Effective Contact Potential of Thin Film Metal-Insulator Nanostructures and Its Role in Self-Powered Nanofilm X-ray Sensors

Abstract

We studied the effective contact potential difference (ECPD) of thin film nanostructures and its role in self-powered X-ray sensors, which use the high-energy current detection scheme. We compared the response to kilovoltage X-rays of several nanostructures made of disparate combinations of conductors (Al, Cu, Ta, ITO) and oxides (SiO, TaO, AlO). We measured current-voltage curves in parallel-plate configuration separated by an air gap and determined three characteristic parameters: current at zero voltage bias I, the voltage offset for zero current ECPD, and saturation current I. We found that the metals' ECPD values measured with our technique were higher than the CPD values measured with photoelectron spectroscopy in situ, i.e., no air contact. These differences are related to natural oxidization and to the presence of photo-/Auger-electron current leaking from the high-Z toward the low-Z electrode, as suggested by additional experiments carried out in vacuum. Further, the deposition of the 40-500 nm oxide layer on the surface of metallic substrates strongly affects their contact potential. This technique exploits ionization and charge carrier transport in both solid insulators and in air, and it opens the possibility of measuring the ECPD between metals separated by a solid insulator in a metal-insulator-metal (MIM) configuration. Additionally, we demonstrated that certain configurations of MIM structures are suitable for X-ray detection in self-powered mode.

Flexible X-ray Detectors Based on Amorphous Ga2O3 Thin Films

“…28 This viewpoint can be further consolidated by the work of D. Brivio et al, where they used high-energy current to detect X-ray signals. 29 Interestingly, a subsequent long turn-on transient appears in D2, D3, D4, and D5, which has also been observed in GaN and diamond X-ray detectors. 17,30 They just ascribed this behavior to defect traps and did not provide any strategy to control these traps.…”

“…If these high-energy electrons locate at a few nanometers below the sample surface, they can escape from the material and make a contribution to the air ionization effect. 29 At the same time, because of the electron escape process, a photochemical reaction will happen at the surface, which will change the stoichiometry of the surface with the occurrence of oxygen deficiency. 46 Most of the high-energy electrons exist in the whole active layer because of the good penetrability of X-rays.…”

Signal enhancement due to high‐Z nanofilm electrodes in parallel plate ionization chambers with variable microgaps

“…(a)]. The contribution from the electron current J(x → 0) for vanishing gap has been found to be of the same order as the current measured by placing the same Al–air–Ta sample in vacuum [i.e., about (1.2 ± 0.1) pA/cm 2 ] …”

Self‐powered nano‐porous aerogel x‐ray sensor employing fast electron current

“…The direction of conduction current depends on the specific values of the work functions. The contact potential can be more than 1V depending on the bulk and surface properties of the materials involved . However, as explained below for polyimide aerogel HEC detector, ionization current is insignificant compared to the fast electron current, and for this reason it is omitted in Fig.…”

“…, which is composed of central high‐Z emission electrode and two external low‐Z receptor electrodes. In general, the signal of a self‐powered HEC detector, operating without any external bias voltage, is composed of two contributions: (a) conduction current due to ionization of the dielectric, generation of charge carriers (electrons, holes, excitons) and their electrical transport in the internal electric field formed by the contact potential of disparate material electrodes (high‐Z and low‐Z) and, (b) fast electron current (high energy particle current) between the electrodes. The direction of conduction current depends on the specific values of the work functions.…”