FG Optische Charakterisierung von Halbleitern

26 Items

Recent Submissions
A 310 nm optically pumped AlGaN vertical-cavity surface-emitting laser

Hjort, Filip ; Enslin, Johannes ; Cobet, Munise ; Bergmann, Michael A. ; Gustavsson, Johan ; Kolbe, Tim ; Knauer, Arne ; Nippert, Felix ; Häusler, Ines ; Wagner, Markus R. ; Wernicke, Tim ; Kneissl, Michael ; Haglund, Åsa (2020-12-17)

Ultraviolet light is essential for disinfection, fluorescence excitation, curing, and medical treatment. An ultraviolet light source with the small footprint and excellent optical characteristics of vertical-cavity surface-emitting lasers (VCSELs) may enable new applications in all these areas. Until now, there have only been a few demonstrations of ultraviolet-emitting VCSELs, mainly optically...

Einfluss der Lichtextraktion auf die Effizienz AlGaN-basierter Leuchtdioden im tiefen ultravioletten Spektralbereich

Guttmann, Martin (2020)

In dieser Arbeit wird die Lichtextraktion AlGaN-basierter Leuchtdioden (LEDs), welche im tiefen ultravioletten (UV) Spektralbereich emittieren, untersucht. Mithilfe von Raytracing-Simulationen soll die Lichtextraktionseffizienz (LEE) von UV-LEDs bestimmt und die gemessene externe Quanteneffizienz (EQE) bezüglich der internen Quanteneffizienz (IQE) und LEE quantifiziert werden. Im ersten Teil de...

Twofold gain enhancement by elongation of QDs in polarization preserving QD-SOAs

Greif, Ludwig Albrecht Thorsten ; Mittelstädt, Alexander ; Jagsch, Stefan Thomas ; Schliwa, Andrei (2019-06-06)

The impact of quantum dot (QD) elongation on key parameters of QD-based semiconductor optical amplifiers (SOAs) is investigated using a combination of 8-band k·p-theory including strain and piezoelectricity up to second order and a rate equation model describing the population of QD ground, excited and wetting layer states. By considering columnar QDs of selected aspect ratios, we show that chi...

Tuning the Emission Directionality of Stacked Quantum Dots

Greif, Ludwig Albrecht Thorsten ; Jagsch, Stefan Thomas ; Wagner, Markus R. ; Schliwa, Andrei (2018-11-02)

The emission directionality of stacks of coupled quantum dots (QDs) is investigated within the framework of 8-band k·p-theory including strain and strain-induced piezoelectricity up to second order. Using an artificial cuboidal QD, we show that the degree of radiation anisotropy can be tuned from −33% to nearly +60% via the structure’s vertical aspect ratio. We then demonstrate that these findi...

Thermal conductivity and air-mediated losses in periodic porous silicon membranes at high temperatures

Graczykowski, Bartlomiej ; Sachat, Alexandros el ; Reparaz, Juan Sebastián ; Sledzinska, Marianna ; Wagner, Markus R. ; Chávez-Ángel, Emigdio ; Wu, Y. ; Volz, Sebastian ; Alzina, Francesc ; Sotomayor Torres, Clivia M. (2017-09-04)

Heat conduction in silicon can be effectively engineered by means of sub-micrometre porous thin free-standing membranes. Tunable thermal properties make these structures good candidates for integrated heat management units such as waste heat recovery, rectification or efficient heat dissipation. However, possible applications require detailed thermal characterisation at high temperatures which,...

Electronic excitations stabilized by a degenerate electron gas in semiconductors

Nenstiel, Christian ; Callsen, Gordon ; Nippert, Felix ; Kure, Thomas ; Schlichting, Sarah ; Jankowski, Nadja ; Hoffmann, M. P. ; Dadgar, Armin ; Fritze, S. ; Krost, A. ; Wagner, Markus R. ; Hoffmann, Axel ; Bechstedt, Friedhelm (2018-07-26)

Excitons in semiconductors and insulators consist of fermionic subsystems, electrons and holes, whose attractive interaction facilitates bound quasiparticles with quasi-bosonic character. In the presence of a degenerate electron gas, such excitons dissociate due to free carrier screening. Despite their absence, we found pronounced emission traces in the below-band-edge region of bulk, germanium...

Suppression of the quantum-confined Stark effect in polar nitride heterostructures

Schlichting, Sarah ; Hönig, Gerald Martin Otto ; Müßener, Jan ; Hille, Pascal ; Grieb, Tim ; Westerkamp, Steffen ; Teubert, Jörg ; Schörmann, Jörg ; Wagner, Markus R. ; Rosenauer, Andreas ; Eickhoff, Martin ; Hoffmann, Axel ; Callsen, Gordon (2018-08-23)

Recently, we suggested an unconventional approach (the so-called Internal-Field-Guarded-Active-Region Design “IFGARD”) for the elimination of the quantum-confined Stark effect in polar semiconductor heterostructures. The IFGARD-based suppression of the Stark redshift on the order of electronvolt and spatial charge carrier separation is independent of the specific polar semiconductor material or...

Size-dependent recombination dynamics in ZnO nanowires

Reparaz, Juan Sebastián ; Güell, Frank ; Wagner, Markus R. ; Hoffmann, Axel ; Cornet, Albert ; Morante, Joan Ramon (2010-02-01)

A deep understanding of the recombination dynamics of ZnO nanowires (NWs) is a natural step for a precise design of on-demand nanostructures based on this material system. In this work we investigate the influence of finite-size on the recombination dynamics of the neutral bound exciton around 3.365 eV for ZnO NWs with different diameters. We demonstrate that the lifetime of this excitonic tran...

Reduction of the transverse effective charge of optical phonons in ZnO under pressure

Reparaz, Juan Sebastián ; Muniz, Luis Antonio ; Wagner, Markus R. ; Goñi, Alejandro R. ; Alonso, Maria Isabel ; Hoffmann, Axel ; Meyer, Bruno K. (2010-06-09)

From Raman scattering on a-plane wurtzite ZnO crystals we obtained a decreasing splitting between longitudinal and transversal optical phonons with A1 and E1 symmetry as a function of hydrostatic pressure up to 5.5 GPa. Consequently, the transverse effective charge (e∗T) exhibits a strong reduction with increasing pressure, yielding 2.17–14.6×10−3 P/GPa and 2.04–13.7×10−3 P/GPa (in units of the...

Recombination dynamics in ZnO nanowires: Surfaces states versus mode quality factor

Reparaz, Juan Sebastián ; Güell, Frank ; Wagner, Markus R. ; Callsen, Gordon ; Kirste, Ronny ; Claramunt, Sergi ; Morante, Juan Ramon ; Hoffmann, Axel (2010-10-01)

In this work, we investigate the influence of finite size on the recombinations dynamics of ZnO nanowires. We demonstrate that diameter as well as length of nanowires determine the lifetime of the neutral donor bound excitons. Our findings suggest that while the length is mainly responsible for different mode quality factors of the cavity-like nanowires, the diameter determines the influence of...

Polariton effects in the dielectric function of ZnO excitons obtained by ellipsometry

Cobet, Munise ; Cobet, Christoph ; Wagner, Markus R. ; Esser, Norbert ; Thomsen, Christian ; Hoffmann, Axel (2010-01-19)

The complex dielectric tensor of ZnO in the regime of the excitonic transitions is determined with ellipsometry and analyzed concerning the quantization of the electromagnetic field in terms of coupled polariton-eigenmodes. Negative sections in the real part indicate the significant formation of polaritons for the dipole-allowed excitons of the three upper valence-bands Γ7,Γ9,Γ7. The transverse...

Lithium related deep and shallow acceptors in Li-doped ZnO nanocrystals

Rauch, Christian ; Gehlhoff, Wolfgang ; Wagner, Markus R. ; Malguth, Enno ; Callsen, Gordon ; Kirste, Ronny ; Salameh, Belal ; Hoffmann, Axel ; Polarz, Sebastian ; Aksu, Ylmaz ; Drieß, Matthias (2010-01-27)

We study the existence of Li-related shallow and deep acceptor levels in Li-doped ZnO nanocrystals using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. ZnO nanocrystals with adjustable Li concentrations between 0% and 12% have been prepared using organometallic precursors and show a significant lowering of the Fermi energy upon doping. The deep Li acceptor with a...

Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN

Kirste, Ronny ; Ramón, Collazo ; Callsen, Gordon ; Wagner, Markus R. ; Kure, Thomas ; Reparaz, Juan Sebastián ; Mita, Seji ; Xie, Jinqiao ; Rice, Anthony ; Tweedie, James ; Sitar, Zlatko ; Hoffmann, Axel (2011-11-02)

We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB)...

Phonon deformation potentials in wurtzite GaN and ZnO determined by uniaxial pressure dependent Raman measurements

Callsen, Gordon ; Reparaz, Juan Sebastián ; Wagner, Markus R. ; Kirste, Ronny ; Nenstiel, Christian ; Hoffmann, Axel ; Phillips, Matthew R. (2011-02-09)

We report the phonon deformation potentials of wurtzite GaN and ZnO for all zone center optical phonon modes determined by Raman measurements as a function of uniaxial pressure. Despite all the structural and optical similarities between these two material systems, the pressure dependency of their vibrational spectra exhibits fundamental distinctions, which is attributed to their different bond...

Decay dynamics of excitonic polarons in InAs/GaAs quantum dots

Werner, Stefan ; Reparaz, Juan Sebastián ; Wagner, Markus R. ; Zimmer, Patrick ; Ledentsov, Nikolai N. ; Kabuß, Julia ; Dachner, M. R. ; Richter, Marten ; Knorr, Andreas ; Thomsen, Christian ; Hoffmann, Axel (2011-10-03)

We present time-resolved studies of the exciton-phonon interaction in self-assembled InAs/GaAs quantum dots. Different scattering and luminescence processes were investigated by time-resolved spectroscopy exciting resonantly into the quantum dot’s electronic structure. By studying the characteristic decay times of the ground state and of several phonon-assisted recombinations we were able to di...

Phonon plasmon interaction in ternary group-III-nitrides

Kirste, Ronny ; Mohn, Stefan ; Wagner, Markus R. ; Reparaz, Juan S. ; Hoffmann, Axel (2012-07-26)

Phonon-plasmon-coupling in the ternary group-III-nitrides InGaN and AlGaN is investigated experimentally and theoretically. Based on the observation of broadening and shifting of the A1(LO) mode in AlGaN upon Si-doping, a lineshape analysis was performed to determine the carrier concentration. The results obtained by this method are in excellent agreement to those from Hall measurements, confir...

Structural and optical investigation of non-polar (1-100) GaN grown by the ammonothermal method

Gogova, Daniela ; Petrov, P. P. ; Bügler, Max ; Wagner, Markus R. ; Nenstiel, Christian ; Callsen, Gordon ; Schmidbauer, Martin ; Kucharski, R. ; Zajac, M. ; Dwilinski, R. ; Phillips, M. R. ; Hoffmann, Axel ; Fornari, Roberto (2013-05-29)

We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 × 104 cm−2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray ...

Spatial mapping of exciton lifetimes in single ZnO nanowires

Reparaz, Juan Sebastian ; Callsen, Gordon ; Wagner, Markus R. ; Güell, F. ; Morante, Joan Ramon ; Sotomayor Torres, Clivia M. ; Hoffmann, Axel (2013-06-06)

We investigate the spatial dependence of the exciton lifetimes in single ZnO nanowires. We have found that the free exciton and bound exciton lifetimes exhibit a maximum at the center of nanowires, while they decrease by 30% towards the tips. This dependence is explained by considering the cavity-like properties of the nanowires in combination with the Purcell effect. We show that the lifetime ...

Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements

Kirste, Ronny ; Hoffmann, Marc P. ; Tweedie, James ; Bryan, Zachary ; Callsen, Gordon ; Kure, Thomas ; Nenstiel, Christian ; Wagner, Markus R. ; Collazo, Ramón ; Hoffmann, Axel ; Sitar, Zlatko (2013-03-08)

Compensation effects in metal organic chemical vapour deposition grown GaN doped with magnesium are investigated with Raman spectroscopy and photoluminescence measurements. Examining the strain sensitive E2(high) mode, an increasing compressive strain is revealed for samples with Mg-concentrations lower than 7 × 1018 cm−3. For higher Mg-concentrations, this strain is monotonically reduced. This...

Triple group-V donors in ZnO

Hegde, Manu ; Mohammadbeigi, F. ; Kure, Thomas ; Senthil Kumar, Eswaran ; Wagner, Markus R. ; Hoffmann, Axel ; Watkins, Simon P. (2020-02-18)

Triple donors have been explored in a few semiconductor materials; however, the conventional effective mass theory treatment fails at short length scales due to the high degree of localization implied by a 3+ nuclear charge. Using density functional theory, we consider the various charge states of group-V elements substituting for the Zn sublattice in ZnO under oxygen-rich conditions. For the c...