Phase and Structural Modifications in Porous Silicon Under Pulse Heating

The phase transitions in crystalline and amorphous porous silicon layers on silicon single crystal under isothermal or laser pulse nanosecond heating were modeled. The pulse heating was described as an adiabatic process by using a quasi-statistical approximation through homogeneous nucleation and growth of a new phase. The calculation of the free energy of porous silicon for cylindrical, spherical, and complex structures of the pores and its dependence on the pore radius, overall porosity, and thermoelastic stresses was made. The equilibrium free energy increased to 0.15 and 0.09 eV, with a corresponding decrease in melting temperature of 400 and 300 K for crystalline and amorphous porous silicon, respectively. The Laplace pressure retards this shift no more than 10 K. The possibility of epitaxial silicon layer formation (0.1 to 1.2 nm thick) on porous silicon after pulse heating (30 ns; beam density from 2 to 10 k J . m - 2 ) is shown.