Kurt Semmelroth

Electrical and Optical Characterization of SiC

Three topics are reported in this paper: (a) the determination of a temperaturedependent Hall scattering factor for holes r H,h(T) in 4H-SiC, (b) the detection of shallow Al-related defect centers in Al-doped, p-type 6H-/3C-SiC; these defects are generated either by i mplantation of any ion species or by an oxidation process and (c) the observation of absorption lines in infra red (IR) spectra, which are due to phosphorus donors in 6H-SiC.

Growth of SiC polytypes by the physical vapour transport technique

A brief survey of the development of the sublimation growth of SiC is given. The growth equipment and especially the hot zone of the furnace for the physical vapour transport (PVT) technique are described in detail. In order to grow micropipe-free SiC crystals, near-thermal-equilibrium growth is developed and the individual processing steps are revealed. The essential parameters for the growth of 4H-, 6H-, 15R- and 3C-SiC single crystals are discussed and a survey of the incorporation of donors (N, P) and acceptors (Al, B) during the PVT growth is presented.

Temperature induced polytype conversion in cubic silicon carbide studied by Raman spectroscopy

In this paper, we study the polytype transformation of cubic silicon carbide (3C-SiC) by micro Raman spectroscopy. Two sets of samples are investigated. First, physical vapor transport (PVT) grown samples, grown on chemical vapor deposition (CVD) substrates at growth temperatures between 1800 and 1950°C. The microscopic images of the cross sections and the Raman spectra taken on these grown crystals show a growing fraction of 6H-SiC inclusions with increasing growth temperature. To decide whether these polytype inclusions are induced by the PVT growth process or by the temperature treatment of the CVD substrates, we studied the temperature induced polytype coversion of cubic CVD substrates grown on undulant Si(100) surfaces which were used as seed crystals for the PVT growth. The CVD substrates were annealed in the temperature range between 1700 and 2100°C. The results of this annealing series are similar to the results we find in the PVT grown samples. Therefore we argue that the polytype conversion is n...

Impurity Conduction Observed in Al-Doped 6H-SiC

6H-SiC single crystals doped with aluminum acceptors at different concentrations were investigated with admittance spectroscopy and Hall effect. Thermally activated impurity conduction was observed at temperatures below 160K. The activation energy of 17meV was obtained from admittance spectroscopy as well as from temperature-dependent resistivity measurements. The Hall coefficient showed a sign reversal in the temperature range where hopping conduction dominates.

Study of the Temperature Induced Polytype Conversion in Cubic CVD SiC by Raman Spectroscopy

Using 3C-SiC crystals grown on undulant Si(100) substrates as seed crystals, bulk growth of 3C-SiC by the modified Lely method has been demonstrated. However, particularly at elevated growth temperatures, macroscopic 6H-SiC inclusions can be observed in the grown crystals. Here we investigate the temperature dependence of the 3Cto 6H-SiC transformation of the CVD substrate by Micro Raman Spectroscopy after annealing at temperatures between 1700◦C and 2100◦C. At temperatures above 1800◦C 3Cto 6H-SiC polytype transformation is observed that proceeds from the top side of the crystals. However, the conversion appears to be stopped at domain boundaries that are oriented along {111} planes.

Phosphorus-Related Centers in SiC

Phosphorus (P) is considered to serve alternatively to nitrogen (N) as a shallow donor in SiC. It is the aim of this paper to report the present status of our knowledge on the optical and electrical properties of P donors. We will not report on results obtained from magnetic resonance experiments in this article and refer to the relevant literature [1]–[4]. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) investigations are predominantly taken on SiC samples, which are P-doped by neutron transmutation. These magnetic resonance experiments and also a theoretical prediction by A. Gali et al. [5] support that P donors are incorporated into the silicon (Si) sublattice. However with respect to the microscopic structure of P-related centers, there exists still a controversial discussion [1]–[4].

Concentration of N and P in SiC Investigated by Time-Of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)

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.

Growth of 3C-SiC Bulk Material by the Modified Lely Method

The growth of cubic 3C-SiC single crystals by the modified Lely method is reported. Free-standing 200μm thick 3C-SiC epilayers were employed as seed crystals. The source material consisted of stoichiometric SiC; in addition, a separate Si source was deposited in the furnace at a temperature of about 1500°C. The temperature of the seed crystal was kept in the range of (18501950)°C. The growth rate is equal to 0.05mm/h and a nitrogen donor concentration of (2-6) x10cm is determined in the grown 3C-SiC.

Electronic raman studies of shallow donors in silicon carbide

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.

Microstructure of Cubic SiC Grown by the Modified Lely-Method

We investigate the microstructure of 3C-SiC by both conventional and high resolution transmission electron microscopy. The samples were grown by the modified Lely-method at temperatures between 1850-1950 °C using 3C-SiC seed crystals grown on undulating silicon. Our investigations show that growth is predominantly cubic. However, transmission electron microscopy reveals that the material contains bands of several micrometer thickness that consist of stacking faults in high density. High resolution transmission electron microscopy shows in addition that these stacking faults are irregularly spaced with maximum densities of one stacking fault every three bilayers. In some small regions the stacking faults have regular spacing and form 6H nanoinclusions with a thickness of up to 15 bilayers. The presence of 6H in these regions could also be confirmed by electron diffraction patterns. In addition we detect macroscopic 6H inclusions with several micrometer thickness.