Investigation of the recombination properties of III-V semiconductors using time-resolved photoluminescence
Supervision: Dr. Peter Kleinschmidt (Tel.: -2577)
The recombination properties of the semiconductor are of crucial importance for the function of the solar cell. The electron-hole pairs generated by absorbed photons must be separated before their energy is lost through non-radiative recombination. The ratio of the rates of radiative to non-radiative recombination ultimately determines the open clamping voltage of the cell. The theoretical Shockley-Queisser limit for the conversion efficiency can only be reached if only radiative recombination occurs, i.e. if non-radiative recombination is minimized. The method of time-resolved photoluminescence established in our department, based on time-correlated single photon counting, directly measures the time-dependent photoluminescence signal in response to a laser pulse. In this bachelor thesis, specially prepared samples, so-called double heterostructures (DHS), from III-V semiconductors will be investigated by means of time-resolved photoluminescence. From the measurements the quality of the layers and interfaces will be evaluated. The results will directly serve to improve the material quality in III-V based solar cell materials.
STM investigations on MOCVD-prepared semiconductors for III-V/IV solar cells
Supervision: Dr. Peter Kleinschmidt (Tel.: -2577)
The semiconductor structures for solar energy conversion developed in the department of photovoltaics are based on epitaxially grown group IV and III/V semiconductors. The properties of interfaces and surfaces are of crucial importance for the function of the structures produced in this way, as they have an important influence on the formation of crystal defects, the control of which is crucial for the optoelectronic properties of the semiconductor. In our department the semiconductors grown by MOVPE (metal organic vapor phase epitaxy) are transferred into the UHV (ultra high vacuum) with a special transfersystem and can be investigated there with surface physical methods. In this master thesis the semiconductor layers prepared by MOVPE will be analyzed by STM (scanning tunneling microscopy). The main focus is on the investigation of silicon surfaces, whose structure is crucial for the subsequent III-V epitaxy aimed at in our department. The focus is on the step structure and the reconstruction of the surfaces depending on the preparation and crystal orientation, but especially on the influence of small amounts of other elements such as arsenic. In addition, surfaces of germanium and III-V semiconductors such as GaP, InP and GaAs are also investigated. The investigations planned for this master thesis will directly influence the development of novel solar cells based on the combination of Si and III-V semiconductors.
GaAsP quantum dot structures on Si(100) for highly efficient multi-junction solar cells
Supervision: Dr. rer. nat. Agnieszka Paszuk (Tel.: -2578)
In a research cooperation with the University of Tokyo, Japan, the Department of Basic Energy Materials is developing novel multiple quantum wells (MQW) solar cells. These are promising candidates for future high-efficiency solar cells of the so-called third generation: The combination of GaAsP with silicon in the form of a tandem solar cell promises conversion efficiencies of more than 40% while simultaneously reducing material consumption. Stress-compensated quantum well structures in the GaAsP top cell allow for a tailored adjustment of band gaps with greater lattice mismatch lattice lattice margin. The focus of the master thesis will be the heteroepitaxial growth of these MQW layers by metal organic vapor deposition (MOVPE) and especially the preparation of abrupt internal interfaces. A unique approach combining sophisticated optical in situ spectroscopy with established vacuum-based surface methods will be used to adjust and characterize the material properties during the actual MOVPE production process.
Modeling and characterization of nanowire structures
Supervision: M.Sc. Juliane Koch (Tel.: -2578)
Low-dimensional structures such as III-V nanowires (NW) are considered extremely promising candidates for future optoelectronic components (LEDs, FETs, sensors or solar cells). The electrical characterization of these NW is one of the basic building blocks to gain access to the electrical properties and thus to realize high efficiencies.For this purpose, a multi-tip STM (Fig. 1) was developed, which can investigate free-standing NW non-destructively.
Bachelor Thesis: To better understand these measurement results and to give predictions for not yet realized semiconductor structures, modeling is necessary. Especially NWs can differ in their properties from macroscopic structures due to their small size.The work focuses on the use of the simulation software "Silvaco ATLAS" (Fig. 2), the determination of suitable physical models, the evaluation of the results and the comparison with real measurement data.
Master thesis: Investigation of the electrical properties of nanowire structures or other low-dimensional structures is possible by appointment.
ATR-IR spectroscopy in ultra high vacuum
Supervision: Dr. Agnieszka Paszuk (Tel.: -2578)
For the development of epitaxially grown III-V/IV solar cells an exact understanding of the surfaces and interfaces is essential. These solar cells are promising candidates for future high-efficiency solar cells of the so-called third generation and potential candidates for direct solar water splitting. The work will focus on the heteroepitaxial growth of nukeation layers by metalorganic vapor deposition (MOVPE) and the spectroscopic characterization of abruptly prepared interfaces with respect to their surface termination and structure by Fourier-transformed infrared spectroscopy (FTIR). This is performed in a special ATR (Attenuated Total Reflection) configuration in ultra-high vacuum (UHV) to significantly increase surface sensitivity and avoid contamination. Since the termination of the surfaces and their configuration are very important for the subsequent epitaxies, a precise understanding is essential. The aim should be to gain a more precise understanding of the first nucleation step and any hydrogen or Group III or Group V termination of the surfaces.