Si Beam Research Articles

Selective area epitaxy of GaAs using very low energy Ga+ focused ion beam deposition combined with molecular beam epitaxial growth

A Ga focused ion beam (FIB) has been used to grow conducting n-type GaAs in areas defined by the FIB irradiation. A low energy (25-100 eV) mass separated 69 Ga + FIB was supplied to a GaAs (1 0 0) substrate, simultaneously with Si and As4 molecular beams. The FIB was rastered over a selected area of the substrate producing a square 200 μm x 200 μm mesa during growth. The 77 K Hall mobility and carrier concentration values of the grown GaAs layers showed that material quality improved as the ion energy was reduced from 100 to 25 eV. At 300 K a peak mobility of 2380 cm 2 V -1 s -1 at a carrier concentration of 2.5 x 10 12 cm -3 was achieved for the 25 eV ion grown sample.

Comparative characteristics of laser and electron beam ion sources at the linac LU-20

The Laser Ion Source (LIS) has been operating at the preinjector of the linac LU-20 since 1976. The linac LU-20 is an injector of the Synchrophasotron and the Nuclotron at the Laboratory of High Energies.The main operating principle of the source is ion extraction from laser plasma produced in a vacuum chamber by focusing laser radiation on a solid target surface. At present the LIS is used to perform experiments on relativistic beams of Li, B, C, N, O, F, Mg and Si nuclei.The EBIS `Krion' has been operating at the linac LU-20 since 1977. 12C6+, 14N7+, 16O8+, 20Ne10+ and 22Ne10+ nuclei were accelerated at first in the linac and then in the Synchrophasotron.40Ar18+, 84Kr35+,36+ and 131Xe53+,54+ ions are produced by the EBIS `Krion-2'. 32S14+, 40Ar14+,15+,16+,17+ and 84Kr29+ ions from the EBIS `Krion-C' injected into the linac were accelerated in 1993. Sulphur nuclei were accelerated in the Synchrophasotron.A comparison of the intensities reached behind the linac for both these sources shows that the LIS has an essential advantage in producing ions of the elements placed in the first three lines of Mendeleev's table.The comparison of the ionization states of heavy ions shows that these states for the EBIS essentially exceed the LIS ones.

Si Surface Preparation with Si Beam Irradiation on the Growth of III-V on Si

AbstractThe preliminary results of the effects of the Si beam irradiation for the Si surface preparation on the growth of GaAs on Si by MBE are reported. The effects are combined with thermal cyclic anneal (TCA). A slight improvement in the crystalline quality is observed on the photoluminescence spectra of the films grown with Si irradiation. In our experimental conditions, Si irradiation during the Si surface preparation has not indicated large effects on the FWHM of XRD. It is also indicated that initial substrate surface treatment affects the quality of thicker film through TCA treatment. Higher substrate temperature during Si beam irradiation is expected to indicate positive Si beam irradiation effects.

MODELING RELATIVISTIC HEAVY ION COLLISIONS AT THE AGS

▪ Abstract A quantitative model, based on hadronic physics and Monte Carlo cascading, is described and applied to heavy ion collisions at BNL-AGS energies (∼ 14 GeV/u). The model was found to be in excellent agreement with particle spectra where data previously existed, for Si beams, and was able to successfully predict the spectra where data were initially absent, for Au beams. For Si + Au collisions, baryon densities of three or four times the normal nuclear matter density (ρ0) are seen in the theory, whereas for Au + Au collisions, matter at densities up to 10 ρ0 is anticipated. The possibility that unusual states of matter may be created by the Au beams and potential signatures for its observation are considered.

Mössbauer study of the Fe-Si phases produced by Fe implantation followed by ion-beam-induced epitaxial crystallization.

Iron silicides were produced by 100-keV $^{57}\mathrm{Fe}$ implantation at room temperature into Si(100), followed by 500-keV Si beam irradiation at 593 K to induce epitaxial crystallization. The iron concentration was varied between 4 and 28 at. %. The M\"ossbauer spectra associated with $^{57}\mathrm{Fe}$ probes can be described by means of a broad quadrupole interaction distribution. After thermal treatments at temperatures up to 793 K the spectra become narrower and quadrupole distribution gets closer to that of \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$. The fitted interactions are discussed in terms of those of \ensuremath{\gamma}-${\mathrm{FeSi}}_{2}$, \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$, and \ensuremath{\alpha}-${\mathrm{FeSi}}_{2}$ phases which have been observed by high-resolution transmission electron microscopy. Low-temperature measurements of \ensuremath{\gamma}-${\mathrm{FeSi}}_{2}$ precipitates prepared by the ion-beam-induced epitaxial crystallization route do not confirm the reported occurrence of magnetic ordering in this system, above 4.2 K. \textcopyright{} 1996 The American Physical Society.

Growth, microstructure, and strain relaxation in low-temperature epitaxial Si1−xGex alloys deposited on Si(001) from hyperthermal beams

Epitaxial Si1−xGex(001) alloy films, with 0.15≤x≤0.30, were grown on Si(001) at temperatures Ts ranging from 300 to 550 °C using hyperthermal Si (average energy 〈ESi〉≂18 eV) and Ge (〈EGe〉≂15 eV) beams. The deposition rate was 0.1 nm s−1 and film thicknesses ranged from 30 nm to 0.8 μm. The energetic Si and Ge beams are generated by bombarding Si and Ge targets with 1 keV Kr+ ions from double-grid, multiaperture, broad ion-beam sources in a system geometry established based upon TRIM simulations of energy-dependent angular distributions of sputtered and backscattered particles. A combination of high-resolution plan-view and cross-sectional transmission electron microscopy, high-resolution x-ray diffraction, Rutherford backscattering spectroscopy, channeling, and axial angular-yield profiles demonstrated that the films are of extremely high crystalline quality. Critical layer thicknesses hc for strain relaxation in these alloys were found to increase rapidly with decreasing growth temperature. For Si0.70Ge0.30, hc ranged from 35 nm at Ts=550 °C to 650 nm at 350 °C compared to an equilibrium value of ≂8 nm. At even lower growth temperatures, hc becomes larger than critical epitaxial layer thicknesses, ≳1 μm at 300 °C. In addition, atomic force microscopy studies showed that strain-induced roughening, which occurs at elevated growth temperatures, is strongly suppressed at Ts between 300 and 400 °C with no indication of kinetic roughening.

Structures of 6H-SiC Surfaces

We have systematically studied reconstructions of the 6H SiC(0001) and (0001) surface under both Si rich and C rich condition using field ion-scanning tunneling microscopy (FI-STM). The sample was cleaned by in situ Si beam etching at 900-1000 °C. The Si rich and C rich phases were produced by annealing the sample in a Si flux and C 2 H 2 , respectively. On the (0001) surface, the as-cleaned surface showed (√3x√3) reconstruction. With increasing Si concentration, (2x2), (2√3x6√3), (3x3), and (7x7) reconstructions were observed. Reaction of the Si rich phases with C 2 H 2 molecule at 1050 °C resulted in the formation of C rich surface, which exhibited (2x2)/(6x6) reconstruction. On the (0001) surface, (2√3x2√3) reconstruction was observed after cleaning. Under Si rich condition, (2x2), (3x3), and (7x7) reconstructions were observed. Annealing the surface in C 2 H 2 beam at 1050 °C led to C rich phase with (1 x 1) structure. Structure model for (√3x√3) and (2√3x2√3) reconstruction are proposed, and possible applications of using the surface reconstruction to selectively grow the polytype SiC is discussed.

Optical and electronic properties of SiGeC alloys grown on Si substrates

Metastable Si 1 − x − y Ge x C y alloys were grown by molecular beam epitaxy on (100) Si substrates. Solid elemental sources were used for the Si and Ge beams, and a resistively heated graphite filament was used for the C beam. Up to 3 at% of C was incorporated in the alloy layers. Optical transmission measurements showed that the absorption edge of thick layers increased to higher energies with increasing C fraction, and revealed the presence of SiC and GeC vibrational modes in the infrared. At low temperatures, the alloys showed significant photoluminescence. The bandgap energies of thick layers increased linearly with the C fraction and followed a linear dependence of the bandgap on composition. Measurements of the valence band density of states using X-ray photoelectron spectroscopy indicated that the valence band energy maximum increased with the C fraction relative to that of SiGe alloys of similar composition. Our results indicated that SiGeC alloys are promising materials for Si-based heterostructure devices.

Statistical analysis of correlated resonance phenomena in two systems: 28Si+20Ne and 24Mg+20Ne.

A differentially pumped windowless {sup 20}Ne gas target was used to measure angle averaged excitation functions for binary decay of the {sup 28}Si+{sup 20}Ne reaction into low-lying states of {sup 28}Si+{sup 20}Ne, {sup 24}Mg+{sup 24}Mg, and {sup 32}S+{sup 16}O, and for binary decay of the {sup 24}Mg+{sup 20}Ne reaction into low-lying states of {sup 24}Mg+{sup 20}Ne and {sup 28}Si+{sup 16}O. The {sup 28}Si+{sup 20}Ne measurements span the region of excitation energy in {sup 48}Cr from 50.6 to 67.7 MeV ({sup 28}Si beam energies from 87.2 to 128.2 MeV). The reaction {sup 24}Mg+{sup 20}Ne was investigated over a region in {sup 44}Ti from 48.75 to 71.0 MeV ({sup 24}Mg beam energies from 70.8 to 119.75 MeV). Angular distribution measurements were made for {sup 20}Ne({sup 28}Si,{sup 24}Mg){sup 24}Mg at excitation energy=56.22 MeV and for {sup 20}Ne({sup 24}Mg,{sup 16}O){sup 28}Si at excitation energy=53.02 and 57.75 MeV. Both systems display evidence of statistically significant correlated resonance phenomena, but the resonances are dominated by spins comparable to the grazing angular momenta of the respective entrance channels, and are not analogs of the molecular configurations that dominate the {sup 24}Mg({sup 24}Mg,{sup 24}Mg{sup *}){sup 24}Mg{sup *} reactions.

Measurement of low mtK0s production from 14.6A GeV/c Si beams on a Pb target.

We present the measurement of low ${\mathit{m}}_{\mathit{tK}\mathit{s}}^{0}$ production in the rapidity range 2.2y2.6 with silicon beams on a lead target. We have selected central events with a cross section of approximately 280 mb and compare with a low kaon ${\mathit{m}}_{\mathit{t}}$ enhancement as reported earlier.

Thermal etching defect structures on Si(001): a comparative study with high-resolution low-energy electron diffraction and scanning tunnelling microscopy

Defect structures formed on Si(001) surfaces heated to different temperatures were studied comparatively with a high-resolution spot profile analysis low-energy electron diffraction (LEED) system and with a scanning tunnelling microscope. Using scanning tunnelling microscopy (STM) we observed tetragonal pyramids after annealing the surface to 1400 K which are formed by a thermal etching process that removes silicon from the surface. With increasing slope of the pyramid planes we observe a transformation from monatomic to DB double steps. For the latter, dimer rows of the (2*1) reconstruction extend perpendicular to the step edges. Pyramid planes with DB double steps that are rotated by 90 degrees transform into each other by the formation of single-stepped planes. For annealing temperatures of 1600 K, very flat surfaces with only monatomic steps are formed. A profile analysis of the specular and half-order beams of the two-domain Si(001) (2*1) LEED pattern was performed after the same preparation procedures were applied to the samples. This analysis leads to step structures that are identical with the results from the real-space STM studies.

H0 candidates from the decay H0 → Σ−p, observed in heavy ion collisions with 14.6 × A GeV/c Si beam on Pb target

We present H 0 candidate events, where the H 0 is observed through the decay H 0 → Σ − p. These decays are reconstructed in the three TPC modules of E810 while triggering on central Si Pb events at 14.6 × A GeV/c Si beam.

Modeling relativistic heavy ion collisions: From AGS to SPS

The importance of hadronic resonances in relativistic heavy ion collisions, from AGS to SPS energies, is now widely recognized. The systematics from Si beams to Au beams, provide overwhelming evidence that the hadronic resonance description of the heavy ion collisions is quite accurate at AGS energies with the possible exception when the highest baryon densities are reached. The authors have seen the importance of resonance rescattering in nucleus-nucleus collisions. However, they have very little direct knowledge on the cross-sections and the rescattering mechanisms of these resonances. They also have the formation of high baryon density regions, but they have no direct knowledge on the medium dependence of the hadronic cross sections involved. In the hadronic cascade model, ARC, they have assumed that baryon resonances rescatter as nucleons and the meson resonances as pions, and the free space cross-sections for nucleons and pions are used through out the collisions. It would be much better if these assumptions could be verified independently through experiments. For details of resonance rescattering, they obtain some information by reconstructing the resonances from the observed hadrons. Determining the hadronic cross sections in a dense medium is a hard problem. The authors describe tests on the sensitivities ofmore » medium dependence to the observed spectra. These tests make very drastic and unrealistic assumptions on the density dependence of the hadronic cross sections, but as sensitivity tests they are nevertheless quite revealing.« less

Low-temperature Si(001) epitaxy using low-energy (〈E 〉≂18 eV) Si atoms

The use of energetic (average energy ≂18 eV), rather than thermal (≂0.2 eV), Si beams during deposition at R=1 Å s−1 was found to increase the Si(001) epitaxial thickness te (100 Å–1.2 μm) by up to an order of magnitude over the growth temperature range Ts=80–300 °C. The overall increase in te is attributed primarily to a more effective filling of interisland trenches which form during growth in the low adatom mobility two-dimensional multilayer mode and provide preferential sites for the nucleation of the terminal amorphous phase. In addition, the behavior of te(Ts) at constant R and te(R) at constant Ts is quite different than that reported for films grown by molecular-beam epitaxy. A decrease in the slope of ln(te) versus −1/Ts at Ts<225 °C indicates an additional increase in the epitaxial thickness at very low growth temperatures while at constant Ts, 150 °C, te increases with decreasing R, reaches a maximum, and then decreases. These latter effects are explained in terms of changes in average island sizes giving rise to corresponding changes in interlayer mass transport.

Heavy ion microprobe for PIXE analysis of iron

Heavy ion microprobes such as a few MeV Si, Ni beams with a beam size of a few microns were applied for micro-analysis of iron using PIXE. Stainless steel foil or iron substrates were analyzed by 5 MeV Ni 3+, Si 3+ and 2 MeV proton beams. The X-ray spectra showed that the X-ray emission strongly depended on the incident ion species. It shows that the Si probe has a higher sensitivity to the Fe-L X-rays than that of the proton beam. On the contrary, the proton has higher sensitivity to the Fe-K X-rays than the Si beam. The Ni probe has more higher sensitivity for both energies of X-rays than the other probes. It is considered that molecular orbital effects took place between Fe and Ni atoms due to very close level matching. The energy shift and broadening due to multi-ionization were observed in the Fe-L X-ray spectra of the Si and Ni impacts.

Structure of a Si epilayer on Ni{100}

Silicon epilayere have been grown on a Ni{100}-p(1 × 1) substrate at 650°C using an effusion cell. The deposited layers exhibit a c(2 × 2) low energy electron diffraction (LEED) pattern. The structure of the c(2 × 2)-Si epilayer was investigated by time-of-flight scattering and recoiling spectrometry (TOF-SARS) using 4 keV Kr + ions to recoil the Si and Ni atoms. The sharp anisotropy of Si recoil intensity as a function of crystal azimuthal angle (δ) demonstrates that the Si epilayer is ordered. The alignment of the azimuthal patterns of the Si and Ni recoils shows that the Si atoms occupy four-fold hollow sites on the Ni{100} surface. Plots of Si recoil intensity versus beam incident angle (α) for monolayer and fractional monolayer coverages exhibit high sensitivity to Si defects. Classical ion trajectory simulations that have been used to model these α-scans allow interpretation of the results in terms of direct and surface recoils. The height of the Si epilayer above the four-fold hollow Ni sites, as determined from the agreement between the trajectory simulations and the α-scans, of 1.7 ± 0.2 Å gives a SiNi bond length of 2.5 Å. TOF-SARS is demonstrated to be a viable technique for monitoring in situ epitaxial film growth.

Structure of the four quasiparticle band in 84Sr.

The high spin levels in [sup 84]Sr have been populated through the [sup 59]Co([sup 28]Si, 3[ital p]) reaction using 98 MeV [sup 28]Si beam. A regular band structure was found to develop above the 12[sup +] state with a deformation parameter [beta]=0.21[plus minus]0.04 and [gamma]=8[degree][plus minus]6[degree]. The projected Hartree-Fock calculations predict a two proton and two neutron aligned configuration for this band. The predicted quadrupole moment for this configuration is also in good agreement with the experimental values obtained from the lifetime measurements. In addition, a large enhancement in [ital B]([ital M]1)/[ital B]([ital E]2) transition rates has been observed in the negative parity band above the 11[sup [minus]] state.

Semi-inclusive hadron spectra with beams of Si and Au at the AGS

Recent results for 14.6 AGeV c Si + A collisions at the BNL-AGS by the E802 collaboration are presented. Systematics in particle rapidity distributions and inverse-slope parameters for central and peripheral collisions are shown. Results from the first Au beams by the E866 collaboration are presented. Some new features with heavy beams are shown, and systematics of particle productions are discussed.

Recent results from E859 using Si beams at 146 A. [formula omitted

Recent results from E859 using Si beams at 146 A. [formula omitted

Results from E-810 concerning strange particles and strangelet search

We present the results of Λ and K 0 5 production from a Si target with a 14.6 × A GeV c Si beam. The measured rapidity distribution and transverse mass exponential slopes are presented and compared with models. We also present a measurement of Ξ − production from a Pb target with the same beam. The rapidity distribution is measured along with a model comparison. Finally we have searched for strangelets which decay in our TPC volume. From a Monte Carlo study with a plasma production model, we have established a upper limit on a given strangelet production cross section.