Piezoresistivity of boron doped CVD diamond films

The piezoresistivity of chemical vapor-deposited (CVD) and boron doped polycrystalline diamond films has been measured for a wide range of resisitivities (from 0.1 Ω-cm up to 106 Ω-cm) at room ambient. Two branches of piezoresistivity of such films are identified—one for highly doped films (resistivities from 0.1 Ω-cm to 102 Ω-cm) and another one for slightly doped films (resistivities from 103 to 106 Ω-cm), the second one being reported for the first time. For both branches of piezoresistivities, the gauge factor increases when the resistivity increases, changing gradually from 7–9 up to 70–75 for the first branch (low resistivities) and from less than 0.1 up to 30–35 for the second branch (high resistivities). The two observed branches of piezoresistivity are attributed to the conduction mechanisms of impurity band conduction (branch 1) and valence band conduction (branch 2). For the first branch gauge factor values of approx. 70–75 are reached at room temperature, which are superior to that of polycrystalline silicon (30) and similar to those achieved for crystallised silicon (120) films. In addition to the instant piezoresistive response of the diamond films, slow creeping piezoresistive effects also have been observed with a time constant of several seconds. Such slow contributions to piezoresistive performance of diamond films may be attributed to the action of intergrain boundaries and in some cases significantly enhances the overall response of the devices.