T. Makimoto

An aluminium nitride light-emitting diode with a wavelength of 210 nanometres

Compact high-efficiency ultraviolet solid-state light sources—such as light-emitting diodes (LEDs) and laser diodes—are of considerable technological interest as alternatives to large, toxic, low-efficiency gas lasers and mercury lamps. Microelectronic fabrication technologies and the environmental sciences both require light sources with shorter emission wavelengths: the former for improved resolution in photolithography and the latter for sensors that can detect minute hazardous particles. In addition, ultraviolet solid-state light sources are also attracting attention for potential applications in high-density optical data storage, biomedical research, water and air purification, and sterilization. Wide-bandgap materials, such as diamond and III–V nitride semiconductors (GaN, AlGaN and AlN; refs 3–10), are potential materials for ultraviolet LEDs and laser diodes, but suffer from difficulties in controlling electrical conduction. Here we report the successful control of both n-type and p-type doping in aluminium nitride (AlN), which has a very wide direct bandgap of 6u2009eV. This doping strategy allows us to develop an AlN PIN (p-type/intrinsic/n-type) homojunction LED with an emission wavelength of 210u2009nm, which is the shortest reported to date for any kind of LED. The emission is attributed to an exciton transition, and represents an important step towards achieving exciton-related light-emitting devices as well as replacing gas light sources with solid-state light sources.

Field emission properties of heavily Si-doped AlN in triode-type display structure

Using heavily Si-doped AlN, a triode-type field emission display is demonstrated. The device consists of the heavily Si-doped AlN field emitter, mesh grid, and phosphor-coated anode screen. The device exhibits a low turn-on electric field of 11 V/μm, and the field emission current exponentially increases as the grid voltage increases. The field emission current reaches 9.5 μA at an electric field strength of 23 V/μm. Luminescence from the phosphor excited by the field-emitted electrons is uniform over the anode screen and is intense enough for the display application. The field emission current is stable over time.

High-pressure and high-temperature annealing as an activation method for ion-implanted dopants in diamond

The authors show that high-pressure and high-temperature (HPHT) annealing is very effective for the activation of ion-implanted dopants in diamond. The HPHT annealing condition is located in the thermodynamically stable region for diamond in the phase diagram and is, therefore, much more efficient for the recovery of implantation-induced damage and for the activation of ion-implanted dopants than thermal annealing in vacuum. The B-implanted film after HPHT annealing showed a high mobility of 632cm2∕Vs with a sheet hole concentration of 4.8×1010cm−2 at 300K and the doping efficiency of ∼7%. The mobility is the highest so far for ion-implanted diamond. In the entire annealing temperature range, the HPHT annealing is more efficient than the thermal annealing in vacuum.

Reduction of threading dislocations in crack-free AlGaN by using multiple thin SixAl1-xN interlayers

Crack-free AlGaN thin films were directly grown on SiC substrates by metalorganic vapor phase epitaxy, and their threading dislocation density was reduced by one order of magnitude using 1–2 nm thick, heavily Si-doped AlN multiple interlayers. The interlayers form SixAl1−xN ternary alloys, where the Si molar fraction ranges typically from 0.07 to 0.17. This technique enables us to grow crack-free AlGaN films, since the film thickness of about 1 μm is much smaller than that required in conventional epitaxial lateral overgrowth techniques. Both termination and looping of threading dislocations were observed near the interlayers using cross-sectional transmission electron microscopy. Light emitting devices with the SixAl1−xN multiple interlayers showed a remarkable improvement in the intensity and spectral width of electroluminescence and the series resistance.

Gate capacitance-voltage characteristics of submicron-long-gate diamond field-effect transistors with hydrogen surface termination

The radio-frequency characteristics of p-type diamond field-effect transistors with hydrogen surface termination were numerically analyzed using an equivalent-circuit model. From the gate-source capacitance (CGS)-voltage (VGS) results extracted from measured s parameters, the authors found a plateau in CGS within a certain VGS range. This means that a two-dimensional hole gas channel forms parallel to the surface and that the channel is separated by a thin energy-barrier layer with an infinite height from the gate metal. At a high negative VGS, as negative VGS is increased, CGS increases steeply. This results from holes penetrating the energy barrier.

Formation of stacking faults containing microtwins in (111) chemical-vapor-deposited diamond homoepitaxial layers

Stacking faults containing microtwins in (111)-oriented diamond layers grown on a high-pressure high-temperature (HPHT)-synthesized diamond substrate by chemical vapor deposition start to form just on the substrate. The microtwins in the stacking faults form on the {111} plane, not on the (111) substrate plane. To explain these results, we propose an atomic-scale model in which a foreign atom remains on the HPHT substrate surface and a C atom on the foreign atom cannot form a covalent bond with it. Therefore, twinning of the C atom occurs on the {111} plane. The next C atoms bond with the twinned C atom in an untwinned (normal crystalline) relation. Consequently, the formation of stacking faults that contain microtwins occurs.

Anisotropic surface morphology of GaAs (001) surfaces passivated with nitrogen radicals

We have studied GaAs (001) surfaces passivated with nitrogen (N) radicals at submonolayer N coverage mainly using scanning tunneling microscopy and transmission electron microscopy. We determined that GaN‐rich regions are elongated in the [110] direction, suggesting that N passivation proceeds in the [110] direction. This can be explained in terms of minimization of the tensile strains in the [110] direction induced when the supplied N atoms replace first‐layer As atoms on the (2×4) surface.

Strained Thick p-InGaN Layers for GaN/InGaN Heterojunction Bipolar Transistors on Sapphire Substrates

Thick p-InGaN layers were grown on GaN by metalorganic vapor phase epitaxy to investigate the strain inside p-InGaN using a reciprocal space map of X-ray diffraction intensity. It was found that a large part of p-InGaN grows coherently on the GaN buffer layer, even though it is much thicker than the calculated critical thickness. This result means that few dislocations are generated at the InGaN/GaN interface. Using this strained thick p-InGaN as a base, a GaN/InGaN heterojunction bipolar transistor was fabricated on a sapphire substrate. Its maximum current gain was as high as 1000 and its offset voltage as low as 0.2 V, which matches that calculated from the conduction-band discontinuity between the n-GaN emitter and the p-InGaN base.