Papers by Author: Kurt Semmelroth

Abstract: 6H-SiC was doped with phosphorus (P) donors by neutron transmutation. IR transmission spectra were taken in the temperature range from 6 K to 300 K. A great number of absorption lines is observed at temperatures below 140 K; these lines are attributed to transitions of the donor electron between ground states and bound excited states of P-related donors. Based on Faulkner's theory, three series of effective-mass-like states (P1, P2, P3) could be identified. The corresponding ground state energies are: E(P1) = EC - 91.5 meV, E(P2) = EC - 81.8 meV, E(P3) = EC - 73.5 meV.

Abstract: We report on admittance spectroscopy (AS) investigations taken on aluminum (Al)- doped 6H-SiC crystals at low temperatures. Admittance spectra taken on Schottky contacts of highly doped samples (NA ≥ 7.2×1017 cm-3) reveal two series of conductance peaks, which cause two different slopes of the Arrhenius plot. The steep slope is attributed to the Al acceptor, while the flatter one - obtained from the low temperature peaks - is attributed to the activation energy ε3 of nearest neighbor hopping. We propose a model, which explains the unexpected sharpness of the low temperature conductance peaks and the disappearance of these peaks for low acceptor concentrations. The model is verified by simulation, and the AS results are compared with corresponding results obtained from resistivity measurements taken on 4H- and the identical 6HSiC samples.

Abstract: We study electronic Raman scattering of phosphorus and nitrogen doped silicon carbide (SiC) as a function of temperature in the range 7K < T < 300K. We observe a series of peaks in the Raman spectra which we assign to electronic transitions at nitrogen and phosphorus donors on different lattice sites. These transitions are identified as valley orbit transitions of the 1s donor ground state. From the polarization dependence of the observed peaks, we find that all electronic Raman signals have E2-symmetry of C6v for the hexagonal polytypes (6H-SiC and 4H-SiC) and E-symmetry of C3v for 15R-SiC. We find a reduction of the intensities of all valley-orbit Raman signals with increasing temperature and ascribe this reduction to the decreasing occupation of donor states.

Abstract: The concentration of nitrogen and phosphorous in SiC bulk material and epitaxial layers was investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The advantage of TOF-SIMS of acquiring a complete mass spectrum in a single run was used to identify the most sensitive atomic ion or ionic cluster for the selected element to be monitored. For the investigation of N with its intrinsic low ionization yield the use of a Cs containing cluster ion is necessary. Selection of a CNCs2 + cluster allows to reach a detection limit of about cN,min » 5×1016 cm-3. In the case of P the elemental ion was used. However, the adjacent mass of 30SiH influences the P peak as well as its background and has to be suppressed. This can be achieved by limiting the residual gas re-adsorption during the measurement resulting in a detection limit of about cP,min » 5×1015 cm-3. These measurement parameters were used to investigate a single crystal SiC bulk sample grown by the modified Lely method with intentional P doping and an N doped epitaxial SiC layer sample.