S. Foss

Single beam determination of porosity and etch rate in situ during etching of porous silicon

A laser reflection method has been developed and tested for analyzing the etching of porous silicon (PS) films. It allows in situ measurement and analysis of the time dependency of the etch rate, the thickness, the average porosity, the porosity profile, and the interface roughness. The interaction of an infrared laser beam with a layered system consisting of a PS layer and a substrate during etching results in interferences in the reflected beam which is analyzed by the short-time Fourier transform. This method is used for analysis of samples prepared with etching solutions containing different concentrations of HF and glycerol and at different current densities and temperatures. Variations in the etch rate and porosity during etching are observed, which are important effects to account for when optical elements in PS are made. The method enables feedback control of the etching so that PS films with a well-controlled porosity are obtainable. By using different beam diameters it is possible to probe inter...

Phase separation in SiGe nanocrystals embedded in SiO2 matrix during high temperature annealing

SiGe nanocrystals have been formed in SiO2 matrix by cosputtering Si, Ge, and SiO2 independently on Si substrate. Effects of the annealing time and temperature on structural and compositional properties are studied by transmission electron microscopy, x-ray diffraction (XRD), and Raman spectroscopy measurements. It is observed that Ge-rich Si(1−x)Gex nanocrystals do not hold their compositional uniformity when annealed at high temperatures for enough long time. A segregation process leading to separation of Ge and Si atoms from each other takes place. This process has been evidenced by a double peak formation in the XRD and Raman spectra. We attributed this phase separation to the differences in atomic size, surface energy, and surface diffusion disparity between Si and Ge atoms leading to the formation of nonhomogenous structure consist of a Si-rich SiGe core covered by a Ge-rich SiGe shell. This experimental observation is consistent with the result of reported theoretical and simulation methods.

Determination of the crystal structure of the π-AlFeMgSi phase using symmetry- and site-sensitive electron microscope techniques

The crystal structure of the complex pi-AlFeMgSi phase, which was previously thought to have the composition Al(8)FeMg(3)Si(6), has been investigated. Microprobe analysis revealed that the phase has a different composition, Al(9)FeMg(3)Si(5). The space group was determined and confirmed to be P62m with the use of parallel-beam electron diffraction (SAD) and convergent-beam electron diffraction (CBED). Owing to symmetry considerations the elements within the unit cell had to be rearranged. The rearrangement was confirmed using electron channelling. The z parameters of the elements were refined by examining the intensities from high-angle convergent-beam electron diffraction. Finally, the x parameters were adjusted slightly to arrive at acceptable interatomic distances.

Properties of Al2O3: nc-Si nanostructures formed by implantation of silicon ions into sapphire and amorphous films of aluminum oxide

Photoluminescence, infrared Fourier spectroscopy, Raman scattering, transmission electron microscopy, and electron diffraction were used to study the luminescent, optical, and structural properties of aluminum oxide layers (sapphire and films of Al2O3 deposited on silicon) implanted with Si+ to produce silicon nanocrystals. It is established that, in both cases, a high-temperature annealing of heavily implanted samples brings about the formation of silicon nanocrystals. However, the luminescent properties of the nanocrystals are strongly dependent on the type of pristine matrix; namely, nanocrystals in Al2O3 films emit light in the spectral range typical of Si quantum dots (700–850 nm), whereas in sapphire this photoluminescence is not observed. This difference is interpreted as being due to the fact that local stresses arise in the nanocrystal/sapphire system and break chemical bonds at the interface between the phases, whereas in Al2O3 films stresses are relaxed.

Segmented nanowires of HgTe and Te grown by molecular beam epitaxy

Heterostructured nanowires consisting of alternating segments of HgTe and Te have been grown by molecular beam epitaxy. The cubic ⟨111⟩HgTe and the hexagonal ⟨001⟩Te directions are oriented along the wire. The 15–70nm wide, 0.5–1.5μm long wires are nucleated at Au particles and grow laterally on Si substrates, but they are not epitaxially coupled to the substrates. An excess of Te relative to Hg during growth could explain the segmentation, as the bulk phase diagram then allows only HgTe and elemental Te. Alternating between these two phases is facilitated by the epitaxial match between the HgTe and Te segments.

Luminescence from silicon nanoparticles in SiO2: atomic force microscopy and transmission electron microscopy studies

Si nanocrystals in thermal oxide films (~250 nm) were fabricated by 100 keV Si ion implantation at various doses followed by high temperature annealing. After annealing a sample implanted with a dose of 1×1017 cm−2 at 1050°C for 2 h, a broad photoluminescence peak centred around 880 nm was observed. A dose of 5×1016 cm−2 yields a considerable blue shift of about 100 nm relative to the higher dose. Transmission electron microscopy and atomic force microscopy (AFM) are used to characterize the microstructures in the SiO2 film. The limitations of these techniques for the study of the nanostructures are addressed in this paper and it is suggested that AFM combined with etching can yield a structural spectroscopy with very good sensitivity.

Structure determination of MnO2 films grown on single crystal α-Al2O3 substrates

Manganese oxide films have been grown by atomic layer deposition and investigated using electron diffraction and high-resolution electron microscopy (HREM). The films were deposited on the (001) surface of monocrystalline α-Al2O3. The films were found to consist of an ordered version of the hexagonal ε-MnO2 (Akhtenskite) type. Using X-ray diffraction, the cell parameters were determined to be a = 2.75(2) Å and c = 4.302(5) Å. The films are epitaxial with a specific orientation relative to the Al2O3 substrate. The [210] and [001] axes of ε-MnO2 are parallel to the [110] and [001] axes of α-Al2O3, respectively. Evidence of cation ordering was found by parallel beam electron diffraction. The ordered domains are needle shaped with widths of 2–10 nm. The unit cell of the ordered structure was found to be orthorhombic with cell dimensions a = 2.75, b = 4.76, c = 4.302 Å and space group Pmnn (No. 58).

Porous Silicon for Chip Cooling Applications

Porous silicon (PSi) was prepared by electrochemical etching with electrolytes consisting of various concentrations of HF and dimethylsulphoxide (DMSO) to control the pore size in the range 0.4µm–8µm as measured by scanning electron microscopy. Pool boiling experiments employing resistive heating of samples with a PSi surface in the dielectric cooling liquid FC-72 was performed and compared to samples with a polished Si surface. It was observed that the PSi surface had favorable cooling behavior and yielding the lowest temperature overshoot upon warming up and the lowest surface temperature during boiling by promoting continuous bubble nucleation which again leads to the most efficient heat removal. The results indicate that PSi could favorably be used in cooling of electronic chips. We also present demonstration of the successful fabrication of a Si porous membrane by a onestep electrochemical etching procedure. The onestep etching technique could be used as an alternative method to the standard Si micro machining such as etched by KOH. The PSi membrane could be used for various closed loop two-phase coolers.

A specimen preparation technique for plane-view studies of surfaces using transmission electron microscopy

A method for preparing plane-view transmission electron microscope (TEM) samples is presented. With this inclined pseudo-plane-view technique, the undisturbed surface of the sample can be studied in plane view. Thus, nanostructures on the surface of a substrate can be studied with TEM in much the same way as with scanning electron microscopy (SEM), but in transmission at a much higher spatial resolution and with the opportunity of performing nanoscale diffraction. A glued sandwich with two surfaces facing each other was thinned at a low angle relative to the surfaces. The resultant construction contained thin wedges of the surfaces upon which it was possible to do TEM analysis. SEM analysis before and TEM analysis after such sample preparation was found to be consistent.

Locating Atoms in Small Crystals by Combining Convergent Beam Electron Diffraction and Electron Channeling

With the current focus on nanotechnology and nanomaterials, there is a growing interest for quantitative determination of composition and atomic arrangement within small volumes of materials. In chemical microanalysis the fast electrons within a subnanometer probe may excite inner shell electrons with accompanying element-characteristic energy losses and x-ray emission that originate in the electron-illuminated volume of a thin specimen. The change, under channeling conditions, of the yield of element characteristic x-ray emission was first observed by Duncumb [1], and the first observation in energy loss spectroscopy was done at Fritz-Haber-Institut [2]. These channeling effects, now often referred to as ALCHEMI, can be used to locate small concentrations of atoms within the unit cell of small crystals [3].