DEPARTMENT OF MATERIAL PHYSICS

The aim of the project is to develop new materials and material combinations with semiconducting properties and for use as conductive buffer layers for semiconductor components. This requires a systematic investigation of the material properties by means of X-ray diffraction, ellipsometry, Raman scattering, photoluminescence investigations and electrical measurements. To this end, the sputter epitaxy process is being further developed. This process makes many new material combinations possible in the first place and enables the investigation of a wide range of materials without extreme costs. This development is partly carried out in conjunction with the established growth process of metal-organic vapor phase epitaxy for the first demonstrators.
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The aim is to develop new materials and material combinations with semiconducting properties and for use as buffer layers for semiconductor components. This requires a systematic investigation of the material properties using X-ray diffraction, ellipsometry, Raman scattering, photoluminescence investigations and electrical measurements. To this end, the process of sputter epitaxy is being further developed, which is still in its infancy worldwide except in Japan.
This process makes many new material combinations possible in the first place and enables the investigation of a wide range of materials without extreme costs. This development is partly carried out in conjunction with the established growth process of organometallic vapor phase epitaxy for the first demonstrators.
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Anionic amidinate ligands of the type [RC(NR')₂]^- have become indispensable tools in the coordination chemistry of virtually every metallic element in the Periodic Table. They enable the synthesis of new homogeneous catalysts as well as the design of volatile orecusors for ALD and CVD processes in materials science (e.g. phase-change and semiconductor materials). The main goal of the current research project is the investigation of alkynylamidinate ligands in the coordination chemistry of transition metals.

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Major goal of this project is the synthesis and full characterization (IR, Raman, NMR, elemental analysis, X-ray structure determination) of polysulfide anions and their metal complexes.

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Goal of this project is to identify specific TM-group-III-N layers with epitaxial quality for a potential application in group-III-nitride electronics. For this we will first study the properties of pure and alloyed group-IIIb-, -IVb-, and -Vb-nitrides (Cr, V, Ti, Sc, Nb, Zr, Ta, Hf) with AlN and in some cases also with GaN. This will result in a database of material parameters, namely crystal structure, lattice parameter, electrical and optical properties for a wide range of compositions.
Their potential should be then evaluated within the framework of thin films applied as active layers, i.e. for polarization optimization in HEMTs, novel HEMT structures as, for example, GaN/ScN/GaN binary high mobility electron channels or as thicker films for an application as highly conductive buffer layer and electrically conducting strain engineering layers, enabling true vertical electronic devices on Si substrates. For the latter pure TMN alloys or TMN alloys with AlN are the most promising candidates, while for active layers, apart from binary TMN layers, also alloys with GaN are interesting.

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Binary metal oxides and their alloys (In,Ga,Al)₂O₃ are among the materials with the greatest adjustability of physical properties. They include insulators, semiconductors and conductors, they are used in magnetic and ferroelectric layers and thus allow the development of a new generation of electronic components. The production of oxide structures with the highest material quality and the understanding of the fundamental physical properties are of fundamental importance for the development of application-oriented technologies. This is the subject of the Leibniz ScienceCampus Growth and fundamentals of oxides for electronic applications - GraFOx . The focus of the work in the Materials Physics department is on the determination of the dielectric function from the mid-infrared to the vacuum-ultraviolet spectral range (also using synchrotron radiation), the determination of fundamental band structure properties and the analysis of multi-particle effects in highly doped transparent conductive oxides (TCOs).
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In addition to UV transparency, materials for the encapsulation of UV light-emitting diodes must also have a defined and reproducibly adjustable refractive index in order to be of technological interest. In this project, various candidate materials for the encapsulation of nitride UV light-emitting diodes will be characterized using spectroscopic ellipsometry. The refractive index and absorption coefficient of the materials are determined.
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To further enhance GaN power electronics and enable new device
structures as well as device designs we will investigate transition
metal based nitrides and their alloys as well as their alloys with AlN
and GaN. This with the goal to enable true vertical electronics on low
cost silicon substrates and lateral enhancement-mode high electron
mobility transistors (HEMT) allowing for higher current densities and,
therefore more compact device size. In addition we will apply a new
growth method, pulsed sputtering epitaxy, which is capable of growing
high quality GaN layers at temperatures below 800 °C and thus offers
a huge potential for Si CMOS integration of GaN electronics. To
identify new materials suited to achieve conducting buffer layers for
subsequent GaN epitaxy as well as to achieve new or better
functionalities of group-III-N based devices we will investigate
transition metal (TM) nitrides also alloyed with AlN and GaN for their
potential in group-III-nitride electronic applications. For this we will first
study the properties of pure and alloyed group-IIIb -IVb and -Vbnitrides
(Cr, V, Ti, Sc, Nb, Zr, Ta, Hf) with AlN and in some cases also
with GaN. Our goal is a database on crystal structure, lattice
parameter, electrical and optical properties for a wide range of
compositions. In detail the potential will then be investigated for thin
films for applications as active layer in electronic devices, e.g. for
polarization optimization in HEMTs, novel HEMT structures with, e.g.
binary, highly conducting GaN/ScN/GaN channels, as thicker highly
conducting film, or as electrically conducting strain engineering layer,
enabling true vertical electronic devices on Si substrates. For the
latter pure TMN alloys or TMN alloys with AlN are the most promising
candidates, while for active layers, apart from binary TMN layers, also
alloys with GaN are interesting. Based on the properties of TMNs
known to date, we expect that fully vertical devices on Si as well as
better HEMT devices are achievable which will primarily result in a
further increase in power density of GaN based devices.

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The aim of the project is the experimental determination of fundamental parameters and the band structure of modern semiconductor materials. The main focus is on gallium nitride (GaN), both in the wurtzite and the zinc-blende modification, indium oxide (In₂O₃), but also other nitrides and oxides.

In addition to the band gap, the most important parameters of any semiconductor are the effective masses of electrons and holes. Surprisingly, these have so far only been known with great imprecision. [The aim of the parameter project is to determine these and other material parameters as comprehensively as possible. In addition to a precise characterization of the investigated systems, the development of methods is a central component of the work. The techniques used should be universally applicable and, in principle, transferable to a wide variety of material systems.
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Polysulfide anions and their metal complexes are synthesized and fundamentally characterized. Raman spectroscopy, infrared spectroscopy, NMR, elemental analysis and X-ray diffraction are used to elucidate the structure.
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Binary metal oxides and their alloys (In,Ga,Al)₂O₃ are among the materials with the greatest adjustability of physical properties. They include insulators, semiconductors and conductors, they are used in magnetic and ferroelectric layers and thus allow the development of a new generation of electronic components. The production of oxide structures with the highest material quality and the understanding of the fundamental physical properties are of fundamental importance for the development of application-oriented technologies. This is the subject of the Leibniz ScienceCampus Growth and fundamentals of oxides for electronic applications - GraFOx . The focus of the work in the Materials Physics department is on the determination of the dielectric function from the mid-infrared to the vacuum-ultraviolet spectral range (also using synchrotron radiation), the determination of fundamental band structure properties and the analysis of multi-particle effects in highly doped transparent conductive oxides (TCOs).
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The technologically, scientifically and commercially extremely important II-nitride semiconductor family allows the production of p-doped, undoped (semi-insulating) up to highly n-doped material. The optical properties are strongly dependent on the doping. In this project, fundamental relationships between doping and linear optical response are systematically investigated. This starts with the effective electron mass, includes the phonon-plasmon coupling and extends to the dependence of the absorption edge on the interacting mechanisms of renormalization and band filling.
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The high-energy semiconductor AlN (E_g ~ 6 eV) has recently undergone enormous quality improvements, which is mainly due to the availability of AlN single crystals. In this project, a comparison of photoluminescence and spectroscopic ellipsometry is used to assign emission bands. The aim is to unambiguously identify defect luminescence contributions chemically in order to further improve sample quality and advance the understanding of the semiconductor material. To this end, homoepitaxial thin films produced under different growth conditions and in different laboratories are being investigated.
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A particularly promising new type of transistor (AlInN HEMT), for example, allows higher current densities than conventional structures. The operational safety of this type of transistor has not yet been sufficiently investigated and will be analyzed by comparing transistor characteristics with thermal imaging methods. Raman spectroscopy will be used in particular.
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Indiumnitrid (InN) besitzt unter allen Nitridhalbleitern die kleinste Bandlücke. Erst seit wenigen Jahren ist bekannt, dass diese nicht im sichtbaren sondern im infraroten Spektralbereich liegt. Dies eröffnet neue Anwendungsfelder der Nitride. Ein Konsortium aus 14 europäischen Universitäten und Forschungseinrichtungen hat es sich zum Ziel gestellt, zum einen die Herstellung  von InN und seinen Legierungen mit GaN und AlN deutlich zu verbessern und zum anderen grundlegende elektronische und optische Eigenschaften des Materialsystems zu untersuchen. Auf dieser Basis sollen erste Demonstratoren neuartiger Bauelemente realisiert werden.

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The conduction band density of states of III-nitrides is to be investigated by means of synchrotron-based photoluminescence excitation spectroscopy. This will take advantage of the fact that the semi-core states of the metal ions have very low dispersion and can therefore be used as the starting state for excitation into the conduction band. Four whole days at the synchrotron are available for measurements as part of this project.
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Mit Synchrotronstrahlung werden Photolumineszenzanregungsspektren von ternären AlGaN Schichten aufgenommen. Dabei werden Aluminiumkonzentrationen zwischen 20% und 100% untersucht. Hochauflösende Tieftemperaturspektren erlauben Einclick in die Valenzbandstruktur der Materialien. Für Messungen stehen 4 ganze Tage am Synchrotron zur Verfügung.

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