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Ordered In x Ga 1 À x P alloys have been grown on to GaAs(001) substrates by metal-organic vapour-phase epitaxy. Lattice-matched compositions of alloys (In x Ga 1 À x P with x % 0.5) were used in order to produce epitaxial layers free of structural defects (threading dislocations). Growth temperature and substrate orientation were adjusted to control the degree , of order, of the alloys in the range 0.3-0.5. Nanoindentation tests were carried out to measure the mechanical response of the heteroepitaxial layers. Transmission electron microscopy was used to characterize the structure as well as the plastic zones generated into the specimen by nanoindentation. Comparison of the alloys with binary references (InP and GaP bare substrates) showed strengthening, with the hardness and flowstress values that are much larger than those of InP and of the same order of magnitude as those of GaP. } 1. Introduction Solid-solution strengthening of III-V semiconductors has been used to reduce the density of dislocations introduced during their growth and to improve their mechanical behaviour (Ehrenreich and Hirth 1985, Rabier andGeorge 1987). The mechanical properties of semiconductors are relevant in connection with optoelectronic applications. The development of heterostructures has shown that the performance of devices is dramatically affected by threading dislocations that extend through the active layers. However, the mechanisms by which such dislocations appear in mismatched structures is still controversial. Recently, by modifying the surface of the substrate (compliant substrates), the structural quality of heteroepitaxial layers has been improved (Patriarche et al. 2000a,b, Patriarche andLe Bourhis 2000). Further improvement in the structure and performance of the devices is expected once the strengthening of the different layers has been engineered.

Nanoindentations have been made on (001) surfaces of InAs single crystals at room temperature and the indents formed observed by transmission electron microscopy. The loading and unloading curves of a Berkovitch indentor submitted to maximum loads ranging between 200 and 10000 mN, were analysed and compared to those previously obtained in GaAs. The plastic zone size was measured as a function of maximum load to determine the yield strength of InAs at room temperature. Finally, the arrangement of the dislocations at the edge of the plastic zone has been analysed and is compared to a previously reported arrangement for GaAs.