Masayoshi Kosaki

Metalorganic vapor phase epitaxy growth of crack-free AlN on GaN and its application to high-mobility AlN/GaN superlattices

We have succeeded in growing crack-free AlN of even 0.5 μm thickness on GaN by metalorganic vapor phase epitaxy. A (0001) sapphire substrate was used. Crack-free AlN was grown on GaN at 1000 °C with N2 carrier gas. An AlN layer was grown on GaN of 2 μm thickness grown at 1050 °C, following the low-temperature deposition of an AlN buffer layer of 30 nm. No cracks were observed in the microphotographs of AlN on GaN grown using N2. X-ray diffraction analysis revealed that AlN/GaN superlattices (SLs) were coherently grown on GaN, and satellite peaks up to the third order were observed. The structure of AlN/GaN SLs on GaN showed a maximum electron mobility of 1580 cm2/V s at room temperature and a nominal sheet carrier density of 8.4×1012 cm−2.

Polarization Engineering on Buffer Layer in GaN-Based Heterojunction FETs

To improve the pinched-off characteristics of an AlGaN/GaN heterojunction field effect transistor (HJFET), the conduction band potential of an incorporated ALxGa1-xN buffer is designed to be upwardly convex in a band diagram. This approach utilizes the polarization effects specific to GaN-based materials by lowering the Al content x from 30% to 5% almost linearly toward the front side. Fabricated field effect transistors (FETs) adopting the designed buffer have demonstrated the following advanced characteristics in comparison to those of a FET adopting a conventional GaN buffer: less than one-tenth of the buffer leakage current, a gate-to-drain breakdown voltage BVgd twice or more as high, and remarkably improved carrier confinement and pinched-off behavior. The FETs are operated in an enhancement mode with a gate-to-channel distance thick enough to prevent tunneling current through the gate.

Electrical properties of strained AlN/GaN superlattices on GaN grown by metalorganic vapor phase epitaxy

We have studied the temperature dependence of electrical properties of crack-free strained AlN/GaN superlattices (SLs) on GaN grown by metalorganic vapor phase epitaxy. A (0001) sapphire substrate was used. A single AlN on GaN and one and five pairs of AlN/GaN superlattices were grown using N2 carrier gas. The thicknesses of AlN and GaN in the superlattices were 1 and 5 nm, respectively. Hall measurements were performed at 295–20 K. The five pairs of AlN/GaN SLs on GaN showed an electron mobility of 9925 cm2/V s and a sheet carrier density of 1.14×1012 cm−2 at 20 K, and 1354 cm2/V s and 1.14×1012 cm−2 at 295 K.

Crystal Growth of High‐Quality AlInN/GaN Superlattices and of Crack‐Free AlN on GaN: Their Possibility of High Electron Mobility Transistor

Al 0.90 In 0.10 N/GaN, AIN/GaN and their superlattices (SLs) grown on GaN by metalorganic vapor phase epitaxy (MOVPE) were studied. A (0001) sapphire substrate was used. X-ray diffraction analysis of 2θ/ω scans and reciprocal space mapping showed that the samples were coherently grown on GaN, and in particular, crack-free AlN of 0.5 μm thickness was grown on GaN. Al 0.90 In 0.10 N/GaN (or AlN/GaN) SLs grown on GaN showed a maximum electron mobility of 946 (or 1422) cm 2 /Vs at room temperature and a nominal sheet carrier density of 2.9 x 10 12 (or 7.5 x 10 12 ) cm -2 .

Control of strain in GaN by a combination of H2 and N2 carrier gases

We study the effect of a combination of N2 and H2 carrier gases on the residual strain and crystalline properties of GaN, and we propose its application to the improvement of crystalline quality of GaN/Al0.17Ga0.83N multiple quantum well (MQW) structures. GaN was grown with H2 or N2 carrier gas (H2– or N2–GaN) on an AlN low-temperature-deposited buffer layer. A (0001) sapphire substrate was used. N2–GaN was grown on H2–GaN. The total thickness was set to be 1.5 μm, and the ratio of N2–GaN thickness to the total thickness, x, ranged from 0 to 1. With increasing x, the tensile stress in GaN increased. Photoluminescence intensity at room temperature was much enhanced. Moreover, the crystalline quality of GaN/Al0.17Ga0.83N MQW was much higher when the MQW was grown with N2 on H2–GaN than when it was grown with H2 on H2–GaN. These results were due to the achievement of control of strain in GaN using a combination of N2–GaN and H2–GaN.

Effect on GaN/Al0.17Ga0.83N and Al0.05Ga0.95N/Al0.17Ga0.83N Quantum Wells by Isoelectronic In-Doping during Metalorganic Vapor Phase Epitaxy

The effects of isoelectronic In-doping on the structural and optical properties of GaN/Al0.17Ga0.83N and Al0.05Ga0.95N/Al0.17Ga0.83N quantum wells (QWs) on GaN were investigated. QWs were grown by atmospheric pressure metalorganic vapor phase epitaxy with either H2 or N2 carrier gas. Without In-doping, QWs grown in N2 carrier gas had highly superior crystalline and optical properties than those grown in H2 carrier gas. X-ray diffraction and photoluminescence studies revealed that In-doping improves the crystalline and optical properties of QWs, irrespective of the carrier gas species used during growth. These improvements are more remarkable for In-doping into the well layers rather than into the barrier layers.

Electrical and crystalline properties of as-grown p-type GaN grown by metalorganic vapor phase epitaxy

P-GaN layer was grown by metalorganic vapor phase epitaxy at 1000°C with an LT-deposited AIN buffer layer. Bis(cyclopentadienyl) magnesium was used as a dopant and trimethyindium (TMIn) was simultaneously supplied. N 2 carrier gas was used during GaN growth. The contact of as-grown GaN:Mg sample was Ohmic only for In-doped GaN. Others showed Schottky behavior. The hole concentration was increased up to 5.0 x 10 17 cm -3 at room temperature for In-doped samples. Moreover, with increasing TMIn flow rate, the biaxial strain in as-grown p-GaN was reduced and accordingly the hole concentration was increased.

Metalorganic Vapor Phase Epitaxial Growth of High-Quality AlInN/AlGaN Multiple Layers on GaN

Al1-xInxN epilayers with a thickness of 20 nm grown on GaN were characterized. It was found that the surface roughness of very thin Al1-xInxN is more sensitive to the growth temperature than to the lattice mismatch to GaN. Multiple layers of (Al0.95In0.05N/Al0.10Ga0.90N)5 were grown at different temperatures. X-ray diffraction measurement revealed that the sample grown at 800°C is of high quality, showing up to five orders of sharp satellite peaks, while no clear satellite peaks are observed from the sample grown at 720°C. Transmission electron microscopy observation revealed that multiple layers grown at 800°C exhibit very sharp interfaces.

Correlation between micropipes on SiC substrate and dc characteristics of AlGaN∕GaN high-electron mobility transistors

We report the influence of the size of hollow cores that extend from micropipes in SiC substrates on the dc characteristics of AlGaN∕GaN high-electron mobility transistors (HEMTs) fabricated on the substrates. Significant deterioration of the dc characteristics of HEMTs fabricated in the vicinity of hollow cores with a diameter of 5μm was observed, while no major deterioration was observed for HEMTs fabricated around hollow cores whose diameters were 1.5 and 3μm. A clear correlation between the size of hollow cores and free carrier densities at the peripheries of the cores was observed using micro-Raman imaging. The high densities of free carriers around relatively large hollow cores were suggested to be the cause of deterioration of the dc characteristics of HEMTs fabricated in the vicinity of the hollow cores. The device deterioration length with respect to the hollow core size was empirically deduced, and the effective decrease in the wafer yield was estimated as well.

Effect of In-Doping on the Properties of as-Grown p-Type GaN Grown by Metalorganic Vapour Phase Epitaxy

As-grown p-GaN was characterized using Hall measurement and X-ray diffraction analysis mainly at room temperature. All samples were grown by metalorganic vapour phase epitaxy. p-type GaN was grown using Cp 2 Mg as a dopant and In-doping during the GaN growth. Only using In-doping, GaN-Mg was activated at room temperature even without a thermal annealing process. A hole concentration of as high as 5 x 10 17 cm -3 was achieved. As a whole trend, decreasing the biaxial strain and the twist in as-grown p-GaN resulted in an increase of the hole concentration.