ETCH TECHNIQUE 



45 



Brazil twinning). Further, twinning exhibiting both opposite-handedness 

 and same-electrical-sense, combines the cr\'stallographic twinning laws of 

 Brazil twinning and Dauphine (electrical) twinning. Hence this uncom- 

 mon variety of twinning may preferably be called combined electrical and 

 optical twinning, or just COMBINED TWINNING. Thus, the boundary 



Fig 5 24— Regions d are electrical twins of the region c. The striated regions a are of 

 opposite handedness and electrical sense to the d region enclosing them, hence optical 

 twins of d. The b regions are small optical twins of c, and/ are flaws. 



between the a and c twins separates combined twins. Note also that the a 

 twin bounds the b twin and the h twin bounds the c twin. Thus, a and b are 

 true electrical twins, and b and c are true optical twins." 



23 It is possible that growth conditions are such that combined twinning cannot occur 

 bv itself without the presence of true optical twinning and true electrical twinning. That 

 is, a region of given handedness and sense can not be entirely bordered by a region of 

 opposite-handedness and same-sense. 



