Daisuke Matsumura

Durability-enhanced two-dimensional hole gas of C-H diamond surface for complementary power inverter applications

Complementary power field effect transistors (FETs) based on wide bandgap materials not only provide high-voltage switching capability with the reduction of on-resistance and switching losses, but also enable a smart inverter system by the dramatic simplification of external circuits. However, p-channel power FETs with equivalent performance to those of n-channel FETs are not obtained in any wide bandgap material other than diamond. Here we show that a breakdown voltage of more than 1600 V has been obtained in a diamond metal-oxide-semiconductor (MOS) FET with a p-channel based on a two-dimensional hole gas (2DHG). Atomic layer deposited (ALD) Al2O3 induces the 2DHG ubiquitously on a hydrogen-terminated (C-H) diamond surface and also acts as both gate insulator and passivation layer. The high voltage performance is equivalent to that of state-of-the-art SiC planar n-channel FETs and AlGaN/GaN FETs. The drain current density in the on-state is also comparable to that of these two FETs with similar device size and VB.

High voltage breakdown (1.8 kV) of hydrogenated black diamond field effect transistor

We fabricated and characterized black polycrystalline diamond field effect transistors. By implementing a C-H bonded channel with a wide gate-drain length up to 20 μm, a breakdown voltage of 1.8 kV was achieved, which is the highest value reported for a diamond field effect transistor (FET) to date. Several of our devices achieved a breakdown voltage/wide gate-drain length ratio > 100 V/μm. This is comparable to the performance of lateral SiC and GaN FETs. We investigated the effects of voltage stress up to 2.0 kV, and showed that the maximum current density fell to 26% of its initial value of 2.42 mA/mm before the device eventually broke down at 1.1 kV.

Space-charge-controlled field emission model of current conduction through Al2O3 films

This study proposes a model for current conduction in metal-insulator-semiconductor (MIS) capacitors, assuming the presence of two sheets of charge in the insulator, and derives analytical formulae of field emission (FE) currents under both negative and positive bias. Since it is affected by the space charge in the insulator, this particular FE differs from the conventional FE and is accordingly named the space-charge-controlled (SCC) FE. The gate insulator of this study was a stack of atomic-layer-deposition Al2O3 and underlying chemical SiO2 formed on Si substrates. The current-voltage (I-V) characteristics simulated using the SCC-FE formulae quantitatively reproduced the experimental results obtained by measuring Au- and Al-gated Al2O3/SiO2MIS capacitors under both biases. The two sheets of charge in the Al2O3 films were estimated to be positive and located at a depth of greater than 4 nm from the Al2O3/SiO2 interface and less than 2 nm from the gate. The density of the former is approximately 1 × 1013 cm−2 in units of electronic charge, regardless of the type of capacitor. The latter forms a sheet of dipoles together with image charges in the gate and hence causes potential jumps of 0.4 V and 1.1 V in the Au- and Al-gated capacitors, respectively. Within a margin of error, this sheet of dipoles is ideally located at the gate/Al2O3 interface and effectively reduces the work function of the gate by the magnitude of the potential jumps mentioned above. These facts indicate that the currents in the Al2O3/SiO2MIS capacitors are enhanced as compared to those in ideal capacitors and that the currents in the Al-gated capacitors under negative bias (electron emission from the gate) are more markedly enhanced than those in the Au-gated capacitors. The larger number of gate-side dipoles in the Al-gated capacitors is possibly caused by the reaction between the Al and Al2O3, and therefore gate materials that do not react with underlying gate insulators should be chosen in order to achieve a low leakage current by suppressing the current enhancement. Although the current conduction in this study is essentially limited by FE, neither the Fowler-Nordheim(FN) nor Poole-Frenkel (PF) plots of the I-V characteristics are fitted by a linear function. The failures of the FN and PF plot methods alert us to the inaccuracies of basing the investigation of current conduction on these traditional plots. Hence, the methodology of a current conduction analysis and the knowledge of Al2O3charging in this study provide a solid foundation for investigating the current conduction in MIS capacitors.

Hydrogen-terminated diamond vertical-type metal oxide semiconductor field-effect transistors with a trench gate

The hydrogen-terminated diamond surface (C-H diamond) has a two-dimensional hole gas (2DHG) layer independent of the crystal orientation. A 2DHG layer is ubiquitously formed on the C-H diamond surface covered by atomic-layer-deposited-Al2O3. Using Al2O3 as a gate oxide, C-H diamond metal oxide semiconductor field-effect transistors (MOSFETs) operate in a trench gate structure where the diamond side-wall acts as a channel. MOSFETs with a side-wall channel exhibit equivalent performance to the lateral C-H diamond MOSFET without a side-wall channel. Here, a vertical-type MOSFET with a drain on the bottom is demonstrated in diamond with channel current modulation by the gate and pinch off.

Isotope analysis of diamond-surface passivation effect of high-temperature H{sub 2}O-grown atomic layer deposition-Al{sub 2}O{sub 3} films

The Al2O3 film formed using an atomic layer deposition (ALD) method with trimethylaluminum as Al precursor and H2O as oxidant at a high temperature (450 °C) effectively passivates the p-type surface conduction (SC) layer specific to a hydrogen-terminated diamond surface, leading to a successful operation of diamond SC field-effect transistors at 400 °C. In order to investigate this excellent passivation effect, we carried out an isotope analysis using D2O instead of H2O in the ALD and found that the Al2O3 film formed at a conventional temperature (100 °C) incorporates 50 times more CH3 groups than the high-temperature film. This CH3 is supposed to dissociate from the film when heated afterwards at a higher temperature (550 °C) and causes peeling patterns on the H-terminated surface. The high-temperature film is free from this problem and has the largest mass density and dielectric constant among those investigated in this study. The isotope analysis also unveiled a relatively active H-exchange reaction between the diamond H-termination and H2O oxidant during the high-temperature ALD, the SC still being kept intact. This dynamic and yet steady H termination is realized by the suppressed oxidation due to the endothermic reaction with H2O. Additionally, we not only observed the kinetic isotope effect in the form of reduced growth rate of D2O-oxidant ALD but found that the mass density and dielectric constant of D2O-grown Al2O3 films are smaller than those of H2O-grown films. This is a new type of isotope effect, which is not caused by the presence of isotopes in the films unlike the traditional isotope effects that originate from the presence of isotopes itself. Hence, the high-temperature ALD is very effective in forming Al2O3 films as a passivation and/or gate-insulation layer of high-temperature-operation diamond SC devices, and the knowledge of the aforementioned new isotope effect will be a basis for further enhancing ALD technologies in general.

Diamond MOSFETs using 2D hole gas with 1700V breakdown voltage

More than 1600V breakdown voltages have been obtained in hydrogen terminated (C-H) diamond planar p-channel MOSFETs with gate-drain distance of 16-22 μm. The drain current density exceeds 100mA/mm in the FETs. The blocking voltage and drain current characteristics are comparable to those of n-channel AlGaN/GaN FETs and planar SiC MOSFETs in a similar device size. Atomic layer deposited Al2O3 works as gate insulator and passivation layer. It also induces the 2 dimensional hole gas ubiquitously on C-H diamond surface not only in planar, but in a trench gate structure. The first diamond vertical MOSFET has also operated using the trench structure.

Effect of atomic layer deposition temperature on current conduction in Al2O3 films formed using H2O oxidant

To develop high-performance, high-reliability gate insulation and surface passivation technologies for wide-bandgap semiconductor devices, the effect of atomic layer deposition (ALD) temperature on current conduction in Al2O3 films is investigated based on the recently proposed space-charge-controlled field emission model. Leakage current measurement shows that Al2O3 metal-insulator-semiconductor capacitors formed on the Si substrates underperform thermally grown SiO2 capacitors at the same average field. However, using equivalent oxide field as a more practical measure, the Al2O3 capacitors are found to outperform the SiO2 capacitors in the cases where the capacitors are negatively biased and the gate material is adequately selected to reduce virtual dipoles at the gate/Al2O3 interface. The Al2O3 electron affinity increases with the increasing ALD temperature, but the gate-side virtual dipoles are not affected. Therefore, the leakage current of negatively biased Al2O3 capacitors is approximately independent of the ALD temperature because of the compensation of the opposite effects of increased electron affinity and permittivity in Al2O3. By contrast, the substrate-side sheet of charge increases with increasing ALD temperature above 210 °C and hence enhances the current of positively biased Al2O3 capacitors more significantly at high temperatures. Additionally, an anomalous oscillatory shift of the current-voltage characteristics with ALD temperature was observed in positively biased capacitors formed by low-temperature (≤210 °C) ALD. This shift is caused by dipoles at the Al2O3/underlying SiO2 interface. Although they have a minimal positive-bias leakage current, the low-temperature-grown Al2O3 films cause the so-called blisters problem when heated above 400 °C. Therefore, because of the absence of blistering, a 450 °C ALD process is presently the most promising technology for growing high-reliability Al2O3 films.

Trench-channel MOSFET using C-H Diamond Surface

Hydrogen-terminated diamond metal –oxide-semiconductor field-effect transistors (C-H diamond MOSFETs) with trench channels were fabricated. The trench structure was formed by inductive coupled plasma etching. The side walls of the trenches can be used for p-channel. This result indicates that, in principal, a vertical diamond MOSFET with trench gate and bottom drain is feasible for power device application.