Study of free GaAs surfaces using a back-gated undoped GaAs/AlGaAs heterostructure

We study the free GaAs surface by using a back-gated undoped AlGaAs/GaAs heterostructure. This structure is suitable for investigating this 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 'mid-gap pinning model', which assumes a constant surface Fermi level, and an alternative approach called the 'frozen surface model', which assumes a constant surface charge density. The experimental results indicate that the frozen surface model appropriately describes free GaAs surfaces at low temperature in spite of the fact that the characteristics deviate from this model at higher temperature or for a shallow channel.