Search Results (10)

{CdO/ZnO}_m superlattices (SLs) have been grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy (PA-MBE). The observation of satellite peaks in the XRD studies of the as-grown and annealed samples confirms the presence of a periodic superlattice structure. The properties of as-grown and annealed SLs deposited on c-oriented sapphire were investigated by transmission electron microscopy, X-ray diffraction and temperature dependent PL studies. The deformation of the SLs structure was observed after rapid thermal annealing. As the thermal annealing temperature increases, the diffusion of Cd ions from the quantum well layers into the ZnO barrier increases. The formation of CdZnO layers causes changes in the luminescence spectrum in the form of peak shifts, broadening and changes in the spacing of the satellite peaks visible in X-ray analysis. Full article

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, In_xGa_1−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. Full article

GaN on Si plays an important role in the integration and promotion of GaN-based wide-gap materials with Si-based integrated circuits (IC) technology. A series of GaN film materials were grown on Si (111) substrate using a unique plasma assistant molecular beam epitaxy (PA-MBE) technology and investigated using multiple characterization techniques of Nomarski microscopy (NM), high-resolution X-ray diffraction (HR-XRD), variable angular spectroscopic ellipsometry (VASE), Raman scattering, photoluminescence (PL), and synchrotron radiation (SR) near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. NM confirmed crack-free wurtzite (w-) GaN thin films in a large range of 180–1500 nm. XRD identified the w- single crystalline structure for these GaN films with the orientation along the c-axis in the normal growth direction. An optimized 700 °C growth temperature, plus other corresponding parameters, was obtained for the PA-MBE growth of GaN on Si, exhibiting strong PL emission, narrow/strong Raman phonon modes, XRD w-GaN peaks, and high crystalline perfection. VASE studies identified this set of MBE-grown GaN/Si as having very low Urbach energy of about 18 meV. UV (325 nm)-excited Raman spectra of GaN/Si samples exhibited the GaN E₂(low) and E₂(high) phonon modes clearly without Raman features from the Si substrate, overcoming the difficulties from visible (532 nm) Raman measurements with strong Si Raman features overwhelming the GaN signals. The combined UV excitation Raman–PL spectra revealed multiple LO phonons spread over the GaN fundamental band edge emission PL band due to the outgoing resonance effect. Calculation of the UV Raman spectra determined the carrier concentrations with excellent values. Angular-dependent NEXAFS on Ga K-edge revealed the significant anisotropy of the conduction band of w-GaN and identified the NEXAFS resonances corresponding to different final states in the hexagonal GaN films on Si. Comparative GaN material properties are investigated in depth. Full article

The van der Waals epitaxy of wafer-scale GaN on 2D MoS₂ and the integration of GaN/MoS₂ heterostructures were investigated in this report. GaN films have been successfully grown on 2D MoS₂ layers using three different Ga fluxes via a plasma-assisted molecular beam epitaxy (PA-MBE) system. The substrate for the growth was a few-layer 2D MoS₂ deposited on sapphire using chemical vapor deposition (CVD). Three different Ga fluxes were provided by the gallium source of the K-cell at temperatures of 825, 875, and 925 °C, respectively. After the growth, RHEED, HR-XRD, and TEM were conducted to study the crystal structure of GaN films. The surface morphology was obtained using FE-SEM and AFM. Chemical composition was confirmed by XPS and EDS. Raman and PL spectra were carried out to investigate the optical properties of GaN films. According to the characterizations of GaN films, the van der Waals epitaxial growth mechanism of GaN films changed from 3D to 2D with the increase in Ga flux, provided by higher temperatures of the K-cell. GaN films grown at 750 °C for 3 h with a K-cell temperature of 925 °C demonstrated the greatest crystal quality, chemical composition, and optical properties. The heterostructure of 3D GaN on 2D MoS₂ was integrated successfully using the low-temperature PA-MBE technique, which could be applied to novel electronics and optoelectronics. Full article

Red, green, and blue light In_xGa_1−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 In_xGa_1-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 (In_xGa_1-xN) solely by tuning the indium content. We also demonstrated that one can fabricate a beautiful In_xGa_1-xN-QW microdisk by choosing an appropriate buffer layer for optoelectronic applications. Full article

Mg-doped GaN was grown by plasma-assisted molecular beam epitaxy (PAMBE) on a Fe-doped GaN template substrate by employing a shutter-controlled process. The transition from n-type to p-type conductivity of Mg-doped GaN in relation to the N/Ga flux ratio was studied. The highest p-type carrier concentration in this series was 3.12 × 10¹⁸ cm⁻³ under the most N-rich condition. By modulating the shutters of different effusion cells for the shutter-controlled process, a wide growth window for p-type GaN was obtained. It was found that the presence of Mg flux effectively prevents the formation of structural defects in GaN epi-layers, resulting in the improvement of crystal quality and carrier mobility. Full article

InGaN quantum dots (QDs) have attracted significant attention as a promising material for high-efficiency micro-LEDs. In this study, plasma-assisted molecular beam epitaxy (PA-MBE) was used to grow self-assembled InGaN QDs for the fabrication of green micro-LEDs. The InGaN QDs exhibited a high density of over 3.0 × 10¹⁰ cm⁻², along with good dispersion and uniform size distribution. Micro-LEDs based on QDs with side lengths of the square mesa of 4, 8, 10, and 20 μm were prepared. Attributed to the shielding effect of QDs on the polarized field, luminescence tests indicated that InGaN QDs micro-LEDs exhibited excellent wavelength stability with increasing injection current density. The micro-LEDs with a side length of 8 μm showed a shift of 16.9 nm in the peak of emission wavelength as the injection current increased from 1 A/cm² to 1000 A/cm². Furthermore, InGaN QDs micro-LEDs maintained good performance stability with decreasing platform size at low current density. The EQE peak of the 8 μm micro-LEDs is 0.42%, which is 91% of the EQE peak of the 20 µm devices. This phenomenon can be attributed to the confinement effect of QDs on carriers, which is significant for the development of full-color micro-LED displays. Full article

AlGaN nanorods have attracted increasing amounts of attention for use in ultraviolet (UV) optoelectronic devices. Here, self-assembled AlGaN nanorods with embedding quantum disks (Qdisks) were grown on Si(111) using plasma-assisted molecular beam epitaxy (PA-MBE). The morphology and quantum construction of the nanorods were investigated and well-oriented and nearly defect-free nanorods were shown to have a high density of about 2 × 10¹⁰ cm⁻². By controlling the substrate temperature and Al/Ga ratio, the emission wavelengths of the nanorods could be adjusted from 276 nm to 330 nm. By optimizing the structures and growth parameters of the Qdisks, a high internal quantum efficiency (IQE) of the AlGaN Qdisk nanorods of up to 77% was obtained at 305 nm, which also exhibited a shift in the small emission wavelength peak with respect to the increasing temperatures during the PL measurements. Full article

We demonstrate the applications of tunnel junctions (TJs) for new concepts of monolithic nitride-based multicolor light emitting diode (LED) and laser diode (LD) stacks. The presented structures were grown by plasma-assisted molecular beam epitaxy (PAMBE) on GaN bulk crystals. We demonstrate a stack of four LDs operated at pulse mode with emission wavelength of 453 nm. The output power of 1.1 W and high slope efficiency of 2.3 W/A is achieved for devices without dielectric mirrors. Atomically flat surface after the epitaxy of four LD stack and low dislocation density is measured as a result of proper TJ design with optimized doping level. The strain compensation design with InGaN waveguides and AlGaN claddings is shown to be crucial to avoid cracking and lattice relaxation of the 5 µm thick structure. Vertical connection of n-LDs allows for cascade emission of photons and increases the quantum efficiency n-times. The two-color (blue and green) LEDs are demonstrated. Application of TJs simplifies device processing, reducing the need for applications of p-type contact. The key factor enabling demonstration of such devices is hydrogen-free PAMBE technology, in which activation of buried p-type layers is not necessary. Full article

Hexagonal boron nitride (hBN) has attracted a great deal of attention as a key component in van der Waals (vdW) heterostructures, and as a wide band gap material for deep-ultraviolet devices. We have recently demonstrated plasma-assisted molecular beam epitaxy (PA-MBE) of hBN layers on substrates of highly oriented pyrolytic graphite at high substrate temperatures of ~1400 °C. The current paper will present data on the high-temperature PA-MBE growth of hBN layers using a high-efficiency radio-frequency (RF) nitrogen plasma source. Despite more than a three-fold increase in nitrogen flux with this new source, we saw no significant increase in the growth rates of the hBN layers, indicating that the growth rate of hBN layers is controlled by the boron arrival rate. The hBN thickness increases to 90 nm with decrease in the growth temperature to 1080 °C. However, the decrease in the MBE temperature led to a deterioration in the optical properties of the hBN. The optical absorption data indicates that an increase in the active nitrogen flux during the PA-MBE process improves the optical properties of hBN and suppresses defect related optical absorption in the energy range 5.0–5.5 eV. Full article