Halbleiterphysik - Research Profile

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Our research topics represent an exciting range of fundamental and applied challenges in semiconductor physics, from deep questions in light-matter interaction to photonic devices and integrated circuits using novel electronic materials. The main streams of our research are sketched below. Please also consult our scientific reports and journal publications for more details!

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	Transparent Electronics Using wide gap electronic materials we can create transparent electronic circuits for the use in smart windows or for invisible devices. Our UV-harvesting transparent solar cells are an invisible source of electrical power. We investigate n-type amorphous semiconductors such as zinc-tin-oxide or zinc-oxynitrides for enhancement of the electron mobility. Also various p-type materials (hole conductors) are fabricated, among them ZnCo₂O₄, NiO and CuI. The amorphous layers are used also for flexible electronic circuits. From these materials we fabricate Schottky and pn-diodes with record high rectifications and the related transistors, MESFETs and JFETs, respectively. Key publications C. Yang, D. Souchay, M. Kneiß, M. Bogner, H. M. Wei, M. Lorenz, O. Oeckler, G. Benstetter, Y.Q. Fu, M. Grundmann Transparent Flexible Thermoelectric Material Based on Non-toxic Earth-Abundant p-Type Copper Iodide Thin Film Nature Commun. 8, 16076 (2017) \| doi C. Yang, M. Kneiß, M. Lorenz, M. Grundmann Room-temperature Synthesized Copper Iodide Thin Film as Degenerate p-Type Transparent Conducting Material with a Boosted Figure of Merit PNAS 113, 12929-12933 (2016) \| doi R. Karsthof, P. Räcke, Z. Zhang, H. von Wenckstern, M. Grundmann Semi-transparent n-ZnO/p-NiO UV solar cells phys. stat. sol. (a) 213, 30 (2016) \| doi H. Frenzel, A. Lajn, M. Grundmann One decade of fully transparent oxide thin film transistors: fabrication, performance and stability phys. stat. sol. RRL 7, 605 (2013) \| doi

	Ultra-wide Gap Semiconductors We investigate gallium oxide, monoclinic β-Ga₂O₃, a material with very large band gap of about 5 eV, and related compounds alloyed with In and Al for the use in high power electronics and as solar-blind deep UV photodetectors. Also inter-subband devices are researched for this material and its heterostructures. Interesting polymorphs like InGaO₃-II or hexagonal ε-Ga₂O₃ are stabilized using epitaxial techniques. The dielectric function of color dispersive materials, such as monoclinic Ga₂O₃, is analyzed in terms of dipoles (phonons and interband transitions). Key publications S. Müller, H. von Wenckstern, F. Schmidt, D. Splith, H. Frenzel, M. Grundmann Method of choice for fabrication of high-quality β-gallium oxide-based Schottky diodes Semic. Sci. Technol. 32, 065013 (2017) \| doi C. Sturm, R. Schmidt-Grund, C. Kranert, J. Furthmüller, F. Bechstedt, M. Grundmann Dipole analysis of the dielectric function of color dispersive materials: Application to monoclinic Ga₂O₃ Phys. Rev. B 94, 035148 (2016) \| doi C. Kranert, M. Jenderka, J. Lenzner, M. Lorenz, H. von Wenckstern, R. Schmidt-Grund, M. Grundmann Lattice parameters and Raman-active phonon modes of β-(Al_xGa_1-x)₂O₃ J. Appl. Phys. 117, 125703 (2015) \| doi R. Schewski, G. Wagner, M. Baldini, D. Gogova, Z. Galazka, T. Schulz, T. Remmele, T. Markurt, H. von Wenckstern, M. Grundmann, O. Bierwagen, P. Vogt, M. Albrecht Epitaxial stabilization of pseudomorphic α-Ga₂O₃ on sapphire (0001) Appl. Phys. Expr. 8, 011101 (2015) \| doi C. Kranert, J. Lenzner, M. Jenderka, M. Lorenz, H. von Wenckstern, R. Schmidt-Grund, M. Grundmann Lattice parameters and Raman-active phonon modes of (In_xGa_1-x)₂O₃ for x < 0.4 J. Appl. Phys. 116, 013505 (2014) \| doi

	Microcavities and Polariton Condensates We fabricate highly reflective dielectric Bragg mirrors for various design wavelengths in the visible and ultraviolet spectral range. In conjunction with cavity materials (ZnO) we investigate microcavities in the strong coupling regime, in particular the condensation of exciton-polaritons at room temperature into Bose-Einstein-like condensates. The polariton dispersion, scattering mechanisms and the influence of disorder on the quantum liquid are subjects of investigation. Also the Purcell enhancement of nanostructures (NV centers in diamond, CuI-based organic molecules, C-dots, ...) is studied. Key publications T. Michalsky, M. Wille, M. Grundmann, R. Schmidt-Grund Spatiotemporal evolution of coherent polariton modes in ZnO microwire cavities Nano Letters XX, XXXXXX (2018) \| doi M. Thunert, A. Janot, H. Franke, C. Sturm, T. Michalsky, M.D. Martín, L. Viña, B. Rosenow, M. Grundmann, R. Schmidt-Grund Cavity Polariton Condensate in a Disordered Environment Phys. Rev. B 93, 064203 (2016) \| doi S. Richter, T. Michalsky, L. Fricke, C. Sturm, H. Franke, M. Grundmann, R. Schmidt-Grund Maxwell consideration of polaritonic quasi-particle Hamiltonians in multi-level systems Appl. Phys. Lett. 107, 231104 (2015) \| doi H. Franke, C. Sturm, R. Schmidt-Grund, G. Wagner, M. Grundmann Ballistic propagation of exciton-polariton condensates in a ZnO-based microcavity New J. Phys. 14, 013037 (2012) \| doi

	Anisotropic Optical Bulk Materials In biaxial materials each of the optic axes splits into two circularly polarized singular axes when absorption or other dissipation processes set in and cause imaginary parts in the dielectric tensor. We investigate this for all possible crystal structures showing this effect (orthorhombic, monoclinic and triclinic) and for modern functional materials (Ga₂O₃, KTP, ...). Also the Raman scattering in optically anisotropic materials and structures is studied. Key publications M. Grundmann, C. Sturm, C. Kranert, S. Richter, R. Schmidt-Grund, C. Deparis, J. Zúñiga-Pérez Optically Anisotropic Media: New Approaches to the Dielectric Function, Singular Axes, Raman Scattering Intensities and Microcavity Modes phys. stat. sol. RRL 11, 1600295 (2017) \| doi C. Sturm, M. Grundmann The Singular Optical Axes in Biaxial Crystals and Analysis of Their Spectral Dispersion Effects in β-Ga₂O₃ Phys. Rev. A 93, 053839 (2016) \| doi C. Kranert, C. Sturm, R. Schmidt-Grund, M. Grundmann Raman Tensor Formalism for Optically Anisotropic Crystals Phys. Rev. Lett. 116, 127401 (2016) \| doi

	Singular Optical Modes in Metamaterials A similar anisotropy effect as singular optic axes in bulk material has been predicted by us for stratified metamaterials. It is investigated experimentally by us for anisotropic microcavities where the mirror axis does not lie parallel to the optic axis of the uniaxial cavity material (ZnO, GaN, ...). The occurrence of generally elliptically polarized modes and singular axes with circularly polarized exceptional points represents a unique feature of such anisotropic metamaterials. Key publications S. Richter, T. Michalsky, C. Sturm, B. Rosenow, M. Grundmann, R. Schmidt-Grund Exceptional points in anisotropic planar microcavities Phys. Rev. A 95, 023836 (2017) \| doi M. Grundmann, C. Sturm, C. Kranert, S. Richter, R. Schmidt-Grund, C. Deparis, J. Zúñiga-Pérez Optically Anisotropic Media: New Approaches to the Dielectric Function, Singular Axes, Raman Scattering Intensities and Microcavity Modes phys. stat. sol. RRL 11, 1600295 (2017) \| doi

	Multiferroic Heterostructures Materials that show both ferroelectric and magnetic response are called multiferroic, and when the two properties are coupled, they are magnetoelectric. Such materials are currently highly interesting due to their potential applications as sensors and memory devices. Epitaxial thin-film structures show a much stronger magnetoelectric coupling than multiferroic bulk materials. Our two-dimensional superlattice composites show the highest reported magnetoelectric voltage coefficients. However, the detailed understanding of the coupling mechanism and of the temperature and DC magnetic field dependence of the magnetoelectric coupling are subjects of current research. Key publications M. Lorenz, J. Barzola-Quiquia, C. Yang, C. Patzig, T. Höche, P. Esquinazi, M. Grundmann, H. Wei Charge transfer-induced magnetic exchange bias and electron localization in (111)- and (001)-oriented LaNiO₃/LaMnO₃ superlattices Appl. Phys. Lett. 110, 102403 (2017) \| doi H. Wei, M. Grundmann, M. Lorenz Confinement-driven metal-insulator transition and polarity-controlled conductivity of epitaxial LaNiO₃/LaAlO₃ (111) superlattices Appl. Phys. Lett. 109, 082108 (2016) \| doi

	Complex Nanostructures With a focus on nanowires and microwires we investigate the growth and optical properties of various nanostructures with the purpose to create miniaturized laser sources. Laser emission on whispering gallery modes has been observed for a variety of materials (ZnO, CuI, ...). The gain mechanisms and the mode properties are investigated with time- and angular-resolved optical spectroscopy. Also the effects of mechanical strain are researched for bent, stretched and twisted wires. Lateral mode confinement is investigated for concentric Bragg mirrors wrapped around the wires. Key publications A. Shkurmanov, C. Sturm, H. Franke, J. Lenzner, M. Grundmann Low temperature PLD-growth of ultrathin ZnO nanowires by using Zn_xAl_1-xO and Zn_xGa_1-xO seed layers Nanoscale Res. Lett. 12, 134 (2017) \| doi C. Sturm, M. Wille, J. Lenzner, S. Khujanov, M. Grundmann Non-linear optical deformation potentials in uniaxially strained ZnO microwires Appl. Phys. Lett. 110, 062103 (2017) \| doi M. Wille, T. Michalsky, E. Krüger, M. Grundmann, R. Schmidt-Grund Absorptive lasing mode suppression in ZnO nano- and microcavities Appl. Phys. Lett. 109, 061102 (2016) \| doi C.P. Dietrich, M. Grundmann Pulsed-laser deposition growth of ZnO NWs Wide Band Gap Semiconductor Nanowires: Low-Dimensionality Effects and Growth, Vincent Consonni, Guy Feuillet eds., p. 303-323 (2014) (Wiley-ISTE, 2014) \| doi

	UV Photodetectors Band gap engineering of (Mg,Zn)O and (Al,Ga,In)₂O₃ thin films is used to fabricate visible-blind and solar-blind photo detectors, respectively, with tailored absorption edge in the spectral range from 3.3 eV to 6 eV. In an advanced device design we integrate optical filter layers in a way that the spectral response band of these detectors is narrow, below 10 nm, enabling wavelength-selective UV and deep-UV photo detector arrays without the need of a dispersive element. Key publications Zh. Zhang, H. von Wenckstern, J. Lenzner, M. Grundmann Wavelength-selective ultraviolet (Mg,Zn)O photodiodes: Tuning of parallel composition gradients with oxygen pressure Appl. Phys. Lett. 108, 243503 (2016) \| doi Zh. Zhang, H. von Wenckstern, J. Lenzner, M. Lorenz, M. Grundmann Visible-blind and solar-blind ultraviolet photodiodes based on (In_xGa_1-x)₂O₃ Appl. Phys. Lett. 108, 123503 (2016) \| doi Zh. Zhang, H. von Wenckstern, M. Schmidt, M. Grundmann Wavelength selective metal-semiconductor-metal photodetectors based on (Mg,Zn)O-heterostructures Appl. Phys. Lett. 99, 083502 (2011)\| doi

	Integrated Circuits On the basis of the novel electronic materials for which a stable technology has been established, we fabricate and investigate integrated circuits (inverters, NAND, NOR, ring oscillators, ...) to test device properties and performance in a more complex environment and for the creation of communication and sensing devices. Key publications S. Vogt, H. von Wenckstern, M. Grundmann MESFETs and inverters based on amorphous zinc-tin-oxide thin films prepared at room temperature Appl. Phys. Lett. 113, 133501 (2018) \| doi F.J. Klüpfel, H. von Wenckstern, M. Grundmann Ring Oscillators based on ZnO Channel JFETs and MESFETs Adv. Electron. Mater. 2, 1500431 (2016) \| doi F.J. Klüpfel, A. Holtz, F.-L. Schein, H. von Wenckstern, M. Grundmann All-Oxide Inverters Based On ZnO channel JFETs with amorphous ZnCo₂O₄ gates IEEE Transact. Electr. Dev. 62, 4004 (2015) \| doi

Experimental setups and methods are available for the fabrication of thin films and nanostructures (pulsed laser deposition, sputter deposition, plasma-enhanced chemical vapor deposition, thermal evaporation, carbo-thermal evaporation), and the detailed characterization of structural, electrical and optical material properties, among them high-resolution X-ray diffraction, scanning force microscopy, transmission electron microscopy, IV- and CV-spectroscopy, admittance spectroscopy, DLTS, optical DLTS, Hall effect, optical transmission and reflection, (micro-) Raman spectroscopy, (micro-) photoluminescence, time-resolved PL, excitation spectroscopy, Fourier transform IR spectroscopy, scanning low temperature cathodoluminescence, photocurrent microscopy, generalized spectroscopic ellipsometry from the mid-infrared to the deep UV. Photonic and electronic devices are processed using photolithography, wet and dry (ICP-RIE) etching, and thermal and sputter deposition of metals. Device properties can be characterized with several set-ups, incl. on-wafer testing up to 30 GHz, the fA-regime and helium temperatures.

Funding is provided through several cooperative projects and numerous individual grants. Most of our research is within the university's research profile area "Complex Matter".

Image credits: Ch. Yang, Zh. Zhang, J. Lenzner, S. Richter, M. Lorenz, all Universität Leipzig.

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