A study of high-resolution ADF imaging in uncorrected and aberration-corrected STEMs was carried out by multislice simulation. The presence of amorphous layers at the surface of a crystalline specimen is shown to significantly alter the visibility of the atomic columns in ADF images. After propagating through an amorphous layer a portion of the beam passes without any alteration while scattered electrons introduce a Gaussian background. An amorphous layer at the beam entry surface appears to have slightly more of an effect on the ADF image contrast than that of an amorphous layer at the exit surface, and this difference increases with increasing atomic number. With constant crystal layer thickness, the reduction of contrast as a function of increasing amorphous layer is found to have the same behavior, regardless of the thickness of the initial crystal layer thickness.

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

Scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) was used to determine the distribution of ZnS shell material on colloidal core-shell CdSe/ZnS quantum dots (QDs). A sub-nm electron probe was placed at various locations on core-shell QDs to ascertain the chemical distribution of the shell material. While a definite shell of ZnS was detected surrounding the CdSe core, the integrated EELS signals from positions around the QD suggest the distribution of the shell material may not be uniform. A non-uniform shell implies a reduced quality of the QD surface passivation.

In 1936, Otto Scherzer published his famous theoretical paper on the correction of aberrations in round magnetic electron lenses. For fundamental reasons, the aberration coefficients are positive and to introduce negative corrections, the electron optical elements have to produce either charges or poles on the optic axis, or contain a mirror or be a multipole system. In 1997, three papers appeared with actual demonstrations of the correction of aberrations in imaging lenses, probe forming lenses and electron mirrors. It took almost two thirds of a century for this seemingly straightforward development to occur and for the promise of wavelength limited electron imaging to emerge as a practical proposition.

AlN/GaN multiple quantum wells (MQWs) were grown on sapphire substrates by plasmaassisted molecular beam epitaxy. Growth temperature, III/V ratio, growth rate, and other growth parameters were optimized for the buffer layer and the MQWs, separately. The growth of AlN buffer was kept as Al-rich as possible while the formation of Al droplets was avoided. A GaN buffer layer was also tried but proved to be inferior to AlN buffer probably due to its larger surface roughness, higher dislocation density, and larger lattice mismatch with the AlN barrier layers in the MQWs. Very flat surfaces with a RMS roughness of 0.7nm were observed by atomic force microscopy (AFM) on the samples with both AlN buffer layer and 20 MQWs deposited under the optimized growth conditions. Abrupt interfaces and excellent periodicities of the MQWs were confirmed by X-ray diffraction (XRD) and reflectivity measurements with MQWs' satellite peaks clearly visible up to the 10th order. Room-temperature intense ultraviolet (UV) photoluminescence (PL) emission with wavelength in the range of 320–350nm was also observed from the MQWs with well width ranging from 1.0 to 1.5nm. These MQW structures can potentially be used for UV light emitters and quantum cascade lasers.

The typical electron micrograph is often rich in detail, sometimes too rich, and it can overwhelm the casual observer. The obvious interpretation may be quickly drawn and the next sample brought under study. Yet the electron interaction with the sample is rarely fully exploited. This may be the reason that electron microscopy may sometimes be dismissed as “just pretty pictures”. While in practice “pretty pictures” are not that easy to record or find, it is still true that the interaction of electrons with the sample is complex and therefore able to give the observer much more understanding than is normally found, provided only that the observer is able to unravel the observations. It is the rapid change in this situation that has been fascinating to watch unfurl during the past decades.

In the Scanning Transmission Electron Microscope (STEM) electrons are focused into a tiny spot that is scanned over a thin film specimen.

The presence of oxygen throughout the nominally AlN nucleation layer of a RF assisted MBE grown III-N HEMT was revealed upon examination by Electron Energy Loss Spectroscopy (EELS) in a Scanning Transmission Electron Microscope (STEM). The nucleation layer generates the correct polarity (gallium face) required for producing a piezoelectric induced high mobility two dimensional electron gas at the AlGaN/GaN heterojunction. Only AlN or AlGaN nucleation layers have provided gallium face polarity in RF assisted MBE grown III-N’s on sapphire. The sample was grown at Cornell University in a Varian GenII MBE using an EPI Uni-Bulb nitrogen plasma source. The nucleation layer was examined in the Cornell University STEM using Annular Dark Field (ADF) imaging and Parallel Electron Energy Loss Spectroscopy (PEELS). Bright Field TEM reveals a relatively crystallographically sharp interface, while the PEELS reveal a chemically diffuse interface. PEELS of the nitrogen and oxygen K-edges at approximately 5-Angstrom steps across the GaN/AlN/sapphire interfaces reveals the presence of oxygen in the AlN nucleation layer. The gradient suggests that the oxygen has diffused into the nucleation region from the sapphire substrate forming this oxygen containing AlN layer. Based on energy loss near edge structure (ELNES), oxygen is in octahedral interstitial sites in the AlN and Al is both tetrahedrally and octahedrally coordinated in the oxygen rich region of the AlN.

Recently there has been increasing interest in nickel induced lateral recrystallization of amorphous silicon because of its potential to improve device performance and to lower the thermal budget during processing. The hypothesis is that the formation of nickel silicide provides a low energy nucleus for the recrystallization of amorphous silicon. The silicide, moving into a-Si, leaves crystalline silicon behind.1 The grains formed, therefore, tend to elongated. In this paper, we attempt to use TEM to investigate in detail the nickel assisted lateral crystallization of amorphous silicon. The sample was prepared by first depositing a 1000A thick low temperature, oxide layer, LTO, on Corning 1737 glass. A 1000A thick amorphous silicon layer, a-Si, and 1000A thick a-Si were deposited subsequently. The sample was pattern and etched with hydrofluoric acid to form lOum x lOum holes in the oxide layer. Next, 200A of nickel was evaporated onto the sample, followed by a 600°C, 6 hours anneal to induce lateral recrystallization.

The presence of oxygen throughout the nominally AlN nucleation layer of a RF assisted MBE grown III-N HEMT was revealed upon examination by Electron Energy Loss Spectroscopy (EELS) in a Scanning Transmission Electron Microscope (STEM). The nucleation layer generates the correct polarity (gallium face) required for producing a piezoelectric induced high mobility two dimensional electron gas at the AlGaN/GaN heterojunction. Only AlN or AlGaN nucleation layers have provided gallium face polarity in RF assisted MBE grown III-N's on sapphire. The sample was grown at Cornell University in a Varian GenII MBE using an EPI Uni-Bulb nitrogen plasma source. The nucleation layer was examined in the Cornell University STEM using Annular Dark Field (ADF) imaging and Parallel Electron Energy Loss Spectroscopy (PEELS). Bright Field TEM reveals a relatively crystallographically sharp interface, while the PEELS reveal a chemically diffuse interface. PEELS of the nitrogen and oxygen K-edges at approximately 5-Angstrom steps across the GaN/AlN/sapphire interfaces reveals the presence of oxygen in the AlN nucleation layer. The gradient suggests that the oxygen has diffused into the nucleation region from the sapphire substrate forming this oxygen containing AlN layer. Based on energy loss near edge structure (ELNES), oxygen is in octahedral interstitial sites in the AlN and Al is both tetrahedrally and octahedrally coordinated in the oxygen rich region of the AlN.

The demands of the National Technology Roadmap for Semiconductors will necessitate measurement of dopant concentrations with greater spatial resolution than now possible. Current experimental and simulation experience indicate that Annular Dark Field (ADF) imaging in a Scanning Transmission Electron Microscope (STEM) should be able to determine dopant distributions with near atomic resolution. The ADF signal is derived from electrons diffusely scattered to high angles, resulting in contrast due to atomic number (Z-contrast) and defects in the crystal lattice. Thus, heavy atoms can be imaged by their Z-contrast and small atoms by the misfit strain induced in the silicon lattice. Atomic number scattering is proportional to Zn where n is between 1.5 and 1.9 depending upon the inner detector angle of the ADF detector.

To obtain insight into the effect of dopants on the bonding and cohesive energy of gram boundaries in Ll2 intermetallic compounds, the chemistry and electronic structure at grain boundaries in B-free and B-doped Ni-23 at % Si alloys were examined, with electron energy loss spectroscopy (EELS) providing information on the former and energy dispersive X-ray spectroscopy (EDX) on the latter. Ni-enrichment was seen at large angle boundaries, both in the absence and presence of B. EELS of the Ni L3 edge showed that the bonding at Ni-rich grain boundaries was similar in both undoped and doped alloys. Comparison of the Ni L3 edge recorded at the grain boundary and in the bulk suggests that reduced hybridization and weaker bonding occurs at Ni-rich grain boundaries in both doped and undoped alloys. These changes in bonding are interpreted in terms of changes in the cohesive energy of the boundaries.

The interfacial structure of CVD diamond grown on silicon was studied using spatially resolved electron energy loss spectroscopy (EELS) in a UHV STEM with a subnanometer probe size. Both the plasmon and core excitations in the bulk appear to be localized on this scale. Spatial maps of the different bonding configurations of carbon were obtained by forming images from transmitted electrons that had undergone energy losses characteristic of threefold and fourfold coordinated carbon. Films grown on both prescratched silicon and intermediate amorphous carbon layers were examined. In the latter case, diamond nucleation on a narrow sp2 a-C occurred. For diamond grown directly on silicon, at some regions of the interface, threefold coordinated defect states smaller than 1 nm are observed on the diamond side of the interface while at other regions along the interface the presence of an intermediate 2nm thick SiC layer preserves the fourfold coordination of the carbon.

Annular dark field scanning transmission electron microscopy (ADF STEM) is chemically sensitive at high spatial resolution (e.g., 1.8ë at 100keV). Images can be digitally acquired and recorded, permitting quantitative analysis. It is particularly powerful when used in combination with complementary analysis modes such as x-ray microanalysis and transmission electron energy loss spectroscopy. Critical to the interpretation of these data is an understanding and determination of the electron probe intensity, shape and propagation characteristics inside the specimen. Quantitative measurements of diffraction patterns and images in comparison with computer-based simulations (including phonon scattering) provide a basis for developing that information. Results of a series of studies are reviewed that address questions such as defocus and other instrumental factors, and also the formation of channeling peaks that appear on the atomic columns along zone axes. For example, along Si(100) a peak forms and penetrates over 500ë whereas along Ge(100) it developes rapidly but disappears in less than 200ë. In higher atomic number elements, the penetration is even less (e.g. 1 O0ë for In).

Electron beam irradiation effects in SiO2 have been studied by STEM (Scanning Transmission Electron Microscopy). Oxygen loss and the corresponding transformation from SiO2 to Si in SiO2 are confirmed and consistent with previous reports 1,2. A “flower-like” Si rich area, which might not be observed in STEM BF (Bright Field) or ADF (Annular Dark Field) images, was found in Si plasmon energy filtered images. Quantification of the observations and the formation mechanisms leading to the Si-rich area are discussed.

Although it has been possible for many years to determine the average position of atoms by using diffraction techniques, finding the positions of specific atoms has been an elusive target. This latter goal is now in sight. Individual atoms on surfaces were first seen in the seventies by the Chicago Group using the then newly invented Scanning Transmission Electron Microscope (STEM). More recently atoms on surfaces have also been seen by Scanning Tunneling Microscopy (STM) and associated variants (e.g. Atomic Force Microscopy). Atomic structures in the interior of a material have been imaged in recent years through atomic resolution transmission electron microscopy of thin film specimens. Progress in these developments is following an acceleration path and the Materials Research Division of the NSF recently commissioned a panel to review the area and provide advice on an appropriate response.

Strained solid phase epitaxial (SPE) regrowth of amorphous Ge xSi1-x on Si (100) substrates was studied using time-resolved reflectivity(TRR). Films of CVD-grown Ge0.13Si0.87 on Si wereamorphized by Si ion implantation, and subsequently regrown at temperaturesbetween 550°C and 610°. Information on regrowth dynamics and interfaceroughness evolution was obtained by accurately modeling the complicated TRRdata for Ge xSi1-x regrowth using a Moving, statistically roughening interface. The SPEregrowth rate slowed as the interface crossed into the Ge xSi1-x layer and the originally planar interface roughened, as confirmed bytransmission electron Microscopy. A Minimum in the regrowth velocity wasobserved after regrowing approximately 60 nm into the Ge xSi1-x layer; the SPE rate subsequently increased to a final,thickness-dependent velocity that was still below that for pure Si. Uponentering the Ge xSi1-x layer, the interface roughened quickly to a 15–20 nm amplitude,increasing only slightly more during the remainder of regrowth. The degreeof roughening and velocity reduction was found to be dependent on the annealtemperature. In contrast, samples with low Ge concentrations (< 3 at.%)prepared by ion implantation exhibited minimal interface roughening andessentially identical SPE velocities as pure Si.

Amorphous GeSi films with different thicknesses and oxygen contents wereelectron beam evaporated onto Si (KK)) wafers and wet oxidized at 900 °C for30 Min. If there was no oxygen in the as-deposited film, an epitaxial GeSifilm would be grown after wet oxidation. For the samples with oxygen,epitaxial growth broke down when the thickness of the epitaxy exceeded about200 A and polycrystalline GeSi films were formed. A dedicated STEM (scanningtransmission electron Microscope) was used to characterize the sample afteroxidation. STEM BF (bright field), ADF (annular dark field), and energyfiltered images revealed the presence of small oxide particles in thepolycrystalline GeSi films. X-ray microprobe analysis with a windowlessdetector was employed to identify the oxide particles. The failure of theepitaxy is explained by the random nucleation and growth of GeSi grains onthe oxide particles.