16 research outputs found
Fe in III-V and II-VI semiconductors
No full textMany theoretical and experimental studies deal with the realization of room-temperature ferromagnetism in dilute magnetic semiconductors (DMS). However, a detailed quantitative understanding of the electronic properties of transition metal doped semiconductors has often been neglected. This article points out which issues concerning electronic states and charge transfers need to be considered using Fe as an example. Methods to address these issues are outlined, and a wealth of data on the electronic properties of Fe doped III-V and II-VI compound semiconductors that have been obtained over a few decades is reviewed thoroughly. The review is complemented by new results on the effective-mass-like state consisting of a hole bound to Fe2+ forming a shallow acceptor state. The positions of established Fe3+/2+ and Fe2+/1+ charge transfer levels are summarized and predictions on the positions of further charge transfer levels are made based on the internal reference rule. The Fe3+/4+ level has not been identified unambiguously in any of the studied materials. Detailed term schemes of the observed charge states in tetrahedral and trigonal crystal field symmetry are presented including hyperfine structure, isotope effects and Jahn-Teller effect. Particularly, the radiative transitions Fe3+(4T 1 → 1A1) and Fe2+(5E → 5T2) are analyzed in great detail. An effective-mass-like state [Fe2+, h] consisting of a hole bound to Fe2+ is of great significance for a potential realization of spin-coupling in a DMS. New insights on this shallow acceptor state could be obtained by means of stress dependent and temperature dependent absorption experiments in the mK range. The binding energy and effective Bohr radius were determined for GaN, GaP, InP and GaAs and a weak exchange interaction between the hole and the Fe2+ center was detected. With regard to the Fe 3+ ground state, 6A1, in GaP and InP, the hyperfine structure level Γ8 was found to be above the Γ7 level. All results are discussed with respect to a potential realization of a ferromagnetic spin-coupling in DMSs. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA
Fe-Centers in GaN as Candidates for Spintronics Applications
No full textAbstractThe potential use of Fe doped GaN for spintronics applications requires a complete understanding of the electronic structure of Fe in all of its charge states. To address these issues, a set of 500 µm thick freestanding HVPE grown GaN:Fe crystals with different Fe-concentration levels ranging from 5×1017 to 2×1020 was studied by means of photoluminescence, photoluminescence excitation (PLE) and Fourier transform infrared (FTIR) transmission experiments. The Fe3+/2+ charge transfer (CT) level was determined to be at 2.86 ± 0.01 eV above the valence band maximum considerably lower than the previously reported value of 3.17 ± 0.10 eV. A bound state of the form (Fe2+, hVB) with a binding energy of 50 ± 10 meV has been established as an excited state of Fe3+. FTIR transmission measurements revealed an internal (5E-5T2) transition of Fe2+ around 400 eV which, until now, was believed to be degenerate with the conduction band. Consequently, a second CT band was detected in PLE spectra.</jats:p
Development of ultra-thin tunneling oxides and Si/SiO<sub>2</sub> nanostructures for the application in silicon solar cells
No full textFe-Centers in GaN as Candidates for Spintronics Applications
No full textABSTRACT The potential use of Fe doped GaN for spintronics applications requires a complete understanding of the electronic structure of Fe in all of its charge states. To address these issues, a set of 400 µm thick freestanding HVPE grown GaN:Fe crystals with different Fe-concentration levels ranging from 5×1
Mn charge states in GaMnN as a function of Mn concentration and co-doping
No full textAbstractIn the context of the pursuit of a dilute magnetic semiconductor for spintronic applications, a set of GaMnN samples with varying Mn concentration and Si or Mg co-doping was investigated by optical and electron spin resonance spectroscopy. The results clearly demonstrate how the charge state of Mn is changed between 2+, 3+ and 4+ by Mg and Si co-doping. For p-type GaMnN we show that the introduction of the Mn3+/4+ donor can be compensated by Mg co-doping lowering the Fermi energy below the Mn3+/4+ level. While our results are in agreement with the hypothesis that the infrared photoluminescence appearing in GaMnN upon Mg doping originates from Mn4+, an unambiguous proof is still to be presented. Under this assumption, our measurements show that the Mn4+ center must be excited via an extra-center process at 2.54 eV.</jats:p
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Fast scatterometric measurement of periodic surface structures plasma-etching processes
No full textTo satisfy the continuous demand of ever smaller feature sizes, plasma
etching technologies in microelectronics processing enable the fabrication of
device structures with dimensions in the nanometer range. In a typical plasma
etching system a plasma phase of a selected etching gas is activated, thereby
generating highly energetic and reactive gas species which ultimately etch
the substrate surface. Such dry etching processes are highly complex and
require careful adjustment of many process parameters to meet the high
technology requirements on the structure geometry. In this context, real-time
access of the structures dimensions during the actual plasma process would be
of great benefit by providing full dimension control and film integrity in
real-time. In this paper, we evaluate the feasibility of reconstructing the
etched dimensions with nanometer precision from reflectivity spectra of the
etched surface, which are measured in real-time throughout the entire etch
process. We develop and test a novel and fast reconstruction algorithm, using
experimental reflection spectra taken about every second during the etch
process of a periodic 2D model structure etched into a silicon substrate.
Unfortunately, the numerical simulation of the reflectivity by Maxwell
solvers is time consuming since it requires separate time-harmonic
computations for each wavelength of the spectrum. To reduce the computing
time, we propose that a library of spectra should be generated before the
etching process. Each spectrum should correspond to a vector of geometry
parameters s.t. the vector components scan the possible range of parameter
values for the geometrical dimensions. We demonstrate that by replacing the
numerically simulated spectra in the reconstruction algorithm by spectra
interpolated from the library, it is possible to compute the geometry
parameters in times less than a second. Finally, to also reduce memory size
and computing time for the library, we reduce the scanning of the parameter
values to a sparse grid
Fast scatterometric measurement of periodic surface structures in plasma-etching processes
No full textTo satisfy the continuous demand of ever smaller feature sizes, plasma etching technologies in microelectronics processing enable the fabrication of device structures with dimensions in the nanometer range. In a typical plasma etching system a plasma phase of a selected etching gas is activated, thereby generating highly energetic and reactive gas species which ultimately etch the substrate surface. Such dry etching processes are highly complex and require careful adjustment of many process parameters to meet the high technology requirements on the structure geometry. In this context, real-time access of the structure's dimensions during the actual plasma process would be of great benefit by providing full dimension control and film integrity in real-time. In this paper, we evaluate the feasibility of reconstructing the etched dimensions with nanometer precision from reflectivity spectra of the etched surface, which are measured in real-time throughout the entire etch process. We develop and test a novel and fast reconstruction algorithm, using experimental reflection spectra taken about every second during the etch process of a periodic 2D model structure etched into a silicon substrate. Unfortunately, the numerical simulation of the reflectivity by Maxwell solvers is time consuming since it requires separate time-harmonic computations for each wavelength of the spectrum. To reduce the computing time, we propose that a library of spectra should be generated before the etching process. Each spectrum should correspond to a vector of geometry parameters s.t. the vector components scan the possible range of parameter values for the geometrical dimensions. We demonstrate that by replacing the numerically simulated spectra in the reconstruction algorithm by spectra interpolated from the library, it is possible to compute the geometry parameters in times less than a second. Finally, to also reduce memory size and computing time for the library, we reduce the scanning of the parameter values to a sparse grid
Mn- and Fe- doped GaN for spintronic applications
No full textAbstractIn the context of ferromagnetic spin-coupling in dilute magnetic semiconductors, we present optical investigations on Mg co-doped GaMnN and Fe doped GaN. A complex luminescence feature occurring in Mg co-doped GaMnN around 1 eV was previously attributed to the internal 4T2(F)—4T1(F) transition of Mn4+ involved in different complexes. Selective excitation studies indicate the presence of at least three different complexes. Photoluminescence excitation spectra suggest that the internal Mn3+ transition may represent an excitation mechanism. Magneto photoluminescence spectra indicate equal g values for the ground and excited state. Low temperature infrared absorption spectra of Fe doped GaN allow to unambiguously establish the electronic structure of the Fe2+ center in GaN. Our results suggest that the Fe2+(5T2) state is stabilized against Jahn-Teller coupling by the reduced site-symmetry of the hexagonal lattice.</jats:p
Fast Scatterometric Measurement of Periodic Surface Structures in Plasma-Etching Processes
No full textTo satisfy the continuous demand of ever smaller feature sizes, plasma etching technologies in microelectronics enable the fabrication of device structures in the nanometer range. To control these processes, real-time access to the structure’s dimensions is needed. We develop a special method of optical critical dimension metrology and evaluate the feasibility of reconstructing the etched dimensions from experimental reflectivity spectra of the surface, taken about every second. For a periodic 2D model structure etched into a silicon, we develop and test a fast algorithm. To reduce the computing time, we generate a library of spectra before the etching. We demonstrate that, by replacing the numerically simulated spectra in the reconstruction algorithm by spectra interpolated from the library, it is possible to compute the geometry parameters in times less than a second. Finally, to also reduce memory size and computing time for the library, we reduce the scanning of the parameter values to a sparse grid
