IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 1, JANUARY 1998 45 Low-Threshold Laterally Oxidized GaInP–AlGaInP Quantum-Well Laser Diodes P. D. Floyd, Member, IEEE, D. Sun, Member, IEEE, and D. W. Treat Abstract—Low-threshold, high-efficiency edge-emitting visible AlGaInP–GaInP laser diodes using a buried AlAs native oxides for carrier and optical confinement are described. The lasers incorporate a thin AlAs layer in the upper cladding region, which when laterally wet oxidized, forms a narrow aperture. The lasers operate with room temperature, continuous-wave (CW) threshold currents of 11 mA with external differential quantum efficiency of 34% per facet for an uncoated 300- m-long 3.5- m-wide device. As-fabricated lasers exhibited modest performance under CW operation. Post-fabrication annealing was shown to dramatically improve the device characteristics. Index Terms— AlGaInP, annealing, lasers, lateral oxidation, native oxide, quantum-well lasers, semiconductor lasers. H IGH-EFFICIENCY AlGaInP visible laser diodes are of interest as light sources for laser printing and optical storage applications. Many AlGaInP laser structures utilize a regrowth of GaAs over an etched ridge to achieve carrier and optical confinement [1]. However, such structures often have low external differential quantum efficiency due to optical absorption loss in the GaAs. This can be addressed by use of wider bandgap regrown layers, such as AlInP, to form an index guided laser structure [2]. However, this results in more complex crystal regrowth. In this letter, we demonstrate the use of thin, buried layers of AlAs native oxides for optical and carrier confinement in AlGaInP visible laser diodes. Such oxide layers are created by mesa etching and lateral wet oxidation of a buried AlAs layer, starting at the mesa edge. This allows creation of narrow stripes, without optical losses caused by rough sidewalls of ridge structures or losses caused by absorption in buried ridge structures. Previously, such layers have been used for confinement in both vertical-cavity lasers [3], [4] and in edge- emitting lasers [5], [6], but not in the AlGaInP-based, edge emitting lasers. The laser heterostructure used in this experiment was grown by low-pressure organometallic vapor phase epitaxy (OMVPE) on a n GaAs substrate, 10 misoriented toward (111) A. The cladding layers are 1- m-thick p- and n-type Al In P and the separate confinement heterostructure (SCH) consists of two 0.12- m-thick Al Ga In layers surrounding an 80- ˚ A Ga In P quantum well. This basic structure is typical Manuscript received August 1, 1997; revised September 11, 1997. This work was supported in part by the Department of Commerce Advanced Technology Program under Grant 70NAN82H1241. The authors are with the Electronic Materials Laboratory, Xerox Palo Alto Research Center, Palo Alto, CA 94304 USA. Publisher Item Identifier S 1041-1135(98)00462-5. Fig. 1. Schematic diagram of the laser heterostructure. of visible laser structures grown in our laboratory. However, an additional 500- ˚ A layer of p-type AlAs is included, located above the SCH. The layer structure is shown schematically in Fig. 1. Laterally oxidized lasers were fabricated by capping the laser structure with SiO , to prevent oxidation of the GaAs contact layer, followed by wet etching of 50- m-wide ridges to expose the edge of the AlAs layer. Wet oxidation of the AlAs is performed in a saturated water vapor environment, in a furnace at 440 C. The AlAs layer is converted to oxide starting at the ridge edge, proceeding toward the center of ridge. The resulting oxidized region is illustrated in Fig. 1. Oxidation rates of 1.2–1.5 m/min were observed over lateral distances of 30–40 m. After oxidation, the SiO layer is removed in CF plasma. Ti–Au contacts are evaporated for p-contacts and AuGe is evaporated on the back of the polished and thinned substrate to form the n-contact. The AuGe contact is alloyed for 2.5 minutes at 350 C, then laser bars are cleaved for testing. Laser testing is performed p-side up on a temperature controlled stage. Pulsed mode testing of a 3.5- m-wide, 625- m-long device resulted in a threshold of 20 mA with 10.8%/facet external differential quantum efficiency . This value of compares poorly with external efficiency values of 18%/facet measured in GaAs-buried ridge waveguide lasers and 45%/facet measured in metal-clad ridge waveguide lasers of similar stripe width [7], [8]. It has been observed that AlGaInP-based laser diode per- formance can be improved by thermal annealing [9], [10]. The observed reduction of threshold current and increase in are related to the dissociation of acceptor-hydrogen complexes. The free carrier concentration of the p-doped cladding increases because the hydrogen no longer reduces the acceptor activation. This increase in free hole concentration in the cladding region results in better electron confinement, 1041–1135/98$10.00  1998 IEEE