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Depth profiles of ultrashallow implanted P in Si at a dose of 3 × 10 14 cm −2 in the energy range of 5–80 keV were investigated by nuclear reaction analysis (NRA) using a 31 P(α,p) 34 S reaction. Both projected ranges ( R p ) and range straggling (Δ R p ) are in good agreement with those calculated by TRIM. The accuracy of the dose measurement including reproducibility of this method is better than ±5%. This method is very useful to improve the quantitative accuracy in ultrashallow depth profiling of P in combination with SIMS.

Capillary optics are valuable tools for concentrating synchrotron radiation [D.J. Tiel, A.E. Stern, D.H. Bilderback and A. Lewis, Physica B 158 (1989) 314]. Single tapered capillaries are used at several facilities. However, most of these optics collect only over a small area. This can be overcome by using larger capillary structures. Polycapillary optics can deflect X-rays by larger angles than other X-ray optics that use only one or two reflections. Conventional X-ray optics that achieve similar deflections are much more energy selective than capillaries. Therefore, capillaries can achieve very short focal distances for a wide range of energies. The measurements shown here represent first tests performed with polycapillaries with large collection area. The performance with respect to transmission efficiency and spot size was evaluated for a set of four very different prototypes. It is shown that for these prototypes a significant gain may be achieved if a spot size of the order of 0.1 mm is required. Further, some characteristics of the different optics are discussed.

Development of techniques for production of carefully controlled, low-loss optical waveguides in solid dielectric materials is essential to development of integrated optical circuits for signal processing in future optical communications systems. Ion implantation offers an attractive possibility because of the refractive index and film thickness control possible by this technique. To evaluate this possibility we have investigated some of the optical properties of ion-bombarded fused quartz. A variety of ions ranging from helium ions to bismuth ions has been used. We have concentrated on refractive index and optical loss variations (on those implants into which a beam could be launched) as effected by (1) ion species and dose, (2) surface preparation, (3) surface temperature during bombardment, and (4) postbombardment annealing. This paper does not attempt to give an inclusive account of all the results obtained but principally discusses the best results so far, which are those using lithium ions. For lithium ion bombardment we have observed approximately linear variation of refractive index at 6328 A with dose n = n(0) + 2.1 x 10(-21)C, where n(0) is the prebombardment value (= 1.458 for fused quartz), and C is the ion concentration in ions/cm(3) (C < 2.2 x 10(19)). The optical absorption decreases significantly with increase in substrate temperature during implantation, and losses less than 0.2 dB/cm have been achieved. The refractive index change appears to be primarily due to disorder produced by the incident particles rather than a chemical doping effect as evidenced by postbombardment annealing studies.

The defect density of 2500 A 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.

Ni was electrodeposited on Ge(111) surfaces from an aqueous solution containing 0.5M Na 2 SO 4 and 10 −3 M NiSO 4 , using a standard three-electrode electrochemical cell. Cyclic voltammetry was used to yield information about the cleanliness of the immersed, p-type Ge(111) surfaces. After deposition at a fixed potential, the samples were removed from the cell and investigated with X-ray standing waves while kept under an inert gas atmosphere. The analysis of the X-ray standing wave data shows that several monolayers of Ni occupy a well defined lattice location. From these results together with the results of the cyclic voltammetric measurements we conclude that the first layers of Ni react with the Ge(111) surface forming a few layers of ordered germanide.

A new x-ray optics has been developed which is capable of focusing, redirecting and collimating x-rays over broad energy and angular ranges. These optics are small light weight systems of hollow capillary tubes based on the principle of total external reflection at small angles. The transmission efficiency of these lenses is greater than 50% in the energy of 10-100 Kev. These lenses are expected to have significant medical applications.

Epitaxial layers of (111) GaAs of approximately 1 μm thickness were grown on epitaxial CaF2 buffer layers that were either 140 or 380 nm thick on Si(111) substrates. The best nucleation temperature for the GaAs on CaF2/Si(111) we have observed was 620 °C. This resulted in high quality GaAs films, which exhibited channeling minimum yields of 4%. The density of threading dislocations in the GaAs layers was observed by transmission electron microscopy (TEM) to be ∼108 cm−2. Double‐crystal x‐ray diffraction measurements showed that the strain (e⊥) was less than 2.2×10−4 in both sets of GaAs samples. However, ion channeling revealed a large tetragonal strain of 3.5×10−3 (e⊥=1.7×10−3) in the thinner (140 nm) CaF2 buffer layers. By doing ion channeling with high energy (2.5 MeV) protons, it was possible to determine strain more accurately. Using this technique, we were able to set an upper limit for the tetragonal strain of 2.5×10−4 in both the GaAs (which implies e⊥

Channeling phenomena are observed for charged particles of momentum up to 250 GeV/c in a germanium crystal. The angular distributions of the channeled particles are compared with theoretical predictions based on a diffusion model. The results indicate additional mechanisms leading to dechanneling of the particles although channeling effects are observed for particles incident at up to several times the critical angle, in contrast with the results from low energy channeling.

This article reviews some aspects of the steering of charged particles using channeling in bent crystals. Crystal angular and spatial acceptance, deflection, dechanneling, and radiation damage are discussed. Examples of possible bent crystal applications are presented including extraction, beam transport, focusing, the possibility of charm particle separated beams, and magnetic moment determination.

The crystal-blocking lifetime technique (BLT) was used to examine several effects in the in-elastic scattering through 90° of ≈ 5 MeV protons from ≈ 1.5 μm thick germanium crystals to various excited states, using the elastic scattering observed simultaneously as a “promptness” monitor: (a) the enhancement effect an isobaric analog resonance has on the underlying fine-structure states in its vicinity; (b) the effect of different level densities and hence level widths at different excitation energies in different Ge isotopes; (c) the effect on the mean lifetime of the final-state spin in the same isotope; and (d) a slight difference in the mean lifetimes for two different proton groups in the same isotope leading to states of the same spin, a result revealing a possible nuclear (intermediate) structure effect, i.e. one that cannot be explained on the basis of the statistical model. The excitation energies reached in the compound nuclei of arsenic range between 10 and 12 MeV. All results were obtained below the respective lowest neutron thresholds; the resulting lifetimes lie in the range 10−16–10−17 sec, corresponding to mean level widths around 20 eV; this is still considerably less than the estimated mean level spacing, even when the fine structure is enhanced within the analog resonance; i.e. Γ/D ⪡ 1. The actual contact between experiment and calculations based on computer simulations of charged-particle penetration through crystal lattices is made at the level of comparisons of the depths of characteristic blocking dips.

The fatigue deformation was characterized by transmission electron microscopy in unimplanted and in ion-implanted metal surface layers. The metals investigated were austenitic 304-stainless steel and copper, annealed and implanted at 3 MeV with nitrogen or neon to a dose of 10 21m −2. The implanted surface layers contain a high density of lattice defects, most likely being Frank-dislocation loops. They are obstacles against glide dislocations. After flexural fatigue at ±0.3% cyclic strain, dislocation channels were formed heterogeneously along crystallographic {111} slip planes. Inside the channels the Frank loops were annihilated. These areas became thus mechanically soften relative to their surrounding volume. The channels assume the role of the well known persistent slip bands in unimplanted metals and lead to extrusions at the free surface. They are the precursors of fatigue crack nucleation. The fatigue lives of the as-implanted samples were not improved. However, an improvement by a factor of two to three was obtained after aging of the nitrogen-implanted stainless steel. Electron diffraction evidence suggests that this improvement was due to the formation of beginning precipitation ( = stronger obstacles against glide dislocations). Stress-induced martensitic transformation and twinning in the stainless steel were also modified by the implantation. Implanted nitrogen prevented the formation of α′-martensite, whereas in neon-implanted samples the martensite formed freely.

Channeling phenomena are observed for positive particles of momentum up to 250 GeV/ c in a germanium crystal. The polar angular distributions of the channeled particles are compared with theoretical predictions based on a diffusion model. The results indicate that at high particle energy there may be additional mechanisms besides those operative at low energy leading to dechanneling of the particles. In spite of this, channeling effects are observed for particles incident at up to several times the critical angle, in contrast with the results from low energy channeling. Statistical equilibrium in the azimuthal angular distribution has also been observed at all measured beam momenta to about twice the calculated channeling critical angle. The breakdown of statistical equilibrium for the 2 cm crystal used occurs at an incident angle 2–3 times smaller than predicted theoretically.

The results of measurements of energy losses of 35-, 100-, and 250-GeV particles channeled through a 2-cm Ge crystal are reported. The energy losses are correlated with the incoming and outgoing particle angles, relative to the channel axis, and the results are compared with existing theories.

The first application of high energy ion channeling to the atomically clean GaAs(110) surface and metal-GaAs interfaces is reported. Questions of sample preparation, background correction and computer simulation are addressed. It is found that the Ga and As atoms at the clean surface are laterally displaced ⩽ 0.1 A from the ideal bulk-like sites. The implications of this result to current LEED models are discussed. Au overlayers, deposited at room temperature, do not seem to produce lateral displacements of the substrate for coverages below ≈ 5 monolayer (ML). However, ≈ 0.9 ML of the substrate are expanded or contracted upon Au deposition; this process is completed at a coverage of 0.5 ML. Neither an indication of any order in the Au film is found, nor seems a significant (≳ 5%) fraction of Au atoms to occupy substitutional sites. In contrast, room-temperature deposition of Pd disorders the substrate substantially, without threshold coverage, even at very small film thicknesses.

Channeling of protons and positive and negative pions has been observed in a single crystal of germanium at beam momenta of 35, 100 and 250 GeV/c. Both energy loss and scattering have been measured. The channeling effects agree with observations at lower energies suitably scaled to these high energies. The suppression of inelastic nuclear interactions has been directly observed by monitoring multiple-track events.

High energy proton channeling (12–120 GeV) and low energy helium ion channeling (1.5–4 MeV) have been used to study the effects of radiation damage induced in silicon by high energy (400 GeV) proton irradiation. It is shown that radiation fluence up to 10 17 /cm 2 does not preclude deflection of high energy charged particle beams in elastically bent silicon crystals. Evidence of macroscopic segregation of residual defects in the strain field of a deformed crystal is demonstrated.

The time development of fission in highly excited Pb nuclei has been studied by the crystal blocking technique. Thin Ta crystals were bombarded with 19 F ions in the energy range 90–120 MeV and the yield of fission fragments was measured for emission directions close to a strong axis. The experimental blocking dips are compared with calculated dips containing a superposition of two components, corresponding to short- and long-lived compound nuclei. The information extracted is the energy dependence of the relative amount of fission which comes from compound nuclei with lifetimes of τ≳3 × 10 −17 s. The total fission cross section and angular distribution of fission fragments were also measured for 19 F bombardment of 181 Ta in the energy range 84.3–114.7 MeV and for 16 O bombardment of 184 W in the energy range 83.4–107.9 MeV. The results of the three types of measurements have been interpreted through comparison with statistical model calculations that follow the spin and energy distribution of compound nuclei through the neutron evaporation cascade. The 19 F + 181 Ta measurements, when compared with the present 16 O + 184 W cross section and angular distribution measurements and earlier lifetime measurements for 16 O + W, yield information on the spin distribution for the compound nucleus and its influence on the fission process.

Using the crystal blocking technique, we have measured the emission distributions of protons near the 〈110〉 axis elastically scattered from a 7000 A thick single nickel crystal, at 5.65 and 6.50 MeV. These distributions are fitted by an analytic multiple string calculation of the blocking process which yields the time-dependent compound-nucleus decay probability curve. Shape- and compound-elastic cross sections are simultaneously determined in a reaction-model-independent way, as is the mean compound nucleus lifetime. The resulting decay curves for the delayed component are consistent with a single exponential at both energies and show no indication of the presence of level-level correlations. Use of such data in testing recent statistical theories is discussed.

An experiment has been carried out to observe the deflection of charged particles by planer channeling in bent single crystals with momenta between 12 and 180 GeV and with angular deflections up to 27 milliradians. Anomalous losses of particles from the center point of a three point bending apparatus have been observed at high incident particle energy. This effect has been employed as a “dechanneling spectrometer” to study dechanneling effects caused by bending. The bending losses generally conform to theories of bending dechanneling based on the classical channeling model.

In an experiment in the CERN SPS charged hyperon beam, a sample of 16 000Ω− decays was collected at an Ω− momentum of 131 GeV/c the Ω− lifetime, the branching ratios and decay asymmetry parameters for the main decay modes were measured, giving: τ Ω − =(0.823±0.015)×10 −10 s , Γ(Ω − →Λ K − ) Γ(Ω − -→ all ) =0.675±0.008 , α(Ω − →Λ K − )= −0.025±0.028, Γ(Ω − →Ξ 0 π − ) Γ(Ω − → all ) = 0.236±0.008 , α(Ω − →Ξ 0 π − )=0.09±0.14, Γ(Ω − →Ξ − π 0 ) Γ(Ω − → all ) =0.089±0.005 , α(Ω − →Ξ − π 0 )=0.05±0.21. For the rare decay modes, the following results for the branching ratios were obtained: if Γ(Ω − →Ξ 0 e − ν ) Γ(Ω − → all ) =(0.56±0.28)×10 −2 , if Γ(Ω − →Ξ − π + π − ) (Γ(Ω→ all ) =(3.7 −1.1 +3.6 )×10 −4 , Γ(Ω − →Ξ − γ) (Γ(ω − → all ) −3 at the 90% confidence level, Γ(Ω − →Λπ − ) (Γ(Ω − → all ) −4 at the 90% confidence level . A sample of 32 000 Ξ− decays collected under the same experimental conditions gave a new measurement of the Ξ− lifetime: τ Ξ − =(1.652±0.051)×10 −10 s . Combining the measurements of the present experiment with those obtained in a previous experiment in the SPS hyperon beam gives: if τ Ω − =(0.823±0.013)×10 −10 s , Γ(Ω − →Λ K − ) Γ(Ω − → all =0.678±0.007 , Γ(Ω − →Ξ 0 π − ) Γ(Ω − → all ) =0.236±0.007 , Γ(Ω − →Ξ − π 0 ) Γ(Ω − → all =0.086±0.004 , Γ(Ω − →Ξ − π + π − ) Γ(Ω − → all =(4.3 −1.3 +3.4 )×10 −4 . The experimental results are compared with theoretical calculations.

Summary Ilford G5 emulsions exposed to a 950 mev proton beam from the Birmingham synchrotron, have been searched for proton-proton elastic collisions. Forty-six such events have been found in 950 metres of proton track, equivalent to a cross section for elastic scattering of 15·5±2·5 mb. The angular distribution of the scattered protons in the centre-of-mass system shows a strong forward peak in agreement with results obtained at Brookhaven by cloud chamber and counter workers.

Summary Photographic plates soaked in heavy water have been used in a study of the photodisintegration of deuterons by γ-rays of energy 6·1 and 7·0 Mev. The principles of the method were described in an earlier paper, but the technique has been improved and the accuracy of the results increased.

Summary A way of simplifying the microscope measurements and the resulting calculations, in the photographic emulsion method of studying neutron spectra, is described. The energy distribution of neutrons from the reaction 10B(d, n)11C has been measured, to show that the saving in time is accompanied by no appreciable loss of accuracy. The method has been used in a study of the angular distribution of neutrons from the reaction 11B(d, n)2C at 8·1 mev. The results are consistent with the theory of the stripping process and confirm that the 4·47 mev level in 12C, like the ground state, has even parity.

Summary The photodisintegration of the deuteron has been observed in photographic plates exposed, while wet with heavy water, to the γ-rays arising from the bombardment of fluorine with 1·4 MeV. protons. Two groups of protons, produced by the known γ-ray lines at 6·15 MeV. and 7·01 MeV., have been clearly resolved. It is shown that γ-ray energies of this order of magnitude can be measured with a probable error of 0·05 MeV., and γ-ray lines with an energy difference of 0·6 MeV. can be resolved. Suggestions are made for a further improvement of the technique which should increase the resolving power of the method. The angular distribution of the protons produced by each of the two 7-ray lines has been measured, but statistical uncertainties must be further reduced before any useful comparison with theory can be made.

A method for producing large-area, several thousand A thick silicon films supported by a frame of thicker material by selective electro-chemical etching is described. Preliminary results on use of the films for channeling studies are presented, especially as they relate to characterization of the film itself. A number of experiments of interest and importance, some of which are presently in progress, are discussed.

Ilford G5 emulsions have been exposed to a 925 mev beam from the Birmingham proton synchroton. Sixty-five examples of the elastic scattering of protons by free protons in the emulsion have been found in 1539 metres of track. After removal of events having azimuthal angles ψ>45° where ψ is the angle between the plane of the two secondary particles and the plane of the emulsion, the remaining 44 events yielded a total elastic cross section of 17±3mb. The angular distribution of all the events shows a strong forward peak in the centre of mass system. The shape of this distribution has been compared with the predictions of a simple optical model and a value of (0·9±0·1) × 10−13 cm has been found for the interaction radius of the proton. The results are compared with other experimental work.

Cross sections were determined radiochemically following bombardments of 234U, 235U, 236U, 238U and 239Pu with 5–25 MeV deuterons and 237Np with 20–50 MeV helium ions. Although fission accounts for most of the reaction cross section for all target isotopes, products from radiative capture and various spallation reactions were observed. Products corresponding to the (d, γ) reaction were observed from 238U and other uranium isotopes, with a cross section of about 1 mb. Both compound-nucleus and direct-interaction characteristics are apparent in the spallation excitation functions. The uranium (d, 2n), (d, 3n) and (d, 4n) excitation functions show a “mass” effect; that is, the heavier target isotopes have generally higher spallation-product yields. This corresponds to a neutron partial-level width ratio Γn/(Γn+Γt) increasing with N. Results from 239Pu+d and 237Np+α, both of which form the compound system 241Am*, are consistent with the compound nucleus theory, in that (d, 2n) and (d, 3n) excitation functions correlate with (α, 2n) and (α, 3n) excitation functions. Direct interaction features include (d, n) stripping and a prominent (α, αn) reaction in 237Np. Yield curves and fission cross sections have been obtained to characterize the fission reaction.

Some electron-capture and α-decay properties of neutron-deficient americium and curium isotopes have been investigated using 2π proportional counting, scintillation spectrometry, and α-pulse analysis. The results for 238Am include a new half-life of 1·86 hr and identification of γ-rays of ∼370 keV (∼12%), 580 keV (∼29%), ∼950 keV (∼2%), 980 keV (∼76%), and ∼1350 keV (∼17%). The data on 239Am (half-life 12·1 hr) indicate γ-rays of 225 keV (∼30%) and 275 keV (∼20%) with a new α-particle energy of 5·77 MeV and abundance of 5·0 × 10−3 per cent. For 240Am (half-life 51·0 hr) γ-rays of 900 keV (∼23%) and 1000 keV (∼77)% and a negatron branching limit of ≤ 6 × 10−6 per cent were established. For 240Cm a new α-particle energy of 6·26 MeV was established. For 241Cm a new alpha particle energy of 5·95 MeV and abundance of 0·96 per cent and a γ-ray of 478 keV (97%) were established. Counting efficiencies for the 2π-geometry methane-flow proportional counter are 60, 90 and 82 per cent for the electron-capture decay of 239Am, 240Am and 241Cm, respectively. These are in the range of values found for other β-unstable isotopes of heavy elements. The data on 238Am and 240Am are consistent with a rule that only levels ∼1000 keV and higher in even-even plutonium isotopes are populated by relatively fast β-transitions (log ft 5–8) in the decay of odd-odd americium and neptunium isotopes.

The crystal blocking effect was used to study the time distribution of 5- MeV protons elastically scattered from a germanium crystal. The time delay associated with compound-elastic scattering is estimated and used to separate the direct and compound-nuclear contributions to the elastic cross section.

The energy loss of 0.3 to 1 MeV H + ions in thin (200–2000 A) carbon films has been measured for ions incident as independent accelerated hydrogen atomic ions (H + ) and for incidence as members of accelerated diatomic hydrogen molecular ions (H 2 + ). For the short traversal times encountered in these experiments, the molecular beam shows an energy loss per atom which is about 1.2 times that observed for an atomic beam of the same velocity. This ratio is less than the value of 1.5 which might be expected by simple equipartition of energy loss into close and distant collisions. Similar measurements have been carried out for O − and O 2 − beams with energies of 2.9 to 2.0 MeV per atom. As in the hydrogen case a ratio of ∼1.2 was observed for the energy loss of the molecular beam relative to the atomic beam for short transit times decreasing to ∼1.0 as the transit time increased. The increased width of the transmitted particle energy spectrum for the molecular beam can be attributed to Coulomb repulsion of the two ions resulting from electron loss by atoms of the diatomic molecule upon entry into the solid. Measurement of the energy width indicates a mean charge of +1 and +4±1 for the hydrogen and oxygen molecular ions respectively.

The cooperative Jahn-Teller phase transition in Nickelchromite (NiCr2O4) has been studied using the channeling method. The angular dependences of the normalized backscattering yields of 1.5 MeV He ions from a NiCr2O4 single crystal close to the [111], [110] and [111] crystallographic axes has been measured for various temperatures above and below the transition temperature. The standard backscattering geometry utilizing an annular detector was employed. Significant changes in the minimum yields and the widths of the angular dependences were found, as well as changes in the angles between the axes. The phase transition temperature of the crystal has been measured.

Implanted dopant ions are used to form the resistive layer in position sensitive p-n junction particle detectors. Position resolution of 0.2 mm fwhm and linearity of ±0.3 mm are obtained.

Ion scattering is shown to be a sensitive technique for determination of lighter than substrate impurities when thin samples are used and particles scattered 90° through the sample (90° forward) are analyzed. The method is applied to the determination of carbon on silicon. The sensitivity is estimated to be a few monolayers.

Supersaturated high-conductivity polycrystalline silicon (polysilicon) formed by rapid thermal processing (RTP) is a promising new material for emitters, contacts and diffusion sources in advanced high-speed bipolar and MOS IC technologies.A matrix of processing conditions was used to evaluate the effect of polysilicon thickness, implant dose, RTP conditions and the nature of the substrate on the dopant diffusion in both the polysilicon and single crystal substrate and also on the interface properties.Results of conductivity measurements, spreading resistance profiling (SRP), secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS)/channeling are presented. The results have proved the formation of shallow, defect-free junctions and epitaxial emitters with low series resistance and improved contact properties.

Metal films such as gold, copper, nickel and palladium exhibit an interfacial intermixing reaction at room temperature with semiconductors with energy gaps Eg less than about 2.5 eV or dielectric constants ε larger than about 81. We have proposed a model2 of the triggering mechanism of this interfacial reaction based on the ability of a metal to screen coulombic interaction by its mobile free electrons. Such a screening may disturb the electron distribution responsible for the covalent bonding, and consequently it may make the semiconductor surface reactive towards the metal. In this respect, there must be a critical film thickness for the deposited metal film to behave as a true metal with sufficient mobile free electrons for the screening; therefore the reactivity of the semiconductor surface for the intermixing reaction must be dependent on the thickness of the deposited metal film. In favour of the screening model, we show that the reactivity of a silicon surface for intermixing or silicide formation with a gold or palladium film depends clearly on the film thickness.

600∼4000Å thick Ag films grown on 3∼4· misoriented Si(111) substrates by molecular beam epitaxy (MBE) technique have been studied by using x-ray pole-figure analysis and MeV He+ Rutherford Backscattering Spectrometry (RBS)/channeling technique. X-ray pole-figure measurements revealed that despite the large lattice mismatch (∼25%) between Ag and Si, Ag films with epitaxial relationship Ag(111)//Si(111):Ag[011]//Si[011] were grown with a small quantity (15∼20%) of twin structure. The axial channeling minimum yield (Xmin) is reduced at the Ag surface as the Ag film thickness increases. These films were thermally stable up to 500°C annealing but the twinning disappeared after annealing

Epitaxial layers of (111) GaAs of approximately 1 µm thickness were grown on epitaxial CaF2 buffer layers which were either 140 or 380 nm thick on Si(111) substrates. The best nucleation temperature for the GaAs on CaF2/Si(111) we have observed was 620 °C. This resulted in high quality GaAs films which exhibited channeling minimum yields of 4%. The density of threading dislocations in the GaAs layers was observed by TEM to be ~108 cm-2. Double-crystal x-ray diffraction measurements showed that the strain (ε┴.) was less than 2.2×10-4 in both sets of GaAs samples. Ion channeling, however, revealed a large tetragonal strain of 3.5×10-3 (ε┴ = 1.7×10-3) in the thinner (140 nm) CaF2 buffer layers. By doing ion channeling with high energy (2.5 MeV) protons, it was possible to determine strain more accurately. Using this technique, we were able to set an upper limit for the tetragonal strain of 2.5×10-4 in both the GaAs (which implies ε┴ < 8×10-5 and CaF2 (ε┴ < 1.5×10-4) layers for the thicker (380 nm) CaF2 buffer layer structure. These results are in good agreement with the strain predicted from previous strain measurements of CaF2 epitaxial layers on Si.

Strains in CaF2 films, grown by molecular beam epitaxy at 700ºC on Si(lll) subsrates, have been measured by MeV 4He+ ion channeling. For CaF2 films thinner than 200nm, the strain parallel to the (111) plane is found to be tensile. No strain is observed for films thicker than 200nm. The observed tensile strain cannot be explained by a simple pseudomorphic growth model because the larger lattice constant of CaF2 relative to Si should result in a compressive misfit strain. The tensile strain is an indication of the relaxation of the compressive misfit strain at the growth temperature due to nucleation of interfacial defects. If these defects are not annealed out as the substrate temperature is lowered after growth, the tensile strain can result because of the larger thermal contraction of CaF2 compared to Si. The final film quality near the interface improves as the film thickness is increased. This indicates that reordering of interfacial disorder is caused by the strain energy accumulated during the cooling process.