Substrates for microelectronic radiofrequency devices may include a substrate comprising a semiconductor material. Trenches may be located in an upper surface of the substrate, at least some of the trenches including a filler material located within the respective trench. A resistivity of the filler material may be 10 kOhms·cm or greater. A piezoelectric material may be located on or above the upper surface of the substrate. Methods of making substrates for microelectronic radiofrequency devices may involve forming trenches in an upper surface of a substrate including a semiconductor material. A filler material may be placed in at least some of the trenches, and a piezoelectric material may be placed on or above the upper surface of the substrate.