U. Bangert

Electron diffraction and Raman studies of the effect of substrate misorientation on ordering in the AlGaInP system

We report a systematic study of the effect of substrate orientation on the ordering in the AlGaInP system, including the Al0.52In0.48P lattice-matched ternary case. Four AlGaInP/GaAs/AlInP samples were grown by metalorganic vapor phase epitaxy under identical growth conditions on [100] substrates orientated 0°, 2°, 10° and 15° either towards the [110] or the [111] axis. The ordering in both the AlInP and the AlGaInP layers was studied by electron diffraction and the Raman scattering technique. The tendency for ordering decreased with increasing misorientation and is less for (AlxGa1−x)0.52In0.48P than for AlInP. The AlInP was found to spontaneously order even when grown after a completely disordered AlGaInP layer. The Raman results show features correlated to the electron diffraction results and hence we conclude that this technique constitutes a reliable nondestructive means of characterizing this system.

In-situ observation and atomic resolution imaging of the ion irradiation induced amorphisation of graphene

Ion irradiation has been observed to induce a macroscopic flattening and in-plane shrinkage of graphene sheets without a complete loss of crystallinity. Electron diffraction studies performed during simultaneous in-situ ion irradiation have allowed identification of the fluence at which the graphene sheet loses long-range order. This approach has facilitated complementary ex-situ investigations, allowing the first atomic resolution scanning transmission electron microscopy images of ion-irradiation induced graphene defect structures together with quantitative analysis of defect densities using Raman spectroscopy.

Formation of N- containing C-nanotubes and nanofibres by carbon resistive heating under high nitrogen pressure

Abstract A variety of nanocarbons of different shapes and compositions (C, C–N) has been formed using a modified high isostatic pressure (HIP) apparatus. It was found that the presence of nitrogen in the background gas increases evaporation of graphite in the hot zone of the heater and increases the yield of carbon nanostructures. Carbon deposits formed by different gas pressures and temperatures were characterised using transmission electron microscopy and electron energy loss analytical microscopy.

Scanning Tunnelling Microscopy of Suspended Graphene

We have obtained for the first time atomic resolution STM images of free-standing graphene, and have found UHV high temperature annealing conditions to controllably achieve atomically clean surfaces. This is an important premise for fundamental studies of the interaction of pristine graphene with foreign atom species, in particular metals.

Assessment of electron energy-loss spectroscopy below 5 eV in semiconductor materials in a VG STEM

Abstract The possibilities of obtaining information about interband scattering processes from electron energy-loss spectra, taken in a VG601UX scanning transmission microscope, are investigated. With the help of precise simulations of the zero-loss peak it is feasible to process, extract and analyse data in the electron volt regime. The accuracy of the results is restricted predominantly by instrumental limitations. It is possible to extract band gaps of > 2 eV (e.g. for GaN) correctly. In the case of band gaps between 1 and 2eV (i.e. in GaAs), the uncertainty is larger, since the tail of the zero-loss peak interferes with the interband losses, and further refinements in the data analysis as well as the measurement techniques are discussed. ‘Band-gap mapping’ in the VG601UX can be carried out with a spatial resolution of a few nanometres.

Optimization of growth conditions for strain compensated Ga0.32In0.68As/Ga0.61In0.39As multiple quantum wells

The results of an investigation into the growth of Ga0.32In0.68As/Ga0.61In0.39As strain compensated multiple quantum well stacks are presented. We show that these structures, which contain alloys that are not prone to compositional clustering, do suffer from thickness modulated growth. A growth strategy has been developed to inhibit this phenomenon and a 50 well, strain compensated stack has been grown with planar and abrupt interfaces.

Electron energy loss spectroscopic studies of brown diamonds

We investigate both experimentally and theoretically, low-loss electron energy losses in brown type IIa monocrystalline diamonds both before and after high-temperature, high-pressure anneals which remove the brown colouration. We find additional losses within and near the band edge for brown diamond which are significantly reduced after treatment. The additional losses are not associated with dislocations. Graphitic inclusions are detected by EELS as well as TEM studies for some brown diamonds before treatment. These lead to pronounced subgap absorption. However, all brown diamonds exhibit additional losses which are due to point defects lying in the regions between dislocations. First principles theoretical modelling shows that common dislocations are not responsible for the brown colouration but a π-bonded vacancy disk lying on {111} planes gives broad bands lying in the diamond band gap, possesses an optical absorption spectrum similar to that of brown diamond, and leads to additional electron energy losses in the band edge region. These and similar defects are suggested to be responsible for the brown colouration. Mechanisms are proposed for their formation and removal.

Imaging of Bernal stacked and misoriented graphene and boron nitride: experiment and simulation

Experimental atomic resolution bright and high angle dark field transmission electron microscopy images of mono‐ and few‐layer graphene and boron nitride, as well as of turbostratic arrangements in both materials, are compared to their simulated counterparts. Changes in the images according to defocus, layer number and accelerating voltage are discussed. It emerges that simulations with realistic microscope parameters accurately depict experimental graphene and boron nitride images and present a reliable tool for their interpretation.

The effect of strain field seeding on the epitaxial growth of Ge islands on Si(001)

The effect of strain, due to a buried, nominally 6 ML Ge quantum dot layer, upon the growth of subsequent Ge layers grown by gas source molecular beam epitaxy has been investigated. A series of samples were grown at 700 °C with a nominally 6 ML Ge layer followed by a 30 nm Si spacer and then a second, thinner Ge layer. In each sample, the thickness of the second Ge layer was varied (2, 3, and 4 ML). Atomic force microscopy shows that in the second Ge layer islands form at thicknesses below the established critical thickness for this material system. This is confirmed by transmission electron microscopy images which also show the quantum dots in the second layers are stacked above those in the first layer, the island growth in the thin Ge layer being seeded by the strain field from the buried Ge islands. Photoluminescence results show a luminescence feature attributed to the strain-controlled quantum dots in the thin Ge layer. This band has properties similar to the frequently observed Ge dot luminescence ...

Transmission electron microscopy studies of nanofibers formed on Fe7C3-carbide

Abstract Carbon nanofibers have been formed on iron details of the high isostatic pressure (HIP) apparatus. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) techniques proved the existence of faceted catalyst particles found in the middle part of fibers and allowed identification of their lattice and composition as the pseudo-hexagonal crystalline lattice of Fe7C3. Two systems of arc-reflections with 0.335-nm spacing were ascribed to two different orientations of graphite layers and a trace analysis showed that the growth direction of the fiber coincides with the [210]-direction of pseudo-hexagonal Fe7C3-lattice. This direction is compared with directions of fibers formed on bcc-Fe and Fe3C.