MeV Ion Channeling Research Articles

Comparative study of the MeV ion channeling implantation induced damage in 6H-SiC by the iterative procedure and phenomenological CSIM computer code

Due to its unique material properties, such as extreme hardness and radiation resistance, silicon carbide has been used as an important construction material for environments with extreme conditions, like those present in nuclear reactors. As such, it is constantly exposed to energetic particles (e.g., neutrons) and consequently subjected to gradual crystal lattice degradation. In this article, the 6H-SiC crystal damage has been simulated by the implantation of 4 MeV C3+ ions in the (0001) axial direction of a single 6H-SiC crystal to the ion fluences of 1.359 1015 cm-2, 6.740 1015 cm-2, and 2.02 1016 cm-2. These implanted samples were subsequently analyzed by Rutherford and elastic backscattering spectrometry in the channeling orientation (RBS/C & EBS/C) by the usage of 1 MeV protons. Obtained spectra were analyzed by channeling simulation phenomenological computer code (CSIM) to obtain quantitative crystal damage depth profiles. The difference between the positions of damage profile maxima obtained by CSIM code and one simulated with stopping and range of ions in matter (SRIM), a Monte Carlo based computer code focused on ion implantation simulation in random crystal direction only, is about 10%. Therefore, due to small profile depth shifts, the usage of the iterative procedure for calculating crystal damage depth profiles is proposed. It was shown that profiles obtained by iterative procedure show very good agreement with the ones obtained with CSIM code. Additionally, with the introduction of channeling to random energy loss ratio the energy to depth profile scale conversion, the agreement with CSIM profiles becomes excellent.

Structural analysis of (Ga,Mn)N epilayers and self-organized dots using MeV ion channeling

Rutherford backscattering (RBS) and particle-induced X-ray emission (PIXE) experiments were performed on (Ga,Mn)N epilayers and self-organized dots grown by plasma-assisted MBE. The combined channeling RBS and Mn PIXE experiments were performed on Ga 0.949 M 0.051 N epilayer, which was confirmed to be of pure diluted phase by X-ray diffraction (XRD) in our previous study, in order to check the presence of Mn atoms in the interstitial site in wurtzite crystal. The axis scan around and axes and the plane scan around (1010) plane revealed that almost all the Mn atoms were in the substitutional site of wurtzite GaN crystal. For self-organized dots of (Ga,Mn)N grown on AlN by MBE, the PIXE spectra were measured and the Mn composition in the dot layer was estimated from the ratio of emission intensity of Mn and Ga K α lines. At a result, it was found that the Mn composition in the dots was higher by two or three times than that in thick epilayers grown with the addition of the same amount of Mn flux.

Using MeV ion backscattering/channeling and MC simulations to characterize the composition and structure of buried metal–metal interfaces

The technique of MeV ion backscattering and channeling has been used to study the interface structure and stoichiometry for thin transition metal films deposited on Al single crystal surfaces. Analysis of the backscattering data is based on the concepts of shadowing and the well-known theory of Rutherford scattering. In some cases we have used X-ray photoemission spectroscopy (XPS), low-energy (keV) He + ion scattering spectroscopy, and Monte Carlo computer simulations of interface evolution to obtain additional information about the structure of buried interfaces. We first describe the growth of epitaxial fcc Ti films on Al single crystal surfaces, and alloy formation at the epitaxial Ag–Al interface. We next describe measurements for alloy formation at the Ni–Al(1 1 0) interface and present Monte Carlo simulations of the interface evolution. Finally, we describe an approach to stabilize metal–metal epitaxial interfaces using an extremely thin metallic interlayer. Specifically, we show that a single ML of Ti metal deposited at the Fe–Al interface, a system well known for considerable intermixing at room temperature, prevents interdiffusion and enables the epitaxial growth of Fe films on the Al(1 0 0) surface. The resulting structure is observed to be stable for temperatures up to about 200 °C.

Planar MeV ion channeling on strained buried nanofilms

Planar MeV ion channeling experiments have been used to characterize a buried Si 1− x Ge x film with a thickness of 2.2 nm in a silicon host crystal. The tetragonal deformation in the film shows up as a translation of the {0 1 1} planes across the film, which was measured as a step in the yield of the planar channeled Rutherford backscattering spectra. The flux distribution between the planes acts as a probe for the position of the planes. The angular dependence of the step height has been measured for different incident ion energies and interpreted with Monte Carlo calculations. Simulations only resemble the measured spectra when the Hartree–Fock potential is used for the ion–atom interaction. The translation of the planes can be measured with an accuracy of approximately 10%, which is comparable to results obtained with axial channeling experiments.

Lattice deformation in InAs/GaAs superlattices characterized by MeV ion channeling

Axial and planar MeV ion-channeling experiments are performed on a InAs/GaAs superlattice with ten coherent buried InAs nanofilms in GaAs. We have measured a step in the channeled Rutherford backscattering spectra, which depends on the incidence angle of the ions. With Monte Carlo simulations, the average tetragonal distortion in the nanofilms can be determined from the axial channeling measurements, although the separate contributions of each individual nanofilm cannot be resolved in the measured spectra. Planar channeling measurements show a higher step, but a detailed resemblance between measurements and simulations cannot be achieved.

Characterization of gas-source molecular beam epitaxial growth of strain-compensated Si 1− x− yGe xC y/Si(0 0 1) heterostructure

Strain-compensated SiGeC ternary alloys have been successfully grown on Si(0 0 1) substrate by gas-source molecular beam epitaxy. Si 2H 6 and GeH 4 are used as Si and Ge sources, respectively, while CH 6Ge (monomethylgermane: CH 3GeH 3, MMGe) is employed as the C source. Compressive strain in the SiGe/Si system is reduced by adding MMGe, which works not only as a C source but also as a Ge source. The magnitude of strain is evaluated by MeV-ion channeling. An almost strain-compensated Si 0.794Ge 0.19C 0.016/Si(0 0 1) heterostructure is grown at 550°C with good uniformity and a flat surface.

Silicon interstitials generation during the exposure of silicon to hydrogen plasma

Interstitials in silicon can be produced by hydrogen ions through chemical action and physical impact. To establish the relative importance of the two processes, wafers of single crystal silicon kept at a temperature of 250 °C were exposed to a hydrogen plasma, with and without an accelerating voltage (500 V). As gettering centers interstitial-type discolation loops were made, before H exposure, by implanting 1.9 × 1015 at. cm−2 110 keV P atoms and annealing at 1000 °C for 30 s. The analysis was performed using MeV ion channeling, elastic recoil detection analysis and cross-section transmission electron microscopy. The results suggest that H is captured by the defects: in the case of unbiased plasma, H goes in the region of the extrinsic defects, while in the case of biased plasma, H is trapped near the surface where the defects are produced by the energetic H ions themselves. In both cases the data are consistent with the formation of hydrogen nanoblisters with the subsequent breaking of silicon bonds and injection of silicon interstitials. The migration of interstitials occurs at a temperature as low as 250 °C.

On the Epitaxy of Metal Films on GaN

ABSTRACTA variety of metal films deposited at room temperature have been found to grow epitaxially under conventional vacuum conditions on GaN grown by metalorganic vapor phase epitaxy on sapphire substrates. The metal films have been characterized by X-ray diffraction using a thin-film Read camera and by MeV ion channeling measurements. Lattice mismatch between the epitaxial metals and the GaN basal planes ranges from ∼ 0.2% to ∼ 22%, and does not appear to be the determining factor in the epitaxy reported here. Surface structure of the epitaxial metal films has been studied by atomic force microscopy and found to differ considerably from that of nonepitaxial metal films grown on similar GaN substrates.

Fabrication and characterization of selectively grown Si 1- xGe x/Si p +/n heterojunctions using pulsed laser induced epitaxy and gas immersion laser doping

A pulsed XeCl excimer laser is used to grow ideally strained heteroepitaxial Si 1- x Ge x /Si layers with Ge fractions up to 21% by intermixing a structure of electron beam evaporated a-Ge on Si(100). The rapid regrowth process induces an interfacial grading of the Ge fraction, which results in unusual stability of the layer strain upon heat treatment, as confirmed by MeV-ion channeling along <100> and <110>. Boron dopant is incorporated during the meltprocess by using a BF 3 gas ambient. Hall/van der Pauw and SIMS analysis reveal that the incorporated dopant dose scales with the number of laser pulses. The junction depth is controlled by the incident laser fluence. The melt time is monitored in-situ utilizing the transient reflectance of the sample during the phase transformations. A patterned reflective aluminum mask is used to obtain spatially selective melting. In-plane Hall mobilities are found to be lower for the heteroepitaxial junctions than for Si homojunctions. We believe this is due to different transport behavior for holes in the observed doping regime of 10 18–10 20 cm -3. Quasiplanar p +/n heterojunction diodes are fabricated and exhibit near-ideal forward I-V characteristics. Heterojunction diodes exhibit lower turn-on voltages than equivalent Si homojunction diodes, indicative of a lowered bandgap. The turn-on voltages also depend on the B junction depth with respect to the Si 1- x Ge x /Si interface. Both quantities are controlled independently by separating the epitaxy from the doping step.

Crystallinity, strain, and thermal stability of heteroepitaxial Si1−xGex/Si (100) layers created using pulsed laser induced epitaxy

Heteroepitaxy of Si1−xGex/Si alloy layers on Si (100) substrates has been achieved using pulsed laser induced epitaxy (PLIE). The energy of 1 to 20 pulses from a spatially homogenized XeCl excimer laser beam is used to melt a structure consisting of electron-beam evaporated Ge on Si (100) substrates. Alloy films with different Ge fractions are investigated and films with up to 21% Ge content are found to exhibit excellent crystallinity, as confirmed by MeV-ion channeling along the 〈100〉 direction. MeV-ion channeling is also used to determine the level of strain in the layers. This is done by comparing angular yield curves around the 〈110〉 direction for the substrate and alloy layer. The strain values obtained match with calculations for an ideally strained layer state. The strain is also measured for layers that have been subjected to different thermal cycles. A high level of strain is preserved in the alloy layer even after heating to 950 °C for 1 h. This unusual thermal stability is believed to be due to two process features, the ultrarapid nature of the process and the interfacial grading of the Ge fraction created during the melt-regrowth process.

A Study of Interdiffusion, Crystallinity, Strain and Thermal Stability of Si1−xGex/Si Created Using Pulsed Laser Induced Epitaxy (PLIE)

ABSTRACTHeteroepitaxy of Sil−xGex alloy layers on Si(100) and Si(111) has been achieved using Pulsed Laser Induced Epitaxy (PLIE). The energy of 1-50 pulses from a spatially homogenized XeCl excimer laser is used to melt a structure of electron beam evaporated Germanium on Silicon substrates. On Si(100) substrates alloys with Germanium fractions between 6 and 22% are investigated and exhibit very good crystallinity, as confirmed by MeV-Ion Channeling along the &lt;100&gt; - direction. Heteroepitaxy on Si(l11) yields similar results. MeV-Ion Channeling is also used to determine the level of strain in the layers. This is done by comparing angular yield curves around the &lt;110&gt; - direction for substrate and alloy layer. The obtained strain values are close to calculations for an ideally strained layer state. The strain is also measured for layers that have been subjected to different thermal cycles. As a result of this, predictions of feasibility for subsequent device fabrication can be made for the layers.

Ion channeling study of nickel metal growth on Ge(111) at room temperature

The room temperature growth of Ni on Ge(111) was studied by MeV ion channeling and Auger electron spectrometry in an ultrahigh vacuum environment. For Ni coverages of up to 3 monolayers (ML), Ni reacts with the Ge substrate and effectively displaces 3 ML of Ge atoms from their original lattice positions forming a possible interfacial layer of NiGe. Further deposition of Ni results in a continuous, uniform metal overlayer. These room temperature results are very similar to those found for Ni and Co on Si(111). The similarities and differences between Ni on Ge and Ni on Si are also discussed.

Demonstration of laser-assisted epitaxial deposition of GexSi1−x alloys on single-crystal Si

We report initial results of a novel technique for epitaxial growth of GexSi1−x alloys on single-crystal Si. During electron beam deposition of amorphous GexSi1−x, an incident XeCl excimer laser, operating at 308 nm with a 30 ns pulse duration, melts and crystallizes the amorphous layer in situ after each ≊5 nm of deposition. This laser-induced melt extends approximately 20 nm and provides epitaxy from the underlying substrate (or previous layers) at each stage of deposition. This melt/solidification process can be repeated continuously until the final desired alloy thickness is achieved. For layers up to 260 nm with Ge concentrations of 1.5–3 at. %, MeV ion channeling and cross-sectional transmission electron microscopy confirm epitaxial growth.

Defect analysis of epitaxial Ag films on silicon by MeV ion channeling

The defect density of 2500 Å thick Ag films grown on Si(111) and Si(100) substrates by molecular beam epitaxy have been studied using MeV ion channeling and Rutherford backscattering spectrometry (RBS). Ion channeling measurements indicate that despite the large lattice mismatch between Ag and Si (~ 25%), the Ag films showed excellent epitaxy (χ min = 6–20%). In the best cases, the minimum yield was close to that expected for a perfect Ag crystal. Dechanneling was observed due to defects, predominantly dislocations which were distributed throughout the Ag film. Near the Ag/Si interface, the dislocation density was an order of magnitude higher than that found for the bulk of the Ag(111) film and produced an interface peak in the channeling spectra. These results show that high quality epitaxy can be achieved for even large mismatched heteroepitaxial systems such as Ag on Si and illustrate the power of the ion channeling technique for studying defects in crystalline films.

Study of the SiNx/Si interface by million electron volt ion channeling

The structure, stoichiometry, and integrity of 3000-Å silicon nitride films on Si(100) substrates were studied using MeV ion channeling. The silicon nitride film was found to be slightly Si rich with a stoichiometry of Si3 N3.6 . The Si substrate contained a thin region (∼7 ML) of disordered Si atoms located immediately below the silicon nitride film.

Studies of surface atomic structure of Al(l00) by MeV ion scattering

An UHV system specially designed for studying surface and interface atomic structure by MeV ion scattering and channeling is described. The vacuum in the UHV chamber is 1 × 10 −10 Torr. The chamber is equipped with an ion gun used for sample cleaning, a translatable four-grid LEED-Auger system used for characterization of the crystal surface, and a three-dimensional goniometer. The crystal preparation and cleaning procedure of Al(100) are presented. The surface peak intensity of Al(100)-〈100〉 and Al(100)-〈110〉 has been measured by MeV ion channeling and scattering. The measured surface peak intensity was compared with that of a Monte Carlo simulation. The experimental results indicate that the thermal vibration amplitude of Al(100) surface atoms is 1.2–1.3 times that of bulk atoms. The relaxation of the first layer for Al(100) is less than −0.05 Å.

Channeling Study of Partially Ionized Beam Deposited Epitaxial Ag Films on Si(111) Substrates

AbstractAg films deposited on Si(111) substrates by partially ionized beam (PIB) under conventional vacuum conditions were studied by MeV ion channeling techniques. In spite of their large lattice mismatch (24.8%), Ag films were still found to be epitaxial. With a deposition temperature of 350°C and without post-annealing, the Xmin value at the surface of a 2550 A° thick Ag film was found to be 10%. The azimuthal angular scan and the measured axial channeling dip showed that the Ag film was (111) oriented. The lattice quality of the films was comaparable to that deposited by MBE techniques. Dislocations were found in the PIB deposited Ag films. Lattice damage due to the bombardment of energetic ions was also observed. The thickness of the Ag film was found to have a pronounced effect on the crystalline quality at the surface. With the thickness increasing from 1240 A° to 2550 A°, the lattice quality at the Ag surface improved significantly, but not much change in the defect density in the Ag films was obseved.

MeV ion channeling studies of (111) CdTe films on (111) GaAs substrates

Group II-VI compound semiconductors such as CdTe can be grown epitaxially on III-V materials such as GaAs. These films possess better physical properties than their bulk-grown counterparts. This work explores the (111) CdTe/(111) GaAs system by means of MeV ion channeling. Good epitaxy is found (backscatter minimum yields of 15%) for a thickness greater than 1000 Å, which is remarkable considering the 14% lattice mismatch between film and substrate. A narrowing of the Cd angular scan suggests Cd atom displacement. A model based on Te vacancies is presented to describe our data.

Room-temperature study of cobalt metal growth on Ge(111)

The room-temperature growth of Co on Ge(111) has been studied by MeV ion channeling and Auger electron spectroscopy in an ultrahigh vacuum environment. It has been found that even at room temperature, Co reacts with the Ge substrate and effectively displaces ∼3 monolayers of Ge atoms from their original lattice positions forming a possible interfacial layer of CoGe. Further deposition of Co resulted in a continuous and uniform metal overlayer. These room-temperature results are very similar to those found for Co on Si(111). After annealing a sample of 35 Å of Co on Ge at 600 °C for 1 h, ion channeling and transmission electron microscopy data showed the annealed film to consist of small crystalline islands with a stoichiometry of Co5Ge7. The similarities and differences between Co on Ge and Co on Si are also discussed.

Channeling study of epitaxial Al and Ag films on Si(111) substrates

Al films formed by the ion cluster beam deposition (ICBD) and Ag films formed by molecular beam epitaxy deposition on Si(111) substrates were studied by MeV ion channeling techniques. Both the Al and Ag films were found to be epitaxial despite their large lattice mismatch to Si substrates (both are close to 25%). Both the Al and Ag films were found to contain dislocation loops, which is attributed to the large lattice mismatch. Displaced Al atoms in the Al film but no displaced Ag atoms in the Ag films were found. The existence of displaced Al atoms is attributed to the bombardments of ion or ionized clusters in ICBD. A large step increase in dechanneling at the Al/Si interface was observed, which decreases with the increasing incident energy. The existence of a “semicoherent” interface in which four Al lattice planes match three Si lattice planes almost perfectly, cannot explain the energy dependence of the step increase. The thickness of the Ag films was found to have an obvious effect on the epitaxial growth. A misoriented growth of the Ag film was also observed.