 0.03, 0.05; xNd: NBT) ferroelectric films were synthesized on indium tin oxide (ITO)/glass substrates via chemical solution deposition. Structural characterization shows the similar phase-pure perovskite structures in all the films and gradually decreased grain sizes with Nd3+ doping amount increasing. For all the films, the leakage behaviors are dominant by the Ohmic conduction in low electric field region and interface-limited Fowler-Nordheim tunneling mechanism in high electric field region. Additionally, the space-charge-limited conduction is involved in 0.03Nd: NBT sample. Compared with the sample of x = 0, the resistivity can be improved through Nd3+-substitution in NBT. Enhanced ferroelectricity can be obtained from the dynamic polarization-electric field test, and the reversible domains switching in film can be confirmed by static dielectric constant-electric field measurement. Especially, the 0.03Nd: NBT possesses optimal electrical performances with a large remanent polarization (Pr = 26.7μC/cm2) and a high dielectric tunability (19.6% at 100kHz).

Rectifying electronic transport and the role of Fowler-Nordheim tunneling in Ag/PVDF/Au capacitor structures

The electron transport through β phase dominant Polyvinylidene fluoride (PVDF) thin films in its intrinsic form is investigated and reports the stable rectification up to ±30 V in Ag/PVDF/Au capacitor structures. A practical resistance ratio of ∼40 between −1 V and 1 V and a maximum ratio of ∼77 at −24 V and 24 V exhibits its potential usage in selector devices. Furthermore, the nonpolar electronic transport is analysed with Schottky, Space Charge Limited Current (SCLC), Poole-Frenkel (PF) and Fowler-Nordheim (FN) tunneling mechanisms with respect to electrode, thickness and temperature variations.

Investigation of Leakage Current Mechanisms in La2O3/SiO2/4H-SiC MOS Capacitors with Varied SiO2 Thickness

In this study, the material and electrical properties of La2O3/SiO2/4H-SiC metal–oxide–semiconductor (MOS) capacitors are systematically characterized. Thermal oxidization SiO2 with varying thickness (0 nm, 3.36 nm, 5 nm, 8 nm, and 30 nm) were coated with La2O3 using atomic layer deposition on n-type 4H-SiC. The stacking oxides were measured using atomic force microscopy, transmission electron microscopy, and x-ray photoelectron spectroscopy, and the MOS capacitors were measured by capacitance–voltage and current–voltage measurements. The results demonstrate that the main gate current leakage mechanisms are dependent on the thickness of the SiO2 oxide under the applied electric field. The primary mechanism for current leakage from the La2O3/4H-SiC MOS capacitor follows the Schottky emission mechanism due to its low conduction band offset. In contrast, the current leakage mechanism for the capacitor with a 3.36 nm SiO2 layer follows the Poole–Frenkel emission mechanism on account of its high trap charge density in the gate dielectric and at the interface. When the thickness of the SiO2 layer increases to 8 nm, lower leakage current is observed by reason of the low trap charge density and high conduction band offset when E ≤ 5 MV/cm. As the electric field strength increases to 5 MV/cm and 5.88 MV/cm (30 nm SiO2: 4.8 MV/cm), the main current leakage mechanism changes to the Fowler–Nordheim tunneling mechanism, which indicates that the La2O3/SiO2 stacking structure can improve the properties of MOS capacitors.

Crystal structure, leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics

Ceramics of pure phase Yttrium (Y) doped BiFeO3 prepared by a solid-state sintering route were characterized by X-ray diffraction, Raman spectroscopy, magnetic and electrical measurements. The results and analysis show that Y substitution greatly reduces the leakage current and enhances the multiferroic properties of BiFeO3. Leakage conduction mechanism is shown to change from space-charge-limited conduction type in pure BiFeO3 to a Poole–Frenkel emission behavior in Bi0.90Y0.10FeO3. A Fowler-Nordheim tunneling mechanism in Bi0.95Y0.05FeO3 ceramic is also evidenced under high electric fields. At the same time, enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. An unexpected and sharp switching behavior in the magnetization under low magnetic fields observed in Bi0.90Y0.10FeO3 ceramic, together with its improved ferroelectric property, may trigger such system for promising magneto-electric applications.

(Invited) Temperature Influence on Current Leakage and Hysteresis of Nc-CdSe Embedded Zr-Doped HfO2 High-k Dielectric Nonvolatile Memory

The nanocrystals embedded high high-k gate dielectric structure can replace the poly-Si floating-gate dielectric structure in nonvolatile memories (NVMs) for advantages of the low leakage current and improved reliability [1]. The Zr-doped HfO2 (ZrHfO) high-k film has better bulk and interface properties than those of the undoped film with respect to the crystallization temperature and interface density of states [2]. The nanocrystalline cadmium selenide (nc-CdSe) embedded ZrHfO NVM has exhibited excellent charge storage and retaining capacitiess [3]. Most studies on this kind of device were done at room temperature [4]. Since the temperature of the high density integrated circuit on a board can be much higher than the room temperature [5], it is imperative to understand the temperature influence on charge transferring mechanisms. In this paper, authors studied the temperature effect on the leakage current characteristics of the nc-CdSe embedded ZrHfO MOS capacitor. The ZrHfO/nc-CdSe/ZrHfO tri-layer stack was sputter deposited on the dilute HF cleaned p-type Si (100) wafer in a one pumpdown process without breaking the vacuum. The post deposition annealing was done at 800°C for 3 min under the N2 condition. The ITO film was deposited and wet etched into the gate electrodes. The final annealing was done at 400°C for 5 min under the H2/N2 atmosphere. The control sample contained the 12-min sputter deposited ZrHfO prepared under the same process condition. The detailed device fabrication process can be found in ref. 3. Figure 1 shows J-V hysteresis curves of the (a) control and (b) nc-CdSe embedded samples with the gate voltage (Vg ) swept from -4 V to +4 V to -4 V at 20°C, 70°C, and 120°C, separately. In the small Vg range of -2 to +2 V, the control sample has a larger leakage current than those of the nc-CdSe embedded sample. The is because the nc-CdSe embedded sample can trap charges in the nc-CdSe bulk or at the nc-CdSe/ZrHfO interface. The bulk ZrHfO film traps negligible amount of charges. In the control sample, charges can be easily transferred through the ZrHfO bulk, which shows as a large leakage current. In the nc-CdSe embedded sample, they are trapped at shallow or deep trapping sites, leading to a smaller leakage current. At the raised temperature, charges gain higher thermal energy and are easily transferred through the bulk ZrHfO film, which enlarges the leakage current. However, in the nc-CdSe embedded sample, the energy is still not high enough to overcome the energy barrier between nc-CdSe and ZrHfO at a small electric field. Therefore, the nc-CdSe embedded sample has a slightly larger leakage current at the high temperature. In the C-V hysteresis study, it was found that when the temperature was increased, the interface state density (Dit ) of the nc-CdSe embedded sample increased but that of the control sample did not change much. At the raised temperature, the Si/high-k interface is damaged by the charge transfer process but the bulk film was little damaged. Figure 2 shows that in the negative Vg range, holes are transferred through the nc-CdSe embedded sample following (a) the Schottky emission mechanism in the low electric field range and (b) the Poole-Frenkel (P-F) mechanism in the large electric field from 20°C to 120°C. This is in agreement of the literature report that the Schottky emission happens under the low electric field but the F-P emission needs the high electric field condition [6]. Figure 3(a) shows that in the positive Vg range, for the nc-CdSe embedded sample, electrons are transferred through the nc-CdSe embedded sample following the Schottky emission mechanism in the low electric field range over the temperature range of 20°C to 120°C. However, it changes to the Fowler-Nordheim (F-N) tunneling mechanism at the high electric field. The transition occurs faster with the increase of the temperature. For example, it is around E1/2 ~ 1700 (V/cm)1/2 at 120oC, while the Schottky emission still dominates around the same region at 20oC and 70oC. [1] C.-H. Lin and Y. Kuo, J AP, 110, 024101 (2011). [2] J. Lu and Y. Kuo, APL, 87,232906 (2005). [3] C.-C. Lin and Y. Kuo, JAP, 115, 084113 (2014). [4] C. H. Yang and Y. Kuo, MRS Symp. Proc., 1071-F02-09 (2008). [5] T. Y. T. Lee et al, IEEE Trans. Comp. Packag. Manufact. Technol., 17, 564 (1994). [6] C.-L. Cheng, et al, APL, 86, 212902 (2005). Figure 1

Current transport mechanisms in lattice-matched Pt/Au-InAlN/GaN Schottky diodes

Lattice-matched Pt/Au-In0.17Al0.83N/GaN hetreojunction Schottky diodes with circular planar structure have been fabricated and investigated by temperature dependent electrical measurements. The forward and reverse current transport mechanisms are analyzed by fitting the experimental current-voltage characteristics of the devices with various models. The results show that (1) the forward-low-bias current is mainly due to the multiple trap-assisted tunneling, while the forward-high-bias current is governed by the thermionic emission mechanism with a significant series resistance effect; (2) the reverse leakage current under low electric fields (<6 MV/cm) is mainly carried by the Frenkel-Poole emission electrons, while at higher fields the Fowler-Nordheim tunneling mechanism dominates due to the formation of a triangular barrier.

Charge tunneling and trapping in Er stabilized β-MnO2 films for memory applications

Recently, the memory effect was observed in Er stabilized β-MnO2 metal oxide semiconductor structure. In this work, the electric properties and mechanism of charge tunneling and trapping are studied by combining frequency-dependent capacitance-voltage curves, variable sweep range capacitance-voltage curves, and current-voltage curves. The charge traps are identified to be deep donors in β-MnO2. The deep donor level is close to the valance band, which results in the asymmetric enlargement in variable sweep range capacitance-voltage curves. Resonant tunneling and Fowler-Nordheim tunneling mechanisms are observed at low and high voltages, respectively. All these phenomena can be explained by the proposed model as a whole. The experimental evidence shows that Er-stabilized β-MnO2 is a good candidate charge storage material in nonvolatile memory devices.

Electrical study of ZrO2/Si system formed at different oxidation/nitridation temperatures for extended duration in N2O ambient

Abstract

Self-limited grain growth, dielectric, leakage and ferroelectric properties of nanocrystalline BiFeO3 thin films by chemical solution deposition

In this study, the growth mechanism of nanocrystalline BiFeO3/Pt/Ti/SiO2/Si thin films by chemical solution deposition has been investigated through isothermal annealing, and the dielectric, leakage and ferroelectric properties have also been studied in detail. The derived thin films show self-limited grain growth, which can be described well by a relaxation model, and the microstrain evolution follows exponential decay behavior. The dielectric constant and the loss behaviors have been investigated, and different contributions are expected for different thin films. The leakage current density behavior shows that conduction mechanisms are dominant, with the ohmic mechanism, the space-charge-limited current mechanism and the Fowler–Nordheim tunneling mechanism in low, middle and high electric fields, respectively. The behaviors of the ferroelectric coercive field and the polarization have also been investigated and found to be dominated by different factors for different samples. The results will provide an instructive route to optimize BiFeO3-based thin films grown using the chemical solution deposition method.

Polarization effects on gate leakage in InAlN/AlN/GaN high-electron-mobility transistors

Lattice-matched InAlN/AlN/GaN high electron mobility transistors offer high performance with attractive electronic and thermal properties. For high-voltage applications, gate leakage currents under reverse bias voltages remain a serious challenge. This current flow is dominated by field enhanced thermal emission from trap states or direct tunneling. We experimentally measure reverse-bias gate leakage currents in InAlN/AlN/GaN transistors at various temperatures and find that the conventional trap-assisted Frenkel-Poole model fails to explain the experimental data. Unlike the non-polar semiconductors Si, Ge, large polarization-induced electric fields exist in III-nitride heterojunctions. When the large polarization fields are accounted for, a modified Frenkel-Poole model is found to accurately explain the measured data at low reverse bias voltages. At high reverse bias voltages, we identify that the direct Fowler-Nordheim tunneling mechanism dominates. The accurate identification of the gate leakage current flow mechanism in these structures leads to the extraction of several useful physical parameters, highlights the importance of polarization fields, and leads to suggestions for improved behavior.

High permittivity 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 relaxor thin films for high-value, wide-temperature capacitor applications

High permittivity 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 (PMNT) relaxor thin films with a nearly pure perovskite structure as well as a dense and uniform microstructure have been prepared on Pb(Zr,Ti)O3-buffered platinized silicon substrates by sol-gel method. Interestingly, the PMNT thin film exhibits high dielectric permittivity, εr ∼ 1200, and high dielectric tunability, ∼70% under a moderate E = 333 kV/cm, over a wide temperature range. These results are explained in terms of a relaxor behavior of the PMNT film. Moreover, the leakage current density of the PMNT thin film is reasonably low, roughly 5.2 × 10−6 A/cm2 at an electric field intensity of 400 kV/cm. As the dc electric field increases, the leakage current mechanism is transformed from ohmic law to Fowler–Nordheim tunneling mechanism. All the results obtained indicate that the PMNT relaxor thin film is a good candidate for high-value, wide-temperature capacitor applications.