Yasuo Koide

Low on-resistance diamond field effect transistor with high-k ZrO2 as dielectric

Although several high-k insulators have been deposited on the diamond for metal-insulator-semiconductor field effect transistors (MISFETs) fabrication, the k values and current output are still not fully satisfactory. Here, we present a high-k ZrO2 layer on the diamond for the MISFETs. The k value for ZrO2 is determined by capacitance-voltage characteristic to be 15.4. The leakage current density is smaller than 4.8 × 10−5u2005A·cm−2 for the gate voltage ranging from −4.0 to 2.0u2005V. The low on-resistance MISFET is obtained by eliminating source/drain-channel interspaces, which shows a large current output and a high extrinsic transconductance. The high-performance diamond MISFET fabrication will push forward the development of power devices.

Normally-off HfO2-gated diamond field effect transistors

A normally-off hydrogenated-diamond (H-diamond) field effect transistor (FET) using a HfO2 gate oxide is demonstrated. The HfO2 gate oxide has a bilayer structure which is fabricated by a sputter-deposition (SD) technique on a thin buffer layer prepared by an atomic layer deposition (ALD) technique. The role of the ALD-HfO2 is found to prevent deterioration of the H-diamond surface by the SD process. The leakage current density of the SD-HfO2/ALD-HfO2/H-diamond structure is smaller than 1.1u2009×u200910−4u2009Au2009cm−2 at gate voltages from −9.0 to 2.0u2009V. The capacitance-voltage characteristic shows that fixed and trapped charge densities are low enough to operate the FET. The HfO2-gated FET has p-type channel and complete normally-off characteristics. The drain-source current maximum, threshold voltage, extrinsic transconductance maximum, and effective mobility of the FET with gate length of 4u2009μm are −37.6u2009mAu2009mm−1, −1.3u2009±u20090.1u2009V, 11.2u2009±u20090.1u2009mSu2009mm−1, and 38.7u2009±u20090.5u2009cm2u2009V−1u2009s−1, respectively.

Band offsets of Al2O3 and HfO2 oxides deposited by atomic layer deposition technique on hydrogenated diamond

High-k oxide insulators (Al2O3 and HfO2) have been deposited on a single crystalline hydrogenated diamond (H-diamond) epilayer by an atomic layer deposition technique at temperature as low as 120u2009°C. Interfacial electronic band structures are characterized by X-ray photoelectron spectroscopy. Based on core-level binding energies and valence band maximum values, valence band offsets are found to be 2.9u2009±u20090.2 and 2.6u2009±u20090.2u2009eV for Al2O3/H-diamond and HfO2/H-diamond heterojunctions, respectively. Band gaps of the Al2O3 and HfO2 have been determined to be 7.2u2009±u20090.2 and 5.4u2009±u20090.2u2009eV by measuring O 1s energy loss spectra, respectively. Both the Al2O3/H-diamond and HfO2/H-diamond heterojunctions are concluded to be type-II staggered band configurations with conduction band offsets of 1.2u2009±u20090.2 and 2.7u2009±u20090.2u2009eV, respectively.

Interfacial band configuration and electrical properties of LaAlO3/Al2O3/hydrogenated-diamond metal-oxide-semiconductor field effect transistors

In order to search a gate dielectric with high permittivity on hydrogenated-diamond (H-diamond), LaAlO3 films with thin Al2O3 buffer layers are fabricated on the H-diamond epilayers by sputtering-deposition (SD) and atomic layer deposition (ALD) techniques, respectively. Interfacial band configuration and electrical properties of the SD-LaAlO3/ALD-Al2O3/H-diamond metal-oxide-semiconductor field effect transistors (MOSFETs) with gate lengths of 10, 20, and 30u2009μm have been investigated. The valence and conduction band offsets of the SD-LaAlO3/ALD-Al2O3 structure are measured by X-ray photoelectron spectroscopy to be 1.1u2009±u20090.2 and 1.6u2009±u20090.2 eV, respectively. The valence band discontinuity between H-diamond and LaAlO3 is evaluated to be 4.0u2009±u20090.2u2009eV, showing that the MOS structure acts as the gate which controls a hole carrier density. The leakage current density of the SD-LaAlO3/ALD-Al2O3/H-diamond MOS diode is smaller than 10−8u2009Au2009cm−2 at gate bias from −4 to 2u2009V. The capacitance-voltage curve in the depletion mode shows sharp dependence, small flat band voltage, and small hysteresis shift, which implies low positive and trapped charge densities. The MOSFETs show p-type channel and complete normally off characteristics with threshold voltages changing from −3.6u2009±u20090.1 to −5.0u2009±u20090.1u2009V dependent on the gate length. The drain current maximum and the extrinsic transconductance of the MOSFET with gate length of 10u2009μm are −7.5u2009mAu2009mm−1 and 2.3u2009±u20090.1u2009mSu2009mm−1, respectively. The enhancement mode SD-LaAlO3/ALD-Al2O3/H-diamond MOSFET is concluded to be suitable for the applications of high power and high frequency electrical devices.

Integration of high-dielectric constant Ta2O5 oxides on diamond for power devices

The authors report on the direct integration of high-dielectric constant (high-k) Ta2O5 films on p-type single crystal diamond for high-power electronic devices. Crystallized hexagonal phase δ-Ta2O5 film is achieved on diamond by annealing the amorphous Ta2O5 film deposited by a sputter-deposition technique. The electrical properties of the Ta2O5 thin films are investigated by fabricating metal-insulator-semiconductor (MIS) diodes. The leakage current of the MIS diode is as low as 10−8 A/cm2 for the as-deposited amorphous Ta2O5 film and 10−2 A/cm2 for the crystallized film, which is 108 and 102 times lower than that of the Schottky diode at a forward bias of −3u2009V, respectively. The dielectric constant of the amorphous Ta2O5 films is measured to be 16 and increases to 29 after annealing at 800u2009u2009°C. Different current leakage mechanisms and charge trapping behaviors are proposed for the amorphous and crystallized Ta2O5 thin films.

Electrical characteristics of hydrogen-terminated diamond metal-oxide-semiconductor with atomic layer deposited HfO2 as gate dielectric

HfO2 films have been deposited on hydrogen-terminated diamond (H-diamond) by an atomic layer deposition (ALD) technique at 120u2009°C. Effect of rapid thermal annealing treatment on electrical properties of Au/Ti/Pd/ALD-HfO2/H-diamond metal-oxide-semiconductor (MOS) diodes has been investigated. The leakage current density of the MOS diode after annealing at 300u2009°C is as small as 10−8 A/cm2 at gate biases from −5.0 to 4.0 V. The capacitance-voltage curve in the depletion mode of the MOS diode after annealing is much sharper than that of the MOS diode before annealing and close to the theoretical dependence, which indicates the small interface state density. The annealed MOS diode is concluded to be more suitable for the fabrication of field effect transistors.

Arbitrary Multicolor Photodetection by Hetero-integrated Semiconductor Nanostructures

The typical photodetectors can only detect one specific optical spectral band, such as InGaAs and graphene-PbS quantum dots for near-infrared (NIR) light detection, CdS and Si for visible light detection, and ZnO and III-nitrides for UV light detection. So far, none of the developed photodetector can achieve the multicolor detection with arbitrary spectral selectivity, high sensitivity, high speed, high signal-to-noise ratio, high stability, and simplicity (called 6S requirements). Here, we propose a universal strategy to develop multicolor photodetectors with arbitrary spectral selectivity by integrating various semiconductor nanostructures on a wide-bandgap semiconductor or an insulator substrate. Because the photoresponse of each spectral band is determined by each semiconductor nanostructure or the semiconductor substrate, multicolor detection satisfying 6S requirements can be readily satisfied by selecting the right semiconductors.

Diamond field effect transistors with a high-dielectric constant Ta2O5 as gate material

A Ta2O5/Al2O3 bilayer gate oxide with a high-dielectric constant (high-k) has been successfully applied to a hydrogenated-diamond (H-diamond) metal-insulator-semiconductor field effect transistor (MISFET). The Ta2O5 layer is prepared by a sputtering-deposition (SD) technique on the Al2O3 buffer layer fabricated by an atomic layer deposition (ALD) technique. The ALD-Al2O3 plays an important role to eliminate plasma damage for the H-diamond surface during SD-Ta2O5 deposition. The dielectric constants of the SD-Ta2O5/ALD-Al2O3 bilayer and single SD-Ta2O5 are as large as 12.7 and 16.5, respectively. The k value of the single SD-Ta2O5 in this study is in good agreement with that of the SD-Ta2O5 on oxygen-terminated diamond. The capacitance–voltage characteristic suggests low interfacial trapped charge density for the SD-Ta2O5/ALD-Al2O3/H-diamond MIS diode. The MISFET with a gate length of 4 µm has a drain current maximum and an extrinsic transconductance of −97.7 mA mm−1 (normalized by gate width) and 31.0 ± 0.1 mS mm−1, respectively. The effective mobility in the H-diamond channel layer is found to be 70.1 ± 0.5 cm2 V−1 s−1.

Diamond logic inverter with enhancement-mode metal-insulator-semiconductor field effect transistor

A diamond logic inverter is demonstrated using an enhancement-mode hydrogenated-diamond metal-insulator-semiconductor field effect transistor (MISFET) coupled with a load resistor. The gate insulator has a bilayer structure of a sputtering-deposited LaAlO3 layer and a thin atomic-layer-deposited Al2O3 buffer layer. The source-drain current maximum, extrinsic transconductance, and threshold voltage of the MISFET are measured to be −40.7u2009mA·mm−1, 13.2u2009±u20090.1u2009mS·mm−1, and −3.1u2009±u20090.1u2009V, respectively. The logic inverters show distinct inversion (NOT-gate) characteristics for input voltages ranging from 4.0 to −10.0u2009V. With increasing the load resistance, the gain of the logic inverter increases from 5.6 to as large as 19.4. The pulse response against the high and low input voltages shows the inversion response with the low and high output voltages.

Control of normally on/off characteristics in hydrogenated diamond metal-insulator-semiconductor field-effect transistors

Normally on/off operation in hydrogenated diamond (H-diamond) metal–insulator–semiconductor field-effect transistors (MISFETs) is reproducibly controlled by annealing at 180u2009°C. The transfer characteristics of the MISFETs reveal that the threshold gate voltage changes from 0.8u2009±u20090.1 to −0.5u2009±u20090.1u2009V after annealing, which indicates the MISFETs switch from normally on to normally off operation. Annealing also shifts the flat-band voltage in the capacitance–voltage curve of MIS capacitors from zero to −0.47u2009V. The mechanism behind the switch of normally on/off characteristics induced by annealing is explained by a change of transfer doping as follows. Adsorbed acceptors at the insulator/H-diamond interface allow the holes to accumulate in the H-diamond channel layer, so the MISFETs before annealing show normally on characteristics. Annealing causes loss of the adsorbed acceptors or provides compensatory positive charge in the insulator oxide, so the hole density in the H-diamond channel layer decreases marke...