Atsushi Hiraiwa

The effect of a heavily boron-doped (p⁺) source/drain (S/D) layers ([B]: 2–8 × 10²⁰ cm^-3) are examined compared with conventional Au/diamond S/D contacts without p⁺ on the same hydrogen-terminated (C-H) diamond metal-oxide-semiconductor field-effect transistors (MOSFETs). The following points such as contact resistance (R_c), subthreshold swing (SS), on-resistance (R_on) and threshold voltage (V_th) are comparatively investigated between with/without p⁺ layers. The selective epitaxial growth of p⁺ S/D regions in (001) enhances the contact performance with the C-H diamond channel. It reduces the R_c between the metal S/D contacts and C-H diamond from 5.8 to 1.4 Ω·mm, which is the lowest reported for (001) diamond MOSFETs. For MOSFETs with a p⁺ layer and source-drain length of 2 μm, the SS is ~560 mV/dec, the on-resistance (R_on) is 0.78 mΩ·cm², and the threshold voltage (V_th) is 3.4 V. In contrast, without the p⁺ layer, the SS is ~1600 mV/dec, the R_on is 1.44 mΩ·cm², and the V_th is 13.7 V. Overall, the p⁺ layer is beneficial for the development of high-performance diamond MOSFETs using the (001) surface.

AlN films formed on (0001) GaN by a plasma-enhanced atomic layer deposition (PEALD) method at 200 and 450 °C without remote-plasma pretreatment (RPP) were crystalline as formed and epitaxially aligned with the GaN substrate. Even a 0.7-nm AlN film formed by high-temperature (HT, 450 °C) PEALD suppressed the bias instability of Al/Al2O3/AlN/GaN capacitors to a practically negligible level. By contrast, the bias instability improvement for low-temperature (LT, 200 °C) PEALD was quite limited. Although RPP was not conducted, the interface-state density of the HT-PEALD capacitors was reduced to 1.4 × 1011 cm−2 eV−1 for AlN thicknesses of ≤1.5 nm, supposedly owing to the RPP effect inherent in HT-PEALD. By contrast, LT-PEALD AlN films generated a considerable number of interface states. The HT-PEALD AlN interlayer possessed a polarization charge of −7.7 × 1013 cm−2 in units of electronic charge, comparable to a reported value of −8.7 × 1013 cm−2. The flatband-voltage shift of an Al/Al2O3/0.7-nm AlN/GaN capacitor caused by this polarization was −1.2 V. Similar to previous results, the net insulator charge, which comprised interface-state and GaN-polarization charges, was considerably smaller in magnitude than the GaN polarization charge, indicating the presence of highly positive interface-state charge compensating for the negative GaN polarization charge. A physical model for the interface-state charge was proposed. As the thin HT-PEALD AlN interlayer effectively suppressed the bias instability of Al2O3/AlN/GaN capacitors, retaining a good interface with the GaN substrate and causing a small flatband-voltage shift, as aforementioned, it will be a key to realizing high-performance and high-reliability GaN metal-insulator-semiconductor field-effect transistors.

The influence of Al2O3 atomic-layer deposition (ALD) temperature on the electric characteristics of Al/Al2O3/(2¯01) β-Ga2O3 capacitors was investigated focusing on the positive-bias instability (PBI) of the capacitors. The current in the capacitors increased with ALD temperature, mostly because of the reduced energy barrier height for the electron field emission from the substrate and less negative Al2O3 charge, as revealed by the analysis conducted assuming a space-charge-controlled field emission process. The PBI tests were conducted for cumulative voltage stressing times vastly ranging from 3 × 10−6 to 4 × 105 s. The capacitance–voltage (C–V) characteristics of the capacitors for an ALD temperature of 100 °C displayed negative shifts in the middle of voltage stressing, unlike those for the other ALD temperatures. The bias stability of the capacitors was found to be considerably improved by high-temperature (450 °C) ALD. Additionally, the C–V characteristic shifts caused by the voltage stressing were theoretically reproduced quite accurately, assuming a model proposed in this study. In the simulations, the trap distributions in the Al2O3 films were assumed to be uniform both spatially and energetically. Importantly, the experimental results for various stressing voltages were excellently fitted by the simulations that assumed the same trap distribution. The trap densities in the Al2O3 films thus estimated reduced from 1.2 × 1020 to 2.2 × 1019 cm−3 eV−1 for ALD temperatures of 100–450 °C. This reduction in the trap densities was a major cause of the bias stability enhancement for high-temperature ALD. Moreover, the trap density as a function of ALD temperature qualitatively agreed with the aforementioned Al2O3 charge generated by the current measurements. This agreement provides a strong basis for the validity of the PBI model proposed in this study.

The current conduction in negatively biased metal/Al2O3/diamond capacitors with various gate materials and diamond surface terminations was comprehensively characterized by assuming a space-charge-controlled field emission (SCC-FE) process, yielding the following results. The current-feeding carriers were field-emitted from the electrode that had a barrier height lower than that of the opposing electrode. Specifically, in an Au-gate oxygen-terminated diamond capacitor, holes were likely to be emitted from the Al2O3/diamond interface states, whereas, in Al-gate oxygen-terminated, Au-gate hydrogen-terminated, and Al-gate hydrogen-terminated diamond capacitors, electrons were emitted from the gate. The barrier heights at the Al/Al2O3 and Au/Al2O3 interfaces were estimated as 1.6–1.7 and 2.8 eV, respectively. However, the analysis of the hole emission from the interface states is left for future studies. Although the substrate of the oxygen-terminated diamond capacitor remained weakly depleted even for highly negative gate voltages, the conduction current was observed as mentioned earlier. To analyze the mechanism for this conduction, the diffusion and drift currents in the depleted substrate were quantitatively estimated, ascribing the current conduction to the diffusion current. As these results could not be obtained by using the conventional graphical methods based on model-specific plots, such as the Fowler–Nordheim and Poole–Frenkel plots, the SCC-FE analysis proved to be a powerful tool for investigating the current conduction in metal-insulator-semiconductor capacitors.

2023 International Conference on Solid State Devices and Materials ,Radiation Effects on 2DHG Diamond MOSFETs with Boron-doped Source and Drain Layers

As diamond possesses only deep dopants, certain conventional physics and characterization methods are not applicable to diamond devices, owing to the explicit or implicit assumption of shallow dopants. To resolve this limitation, the capacitance–voltage (C–V) characteristics of metal–insulator–semiconductor (MIS) capacitors formed on a semiconductor substrate with deep and compensating dopants were successfully formulated. Based on these equations, methods for accurately estimating the MIS capacitor properties were developed and validated through their application in the analysis of an actual MIS capacitor formed on a boron-doped hydrogen-terminated diamond substrate. The high-frequency C–V characteristic of the capacitor exhibited a prominent dip specific to deep dopants. However, the dip depth was considerably shallower than theoretically expected. This C–V characteristic was accurately reproduced theoretically, assuming the presence of a surficial diamond layer that contains acceptors with an activation energy of 0.23 eV, which is less than the value 0.37 eV for boron, and has a thickness of the extrinsic Debye length (40 nm in this study) or larger. The insulator charge of the MIS capacitor was estimated as −4.6 × 10¹² cm⁻² in units of electronic charge, which is sufficiently large to induce two-dimensional hole gas. The interface-state density was 1.4 × 10¹² cm⁻² eV⁻¹ for interface-state energies of 0.3–0.5 eV above the valence band maximum. Hence, the proposed methodology and the possible presence of the reduced activation energy layer will guide the development of diamond-based devices.