M.Sc. Engg.

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    Numerical Simulation of Dislocation Reduction in InGaN and InGaAs Heteroepitaxy with Step-graded Interlayers
    (Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh., 2012-07) Hossain, Md. Arafat; Islam, Prof. Dr. Md. Rafiqul
    In the recent years, the InGaN and lnGaAs heteroepitaxy with low dislocation density have become crucial important for high performance electronic and optoelectron Ic devices, especially for multi junction solar cell. Theoretical efforts on dislocation reduction have become a potential issue to realize the future novel device using these materials. In this dissertation, a numerical simulation has been carried out for the reduction of dislocation density in wuzrite InGaN as well as cubic lnGaAs heteroepitaxy with step-graded interlayers. An energy balance model has been developed for evaluating the misfit dislocation (MD) density in the step-graded structure of these heteroepitaxy. The residual strain from previous interlayer has been taken into account with misfit strain to calculate the MD density in each interlayer. To obtain a detail understanding of dislocation interactions and their propagation through the material, a reaction model has been developed considering the geometrical parameters of the step-graded heteroepitaxy. The reaction equations in each model are developed considering the possible annihilation and fusion reactions between each pair of threading dislocations (TDs) and blocking of TDs by MDs on their gliding paths. The evaluations of TI) densities have been done using the numerical simulation of the reaction model by Euler method. The simulation results confirm a significant improvement of epilayer quality due to the use of step-graded interlayers for the heteroepitaxy. The calculations have been done for 3 step-graded interlayers each containing 10% composition difference. Each interlayer and the total thickness of the film are 0.2 im and 1.5 tm respectively. The edge, screw and mixed type MDs are found to be 1.14x10, l.4x10'° and 9.9x1011 cm-2 respectively on the 1/3<11-23>(1 1-22) slip of the lno 4Ga06N epilayer. The MDs are also estimated in 1/3<1 1-23>(1-101) and 1/3<1 1-20>(0001) slip systems. Significant decreases in MD densities from first interlayer to second interlayer, second interlayer to third interlayer and third interlayer to final epilayer have been evaluated. The edge, screw and mixed MDs are found to be decreased from 1.6x1010 to 1.14x1011 cm-2. 2.0x lO'° to 1.4x I 0 cm-2 and 1.4x 1012 to 9.9x 1011 cm-2 in the 1/3<1 1-23>( 11-22) slip system from first interlayer to final InGaN epilayer. On the other hand, due to use of 3 interlayers for 11104Ga06As the total edge and mixed type MDs are found to be decreased from l.lx10 to 8.8x 108 cm-2 and 9.2x 10'' to 7.6x 109 cm-2 respectively. These step wise decrease in dislocation densities at each interlayer and final epilayer have good agreement with experimentally observed results. The numerical solutions of TD densities also confirm the improved epilayer quality using step-graded interlayer. Due to more relative motion and step inclination at each interlayer a higher rate of reduction with film thickness have been reported for mixed type TDs. The average edge, screw and mixed type TDs densities for the step graded structures are found to be 1.48x 1010, 3.7x1010 and 1.1x 109 cm-2 respectively at the top surface of the In0.4Ga0.6N epilayer. In contrast, these values are 7.6x 10'°, 1.89x10'' and 6.26x109 cn12 respectively for the without graded structure. In the same way, the average edge and mixed type TD densities decreased from 4.9x109 to 2.05x 109 cm-2 and 2.1x 1010 to 2.28x 109 cm-2 respectively for step grading in lnGaAs heteroepitaxy. The above performance analysis of the proposed step-graded technique suggests that it will be very promising and superior for improving the material quality in case of heteroepitaxial film.
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    Modeling and Performance Analysis of 1.55 µm Quantum Well Edge Emitting Laser Based on InGaN
    (Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh., 2012-01) Islam, Md. Jahirul; Islam, Dr. Md. Rafiqul
    The advent of research work for longer wavelength 1.55 µm lasers, InGaN advances fast due to its compatible band gap energy and excellent properties. These lasers are well-suited and ideally matched with the existing fiber optic infrastructures as well as bandwidth of the modern ultra speed communication system with substantially tower attenuation, transmission losses, waveform degradation and dispersion penalty. In this thesis, a study is conducted on the design of 1.55 µm InGaN quantum well laser to investigate the effects on various design parameters for the better performance and to deduce an optimum laser structure. The study is separated into stages; it begins with an extensive review on the lasers in chronological order, followed by the development of the laser model with the analysis of band structure interpolation model as well as laser characterization Next, the thermal and equivalent circuit modeling is presented to investigate the temperature effect and electrical properties of the laser respectively. Careful analysis of the band profile by solving one-dimensional time independent Schrodinger and Poisson's equations using finite difference method is 4 demonstrated. The thermal and circuit-level laser modeling is also developed by solving the respective rate equations. Matlab as well as PSPICE simulation programming languages are employed for numerical analysis. The simulation and analysis results reveal the energy separation between different band and subbands for quantum well laser. It is observed that the electron density in conduction band is 1018cm-3 It is also found a better efficiency (59%), reduced threshold current density (1,1 19A/cm2), and bias voltage (1.1 volts), high optical gain (9,000 cm-1), moderate material gain (3,660 cm-1) and modal gain (45 cm-1). In addition, threshold current, 5.1 mA, output power, 5mW, and slope efficiency, 0.695W/A are obtained. Further understanding of the laser performance with different ambient temperatures the thermal effects has been analyzed. It is found that the threshold current has been increased to 5.5mA at the same ambient. Finally, the circuit level equivalent circuit demonstrates the electrical properties of the laser.
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    Impact of Strain on the Performance of InGaN-based Multij unction Solar Cell
    (Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh., 2014-09) Rahman, Md. Aminur; Islam, Dr. Md. Rafiqul
    Strain issue and its impact on the performance of InGaN-based multijunction solar cell (MJSC) is addressed for the first time in the present research work. The route of strain in MJSC is identified to be due to the change in lattice constants in different layers of subcell grown epitaxially with bandgap stepping. Using multi-layered strain model, the state of strain and its magnitude is determined for three kinds of MJSC structures named as MJSC- 1, MJSC-2 and MJSC-3. The results are expressed in terms of subcell thickness 80nm, lOOnm and 120nm and number of subcells 3, 5 and 7. It is found that the magnitude of MJSC position-dependent strain is strongly dependent on the subcell thickness and decreases with increasing the layer thickness. Further, the position-dependent strain increases with increasing the number of subcells. With the combination of MJSC position-dependent strain and deformation potentials, the strain-induced energy bandgap modification is determined under tensile strain condition. Finally, including the strain effect the efficiencies of different MJSC structures are evaluated and found to be lower with that of reported without taking into account of strain. The detraction of efficiency is identified to be due to the open circuit voltage which decreases under tensile strain condition. Among the MJSC structures studied here, MJSC-3 with 7-layers is less efficient and it's efficiency decreases upto 3.01% when strain effect is taken into consideration.
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    Crystallographic Orientation-Dependent Performance of InGaN Blue Laser
    (Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh., 2017-12) Roy, Sourav; Islam,Dr. Md. Rafiqul
    This thesis attempts to address a comprehensive in-depth study on the crystal orientation and strain-dependent optoelectronic performance and frequency response of 445nm InGaN/GaN single quantum well (QW) Blue Laser operating at 300K by solving a six-band k.p Hamiltonian at the Brillouin zone center point using finite difference method. Here 8nm In0.17Ga0.83N is used in the active layer and 10nm GaN is used in the guiding/barrier layer. The well is compressively strained due to lattice mismatch between well and barrier layer. The simulation is carried out in MATLAB/Simulink environment along (0001), (1010), (1012), (1122) and (1011 ) crystal orientations. Tensor rotation scheme is applied to modify the wave vector and Hamiltonian matrix from conventional (0001) crystal orientation. The compositions of well materials are selected as In0.15Ga0.85 N, In0.17Ga0.83 N, In0.19Ga0.81 N and In0.21Ga0.79 N for strain-dependent performance analysis along (1122 ), (1012) and (0001) crystal orientation. In crystal orientation-dependent analysis for 17% Indium composition, it is found that there is a substantial correlation of the energy band dispersion profile, momentum matrix, piezoelectric (PZ) field, peak gain and peak emission wavelength with change of crystal orientation. The PZ field is found to be zero in (1122) and (1010) crystal orientation (growth angle of 58.4° and 90° w.r.t z-axis). The optical gains are inspected as 3845, 4460, 4880, 4750 and 4178 cm-1 corresponding to peak emission wavelength of 447, 455, 446, 441 and 438nm at (0001), ( 1012), (1122), (1011) and (1010) orientations when the injection carrier density is of 3.5×1019 cm-3. Moreover, the optical gain in (1122) orientation is always higher than other orientation in any injection carrier density. During compressive strain-dependent performance evaluation, significant amount of optical gain and emission wavelength variations are found for the value of In composition from 15% to 21%. For example, in (1122)-oriented QW; optical gain changes from 4730 to 5190 cm-1 with a corresponding emission wavelength shifting from 452 to 430 nm for changing the indium composition from 15% to 21%. Output optical power characteristics are analyzed by developing a Simulink model with the help of 2- level rate equations and following the concept of signal-flow diagram. In (1122), (1012) and (0001)-oriented structure, higher optical power is found when the indium composition is 21% in the active region. Among arbitrary crystal orientated structure, maximum optical power of 4.35mW and minimum threshold current of 0.74mA is obtained in (1122) crystal orientation. A state-space model is formed for each (hkil)-orientated crystal structure in order to observe their frequency responses (Bode Plot) close to corner frequencies. The laser system for each orientation and indium composition is found to be stable as positive gain and phase margin is achieved. Highest magnitude (dB) response is obtained in semipolar (1122) crystal orientation for 21% indium composition in well. This numerical result demonstrates that semipolar (1122)crystal orientation is of special interest and can be incorporated into the active layer of blue laser in order to have best performance for high-speed lightwave communication technologies which can be used for medical & industrial purpose, precision measurement, military defence & security, thermal imaging, DVD and Blu-ray players etc.
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    Design and Analysis of Grid-Connected Hybrid Power Systems Based on Renewable Energy: Bangladesh Perspective
    (Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh., 2016-01) Nurunnabi, Md.; Roy, Dr. Naruttam Kumar
    Current power generation scenarios all over the world are not climate friendly as the generation systems are mainly dependent on fossil fuels that produce greenhouse gas (GHG) which contributes to global warming. This thesis presents an economical expediency of gridconnected hybrid (PV/Wind turbine) power system model by investigating the potentials of the wind and solar energy. It also conducts a feasibility analysis to explore the potentialities of green energy at different locations namely Kuakata, Sitakunda, Magnamaghat, Dinajpur, Rangpur and Khulna in Bangladesh. Initially, a flowchart of the proposed hybrid power system model is developed and then a hybrid model is designed with varying the contributions of renewable resources for the considered coastal region and the northern part of Bangladesh using a software tool named Hybrid Optimization of Multiple Energy Resources (HOMER). The simulation results are calculated for finding the cost of energy (COE), net present cost (NPC), total annualize cost, annual real interest rate, capital recovery factor (CRF), fraction of renewable energy (RE) contribution and greenhouse gas emission in terms of tons/year from which an optimum combination of RE sources and fraction of different RE sources in the designed hybrid power plant are determined. Sensitivity variables, such as range of wind speed, solar radiation, PV panel price, wind turbine hub height, are defined as inputs during simulation. The optimization process is carried out repeatedly for the sensitivity variables and the results are refined accordingly. Also, a comparison is made between off-grid and grid connected models on the basis of COE and GHG emission. The simulation results show that the proposed grid-PV-wind hybrid power system model is most suitable, economical and eco-friendly for the considered regions in Bangladesh.