A new project on crystalline defect science maned “singularity project” has started from the fisical year 2016. Traditionally, any disorders found in crystals have been regarded as structural defects that ought to be eliminated from the materials. However, the singularity project is trying to introduce a Copernican revolution in this notion: it aims to focus on imperfect crystals which contain intentionally introduced defects. Furthermore, we try to fabricate functional electronic devices that cannot be achieved with conventional perfect crystals by having these devices take advantage of the versatile physical properties of defects. In this volume, several members of the singularity project show the newst results on widegap semiconductors, which are the exellect materias to test our new idea on crystalline defects.
N-polar III-nitride materials have been attracted attention for the application to InGaN-based optical devices and AlGaN-based electronic devices. Most of the III-nitride-based device structures have been grown using metalorganic vapor phase epitaxy. In this report, recent progress of crystal growth technology of N-polar III-nitride materials using metalorganic vapor phase epitaxy is reviewed. The growth of N-polar materials shows different behavior from that of Ga-polar materials, several optimizations of growth conditions are necessary. N-polar GaN often shows hillock structure at the surface. The use of vicinal substrates helps the suppression of hillock formation. InGaN growth often suffers from the inclusion of zinc-blende crystal phase. Growth at low supersaturation conditions is one of the solutions to obtain pure wurtzite InGaN films. To prevent hillock formation during the growth of AlGaN on N-polar GaN, lowering the growth rate is effective.
We demonstrated the growth of device-quality N-polar GaN films on sapphire (0001) substrates. Specifically, we investigated the crystalline quality of N-polar GaN and basic electrical properties of Mg-doped N-polar GaN films. We found that the dislocation density of PSD-grown N-polar GaN could be reduced with increase in the film thickness. By Mg doping, p-type conductivity in N-polar GaN could be well controlled in the hole concentration range between 8 × 1016 and 2 × 1018 cm−3. With the use of these materials, we demonstrated the successful operation of N-polar InGaN LEDs with a long wavelength up ot 609 nm. The results presented in our manuscript indicate that the PSD growth technique is quite promising for fabricating N-polar devices such as high-efficiency InGaN-based long-wavelength LEDs or solar cells.
We report a direct approach for calculating individual energy of polar semiconductor surfaces and interfaces using density functional theory calculations. This approach is applied to polar surfaces and interfaces of group-III nitrides (AlN, GaN, and InN) and clarifies the interplay of chemical bonding and charge neutrality at the interface, which is crucial for the stability and polarity of group-III nitrides. Furthermore, using the surface and interface energies we demonstrate phase diagrams as functions of temperature and pressure. For AlN on Si-face SiC substrate under H-rich condition, we find that Al-polar AlN surfaces with substitutional Al atoms at AlN/SiC interface are stabilized over the wide range of Al pressure. In contrast, the H-terminated Ga-polar GaN surface on N-polar AlN(0001) substrate with two monolayers of Al olverlayers is favorable over the entire growth conditions. These results suggest that the stability of interface between III-nitride and substrate rather than surface stability is crucial for the polarity of III-nitrides.
We report recent studies on the three-dimensional (3D) analysis of singularity structures in semiconductor materials, especially for nitride semiconductor epitaxial thick films and group IV semiconductor heteroepitaxial films. Nanobeam X-ray diffraction (nanoXRD) with a probe size of a few hundred nanometer performed at the beam line of SPring-8 enables us to reveal the lattice plane microstructures and their modulations caused by the singularity structures. 3D reciprocal space mapping (3D-RSM) analysis has recently been realized and, by using this, not only the lattice tilting and spacing but also the lattice rotation (or twisting) can simultaneously be characterized for exactly the same position in samples. Furthermore, we have newly developed the tomographic mapping method for analysing stacked epitaxial films. This technique allows us to observe not only in-plane but also in depth variations of microstructures, thereby to perform true 3D analysis. A methodology of the nanoXRD 3D-RSM and the tomographic mapping and experimental results revealing the correlation among the singularity structures, defect distribution, and 3D lattice morphology in the films are presented.
Single-crystal AlN/diamond heterojunction with high-density interface hole channel was successfully obtained by metal-organic vapor phase epitaxy. AlN layer was epitaxially grown on hydrogen-terminated (H-)diamond(111) substrate. Thermal treatment of diamond substrate was carried out under hydrogen and ammonia mixture environment at 1250 ℃ just before AlN growth. This thermal treatment led to surface sheet hole density as high as ~1.0 × 1014 cm-2 without structural reconstruction of diamond surface. In addition, the use of smaller off-cut angle (0.20 ± 0.25°) H-diamond(111) substrate combined with this treatment enabled to obtain single-crystal epitaxial AlN layer, which simultaneously acted as passivation of the surface hole channel with such a high density. The AlN/H-diamond(111) hetero-junction revealed type-II staggered energy band configuration with valence band offset of ~2.0 eV, which is suitable for the fabrication of p-channel field-effect transistor using AlN-gate-insulator/ diamond heterojunction. These results are promising for the development of AlN/diamond hybrid power electronic devices.
Growth and optical properties of GaN nanowires (NWs) and related technologies are discussed, in terms of their applications to optoelectronic devices. Although few reports regarding GaN-based NWs applied to laser diodes have been published, it will atract much attentions in the near future. This paper presents that GaN NWs and surounding multi-quantum-shell (MQS) active medium have a high potential to improve the performance of lasers because of its high 3D optical cofinement factor. To grow GaN NWs with (0001) tip-plane, pulsed mode MOVPE, which provides effectively very low V/III ratios, and relatively high growth temperature are essential. The formation of (0001) tip-plane improves the composition uniformity of GaInN/GaN MQS. PL intentisy per excited volume from MQS is comparable to that from conventional 2D MQW. The GaN NWs and MQS will provide us a next generation platform of optoelectronic devices.