Quantum-Dot Optoelectronic Devices

Self-organized In(Ga)As/Ga(Al)As quantum dots have emerged as useful nanostructures that can be epitaxially grown and incorporated in the active region of devices. The near pyramidal dots exhibit properties arising from the three-dimensional quantum confinement and from the coherent built-in strain. The properties and current state-of-the-art characteristics of quantum-dot junction lasers, intersublevel infrared detectors, optical amplifiers, and microcavity devices are briefly reviewed. It is evident that self-organized quantum-dot optoelectronic devices demonstrate properties that are sometimes unique and often surpass the characteristics of existing devices.     

Room temperature far infrared (8~10 μm) photodetectors usingself-assembled InAs quantum dots with high detectivity

Room temperature operation of far-infrared detectors made of self-assembled quantum dots embedded in the channel region of modulation-doped heterostructures is demonstrated. At room temperature, the detector shows a low dark current ranging in the nano-amperes at a bias voltage of 10 V. After the optimization of the separation between the quantum dot region and the 2DEG, a peak responsivity of 5.3 A/W is obtained at 9.0 μm. The high detectivities of 6×10⁸ and 5×10¹⁰ cmHz^1/2/W are obtained at room temperature and 80 K, respectively     

Self-assembled InAs-GaAs quantum-dot intersubband detectors

The use of self-assembled InAs-GaAs quantum dots in photoconductive intersubband detectors in the far-infrared is presented. Far-infrared absorption is observed in self-assembled quantum dots in the 6-18-μm range for subband-subband and subband-continuum transitions. Photoconductive quantum-dot intersubband detectors were fabricated and demonstrate tunable operating wavelengths between 6-18 μm using subband-subband or subband-continuum transitions. The use of AlAs barriers allows further tuning to shorter wavelengths of 3-7 μm. Subband-continuum quantum dot intersubband detectors show encouraging normal incidence performance characteristics at T=40 K, with responsivities of 10-100 mA/W, detectivities of 1-10 ×10⁹ cm·Hz^1/2/W and large photoconductive gain up to g=12 for a ten-layer quantum-dot heterostructure. With improvements in device structure, self-assembled quantum dots can be expected to provide intrinsic normal incidence broad-band detectors with advantages over quantum wells     

640 × 512 Pixels Long-Wavelength Infrared (LWIR) Quantum-Dot Infrared Photodetector (QDIP) Imaging Focal Plane Array

Epitaxially grown self-assembled InAs-InGaAs-GaAs quantum dots (QDs) are exploited for the development of large-format long-wavelength infrared focal plane arrays (FPAs). The dot-in-a-well (DWELL) structures were experimentally shown to absorb both 45deg and normal incident light, therefore, a reflection grating structure was used to enhance the quantum efficiency. The devices exhibit peak responsivity out to 8.1 mum, with peak detectivity reaching ~1times10¹⁰ Jones at 77 K. The devices were fabricated into the first long-wavelength 640times512 pixel QD infrared photodetector imaging FPA, which has produced excellent infrared imagery with noise equivalent temperature difference of 40 mK at 60-K operating temperature     

Structural and electrooptical characteristics of quantum dotsemitting at 1.3 μm on gallium arsenide

We present a comprehensive study of the structural and emission properties of self-assembled InAs quantum dots emitting at 1.3 μm. The dots are grown by molecular beam epitaxy on gallium arsenide substrates. Room-temperature emission at 1.3 μm is obtained by embedding the dots in an InGaAs layer. Depending on the growth structure, dot densities of 1-6×10¹⁰ cm^-2 are obtained. High dot densities are associated with large inhomogeneous broadenings, while narrow photoluminescence (PL) linewidths are obtained in low-density samples. From time-resolved PL experiments, a long carrier lifetime of ≈1.8 ns is measured at room temperature, which confirms the excellent structural quality. A fast PL rise (τ_rise=10±2 ps) is observed at all temperatures, indicating the potential for high-speed modulation. High-efficiency light-emitting diodes (LEDs) based on these dots are demonstrated, with external quantum efficiency of 1% at room temperature. This corresponds to an estimated 13% radiative efficiency. Electroluminescence spectra under high injection allow us to determine the transition energies of excited states in the dots and bidimensional states in the adjacent InGaAs quantum well     

Low-threshold current density 1.3-μm InAs quantum-dot laserswith the dots-in-a-well (DWELL) structure

The wavelength of InAs quantum dots in an In_0.15Ga_0.85As quantum-well (DWELL) lasers grown on a GaAs substrate has been extended to 1.3-μm. The quantum dot lasing wavelength is sensitive to growth conditions and sample thermal history resulting in blue shifts as much as 73 nm. The room temperature threshold current density is 42.6 A cm^-2 for 7.8-mm cavity length cleaved facet lasers under pulsed operation     

Influence of Injector Doping Concentration on the Performance of InP-Based Quantum-Cascade Lasers

The influence of injector doping concentration N_inj on the performance of InP-based quantum-cascade (QC) lasers is investigated for devices emitting around 9.2-mum wavelength and injector doping concentrations between 1times10¹⁷ cm^-3 and 3times10¹⁷ cm^-3. The threshold current density, the dynamic range, the maximum emitted output power as well as the maximum operating temperature are found to increase with increasing N _inj. All in all, there exists no optimal value of N_inj per se. In fact, the injector doping concentration has to be adjusted individually depending whether emphasis is placed on obtaining low threshold current densities or on high-power operation     

Room Temperature Near-Infrared Photoresponse Based on Interband Transitions in In0.35Ga0.65As Multiple Quantum Dot Photodetector

Near-infrared photoresponse is observed in the temperature range of 77-300 K for a photodetector fabricated from undoped In_0.35Ga_0.65As/GaAs multiple quantum dots grown in a molecular beam epitaxy system. The detectivity is estimated to be on the order of 3.70 times 10⁹ and 2.70 X 10⁷ cm .radicHz/W at 77 and 300 K, respectively. The reduction of the detectivity is attributed to the increase of the dark current as the temperature is increased. The photoresponse is explained in terms of several interband transitions. These transitions are found to be in good agreement with the self-consistent theoretical calculations.     

Intersubband gain and stimulated emission in long-wavelength(λ=13 μm) intersubband In(Ga)As-GaAs quantum-dotelectroluminescent devices

The dynamics of injected carriers and the conditions for intersubband gain and population inversion in In(Ga)As-GaAs self-organized quantum dots have been studied. Direct femtosecond pump-probe spectroscopy as a function of temperature and excitation density confirms earlier results and shows a long (>100 ps) electron relaxation time between the excited states and ground state in the dots. Intersubband gains as high as 170 cm^-1 are calculated in the dots. Far-infrared spontaneous emission centered around 13 μm is observed in edge-emitting light-emitting diodes. Stimulated emission, with a distinct threshold around 1.1 kA/cm² in the light-current characteristics, is observed in plasmon-enhanced waveguide devices. The intersubband threshold occurs after a threshold is observed for interband lasing (~1 μm) in the same device     

High-performance self-aligned (Al,Ga)As/(In,Ga)As pseudomorphicHIGFETs

Transconductance as high as 676 mS/mm at 300 K was observed to 0.7×10-μm² n-channel devices (HIGFETs) made on epilayers with Al_0.3Ga_0.7As insulator thickness of 200 Å and In_0.15Ga_0.85As channel thickness of 150 Å. An FET K value (K=W_g Uε/2aL_g) as large as 10.6 mA/V ² was also measured from another device with transconductance of 411 mS/mm. The high K values are achieved under normal FET operation without hole-injection or drain-avalanche breakdown effects. These results demonstrate the promise of pseudomorphic (Al,Ga)As/(In,Ga)As HIGFETs for high-performance circuit applications     

In(Ga)As/GaAs self-organized quantum dot lasers: DC andsmall-signal modulation properties

Self-organized growth of InGaAs/GaAs strained epitaxial layers gives rise to an ordered array of islands via the Stranski-Krastanow growth mode, for misfits >1.8%. These islands are pyramidal in shape with a base diagonal of ~20 nm and height of ~6-7 nm, depending of growth parameters. They therefore exhibit electronic properties of zero-dimensional systems, or quantum dots. One or more layers of such quantum dots can be stacked and vertically coupled to form the gain region of lasers. We have investigated the properties of such single-layer quantum dot (SLQD) and multilayer quantum dot (MLQD) lasers with a variety of measurements, including some at cryogenic temperatures. The experiments have been complemented with theoretical calculations of the electronic properties and carrier scattering phenomena in the dots. Our objective has been to elucidate the intrinsic behavior of these devices. The lasers exhibit temperature independent threshold currents up to 85 K, with T₀⩽670 K. Typical threshold currents of 200-μm long room temperature lasers vary from 6 to 20 mA. The small-signal modulation bandwidths of ridge waveguide lasers are 5-7.5 GHz at 300 K and increased to >20 GHz at 80 K. These bandwidths agree well with electron capture times of ~30 ps determined from high-frequency laser impedance measurements at 300 K and relaxation times of ~8 ps measured at 18 K by differential transmission pump-probe experiments. From the calculated results we believe that electron-hole scattering intrinsically limits the high-speed performance of these devices, in spite of differential gains as high as ~7×10^-14 cm² at room temperature     

Self-organized anisotropic strain engineering: a new concept for quantum dot ordering

We have established a new concept for creating ordered arrays of quantum dots by self-organized epitaxy. The concept is based on self-organized anisotropic strain engineering of strained layer templates and is demonstrated for (In,Ga)As/GaAs superlattice structures on GaAs (100) and strain-induced (In,Ga)As growth instability on GaAs (311)B. The well-defined one- and two-dimensional networks of InAs quantum dots grown on top of these templates are of excellent structural and optical quality. This breakthrough, thus, allows for novel fundamental studies and device operation principles based on single and multiple carrier- and photon-, and coherent quantum interference effects.     

Atomic vapor quantum counter: Narrow-band infrared upconverter

A novel device for the upconversion of narrow-band infra-red (IR) radiation in the 1.5-20-µm spectral region is described. The atomic-vapor quantum counter (AVQC) takes advantage of the efficient incoherent upconversion of narrow-band IR radiation in an optically excited atomic vapor. In a 300 K background and a 2π-sr field of view, a D^*is projected that is two orders of magnitude greater than that of conventional semiconductor devices. The device offers the extremely high Q's (>10⁵) associated with heterodyne detection while simultaneously offering a broad tuning range and an unlimited collection aperture. The anticipated performance characteristics of the AVQC are discussed, Experimental results in sodium and potassium have demonstrated the high Q and the tunability of the device. Electric field tuning of the wavelength response to CO2laser wavelengths spanning 23 cm^-1has been accomplished. Applications of the AVQC involving the detection of low level laser radiation and molecular emission are discussed.     

High-Performance Quantum Dot Lasers and Integrated Optoelectronics on Si

This paper provides a review of the recent developments of self-organized In(Ga)As/Ga(Al)As quantum dot lasers grown directly on Si, as well as their on-chip integration with Si waveguides and quantum-well electroabsorption modulators. A novel dislocation reduction technique, with the incorporation of self-organized In(Ga,Al)As quantum dots as highly effective three-dimensional dislocation filters, has been developed to overcome issues associated with the material incompatibility between III-V materials and Si. With the use of this technique, quantum dot lasers grown directly on Si exhibit relatively low threshold current (J _th=900 A/cm²) and very high temperature stability (T ₀=278 K). Integrated quantum dot lasers and quantum-well electroabsorption modulators on Si have been achieved, with a coupling coefficient of more than 20% and a modulation depth of ~100% at a reverse bias of 5 V. The monolithic integration of quantum dot lasers with both amorphous and crystalline Si waveguides, fabricated using plasma-enhanced chemical-vapor deposition and membrane transfer, respectively, has also been demonstrated.     

Wide operating wavelength range and low threshold currentIn0.24Ga0.76As/GaAs vertical-cavitysurface-emitting lasers

Very low threshold current density operation of triple quantum well vertical-cavity surface-emitting lasers (VCSELs) is reported. The active wells are strained In_0.24Ga_0.76As in GaAs. Devices from the same wafer operate at room temperature over a wavelength range of 958-1042 nm, with a minimum threshold current density of 366 A-cm^-2 at 1018 nm. The dependence of threshold current on wavelength gives an insight into the optical gain spectrum of the quantum wells. It was shown that 50-μm-diameter devices operate CW without heatsinking     

InAs/InGaAs/GaAs quantum dot lasers of 1.3 μm range with high(88%) differential efficiency

Multiple layers (up to 10) of InAs/InGaAs/GaAs quantum dots considerably enhance the optical gain of quantum dot lasers emitting around 1.3 μm. A differential efficiency as high as 88% has been achieved in these lasers. An emission wavelength of 1.28 μm, threshold current density of 147 A/cm², differential efficiency of 80%, and characteristic temperature of 150 K have been realised simultaneously in one device     

Electrical properties of Ga-implanted Si p+-n shallowjunctions fabricated by low-temperature rapid thermal annealing

p⁺-n shallow-junction diodes were fabricated using on-axis Ga⁶⁹ implantation into crystalline and preamorphized Si, at energies of 25-75 keV for a dose of 1×10¹⁵/cm ², which is in excess of the dosage (2×10¹⁴/cm²) required to render the implanted layer amorphous. Rapid thermal annealing at 550-600°C for 30 s resulted in the solid-phase epitaxial (SPE) regrowth of the implanted region accompanied by high Ga activation and shallow junction (60-130 nm) formation. Good diode electrical characteristics for the Ga implantation into crystalline Si were obtained; leakage current density of 1-1.5 nA/cm² and ideality factor of 1.01-1.03. Ga implantation into preamorphized Si resulted in a two to three times decrease in sheet resistance, but a leakage current density orders of magnitude higher     

Electrical properties of Ge:Ga near the metal-insulator transition

The results of the metal-insulator transition (MIT) induced by impurity concentration are presented in the case of metallic and insulating samples ⁷⁰Ge:Ga p-type. The eight samples studied have Ga concentrations N ranging from 1.848 × 10¹⁷ to 1.912 × 10¹⁷cm^-3. The conductivity measurements were carried out at low temperature in the range 1 to 0.019 K. We provide physical explanations to explain the behaviors of the temperature dependence of the electrical conductivity in both sides of the MIT. The data are for a ⁷⁰Ge:Ga sample prepared and reported by Itoh et al. in Ref. [Itoh K M, Watanabe M, Ootuka Y, et al. J Phys Soc Jpn, 2004, 73(1): 173].     

Electrically pumped lasing from CdSe quantum dots

Fabrication of a ZnSe-based laser diode which employs a fivefold CdSe quantum dot stack separated by ZnSSe spacer layers of high S content is reported. For the first time, electrically pumped room-temperature lasing from such quantum dots was obtained at a wavelength around 560.5 nm. The threshold current density is 7.5 kA/cm ²     

Radiation effects in transit-time microwave diodes

There has been considerable progress in the direct generation of microwave power using two-terminal semiconductor devices during the last decade. Permanent and transient radiation effects on bulk (Gunn and LSA) and junction (IMPATT, TRAPATT, and BARITT) transit-time microwave diodes are reviewed. Emphasis is placed upon relating the primary effects of radiation to the physics of device operation. The principal permanent damage is attributed to carrier removal effects, impairing the RF performance of bulk diodes below 10¹⁴neutrons/cm²and junction transit-time diodes at fluences near 10¹⁵neutrons/cm². The principal transient effect is the generation of free carriers by ionizing radiation, affecting the RF performance of bulk diodes above 10⁹rad/s and junction transit-time diodes at dose rates near 10⁸rad/s.