1. Field of the Invention
The present invention relates, in general, to photonic devices, and more particularly to improved laser devices and methods for fabricating them.
2. Description of the Background Art
One of the remaining problems in obtaining low-cost single lateral mode semiconductor lasers is controlling the lateral beam propagating in the laser cavity so that only a single lateral mode is allowed to operate over a wide range of currents and temperatures. One type of laser that has successfully solved this problem is the buried heterostructure laser; however, this success is at the expense of the need to regrow the semiconductor material after an initial patterning. Ridge lasers have eliminated the need for this regrowth; however, there are limitations placed on the width of the ridge as well as the depth to which the ridge can be etched while still allowing a single lateral mode behavior.
Semiconductor, or solid state, lasers are typically fabricated by growing the appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an active layer parallel to the substrate surface. The material is then processed with a variety of semiconductor processing tools to produce a laser cavity incorporating the active layer and incorporating metallic contacts attached to the semiconductor material. Laser mirror facets are typically formed at the ends of the laser cavity by cleaving the semiconductor material to define edges or ends of a laser optical cavity so that when a bias voltage is applied across the contacts, the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow. Since the wafer is cleaved into bars to form the laser facets, conventional lithographical techniques on the wafer cannot be used to further process the lasers.
The problems encountered in semiconductor processing due to the need to cleave laser facets are overcome in U.S. Pat. No. 4,851,368, which discloses a process for forming the mirror facets of semiconductor lasers through etching, allowing lasers to be monolithically integrated with other photonic devices on the same substrate. This work was further extended and a process for fabricating a ridge laser process based on etched facets was disclosed in “Monolithic AlGaAs—GaAs Single Quantum-Well Ridge Lasers Fabricated with Dry-Etched Facets and Ridges”, IEEE Journal of Quantum Electronics, volume 28, No. 5, pages 1227-1231, May 1992. These processes were further improved upon in U.S. patent application Ser. No. 11/356,203 of Alex A. Behfar, filed Feb. 17, 2006, and entitled “High Reliability Etched-Facet Photonic Devices” in which high reliability etched-facet photonic devices are described. However, there is a need for a process for fabricating a photonic device such as a laser without the need for regrowth of the semiconductor material, while providing spatial lateral control of the laser output apart from the etch depth and width of the ridge, and such a laser is extremely desirable.