Lateral ordering of quantum dots and wires in the (In,Ga)As/GaAs(100) multilayer structures

The surface morphology and optical properties of the (In,Ga)As/GaAs(100) multilayer structures with self-organized quantum dots and quantum wires, which were grown by molecular-beam epitaxy, are investigated. It is found that the ordered arrangement of quantum dots in the heterointerface plane starts to form during the growth of the first periods of the multilayer structure. As the number of periods increases, quantum dots line up in series and form wires along the \([0\bar 11]\) direction. An increase in the lateral ordering of the structures under consideration correlates with an increase in the optical emission anisotropy governed by relaxation anisotropy of elastic strains and by the shape of nano-objects. A possible mechanism of lateral ordering of quantum dots and wires in multilayer structures, which includes both anisotropy effects of the strain fields and adatom diffusion, as well as the elastic interaction of neighboring quantum dots, is discussed.     

GaAs-based long-wavelength InAs bilayer quantum dots grown by molecular beam epitaxy

Molecular beam epitaxy growth of a bilayer stacked InAs/GaAs quantum dot structure on a pure GaAs matrix has been systemically investigated.The influence of growth temperature and the InAs deposition of both layers on the optical properties and morphologies of the bilayer quantum dot（BQD） structures is discussed.By optimizing the growth parameters,InAs BQD emission at 1.436μm at room temperature with a narrower FWHM of 27 meV was demonstrated.The density of QDs in the second layer is around 9×10⁹ to 1.4×10¹⁰ cm^-2. The BQD structure provides a useful way to extend the emission wavelength of GaAs-based material for quantum functional devices.     

Photoluminescence characterization technique for resonant-tunneling structures based on a long-period GaAs/AlGaAs superlattice,applicable at different stages of fabrication

A technique is developed for the photoluminescence-spectroscopy characterization of resonant-tunneling structures based on a long-period GaAs/AlGaAs superlattice that can be used for quality evaluation at all the stages of fabrication, including molecular-beam epitaxy, photolithography, and annealing. Factors such as the small energy difference between the quantum confined states in wide quantum wells, which make the photoluminescence characterization of such structures more difficult are taken into account. The long-period multiquantum-well structures are promising for the development of a new kind of solid-state intersub-band-transition devices emitting the narrow band radiation in far infrared. Their potential is essentially based on the fact that the scattering and the decay of carriers in the lower quantum-confined states may or may not involve optical phonons. The technique works at both liquid-helium and room temperature. It helps one optimize the process conditions to fabricate high-quality wide-quantum-well structures with excellent uniformity and desired parameters.     

Charge accumulation of quantum dots at room temperature

The accumulation of charge in InGaAs quantum dots has been measured at room temperature by the photoelectrochemical capacitance-voltage (CV) technique for the first time. A carrier per quantum dot ratio greater than four has been observed. The use of atomic force microscopy and low temperature and room temperature photoluminescence (PL) confirm the existence of quantum dots. Also, a possible excited state is indicated by room temperature PL in a sample with small quantum dots.     

Quantum phenomena in field-effect-controlled semiconductornanostructures

Quantum effects resulting from sub-100 nm features in planar, field-effect-controlled semiconductor structures or devices are discussed, and experimental results are compared with calculations. These devices are based on the GaAs-AlGaAs modulation-doped field-effect transistor (MODFET) and include grating-gate lateral surface superlattices. (LSSLs), grid-gate LSSLs, planar-resonant-tunneling field-effect transistors (PRESTFETs), multiple parallel quantum wires (MPQWs), and arrays of quantum dots (QDs). In contrast to conventional, epitaxially grown vertical quantum structures, planar structures offer the opportunity for electron confinement in three, two, and one dimensions and the flexibility of electrical tuning of quantum effects     

Optoelectronic devices based on lateral p-n junctions fabricated bymolecular-beam epitaxy growth of silicon-doped GaAs on patterned(311)A-oriented substrates

We demonstrate light-emitting diodes, a vertical-cavity surface-emitting laser (VCSEL), and a photodiode fabricated using a lateral p-n junction. The lateral p-n junction is formed in a GaAs-silicon doped layer grown by molecular-beam epitaxy on a patterned GaAs (311)A-oriented substrate. Lateral p-n junctions have particular properties (i.e., small junction area, coplanar contact geometry, can be clad between electrically insulating layers, allow carrier transport in the plane of multilayer structures, etc.) that are promising for application in new devices. Light-emitting diodes exhibit good electroluminescence at room temperature for both GaAs single layers and GaAs-AlGaAs multiple-quantum-well structures. The VCSEL has electrically insulating distributed Bragg reflectors and coplanar contacts which simplify the device fabrication process. Pulsed-mode operation at room temperature was obtained with a threshold current of 2.3 mA. The light-emission spectrum has a single peak at 942 nm with a full-width at half-maximum of 0.15 nm. The photodiode design allows a reduction of the junction capacitance and an increase of the response speed. A nonoptimized device exhibited a time constant of 10 ps     

Possibility of room temperature intra-band lasing in quantum dot structures placed in high-photon density cavities

We study the possibility of room temperature intraband lasing in quantum dots placed in high-photon density cavities. In general, if intra-band population inversion is to created in a quantum well by carrier injection at the barrier energy, it is necessary that the electron intra-band energy relaxation times are long. Additionally the bandedge electron-hole recombination times should be short. The use of sub-two-dimensional structures (quantum dots) allows us to increase the intra-band energy relaxation time from about a picosecond for bulk or quasi-two-dimensional systems to several hundred picoseconds at room temperatures. Also, by placing these structures in a high coherent photon density optical cavity, it is possible to decrease the bandedge electron-hole recombination times through stimulated emission. Our studies show that strong population inversion is possible at room temperature in such systems. Gain versus injection curves are also calculated.     

Uniform wafer-bonded oxide-confined bottom-emitting 850-nm VCSELarrays on sapphire substrates

Uniform bottom-emitting 850-nm vertical-cavity surface-emitting laser (VCSEL) arrays on sapphire substrates have been demonstrated using wafer bonding technology to transfer the epitaxially-grown VCSEL structures from GaAs substrates onto sapphire substrates. The uniformity of the VCSEL arrays were improved by placing thin oxide aperture at the standing wave node to reduce scattering loss for small aperture devices. The averaged threshold current of a 5×5 VCSEL array is as low as 346 μA, while the averaged external quantum efficiency approaches 57%. The maximum wall-plug efficiency is 25% and the single-mode output power is more than 2 mW under continuous-wave current excitation at room temperature. We have also demonstrated a large (10×20) VCSEL array with variations of threshold current and external quantum efficiency less than 4% and 2%, respectively     

Low-threshold patterned quantum well lasers grown by molecular beamepitaxy

GaAs/AlGaAs patterned quantum well lasers were grown by molecular beam epitaxy on grooved substrates. The carrier confinement and the real-index waveguiding in these lasers rely on lateral thickness variations in the quantum well active layer. Very low threshold currents, as low as 1.8 mA for uncoated devices at room temperature, with 63% differential efficiency have been obtained     

Self-Consistent SchrÖdinger–Poisson Simulations on Capacitance–Voltage Characteristics of Silicon Nanowire Gate-All-Around MOS Devices With Experimental Comparisons

We simulate room temperature capacitance-voltage characteristics of silicon (Si) nanowire gate-all-around MOS structures with radius les 10 nm using a self-consistent Schrodinger- Poisson solver in cylindrical coordinates with full treatment of the transverse quantum confinement. In this paper, we compare our simulation results with the latest capacitance measurements on single Si nanowire pMOS and nMOS devices in the subfemtofarad range. We also propose to probe the density-of-states features of the Si channel from the capacitance-voltage characteristics at room temperature measurements using dC/dV dependence and illustrate the idea by employing the latest measurements, our quantum and Medici (Synopsys) simulations, as well as a simplified analytical model.     

Encoding Information on the Excited State of a Molecular Spin Chain

The quantum states of nano-objects can drive electrical transport properties across lateral and local-probe junctions. This raises the prospect, in a solid-state device, of electrically encoding information at the quantum level using spin-flip excitations between electron spins. However, this electronic state has no defined magnetic orientation and is short-lived. Using a novel vertical nanojunction process, these limitations are overcome and this steady-state capability is experimentally demonstrated in solid-state spintronic devices. The excited quantum state of a spin chain formed by Co phthalocyanine molecules coupled to a ferromagnetic electrode constitutes a distinct magnetic unit endowed with a coercive field. This generates a specific steady-state magnetoresistance trace that is tied to the spin-flip conductance channel, and is opposite in sign to the ground state magnetoresistance term, as expected from spin excitation transition rules. The experimental 5.9 meV thermal energy barrier between the ground and excited spin states is confirmed by density functional theory, in line with macrospin phenomenological modeling of magnetotransport results. This low-voltage control over a spin chain's quantum state and spintronic contribution lay a path for transmitting spin wave-encoded information across molecular layers in devices. It should also stimulate quantum prospects for the antiferromagnetic spintronics and oxides electronics communities.     

Recent results on metalorganic vapor phase epitaxially grown HgCdTe heterostructure devices

The ability to grow complex multilayer structures in Hg_1-xCd_xTe by epitaxial techniques has made it possible to produce a range of new devices such as infrared LEDs, lasers, and two-color infrared detector arrays. The devices described here, however, are designed to operate at temperatures above 145K and include both infrared sources and detectors. Three layer ppn structures, where the underlined symbols mean wider gap, have close to Auger limited R_oAs at temperatures above 145K. Under reverse bias, the devices exhibit Auger suppression leading to useful detectivities at room temperature. The diodes exhibit forward biased electroluminescence at room temperature although the efficiency of this emission is found to fall rapidly as the peak wavelength is increased toward 9 μm due to increased Auger recombination rates. By reverse biasing them, however, the devices show negative luminescence as a result of reducing the electron and hole densities below their thermal equilibrium value. The diode emitters have a higher quantum efficiency when used in this mode due to Auger suppression of the dark current.     

Step and flash imprint lithography for quantum dots based room temperature single electron transistor fabrication

In this work we demonstrate the successful fabrication using step and flash imprint lithography – reverse tone (SFIL-R)? coupled with a novel Focus ion beam (FIB) quantum dot (QD) deposition technique to produce of a full array of room temperature single electron transistors (RT-SET) based on tungsten quantum dot arrays. The integration of SFIL-R and FIB technology process flow has been developed in order to explore the possibility of an ultra low power, monolithically integrated nano-electronics circuits using RT-SET. We describe the parallel production of RT-SET devices using SFIL-R. The yield of the mass produced devices are examined. These QD based devices are characterized and initial results are evaluated.     

Applications of colloidal quantum dots

This paper addresses a number of major trends underlying the continuing effort to realize practical optoelectronic, electronic, and information-processing devices based on ensembles of quantum dots assembled in a variety of matrix materials. The great diversity of such structures makes it possible to fabricate numerous ensemble-based devices for applications underlying photoluminescent devices, light-emitting diodes, displays, photodetectors, photovoltaic devices, and solar cells. In addition, the application of colloidal quantum dots to allied technologies such as nanobiotechnology is considered for the case of monitoring conformational changes in biomolecules using luminescent quantum dots.     

Numerical Simulation of ZnO-Based Terahertz Quantum Cascade Lasers

In this work, a particle-based Monte Carlo model is used to quantify the potential of terahertz sources based on the ZnO-based material system relative to existing devices based on GaAs/AlGaAs quantum wells. Specifically, two otherwise identical quantum cascade structures based on ZnO/MgZnO and GaAs/AlGaAs quantum wells are designed, and their non- equilibrium carrier distributions are then computed as a function of temperature. The simulation results show that, because of their larger optical phonon energy, ZnO/MgZnO quantum cascade laser structures exhibit weaker temperature dependence of the population inversion than in the case of similar structures made of GaAs/AlGaAs. In particular, as the temperature is increased from 10 K to 300 K, population inversion is found to decrease by a factor of 4.48 and 1.50 for the AlGaAs and MgZnO structure, respectively. Based on these results, the MgZnO devices are then predicted to be, in principle, capable of laser action without cryogenic cooling.     

Controlling Phase-Coherent Electron Transport in III-Nitrides: Toward Room Temperature Negative Differential Resistance in AlGaN/GaN Double Barrier Structures

Resonant tunneling of electrons is important for the manufacture of high-speed electronic oscillators and the electron injection control in quantum cascade lasers. In this work, room temperature negative differential resistance (NDR) in AlGaN/GaN double barrier structure with AlN/GaN digital alloy (DA) barriers is demonstrated. The peak-to-valley current ratio (PVCR) ranges from 1.1 to 1.24 at room temperature and becomes 1.5 to 2.96 at low temperatures, whereas no NDR is observed in double barrier structures with conventional ternary AlGaN barriers. The room temperature NDR together with the high PVCR at low temperature is attributed to the suppression of alloy disorder scattering by introducing AlN/GaN DA barriers. This work presents the successful control of phase-coherent electron transport in III-nitride heterostructures and is expected to benefit the future design of nitride-based resonant tunneling structures and high-speed electronic devices.     

High‐Quality Whispering‐Gallery‐Mode Lasing from Cesium Lead Halide Perovskite Nanoplatelets

Semiconductor micro/nano‐cavities with high quality factor (Q) and small modal volume provide critical platforms for exploring strong light‐matter interactions and quantum optics, enabling further development of coherent and quantum photonic devices. Constrained by exciton binding energy and thermal fluctuation, only a handful of wide‐band semiconductors such as ZnO and GaN have stable excitons at room temperature. Metal halide perovskite with cubic lattice and well‐controlled exciton may provide solutions. In this work, high‐quality single‐crystalline cesium lead halide CsPbX₃ (X = Cl, Br, I) whispering‐gallery‐mode (WGM) microcavities are synthesized by vapor‐phase van der Waals epitaxy method. The as‐grown perovskites show strong emission and stable exciton at room temperature over the whole visible spectra range. By varying the halide composition, multi‐color (400–700 nm).WGM excitonic lasing is achieved at room temperature with low threshold (~ 2.0 μJ cm^?2) and high spectra coherence (~0.14–0.15 nm). The results advocate the promise of inorganic perovskites towards development of optoelectronic devices and strong light‐matter coupling in quantum optics.     

Observation and Modeling of a Room-Temperature Negative Characteristic Temperature 1.3-μm p-Type Modulation-Doped Quantum-Dot Laser

A room-temperature negative characteristic temperature (T₀ ) and ultralow threshold current density (J_th) of 48 Amiddotcm^-2 are demonstrated for a 1.3-mum InAs quantum dot laser. These characteristics are obtained by combining a high-growth-temperature GaAs spacer layer with p-type modulation doping of the quantum dots in multiple layer dot-in-a-well structures. Through a comparison of p-doped and undoped devices, a photon coupling mechanism is proposed to account for the different temperature dependences of Jth for the two devices. Numerical simulations based on a rate equation model, which includes photon coupling between ground and excited quantum dot states, are performed. The simulations are able to account for the very different temperature-dependent Jth behavior of the doped and undoped device     

Large room-temperature magnetoresistance in lateral organic spin valves fabricated by in situ shadow evaporation

We report the successful fabrication of lateral organic spin valves with a channel length in the sub 100 nm regime. The fabrication process is based on in situ shadow evaporation under UHV conditions and therefore yields clean and oxygen-free interfaces between the ferromagnetic metallic electrodes and the organic semiconductor. The spin valve devices consist of Nickel and Cobalt–Iron electrodes and the high mobility n-type organic semiconductor N,N^′-bis(heptafluorobutyl)-3,4:9,10-perylene diimide. Our studies comprise fundamental investigations of the process’ and materials’ suitability for the fabrication of lateral spin valve devices as well as magnetotransport measurements at room temperature. The best devices exhibit a magnetoresistance of up to 50%, the largest value for room temperature reported so far.