An MBE and Modeling Study of Pulsed Growth on Ge(001)
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An MBE and Modeling Study of Pulsed Growth on Ge(001) M. A. Gallivan and H. A. Atwater Division of Engineering and Applied Science, California Institute of Technology Pasadena, CA 91125 ABSTRACT Ge molecular beam epitaxy (MBE) and kinetic Monte Carlo (KMC) simulations are used to study time-varying processing parameters and their effect on surface morphology. We focus here on Ge growth on highly-oriented Ge(001) substrates with reflection high-energy electron diffraction (RHEED) as a real-time sensor. KMC simulations are used as the physical model, and physical parameters are determined from growth under pulsed flux. A reduced version of the simulations is generated, and temperature trajectories are computed that minimize surface roughness subject to experimental constraints. INTRODUCTION The processing history during thin film deposition strongly influences the final properties of a film. Time-varying conditions may be beneficial [4], but it is not practical to try all possibilities in experiment. If a systematic optimization could be applied to a mathematical model of the physics, optimal time-dependent processing conditions could be computed. We model the interplay among nucleation, coarsening, and coalescence of islands with a cubic lattice KMC simulation, using RHEED data during submonolayer growth and subsequent recovery to infer the diffusion and detachment rates. A low-dimensional differential equation is then generated that captures the behavior seen in the KMC simulations. See [2] for more details on the experiments, the KMC model, and the model reduction. EXPERIMENTAL Germanium films were deposited on highly-oriented Ge(001) wafers, specified by Eagle-Picher as 0.05◦ ±0.02◦ . The wafers were prepared by sonicating in acetone and methanol, UV-ozone exposure, and a dip in 5% HF. A typical base pressure was 1×10−10 torr, with a growth pressure of 5×10−9 torr. After the growth of a buffer layer at 550◦ C, the RHEED pattern consisted of the Ewald sphere, indicating a smooth clean surface. The temperature was then lowered into the range of 230–305◦ C, after which submonolayer doses were deposited at rates of 0.05–0.8 ˚ A/s. Between each submonolayer ◦ A/s. RHEED was dose, the temperature was raised to 550 C for buffer layer growth at 1 ˚ used as a real-time diagnostic. The intensity of the spectral spot was monitored using a photodiode, with an off-Bragg angle of incidence of 5◦ and an azimuthal angle of 3◦ from the (110) direction. Figure 1 shows the normalized intensity of the spectral spot during growth and subsequent recovery. Typical intensity data is shown in Figure 1(a), in which the intensity
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Figure 1: Normalized intensity of RHEED spectral spot. (a) During deposition at 0.8 ˚ A/s and 290◦ C. (b) Immediately after the deposition of 1/2 mL (‘x’s) and after a further 40 s of annealing (‘o’s), at 0.4 ˚ A/s (solid line) and 0.05 ˚ A/s (dashed line
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