The characteristics and growth of III-nitride materials on ferroelectric lithium niobate with varying Li mole composition have been investigated to achieve a better device performance of AlGaN/GaN heterojunction structures. III-nitride growth on lithium niobate has been performed after high temperature (1000 degrees C) furnace thermal treatments in dry air environment resulting in atomically flat surfaces on lithium niobate (LN). However, while this furnace thermal treatment results in improved surface smoothness and III-nitride adhesion, it also causes repolarization, ferroelectric domain reversal from a+z spontaneous polarization to a -Z spontaneous polarization in the surface of congruent LN (48.39 mole % of Li2O). On the other hand, near-stoichiometric LN (49.9 mole % of Li2O) did not develop repolarization during the identical thermal treatment. Furthermore, as determined in situ by spectroscopic ellipsometry, congruent LN (CLN) shows a bigger variation of the pseudorefractive index and pseudoextinction coefficient during annealing at various temperatures in the range of 100-800 degrees C, indicating surface modification at those temperatures. On the other hand, near-stoichiometric LN (SLN) shows little change in these indexes implying that SLN provides better surface stability at elevated temperatures in the vacuum environment. This improved vacuum tolerance of SLN has allowed Al0.20Ga0.80N/GaN heterostructures to be grown on CLN and SLN substrates, with two times higher channel mobility resulting on SLN substrates. Thus, SLN appears to be a viable substrate for molecular beam epitaxial growth of III-nitrides in various applications. (c) 2006 American Vacuum Society.