揭开Cs控制的二次离子形成的秘密:位置特定表面化学、合金化和离子键主导的证据

IF 8.2 1区 化学 Q1 CHEMISTRY, PHYSICAL Surface Science Reports Pub Date : 2013-03-01 DOI:10.1016/j.surfrep.2012.11.001
Klaus Wittmaack
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If SIMS is applied to characterise the composition of solid materials, the simplest approach to achieving sample erosion as well as high negative-ion yields is bombardment with primary ions of Cs. Two other methods of sample loading with Cs provide more flexibility, (i) exposure to a collimated beam of Cs vapour and concurrent bombardment with high-energy non-Cs ions and (ii) the mixed-beam approach involving quasi-simultaneous bombardment with Cs and </span></span>Xe ions<span>. Both concepts have the advantage that undesirable sample overload with Cs can be avoided. High Cs concentrations reduce the formation probability of target specific molecular ions<span> and lower the yields of all types of positive secondary ions, including Cs</span></span></span><sup>+</sup>, M<sup>+</sup>, X<sup>+</sup>, MCs<sup>+</sup> and XCs<sup>+</sup> (M and X denoting matrix and impurity elements). Quantitative SIMS analysis using MCs<sup>+</sup> and XCs<sup>+</sup> ions appears feasible, provided the Cs coverage is kept below about 5%.</p><p><span>The semi-classical model of resonant charge transfer, also known as the tunnelling model, has long been considered a solid framework for the interpretation of Cs and Li based SIMS data. The model predicts ionisation probabilities for cases in which, at shallow distances from the surface, the affinity (ionisation) level of the departing atom is shifted below (above) the Fermi level. Ion yields should be controlled by the work function (WF) of the sample, </span><em>Φ</em><span>, and the normal velocity of the ejected ions. To explore the predicted velocity dependence, the performance characteristics of the employed SIMS instrument need to be known. The Cs induced negative-ion yield enhancement observed with pure metal and alloy targets often exceeded five orders of magnitude, with enhancement factors essentially independent of the emission energy. This absence of a velocity dependence is at variance with the predictions of the tunnelling model.</span></p><p>Previous theoretical attempts to model the <em>Φ</em>-dependence and the apparent velocity effect for the overrated case of O<sup>−</sup><span><span><span>emission from Li and Cs exposed oxidised metal surfaces must be considered a meander. The experimental data, recorded with a </span>quadrupole based instrument of inadequate extraction geometry, may alternatively be rationalised in terms of alkali induced changes in the </span>energy spectrum of sputtered atoms. Another important finding is that secondary ion yield changes do not correlate with the absolute magnitude of the (macroscopic) WF but often with WF changes, Δ</span><em>Φ</em>. The frequently used method of determining Δ<em>Φ</em><span> in situ from the shift of the leading edge of secondary ion energy spectra rests on the assumption, taken for granted or not even appreciated, that Cs induced yield changes are independent of the ion's emission<span><span> velocity. Hence the approach is only applicable if the tunnelling model is not valid. The local character of alkali induced WF changes, which might provide a route to an understanding of previously unexplained phenomena, has been explored using photoemission<span> of adsorbed inert gases, </span></span>scanning tunneling microscopy<span> and low-energy ion scattering<span> spectrometry.</span></span></span></span></p><p><span>At room temperature<span><span><span>, the Cs coverage is limited to one layer of adatoms. Close similarities are identified between WF changes generated by Cs </span>vapour deposition and by bombardment with </span>Cs ions. This finding implies that sub-monolayer quantities of Cs adatoms grow at the surface of Cs bombarded samples. The process has been studied in-situ by medium-energy ion scattering spectrometry. The stationary Cs coverage, </span></span><em>N</em><sub>Cs</sub><span>, is controlled by the efficiency of active transport of implanted atoms to the surface, the bulk retention properties of the sample and the cross section for sputtering of adatoms. Unearthing immobile implanted Cs atoms by sputter erosion usually provides only a minor contribution to the stationary coverage. Cs adatoms are mobile; the time required for final adatom rearrangement may be on the order of minutes at room temperature. Exposure of Cs bombarded samples to oxygen gives rise to oxidation<span> of the substrate as well as to the formation of oxide layers of complex composition. Intercalation should be taken into account as a possible route of alkali transport into analysed samples. An important aspect ignored in prior work is that the alkali coverage required to produce a certain WF change is five to seven times higher if Li is deposited instead of Cs. Studies involving the use of Li thus provide no advantage compared to Cs. Furthermore, migration of the tiny Li atoms into the sample and metallisation effects aggrevate data interpretation.</span></span></p><p>Literature data for Δ<em>Φ</em> (<em>N</em><sub>Cs</sub><span>), measured using Cs vapour deposition, can be converted to calibration curves, </span><em>N</em><sub>Cs</sub> (Δ<em>Φ</em>), for calculating the coverage established in implantation studies, a method referred to as Δ<em>Φ</em>→<em>N</em><sub>Cs</sub><span> conversion. This concept may be carried even further, as shown convincingly for silicon, the material examined most frequently in basic SIMS studies: Si</span><sup>−</sup> ion fractions, <em>P</em>(Si<sup>−</sup>), derived from yields measured under vastly different conditions of Cs supply, exhibit essentially the same Δ<em>Φ</em> dependence. Inverting the data one can produce calibration functions for Δ<em>Φ</em> versus <em>P</em>(Si<sup>−</sup>), denoted <em>P</em>(Si<sup>−</sup>)→Δ<em>Φ</em>, or, more generally, <em>P</em>(M<sup>−</sup>)→Δ<em>Φ</em> conversion. On this basis, transient yields measured during Cs implantation can be evaluated as a function of Cs coverage.</p><p><span><span>The summarised results imply that secondary ions are commonly not formed by charge transfer between an escaping atom and the electronic system of the sample but are already emitted as ions. The probability of ion formation appears to be controlled by the local ionic character of the alkali-target atom bonds, i.e., by the difference in electronegativity between the involved elements as well as by the </span>electron affinity<span> and the ionisation potential of the departing atom. This idea is supported by the finding that Si</span></span><sup>−</sup> yields exhibit the same very strong dependence on Cs coverage as Si<sup>+</sup> and O<sup>−</sup><span> yields on the oxygen fraction in oxygen loaded Si. Most challenging to theoreticians is the finding that the ionisation probability is independent of the emission velocity of sputtered ions. This phenomenon cannot be rationalised along established routes of thinking. Different concepts need to be explored. An old, somewhat exotic idea takes account of the heavy perturbation created for a very short period of time at the site of ion emission (dynamic randomisation). Molecular dynamics simulations are desirable to clarify the issue. Ultimately it may be possible to describe all phenomena of enhanced or suppressed secondary ion formation, produced either by surface loading with alkali atoms or by enforced surface oxidation, on the basis of a single universal model. There is plenty of room for exciting new studies.</span></p></div>","PeriodicalId":434,"journal":{"name":"Surface Science Reports","volume":null,"pages":null},"PeriodicalIF":8.2000,"publicationDate":"2013-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.surfrep.2012.11.001","citationCount":"39","resultStr":"{\"title\":\"Unravelling the secrets of Cs controlled secondary ion formation: Evidence of the dominance of site specific surface chemistry, alloying and ionic bonding\",\"authors\":\"Klaus Wittmaack\",\"doi\":\"10.1016/j.surfrep.2012.11.001\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><p><span>Exposure of ion bombarded solids to Cs gives rise to a very strong enhancement of the yields of negatively charged secondary ions and, concurrently, to a lowering of </span>positive ion<span> yields. The phenomena have been explored in a large number of experimental and theoretical studies but attempts to clarify the mechanism of ion formation were not as successful as assumed. This review examines the state of the art in Cs controlled secondary ion mass spectrometry (SIMS) in great detail, with due consideration of low-energy alkali-ion scattering.</span></p><p><span><span>In very basic studies on alkali induced secondary ion yield changes, sub-monolayer quantities of Cs or Li were deposited on the sample surface, followed by low-fluence ion bombardment<span>, to avoid significant damage. If SIMS is applied to characterise the composition of solid materials, the simplest approach to achieving sample erosion as well as high negative-ion yields is bombardment with primary ions of Cs. Two other methods of sample loading with Cs provide more flexibility, (i) exposure to a collimated beam of Cs vapour and concurrent bombardment with high-energy non-Cs ions and (ii) the mixed-beam approach involving quasi-simultaneous bombardment with Cs and </span></span>Xe ions<span>. Both concepts have the advantage that undesirable sample overload with Cs can be avoided. High Cs concentrations reduce the formation probability of target specific molecular ions<span> and lower the yields of all types of positive secondary ions, including Cs</span></span></span><sup>+</sup>, M<sup>+</sup>, X<sup>+</sup>, MCs<sup>+</sup> and XCs<sup>+</sup> (M and X denoting matrix and impurity elements). Quantitative SIMS analysis using MCs<sup>+</sup> and XCs<sup>+</sup> ions appears feasible, provided the Cs coverage is kept below about 5%.</p><p><span>The semi-classical model of resonant charge transfer, also known as the tunnelling model, has long been considered a solid framework for the interpretation of Cs and Li based SIMS data. The model predicts ionisation probabilities for cases in which, at shallow distances from the surface, the affinity (ionisation) level of the departing atom is shifted below (above) the Fermi level. Ion yields should be controlled by the work function (WF) of the sample, </span><em>Φ</em><span>, and the normal velocity of the ejected ions. To explore the predicted velocity dependence, the performance characteristics of the employed SIMS instrument need to be known. The Cs induced negative-ion yield enhancement observed with pure metal and alloy targets often exceeded five orders of magnitude, with enhancement factors essentially independent of the emission energy. This absence of a velocity dependence is at variance with the predictions of the tunnelling model.</span></p><p>Previous theoretical attempts to model the <em>Φ</em>-dependence and the apparent velocity effect for the overrated case of O<sup>−</sup><span><span><span>emission from Li and Cs exposed oxidised metal surfaces must be considered a meander. The experimental data, recorded with a </span>quadrupole based instrument of inadequate extraction geometry, may alternatively be rationalised in terms of alkali induced changes in the </span>energy spectrum of sputtered atoms. Another important finding is that secondary ion yield changes do not correlate with the absolute magnitude of the (macroscopic) WF but often with WF changes, Δ</span><em>Φ</em>. The frequently used method of determining Δ<em>Φ</em><span> in situ from the shift of the leading edge of secondary ion energy spectra rests on the assumption, taken for granted or not even appreciated, that Cs induced yield changes are independent of the ion's emission<span><span> velocity. Hence the approach is only applicable if the tunnelling model is not valid. The local character of alkali induced WF changes, which might provide a route to an understanding of previously unexplained phenomena, has been explored using photoemission<span> of adsorbed inert gases, </span></span>scanning tunneling microscopy<span> and low-energy ion scattering<span> spectrometry.</span></span></span></span></p><p><span>At room temperature<span><span><span>, the Cs coverage is limited to one layer of adatoms. Close similarities are identified between WF changes generated by Cs </span>vapour deposition and by bombardment with </span>Cs ions. This finding implies that sub-monolayer quantities of Cs adatoms grow at the surface of Cs bombarded samples. The process has been studied in-situ by medium-energy ion scattering spectrometry. The stationary Cs coverage, </span></span><em>N</em><sub>Cs</sub><span>, is controlled by the efficiency of active transport of implanted atoms to the surface, the bulk retention properties of the sample and the cross section for sputtering of adatoms. Unearthing immobile implanted Cs atoms by sputter erosion usually provides only a minor contribution to the stationary coverage. Cs adatoms are mobile; the time required for final adatom rearrangement may be on the order of minutes at room temperature. Exposure of Cs bombarded samples to oxygen gives rise to oxidation<span> of the substrate as well as to the formation of oxide layers of complex composition. Intercalation should be taken into account as a possible route of alkali transport into analysed samples. An important aspect ignored in prior work is that the alkali coverage required to produce a certain WF change is five to seven times higher if Li is deposited instead of Cs. Studies involving the use of Li thus provide no advantage compared to Cs. Furthermore, migration of the tiny Li atoms into the sample and metallisation effects aggrevate data interpretation.</span></span></p><p>Literature data for Δ<em>Φ</em> (<em>N</em><sub>Cs</sub><span>), measured using Cs vapour deposition, can be converted to calibration curves, </span><em>N</em><sub>Cs</sub> (Δ<em>Φ</em>), for calculating the coverage established in implantation studies, a method referred to as Δ<em>Φ</em>→<em>N</em><sub>Cs</sub><span> conversion. This concept may be carried even further, as shown convincingly for silicon, the material examined most frequently in basic SIMS studies: Si</span><sup>−</sup> ion fractions, <em>P</em>(Si<sup>−</sup>), derived from yields measured under vastly different conditions of Cs supply, exhibit essentially the same Δ<em>Φ</em> dependence. Inverting the data one can produce calibration functions for Δ<em>Φ</em> versus <em>P</em>(Si<sup>−</sup>), denoted <em>P</em>(Si<sup>−</sup>)→Δ<em>Φ</em>, or, more generally, <em>P</em>(M<sup>−</sup>)→Δ<em>Φ</em> conversion. On this basis, transient yields measured during Cs implantation can be evaluated as a function of Cs coverage.</p><p><span><span>The summarised results imply that secondary ions are commonly not formed by charge transfer between an escaping atom and the electronic system of the sample but are already emitted as ions. The probability of ion formation appears to be controlled by the local ionic character of the alkali-target atom bonds, i.e., by the difference in electronegativity between the involved elements as well as by the </span>electron affinity<span> and the ionisation potential of the departing atom. This idea is supported by the finding that Si</span></span><sup>−</sup> yields exhibit the same very strong dependence on Cs coverage as Si<sup>+</sup> and O<sup>−</sup><span> yields on the oxygen fraction in oxygen loaded Si. Most challenging to theoreticians is the finding that the ionisation probability is independent of the emission velocity of sputtered ions. This phenomenon cannot be rationalised along established routes of thinking. Different concepts need to be explored. An old, somewhat exotic idea takes account of the heavy perturbation created for a very short period of time at the site of ion emission (dynamic randomisation). Molecular dynamics simulations are desirable to clarify the issue. Ultimately it may be possible to describe all phenomena of enhanced or suppressed secondary ion formation, produced either by surface loading with alkali atoms or by enforced surface oxidation, on the basis of a single universal model. 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引用次数: 39

摘要

离子轰击固体暴露于Cs会导致带负电荷的二次离子的产率大大提高,同时降低正离子的产率。这种现象已经在大量的实验和理论研究中进行了探索,但试图澄清离子形成的机制并不像假设的那样成功。本文详细介绍了Cs控制的二次离子质谱(SIMS)的现状,并适当考虑了低能碱离子散射。在碱诱导二次离子产率变化的非常基础的研究中,在样品表面沉积亚单层数量的Cs或Li,然后进行低通量离子轰击,以避免明显的损伤。如果应用SIMS来表征固体材料的组成,实现样品侵蚀和高负离子产量的最简单方法是用Cs的初级离子轰击。另外两种Cs加载样品的方法提供了更多的灵活性,(i)暴露于Cs蒸汽的准直光束并同时轰击高能非Cs离子和(ii)混合光束方法,涉及Cs和Xe离子的准同时轰击。这两个概念都有一个优点,即可以避免不希望的Cs样本过载。高Cs浓度降低了目标特定分子离子的形成概率,降低了Cs+、M+、X+、MCs+和XCs+ (M和X分别表示基体元素和杂质元素)等各类正二次离子的产率。在Cs覆盖率低于5%的条件下,使用MCs+和XCs+离子进行定量SIMS分析是可行的。谐振电荷转移的半经典模型,也称为隧穿模型,长期以来一直被认为是解释基于Cs和Li的SIMS数据的坚实框架。该模型预测了在离表面较浅距离处,离开原子的亲和(电离)能级低于(高于)费米能级的情况下的电离概率。离子产量应由样品的功函数(WF) Φ和喷射离子的正常速度来控制。为了探索预测的速度相关性,需要了解所使用的SIMS仪器的性能特性。在纯金属和合金靶上观察到的Cs诱导的负离子产率增强通常超过5个数量级,增强因子基本上与发射能量无关。这种速度依赖性的缺失与隧道模型的预测是不一致的。以前的理论尝试模拟Φ-dependence和明显的速度效应,对于Li和Cs暴露于氧化金属表面的O−发射过高的情况,必须被认为是一种迂回。实验数据,记录的四极杆为基础的仪器不充分的提取几何,可以选择合理化的条件下碱诱导的变化的能谱溅射原子。另一个重要的发现是,二次离子产率的变化与(宏观)WF的绝对大小无关,但通常与WF的变化有关,ΔΦ。从二次离子能谱前缘的位移中确定ΔΦ原位的常用方法依赖于这样一个假设,即Cs诱导的产率变化与离子的发射速度无关,这一假设被认为是想当然的,甚至没有得到重视。因此,该方法仅适用于隧道模型不成立的情况。利用吸附惰性气体的光电发射、扫描隧道显微镜和低能离子散射光谱法探索了碱诱导WF变化的局部特征,这可能为理解以前无法解释的现象提供了一条途径。在室温下,Cs的覆盖仅限于一层附着原子。Cs气相沉积和Cs离子轰击所产生的WF变化非常相似。这一发现表明,亚单层数量的碳原子生长在碳轰击样品的表面。用中能离子散射光谱法对该过程进行了原位研究。固定碳的覆盖率由注入原子主动迁移到表面的效率、样品的体积保留特性和溅射原子的横截面控制。通过溅射侵蚀挖掘出不动的植入铯原子通常只对静止覆盖做出很小的贡献。碳原子是可移动的;在室温下,最终原子重排所需的时间可能在几分钟左右。将Cs轰击样品暴露于氧中会引起衬底氧化以及形成复杂成分的氧化层。应考虑插层作为碱进入分析样品的可能途径。 在以前的工作中忽略的一个重要方面是,如果沉积Li而不是Cs,则产生一定WF变化所需的碱覆盖率要高5到7倍。因此,涉及使用Li的研究与Cs相比没有任何优势。此外,微小的Li原子向样品中的迁移和金属化效应聚集了数据解释。使用Cs气相沉积测量的ΔΦ (nc)的文献数据可以转换为校准曲线nc (ΔΦ),用于计算植入研究中建立的覆盖率,该方法称为ΔΦ→nc转换。这一概念甚至可以进一步推广,正如硅所显示的那样,硅是基本SIMS研究中最常检查的材料:硅离子分数,P(Si -),在Cs供应的不同条件下测量的产量,表现出本质上相同的ΔΦ依赖关系。对数据进行反求可以得到ΔΦ与P(Si−)的校准函数,表示为P(Si−)→ΔΦ,或者更一般地说,P(M−)→ΔΦ转换。在此基础上,可以将Cs注入过程中测量的瞬时产量作为Cs覆盖的函数进行评估。总结的结果表明,二次离子通常不是由逸出原子和样品的电子系统之间的电荷转移形成的,而是已经作为离子发射的。离子形成的可能性似乎是由碱-靶原子键的局部离子特性控制的,即由所涉及的元素之间的电负性差异以及离开原子的电子亲和和电离势控制的。这一观点得到了以下发现的支持:Si -产率对Cs覆盖的依赖性与Si+和O -产率对氧负载Si中氧分数的依赖性相同。对理论学家来说,最具挑战性的是发现电离概率与溅射离子的发射速度无关。这种现象不能按照既定的思维方式加以合理化。需要探索不同的概念。一个古老的,有点奇特的想法考虑到在离子发射的地方在很短的时间内产生的重扰动(动态随机化)。分子动力学模拟有助于澄清这一问题。最终,在一个单一的通用模型的基础上,有可能描述所有由碱原子表面负载或强制表面氧化产生的增强或抑制二次离子形成的现象。有足够的空间进行令人兴奋的新研究。
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Unravelling the secrets of Cs controlled secondary ion formation: Evidence of the dominance of site specific surface chemistry, alloying and ionic bonding

Exposure of ion bombarded solids to Cs gives rise to a very strong enhancement of the yields of negatively charged secondary ions and, concurrently, to a lowering of positive ion yields. The phenomena have been explored in a large number of experimental and theoretical studies but attempts to clarify the mechanism of ion formation were not as successful as assumed. This review examines the state of the art in Cs controlled secondary ion mass spectrometry (SIMS) in great detail, with due consideration of low-energy alkali-ion scattering.

In very basic studies on alkali induced secondary ion yield changes, sub-monolayer quantities of Cs or Li were deposited on the sample surface, followed by low-fluence ion bombardment, to avoid significant damage. If SIMS is applied to characterise the composition of solid materials, the simplest approach to achieving sample erosion as well as high negative-ion yields is bombardment with primary ions of Cs. Two other methods of sample loading with Cs provide more flexibility, (i) exposure to a collimated beam of Cs vapour and concurrent bombardment with high-energy non-Cs ions and (ii) the mixed-beam approach involving quasi-simultaneous bombardment with Cs and Xe ions. Both concepts have the advantage that undesirable sample overload with Cs can be avoided. High Cs concentrations reduce the formation probability of target specific molecular ions and lower the yields of all types of positive secondary ions, including Cs+, M+, X+, MCs+ and XCs+ (M and X denoting matrix and impurity elements). Quantitative SIMS analysis using MCs+ and XCs+ ions appears feasible, provided the Cs coverage is kept below about 5%.

The semi-classical model of resonant charge transfer, also known as the tunnelling model, has long been considered a solid framework for the interpretation of Cs and Li based SIMS data. The model predicts ionisation probabilities for cases in which, at shallow distances from the surface, the affinity (ionisation) level of the departing atom is shifted below (above) the Fermi level. Ion yields should be controlled by the work function (WF) of the sample, Φ, and the normal velocity of the ejected ions. To explore the predicted velocity dependence, the performance characteristics of the employed SIMS instrument need to be known. The Cs induced negative-ion yield enhancement observed with pure metal and alloy targets often exceeded five orders of magnitude, with enhancement factors essentially independent of the emission energy. This absence of a velocity dependence is at variance with the predictions of the tunnelling model.

Previous theoretical attempts to model the Φ-dependence and the apparent velocity effect for the overrated case of Oemission from Li and Cs exposed oxidised metal surfaces must be considered a meander. The experimental data, recorded with a quadrupole based instrument of inadequate extraction geometry, may alternatively be rationalised in terms of alkali induced changes in the energy spectrum of sputtered atoms. Another important finding is that secondary ion yield changes do not correlate with the absolute magnitude of the (macroscopic) WF but often with WF changes, ΔΦ. The frequently used method of determining ΔΦ in situ from the shift of the leading edge of secondary ion energy spectra rests on the assumption, taken for granted or not even appreciated, that Cs induced yield changes are independent of the ion's emission velocity. Hence the approach is only applicable if the tunnelling model is not valid. The local character of alkali induced WF changes, which might provide a route to an understanding of previously unexplained phenomena, has been explored using photoemission of adsorbed inert gases, scanning tunneling microscopy and low-energy ion scattering spectrometry.

At room temperature, the Cs coverage is limited to one layer of adatoms. Close similarities are identified between WF changes generated by Cs vapour deposition and by bombardment with Cs ions. This finding implies that sub-monolayer quantities of Cs adatoms grow at the surface of Cs bombarded samples. The process has been studied in-situ by medium-energy ion scattering spectrometry. The stationary Cs coverage, NCs, is controlled by the efficiency of active transport of implanted atoms to the surface, the bulk retention properties of the sample and the cross section for sputtering of adatoms. Unearthing immobile implanted Cs atoms by sputter erosion usually provides only a minor contribution to the stationary coverage. Cs adatoms are mobile; the time required for final adatom rearrangement may be on the order of minutes at room temperature. Exposure of Cs bombarded samples to oxygen gives rise to oxidation of the substrate as well as to the formation of oxide layers of complex composition. Intercalation should be taken into account as a possible route of alkali transport into analysed samples. An important aspect ignored in prior work is that the alkali coverage required to produce a certain WF change is five to seven times higher if Li is deposited instead of Cs. Studies involving the use of Li thus provide no advantage compared to Cs. Furthermore, migration of the tiny Li atoms into the sample and metallisation effects aggrevate data interpretation.

Literature data for ΔΦ (NCs), measured using Cs vapour deposition, can be converted to calibration curves, NCsΦ), for calculating the coverage established in implantation studies, a method referred to as ΔΦNCs conversion. This concept may be carried even further, as shown convincingly for silicon, the material examined most frequently in basic SIMS studies: Si ion fractions, P(Si), derived from yields measured under vastly different conditions of Cs supply, exhibit essentially the same ΔΦ dependence. Inverting the data one can produce calibration functions for ΔΦ versus P(Si), denoted P(Si)→ΔΦ, or, more generally, P(M)→ΔΦ conversion. On this basis, transient yields measured during Cs implantation can be evaluated as a function of Cs coverage.

The summarised results imply that secondary ions are commonly not formed by charge transfer between an escaping atom and the electronic system of the sample but are already emitted as ions. The probability of ion formation appears to be controlled by the local ionic character of the alkali-target atom bonds, i.e., by the difference in electronegativity between the involved elements as well as by the electron affinity and the ionisation potential of the departing atom. This idea is supported by the finding that Si yields exhibit the same very strong dependence on Cs coverage as Si+ and O yields on the oxygen fraction in oxygen loaded Si. Most challenging to theoreticians is the finding that the ionisation probability is independent of the emission velocity of sputtered ions. This phenomenon cannot be rationalised along established routes of thinking. Different concepts need to be explored. An old, somewhat exotic idea takes account of the heavy perturbation created for a very short period of time at the site of ion emission (dynamic randomisation). Molecular dynamics simulations are desirable to clarify the issue. Ultimately it may be possible to describe all phenomena of enhanced or suppressed secondary ion formation, produced either by surface loading with alkali atoms or by enforced surface oxidation, on the basis of a single universal model. There is plenty of room for exciting new studies.

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来源期刊
Surface Science Reports
Surface Science Reports 化学-物理:凝聚态物理
CiteScore
15.90
自引率
2.00%
发文量
9
审稿时长
178 days
期刊介绍: Surface Science Reports is a journal that specializes in invited review papers on experimental and theoretical studies in the physics, chemistry, and pioneering applications of surfaces, interfaces, and nanostructures. The topics covered in the journal aim to contribute to a better understanding of the fundamental phenomena that occur on surfaces and interfaces, as well as the application of this knowledge to the development of materials, processes, and devices. In this journal, the term "surfaces" encompasses all interfaces between solids, liquids, polymers, biomaterials, nanostructures, soft matter, gases, and vacuum. Additionally, the journal includes reviews of experimental techniques and methods used to characterize surfaces and surface processes, such as those based on the interactions of photons, electrons, and ions with surfaces.
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