OPTICAL OBSERVATION OF TWINNING PATTERNS CHARACTERISTIC OF CUPTB ATOMIC ORDERING IN GA1-XINP

Patterns of alternating bright and dim stripes, a few micrometers wide and a few tens of micrometers long, parallel to [110], appear at the surface of (001) Ga1-xInxP epitaxial layers displaying atomic ordering , alongside cleavage planes, when observed between crossed polarizers under a microscope. It is shown that these patterns associated to twin ned regions of the sample (each stripe corresponding to a single varia nt large domain) are a consequence of the spontaneous trigonal strains resulting from atomic ordering. The strains being differently oriente d in the two variants of the CuPtB structure give rise to two index el lipsoids, distinctly oriented in each variant domain. The boundaries b etween two adjacent variants are 1 (1) over bar 0 planes in order to m aintain a strain compatibility between them. Under the polarized micro scope these boundary planes appear as [110] directions at the surface of the sample. Obviously the same 1 (1) over bar 0 planes separate lar ge adjacent CuPtB domains produced by Chen and Stringfellow, starting from substrates patterned with [110]-oriented groves. The (001) planes which also fulfill the strain compatibility between two CuPtB variant s have been observed as boundaries between two variant domains by tran smission-electron microscopy. The strong analogy between atomic orderi ng and ferroelasticity is pointed out and is exploited here, to give a straightforward solution to problems related to variant domain struct ure in ''ordered'' alloys.