Two general criteria were proposed for the situation where a surface defect state acted as an efficient non-radiative recombination center. The first criterion was that the thermal ionization energy should not deviate from the mid-gap energy by more than the relaxation energy of the defect. In this case, the activation energy for recombination was given by the barrier to the capture of the first carrier, whereas the second carrier was captured athermally. The second criterion was related to the position of the average dangling-bond energy relative to the band edges. If, as in the case of InP or InAs, it was located close to a band edge, a low surface recombination velocity was expected. However, much more rapid recombination was predicted, and experimentally observed, in materials where the average dangling-bond energy was located close to the mid-gap.

J.M.Langer, W.Walukiewicz: Materials Science Forum, 1995, 196-201, 1389-94