Nano-indentation measurements on freestanding GaN substrates clearly showed, for the first time, that the hardness of the GaN crystal can be controlled by changing the growth conditions for hydride vapor phase epitaxy (HVPE). The hardness of the GaN crystal is probably governed by heterogeneous nucleation of dislocations through a nitrogen vacancy-related mechanism. The observed changes in the nano-indentation hardness can be explained in terms of the dependence on growth condition of the concentration of nitrogen vacancies in the GaN crystal. This control of the crystal hardness has a significant effect on the dislocation-reducing process during the HVPE-growth of freestanding GaN substrates. According to the theory, the threading dislocation density (TDD) should decrease continuously with increasing growth thickness. However, as a result of the accumulation of growth-induced stress, the reduction of TDD for a freestanding GaN substrate with a less-hard crystal stopped at a certain critical thickness and became saturated at around the mid-10(6)/cm(2) range. This saturation behavior of TDDs can be overcome by making the GaN crystal harder by changing the HVPE conditions, giving freestanding GaN substrates with extremely low TDDs in the range 10(5)/cm(2). (C) 2011 Elsevier B.V. All rights reserved.