We investigated MOVPE growth conditions for AlInN layers with high growth rates and obtained 0.5µm/h with smooth surfaces. We found that short gas mixing time, relatively high growth temperature, and very low In/Al supply ratio were key growth parameters in order to obtain the AlInN layers with high growth rate and smooth surface simultaneously. AlInN/GaN DBRs grown under such growth conditions showed smooth surfaces and a reflectivity of over 99%.

We discovered that Si-doped Al0.05Ga0.95N can be used to realize an extremely low-resistivity n-type layer at room temperature. In Si-doped GaN, a resistivity of 2.7×10-3 Ω cm with a carrier concentration of 4.0×1019 cm-3 was almost saturated. In contrast, Si-doped Al0.05Ga0.95N with a minimum resistivity of 5.9×10-4 Ω cm was produced with an electron concentration and electron mobility of 1.4×1020 cm-3 and 70 cm2/V s, respectively. We confirmed a reduction in the differential resistance of a violet light-emitting diode with a high external quantum efficiency by using this Si-doped Al0.05Ga0.95N.

We investigated electron and hole accumulations at GaN/AlInN/GaN interfaces by Hall effect measurement. The InN mole fraction and temperature dependences on the sheet carrier densities at the interfaces reveal that electrons and holes were induced by large positive and negative polarization charges to satisfy the charge neutrality conditions, respectively. On the basis of the above results, we then designed and demonstrated a low-resistity 10-pair Si-doped n-type AlInN/GaN distributed Bragg reflector (DBR) by using high Si doped and graded layers at the GaN/AlInN interfaces. The low-resistity n-type AlInN/GaN DBR will reduce the resistance and the internal loss in blue vertical-cavity surface emitting lasers.

We investigated hole concentrations induced by spontaneous and piezoelectric polarization charges in GaN/AlGaN/GaN heterostructures. An AlN mole fraction dependence reveals that the induced hole concentrations were accurately controlled by the AlN mole fractions up to 0.34 in the GaN/AlGaN/GaN structures. We then demonstrated vertical hole conduction across an undoped graded AlGaN layer and hole injection to an active region in a LED structure to some extent. These results suggest that the controlled polarization charges by AlN mole fractions have potential as an alternative doping method for high hole concentrations in nitride semiconductors.

We investigated polarization doping for hole generation in abrupt and graded GaN/Al0.7Ga0.3N interfaces on Al0.99Ga0.01N templates. The abrupt interface exhibited hole generation, whereas the graded interface exhibited electron generation. In the graded AlxGa1−xN (x = 0.65–0), a graded part with an AlN mole fraction ranging from 0.2 to 0 showed a large relaxation. Theoretical estimation revealed that this part contained positive polarization charges, accumulating electrons. Via Mg doping in the graded AlGaN layer, we obtained a high hole concentration of 3 × 1013 cm−2. These results indicate that understanding the relaxation conditions in the graded layer is indispensable for polarization doping.