Growth Kinetics and Morphology of ZnO Single Crystal Grown under Hydrothermal Conditions

Abstract

Anisotropic growth rates, interface processes during growing and occurrence of an inversion twin of ZnO crystals have been studied under a hydrothermal condition as growth temperature 360°-380°C, temperature difference 10°-25°C, pressure 600-1000 bar, solvent 3.0mol KOH with 0-2.0mol LiOH.The results showed:(1) The growth rate was a function of growth temperature, concentration of LiOH added and temperature difference between dissolution and growth zones; under Li-free condition the activation energies in normal direction of basal and prismatic face, (0001) and (1010), were 20 and 80kcal/mol, respectively, whereas these energies almost reversed in 2.0mol LiOH.(2) The concentration of Li adsorbed on -c (0001) or O-layer side was higher than that on c (0001) or Zn-layer side. This anisotropic adsorption of Li seems to be caused by the growth environment or reductive atmosphere.(3) The growth mechanism on (0001) face distinctly changed from a rough pattern to a spiral step with the increase of adsorbed Li content.(4) The rate functions of Rg∝tm and Rg∝ΔTn had the transition point which can be regarded as the alteration of processes at the interface, where Rg is the growth rate of linear extention of a particular face, t the growth time, ΔT the temperature difference between dissolution and growth zones, m and n constants. It is phenomenologically concievable that a interface of the initial crystal growth from the seed crystals, and the one of the following principal growth can be corresponded to the processes of the growth on the rough surface or kink face and on the smooth surface or spiral step face, respectively. From the results obtained m=1/2 and n=1 for the initial growth, and m=-1/2 and n=1.5-1.7 for the principal growth.(5) A polar twin easily occurred from spontaneous nucleation. This twin was an inversion contact twin at the initial state of the growth. With progressing of the growth, the twinning shape which changed from the inversion contact to the inversion core twin was observed by chemical etch pattern on the cross section in parallel with the c-axis.Thus the growth kinetics and morphology of ZnO single crystal are closely correlated with calculated activation energies, adsorbed Li content in the crystal, the rate functions and micro-growth mechanism of a particular surface.