Lateral Ordering, Position, and Number Control of Self-Organized Quantum Dots: The Key to Future Functional Nanophotonic Devices

Lateral ordering, position, and number control of self-organized epitaxial semiconductor quantum dots (QDs) are demonstrated. Straight linear InAs QD arrays are formed by self- organized anisotropic strain engineering of an InGaAsP/InP (10 0) superlattice template in chemical beam epitaxy. The QD emission wavelength at room temperature is tuned into the important 1.55 mum telecom wavelength region through the insertion of ultrathin GaAs interlayers. Guided self-organized anisotropic strain engineering is demonstrated on shallow- and deep-patterned GaAs (3 1 1)B substrates by molecular beam epitaxy for the formation of complex InGaAs QD arrays. Lateral positioning and number control of InAs QDs, down to a single QD, are demonstrated on truncated InP (100) pyramids by selective-area metal-organic vapor phase epitaxy. Sharp emission around 1.55 mum is observed well above liquid nitrogen temperatures. The regrowth of a passive waveguide structure establishes submicrometer-scale active- passive integration. The demonstrated control over QD formation is the key to future functional nanophotonic devices and paves the way toward the ultimates of photonic-integrated circuits operating at the single and multiple electron and photon level with control of the quantum mechanical and electromagnetic interactions.     

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.     

Characteristics of spectral-hole burning of InAs self-assembledquantum dots

Spectral-hole burning of InAs self-assembled quantum dots (QDs) embedded in pin-diode was observed. At 5 K, a narrow hole with width of less than 1 mm was observed and the hole depth increased as electric field increased with the writing light power of 8 mW. The hole was observed up to 40 K. The spectral hole was broadened as the writing light power increases from 8 to 20 mW. Spectral-hole width at the 8 mW was well fitted with the convolution integral of Gaussian distribution for reading light and Lorentzian distribution for absorption change taking into account homogeneous broadening of InAs QDs of ⩽80 μeV. Spectral-hole lifetime at the 8 mW was estimated to be in the order of 10^-6 s. Optical absorption spectrum of 15-stacked InAs QD structure was also observed at 77 K and 300 K     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

Monte Carlo simulation of resonant phonon THz quantum cascade lasers

We report on Monte Carlo (MC) simulations aimed at the design and optimization of GaAs-based THz quantum cascade lasers. Results are presented for a GaAs/Al_0.15Ga_0.85As quantum cascade laser design based on LO phonon scattering depopulation, which operates at 2.8 THz. The obtained electron distribution functions in the subbands and the photoluminescence spectra are compared to experimental results. Also the dependence of the inversion and current density on the applied field is investigated, and the parasitic channels are identified based on the intersubband lifetimes.     

Lasing at 1.28 /spl mu/m of InAs-GaAs quantum dots with AlGaAs cladding layer grown by metal-organic chemical vapor deposition

We report the device characteristics of stacked InAs-GaAs quantum dot (QD) lasers cladded by an Al/sub 0.4/Ga/sub 0.6/As layer grown at low temperature by metal-organic chemical vapor deposition. In the growth of quantum dot lasers, an emission wavelength shifts toward a shorter value due to the effect of postgrowth annealing on quantum dots. This blueshift can be suppressed when the annealing temperature is below 570/spl deg/C. We achieved 1.28-/spl mu/m continuous-wave lasing at room temperature of five layers stacked InAs-GaAs quantum dots embedded in an In/sub 0.13/Ga/sub 0.87/As strain-reducing layer whose p-cladding layer was grown at 560/spl deg/C. From the experiments and calculations of the gain spectra of fabricated quantum dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots. We also discuss the device characteristics of fabricated quantum dot lasers at various growth temperatures of the p-cladding layer.     

Effects of substrate orientation on opto-electronic properties in self-assembled InAs/GaAs quantum dots

Electronic structure and optical transition characteristics in (100), (110), and (111) oriented InAs/GaAs quantum dots (containing \({\sim }2\) million atoms) were studied using a combination of valence force-field molecular mechanics and 20-band \(sp^{3}d^{5}s^{*}\) atomistic tight-binding framework. These quantum dots are promising candidates for non-traditional applications such as spintronics, quantum cryptography and quantum computation, but suffer from the deleterious effects of various internal fields. Here, the dependence of strain and polarization fields on the substrate orientation is reported and discussed. It is found that, compared to the (100) and (110) oriented counterparts, quantum dots grown on the (111) oriented substrate exhibit a smaller splitting (non-degeneracy) in the excited \(P\) states and enhanced isotropy in the interband optical emission characteristics.     

Progress in GaN-based quantum dots for optoelectronics applications

Our recent progress in GaN-based quantum dots (QDs) for optoelectronics application is discussed. First, we discussed an impact of the use of GaN-based QDs on semiconductor lasers, showing theoretically that reduction of threshold current by using the QDs in GaN-based lasers is much more effective compared to those in GaAs-based or InP-based lasers. Then discussed are our growth technology including self-assembling growth of InGaN QDs on sapphire substrates by atmospheric-pressure metalorganic chemical vapor deposition. Using the self-assembling growth technique, we have succeeded in obtaining lasing action in an edge-emitting laser structure with the InGaN QDs embedded in the active layer under optical excitation with the emission wavelength of 410 nm. Toward UV light wavelength emission, we have recently established self-assembled GaN QDs of high quality and high density under very low V-III ratio. We clearly observed two photoluminescence peaks from both the QDs and the wetting layer at room temperature, which clearly shows the nanostructures are formed with the Stranski-Krastanow growth mode.     

Microscopic photoluminescence spectroscopy of self-organizedCdSe-ZnSe quantum dots grown on the GaAs (110) cleaved surface

Self-organized CdSe-ZnSe quantum dots (QDs) were fabricated on the cleavage-induced GaAs (110) surface in ultra high vacuum (UHV) by molecular beam epitaxy (MBE). CdSe layer showed the Stranski-Krastanow (S-K) growth mode, QWs and QDs emissions originated from the wetting layer and island structures, respectively, were observed in photoluminescence (PL) spectra. This is a evidence of S-K type where island structures are self-formed on the two-dimensional wetting layer as a result of the transition of the growth mode. The state filling effect in the QDs was also observed by employing excitation power dependence on the PL intensity. By using the microscopic PL spectroscopy, the broad PL peak of QDs was resolved into a number of sharp peaks. These peaks are attributed to the recombination of excitons localized at the individual QDs indicating that the fabricated CdSe islands have quasi-zero-dimensional δ-function like density of states     

Self-consistent calculation for valence subband structure and hole mobility in <Emphasis Type="Italic">p</Emphasis>-channel inversion layers

Using six- and eight-band k⋅p models—with parameters calibrated against the bulk band structure obtained using non-local empirical pseudopotentials—we have employed a new hybrid self-consistent method to calculate the valence subband structure in p-channel inversion layers of InAs, InSb, GaAs, In_0.53Ga_0.47As, and GaSb. This method involves two separate stages: first, density-of-states (DOS) of the three lowest-energy subbands (heavy, light, and split-off holes) is calculated using the triangular-well approximation. Then, the self-consistent calculation is performed using the DOS previously obtained, but shifting each subband by the amount obtained from the self-consistent eigenvalues obtained during the self-consistent iteration. Finally, we present results regarding the hole mobility in Ge p-channel inversion layers. The results are compared to those obtained employing the subband structure computed with the triangular-well approximation and also with experimental data.     

Growth of InGaN self-assembled quantum dots and their applicationto lasers

We have successfully grown InGaN self assembled quantum dots (QD's) on a GaN layer, using atmospheric-pressure metalorganic chemical vapor deposition (MOCVD). The average diameter of the QD's was as small as 8.4 nm, and strong emission from the QD's was observed at room temperature. Next, we have investigated a structure in which InGaN QD's were stacked to increase the total QD density. InGaN QD's were formed even when the number of stacked layers was ten. As the number of layers increased, the photoluminescence (PL) intensity increased drastically. Moreover, we have fabricated a laser structure with InGaN QD's embedded into the active layer. A clear threshold of 6.0 mJ/cm² was observed in the dependence of the emission intensity on the excitation energy at room temperature under optical excitation. Above the threshold, the emission was strongly polarized in the transverse electric (TE) mode, and the linewidth of the emission spectra was reduced to below 0.1 nm (resolution limit). The peak wavelength was around 405 nm. These results indicate lasing action at room temperature     

Oscillator strengths of the intersubband electronic transitions in the multi-layered nano-antidots with hydrogenic impurity

In this study, we have obtained the exact solutions of the Schr?dinger equation for a multi-layered quantum antidot (MLQAD) within the effective mass approximation and dielectric continuum model for the spherical symmetry. The MLQAD is nano-structured semiconductor system that consists of a spherical core (e.g. Ga_1?x Al _x As) and a coated spherical shell (e.g. Ga_1?y Al _y As) as the whole anti-dot is embedded inside a bulk material (e.g. GaAs). The dependence of the electron energy spectrum and its radial probability density on nano-system radius are studied. The numeric calculations and analysis of oscillator strength of intersubband quantum transition from the ground state into two first allowed excited states at the varying radius, for both the finite and infinite confining potential (CP) as well as constant shell thickness, are performed. It is shown that, in particular, the binding energy and the oscillator strength of the hydrogenic impurity of a MLQAD behave differently from that of a single-layered quantum antidot (SLQAD). For a MLQAD with finite core and shell CPs, the state energies and the oscillator strengths of the impurity are found to be dependent on the shell thickness. At the large core radius and very small shell thickness, our results are closer to respective values for a SLQAD that previously reported.     

Monte Carlo modeling of X-valley leakage in quantum cascade lasers

This paper presents the first comprehensive Monte Carlo simulation of GaAs/AlGaAs quantum cascade lasers (QCLs) that takes both Γ- and X-valley transport into account and investigates the effect of X-valley leakage on the QCL performance. Excellent agreement with experimental data is obtained for the GaAs/Al_0.45Ga_0.55As QCL at cryogenic and room temperatures. The model reveals two carrier-loss mechanisms into the X valley: coupling of the Γ continuum-like states with the X states in the same stage, and coupling between the Γ localized states in the simulated stage with the X states in the next stage. Simulation results demonstrate that the 45% Al QCL has small X-valley leakage at both 77 K and 300 K, due to the very good confinement of the Γ states, stemming from the high Al content.     

NEGF simulation of the RTD bistability

The simulation of I-V characteristics of Al_0.3Ga_0.7As-GaAs and AlAs-GaAs resonant tunneling diodes (RTD) is presented. The nonequilibrium Green function (NEGF) based 1D quantum transport simulator Wingreen is used in our case. The plateau region on the IV characteristics usually present only by the Wigner function equation (WFE) based simulation appeared now by the NEGF simulation of our AlAs-GaAs RTD and its shape is comparable with our experimental measurements. Analysis of our results from point of view of the scattering and geometrical parameters of the RTD structure is presented.     

High‐responsivity AlGaN–GaN multi‐quantum well UV photodetector

In this paper, we propose a set of AlGaN–GaN multi‐quantum well (MQW) photodetectors based on p‐i‐n heterostructures with 14 AlGaN–GaN MQW structures in i‐region, where GaN quantum well has 6 nm thickness and Al_xGa_1−xN barrier thickness is 3 nm. In this structure, the peak responsivity of 0.19 A/W at 246 nm is reported. In addition, we investigate effects of various parameters on responsivity, and we show that responsivity of MQW‐based photodetectors strongly depends on proper device design, that is, number of quantum wells, well thickness, barrier thickness, and mole fraction. We also show that increasing number of quantum wells, thickness of wells, and mole fraction as well as decreasing thickness of barriers, increase the responsivity. Using obtained results, we proposed optimal structure. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of seed layers on dielectric properties of Ba(Zr0.3Ti0.7)O3 thin films

The Barium zirconium titanate Ba(Zr_0.3Ti_0.7)O₃ thin films were prepared on Pt/Ti/SiO₂/Si substrates with seed layers at the BZT/Pt interface by sol–gel process. Microstructure and structure of thin films were examined. Dielectric properties of thin films with various seed layers thicknesses were investigated as a function of frequency and direct current electric field. The tunability and dielectric constant of BZT thin films increased with increasing seed layer thickness from 0 to 20 nm, while it decreased with a further increase in thickness above 20 nm, meanwhile, the leakage current showed the similar tendency at applied electric field of 250 kV/cm. The optimized seed layer thickness for BZT thin films plays an important role in maintaining the high tunability and low leakage current, which are suitable for microwave device applications.     

Blinking mechanism of colloidal semiconductor quantum dots: Blinking mechanisms

Nanoscale semiconductor quantum dots in colloidal suspensions are observed to blink with off times, τ_off, that scale as 1/τ < eqid2 > ^3/2. In this paper, it is shown by relating the fluctuations of the surface charge density on the quantum dot to fluctuations in the double layer potential, that the related fluctuations in the barrier potential have exactly the functional form needed to result in an inverse-power-law distribution for off times.     

Improved luminescence from quantum-dot nanostructures embedded in structurally engineered (In,Ga)As confining layers

Efficient luminescence of quantum-dot nanostructures embedded in active regions of lasers is important for low-threshold current density devices. This paper discusses an approach for structurally engineering confining (In,Ga)As layers into which InAs quantum dots are inserted to enhance their emission efficiency. It is shown that by inserting the dots at the center of compositionally graded In/sub x/Ga/sub 1-x/As layers, the relative emission efficiency can be increased by nearly an order of magnitude over the emission of dots inside a constant composition (In,Ga)As structure. This enhancement is thought to be a result of the high structural and optical quality of the confining layers.     

Operational Analysis of a Quantum Dot Optically Gated Field-Effect Transistor as a Single-Photon Detector

We report on the operation of a novel single-photon detector, where a layer of self-assembled quantum dots (QDs) is used as an optically addressable floating gate in a GaAs/Al_0.2Ga_0.8As delta-doped field-effect transistor. Photogenerated holes charge the QDs, and subsequently, change the amount of current flowing through the channel by screening the internal gate field. The photoconductive gain associated with this process makes the structure extremely sensitive to light of the appropriate wavelength. We investigate the charge storage and resulting persistent photoconductivity by performing time-resolved measurements of the channel current and of the photoluminescence emitted from the QDs under laser illumination. In addition, we characterize the response of the detector, and investigate sources of photogenerated signals by using the Poisson statistics of laser light. The device exhibits time-gated, single-shot, single-photon sensitivity at a temperature of 4 K. It also exhibits a linear response, and detects photons absorbed in its dedicated absorption layer with an internal quantum efficiency (IQE) of up to (68 plusmn18)%. Given the noise of the detection system, the device is shown to operate with an IQE of (53 plusmn 11)% and dark counts of 0.003 counts per shot for a particular discriminator level.     

3D Parallel Simulations of Fluctuation Effects in pHEMTs

The use of 3D simulations is essential in order to study the effects of fluctuations when devices are scaled to deep submicron dimensions. A 3D drift-diffusion device simulator has been developed to effectively simulate pseudomorphic high electron mobility transistors (pHEMTs) on a distributed memory multiprocessor computer. The drift-diffusion equations are discretized using a finite element method on an unstructured tetrahedral mesh. The obtained set of equations is solved in parallel on an arbitrary number of processors using the message-passing interface library. We have applied our simulator to a 120 nm pHEMT based on the Al_0.3Ga_0.7As/In_0.2Ga_0.8As interface and carried out a calibration to real experimental data.