RIE-induced damage and contamination in silicon

Abstract

Reactive ion etching always causes a dynamic radiation effect to crystalline silicon, beacause of an energetic particle bombardment. RIE induced radiation effects are mostly confined to the near surface within a projected range of impinging ions, but point defects, which are highly mobile at room temperatures, can migrate further into the bulk before a damaged surface layer is etched away. Competition between etch rates and damage rates ultimately determines a degree of the RIE damage residue: the slower the etch rate, the heavier the damage may be accumulated at the near surface, eventually leading to amorphization of the surface region. Also, a removal of the surface layer due to etching or sputtering enhances a chemical reaction between a bare surface and incoming radicals. This easily forms a foreign material on the surface which gives rise to a serious contamination problem. A post-cleaning at a low temperature is highly desirable whenever the surface of active devices must be exposed to reactive plasmas.     

Influence of ion bombardment induced radiation damages in the crystal lattice on the angular regularlties in ion scattering

The paper examines the angular, spatial and energy distribution of ions scattered by various faces of silicon and germanium single crystals. It is shown that, due to the orderly arrangement of atoms in the crystal lattice, the orientation effects in the angular, spatial and energy distributions are observed at temperatures exceeding the annealing temperature of ion bombardment induced radiation damage. At high temperatures of the sample a reduction of the anisotropy in the angular, spatial and energy distributions of scattered ions was observed which is due to the changing transparency of the crystal resulting from the thermal vibrations of atoms in the lattice.

Results of investigations testify to the possibility of using the orientation dependences of phenomena occurring during the interaction of ions with crystals for determining the annealing temperature of ion bombardment damage and for studying the kinetics of the process of ion bombardment. It is shown that further studies along the same lines as conducted by the authors of this paper could develop a technique for the quantitative estimation of ion bombardment damage in crystals and also in thin epitaxial films.     

Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

Recent X-ray diffraction studies on α-quartz (SiO₂) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO₂ and α-AlPO₄ (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO₂.     

Laser annealing of silicon layers amorphized by molecular ions

The behaviour, after laser beam annealing, of heavily doped silicon layers obtained by a high current density atomic and molecular ion bombardment is investigated. The ion beam is realized by glow discharge of a gas containing the dopant, acceleration towards the sample, without any magnet.

The annealing is performed by using a high power (3.5 J/cm²) pulsed laser and the surfaces are studied by Rutherford backscattering, secondary ions mass spectroscopy and conductivity measurements. Comparison with a classical thermal annealing shows the advantage of the laser pulse to restore completely the original cristallinity, even if the layer is im-planted at doses in excess the solubility limit of the dopant, leading to a full surface amorphization.     

Surface sensitive spectroscopic study of the interaction of oxygen with Al(111) - low temperature chemisorption and oxidation

The interaction of an atomically clean Al(111) surface with O₂ has been studied using a combination of electron energy loss spectroscopy (EELS) and Auger spectroscopy (AES). Oxygen dissociatively adsorbs and occupies both surface and subsurface binding sites under all exposure conditions in the temperature range 122–700 K. Surface sites are preferentially occupied at low exposures, while higher exposures increasingly favor population of subsurface sites. Studies of O₂ adsorption at temperatures as low as 131 K have shown that formation of Al₂O₃ occurs at high oxygen exposures. The Al₂O₃ produced exhibits a 54 eV Auger transition and a characteristic vibrational spectrum with loss features at 430, 645, and 880 cm⁻¹. Argon ion bombardment of thin monolayer level Al₂O₃ layers leads to preferential loss of Al₂O₃ and a reduction in the subsurface-to-surface oxygen ratio. Electron bombardment of similar, thin Al₂O₃ layers is ineffective in inducing desorption of surface species, whereas thick Al₂O₃ layers are strongly influenced by electron bombardment, as judged from AES behavior. Qualitative models for O₂ adsorption, oxidative annealing, and damage by ion and electron bombardment are given.     

Linking high- and low-temperature plasticity in bulk metallic glasses: thermal activation,extreme value statistics and kinetic freezing

Abstract

At temperatures well below their glass transition, the deformation properties of bulk metallic glasses are characterized by a sharp transition from elasticity to plasticity, a reproducible yield stress and an approximately linear decrease of this stress with increasing temperature. In the present work, it is shown that when the well-known properties of the undercooled liquid regime, in terms of the underlying potential energy landscape, are assumed to be also valid at low temperature, a thermal activation model is able to reproduce the observed onset of macroscopic yield. At these temperatures, the thermal accessibility of the complex potential energy landscape is drastically reduced, and the statistics of extreme value and the phenomenon of kinetic freezing become important, affecting the spatial heterogeneity of the irreversible structural transitions mediating the elastic-to-plastic transition. As the temperature increases and approaches the glass transition temperature, the theory is able to smoothly transit to the high-temperature deformation regime where plasticity is known to be well described by thermally activated viscoplastic models.     

Modification of engineering silicon nitride ceramics by energetic-ion bombardment

The microstructural and mechanical properties of hot-pressed Si₃N₄ ceramics after Si⁺ ion bombardent and annealing in N₂ atmosphere have been investigated as a function of the ion fluence and the annealing temperature. The irradiations were carried out at target temperatures of about 80 K and 450 K with ion energies of 0.5 MeV and 1.0 MeV. In all cases the fluence range was subdivided into two regimes: a low-fluence regime with improved microhardness and fracture toughness, and a high-fluence regime with an absolute degradation of these properties. The transition fluence was found to strongly depend on the ion energy and implantation temperature. This property transition coincides with a microstructural transition from a highly damaged, but still crystalline material, to the formation of a buried amorphous layer. The amorphization results in a strong volume swelling which causes a closure of surface flaws. The latter process significantly enhances the fracture strength of the implanted material. Thermal relaxation of the modified mechanical properties was found to occur at temperatures above 800° C. The relationships between the ion-induced changes of the mechanical properties and the microstructural modifications will be discussed.     

Orbital and spin effects in the low-temperature behavior of the magnetoresistance of doped CdTe crystals

An observation of the suppression of negative magnetoresistance in samples of doped CdTe that are far from the metal-insulator transition as the temperature is lowered in the temperature range 3–0.4 K was previously reported [N. V. Agrinskaya, V. I. Kozub, and D. V. Shamshur, JETP 80, 1142 (1995)]. The results of an investigation of samples that are closer to the transition in the low-temperature region below 36 mK are presented. It is discovered that the samples investigated (which do not exhibit the suppression of negative magnetoresistance at comparatively high temperatures) display this effect at low temperatures and that, as previously, the suppression of the negative magnetoresistance correlates with the transition to conduction via Coulomb-gap states. A plateau-like magnetoresistance feature is displayed at low temperatures for the sample that is closest to the metal-insulator transition. The results obtained are analyzed within existing theoretical models that take into account the role of both the orbital and spin degrees of freedom. In particular, the low-temperature feature indicated is interpreted as a manifestation of positive magnetoresistance caused by spin effects. Nevertheless, it is shown within a detailed analysis supplemented by numerical calculations that the observed suppression of the negative magnetoresistance cannot be attributed only to the appearance of spin positive magnetoresistance. Moreover, the possibility of observing spin positive magnetoresistance is determined to a certain extent specifically by the suppression of the negative magnetoresistance competing with it. Zh. éksp. Teor. Fiz. 111, 1477–1493 (April 1997)     

Observation of the transition from diffusive regime to ballistic regime of the 2DEG transport property in AlxGa1−xN/GaN heterostructures

Electron–electron interaction effect of the two-dimensional electron gas (2DEG) in Al_xGa_1−xN/GaN heterostructures has been investigated by means of magnetotransport measurements at low temperatures. From the temperature dependence of the longitudinal conductivity of the heterostructures, a clear transition region has been observed. Based on the theoretical analysis, we conclude that this region corresponds to the transition from the diffusive regime to the ballistic regime of the 2DEG transport property. The interaction constant is determined to be −0.423, which is consistent with the theoretical prediction. However, the critical temperature for the transition, which is 8 K in Al_xGa_1−xN/GaN heterostructures, is much higher than the theoretical prediction.     

Au5+ ion implantation induced structural phase transitions probed through structural,microstructural and phonon properties in BiFeO3 ceramics,using synergistic ion beam energy

Implanted Au⁵⁺-ion-induced modification in structural and phonon properties of phase pure BiFeO₃ (BFO) ceramics prepared by sol–gel method was investigated. These BFO samples were implanted by 15.8?MeV ions of Au⁵⁺ at various ion fluence ranging from 1?×?10¹⁴ to 5?×?10¹⁵?ions/cm². Effect of Au⁵⁺ ions’ implantation is explained in terms of structural phase transition coupled with amorphization/recrystallization due to ion implantation probed through XRD, SEM, EDX and Raman spectroscopy. XRD patterns show broad diffuse contributions due to amorphization in implanted samples. SEM images show grains collapsing and mounds’ formation over the surface due to mass transport. The peaks of the Raman spectra were broadened and also the peak intensities were decreased for the samples irradiated with 15.8?MeV Au⁵⁺ ions at a fluence of 5?×?10¹⁵?ion/cm². The percentage increase/decrease in amorphization and recrystallization has been estimated from Raman and XRD data, which support the synergistic effects being operative due to comparable nuclear and electronic energy losses at 15.8?MeV Au⁵⁺ ion implantation. Effect of thermal treatment on implanted samples is also probed and discussed.     

Correlation of light scattering and molecular dynamics in superionic Ba1−xLaxF2+x

Abstract

The squares of the Brillouin frequencies (Δω_B)² of the LA mode in the [100] direction related to the C₁₁ elastic constant show a linear decrease with temperature followed by significant deviations that occur above the corresponding transition temperatures (T_c), being at 850, 800, 970, 950 and 920 K for samples with x = 0.05, 0.1, 0.2, 0.3 and 0.5, respectively. The Raman linewidths show linear increases with temperature followed by rapid increases around the same temperatures at which the elastic constants start to show marked decreases. The complementary studies using Molecular Dynamics show that the diffusion coefficients start to increase markedly around the same temperatures observed experimentally.     

Ferroic phase transitions in β-LiNH4SO4

Abstract

The paper reviews the results of experimental and theoretical studies of ferroic phase transitions in β-LiNH₄SO₄ and its deuterated analogue. β-LiNH₄SO₄ undergoes succesive phase transitions: a paraelectric - ferroelectric phase transition at T₁ ? 462 K, a ferroelectric - ferroelastic phase transition at T₂ ? 283 K and a transition from one ferroelastic phase to the other at T₃ ? 28 K. Attention is focused on the influence of the order of phase transitions on the pattern of ferroelectric and ferroelastic domain structure, and also on the role played by the dynamics of molecular groups in the mechanism of transitions. The pre-transition effect connected with the ferroelectric-paraelectric transition: heterophase, capable of accounting for anomalies in different physical properties present 1-3 K below T₁ is shown. The anomalous temperature variation of spontaneous polarisation of the crystal is discussed within the framework of the phenomenological model of weak ferroelectrics.     

Temperature dependence of compositional changes of SiO2 surface during ion and electron bombardment

The influence of ion (Ar⁺ 0.5 keV, 2 microA/cm²) and electron (2 keV, 2 mA/cm²) bombardment on the elemental composition of SiO₂ was investigated in the temperature range of 270–790 K. Elemental composition was controlled by AES. It was found that both ion and electron bombardment resulted in an increasing amount of Si₉₂ (elemental silicon) and in decreasing amounts of both O₅₁₀ and Si₇₈ (silicon bound to oxygen). The temperature influence on the composition of SiO₂ is negligible under ion bombardment while the amount of Si₉₂ strongly increases under electron bombardment at temperatures exceeding 600 K. The mechanism of temperature dependence is discussed.     

Nanostructuring Germanium Nanowires by In Situ TEM Ion Irradiation

Once nanomaterials have been synthesized, inducing further structural modifications is challenging. However, being able to do so in a controlled manner is crucial. In this context, germanium nanowires are irradiated in situ within a transmission electron microscope (TEM) by a 300 keV xenon ion beam at temperatures ranging from room temperature (RT) to 500 °C. The ion irradiation is performed in situ and the evolution of nanowires during irradiation is monitored. At 300 °C and below, where the temperature is low enough to allow amorphization, the ion beam causes the formation of nanostructures within the nanowires. Formation of nanopores and swelling of nanowires is observed for a very low fluence of 2.2 × 10¹⁴ and up to 4.2 × 10¹⁵ ions cm⁻². At higher fluences, the thickness of the nanowires decreases, the nanowires lose their wire-like cylindrical shape and the nanostructuring caused by the ion beam becomes more complex. The nanostructures are observed to be stable upon crystallization when the nanowires are annealed at 530 °C. Furthermore, in situ imaging allows the growth of nanopores during irradiation to be followed at RT and at 300 °C providing valuable insights into the mechanism responsible for the nanostructuring.     

A luminescence study of B-type Eu2O3 under pressure

Abstract

Luminescence spectra from Eu³ + ion in B-type (monoclinic) ₂O₃ powder have been recorded at room temperature as a function of pressure using a diamond anvil cell. Changes in the spectral pattern of the Eu³ + ion emission at about 4 GPa indicated that a phase transition to the A-type (hexagonal) structure had taken place. Upon release of the applied pressure, the B-type structure was regained with hysteresis. The spectral shifts with pressure have been used to study the effect of pressure on the spin-orbit interaction of the 4f electrons in the Ed + ion. The relationship between the relative changes in the spin-orbit coupling constant, ζ_4f, and the volume accompanying the phase transition is also discussed.     

Magnetic and structural phase transitions in the shape-memory ferromagnetic alloys Ni2+x Mn1−x Ga

The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni_2+x Mn_1−x Ga (x=0–0.20) are reported. A T−x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) T _M and magnetic T _C (Curie point) phase transitions to converge. In the region where T _C =T _M the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T−x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment. Zh. éksp. Teor. Fiz. 115, 1740–1755 (May 1999)     

Amorphization of metals by ion implantation and ion mixing methods

In this paper, we present a review of the experimentally determined characteristics of solid-phase amorphization of metals and alloys under ion implantation and ion mixing conditions. For the first time we systematically consider the characteristic features and the thermodynamic, structural, and kinetic criteria for amorphization in different metallic systems, and we demonstrate the expediency of the following classification of alloys undergoing amorphization: metal-metalloid systems, intermetallics, heterophase alloys, alloys with high positive heat of mixing. We compare solid-phase amorphization under conditions of bombardment by beams of charged particles, thermal mixing of the alloy components, and mechanical alloying. We consider the phase and structural states preceding amorphization and the possibilities for predicting metallic systems which can undergo amorphization.V. D. Kuznets Siberian Physicotechnical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 3–30, August, 1994.     

Some of the current trends in the studies of sputtering

Abstract

The main directions in experimental, theoretical and computer studies of sputtering of solids under ion bombardment are analyzed. It is emphasized that there is a close relation between these studies and practical applications of sputtering.     

On the theory of recoil implantation

Abstract

EPR experiments on photoholes in both AgCl :Cu and AgCl :Pd show the existence of an activation energy barrier, of height near 1.8 meV, in the transition to the localized self-trapped state. The self-trapped hole then migrates athermally via the small-polaron band, for temperatures below 30 K, and by phonon-assisted hopping for temperatures above 35 K. In the hopping régime, the activation energy is 61 meV, suggesting that the binding energy is about 0.1 eV. From the pre-exponential factor, one estimates that the electron transfer integral is about 1% of the energy of the pertinent acoustical phonon. Experiments on AgX:Cu in which 1% of the halide is Br^? show that all of the self-trapped holes ultimately migrate to sites with 2 bromide neighbors. Upon annealing, these disappear with complex kinetics and an activation energy of about 180 meV. In AgCl :Pd, Fe, the temperature-dependence of the efficiencies of production of trapped photocarrier centers is determined and correlated with the dynamics of migration of the self-trapped hole and the cation vacancy.     

The behaviour of mosaic blocks and electrical properties of polysilicon under ion implantation

Abstract

The change in microstrains ε, block sizes L and in the temperature dependences of conductivity of polysilicon with the grain size 30-40nm at N⁺, Ne⁺, P⁺ ion irradiation has been studied. It is shown that ε increases while L practically is not changing up to amorphization. The change in conductivity is governed by an increase in the density of states near the Fermi level and depends both on the damage rate for the given ions and their chemical activity.