Combined Local DLTS/CV Measurement of Al₂O₃/OH-Diamond (111) Interface by Time-Resolved Scanning Nonlinear Dielectric Microscopy

Diamond is a wide bandgap semiconductor material promising for power device applications. Al₂O₃/OH-diamond (111), where an Al₂O₃ layer is deposited on an OH-terminated diamond (111) surface by ALD, realizes a high-quality MOS interface permitting an inversion-type MOSFET with normally-off operation [1]. However, the obtained channel mobility is insufficient and improving the interface quality remains a challenge. To investigate the interface at the microscopic level, here we performed nanoscale imaging of the interface charge states using time-resolved scanning nonlinear dielectric microscopy (time-resolved SNDM). Time-resolved SNDM is a scanning probe microscopy technique that detects electrostatic capacitance with high sensitivity [2]. Because this technique can measure microscopic MOS capacitance change for an arbitrary voltage pulse sequence, we can perform microscopic DLTS (deep level transient spectroscopy) and CV (capacitance-voltage) profiling at the same position with nanometer precision. This unique capability allows us to obtain simultaneous images reflecting interface defect density (<i>D</i>_it) and surface potential fluctuations.<br/>The sample consists of an Al₂O₃ layer (50 nm) deposited by ALD, a B-doped p⁺ CVD diamond (111) layer (200 nm), an underlying high-temperature high-pressure synthesized p-doped (111) substrate (300 μm) for ohmic contact formation, and a gold electrode layer. The diamond (111) surface was atomically flattened by using etching by Ni carbon solid solution reaction and subsequent hydrogen plasma etching treatment prior to OH-termination and ALD. At each measurement point, we applied a rectangular voltage pulse (height: 7 V, width: 5 μs) for local DLTS and a subsequent triangular pulse (1.5 cycles, amplitude: 50 V_pp, length of 1 cycle: 100 μs) for local CV profiling, both superimposed on a common dc bias of 2 V_dc. For noise reduction, capacitance response was averaged over 160 repetitions of this pulse sequence. The <i>D</i>_itimage was obtained by analyzing the response from local DLTS. In addition, by analyzing the local CV profiles, feature voltages <i>V</i>_d and <i>V</i>_a were extracted for obtaining spatial fluctuations of the local CV profiles near the depletion and accumulation states, respectively. The measurement was performed in air at room temperature.<br/>By using our technique, we simultaneously obtained <i>D</i>_it, <i>V</i>_d, and <i>V</i>_a images of the Al₂O₃/OH-diamond(111). We found that the fluctuations of <i>V</i>_d and <i>V</i>_a were approximately 0.4 V and 0.6 V, respectively. This means that the surface potential fluctuations are higher in the accumulation state compared to the depletion state. In addition, through the correlation analysis of the images, we found that the <i>D</i>_it image has significantly higher correlation coefficients with the <i>V</i>_a images than the <i>V</i>_d image. In fact, the spatial features of the fluctuations in the <i>D</i>_it and <i>V</i>_a images show a high degree of correspondence. This implies that observed fluctuation increase from the depletion to the accumulation is mainly caused by the capture of dominant carriers (holes) by interface defects. Device simulations have shown that the observed non-uniform interface charge states can lead to a decrease in channel mobility through Coulomb scattering. Our results demonstrate that time-resolved SNDM imaging is useful for microscopic understanding of interface charge states and evaluating their impact on the carrier transport properties at the interface.<br/><br/>Acknowledgments: This research was partially supported by JSPS KAKENHI Grant Number 24H00414.<br/><br/>References:<br/>[1] T. Matsumoto et al., Sci. Rep. 6, 31585 (2016).<br/>[2] Y. Cho, Scanning Nonlinear Dielectric Microscopy: Investigation of Ferroelectric, Dielectric, and Semiconductor Materials and Devices. Elsevier, ISBN 978-0-08-102803-2 (2020).