The detailed morphological evolution during the transition between hut clusters and dome clusters is examined for Si 0.8 Ge 0.2 ͞Si͑001͒ heteroepitaxy. Simultaneous real-time light scattering and stress measurements directly demonstrate the correlation between island impingement and the shape transformation. We show that elastic interactions between islands can significantly reduce the equilibrium transition volume and may also modify the activation barrier for the transition. [S0031-9007(98)06223-1] PACS numbers: 68.35.Bs, 68.55. -a, 81.15. -zCoherent island formation provides a pathway to strain relaxation during lattice-mismatched heteroepitaxy that can bypass, or at least precede, dislocation introduction [1-6]. Relaxation results from the evolution of the film geometry rather than through shear, and is kinetically mediated by surface diffusion. Strain not only drives the initial 2D-to-3D transition during growth, but can also drive transitions from one island shape to another [6][7][8][9][10][11][12]. Island shape transitions are of interest because they probe the energetics and kinetic pathways associated with deterministic morphological evolution arising from the competition between strain energy and surface energy. These issues are critical to producing dense arrays of islands of controlled size for quantum dot applications.In a previous publication, we showed that the sequence of island transitions during Si 0.8 Ge 0.2 ͞Si͑001͒ heteroepitaxy (0.8% lattice mismatch strain) mirrors that observed for Ge͞Si(001) heteroepitaxy (4% strain), as long as the growth temperature is high enough to provide sufficient adatom mobility [11]. Longer surface diffusion lengths are required to overcome kinetic limitations imposed by the low strain. In particular, there is a natural length scale associated with strain-driven islanding that is proportional to DG͞M´2 coh , where DG is the change in surface energy, M is an elastic modulus, and´c oh is the lattice mismatch strain. Length scaling in the low strain regime can be exploited in the study of islanding phenomena, since transitions are a more gradual function of film thickness, allowing easier observation of intermediate transition stages, and since the increased length scales allow use of real-time optical probes.In this Letter, we examine the detailed transition from the hut cluster morphology (pyramidal islands bound by ͓501͔ facets [1]) to the dome cluster morphology (more isotropic island predominantly bound by ͓311͔ facets [8]) during Si 0.8 Ge 0.2 ͞Si͑001͒ heteroepitaxy. We show that island-island elastic interactions can strongly modify the transition energetics.Si 0.8 Ge 0.2 films were grown by molecular beam epitaxy (MBE) on Si(001) at 755 6 10 ± C and 0.
Real-time light scattering measurements of coherent island coarsening during SiGe/Si heteroepitaxy reveal unusual kinetics. In particular, the mean island volume increases superlinearly with time, while the areal density of islands decreases at a faster-than-linear rate. Neither observation is consistent with standard considerations of Ostwald ripening. Modification of the standard theory to incorporate the effect of elastic interactions in the growing island array reproduces the observed behavior.
We have investigated reactive phase formation in magnetron sputter-deposited NiyAl multilayer films with a 1 : 3 molar ratio and various periodicities, L, ranging from 320 nm down to a codeposited film with zero effective periodicity. The films were studied by x-ray diffraction, differential scanning calorimetry, electrical resistance measurements, and transmission electron microscopy. We find that Ni and Al have reacted during deposition to form the B2 NiAl phase and an amorphous phase. The formation of these phases substantially reduces the driving force for subsequent reactions and explains why nucleation kinetics become important for these reactions. Depending on the periodicity, these reactions result in the formation of NiAl3 or Ni2Al9 followed by NiAl3. Detailed calorimetric analysis reveals differences in the nucleation and growth behavior of NiAl3 compared with other studies.
Ni/Al multilayer films with pair thicknesses of 10 and 20 nm and with overall compositions in the range 48–88 at. % Al were prepared by sputtering. For comparison, Ni-Al alloy films in the same concentration range were prepared by co-deposition of the elements. The films were studied by x-ray diffraction, electron diffraction, and differential scanning calorimetry. It was found that the B2 NiAl phase with a metastable concentration of approximately 63 at. % Al was the first phase to grow upon annealing of the multilayer films. The growth of this phase could be described by Johnson–Mehl–Avrami kinetics with an activation energy of 0.8 eV and an Avrami exponent of 0.5. This low activation energy was consistent with the observation that the phase had formed during deposition and continued to grow upon annealing at low temperatures to thicknesses of a few nanometers. If the reactant phases were not fully consumed by the B2 phase growth, the subsequent reaction was the formation of NiAl3, previously thought to be the first product phase in the Ni-Al system. The reduction of driving force by the preceding B2 phase growth explains why the formation of NiAl3 takes place by a nucleation-and-growth process, an observation that has been discussed controversially in the recent literature. The nucleation and growth of NiAl3 had an activation energy of 1.5 eV in agreement with previous studies.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.