Master- & Bachelorarbeiten
Contact: Prof. Dr. E. Lavrov
1.Hydrogen in delafossite CuAlO2
Description: Most transparent conductive oxides (TCOs) exhibit only n-type conductivity, which has largely confined their use to transparent electrodes and coating films. To fully exploit the potential of TCOs - particularly for emerging optoelectronic applications - the development of reliable p-type TCOs is essential.
In this context, the discovery of p-type conductivity in CuAlO2 generated considerable interest, as it identified the family of so-called delafossite compounds as promising candidates for active device applications.
Hydrogen is a ubiquitous impurity in oxides and can strongly influence their electrical and optical properties. At sufficiently high concentrations, hydrogen incorporation in CuAlO2 can significantly modify both its electrical behavior and optical response. However, the properties of hydrogen in CuAlO2 remain largely terra incognita, as its behavior is still poorly understood.
Tasks: The student will investigate hydrogen-related defects in CuAlO2 by means of IR absorption spectroscopy.
Skills needed: Basic knowledge of physics of semiconductors, optics, and quantum mechanics, inclination to experimental work.
Gained experiences: Fourier transform IR absorption measurements, vibrational mode spectroscopy of light impurities in solids. Semiconductor chemistry. Optical characterization of solids.
2.Oxygen in antimony triselenide (Sb2Se3)
Description: Antimony triselenide (Sb2Se3) is an emerging chalcogenide semiconductor material considered as a promising photovoltaic absorber. It possesses an unique orthorhombic quasi-1-dimensional crystal structure in which covalently bonded [Sb4Se6]n `nano-ribbons' are linked by weak van der Waals interaction.
Oxygen plays an important role as key contaminant in many solids. Its decisive impact on the performance of various semiconductor devices has attracted considerable research interest over the decades leading to identification of fundamental microscopic configurations of oxygen defects in the host lattice and revealing their electrical activity.
Research related to the role of oxygen in antimony triselenide started quite early, after it has been recognized that oxygen doping has a strong influence - a combination of both beneficial and detrimental aspects - on the performance of Sb2Se3 thin film solar cells. Yet, the nature, microscopic configuration, and electrical activity of basically all oxygen-related complexes in this material remain experimentally undiscovered.
Tasks: The student will investigate oxygen-related defects in Sb2Se3 preferentially by means of IR absorption spectroscopy and photoconductivity.
Skills needed: Basic knowledge of physics of semiconductors and quantum mechanics, inclination to experimental work.
Gained experience: Fourier transform IR absorption and photoconductivity measurements, vibrational mode spectroscopy of light impurities in solids. Semiconductor chemistry. Optical characterization of solids.
3. Hydrogen-related centers in ZnS
Description: Zinc sulfide (ZnS) is a wide-bandgap II-VI semiconductor that has long demonstrated remarkable versatility in its fundamental properties, showing great potential for a wide range of applications. It is one of the earliest semiconductors to be discovered; the luminescence of a zinc sulfide-coated screen was used in experiments conducted by Rutherford between 1907 and 1911 to study the scattering of charged particles by the nuclei of various elements.
Hydrogen is ubiquitous in semiconductors, including ZnS, and it is very difficult to remove from the crystal growth environment and during post-growth treatment. The technological importance of hydrogen lies in its chemical reactivity, which typically leads to the healing of lattice imperfections and the passivation of dopants.
Despite significant progress over the past decade, comparatively little is known about the properties of hydrogen in ZnS relative to other important semiconductors.
Tasks: The student will investigate hydrogen-related defects (preferentially interstitial species and complexes with oxygen) in ZnS by means of IR absorption spectroscopy.
Skills needed: Basic knowledge of physics of semiconductors, optics, and quantum mechanics, inclination to experimental work.
Gained experiences: Fourier transform IR absorption measurements, vibrational mode spectroscopy of light impurities in solids. Semiconductor chemistry. Optical characterization of solids.
4. Photodissociation of H2 in Si
Description: Photodissociation is a process of fragmentation of a molecule through absorption of one or more photons. It is at heart of photochemistry and plays a fundamental role in interstellar clouds, planetary atmospheres, and plasma physics.
In spite of its `simplicity', the hydrogen molecule has fundamental importance in quantum mechanics, molecular physics, and chemistry. Because the closest dipole allowed electronic transition lies 10 eV above the ground state of H2, all optical excitations leading to photodissociation of hydrogen require either a single UV photon or multiple photons of less energies. The latter mechanism demands significant power densities ranging from 1011 to 1015 W/cm2.
Recently, it was shown that, contrary to free molecule, H2 embedded in silicon dissociates under photoexcitation with green light of power densities below 106 W/cm2, i.e. at least five orders of magnitude below the typical values. The physical mechanisms of photodisscociation, however, remain unclear.
Tasks: The student will investigate photodissociation of H2, D2, and HD molecules embedded in silicon by means of Raman scattering spectroscopy at different temperatures and photoexcitation conditions.
Skills needed: Basic knowledge of physics of semiconductors, optics, and quantum mechanics, inclination to experimental work.
Gained experiences: Raman scattering measurements, vibrational mode spectroscopy of light impurities in solids. Optical characterization of solids.