UNIVERSITY OF CINCINNATI Date:_February 07, 2006__ I, _ Rajasundaram Rajasekaran____________________, hereby submit this work as part of the requirements for the degree of: Master of Science in: Electrical & Computer Engineering and Computer Science It is entitled: Dependence of LASER Performance on Number of Quantum Wells in InAlGaAs Semiconductor LASERS This work and its defense approved by: Chair: _Dr.David J.Klotzkin__________ _Dr.Kenneth P. Roenker _______ _Dr.Fred Beyette_____________ _______________________________ _______________________________ Dependence of LASER Performance on Number of Quantum Wells in InAlGaAs Semiconductor LASERS A thesis submitted to the Division of Graduate Studies and Research of the University of Cincinnati in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in the Department of Electrical and Computer Engineering and Computer Science of the College of Engineering February 2006 By Rajasundaram Rajasekaran Bachelor of Engineering, Bharathidasan University, India, 2001 Thesis Advisor and Committee Chair: Dr. David J. Klotzkin Abstract This thesis is undertaken to understand the trade-off in the performance of semiconductor lasers with the increase in well number. The five and seven quantum well InAlGaAs lasers operating at an optical communication wavelength of 1300nm were used in this investigation. The dependence of laser performance on the waveguide structure and the geometry of the laser device were also researched. Experimental setup were designed and implemented to perform DC and dynamic characterization. Numerical simulations were carried out using Mode Solver to compare the optical confinement in five and seven quantum wells. The seven quantum well InAlGaAs lasers showed better optical confinement and characteristic temperature compared to the five quantum well lasers. The increase in well number aids in lowering threshold current and improving the optical confinement factor, characteristic temperature, net gain, material gain and d-factor with trade off in slope efficiency, differential quantum efficiency and cavity loss. The design curves were plotted to aid the laser designer to understand the performance trade off in increasing the well number. The comparison plots would aid the designer to optimize the number of quantum wells when designing uncooled semiconductor lasers for the application in optical communication. Seven quantum well InAlGaAs lasers exhibited low threshold current of 13.8mA and high characteristic temperature of 71°c. The five quantum well lasers showed high slope efficiency of 0.198W/A and a low cavity loss of 18/cm. The better modulation responses of the seven quantum well lasers were shown by determining the D-factor. This study also helps in optimizing the number of quantum wells for the given cavity length. To mom, dad and Rachana ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude to my advisor Dr. David Klotzkin for his valuable guidance and constant encouragement throughout my graduate studies. His patience, generosity and wit have all made working under him a very memorable and thoroughly enjoyable experience. I would also like to thank him especially for creating a good learning environment in the research group. He has been a true mentor, academically as well as in real life. I would also like to thank Dr. Kenneth P. Roenker and Dr. Fred Beyette along for reviewing my thesis and obliging to be in my defense committee I would also like to express my deep appreciation to my colleagues for helping me out in times of need, and for making the Optoelectronic Devices Lab such a wonderful place to work in. Many thanks go to Hua Tan, Ajit Balagopal, Ashwin Chincholi, Siddartha Banerjee, Hui Shen, Haichuan Mu, Yaling Zhou and Ansuman Banerjee. I would like to especially mention the help and guidance Hua Tan provided during the experimental setup and measurements. Finally, I am grateful to my family in India and to all my friends for their unending love, support and encouragement throughout my academic career. Table of Contents 1. Introduction…………………………………………………......8 1.1 Thesis Overview…………………………………………………………………..8 1.2 Quantum well Lasers ……………………………………………………………..8 1.3 Advantage of introducing quantum wells in semiconductor lasers……………...10 1.4 Multi quantum well lasers……………………………………………………......13 1.5 Advantage of graded index separate confinement heterostructure………………14 1.6 Effect of strain in quantum well lasers…………………………………………..15 1.7 Long wavelength quantum well lasers in optical communication……………….17 1.8 InAlGaAs/InP and Conventional InGaAsP/InP 1.3 µm Quantum well lasers…..18 2. Fabery- Perot In1-x-yAlxGayAs RWG MQW laser structure and determination of optical confinement factor……………………..23 2.1 Introduction………………………………………………………………………23 2.1.2 Design of Fabery-Perot 1.3µm InAlGaAs-InP MQW laser…………………...24 2.1.3 Device Structure of the fabricated InAlGaAs RM-RWG MQW laser 1………25 2.1.3.1 InAlGaAs-InP reverse mesa (RM) RWG lasers……………………………..26 2.1.3.2 Effect of the structural parameters of the active layers………………………29 2.1.3.3 In1-x-yAlxGayAs and In1-xGaxAsyP1-y energy band gap……………………….30 2.1.3.4 In1-xGaxAs energy bandgap…………………………………………………..31 2.1.4 Determination of Refractive index of each layer………………………………32 2.1.5 Optical Confinement Factor Theory…………………………………………...33 2.1.6 Simulation using MODE solutions for InAlGaAs MQW lasers……………….34 1 2.1.7 Effect of well number on optical confinement factor………………………….36 2.1.8 Conclusions…………………………………………………………………….38 3. Effects of well number and the waveguide structure on the performance of InAlGaAs MQW lasers…………………………39 3.1 Introduction………………………………………………………………………39 3.2.1 L-I characteristics plot of 1.3µm RM–RWG InAlGaAs MQW laser………….40 3.2.2 Threshold current dependence on well number for InAlGaAs MQW Laser………………………………………………………………………………….41 3.3 Determination of Slope Efficiency, External Differential Quantum Efficiency an Internal Quantum Efficiency of InAlGaAs MQW laser……………………………..44 3.4 Threshold current dependence on temperature for InAlGaAs MQW Laser……..45 3.5 Conclusion……………………………………………………………………….49 4. Effect of well number in Gain and Intrinsic modulation responses in InAlGaAs MQW lasers ……………………………62 4.1 Introduction………………………………………………………………………62 4.1.1 Experimental details – Gain measurement for InAlGaAs MQW lasers……….63 4.2 Gain measurement of InAlGaAs MQW lasers…………………………………..64 4.3 Gain measurement at various temperatures for InAlGaAs MQW lasers………..66 4.4 Measurement of Optical loss…………………………………………………….71 4.5 Material gain determination for InAlGaAs MQW lasers………………………..73 4.6 Experimental setup for modulation response of InAlGaAs MQW lasers………..74 4.7 Determination of Frequency response for InAlGaAs MQW lasers……………...76 4.8 Conclusion……………………………………………………………………….80 2 5. Results and Future Work……………………………………...81 5.1 Effect of well number and waveguide structure in the optical confinement factor…………………………………………………………………………………81 5.2 Effect of well number and waveguide structure on the performance of the InAlGaAs MQW lasers………………………………………………………………81 5.3 Effect of well number on Gain and Intrinsic modulation response in InAlGaAs MQW lasers………………………………………………………………………….82 5.4 Scope for future work……………………………………………………………82 3 List of Figures Fig. 1.1 Schematic diagram of Multi Quantum well lasers Fig. 2.1 Schematic diagram of 1.3µm Fabery-Perot RM RWG InAlGaAs MQW Fig. 2.2 SEM of 1.3µm InAlGaAs seven QW RM RWG laser Fig. 2.3 SEM of a RWG MQW laser Fig. 2.4 Fabery-Perot 1.3µm InAlGaAs RM RWG Lasers Fig. 2.5 Modal confinement in Fabery–Perot InAlGaAs RM RWG MQW lasers Fig. 3.0 Schematic Setup for Characterization of InAlGaAs MQW Lasers Fig. 3.1 L-I plot for InAlGaAs MQW lasers at room temperature Fig. 3.2 (a) L-I plot for 7 QW InAlGaAs- Device 1 at various temperatures Fig. 3.2 (b) L-I plot for 7 QW InAlGaAs- Device 2 at various temperatures Fig. 3.3 (a) L-I plot for 5 QW InAlGaAs- Device 1 at various temperatures Fig. 3.3 (b) L-I plot for 5 QW InAlGaAs- Device 2 at various temperatures Fig. 3.4 (a) Characteristic Temperature for device-1 7QW InAlGaAs Fig. 3.4 (b) Characteristic Temperature for device-2 7QW InAlGaAs Fig. 3.5 (a) Characteristic Temperature for device-1 5QW InAlGaAs Fig. 3.5 (b) Characteristic Temperature for device-2 5QW InAlGaAs Fig. 3.6 Slope efficiency dependence on temperature for InAlGaAs MQW lasers Fig. 4.1 Schematic representation of the experimental setup for gain measurement Fig. 4.2 Sample plot of modulation response at ASE Fig 4.3 Net Gain at various current below threshold at room temperature (a) 7Qw (b) 5Qw Fig. 4.4 Net Peak gain at various currents below threshold at room temperature 4 Fig. 4.5 (a) Net peak gain (G) versus current – 7 Qw InAlGaAs lasers Fig. 4.5 (b) Net peak gain (G) versus current – 5 Qw InAlGaAs lasers Fig. 4.6 Material Gain of InAlGaAs MQW lasers Fig. 4.7 Schematic diagram of the experimental setup for frequency response of InAlGaAs Fig. 4.8 Frequency response for InAlGaAs MQW lasers Fig. 4.9 (a) Frequency responses at varying temperatures – 5Qw InAlGaAs Lasers Fig. 4.9 (b) Frequency responses at varying temperatures – 7Qw InAlGaAs Lasers Fig. 4.10 D-factor versus temperature for InAlGaAs Lasers 5 List of Tables Table 2.1 Refractive index of the layers of the InAlGaAs MQW laser Table 2.2 Optical Confinement factor for five and seven InAlGaAs quantum well laser Table 3.1 Threshold current and threshold current density of InAlGaAs MQW lasers Table 3.2 Slope efficiency and differential quantum efficiency of InAlGaAs MQW lasers Table 3.3 Characteristic temperature for InAlGaAs MQW lasers Table 3.4 Summary of the DC characteristic measurement of InAlGaAs lasers Table 4.1 Material property of InAlGaAs MQW laser dependence on Well number Table 4.2 Parameters as function of well numbers for InAlGaAs MQW lasers 6 7 Introduction 1.1 Thesis Overview This research is on the study of the effects of the well number in the laser function and intrinsic modulation in both five and seven fabry-perot 1.3µm In1-x-yAlxGayAs multi quantum well reverse mesa ridge and tapered waveguide lasers.