InAs/InP quantum dot (QD) stacks were grown by low-pressure metalorganic chemical vapor deposition and the entire growth procedure was monitored by real time in situ spectral reflectance (SR). Intensity of 2.4 eV peak in subtraction SR spectra increased during 30s growth interruption (GI) under AsH3 atmosphere, suggesting that morphological evolution for InAs QD formation occurred. The slopes in reflectance signals at 470 nm during AsH3 GIs increased for the second stack (2.5 times increase) and increased further for the third QD stacks (5 times increase) when 10 nm thick spacer layers were used. For the 10 nm spacer layers, the dot size increased and the density decreased in the second and the third QD stacks, as measured by atomic force microscopy, However, little changes in QD size, density and uniformity in the second and the third QD stacks were observed with 50 nm spacers. The formation of excess InAs due to the enhanced As/P exchange reaction with the thin InAs spacer is responsible for the changes in reflectance slopes during AsH3, GIs. (C) 2002 Elsevier Science B.V. All rights reserved.