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«Design and Analysis of Whispering Gallery Mode Semiconductor Lasers Ali T. Hajjiah Dissertation submitted to the faculty of the Virginia Polytechnic ...»

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Design and Analysis of Whispering Gallery

Mode Semiconductor Lasers

Ali T. Hajjiah

Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State

University in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

In

Electrical Engineering

Professor Louis J. Guido Chair

Professor Robert Hendricks

Professor Guo Quan Lu

Professor Ahmad Safaai-Jazi

Professor Levon V. Asryan

January 19th, 2009

Blacksburg, Virginia

Keywords: Semiconductor laser, Whispering Gallery-Mode Copyright 2009, Ali Hajjiah Design and Analysis of Whispering Gallery Mode Semiconductor Lasers Ali T. Hajjiah Abstract Significant technical barriers currently prevent the wide spread adoption of WGM lasers as building blocks in large-scale photonic integrated circuits. The first challenge is to reduce the electrical power consumption at desirable levels of light output power. The second target is to obtain directional light emission without sacrificing other laser performance metrics. The best opportunity for success lies in the pursuit of small µ-Pillar lasers with spiral-geometry cavities.

Process technology has been demonstrated for making high-performance WGM lasers including a refined ICP etching process for fabricating µ-Pillar cavities with sidewall roughness less than 10 nm and a new hydrogenation based approach to achieving current blocking that is compatible with all other processing steps and robust in comparison with earlier reports. A comprehensive photo-mask has been designed that enables investigation of the interplay between device geometry and WGM laser performance. Emphasis has been placed on enabling experiments to determining the impact of diffraction and scattering losses, current and carrier confinement, and surface recombination on electrical/optical device characteristics. In addition, a methodology has been developed for separating out process optimization work from the task of identifying the best means for directional light out-coupling. Our device fabrication methods can be proven on WGM lasers with pure cylindrical symmetry, hence results from these experiments should be independent of any specific light output coupling scheme. Particular attention has been paid to the fact that device geometries that give the best performance for purely symmetrical cavities may not yield the highest level of light emissionfrom the spiral output notch. Such considerations seem to be missing from much of the earlier work reported in the literature.

Finally, our processing techniques and device designs have resulted in individual WGM lasers that outperform those made by competitors. These devices have been incorporated into multielement, coupled-cavity optical circuits thereby laying the groundwork for construction of digital photonic gates that execute AND, OR, and NOT logic functions.

–  –  –

I would like to express my gratitude to the following people:

• My thesis advisor, Prof. L. Guido, whose mentorship and support made my graduate career a truly remarkable experience. Despite his busy schedule, he has always made time for discussion. In addition to the scientific side, he has also provided me with invaluable help in carefully planning and managing the different steps of my research. I have truly learned a lot during my thesis, which goes beyond science itself.

• The members of my thesis committee: Prof. G.Q.Lu, Prof. Ahmad Safaai-Jazi, Prof.

Robert Hendricks, and Prof. Levon Asryan for their guidance and support throughout my Ph.D. study.

• The other members of Prof. Guido’s group, Kevin and Tim for all the support they have given me during my stay here in Virginia Tech.

• All my friends in Blacksburg for their friendship over the past 7 years. They have helped me many times through good and bad times.

• I would like to thank Kuwait University for all the financial support and help they provided me. Without them, it would not have been possible to finish this very difficult journey.

• I would like to give a special thanks to my lovely wife, Alya, who has always supported me during my difficult journey. Her love, understanding and support have made my journey easy.

• Last but not least, I wish to thank my parents, my mother Sara and my father Talib, for their support, love and patience during my entire time at Virginia Tech. In particular my father Talib, who I deeply admire, has provided me with advice and helped me overcome many challenging situations. He has always been an inspiration and motivation source to me. It is to my wife and parents; I wish to dedicate this thesis.

–  –  –

Abstract

Acknowledgments

Table of Contents

List of Figures

List of Tables

1. Introduction

1.1. Historical Perspective

1.2. Problem Statement

1.3. Technical Objectives

1.4. Chapter Overview

References

2. Technical Background

2.1. Design Considerations for Diode Lasers

2.1.1. Compound Semiconductor Alloys

2.1.2. Carrier Injection via P-N Heterojunctions

2.1.3. Separate Confinement Heterostructures





2.1.4. Low Dimensional Gain Media

2.2. Optical Modes in Dielectric Structures

2.2.1. Transverse Modes in Slab Waveguides

2.2.2. Lateral Modes in Ridge Waveguides

2.2.3. WGMs in Cylindrical Cavities

iv Reference

3. Material Structure and Device Processing

3.1. Description of Material Structures

3.2. Overview of Device Processing

Design of µ-Pillar Optical Cavities

3.3.

Layout of µ-Pillar Photomask

3.4.

References

4. Measurement Techniques and Device Analysis

4.1. Instrument Design and Calibration

4.1.1. Apparatus for I-V Measurements

4.1.2. Apparatus for L-I Measurements

4.1.3. Apparatus for Spectral Measurements

4.2. Performance Metrics for Diode Lasers

4.3. Parameter Extraction for Diode Lasers

4.3.1. Analysis of I-V Characteristics

4.3.2. Analysis of L-I Characteristics

4.3.3. Analysis of Emission Spectra

References

5. Process Development for WGM Lasers

Formation of µ-Pillar Optical Cavities

5.1.

5.1.1. Evaluation of Fabrication Methods

5.1.2. Optimization of Dry Etching Process

–  –  –

5.3. Current Blocking via Hydrogen Passivation

References

6. Design Considerations for WGM Lasers

6.1. Optical Losses via Diffraction

6.2. Optical Losses via Scattering

6.3. Electrical Losses via Current Leakage

6.4. Electrical Losses via Carrier Recombination

6.5. Output Coupling from WGM Lasers

6.5.1. Output Coupling via Y-Junctions

6.5.2. Output Coupling via Evanescent Waves

6.5.3. Output Coupling via Spiral Cavities

References

7. Summary and Conclusions

Appendix A: Analysis of Optical Modes in Slab Waveguides

Appendix B: Analysis of Optical Modes in Cylindrical Cavities

Appendix C: Procedures for WGM Laser Fabrication

Appendix D: Definition of Optical Confinement Factor

Appendix E: Design Parameters for WGM Lasers

–  –  –

Figure 1.1: Schematics for the three basic radiative transitions envisioned by Einstein in 1929.

Figure 1.2: Plan view image of St.

Paul’s Cathedral in London. The solid white line traces out the circular propagation path of an acoustic wave confined at the periphery of the dome (http://www.explorestpauls.net/oct03/textMM/DomeConstructionN.htm).

Figure 2.1: Schematic of a broad-area, edge-emitting Fabry-Perot laser................14

Figure 2.2: Bandgap energy vs.

lattice constant for some III-V semiconductors.

The plotted data correspond to room temperature values of these physical quantities (Materials catalogue, EPI Materials Ltd., Lancaster Way, Ely, Cambridge-shire, (CB6 3NW UK)).

Figure 2.3: Energy vs.

distance diagram for a separate-confinement p-n junction heterostructure laser under high forward bias. This state of operation is referred to as “flat-band” since the built-in diode potential is offset by the applied voltage giving rise to a spatially independent energy band structure

Figure 2.4: Energy vs.

position diagram (upper panel) and corresponding refractive index profile (lower panel) for a separate-confinement heterostructure laser. The optical intensity pattern is also shown for the fundamental transverse mode (lower panel).

Figure 2.5: Density of states vs.

energy relationships for laser active regions with different dimensionality. Carriers are free to move without quantum confinement in three (bulk), two (well), one (wire), or zero (dot) dimensions. These spatial configurations are referred to as 3-D, 2-D, 1-D, and 0-D material structures (from left to right).

Figure 2.6: Schematic of a three-layer slab waveguide.

–  –  –

Figure 2.8: Narrow-stripe ridge waveguide laser.

Figure 2.9: Ray optics picture of a WGM as it travels around a dielectric cavity (n2 n1) and in so doing experiences multiple TIR events.

Figure 2.10: Geometry and parameters for a µ-Disk laser cavity.

Figure 2.11: Radial intensity profiles for two different TM polarized modes confined within a Gallium Nitride µ-Disk cavity with R0 = 2.

5 µm and n = 2.65..29 Figure 3.1: Schematic cross-section of AlGaAs/GaAs QW laser structure (right hand side) and corresponding SEM image (left hand side). The SEM image was taken (after cleaving and staining the sample) using the LEO 1550 instrument at Virginia Tech.

Figure 3.2: SIMS depth profiles of the alloy constituents (Al, Ga, As) and the doping species (Mg, Se) in the AlGaAs/GaAs QW laser structure.

These measurements were made using the Cameca IMS 7f GEO instrument at Virginia Tech.

Figure 3.3: Cross-section of the WGM laser structure with oxide as the current blocking region.

Figure 3.4: Plan-view drawing of full-circle (FC) optical cavity.

Figure 3.5: Plan-view drawing of partial-circle (PC) optical cavities: (a) ¾-circle, (b) ½-circle, and (c) ¼-circle dielectric structure.

Figure 3.6: Plan-view drawing of spiral (Sp) optical cavity.

Figure 3.7: Optical image of WGM laser mask designed at Virginia Tech using Ledit software from Tanner Research.

The photo-mask was manufactured by Image Technology using advanced optical lithography techniques.

–  –  –

Figure 4.3: Comparison of I-V data acquired under both CW and PW conditions at 300 K.

The DUT was a commercial laser diode (NVG D650-5). The resistance values shown in the legend correspond to R in the current divider circuit..............49 Figure 4.4: Apparatus for measuring L-I curves under CW excitation..................50 Figure 4.5: Apparatus for measuring L-I curves under PW excitation..................52 Figure 4.6: Input (current) and output (detector) waveforms from a typical pulsedwave L-I measurement run. The current source pulses were 2 µs in duration with a repetition frequency of 200 Hz. The pulses delivered to the resistor network were swept from 0.8 to 3.2 A which corresponds to bias currents ranging from 8 to 32 mA. The DUT was a NVG D650-5 laser diode

Figure 4.7: L-I data recorded under both CW and PW conditions at 300 K.

The DUT was a NVG D650-5 laser diode. The reference CW data were plotted using the Y-axis labeled “light power” on the left hand side. The original PW data were plotted against “detector voltage” (right hand side). The solid line represents the corrected PW curve with its Y-axis values in mW.

Figure 4.8: Apparatus for measuring optical spectra under CW excitation...........55

Figure 4.9: Spectral radiant flux measurements under PW excitation at 300 K.

The DUT is an oxide-confined AlGaAs/GaAs QW spiral laser fabricated at Virginia Tech. The nominal radius of the spiral cavity is 250 µm and the width of the p-metal contact ring is 25 µm. The laser material used to fabricate this device is identical to that described in Chapter 3 of this manuscript.

Figure 4.10: I-V data recorded using CW drive conditions at 300 K.

The DUT is an oxide-confined AlGaAs/GaAs QW spiral laser fabricated at Virginia Tech. The nominal radius of the spiral cavity is 250 µm and the width of the p-metal contact ring is 25 µm. The laser material used to fabricate this device is identical to that described in Chapter 3 of this manuscript.

Figure 4.11: I-V data recorded using CW drive conditions at 300 K.

The DUT is an oxide-confined AlGaAs/GaAs QW spiral laser fabricated at Virginia Tech. The ix nominal radius of the spiral cavity is 250 µm and the width of the p-metal contact ring is 25 µm. The laser material used to fabricate this device is identical to that described in Chapter 3 of this manuscript.

Figure 4.12: L-I data recorded under PW (symbols) and CW (solid line) excitation at 300 K.

The DUT is an edge-emitting AlGaAs/GaAs QW laser diode with a pcontact stripe width of 100 µm and a cavity length of 1000 µm.

Figure 4.13: Optical spectra recorded under CW excitation at 300 K.

This laser diode was fabricated in 1995 by an industrial collaborator using material nearly identical to that described in Chapter 3 of this manuscript.

Figure 5.1: 3-D view of test structure used during wet and/or dry etching runs.



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