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AlN and SiC in Erlangen

AlN - Research Summary

Die Entwicklung von AlN in Erlangen

Growth and characterization of AlN bulk single crystals (my habilitation thesis 2008)

In recent years, single-crystalline aluminum nitride (AlN) became a candidate as substrate material for group-III nitride epilayers. Devices based on such epilayers constitute a rapidly emerging market in the field of optoelectronics and high-frequency communications. Particularly UV optoelectronics would greatly benefit from the use of AlN substrates, if these were available in sufficient amount, quality, and size. That is still not the case.

Bulk AlN crystals are most successfully grown by a sublimation-recondensation process (PVT growth) at temperatures exceeding 2000°C. A set-up based on tungsten parts was found to be most stable against the aggressive vapor and provides very low contamination of the growing crystal. We show that mass transport and unidirectional solidification may be easily established. However, grain selection and enlargement is very slow, as the crystals tend to form a columnar structure. Under nearly isothermal conditions, freestanding crystals of up to 25 mm in size and high structural quality have been fabricated. But further enlargement seems infeasible, and their special habit renders them unsuitable for preparation of substrates.

Large-area AlN single crystals are obtained by seeding on SiC substrates, alas at the cost of severe problems in material compatibility which lead to highly contaminated AlN crystals. As the set-up degrades in the presence of SiC during long-time growth runs, the achievable crystal thickness is limited. Still, such large-area AlN crystals have been successfully used as templates for further growth of AlN single crystals up to 30 mm in diameter and 10 mm in height.

In the main volume of such crystals, the density of dislocations threading the basal plane is lower than 105 cm-2. However, low angle grain boundaries are inherited from the SiC seed and the template, leading to a mosaic structure of elongated subgrains which may deviate by more than 1° from the main orientation. Structural quality improves in crystals that were grown under optimized conditions.

According to chemical analysis, oxygen is found to be the dominating impurity regardless of growth orientation. Additionally, silicon and carbon are clearly detectable even in bulk AlN grown homoepitaxially on the templates. The wurtzite structure of AlN is highly anisotropic. As growth takes place on different facets simultaneously, the resulting crystal shows a zonal structure. Different zones of AlN single crystals showed very similar threading dislocation densities and topographies. On the other hand, impurity incorporation and formation of intrinsic defects clearly depends on the zone or growth orientation. The differences are evidenced by optical absorption and cathodoluminescence.

We show that both intensity and peak position of UV optical absorption are closely linked to the 'violet luminescence' in the 3-4 eV energy range. Comparing our data with experimental results and first principles calculations reported in literature, we establish a model of the defect content in nominally undoped AlN crystals. As a conclusion, oxygen contamination as well as aluminum vacancy formation increases in the order Al-polar (0001) zone, rhombohedral {1012} zones, prismatic {1010} zones, N-polar (0001) zone. In the same sequence, the absorption at 2.8 eV and thus the yellowish coloration of the crystal areas increases.

The model further suggests a DX-center formation of oxygen, whose thermal activation energy of 0.6-0.8 eV is evidenced by thermally stimulated luminescence and high-temperature resistivity measurements. In contrast, the existence of nitrogen vacancies in significant concentrations is disputed at least for samples in which the oxygen content governs the electrical behavior.

In conclusion, a procedure for homoepitaxial growth of AlN bulk crystals is developed. The structural quality of the resulting substrates is very promising for group-III nitride epitaxy. Additionally, important insights into the defect occurrence and distribution in this novel material are obtained, which may aid in further optimization of crystal growth in regard to applicability e.g. in UV optoelectronics.

SiC - Research Summary

Various doped SiC crystal slices from my research in Erlangen

Growth of semi-insulating SiC bulk single crystals (my PhD thesis 2002)

The goal of this work, the production of semi-insulating silicon carbide single crystals, was realized by vanadium doping. By the appropriate addition of vanadium to the SiC source material during the growth of SiC crystals according to the modified-Lely method, the reproducible production of SiC crystal discs with homogeneous electrical properties, i.e. specific resistances at room temperature of more than 1010 Ohm-cm, with a high yield is possible. As a substrate material, these crystal discs can significantly improve the electrical properties of high-performance and high-frequency components.

A detailed investigation of all known impurities in SiC leads to the conclusion that the vanadium donor is best suited to compensate flat dopants by its band-centered impurity and to generate semi-insulating behavior with more than 1015 ohm-cm at room temperature. Since vanadium acts as an amphoteric dopant in SiC, it is necessary to generate a slight excess of flat acceptors in the crystal to compensate them with vanadium. On the other hand, the activation of the vanadium acceptor level at compensation with flat donors also leads to semi-insulating behavior, but at lower specific resistances at room temperature in the order of 1011 Ohm-cm.

The impurities introduced during nominally undoped growth are decisive for the development of the manufacturing process for semi-insulating SiC. Nitrogen, which is molecularly adsorbed on the materials used in the cultivation process, dominates the electrical properties as a flat donor. It desorbs during growth and is built into the crystal in a time-inhomogeneous manner, decreasing exponentially with growth time. Through the development and improvement of a high vacuum heating step prior to cultivation, it was possible to reduce the nitrogen concentration in crystal regions remote from the germ to up to 4 x 1016 cm-3, but at the beginning of cultivation more than 1 x 1018 cm-3 was measured.

With the aim of producing a slight excess of acceptors in the crystal, the doping with boron was investigated by adding boron carbide directly to the SiC powder. This process is efficient - the transfer coefficients are k = 0.1 depending on the configuration ... 0.22 - and enables constant boron incorporation during growth. This was demonstrated by the evaluation of temperature-dependent Hall effect measurements on several crystals with different boron additions. The electrical properties of boron-doped crystals are clearly influenced by the time inhomogeneous nitrogen incorporation, especially with low boron addition. Nevertheless, the production of boron-doped crystal discs with homogeneous electrical properties has been successful.

Doping with vanadium was also realized by adding vanadium carbide to the SiC powder. This process is inefficient because the vanadium species evaporate rapidly and the vanadium concentration in the crystal decreases by up to three orders of magnitude during growth. By the addition in an inner crucible and a slight lowering of the growth temperature the evaporation of the vanadium species can be limited and an almost homogeneous vanadium incorporation during the growth can be achieved. The formation of vanadium carbide precipitates can also be limited, which occur due to the exceeding of the maximum solubility of vanadium of 5 x 1017 cm-3 especially at the beginning of the growing process in the crystals.

In vanadium-doped crystals, the electrical properties are determined by the vanadium acceptor, which compensates the flat nitrogen donors. Absorption and ESR measurements can be used to identify the active vanadium charge states. Measurements of the Hall effect and the specific resistance confirm homogeneous electrical properties as well as semi-isolating behaviour with charge carrier concentrations in the range of 108 cm-3, specific resistance values around 1011 Ohm-cm and an activation energy of EA > 700 meV at room temperature in the crystal disks.

Crystals with co-doping of vanadium and boron were produced in which the electrical properties are determined by the vanadium donor. The usable semi-insulating range has activation energies between 1.2 eV and 1.8 eV and exhibits extremely high specific resistances limited at room temperature by leakage currents and poor contacts. It is limited by the high nitrogen concentration and precipitate formation at the beginning of cultivation and by the decreasing vanadium incorporation at the end of cultivation. By optimizing the co-doping, it has been possible to significantly increase the semi-insulating range in the crystals.

The highly pure semi-insulating SiC without vanadium doping developed by some research groups, in which the compensation is performed by intrinsic defects, shows ways to further optimize the production of semi-insulating SiC single crystals: A further reduction of the impurity concentrations and the investigation of alternative impurities for compensation in volume crystal growth, together with the improvement of the homogeneity of the vanadium incorporation, can be regarded as the most urgent tasks.