Crystal growth in c direction and crystallographic polarity in ZnO crystals

The c-axis polarity of ZnO crystals growing in the c direction was investigated for crystals grown by chemical reaction in the vapor, by VLS mechanism and by chemical vapor deposition. In the initial growth stage ZnO crystals nucleate both in the +c and -c directions in each type of growth. In growth by chemical reaction in the vapor, a higher growth rate of the crystal in the +c direction results in dominant growth of large crystals in this direction. The difference in the growth rate is attributed to a higher effective surface energy of the Zn atom surface than that of the O atom surface. On the other hand, in VLS growth long whiskers and needles grow in both c directions. This fact indicates that the difference in the growth rates between the two c directions decreases in VLS growth.