Under certain growth conditions InAs/Si heteroepitaxial growth proceeds via Stranski-Krastanow or Volmer-Weber growth modes depending on the growth parameters. Critical thickness at which three dimensional InAs islands start to appear at the Si(100) surface is within the range of 0.7 - 4.0 monolayers (substrate temperature range is 350°C - 430 °C). Their size depends critically on the growth conditions and is between 5 nm and 80 nm (uncapped islands). Critical lateral size of the coherent (Si capped) dislocation-free island is equal to 2 - 5 nm depending on the island height. Islands having larger size are dislocated. Optical properties of InAs nanoscale islands capped with Si reveal a luminescence band in the 1.3 μm region.