Thin layers of GaInP, GaP and GaAsP in metalorganic vapour phase epitaxy-grown resonant tunnelling diodes

We have studied the epitaxial growth and electrical performance of Al-free, GaAs-based, resonant tunnelling diodes (RTDs) including thin barriers of GaInP, GaP, or GaAs_xP_1−x. n-Type tunnelling diodes have been fabricated and the symmetry in the current–voltage (I–V) characteristics, as well as the peak-to-valley ratios, are found to be sensitive probes for the interface quality in the heterostructures. For GaInP RTDs, we show that the introduction of GaP intermediate layers is crucial for the realisation of a useful tunnelling current. RTDs including thin barriers (less than about 10 monolayers (ML)) of GaP are realised, but the strong mismatch between the materials limit the useful thickness. Finally, RTDs with GaAs_xP_1−x alloys are fabricated showing the best peak-to-valley ratio of the diodes (about 5), as well as a symmetric I–V characteristics. The electrical data are further compared to studies by transmission electron microscopy (TEM) in the various material systems.     

Comparison of resonant tunneling diodes grown on freestanding GaN substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy

AlN/GaN resonant tunneling diodes (RTDs) were grown separately on freestanding GaN (FS-GaN) substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy (PA-MBE). Room temperature negative differential resistance (NDR) was obtained under forward bias for the RTDs grown on FS-GaN substrates, with the peak current densities (J_p) of 175-700 kA/cm² and peak-to-valley current ratios (PVCRs) of 1.01-1.21. Two resonant peaks were also observed for some RTDs at room temperature. The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically, showing that lower dislocation densities, flatter surface morphology, and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.     

Influence of spacer layer thickness on the current-voltage characteristics of pseudomorphic AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes

This paper investigates the dependence of current voltage characteristics of AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes （RTDs） on spacer layer thickness. It finds that the peak and the valley current density J in the negative differential resistance （NDR） region depends strongly on the thickness of the spacer layer. The measured peak to valley current ratio of RTDs studied here is shown to improve while the current density through RTDs decreases with increasing spacer layer thickness below a critical value.     

Extrinsic tristability as the cause of bistability in resonant-tunneling diodes

The problem of extrinsic bistability in the current-voltage (I-V) curves of resonant-tunneling diodes (RTDs) is examined with the aid of a simple lumped-circuit model. It is shown that the presence of series resistance causes the I-V curve to fold over in the region that would normally exhibit negative differential resistance (NDR). Thus for each value of bias voltage, there are three possible values of current. This phenomenon is labelled extrinsic tristability and shown to be the underlying cause of extrinsic bistability in RTDs.     

Magnetic field effects and k?-filtering in BEEM on GaAsAlGaAs resonant tunneling structures

In this work, GaAsAlGaAs double barrier resonant tunneling diodes (RTDs) are investigated by ballistic electron emission microscopy (BEEM). RTDs grown directly below the sample surface exhibit characteristic steplike features in the BEEM spectrum, whereas for buried RTDs, a linear spectrum is observed. Moreover, the BEEM spectra of sub-surface RTDs show Shubnikov-de Haas-like oscillations in magnetic fields. To investigate the origin of these effects, the BEEM spectra were calculated using a scattering formalism within the framework of a semi-empirical tight binding method. As a main result we found that, independent of the applied bias, only electrons within a narrow k_ distribution are transferred resonantly through the RTD. Hence, a k_ filter is established for ballistic electrons close to k_=0. The calculated filter width is consistent with the magnetic field data.     

Electrical characteristics of hole resonant tunneling diodes with high Ge fraction (x > 0.4) Si/strained Si1−xGex/Si(1 0 0) heterostructure

Effectiveness of a Ge fraction modulated spacer in hole resonant tunneling diodes (RTDs) with Si/strained Si_1−xGe_x heterostructures epitaxially grown on Si(1 0 0) was investigated to improve the electrical characteristics at higher temperatures. Electrical characteristics measured for 30 RTDs, with the modulated spacer at higher Ge fraction (x = 0.48) on a single wafer, show that the deviation of the peak current and voltage at the resonant peak falls in ranges of ±25% and ±10%, respectively. For the RTDs, negative differential conductance (NDC) characteristics are obtained even at higher temperatures around 230 K than that for the RTDs with x = 0.42. The result indicates that the introduction of higher Ge fraction is effective for NDC in RTD at higher temperature.     

A novel micro accelerometer with adjustable sensitivity based on resonant tunneling diodes

Resonant tunnelling diodes (RTDs) have negative differential resistance effect, and the current--voltage characteristics change as a function of external stress, which is regarded as meso-piezoresistance effect of RTDs. In this paper, a novel micro-accelerometer based on AlAs/GaAs/In_0.1Ga_0.9As/GaAs/AlAs RTDs is designed and fabricated to be a four-beam-mass structure, and an RTD-Wheatstone bridge measurement system is established to test the basic properties of this novel accelerometer. According to the experimental results, the sensitivity of the RTD based micro-accelerometer is adjustable within a range of 3 orders when the bias voltage of the sensor changes. The largest sensitivity of this RTD based micro-accelerometer is 560.2025 mV/g which is about 10 times larger than that of silicon based micro piezoresistive accelerometer, while the smallest one is 1.49135 mV/g.     

Pressure effects in AlAs/Inx Ga1-xAs/GaAs resonant tunnelling diodes for application in micromachined sensors

This paper reports the current-voltage characteristics of [001]-oriented AlAs/InxGa1-xAs/GaAs resonant tunnelling diodes （RTDs） as a function of uniaxial external stress applied parallel to the [110] and the [1^-10] orientations, and the output characteristics of the GaAs pressure sensor based on the pressure effect on the RTDs. Under [110] stress, the resonance peak voltages of the RTDs shift to more positive voltages. For [1^-10] stress, the peaks shift toward more negative voltages. The resonance peak voltage is linearly dependent on the [110] and [1^-0] stresses and the linear sensitivities are up to 0.69 mV/MPa, -0.69 mV/MPa respectively. For the pressure sensor, the linear sensitivity is up to 0.37 mV/kPa.     

Device structures based on resonant tunneling diodes: A theoretical consideration

Simple device structures incorporating resonant tunneling diodes (RTDs) are considered in terms of electrical models and the EC-RTS-NANODEV software suite. It is shown that the structures can be used in multilevel logic, frequency converters, and generators of harmonic, relaxation, and chaotic signals.     

The role of emitter quasi-bound state and scattering on intrinsic bistability in resonant tunneling diodes

Usually, numerical self-consistent calculations predict a much larger intrinsic bistability region than actually is measured in resonant tunneling diodes (RTDs). In addition, numerical calculations have shown that scattering in the well reduces bistability. We used a unified treatment of current flowing from continuum states and emitter quasi-bound states to show numerically and analytically that not only the scattering in the quantum well but also the scattering in the emitter reduces bistability. Moreover, within the Hartree approximation, bistability occurs by tunneling resonantly between emitter quasi-bound state and well quasi-bound state as a pitchfork bifurcation.     

Electronic Transport and Thermopower in 2D and 3D Heterostructures—A Theory Perspective

The impact of interfaces and heterojuctions on the electronic and thermoelectric transport properties of materials is discussed herein. Recent progress in understanding electronic transport in heterostructures of 2D materials ranging from graphene to transition metal dichalcogenides, their homojunctions (grain boundaries), lateral heterojunctions (such as graphene/MoS₂ lateral interfaces), and vertical van der Waals heterostructures is reviewed. Work on thermopower in 2D heterojunctions, as well as their applications in creating devices such as resonant tunneling diodes (RTDs), is also discussed. Last, the focus turns to work in 3D heterostructures. While transport in 3D heterostructures has been researched for several decades, here recent progress in theory and simulation of quantum effects on transport via the Wigner and non‐equilibrium Green's functions approaches is reviewed. These simulation techniques have been successfully applied toward understanding the impact of heterojunctions on transport properties and thermopower, which finds applications in energy harvesting, and electron resonant tunneling, with applications in RTDs. In conclusion, tremendous progress has been made in both simulation and experiments toward the goal of understanding transport in heterostructures and this progress will soon be parlayed into improved energy converters and quantum nanoelectronic devices.     

Observation of the current－voltage plateau-like structure of resonant tunnelling diode with prewells under different magnetic fields

We have grown resonant tunnelling diodes (RTDs) with different sized emitter prewells and without a prewell. The current－voltage (I－V) characteristics of them in different magnetic fields were investigated. Two important phenomena were observed. First, a high magnetic field can destroy the plateau-like structure in the I－V curves of the RTD. This phenomenon is ascribed to the fact that the high magnetic field will demolish the coupling between the energy level in the main quantum well and that in the emitter quantum well or in the prewell. Secondly, the existence and size of the prewell are also important factors influencing the plateau-like structure.     

A new method for characterizing ultrafast resonant-tunnelling diodes with electrooptic sampling

A new electrooptic sampling technique for characterizing ultrafast resonant-tunnelling diodes (RTD) is presented, in which the RTD is driven by the output of the photodiode irradiated by the same laser pulse that probes the output of the RTD. This method features a high time resolution, moderate slew rate and low heat load, which are the keys to characterizing RTD switching time. From the investigation of several factors, such as laser pulse width, interaction time between the probe pulse and electrical signal, and triggering jitter, the overall time resolution was found to be less than 1 ps. The measured switching times for In_0.53Ga_0.47As/AIAs RTDs were compared with the resistance-capacitance time constant for each device, and this confirmed that this driving method accurately measured RTD switching time at the order of 1 ps.     

Room-temperature observation of current bistability and fine structures in germanium quantum dots/SiO2 resonant tunneling diodes

The steady-state and time-dependent current–voltage (I–V) characteristics are experimentally investigated in Ge quantum dot (QD)/SiO₂ resonant tunneling diodes (RTDs). Ge QDs embedded in a SiO₂ matrix are naturally formed by thermal oxidation of Si_0.9Ge_0.1 nanowires (30 nm×50 nm) on silicon-on-insulator substrates. The average dot size and spacing between dots are 9±1 and 25 nm, respectively, from TEM observations, which indicate that one or two QDs are embedded between SiO₂ tunneling barriers within the nanowires. Room-temperature resonant oscillation, negative differential conductance, bistability, and fine structures are observed in the steady-state tunneling current of Ge-QD/SiO₂ RTDs under light illumination. Time-dependent tunneling current characteristics display periodic seesaw features as the Ge-QDs RTD is biased within the voltage regime of the first resonance peak while they exhibit harmonic swing behaviors as the RTD is biased at the current valleys or higher-order current peaks. This possibly originates from the interplay of the random telegraph signals from traps at the QD/SiO₂ interface as well as the electron wave interference within a small QD due to substantial quantum mechanics effects.     

Piezoelectric polarization and quantum size effects on the vertical transport in AlGaN/GaN resonant tunneling diodes

In this work,the electronic properties of resonant tunneling diodes(RTDs) based on GaN-AlxGa(1-x)N double barriers are investigated by using the non-equilibrium Green functions formalism(NEG).These materials each present a wide conduction band discontinuity and a strong internal piezoelectric field,which greatly affect the electronic transport properties.The electronic density,the transmission coefficient,and the current–voltage characteristics are computed with considering the spontaneous and piezoelectric polarizations.The influence of the quantum size on the transmission coefficient is analyzed by varying GaN quantum well thickness,Al_xGa_(1-x)N width,and the aluminum concentration x_(Al).The results show that the transmission coefficient more strongly depends on the thickness of the quantum well than the barrier;it exhibits a series of resonant peaks and valleys as the quantum well width increases.In addition,it is found that the negative differential resistance(NDR) in the current–voltage(I–V) characteristic strongly depends on aluminum concentration xAl.It is shown that the peak-to-valley ratio(PVR) increases with xAlvalue decreasing.These findings open the door for developing vertical transport nitrides-based ISB devices such as THz lasers and detectors.     

Schottky barrier height lowering induced by CoSi2 nanostructure

CoSi₂ nanostructures were formed through thermal agglomeration by annealing ultrathin Co film on Si substrate at high temperatures. The characteristics of the Schottky diodes with CoSi₂ nanostructures capped by a Pt layer were measured and fitted using thermionic emission theory. All the diodes have a ideality factor less than 1.1. The results show that the Schottky barrier height of these diodes significantly decreases as the annealing temperature for CoSi₂ agglomeration increases. The barrier height lowering is correlated with the agglomeration of CoSi₂ film and the formation of CoSi₂ nano-islands. The thermal field emission may be the major reason to cause barrier lowering. Although the Schottky contact interface consists of both CoSi₂ nano-islands and Pt film whose individual contact barrier height to Si is very different, the current-voltage-temperature measurements reveal that the interface homogeneity is not degraded as expected. The study demonstrates that the CoSi₂ nanostructures can both lower the Schottky barrier height and form an ideal Schottky contact with a Pt capping layer.     

The phosphor’s optical properties—chromaticity coordinate relationship of phosphor converted white LEDs

Based on optical ray-tracing simulations we discuss the effect of the extinction coefficient and the quantum efficiency of the phosphors on the angular homogeneity of the white light emitted from phosphor converted light-emitting diodes. In particular variations of the extinction coefficient are prone to affect diverse CIE chromaticity coordinates for different viewing angles. Contrarily, the impact of the quantum efficiency on angle dependent variations of the chromaticity coordinates turns out to be of minor importance.     

Track-etch membranes as templates enabled nano/micro technology: a review

Many techniques are being used in order to synthesize nano-micro materials falling under the realm of nanotechnology. It need not be overemphasized that the miniaturization of devices and synthesis of new materials have a tremendous role in the development of powerful electronics as well as material based technologies in other areas but for the laws of quantum mechanics posing limitations besides the increasing cost and difficulties in manufacturing in such a small scale. The quest, therefore, for the alternative technologies, have stimulated a surge of interest in nano-meter scale materials and devices in the recent years. Metallic as well as semiconducting nano wires are the most attractive materials because of their unique properties having myriad of applications like interconnects for nano-electronics, magnetic devices, chemical and biosensors, whereas the hollow tubules are equally considered to be candidates for more potent applications — both in physical as well as biosciences. Materials’ processing for nano-structured devices is indispensable to their rational design. The technique, known as “Template Synthesis”, using electrochemical-electro less deposition is one of the most important processes for manufacturing nano-micro structures, nano-composites and devices and is relatively inexpensive and simple. The technique involves using membranes — ion crafted ones (popularly known as Particle Track-Etch Membranes or Nuclear Track Filters), alumite substrate membranes, besides other types of membranes as templates. The parameters viz., diameter as well as length i.e., aspect ratio, shape and wall surface traits in these membranes are controllable. In the present article a detailed review of this technique using track-etch membranes as templates in synthesis of nano-micro materials including hybrid materials and devices like field-ion emitters, resonant tunneling diodes (RTDs) etc. is presented including most of the results obtained in our laboratory.      

基于改性空穴注入层与复合发光层的高效钙钛矿发光二极管

有机金属卤化钙钛矿作为发射体具有极高的色纯度和极低的成本,但钙钛矿层普遍较差的形貌制约了器件的性能.引入合适的聚合物可有效改善旋涂型钙钛矿薄膜的均匀性.本文引入聚(4-苯乙烯磺酸盐)(PSS)改性的聚(3,4-乙撑二氧噻吩):PSS(PEDOT:PSS)作为空穴注入层(HIL),结合一步旋涂制备的三溴化铅甲基胺(MAP...     

Analysis of reliability of semiconductor emitters with different designs of cavities

We have reported on the results of analysis of the operating time of conventional laser diodes and diodes with noninjecting output sections. The reasons for shorter operating time of diodes with a single anti-reflection face of the cavity compared to diodes with two protecting coatings and emitters equipped with a fiber Bragg grating have been considered.