Coulomb enhancement in InGaAs-GaAs quantum-well lasers

It is shown that Coulomb enhancement (CE) has a significant influence on the spectral characteristics of optical gain and spontaneous emission in strained InGaAs quantum wells. CE-modified gain spectra are utilized to make an accurate prediction of the dependence of lasing wavelength on cavity length, Threshold-current predictions using the CE-modified gain-current relation show improved agreement with experiment     

Charge neutrality violation in quantum-dot lasers

Theory of quantum-dot (QD) lasers is augmented to include, in a self-consistent manner, the QD-layer charge. The electron- and hole-level occupancies in QDs are obtained through the solution of the problem for the electrostatic-field distribution across the junction. They are shown to differ from each other. As a result, the local neutrality is broken down in each QD, i.e., the QD layer is charged. The key dimensionless parameters controlling the difference of the hole- and electron-level occupancies are revealed. The detailed analysis of the gain and spontaneous radiative recombination current density is given, having regard to the fact of violation of the charge neutrality in QDs. The gain-current density dependence is calculated, The voltage dependences of the electron- and hole-level occupancies, gain, and current density are obtained. Particular emphasis is given to the transparency and lasing threshold characteristics. Optimization of the QD-laser structure is carried out. The optimum surface density of QDs, minimizing the threshold current density, is shown to be distinctly higher than that calculated without regard for the lack of the charge neutrality in QDs     

Modification of modal gain in InGaAs-GaAs quantum-well lasers dueto barrier-state carriers

This paper describes the effects of barrier-state carriers on the modal gain of InGaAs-GaAs quantum-well (QW) lasers emitting at 980 nm. Experimental studies and numerical simulations are used to examine several drive configurations, each having a unique effect on the laser response. These include compound drive current shapes, optical excitations and fast electrical drives with rise times shorter than 100 ps. We demonstrate that a large barrier-state carrier density affects the index of refraction sufficiently so as to cause a reduction in the confinement factor and modal gain which is large enough to turn the laser off     

InP-GaInP quantum-dot lasers emitting between 690-750 nm

We describe the growth, material characterization, and device characterization of InP-GaInP quantum-dot lasers for operation in the wavelength range 690-750 nm. We show that the growth conditions have a major influence on the form of the gain spectrum. Relatively flat gain can be achieved over a spectral width of 90 nm at 300 K using samples containing a bimodal distribution of dot sizes, or narrower gain spectra at shorter wavelength can be achieved by suppressing the bimodal distribution by using (211)B substrates. Optimization of samples grown on substrates with the growth surface of (100) misorientated by 10/spl deg/ toward [111] results in laser operation between 729 and 741 nm and with a room temperature threshold current density as low as 190 A/spl middot/cm/sup -2/ for a 2000-/spl mu/m-long device with uncoated facets.     

Lasing characteristics of self-formed quantum-dot lasers withmultistacked dot layer

Room-temperature continuous-wave (CW) operation at the ground state has been achieved in self-formed quantum-dot lasers with multistacked dot layer. By systematic investigation, discontinuous shifts of lasing wavelength from the high-order subbands to the ground state are clearly demonstrated for the first time by varying the number of dot layers and the cavity loss. Lasers oscillating at different subbands exhibit different behaviors against temperature both in the spectral characteristics and in the threshold currents, which are strongly related to emission efficiency of quantum dots and thermal excitation of carriers to higher order subbands. High characteristic temperature over 300 K has been achieved in a laser with high-reflection coating on both facets in the temperature range 60-200 K. Future prospects of improvement in the laser characteristics are also discussed     

Long-wavelength quantum-dot lasers on GaAs substrates: from media to device concepts

Recent progress in semiconductor quantum-dot (QD) lasers approaches qualitatively new levels, when dramatic progress in the development of the active medium already motivates search for new concepts in device and system designs. QDs, which represent coherent inclusions of narrower bandgap semiconductor in a wider gap semiconductor matrix, offer a possibility to extend the wavelength range of heterostructure lasers on GaAs substrates to 1.3 /spl mu/m and beyond and create devices with dramatically improved performance, as compared to commercial lasers on InP substrates. Low-threshold current density (100 A/cm/sup 2/), very high characteristic temperature (170 K up to 65/spl deg/C), and high differential efficiency (85%) are realized in the same device. The possibility to stack QDs (e.g., tenfold) without an increase in the threshold current density and any degradation of the other device parameters allow realization of high modal gain devices suitable for applications in 1.3-/spl mu/m short-cavity transmitters and vertical-cavity surface-emitting lasers (VCSELs). The 1.3-/spl mu/m QD GaAs VCSELs operating at 1.2-mW continuous-wave output power at 25/spl deg/C are realized, and long operation lifetime is manifested. Evolution of GaAs-based 1.3-/spl mu/m lasers offers a unique opportunity for telecom devices and systems. Single-epitaxy VCSEL vertical integration with intracavity electrooptic modulators for lasing wavelength adjustment and/or ultrahigh-frequency wavelength modulation is possible. Arrays of wavelength-tunable VCSELs and wavelength-tunable resonant-cavity photodetectors may result in a new generation of "intelligent" cost-efficient systems for ultrafast data links in telecom.     

Recombination and loss mechanisms in low-threshold InAs-GaAs 1.3-/spl mu/m quantum-dot lasers

We show that even in quantum-dot (QD) lasers with very low threshold current densities (J/sub th/=40--50 A/cm/sup 2/ at 300 K), the temperature sensitivity of the threshold current arises from nonradiative recombination that comprises /spl sim/60% to 70% of J/sub th/ at 300 K, whereas the radiative part of J/sub th/ is almost temperature insensitive. The influence of the nonradiative recombination mechanism decreases with increasing hydrostatic pressure and increasing band gap, which leads to a decrease of the threshold current. We also studied, for the first time, the band gap dependence of the radiative part of J/sub th/, which in contrast increases strongly with increasing band gap. These results suggest that Auger recombination is an important intrinsic recombination mechanism for 1.3-/spl mu/m lasers, even in a very low threshold QD device, and that it is responsible for the temperature sensitivity of the threshold current.     

High-speed modulation and switching characteristics ofIn(Ga)As-Al(Ga)As self-organized quantum-dot lasers

The dynamic characteristics, and in particular the modulation bandwidth, of high-speed semiconductor lasers are determined by intrinsic factors and extrinsic parameters. In particular, carrier transport through the heterostructure and thermalization, or quantum capture in the gain region, tend to play an important role. We have made a detailed study of carrier relaxation and quantum capture phenomena in In(Ga)As-Al(Ga)As self-organized quantum dots (QD's) and single-mode lasers incorporating such dots in the gain region through a variety of measurements. The modulation bandwidth of QD lasers is limited to 5-6 GHz at room temperature and increases to ~30 GHz only upon lowering the temperature to 100 K. This behavior is explained by considering electron-hole scattering as the dominant mechanisms of electron relaxation in QD's and the scattering rate seems to decrease with increase of temperature. The switching of the emission wavelength, from the ground state to an excited state, has been studied in coupled cavity devices. It is found that the switching speed is determined intrinsically by the relaxation time of carriers into the QD states. Fast switching from the ground to the excited state transition is observed. The electrooptic coefficients in the dots have been measured and linear coefficient τ=2.58×10^-11 m/V. The characteristics of electrooptic modulators (EOM's) are also described     

Performance and physics of quantum-dot lasers with self-assembledcolumnar-shaped and 1.3-μm emitting InGaAs quantum dots

This paper reports recent developments of our self-assembled InGaAs quantum-dot (QD) lasers and their unique physical properties. We achieved a low-threshold current of 5.4 mA at room temperature with our originally designed columnar-shaped QD's, and also, room-temperature 1.3-μm continuous-wave (CW) lasing with self-assembled dots grown at a decreased growth rate and covered by a strained InGaAs layer. We discuss influence of homogeneous broadening of single-dot optical gain on lasing spectra, influence of nonradiative carrier recombination on temperature characteristics of threshold currents, a model for the origin of the homogeneous broadening, a finding of random telegraph signals, and suppression of temperature sensitivity of interband emission energy by covering dots with a strained InGaAs layer     

Progress in InGaAs-GaAs selective-area MOCVD toward photonicintegrated circuits

The progress toward integrated photonic devices by selective-area metalorganic chemical vapor deposition (MOCVD) is reviewed. Processing steps involved with fabricating buried heterostructures (BHs) by a three-step technique are outlined, and a computational model is presented that predicts the enhancement behavior of selective-area MOCVD. Results are reviewed for several discrete and integrated photonic devices. These include low-threshold BH lasers, laser diodes integrated with either intracavity or external cavity modulators, dual-channel emitters integrated with both modulators and passive y-junction waveguides, and broad-band light-emitting diodes (LEDs)     

Development of IR-emitting colloidal II-VI quantum-dot materials

The current state-of-the-art of colloidal II-VI nanocrystal formation using the aqueous/thiol synthesis route is described. Work on single component and heterostructures and mixed compound quantum dots is discussed. The purpose of the work is to provide a range of infrared (IR)-emitting materials with high quantum efficiency (QE) as potential gain media for future ultrawideband optical amplifiers for high-capacity wavelength-division multiplexing (WDM) telecommunications systems. Physical and chemical factors influencing particle sizes are described     

Light scattering in semiconductor quantum-dot cavity systems

Scattering theory is used to investigate the interaction of a coherent light beam with two distant one-sided semiconductor double micro-cavities. Each cavity contains a single quantum dot charged by one extra electron. The polarization and phase shift of the scattered light is studied as a function of the initial spin state of the two electrons, as well as differences in structural properties of the two cavities. It is shown that the Faraday rotation of the transmitted light is sensitive to electronic and structural properties of the cavities making careful calibration mandatory, when entanglement generation between the electron spin states is to be achieved.     

A signal distribution grid for quantum-dot cellular automata

Coplanar wire crossing has been a major challenge for quantum-dot cellular automata systems since their development. Several possible solutions have been presented, but they have either relied on non-adjacent cell interactions or have required switching time that scales with the number of inputs or outputs. In this paper, the authors present a signal distribution grid that enables multiple parallel crossings, while doing so with only adjacent cell interactions, a constant time for signal distribution regardless of the number of inputs or outputs, and regularly shaped and contiguous clocking regions that will be relatively easier to fabricate. The utility of this device is demonstrated by the design of a one-bit full adder that meets all of the listed requirements.     

A novel quantum-dot cellular automata CLB of FPGA

Quantum-dot cellular automata (QCA) is a promising, emerging nano-technology based on single electron effects in quantum dots and molecules. This paper presents design, implementation and simulation of a configurable logic block for a field programmable gate arrays (FPGA) by QCA. Previous works focus on QCA-based FPGA that have fixed logic and programmable interconnection or programmable logic and fixed interconnection; however, proposed structures in this paper have programmable logic and programmable interconnection. The presented look-up table implemented with novel structure which has been allowed as frequently as the read/write operation occurs, also acts as a pipeline. In this paper, we presented novel decoders and multiplexers and implemented with QCA, designed with the minimum number of majority gates and cells. Finally, a new configurable logic block (CLB) is designed, implemented and simulated in the QCA, which used signal distribution network method to avoid the coplanar problem of crossing wires. Also, QCADesigner software is used for detailed layout and QCADesigner attend with HDLQ verilog are used for circuit simulation. The proposed CLB is simulated with programming by the QCADesigner software. The area and delay of QCA-based CLB presented in this paper compared to the CLB based on CMOS, nanomaterial and CNT (32 nm). Results show that proposed CLB will do the task with a minimum clock and can be configured as a FPGA.     

Single-mode distributed feedback and microlasers based on quantum-dot gain material

Quantum-dot gain material fabricated by self-organized epitaxial growth on GaAs substrates is used for the realization of 980-nm and 1.3-/spl mu/m single-mode distributed feedback (DFB) lasers and edge-emitting microlasers. Quantum-dot specific properties such as low-threshold current, broad gain spectrum, and low-temperature sensitivity could be demonstrated on ridge waveguide and DFB lasers in comparison to quantum-well-based devices. 980-nm DFB lasers exhibit stable single-mode behavior from 20/spl deg/C up to 214/spl deg/C with threshold currents < 15 mA (1-mm cavity length). Utilizing the low-bandgap absorption of quantum-dot material miniaturized monolithically integrable edge-emitting lasers could be realized by deeply etched Bragg mirrors with cavity lengths down to 12 /spl mu/m. A minimum threshold current of 1.2 mA and a continuous-wave (CW) output power of >1 mW was obtained for 30-/spl mu/m cavity length. Low-threshold currents of 4.4 mA could be obtained for 1.3-/spl mu/m emitting 400-/spl mu/m-long high-reflection coated ridge waveguide lasers. DFB lasers made from this material by laterally complex coupled feedback gratings show stable CW single-mode emission up to 80/spl deg/C with sidemode suppression ratios exceeding 40 dB.     

Lasing characteristics of InGaAs-GaAs polarization controlledvertical-cavity surface-emitting laser grown on GaAs (311) B substrate

We have demonstrated an oxide confinement polarization controlled vertical-cavity surface-emitting laser (VCSEL) grown on a GaAs (311)B substrate. The polarization state was well controlled along the [2¯33] crystal direction due to an anisotropic gain in the (311)B plane. We fabricated a small oxide aperture VCSEL with a threshold of 260 μA and realized single-transverse mode and single-polarization operation for the first time. The sidemode suppression ratio (SMSR) was 35 dB and the orthogonal polarization suppression ratio (OPSR) was 25 dB. In addition, we have measured polarization and transverse mode characteristics of multi- and single-transverse mode devices under high-speed modulation. In the multimode device of 12 μm×12 pm oxide aperture, we have achieved stable polarization operation of over 25-dB OPSR up to 10 Gb/s and have observed no power penalty due to polarization instability under 2.5-Gb/s pseudorandom modulation. The single-mode device showed stable single-transverse mode and polarization under the modulation conduction up to 5 GHz of sinusoidal modulation. SMSR and OPSR were over 30 and 10 dB, respectively     

Self-assembled nanoholes, lateral quantum-dot molecules, and rolled-up nanotubes

We present a detailed investigation of novel strain-driven semiconductor nanostructures. Our examinations include self-assembled nanoholes, lateral quantum-dot (QD) molecules, and rolled-up nanotubes. We overgrow InAs QDs with GaAs and apply atomically precise in situ etching to fabricate homogeneous arrays of nanometer-sized holes with diameters of 40 to 60 nm and depths up to 6.2 nm. The structural properties of the nanoholes can be precisely tuned by changing the QD capping thickness and the in situ etching time. We show that strain fields surrounding the buried quantum dots drive the nanohole formation process. We overgrow the nanoholes with 0.2- to 2.5-ML InAs and observe the formation of compact lateral InAs QD molecules. The number of QDs involved in a lateral QD molecule can be tuned from two to six by changing the growth temperature. Our systematic photoluminescence study documents the QD molecule formation process step by step and helps to interpret our structural results. We also present the fabrication of laterally aligned lateral QD bimolecules by growing InGaAs on a GaAs [001] substrate patterned with a square array of nanometer sized holes. Charge carriers in such bimolecules might serve as quantum gates in a future semiconductor based quantum computer. Furthermore, we release strained semiconductor bilayers from their surface to fabricate individual rolled-up semiconductor micro- and nanotubes. We control the diameter of strain-driven In(Ga)As-GaAs tubes from the nanometer to micrometer range by simply changing the layer thicknesses and built-in strain. We propose to roll in metal strip lines to fabricate nanocoils and nanotransformers. To support our proposition, we fabricate homogeneous single and twin GaInP tubes. We present a straight GaInP microtube of more than 2 mm in length and a length-to-diameter ratio of about 2000, thus, elucidating the great potential of this technology.     

New approaches for modeling and simulation of quantum-dot cellular automata

In order to further development of the microelectronic systems and to achieve the circuits with higher speed, higher density and lower power consumption, new technologies to replace the conventional CMOS technology must be introduced. Quantum-dot cellular automata (QCA) is an emerging nanotechnology that provides a new method for computation at the nanoscale regime. In this paper, two methods e.g. artificial neural network and a mathematical algorithm based on the QCA cell–cell response function named Tansig method are used for the modeling and simulation of QCA circuits at the cell level. The accuracy and performance of the proposed methods are analyzed through few circuits. The results of these two approaches are compared with each other and QCADesigner software. The results show the feasibility and acceptable accuracy of these types of simulations. Also, these methods enable the simulation of large QCA circuits at the cell level with acceptable precision in a short time with the ability to implement in other circuit simulators such as HSPICE and so on.     

Power dissipation in clocking wires for clocked molecular quantum-dot cellular automata

In the molecular quantum-dot cellular automata (QCA) paradigm clocking wires are used to produce an electric field which is perpendicular to the device plane of surface-bound molecules and is sinusoidally modulated in space and time. This clocking field guides the data flow through the molecular QCA array. Power is dissipated in clocking wires due to the non-zero resistance of the conductors. We analyze quantitatively the amount of power dissipated in the clocking wires and find that in the relevant parameter range it is fairly small. Dissipation in the molecular devices themselves will likely dominate the energy budget.