Improvements Toward High-Efficiency Vertical-Cavity Surface-Emitting Lasers

Vertical-cavity surface-emitting lasers (VCSELs) have been proposed for many applications including communications, two-dimensional optical interconnects, optical computing, and image processing. The vertical optical cavity and surface-normal output provide advantages in two-dimensional packing density, output beam size and quality, and fabrication ease as compared with edge-emitting designs. Furthermore, these devices inherently operate in a single longitudinal mode due to the very short optical cavities. The vertical geometry, however, places severe demands on the growth and design of high-efficiency lasers. The large longitudinal mode spacing, for example, places the tolerances on layer thickness control at or beyond the limits of conventional crystal growth calibration methods. If the spectral position of the cavity mode, determined by the cavity thickness, does not overlap correctly with the material gain, the laser will have a high threshold current if it operates at all. Additionally, the multiple heterointerfaces of the distributed Bragg reflector (DBR) mirrors cause the devices to exhibit high series resistance. This reduces efficiency not only due to the electrical power lost, but also because the ohmic heating; seriously degrades the intrinsic optical performance of these lasers. This thesis describes improvements in crystal growth techniques and design of VCSELs to achieve high -efficiency operation. An in-situ growth characterization technique was developed to accurately position the cavity mode of a VCSEL at the designed wavelength during growth. Compositional grading and modulation doping are utilized to decrease the resistance of the p-type DBR mirror. A wet-etching process was developed for realizing an intracavity contact to bypass the resistance of the n-type mirror. These improvements should combine to yield high-efficiency vertical-cavity surface-emitting lasers.