Quantum size effects in epitaxial ErAs on GaAs(001)

The electronic structure of very thin epitaxial ErAs layers on GaAs(100) is studied with angle resolved photoelectron spectroscopy. Clear evidence is found for confinement induced quantization of states around the Fermi level. From the dispersive properties of the quantum well states effective masses are obtained, representing electron motion parallel to the surface layers and orthogonal to the layers. We find, for the first time, that effective masses along equivalent bulk directions (XW) are significantly different in the thin layers. Furthermore, the bottom of the highest occupied band shifts towards the Fermi level when going from very thin to thick ErAs layers.