Papers by Author: Martin Rambach

Abstract: We present a nanoscale morphological and structural characterization of few layers of graphene grown by thermal decomposition of off-axis 4H-SiC (0001). A comparison between transmission electron microscopy (TEM) in cross-section and in plan view allows to fully exploit the potentialities of TEM. Such a comparison was used to get information on the number of graphene layers as well as on the rotational order between the layers and with respect to the substrate. Some peculiar structures observed by TEM (wrinkles) could only be systematically measured by atomic force microscopy (AFM). In particular, the density and the height of the wrinkles in the few layers of graphene was investigated.

Abstract: In this study we examined the structural evolution of graphene grown on 8° off-axis 4H-SiC(0001) substrates at temperatures from 1600°C to 1700°C in Ar ambient. Morphological transformation of SiC substrate after annealing was examined by Tapping Mode Atomic Force Microscopy. Moreover, by etching-out graphene layers from graphitized SiC substrates in selective trenches we determined the number of graphene layers. Numbers of graphene layers were then independently confirmed by Transmission Electron Microscopy imaging.

Abstract: We studied the evolution of the electrical activation with annealing temperature and time in 4H-SiC implanted with Al ions at room temperature (RT). An accurate comparison between the electrical activation data obtained by FPP and SCM was carried out. The dependence of the electrically active profiles on annealing time was studied during isothermal (Tann=1600 °C) annealings for times ranging from 0 (spike anneal) to 30 min. By performing isochronal (t=30 min) processes at temperatures from 1550 to 1650 °C, the effect of the annealing temperature on the net doping concentration profiles was studied. Moreover, the activation energy (6.30.3 eV) associated to the process was extracted from the Arrhenius plot of the net active dose. Finally, the effect of the different thermal budgets on the roughening of the Al implanted 4H-SiC surface was also investigated in details by atomic force microscopy.

Abstract: The influence of the implantation temperature on the surface roughness and the resistivity of aluminum implanted 4H-silicon carbide was determined. A dose of 1.2 ⋅1015cm-2 aluminum ions was implanted at temperatures between room temperature and 1000°C. A decrease of the surface roughness down to an rms-value of 12nm and a decrease in the resistivity down to 0.35Wcm were found with increasing implantation temperature. The influence of the implantation temperature on the resistivity was identified by modeling temperature dependent resistivity data. The results showed an increase in mobility with temperature due to the reduction of compensation centers.

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: Characterization of post implantation annealing steps is done by extracting the activation and compensation data of implanted Al atoms. Usually, this is done by Hall measurements. The preparation of Hall samples and temperature dependent Hall measurements, however, are rather complex compared to, e.g., temperature dependent resistivity measurements by 4-point probing. Therefore, a model for extracting relevant electrical parameters from resistivity data has been developed. The model is based on the neutrality equation and a temperature dependent mobility model.

Abstract: Furnace annealing and lamp annealing of aluminum implanted layers in 4H silicon carbide (SiC) were investigated with respect to surface degradation and electrical parameters. A sheet resistance of about 20kW/ı was obtained for an aluminum implantation dose of 1.2×1015cm-2 and annealing in the furnace at 1700°C for 30min. For the same implantation dose, lamp annealing at 1770°C for 5min resulted in a three times higher sheet resistance of 60kW/ı. The surface roughness was best for the lamp system and stayed below 1nm for Al doses lower than 1×1015cm-2.