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Design and Analysis of Quantum Dot Laser Diode for Communication Applications

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Design and Analysis of Quantum Dot Laser Diode for Communication Applications International Journal of Advancements in Research & Technology, Volume 3, Issue 7, July-2014 94 ISSN 2278-7763 Design and Analysis of Quantum Dot Laser diode for Communication Applications Dr. Thaira Zakaria Al-Tayyar*, and Jassim Mohammed Sahan** ABSTRACT Semiconductor laser diodes are important components in advanced optical fiber communication systems. Recent progress in nanostructure technology of quantum dot (QD), as active region, has led to the development of QD laser diodes which are superior to bulk, quantum-well or quantum wire laser diodes. Design model of InAs QD laser diode structure is proposed in this paper; the proposed model of QD is disk shaped its height is 2 nm and diameter is 14 nm, surface density per layer is 7×1012 cm-2, number of QD layers are five, wetting layer thickness is 1nm and barrier thickness is 90 nm, the dimensions of the proposed model have an active region of length 800 µm, width 12 µm, and a height of 375 nm. The evaluation of the proposed model is based on rate equations model. The results show that the QD laser diode has a lower threshold current 2.93 mA, threshold current density 30.42 A/cm2, bias voltage 0.97 V, while output optical power range is (0-25 mW) under wavelength range (1298-1305 nm). The model gain of the ground state is 44.6 cm-1. Keywords: Quantum dots, InAs, laser diode, rate equations. * Electrical Engineering Department, University of Technology, Email: [email protected] ** College engineering, Al-Nahrain University, Email: [email protected] IJOART 1-INTRODUCTION Arsenide in the reported research) which Semiconductor nano-particles, often presents a potential barrier to the motion of referred as quantum dots (QDs), are the carriers out of the box. Researchers semiconductor particles with physical call QDs as "artificial atoms", because the dimensions smaller than the exciton Bohr charge carriers in them behave the same radius (which is defined as the distance way that orbiting electrons behave around between the proton and electron in a the nucleus of an atom. In other words, the hydrogen atom in its ground state (GS)) charge carriers in QDs can absorb or emit and are confined in three spatial directions discrete amounts of energy as atoms do. [1]. Essentially, a semiconductor QD is a [13] region of material a few nanometers in size The discovery of self-assembling of QDs where the carriers are confined. These opened a possibility of mass-production of carriers can be electrons, holes (missing QDs and gave an additional impact to their electrons), or excitons (bound electron investigation. The self-assembling hole pairs). principle is based on the fact that during The charge carriers are confined in a epitaxial by some growth conditions planar nanoscale box of one semiconducting growth changes to 3D growth, forming material (in the case of the reported objects with quantum confinement in all research, Indium Arsenide). That box or 3Ds. The InGaAs QDs on GaAs substrate ring is surrounded by another open a possibility to obtain an emission semiconducting material GaAs (Gallium wavelength over a very wide range from Copyright © 2014 SciResPub. IJOART International Journal of Advancements in Research & Technology, Volume 3, Issue 7, July-2014 95 ISSN 2278-7763 0.95 μm up to 1.55 μm. Such long the barrier towards the QD-GS as follows wavelengths could not be achieved using [4], InGaAs quantum well due to the formation • Carriers are injected in the SCH barrier of misfit dislocations caused by a larger at rate I/q, where I is the injection misfit strain [2]. current, q is the elementary charge, In QDs carriers are three dimensions (3D) • Relax in the WL state at rate l/τc,sch or confined and the modification of electronic escape back in the barrier at rate l/τe,wl , properties is quite stronger than quantum • From the WL they can be captured in well and quantum wire. Also, in QDs, the the dot. Therefore, the WL state acts as energy levels are discrete and transitions a common reservoir from which the between electrons and holes are carriers are captured in the ES at rate comparable with transitions between l/τc,wl and from the ES to the GS at rate discrete levels of single atoms. The QD l/τc,ES where, the times τc,sch, τc,wl and lasers have demonstrated both τc,ES are the average capture times theoretically and experimentally many from the SCH to the WL, from the WL properties, such as high optical gain, high- to the ES and from the ES to the GS speed operation, ultra-low and respectively, temperature-stable threshold current • Carriers escape also from the GS back density, broad modulation bandwidth, and to the ES at rate l/τe,GS or from the ES narrow spectrum linewidth primarily due back to the WL at rate l/τe,ES where the to the delta-function like density of states emission times τe,GS and τe,ES are the [3]. escape time from the GS back in the ES and the escape from the ES back 2-Four-Level Rate Equations Model into the WL, As with any other semiconductor light • Carriers can also recombine with source, also in QD lasers, there are the radiative and nonradiative processes well-known electronic transitions between from the SCH, the WL and the various the conduction band (CB) and valence confined states with rates l/τrs , l/τrw , band (VB). TheIJOART model can be divided into l/τr respectively. two parts, photon equations and electron The rate of photons emitted out of the equations. Thus, QD lasers are modeled cavity is Sp/τp, with τp the corresponding with four carrier rate equations, two for photon lifetime of the mode and Sp is the GS, Excitation state (ES) and the latter is photon occupation probability. It is for wetting layer (WL) and separate assumed that the stimulated emission can confinement heterostructure (SCH) barrier take place only due to recombination layer with one rate equations related to between the electron and hole in the GS. photon populations. The presence of more The diffusion, relaxation, recombination, than one state in the dot, is then, taken into and escape processes of carriers are shown account by including the ES. in Figure (1) [4]. The numerical model of the QD laser Since there are a small number of carriers holds under the assumption that the active in the QD, depending on the number of region consists of only one QD ensemble, states, it is beneficial to represent the where QDs are interconnected by WL. It carrier rate equations in the dot by the rate has been assumed that each QD has only equations of the occupation probability of two confined energy states, the GS and the the QD states. The resulting REs system ES. The carrier energy levels are shown in for the carrier occupation probability is as Figure (1). follows [5], The carrier dynamics of the initial excitation position and energy level within Copyright © 2014 SciResPub. IJOART International Journal of Advancements in Research & Technology, Volume 3, Issue 7, July-2014 96 ISSN 2278-7763 (1 ) where, nR R is the number of electrons in = + GS the GS, nRES R is the number of electrons in 푠푐ℎ 푠푐ℎ , 푤푙 푤푙, 푑푃 퐼 푃 − 푃 푃 the ES, nRsch R is the carrier number in the − 푐 푠푐ℎ 푒 푤푙 푑푡 푒 휏 휏 SCH, nRwlR is the carrier number in the WL, (1) nR R is the photon number, NR R is the total 푃푠푐ℎ p D number of QDs, PR wlR is the WL population, − 4푠푟 = + 휏 (1 ) PRschR is the SCH population and S RpR is the 푤푙 푠푐, ℎ ,퐸푆 푤푙, photon occupation. 푑푃 푃 푃 푤푙 푃 � 푐 푠푐ℎ 푒 퐸푆 � − 푃 − 푒 푤푙 The carrier escape times relate to the 푑푡 휏 휏 (1 ) 휏 carrier capture times as follows [4], 푤푙, 푃 퐸푆 − − 푃 , = , (11) 휏푐 푤푙 퐸퐸푆−퐸퐺푆 (2) 퐺푆 퐾퐵푇 푒 퐺푆 푐 퐸푆 퐷 푃푤푙 휏 = 휏 퐸푆 푒 (12 , 퐷 , 퐸푤푙−퐸퐸푆 ) − 퐸푆 푑 푤푟 퐷 푁 퐾퐵푇 휏 where, the DR R and DR R are the = + (1 ) 푒 퐸푆 휌푤푙 SG푐 푤푙 ES 4 , 2 , 휏 � � 휏 푒 푑푃퐸푆 푃푤푙 푃퐺푆 degeneracies of the GS and ES levels, � � − 푃퐸푆 respectively. 푑푡 휏푐 푤푙 휏푒 퐺푆 (1 ) The ERGSR, ERES R and ER wl R are the energies of 퐸푆, 푃 푤푙 the GS, ES and WL level, respectively, the − 푒 퐸푆 − 푃 KRB R is the Boltzmann constant, and T is the 휏 (1 ) o degree of temperature in KP .P The carrier 퐸푆, 푃 퐺푆 escape time from the ES depends on the − 푐 퐸푆 − 푃 휏 (3) ratio between the number of available states in the QDs and in the WL. 푃퐸푆 2 (1− 푟 ) (1 ) The escape time from the WL to the SCH = 휏 is given by [7], , , 푑푃퐺푆 푃퐸푆 − 푃퐺푆 푃퐺푆 − 푃퐸푆 , = , (13) 푐 퐸푆 − 푒 퐺푆 퐸푠푐ℎ−퐸푤푙 푑푡 휏 ѵ Г ɡ휏 (4) 휌푤푙푁푤 퐾퐵푇 IJOART푒 푤푙 R R 푐 푠푐ℎ where Lsch is the total thickness of the 퐺푆 휏 � 푠푐ℎ 푠푐ℎ� 휏 푒 푃 R R 푔 퐺푆 푝 SCH, ρwl휌 is the퐿 Density of states per unit − 푟 − 2 푆 = ѵ Г ɡ 휏 + (5) area in the WL and ρRschR is the density of 푑푆푝 푆푝 푃퐺푆 states per unit volume in SCH are given by 푔 퐺푆 푆푝 − 훽 [7], where푑푡 PRES R and P휏R GS푝 R are 휏푟occupation probabilities of ES, and GS of a QD = (14) R R R R R R ћ respectively and Pwl , Psch and Sp the WL 푚푒푤푙퐾퐵푇 and SCH population and the photon 휌푤푙 2 2 occupation respectively.
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