Papers by Author: Erik Janzén

Abstract: The diffusion of the carbon vacancy (V_C) in n-type 4H-SiC has been studied using Deep Level Transient Spectroscopy (DLTS). Samples grown along two different crystallographic planes, (0001) or c-cut and (11-20) or a-cut, have been utilized. The samples were implanted with 4.0 MeV C ions to generate V_C’s and subsequently annealed at temperatures between 200 and 1500 °C. Following each annealing stage, concentration versus depth profiles of the V_C were obtained. The V_C is essentially immobile in both the c-cut and a-cut samples up to at least 1200 °C. The 1400 °C annealing stage, however, resulted in considerable migration, predominantly along the a-direction. Using half the difference in the Full Width at Half Maximum (FWHM) of the initial and diffused concentration profiles as a measure of the diffusion length, we deduced the diffusivity of the V_C at 1400 °C to be approximately (3.8±1.1)×10^-14 cm²/s along the c-axis and (4.1±1.2)×10^-13 cm²/s along the a-axis, indicating a substantial anisotropy for the V_C diffusion in 4H-SiC.

Abstract: In this study we have grown thick 4H-SiC epitaxial layers with different n-type doping levels in the range 1E15 cm^-3 to mid 1E18 cm^-3, in order to investigate the influence on carrier lifetime. The epilayers were grown with identical growth conditions except the doping level on comparable substrates, in order to minimize the influence of other parameters than the n-type doping level. We have found a drastic decrease in carrier lifetime with increasing n-type doping level. Epilayers were further characterized with low temperature photoluminescence and deep level transient spectroscopy.

Abstract: We report on a very low density (<5×10¹¹ cm^-2) of near-interface traps (NITs) at the Al₂O₃/4H-SiC interface estimated from capacitance-voltage (CV) analysis of MOS capacitors at different temperatures. The aluminum oxide (Al₂O₃) is grown by repeated deposition and subsequent low temperature (200°C) oxidation for 5 min of thin (1-2 nm) Al layers using a hot plate. We refer to this simple method as hot plate Al₂O₃. It is observed that the density of NITs is significantly lower in the hot plate Al₂O₃ samples than in samples with Al₂O₃ grown by atomic layer deposition (ALD) at 300°C and in reference samples with thermally grown silicon dioxide grown in O₂ or N₂O ambient.

Abstract: Over 150 μm thick epilayers of 4H-SiC with long carrier lifetime have been grown with a chlorinated growth process. The carrier lifetime have been determined by time resolved photoluminescence (TRPL), the lifetime varies a lot between different areas of the sample. This study investigates the origins of lifetime variations in different regions using deep level transient spectroscopy (DLTS), low temperature photoluminescence (LTPL) and a combination of KOH etching and optical microscopy. From optical microscope images it is shown that the area with the shortest carrier lifetime corresponds to an area with high density of structural defects.

Abstract: We investigated Molybdenum (Mo) defects in 4H silicon carbide (SiC). This system can be suitable candidate for in vivo biomarker applications since it shows photoluminescence (PL) in the near-infrared (NIR) region. In order to reveal the origin of this Mo-related PL center we carried out ab initio density functional theory (DFT) calculations on two microscopic models.

Abstract: We demonstrate optically pumped dynamic nuclear polarization (DNP) of ²⁹Si nuclear spins that are strongly coupled to paramagnetic color centers in 4H- and 6H-SiC. We observe 99%±1% degree of polarization. By combining ab initio theory with the experimental identification of the color centers’ optically excited states, we quantitatively model how the polarization derives from hyperfine-mediated level anticrossings. In addition, we developed a general model for these optical DNP processes that allows the effects of many microscopic processes to be integrated. Applying this theory, we gain a deeper insight into dynamic nuclear spin polarization. In particular, our findings show that the defect electron spin coherence times and excited state lifetimes are crucial factors in the entire DNP process. These results lay a foundation for SiC-based quantum memories, nuclear gyroscopes, and hyperpolarized probes for magnetic resonance imaging.

Abstract: 4H-SiC epilayers with very smooth surfaces were grown with high growth rates on 4° off-cut substrates using standard silane/propane chemistry. Specular surfaces with RMS values below 0.2 nm are presented for epilayers grown with growth rates up to 30 μm/h using horizontal hot-wall chemical vapor deposition, with up to 100 μm thickness. Optimization of in-situ etching conditions and C/Si ratio are presented.

Abstract: Nowadays, computational techniques can greatly facilitate the identification of point defect related photoluminescence and EPR centers in semiconductors. Once the identification has been achieved, one can gain a detailed description of the microstructure and the electron configuration of the defect, providing a basis for further understanding and development. Recently, the importance of divacancy and related point defects in different polytypes of SiC has substantially increased due to their possible quantum bit application. However, their different configurations have not been satisfactorily identified yet. In our study, we carry out large-scale first principles supercell calculations to identify the divacancy related point defects in 4H and 6H-SiC. By resolving some general accuracy issues of usual ab initio supercell techniques, we are able to obtain convergent photoluminescence (PL) energies, zero-field-splitting, and hyperfine parameters. Our results confirm the previous assignment of the PL1-4 PL lines in 4H-SiC (also known as UD-2 luminescence lines previously) to the four possible divacancy configurations and provide the identification of QL1,QL2, and QL6 PL lines in 6H-SiC. In all cases the calculated zero-field and hyperfine tensors’ parameters are provided.

Abstract: The influence of chlorine has been investigated for high growth rates of 4H-SiC epilayers on 4o off-cut substrates. Samples were grown at a growth rate of approximately 50 and 100 μm/h and various Cl/Si ratios. The growth rate, net doping concentration and charge carrier lifetime have been studied as a function of Cl/Si ratio. This study shows some indications that a high Cl concentration in the growth cell leads to less availability of Si during the growth process.

Abstract: Chemical vapor deposition of silicon carbide (SiC-CVD) is a complex process involving a Si-C-H system wherein a large number of reaction steps occur. To simulate such a system requires knowledge of thermochemical and transport properties of all the species involved in the process. The accuracy of this information consequently becomes a crucial factor toward the correctness of the outcome prediction. The database on thermochemical properties of well-known species such as small hydrocarbons has been established over decades and it is accurate and easily accessible. On the other hand, the database for less frequently used species such as organosilicons is still under development. Apart from the accuracy issue, a consistency in acquiring procedures, whether theoretical or experimental, is another factor controlling the final error of the simulated outcome. In this work, the thermochemical data for several important growth species for SiC CVD using the SiH₄/C_xH_y/H₂ system has been calculated. For the most part an excellent agreement is seen with previously reported data, however for the organosilicons a larger deviation is detected and in particular for the CH₃SiH₂SiH species which shows a stark deviation from the CHEMKIN database.