Design And Fabrication Of 1310nm Vertical-Cavity Surface-Emitting Lasers

Vertical-cavity surface-emitting lasers (VCSELs) are important optical sources for fiber-optic communication. 1310nm VCSELs are adapted to the demands of high-capacity Gigabyte Ethernet-based Metro Area Networks (MAN). The development of LW-VCSELs, however, is confined by the poor performances of InP-based distributed Bragg reflectors (DBRs). To solve this problem, several methods such as InP/GaAs hetero-integration, epitaxy of InP-based Sb-compound DBRs and GaAs-based active materials are applied to the fabrication of 1310nm VCSELs. This thesis has designed a composite structure of 1310nm VCSELs in which InP-based InAsP/InGaAsP strain-compensated multi-quantum wells (SC-MQWs) are sandwiched between SiO₂/TiO₂ dielectric DBRs and GaAs/AlAs DBRs. The lasers have been fabricated by structure design, materials growth, wafer direct-bonding and device processing. Main research works are as follows:1. We design the structure of VCSELs in three aspects: active region, resonant cavity and effective carrier injection. First, energy bands of InAsP/InGaAsP SC-MQWs are calculated by employing effective mass model, and structure parameters such as thickness and element component of the well and barrier are optimized so that maximum electron-confinement energy can be obtained. Second, we provide an intuitive method to qualitatively evaluate the reflectivity of periodic structures. The cavity length and DBRs' sequence of VCSELs with the structure of SiO₂/TiO₂ DBR-cavity-GaAs/AlAs DBR are then determined by this method for obtaining F-P resonance and effective reflection from DBRs. The resonance features are confirmed by calculating optical distribution using transfer-maxtrix method. The threshold conditions of VCSELs are analyzed and the relationships of DBR reflectivity and well numbers are obtained to achieve lower threshold current. We then provide an optimizing scheme for high device perfomances. The carrier injection efficiency is of vital importance for VCSELs. Based on the self-consitent solution to Poisson equation and carrier diffusion, we calculate carrier distribution in active layer under different structures of current-injection region, and propose the approaches for effective carrier injection into the active region.2. To obtain lower optical absorption and higher conductivity of materials, we optimize the doping of M-GaAs/AlAs DBRs. Then we grow GaAs/AlAs DBRs and InP-based resonant cavities including InAsP/InGaAsP MQWs using gas-souce molecular-beam epitaxy (GSMBE). Considering the requirement of precisely control of DBR central wavelength and resonant cavity length, we can easily adjust the DBR central wavelength by employing Fabry-Perot (F-P) resonators, and grows InP/InGaAsP superlattice in the InP-based cavity so that specific resonance wavelength can be realized in device processing.3. We develope the technologies of InP/GaAs wafer direct-bonding. By designing a experimental fixture and optimizing the process of wafer-bonding, we fabricate uniformly bonded InP/GaAs wafer-pairs. In process, infrared-spectra method and high-resolution X-ray diffraction (HRXRD) are used to characterize bonding quality and the InP epilayer on GaAs substrate respectively. Results show that 0.044% of residual strain and 0.11°of <001> tilt angle exist in the sample with bonding process of (001) InP layer and GaAs substrate. This result confirms uniform bonding between InP and GaAs materials.4. We evaluate the effects of high-temperature bonding process on material performances. Experimential results show that the barrier height of bonded InP/GaAs hetero-junction is close to the theoretic value. By employing superlattice structures in InP-based materials and higher bonding temperatures, electrical performances of bonding samples can be improved. Owing to the high-temperature stability, InAsP/InGaAsP MQWs upon bonding process has comparable luminescent performances with those of as-grown ones even when the bonding process is up to 650°C. Thus this material system is adapted to hetero-integration using wafer direct-bonding techniques. However, high-temperature annealing triggers the disordering of InP/GaAs interface, which introduces excess optical loss and blue-shift of cavity mode in VCSELs. We evaluate optical loss by using wafer-bonded F-P structures. Simulated Results show that the vaule of optical loss introdued by bonding interface is comparable to the loss of tunnel junction and aperture scattering etc. Lowering annealing temperatures and employing superlattice at the interface is beneficial to lower optical loss of bonding interface.5. Bottom-emitting devices of 1310nm VCSELs are fabricated by device processing including high-temperature wafer direct-bonding of InP-based active layer and GaAs-based DBRs, circular mesa etching, current aperture definition, metal contact and dielectric DBRs' deposition. This device is lasered under room-temperature pulse condition with threshold current density of 7.6 kA/cm². Single mode with wavelength of 1288.6 nm and linewidth of 0.38nm is obtained at the threshold. While large current injection inspires multimode features, which is ascribed to the large aperture (15μm) in our devices. Analytical results show that optimization of the structure and device with smaller current aperture (<8μm), high reflectiviy of GaAs-based DBR and specific control of gain-offset should be account for the room-temperature continuous-wave laser performances.The basic design of 1310nm VCSELs, key technologies developed in this thesis such as material growth of tunnel junction, wafer direct-bonding and fabrication of current aperture etc are the groundworks of future VCSEL devices for performances optimizing.