Many semiconductor materials cannot be grown as ingots that are later cut into wafers, like it is done with silicon or germanium, but they are grown epitaxially

Many semiconductor materials cannot be grown as ingots that are later cut into wafers, like it is done with silicon or germanium, but they are grown epitaxially as thin layers on wafers made of other semiconductor or insulator materials that have a compatible crystal structure. Most gallium nitride devices, like those that are used now in miniature chargers for laptops/smartphones, are made like this. Using this technique for indium selenide is a standard procedure, not something surprising. The "wells" are made by doping with various kinds of atoms, which is normally done by ion implantation, i.e. a ion beam inserts the desired impurities into the crystal, at the desired depth. In very thin devices, like in most modern CMOS technologies, the doped zones no longer look like "wells". For an N-channel MOSFET, you just have from source to drain 3 zones of alternating polarity, n-p-n. The middle zone is surrounded partially or even totally by the gate insulator.