Hiroshi Fujioka

High hole mobility p-type GaN with low residual hydrogen concentration prepared by pulsed sputtering

We have grown Mg-doped GaN films with low residual hydrogen concentration using a low-temperature pulsed sputtering deposition (PSD) process. The growth system is inherently hydrogen-free, allowing us to obtain high-purity Mg-doped GaN films with residual hydrogen concentrations below 5 × 1016 cm−3, which is the detection limit of secondary ion mass spectroscopy. In the Mg profile, no memory effect or serious dopant diffusion was detected. The as-deposited Mg-doped GaN films showed clear p-type conductivity at room temperature (RT) without thermal activation. The GaN film doped with a low concentration of Mg (7.9 × 1017 cm−3) deposited by PSD showed hole mobilities of 34 and 62 cm2 V−1 s−1 at RT and 175 K, respectively, which are as high as those of films grown by a state-of-the-art metal-organic chemical vapor deposition apparatus. These results indicate that PSD is a powerful tool for the fabrication of GaN-based vertical power devices.

Electrical properties of Si-doped GaN prepared using pulsed sputtering

In this study, we investigated the basic electrical properties of Si-doped wurtzite GaN films prepared using a low-temperature pulsed sputtering deposition (PSD) process. We found that the electron concentration can be controlled in the range between 1.5 × 1016 and 2.0 × 1020 cm−3. For lightly Si-doped GaN ([Si] = 2.1 × 1016 cm−3), the room temperature (RT) electron mobility was as high as 1008 cm2 V−1 s−1, which was dominantly limited by polar optical phonon scattering. Moreover, we found that heavily Si-doped GaN prepared using PSD exhibited an RT mobility as high as 110 cm2 V−1 s−1 at an electron concentration of 2 × 1020 cm−3, which indicated that the resistivity of this film was almost as small as those of typical transparent conductive oxides such as indium tin oxide. At lower temperatures, the electron mobility increased to 1920 cm2 V−1 s−1 at 136 K, and the temperature dependence was well explained by conventional scattering models. These results indicate that Si-doped GaN prepared using PSD is pr...

Fabrication of InGaN thin-film transistors using pulsed sputtering deposition.

We report the first demonstration of operational InGaN-based thin-film transistors (TFTs) on glass substrates. The key to our success was coating the glass substrate with a thin amorphous layer of HfO2, which enabled a highly c-axis-oriented growth of InGaN films using pulsed sputtering deposition. The electrical characteristics of the thin films were controlled easily by varying their In content. The optimized InGaN-TFTs exhibited a high on/off ratio of ~108, a field-effect mobility of ~22 cm2 V−1 s−1, and a maximum current density of ~30 mA/mm. These results lay the foundation for developing high-performance electronic devices on glass substrates using group III nitride semiconductors.

Epitaxial growth of GaN films on nearly lattice-matched hafnium substrates using a low-temperature growth technique

We demonstrated epitaxial growth of GaN (0001) films on nearly lattice-matched Hf (0001) substrates by using a low-temperature (LT) epitaxial growth technique. High-temperature growth of GaN films results in the formation of polycrystalline films due to significant reaction at GaN/Hf heterointerfaces, while LT-growth allowed us to suppress the interfacial reactions and to obtain epitaxial GaN films on Hf substrates with a GaN1120//Hf1120 in-plane orientation. LT-grown GaN films can act as buffer layers for GaN growth at high temperatures. The interfacial layer thickness at the LT-GaN/Hf heterointerface was as small as 1 nm, and the sharpness of the contact remained unchanged even after annealing up to approximately 700 °C, which likely accounts for the dramatic improvement in GaN crystalline quality on Hf substrates.

InN thin-film transistors fabricated on polymer sheets using pulsed sputtering deposition at room temperature

Indium nitride (InN) is potentially suitable for the fabrication of high performance thin-film transistors (TFTs) because of its high electron mobility and peak electron velocity. However, InN is usually grown using a high temperature growth process, which is incompatible with large-area and lightweight TFT substrates. In this study, we report on the room temperature growth of InN films on flexible polyimide sheets using pulsed sputtering deposition. In addition, we report on the fabrication of InN-based TFTs on flexible polyimide sheets and the operation of these devices.

Spatial distribution of transferred charges across the heterointerface between perovskite transition metal oxides LaNiO3 and LaMnO3

To investigate the interfacial charge-transfer phenomena between perovskite transition metal oxides LaNiO3 (LNO) and LaMnO3 (LMO), we have performed in situ x-ray absorption spectroscopy (XAS) measurements on LNO/LMO multilayers. The Ni-L2,3 and Mn-L2,3 XAS spectra clearly show the occurrence of electron transfer from Mn to Ni ions in the interface region. Detailed analysis of the thickness dependence of these XAS spectra has revealed that the spatial distribution of the transferred charges across the interface is significantly different between the two constituent layers. The observed spatial distribution is presumably described by the charge spreading model that treats the transfer integral between neighboring transition metal ions and the Coulomb interaction, rather than the Thomas–Fermi screening model.

Fabrication of full-color GaN-based light-emitting diodes on nearly lattice-matched flexible metal foils

GaN-based light-emitting diodes (LEDs) have been widely accepted as highly efficient solid-state light sources capable of replacing conventional incandescent and fluorescent lamps. However, their applications are limited to small devices because their fabrication process is expensive as it involves epitaxial growth of GaN by metal-organic chemical vapor deposition (MOCVD) on single crystalline sapphire wafers. If a low-cost epitaxial growth process such as sputtering on a metal foil can be used, it will be possible to fabricate large-area and flexible GaN-based light-emitting displays. Here we report preparation of GaN films on nearly lattice-matched flexible Hf foils using pulsed sputtering deposition (PSD) and demonstrate feasibility of fabricating full-color GaN-based LEDs. It was found that introduction of low-temperature (LT) grown layers suppressed the interfacial reaction between GaN and Hf, allowing the growth of high-quality GaN films on Hf foils. We fabricated blue, green, and red LEDs on Hf foils and confirmed their normal operation. The present results indicate that GaN films on Hf foils have potential applications in fabrication of future large-area flexible GaN-based optoelectronics.

N-polar InGaN-based LEDs fabricated on sapphire via pulsed sputtering

High-quality N-polar GaN epitaxial films with an atomically flat surface were grown on sapphire (0001) via pulsed sputtering deposition, and their structural and electrical properties were investigated. The crystalline quality of N-polar GaN improves with increasing film thickness and the full width at half maximum values of the x-ray rocking curves for 0002 and 101¯2 diffraction were 313 and 394 arcsec, respectively, at the film thickness of 6μm. Repeatable p-type doping in N-polar GaN films was achieved using Mg dopant, and their hole concentration and mobility can be controlled in the range of 8 × 1016–2 × 1018 cm−3 and 2–9 cm2V−1s−1, respectively. The activation energy of Mg in N-polar GaN based on a temperature-dependent Hall measurement was estimated to be 161 meV, which is comparable to that of the Ga-polar GaN. Based on these results, we demonstrated the fabrication of N-polar InGaN-based light emitting diodes with the long wavelength up to 609 nm.

High-current-density indium nitride ultrathin-film transistors on glass substrates

We investigated basic characteristics of InN films grown on glass substrates and fabricated thin-film transistors with them. It was found that highly c-axis oriented InN films can be obtained by the surface modification of glass substrates with amorphous HfO2 layers. The electron mobility of the c-axis-oriented InN on HfO2/glass reached 330 cm2 V−1 s−1. We demonstrated that a field effect transistor based on the ultrathin film of highly c-axis-oriented InN exhibited an on/off current ratio as high as 106; in addition, this InN-on-glass device supported current densities greater than 14 mA mm−1. The results indicate that InN-based ultrathin-film transistors are promising electronic devices that enable high-current densities to be achieved on glass substrates.

Pulsed sputtering epitaxial growth of m -plane InGaN lattice-matched to ZnO

Abstractm-Plane GaN and InGaN films were grown on m-plane ZnO substrates at ~350 °C by pulsed sputtering deposition. It was found that the critical thickness of the m-plane GaN films grown on ZnO lies between 25 and 62 nm, whereas 180-nm-thick m-plane In0.12Ga0.88N can be coherently grown on ZnO substrates, which is explained well by theoretical calculations based on an energy-balance model. The coherently grown m-plane InGaN on ZnO exhibited narrow X-ray rocking curves compared with the m-plane GaN grown on ZnO. These results demonstrate the benefit of lattice-matched ZnO substrates for epitaxy of high-quality nonpolar InGaN films.