I. Introduction
As silicon devices have reached their material physical limits, the emergence of new semiconductors is mandatory for power electronic applications. Wide bandgap semiconductors like SiC, GaN, Ga2O3, and diamond are attractive candidates due to their high critical electric field, high mobility, and/or high thermal conductivity [1]–[3]. Among them, diamond is widely accepted as the best material for high voltage, high power, high temperature, and high frequency devices [1]–[4]. Recently, numerous diamond devices with promising performances have been realized such as diamond Schottky diode [5]–[8], PIN diode [9], [10], JFET [11]–[13], MESFET [14], depletion-mode hydrogen-terminated diamond (H-diamond) MOSFETs [15]–[18], inversion-mode oxygen terminated diamond (O-diamond) MOSFET [19], and deep depletion O-diamond MOSFET [20]–[22]. As demonstrated from SiC devices, along with device performances [23]–[25], devices reliability [26], [27] also play a critical role. Performance variability, device toleration, and degradation are generally associated with structural defects in epilayer [27]–[30]. Therefore, to fully exploit the potential of diamond devices, a clear correlation between device electrical characteristics and epitaxial structural defects are critical. The correlation between structural defects and electrical characteristics in diamond devices have been demonstrated in two of the three fundamental building blocks of electronic devices: in diamond Schottky diode by using either cathodoluminesence measurements [31] or kelvin probe force microscopy measurements [32] and in diamond p-n junction using transmission electron microscopy (TEM) measurements [33]. The structural defects and the electrical characteristics correlation for the third fundamental building block, i.e., diamond metal–oxide–semiconductor capacitors (MOSCAPs) have not been done. Another issue is that, most of the investigations are generally focused only on the dc electrical characteristics of the devices. Even if ac measurement, i.e., capacitance measurement, is an important measurement, a clear correlation between structural defects and ac electrical characteristics of the diamond devices has not been well established at the moment.