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We investigate transport measurements on all II-VI semiconductor resonant tunneling diodes (RTDs). Being very versatile, the dilute magnetic semiconductor (DMS) system (Zn,Be,Mn,Cd)Se is a perfect testbed for various spintronic device designs, as it allows for separate control of electrical and magnetic properties. In contrast to the ferromagnetic semiconductor (Ga,Mn)As, doping ZnSe with Mn impurities does not alter the electrical properties of the semiconductor, as the magnetic dopant is isoelectric in the ZnSe host.
In the context of the ongoing discussion about a carrier-induced ferromagnetic phase transition in diluted-magnetic II-VI semiconductors (DMS), theoretical studies on coherent dynamics of localized spins coupled with a two-dimensional hole gas (2DHG) in DMS quantum wells (QWs) were done by K.V. KAVOKIN. His key for studying the exchange interaction of the localized spin ensemble (e.g. Mn2+) with the 2DHG is the Larmor frequency of the localized Mn-ion spins and thus their Mn-g-factor. It was shown that the 2DHG affects a time evolution of the (Mn-) spin system in an in-plane magnetic field resulting in the reduction of its Larmor frequency (Mn-g-factor) under the influence of an oscillating effective field of holes. This is called magnetic soft mode (behaviour). The experimental access for demonstrating this Mn-g-factor reduction with increasing hole concentration is the method of Multi-Spin-Flip (SF) Raman scattering combined with the variation of the carrier concentration by photo-excitation with an additional light source (two-colour experiment). The main motivation for this thesis was the experimental confirmation of the theoretically predicted magnetic soft mode and the analysis of its dependence on the hole-concentration and external B-field, as well as its disappearance with increasing sample temperature. For that purpose, CdMnTe/CdMgTe QWs (Mn: 0.6%, 1.0%) positioned close to the sample surface (13−19nm) were investigated in an in-plane applied external magnetic field (up to 4.5T in Voigt-geometry) via a two-colour experiment i.e. using two light sources. This allows the spin excitation of Mn-ions by simultaneously tuning the hole-concentration towards the ferromagnetic phase transition by photo-generated carriers. Thus, one tuneable laser is responsible for resonant below-barrier excitation as a probe for Multi-SF Raman scattering. The other laser excites photo-generated carriers from above barrier (2.41eV) for tuning the hole concentration in the QW. Positioning the QW close to the sample surface causes a surface-induced p-doping of the QW (intrinsic hole concentration in the QW) and enables the active tuning of the hole concentration by photo-generated carriers due to different tunnelling behaviour of electrons and holes from the QW to the surface. The Mn-g-factor was decreased by quasi-continuously increasing the above-barrier illumination (and thus the hole concentration), while the below-barrier excitation (Multi-PR probe) was kept at a constant low power. This results in a Mn-g-factor reduction starting from its atomic value g=2.01 to lowest evaluated Mn-g-factor in this thesis g=1.77. This is a magnetic softening of 12%. Apart from the general magnetic soft mode behaviour at low temperatures, one of the main experimental results in this thesis is the confirmation of the theoretical prediction that the magnetic soft mode behaviour in the external B-field does not only depend on the carrier concentration but also on the B-field strength itself. An additional aspect is the temperature dependence of the magnetic soft mode. The Mn-g-factor decrease is suppressed with increasing temperature almost reaching the atomic Mn-g-factor at 4.2K (g=1.99). This behaviour is due to the T-induced weakening of the transverse 2DHG spin susceptibility. The results of the investigations concerning the cap layer thickness impact on the QW carrier characteristics were investigated in the cap thickness range of 13nm to 19nm. The cap thickness configures on the one hand the intrinsic hole concentration of the QW ("2DHG offset") due to the surface-induced p-doping and sets the "starting point" for the Mn-g-factor reduction. On the other hand the cap thickness determines the probability of electron tunnelling to the surface and thus the efficiency of the hole tuning by light. The latter is the criterion for the range of Mn-g-factor reduction by light. This two dependences were pointed out by the photo-generated hole influence on the QW PL-spectra which results in tuning the exciton-trion ratio. In summary both mechanisms are of relevance for the hole tuning and thus for the magnetic soft-mode behaviour. The mechanism of tunnelling time prevails at small cap layer thicknesses while the surface-induced p-doping plays the major role for larger cap thicknesses (> 25nm). In conclusion, the presented method in this thesis is a sensitive tool to study the dynamics of the spin excitations and the paramagnetic susceptibility in the vicinity of the hole-induced ferromagnetic phase transition.
ZnO-based semiconductors were studied by Raman spectroscopy and complementary methods (e.g. XRD, EPS) with focus on semimagnetic alloying with transition metal ions, doping (especially p-type doping with nitrogen as acceptor), and nanostructures (especially wet-chemically synthesized nanoparticles).
In pursuit of a novel generation of devices, exploration of spin properties of the particles is needed. Spintronics is a modern field in physics which exploits spin properties to be used in addition to the charge degree of freedom. Since the conductivity mismatch problem presents a fundamental obstacle for electrical spin injection from a ferromagnetic metal into a diffusive semiconductor [SFM+00], other means for injecting spin-polarized carriers must be used. With a tunnel contact, it is possible to achieve a highly spin-polarized room-temperature tunnel injection [JWS+05]. We used a novel approach and applied magnetic RTDs for spin manipulation. In this work, properties of all-II-VI magnetic resonant tunneling diodes (RTDs), as applied to spintronics, were reported. Growth conditions were optimized to increase the peak-to-valley ratio, and the design of the RTDs was optimized for observation of spin related transport effects. When an external magnetic field was applied, spin manipulation became possible. Selforganized CdSe quantum structures were grown and investigated using optical means. After embedding them into a (Zn,Be)Se tunneling barrier, the properties were assessed by the resonant tunneling.
In this work we investigate magnetic resonant tunneling diode (RTD) structures for spin manipulation. All-II-VI semiconductor RTD structures based on [Zn,Be]Se are grown by molecular beam epitaxy. We observe a strong, magnetic field induced, splitting of the resonance peaks in the I-V characteristics of RTDs with [Zn,Mn]Se diluted magnetic semiconductors (DMS) quantum well. The splitting saturates at high fields and has strong temperature dependence. A phonon replica of the resonance is also observed and has similar behaviour to the peak. We develop a model based on the giant Zeeman splitting of the spin levels in the DMS quantum well in order to explain the magnetic field induced behaviour of the resonance.