multiple-quantum-well active regions

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A study of distributed Bragg reflector stacks and quantum wells for 1.55 micron Fabry-Perot modulators

A study of distributed Bragg reflector stacks and quantum wells for 1.55 micron Fabry-Perot modulators

In chapter five, aluminium based multiple quantum well active regions were studied. All samples showed an initial increase in photocurrent, up to at least 2V, before any red shift was seen. This was thought to be due to residual doping in the active region o f the device. As would be expected, the samples using quaternary wells or barriers showed a larger Stark shift than the ternary/ternary sample, which had its maximum change in photocurrent at lOV, whereas the other three samples showed a maximum change in photocurrent at about half this value. One sample in particular, M792, demonstrated that high quality multiple quantum well systems can be obtained in these materials. This sample showed a PL line width o f only 32nm (17meV FHWM) at room temperature, the best so far obtained by Sheffield in these materials, and on a par with the best materials reported in the literature. We expect that these materials would demonstrate better characteristics, in terms o f Stark shifts and strength o f the exciton feature, if the photocurrent was saturated at OV. Using M792, absorption measurements were carried out, and an absorption coefficient in the well o f 4.9xl0^cm'^ at the operating voltage was calculated. This figure was felt to be a worse case scenario, and it is therefore expected that the operating parameters assumed from this
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Cathodoluminescence studies of chevron features in semi-polar (11-22) InGaN/GaN multiple quantum well structures

Cathodoluminescence studies of chevron features in semi-polar (11-22) InGaN/GaN multiple quantum well structures

etching of GaN microrods in a regular array. GaN over- growth begins from the sidewalls of the microrods, along the [0001] direction and the ½11 20 direction. Growth continues in these directions until they coalesce, and then continued to a thickness of 5 lm. After that the LED structure was grown, which includes a 1 lm n-GaN layer, three periods of InGaN/ GaN QWs, and was finished with a 150 nm layer of p-GaN. The InN content of the InGaN QWs is 40% to achieve light emission in the amber spectral region and the well and bar- rier widths are 3.8 nm and 7.8 nm (nominal values), respec- tively. LEDs were fabricated by etching down to the n-GaN to apply a Ti/Au n-contact, and applying a Ti/Au p-contact above a layer of indium tin oxide (ITO), used to assist cur- rent spreading. 21 The samples were analysed using a variable pressure scanning electron microscope (SEM) to which a custom-built CL system has been added. 22,23 The axis of the light collection optics is situated at 90 with respect to the electron beam, and the sample is tilted by 45 . The light emitted at room temperature is collected by a reflecting objective and focussed on to the entrance slit of a spectro- graph, and the light is detected using an electron multiplying charged coupled device. The beam scans across the sample surface, and a 1600 pixel emission spectrum from 300 to 800 nm is recorded for every pixel with a spatial resolution approaching 10 nm. 24 Electron beam energies of up to 10 kV have been used to probe light emission from up to 300 nm below the sample surface. This depth was calculated by Monte Carlo simulations using the CASINO 25 software to estimate the beam voltages required to excite the active regions of the samples. For contacted LEDs, it is possible to simultaneously probe the light emission and the EBIC in a sample. When the carriers generated by the electron beam reach the active region, one of three processes can occur: radiative recombination (CL), non-radiative recombination, or a flow of current. Therefore, the EBIC signal, measured via an external circuit, provides a pathway to investigate the non-radiative recombination occurring in a sample when cor- related with the CL. 26,27 AFM was carried out, in PeakForce tapping mode, on the samples to provide additional informa- tion about their surface morphology.
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The Effects of Strained Multiple Quantum Well on the Chirped DFB-SOA All Optical Flip-Flop

The Effects of Strained Multiple Quantum Well on the Chirped DFB-SOA All Optical Flip-Flop

quasi static condition. In order to characterize the device behavior, we need to calculate the ratio of the output power to the small signal input power, known as the optical transmittivity, as a function of the wavelength. To trace out the transmittivity curve, a single CW light signal is injected into QW or MQW liner chirped DFB-SOA from the left. Then, after 10.8ns, the output power is calculated as a function of wavelength. Figure 4a demonstrates the transmittivity curves for various active layer regions, QW and MQW with and without strained, while grating coupling coefficient, κL and the chirp coefficient (C) are kept constant; e.g. κL=2 and C=6 that corresponding to chirped DFB-SOA. Figure 4b show the dependence of optical output power versus optical input power to show the bistability behavior of an DFB-SOA AOFF. An alternative method to latch the flip-flop operation is the switching action based on XPM. This is done as the set and reset signals are separately injected to DFB-SOA along with the holding beam (CW input light). A XPM based switching process is similar to that of SPM based, except that the holding beam power is adjusted in the center of the hysteresis loop and control signal (set or reset pulses) are injected to the device separately.
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Efficiency of True-Green Light Emitting Diodes: Non-Uniformity and Temperature Effects

Efficiency of True-Green Light Emitting Diodes: Non-Uniformity and Temperature Effects

[5] Titkov, I.E.; Karpov, S.Yu.; Yadav, A.; Zerova, V.L.; Zulonas, M.; Galler, B.; Strassburg, M.;Pietzonka, I.; Lu- gauer, H.-J.; and Rafailov, E.U. Temperature-Dependent Internal Quantum Efficiency of Blue High-Bright- ness Light-Emitting Diodes. IEEE J. Quantum Electron. 2014, 50, 189-129, doi: 10.1109/JQE.2014.2359958. [6] Karpov, S.Yu.; Binder, M.; Galler, B.; and Schiavon, D. Spectral dependence of light extraction efficiency of

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A micro finite element model for soil behaviour: experimental evaluation for sand under triaxial compression

A micro finite element model for soil behaviour: experimental evaluation for sand under triaxial compression

Improving computational modelling towards a more realistic description of granular behaviour is a long standing challenge (e.g. O’Sullivan, 2011; Matsushima & Chang, 2011; Andrade, et al., 2012; Kawamoto, et al., 2018). Nadimi & Fonseca (2018a, 2018b) proposed a micro Finite Element (µFE) model that virtualises the fabric of a natural sand obtained from micro Computed Tomography (μCT) to simulate the mechanical response under loading. In this model, the grain- to-grain interactions are modelled in a framework of combined discrete-finite element method (Munjiza, 2004). The underlying idea for the development of this μFE model was the need to better represent soil fabric into numerical modelling. The effect of fabric on the mechanical response of soil is well-known and has been repeatedly demonstrated in both experiments and numerical analysis (e.g., Cuccovillo & Coop, 1999; Oda & Iwashita, 1999; Kuwano & Jardine, 2002; Ng, 2004; Fonseca, et al., 2016).
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Genetic algorithm applied to the optimization of quantum cascade lasers with second harmonic generation

Genetic algorithm applied to the optimization of quantum cascade lasers with second harmonic generation

Genetic Algorithms are a family of computational mod- els created with the purpose of solving complex problems by imitating the process that happen in nature during the course of natural evolution. In these algorithms, the solutions to optimization problems are found using a combination of selection, recombination, and mutation. 22 An implementa- tion of a genetic algorithm begins with a population of typi- cally random chromosomes into which a potential solution to a specific problem is encoded. 23 At each step, the genetic algorithm randomly selects individuals from the current pop- ulation and uses them as parents to produce the children for the next generation. Over successive generations, the popula- tion “evolves” toward an optimal solution and the “fitness” of a solution is typically defined with respect to the current population. The manner in which the algorithm searches the parameter space with the purpose of finding the optimal solu- tion, as well as its independence on the initial conditions, makes it particularly suitable for applications in which other optimization techniques would have little or limited success, that is problems with discontinuous, nondifferentiable, sto- chastic, or highly nonlinear objective (target) functions. Various production constraints regarding design parameters can easily be included in the optimization procedure, which make the resulting structures more convenient for further fabrication than the ones obtained with more rigid methods such as, for instance, SYSQM, which results in a potential profile that needs to be further discretized in order to be pro- duced, which can result in the deterioration of the output characteristics.
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Research on Some New Results Arising from Multiple q-Calculus

Research on Some New Results Arising from Multiple q-Calculus

In this paper, we develop the theory of the multiple q-analogue of the Heine’s binomial formula, chain rule and Leibnitz’s rule. We also derive many useful de…nitions and results involving multiple q-antiderivative and multiple q-Jackson’s integral. Finally, we list here multiple q-analogue of some elementary functions including trigonometric functions and hy- perbolic functions. This may be a good consideration in developing the multiple q-calculus in combinatorics, number theory and other …elds of mathematics.

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Quantum well intermixing by ion implantation

Quantum well intermixing by ion implantation

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Graphene Electron States in a Quantum Well

Graphene Electron States in a Quantum Well

Graphene is a mono-atomic layer of carbon atoms arranged on a 2D honeycomb lattice. Near each corner of the hexagonal first Brillouin zone (also called Dirac points or K-points) the quasiparticle excitations obey a 2D linear dispersion relation and behave like massless “relativistic” particles. That leads to a number of unusual electronic properties; one of them is so called Klein paradox [5] well known in quantum electrodynamics. It predicts that the electron can perfectly pass through the potential barrier independently of its height in contrast to the conventional nonrelativistic tunneling where the transmission probability exponentially decays with the barrier height increasing. As was shown in [6,7] the similar phenomenon takes place in graphene (see also [8]). So some electron states in graphene with a quantum well (which can be considered as a barrier for holes) are those corresponding just to that Klein (or “chiral”) tunneling in both directions. Klein chiral tunneling
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Effect of triplet multiple quantum well structures on the performance of blue phosphorescent organic light emitting diodes

Effect of triplet multiple quantum well structures on the performance of blue phosphorescent organic light emitting diodes

Another way to achieve high efficiency in OLEDs is to confine excitons inside the EML using the multiple quantum well [MQW] structure [7]. Only a few reports concerning the MQW structure with good carrier and exciton confinement ability have been presented on OLEDs until quite recently. For example, Qiu et al. [7] improved the charge balance by utilizing an organic MQW structure to decelerate hole transportation. Huang et al. [8] used MQW structures to increase the carrier recombination efficiency, where both charges and excitons were confined to the EMLs. Park et al. [9] and Kim et al. [10] also reported similar triplet MQW structures. Recently, Liu et al. [11] proposed a non-dop- ing EML method, instead of a host-emitter doping method, to improve the efficient triplet exciton confine- ment effect and the suppression of triplet-triplet annihi- lation that occurs via a single-step long range (Forster- type) energy transfer between excited molecules.
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Chiroptically Active Quantum Nanostructures

Chiroptically Active Quantum Nanostructures

As these ZnS:Mn QDs were intended for biomedical applications they first had to be transferred into the aqueous phase. In order to do this, a ligand exchange was performed where the organic oleylamine and any residual dibenzylamine was replaced with a chiral, water-soluble ligand such as cysteine or penicillamine. This process was carried out primarily using the method outlined in section 2.2.2 of chapter 2. Once the excess organic ligand was washed off, the QDs were mixed with an acidified solution of the ligand of choice. Once the ligand exchange had taken place, the QDs were no longer soluble in chloroform and so precipitated from solution. The QDs were centrifuged, washed with methanol several times and then re-dispersed in alkaline (pH≈11) water. The resulting aqueous particles were characterised using various instrumental techniques: UV-Vis spectroscopy, PL spectroscopy, circular dichroism spectroscopy and transmission electron microscopy. While it was interesting to prepare both doped and undoped QDs, for biological purposes, it was advantageous only to continue using the manganese- doped quantum dots. This is due to the undoped ZnS QDs emitting in the ultraviolet range which is damaging to biological media. The absorbance is also in the UV however it has been shown that 3 photon excitation circumvents this issue 17 .
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Aperiodic and Asymmetric Multiple Quantum Well Photonic Devices: A Novel Transfer Matrix Based Model

Aperiodic and Asymmetric Multiple Quantum Well Photonic Devices: A Novel Transfer Matrix Based Model

This is a most general mathematical model that helps in computing the energy Eigen values inside asymmetric and aperiodic MQW structures and predicts the carrier distribution for each of the energy states in terms of the eigen function. The model has been tested on experimental data for two different structures; the first a 16.5 µm QCD and the second one GaAs/AlGaAs MQW [1, 2]. This model also helps in studying the extent of carrier tunnelling through the quantum barrier. Tunnelling depends significantly on the barrier width. Scaling of structure dimension affects this variation very sharply. [5-8, 12].
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Enhancing the Efficacy of the EuroMed Partnership through intensified Iterations. EUMA Papers, Vol. 5, No.1 January 2008

Enhancing the Efficacy of the EuroMed Partnership through intensified Iterations. EUMA Papers, Vol. 5, No.1 January 2008

socio-cultural understanding within and among regions. While there has been no shortage of initiatives to contribute to the socio-economic and political stabilization of the Magreb and Mashriq in past years, such as the G-8’s Broader Middle East and North Africa Fora for the Future in 2004 - 2006 (in which the EMP was included), for example, as cooperative efforts for regional civil society and business groups to express their reform goals to their governments and to “advance the universal values of human dignity, democracy, economic opportunity and social justice” (U.S. Department of State November 7. 2005, 1), few have succeeded in contributing to a unified zone of peace, not to mention in establishing a security community in Karl Deutsch’s terms, and much less to establishing a Euro-Mediterranean security complex in Buzan and Waever’s terminology, aside from NATO’s Mediterranean Dialogue. In fact, the International Crisis Group calls the Broader Middle East and North Africa Initiative “imperiled at birth” (ICG briefing 7 June 2004), 2). Nevertheless, regional integration in the Middle East has made strides in the past decade especially, e.g. in a broader context with the Gulf Cooperation Council 1 taking an active role in balancing between trade with Iran and their support of Sunnis in Iraq (Kerr and Bozorgmehr, 2007).
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Theory of terahertz intervalence band polaritons and antipolaritons

Theory of terahertz intervalence band polaritons and antipolaritons

Terahertz time domain spectroscopy (THz-TDs) has application in Chem­ istry and Biochemistry, security, imaging in medicine, communications as well as manufacturing processes [39-41]. The system involves directing sub­ picoseconds THz-pulses using an ultrashort near infrared pulsed laser at the surface of a sample to be analysed, and therefore becomes transmitted, re­ flected or absorbed after they interact. W ith the used of a second femtosec­ ond laser pulse, the emitted radiation is then detected and analysed. THz radiation generates images of samples that are opaque in both visible and infra red regions. However, its application (in THz-TDs) is limited to very thin samples or samples with low absorbance. This is due to difficulty to dis­ tinguish the coherently generated THz pulses resulting from the samples and that caused by the driven lasers. The spectroscopic or imaging techniques of THz-radiation have the potential to improve the quality and uniformity of pharmaceutical products such as: 3D-chemical mapping [42], tablet coating and chemical fingerprint.
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Finite-Difference Time-Domain Simulations on Overgrown InGaN/GaN Multiple Quantum Well (MQW) Light Emitting Diode (LED).

Finite-Difference Time-Domain Simulations on Overgrown InGaN/GaN Multiple Quantum Well (MQW) Light Emitting Diode (LED).

In the past few years, there has been an increasing trend to build blue LEDs based on GaN (wurtzite) nanostructures because the facets of GaN nanostructures expose their semi-polar and non-polar planes for the active layer growth. Specifically, InGaN/GaN multiple quantum wells are widely used as active layers because of its tunable band gap with indium concentration. As a result, this helps in eliminating the effects like Quantum Confined Stark Effect (QCSE) in the active layers and enhances the radiative recombination rate in them. One such approach was successfully demonstrated by Dr. Hosalli et al. in Dr. Bedair’s group at NCSU by conformally overgrowing In x Ga 1−x N
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Quantum Multiple Q Learning

Quantum Multiple Q Learning

With the increase in interest in quantum computing has come interest in apply- ing it to reinforcement learning. As the application of reinforcement learning to real-world problems generally requires a very large state-space, the hope is that the application of quantum computing would significantly reduce the amount of time for the algorithm to reach convergence. A generalized framework for quantum reinforcement learning is described in detail by Cárdenas-López, et al. [19]. In this framework, the goal is to maximize the overlap between quantum states stored in registers and the environment through a rewarding system. Oth- er efforts have been directed toward evaluating the adaptability of quantum reinforcement learning agents, as one key component of a reinforcement learn- ing algorithm is its ability to adapt to a changing environment [20] [21]. Initial evidence indicates that quantum computing can improve the agent’s decision making in a changing environment.
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Physical model of quantum-well infrared photodetectors

Physical model of quantum-well infrared photodetectors

In the past decade quantum well infrared photodetectors (QWIPs) have reached a technologi- cal maturity as devices offering excellent performance in the mid- (3 − 5µm) and long-wavelength (8 − 14µm) infrared spectral range [1]. A large number of papers have been published, covering different aspects of QWIP design, modelling and characterization, delivering tunable, broadband and multicolor operation [2–8]. Moreover, large, highly uniform QWIP focal plane arrays have been reported with a wide range of possible applications [9, 10]. However, despite the extensive amount of experimental and theoretical efforts, not much work has been done on a microscopic quantum description of the processes that govern both vertical and parallel electron transport in periodic quantum structures, involving bound-bound and bound-continuum intersubband transi- tions [6, 11–13]. In order to ensure further improvement of the QWIP technology, primarily by using novel structures and material systems, a thorough understanding of fundamental physical procesess in QWIPs, as well as a first principles simulation tool are neccesary.
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Study of GaN-based Materials for Light-emitting Applications

Study of GaN-based Materials for Light-emitting Applications

It is important to note that Figure 4.5 was calculated for bulk InGaN. The situation may actually be somewhat less severe when dealing with InGaN/GaN quantum wells. This can be understood by referring to Cahn’s theory of spinodal decomposition [32], which deals with three energy terms: the free energy per unit volume (i.e., the driving force of the spinodal); the gradient energy term (which arises from the non- constant composition); and the strain energy term [33]. Whereas the free energy per unit volume is necessary for the spinodal to take place, the gradient and strain energy terms work against its presence. Therefore, in a highly-strained structure (such as an InGaN well sandwiched between GaN barriers), the spinodal may perhaps be suppressed relative to the unstrained case. For this reason, it has been suggested [34] that one can achieve somewhat higher In content in InGaN/GaN structures without phase separation than what is predicted by the standard models. This theory, however, is not universally accepted.
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Hitting statistics from quantum jumps

Hitting statistics from quantum jumps

We define the hitting time for a model of continuous-time open quantum walks in terms of quantum jumps. Our starting point is a master equation in Lindblad form, which can be taken as the quantum analogue of the rate equation for a classical continuous-time Markov chain. The quantum jump method is well known in the quantum optics community and has also been applied to simulate open quantum walks in discrete time. This method however, is well-suited to continuous-time problems. It is shown here that a continuous-time hitting problem is amenable to analysis via quantum jumps: The hitting time can be defined as the time of the first jump. Using this fact, we derive the distribution of hitting times and explicit exressions for its statistical moments. Simple examples are considered to illustrate the final results. We then show that the hitting statistics obtained via quantum jumps is consistent with a previous definition for a measured walk in discrete time [Phys. Rev. A 73, 032341 (2006)] (when generalised to allow for non-unitary evolution and in the limit of small time steps). A caveat of the quantum-jump approach is that it relies on the final state (the state which we want to hit) to share only incoherent edges with other vertices in the graph. We propose a simple remedy to restore the applicability of quantum jumps when this is not the case and show that the hitting-time statistics will again converge to that obtained from the measured discrete walk in appropriate limits.
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Double-barrier Superlattice Infrared Photodetector Integrated with Multiple Quantum-Well Infrared Photodetector to Improve Performance

Double-barrier Superlattice Infrared Photodetector Integrated with Multiple Quantum-Well Infrared Photodetector to Improve Performance

An infrared photodetector which has a 15-period superlattice (SL) sandwiched by double barriers with multiple quantum wells (MQWs) inserted in the right side of them has investigated. Photoelectrons can bounce back and forth in the second miniband between two barriers and then inject through the graded barrier to enhance the photocurrent. MQWs is utilized to reduce the noise current power and add response range. Because of enhanced photocurrent and low noise gain, this detector shows satisfactory detectivity (D*) 1.85 × 10 10 cm Hz 0.5 /W (at 8.2 µm and 0.7 V under 80 K). Due to combination of superlattice and multiple quantum wells infrared photodetectors, multi-color detection can be achieved by bias magnitude modification. From the experimental results, this device is the promising candidate of a pixel in the focal plane array.
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