Direct Recoil Research Articles

Negative direct recoil spectroscopy study of the initial oxidation of Al(111)

The submonolayer oxygen-covered Al(111) surface has been studied by Negative Direct Recoil Spectroscopy (DRS) with the aim of detecting or dismissing the presence of subsurface oxygen atoms. Experiments were performed at room temperature and with a coverage of 75% of saturation as measured by DRS. Four possible subsurface adsorption sites have been checked selectively and results compared with those from computer simulations. No subsurface oxygen was found in any of these sites down to the detection limit of DRS, which has been determined to be as small as 2.5% here. This is much less than that previously found by MEIS. On the basis of our results, the decrease of the work function, while the surface approaches the monolayer coverage (result reported by several groups in the past), cannot be attributed to the penetration of oxygen atoms below the surface.

Negative Direct Recoil Spectroscopy Study of the Initial Oxidation of the Mg(0001) Surface

The sub monolayer oxygen-covered Mg(0001) surface has been studied by negative direct recoil spectroscopy (DRS) with the aim of detecting or dismissing the presence of subsurface oxygen atoms. Recorded incidence scans show that at room temperature and with the coverage of 70% of saturation, as measured by DRS, one of two subsurface sites (oC and tA) may be populated. This supports a recent investigation of this surface using forward focusing of Auger electrons, where the most likely occupied subsurface site was found to be the oC site. Furthermore, it was found that about 90% of the oxygen still remains above the surface, while only about 10% penetrates below the surface.

Surface-recoil processes of hydrogen on Si (1 0 0)-2 × 1:H and Si (1 0 0)-1 × 1:2H surfaces studied by low-energy He ion beams

For the hydrogen atoms adsorbed on a Si (1 0 0) surface, the processes of direct recoil by incident ions and the surface-recoil process (surface scattering of the direct recoils) are simultaneously observed at relatively low incident ion energies below 2000 eV using a He ion beam. From computer simulations, the surface-recoil processes of hydrogen atom is found to be sensitive to the bonding parameters of hydrogen atom on the surface. Comparing the experimental results with computer simulation, we discuss the surface-recoil processes of hydrogen on Si (1 0 0)-2 × 1:H and Si (1 0 0)-1 × 1:2H surfaces, and suggest the bond angle of hydrogen on both surfaces to be 110 ∘ and 125 ∘, respectively.

Evolution of clusters in energetic heavy ion bombarded amorphous graphite

Carbon clusters have been generated by a novel technique of energetic heavy ion bombardment of amorphous graphite. The evolution of clusters and their subsequent fragmentation under continuing ion bombardment is revealed by detecting various clusters in the energy spectra of the direct recoils emitted as a result of collision between ions and surface constituents.

Hydrogen analysis of silicon surfaces by low-energy ion beams

For the analysis of hydrogen adsorption on silicon surfaces, low-energy recoil ion spectroscopy (LE-RIS) is one of the most powerful methods because it directly provides information over ‘real-space’ interatomic distance and the adsorbed coordinates. Observing both the surface recoils and the direct recoils which are created by He ion beams, we investigate the structure of Si(100)-1 × 1:2H dihydride and Si(100)-2 × 1:H monohydride surfaces. Comparing our experimental results with computer simulations, we conclude that the H Si bond angle in the Si(100)-2 × 1:H monohydride surface is 65–70° and that in the Si(100)-1 × 1:2H dihydride surface is 55–60°. For this dihydride surface, a canted dihydride structure is suggested rather than a symmetric dihydride structure.

Adsorption of hydrogen on clean and oxidized beryllium studied by direct recoil spectrometry

Direct monitoring of hydrogen adsorption on clean and on pre-oxidized polycrystalline beryllium by means of direct recoil spectrometry (DRS) is reported. While no adsorption occurs for molecular hydrogen either on clean or on oxidized beryllium, atomic hydrogen adsorbs on the surface of clean Be, as well as at the sites available between oxygen clusters, formed by low oxygen pre-exposure. For higher oxygen pre-exposures, when oxide islands are formed, the apparent coverage of hydrogen is significantly lower than the amount needed for complementary full coverage on the Be surface. Possible origins for this observation are discussed.

Hydrogen interactions with polycrystalline and with deposited titanium surfaces

The room temperature kinetics of hydrogen chemisorption and adsorption on polycrystalline and on deposited (sputter-deposited and evaporation-deposited) titanium surfaces were studied. Measurements of hydrogen surface accumulation were performed in a combined surface analyses system incorporating direct recoils spectrometry and Auger electron spectroscopy (AES). There, three different types of surface cleaning procedure were applied: heat-flashing, sputtering and sputter-deposition of titanium on a polycrystalline titanium substrate. The surface chemisorption kinetics obtained for the deposited samples were compared with the total kinetics of the gas phase consumption, performed in a volumetric Wagener system. From this comparison it was possible to distinguish between topmost surface chemisorption and subsurface (or bulk) absorption kinetics. It was concluded that, for all types of surface studied, hydrogen chemisorbed according to a Langmuir-type random two-sites chemisorption model, with high (close to unity) zero-coverage sticking probabilities. The only difference between these surfaces was in their roughness factors, which increased going from the heat-flashed, through the sputtered, to the deposited surfaces. Following the initial stage of a chemisorbed surface layer formation, constant-rate absorption of hydrogen proceeded over a very wide range of exposures (greater than 10 4 Langmuirs). The accomodation probability of hydrogen during this linear stage was about 10 −2. It is possible that this absorption process is controlled by the chemisorption of the H 2 on the surface hydride phase, formed by the earlier hydrogen chemisorption.

Multiply charged direct recoil spectra from Ar + and Kr + bombarded graphite

Measurements of the energy spectra of multiply charged positive and negative carbon ions (C n ± ) recoiling from graphite surface under 100 and 150 keV argon and krypton ion bombardment are presented. With the energy spectrometer set at recoil angle of 79.5°, direct recoil (DR) peaks have been observed with singly as well as multiply charged carbon ions C n ± , where n = 1 to 6. These C n ± ions have been seen recoiling with the characteristic recoil energy E DR = kE 0cos 2 θ DR, where θ DR is the direct recoil angle, k = 4m 1m 2 (m 1 + m 2) 2 , m 1 and E 0 are projectile mass and energy and m 2 is carbon mass. We have observed sharp DR peaks. A collimated projectile beam with divergence ∼ ±0.2° is supplemented with a similar collimation before the energy analyzer to reduce the background of sputtered ions due to scattered projectiles.

The initial interactions of oxygen with polycrystalline titanium surfaces

The interactions of gaseous oxygen and different types of polycrystalline titanium surfaces were studied at room temperature within the exposure range of 0–1000 L. Combined measurements utilizing direct recoils spectrometry (DRS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and work function variations enabled the distinction between processes occurring on the topmost atomic layer and those associated with subsurface incorporation of oxygen. Also, the different chemical forms (oxidation states) developing during the exposure course were identified. The results were compared for three types of surfaces, each prepared by a different cleaning procedure. It has been concluded that: 1. (i) Oxygen initially accumulates on the topmost atomic layer, regardless of the type of the studied surface. No preferred subsurface occupation has been observed. 2. (ii) The kinetics of initial accumulation (up to a complete surface coverage) are similar for all the different types of surfaces. 3. (iii) Mixtures of different oxidation states of titanium (0, +2, +3, +4) are present during the whole course of exposure. Qualitatively, increasing proportions of the higher valence states are displayed for higher oxygen exposures. However, the quantitative estimates of their relative amounts indicate a strong dependence on the type of surface, with preferred high oxidation (+4) states obtained for high temperature annealed samples (as compared with room temperature sputtered surfaces). 4. (iv) Topmost oxygen atoms which terminate the oxides surfaces are less negatively charged than the underlying (i.e., subsurface) “oxidic” atoms. These results may account for some of the controversies presented in the literature.

In situ analysis of thin film deposition process using time of flight (TOF) ion beam analysis methods

The use of non-destructive, in situ methods for the characterization of thin film growth phenomena is the key both to obtaining a better understanding of thin film growth processes and to the development of more reliable deposition procedures, especially for complex layered structures involving multi-phase materials. However, surface characterization methods that utilize either electrons (e.g. AES or XPS) or low energy ions (SIMS) for the signal require an ultra-high vacuum (UHV) environment and utilize instrumentation which obstructs line of sight access to the substrate. These methods are therefore incompatible with line of sight deposition methods and thin film deposition processes which introduce gas, either as an intrinsic part of the deposition procedure or in order to produce the desired composition. We have developed a means of differentially pumping both the ion beam source and detectors of a TOF ion beam surface analysis spectrometer that does not interfere with the deposition process and permits compositional and structural analysis of the growing film in the present system, at pressures up to several mTorr. Higher pressures are feasible with modified source-detector geometry. In order to quantify the sensitivity of Ion Scattering Spectroscopy (ISS) and Direct Recoil Spectroscopy (DRS), we have measured the signal intensity for stabilized clean metals in a variety of gas environments as a function of the ambient gas species and pressure, and ion beam species and kinetic energy. The results are interpreted in terms of collision cross sections which are compared with known gas phase scattering data and provide an apriori basis for the evaluation of time-of-flight ion scattering and recoil spectroscopies (ToF-ISARS) for various industrial processing environments which involve both inert and reactive gases. The cross section data for primary ion-gas molecule and recoiled atom-gas molecule interactions are also provided, from which the maximum operating pressure in any experimental configuration can be obtained.

Effects of Intraparticle Porosity on the Radon Emanation Coefficient

An analysis of the effects on the radon emanation from individual material particles by the direct recoil process and the intraparticle porosity is presented. The purpose is to fill the gap in knowledge for a comprehensive theoretical explanation of the nature of the radon emanation processes. The overall fractional release at the particle boundaries is mechanistically described as a diffusion process. In this model, the solid and the pore diffusion as well as the direct recoil processes are essentially working in parallel. The net generation rate of the emanation atoms within the particle domain is treated as an effective generation rate βeff. An analytical solution of radon release from the diffusion-based equation is derived for short and long times. By employing this analysis as a re-interpretive device for the existing experimental data, new or improved values of diffusion coefficients may be determined. The diffusion coefficient of 222Rn for fly ash samples from various source and for different part...

Isothermal hydrogen desorption from the diamond (100)2×1 surface

Isothermal desorption experiments to study the kinetics of the desorption of hydrogen from the diamond (100)2×1 surface are described. The surface hydrogen concentrations are measured using direct recoil spectroscopy, an ion spectroscopy in which ions and neutral species recoiled from the surface are detected by time-of-flight measurements. Results indicate that the desorption of hydrogen from diamond (100) is zeroth order at temperatures below 800 °C and changes to first order above 825 °C. In the first-order region, the measured activation barrier is 39 kcal/mol and the pre-exponential factor is 3.2×105 s−1.

The investigation of GaN growth on silicon and sapphire using in-situ time-of-flight low energy ion scattering and RHEED

In-situ time-of-flight low energy ion scattering (TOF-LEIS) and reflection high energy electron diffraction (RHEED) is used effectively to monitor surface structure, major elements, and impurities during substrate annealing, nitridation and thin film growth of GaN on Si and Al 2O 3 substrates by electron-cyclotron-resonance (ECR) assisted MBE. During the annealing process of silicon, a combination of time-of-flight direct recoil spectroscopy (TOF-DRS) and mass spectroscopy of recoiled ions (MSRI) was used to monitor the desorption of surface contaminants such as H, C, and O. A change from a 1 × 1 RHEED pattern of oxygen-covered Si(001) to a 1 × 2 + 2 × 1 RHEED pattern, evidence of a clean Si(001) substrate, was observed. For the case of Al 2O 3(0001), a 1 × 1 RHEED pattern of the hexagonal structure was seen when all major contaminants were removed, as indicated by MSRI. The nitridation of a clean Al 2O 3 surface was followed by RHEED and the formation of an AlN overlayer was confirmed by MSRI. A 1 × 1 RHEED pattern was observed after GaN thin film growth on sapphire, indicating a good crystalline structure. Si nitridation and subsequent deposition of columnar GaN on Si, resulted in an amorphous RHEED pattern. MSRI spectra showed the presence of Si in addition to Ga and N. When nitridation of Si was avoided the resulting GaN film was polycrystalline as observed by RHEED and complete coverage occurred, evidenced by the absence of Si in the MSRI spectra. The results show that a combination of RHEED and TOF-LEIS permit real-time optimization of substrate preparation and thin film growth parameters.

Study of nitrogen location on ZnSe surface using ion scattering and recoiling spectrometry

We have applied ion scattering and recoiling spectrometry to study the nitrogen location on radical nitrogen doped ZnSe surface by molecular beam epitaxy. In the recoiling nitrogen measurements of heavy doped ZnSe:N surface, a two-peak structure is observed corresponding to two recoil processes, i.e., direct recoil and surface recoil. Direct recoil events occur due to the single-collision recoiling from Se site nitrogen. On the other hand, surface recoil events occur due to the nitrogen recoiling scattered by second layers and is highly sensitive to the nitrogen location on the ZnSe:N surface during molecular beam epitaxy growth. The experimental results and calculation suggest that the optimum doped nitrogen atoms locate in the substitutional Se sites but excess nitrogen atoms locate in the interstitial sites.

Surface analysis at low to ultrahigh vacuum by ion scattering and direct recoil spectroscopy

The surface analytical methods designated as mass spectroscopy of recoiled ions (MSRI) and direct recoil spectroscopy (DRS) are characterized by the incidence of energetic ions on the sample and the detection of scattered primary beam particles or recoil-sputtered surface atoms that are energy and/or mass analyzed. The use of time-of-flight for energy analysis of scattered primary particles and recoiled surface atoms creates a powerful technique for structural and elemental analysis of surfaces. Time-of-flight ion scattering and recoil spectroscopy are emerging as viable surface analysis tools with particular strength for in situ monitoring during low pressure (<few mTorr) thin film processing. DRS has the distinct advantage over most other surface spectroscopies of being able to measure surface hydrogen. MSRI is particularly useful for elemental and isotopic surface analysis and, in several important cases (e.g., N), is considerably more sensitive than secondary ion mass spectroscopy. This review covers the latest applications of MSRI and DRS in a variety of thin film growth and nucleation studies, highlighting the many advantages presented by these unique spectroscopies.

Time-of-Flight, Ion-Beam Surface Analysis for In Situ Characterization of Thin-Film Growth Processes

Low-energy (5–15 keV) ion scattering spectroscopy (ISS) and direct recoil spectroscopy (DRS) are well-established analysis techniques that have been used for several years to characterize surfaces of materials. ISS utilizes a beam of ions in the 1–15 keV range which are scattered by a surface atom at an angle θ1 relative to the direction of incidence. In this energy range, the collisions are elastic and are described by classical two-body collision kinematics. The kinetic energy E1 of the scattered primary ion is given bywhere E0 is the primary ion kinetic energy and α = M2/M1. Note that Equation 1 has real values only if M2 > M1, and ISS is not able to detect surface atoms which are lighter than the primary ion beam.The kinetic energy of the backscattered primary particle may be measured either by using an electrostatic energy analyzer (ESA) or by pulsing the beam and using a time-of-flight (TOF) detection scheme. The ESA only detects the ion fraction (typically 10 2−10−4) of the scattered primary beam and transmits only a narrow portion of the energy distribution of these ions, whereas the TOF detection scheme simultaneously detects the full energy range of both ions and neutrals. Consequently, the required beam dose for TOF detection is 3–4 orders of magnitude smaller than that required by the ESA. A typical analysis beam dose for TOF detection is ∼1011−1012 ions/cm2, removing or displacing one part in 103−104 of the near-surface atoms and making the TOF scheme essentially nondestructive.

Oxygen enhanced secondary ion emission of Fe and Co by TOF-SIMS and ISS/DR

Using secondary ion mass spectrometry (SIMS), ion scattering spectroscopy (ISS), direct recoil (DR) and X-ray photoelectron spectroscopy (XPS), we have studied the mechanism of oxygen enhancement of secondary ion emission. The secondary ion intensities were measured as a function of oxygen exposure for Fe and Co samples under static- and dynamic-SIMS conditions. In the static-SIMS, the intensity of Fe+ increased up to 60 L oxygen exposure and then slowly saturated. However, the intensity of Co+ had an additional peak at low oxygen coverage when compared with the result of Fe+. It sharply increased up to 5 L, dropping off around 20 L and increased again to saturation. Also the intensities of Fe+ and Co+ under dynamic-SIMS conditions showed similar changes to those under static-SIMS conditions. According to XPS spectra, the polycrystalline Fe surface was oxidized immediately on oxygen exposure. However the oxygen on the Co surface was changed from the chemisorbed state at low oxygen doses (0–5 L) to the oxide state at oxygen doses above 5 L. The relative oxygen coverages on Fe and Co samples estimated by DR and ISS intensities showed trends similar to the secondary ion intensities of Fe+ and Co+ as a function of oxygen dose. We applied the bond breaking model and the electron tunneling model for interpretation of the oxygen enhancement effect in the positive secondary ion emission for Fe and Co.

Mechanisms of Argon Retention in Clays Revealed by Laser 40 Ar- 39 Ar Dating

A method for dating clays is important for studies of weathering, diagenesis, hydrocarbon migration, and the formation of major metalliferous deposits. However, many attempts have produced imprecise or inaccurate results. Data from shales show that, contrary to expectations, the (40)Ar-(39)Ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because (39)Ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic (40)Ar in nature, rather than by direct recoil as is generally assumed.

Recoil implantation of alpha sources for thickness measurement of thin films

A sequence of radioactive decays can be used to implant alpha-emitting sources in substrates for thickness measurements of films grown on them. Starting with 228Th, both direct recoil implantation of 224Ra and a two-stage recoil implantation of 212Pb were performed. The thickness of germanium layers grown on gallium arsenide was determined by measuring the energy loss of the alpha particles traversing them. The results were found to be consistent with those obtained by other methods. The special advantages of the present procedure are discussed.

Atmosphere influence on in-situ ion beam analysis of thin film growth

In-situ non-destructive surface characterization of thin film growth processes in an environment of chemically active gas at pressures of several millitorrs is required both for the understanding of growth processes in multicomponent films and layered heterostructures and for the improvement of process reproducibility and device reliability. We have developed a differentially pumped pulsed ion beam surface analysis system that includes ion scattering spectroscopy (ISS) and direct recoil spectroscopy (DRS), coupled to an automated ion beam sputter deposition system, to study film growth processes in an environment of chemically active gas, such as required for the growth of oxide, nitride or diamond thin films. The influence of gas-phase scattering and gas-surface interactions on the ISS and DRS signal intensity and peak shape has been studied. From the intensity variation as a function of ambient gas pressure, we have calculated the mean free path and the scattering cross-section for several combinations of primary ion species and ambient gas. Depending on the system geometry and the combination of primary beam and background gas, it is shown that surface-specific data can be obtained during thin film growth at pressures ranging up to several millitorrs. Detailed information such as composition, structure and film growth mechanism may be obtained in real time, making ion beam analysis an ideal non-destructive in-situ probe of thin film growth processes.