We investigate energy band structures of the Si1−xSnx compound alloy in the zincblende structure using interacting quasi‐band (IQB) theory. We first extend the IQB theory for four‐element compounds and subsequently calculate the electronic structures of the virtual Si1−xSnxSi1−ySny alloy where x = y. Diagonalizing a 20 × 20 non‐Hermitian Hamiltonian matrix using sp3s* empirical tight‐binding model with parameters obtained for the Si, Sn, and SiSn (zincblende) crystals, we obtain the electronic energy spectrum of the Si1−xSnx alloy for arbitrary x. Comparing the band structures, we reveal that the indirect–direct gap crossover in Si1−xSnx alloys occurs around x = 0.67 with Eg = 0.87 eV. Calculated Sn concentration dependence of energy gaps in Si1−xSnx alloy.

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