Ulrike Künecke

Tuning the emission colour by manipulating terbium-terbium interactions: Terbium doped aluminum nitride as an example system

Terbium-terbium interactions in terbium doped semiconductors and insulators may lead to the so-called cross-relaxation process, which increases the D54 (green) emission of the terbium ions at the cost of the D53 (blue) luminescence intensity. This effect can generally be reduced by increasing the distance between an excited ion and the nearest ion in the ground state. A straightforward measure is to use a specimen with a decreased terbium concentration. The alternative is to increase the intensity of the excitation (either by photons or electrons) and thereby to reduce the population of terbium ions in the ground state. This paper works this process out with the example of AlN:Tb on the basis of a model and respective experimental results. As will be seen, stronger excitation causes in essence more Tb ions to be excited, thus less ions in the ground state which increases the distance between an excited and the nearest ground state ions. This hinders energy transfer between the terbium ions and thus counte...

High Al-Doping of SiC Using a Modified PVT (M-PVT) Growth Set-Up

Several highly aluminum doped SiC bulk crystals were grown with a modified PVT (MPVT) method. To facilitate 4H-SiC formation, growth was conducted on the C-face. The samples were investigated using Hall measurements in the Van-der-Pauw geometry. Lowest room temperature values for specific resistivities were 0.09 Ωcm for 6H-SiC and 0.2 Ωcm for 4H-SiC, which are to our knowledge the lowest values yet reported in literature. Thus, resistivity values of < 0.2 Ωcm, which are required for substrates in high power device applications, could be demonstrated for 4HSiC. Remarkably, in very highly doped samples the type of conduction could not be determined by Hall measurements.

Results of SIMS, LTPL and temperature-dependent hall effect measurements performed on al-doped α-SiC substrates grown by the M-PVT method

We report on investigation of p-type doped, SiC wafers grown by the Modified- Physical Vapor Transport (M-PVT) method. SIMS measurements give Al concentrations in the range 1018 to 1020 cm-3, with weak Ti concentration but large N compensation. To measure the wafers’ resistivity, carrier concentration and mobility, temperature-dependant Hall effect measurements have been made in the range 100-850 K using the Van der Pauw method. The temperature dependence of the mobility suggests higher Al concentration, and higher compensation, than estimated from SIMS. Additional LTPL measurements show no evidence of additional impurities in the range of investigation, but suggest that the additional compensation may come from an increased concentration of non-radiative centers.

Basal Plane Dislocation Dynamics in Highly p-Type Doped versus Highly n-Type Doped SiC

The long term performance of today’s SiC based bipolar power devices suffer strongly from stacking fault formation caused by slip of basal plane dislocations, the latter often originating from the n-type doped SiC substrate wafer. In this paper, using sequentially p-type / n-type / p-type doped SiC crystals, we address the question, whether basal plane dislocation generation and annihilation behaves differently in n-type and p-type SiC. We have found that basal plane dislocations are absent or at least appear significantly less pronounced in p-type doped SiC, which may become of great importance for the stacking fault problem in SiC.

Impact of Compensation on Optical Absorption Bands in the Below-Bandgap Region in n-Type (N) 6H-SiC

We present an optical method for the determination of the charge c ri r concentration as well as the compensation level based on absorption measurements at roo m temperature in n-type 6H-SiC. Below band-gap absorption bands (BBGA) are best fitted by a Fano like shape. Calibration plots are provided for evaluation of the charge carrier concentration f rom the peak area of the BBGA. The compensation level is derived from the comparison of th e peak area of the BBGA and the absolute peak value.

Bulk Growth of SiC

The paper reviews the basics of SiC bulk growth by the physical vapor transport (PVT) method and discuss current and possible future concepts to improve crystalline quality. In-situ process visualization using x-rays, numerical modeling and advanced doping techniques will be briefly presented which support growth process optimization. The “pure” PVT technique will be compared with related developments like the so called Modified-PVT, Continuous-Feeding-PVT, High-Temperature-CVD and Halide-CVD concepts. Special emphasis will be put on dislocation generation and annihilation and concepts to reduce dislocation density during SiC bulk crystal growth. The dislocation study is based on a statistical approach. Rather than following the evolu-tion of a single defect, statistic data which reflect a more global dislocation density evolution are interpreted. In this context a new approach will be presented which relates thermally induced strain during growth and dislocation patterning in networks.

Germanium Incorporation during PVT Bulk Growth of Silicon Carbide

Silicon carbide as a material for electronic devices still has substantial problems concerning its structural quality and defects. It has been shown that dopants can have a big influence on structural properties like polytype stability and dislocation statistics [1]. We will discuss the effect of an isoelectronic dopant in silicon carbide. Germanium, being a member of the 4th group in the periodic table of elements like silicon and carbon, will not influence the electrical properties of the material such as e.g. aluminum. In our experiments we reached concentrations of up to 1*1020 cm-3. We have observed an impact on the polytype stability during sublimation growth with in-situ germanium incorporation. We investigated an influence on the dislocation statistics during growth and, hence, varying germanium concentration. We found only a slight decrease in mobility during Hall measurements but no severe changes in electrical properties of the material.

Aluminum p-type Doping of Bulk SiC Single Crystals by Tri-Methyl-Aluminum

We present p-type doping of bulk SiC crystals by the modified physical vapor transport (M-PVT) technique using TMA (Tri-Methyl-Aluminum). Using TMA as a dopant precursor allows a quite well defined crystal growth process control. The issue of improvement of conductivity (reduction of substrate resistivity) by reduction of unintentional acceptor compensation by nitrogen is addressed. It is shown that a decrease of compensation from approx. 3%...10% to approx. 0.5%...2.5% leads to a charge carrier mobility and, hence, conductivity increase of about factor two.

Uniform Axial Charge Carrier Concentration in PVT-Grown p-Type 6H SiC by Non-Uniform Distribution of Boron in the Powder Source

We have investigated the influence of boron distribution within the powder source during PVT growth of p-type 6H-SiC on the axial homogeneity of the charge carrier concentration. It is shown that a rather homogeneous hole concentration in the grown crystals can be achieved if the boron doped portion of the powder is located at the upper crucible part due to the compensation with nitrogen. In this case a simultaneous decrease of NA and ND concentration is observed during growth. Homogeneity with respect to the optical absorption in the visible was also achieved.