The high-frequency tunnel capacitance overload phenomenon of semiconductor-insulator-semiconductor heterojunction caused by the ultra-thin interfacial layers

Abstract

The SIS (semiconductor-insulator-semiconductor) structure heterojunction derived from the MOS (metal-oxide- semiconductor) devices, and inherit the advantages of low cost, simple structure, stable performance and high theoretical efficiency, which drew tons of attention for solar cell and integrated circuit applications researches. SIS device consist of TCO (transparent conduction oxide) layer, ultra-thin oxide layer and silicon substrate, in which the carriers can tunnel through the oxide layer when the thickness of the interface oxide layer is reduced to 1–5 nm. Inside the SIS solar cell, the ultra-thin passivation layer has multiple functions including tunneling contact and buffer the crystalline lattice mismatch. Capacitance reflects a device’s ability to charge and discharge, and the capacitance voltage ( CV ) characteristic is critical in the estimate of the passivation quality no matter in the high-low frequency method or DLTS (deep level transient spectroscopy system) method, and the passivation quality manifest as interface state density ( D it). When examining the high frequency (1 MHz) CV characteristic of the SIS which was fabricated by depositing ITO upon n-type silicon directly by RF magnetron sputtering, it is found that once the applied voltage ( V g) exceeds a certain value V T (tunneling voltage), the differential capacitance value of the device would far beyond the range of the instrument and tends to infinity, which can be summarized as tunnel capacitance overload phenomenon (TCOP). As the description and explanation of this phenomenon have not been found in previous studies, the high-frequency TCOP of the SIS heterojunction with ultra-thin interfacial layers was investigated in details in this paper. The current-voltage characteristics of the SIS devices were obtained from the solar simulator and shown good photovoltaic conversion efficiency. The built-in electric field of the SIS with different ITO thickness was obtained by fitting the CV curves, which was measured by the Keysight E4980A CV system. The XPS depth profiling showed that the composition and percentage content of the interface, including In, Sn, O, Si, and the oxide silicon compound of Si2O, SiO, Si2O3, SiO2 which can be summed as SiO x (1≤ x ≤2). The TEM images showed that the thickness of the SiO x layer increases with the thickness of ITO layer, and the V T in TCOP is apparently linked to the SiO x thickness. When the applied V g exceed V T, a huge amount of electrons would enter the E C of the silicon surface, unlike the MOS with thick and well passivated insulator layer, the ultra-thin SiO x layer can’t hold these mass of electrons at the SIS interface, so the electrons would directly tunnel through the SiO x into the ITO layer and the TCOP appears. Capacitance voltage characteristics are critical for estimating the properties of heterojunction interfaces, and high frequency CV is indispensable for measuring the D it. The existence of the TCOP is the main reason why other methods which based on the CV characteristics are difficult to estimate the interface state. But from another point of view, the TCOP is repeatable in the same device, which implied that the solar cells that fabricated by the directly depositing ITO thin films on n-type silicon is stable.