Fowler-Nordheim Tunneling Of Electrons Research Articles

Demonstration of Fowler-Nordheim Tunneling in Simple Solution-Processed Thin Films.

The production of high-quality thin-film insulators is essential to develop advanced technologies based on electron tunneling. Current insulator deposition methods, however, suffer from a variety of limitations, including constrained substrate sizes, limited materials options, and complexity of patterning. Here, we report the deposition of large-area Al2O3 films by a solution process and its integration in metal-insulator-metal devices that exhibit I- V signatures of Fowler-Nordheim electron tunneling. A unique, high-purity precursor based on an aqueous solution of the nanocluster flat-Al13 transforms to thin Al2O3 insulators free of the electron traps and emission states that commonly inhibit tunneling in other films. Tunneling is further confirmed by the temperature independence of device current.

Current-Fluctuation Mechanism of Field Emitters Using Metallic Single-Walled Carbon Nanotubes with High Crystallinity

Field emitters can be used as a cathode electrode in a cathodoluminescence device, and single-walled carbon nanotubes (SWCNTs) that are synthesized by arc discharge are expected to exhibit good field emission (FE) properties. However, a cathodoluminescence device that uses field emitters radiates rays whose intensity considerably fluctuates at a low frequency, and the radiant fluctuation is caused by FE current fluctuation. To solve this problem, is very important to obtain a stable output for field emitters in a cathodoluminescence device. The authors consider that the electron-emission fluctuation is caused by Fowler–Nordheim electron tunneling and that the electrons in the Fowler–Nordheim regime pass through an inelastic potential barrier. We attempted to develop a theoretical model to analyze the power spectrum of the FE current fluctuation using metallic SWCNTs as field emitters, owing to their electrical conductivity by determining their FE properties. Field emitters that use metallic SWCNTs with high crystallinity were successfully developed to achieve a fluctuating FE current from field emitters at a low frequency by employing inelastic electron tunneling. This paper is the first report of the successful development of an inelastic-electron-tunneling model with a Wentzel–Kramers–Brillouin approximation for metallic SWCNTs based on the evaluation of FE properties.

Mechanism of oxide thickness and temperature dependent current conduction in n+-polySi/SiO2/p-Si structures — a new analysis

The conduction mechanism of gate leakage current through thermally grown silicon dioxide (SiO2) films on (100) p-type silicon has been investigated in detail under negative bias on the degenerately doped n-type polysilicon (n+-polySi) gate. The analysis utilizes the measured gate current density JG at high oxide fields Eox in 5.4 to 12 nm thick SiO2 films between 25 and 300 °C. The leakage current measured up to 300 °C was due to Fowler–Nordheim (FN) tunneling of electrons from the accumulated n +-polySi gate in conjunction with Poole Frenkel (PF) emission of trapped-electrons from the electron traps located at energy levels ranging from 0.6 to 1.12 eV (depending on the oxide thickness) below the SiO2 conduction band (CB). It was observed that PF emission current IPF dominates FN electron tunneling current IFN at oxide electric fields Eox between 6 and 10 MV/cm and throughout the temperature range studied here. Understanding of the mechanism of leakage current conduction through SiO2 films plays a crucial role in simulation of time-dependent dielectric breakdown (TDDB) of metaloxide–semiconductor (MOS) devices and to precisely predict the normal operating field or applied gate voltage for lifetime projection of the MOS integrated circuits.

Mechanistic analysis of temperature-dependent current conduction through thin tunnel oxide in n+-polySi/SiO2/n+-Si structures

We present a detailed investigation on temperature-dependent current conduction through thin tunnel oxides grown on degenerately doped n-type silicon (n+-Si) under positive bias (VG) on heavily doped n-type polycrystalline silicon (n+-polySi) gate in metal-oxide-semiconductor devices. The leakage current measured between 298 and 573 K and at oxide fields ranging from 6 to 10 MV/cm is primarily attributed to Poole-Frenkel (PF) emission of trapped electrons from the neutral electron traps located in the silicon dioxide (SiO2) band gap in addition to Fowler-Nordheim (FN) tunneling of electrons from n+-Si acting as the drain node in FLOating gate Tunnel OXide Electrically Erasable Programmable Read-Only Memory devices. Process-induced neutral electron traps are located at 0.18 eV and 0.9 eV below the SiO2 conduction band. Throughout the temperature range studied here, PF emission current IPF dominates FN electron tunneling current IFN at oxide electric fields Eox between 6 and 10 MV/cm. A physics based new analytical formula has been developed for FN tunneling of electrons from the accumulation layer of degenerate semiconductors at a wide range of temperatures incorporating the image force barrier rounding effect. FN tunneling has been formulated in the framework of Wentzel-Kramers-Brilloiun taking into account the correction factor due to abrupt variation of the energy barrier at the cathode/oxide interface. The effect of interfacial and near-interfacial trapped-oxide charges on FN tunneling has also been investigated in detail at positive VG. The mechanism of leakage current conduction through SiO2 films plays a crucial role in simulation of time-dependent dielectric breakdown of the memory devices and to precisely predict the normal operating field or applied floating gate (FG) voltage for lifetime projection of the devices. In addition, we present theoretical results showing the effect of drain doping concentration on the FG leakage current.

Current conduction mechanisms through thin tunnel oxide during erase operation of flash electrically erasable programmable read-only memory devices

The mechanism of current conduction through thin tunnel oxide during erase operation of flash electrically erasable programmable read-only memory devices has been studied both theoretically and experimentally. The floating gate (FG) leakage current measured between 25 and 300 °C and at oxide fields ranging from 6 to 10 MV/cm is primarily attributed to Poole–Frenkel (PF) emission of trapped electrons from the electron traps located at about 1.0 eV below the silicon dioxide conduction band in addition to Fowler–Nordheim (FN) tunneling of electrons from the degenerately doped n-type polycrystalline silicon (n+-polySi) FG. It is observed that PF emission current IPF dominates FN electron tunneling current IFN at oxide electric fields Eox between 6 and 10 MV/cm and throughout the temperature range studied here. The observed thickness dependence of FG leakage current at a given applied oxide field arises due to different electron trap concentrations in the oxide. A physics based new temperature dependent analytical formula has also been developed for FN tunneling of electrons from the accumulation layer of semiconductors. In addition, the authors present theoretical results showing the effect of the FG doping concentration on the leakage current.

The Role of Field Electron Emission in Polypropylene/Aluminum Nanodielectrics Under High Electric Fields.

Polymer/metallic particle nanocomposites or nanodielectrics can exhibit colossal dielectric constants with a relatively low dissipation factor under low electric fields and thus seem to be promising for high-energy density dielectric capacitors. To study this possibility, this work focused on the dielectric performance and loss mechanisms in polypropylene (PP)/aluminum nanoparticle (nAl NP) composites under high electric fields. Phosphonic acid-terminated poly(ethylene-co-1-butene) was grafted to the Al2O3 surface layer on the nAl NPs in order to achieve reasonable dispersion in the PP matrix. The dielectric breakdown study showed that the breakdown strength decreased to nearly 1/20 that of the neat PP film as the nAl content increased to 25.0 vol %. The leakage current study revealed three electronic conduction mechanisms in the PP/100 nm nAl nanocomposites, namely, ohmic conduction at low fields, hopping conduction at intermediate fields, and Fowler-Nordheim (FN) field electron emission above a critical field, depending on the filler content. Compared to the 100 nm nAl NPs, smaller (e.g., 18 nm) nAl NPs needed a much higher electric field to exhibit FN field electron emission. It was the FN electron tunneling that induced a substantial reduction in breakdown strength for the PP/nAl nanocomposites. Meanwhile, electron-tunneling injected space charges (electrons) from nAl NPs into the PP matrix, and internal electronic conduction led to significant dielectric nonlinearity at high poling fields. Although polymer/metallic NP composites are not suitable for high-field electric applications, they can be good candidates for electrical switches and quantum tunneling composites operated at relatively low electric fields.

Leakage current conduction, hole injection, and time-dependent dielectric breakdown of n-4H-SiC MOS capacitors during positive bias temperature stress

The conduction mechanism(s) of gate leakage current JG through thermally grown silicon dioxide (SiO2) films on the silicon (Si) face of n-type 4H-silicon carbide (4H-SiC) has been studied in detail under positive gate bias. It was observed that at an oxide field above 5 MV/cm, the leakage current measured up to 303 °C can be explained by Fowler-Nordheim (FN) tunneling of electrons from the accumulated n-4H-SiC and Poole-Frenkel (PF) emission of trapped electrons from the localized neutral traps located at ≈2.5 eV below the SiO2 conduction band. However, the PF emission current IPF dominates the FN electron tunneling current IFN at oxide electric fields Eox between 5 and 10 MV/cm and in the temperature ranging from 31 to 303 °C. In addition, we have presented a comprehensive analysis of injection of holes and their subsequent trapping into as-grown oxide traps eventually leading to time-dependent dielectric breakdown during electron injection under positive bias temperature stress (PBTS) in n-4H-SiC metal-oxide-silicon carbide structures. Holes were generated in the heavily doped n-type polycrystalline silicon (n+-polySi) gate (anode) as well as in the oxide bulk via band-to-band ionization by the hot-electrons depending on their energy and SiO2 film thickness at Eox between 6 and 10 MV/cm (prior to the intrinsic oxide breakdown field). Transport of hot electrons emitted via both FN and PF mechanisms was taken into account. On the premise of the hole-induced oxide breakdown model, the time- and charge-to-breakdown (tBD and QBD) of 8.5 to 47 nm-thick SiO2 films on n-4H-SiC were estimated at a wide range of temperatures. tBD follows the Arrhenius law with activation energies varying inversely with initial applied constant field Eox supporting the reciprocal field (1/E) model of breakdown irrespective of SiO2 film thicknesses. We obtained an excellent margin (6.66 to 6.33 MV/cm at 31 °C and 5.11 to 4.55 MV/cm at 303 °C) of normal operating field for a 10-year projected lifetime of 8.5 to 47 nm-thick SiO2 films on n-4H-SiC under positive bias on the n+-polySi gate. Furthermore, the projected maximum operating oxide field was little higher in metal gate devices compared to n+-polySi gate devices having an identically thick thermal SiO2 films under PBTS.

Verification of Fowler–Nordheim electron tunneling mechanism in Ni/SiO2/n-4H SiC and n+ poly-Si/SiO2/n-4H SiC MOS devices by different models

This article emphasizes verification of Fowler–Nordheim electron tunneling mechanism in the Ni/SiO2/n-4H SiC MOS devices by developing three different kinds of models. The standard semiconductor equations are categorically solved to obtain the change in Fermi energy level of semiconductor with effect of temperature and field that extend support to determine sustainable and accurate tunneling current through the oxide layer. The forward and reverse bias currents with variation of electric field are simulated with help of different models developed by us for MOS devices by applying adequate conditions. The latter is quite different from former in terms of tunneling mechanism in the MOS devices. The variation of barrier height with effect of quantum mechanical, temperature, and fields is considered as effective barrier height for the generation of current–field (J–F) curves under forward and reverse biases but quantum mechanical effect is void in the latter. In addition, the J–F curves are also simulated with variation of carrier concentration in the n-type 4H SiC semiconductor of MOS devices and the relation between them is established.

Fowler-Nordheim Tunneling, Photovoltaic Applications and New Band Structure Models of Electroconductive a-CN<sub>x</sub>:H Films Formed by Supermagnetron Plasma CVD

Hydrogenated amorphous carbon nitride (a-CNx:H) films were formed on Al films deposited on Si or glass (SiO2) substrates, using pulsed radio frequency (PRF) supermagnetron plasma (SMP) chemical vapor deposition (CVD) with N2/i-C4H10 mixed gases. a-CNx:H films were grown under the upper and lower electrode RF powers (13.56 MHz) of continuous and pulsed conditions, respectively, which showed low band gap of about 0.7 eV. a-CNx:H films deposited on the Al/Si or Al/SiO2 substrates showed same low threshold emission electric field (ETH) of 12 V/μm. Multiple layer of Al or ITO (anode)/50nm-SiO2/a-CNx:H/Al (cathode)/Si structures showed Fowler-Nordheim (FN) electron tunneling effect in both forward and reverse current directions. 12.5 nm a-CNx:H film on p-Si substrate showed a photoelectric conversion. Energy band structure and electron conduction models were proposed for the active states of both the field emission and FN tunneling devices and photovoltaic cells.

Field Emission Characteristics of Regular Arrays of Carbon Nanotubes

The developments of electronic devices based on micron-sized vacuum electron sources during the last decades have triggered intense research on highly efficient carbon based thin film electron emitters. The synthesis of massive arrays of carbon nanotubes that are oriented on patterned Fe catalyst deposited on quartz substrates is reported. The well-ordered nanotubes can be used as electron field emission arrays. Scaling up of the synthesis process should be entirely compatible with the existing semiconductor processes, and should allow the development of nanotubes devices integrated into future technology. The emission from carbon nanotubes array is explained by Fowler-Nordheim tunneling of electrons from tip-like structures in the nanometer range, which locally amplify the applied field by the field enhancement factor beta. We found that the low pressure chemical vapour deposition (LPCVD) system can produce nanotubes capable of excellent emission currents at lower voltages. The carbon nanotubes array shows good field emission with turn on field E(alpha) = 1.30 V/microm at the current density of 3.50 mA/cm2 with enhancement factor beta = 1.22 x 10(2).

Bipolar pulsed excitation of erbium-doped nanosilicon light emitting diodes

High quantum efficiency erbium doped silicon nanocluster (Si-NC:Er) light emitting diodes (LEDs) were grown by low-pressure chemical vapor deposition (LPCVD) in a complementary metal-oxide-semiconductor (CMOS) line. Erbium (Er) excitation mechanisms under direct current (DC) and bipolar pulsed electrical injection were studied in a broad range of excitation voltages and frequencies. Under DC excitation, Fowler-Nordheim tunneling of electrons is mediated by Er-related trap states and electroluminescence originates from impact excitation of Er ions. When the bipolar pulsed electrical injection is used, the electron transport and Er excitation mechanism change. Sequential injection of electrons and holes into silicon nanoclusters takes place and nonradiative energy transfer to Er ions is observed. This mechanism occurs in a range of lower driving voltages than those observed in DC and injection frequencies higher than the Er emission rate.

Novel Double-Gate 1T-DRAM Cell Using Nonvolatile Memory Functionality for High-Performance and Highly Scalable Embedded DRAMs

We proposed for the first time a new double-gate 1T-DRAM cell to be applicable to sub-80-nm DRAM technology that has a silicon-oxide-nitride-oxide-silicon type storage node on the back gate (control gate) for nonvolatile memory (NVM) functionality. An NVM functionality is achieved by Fowler-Nordheim tunneling of electrons into the nitride storage node. Then, holes are accumulated on the back-channel, which makes 1T-DRAM operation in fully depleted silicon-on-insulator (SOI) MOSFETs possible and enhances retention characteristics. We investigated the effect of the NVM functionality on 1T-DRAM performance in nanoscale 1T-DRAM cells through device simulation and verified the effect in 0.6-¿m devices fabricated on SOI wafers.

Fowler–Nordheim electron tunneling under very intense electric field

In the context of the Fowler–Nordheim tunneling under an electric field of very high strength, electron velocity and drift mobility are calculated as functions of the above strength. In addition, carrier relaxation time is determined.

Electrical behavior of 4H-SiC metal-oxide-semiconductor structures with Al2O3 as gate dielectric

The electrical properties of Al2O3 as a gate dielectric in metal-oxide-semiconductor structures based on n- and p-type 4H-SiC grown by sublimation method have been investigated and compared to the properties of similar structures utilizing SiO2. The electrically active defects in the structures are studied by capacitance–voltage (C–V) and current–voltage (I–V) methods. The results show that the type as well as spatial and energy distributions of defects in Al2O3∕SiC and SiO2∕SiC samples are different. The structures with Al2O3 on p-type 4H-SiC demonstrate much better electrical characteristics than the p-type 4H-SiC∕SiO2 structures. It is demonstrated that the conduction process in the former is governed by Fowler–Nordheim electron tunneling from the Al gate whereas in the latter the hole tunneling from SiC is the more probable process. This difference combined with the higher defect density in p-type SiC∕SiO2 structures defines the higher leakage currents compared to the structures utilizing Al2O3.

Evolution of quasi-breakdown in thin gate oxides

The mechanism for quasi-breakdown (QB) in thin gate oxides was studied using bipolar current stress and unipolar constant current stress. Continual bipolar current stressing on the gate oxide shows two distinct stages of QB—recoverable and unrecoverable QB. During the recoverable QB stage, the gate leakage current recovers to the stress-induced leakage current level upon application of a proper reverse bias. In contrast, no electrical recovery is observed within the unrecoverable QB stage. This stage is characterized by a higher gate leakage current than that of the recoverable QB stage and a very stable gate voltage during stressing. Carrier separation measurements further demonstrate that two different modes of conduction can occur during the recoverable QB stage. In the early stage, Fowler–Nordheim electron tunneling dominates the conduction mechanism although a small hole current is observed. With prolonged electrical stress, the hole direct tunneling current becomes dominant. Based on the aforementioned observations and the monitoring of the generation of oxide traps using the direct-current current–voltage technique, a QB model of positive hole trapping at the anode is proposed.

Ultra-thin SiO2 film studies: index, thickness, roughness and the initial oxidation regime

Special techniques are needed to reliably measure the ultra-thin film and interfacial properties. In this review several novel ultra-thin film techniques that were developed and/or adapted are discussed: Fowler–Nordheim electron tunneling current oscillations; spectroscopic ellipsometry (SE); a novel application of SE called spectroscopic immersion ellipsometry; atomic force microscopy along with fractal analysis. Recent results using these novel techniques are reviewed on ultra-thin SiO2 films for film thickness measurements, the kinetics of the initial regime of Si oxidation, and interfacial roughness and the effects of roughness on Si oxidation and interfacial electronic properties.

Consistent refractive index parameters for ultrathin SiO2 films

Refined values are obtained in the present study for the refractive index parameter (n) for ultrathin SiO2 films (less than 8 nm in film thickness, Lox) and summarized as n=2.139−8.991×10−2×Lox+1.872×10−3×Lox2. This formula was obtained starting from variable angle of incidence ellipsometry data on ultrathin SiO2 films, and a previously derived refractive index parameter formula that was based on independently measured thickness’ using Fowler–Nordheim electron tunneling measurements. The new n values are useful for extracting more accurate ultrathin SiO2 film thicknesses from ellipsometry measurements.

Self-Convergent Programming Scheme for Multilevel P-Channel Flash Memory

We propose a novel operation scheme for multilevel p-channel flash memory cell with a self-convergent programming process. By utilizing the simultaneous Fowler-Nordheim electron tunneling out of floating gate and channel hot electron injection into floating gate, the threshold voltage of memory cell can be converged to a specific value. The gate pulse level can be varied to result in different converged threshold voltages such that multilevel can be achieved. Owing to the nature of self-convergence, the possibility of eliminating or reducing the verification operation in multilevel applications increases considerably by using the proposed scheme. In this study the reliability considerations of this programming technique for long-term operations are also addressed.

Temperature dependent hole fluence to breakdown in thin gate oxides under Fowler–Nordheim electron tunneling injection

The dielectric breakdown mechanism in thin gate oxides is discussed based on the temperature dependence of hole fluence to dielectric breakdown through gate oxides. It has been demonstrated for the first time that the hole fluence to dielectric breakdown is not a constant value for different oxide fields but has a strong oxide field dependence at low temperatures. It has been also found that the dominant dielectric breakdown mechanism changes at around 150 K. At higher temperatures than 150 K, the dielectric breakdown mechanism in silicon-dioxide is not dependent on the oxide field. On the other hand, the dielectric breakdown at lower than 150 K is dominated by the mechanism strongly dependent on the oxide field.

Model for the substrate hole current based on thermionic hole emission from the anode during Fowler–Nordheim electron tunneling in n-channel metal-oxide-semiconductor field-effect transistors

A model is proposed to explain the dependence of the substrate hole current in n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) on applied electric field and on oxide thickness. Two types of devices were prepared: n-channel MOSFETs with gate oxides of 67, 86, and 131 Å and p-channel MOSFETs in which gate oxide thicknesses were almost equal to those in the n-channel MOSFETs. The carrier-separation technique was used in the p-channel MOSFETs, and the average energy of hot electrons entering the silicon substrate was obtained. The average energy of the hot electrons is related to the energy distribution of hot holes created by hot electrons emitted from the oxide into the n+ polysilicon gate during the Fowler–Nordheim electron tunneling in the n-channel MOSFETs. The substrate hole current is numerically modeled as thermionic emission of the hot holes overcoming the energy barrier at the oxide-n+ polysilicon interface. For the gate oxides ranging from 67 to 131 Å, the dependence of the substrate hole current on the electric field and on oxide thickness is explained by using the average energy of the hot electrons and the thermionic hole emission model.