Understanding Dark Spot Formation and Growth in Organic Light‐Emitting Devices by Controlling Pinhole Size and Shape

We report on our in‐situ experimental observations of dark spots in organic light‐emitting diodes using optical microscopy. Uniformly sized silica microparticles are used to intentionally create size‐ and shape‐controllable pinholes on the cathode protective layer. Subsequently, the pinholes trigger the initial formation of dark spots, which we then monitor. Due to the use of particles of various diameters, we are able to linearly associate the growth rate with pinhole size. This allows us to estimate the original pinhole sizes that give rise to the dark spots and to study their distribution. Our studies verify that pinholes on the protective layer create pathways for water or oxygen diffusion, which controls the dark spot growth rate. The pinhole size dependence illustrates that the pinhole perimeter (not the area) determines the amount of water or oxygen diffusing into the diodes at a certain time.     

New method for the determination of the surface barrier heights in MIS tunnel diodes (solar cells)

The methods commonly applied for the determination of the surface barrier heights of Schottky diodes are usually extended for the MIS tunnel structures characterization, although, in such cases both, their range of application and accuracy are often strongly reduced.In this paper a new photoelectric method for the determination of the surface barrier heights in MIS tunnel diode is proposed together with experimental results supporting its validity. The presence of the ultra-thin oxide layer between the metal and semiconductor does not reduce method's applicability as long as the direct semiconductor-metal tunneling is dominant current flow mechanism through the oxide layer. In the contrary to the previous methods it allow us to measure directly the barrier height for minority carriers, and therefore, is especially recommended for minority carrier dievices.     

Minority carrier MIS tunnel diodes and their application to electron- and photo-voltaic energy conversion—I. Theory

If the insulating layer in a metal-insulator-semiconductor (MIS) diode is very thin ($\text{<60}\text{A}\text{?}$ for AlSiO₂Si), measureable tunnel current can flow between the metal and the semiconductor. If the insulating layer is even thinner ($\text{<30}\text{A}\text{?}$), tunnel currents are so large that they can significantly disturb the semiconductor from thermal equilibrium. Under such conditions, MIS diodes exhibit properties determined by which of the following tunneling processes is dominant; tunneling between the metal and the majority carrier energy band in the semiconductor, between the metal and the minority carrier energy band, or between the metal abd surface state levels. In the present paper, minority carrier MIS tunnel diodes are analysed using a very general formulation of the tunneling processes through the insulator, transport properties in the semiconductor, and surface state effects. Starting from solutions for diodes with relatively thick insulating layers where the semiconductor is essentially in thermal equilibrium, solutions are obtained for progressively thinner insulating layers until non-equilibrium effects in the semiconductor are observed. It is shown that such minority carrier MIS tunnel diodes with very thin insulating layers possess properties similar to p-n junction diodes including exponential current-voltage characteristics which approach the “ideal diode” law of p-n junction theory. The theory adequately describes the observed properties of experimental devices reported in a companion paper. The diodes have application as injecting contacts, as photodiodes or elements of photodiode arrays, and as energy conversion devices employing the electron- or photo-voltaic effects.     

Mechanism of the operational effect of black spot growth in OLEDs

When organic light-emitting diodes (OLEDs) are exposed to the ambient atmosphere, water penetration through pinholes in the cathode results in the formation of non-emissive areas (black spots) due to local oxidation of the cathode around the pinhole. This degradation effect severely limits the lifetime of OLED devices and requires high performance encapsulation in order to delay its onset. We have investigated the process of degradation of OLED devices by water penetration through cathode pinholes, and have discovered that this is not just a simple oxidation of the cathode occurring as water diffuses within the device. We have observed that other layers within the device interact with the water. In solution processed OLEDs, the hole-injection layer (PEDOT:PSS) absorbs water due to the presence of the PSS acid (formation of H₃O⁺). This results in a slight local reduction of the luminance of the device (grey spot) around the pinhole location and actually a reduction of the cathode black spot growth rate at the pinhole during a shelf lifetime test. When the device is not operated, the PEDOT:PSS layer is acting as a local reservoir for water keeping it away from the cathode and slowing down the cathode degradation. However, when the device is operated, water that is captured by the PEDOT:PSS layer to form H₃O⁺ can be transported to the cathode under the influence of the applied electric field. This then increases the black spot growth rate. Experimental indications for this operational effect are provided by the threshold voltage behaviour of the effect and transient behaviour of the black spot growth after switching off the OLED. Direct evidence for the role of PEDOT:PSS in the operational effect has been provided by Raman Spectroscopy.     

Interband tunneling in heterostructure tunnel diodes

A particular type of tunnel diode, incorporating a wideband tunnel barrier, is studied. Simple analytic expressions are developed for estimating the tunneling coefficients to guide and optimize the design of heterostructure interband tunnel devices. The interband tunneling in such heterostructure tunnel diodes is modeled by a two-band Schrodinger equation. For a certain family of InGaAs/InA/GaAs p-n junction tunnel diodes, the interband transmission coefficients are calculated. The estimated peak currents are shown to compare very favorably with experimental results     

On the current transport mechanism in a metal—insulator—semiconductor (MIS) diode

The current transport mechanism in an MIS-tunnel diode has been studied by considering both the process of tunneling and the effect of pinholes in the insulating layer. It has been shown that in order to explain the experimental J-V characteristics of MIS-diodes, presence of a thin interfacial layer of thickness δ_p within the pinholes should be considered. From an analysis of the tJ-V and C-V characteristics, a method has been suggested for the estimation of the value of δ_p. The values of interface trap density and barrier height for the MOS-part of the diodes are also calculated. The dependence of barrier height on oxide thickness for the diodes is found to obey the barrier height model of Cowley and Sze.     

Condensation Induced Delamination of Nanoscale Hydrophobic Films

Vapor condensation is a crucial phenomenon governing the efficiency of many processes. In particular, dropwise condensation on hydrophobic thin films (≈100 nm‐thick) has the potential to achieve remarkable heat transfer. However, the lack of durability of these thin films has limited applications for a century. Although degradation due to steam condensation has been described as “blistering,” no satisfactory insight exists capable of elucidating the driving force for film delamination. Here, it is shown that nanoscale pinholes in hydrophobic films are the source of blister formation. By creating artificial pinholes via nanoindentation on thin (30 to 500 nm‐thick) fluorinated hydrophobic films, it is demostrated that water blisters can be initiated at the pinholes during condensation. It is experimentally demonstrated that vapor is transferred to the blister through the nanoscale pinhole, and the driving force for delamination is capillary pressure generated at the pinhole by the pinned liquid–vapor interface. The techniques and insights presented here will inform future work on polymeric thin film and enable their durable design for a variety of applications.     

High-speed tunneling-barrier GaAs light emitters

High-speed GaAs light-emitting diodes are described that contain a single-AlAs tunneling barrier for localization of the carriers. These devices are identical to double-barrier resonant tunneling light emitters in speed and efficiency, but are not dependent on resonant tunneling of charge carriers. An external quantum efficiency of 0.18% and a 3-dB modulation bandwidth of 1.3 GHz are reported. These devices are less affected by nonradiative recombination compared to the conventional heavy-doping approach, in which a high-modulation bandwidth is obtained at the expense of a more than proportional reduction in quantum efficiency     

Generation-Recombination Effect in High-Temperature HgCdTe Heterostructure Nonequilibrium Photodiodes

The dark current of near-room-temperature long-wavelength heterojunction photodiodes was studied. The dark current of the devices is much greater than that calculated from the Auger generation mechanisms. A model of trap- assisted tunneling via traps located at dislocation cores is proposed as the mechanism of enhanced thermal generation of charge carriers in reverse-biased diodes. Field-induced reduction of trap activation energies can increase thermal generation and create conditions for tunneling currents. The model qualitatively explains experimental current−voltage characteristics of the diodes assuming a dislocation density of approximately 10⁸ cm⁻² at the graded gap interface between absorber and contact regions of the photodiode.     

Tunneling-based SRAM

This paper describes a new high-density low-power circuit approach for implementing static random access memory (SRAM) using low current density resonant tunneling diodes (RTDs). After an overview of semiconductor random access memory architecture and technology, the concept of tunneling-based SRAM (TSRAM) is introduced. Experimental results for a compound semiconductor 1-bit 50-nW TSRAM gain cell using low current density (~1 A/cm²) RTDs and low-leakage heterostructure field effect transistors are presented. We describe a one-transistor TSRAM cell which could convert silicon dynamic RAM (DRAM) to ultradense SRAM if an ultralow current density (~1 μA/cm² ) silicon bistable device is developed. Finally, we present experimental and simulation results for a TSRAM cell using multipeaked I-V curve devices and a multivalued word line. This approach aims at increasing storage density through vertical integration of bistable devices such as RTD's     

Digital circuit applications of resonant tunneling devices

Many semiconductor quantum devices utilize a novel tunneling transport mechanism that allows picosecond device switching speeds. The negative differential resistance characteristic of these devices, achieved due to resonant tunneling, is also ideally suited for the design of highly compact, self-latching logic circuits. As a result, quantum device technology is a promising emerging alternative for high-performance very-large-scale-integration design. The bistable nature of the basic logic gates implemented using resonant tunneling devices has been utilized in the development of a gate-level pipelining technique, called nanopipelining, that significantly improves the throughput and speed of pipelined systems. The advent of multiple-peak resonant tunneling diodes provides a viable means for efficient design of multiple-valued circuits with decreased interconnect complexity and reduced device count as compared to multiple-valued circuits in conventional technologies. This paper details various circuit design accomplishments in the area of binary and multiple-valued logic using resonant tunneling diodes (RTD's) in conjunction with high-performance III-V devices such as heterojunction bipolar transistors (HBT's) and modulation doped field-effect transistors (MODFET's). New bistable logic families using RTD+HBT and RTD+MODFET gates are described that provide a single-gate, self-latching majority function in addition to basic NAND, NOR, and inverter gates     

Current transport in the Me-n-n + Schottky-barrier structures

A model of current transport in Schottky-barrier diodes based on the concept of ballistic electron transport through a thin base is proposed. The method of transfer matrix was used in order to obtain tunneling probabilities, which were used in calculation of the forward and reverse current-voltage (I-V) characteristics, as well as of the transit time. It is demonstrated that by considering a potential in full form, a good agreement between the experimental and calculated I-V characteristics is obtained. It is found that a consideration of the role of a thin base causes the current to decrease; the probability of tunneling through the n-base can be close to unity. It is demonstrated that the tunneling probability has a large number of local resonances and that the energy dependence of the transit time is nonmonotonic. This is caused by the influence of the base region. The boundary operating frequency of diodes is evaluated and is found to be 10–100-fold higher than that obtained from the classical concept.     

Effect of minority carrier injection on lateral current in MIS tunnel structures

In this work it is shown that the MIS structure with an ultra-thin $\text{(20}\text{A}\text{?}\text{< x}{}_{\mathrm{i}}\text{< 40}\text{A}\text{?}\text{)}$ oxide is distinguished by a unique mechanism of lateral conduction. The flow of the lateral current between the two metal electrodes formed on the surface of an ultra-thin silicon dioxide is found to take place through the silicon substrate and the two non-equilibrium MIS tunnel diodes formed by the metal electrodes, the ultra-thin oxide, and the silicon. One of these diodes is forward biased and the other reverse biased. It is shown that the main limitation to the lateral current comes from the reverse biased diode. A structure is proposed in which such a lateral current can be efficiently controlled by the injection of minority carriers from the gate of an appropriately formed, conventional MIS tunnel diode into the depletion region of the reverse biased diode, so limiting the lateral current. Various physical phenomena related to this effect are studied and discussed.The MIS lateral tunnel structure considered in this work was fabricated in order to investigate the influence of minority carrier injection on the lateral conduction in MIS tunnel devices. However, the ideas proposed in this work can be developed into useful device structures.     

Simulation and optimization of metal-insulator-semiconductorinversion-layer silicon solar cells

Metal-insulator-semiconductor inversion-layer (MIS-IL) silicon solar cells are promising devices for photovoltaic energy conversion due to the ease of junction fabrication. In order to improve the fundamental understanding of these devices, this paper presents a detailed three-dimensional analysis of existing MIS-IL cells by means of two-dimensional (2-D) numerical modeling and circuit simulation. We implement a physical model suggested in the literature for the tunneling current through the MIS tunnel contact into a device simulator and solve the complete set of drift-diffusion equations for electrons and holes within the silicon in two dimensions. Based on experimentally determined device parameters, a good agreement between simulated and experimental current-voltage (I-V) characteristics is obtained, enabling the spatially resolved determination of resistive and recombinative losses. Furthermore, an optimization study is performed to reveal the efficiency limit of MIS-IL silicon solar cells     

MIS隧道器件电流-电压特性的数值模拟

本文建立了MLS隧道器件的电流-电压特性的数值模拟程序,提出了一种新的计算方法:龙格-库塔数值积分与边界条件的预估-校正处理相结合的算法.利用建立的程序模拟计算了两种不同氧化层厚度的MIS隧道器件的电流-电压特性.对TiW/Si肖特基二极管,考虑了界面态的静态和动态影响,模拟特性和实验结果相比,令人满意的一致.     

Spatial integration of direct band-to-band tunneling currents ingeneral device structures

A simplified integration technique for direct band-to-band tunneling current calculation in semiconductor devices of 1- or 2-D general device structures is described. The integration, along part of the depletion region, is of a tunneling generation function which depends on the local electric field. The simplified integration scheme relies on Kane's parabolic shaped gap barrier which accurately applies to such narrow-bandgap semiconductors as InSb and Hg_1-xCd_xTe. Tunneling current and zero bias resistance calculations in 1-D Hg_1-xCd_xTe p-n junctions using the proposed technique are presented. The extension of the technique to 2-D potential structures is demonstrated by modeling peripheral surface tunneling currents. The results compare well with measured reverse breakdown currents of InSb gate-controlled diodes     

TFT-LCD取向层表面的针孔缺陷分析(英文)

在TFT-LCD的生产过程中,取向层表面针孔缺陷是造成产品不良的常见原因。应用聚焦离子束(FIB)、扫描电子显微镜(SEM)和光学测量系统(OMS)工具,并结合数据统计软件Business Objects(BO)对实际生产过程中的一种典型产品不良——黑点,进行了测试和分析。结果表明,取向层表面针孔缺陷是产生黑点不良的根本原因。在此基础上,进一步通过理论分析和实验研究证明,成膜过程中膜液的流体力学不稳定性是导致取向层表面针孔缺陷的重要原因,而固化时间则是影响流体力学不稳定性的重要参数。膜液流体力学不稳定性的充分发展并最终对膜结构产生影响需要一定时间,当固化时间接近甚至小于不稳定性充分发展的时间时,取向层表面产生针孔缺陷的机会将大大减小甚至消除。     

纳米电子器件及其集成

对基于Top-Down加工技术的纳米电子器件如：单电子器件、共振器件、分子电子器件等的研究现状、面临的主要挑战等进行了讨论. 采用CMOS兼容的工艺成功地研制出单电子器件，观察到明显的库仑阻塞效应；在半绝缘GaAs衬底上制作了AlAs/GaAs/In0.1Ga0.9As/GaAs/AlAs双势垒共振隧穿二极管,采用环型集电极和薄势垒结构研制的共振隧穿器件，在室温下测得其峰谷电流比高达13.98，峰电流密度大于89kA/cm2；概述了交叉阵列的分子存储器的研究进展.     

An investigation of multiple domain Gunn effect oscillators

The quenched multiple domain mode of oscillation is proposed in GaAs Gunn diodes. Operating conditions for this mode are explored by means of a computer simulation and a simplified linearized-lumped analysis that was developed to generate waveforms for quenched domain devices. Optimum efficiency is seen when a maximum number of "hybrid sized" domains are packed into a device which is operated at the transit frequency associated with the width of a domain. By reducing the voltage across a device, nonhybrid sized domains form and the device operates at less than peak efficiency. The investigation includes a discussion of device performance when nonidentical but comparably sized domains are present in the device. Experimentally, multiple domain devices have been fabricated using diodes with coplanar contacts. The experimental results tend to confirm the existence of multiple domains in the diodes as well as the results of our analysis.     

Tunneling hot electron transfer amplifiers (theta): Amplifiers operating up to the infrared

A family of novel three-terminal devices which relies on the transfer of a quasi-monoenergetic hot electron beam through a thin base is described. The devices are similar in principle to the proposed tunneling amplifier by Mead in the early sixties (“Cold Cathode” or “Metal Base” amplifiers). Results are reviewed and the probable reasons for the poor performances are pointed out. It is predicted that, with a proper choice of parameters, metal-base amplifiers can operate as switches, negative resistance devices and continuous amplifiers in the subpicosecond range.Two subclasses are described: The tunneling emitter (THETA), in the major part of the work, and the nontunneling emitter (BHETA) amplifiers. In the THETA family the metal-oxide-metal-oxide-metal (MOMOM), the MOM-semiconductor (MOMS), and the heterojunctions devices are described. Members of the BHETA family generate quasi-monoenergetic electron beams by injecting electrons by an n⁺n^? or a metal-n^? junctions, and include a variety of metals and semiconductor combinations.Very thin films are required in these devices (oxides ～15 Å, metals ～100 Å, semiconductors ～100 Å). The molecular beam epitaxy technique and lattice matching considerations are required for pinhole free semiconductors and metal films with minimum interface states. Sputter-oxidation methods are needed for thin oxide growth. Systems which combine these features with availability of microfabrication make these devices feasible today.