Yoshitaka Taniyasu

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 6 eV. This doping strategy allows us to develop an AlN PIN (p-type/intrinsic/n-type) homojunction LED with an emission wavelength of 210 nm, 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.

Intentional control of n-type conduction for Si-doped AlN and AlXGa1−XN (0.42⩽x<1)

We have obtained n-type conductive Si-doped AlN and AlXGa1−XN with high Al content (0.42⩽x<1) in metalorganic vapor phase epitaxy by intentionally controlling the Si dopant density, [Si]. Si-doped AlN showed the n-type conduction when [Si] was less than 3×1019 cm−3. When [Si] was more than 3×1019 cm−3, it became highly resistive due to the self-compensation of Si donors. This indicates that the self-compensation plays an important role at higher [Si] and determines the upper doping limit of Si for the AlN and AlXGa1−XN. For x⩾0.49, the ionization energy of Si donors increased sharply with increasing Al content. These resulted in a sharp decrease in the highest obtainable electron concentration with increasing Al content for the Si-doped AlXGa1−XN.

Electrical conduction properties of n-type Si-doped AlN with high electron mobility (>100cm2V−1s−1)

For n-type Si-doped AlN, we have obtained an electron mobility and concentration of 125cm2V−1s−1 and 1.75×1015cm−3 at 300K, respectively. At 250K, the mobility reached the maximum of 141cm2V−1s−1. To explain the temperature dependence of the mobility, we calculated mobilities limited by specific scattering mechanisms. We found that the mobility is limited by neutral impurity scattering rather than ionized impurity scattering or lattice scattering because of a large donor ionization energy (∼250meV).

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.

Increased electron mobility in n-type Si-doped AlN by reducing dislocation density

For n-type Si-doped AlN with a low Si doping concentration of 3×1017cm−2, a high room-temperature electron mobility of 426cm2V−1s−1 was achieved, and at 220K the mobility reached 730cm2V−1s−1, the highest value ever reported for AlN. At Si doping concentrations lower than 1018cm−3, dislocation scattering is the most dominant scattering mechanism, and the mobility can therefore be increased significantly by reducing the dislocation density.

RF High-Power Operation of AlGaN/GaN HEMTs Epitaxially Grown on Diamond

We epitaxially grow AlGaN/GaN high-electron-mobility transistors (HEMTs) on IIa-type single-crystal diamond (111) substrates. A 0.4-μm gate-length HEMT showed a dc drain-current density IDS of 770 mA/mm and a breakdown voltage of 165 V. In the RF large-signal measurements at 1 GHz, an RF output-power density POUT of 2.13 W/mm was obtained. This is the first report of RF power operation of AlGaN/GaN HEMTs epitaxially grown on diamond. The AlGaN/GaN HEMTs epitaxially grown on diamond showed a low thermal resistance of 1.5 K·mm/W.

Formation of Solid Solution of Al1-xSixN (0<x≤12%) Ternary Alloy

When Si was doped into an AlN layer during metalorganic vapor-phase epitaxial growth, the Al density (NAl) in the AlN layer decreased but the N density (NN) did not change. The decrease in NAl was almost the same as the Si density (Nsi). As Nsi increased in Si-doped AlN, the lattice constant decreased. These results can be explained by Si atoms replacing Al atoms in Si-doped AlN and subsequent Si–N bond formation. Thus, Si-doped AlN becomes a substitutional solid solution of Al1-xSixN ternary alloy. The highest Si density at which the x-ray diffraction peak still appears was 5.8×1021 cm-3 (x=12%).

Mg doping for p-type AlInN lattice-matched to GaN

P-type AlInN layers lattice-matched to GaN are achieved by Mg doping. The net acceptor concentration NA – ND is 5 × 1018 cm−3 at a Mg concentration [Mg] of ∼2 × 1019 cm−3. Mg acceptors are partly compensated and one of the compensating defects is related to the occurrence of surface pits. At [Mg]   2 × 1019 cm−3, as [Mg] increases, the pit density increases and the NA – ND decreases. By decreasing the pit density, a higher NA – ND value is obtained and light-emitting diodes with p-type AlInN layer show improved emission intensity.

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.

Lattice parameters of wurtzite Al1−xSixN ternary alloys

Lattice constants and bond lengths of wurtzite Al1−xSixN ternary alloys (0⩽x⩽0.08) were determined by reciprocal lattice maps around Al1−xSixN (0002) and (11–24) reflections. The measured lattice constants obtained directly from as-grown Al1−xSixN layers were scattered because they include the factor of residual strain. Therefore, the lattice constants in the strain-free case were calculated from the measured lattice constants taking the residual strain into account. We found that the a-axis and c-axis lattice constants of the strain-free Al1−xSixN linearly decreased with the Si content as a0=3.1113−0.1412x (A) and c0=4.9814−0.2299x (A). Further, we obtained the bond length as d0=1.86818−0.0862x (A). The bond length is nearly equal to the interpolation between the Al–N bond and the Si–N bond.