In this paper we experimentally determined the critical layer thickness for highly strained 1.2-mum GaInAs/GaAs quantum wells of good crystal quality. The dependence of the critical layer thickness on the indium content indicates that the observed quality degradation is caused by a growth mode transition. This is also supported by transmission electron microscopy measurements. We discuss the possibility of extending the wavelength of highly strained GaInAs/GaAs quantum wells toward 1.3 mum by delaying the growth mode transition. As a first step, a wavelength extension to 1.225 mum is achieved by using the presented technique.