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A scanning tunneling microscope (STM) is used to measure the piezoelectric constant of thin films including its sign with high lateral spatial resolution. Since a STM offers an extraordinarily high vertical resolution, even weak piezoeffects can be studied. Test measurements with an X-cut quartz in air at room temperature showed qualitatively correct behavior, but the measured value of the piezoelectric constant varied both temporally and spatially due to the nonideal characteristics of the tunneling contact. In order to circumvent this difficulty we used a novel compensation technique, in which the piezoelectric sample is mounted on an X-cut quartz. We present measurements of the piezoelectric constant of sputtered ZnO films which yielded two different values, one of them being considerably higher than the piezoelectric constant of bulk ZnO.

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