Direct-gap group IV semiconductors based on tin

The MBE growth on InSb (and CdTe) of thin films of tin with diamond structure led to two unexpected findings: (i) photovoltaic behaviour suggesting an energy gap of 0.12eV, (ii) stabilisation of the diamond structure to about 70°C (compared with 13°C for bulk material). One explanation of semiconductivity is a strain-induced splitting of the Groves-Paul degeneracy which has been taken to confer semimetallic behaviour. Alternatively, if less probably, MBE material is free from Hg contamination (present in almost all earlier material), and Hg could act as both donor (interstitial) and acceptor (substitutional), which could compensate to give quasi-gap-bridging impurity bands in an intrinsically semiconducting material. Piezoresistance or other band measurements sensitive to band symmetry on MBE ¿-Sn are urgently needed to clarify the situation. The substrate stabilisation effect suggests that it could also be possible to stabilise films of the previously unknown (Sn, Ge) solid solutions by using appropriate substrates, e.g. InAs for compositions near Sn0.5Ge0.5 Such material could have considerable device interest: a direct gap of about 0.3 eV should be paired with higher carrier mobility (because of absence of polar scattering) than III-V or II-VI materials with a similar gap.