Cascaded homojunction avalanche photodiodes

The structure and the performance of a cascaded avalanche photodiode (APD) based on cascaded home-junctions are discussed. Similar to the hetero-structure superlattice APD, such a device has a lower excess noise and a higher gain at lower bias voltages than conventional APDs. In addition to the ionization ratio, several parameters also affect the device's performance.     

The electron ionization rate in multilayered semiconductor structures

We present theoretical results of the electron impact ionization rate in GaAs/AlGaAs multiquantum well structures as a function of applied electric field for various geometries, i.e., well and barrier widths. In addition, we present preliminary measurements of the current-voltage characteristics of MBE grown devices which demonstrate very low leakage current as well as sharp breakdwon behavior. It is found that the net ionization rate, determined by averaging over the constitutent GaAs and AlGaAs layers, approaches the weighted average of the constituent bulk rates at high electric field strengths; the potential discontinuity is relatively unimportant. The electron ionization rate within the well regions alone is still higher than that in bulk GaAs, but is insufficiently enhanced to compensate for the much lower rate in the AlGaAs layers. As the field is lowered to 250.0 kV/cm, the average ionization rate in the multiquantum well structure becomes larger than in the bulk.     

Avalanche mechanism analysis of 4H-SiC n-i-p and p-i-n avalanche photodiodes working in Geiger mode

Understanding detailed avalanche mechanisms is critical for design optimization of avalanche photodiodes(APDs). In this work, avalanche characteristics and single photon counting performance of 4 H-Si C n-i-p and p-i-n APDs are compared. By studying the evolution of breakdown voltage as a function of incident light wavelength, it is confirmed that at the deep ultraviolet(UV) wavelength region the avalanche events in 4 H-Si C n-i-p APDs are mainly induced by hole-initiated ionization,while electron-initiated ionization is the main cause of avalanche breakdown in 4 H-Si C p-i-n APDs. Meanwhile, at the same dark count rate, the single photon counting efficiency of n-i-p APDs is considerably higher than that of p-i-n APDs. The higher performance of n-i-p APDs can be explained by the larger impact ionization coefficient of holes in 4 H-Si C. In addition, this is the first time, to the best of our knowledge, to report single photon detection performance of vertical 4 H-Si C n-i-p-n APDs.     

Infrared absorption in parabolic multiquantum well structures

The absorption constant for intraband transitions in parabolic multiquantum well structures is calculated and compared with intraband absorption in a square well superlattice. The parabolic multiple quantum well structure may be used as an Infrared detector with the possibility of lower leakage current compared to one made of square wells.     

Phenomena in silicon nanostructure devices

In nanostructures, whenever the electron mean-free-path exceeds the appropriate dimensions of the device structure, quantum natures may dictate the physical properties of devices. Among many important issues, some are selected in this work, whereas others, such as the reduction of dielectric constant, the increased binding energy of dopants, etc., are discussed briefly with references for further considerations. In the past several years, resonant tunneling via nanoscale silicon particles imbedded in an oxide matrix has shown striking similarity to the so-called soft breakdown (SBD), an important current subject in devices with ultrathin oxide gates. The relevance in applying results discussed here to SBD is discussed. A Si/O superlattice, a particular form of a new type of superlattice, semiconductor-atomic superlattice (SAS), is fully discussed. This Si/O superlattice can be used in silicon quantum and light-emitting devices. A diode structure with green electroluminescence has been life-tested for more than one year without degradation. High-resolution TEM shows defect density below 10⁹/cm². Preliminary calculation shows that the Si/O complexes result in a barrier height of 0.9 eV for silicon, sufficient for an epitaxially grown SOI, which is potentially far better than the SOI using buried oxide implantation followed by high temperature anneal. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

The variably spaced superlattice energy filter

A new superlattice device concept which provides for high energy injection of electrons into a semiconductor layer is presented. The device is based on resonant tunneling of electrons between adjacent aligned quantum well levels in a variably spaced superlattice structure. By a judicial choice of well and barrier widths the energy levels under reverse bias become aligned such that resonant tunneling of electrons through the structure can occur. Thus, electrons are injected into a semiconductor layer at an energy corresponding to the energy of the first subband in the last quantum well. This structure has significant advantages over the conventional method of producing hot electrons in that a nearly monoenergetic high-energy electron distribution is created at low reverse bias and with high efficiency, since energy loss to phonons is inhibited as a consequence of the channeling of electrons through a narrow band of quantum states. Applications of the VSSEF structure to avalanche photodiodes, IMPATT diodes and electroluminescent devices are discussed.     

Investigation of InP/InGaAs superlattice-emitter resonant tunneling bipolar transistors (RTBTs)

An InP/InGaAs superlattice-emitter resonant tunneling bipolar transistor (SE-RTBT) has been fabricated and demonstrated. The influence of the superlattice and emitter thickness on the device characteristics is studied. The insertion of the superlattice and a well-designed emitter improve the characteristics of the SE-RTBT. Common-emitter current gains up to 170 and 54 are obtained for the studied devices with emitter thicknesses of 800 Å and 150 Å, respectively. Based on the specified structure, the lower offset voltage and saturation voltage (  ≤ 1.5 V) are obtained. Experimentally, the device with a 5-period superlattice and an emitter thickness of 800 Å provides higher dc performance and stable temperature-dependent characteristics.     

Thermal resistance of GaAs/AlAs superlattices used in modern light-emitting diodes

Superlattices are used in modern light-emitting diodes to modify intentionally electron, phonon and/or photon transport within their volumes, which leads to their expected performance characteristics. In particular, superlattices may have a dramatic impact on device thermal properties. Superlattice thermal resistance is anisotropic and usually distinctly higher than its values in constituent bulk materials, which results from phonon reflections and/or phonon scatterings at numerous layer interfaces. In the present paper, thermal resistance of a typical superlattice of layer thicknesses neither much higher nor much lower than the phonon free path is discussed. Besides, as an important example, thermal resistance of the typical GaAs/AlAs superlattice is determined theoretically and compared with its measured values known from literature.     

Surface and interface issues in wide band gap semiconductor electronics

Wide band gap (WGB) materials are the most promising semiconductors for future electronic devices, and are candidates to replace the conventional materials (Si, GaAs, …) that are approaching their physical limits. Among WBG materials, silicon carbide (SiC) and gallium nitride (GaN) have achieved the largest advancements with respect to their material quality and device processing. Clearly, the devices performances depend on several surface and interface properties, which in turn are often crucially determined by the quality of the available material, as well as by the device processing maturity. In this paper, some surface and interface issues related to SiC and GaN devices processing are reviewed. First, the control of metal/SiC barrier uniformity and surface preparation will be discussed with respect to the performance of Schottky-based devices. Moreover, the impact of high-temperature annealing required for high-voltage Schottky diodes and MOSFETs fabrication, on the surface morphology and device performances will also be briefly presented. In the second part, it will be shown that for GaN the material quality is still the main concern, since dislocations have a severe influence on the current transport and barrier homogeneity of metal/GaN interfaces. Other practical implications of thermal annealing and surface passivation during GaN-based devices fabrication will also be addressed.     

Nonvolatile memory devices based on self-assembled nanocrystals

Nonvolatile memory devices are one of the most important components in modern electronic devices. Many efforts have been made to fabricate high-density, low-cost, nonvolatile solid-state memory devices for use in portable/mobile electronic devices such as laptop computers, tablet devices, smart phones, etc. Among the many available nonvolatile memory devices, flash memory devices are of great interest to the electronics industry owing to their simple device structure, enabling high-density memory applications. Flash memory devices in which nanoparticles or nanocrystals are used as the charge-trapping elements have advantages over conventional flash memory devices because the charge-trapping layer and memory performance of the former can be readily optimized. Active research has recently been conducted to fabricate and characterize self-assembled-nanocrystal-based nonvolatile memory devices. We reviewed various strategies for fabricating nanocrystal-based nonvolatile memory devices and discussed the programmable memory properties and the device reliability characteristics of nanocrystal-based memory devices to possibly apply nanocrystal-based memory devices to those used in portable/mobile electronic devices. Finally, novel device applications such as printed/flexible/transparent electronic devices were explored based on nanocrystal-based memory devices.     

Non-covalent interaction controlled 2D organic semiconductor films: Molecular self-assembly,electronic and optical properties,and electronic devices

The establishment of electronic and opto-electronic products relying on organic semiconductors (OSCs) has been intensely explored over the past few decades due to their great competitiveness in large area, low cost, flexible, wearable and implantable devices. Many of these products already entered our daily lives, such as organic light-emitting diodes-based displays, portable organic solar cells and organic field-effect transistors. The device performance of OSC devices are determined by the supramolecular organization (orientation, morphology) as well as the supramolecular organization dependent energy level alignment at various interfaces (organic/electrode, organic/dielectric, organic/organic). This review focuses on the impact of non-covalent interaction on the molecular self-assembly of organic thin films, their electronic and optical properties, as well as the device performance. Beginning with the growth of multiple OSCs on substrates with different interfacial interaction strengths (metals, insulators, semiconductors), the critical roles of molecule-substrate and intermolecular interactions in determining the thin film organization have been demonstrated. Several non-covalent interactions that contribute to the energy levels of organic materials in solid phase are summarized, mainly including the induction contributions, electrostatic interactions, band dispersions and interface dipoles. The excitonic coupling in specific aggregations of organic molecules and the corresponded effect on their optical properties are also discussed. Finally, the influences of weak intermolecular interactions on the device performance are presented.     

半导体超晶格声子激光器的研究进展

太赫兹频率的相干声子在纳米尺度器件的探测和操控领域具有重要的应用价值。半导体超晶格声子激光器是实现太赫兹频率相干声子源稳定输出的重要途径。本文首先回顾了GHz到THz频率范围声学放大的多种方法,然后详细阐述了超晶格声子放大、超晶格声学布拉格镜的工作原理与设计方法以及声子激光器的阈值条件,同时总结了电抽运和光抽运结构器件的研究现状,最后简要讨论了亚太赫兹声子激光器在声-电子领域的应用。分析表明,这种能够产生强相干太赫兹声子的半导体超晶格声子激光器在纳米尺度器件的探测与成像等方面具有广阔的发展前景。     

Temperature dependent characteristics of submicron GaAs avalanche photodiodes obtained by a nonlocal analysis

In this paper, using a nonlocal analysis we have extracted the temperature dependent ionization coefficients and threshold energies of submicron GaAs avalanche photodiodes (APDs) with multiplication region thicknesses as narrow as 49 nm, from electron and hole injection photo-multiplication processes. These extracted parameters have been used to predict the temperature dependence of APDs characteristics, such as mean gain, 3 dB-bandwidth, gain-bandwidth product, excess noise factor, performance factor, and breakdown field, over a temperature range of 20 K to 290 K. In the nonlocal analysis we have taken the effects of nonuniform electric filed within the multiplication region and its surrounding depletion regions, injected carrier’s initial ionization energy, carrier’s spatial ionization rate as well as the carrier’s dead space and its previous ionization history into account. We have shown that our predicted gain values are in excellent agreement with existing experimental data measured by others.     

Tactile and temperature sensors based on organic transistors:Towards e-skin fabrication

Tactile and temperature sensors are the key components for e-skin fabrication.Organic transistors,a kind of intrinsic logic devices with diverse internal configurations,offer a wide range of options for sensor design and have played a vital role in the fabrication of e-skin-oriented tactile and temperature sensors.This research field has attained tremendous advancements,both in terms of materials design and device architecture,thereby leading to excellent performance of resulting tactile/temperature sensors.Herein,a systematic review of organic transistor-based tactile and temperature sensors is presented to summarize the latest progress in these devices.Particularly,we focus on spotlighting various device structures,underlying mechanisms and their performance.Lastly,an outlook for the future development of these devices is briefly discussed.We anticipate that this review will provide a quick overview of such a rapidly emerging research direction and attract more dedicated efforts for the development of next-generation sensing devices towards e-skin fabrication.     

Double superlattice structure for improving the performance of ultraviolet light-emitting diodes

The novel AlGaN-based ultraviolet light-emitting diodes(UV-LEDs) with double superlattice structure(DSL) are proposed and demonstrated by numerical simulation and experimental verification. The DSL consists of 30-period Mg modulation-doped p-AlGaN/u-GaN superlattice(SL) and 4-period p-AlGaN/p-GaN SL electron blocking layer, which are used to replace the p-type GaN layer and electron blocking layer of conventional UV-LEDs, respectively. Due to the special effects and interfacial stress, the AlGaN/GaN short-period superlattice can reduce the acceptor ionization energy of the ptype regions, thereby increasing the hole concentration. Meanwhile, the multi-barrier electron blocking layers are effective in suppressing electron leakage and improving hole injection. Experimental results show that the enhancements of 22.5%and 37.9% in the output power and external quantum efficiency at 120 m A appear in the device with double superlattice structure.     

Type-II InAs/GaSb photodiodes and focal plane arrays aimed at high operating temperatures

Recent efforts to improve the performance of type II InAs/GaSb superlattice photodiodes and focal plane arrays (FPA) have been reviewed. The theoretical bandstructure models have been discussed first. A review of recent developments in growth and characterization techniques is given. The efforts to improve the performance of MWIR photodiodes and focal plane arrays (FPAs) have been reviewed and the latest results have been reported. It is shown that these improvements has resulted in background limited performance (BLIP) of single element photodiodes up to 180 K. FPA shows a constant noise equivalent temperature difference (NEDT) of 11 mK up to 120 K and it shows human body imaging up to 170 K.     

21世纪的光学和光电子学讲座 第二讲 硅基发光材料和器件研究

硅基发光材料和器件是实现光电子集成的关键．文章评述了目前取得较大进展的几种主要硅基发光材料和器件的研究,包括掺饵硅,多孔硅,纳米硅以及Ｓｉ／ＳｉＯ２ 等超晶格结构材料．展望了这些不同硅基发光材料作为发光器件和在光电集成中的发展前景     

Broad-band microwave detection with a novel 2D hot-electron device

We present the operation of a new AlGaAs–GaAs multiquantum well hot-electron microwave detector. The working principle of this device is based on the interaction of two-dimensional free carriers inside the wells with the in-plane electric field.We shall report room-temperature responsivity of several 10³V W^− 1, comparable to that of conventional solid-state devices. The different physical principle of operation, however, yields for the present detector a broader frequency range, extending up to the submillimetre band, and short response times which can be estimated around 10 ps. Finally, we report a characterization of the device from the point of view of noise.     

Recent progress on two-dimensional neuromorphic devices and artificial neural network

Mimicking biological synapses with microelectronic devices is widely considered as the first step in hardware building artificial neuromorphic networks, which is also the basis of brain-inspired neuromorphic computing. Numerous artificial neurons and synapses making up an artificial neuromorphic network have been gained wide attention due to their powerful and efficient data processing capabilities. Recently, artificial synapses, especially memristor-type and transistor-type synapses based on multifarious two-dimensional (2D) materials have been paid much attention. The unique properties of 2D materials make devices perform well in learning ability and power efficiency when mimicking synaptic behaviors, which highlights the feasibility of 2D neuromorphic devices in constructing artificial neuromorphic networks. Herein, the basic structures and principles of biological synapses are introduced, and the definitions of synaptic behaviors in synaptic electronic devices are discussed. Then, the progress of 2D memristor-type and transistor-type neuromorphic devices involving their device architecture, neuromorphic operational mechanism, and promising applications is reviewed. Finally, the future challenges of artificial synaptic devices based on 2D materials are discussed briefly.     

Resonant tunneling in double superlattice barrier heterostructures

We have investigated resonant tunneling in double barrier heterostructures in which the tunnel barriers have been replaced by short period superlattices, and have shown for the first time quantum well confinement in a single quantum well bounded by superlattices. These results also demonstrate the first utilization of short period binary superlattices as effective tunnel barriers to replace the conventional Al_xGa_1−xAs barriers. The superlattice structure does not exhibit the asymmetry around zero bias in the electrical characteristics normally observed in the conventional Al_xGa_1−xAs barrier structures, suggestive of reduced roughness at the inverted interface by superlattice smoothing. The superlattice barrier also exhibits an anomalously low barrier height. The performance of this symmetric superlattice structure is compared with an intentionally constructed asymmetric double barrier superlattice structure, which exhibits pronounced asymmetry in the electrical characteristics. The observed behavior supports the view that resonant enhancement occurs in the quantum well.