ALBERT

Notes: Soft x-ray magnetic circular dichroism (SXMCD) of the L2 and L3 edges is used to independently determine magnetic information for single crystal FexCo1−x alloy thin films (0≤x≤1) grown on ZnSe(001). Similar to surface magneto-optical Kerr effect (SMOKE) but with element and site specificity, SXMCD, which is the difference between the absorption of left and right circularly polarized soft x rays, can be used to determine a variety of magnetic parameters including spin-orbit and exchange interaction energies. Measurements made at the National Synchrotron Light Source (NSLS) U4B beamline on the Fe–Co alloy system exhibit unique changes in SXMCD signal as a function of alloy concentration. The SXMCD signal is found to rise linearly with reducing atomic concentration for both the Co and Fe. At the dilute limit of 10% FE (x=0.1), the Fe L3 SXMCD is found to increase by almost a factor of three to near 40%. In some Fe/Co structures, the largest reported SXMCD values in excess of 70% are observed! This example shows the utility of SXMCD in obtaining magnetic information in alloy systems in an element specific and even site specific manner.

Notes: The temperature dependence of the unusual magnetic 90° coupling in epitaxial Fe/Al/Fe(001) trilayers has been determined. For specific values of applied field and temperature, in-plane anisotropy and the interlayer coupling drive abrupt first-order changes in the temperature-dependent moment of these trilayers. The occurrence of such a first-order transition can be used to determine the value of the interlayer coupling constant, JQ, at the transition temperature. We show that Fe/Al/Fe(001) trilayers also exhibit distinct second-order transitions in the temperature-dependent moment, and that the occurrence of a second-order transition can similarly be used to specify JQ(T). A comparison is made of the values of JQ(T) determined by these two approaches, and the dependence of JQ on T is analyzed.

Notes: Using a multiparticle Monte Carlo method, a theoretical analysis of the spectral density of velocity fluctuations in semiconductors has been performed, under both stationary and transient conditions (when the electric field applied to a semiconductor changes). In the case of the transient analysis a general method has been developed and applied to N-type GaAs and InP. The results obtained are interpreted in terms of the microscopic processes occurring during the transient. Significant differences between these materials have been observed. The following results were found: (i) The main source of noise is the presence of carriers in the Γ valley subject to the action of high fields, and the velocity-disorienting effect of the intervalley mechanisms; (ii) the maximum in the spectral density is essentially due to the presence of intervalley mechanisms; (iii) the dominant frequencies in the spectral density are strongly affected by the duration of the free flights; (iv) for long times, the transient spectral density converges on the steady-state one of the final field.

Notes: p-type fluorinated and hydrogenated amorphous silicon carbide (a-Si:C:F:H) thin films have been prepared by plasma-enhanced chemical vapor deposition, using mixtures of silane (SiH4), methane (CH4), and boron trifluoride (BF3). The influence of preparation conditions on the optical and electrical properties of the films has been systematically studied with the aim of confirming the validity of BF3 to make p-type amorphous silicon carbide alloys and getting a better understanding of this material for its use in the preparation of p-type window layers of amorphous silicon solar cells. It has been found that the use of a low process pressure tends to prevent film growth as a consequence of the combination of a low density of radicals in the plasma and a high probability of desorption of species. High RF-power densities favor the incorporation of relatively large amounts of fluorine and the formation of defects. These two factors determine a poor doping efficiency and film quality. It has been shown that it is possible to prevent optical gap degradation if BF3 is used instead of B2H6. The effect is attributed to the incorporation of fluorine. Optimum dopant-gas concentrations seem to be of the order of 100%, instead of 1% as is the case for B2H6. This is due to the high tightness of the B-F bond. Our best films have an optical gap of 2.1 eV, a conductivity of 2.7×10−7 (Ω cm)−1 and an extended-state conductivity activation energy of 0.49 eV. Such properties make them suitable for their use as p-type window layers for amorphous silicon solar cells.

Notes: Fe(110)/Ag(111) heterostructures, composed of fixed 3 monolayer (ML) Fe bilayers and variable-thickness Ag bilayers, were grown by molecular-beam epitaxy and investigated by transmission Mössbauer spectroscopy in the temperature range from 4.2 to 300 K. We found that as the Ag layer is thick enough ((approximately-greater-than)17 ML) to magnetically isolate the neighboring Fe layers, a quasilinear temperature dependence of the Mössbauer hyperfine field results due to the two-dimensional spin-wave excitations. As the Ag layer is reduced to 4 ML, a dimensional crossover in the spin-wave excitations is induced by the magnetic interaction between neighboring Fe layers which makes the magnetic temperature dependence change from a two-dimensional T-linear relation to a three-dimensional T3/2 dependence.

Notes: The magnetic structure of Cr films deposited on single crystal Fe(001)/GaAs(001) substrates is investigated by soft x-ray magnetic circular dichroism (SXMCD) of the L2 and L3 absorption edges. We find that 0.25 ML Cr coverages result in Cr moments aligned with each other and antialigned to the underlying Fe moment direction. The Cr moment is measured to be 0.6±0.2 μB. Additional Cr deposition results in a monotonically reducing averaged moment consistent with the island growth of ferromagnetic Cr sheets of a single layer, antialigned with adjoining sheets. SXMCD measurements of the trilayer structures clearly show the antiferromagnetic coupling of the two Fe films through the interlying Cr film.

Notes: Ferromagnetic alloy films of bcc FexCo1−x(001) were epitaxially grown on ZnSe-epilayered GaAs substrates spanning both the thermodynamically allowed bcc regime (0.25〈x≤1) and the epitaxy-extended metastable bcc regime (0≤x≤0.25). Conversion-electron extended x-ray absorption fine-structure (EXAFS) and reflection high-energy electron-diffraction (RHEED) measurements verify that metastable bcc FexCo1−x films with 0≤x≤0.25 are stabilized by epitaxy on ZnSe. Additionally, the composition-dependent magnetic properties of these films were characterized by ferromagnetic resonance and vibrating sample magnetometry.

Notes: Three heterostructures were grown, each with 40 monolayer (ML) Ag bilayer thicknesses. The Fe(100) bilayers had thicknesses of 3, 6, and 9 ML. All growths were performed with a Perkin-Elmer PHI 430B molecular-beam-epitaxy (MBE) system equipped with reflection high-energy electron diffraction (RHEED) and a quadrupole mass analyzer. The growth region consistently achieved a base pressure of less than 5×10−10 Torr, and a growth pressure of less than 1.5×10−9 Torr. The base for all of our heterostructures consisted of 5 kA(ring) Ag(100) grown on polished single-crystal NaCl(001) substrates. Before the Ag base growth, a 200-A(ring) NaCl epilayer was deposited on the previously out-gassed NaCl substrate at 250 °C. Excellent Ag(100) RHEED patterns were obtained after a 3-h post-growth anneal of the base at 375 °C. Typical heterostructure growth rates were 2 ML/min for Fe and 15 ML/min for Ag. All the heterostructures were capped by a 5-kA(ring) Ag protective cover.Our growths experienced a ramped substrate growth temperature between 30 and 75 °C caused by radiant heating from our effusion cells (due to our present inability to cool the substrate). Since earlier work1 reported that layer-by-layer growth of Fe(100) on Ag(100) (indicated by RHEED oscillations) occurs at substrate temperatures far below room temperature, the growth of optimally flat Fe(100) films was hindered in our work. Removal of the NaCl substrate allowed 57Fe transmission Mössbauer spectroscopy to be performed. Only the 6- and 9-ML Fe bilayer films showed sextet features at room temperature (RT). The 9-ML film spectrum at RT consisted of a broadened sextet with in-plane bulklike magnetization. The 6-ML spectrum at RT had a large (65%) single-line central feature together with the sextet component. A small external field (5 kOe) applied to the 6-ML film at RT almost totally removed the central feature in the spectrum revealing a broadened two-site sextet spectrum. The indicative presence of superparamagnetism is expected from our islandlike growth of Fe(100) at warmer substrate temperatures. The 3-ML spectrum at RT consisted of two differently isomer-shifted single lines. At 4.2 K, all of the Mössbauer spectra consisted only of sextets.A two-site sextet nature in the 3- and 6-ML films was apparent, being more pronounced in the 3-ML film. The 3-ML film magnetization was heavily canted out of plane, and was virtually identical in appearance and Mössbauer fit parameters to the 2.4-ML Fe(100)/Ag(100) superlattice at 15 K reported by Volkening et al.2 at NRL. Striking differences in the magnetic behavior of ultrathin epitaxial multilayers of the Fe(100)/Ag(100) system and the Fe(110)/Ag(111) system had been previously observed by various groups using Mössbauer spectroscopy.2–5 These differences caused a lively discussion, especially since no single group had yet studied both systems with Mössbauer spectroscopy. This work, together with our previous work in the Fe(100)/Ag(111) system, allows our group to be the first to compare these systems first-hand with Mössbauer spectroscopy. Because the Fe(100)/Ag(100) series we studied closely agreed with previous experimental results2 despite differences in substrate growth temperature, increased superparamagnetism, and choice of substrate, there is no doubt that the observation of perpendicular magnetization at small Fe(100) thicknesses is a real effect. There appear to be great differences in the growth behavior and hyperfine-field characteristics between the Fe(100)/Ag(100) and Fe(110)/Ag(111) systems.

Notes: Epitaxial Fe(100)/Ni and Fe(110)/Ni heterostructures were grown using a Perkin-Elmer PHI 430B molecular-beam-epitaxy system equipped with (RHEED) and quadrupole mass analysis. The growth system typically achieved a base pressure of less than 5×10−10 Torr, and a growth pressure of less than 3×10−9 Torr. Typical growth rates were 3 A(ring)/min for Fe and 2 A(ring)/min for Ni. For all the heterostructures, the Ni thickness was held at 14 A(ring), the number of repetitions varied between 8 and 15 cycles, and growth always began with the Fe bilayer. Protective Ag covers were grown on all films. Three Fe (100)/Ni heterostructures were grown on 5-kA(ring) single-crystal Ag(100) bases grown on NaCl(001).1 The single-crystal Fe(100) bilayer thicknesses were 3, 8, or 12 monolayers (ML). The substrate growth temperature for this series was ramped from 40 to 80 °C due to radiant heating from the effusion cells. Four Fe(110)/Ni heterostructures were grown with Fe bilayer thicknesses of 2, 4, 8, and 12 ML. These heterostructures were grown on 5-kA(ring) Ag(111) single-crystal bases grown on single-crystal natural muscovite mica.An intervening epilayer of NaCl (150 A(ring)) deposited between the mica and Ag base facilitated film removal from the Fe-contaminated mica for ex situ transmission 57Fe Mössbauer analysis. The substrate growth temperature for this series was held at 180 °C, since this appears to be optimal for Fe(110) growth on Ag(111).2 Note that the resultant Fe(110) growth is mosaic with Fe[001] parallel to Ag〈110〉 (threefold symmetry). The RHEED observation of the growth of Ni on Fe(100) always resulted in the Ni RHEED pattern closely following that of the Fe (100) pattern, with broader Ni RHEED lines apparent. The characteristic behavior of our Ni RHEED patterns mimicked that observed by Heinrich et al. for bcc Ni(100),3 and did not match that of fcc Ni. The Ni-on-Fe(110) growth was analogous in RHEED characteristics to that of the (100) case. The Ni RHEED patterns again closely matched that of Fe(110), the only real difference being the broadening of the Ni RHEED streaks. Note that fcc Ni(111) was seen to grow on Ag(111) under similar growth parameters. It is likely that a metastable bcc Ni(110) structure analogous to bcc Ni(100) was observed. The quality of the Fe/Ni RHEED patterns did not seem to significantly worsen from bilayer to bilayer throughout the growths of either series. Furthermore, the respective Ag cover layers for all films showed excellent RHEED patterns. All the observed Mössbauer spectra for both series of Fe/Ni multilayers show sextets at room temperature, except for the 2-ML Fe(110) film, which exhibited a very small additional single-line central feature. At 4.2 K, the 2-ML Fe(110) film had no change in central feature, ruling out superparamagnetism as a cause. All films exhibited in-plane magnetization, and thinner Fe bilayers exhibited a growing isomer-shifted second sextet-site presence, suggestive of an interfacial Fe site at the Fe/Ni interface.An enhanced hyperfine field is seen for the thinnest Fe bilayer films at 4.2 K. This enhancement is greatest for the Fe(100) system [most enhanced Fe(100) site=365 kOe vs most enhanced Fe(110) site=351 kOe, compared to 341 kOe for bulk]. The thickest Fe bilayer films for both series showed nearly-single-site, bulklike hyperfine-field behavior. The Mössbauer spectra observed for these epitaxial Fe/Ni heterostructures are different than that previously reported for polycrystalline fcc Fe/fcc Ni films.4 More detailed structural and magnetic studies of the novel bcc Ni reported here should be pursued.

Notes: The influence of the rf-power density and the silane mass flow rate on the properties and growth rate of glow-discharge amorphous silicon has been investigated. The silane deposition efficiency during the preparation process has been deduced using mass spectrometry. It has been found that this deposition efficiency is correlated with film quality and growth rate. Such a correlation and its dependence on the preparation parameters has allowed to find out a way to make films at increasing growth rates while keeping their properties essentially unchanged. The results are interpreted in terms of selective etching of weakly bonded radicals owing to ion bombardment. The research work undertaken has allowed to obtain intrinsic amorphous silicon of excellent photovoltaic properties with growth rates up to 10 A(ring) s−1.