* Corresponding author. Fax: #886-2-2363-3917. E-mail address: lan@ruby.che.ntu.edu.tw (C.W. Lan) Journal of Crystal Growth 203 (1999) 286}296 E!ects of ampoule rotation on vertical zone-melting crystal growth: steady rotation versus accelerated crucible rotation technique (ACRT) C.W. Lan*, J.H. Chian Chemical Engineering Department, National Taiwan University, Taipei 10617, Taiwan, ROC Received 23 November 1998; accepted 4 February 1999 Communicated by D.T.J. Hurle Abstract Computer simulation is performed to illustrate the signi"cance of ampoule rotation on the #ows, interface shapes, and the growth rate in vertical zone-melting (VZM) crystal growth. For the growth of a 2 cm GaAs crystal in a quartz ampoule, simulation results show that even low-speed ampoule rotation can signi"cantly a!ect the #ows and further the growth interface. The concave growth front due to buoyancy convection can be inverted easily to a convex one by steady ampoule rotation. The accelerated crucible rotation technique (ACRT) also has a similar e!ect on the growth interface, but less e!ective. In addition, severe periodic growth and remelting is induced by ACRT, and it is enhanced signi"cantly by natural convection.  1999 Elsevier Science B.V. All rights reserved. PACS: 44.25.#f; 47.27.Te; 81.10.Fq; 02.60.cb; 02.70.Fj Keywords: Ampoule rotation; Zone-melting; Interface control; Buoyancy convection; Growth rate; ACRT 1. Introduction The control of melt #ow, the interface shape, and the growth rate is important in crystal growth pro- cesses. Especially, for crystal growth in an ampoule, a #at or convex growth front is usually required to minimize the parasitic nucleation of grains from the ampoule wall [1]. The convection and its asso- ciated heat and mass transfer are key factors for the interface control. Therefore, "nding suitable opera- tion parameters for a better control the transport processes is important for crystal growth. In addi- tion to the interface shape, the melt #ow is also responsible for the growth rate (solidi"cation or melting) and dopant mixing. An unstable growth rate due to time-dependent #ows and unfavorable solute segregation are also detrimental to crystal quality. Tuning heating pro"les and applying mag- netic "elds are well-known approaches for control- ling the melt #ow [2}6]. Vibration can also be used [7,8]. For vertical Bridgman (VB) or zone-melting 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 9 ) 0 0 0 9 2 - 5