Multi-alloy structures for injectorless quantum cascade lasers [Elektronische Ressource] / Casimir Richard Simeon Katz

Katz H.

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• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2010
• Nombre de lectures: 32
• Poids de l'ouvrage: 6 Mo

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TECHNISCHE UNIVERSITÄT MÜNCHEN
Lehrstuhl für Halbleitertechnologie
Walter Schottky Institut



Multi-Alloy Structures for Injectorless
Quantum Cascade Lasers


Casimir Richard Simeon Katz



Vollständiger Abdruck der von der Fakultät für Elektrotechnik und Informationstechnik
der Technischen Universität München zur Erlangung des akademischen Grades eines
Doktor-Ingenieurs
genehmigten Dissertation.



Vorsitzender: Univ.-Prof. Dr. E. Biebl
Prüfer der Dissertation: 1. Univ.-Prof. Dr. M.-Chr. Amann
2. Univ.-Prof. P. Lugli, Ph.D.



Die Dissertation wurde am 04.03.2010 bei der Technischen Universität München eingereicht
und durch die Fakultät für Elektrotechnik und Informationstechnik
am 24.11.2010 angenommen. Index


Index
ABSTRACT  4 
INTRODUCTION TO QUANTUM CASCADE LASERS  7 
Applications for lasers  8 
Operation principles of quantum cascade lasers  10 
State of the art devices at the beginning of this thesis  12 
Focus of this thesis  13 
THEORY AND DESIGN OF INJECTORLESS QUANTUM CASCADE LASERS  14 
2.1  Electrical Model and Theory  15 
2.1.1  Self‐consistent Schrödinger equation in one dimension  17 
2.1.2  Intersubband material gain  18 
2.1.3  Scattering by optical phonons  19 
2.1.4 g by acoustical phonons  20 
2.1.5  Scattering by interface defects  21 
2.1.6  Summary of scattering mechanisms  21 
2.1.7  Electrical losses  22 
2.2  Materials for the electrical design  24 
2.2.1  Conduction band, effective mass and band gap  25 
2.2.2  Lattice matched and strain balanced  26 
2.3  Design and optimization  28 
2.3.1  Design optimization by evolution algorithm  29 
2.3.2  Designs for mid infrared devices based on InP  31 
2.4  Optical Model and Theory  33 
2.4.1  Optical Resonator  33 
2.4.2  TM‐Wave propagation in a slab waveguide  34 
2.4.3  Plasmon waveguides for TM polarized waves  36 
2.4.4  Confinement factor and modal gain  37 
2.4.5  Optical losses  37 
2.5  Materials for the optical design  40 
2.5.1  InP based waveguide materials for mid infrared devices  41 
2.5.2  GaSb based waveguide materials for the mid infrared devices  41 
2.6  Laser characteristics  43 
 
‐ 1 ‐ Index


2.7  Thermal Model and Theory  45 
2.7.1  Thermal conductivity  45 
2.7.2  Heating effects on device performance  45 
2.7.3  Material dependent thermal conductivity  46 
2.7.4  Finite element analysis of thermal designs  47 
PROCESS TECHNOLOGY  49 
3.1  Molecular Beam Epitaxy of Devices  49 
Growth control during and after the process  51 
3.2  Process Technology of Devices for the mid and far infrared  53 
3.2.1  Standard process for mid infrared devices  53 
3.2.2  Continuous wave process for mid infrared  54 
3.2.3  Facet treatment for high reflective coatings  56 
3.3  Setup Technology of Devices  57 
3.3.1  Standard setup for pulsed devices in the mid infrared  57 
3.3.2  Continuous wave setups  57 
3.4  Characterization of Devices  59 
3.4.1  Measurement setup for mid infrared devices  59 
3.4.2  Measurement techniques for mid infrared devices  60 
RESULTS AND DISCUSSION  64 
4.1  Low threshold devices  64 
4.1.1  Evolution optimized simple four alloy device  65 
4.1.2  Reproducibility of Growths and Sample Statistics  67 
4.1.3  Gain Spectra of Injectorless Devices  69 
4.2  Continuous wave devices  74 
4.2.1  Double‐trench process of two alloy reference sample  74 
4.2.2  p‐doped InP overgrowths  75 
4.2.3  Simple ridge process of optimized four alloy sample  75 
4.2.4  Broad Double Channel Process of Four Alloy Sample  77 
4.3  High performance devices  79 
4.3.1  Optimization of performance by emission wavelength and upper state lifetime  79 
4.3.2  Beam propagation and power collection efficiency  82 
4.4  Voltage defect and injection behavior  86 
4.4.1  Voltage defect and electric field  86 
4.4.2  Injection behavior investigations  89 
4.4.3  Transit time and negative differential resistance  92 
‐ 2 ‐


CONCLUSION  97 
APPENDIX A ‐ ABBREVIATIONS AND SYMBOLS  100 
APPENDIX B ‐ REFERENCES  104 
APPENDIX C ‐ PROCESS DETAILS FOR CW DEVICES  112 
APPENDIX D ‐ EVOLUTION ALGORITHM  117 
APPENDIX E ‐ DETAILED DIELECTRIC FUNCTION  120 
APPENDIX F – PREEXAMINATIONS IN RELATED FIELDS  123 
GaSb based devices  123 
Far infrared devices  126 
APPENDIX G ‐ PUBLICATIONS  128 
ACKNOWLEDGMENTS  131 


‐ 3 ‐ Abstract


Abstract
Since the first realization of quantum cascade lasers in 1994, they have been steadily improved
and reach more than 3 W continuous-wave output power at room temperature. Nearly all devices today
use an intermediate superlattice, which creates an artificial miniband for moderating the electron
injection. This injector was necessary for the first successful devices, as all concepts without had failed.
The original concept, as suggested by Suris and Kazarinov, discards this injector and requires direct
injection of electrons from the ground state into the subsequent upper laser level, and is therefore called
injectorless. With this more compact design, the gain and the slope efficiency can be strongly improved,
although the complexity of designs increases.
This work continues the successful work of the previous doctoral candidates G. Scarpa and A.
Friedrich, who developed very good pulsed injectorless devices, with pulsed threshold current densities
3
of 0.73 kA/cm² and threshold power densities of 70 MW/cm at room temperature for an emission
wavelength of 6.8 µm. Samples with higher threshold current density reached pulsed optical output
powers of 240 mW, yielding an overall efficiency of 2.4 %. Devices with shorter wavelengths achieved
2 2pulsed threshold current densities of 2.75 kA/cm at 5.8 µm and more than 3 kA/cm at 4.6 µm, as high
electric fields decrease the performance. At 7.9 µm the best pulsed threshold current density was found
2
to be 2.4 kA/cm at 300 K.
As all of the previous designs only used the two alloys Al In As and Ga In As, with the x 1-x x 1-x
exception of AlAs enhanced barriers, the concept of structures using multiple materials was the starting
idea of this thesis. Using more than two alloys for the design increases the degree of freedom and should
therefore lead to an improvement in performance. Similar approaches with injectorbased devices could
improve neither threshold current density nor output power performance.
With the full range of materials in the AlGaInAs system being in consideration, the complexity
in design strongly increases and the growth becomes more challenging. Therefore the first goal of this
thesis was t