Jo Malm

SCANNING TUNNELING MICROSCOPE AND ELECTRON-BEAM-INDUCED LUMINESCENCE IN QUANTUM WIRES.

Quantum wire structures of GaAs/AlGaAs have been grown by metalorganic vapor phase epitaxy in V grooves using pre‐etched corrugated substrates and have been characterized by high‐resolution transmission electron microscopy. Low‐temperature cathodoluminescence (CL) identifies luminescence peaks with the spatial distributions of the different recombinations, achieving a top view spatial resolution of ≊0.2 μm in the CL images. Principally we report how a scanning tunneling microscope (STM) induces local luminescence in the sample structure, and we spectrally resolve STM‐induced luminescence for the tip in different positions relative to the wires. We have recorded the luminescence from a single wire and observed band‐filling effects resulting from varying levels of excitation into a wire. We have demonstrated the difference between recombination of electron‐hole pairs generated in CL and the recombination of injected holes from the STM tip with a thermalized distribution of accumulated electrons in scanning ...

Deposition of Transition Metal Carbides and Superlattices Using C 60 as Carbon Source

Thin films of TiC, VC, and NbC have been deposited on MgO(001) by coevaporation of the metals and C-60 It was found that these metals induced a decomposition of the C-60 molecule and that carbide f ...

Low-temperature epitaxial growth of metal carbides using fullerenes

Epitaxial transition metal carbides can be deposited at low temperatures by simultaneous evaporation of C60 and either metal e-beam evaporation or metal d.c. magnetron sputtering. Hitherto, epitaxial films of TiC, VC, NbC, MoC, W2C and WC have been deposited on MgO(100), MgO(111) and in some cases 6H- and 4H-SiC(0001). Epitaxial TiC films with a good quality have been deposited at temperatures as low as 100°C with metal sputtering, while somewhat higher temperatures (>200°C) are required for the other metals. In general, the plasma-assisted process allows lower deposition temperatures than the co-evaporation process. Most carbides can be deposited in a wide range of compositions within their homogeneity ranges by a fine-tuning of the Me/C60 flux. However, the results suggest that the formation of free surface carbon can be a limiting factor. The processes have also been used to deposit superlattices of TiC/NbC and TiC/VC at 400–500°C as well as epitaxial ternary TixV1−xCy films. Furthermore, epitaxial films of ternary carbides with well-controlled metal concentration profiles can be deposited at temperatures below 500°C.

Characterization of a single‐layer quantum wire structure grown directly on a submicron grating

A single AlGaAs/GaAs quantum well (QW)/quantum wire (QWR) structure was grown by metalorganic vapor phase epitaxy on a submicron period grating of V grooves. High resolution transmission electron microscopy studies of the sample identifies three regions of the QW; between the grooves, approximately 3.5 nm thick and oriented along (100), on the sidewalls of the V groove slightly thinner and oriented along {111}. At the bottom of the groove an approximately 70 nm wide crescent shaped region forms a QWR. In addition, a vertical quantum well (VQW) extends from the bottom of each V groove in the GaAs substrate to the surface. The luminescence spectra of the sample are dominated by a peak originating in the QW, with additional peaks of the QWR, the VQW and the AlGaAs barrier. The striped nature of the sample is revealed in the top view cathodoluminescence (CL) images of all four peaks. In side view CL images, the QWR emission appears spot like, whereas the emission of the VQW is elongated in the direction perpe...

Cathodoluminescence of single quantum wires and vertical quantum wells grown on a submicron grating

We present cathodoluminescence (CL) investigations of a corrugated GaAs/AlGaAs single quantum well (QW) structure grown on a submicron grating. The CL spectra have four distinct emission peaks. Using plan‐view and cross‐sectional CL imaging together with cross‐sectional transmission electron microscope imaging, we have assigned the four peaks: They originate in the nominal QW, a quantum wire (QWR), a vertical quantum well (VQW), and the barrier, respectively. We have CL‐imaged and ‐characterized single QWRs and VQWs.

High-resolution transmission electron microscopy studies of quantum wire structures grown on submicrometre gratings of V-grooves

We present an extensive characterization of a quantum wire (QWR) structure using transmission electron microscopy (TEM). The structure consisted of a single GaAs layer in between AlGaAs barriers, grown on a GaAs substrate patterned with a submicrometre grating of V-grooves. For reference we also studied other QWR, as well as, quantum well (QW) samples, fabricated under similar conditions. We used bright field and dark field imaging to study the overall structure, high-resolution TEM to study the layer thickness and the interface quality, and chemical lattice imaging to study the compositional variations across the interfaces. In the QWR sample, there were mainly two distinctly different areas of the QW: on the (100) planes between the V-grooves, the QW was flat, whereas the QW on the near side walls of the V-grooves had a flat lower interface and a saw-tooth shaped upper interface. The QWRs at the bottom of the V-grooves were crescent shaped. We also observed a fundamental difference in growth of the GaAs and the AlGaAs on the side wall, where the AlGaAs formed straight interfaces, determined by high-index planes, whereas the GaAs tended to form alternating low-index planes giving a saw-tooth appearance of the GaAs QW.

Deposition of Epitaxial Carbide Films and Superlattices by Co-Evaporation of C 60 and Transition Metals

Thin films of TiC, VC and NbC have been deposited on MgO(001) by co-evaporation of C 60 and the metal. The metal induced a decomposition of the C 60 cage and a subsequent carbide formation at 100 °C. Epitaxial TiC films were easily obtained at 250 °C, while higher deposition temperatures were required for epitaxial growth of VC(400 °C) and NbC(500 °C). The films grew with the relation MeC(001)//MgO(00 1) and MeC[ 100]//MgO[ 100]. It was also possible to deposit TiC/NbC/MgO polycrystalline multilayers and TiC/VC/MgO superlattices structures by a sequential evaporation of the metals.