48 Iranian Journal of Materials Science & Engineering Vol. 8, Number 1, Winter 2011 1. INTRODUCTION ZnS, a II-IV wide band gap materials, has attracted considerable attention due to many potential applications in the field of short-wave length photoelectronic devices, especially for ultra-violet laser diodes and photodetectors [1,2]. ZnS crystals have been grown by variety of techniques [3-6]. Among these techniques, chemical vapor transport (CVT) has received grate interest due to several advantages such as lower growth temperatures than melting and sublimation points of materials [7]. It is difficult to grow ZnS crystals by sublimation because of the low vapor pressure at the temperatures where the cubic modification is stable. In addition, these compounds crystallize in a mixed zincblende- wurtzite structure with the presence of twinning and stacking faults when they are grown at high temperatures from melt. CVT has some disadvantages; as an example, controlling the nucleation and establishing optimum growth conditions to obtain high quality and large crystals is to some extend difficult. Experiments have shown that transport rate inside the closed CVT tube depends on the temperature difference between growth and deposition zones which is called undercooling. Literature survey shows that ZnS single crystals have been grown by CVT technique in broad range of temperature from 700 °C to 1150 °C using iodine with concentration of 1-7 mg/cm 3 [8-15]. This survey also shows that these ZnS crystals have been grown in the wide range of 10-300 °C undercooling. 10 °C is the lowest undercooling for which the nucleation of zinc sulfide on the quartz has been reported [9]. For T> _ 10 °C the nucleation begins so vigorously that no control is possible inside the quartz ampoule and a number of crystals grow simultaneously. However, 10 °C undercooling results in the growth of ZnS crystals with bigger size and better quality when seeding procedure is performed. We have shown that in addition to undercooling the stability in flow of material inside the tube is another important factor which affects the nucleation and partially controls this process [7]. The flow of material is temperature dependent process and it has been shown that the exact temperature distribution inside the ampoule is different from that of the heating arrangement and temperature oscillations of up to ±25 °C under typical growth conditions have been measured [17]. In present work, we have grown ZnS crystals with different undercooling values and mass transport stability conditions. The quality of the crystals grown under different conditions has been compared with each other.  EFFECT OF MASS TRANSPORT STABILITYAND UNDERCOOLING ON NUCLEATION OF ZNS CRYSTALS IN CLOSED AMPOULE CVT M. J. Tafreshi 1,* , B. Dibaie 1 , M. Fazli 2 , M. Alidaie 1 * m_jtafreshi@yahoo.com Received: September 2010 Accepted: January 2011 1 Physics Department, Faculty of Science, Semnan University, Semnan,Iran. 2 Chemistry Department, Faculty of Science, Semnan University, Semnan,Iran. Abstract: ZnS crystals were grown by CVT technique using iodine as a transporting agent under different mass transport stability conditions. Theoretically predicted optimum growth temperature and lower undercooling ( T) were applied for getting better control on mass transport and nucleation inside the quartz ampoule. ZnS crystals were grown at both optimum and non-optimum temperatures under different undercooling conditions and their characteristics were compared with each other. For 2 mg/cm 3 concentration of transporting agent ( C ) and T=15 °C growth at optimum temperature (1000 °C) yielded ZnS crystals with volume of 12x4x2 mm 3 and proper quality. For C= 2 mg/cm 3 and T=50 °C growth at non optimum temperature (1050 °C) resulted to the formation of ZnS polycrystallites with faulty structures. Keywords: ZnS, CVT, Nucleation.    Technical Note [ Downloaded from ijmse.iust.ac.ir on 2021-12-07 ] 1 / 5