Intrinsic a-Si:H samples were grown with and without hydrogen (H2) dilution of silane at different growth rates. We find that the dilution leads to a considerable reduction in the defect density, in particular at high growth rates. The defect density is particularly low for samples grown using H2 dilution conditions at growth rates as high as 10 Å/sec. Using transient photocapacitance measurements we find evidence for a small concentration of microcrystallites embedded in the amorphous films. An increase in the microcrystalline fraction correlates with a decrease in the defect density.