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Becker, Thomas; Sonnenberg, Klaus
Defektspezifische Kristallzucht von Galliumarsenid mit den vertikalen Bridgman-Verfahren und Untersuchung von versetzungsnahmen Defektzonen
104 S., 1998

GaAs crystals have been grown by the vertical Bridgman technique and have been analysed by various methods, particularly by near infrared microscopy. The main objective of the present work was to study defect zones which form in doped GaAs around dislocations as a result of reactions between impurities and dislocations. A new model for the formation of defect zones is proposed and experimental results are presented which provide strong evidence for the defect model. For this purpose crystals were doped with different impurities and the zone formation was also studied for different stoichiometric conditions. The new defect model assumes that impurities occupying the As sublattice exchange their substitutional position with As interstitials. This process provides fast diffusing interstitial impurities which can be gettered (probably by forming precipitates) at the dislocation. As a result a diluted defect zone is generated around dislocations. The zone becomes visible by NIR phase contrast through the change of the refractive index in the zone. This change in turn is due to the variation of the free carrier concentration which is proportional to the impurity concentration. According to our model we expect only those impurities to contribute to the formation of the defect zone which occupy As lattice sites (e. g. BAs, SiAs, GeAs, SAs, SeAs, TeAs) but not those on Ga lattice sites (ZnGa, SnGa, SiGa, GeGa). This is confirmed by the experimental results. Also confirmed is the expected change in the carrier concentration in the defect zone which depends on the electrical character of the impurities, i. e. [Delta]ne < 0 for SAs, SeAs and TeAs (donors) and [Delta]ne > 0 for GeAs, BAs and SiAs (acceptors). Since the diffusion mechanism is initiated by As interstitials the formation of defect zones is found to be strongly suppressed if the crystals are grown from a Ga-rich melt (S, Se, Te and Ge-doped GaAs). The dilution of impurities occupying As lattice sites within the defect zone is also confirmed by the study of photoluminescence spectra of Ge- and Si-doped GaAs. By doping GaAs with various impurities the following "spin-off" results were obtained:
(i) Since the crystal growth takes place in a liquid encapsulation with B2O3 unintentional co-doping with boron is observed particularly for Si and Al dopants. The effect is much smaller for S, Se, Te, Ge, Sn and Zn dopants.
(ii) For Si doping the codoping effect depends on the material of the crucible. It is reduced for quartz crucibles, since the B2O3 encapsulation is enriched with SiO2 through the corrosion of the quartz crucible.
(iii) The double acceptor BAs, usually found in "Ga-rich" GaAs, was also observed for stoichiometric GaAs, if this is doped with Si.


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