Refine
Document Type
- Doctoral thesis (2)
Has Fulltext
- yes (2)
Is part of the Bibliography
- no (2)
Language
- English (2)
Keywords
- Versetzungsnetzwerk (2) (remove)
Institute
The thesis presents the results of the investigations of electronic properties and defect states of dislocation networks (DNs) in silicon produced by wafers direct bonding technique. Practical interest for the investigations in this area issued – first of all – from the potential application of such dislocation networks in microelectronics as all-Si light emitter for on-chip interconnection. Besides, dislocation networks may serve as a perfect model object to get new information about the fundamental properties of dislocations and grain boundaries in Si, what is of particular importance for multicrystalline silicon solar cells performance. Despite of a long story of studying of dislocations in silicon, a new insight into the understanding of their very attractive properties was succeeded due to the usage of a new, recently developed silicon wafer direct bonding technique, allowing to create regular dislocation networks with predefined dislocation types and densities. Samples for the investigations were prepared by hydrophilic bonding of p-type Si (100) wafers with same small misorientation tilt angle (~0,5°), but with four different twist misorientation angles Atw (being of <1°, 3°, 6° and 30°, respectively), thus giving rise to the different DN microstructure on every particular sample. The main experimental approach of this work was the measurements of current and capacitance of Schottky diodes prepared on the samples which contained the dislocation network at a depth that allowed one to realize all capabilities of different methods of space charge region spectroscopy (such as CV/IV, DLTS, ITS, etc.). The key tasks for the investigations were specified as the exploration of the DN-related gap states, their variations with gradually increasing twist angle Atw, investigation of the electrical field impact on the carrier emission from the dislocation-related states, as well as the establishing of the correlation between the electrical (DLTS), optical (photoluminescence PL) and structural (TEM) properties of DNs. The most important conclusions drawn from the experimental investigations and theoretical calculations can be formulated as follows: - DLTS measurements have revealed a great difference in the electronic structure of small-angle (SA) and large-angle (LA) bonded interfaces: dominating shallow level and a set of 6-7 deep levels were found in SA-samples with Atw of 1° and 3°, whereas the prevalent deep levels – in LA-samples with Atw of 6° and 30°. The critical twist misorientation angle separating SA- and LA- interfaces was estimated as Atw*≈ 3,5±0,5°, what agrees quiet well with the results of previous PL and TEM investigations. - For the dominating shallow traps in SA-samples (denoted as ST1/ST3 traps) a new phenomenon – that is ‘giant Poole-Frenkel effect’ of enhanced carrier emission due to dislocations elastic strain field was observed for the first time. Performed theoretical calculations have shown that in the investigated samples such an effect should be ascribed to the row of 60° dislocations rather than to the mesh of screw ones. In this respect, shallow traps ST1/ST3 were identified either with shallow 1D bands (directly or as being coupled with them) or with shallow stacking fault states on splitted 60° dislocation. - From the comparison and correlations of measured DLTS spectra with the results of PL and TEM investigations it was established, that shallow ST1/ST3 traps participate in D1 radiative recombination and that the structural elements, responsible for D1 luminescence of small-angle DNs, are the triple knots (intersections with screw dislocations) along the 60° dislocations. However, the optimal density of 60° dislocations as well as of triple knots, in other words – the optimal tilt and twist misorientation angles for maximal D1 intensity – needs further clarification.
In this work, the electrical and luminescence properties of a series of Si based materials used for photovoltaics, microelectronics and nanoelectronics have been investigated by means of electron beam induced current (EBIC), cathodoluminescence (CL), photoluminescence (PL) and electroluminescence (EL). Photovoltaic Si produced by block casting has been investigated by EBIC on wafers sliced from different parts of the ingot. The impact of selected solar cell processing steps on the material properties has been evaluated by EBIC utilizing adjacent wafers from the ingot. The temperature dependence of dislocations’ EBIC contrast was measured to assess the degree of dislocation contamination with impurities, yielding low dislocation contamination for the middle of the block and high contamination in the top and bottom regions. This is in agreement with the impurity distribution in the block. It was found that phosphorus diffusion gettering (PDG) followed by SiN firing greatly reduces the recombination activity of extended defects at room temperature, and improves the bulk property simultaneously. The improvement is attributed to both PDG of metal impurities and a passivation effect of SiN firing. In order to better understand the factors limiting the properties of thin polycrystalline Si layers prepared by the Aluminum induced layer exchange (Alile) technique, epilayers grown on (111) and (100) monocrystalline Si substrates were used as a model system to investigate the impact of processing temperature (Ts) and type of substrate. It was found that no dislocations are formed for epilayers on (100) Si, while a high density of dislocations was detected on epilayers prepared on (111) Si at 450 °C. The dislocation density decreases with increasing TS. The diffusion lengths extracted from the energy dependent EBIC collection efficiencies reveal an improvement of the epilayer quality with increasing TS during growth from 450 °C to 650 °C, and a decrease of the epilayer quality at 700 °C. This is attributed to a reduction of the dislocation density with increasing TS and a formation of precipitates during the process. Precipitate formation of at 700 °C is limited because the metal impurities are very mobile at high TS, resulting in a homogeneous distribution of the impurities. Because the impurities are effective lifetime killers of the minority carriers, so the diffusion length decreases. PL measurements on epilayers grown on Si substrates revealed no characteristic dislocation-related luminescence (DRL) lines at room temperature and 77 K, while intense characteristic DRL lines D1 - D4 have been detected in the sample prepared by the Alile technique. This indicates that dislocations in the Alile sample are relatively clean. The possible reason for the purification of the Alile samples is Al induced gettering during the polycrystalline Si layer growth. The diffusion length in the thin top layer of Si-on-insulator (SOI) samples has been successfully measured by EBIC employing suppression of the surface recombination at the buried oxide layer and at surface of the top layer by biasing. The measured diffusion length is several times larger than the layer thickness. Dislocation networks produced by Si wafer direct bonding have been investigated with regard to their electrical properties by EBIC. The networks were observed to show charge carrier collection and electrical conduction. Inhomogeneities in the charge collection were detected in n- and p-type samples under appropriate beam energy. The EBIC contrast behavior can be understood under the consideration of the positively charged oxide precipitates along with dislocations charged with majority carriers, where the appearance of the contrast in dark or bright depends strongly on the ratio of the collection and the recombination loss of the carriers.The luminescence properties of Si nanostructures (Si nanowires, Si nano rods, porous Si, and Si/SiO2 multi quantum wells (MQWs)) are another important subject of this work. Sub-bandgap infrared (IR) luminescence around 1570 nm has been found in Si nanowires, nano rods and porous Si. PL measurements with samples immersed in different liquid media, for example, in aqueous HF (50%), concentrated H2SO4 (98%) and H2O2 established that the sub-bandgap IR luminescence originates from the Si/SiOx interface. Its origin was explained in terms of a simple recombination model through radiative interface states. EL in the sub-bandgap IR range has been observed in simple diodes prepared on porous Si and MQWs at room temperature. The results show the possibility to fabricate an efficient light emitter around 1570 nm wavelength based on the radiative recombination at the Si/Si oxide interface. Based on the knowledge about radiative transitions via the interface states, an improved understanding of luminescence in dislocated samples was proposed.