Materials Science Forum Vols. 778-780

Abstract: In this work we analyzed the variation of wafer curvature due to the growth of thin Si layers on top of 3C-SiC/Si films. The final Si/3C-SiC/Si hetero-structure, allows not only to have a deeper understanding of the stress within the different layers, but can also be used for MEMS applications, using the Si film as sacrificial layer in order to obtain 3C-SiC free-standing structure, or for electronic application, e.g. using the thin Si layer as high quality MOSFET channel and the SiC layer as the drift region. In details, the influence on wafer curvature by the growth of thin Si layer on top on the 3C-SiC/Si film as been studied by optical profilometer. A theoretical model was also applied in order to fit the measured curvature of the hetero-structure and optimize the system. Finally, in order to study the morphology of the hetero-structure micro-Raman spectroscopy and Transmission Electron Microscopy (TEM) measurements has been performed.

Abstract: We have investigated the electrical properties of n-type 4H-SiC in-situ germanium-doped homoepitaxial layers grown by chemical vapor deposition. Germanium is an isoelectronic impurity and, therefore, not expected to contribute to the conductivity. However, Hall effect measurements taken on samples with and without germanium revealed an enhanced mobility by a factor of ≈2 at T ≈ 55 K in the germanium-doped sample despite equal free electron concentration and equal compensation. Deep level transient spectroscopy (DLTS) measurements taken on germanium-doped samples reveal negative peaks indicating the presence of charged extended defects.

Abstract: Persistent conductivity in n-type 3C-SiC is investigated in a wide temperature range down to 3 K by Hall effect, admittance spectroscopy, low temperature photoluminescence (LTPL) and Raman spectroscopy. We propose a model, which clearly explains the persistent behavior of the electron density n below 50 K. It is experimentally verified that the persistent conductivity results from doped SF bunches, which can be considered as nanopolytype inclusions in 3C-SiC.

Abstract: A total of nine electrically active levels have been detected in as-grown and electron irradiated p-type 4H-SiC epilayers. These traps are found in the 0.32-2.26 eV energy range, above the valence band edge (E_V). Of these, six are majority carrier traps whereas three are minority carrier traps. We found that thermal oxidation affects the concentrations of two midgap levels, the majority carrier trap, labeled HK4 and the minority carrier trap identified as EH_6/7. The analysis of the irradiation energy and dose dependence of the concentration of these two traps, rules out the possibility that they may share the same origin.

Abstract: Ultra-pure n-type (8×10¹³ cm^-3), 99 μm thick epitaxial films of 4H SiC were electron irradiated at 170 keV with a fluence of 5×10¹⁶ cm^-2 or at 1 MeV with a fluence of 1×10¹⁵ cm^-2 in various geometries. Low temperature photoluminescence (LTPL) spectra and microwave photoconductance (μPCD) lifetime measurements were obtained on all samples prior to annealing and after annealing in Argon in free standing mode or on a POCO carbon platform, every 50°C from 1100°C to 1500°C. No improvement in carrier lifetime was obtained. Spurious lines attributable to the use of a Genesis CX 3550Å laser are also reported.

Abstract: We investigated annealing behavior of the carrier lifetime and the deep levels in electron irradiated n-type and semi-insulating 4H-SiCs. We observed two peaks for each sample by photo induced current transient spectroscopy (PICTS) measurements, and their heights depended on annealing temperature. By comparing the annealing behavior of the peak height with reported temperature dependence of concentrations of various defects, we speculated that the observed peaks originate from either V_SiV_C, C_SiV_C or EI4.

Abstract: The deep levels ON₁ and ON_2a/b introduced by oxidation into 4H-SiC are characterized via standard DLTS and via filling pulse dependent DLTS measurements. Separation of the closely spaced ON_2a/b defect is achieved by using a higher resolution correlation function (Gaver-Stehfest 4) and apparent energy level, apparent electron capture cross section and filling pulse measurement derived capture cross sections are given.

Abstract: In freestanding n-type 4H-SiC epilayers irradiated with low-energy (250 keV) electrons at room temperature, the electron paramagnetic resonance (EPR) spectrum of the negative carbon vacancy at the hexagonal site, VC- (h), and a new signal were observed. From the similarity in defect formation and the spin-Hamiltonian parameters of the two defects, the new center is suggested to be the negative C vacancy at the quasi-cubic site, VC- (k). The identification is further supported by hyperfine calculations.

Abstract: Defect levels in high purity semi-insulating 4H silicon carbide substrates are studied by charge transient spectroscopy using 5.5 MeV alpha particles. A shallow defect level with the activation energy around 0.3 eV is found in all samples annealed at temperatures from 1400 to 1600 °C. Some other defect levels lying at deeper in the bandgap are found in samples annealed at 1400 and 1500 °C. As these deep levels are annealed out by 1600 °C, the series resistance of samples is decreased.

Abstract: Carrier lifetime in a high injection condition is a key parameter for design of bipolar devices. Microwave photoconductivity decay (μ-PCD) is a popular method to evaluate the carrier lifetime in silicon carbide (SiC). For accurate evaluation of the carrier lifetime by μ-PCD measurements, the microwave reflectivity needs to be proportional to the excess carrier concentration. In this study, we observed microwave reflectivity from 4H-SiC as a function of injected photon density and suggested a method to keep proportionality of the reflectivity to the excess carrier concentration.