The resistivity, electron concentration, mobility, and luminescence property of AlGaN epilayers with various AlN contents ( x ) are systematically investigated as a function of Si‐doping concentration ([Si]). In x of 59–78%, the resistivity decreases with increasing [Si] up to 4 × 10 19 cm −3 . The temperature dependence of the electron concentration and mobility disappears in the optimum doping range, characteristic of electric degeneracy. The ionization energy of isolated Si donor ( E d0 ) is slightly increased in the range of 60–80 meV with increasing x (59–78%). At higher [Si] than 4 × 10 19 cm −3 , self‐compensation defects such as III vacancy–Si complexes are formed, resulting in higher resistivity. When x is 89%, the resistivity is approximately one order of magnitude higher than that for lower x , and the E d0 is drastically increased to ≈120 meV, which is a feature of a highly compensated semiconductor. Based on the increased oxygen incorporation and the distinctive defect luminescence peak for the sample with x of 89%, the formation of acceptor‐type O‐related defects such as O‐ DX and V III ‐ n O N is indicated and can contribute to a part of the electron compensation.

Deep-ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN crystals have low light-emission efficiency; therefore, there is a need to improve this light-emission efficiency for a wide range of applications such as water and air sterilizations. UV-light-transparent device structures are considered one of the many solutions toward increasing light output power. To this end, the present study focused on developing a transparent AlGaN-based tunnel junction (TJ) as the anode of a deep-UV LED. Deep-UV LEDs composed of n+/p+-type AlGaN TJs were fabricated under the growth condition that reduced the carrier compensation in the n+-type AlGaN layers. The operating voltage was 10.8 V under the direct current (DC) operation of 63 A/cm2. In addition, magnesium zinc oxide (MgZnO)/Al reflective electrodes were fabricated to enhance the output power of the AlGaN homoepitaxial TJ LED. The output power was 57.3 mW under a DC operation of 63 A/cm2, and it was 1.7 times higher than that realized using the conventional Ti/Al electrodes. The combination of the AlGaN-based TJ and MgZnO/Al reflective contact allows further improvement of the light output power. This study confirms that the AlGaN TJ is a promising UV-transmittance structure that can obtain a high light-extraction efficiency.

Conventional deep-ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN crystals have extremely low light-emission efficiencies due to the absorption in p-type GaN anode contacts. UV-light-transparent anode structures are considered as one of the solutions to increase a light output power. To this end, the present study focuses on developing a transparent AlGaN homoepitaxial tunnel junction (TJ) as the anode of a deep-UV LED. Deep-UV LEDs composed of n+/p+-type AlGaN TJs were fabricated under the growth condition that reduced the carrier compensation in the n+-type AlGaN layers. The developed deep-UV LED achieved an operating voltage of 10.8 V under a direct current (DC) operation of 63 A cm−2, which is one of the lowest values among devices composed of AlGaN tunnel homojunctions. In addition, magnesium zinc oxide (MgZnO)/Al reflective electrodes were fabricated to enhance the output power of the AlGaN homoepitaxial TJ LED. The output power was increased to 57.3 mW under a 63 A cm−2 DC operation, which was 1.7 times higher than that achieved using the conventional Ti/Al electrodes. The combination of the AlGaN-based TJ and MgZnO/Al reflective contact allows further improvement of the light output power. This study confirms that the AlGaN TJ is a promising UV-transmittance structure that can achieve a high light-extraction efficiency.

To reduce the operating voltage, we analyzed the p–n junction of an aluminum gallium nitride (AlGaN) homojunction Tunnel Junction (TJ) deep-ultraviolet light-emitting diode using phase-shifting electron holography. We obtained a phase image reflecting the band alignment of the p–n homojunction and derived a depletion layer width of approximately 10 nm. We found the AlGaN homojunction TJ forms a p-n junction. Furthermore, the operating voltage reached 8.8 V at 63 A cm-2 by optimizing the structural characteristics of the AlGaN TJ, such as the thickness and impurity concentration, where the thickness of the TJ was 23 nm. We found that the TJ thickness should be at least the same as the depletion layer width at the AlGaN TJ.

We demonstrated the structural optimization of AlGaN tunnel junction (TJ) deep-ultraviolet light-emitting diodes by changing the thickness and impurity concentrations of p+-type and n+-type AlGaN constituting the TJ. By decreasing the total thickness of the TJ to 23 nm, the operating voltage reached a minimum of 8.8 V at 63 A cm−2. Further decrease in TJ thickness markedly increases the operating voltage. This finding implies that the depletion layer width becomes greater than the TJ thickness if it is smaller than 12 nm. Therefore, we conclude that the TJ thickness must be greater than the depletion layer width.

We investigated light output power (LOP) values of AlGaN-based deep ultraviolet LEDs with transparent $\mathrm{p}^{+}$ -AlGaN contact layers and inexpensive ITO/Al electrodes as a function of p-layer thickness. The measured LOP values were increased as the p-layer thickness was decreased. This increase was due to the interference effect. Further reduction of the internal absorption could improve the LOP of the LEDs.

In this study, we enhanced the emission power of AlGaN-based tunnel junction deep-ultraviolet LEDs (TJ LEDs) by using a MgZnO and aluminum stacked structure as UV reflective electrodes on the anode side. The emission wavelength of the fabricated TJ LED was 284 nm, and the emission power was 57.3 mW at 63 A cm −2 . The polycrystalline MgZnO enabled both conductivity and UV transmittance to be achieved, approximately doubling with a reflectance of the fabricated TJ LED at 284 nm compared to conventional TJ LED with Ti/Al. These factors contributed to the increased emission power of TJ LEDs.

We analyzed the p–n junction of an aluminum gallium nitride (AlGaN) homojunction tunnel junction (TJ) deep-ultraviolet light-emitting diode by phase-shifting electron holography. We clearly obtained a phase image reflecting the band alinement of the p–n homojunction and derived a depletion layer width of approximately 10 nm. In addition, the observed depletion layer width for the AlGaN TJ was in good agreement with the simulated one reflecting the diffusion profile of Mg and Si, thus enabling a discussion on the electrical conduction mechanism for an AlGaN p–n junction.

Oral malignant melanoma, which frequently invades the hard palate or maxillary bone, is extremely rare and has a poor prognosis. Bone morphogenetic protein (BMP) is abundantly expressed in bone matrix and is highly expressed in malignant melanoma, inducing an aggressive phenotype. We examined the role of BMP signaling in the acquisition of an aggressive phenotype in melanoma cells in vitro and in vivo. In five cases, immunohistochemistry indicated the phosphorylation of Smad1/5 (p-Smad1/5) in the nuclei of melanoma cells. In the B16 mouse and A2058 human melanoma cell lines, BMP2, BMP4, or BMP7 induces morphological changes accompanied by the downregulation of E-cadherin, and the upregulation of N-cadherin and Snail, markers of epithelial–mesenchymal transition (EMT). BMP2 also stimulates cell invasion by increasing matrix metalloproteinase activity in B16 cells. These effects were canceled by the addition of LDN193189, a specific inhibitor of Smad1/5 signaling. In vivo, the injection of B16 cells expressing constitutively activated ALK3 enhanced zygoma destruction in comparison to empty B16 cells by increasing osteoclast numbers. These results suggest that the activation of BMP signaling induces EMT, thus driving the acquisition of an aggressive phenotype in malignant melanoma.

AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) are key components for the inactivation of viruses. Highly efficient and high-power UV-LEDs, capable of inactivating viruses in a short time, are in demand. For this purpose, the growth technologies of n-type AlGaN contact layers were developed from two points of view: first, to decrease the resistivity of n-type Al0.62Ga0.38N by minimizing the electron compensation, resulting in electronic degeneracy with metallic conduction; second, to improve the light emission uniformity in AlGaN multiquantum wells (MQWs) by controlling the morphology of the underlying n-type AlGaN layer to inhibit macrostep formation. A UV-LED module emitting at 275 nm was demonstrated with the developed growth technology, and illuminated with an irradiation power of 2.6 mW cm−2 on SARS-CoV-2 samples. Over 99.999 % of viruses were inactivated within 5 s owing to the high power of this module.

We reduced the operating voltage of AlGaN homojunction tunnel junction (TJ) deep-ultraviolet (UV) light-emitting diodes (LEDs) by two approaches: the suppression of carbon incorporation and the doping of a high concentration of silicon in an n(+)-AlGaN layer. The AlGaN homojunction TJ deep-UV LEDs had a significantly reduced forward voltage upon suppressing the incorporation of carbon in the n(+)-AlGaN layer. The suppression of electron compensation by carbon in nitrogen sites and the doping of a high concentration of silicon in an n(+)-AlGaN layer are important for reducing the operating voltage of AlGaN homojunction TJ deep-UV LEDs.

Bone invasion is a critical factor in determining the prognosis of oral squamous cell carcinoma (OSCC) patients. Transforming growth factor β (TGF-β) is abundantly expressed in the bone matrix and is involved in the acquisition of aggressiveness by tumors. TGF-β is also important to cytoskeletal changes during tumor progression. In this study, we examined the relationship between TGF-β signaling and cytoskeletal changes during bone invasion by OSCC. Immunohistochemical staining of OSCC samples from five patients showed the expression of p130Cas (Crk-associated substrate) in the cytoplasm and phosphorylated Smad3 expression in the nucleus in OSCC cells. TGF-β1 induced the phosphorylation of Smad3 and p130Cas, as well as epithelial-mesenchymal transition (EMT) accompanied by the downregulation of the expression of E-cadherin, a marker of epithelial cells, and the upregulation of the expression of N-cadherin, or Snail, a marker of mesenchymal cells, in human HSC-2 cells and mouse squamous cell carcinome VII (SCCVII) cells. SB431542, a specific inhibitor of Smad2/3 signaling, abrogated the TGF-β1-induced phosphorylation of p130Cas and morphological changes. Silencing p130Cas using an short hairpin RNA (shRNA) or small interfering RNA in SCCVII cells suppressed TGF-β1-induced cell migration, invasion, EMT and matrix metalloproteinase-9 (MMP-9) production. Compared with control SCCVII cells, SCCVII cells with silenced p130Cas strongly suppressed zygomatic and mandibular destruction in vivo by reducing the number of osteoclasts, cell proliferation and MMP-9 production. Taken together, these results showed that the expression of TGF-β/p130Cas might be a new target for the treatment of OSCC bone invasion.

The lowest resistivity of highly Si-doped Al0.62Ga0.38N was achieved using metalorganic vapor epitaxy. The resistivity strongly depended on the Si concentrations and reached a minimum value of 6.6 x 10(-3) Omega cm at a Si concentration of 3.2 x 10(19) cm(-3), where the carrier concentration was close to the Si one. Above this concentration, luminescence bands around 2.4 eV originating from group-III-vacancy-Si complexes (V-III-nSi) were observed, whereas carrier concentrations and mobilities decreased. Growth conditions that avoid high temperatures and V/III ratios result in suppressed formation of V-III-nSi, playing a key role in achieving low resistivity. (C) 2020 The Japan Society of Applied Physics

Electronic degeneracy to express metallic conduction in Al-rich AlGaN for the electron injection layer enhances the efficiencies of deep ultraviolet light emitters. This study systematically demonstrates the Si doping range and conditions to realize degenerate n-type Al0.6Ga0.4N layers based on the electron compensation effect. The temperature-independent electron concentrations resulting from the degenerate band appear in high Si doping conditions to overcome the electron compensation due to carbon on nitrogen sites (CN). However, excessive Si doping of over 4.0 × 1019 cm−3 leads to the collapse of the electronic degeneracy and a switch to the temperature-dependent electron transport via the impurity bands, where the luminescence bands originating from III vacancy-Si complexes (VIII-nSi) are dominant. The key parameter is the effective donor concentration, Nd − Na, based on the reduction in electron concentrations via acceptor-like deep levels such as CN and VIII-nSi. The Hall-effect analyses for n-type Al0.6Ga0.4N layers with various Si concentrations yielded an Nd − Na value of (9.5 ± 2.9) × 1018 cm−3 to vanish the ionization energy of Si donors, which is approximately six times higher than that in GaN. The results suggest not only the optimal doping range to obtain an Al-rich AlGaN layer with metallic conduction but also the necessity of the growth condition to minimize electron compensation.

The formation mechanism of Ga-rich streaks in the AlGaN layers formed at macro-step edges was investigated by analyzing AlGaN layers grown by metalorganic vapor phase epitaxy on AlN/sapphire-templates. Energy dispersive X-ray spectroscopy in the cross-sectional scanning transmission electron microscopy revealed that the macro-steps formed during AlN growth were maintained through the upper AlGaN layer. This can be explained by the less interaction between adjacent macro-steps due to the sufficiently smaller surface diffusion lengths of Ga and Al species than the inter-macro-step distance of submicron to a few microns. Moreover, atomic force microscopy indicated that the apparent smooth region between macro-steps involved many atomic steps and terrace width of a few ten nanometers which may be the actual scale of the diffusion lengths. At the macro-step edge, facets that are different from a c-plane are formed. The higher incorporation efficiency of Ga atoms on these facets compared with that of the c-plane likely led to the formation of Ga-rich streaks due to the less inter-surface diffusion between the facets and c-plane regions. We demonstrated AlGaN growth without Ga-rich streaks using the flattened AlN/sapphire-template by polishing; this resulted in the achievement of homogeneous emission energies in AlGaN-based quantum wells.

We deposited a 4-inch crack-free aluminum nitride (AlN) layer on a flat sapphire substrate and a patterned sapphire substrate grown using metalorganic vapor phase epitaxy method. To use the 4-inch sapphire substrate, we introduce air voids in the AlN layer in the initial growth region. Moreover, we discovered that optical absorption exists in the AlN layer with air voids. The secondary ion mass spectrometry (SIMS) results show that the impurity concentrations are high at the regions where the air voids exist, though the growth conditions are same throughout the AlN layer. We considered that it is most probable that the optical absorption is mainly attributed to the complexes of Al and O. The origin of the optical absorption was identified as a deep emission band due to impurities such as carbon and oxygen around the air voids. In this study, it was possible to obtain a transparent AlN template without air voids by changing the growth conditions and the layer structure. In addition, we could further suppress the underlying layer with the air voids. (C) 2019 The Japan Society of Applied Physics