Ion-bombarded Si Research Articles

Effect of beam flux on radiation damage accumulation in ion-bombarded Si

Effect of beam flux on radiation damage accumulation in ion-bombarded Si

Surface roughness of ion-bombarded Si(1 0 0) surfaces: Roughening and smoothing with the same roughness exponent

We have carried out scanning tunneling microscopy experiments under ultrahigh vacuum condition to study the roughness of pristine as well as ion-bombarded Si(100) surfaces and of ultrathin Ge films deposited on them. One half of a Si(100) sample (with native oxide layer) was irradiated at room temperature using 45keV Si− ions at a fluence of 4×1015ions/cm2 while the other half was masked. STM measurements were then carried out on the unirradiated as well as the irradiated half of the sample. Root-mean-square (rms) roughness of both the halves of the sample has been measured as a function of STM scan size. Below a length scale of ∼30nm we observe surface smoothing and surface roughening is observed for length scales above this value. However, the surface is self-affine up to length scales of ∼200nm and the observed roughness exponent of 0.46±0.04 is comparable to earlier cases of ion sputtering studies where only roughening [J. Krim, I. Heyvart, D.V. Haesendonck, Y. Bruynseraede, Phys. Rev. Lett. 70 (1993) 57] or only smoothing [D.K. Goswami, B.N. Dev, Phys. Rev. B 68 (2003) 033401] was observed. Preliminary results involving morphology for Ge deposition on clean ion-irradiated and pristine Si(100) surfaces are presented.

Temperature and fluence effects on the evolution of regular surface morphologies on ion-sputtered Si(111)

Results of a survey of the morphological evolution of the surface of Si(111) are presented for conditions in which regular patterned morphologies are spontaneously created during low energy $(250--1200\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, oblique incidence (60\ifmmode^\circ\else\textdegree\fi{} from normal), ${\mathrm{Ar}}^{+}$ ion beam etching at elevated temperature $(500--750\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$. The morphological evolution was found to vary with sputtering time (fluence), ion flux, ion energy, and sample temperature. Experimental regimes in which it is appropriate to analyze ripple evolution in terms of the linear Bradley-Harper theory are identified, through which we find that the activation energy for surface mass transport on ion-bombarded Si(111) is $1.7\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, in general agreement with measurements made in other manners. At higher fluence, nonlinear pattern forming effects are observed, including the observation of the formation of three distinct regular morphologies: two types of one-dimensional ripple arrays that differed both in orientation and regularity, and a two-dimensional array consisting of dots, all of which appear in different fluence regimes. All patterns possessed submicron periodicity and nanometer scale amplitudes. In addition, the wavelengths of one-dimensional ripples were often observed to coarsen with fluence, an effect potentially attributable to step-edge dynamics.

Ion bombardment as the initial stage of diamond film growth

It is believed that during the initial stage of diamond film growth by chemical-vapor deposition (CVD), ion bombardment is the main mechanism in the bias-enhanced-nucleation (BEN) process. To verify such a statement, experiments by using mass-separated ion-beam deposition were carried out, in which a pure carbon ion beam, with precisely defined low energy, was selected for investigating the ion-bombardment effect on a Si substrate. The results are similar to those of the BEN process, which supports the ion-bombardment-enhanced-nucleation mechanism. The formation of sp3 bonding is based on the presumption that the time of stress generation is much shorter than the duration of the relaxation process. The ion-bombarded Si is expected to enhance the CVD diamond nucleation density because the film contains amorphous carbon embedded with nanocrystalline diamond and defective graphite.

Nonlinear amplitude evolution during spontaneous patterning of ion-bombarded Si(001)

The time evolution of the amplitude of periodic nanoscale ripple patterns formed on Ar+ sputtered Si(001) surfaces was examined using a recently developed in situ spectroscopic technique. At sufficiently long times, we find that the amplitude does not continue to grow exponentially as predicted by the standard Bradley–Harper sputter rippling model. In accounting for this discrepancy, we rule out effects related to the concentration of mobile species, high surface curvature, surface energy anisotropy, and ion-surface interactions. We observe that for all wavelengths the amplitude ceases to grow when the width of the topmost terrace of the ripples is reduced to approximately 25 nm. This observation suggests that a short circuit relaxation mechanism limits amplitude growth. A strategy for influencing the ultimate ripple amplitude is discussed.

Roughening and ripple instabilities on ion-bombarded Si.

Experimental studies of 10-40-keV ${\mathrm{Xe}}^{+}$ ion bombardment of Si at polar incidence angles between 0\ifmmode^\circ\else\textdegree\fi{} and 45\ifmmode^\circ\else\textdegree\fi{} to the surface normal at temperatures between 100 and 300 K show little roughening for near normal incidence but ripple production for 45\ifmmode^\circ\else\textdegree\fi{} incidence. It is shown that inclusion of the directed flux of atoms parallel to the surface and generated by ion bombardment in a stochastic differential equation description of the dynamics of surface evolution during sputtering erosion can induce smoothing for near-normal ion incidence. For oblique incidence, roughening and ripple production occurs with a late stage dynamics dictated by the competition between curvature-dependent sputtering processes and surface relaxation (which is also, probably, irradiation motivated), gradient-dependent sputtering, and other higher-order effects.

Application of x-ray interference method for residual strain measurement in low energy Ar ion-bombarded Si (001)

We have employed the x-ray interference method for measurement of the residual strains in Si (001) surfaces bombarded by low energy Ar+. This method, combined with transmission electron microscopy, permits determination of the average residual strain in very thin bombarded layers. The residual strain in the bombarded layers was found to monotonically increase as the density of pointlike defects increases and saturates upon the formation of extended defects. Annealing data also suggest that defects formed by low energy ion bombardment at moderate temperature (≊450 °C) require high temperature (≳800 °C) annealing for their removal.

Scanning Tunneling Microscopy Observation of Ar-Ion-Bombarded Si(001) Surfaces and Regrowth Processes by Thermal Annealing

The surface morphology of Ar+-ion-bombarded Si(001) surfaces and its regrowth processes during thermal annealing are studied “in situ” using a scanning tunneling microscope (STM). The amorphous layer consists of grains 0.63∼1.6 nm in diameter. The grains gradually coalesce and forms clusters 2∼3.6 nm in diameter at the annealing temperature of 245°C. The (2×1) and (1×2) reconstructed regions surrounded by amorphous regions are partially observed on the surface after prolonged annealing, which suggests the onset of solid phase epitaxy (SPE). Real-time observation reveals that the smoothing of the surface occurs layer by layer. As the annealing temperature is raised to 445°C, the amorphous layer epitaxially crystallizes up to the topmost surface, and the (2×1) reconstructed surface with monatomic-height steps is observed.

Random and ordered defects on ion-bombarded Si(100)-(2 x 1) surfaces.

Scanning tunneling microscopy STM images of Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surfaces bombarded by low-dose 3-keV ${\mathrm{Ar}}^{+}$ ions showed random defects which ordered into line defects perpendicular to the dimer rows upon annealing at elevated temperatures. Molecular-dynamics simulations were performed to explain the shapes and sizes of the observed random defects and also to examine the stability of ordered defects. Our simulations showed good agreement with STM observations.

Scanning tunneling microscopy and spectroscopy of ion-bombarded Si(111) and Si(100) surfaces.

Surfaces of Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) and Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) were bombarded by 3-keV ${\mathrm{Ar}}^{+}$ ions at doses of \ensuremath{\le}${10}^{12}$ ions ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ to study the effect of individual ion impacts on the atomic structure of surfaces. Atom-resolved images show damaged regions of missing and displaced atoms. Current-imaging tunneling spectroscopy shows rest-atom states in the bombarded areas on Si(111), in agreement with line-scan measurements indicating a monatomic step-height difference between bombarded and unbombarded areas. Upon annealing to 750 \ifmmode^\circ\else\textdegree\fi{}C for 2 min, complete (7\ifmmode\times\else\texttimes\fi{}7) was restored for the bombarded Si(111) surface whereas ordering of vacancies into line defects was observed for Si(100).

Reaction chemistry at the Si (100) surface—control through active-site manipulation

Thermal desorption methods have been used to investigate the interaction of propylene (C3H6) with Si(100)-(2×1). The adsorption characteristics depend strongly on the availability of active sites at the Si(100) surface. Reactivity is enhanced by production of active sites during ion prebombardment. Adsorption of C3H6 to a disordered, ion-bombarded Si(100) surface results in nearly complete dissociation of C3H6 for ion fluences as small as 1015 Ar+ ions/cm2. In contrast, for a thermally annealed and ordered Si(100) surface, only 65% of the C3H6 dissociates. The remainder of the propylene chemically bonds to the surface as an undissociated molecule which desorbs intact at 550 K. The increase in reactivity is due to an increase in dissociative chemisorption which occurs at defect sites produced by ion bombardment. Reactivity is suppressed by capping of active sites using atomic hydrogen preadsorption. Hydrogen passivates the Si(100) surface by occupation of silicon dangling bonds, which prevents adsorption of C3H6. By controlling the number and kind of active surface sites in this way, it is possible to manipulate the reactive ability of the Si(100) surface.

Surface temperature measurements for ion-bombarded Si and GaAs at 1.0 to 2.0 MeV

Surface temperatures of ion-bombarded silicon and gallium arsenide have been measured using an infrared detector. Ion beams of N+, N+2, O+, O+2, C+, CO+, and H+ were used at energies from 1–2.0 MeV and at current densities up to 12 μA⋅cm−2. No temperature dependence was found on ion species, energy, or current. The change in temperature depended only on beam power, target material, and sample mounting technique. With proper mounting temperature increases of 20 °C for silicon and 65 °C for gallium arsenide were observed for a beam power density of 1.0 W⋅cm−2.

Study of silicon-oxygen interaction with the statical method of secondary ion mass spectroscopy (SIMS)

The interaction of O2 with atomically clean Si surfaces has been studied by UHV secondary ion mass spectroscopy (SIMS). Thermally cleaned and annealed Si (111) surfaces, as well as ion-bombarded Si (111) surfaces, have been investigated. The silicon-oxygen interaction was found to take place in two steps. In step (i) a monoatomical oxide layer on the surface is formed with an initial sticking coefficient for O2 of about 1 and 0.05 for Si (111) and ion-bombarded Si (111), respectively. In step (ii) this monoatomical layer grows very slowly into greater depth.

The use of dissolution curves to determine amorphization depths and damage ranges in silicon

Abstract Amorphization depths in ion-bombarded Si can be readily measured with dissolution curves, i.e. curves giving the residual activity of the implanted ions versus the time of exposure to an HF-H2O2 mixture. The depth calibration is established by comparison with integral ion distributions. Amorphization depths have been determined in this way both for room-temperature Kr implants (where they range from 35–60 A, at 2 keV, to 105–205 A at 10 keV, to 395–570 A, at 40 keV) and for Kr implants at 95 to 360°C (where they are consistently smaller). An important application of amorphizationdepth (R α) measurements lies in deducing mean projected damage ranges ( ). Thus it is argued that the following quantity must be independent of dose (Et)for a given energy: where the constant “1.28” is an approximation which is valid for intermediate mass ratios and for energies such that (⟨x d ⟩) 1. Solution of this relation gave the result that ⟨x d⟩was 25 ± 2.5 A, at 2 keV, 70 ± 10 A at 10 keV, and 190 ± 15 A at 4...

Lattice strain in ion-bombarded Si studied by X-ray Moiré technique

Lateral strain produced by ion bombardment (Ar, 80 keV, 10 15 ions/cm 2) of the analyser crystal of a Si X-ray interferometer is investigated by Moiré technique. A relative lattice expansion of (5–6) x 10 -7 in the bombarded area is observed.