Stress evolution during growth of InAs on GaAs measured by an in-situ cantilever beam setup
Abstract (Summary)The influence of stress on the growth of InAs on GaAs(001) by molecular beam epitaxy (MBE) is investigated in this thesis. An in-situ cantilever beam measurement (CBM) setup was used to measure the evolution of the film force during deposition and subsequent annealing at the growth temperature. The slope in a plot of film force versus film thickness is equal to the stress that builds up during heteroepitaxial growth. Film force curves were measured for InAs deposition under As-rich as well as In-rich growth conditions. The growth under As-rich conditions proceeds in the Stranski- Krastanov growth mode, meaning that quantum dots are formed after the initial growth of a wetting layer. During subsequent growth interruptions or intentional annealing at the growth temperature, the quantum dots undergo ripening. This growth mode of InAs films and the subsequent annealing behavior were studied in detail in this thesis. To understand the influence of strain on the growth mechanisms, the film force curves were analyzed and correlated to the morphological evolution of the InAs films during deposition and especially during annealing. This was achieved by developing new models for the evolution of the film force during annealing. During the growth of InAs on GaAs(001) under As-rich conditions, the film force shows a linear slope up to a value of 2.3 N/m. This linear increase in film force corresponds to the deposition of the wetting layer. Beyond the critical thickness of 1.5-1.6 monolayers, the film force proceeds with a decreasing slope, indicating a strain release by the formation of quantum dots. When the samples are subsequently annealed, the film force decreases again due to the ripening of the quantum dots and the desorption of InAs. Models were developed to fit and explain the relaxation of the film force measured during the annealing of InAs quantum dots. At temperatures lower than 470 ?C, quantum dots undergo standard Ostwald ripening. Different mechanisms, such as kinetic and diffusion limited, determine the ripening process. Fits of models based on these mechanisms to the film force relaxation curves, show, that although the relaxation curve for annealing at 440 ?C can be fitted reasonably well with all the models, the model describing ripening limited by the diffusion along dot boundaries yields a slightly better fit. The relaxation curves obtained at 455 ?C and 470 ?C can be fitted very well only with the model in which the ripening is controlled by the attachment/detachment of atoms on the dot surface. iv Annealing of quantum dots at temperatures higher than 500 ?C shows a very different behavior. The film force accumulated during the quantum dot formation relaxes below the value which was built-up by the wetting layer growth. Atomic force microscopy images reveal that the quantum dots ripen first and then dissolve after 450 s - 600 s annealing. These observations are explained by a combination of In desorption and interdiffusion between Ga and In. The photoluminescence spectra of quantum dots at different annealing stages corroborate our explanation. In contrast to the growth under As-rich conditions, the growth under Inrich conditions does not lead to the formation of quantum dots but proceeds rather in a layer-by-layer growth mode. The film force curves were also measured during this deposition mode. As expected, the curves are quite different from those obtained during Stranski-Krastanov growth. It is observed that the stress is relieved early on during the deposition. Further, the film force curves are anisotropic as compared to the growth under As-rich conditions. A preliminary analysis of the film force curves is presented.
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication: