Anisotropy in the wet thermal oxidation of AlGaAs: influence of process parameters

Abstract

The crystallographic anisotropy of the lateral selective thermal oxidation of AlGaAs alloys is experimentally studied. The anisotropic behavior of this oxidation process, used primarily for building a lateral confinement in vertical surface emitting lasers (VCSEL), is quantified by varying different process parameters and the geometrical shapes of laterally oxidized mesa structures. This experimental study aims to have a better control of the oxide aperture shape used in oxide-confined photonics devices.

Modeling the Lateral Wet Oxidation ofAlxGa1−xAsinto Arbitrary Mesa Geometries

“…The sample is then oxidized in a furnace equipped with an in situ infrared reflectometry imaging system, allowing us to monitor the time-dependent evolution of the oxidation front with a spatial resolution of 0.8 μm. The sample is oxidized in conditions leading to anisotropic oxidation [11], more precisely, using a reduced-pressure (0.5 atm) environment, a substrate temperature of 400 • C, and a mixed H 2 , N 2 and H 2 O gas steam generated by an evaporator-mixer system operating at 95 • C.…”

“…Interestingly, the wet oxidation process is not an isotropic process and the speed of the oxidation front depends on the crystallographic orientation, as shown in Refs. [10] and [11]. With present-day techniques, current apertures with micron-size diameters can be obtained, but they are difficult to control and to manipulate [12].…”

Modelling anisotropic lateral oxidation from circular mesas

“…The oxidation was carried out in a furnace with an in-situ infrared reflectometry imaging system allowing the time-dependent evolution of the oxidation front to be monitored with a spatial resolution of 0.8 µm. The sample was oxidized in conditions leading to anisotropic oxidation [28], more precisely, using a reduced-pressure (∼0.5 atm.) environment, a substrate temperature of 400 °C and a mixed H 2 /N 2 /H 2 O gas steam generated by an evaporator-mixer system operating at 95 °C.…”

“…In all these models, the process is assumed to be isotropic and, therefore, the shape of the oxide aperture would then readily be inferred from the etched mesa geometry by a homothetic transformation. It has, however, been experimentally established that the Al-III-V semiconductor oxidation is anisotropic [24] [25], which, in turns, means that the shape oxide aperture can significantly differ from the scaled etched mesa contour [26] [27] [28].…”

“…In this paper, we introduce a perturbative refinement to include the process anisotropy into an existing bi-dimensional model capable of simulating the temporal characteristics of the oxidation of circular mesas [15]. We subsequently validate this approach by reproducing the gradual deformation of a thin-layer oxide aperture starting from a circular mesa and progressively morphing into a diamond-like shape resulting from the oxidation of Al(Ga)As disk patterns etched into (100)-oriented wafers [26] [27] [28] as commonly observed when fabricating VCSELs (see Fig. 1).…”