for direct recombination, electrons go from up to down directly and vertically. However, for indirect recombination, it has to go through two-step process( go left and down) Here is my question. It makes sense to use for direct recombination, but why does indirect recombination have to indirectly go to two step instead of one. It's like if you are going from LA to Tokyo and why do you have to stop in Hawaii? As far as I concern, this is a mysterious question and only it only has plausible explanation from some journals or scholar book. Can you at least give me some scholar books that may have answer. Thank you !!!!!!