Field of Research

Major Research Areas

III-nitride based materials (i.e. GaN, AlN, InN, and their ternaries) are currently one of the most important families of semiconducting materials for light emitting diodes and laser diodes operating at the wavelengths ranging from deep ultraviolet to the infra-red region. Furthermore, these materials are considered for high frequency and high power transistors. Good stability of GaN-based materials under high voltages, high temperatures and cosmic radiation makes them attractive for space applications.

This work is carried out at Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik

• Predominant formation of a-plane (Al,Ga)N layers on patterned c-plane sapphire using HVPE
• Analysis of defect behaviour in highly doped (Al,Ga)N layers
• Defect structure and compositional homogeneity of (Al,Ga)N layers on laterally overgrown AlN/sapphire templates
• Compositional analysis of self-organisation effects in AlGaAs-based DFB laser diodes
• Structure and temperature stability of InGaN quantum wells and InGaN barriers in blue laser diodes grown by MOVPE
• Defect and strain reduction in III-N-based heterostructures on SiC substrates
• Structure of AlN/AlGaN superlattices grown on high-temperature AlN layers on sapphire by metalorganic vapour phase epitaxy

see AlGaN_poster

Cooperation with the group of Prof. Saskia F. Fischer Novel Materials

This research field includes structural analysis of a variety of nanostructures, such as:

• Ag nanowires
• exfoliated thin Bi₂Se₃ flakes
• ferecrystals - a new type of layered intergrowth compounds
• Bi₂Te₃ nanowires
• porous Si nanowires obtained by electro-less etching

In particular, Bi₂Te₃ and Bi₂Se₃ as topological insulators have attracted a lot of attention in the past few years. Topological insulators are materials which are insulating, when they appear as bulk crystals, but have conducting states at their surfaces. Experimentally discovered just a few years ago topological insulators evoked great scientific interest, since their novel properties may make them attractive for spintronics and quantum computation.

This work has been carried out in cooperation with Chemnitz University of Technology (TU Chemnitz), Chair of Surface and Interface Physics . The work was financially supported by DPG (German Physical Society)

It has been shown, that substitution of Si by Al or Ga results in a change of the lattice parameter of bulk NiSi₂. This reveals the possibility to fabricate nearly defect-free unstrained epitaxial layers. Typically, the up-to-date Si-based microelectronic devices are realised in 2D-planar technology. The 3D-integration is hindered, because it is not possible to fabricate a defect-free epitaxial Si layer over the transistor structure. Thus, the possibility to obtain thin defect-free epitaxial silicide layers with zero mismatch is challenging for both technological and fundamental research.

Our study clarified the structure of ternary epitaxial NiSi_2-xAl_x and NiSi_2-xGa_x layers on (001)Si as a function of Al resp. Ga content and formation temperature using energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS). Additionaly, the influence of substitutional elements (Al or Ga) on the lattice parameters of thin films, strain relaxation through the defect formation and influence of substitutional elements on the layer morphology and orientation was analysed by conventional and high-resolution transmission electron microscopy (HRTEM).

For results see posters:

• NiAlGaSi_poster1.pdf
• NiAlGaSi_poster2.pdf

 

Former activities

and topics studied in the past

Since there is no commercially available and cheap large-scale GaN substrates for the homoepitaxy, fabrication of GaN-based optoelectronic and electronic devices requires deposition of thin III-nitride layers onto foreign substrates. This initiated a search for novel substrates for the III-nitride epitaxy. Recently, γ-LiAlO₂ has been proposed and tested as a novel substrate for the growth of GaN layers. Compared to conventional Al₂O₃ (sapphire) substrates commonly used for GaN epitaxy, the distinct advantages of γ-LiAlO₂ substrates are the possibility to grow GaN in two different orientations (called c- and m-plane) as well as self-separation of thick GaN films from the γ-LiAlO₂ substrates. Furthermore, the GaN/γ-LiAlO₂ system shows a significantly smaller lattice mismatch compared to conventional substrates.

The following topics were analysed within this project:

• Structural and compositional analysis of inclusions in bulk γ-LiAlO₂ single crystals
• ELNES evidence of electron beam induced oxygen desorption in γ-LiAlO₂
• Characterization of defect structure and polarity of c-plane oriented GaN layers grown by HVPE on (100) γ-LiAlO₂
• Defect analysis of m-plane GaN layers grown on (100) γ-LiAlO₂

This work was carried out at Humboldt-University of Berlin, Institute of Physics, Chair of Crystallography in cooperation with Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik and Leibniz-Institut für Kristallzüchtung

Posters:

• lialo2_GaN_poster
• c_plane_GaN_poster
• m_plane_GaN_poster

• Growth and structure of semiconducting Higher Manganese Silicides (HMS) layers (as PhD thesis)
• Surfactant (Sb) mediated growth of silicides on Si
• Characterisation of semiconducting CrSi₂ layers
• Growth of metallic nickel silicides NiSi and NiSi₂
• Solid phase reaction in Ni/C/Si(001), Ni/Ti/Si(001), Ti/Ni/C/Si(001) as well as Co/C/Si(001) and Ti/Co/Si(001) systems
• Growth of monosilicide MnSi layers on Si(111)

These topics were studied at Technical University of Chemnitz, Chair of Surface and Interface Physics