Evaluation of metal-organic frameworks in electronic devices for gas sensing

Abstract

Integrating nano-porous metal-organic frameworks (MOFs) in electronic devices such as capacitors, transistor or memristor enables sensing applications for a wide variety of guest molecules. Particularly, the incorporation of thin MOF films in metal-insulator-semiconductor (MIS) capacitor structures allows real-life applications because of its low voltage operation. Additionally, MIS capacitors offer a thorough study of interfacial defects such as interface traps and border traps distributed within the device. In electronic devices, a low concentration of interfacial defects is required to avoid threshold-voltage instabilities. This fact guarantees good stability and performance of electrical devices. This research work provides detailed investigation about charges and defects in MOFs-based MIS capacitors by impedance spectroscopy. Cu3(BTC)2 was coated directly on silicon, and on thermally grown silicon dioxide surfaces in a layer-by-layer fashion. The layer thickness was easily handled by varying the number of spray cycles in the coating process. In addition, Si/SiO2/Al MIS capacitors were investigated for comparison reasons. The successful growth of ultra-thin Cu3(BTC)2 films on silicon substrates was verified via X-ray diffraction (XRD) experiments. The function of MOFs within MIS capacitors was investigated via capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics measured at different frequencies and temperatures. The results show evidence of positive and negative fixed charges in the Cu3(BTC)2 dielectric layer as well as of the presence of border traps which cause hysteresis in the C-V characteristics. Evidence of interface traps is directly observed by the peak on the conductance curve. Analysis of the data demonstrates that ultra-thin Cu3(BTC)2 films prepared without ultrasonication exhibit a relatively low density of border traps (~1011cm-2), interface traps (~1011eV-1cm-2) and time response in the order of µs. Temperature-dependent measurements degrade the electrical quality of the MOFs. The addition of ultrasonication steps on the coating process decreases considerably the density of border traps. Additionally, the layers are more stable under heating experiments, and after cooling they almost recover the initial state. The experimental results show that MOF-based capacitors exhibit interface quality comparable to inorganic materials, making them suitable for sensing applications.