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Self-assembled growth of blue-green-yellow-red InGaN quantum dots (QDs) on GaN templates using plasma-assisted molecular beam epitaxy were investigated. We concluded that growth conditions, including small N2 flow and high growth temperature are beneficial to the formation of InGaN QDs and improve the crystal quality. The lower In/Ga flux ratio and lower growth temperature are favorable for the formation of QDs of long emission wavelength. Moreover, the nitrogen modulation epitaxy method can extend the wavelength of QDs from green to red. As a result, visible light emissions from 460 nm to 622 nm have been achieved. Furthermore, a 505 nm green light-emitting diode (LED) based on InGaN/GaN MQDs was prepared. The LED has a low external quantum efficiency of 0.14% and shows an efficiency droop with increasing injection current. However, electroluminescence spectra exhibited a strong wavelength stability, with a negligible shift of less than 1.0 nm as injection current density increased from 8 A/cm2 to 160 A/cm2, owing to the screening of polarization-related electric field in QDs.

We report on the microstructure of high Al content (0.65 < x < 0.78) AlxGa1−xN planar films grown in the III-rich regime by plasma-assisted molecular beam epitaxy (PA-MBE) at elevated growth temperatures from 800°C to 950°C. Films grown at or above 900°C have lateral periodic contrast oscillations in high-angle annular dark field (HAADF) images from transmission electron microscopy (TEM). The features are perpendicular to the growth direction and arise from spatial variations in the ratio of Al to Ga atoms. These oscillations begin at the AlxGa1−xN/AlN interface and are different from previously observed inhomogeneities that arose from spontaneous ordering and random compositional fluctuations. As prior studies have shown, compositional anomalies can improve certain optical properties, these vertical periodic structures observed here merit further investigation.

Gallium nitride (GaN) semiconductors and their broadband InGaN alloys in their hexagonal phase have been extensively studied over the past 30 years and have allowed the development of blue-ray lasers, which are essential disruptive developments. In addition to high-efficiency white light-emitting diodes, which have revolutionized lighting technologies and generated a great industry around these semiconductors, several transistors have been developed that take advantage of the characteristics of these semiconductors. These include power transistors for high-frequency applications and high-power transistors for power electronics, among other devices, which have far superior achievements. However, less effort has been devoted to studying GaN and InGaN alloys grown in the cubic phase. The metastable or cubic phase of III-N alloys has superior characteristics compared to the hexagonal phase, mainly because of the excellent symmetry. It can be used to improve lighting technologies and develop other devices. Indium gallium nitride, InxGa1−xN alloy, has a variable band interval of 0.7 to 3.4 eV that covers almost the entire solar spectrum, making it a suitable material for increasing the efficiencies of photovoltaic devices. In this study, we successfully synthesized high-quality cubic InGaN films on MgO (100) substrates using plasma-assisted molecular beam epitaxy (PAMBE), demonstrating tunable emissions across the visible spectrum by varying the indium concentration. We significantly reduced the defect density and enhanced the crystalline quality by using an intermediate cubic GaN buffer layer. We not only developed a heterostructure with four GaN/InGaN/GaN quantum wells, achieving violet, blue, yellow, and red emissions, but also highlighted the immense potential of cubic InGaN films for high-efficiency light-emitting diodes and photovoltaic devices. Achieving better p-type doping levels is crucial for realizing diodes with excellent performance, and our findings will pave the way for this advancement.

An effective-area photovoltaic efficiency of 1.27% in power conversion, excluding the grid metal contact area and under 1 sun, AM 1.5G conditions, has been obtained for the p-GaN/i-InGaN/n-GaN diode arrays epitaxially grown on (111)-Si. The short-circuit current density is 14.96 mA/cm2 and the open-circuit voltage is 0.28 V. Enhanced light trapping acquired via multiple reflections within the strain and defect free III-nitride nanorod array structures and the short-wavelength responses boosted by the wide bandgap III-nitride constituents are believed to contribute to the observed enhancements in device performance.

We have grown nonpolar nitrogen (N) doped a-plane zinc oxide (ZnO) films on r-plane sapphire substrates in order to eliminate the self-polarization component in the growth direction, which decreases the doping efficiency of N acceptors. Nonpolar a-ZnO:N films were grown by plasma-assisted molecular beam epitaxy (PA-MBE) using Zn metal and a plasma source of O2 and NO mixed gas. It was confirmed by reflection high-energy electron diffraction and x-ray diffraction that single phase a-plane ZnO:N films were grown on the r-plane sapphire substrates. After the PA-MBE growth, the post-annealing was performed in an oxygen atmosphere. Photoluminescence experiments showed donor–acceptor pair emissions increase with increasing the annealing temperature (≤ 700°C). AC magnetic field Hall effect measurements revealed that n-type conduction of the as-grown films clearly changed to the p-type at the annealing temperature of 650°C. The resistivity, hole concentration, and mobility were ρ = 3.4 Ω cm, p = 8.0 × 1017 cm−3, and μ = 2.3 cm2/Vs, respectively.

Red, green, and blue light InxGa1−xN multiple quantum wells have been grown on GaN/γ-LiAlO2 microdisk substrates by plasma-assisted molecular beam epitaxy. We established a mechanism to optimize the self-assembly growth with ball-stick model for InxGa1-xN multiple quantum well microdisks by bottom-up nanotechnology. We showed that three different red, green, and blue lighting micro-LEDs can be made of one single material (InxGa1-xN) solely by tuning the indium content. We also demonstrated that one can fabricate a beautiful InxGa1-xN-QW microdisk by choosing an appropriate buffer layer for optoelectronic applications.

Lattice relaxation on wurtzite GaN microdisks grown by plasma-assisted molecular beam epitaxy was systematically studied. The lattice constants of GaN microdisks were evaluated from high-resolution transmission electron microscopy, and the anisotropic strain was then analyzed by observing the microscopic atomic layers. We found that the vertical lattice strain along the c-axis followed a linear relationship, while the lateral lattice strain along the a-axis exhibited a quadratic deviation. The lattice mismatch is about 0.94% at the interface between the GaN microdisks and the γ-LiAlO2 substrate, which induces the anisotropic strain during epi-growth.

Plasma-assisted molecular beam epitaxy (PAMBE) was used to grow Ga 2 O 3 films on oxidized GaN layers on nitrided sapphire substrates. The GaN layer was grown by PAMBE, and the in situ oxidation of the GaN layer was achieved through exposure to oxygen plasma, which resulted in the formation of monoclinic β-Ga 2 O 3 . Crystalline monoclinic β-Ga 2 O 3 films were grown on the GaN layers, with and without oxidation. The orientation relationships were [ 11 2 ¯ 0 ] Al 2 O 3 //[ 1 1 ¯ 00 ] AlN//[ 1 1 ¯ 00 ] GaN//[102] β-Ga 2 O 3 and [ 1 1 ¯ 00 ] Al 2 O 3 //[ 11 2 ¯ 0 ] AlN//[ 11 2 ¯ 0 ] GaN//[010] β-Ga 2 O 3 . The grown β-Ga 2 O 3 films were not single-crystalline but showed rotational domains along the growth direction with three variations, which resulted in six-fold rotational symmetry instead of two-fold rotational symmetry. The surface roughness of the grown β-Ga 2 O 3 film was closely reflected to that of as-grown GaN and oxidized GaN. By analyzing the x-ray omega rocking curves for the on-axis ( 2 ¯ 01 ) and off-axis (002) reflections, it was concluded that rotational domains dominantly affected the crystal quality of the β-Ga 2 O 3 films.

Indium-incorporation with InxGa1-xN layers on GaN-microdisks has been systematically studied against growth parameters by plasma-assisted molecular beam epitaxy. The indium content (x) of InxGa1-xN layer increased to 44.2% with an In/(In + Ga) flux ratio of up to 0.6 for a growth temperature of 620 °C, and quickly dropped with a flux ratio of 0.8. At a fixed In/(In + Ga) flux ratio of 0.6, we found that the indium content decreased as the growth temperature increased from 600 °C to 720 °C and dropped to zero at 780 °C. By adjusting the growth parameters, we demonstrated an appropriate InxGa1-xN layer as a buffer to grow high-indium-content InxGa1-xN/GaN microdisk quantum wells for micro-LED applications.