J.B. Malherbe

Preferential sputtering of oxides: a comparison of model predictions with experimental data

Abstract Experimental data on the composition of the altered layer of some oxides bombarded with noble gas ions are compared with different model predictions based on Sigmunds sputtering theory. It is shown that although the difference in mass of the metal atoms is the dominant factor in the preferential sputtering of oxygen, the additional consideration of the surface binding energies yields an improved agreement between theory and experiment.

Preferential sputtering of InP: an AES investigation

Abstract AES was used to determine the surface composition of InP bombarded with 0.5 to 5 keV Ar + ions. The (100), (110) and some vicinal planes were sputtered. On all of these surfaces the phosphorus atoms were preferentially sputtered leaving an indium-enriched surface. The equilibrium surface composition showed no dependence on the argon ion energy or on the crystal orientation. A dependence on the angle θ of incidence was observed. The following steady state surface compositions were obtained: θ = 18°, InP 0.68 ± 0.07 ; θ = 40°, InP 0.67 ± 0.07 ; θ = 42°, InP 0.78 ± 0.07 ; θ = 70°, InP 0.94 ± 0.09 . Our results are compared to other published measurements. There is a very large scatter between the results of different studies. This difference can be attributed to several factors. Predictions of several well-known preferential sputtering theories also exhibit large differences. These predictions show that the mass difference between the In and P atoms is the major factor contributing to the preferential sputtering of InP, with the surface binding energies of secondary importance. This conclusion is supported by Monte Carlo calculations using the TRIM-89 computer code.

Sputtering of compound semiconductor surfaces. I. Ion-solid interactions and sputtering yields

Abstract Several phenomena occur on the surface of a solid when being bombarded by energetic ions. A short general review is given of the major ion-solid interactions on compound semiconductor surfaces. An in-depth discussion is presented of the total sputtering yields of component semiconductors. For this discussion, GaAs is assumed to be the prototype compound semiconductor because most experimental measurements exist for GaAs. To exclude any chemical effects in the sputter yields, only the total sputtering yield data for argon ion bombardment of GaAs are compared with the predictions of the major sputtering theories, with particular attention to the Sigmund theory for linear cascade sputtering. Different proposals of each of the parameters in this theory are presented and compared with the GaAs data. These parameters are the surface binding energy, the nuclear stopping power, and the factor α, which represents the fraction of energy available for sputtering. Use of the different parameters results in a...

Sputtering of compound semiconductor surfaces. II. Compositional changes and radiation-induced topography and damage

Abstract Ion bombardment often leads to compositional changes in the surface layers of multicomponent targets. Such changes due to noble gas ion sputtering are discussed for InP and GaAs. The analyses show that the compositional change in InP (i.e., indium enrichment) is mainly due to preferential sputtering. In the case of GaAs. the changes are due to radiation-induced diffusion and segregation effects. Brief mention is made of compositional changes in a few other systems. The discussion on sputter-induced topography development deals mainly with InP because ion bombardment leads to dramatic topographical effects in this material. Ripple development on GaAs is also briefly discussed. Radiation damage has been well researched, and its mechanism and effects usually differ substantially when going from one semiconductor group to another. Bombardment-induced damage is briefly discussed for InP, GaAs, SiC, some II-VI semiconductors, and HgCdTe.

Projected changes in tropical cyclone climatology and landfall in the Southwest Indian Ocean region under enhanced anthropogenic forcing

The conformal-cubic atmospheric model, a variable-resolution global model, is applied at high spatial resolution to perform simulations of present-day and future climate over southern Africa and over the Southwest Indian Ocean. The model is forced with the bias-corrected sea-surface temperatures and sea-ice of six coupled global climate models that contributed to Assessment Report 4 of the Intergovernmental Panel on Climate Change. All six simulations are for the period 1961–2100, under the A2 emission scenario. Projections for the latter part of the 21st century indicate a decrease in the occurrence of tropical cyclones over the Southwest Indian Ocean adjacent to southern Africa, as well as a northward shift in the preferred landfall position of these systems over the southern African subcontinent. A concurrent increase in January to March rainfall is projected for northern Mozambique and southern Tanzania, with decreases projected further south over semi-arid areas such as the Limpopo River Basin where these systems make an important contribution as main cause of widespread heavy rainfall. It is shown that the projected changes in tropical cyclone attributes and regional rainfall occur in relation to changes in larger scale atmospheric temperature, pressure and wind profiles of the southern African region and adjacent oceans.

Atomic force microscopy investigation of noble gas ion bombardment on InP: Effect of ion energy

Mirror-polished InP(100) samples, n-doped with S atoms to 4 x 10 18 cm -3 , were bombarded with neon, argon and krypton ions. The energy of the ions was varied from 0.5-5 keV at a constant angle of incidence at 41° with respect to the sample normal. The ion dose density for Ne + was 1.8 x 10 16 ions cm -2 and for Ar + and Kr + was 5 x 10 18 ions cm -2 . A low ion current density of 5 x 10 13 ions cm -2 s -1 was used to minimize sample heating. The resulting topography development was investigated by atomic force microscopy (AFM). The topography development was quantified in terms of RMS (root-mean-square) roughness using the software of the AFM and analysed as a function of the energy of the bombarded ions. Atomic force micrographs showing various types of topography for some specific ion energies are presented. For each ion species there is a critical energy for which the surface roughness attains a maximum value.

Ion sputtering, surface topography, SPM and surface analysis of electronic materials

Abstract Two side effects introduced on surfaces of electronic materials by ion bombardment, namely compositional changes (for compound semiconductors) and topography changes are discussed. Based on the relative elemental sensitivity factor method with matrix corrections for quantitative AES or XPS analysis, a sputter correction factor is defined to compensate for bombardment induced surface compositional changes. Using several popular preferential sputter models and comparing their predictions to a synopsis of published experimental AES and XPS measurements on argon bombarded binary compound semiconductors, a sputter correction factor for these materials are proposed. The extent of bombardment-induced topography depends primarily on the substrate material while the ion beam characteristics play only a secondary role. Due to the complexity of and the many processes involved in ion/solid interactions, bombardment-induced topography is not well understood. Several quantitative and qualitative theories have been proposed to explain the experimental data. Most of these theories are based on SEM or TEM data. The major disadvantage of these data is the lack of quantitative information. The advent of SPM (scanning probe microscopy) and the subsequent development of software have reversed this. A brief summary of SPM (AFM and STM) investigations of bombardment induced topography on semiconductor surfaces is given. Most studies have concentrated on the topography on Si, Ge, GaAs and InP surfaces with special emphasis on ripple development.

Bombardment-induced ripple topography on GaAs and InP

Abstract Bombardment-induced ripples have possible application in nano-technology as nano-wires. This paper reviews some of the published experimental data of ripple formation on GaAs and InP resulting from noble gas, oxygen, nitrogen and cesium ion bombardment. The dependences of the ripples and their corresponding wavelengths on substrate temperature, areic dose and (to a lesser extent) angle of incidence were considered. The experimental results were tested against the predictions of the Bradley–Harper theory [J. Vac. Sci. Technol. A 6 (1988) 2390]. Surprisingly good agreement was obtained between the experimental data and this theory. This is probably due to the amorphisation of the substrates under ion bombardment and, thereby, minimising the effect of crystallographic-orientated self-diffusion on metals, and of energy barriers (Ehrlich–Schwoebel barriers) to interlayer surface diffusion. From the data of MacLaren et al. [J. Vac. Sci. Technol. A 10 (1992) 469], the activation energy for surface self-diffusion of GaAs in the temperature range 60–100 °C was determined as 0.26 eV.

High dose implantation of nitrogen in tool steel: Auger electron spectroscopy and hardness measurements

Abstract Tool steel samples were implanted with 100 keV N + ions at liquid nitrogen temperature to doses of 3 × 10 17 and 1 × 10 18 cm −2 . Only the dose of 1 × 10 18 cm −2 caused a significant hardening effect. Two mechanisms contributed to this hardness increase, namely nitride formation and radiatin damage. Cooling during implantation caused additional hardening, owing to a martensitic phase transformation. The projected range ( R p = 110 nm) for the implanted species was obtained by Auger sputter depth profiling. If the hardened layer thickness was taken as 2 R p , then the Jonsson-Hogmark model gave an average hardness value of 2010 HV for the implanted layer. However, a more realistic value of 900 HV was calculated under the assumption that typical radiation damage profiles ( R d = 3 R p ) contributed to the hard film thickness. It is difficult to judge the accuracy of the model for predicting the correct absolute hardness of the implanted layer but it is shown that deep radiation-induced damage plays a major role in surface hardening.

Ion angle dependence in the preferential sputtering of InP

Abstract The dependence of the equilibrium Ar+-sputtered surface composition of InP on the angle of incidence of the Ar+ ions was investigated by means of Auger electron spectroscopy. Phosphorus atoms were preferentially sputtered from InP leaving an In-enriched surface. There is a correlation between the angle of incidence of the bombarding Ar+ ions and the final surface composition: The preferential sputtering effect became less with increasing angle of incidence θi (measured from the surface normal). The results show that the ratio of the phosphorus-to-indium compositions, viz. C s p C s In ∝( cos θ i ) −a with a = 1.7. This ratio was independent of crystal orientation and ion energy (between 0.5 and 5 keV). The results are in qualitative agreement with other published data. A similar fit to another set of reported data gave a = 1.1.