Conduction mechanisms in thermal nitride and dry gate oxides grown on 4H-SiC

The charge conduction mechanisms in Metal-Oxide-Semiconductor (MOS) capacitors formed on n-type 4H-silicon carbide (SiC) using thermally grown silicon dioxide (SiO₂) as gate dielectrics are analyzed. The possible conduction mechanisms have been identified in the whole measurement range. At high electric fields, the charge conduction is dominated by Fowler–Nordheim tunneling. In addition, trap assisted tunneling and ohmic type conduction are considered to explain the cause of leakages detected at intermediate and low oxide electric fields. Various electronic parameters are extracted. The oxide breakdown strengths are higher than 8 MV/cm. Fowler–Nordheim tunneling barrier height was found to be 2.74 eV for nitride oxides and 2.54 eV for dry oxides at high electric field regions and the trap energy level extracted using trap assisted tunneling emission model was estimated to be about 0.3 eV for both oxides. The possible contribution of the Poole–Frenkel effect to the conduction mechanism was also considered, and it was found that it does not play a dominant role.     

High-reflectivity Al-Pt nanostructured Ohmic contact to p-GaN

The effect of nanoscale Pt islands on the electrical characteristics of contacts to p-type gallium nitride (GaN) has been investigated to explore the feasibility for the flip-chip configuration light-emitting diodes (LEDs) using an Al-based reflector. An as-deposited Al contact to p-GaN with a net hole concentration of 3/spl times/10/sup 17/cm/sup -3/ was rectifying. However, an Al contact with nanoscale Pt islands at the interface exhibited ohmic behavior. A specific contact resistivity of 2.1/spl times/10/sup -3//spl Omega//spl middot/cm/sup 2/ and a reflectance of 84% at 460 nm were measured for the Al contact with nanoscale Pt islands. Current-voltage temperature measurements revealed a Schottky barrier height reduction from 0.80 eV for the Al contact to 0.58 eV for the Al contact with nanoscale Pt islands. The barrier height reduction may be attributed to electric field enhancement and the enhanced tunneling due to the presence of the nanoscale Pt islands. This will offer an additional silver-free option for the p-type ohmic contact in flip-chip configuration LEDs. Theory suggests that the ohmic contact characteristics may be improved further with smaller Pt islands that will enhance tunneling across the interface with the GaN and in the vicinity of the Pt-Al interface.     

Enhancing the electron injection in polymer light-emitting diodes using a sodium stearate/aluminum bilayer cathode

In this contribution the molecule sodium stearate (NaSt) is used for the first time as electron injection layer in combination with the fluorescent polymer phenylene substituted poly (para-phenylenevinylene) (Ph-PPV) in organic light-emitting diodes (OLEDs). The fabricated devices show current efficiencies up to 8.4 cd/A, indicating that the employed NaSt/aluminum (Al) bilayer cathode has adequate electron injection capabilities in conjunction with Ph-PPV and, therefore, NaSt has the potential to become a non-toxic alternative to the widely-used alkali halide lithium fluoride (LiF).Numerical simulations of the device structure are performed which are in good agreement with the experiments. Additionally, it is shown that the NaSt/Al cathode of the presented device cannot be simply modeled by using a low work function contact, as it is commonly done for the LiF/Al cathode in simulations of multilayer devices. Instead, an alternative approach is introduced in which an insulator in combination with the Fowler–Nordheim tunneling and the direct tunneling model is chosen to describe the charge carrier injection through the NaSt layer.     

Current transport in metal/hafnium oxide/silicon structure

Based on the experimental results of the temperature dependence of gate leakage current and Fowler-Nordheim tunneling characteristics at 77 K, we have extracted the energy band diagrams and current transport mechanisms for metal/HfO₂/Si structures. In particular, we have obtained the following quantities that will be useful for modeling and simulation: i) HfO₂/Si conduction band offset (or barrier height): 1.13 ± 0.13 eV; ii) Pt/HfO₂ barrier height: ~ 2.48 eV; iii) Al/HfO₂ barrier height: ~ 1.28 eV; iv) electron effective mass in HfO₂: 0.1 m_o, where m_o is the free electron mass and v) a trap level at 1.5 ± 0.1 eV below the HfO₂ conduction band which contributes to Frenkel-Poole conduction     

Carrier transport mechanism in La-incorporated high-k dielectric/metal gate stack MOSFETs

In this paper, carrier transport mechanism of MOSFETs with HfLaSiON was analyzed. It was shown that gate current is consisted of Schottky emission, Frenkel-Poole (F-P) emission and Fowler-Nordheim (F-N) tunneling components. Schottky barrier height is calculated to be 0.829 eV from Schottky emission model. Fowler-Nordheim tunneling barrier height was 0.941 eV at high electric field regions and the trap energy level extracted using Frenkel-Poole emission model was 0.907 eV. From the deviation of weak temperature dependence for gate leakage current at low electric field region, TAT mechanism is also considered.     

超薄栅氧化物pMOSFET器件在软击穿后的特性

研究了在软击穿后MOS晶体管特性的退化.在晶体管上加均匀的电压应力直到软击穿发生的过程中监控晶体管的参数.在软击穿后,输出特性和转移特性只有小的改变.在软击穿发生时,漏端的电流和域值电压的退化是连续变化的.但是,在软击穿时栅漏电流突然有大量的增加.对软击穿后的栅漏电流增量的分析表明,软击穿后的电流机制是FN隧穿,这是软击穿引起的氧化物的势垒高度降低造成的.     

待定系数法计算场致发射体阴极表面场强和发射电流密度

在计算场致发射体阴极发射电流时 ,影响发射电流的主要因素是阴极表面的电场强度。由于发射电流密度的大小和阴极表面场强呈指数关系 ,阴极表面场强很小的变化将引起发射电流的剧烈变化 ,所以提高阴极表面场强的计算精度是至关重要的。本文提出了一种计算阴极表面场强的新方法——待定系数法 ,并用该方法计算了一个二极管场致发射体的阴极表面电场强度和电流密度。     

Analytical Modeling of High-Voltage 4H-SiC Junction Barrier Schottky (JBS) Rectifiers

We develop a new analytical model for the junction barrier Schottky (JBS) rectifier and apply it to high-voltage 4H-SiC JBS rectifiers. This model uses a novel method to approximate the electric field at the Schottky contact, which is together with the Fowler–Nordheim tunneling equation to accurately calculate the reverse leakage current of a high-voltage 4H-SiC JBS rectifier. The forward on-resistance of a high-voltage 4H-SiC JBS rectifier consists of several components, which are dominated by the spreading resistances in the drift layer. Moreover, this model has been verified by comparing the simulation and experimental results, and they are shown to be in good agreement.      

Electrical bistability,negative differential resistance and carrier transport in flexible organic memory device based on polymer bilayer structure

Bistable nonvolatile memory devices containing two different layers of polymers, viz. MEH-PPV (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenyl vinylene]) and PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) has been fabricated by a simple spin-coating technique on flexible polyimide (PI) substrates with a structure Al/MEH-PPV/PEDOT:PSS/Ag-Pd/PI. The current–voltage measurements of the as-fabricated devices showed a nonvolatile electrical bistability with electric field induced charge transfer through the polymer layers and negative differential resistance (NDR) which is attributed to the charge trapping in the MEH-PPV layer. The current ON/OFF ratio between the high-conducting state (ON state) and low-conducting state (OFF state) is found to be of the order of 10³ at room temperature which is comparable to organic field effect transistor based memory devices. We propose that such an improvement of rectification ratio (ON/OFF ratio) is caused due to the inclusion of PEDOT:PSS, which serves as a conducting current path for carrier transport; however, NDR is an effect of the trapped charges in the MEH-PPV electron confinement layer. The device shows excellent stability over 10⁴ s without any significant degradation under continuous readout testing in both the ON and OFF states. The carrier transport mechanism of the fabricated organic bistable device has been explained on the basis of different conduction mechanisms such as thermionic emission, space-charge-limited conduction, and Fowler–Nordheim tunneling. A band diagram is proposed to explain the charge transport phenomena. These bilayer structures are free from the drawbacks of the single organic layer based memory devices where the phase separation between the nanoparticles and polymers leads to the degradation of device stability and lifetime.     

Model for Charge Accumulation in <Emphasis Type="Italic">n</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-MOS Transistors during Tunneling Electron Injection from a Gate

A quantitative model for charge accumulation in an undergate dielectric during tunneling electron injection from a gate according to the Fowler–Nordheim mechanism is developed. The model takes into account electron and hole capture at hydrogen-free and hydrogen-related traps as well as the generation of surface states during the interaction of holes with hydrogen-related centers. The experimental dependences of the threshold voltage shift and gate voltage shift of n- and p-channel MOS (metal–oxide–semiconductor) transistors on the injected charge in the constant current mode are analyzed based on the model.     

Electrical and optoelectrical properties of CdS-Ge heterojunctions

The electrical and optoelectrical characteristics are presented for n-CdS/p-Ge heterojunctions fabricated by depositing Ge onto (0001) faces of CdS monocrystals. The experimental results are interpreted in terms of an energy band model proposed for this heterojunction. It is found that a Schottky barrier is present on the CdS side of the interface. With an In (injecting) contact to the CdS and an ohmic contact to the Ge, the device acts as a photoconductor in which the current is space-charge-limited for forward bias (Ge positive, CdS negative). For the opposite bias polarity the photoconductivity is reduced substantially and the current is limited by tunneling through the barrier spike in the conduction band. The photoconductivity at room temperature results from electrons being excited in the CdS bulk by photons of 2·34 eV energy. For forward bias the photocurrent is found to be proportional to the light intensity at moderate voltages, as for a normal photoconductor. However, at low voltages the photocurrent is found to be proportional to the two-thirds power of the light intensity.     

Field acceleration factor for dielectric breakdown of MOS devices

In this paper a model is presented for estimating the field acceleration factor for dielectric breakdown of SiO₂. Using the observation that the total charge through the oxide is invariant (for MOS capacitors fabricated by identical process steps) and assuming that the I-V characteristic is dominated by Fowler Nordheim tunneling prior to breakdown, a simple expression for the acceleration factor can be obtained. The expression gives a good fit to experimentally obtained acceleration factors in the literature. It also indicates that the acceleration factor is field dependent such that the logarithm of acceleration factor vs field is not a straight line and care is required while extrapolating the results of accelerated testing.     

利用FN振荡电流测量薄栅MOS结构栅氧化层中隧穿电子的有效质量

给出了一种利用 FN振荡电流的极值 ,测量电子在薄栅 MOS结构的栅氧化层中的平均有效质量方法 .利用波的干涉方法来处理电子隧穿势垒的过程 ,方便地获得了出现极值时外加电压和电子的有效质量之间的分析表达式 .用干涉方法计算所得到的隧穿电子在不同的 MOS结构的二氧化硅介质层中的有效质量表明 :它一般在自由电子质量的 0 .5 2— 0 .84倍的范围 .实验结果表明 :电子有效质量的值不随外加电压的变化而变化 ,并且对于相同的MOS结构 ,电子可能具有相同的有效质量     

Temperature dependence of the reverse current in Schottky barrier diodes

The temperature dependences of the current I in reverse-biased Al/SiO₂/n-Si, Al/SiO₂/n-GaAs and Al/n-GaAs (with the native oxide) structures are measured. It is established that these dependences all have the property that the thermal activation energy decreases with increasing applied voltage and that at higher voltages the plots of ln I versus 1/T deviate from straight lines. The results can be explained on the basis of the fact that the current through the barrier is due to electron tunneling from surface states into the conduction band of the semiconductor. The field intensity in the Schottky barrier and the density of surface electron states in the interfacial layer of the semiconductor are estimated by comparing the experimental results with a tunneling theory that takes into account the effect of the semiconductor lattice phonons on the tunneling probability. Fiz. Tekh. Poluprovodn. 32, 882–885 (July 1998)     

First-principles study of the effects of oxygen vacancy on hole tunneling current

The effects of oxygen vacancies on the electronic structure of silicon dioxide and the hole tunneling current were investigated using first-principles calculations. A level related to oxygen vacancy was obtained to be nearly 2.0 eV from the top of valence band within the bandgap of the α-quartz supercell with one oxygen vacancy. And therefore the defect assisted hole (electron) tunneling currents were calculated. The results shows that the hole tunneling current will be dominant for a thinner oxide thickness at low oxide field and the contribution of trap assisted hole tunneling to the total tunneling current decreases with oxide thickness and oxide field increasing. It is concluded that the effects of the oxygen vacancies on the hole tunneling current become smaller with larger oxide thickness and higher electric field.     

利用FN振荡电流测量超薄栅MOS结构的栅氧化层厚度

给出了一种利用 FN振荡电流的极值测量超薄栅 MOS结构的栅氧化层厚度和电子在栅氧化层导带中的有效质量方法 .利用波的干涉方法来处理电子隧穿势垒的过程 ,方便地获得了出现极值时外加电压和栅氧化层厚度、势垒高度、电子的有效质量之间的关系 .这种方法的最大优点是精确和简便 ,并可方便地应用于任意形状的势垒和势阱     

Transport Mechanism of SiGe Dot MOS Tunneling Diodes

The blockage of hole transport due to excess holes In SiGe dots was observed in the MOS tunneling diodes for the first time. The five layers of self-assembled SiGe dots are separated by 74-nm Si spacers and capped with a 130-nm Si. The hole tunneling current from Pt gate electrode to p-type Si dominates the inversion current at positive gate bias and is seven orders of magnitude higher than the Al gate/oxide/p-Si device. The large work function of Pt is responsible for the hole transport current from Pt to p-Si. The incorporation of SiGe dots confines the excess holes in the valence band and forms a repulsive barrier to reduce the hole transport current from Pt to SiGe dots by 2-3 orders of magnitude in comparison with the Pt/oxide/p-Si device. This repulsive barrier also reduces the hole tunneling current from SiGe dots to Pt at negative gate bias.     

Temperature-sensitive asymmetrical bipolar resistive switches of polymer:nanoparticle memory devices

The interface between two bulk electronic materials can significantly affect the electrical behavior of electronic devices. But the interface between a bulk metal and metal nanoparticles has been rarely explored. This paper reports significant temperature effect on the asymmetrical resistive switches of polymer:nanoparticle memory devices. The devices have architecture of a polystyrene layer admixed with gold nanoparticles capped with conjugated 2-naphthalenethiol sandwiched between Au and Al electrodes. The devices exhibit significant resistive switches at room temperature. However, the resistive switches become less significant at temperature below 200 K, and they are not noticeable at 103 K. The temperature effect suggests that the resistive switches are assisted by the thermal energy. The charge transport through the devices has different mechanisms at high and low temperatures. At temperature above 220 K, the Poole–Frenkel emission is an important mechanism for the charge transport. At temperature below 220 K, the temperature-independent Fowler–Nordheim tunneling becomes an important process.     

Write-Once Read-Many-Times Memory Based on a Single Layer of Pentacene

We realized an organic electrical memory device with a simple structure based on single-layer pentacene film embedded between Al and ITO electrodes. The optimization of the thickness and deposition rate of pentacene resulted in a reliable device with an on/off current ratio as high as nearly $ hbox{10}^{6}$, which was two orders of magnitude higher than previous results, and the storage time was more than 576 h. The current transition process is attributed to the formation and damage of the interface dipole at different electric fields, in which the current conduction showed a transition from ohmic conductive current to Fowler–Nordheim tunneling current. After the transition from on - to off-state, the device tended to remain in the off-state even when the applied voltage was removed, which indicated that the device was very promising for write-once read-many-times memory.      

Charge retention improvement of charge-trapping type flash device by plasma immersion ion implantation

Electrical characteristics of charge trapping-type flash devices with HfAlO charge trapping layer nitrided by plasma immersion ion implantation (PIII) technique with different implantation energies and time are studied. Utilizing Fowler–Nordheim (FN) operation, the programming speed of flash memory with charge trapping layer nitrided at low implantation energy is faster than that of control sample. The erasing speed of PIII-treated sample is slightly slower than that of control one, which might be due to the formation of silicon nitride in the tunneling oxide. The retention characteristics of all PIII-treated samples are significantly improved. Different peak locations of implanted nitrogen concentrations are formed by different implantation energies, which cause various electrical characteristics of flash devices.