This study has focused on the design of active layers, the growth of the DBRs, the process of the LW-VCSEL structures. Two different material systems have been studied. The monolithically InP-based VCSELs are chosen since the potential of single epitaxial growth ensures the practical devices for mass production. Although the capability of mass production for wafer fused VCSELs is still questionable, the excellent thermal and optical properties of GaAs/AlAs DBRs as well as the capability of the oxidation process make the wafer fused VCSELs worth of studying.
Chapter 2 reviews fundamentals in semiconductor lasers at beginning. The origin of differences between EELs and VCSELs will be discussed. Then, general operation principles of VCSELs including light-current characteristics, the relationship between gain and current, the gain peak and cavity mode alignment, the characteristics of DBRs, the analysis of the heat flow will be introduced and characterized with the use of simulation software. With specific conditions in
requirement of LW-VCSELs, the fundamental issues in design of LW-VCSELs will be discussed at the end of the chapter.
Chapter 3 mainly describes the fabrication method for LW-VCSELs. Metal organic chemical vapor deposition (MOCVD) systems have been used for growing all the epitaxial structures in this study. Since the epitaxial equipment and process determine most of the characteristics of LW-VCSELs, detailed descriptions and specific functions of this MOCVD system are given. The growth process, regrowth process and in-situ monitoring will also be addressed. Wafer fusion technique, which is the other special and important process step in fabrication of LW-VCSELs, will also be introduced.
The characteristics of gain medium suitable for LW-VCSEL have been discussed at the beginning of chapter 4. By considering material quality and limitations of process equipment, the InGaAlAs system lattice-matched to InP has been chosen as the active layers in this study. The optimized layer structures have been determined by investigating performance of EELs with InGaAlAs multiple quantum wells (MQW) as the active layers.
Chapter 5 reports several different fabrication methods for DBRs used in long wavelength range. The optical and electrical properties of different DBRs will also be studied. Followed by the comparisons of novel InP/InGaAlAs and conventional InAlAs/InGaAlAs DBRs, the extremely high reflectivity mirror made by InP/Air-gap DBRs will be discussed.
The feasibility of the active layers and DBRs is first examined by the performance of optically pumped LW-VCSELs. Chapter 6 reports the structures of LW-VCSELs for optical pumping, including the InP-lattice-matched and wafer-fused structures.
Chapter 7 reports several different approaches to make electrically driven
LW-VCSELs, including monolithically InP-based and wafer-fusion type devices. The ion-implantation or buried tunnel junction were used to make current apertures in devices. Although the goal to make electrically driven devices has not yet accomplished, efforts resulted from this study have led to develop other interesting devices. In addition, basic physical phenomenon and material issues observed in this study will turn into useful information in making electrically driven LW-VCSELs.
Chapter 8 is the summary of this thesis.
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Table 1-1 Comparison of features for EEL vs VCSEL [8]
Feature EEL VCSEL Spectral bandwidth Very narrow Narrow
Size of active area Typically 0.5-1×2-10 µm Variable, 5-50 µm in diameter Beam geometry Strong elliptic Circular
Beam divergence High, up to 60°×20° Low, 5°
Number of modes Typically 1 or few 1 or even up to many 10s Coupling to fiber Difficult and sensitive Easy
Coupling efficiency Moderate High
Threshold current Approximately 10 mA Some mA Direct modulation
bandwidth
High, up to 10 Gbit/s High, up to 10 Gbit/s
Temperature drift of Pop Fairly high Tendentially low Environmental
sensitivity
Extremely high Moderate
Processing of chip Very specific Similar to LED
Final processing Single bar On wafer
Burn-in and functional test
Single on heatsink On wafer