Sputtering of compound semiconductor surfaces. I. Ion-solid interactions and sputtering yields

Several phenomena occur on the surface of a solid when being bombarded by energetic ions. A short general review is given of the major ion-solid interactions on compound semiconductor surfaces. An in-depth discussion is presented of the total sputtering yields of component semiconductors. For this discussion, GaAs is assumed to be the prototype compound semiconductor because most experimental measurements exist for GaAs. To exclude any chemical effects in the sputter yields, only the total sputtering yield data for argon ion bombardment of GaAs are compared with the predictions of the major sputtering theories, with particular attention to the Sigmund theory for linear cascade sputtering. Different proposals of each of the parameters in this theory are presented and compared with the GaAs data. These parameters are the surface binding energy, the nuclear stopping power, and the factor α, which represents the fraction of energy available for sputtering. Use of the different parameters results in a large variation in the predictions. Topics also considered are the angle dependence of the sputtering yields, sputter threshold energy, and channeling effects in the sputter yields of compound semiconductors. Spike sputtering effects are evident in the sputtering yields of GaAs by krypton and xenon ions.     

Early stage in low energy ion-induced damage on InP(110) surface

The change in the short-range order created by ion milling in the near surface region of InP single crystals was investigated by primary beam diffraction modulated electron emission (PDMEE). The very early stage of the damage creation by low energy (0.6–1 keV) Ar ions in normal and oblique incidence was studied. A simple model based on the weighted combination of perfectly crystalline and completely amorphous regions was used to model the experimental results. Evidence of a subsurface nucleation of the amorphization process was found. We also found that the total sputtering yield is markedly dependent on the ion dose, being on the undamaged surface much larger than its steady state value. Low energy electron diffraction (LEED) measurements were also performed to correlate long-range and short-range order removal by ion bombardment. Finally, the ion damage on the GaAs and InP surfaces was comparatively discussed.     

Sputtering of A 3 B 5 materials (GaP, GaAs, GaSb, InP, and InSb) by 2-to 14-keV N 2 + ions

Investigations of the general characteristics and distinctive features of sputtering of A ³ B ⁵ materials (GaP, GaAs, GaSb, InP and InSb) under bombardment with N ₂ ⁺ ions have been carried out. From the experimental data, dependences of the sputtering yield of these materials on the incidence angle and ion energy have been obtained and the surface relief patterns produced by target etching have been studied. It has been shown that the dependence on energy of the sputtering yield for GaP, GaAs, and InP can be adequately described by the Haffa-Switkovski formula for binary materials and Yudin’s approximation for elemental targets. Sputtering of GaSb and InSb proceeds in the surface layer recrystallization mode, and the sputtering yield agrees with calculations based on Onderlinden’s model. From a comparison of the experimental and calculated dependences, the surface bonding energies have been determined.     

SEM and XPS studies of nanohole arrays on InP(1 0 0) surfaces created by coupling AAO templates and low energy Ar ion sputtering

The aim of the present study is to demonstrate the feasibility to form well-ordered nanoholes on InP(1 0 0) surfaces by low Ar⁺ ion sputtering process in UHV conditions from anodized aluminum oxide (AAO) templates. This process is a promising approach in creating ordered arrays of surface nanostructures with controllable size and morphology. To follow the Ar⁺ ion sputtering effects on the AAO/InP surfaces, X-ray photoelectron spectroscopy (XPS) was used to determine the different surface species. In_4d and P_2p core level spectra were recorded on different InP(1 0 0) surfaces after ions bombardment. XPS results showed the presence of metallic indium on both smooth InP(1 0 0) and AAO/InP(1 0 0) surfaces. Finally, we showed that this experiment led to the formation of metallic In dropplets about 10 nm in diameter on nanoholes patterned InP surface while the as-received InP(1 0 0) surface generated metallic In about 60 nm in diameter.     

Low energy Ar ion bombardment damage of Si,GaAs, and InP surfaces

Argon bombardment damage to (100) surfaces of Si, GaAs, and InP for sputter ion-gun potentials of 1, 2, and 3 kilovolts was studied using Rutherford backscattering. Initial damage rates and saturation damage levels were determined. Bombardment damage sensitivity increased for the sequence Si, GaAs, and InP. Saturation damage levels for Si and GaAs correspond reasonably to LSS projected range plus standard deviation estimates; damage to InP exceeded this level significantly. For an ion-gun potential of 3 keV, the initial sputter yield of P from an InP surface exceeded the sputter yield of In by four atoms per incident Ar projectile.     

Surface chemistry and optimization of focused ion beam iodine-enhanced etching of indium phosphide

Focused ion beam physical sputtering and iodine-enhanced etching of indium phosphide (InP) were performed. Up to 15× enhanced etching rates over sputtering were measured at room temperature, due to the addition of iodine to the sputter-process. Reaction mechanisms and products are discussed and characterized. The reaction is limited by the desorption of indium triiodide (InI₃) at room temperature. InI₃ has to be removed by sputtering, which simultaneously amorphizes the underlying substrate. Surface roughness and stoichiometry of InP are compared for sputtering and etching. Gallium-contamination and the damaged zone in InP are significantly reduced by iodine-enhanced etching. Based on the reaction mechanisms, an optimum beam scanning strategy is proposed which allows precise microfabrication in reduced time and minimizes damage to the substrate. The method is also applicable for other halide gas etching processes of III-V semiconductors.     

On the problem of whether mass or chemical bonding is more important to bombardment-induced compositional changes in alloys and oxides

The bombardment of alloys, oxides, and halides often leads to marked compositional changes at the surface, and these changes have been attributed to an interplay of mass-dependent effects, chemical bonding, electronic processes, and diffusion. We attempt here to answer the limited question of whether, considering only alloys and oxides, mass or bonding is normally more important. The relevant theory is reviewed and extended, with mass effects being shown to be associated most explicitly with recoil sputtering and bonding effects being shown to be associated with all three of cascade sputtering, thermal sputtering, and surface segregation. As far as experimental examples are concerned, mass correlations are found to be quite unsuccessful, whereas most observations can be understood rather well in terms of bonding. Nevertheless, there is a basic problem in that the cascade component of sputtering, normally judged to be predominant, should give significantly less compositional change than is observed. Thermal sputtering would lead to more significant changes, but there is a new problem that, at least with alloys, the absolute yields are probably rather small. A combination of surface segregation with sputtering would also lead to more significant changes, but it is unclear whether segregation is rapid enough to be important in room-temperature bombardments.     

Highly ordered Ga nanodroplets on a GaAs surface formed by a focused ion beam

The morphological evolution of a GaAs surface induced by a focused ion beam (FIB) has been investigated by in situ electron microscopy. Under off-normal bombardment without sample rotation, Ga droplets with sizes from 70 to 25 nm in diameter on the GaAs surface can self-assemble into a highly ordered hexagonal pattern instead of Ostwald ripening or coalescence. The mechanism relies on a balance between anisotropic loss of atoms on the surface of droplets due to sputtering and an anisotropic supply of atoms on the substrate surface due to preferential sputtering of As. The ratio of wavelength to the droplet diameter predicted by this model is in excellent agreement with experimental observations.     

Simulation of radiation damage in materials under high fluence ion bombardment

The results of analytical estimations and computer simulation of a radiation damage level dpa are presented and discussed taking into account the material sputtering under ion bombardment. It is shown that the calculations of a stationary level of radiation damage are necessary for the interpretation of regularities of a radiation damage in materials under high fluence ion bombardment.     

Energy deposition and penetration depth of heavy ions in the electronic stopping region

Previous calculations of spatial moments over the distribution of ion ranges and bombardment damage have been extended into the range of ion energies where electronic stopping is important. Numerical solutions are given of well-known integral equations, under the assumption of Thomas-Fermi scattering and velocity-proportional electronic stopping, for equaI masses of ion and target and five values of the electronic stopping constant, over a range of four decades of ion energy. The results are compared with experimental damage distributions, with good success. Implications on sputtering are mentioned briefly.     

Physical properties of hydrogenated amorphous gallium arsenide

Summary The elemental composition and the optical properties of hydrogenated amorphous GaAs prepared by r.f. reactive sputtering at different hydrogen and argon pressure and substrate temperature have been determined. From the dependence of the absorption coefficient on photon energy the optical gap has been deduced according to the Tauc law. The data obtained for stoichiometric samples are compared with similar data obtained by different authors. The influence of various deposition parameters on stoichiometry and on the optical properties is briefly discussed.     

Properties of indium phosphite and selected compounds under irradiation with swift heavy ions

Surface and bulk properties of indium phosphate single crystals with initial and previously irradiated by 25 MeV electrons structures were irradiated with ⁸⁶Kr (253 MeV) and ¹⁹⁷Au (200 MeV) up to various fluences. The modern methods of condensed matter studies were used for research of InP property changes before and after irradiation as scanning (SEM) and high resolution transmission electronic microscopy (HTEM), Rutherford backscattering spectroscopy (RBS/C) and atomic force microscopy (AFM). The comparison of obtained results with the results of other authors is carried out. The surface structure change of InP single crystal irradiated by high-energy 86Kr ions and electrons is studied. It is shown the changes of the InP surface have complicated character and caused by inelastic sputtering processes. It is observed the twice irradiated layer swells with the cracks creation on the surface. The swelling with cracks and strong sputtering of twice irradiated by electrons and ions with high energy layers of the InP and GaAs surfaces are explained using the model based on the influence of ionizing energy loss of swift ⁸⁶Kr ions. The small crystalline objects are detected on the InP surface irradiated with ⁸⁶Kr ions which may be nano- and micro-crystals of InP. All obtained effects are discussed in frame of models based on ionizing energy loss of swift heavy ions.     

Low sputter damage of metal single crystalline surfaces investigated with medium energy ion scattering spectroscopy

It was observed clearly that the sputter damage due to Ar⁺ ion bombardment on metal single crystalline surfaces is extremely low and the local surface atomic structure is preserved, which is totally different from semiconductor single crystalline surfaces. Medium energy ion scattering spectroscopy (MEIS) shows that there is little irradiation damage on the metal single crystalline surfaces such as Pt(111), Pt(100), and Cu(111), in contrast to the semiconductor Si(100) surfaces, for the ion energy of 3–7 keV even above 10¹⁶–10¹⁷ ions/cm² ion doses at room temperature. However, low energy electron diffraction (LEED) spots became blurred after bombardment. Transmission Electron Microscopy (TEM) studies of a Pt polycrystalline thin film showed formation of dislocations after sputtering. Complementary MEIS, LEED and TEM data show that on sputtered single-crystal metal surfaces, metal atoms recrystallize at room temperature after each ion impact. After repeated ion impacts, local defects accumulate to degrade long range orders.     

Oxidation and annealing of GaP and GaAs (111)-faces studied by AES and UPS

The annealing behaviour after argon sputtering and the first steps of oxidation of the polar GaP and GaAs (111)-faces are studied by AES and UPS. The Auger spectrum of GaP is briefly discussed. In contrast to GaAs, the GaP surfaces show a gallium accumulation after argon sputtering, but they become stoichiometric by annealing. The photoemission spectra show an additional emission due to oxygen with its maximum at about 5 eV below the valence band edge. The decrease of the emission near the valence band edge by oxidation shows the existence of surface sensitive states. Comparing the results of the two polar faces and for different surface compositions, it is concluded that these states are mainly As- and P-derived, respectively. A linear relationship is found between the UPS and Auger signal of oxygen.     

AES, EELS and TRIM investigation of InSb and InP compounds subjected to Ar ions bombardment

The interaction of ions with matter plays an important role in the treatment of material surfaces. In this paper we study the effect of argon ion bombardment on the InSb surface in comparison with the InP one. The Ar⁺ ions, accelerated at low energy (300 eV) lead to compositional and structural changes in InP and InSb compounds. The InP surface is more sensitive to Ar⁺ ions than that of InSb. These results are directly inferred from the qualitative Auger electron spectra (AES) and electron energy loss spectroscopy (EELS) analysis. However, these techniques alone do not allow us to determine with accuracy the disturbed depth in Ar⁺ ions of InP and InSb compounds. For this reason, we combine AES and EELS with the simulation method TRIM (transport and range of ions in matter) to show the mechanism of interaction between the ions and the InP or InSb and hence determine the disturbed depth as a function of Ar⁺ energy.     

Damage of low-energy ion irradiation on copper nanowire: molecular dynamics simulation

Physical and chemical phenomena of low-energy ion irradiation on solid surfaces have been studied systematically for many years, due to the wide applications in surface modification, ion implantation and thin-film growth. Recently the bombardment of nano-scale materials with low-energy ions gained much attention. Comared to bulk materials, nano-scale materials show different physical and chemical properties. In this article, we employed molecular dynamics simulations to study the damage caused by low-energy ion irradiation on copper nanowires. By simulating the ion bombardment of 5 different incident energies, namely, 1~keV, 2~keV, 3~keV, 4~keV and 5~keV, we found that the sputtering yield of the incident ion is linearly proportional to the energies of incident ions. Low-energy impacts mainly induce surface damage to the nanowires, and only a few bulk defects were observed. Surface vacancies and adatoms accumulated to form defect clusters on the surface, and their distribution are related to the type of crystal plane, e.g. surface vacancies prefer to stay on (100) plane, while adatoms prefer (110) plane. These results reveal that the size effect will influence the interaction between low-energy ion and nanowire.     

Raman scattering study of ion bombardment induced amorphization of SiC

Abstract

Lattice disorder induced by ion bombardment of SiC surfaces has been studied using Raman spectroscopy. After bombardment with 15 keV H⁺, D⁺ or He⁺ to fluences of 10¹⁹ cm^?2 the SiC surface was found to amorphize as indicated by changes in the Raman spectra. Raman studies of the annealing behavior of the damaged surface showed that no recrystallization of the amorphous layer occurred after 12 hours at 1000°C. By using different wavelengths of the exciting radiation the spatial distribution of the ion induced damage was investigated. Evidence for the preferential sputtering of Si resulting in a carbon rich surface layer is discussed.

Based on work performed under the auspices of the US Energy Research and Development Administration.

By acceptance of this article, the publisher and/or recipient acknowledges the US Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper.     

An upgraded TOF-SIMS VG Ionex IX23LS: Study on the negative secondary ion emission of III-V compound semiconductors with prior neutral cesium deposition

A TOF-SIMS VG Ionex IX23LS with upgraded data acquisition and control system was used to study the secondary emission of negative atomic and cluster ions of non-metallic elements (P, As and Sb) upon a 19 keV Ga⁺ bombardment of non-degenerated III-V semiconductors (GaP, GaAs, GaSb, InP, InAs and InSb) with prior neutral Cs deposition from a getter dispenser. It was found that surface cesiation enhances the peak intensity of all negative ion species; in the case of atomic ions, the greatest increase (360) was observed for P⁻ emitted from InP. Such an enhancement was larger for In-based than for Ga-based compounds. We explained that in terms of an electronegativity difference between the composing atoms of III-V semiconductors. The greater electronegativity difference (bond ionicity) of In-based compounds resulted in the greater Cs-induced work function decrease leading to a higher increase in the ionization probability of secondary ions.     

Relation between surface structures and sputtering ratios of copper single crystals

Polished (100) Cu crystals have been bombarded at target temperatures of 204 K, 294 K and 456 K by 10 and 20 keV Ne⁺ ions up to a total dose of 1.7 × 10¹⁹ ions/cm². The plane of incidence was chosen to be a {100} plane perpendicular to the surface. Measurements have been performed for incident angles between 36° and 44° with respect to the surface normal. In this angular interval the sputtering ratio and the surface structure have been studied by weightloss and replica electron microscope techniques respectively. At target temperatures of 204 K and 294 K an anomaly was observed in the curve of the sputtering ratio versus angle of incidence. A small peak appears where the curve slopes towards the 〈110〉 minimum. The position and height of the peak is a function of target temperature and ion energy.

This sputtering submaximum is accompanied by the formation of {100} orientated furrows perpendicular to the ion beam. The nucleation of this relief is tentatively discussed in terms of local deviations from perfection of the surface, which might be due to a singularity in the production of focusing collisions influencing the damage structure. The growth of the furrows and the submaximum in the sputtering ratio are discussed in terms of the angle between the ion beam and the characteristic {110} side of the furrows.

These sputtering and faceting phenomena have not been observed at 20 keV Ar⁺ ion bombardment nor generally under bombardment at a target temperature of 456 K.     

Simulation of sputtering from ion-beam-roughened carbon surfaces

Effects of ion-induced surface roughness on sputtering of amorphous carbon under ion bombardment are studied by means of binary-collision computer simulation in a wide range of incidence angles. Most simulations refer to 1–10?keV Ar ion bombardment, and sinusoidal ripple morphology is assumed. It is shown that surface roughness is a key factor to achieve quantitative agreement with experiment. The simulation results are compared with the analytical estimates of the yield from sine-shaped and ridged surfaces based on continuum models of ion sputtering. Some deviations between the results are discussed.