I am reading Shankar's "Principles of Quantum Mechanics" and am up to the part where he uses Schrodinger's equation to derive the wave function for various 'simple' scenarios in one spatial dimension. The first few were fine but his presentation of the step potential problem (specifically, the time evolution of a Gaussian wave packet encountering a step potential) is appalling. It's as though he dashed it off in a hurry one morning when he was late for his train to work. It leaves huge gaps, doesn't explain the steps, uses undefined terms and unintroduced concepts, to the extent that I find parts of it completely impenetrable. This is a great pity as the step potential problem appears to explain how the amazing phenomenon of quantum tunnelling can occur, and I was really looking forward to understanding that. Can anybody direct me to a derivation of the wave function for a step potential that is both clear and thorough? Thank you very much.