D. Schulz

SixGe1 − x single crystals grown by the RF-heated float zone technique

Abstract Single crystals of silicon-germanium alloys have been prepared over the composition range of 0.78 ≤ x ≤ 1. Using the crucible-free RF-heated float zone technique a special method was developed for the alloying. Growth has been carried out on a pure silicon seed using growth rates of 0.5 mm/min. Complete dislocation free crystals containing up to 5.5 at% Ge could be obtained.

Growth of oxide compounds under dynamic atmosphere composition

Abstract Commercially available gases contain residual impurities leading to a background oxygen partial pressure of typically several 10 - 6 bar , independent of temperature. This oxygen partial pressure is inappropriate for the growth of some single crystals where the desired oxidation state possesses a narrow stability field. Equilibrium thermodynamic calculations allow the determination of dynamic atmosphere compositions yielding such self-adjusting and temperature dependent oxygen partial pressures, that crystals like ZnO, Ga 2 O 3 , or Fe 1 - x O can be grown from the melt.

Extremely reduced dislocation density in SixGe1 − x single crystals grown by the float zone technique

Abstract Using the crucible-free float zone technique single crystals were grown from silicon-germanium solid solutions in the 〈111〉 direction. No dislocations could be observed either by epd measurements or by X-ray topography or by IR-transmission microscopy of copper-decorated samples. The diameter was about 16 mm and the maximum germanium concentration ran up to 5.4 at%.

Growth related distribution of secondary phase inclusions in 6H–SiC single crystals

The formation of grown in defects like misoriented regions and micropipes is correlated with second phase inclusions such as silicon droplets or carbon particles. Crystals up to 35 mm diameter were grown on the silicon face by the modified Lely method. Inclusions could be identified as carbon particles by SEM and EELS. Their distribution along the growth direction and at the phase boundary were investigated by optical microscopy. It was found that their density varies in axial as well as radial direction. The dependency of the carbon particle concentration on growth parameters such as seed temperature, time, pressure and the distance between source and seed is shown. A concentration model will be discussed. Therefore any change of the vapor phase composition characterized by the loss of silicon and caused by changing of a growth parameter may result in a drastic local increase of the carbon particle concentration. If a critical value is exceeded the lattice information of the substrate is lost and defects may be formed.

Evolution of domain walls in 6H- and 4H-SiC single crystals

Abstract From the study of 6H and 4H single crystals, grown under various conditions on Si- and C- terminated seed surfaces, two types of defects with different slit length are discussed. Short slits with a length in the submillimetre range are spread between micropipes in an advanced growth stage. Their generation is correlated to micropipe agglomeration. The long slits up to some millimetres, which are associated with dislocation bundles in the basal plane, are correlated to polytype domain walls. In this region the slit formation seems to be favourable similar to the Frank mechanism of micropipe generation (Acta Crystallogr. 4 (1951) 479).

Computational study on the SiC sublimation growth

Abstract Using a global two-dimensional model of SiC crystal growth by the Modified Lely method the temperature field of the furnace, the multi-species fluid flow between source and crystal, and the transport rate of the crystal building species at the gas–crystal interface are analysed. Finite element codes are used for the field computations while a free energy minimizing software is used to determine the thermodynamic equilibrium composition of the gas phase. The computed species transport rates meet the range of experimental values.

Impurity incorporation during sublimation growth of 6H bulk SiC

Secondary ion mass spectroscopy (SIMS) and inductively coupled plasma spectroscopy (ICP-OES and ICP-MS) have been used to study the impurity concentration within different stages of the SiC crystal growth technology. The pure constituents silicon and carbon, the synthesized SiC powder as well as sublimation grown crystals of 6H polytype were investigated. It was found that the main impurities are iron, aluminium, tungsten, vanadium, nickel and copper. Although high purity silicon and carbon were used as starting materials the impurity level increases due to the preparation technique. However, in comparison to the SiC source powder the crystals show a reduction in the impurity concentration by one order of magnitude.

High Nitrogen Doping During Bulk Growth of SiC

Nitrogen is the most important shallow donor impurity in SiC and the ionization energy is lower in the 4H polytype compared to 6H-SiC. Dependent on the lattice site the activation energy is in the 60 meV range for the hexagonal site and is twice as high for the cubic site [2]–[5]. Highly conducting 4H-SiC substrates of high crystalline quality are one of the most important prerequisites for power electronic devices. The specific resistance should be as low as possible and n-type resistivities lower than 10 mΩcm have been reported for both 6H- and 4H-SiC [1].

Structural Improvement of Seeds for Bulk Crystal Growth by Using Hot-Wall CVD of 4H-SiC

Growth of 4H-SiC epitaxial layers has been performed in a horizontal hot-wall CVD (chemical vapor deposition) reactor using the silane-propane-hydrogen system. Two inch 4H-SiC, C-face wafers with an off-cut angle of about 7° towards <11 2 0> direction have been used as substrates. Micropipe dissociation has been investigated by varying the carbon-silicon (C/Si) ratio in the source gas atmosphere. Depending on the C/Si ratio the micropipes propagate into the layer without changing their image (C/Si > 1) or they dissociate in separate dislocations leaving a scar like formed surface region (C/Si £ 1). The substrates including epitaxial layers of reduced micropipe density were used as seeds for bulk crystal growth. If a micropipe is once closed in an epilayer grown at a low C/Si ratio, it is not opened in the subsequent growth process at high temperature.

Back‐Scattered Electron Imaging of Microscopic Segregation in (Si,Ge) Single Crystals

Compositional inhomogeneities of (Si,Ge) single crystals grown by the radio frequency (RF) heated float zone technique have been studied using the back-scattered electron (BSE) mode of a scanning electron microscope. Numerical analysis of the images by Fast Fourier Transformation (FFT) showed that the number of spatial frequencies with substantial amplitudes is increased when investigating longitudinal sections of crystals containing dislocations instead of dislocation-free crystals. This can be attributed to different growth conditions in terms of super-cooling.