Ion-implantation- and thermal-anneal-induced intermixing in thin Si/Ge superlattices

Abstract

Superlattices (SL s) composed of thin Si and Ge layers (Si»Ge», Si»Ge9) have been implanted with As, Ge, and Ga ions with doses ranging from 1 X 10" to 1 X 10' ions cm, and thermally annealed at 600 C for 30 min. The disordering and the intermixing of these SL s have been studied by the Ramanscattering technique and model calculations. The damage created by ion implantation has been estimated using TRIM simulations and a model. We found that when a thin symmetric Si»Ge» SL was rendered amorphous by ion implantation at high doses 5 X 10' ions cm, a mixed Sio &Geo 5 material was produced by thermal annealing, but the crystalline structure of the asymmetric Si»Ge9 SL equally disordered and annealed returns to a different SL structure with very little intermixing between the layers. Using a kinetic model, we calculated the interdiffusion coefficients and it was found that the recrystallization of the Ge layer is a fast process but that of the Si one is slow with respect to the time needed for intermixing. As a result, Ge diffuses mainly in disordered Si layers and Si in ordered Ge layers. In order to explain our experimental results, we equate the diffusion of Si into crystalline Ge to that of Ge into amorphous Si to minimize the effect of interlayer stress. Model calculations explain the difference in behavior between the two types of SL's, and are in good agreement with the Raman data.

Interface broadening and Raman scattering inSi1−xGeLiu1, Huang2, Wang3 1996Phys. Rev. B4031View full textAdd to dashboardCite

“…In addition, the optical modes have been used predominantly to study the interfaces. [11][12][13][14][15] The light scattering by folded longitudinal acoustic ͑FLA͒ phonons in Si 1Ϫx Ge x /Si superlattices with sharp interfaces has been investigated extensively by Lockwood and co-workers 3,6,7 and by Brugger et al 9,10 Both measured dispersion curves including the phonon gap energies and scattering intensities have been demonstrated to be in quantitative agreement with the theoretical calculations in which the elastic wave equation is solved exactly for superlattice structures in the whole Brillouin zone. 16,17 In this paper, we extend the theoretical calculations 16,17 to the case of superlattices with broadened interfaces.…”

“…It has also been widely used to study the structures 2-10 and interfaces [11][12][13][14][15] of Si 1Ϫx Ge x /Si superlattices. The investigations on interface broadening for this system, however, are mostly concentrated on Si/Ge short-period superlattices [11][12][13][14] for the purpose of searching for Si-based quasi-direct-gap material. Since the dimension of the Brillouin zone in Si/Ge short-period superlattices is much larger than the wave vector of the excitation laser, the phonons being probed are located mainly near the center of the zone.…”

Kinetics of laser-induced low-temperature crystallization of amorphous silicon

“…These applications include continuous wave laser-induced structural changes in a-Si:H ͑and other amorphous materials͒, 4,6 the thermal crystallization in a-Si:H 5,6 and amorphous silicon (a-Si), 9 as well as the thermal recrystallization and intermixing in Si/Ge superlattices of nanometer periods disordered by ion implantation. 10 The aforementioned structural changes in metastable amorphous materials include the following phenomena which can also be extended to the LIC kinetics in a-Si:H. First, the observed crystallization includes a huge number of diffusionlike hoppings of Si atoms toward more ordered states related to a lower free energy. In the course of the crystallization, each of the Si atoms involved experiences on the average A ϭ5 -10 hoppings.…”

Ion-irradiation-induced effects inSimGensuperlattices

“…This, of course, induces some mixing 31,32 and it has been also observed that defects created by ion implantation enhance the diffusion. 7,8 However, during thermal treatment it is the Si atoms that diffuse into Ge layers 32 and this probably evens out the unbalance after the implantation. Unfortunately, studies of diffusion are out of reach of our simulations.…”

“…One could expect that a similar thing would happen also in the layered structures. However, it has been experimentally shown that the outcome of a collision cascade is different in superlattices consisting of thin layers than in thick-film structures, 8 and thus the results from a study with one isolated interface cannot straightforwardly be used to predict the irradiation effects in layered structures.…”