P.J. Roksnoer

Dislocation-free GaAs and InP crystals by isoelectronic doping

Abstract The influence of isoelectronic doping on the properties of LEC GaAs and InP crystals has been studied. These crystals exhibit the usual electrical properties. A decrease of the dislocation density is obtained using In and Sb as dopants in GaAs crystals, and Sb, As and Ga as dopants in InP crystals. GaAs: In crystals in the range of 20–35 mm diameter and 100–300 g in weight have been obtained nearly dislocation free.

Microdefects in a non-striated distribution in floating-zone silicon crystals

Abstract A new type of microdefect has been found in fast-grown dislocation-free floating-zone silicon crystals. The distribution, the concentration and the size distribution have been determined using techniques such as copper and lithium decoration and X-ray topography. The defects in as-grown crystals could not be revealed by means of preferential etching or high voltage electron microscopy. It is concluded that these microdefects are formed by condensation of vacancies which only can become supersaturated when the cooling rate of the crystal is high.

Effect of growth parameters on formation and elimination of vacancy clusters in dislocation-free silicon crystals

Abstract Dislocation-free Si crystals grown by floating-zone techniques generally contain two types of vacancy clusters located in a striated pattern (‘wisrls’). By means of a series of special growth experiments it was established that the formation sequence of both types of clusters predominantly depends on the crystal cooling rate, while the formation of one particular type is also related to the occurrence of remelt. It is suggested that remelt, due either to crystal rotation in an asymmetrical thermal environment or to melt turbulence promotes the trapping of non-equilibrium vacancy concentrations at the solid-liquid interface. The possible mechanisms of this trapping process are discussed.

The mechanism of formation of microdefects in silicon

Abstract Three types of microdefects, formed by the agglomeration of either self-interstitials or vacancies, have been observed in dislocation-free silicon crystals grown by the pedestal pulling technique. From the microdefect distribution in quenched crystals it is concluded that the dominant type of point defect at the melting point of silicon is the vacancy, whereas an excess of self-interstitials is usually present at lower temperatures. It is suggested that a fraction of the carbon atoms in the crystal is incorporated at interstitial lattice sites and that the interaction of interstitial carbon with vacancies is responsible for the change in the dominant type of point defect. A model is described, which is based on this assumption, and it is shown that the influence of different growth conditions on the formation, the elimination and the spatial distribution of all three types of microdefects can be explained with this model.

Effect of low cooling rates on swirls and striations in dislocation-free silicon crystals

Abstract The occurrence of swirls and the amplitude of dopant striations in dislocation-free silicon is shown to be strongly influenced by the cooling rate during crystal growth. It has been found that the formation of swirls can be prevented if the cooling rate does not exceed 5°C/min. It is argued that at this cooling rate the concentration of thermal defects decreases below the critical value necessary for swirl formation by diffusion to the crystal surface. In addition the amplitude of dopant striations is reduced by solid-state diffusion, which permits the growth of more homogeneous silicon crystals.

The introduction of dislocations during the growth of floating-zone silicon crystals as a result of point defect condensation

Abstract During growth of dislocation-free floating-zone crystals two types of “swirl defects” (A- and B-clusters) are formed as a result of point defect condensation. The diffusion coefficient of these point defects (vacancies, silicon interstitials) in the temperature interval 1050–1100 °C, is found to be 2 × 10 -5 cm 2 /sec. Section topographic analysis showed that the A-clusters (dislocation loops) rapidly increase in size with decreasing crystal cooling rate. This is attributed to a combined climb-glide process. Evidence is presented that the predominant cause for the loss of dislocation-free growth, particularly of large-diameter crystals, is due to emission of dislocation arrays from the A-clusters.

Dislocations in GaAs

Abstract Dislocation-free GaAs crystals have been obtained by two different ways (doping and necking procedure) in the LEC growth technique. Undoped GaAs crystals with a diameter as high as 15 mm have been grown free of dislocations. It is shown that if thermal stresses determine the dislocation distribution, some important phenomena cannot be explained, and it is suggested that the stoichiometry of the crystal, the behaviour of point defects, the interaction between impurities and point defects at temperatures just below the melting point play an important role.

Formation and elimination of growth striations in dislocation-free silicon crystals

Abstract Growth striations due to inhomogeneous incorporation of dope impurities as well as residual carbon in dislocation-free floating-zone and Czochralski silicon crystals have been studied. The dope striations are detected by means of spreading resistance measurements and preferential etching, while the carbon striations are analysed using among other things a special type of X-ray transmission section topography. The relationship between various growth parameters and striation formation is determined. The influence of remelt phenomena and solid-state diffusion on striation formation is analysed and directives are obtained for the elimination of striations during crystal growth. Carbon striations are also eliminated by means of annealing treatments of the as-grown crystals.

Analysis of the temperature distribution in FZ silicon crystals

An analytical solution for the temperature distribution in freely radiating crystals has been developed. It was possible to solve this problem, within the growing crystal, because the heat transfer due to radiation was small as compared with the heat transfer due to conduction. The solution was applied to the growth of silicon crystals grown by the floating-zone method and the results were found to be in good agreement with experimental data. It is shown that the influence of the crystal growth rate on the temperature profile must be included, when it is higher than 1–2 mm/min. The longitudinal temperature gradient at a concave solid- liquid interface was found to vary in a characteristic manner with the distance from the axis. A similar variation was observed for the height of the defect area formed at the solid-liquid interface in quenched silicon crystals.

Growth of dislocation-free gallium-phosphide crystals from a stoichiometric melt

Abstract The conditions required for dislocation-free growth of gallium-phosphide crystals by means of the LEC technique are investigated. Using X-ray topography of longitudinal crystal sections it is found that growth of the crystal cone after the necking-in procedure is the most critical stage. Dislocation-free crystals are grown in the Ga 〈111〉 direction up to a diameter of 15 mm at a pulling speed of 0.20 to 0.25 mm min -1 . Doped as well as undoped dislocation-free crystals show a striated distribution of microdefects after preferential etching of P {111} surfaces.