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

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

The influence of capping layers on pore formation in Ge during ion implantation

Ion induced porosity in Ge has been investigated with and without a cap layer for two ion species, Ge and Sn, with respect to ion fluence and temperature. Results without a cap are consistent with a previous work in terms of an observed ion fluence and temperature dependence of porosity, but with a clear ion species effect where heavier Sn ions induce porosity at lower temperature (and fluence) than Ge. The effect of a cap layer is to suppress porosity for both Sn and Ge at lower temperatures but in different temperatures and fluence regimes. At room temperature, a cap does not suppress porosity and results in a more organised pore structure under conditions where sputtering of the underlying Ge does not occur. Finally, we observed an interesting effect in which a barrier layer of a-Ge that is denuded of pores formed directly below the cap layer. The thickness of this layer (∼ 8 nm) is largely independent of ion species, fluence, temperature, and cap material, and we suggest that this is due to viscous flow of a-Ge under ion irradiation and wetting of the cap layer to minimize the interfacial free energy

Suppression of ion-implantation induced porosity in germanium by a silicon dioxide capping layer

Ion implantation with high ion fluences is indispensable for successful use of germanium (Ge) in the next generation of electronic and photonic devices. However, Ge readily becomes porous after a moderate fluence implant (∼1×1015 ion cm−2) at room temperature, and for heavy ion species such as tin (Sn), holding the target at liquid nitrogen (LN2) temperature suppresses porosity formation only up to a fluence of 2×1016 ion cm−2. We show, using stylus profilometry and electron microscopy, that a nanometer scale capping layer of silicon dioxide significantly suppresses the development of the porous structure in Ge during a Sn − implant at a fluence of 4.5×1016 ion cm−2 at LN2 temperature. The significant loss of the implanted species through sputtering is also suppressed. The effectiveness of the capping layer in preventing porosity, as well as suppressing sputter removal of Ge, permits the attainment of an implanted Sn concentration in Ge of ∼15 at.%, which is about 2.5 times the maximum value previously attained. The crystallinity of the Ge-Sn layer following pulsed-laser-melting induced solidification is also greatly improved compared with that of uncapped material, thus opening up potential applications of the Ge-Sn alloy as a direct bandgap material fabricated by an ion beam synthesis technique

Morphology of ion irradiation induced nano-porous structures in Ge and Si1-xGex alloys

Crystalline Ge and Si1−xGex alloys (x = 0.83, 0.77) of (100) orientation were implanted with 140 keV Ge− ions at fluences between 5 × 1015 to 3 × 1017 ions/cm2, and at temperatures between 23 °C and 200 °C. The energy deposition of the ions leads to the formation of porous structures consisting of columnar pores separated by narrow sidewalls. Their sizes were characterized with transmission electron microscopy, scanning electron microscopy, and small angle x-ray scattering. We show that the pore radius does not depend significantly on the ion fluence above 5 × 1015 ions/cm2, i.e., when the pores have already developed, yet the pore depth increases from 31 to 516 nm with increasing fluence. The sidewall thickness increases slightly with increasing Si content, while both the pore radius and the sidewall thickness increase at elevated implantation temperaturesWe acknowledge access to NCRIS facilities (ANFF and the Heavy Ion Accelerator Capability) and the Center for Advanced Microscopy, both at the Australian National University. P.K. and M.C.R. thank the Australian Research Council, and Imam Abdulrahman Bin Faisal University for financial support. This research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron

Suppression of ion-implantation induced porosity in germanium by a silicon dioxide capping layer

Ion implantation with high ion fluences is indispensable for successful use of germanium (Ge) in the next generation of electronic and photonic devices. However, Ge readily becomes porous after a moderate fluence implant (∼1×1015 ion cm−2) at room temperature, and for heavy ion species such as tin (Sn), holding the target at liquid nitrogen (LN2) temperature suppresses porosity formation only up to a fluence of 2×1016 ion cm−2. We show, using stylus profilometry and electron microscopy, that a nanometer scale capping layer of silicon dioxide significantly suppresses the development of the porous structure in Ge during a Sn− implant at a fluence of 4.5×1016 ion cm−2 at LN2 temperature. The significant loss of the implanted species through sputtering is also suppressed. The effectiveness of the capping layer in preventing porosity, as well as suppressing sputter removal of Ge, permits the attainment of an implanted Sn concentration in Ge of ∼15 at.%, which is about 2.5 times the maximum value previously attained. The crystallinity of the Ge-Sn layer following pulsed-laser-melting induced solidification is also greatly improved compared with that of uncapped material, thus opening up potential applications of the Ge-Sn alloy as a direct bandgap material fabricated by an ion beam synthesis technique

Change of daily process measures over the study period among the whole cohort and in subgroups of ICUs with baseline spontaneous trial (SAT) compliance of >50% and ≤50.

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