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Projects

In addition to teaching, research is one of the most important priorities here in our department, which is often rewarded with remarkable results. You can find an overview of all publications and projects in the research report.

enlarge the image: Detailaufnahme eines technischen Messgeräts
Messungen im Physiklabor, Foto: Christian Hüller

Funding

DFG

VolkswagenStiftung

Sachsen

EU/BMBF

The Danish Council for Strategic Research

IAEA

Funding still unknown

High resolution focused ions with MeV kinetic energies (LIPSION lab)

The LIPSION lab runs a 3 MV single ended accelerator and is equipped a micro beam system and a RBS and Channeling stage. Especially the micro beam system is worldwide unique and allows to focused the MeV beam down to sub micrometer lateral resolution with sufficient ion current to run nuclear analytics techniques like proton X-Ray emission (PIXE), Rutherford back scattering spectroscopy (RBS) or nuclear reaction analytics (NRA). This system are applied in a large number of material science fields and leads to the discovery of ferromagnetic behaviour in graphite by the group of Prof. Butz (Daniel Spemann together with Prof. Esquinazis group) or the investigation of several biological samples.

Project Data

Proton induced X-ray emission (PIXE) allows the qualitative detection and identification of elements inside a matrix on ppm level or below. The advantage is that this analysis is destruction free and based on physical effects, chemical properties play a secondarily role and can be neglected. Cole related to PIXE is energy dispersive X-ray spectroscopy (EDX) using an electron beam. The advantage of PIXE compare to EDX is the very low background by three orders of magnitude allows the detection limit of ppm compare to ppt for EDX. However, the main drawback of PIXE is the requirement of a MeV proton beam delivers by an expensive accelerator. Therefore it is necessary to optimize the detection systems. In the last decade new X-ray detectors are developed that allow an increase of the signal detection by a factor of 20. This allows to reduce the measurement time from hours to a few minutes and make PIXE to a competitive analytical tools compare to secondary ion mass spectroscopy (SIMS) or the widely used inductively coupled plasma mass spectrometry (ICP-MS) techniques.
First aim of the new detection system is to measure iron in brain slices to investigate the nature of Parkinson's disease.

Project Data

We will install a new ion source provides a beam with highly charged ions by a electron cyclotron resonance ion source (ECRIS) at LIPSION. This type of source produces a large number of different ions with very high current, e.g. He2+ with 50 µA or Ar8+ with 5 µA. This allows to enlarged the range of ion species and the energy range of the system, e.g. Ar8+ can be accelerate to 24 MeV. Additional we will use the system to investigate the interaction of fast highly charged ions with materials.

Project data

The focused proton beam can be used to create defects with very high lateral resolution. This can be used to investigate radiation hardness of high power devices or detectors but also to create, e.g., buried wires into diamond. Crystal defects are produced mainly by a collision of the proton with the nucleus of the crystal atom. The chance for this collision is high at low energy. Diamant will be convert into graphite if the number of vacancies produced by this collision exceeds 1021 cm2. The figure shows an example of a buried coil in diamond. By changing the energy or using of stopping foils a three dimensional structuring of diamond became possible. This technique allows also to expose thick Polymethylmethacrylat (PMMA) with very high aspect ratio, since the penetration depth of MeV protons in PMMA reach several hundred µm. This technique is known as proton beam writing (PBW).

High resolution focused ions with low kinetic energies (Leibniz Joint Lab)

Ion beam techniques like ion implantation are used for, e.g., complementary metal-oxide-semiconductor (CMOS) fabrication processes in semiconductor technology and since decades constitute the standard tool to introduce impurity atoms into host materials. Using collimated or focused ion beams, ion implantation allows addressing single atoms inside a given solid with nm precision and is, therefore, ideally suited for the fabrication of future quantum devices. Indeed, ion implantation has been used, e.g., to fabricate a quantum register with nitogen-vacancy (NV) centre or 31P spin qubits in Si. Furthermore, ion beams are powerful tools for materials characterisation and modification, e.g., for ultra-sensitive trace element analysis or diamond-to-carbon conversion. However, due to the statistical nature of ion beams, the highest probability to obtain exactly one ion in an “one-shot” implantation is only 37%. As a consequence, ordered arrays of hundreds or thousands of atoms, essential for the fabrication of a solid-state based quantum computer, are impossible to fabricate with this technology. Therefore, efforts have been made to develop a deterministic ion implantation, i.e. techniques to count each individual ion that is implanted.
Deterministic single atom implantation relies on counting each ion before, or during, or in its implantation event. However, up to now, none of the approaches has gained sufficient maturity to allow fabrication of quantum devices beyond the few-atoms scale so that further technical development is required to exploit the potential of deterministic ion implantation in the field of quantum technologies. A second key requirement is a spatial precision of implantation sites of the order of a few nm. Implantation through nm-scaled holes or masks, but also steering laser-cooled ions from a Paul trap, have been able to reach a spatial resolutions below 10 nm and further progress is expected.
In summary, ion beam techniques are essential tools for the fabrication of quantum devices at all.

Project data

Our so called nanobeam implanter uses a combination of low energy ion source and an atomic force microscope (AFM) with a pierced hollow tip as a nano aperture. The goal is to implant single ions with a resolution below 10 nm.
A new method to make single ions visible on a sensitive resist film in cooperation with the group of I. Rangelow, Institute of Microelectronics and Nanoelectronics at TU Ilmenau. The resist is very sensitive to ions or electrons. First implantations were done through a hole in an AFM tip of about 70 nm. Without developing the resist, we found a sensitivity of about 1000 ions/spot which can be resolved in subsequent scanning with the AFM system (Figure (a)). The next step is to develop the resist and go to spots which were implanted with lower fluences until we will resolve single ion spots. Furthermore, tests will be with molecule ions in cooperation with the University of Melbourne. We also continued implanting single N ions in photonic crystals in cooperation with the group of C. Becher, Quantum Optics Group at Saarland University. A few years ago some nitrogen-vacancy (NV) centers were successfully found in the cavity at the centre of the photonic crystals. To have more success in the formation of NV centres which couple to the cavity modes, we implanted N ions with the help of the AFM non contact mode into a cavity. The initial AFM scan without contact reduces the risk of damaging the AFM tip and the closing of the hole in the tip due to dirt on the sample surface (Figure (b)). Subsequent confocal imaging showed fluorescence of one or more NV centres which are located in the middle of the cavities. In 2016 we also worked on finding standard conditions to produce very small apertures in Si3N4 masks. Therefore, we have thin membranes which are Au coated. To have reproducible apertures, it is necessary to use very low ion beam current. The Au coating allows the discharging of the sample as well as the focusing with very low ion beam current due to the typical island structure. We found the best results for an ion beam current of Iion = 1 pA, an acceleration voltage of UB = 30 kV, and an irradiation time of t = 20 s for a hole about 50 nm or smaller in diameter. The masks are successfully used for creating single photon sources due to ion implantation at the University of Melbourne within the Centre of Excellence for Quantum Computation and Communication Technology.

Project data

For the creation of NV centres in diamond and other important applications, ion beams with a very small diameter are needed. This is commonly done with microprobe devices. A less expensive alternative to this is nano collimation which can be applied to several applications. Especially for applications with very low ion currents and single ion implantation this method is useful. Another possible application is the focused ion beam (FIB) technique where nowadays liquid metal ion sources are used. There a liquid metal gets ionized due to an electron spray on a very sharp tip. The advantage of this source is that it emits ions on a very small area which can be demagnified down to a few nanometres but the disadvantage is that it works just for a few chemical elements which have their melting point in a useable region. This disadvantage can be overcome for low current applications with nano collimators which are mounted behind the ion source. So there is still a small point where the ions are emitted but it is possible to use different types of ion sources, so nearly every element can be ionized. We used two different ways to create such nano apertures for different scopes. One way is the ion track etching technique. There muscovite sheets were prepared at the GSI Darmstadt with high energetic Sm ions that penetrated the sheet and left a ion track of amorphous material. Subsequent etching with hydrofluoric acid removed this material and created diamond shaped pores. The shape of this pores origins from the crystal structure of the muscovite where are O planes that have the slowest etching rate and thus this pores have a very well defined shape and orientation, also the cross section of the pores is nearly identical. The pore size depends on the etching time and can be controlled in a certain rage. We got for one hour etching time that the two diagonals had the average length of 123 nm and 231 nm. Furthermore, the mean distance of the pores is controlled by the number of ions which penetrate the sheet because every ion leaves one track. So, this can be controlled by the irradiation time. The advantage of this method is that one can archive very high aspect ratios up to 1000. This leads to nano apertures with a thickness sufficient to stop ions in the low MeV range. But the disadvantage is that the pores are randomly distributed over the irradiated area of the sheet and further collimation is needed to select a single pore. The second method is FIB milling. There thin membranes are irradiated with a well focused beam of Ga ions. This removes material due to sputtering and thus structures can be milled in the surface (Fig.). One big advantage is that therefore arbitrary shapes can be written in the material and used as collimators. Furthermore, the only limitation on the collimator material is that one needs to obtain a certain sputter yield. But a limitation of this method is that for small deep pores the material that is sputtered away from the bottom of the pore is deposited at the side walls and thus the aspect ratio is limited. Meaning that for pores in the range of a few ten nanometres, the material needs to have a thickness in the order of a few hundred nanometres and thus the energy of the particles that should be collimated is limited to the lower keV range.

Publications

Project data

The nanometre-scale engineering of single nitrogen−vacancy (NV) centres in diamond can be obtained by low-energy (keV) N implantation with limited straggling. In this energy range, the ion straggling is typically in the order of a few nanometres or less. This is necessary for application in quantum information processing based on coupled NV centres in diamond which require NV−NV distance to be in the range (10 − 30) nm. A high level of precision in the formation of the NV defect by ion implantation is needed. The necessary use of keV energies implies however that the NV centres are produced close to the surface, a few nm deep. For such shallow NV centres, there are issues concerning the charge state stability and the spin coherence properties of the negatively charged NV-. As a consequence, shallow NV centres generally have inferior overall properties than deeply implanted or deep native NV centres, due to the surface proximity. In this work, we aim to overgrow a thin layer of diamond on top of the sample in order to bury the NV centres and improve their overall properties. We have continued our search for optimised overgrowth conditions (in collaboration with the LSPM laboratory) in which the shallow NV centres survive after the overgrowth and have improved properties. It has already been shown that the spin coherence time of shallow NVs is improved by overgrowth of a thin diamond layer. This demonstration is an encouraging result, however the influence of the overgrowth on the optical properties and charge state of the centres has not been studied in this reference, and neither the survival rate of the NV centres in optimised conditions, we have shown that a pattern of very shallow NV centres (2 nm deep) could be successfully buried and found again after a few µm thick CVD overgrowth. Furthermore, we could demonstrate the stabilisation of the charge state of the NV centres into the negative charge state, which is the one of interest for most of NV-based application. The coherence properties are currently under measurement and a new sample is under preparation.

Project Data

In the Leibniz Joint Lab “Single Ion Implantation” (a cooperation between the Leibniz Institute of Surface Engineering (IOM) and the Universität Leipzig) we have continued building the new set-up for high precision ion implantation. A commercial focused ion beam system (ionLINE made by Raith) was equipped with an electron beam ion source (EBIS made by DREEBIT). This enables a great variety of elements to be ionised to high charge states and accelerated. Furthermore, all structural changes are now incorporated, including the section for the image charge detector to be installed. It is planned to achieve a beam spot resolution in the nanometre range and to implant deterministically by counting single ions with the principle of image charge detection. Simultaneously, a separate new set-up has been realised to be able to measure image charge signals from a large number of moving charges. For the deterministic ion implanter, sensitivity of the detector down to a single elementary charge is necessary. Before this can be realised, theoretical and practical limitations of the signal amplification and analysis process need to be evaluated. Bunches of singly charged ions appear as objects with several thousand charges. Their image charge signal can be recorded through state of the art room temperature pre-amplifier technology (Fig.). The ion gun ionises gas (e.g. Ar) with electrons and accelerates with a potential difference of (1 − 5) kV. Scanning the beam over the aperture with the beam blanker, bunches of ions are sent through the image charge detector (ICD) pick-up tubes. The number of ions per bunch can be calibrated by measuring the continuous beam current in the Faraday cup. Preliminary results verify that the detection of the image charge signal is effectively a non-destructive measurement of the time of flight and the number of ions per bunch.

Publications

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