Mn DOPING OF GaN LAYERS GROWN BY MOVPE

In this contribution we present a growth of Ga1-xMnxN layers by MOVPE. Mn doped GaN layers were grown with and without undoped GaN templates on (0001) sapphire substrates in a quartz horizontal reactor. For the deposition of Ga1-xMnxN layers (MCp)2Mn was used as a Mn – precursor. The flow of the Mn precursor was 0.2-3.2 μmol.min-1. The deposition of Ga1-xMnxN layers was carried out under the pressure of 200 mbar, the temperature 1050 °C and the V/III ratio of 1360. For the growth of high quality GaN:Mn layers it was necessary to grow these layers on a minimally partially coalesced layer of pure GaN. The direct deposition of GaN:Mn layer on the low temperature GaN buffer layer led to a three-dimensional growth during the whole deposition process. Another investigated parameter was the influence of nitrogen on the layer’s properties. A nearly constant ferromagnetic moment persisting up to room temperature was observed on the synthesized thin films

Quantum transport in topological surface states of Bi2_2Te3_3 nanoribbons

Quasi-1D nanowires of topological insulators are emerging candidate structures in superconductor hybrid architectures for the realization of Majorana fermion based quantum computation schemes. It is however technically difficult to both fabricate as well as identify the 1D limit of topological insulator nanowires. Here, we investigated selectively-grown Bi$_2$Te$_3$ topological insulator nanoribbons and nano Hall bars at cryogenic temperatures for their topological properties. The Hall bars are defined in deep-etched Si$_3$N$_4$/SiO$_2$ nano-trenches on a silicon (111) substrate followed by a selective area growth process via molecular beam epitaxy. The selective area growth is beneficial to the device quality, as no subsequent fabrication needs to be performed to shape the nanoribbons. Transmission line measurements are performed to evaluate contact resistances of Ti/Au contacts applied as well as the specific resistance of the Bi$_2$Te$_3$ binary topological insulator. In the diffusive transport regime of these unintentionally $n$-doped Bi$_2$Te$_3$ topological insulator nano Hall bars, we identify distinguishable electron trajectories by analyzing angle-dependent universal conductance fluctuation spectra. When the sample is tilted from a perpendicular to a parallel magnetic field orientation, these high frequent universal conductance fluctuations merge with low frequent Aharonov-Bohm type oscillations originating from the topologically protected surface states encircling the nanoribbon cross section. For 500 nm wide Hall bars we also identify low frequent Shubnikov-de Haas oscillations in the perpendicular field orientation, that reveal a topological high-mobility 2D transport channel, partially decoupled from the bulk of the material.Comment: 13 pages, 5 figures, 6 pages supplementary information, 5 supplementary figure

Preparation and optimization of low-temperature grown GaAs photomixers

The aim of this work was to design and prepare photomixer devices based on LT GaAs and to optimize them with respect to the maximal output power. Essential part of the photomixer is the MSM photodetector structure. For this reason a major part of the optimization process was done on photodetector structures. One set of our photomixers was optimized for the 460 GHz output frequency employing single dipole antenna. The second set of photomixers used broadband bow-tie and spiral antennas designed for the frequency range up to 1.6 THz and 3.7 THz, respectively. The influence of the growth temperature of GaAs on its desired properties (high resistivity, high electric breakdown field, low carrier lifetime, etc.) is presented in Chapter 2. It is well know that with increasing the growth temperature the carrier lifetime in this material increases as well. This is a major parameter for limitation of the maximal output frequency of a photomixer device, that is not RC-constant limited. The second important factor for optimization of the maximal photomixer output power is the mobility of photogenerated carriers. It is a well established fact that with increasing the growth temperature the mobility also increases in GaAs material. The output power of a photomixer device depends on both above mentioned parameters which are in trade-off. Thus, the growth temperature is one of the most important parameters contributing to the output power increase of our photomixers. Photomixers designed for 460 GHz require partially higher growth temperature of LT GaAs, 275-300°C, as shown in Chapter 6. Ion implantation is another possibility how to decrease carrier lifetime in GaAs. In this work we present results showing that employing this technique fabrication of materials with subpicosecond carrier lifetime is possible. Materials implanted with various ion doses (1012-1016 ions/cm2) and implantation energies in the range 82 keV-880 keV were prepared and their properties and dynamics before and after annealing were studied. Photodetectors based on 880 keV nitrogen-implanted GaAs show 50% higher sensitivity than our best LT GaAs photodetectors. These results outline new possibilities for increasing of photomixer output power because with right choice of implantation energy and dose it is possible to prepare materials with subpicosecond carrier lifetime and with higher mobility of photogenerated carriers than in previously reported materials. Next improvement, necessary for photomixer devices, is that MSM contact could be also fabricated not only on the surface of photoconductive material, but also in the material. Recessed contacts exhibit higher breakdown voltage, which depends on the depth of recession. In this work we also observed that photodetectors with recessed contacts are more sensitive in comparison to MSM fabricated on the surface. Recessed contacts fabricated by wet etching and IBE are presented in Chapter 3. In both cases we observed sensitivities from 40% to 200% higher in dependence on the recession depth than for non-recessed structures. Very important fact is that recessed contacts could for optimal recession depth more efficiently collect photogenerated carriers from deeper regions and so not only increase total number of photogenerated carriers, but also in shorter time, because of decreasing of distance to the collecting electrodes. This was also observed in this work. Our first results from the photomixers with broadbanding antenna show in frequency range from 100 GHz to 1 THz approximately 2 times higher output power than photomixers with non-recessed contacts. In the future combination of here presented improvements will be used to reach next improvement of electrical properties of photodetector and photomixer devices

COMPONENT HAVING OPTICALLY ACTIVE MATERIALS

The invention relates to a component, comprising at least one memory area containing optically active material, a control arrangement with at least one control signal for changing the optical properties of the optically active material, and means for detecting the change in the optical properties of the optically active material, and comprising an evaluation input region which has at least one evaluation input signal and an evaluation output region which has an evaluation output signal, wherein the memory area is arranged between the evaluation input region and evaluation output region and the control arrangement adjoins the memory region. The invention also relates to a method for processing and/or storing information, in which method the component according to the invention is used

Emerging technologies for future low energy consumption nano-opto-electronics

During the last decades rapid progress in the development of nanometer sized and mesoscopic devices and their fabrication brought about new insights into applied physics resulting in a number of new applications for physics, electronics, medicine, optics or even main stream electronics. Low dimensional nanometer sized objects exhibit unique material properties, which allow the fabrication of novel devices, such as highly sensitive sensors, ultra-fast transistors or optoelectronic devices. Conventionally used approaches for nano-device fabrication have already reached their limits and further improvements regarding low energy consumption could be achieved only by alternative device concepts and novel system architectures. The next generation of such devices, primarily needed for highly secure and high information rate communication systems as well as spectroscopic applications should also take the potential for mass-production and environmental compatibility into account. Our current achievements are focused on so-called “Emerging Technologies” which do not represent a steady development of the existing techniques but rather constitute a huge advancement step forward. The three novel approaches we will introduce in this contribution are based on group III nitride light emitting diode structures as the primary excitation sources. In the first approach, mesoscopic InGaN structures are used to tune the emission in the telecom band [1]. In the second, a hybrid approach will be introduced in which electrically driven nano-LEDs are utilized as the primary excitation sources to directly optically pump emission from the nanoparticles as the secondary sources – a concept suitable for mass production of few photon sources [2,3]. At last a new concept for a future mask-less lithography based on nano-LED arrays is presented [4,5]. [1] M. Mikulics, A. Winden, M. Marso, A. Moonshiram, H. Lüth, D. Grützmacher, and H. Hardtdegen, to be submitted (2016).[2] M. Mikulics, Y. C. Arango, A. Winden, R. Adam, A. Hardtdegen, D. Grützmacher, E. Plinski, D. Gregušová, J. Novák, P. Kordoš, A. Moonshiram, M. Marso, Z. Sofer, H. Lüth, and H. Hardtdegen, ‘Direct electro-optical pumping for hybrid CdSe nanocrystal/III-nitride based nano-light-emitting diodes’, Appl. Phys. Lett., vol. 108, no. 6, p. 061107, Feb. 2016, DOI: 10.1063/1.4941923.[3] M. Mikulics and H. Hardtdegen, ‘Single Photon Source Suitable for Mass Production and Production Method’, patent specification WO2014094705 A1, 2013.[4] M. Mikulics and H. Hardtdegen, ‘Nano-LED array fabrication suitable for future single photon lithography’, Nanotechnology, vol. 26, no. 18, p. 185302, 2015, DOI: 10.1088/0957-4484/26/18/185302.[5] M. Mikulics and H. Hardtdegen, ‘Method for Optical Transmission of a Structure into a Recording Medium’, patent specification DE20121016178 20120816, 2012

Towards III-nitride nano-LED based single photon emitters: technology and applications

Single photon emitters based on InGaN nano-LEDs (light emitting diodes) have the potential to operate at room temperature. They are therefore the key to enabling future low energy consumption, highly secure and ultrafast optoelectronics. Different technological concepts for emitting sources were developed and will be presented together with their realization. It will be discussed how their wavelength can be tuned for future applications