Structure and interface-controlled growth kinetics of ZnAl2O4 formed at the (11(2)over-bar0) ZnO/(01(1)over-bar2) Al2O3 interface

The solid state reaction between metalorganic chemical vapor deposition gro wn epitaxial ZnO films and the R-plane sapphire substrate after annealing a t 1000 degrees C for various times in an O-2/N-2 atmosphere was studied in detail. Multiple epitaxial relationships between the reaction product (ZnAl 2O4) and the reactants were observed, as determined by cross-sectional tran smission electron microscopy. In the dominant epitaxial relationship (A1), the (2 (2) over bar 0) plane of ZnAl2O4 was parallel to the ((1) over bar 1 01) plane of Al2O3. A twin (A2) of orientation A1, i.e. (2 (2) over bar 0) ZnAl2O4//(10 (1) over bar 1) Al2O3, and a closely related orientation (B) w herein the (2 (2) over bar 0) ZnAl2O4 plane is parallel to the ((1) over ba r 2 (1) over bar 0) ZnO plane (which is equivalent to a 5 degrees clockwise rotation about the [<(11)over bar>2] ZnAl2O4 or [0001] ZnO zone axis relat ive to A2), were also observed. Enhanced growth was observed at grain bound aries. It was necessary to measure the spinel growth rate from grains with the same orientation far away from grain boundaries because the growth rate was observed to be influenced by the orientation of the grains in addition to the enhanced growth at grain boundaries. The growth rate was observed t o follow a linear rate law during early stages (for grains with orientation A1), suggesting an interface-controlled reaction. The structures of the Zn O/Al2O3, ZnO/ZnAl2O4 and ZnAl2O4/Al2O3 interfaces were studied for grains w ith this orientation (A1). The 13.7% lattice mismatch between ZnO and ZnAl2 O4 was relieved by a series of misfit dislocations spaced five to six (1 (1 ) over bar 00) ZnO planes apart. Due to the small lattice misfit (2.1%) at the ZnAl2O4/Al2O3 interface, very few misfit dislocations were present. Thi s interface was faceted and the sapphire surface had a series of single ste ps. It is expected that the reaction at the ZnAl2O4/ZnO interface is the ra te-controlling step due to the necessity for a dislocation climb (of a larg e number of misfit dislocations) for movement of this interface. (C) 2000 A merican Institute of Physics. [S0021-8979(00)06808-0].