4. If a layer of unstrained 51 of only & few nanometers thickness s sandwiched between two layers of 510; such that quantum confinement effects are significant (that is, a \"quantum well\" is formed), what would you expect the conductivity effective mass for electrons to be roughly for transport in the plane of the well in the low carmier temperature limit where only the lowest energy-degenerate subbands are occupied? Assume a [100] 51 ervstal plane 15 in the plane of the well. (Hint: the confinement energy vanes inversely with the effective mass component in the direction normal 1o the mterfaces.)