SAGA UNIVERSITY
POWER ELECTRONICS LABORATORY (Prof. Kasu)

[Achievement 1] Y. Taniyasu, M. Kasu, and T. Makimoto,
"An aluminium nitride light-emitting diode with a wavelength of 210 nanometres",
Nature vol.441, p.325-328, (2006), Citation time 306.

Aluminum nitride (AlN) is a direct-bandgap semiconductor with the widest energy gap (6.0 eV). First we found that its electron mobility is limited by dislocations in crystal, and then we reduced dislocation density in AlN crystal, and achieved high mobility for n-type semiconductor. Next, we achieved p-type AlN and fabricated LED structure, and demonstrated light emission at the shortest wavelength of 210 nm.

[Achievement 2] K. Ueda, M. Kasu, Y. Yamauchi, T. Makimoto, M. Schwitters, D. J. Twitchen, G. A. Scarsbrook, and S. E. Coe,
"Diamond FET using high-quality polycrystalline diamond with fT of 45 GHz and fmax of 120 GHz",
IEEE Electron Device Letters vol.27, p.570-572 (2006), Citation time 35.

We fabricated p-type diamond field-effect transistors (FETs) of hydrogen-terminated diamond and achieved the highest power-gain cut-off frequency (fmax) and demonstrated power amplification in millimeter wave range.

[Achievement 3] M. Kasu, M. Kubovic, A. Aleksov, N. Teofilov, Y. Taniyasu, R. Sauer, E. Kohn, T. Makimoto, and N. Kobayashi,
"Influence of epitaxy on the surface conduction of diamond film"
Diamond and Related Materials vol.13, p.226-232 (2004), Citation time 35.

We achieved high power-gain cut-off frequency of H-terminated diamond transistors, and specified how crystal imperfections, such as defects and impurities, influence device properties.

[Achievement 4] Y. Taniyasu, M. Kasu, and T. Makimoto,
“Field emission properties of heavily Si-doped AlN in triode-type display structure“,
Appl. Phys. Lett. vol.84, p.2115-2117 (2004) , Citation time 43.

Aluminum nitride (AlN) has a property of negatively electron affinity (NEA). By silicon (Si) doping, we supplied electron to AlN crystal, and achieved high-efficient electron field emission.

[Achievement 5] Y. Taniyasu, M. Kasu, and N. Kobayashi,
"Intentional control of n-type conduction for Si-deoped AlN and AlGaN (0.42<x<1)",
Appl. Phys. Lett. vol.81 p.1255-1257 (2002), Citation time 69.

First report of n-type conductive aluminum nitride (AlN). By Si doping we achieved n-type conduction. We clarified Si doping mechanism in AlN When Si concentration exceeds a specific value, Si atoms enter nitrogen site, and compensate Si donors.

[Achievement 6] M. Kasu and N. Kobayashi,
"Surface-diffusion and step-bunching mechanisms of metalorganic vapor-phase epitaxy studied by high-vacuum scanning tunneling microscopy"
J. Appl. Phys. vol.78, p.3026-3035 (1995) [Errata; J. Appl. Phys. vol.79, p.1822-1823 (1996)] , Citation time 53.

We connected high-vacuum chamber to metalorganic vapor phase epiaxy (MOVPE) system, enabling to investigate MOVPE as-grown surface atomic structures by high-vacuum scanning tunneling microscopy (STM) without exposing the surface to air. We observed two-dimensional nuclei on the MOVPE as-grown surface. From the measurement of two-dimensional nucleus density, we estimated surface diffusion coefficient.

[Achievement 7] M. Kasu and N. Kobayashi,
"Equilibrium multiatomic step structure of GaAs (001) vicinal surfaces grown by metalorganic chemical vapor deposition",
Appl. Phys. Lett. vol.62, p.1262-1264 (1993), Citation time 88.

We observed step-bunching phenomena on MOVPE-grown vicinal surface, and clarify the mechanism.

[Achievement 8] M. Kasu and T. Fukui,
"Multi-atomic steps on metalorganic chemical vapor deposition-grown GaAs vicinal surfaces studied by atomic force microscopy",
Jpn. J. Appl. Phys. vol.31, p.L864-866 (1992) ,Citation time 53.

We observed step bunching phenomena on MOVPE-grown vicinal surface and found that B-type steps are straighter than A-type steps. The tendency is the opposite to MBE case. This phenomenon is explained in terms of c(4x4) surface reconstruction on MOVPE-grown surface, and anisotropic surface diffusion and sticking coefficient.

[Achievement 9] M. Kasu, T. Yamamoto, S. Noda, and A. Sasaki,
"Absorption spectra and photoluminescent processes of AlAs/GaAs disordered superlattices",
Jpn. J. Appl. Phys. vol.29, p.828-834 (1990) ,Citation time 41.

In disordered superlattices, we observed increased luminescence intensity by carrier localization effect.

[Achievement 10] T. Yamamoto, M. Kasu, S. Noda, and A. Sasaki,
"Photoluminescent properties and optical absorption of AlAs/GaAs disordered superlattices",
J. Appl. Phys. vol.68, p.5318-5323 (1990) ,Citation time 99.

We proposed disordered superlattice, which is atomically repeated layers with irregular thicknesses, enabling to investigate atomic disordering effect in alloy semiconductors.