Free GaAs surfaces studied using a back-gated undoped GaAs/Al_xGa_1-xAs heterostructure

We study the free GaAs surface by using a back-gated undoped GaAs/Al_xGa_1-xAs heterostructure. This structure is suitable in investigating the free GaAs surface since a two-dimensional electron gas is induced by the back-gate bias in the undoped heterostructure. We compare the channel depth dependence of the transport characteristics with two different models of the free GaAs. The "midgap pinning model" assumes a constant surface Fermi level and an alternative approach called the "frozen surface model" assumes a constant surface charge density. The experimental results indicate that the frozen surface model appropriately describes free GaAs surfaces at low temperatures although the midgap pinning model is widely accepted. This is because charges cannot be transferred to the free GaAs surface at low temperatures.