Molecular Beam Epitaxy “MBE”

Molecular Beam Epitaxy - MBE

Carlo Ottaviani -

Laboratory IC11

Roma - Tor Vergata

In an epitaxy (from ancient Greek έπί, epì, "Top" e τάξίξ, tàxis, ”order-arrangement trough ordering”) process, atoms or molecules are deposited on a substrate and some structures evolve as a result of a multitude of processes. This is a non-equilibrium phenomenon and any synthesized growth is governed by the competition between kinetics and thermodynamics. Self-assembly and self-organization are modes through which nanometer-size structures, called here nanostructures, grow on a surface. In the simplest case, the growth proceeds in a 2D fashion, one atomic layer after the next, up to some required film thickness. This is called layer-by layer Frank-Van der Merwe (FV) growth. Very often the deposited material coagulates into islands/clusters, which at a first stage may form a polycrystalline layer. This is the Volmer-Weber (VW) type growth: 3D crystallites form upon deposition and some surface areas remain uncovered since the initial stages of deposition. The Stranski-Krastanov (SK) growth is in-between: a few layers may grow in FV mode before 3D clusters begin to form.

TECHNICAL SPECIFICATIONS

Working pressure ~10^-11mbar
Si(C-BN-1400 °C), Ge(BN-1200 °C, Mn(BN-1200 °C) K-cells from RIBER; Si (flux=0.04 Å/min); Ge (flux=0.16 Å/min);
Sb, As, Bi- Surfactants effusion cells;
Ag, Au- Capping Layer effusion cells;
DC direct sample heating (RT-1200 °C) and Indirect heating (RT-450 °C ) systems;
Air-vacuum Fast Load-lock Sample Transfer System;

AVAILABLE TECHNIQUES

STAIB EK-3315-R RHEED Ultra-High Vacuum (UHV) System for Surface Science Investigations;
Cleaning Semiconductor (SC), Metal (M)-Surfaces reconstruction;
Epitaxial growth SC/SC, SC/Metal/SC;
Homo- and Hetero-structures growth: 1D, 2D and 3D Materials.

SAMPLES

Sample lateral dimensions: 10 x 5 mm (ideal), 3 x 3 mm (minimal), 10 x 10 mm (maximal);
Sample thickness: ideally up to 2 mm (thicker and/or smaller samples also feasible).

USED FOR

Fundamental Surface Science study;
Artificial Atomic Epitaxial Growth;
Discovery of new 1D, 2D and 3D epitaxial SC/SC; M/SC for micro-nanoelectronics and solar cells purposes;
Semiconductors for Microelectronics;
Microcircuits;
Ultra-thin Films;
Samples Cleaning;
Thin-film Stability;
Barrier Layers;
Lubrication;
Chemical Industry;
Coatings/Catalysis.

CASE STUDIES

Cross-sectional HRTEM Mn_0.06Ge_0.94on Ge(001)2x1

The structural, electronic, and magnetic properties of the Mn_0.06Ge_0.94diluted magnetic semiconductor, grown at 520 K by molecular-beam epitaxy on Ge(001)2✕1, have been investigated. Diluted and highly ordered alloys, containing Mn₅Ge₃nanocrystals, were grown. The valence band photoelectron spectrum of Mn_0.06Ge_0.94 shows a feature located at −4.2 eV below the Fermi level, which is the fingerprint of substitutional Mn atoms in the Ge matrix. Magnetization measurements show the presence of a paramagnetic component due to substitutional Mn atoms and of a ferromagnetic like component due to Mn₅Ge₃nanocrystallites. The Mn L2,3 x-ray absorption spectrum of this polyphase film shows no marked multiplet structure, but a bandlike character.

See: P. De Padova, et al., Phys. Rev. B 77, 045203 (2008).

Cross-sectional high-resolution transmission electron microscopy (HRTEM) image of a Mn_0.06Ge_0.94 film grown on a Ge(001)2x1 substrate held at 520 K.

HRTEM cross-sectional image of a Mn₅Ge₃ film grown on Ge(1 1 1), taken along the [-1- 1 2] zone axis of the Ge(1 1 1) substrate. (b) Ball and stick side-view of a coherent epitaxy between Mn₅Ge₃ film and Ge substrate.

Mn₅Ge₃ film on Ge(111)

An investigation of the structural, magnetic and electronic properties of≈3 nm thick Mn₅Ge₃ films epitaxially grown on a Ge(111)-c(2✕8) reconstructed surface is reported. High resolution transmission electron microscopy and selected area electron diffraction give evidence of 2.2% in-plane compressive strain between the Mn₅Ge₃ film and the Ge substrate. Magneto optical Kerr effect measurements show that the films are ferromagnetic with a Curie temperature of ≈325 K. The analysis of Ge 3d core level photoelectron spectra of the Mn5Ge3 films allows determining an upper limit of 76 meV for the Ge 3d5/2 core-hole lifetime broadening. The Ge 3d3/2 core-hole lifetime broadening is found to be 15 meV larger than that of the Ge 3d5/2 core hole, because of the existence of a Coster–Kronig decay channel due to the metallic character of Mn₅Ge₃.

See: P. De Padova, et al., Phys. Rev. B 77, 045203 (2008).

Molecular Beam Epitaxy “MBE”