The flexibility of sputter-sectioning, in combination with the principal advantages of SIMS technique, such as large dynamic range both in the mass separation and detection systems, enables the SIMS technique to be extremely sensitive for the quantitative analysis of solid surfaces and thin films. With optimized experimental conditions, like extremely low primary beam current (down to ~ 10 nA) and low impact energy (below 1 keV), appropriate impact-angle and with effective profile reconstruction approaches such as MRI (mixing, roughness and information depth), the ultra-high resolution SIMS depth profiling has enabled quantitative analysis of ultra-thin films, multilayers, superlattices, hetero-structures, etc. with extreme high precision [1,2,3]. SIMS study in various disciplines using “Hiden SIMS instrument” has been briefly discussed.
Hiden SIMS Workstation at Saha Institute of Nuclear Physics, Kolkata, India
MCs+-SIMS: Concept, Mechanism and Quantitative Analysis
- Despite the highest detection sensitivity, high dynamic range and significant depth-resolution, the technique suffers from ‘‘matrix effect’’, thus hindering the SIMS quantification to a large extent. No conventional methods existing so far can fully compensate the matrix effect. Working in the MCs+-SIMS mode (M – element to be analysed, Cs+ – bombarding ions) has circumvented the matrix effect by almost 100% [4].
- Intensity of MCsn+ (n = 1, 2, ….) molecular-ions has been found to be strongly dependent on the instantaneous local surface-chemistry, while monitoring these molecular-ions under the combined influence of electropositive and electronegative elements [5].
- A unique procedure has been proposed for the accurate quantification of Ge concentration in MBE-grown Si1−xGex alloys through estimation of Relative Sensitivity Factor (RSF) using MCs+-SIMS MCs+-SIMS methodology has successfully been applied for the first time for direct quantitative composition analysis of MBE grown Si/Ge multilayer structures [6].
- Exponential dependence of the integrated ion-yields of M+, MCs+, and MCs2+ species on instantaneous local work-functions has suggested that an MCs2+ molecular-ion forms via the recombination of M− and two sputtered Cs+ ions in the close proximity of the Cs-covered surface [7,8].
- This paper gives a comprehensive discussion on secondary ion-emission phenomena, “matrix effect”, its compensation, and the potential application of the “MCsn+-SIMS” (n=1,2,…) in the effective quantitative chemical analysis of materials [9].
Multilayers, Superlattices and Quantum-Wells
- A new D023 metastable phase of Cu3Au was found to grow at the interfaces of Au/Cu multilayers deposited by magnetron sputtering. The extent of formation of this novel alloy-phase depends upon an optimal range of interfacial-width primarily governed by the deposition wattage of the direct current magnetron used [10]. Such interfacial-confined growth was utilized to grow a ∼300-nm-thick Au/Cu multilayer with thickness of each layer nearly equal to the optimal interfacial-width obtained from SIMS. The SIMS depth profile also indicates that the mass fragment corresponding to Cu3Au occupies the whole film while x-ray diffraction (XRD) shows almost all the strong peaks belonging to the D023
- Dynamical diffraction calculations in MBE-grown Si/Ge superlattices have revealed the Ge layer to be under compressive strain (~6%). The experimental data has been fitted using Dynamical Diffraction Theory. The estimated periodicity of the multilayer (9.2 nm), with Si (6.8 nm) and Ge (2.4 nm), have been found to be in good agreement with that obtained from XTEM and SIMS data [11].
- Recent investigations in thin polycrystalline film have shown the formation of compressive stress, increasing the density of materials. We have discovered the existence of ultrathin superdense nonmagnetic cobalt layers in a polycrystalline cobalt thin film. The densities of these layers are about 1.2–1.4 times the normal density of Co. The evidence of a superdense cobalt phase is obtained from X-ray reflectivity (XRR) experiment and the result is corroborated by RBS, SIMS and TEM experiments. Additionally, the magnetic and the chemical depth profile analysis from polarized neutron reflectometry (PNR) experiments corroborate the high density and also show that this superdense cobalt layer is nonmagnetic [12].
- We have studied interdiffusion processes occurring in a metal organic chemical vapour deposition (MOCVD) grown InP/In0.33Ga0.67As/InP/In0.33Ga0.67As/InP double quantum well structure through secondary ion mass spectrometry and high-resolution x-ray diffraction measurements along with a simulation programme. Results show an interdiffusion of phosphorus into the quantum wells and the presence of a 10 nm thick intermixed zone of In, As and P formed in-between the cap layer and the subsequent quantum well. Combination of SIMS and XRD along with the simulation was found to be a novel approach towards the understanding and quantification of in-depth compositional variation in thin films and quantum-wells [13].
Photoluminescent and Photocatalytic Materials
- Oxygen vacancies were introduced into hydrothermally processed TiO2 nanotubes by vacuum calcination. Formation of oxygen vacancies modifies the local coordination in TiO2, as evident from Raman spectroscopy and SIMS results. The mid-band gap electronic states created by oxygen vacancies were mostly responsible for the effective narrowing of band gap. Charge carrier separation was sufficiently prolonged as the charged oxygen defect states inhibited facile carrier recombination. With high surface area, narrowed band gap and separated charge carriers defective TiO2 nanotube was a suitable candidate in the photodegradation of methylene blue (MB) and phenol under visible light illumination [14].
- We have reported the synthesis of tin oxide (SnO)-coated zinc oxide ZnO) needle-like nanostructures and their modified light emission and detection features. The formation of SnO phase on ZnO surface has been revealed from energy dispersive x-ray analysis and secondary ion mass spectrometry studies. The luminescence response of the SnO-coated ZnO nanoneedles gets lowered compared to that of bare ZnO and is assigned to the lowering of radiative emission due to the occurrence of charge-carrier separation [15].
- ZnO Nanowalls have been found to exhibit higher photocatalytic activity in contrast to ZnO nanowires and this has been attributed to higher adsorption of oxygen. The photocatalytic activity under visible light is originated from the native defect-states and the photocatalytic efficiency is largely influenced by the surface-adsorbents. A correlation has been established between MCsn+-SIMS analysis of ZnO nanowalls and their photocatalytic responses [16]. Control of surface-adsorption characteristics of the nanowalls upon tuning wall thicknesses can lead to the development of futuristic efficient photocatalytic devices.
- The structural features and elemental constituents of ZnS/ZnO heterostructures have been extensively studied using electron microscopy, EDX and MCsn+-SIMS. ‘MCs+-SIMS’ approach has been found to be highly effective in making a quantitative estimation of Zn and O surface–atom concentrations in both systems without the aid of ‘standard’. The luminescence responses of the ZnS/ZnO heterostructures were found to be strongly dependent on the extent of ZnS phase over ZnO. The higher luminescence responses in ZnS/ZnO heterostructures fabricated with smaller ZnS nanoparticles were explained in terms of a mechanism of charge-carrier transfer from ZnS to ZnO [17].
- Microstructural characteristics of chemically synthesized ZnO nanowalls grown on Al surfaces have been investigated through SIMS. The luminescence responses of the chemically grown ZnO nanowalls on Al surfaces have been found to be dependent on the surface-adsorbant species like OH−, O2−, etc., whose existence on the nanowall surface and oxygen deficiency in the vacuum-annealed nanowalls have been detected through SIMS. Apart from this microstructural alteration of the nanowalls, annealing also aggravated the dominance of insulating nature of the Al2O3 interface [18].
- The structural organization and elemental constituents of the bare ZnO and ZnS/ZnO heterostructure nanowalls have been investigated extensively. The lowering of local surface work function in the heterostructure, as extracted through MCs+-SIMS, has supported the presence of sulfur species of ZnS on the top of ZnO nanowalls. Furthermore, the MCs+-SIMS technique has revealed the higher Zn concentration compared with O on the heterostructure surface confirming the presence of ZnS phase over ZnO. Finally, from the luminescence responses of the two types of nanowalls, it has been found that the emission pattern gets modified and improved upon ZnS loading on the ZnO nanowalls [19].
Polymers
- Erbium ion implantation in polystyrene thin films has been performed with 40 and 60 keV ions to a dose range between 1×1014 and 1×1016 ions/cm2. The x-ray reflectivity and SIMS techniques were applied to determine the ion-induced eroded layer thickness. The erosion rate was found to decrease with increasing ion doses exhibiting simple power-law behaviour of the form (dose)−b. We proposed the formation of a carbonaceous network at the top surface, preventing further erosion of the polymer with increasing the duration of implantation time. These findings opened up a possibility of loading a large amount of erbium in a polymer matrix to make it suitable for various optoelectronic applications [20].
Doubly-Charged ions in SIMS
- Measurements of Si2+and Si+ ions sputtered due to bombardment of 3–5 keV Ar+ ions on silicon substrate have been performed for understanding exact charge-state formation mechanisms. Examination on the penetration depth dependence of incident particle on secondary ion formation has been performed. Kinetic energy distributions suggests that Si+ ions are predominantly formed in the upper surface layer and Si2+ ions are produced due to target-target symmetric collision-induced Si 2p shell vacancy creation following the Auger electron emission. Furthermore, the increase in the oxygen-induced impurity in the silicon substrate enables us to explore the gradual transition from the dominating symmetric to asymmetric collision channel for production of Si2+ ions [21].
References
- Nuclear Instruments and Methods in Physics Research B 266, 1858 (2008);
- Book “Ion Beam Analysis of Surfaces and Interfaces of Condensed Matter Systems” (Ed: Purushottam Chakraborty), Nova Science, New York, USA (2002)
- High-resolution molecular secondary ion mass spectrometry for Absolute quantification of materials in low-dimensional structures: Foundation, Perception and Challenges, Handbook of Materials Science, Volume 1 (Optical Materials), 2024 ISBN : 978-981-99-7144-2 https://doi.org/10.1007/978-981-99-7145-9_21
- Energy Procedia 41, 80 (2013)
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- Phys A: Materials Science and Processing 108, 671 (2012)
- Rev. B 70, 195427 (2004)
- Nuclear Instruments and Methods in Physics Research B 258 (2007) 246–249
- J. Mod. Phys. B (online); https://doi.org/10.1142/S021797922450440X
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- IEEE Xplore (2009), Online resource; https://ieeexplore.ieee.org/document/5407332
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Project summary by: Purushottam Chakraborty, former senior professor of Physics at Saha Institute of Nuclear Physics, Kolkata, India & former adjunct professor of Physics at University of Pretoria, South Africa
Hiden Product: SIMS Workstation
Reference Number: AP-EQS-202010 –