We have studied critical layer thickness (CLT) in an In0.75Ga0.25As resonant tunneling diode structure grown on a GaAs substrate, ia an InAlAs step-graded buffer (SGB) with two types of inverse step(IS)-SGB. We have observed red or blue shift in photoluminescence spectra and CLT change depending on SGB condition. This change reflects from the difference of residual strain InAlAs SGBs adopted here.,(C) 2001 Elsevier Science B.V. All rights reserved.