upper tier 106 a of trench 106 is formed by reactive ion etching in this example upper tier 106 a has a depth equal to about half of the total depth of trench 106 iii the top surface of the lateral wall material that is inside the trench is further selectively masked off that is masks are placed according to the plan view footprints of the defect members to be formed iv lower tier 106 a of trench 106 is formed by further reactive ion etching and v the masking is removed to yield trench cut intermediate sub assembly 100 in this example the lower tier 106 b of trench 106 can be described as u201c zig zag u201d because of the pattern formed by the defect blocking members as shown in fig5 to 8 epitaxially grown intermediate sub assembly 200 includes lateral wall sub layer 102 base sub layer 104 defect blocking members 108 110 and epitaxial layer 202 in this example epitaxial layer 202 is made of a iii v semiconductor material specifically gaas alternatively any material now known or to be developed in the future to which an aspect ratio trapping trench is applicable could be used for example other iii v semiconductor materials such as ingaas gaas or ingaas epitaxial layer 202 is epitaxially grown to an extent that epitaxial.