  Elastic energy absorption measurements versus temperature on semiconducting,
semi-insulating (SI) and Fe-doped InP are reported. A thermally activated
relaxation process is found only in the SI state, which is identified with the
hopping of H atoms trapped at In vacancies. It is proposed that the presence of
In vacancies in InP prepared by the liquid encapsulated Czochralski method is
due to the lowering of their energy by the saturation of the P dangling bonds
with H atoms dissolved from the capping liquid containing H2O. The conversion
of iron-free InP to the SI state following high temperature treatments would be
due to H loss with the transformation of the H-saturated In vacancies, V_In-H_4
donors, into neutral and acceptor V_In-H_n complexes with n < 4. Such complexes
would produce the observed anelastic relaxation process and may also act as
deep acceptors which neutralize unwanted donor impurities.
