Low temperature humidity sensor based on Ge nanowires selectively grown on suspended microhotplates

•Monocrystalline p-type Ge NWs have been locally grown on top of microhotplates.•Gas sensor based-on Ge NWs have been studied in a strict low temperature range.•Pre-adsorbed O2 is required for gas sensing towards reducing species.•The depicted sensing mechanism is comparable to the model for metal oxide. The gas sensing properties of germanium (Ge) monocrystalline nanowires (NWs) at temperatures up to 100°C have been demonstrated for the first time. The devices have been fabricated based on an energy efficient and site-specific vapor-liquid-solid growth of NW meshes on top of microhotplates, which contain a buried heater and top electrodes. The devices have been investigated for the detection of oxygen, nitrogen dioxide and carbon monoxide gases, showing the important effect played by pre-adsorbed surface oxygen in the response to the different gases. The Ge NW-based devices exhibit p-type conductivity and show high selectivity in their response towards water vapor. Water vapor interaction is not dependent on the presence of oxygen and the adsorption leads to electron donation in the Ge nanowires. TEM analysis of the NWs proves that they are covered by a thin, outer germanium oxide layer, which is stable and does not grow upon exposure to these gases and operation temperatures up to 100°C. The presence of this oxide layer plays a key role in the sensing mechanisms.