Fluctuation electron microscopy can reveal the nanoscale order in amorphous materials via the statistical variance in the scattering intensity as a function of position, scattering vector, and resolution. However, several sources of experimental artifacts can seriously affect the magnitude of the variance peaks. The use of a scanning transmission electron microscope for data collection affords a convenient means to check whether artifacts are present. As nanodiffraction patterns are collected in serial, any spatial or temporal dependence of the scattering intensity across the series can easily be detected. We present examples of the major types of artifact and methods to correct the data or to avoid the problem experimentally. We also re-cast the statistical formalism used to identify sources of noise in view of the present results. The present work provides a basis on which to perform fluctuation electron microscopy with a high level of reliability and confidence in the quantitative magnitude of the data.

Thin ruthenium films were deposited using chemical vapor deposition from the single-source precursor tricarbonyl(1,3-cyclohexadiene)Ru(0) onto silicon, silicon dioxide and c-plane sapphire substrates in the absence of a carrier gas by thermolysis. Growth rate, resistivity, purity, crystallinity and microstructure were determined. Tricarbonyl(1,3-cyclohexadiene)Ru(0) gave metallic ruthenium films with near bulk resistivities (11-21μΩ-cm), high growth rates (up to 20 nm/min), and nearly featureless microstructures. Nucleation was rapid on all substrates tested. These results suggest that tricarbonyl(1,3-cyclohexadiene)Ru(0) is an excellent, practical precursor to use for practical applications that require depositing thin ruthenium films.

Ge2Sb2Te5 is under intense investigation for phase-change memory devices, including rewriteable DVDs where optical illumination is used to switch between the glassy and crystalline states. We investigate the influence of optical irradiation on amorphous phase. Many chalcogenides display photo-oxidation, photodarkening or photo-bleaching, but little has been reported on the Ge-Sb-Te system. Using spectroscopic ellipsometry (SE) and secondary ion mass spectrometry, we determine that the samples have a strong tendency to photo-oxidize; if this is not accounted for, then the analysis of SE data appears to show photodarkening. Other authors recently reported photodarkening in nonstoichiometric GexSb20-xTe80 [Pamukchieva et al., Proc. SPIE 5581, 608 (2004); Pamukchieva et al., J. Optoelectron. Adv. Mater 7, 1277 (2005)], but our analysis suggests that the changes were also the result of photo-oxidation. The oxide has lower value of (n, k) than Ge2Sb2Te5, and can be etched by hydrofluoric acid or water. The photo-oxidation is presumably the result of free carrier generation in the Ge2Sb2Te5. Our observation of negligible photodarkening is consistent with previous works that found less photodarkening in tellurides compared with selenides or sulfides, and that an increase in the mean coordination number, here by Ge addition, further reduces the photodarkening effect.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Using plasma growth sources with concurrent particle bombardment, silicon thin films can be deposited with various phases and microstructures. DC Reactive Magnetron Sputtering (RMS), in particular, can produce amorphous, mixed-phase, nanocrystalline, polycrystalline, porous columnar, and epitaxial Si films. In particular, a large flux of low energy, heavy ions strongly affects the phase and microstructure, and therefore the resulting film qualities. Lowpressure (1.6 mTorr) RMS is particularly suited for this type of plasma manipulation: we bias the substrate to produce the ion energy of choice, and use an external magnetic field to control the ion/neutral flux ratio, therefore decoupling the parameters of bombardment energy and flux. In this work, we study the influence of slow (<40eV), heavy (Ar+), ions in RMS deposition on the formation kinetics and microstructures of microcrystalline, mixed-phase, and poly crystalline Si films. The analytical methods are ellipsometry, Raman scattering, and AFM. We will show how ion bombardment influences the direct nucleation of extremely smooth polycrystalline Si on glass at temperatures below 400°C, enhances the crystallinity of this polycrystalline Si in the bulk, and produces nanocrystalline Si with tailored grain sizes at temperatures below 200°C.

Amorphous silicon films were prepared by dc reactive magnetron sputtering under conditions approaching the phase transition to microcrystallinity. Using TEM imaging these films were found to contain clusters of 5 to 50 nm sized Si crystallites embedded in an amorphous silicon matrix. Photocapacitance and transient photocurrent sub-band-gap optical spectra of this material appear to consist of a superposition of a spectrum typical of amorphous silicon together with an optical transition, with a threshold near 1. 1eV, that exhibits a very large optical cross section. This transition arises from valence band electrons being optically inserted into empty levels lying within the amorphous silicon mobility gap. Using modulated photocurrent methods we have determined that these states also dominate the electron deep trapping in this material. We argue that these states arise from defects at the crystalline-amorphous boundary.

We have studied the degradation kinetics of undoped a-Si:H films which contain a significant fraction of silicon microcrystallites. The degradation rate is found to be exceptionally slow in the first stage of degradation, then the defect density follows the “normal” t1/3rate and finally saturates. We present a model which relates this abnormal kinetics to the microcrystallites which are embedded in the amorphous matrix.

We present a study of depletion width effects on the photocarrier collection efficiency in reactive magnetron sputtered a-Si:H films. Results are presented for as-deposited and light soaked 1.75 eV optical gap samples, and an as-deposited 1.60 eV gap film. The depletion width behavior with reverse bias is inferred from capacitance measurements. Comparison with photocurrent collection versus reverse bias voltage suggests that space charge effects can have an important role in the interpretation of collection efficiency on Schottky barriers.

Hydrogenated amorphous silicon (a-Si:H) films have been deposited by dc magnetron reactive sputtering in an Ar+H2 working gas. In this process, the hydrogen content A controlled by two independent variables: the substrate temperature and the hydrogen partial pressure in the deposition chamber. For a wide range of deposition conditions studied, it is shown that total hydrogen content of the films, as inferred from IR absorption, is uniquely related to the hydrogen bonding and the microstructure. We report films of high electronic quality, deposited between 200–300 °C, in which hydrogen bonding varies from predominantly SiH to mixed SiH and SiH2 modes as the hydrogen content is increased. This struciral change is reflected in hydrogen evolution spectra. In addition to peaks at ˜500 and ˜600 ° C, a low-temperature peak (350–400 C) has been observed for the films which have high hydrogen content and consequently are rich in SiH2. The low temperature peak is absent in SiH dominant films. However, the low-temperature peak is not associated with a columnar morphology because SEM images of fracture cross-sections are featureless for SiH dominant films and have a “bumpy” texture for high SiH2 content films. We propose that in our SiH2 rich films, excess hydrogen leads to a void rich but non-columnar morphology. Preliminary measurements of the Staebler-Wronski effect using AM-1 light show a low susceptibilty to degradation in both types of films. The different electronic properties of the films indicate that the quality of magnetron sputtered films does not necessarily correlate with the IR absorption peaks in a simple manner.

We demonstrate the fabrication of high quality GexSi1−x heteroepitaxial alloy layers on Si(100) substrates by pulsed laser melting of a-Ge/Si(100) layered structures. First, 50–200Å of germanium are evaporated onto a chemically cleaned silicon wafer. Next, an excimer laser pulse is used to melt the sample surface to a depth of 200–2500Å. Finally, the melt solidifies in 20–100ns producing a heteroepitaxial alloy layer. By varying the thickness of the evaporated Ge film and the laser energy, alloys of 500–1500Å thickness and Ge fraction x=3–20% are obtained. The crystallinity of the layer and interface are found to be excellent as evaluated by Rutherford Backscattering Spectrometry (RBS), ion channeling, and high resolution cross sectional TEM. This technique for producing GexSi1−x alloy layers is insensitive to minor levels of contamination because the original Ge/Si interface is melted through during the laser processing

Transmission channeling Rutherford scattering of MeV ions is one of the only techniques which can probe the atomic registry at a “buried” internal interface between a thin film and single-crystal substrate. Interfacial intermixing, dislocations, and reconstruction can be observed at concentrations above ∼ 3×1015 atoms/cm. The physical basis for this measurement is the “flux-peaking” effect, which is well known in conventional ion channeling as a method to determine the lattice location of dilute impurities. Transmission channeling is conceptually similar, but the scattering arises from the interface at the exit side of a thin single crystal sample rather than from a volume effect.

In this work, we apply transmission channeling to measure the low temperature intermixing between Pt and Si. In a previous study, we suggested that a disordered interface forms prior to crystalline suicide formation. The present results are a quantitative measurement of the area density of Si which intermixes with Pt at 150°C. Experiments are performed using 80A Pt layers e-béam deposited onto 1–2μm thick single crystals of Si(100) and (111) following various interfacial cleaning procedures. We find that >1×1016 Si atoms/cm2 are displaced from their lattice sites after Pt deposition, increasing to ∼2×1016/cm2 upon annealing to 150°C. The room temperature intermixing of >6 monolayers of Si is large, but not inconsistent with existing studies of the Pt - Si reaction. The transmission channeling measurement includes any Si segregated at grain boundaries or the free surface as well as the interface. No systematic differences are observed as a function of substrate orientation or cleaning procedure.

Grazing angle configurations improve the depth resolution of Rutherford Backscattering Spectrometry (RBS) because the path length of scattered particles is geometrically increased. But in the limit of very grazing angles the resolution degrades due to the finite detector acceptance angle and low angle forward scattering of the beam, considerations which are not present in near-normal incidence geometries. The forward scattering, for example, worsens the depth resolution approximately as the square of the depth, rather than the familiar square root dependence of energy straggling. We present a simple computational scheme which predicts the depth resolution in any target as a function of scattering depth, beam energy, and the grazing angle. The calculations are simpler than previous studies because we use analytic expressions in place of numerical inputs without loss of accuracy. The results lead directly to an optimum depth resolution. We find good agreement between the predicted resolution and measurements on thin silicon dioxide and amorphous silicon films. Finally, we calculate the resolution expected with the use of ion beams heavier than He+, and find improved near surface depth resolution if a low noise detector is used.