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November, 2004 EEEL Models AlGaAs Oxide Formation for Vertical Cavity Lasers |
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Researchers in the Optoelectronics Division of EEEL have developed a model of AlxGa1-xAs oxidation kinetics and the resulting oxide structure, which will assist manufacturers in improving reproducibility and yield of devices containing AlGaAs oxide layers. Due to its ability to provide both electrical and optical confinement, the "native" oxide of AlGaAs has played a major role in compound semiconductor devices, particularly vertical-cavity surface-emitting lasers (VCSELs), which constitute a large fraction of the lasers used in telecommunications. Yet in spite of the importance of this oxide and the numerous investigations it has undergone, a detailed understanding of the oxide phases and microstructure has remained elusive. The structure of the oxide is important because the contraction of AlGaAs layers during oxidation causes strain in the surrounding lattice, which can lead to device failure. The oxide structure also affects the oxidation kinetics, which manufacturers have found difficult to control. Through a combination of kinetic and thermodynamic modeling, EEEL researchers in collaboration with VCSEL manufacturers and the Colorado School of Mines have developed a model of the oxide structure and of the kinetics controlling the oxide formation in water vapor. The oxidation kinetics of AlGaAs epitaxial layers were measured and the results analyzed with a modified Deal-Grove model. This allowed separation of the surface reaction and diffusion components of the oxidation rate. The computed effective diffusion coefficients are large and temperature-insensitive, suggesting gas diffusion through a porous oxidation product layer. Knudsen diffusion in the porous product layer was predicted, and the pore size in the product layer was computed to be about 0.5 nm, consistent with atomic force microscopy measurements. The thermodynamic calculations show that the equilibrium products of oxidation are Al2O3, Ga2O3, and As, and the main gaseous species containing arsenic is As4. This is in opposition to earlier reports which proposed As2O3 and AsH3 to be the primary oxidation products for As. Identification of the primary oxide phases allowed development of a model for the resulting oxide structure, which includes regions of different phases and densities. Diffusion only becomes important and gives nonlinear kinetics when an As-containing layer forms, which depends on the ratio of the reaction rate constant, k, and the equilibrium As4 pressure, both of which are exponentially temperature dependent. Since the activation energy for k varies with composition, the role of temperature and the onset of the importance of diffusion vary as the Al content changes. Controlling the relative thicknesses of the different phase regions determines the rate of oxidation and, because of the different densities in the different regions, can be used by VCSEL manufacturers to minimize the strain associated with the oxide formation.
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