University of Central Florida STARS Electronic Theses and Dissertations, 2004-2019 2010 New Laser Technologies Analysis Of Quantum Dot And Lithographic Laser Diodes Abdullah Demir University of Central Florida Part of the Electromagnetics and Photonics Commons, and the Optics Commons Find similar works at: https://stars.library.ucf.edu/etd University of Central Florida Libraries http://library.ucf.edu This Doctoral Dissertation (Open Access) is brought to you for free and open access by STARS. It has been accepted for inclusion in Electronic Theses and Dissertations, 2004-2019 by an authorized administrator of STARS. For more information, please contact [email protected]. STARS Citation Demir, Abdullah, "New Laser Technologies Analysis Of Quantum Dot And Lithographic Laser Diodes" (2010). Electronic Theses and Dissertations, 2004-2019. 1561. https://stars.library.ucf.edu/etd/1561 NEW LASER TECHNOLOGIES: ANALYSIS OF QUANTUM DOT AND LITHOGRAPHIC LASER DIODES by ABDULLAH DEMIR B.S. and M.Sc. Koc University 2002, 2005 M.Sc. University of Central Florida 2009 A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the College of Optics and Photonics at the University of Central Florida Orlando, Florida Summer Term 2010 Major Professor: Dennis G. Deppe © 2010 Abdullah Demir ii ABSTRACT The first part of this dissertation presents a comprehensive study of quantum dot (QD) lasers threshold characteristics. The threshold temperature dependence of a QD laser diode is studied in different limits of p-doping, hole level spacing and inhomogeneous broadening. Theoretical analysis shows that the threshold current of a QD laser in the limit of uniform QDs is not temperature independent and actually more temperature sensitive than the quantum well laser. The results also explain the experimental trends of negative characteristic temperature observed in QD lasers and clarify how the carrier distribution mechanisms inside and among the QDs affect the threshold temperature dependence of a QD laser diode. The second part is on the experimental demonstration of lithographic lasers. Today’s vertical-cavity surface-emitting lasers (VCSELs) based on oxide-aperture suffer from serious problems such as heat dissipation, internal strain, reliability, uniformity and size scaling. The lithographic laser provides solutions to all these problems. The transverse mode and cavity are defined using only lithography and epitaxial crystal growth providing simultaneous mode- and current-confinement. Eliminating the oxide aperture is shown to reduce the thermal resistance of the device and leading to increased power density in smaller lasers. When it is combined with better mode matching to gain for smaller devices, high output power density of 58 kW/cm2 is possible for a 3 μm VCSEL with threshold current of 260 μA. These VCSELs also have grating- free single-mode single-polarization emission. The demonstration of lithographic laser diodes with good scaling properties is therefore an important step toward producing ultra-small size laser diodes with high output power density, high speed, high manufacturability and high iii reliability. Lithographic VCSELs ability to control size lithographically in a strain-free, high efficiency device is a major milestone in VCSEL technology. iv To my nieces Fatma and Derya v TABLE OF CONTENTS LIST OF FIGURES..........................................................................................................viii LIST OF TABLES...............................................................................................................x LIST OF ACRONYMS/ABBREVIATIONS.....................................................................xi CHAPTER 1: INTRODUCTION AND OUTLINE .......................................................1 CHAPTER 2: QUANTUM DOT LASER MODEL.......................................................4 2.1 Introduction......................................................................................................................... 4 2.2 Analysis of the Temperature Dependence of Threshold..................................................... 7 2.3 Non-Equilibrium Rate Equation Model............................................................................ 10 2.4 Threshold Temperature Dependence for a QD Laser....................................................... 13 2.5 Summary........................................................................................................................... 16 CHAPTER 3: MODEL IN THE LIMIT OF UNIFORM QUANTUM DOTS ............18 3.1 Introduction....................................................................................................................... 18 3.2 Uniform QDs with Energetically Isolated Ground State Transitions ............................... 21 3.3 Uniform QDs with Multiple Discerete Levels.................................................................. 25 3.4 Summary........................................................................................................................... 30 CHAPTER 4: DESIGN PRINCIPLES OF LITHOGRAPHIC LASER.......................32 4.1 Introduction....................................................................................................................... 32 4.2 Oxide-VCSEL Issues and Limitations.............................................................................. 34 4.3 Optical Mode Confinement............................................................................................... 37 4.4 Current Confinement ........................................................................................................ 40 4.5 Summary........................................................................................................................... 44 CHAPTER 5: EXPERIMENTAL RESULTS ON LITHOGRAPHIC VCSEL ...........45 5.1 Introduction....................................................................................................................... 45 5.2 Device Structure................................................................................................................ 45 vi 5.3 Results............................................................................................................................... 48 5.3.1 Lasing Characteristics...................................................................................... 48 5.3.2 Low Thermal Resistance of Lithographic Structure........................................ 50 5.3.3 High Output Power Density of Small Devices................................................ 52 5.3.4 Single-Mode and Single-Polarization Lasing.................................................. 53 5.3.5 Reliability ........................................................................................................ 57 5.4 Summary........................................................................................................................... 59 CHAPTER 6: SUMMARY...........................................................................................60 LIST OF REFERENCES ..................................................................................................62 vii LIST OF FIGURES Figure 2-1: Atomic force microscope photograph of crystal surface right after QD formation. ..................4 Figure 2-2: Schematic illustration of the energy structure and Fermi levels of two different QDs. Non-equilibrium distribution of electrons is created due to finite carrier relaxation from the barrier and carrier transport between the QDs through wetting layer......................................................5 Figure 2-3: The effect of inhomogeneous broadening on threshold temperature dependence for undoped (squares) and p-doped (circles) QD laser with (a) uniform QDs and (b) inhomogeneously broadened QDs.........................................................................................................15 Figure 3-1: Temperature dependence of dephasing rate. The inset shows the calculated homogeneous broadening at T = −40 °C and T = 40 °C. ......................................................................20 Figure 3-2: Characteristic temperature versus threshold population inversion of p-doped and undoped QD lasers. The inset shows the schematic of QD energy levels with single electron and hole states with acceptor doping. ....................................................................................................22 Figure 3-3: Temperature dependence of normalized threshold current for p-doped planar quantum well and QD lasers.................................................................................................................................24 Figure 3-4: Temperature dependence of threshold current for a multistate undoped QD laser at the 300 K transparency and threshold population inversions of 0.042, 0.113 and 0.182. The T0’s are calculated for 20 °C ≤ T ≤ 80 °C. The inset shows the schematic of energy levels in a multistate QD laser. ...............................................................................................................................26 Figure 3-5: Temperature dependence of threshold current for a multistate undoped QD laser at the 300 K transparency and threshold population inversions of 0.042, 0.113 and 0.182. The T0’s are calculated for 20 °C ≤ T ≤ 80 °C. The inset shows the schematic of energy levels in a multistate QD laser. ...............................................................................................................................28