On the piezoelectric coupling constant of epitaxial Mg-doped GaN

The electromechanical coupling coefficient (k2) of an Mg-doped GaN epilayer has been evaluated by Lee et al. [IEEE Trans Electron Dev 2001;48:524–9] as (4.3 ± 0.3)%, which is much higher than those of other comparable III–V materials. As this is a surprising result, the present paper reports an independent determination of k2 from a similar Mg-doped GaN epilayer for comparison. A 2 μm thick Mg-doped GaN epilayer ((0 0 0 1) orientation) was grown on a c-plane sapphire substrate. The atomic concentration of Mg was 1.2 × 1018/cm3. Surface-acoustic wave (SAW) delay-line filters were fabricated using evaporated aluminium and “lift-off” techniques, and consisted of pairs of interdigital transducers (IDTs). Each IDT had 150 pairs of double-fingers. The SAW propagation direction in the sapphire substrate was [1¯100]. Acoustic wavelengths of 32 μm, 40 μm, 48 μm and 56 μm were used in the present work. The insertion loss of these filters was found to be more than 80 dB. This allows an upper bound of the electromechanical coupling coefficient (k2) of the Mg-doped GaN epilayer to be calculated as 1 × 10−4%. Our value, significantly lower than previously reported, shows that an Mg-doped GaN epilayer may not be such a promising material for SAW devices as appeared at first sight.