* Corresponding author. Fax: #49 9131 85 8495; e-mail: georg.mueller@ww.uni-erlangen.de. Journal of Crystal Growth 198/199 (1999) 367—373 Analysis of types of residual dislocations in the VGF growth of GaAs with extremely low dislocation density (EPD1000 cm) J. Amon, J. Ha¨rtwig, W. Ludwig, G. Mu¨ller*  Department of Materials Science, WW 6, Crystal Growth Laboratory, University of Erlangen-Nu ( rnberg, D-91058 Erlangen, Germany  European Synchrotron Radiation Facility (ESRF), BP 220, F-38043 Grenoble Cedex, France Abstract GaAs single crystals (silicon-doped, 2 inch) with extremely low dislocation densities (etch pit density (EPD) 50—1000 cm) were grown by the vertical gradient freeze method. In these crystals we characterised different types of dislocations by the aid of white beam X-ray diffraction topography and infrared transmission microscopy. It was found for decreasing dislocation densities (EPD(200 cm), that dislocations having a line vector l, which is parallel to the [0 0 1] growth direction of the crystals, become more and more dominant. These residual dislocations are induced by the seeding process (so far we were using LEC-grown seed crystals). These residual dislocations cannot be avoided by minimising the thermal stress during the crystal growth process.  1999 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 61.72.F Keywords: Vertical gradient freeze; GaAs; Dislocations; Low EPD 1. Introduction Silicon-doped, n-type gallium arsenide crystals are widely used as substrates for the fabrication of optical devices, such as light emitting diodes (LED), photo-diodes and laser diodes. In the last few years, big efforts have been undertaken to produce laser diodes with high optical outputs of some watts from single diodes and some 10 W from diode arrays. The reduction of the dislocation density of the substrate material is essential, since dislocations in substrates seriously affect the performance and the life-time of laser devices [1—4]. Since the redis- covery of the Vertical Gradient Freeze (VGF) and the Vertical Bridgman (VB) method for the growth of III—V semiconductors by Gault et al. [5], it has been shown by a number of authors (e.g. [6—11]), 0022-0248/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 2 1 9 - 6