Visualization and Theoretical Modelling of the Atomistic Structure of Semiconductor Quantum Well Interfaces
The atomic scale crystallographic and chemical properties of interfaces between semiconductors are of decisive importance for the performance of novel generations of electronic and photonic devices and are in addition of large fundamental interest. Optical methods like luminescence and absorption have recently emerged to yield quantitative information on these properties, if the corresponding lineshape are carefully analyzed. We emphasize here luminescence. The natural lineshape of luminescence from a quantum well shows Gaussian broadening if its interfaces are not ideally abrupt. A detailed lineshape theory is outlined, allowing for a quantitative determination of the interface roughness distribution function. We find this function to depend in a delicate way on growth rates, temperature, interruption time and chemical compositon of the growth surface. The results of an experimental study of the model quantum well system AlGaAs/GaAs/AlGaAs grown by molecular beam epitaxy with and without interruption of the growth at the interfaces is presented. Roughness reduction upon growth interruption is analyzed in detail. For specific growth conditions and interruptions of 2 min at both interfaces formation of up to 7 µm large interface islands differing by a one monolayer step (2.8 A) are observed. Consequently such quantum wells have a columnar structure, which can be directly visualized using cathodoluminescence imaging. Strong reduction of island size indicating transition from planar growth to three-dimensional growth is observed by CLI upon an increase of growth temperature from Tg = 600°C to 660°C.
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