Porosity in Group IV-IV and III-V Alloys Induced by Ion Irradiation in the Nuclear Stopping Regime

Abstract

The motivation for this study is twofold: i) from a fundamental perspective, to further investigate and understand porosity in group IV-IV and group III-V semiconductors and its dependence on ion fluence, temperature, and stoichiometry using multi-characterization techniques including electron microscopy (SEM and TEM), surface profiling, Rutherford backscattering (RBS), Raman Spectroscopy (RS) and Small Angle X-ray scattering (SAXS); and ii) to assist in opening up potential exploitation in applications such as lithium ion batteries as an anode, in gas sensors, in thermoelectrics, and in optoelectronics applications. Firstly, pore formation in Ge and Si1-xGex alloys (x= 0.83, 0.77, and 0.65) was investigated under keV Ge ion irradiation. The initiation of porosity and the evolution of near-surface microstructure highly depend on ion fluence and irradiation temperature, as well as the substrate stoichiometry. Porosity is only observed up to 23 % Si in the alloy, higher Si concentration does not result in porosity for samples implanted at room temperature even when the ion fluence is increased to above 1018 ions/cm2. Additionally, in order to produce porosity in Ge and Si1-xGex alloys, the matrix has to be rendered amorphous during ion bombardment prior to porous formation. Increasing the Si content leads to an increase in the threshold condition for porosity, with higher ion fluence and higher implantation temperature required. Moreover, we observe at a 35 % Si content an unusual surface topography at elevated temperatures, which is largely irreproducible. This is explained in terms of oxidation and contamination due to poor vacuum in the implant chamber. Although, the observation of porosity in Ge is dominated by swelling of amorphous Ge, in Si1-xGex alloys preferential sputtering and segregation of Si/Ge play a significant role. All the data suggest a model of vacancy migration and clustering in an amorphous matrix is appropriate to explain pore formation. SAXS provides complementary information to electron microscopy, giving an estimation of pore size and sidewall thickness. Indeed, SAXS provides a better statistical average of the radius of pore features as it gives the average of the entire bulk porous structure compared with only surface sensitivity of SEM. We find that using an appropriate core shell cylinder model to fit SAXS data, there is good agreement with cross-section TEM (XTEM) results. Results show that the pore size increases with ion fluence until the porous structure fully develops, and then no longer depends on ion fluence. Some differences in pore size between the different techniques is observed and these are explained as follows: SEM images consider only surface effects while XTEM and SAXS take into account the underlying bulk. However, both pore radii and sidewall thicknesses increase at elevated temperature by 8 and 2 nm, respectively, as expected as the point defect diffusivity increases with temperature. The third part of the thesis investigates the effect of a cap layer of SiO2 on pore formation. When porosity is observed, a cap results in a more developed and well-ordered porous layer compared to uncapped samples. This is explained in terms of suppression of sputtering in a capped sample and the resulting protection to the matrix, hence porosity becomes uniform and more developed. For samples implanted below room temperature, the porosity is completely suppressed by a cap. A large occasional void with a mostly intact surface devoid of pores is observed. When a porous layer does not form at higher temperatures there is a continuous amorphous Ge layer denuded of pores formed directly under, and in contact with, the cap. Interestingly, this layer remains constant at about 8 nm in thickness regardless of the ion fluence and temperature. Different possibilities that could explain the formation of this barrier layer are discussed. Firstly, ion irradiation can induce intermixing of O and Si from the cap with the underlying Ge as shown by an x-ray elemental distribution map and this could inhibit vacancy clustering and pore formation. However, this explanation is not the whole story since it would be expected that the barrier layer thickness should increase with ion fluence but the barrier layer thickness is always constant. A more reasonable explanation for a barrier layer denuded of pores is based of viscous flow of amorphous Ge under ion irradiation and the wetting of the cap to minimize interfacial free energy. In addition, a cap layer on Si0.17Ge0.83 alloy always suppresses the porous structure. This may indicate the importance of preferential sputtering in inducing porosity in the alloy when a cap is not present, whereas the cap layer prevents sputtering and hence porosity. The last topic covered in this thesis is porosity and void evolution in GaSb and GaAs1-xSbx alloys (x=0.75, and 0.50) under the same implantation conditions as for Ge. Compared to GaSb behaviour, GaAs0.25Sb0.75 shows small swelling with void formation and sputtering both playing important roles. The formation of voids is strongly depending on ion fluence, temperature and stoichiometry. Indeed, the transformation from crystalline to amorphous xiii and to void formation occurs at the same ion fluence. For GaAs0.5Sb0.5 no void formation was observed, only the formation of an amorphous layer and associated significant sputtering