Zhixin Qin

Polarization of edge emission from III-nitride light emitting diodes of emission wavelength from 395to455nm

Polarization-resolved edge-emitting electroluminescence of InGaN∕GaN multiple quantum well (MQW) light emitting diodes (LEDs) from 395to455nm was measured. Polarization ratio decreased from 3.2 of near-ultraviolet LEDs (395nm) to 1.9 of blue LEDs (455nm). Based on TE mode dominant emissions in InGaN∕GaN MQWs, compressive strain in well region favors TE mode, indium induced quantum-dot-like behavior leads to an increased TM component. As wavelength increased, indium enhanced quantum-dot-like behavior became obvious and E‖C electroluminescence signal increased thus lower polarization ratio. Electroluminescence spectrum shifts confirmed that quantum dotlike behaviors rather than strain might be dominant in modifying luminescence mode of InGaN∕GaN MQWs from near ultraviolet to blue.

Lattice-Polarity-Driven Epitaxy of Hexagonal Semiconductor Nanowires

Lattice-polarity-driven epitaxy of hexagonal semiconductor nanowires (NWs) is demonstrated on InN NWs. In-polarity InN NWs form typical hexagonal structure with pyramidal growth front, whereas N-polarity InN NWs slowly turn to the shape of hexagonal pyramid and then convert to an inverted pyramid growth, forming diagonal pyramids with flat surfaces and finally coalescence with each other. This contrary growth behavior driven by lattice-polarity is most likely due to the relatively lower growth rate of the (0001̅) plane, which results from the fact that the diffusion barriers of In and N adatoms on the (0001) plane (0.18 and 1.0 eV, respectively) are about 2-fold larger in magnitude than those on the (0001̅) plane (0.07 and 0.52 eV), as calculated by first-principles density functional theory (DFT). The formation of diagonal pyramids for the N-polarity hexagonal NWs affords a novel way to locate quantum dot in the kink position, suggesting a new recipe for the fabrication of dot-based devices.

High-quality AlN epitaxy on nano-patterned sapphire substrates prepared by nano-imprint lithography

We report epitaxial growth of AlN films with atomically flat surface on nano-patterned sapphire substrates (NPSS) prepared by nano-imprint lithography. The crystalline quality can be greatly improved by using the optimized 1-μm-period NPSS. The X-ray diffraction ω-scan full width at half maximum values for (0002) and (102) reflections are 171 and 205 arcsec, respectively. The optimized NPSS contribute to eliminating almost entirely the threading dislocations (TDs) originating from the AlN/sapphire interface via bending the dislocations by image force from the void sidewalls before coalescence. In addition, reducing the misorientations of the adjacent regions during coalescence adopting the low lateral growth rate is also essential for decreasing TDs in the upper AlN epilayer.

Zero-field spin splitting in AlxGa1−xN/GaN heterostructures with various Al compositions

The zero-field spin splitting in AlxGa1−xN/GaN heterostructures with various Al compositions has been investigated at low temperatures and high magnetic fields. The zero-field spin-splitting energy and the spin-orbit coupling parameter are obtained by means of beating pattern Shubnikov–de Haas measurements. It is found that the spin-orbit coupling parameter can be tuned by the polarization-induced electric field. The AlxGa1−xN/GaN heterostructure is one of the promising materials for the spin-polarized field effect transistor.

Study of the leakage current mechanism in Schottky contacts to Al0.25Ga0.75N/GaN heterostructures with AlN interlayers

The leakage current mechanism in Schottky contacts (SCs) to Al0.25Ga0.75N/GaN heterostructures incorporated by a thin high-temperature (HT) AlN interlayer has been investigated using current–voltage measurements, atomic force microscopy and deep level transient spectroscopy. It is found that the HT AlN interlayer thickness has a significant effect on the leakage current in SCs. The leakage current density decreases to 1.1 × 10−4 A cm−2 when the growth time of the AlN interlayer increases from 0 to 10 s, and then changes to increase with increasing growth time. Correspondingly, the heterostructure with the AlN growth time of 10 s has the least number of surface pinholes. The thickness of the HT AlN also influences the density of electron traps with the activation energy of 0.762 eV in an Al0.25Ga0.75N barrier. It is suggested that the HT AlN interlayer adjusts the microstructure and the defect state density in the Al1−xGaxN barrier, and the leakage via these defect states makes the main contribution to the leakage current in SCs to Al1−xGaxN/GaN heterostructures.

Free and bound excitonic effects in Al0.5Ga0.5N/Al0.35Ga0.65N MQWs with different Si-doping levels in the well layers.

Free exciton (FX) and bound exciton (BX) in Al0.5Ga0.5N/Al0.35Ga0.65N multiple quantum wells (MQWs) with different Si-doping levels in the well layers are investigated by photoluminescence (PL) spectra. Low temperature (10 K) PL spectra identify a large binding energy of 87.4 meV for the BX in undoped sample, and 63.6 meV for the BX in Si-doped (2 × 1018 cm−3) sample. They are attributed to O-bound and Si-bound excitons, respectively. The large binding energies of BX are assumed to originate from the strong quantum confinement in the quantum wells, which also leads to a stronger FX PL peak intensity in comparison with BX at 10 K. Si-doping is found to suppress the FX quenching by reducing threading dislocation density (TDD) in the well layers, leading to a significant improvement of IQE from 33.7% to 45%.

Positive temperature coefficient of photovoltaic efficiency in solar cells based on InGaN/GaN MQWs

We report a 23.4% improvement of conversion efficiency in solar cells based on InGaN/GaN multiple quantum wells by using a patterned sapphire substrate in the fabrication process. The efficiency enhancement is due to the improvement of the crystalline quality, as proven by the reduction of the threading dislocation density. More importantly, the better crystalline quality leads to a positive photovoltaic efficiency temperature coefficient up to 423 K, which shows the property and advantage of wide gap semiconductors like InGaN, signifying the potential of III-nitride based solar cells for high temperature and concentrating solar power applications.

High-Output-Power Ultraviolet Light Source from Quasi-2D GaN Quantum Structure.

Quasi-2D GaN layers inserted in an AlGaN matrix are proposed as a novel active region to develop a high-output-power UV light source. Such a structure is successfully achieved by precise control in molecular beam epitaxy and shows an amazing output power of ≈160 mW at 285 nm with a pulsed electron-beam excitation. This device is promising and competitive in non-line-of-sight communications or the sterilization field.

Effect of injection current on the optical polarization of AlGaN-based ultraviolet light-emitting diodes.

The injection current dependence of optical polarization of ultraviolet (UV) light-emitting diodes (LEDs) emitting at wavelength of 310 nm and 277 nm was investigated by electroluminescence (EL) measurements. For both diodes, it was found that the degree of polarization (DOP) decreased obviously as the injection current increased. We attribute the decrease in DOP to the different changing trend of the intensity of the light emission from transverse electric (TE) polarization (E⊥c) and transverse magnetic (TM) polarization (E∥c) as the injected carriers occupy higher states above k = 0 with increasing the injection current. For the 277 nm LED, even the polarization switching from TE to TM mode was observed.

High quality AlN epilayers grown on nitrided sapphire by metal organic chemical vapor deposition

Influence of sapphire pretreatment conditions on crystalline quality of AlN epilayers has been investigated by metal organic chemical vapor deposition (MOCVD). Compared to alumination treatment, it is found that appropriate sapphire nitridation significantly straightens the surface atomic terraces and decreases the X-ray diffraction (0002) full width at half maximum (FWHM) to a minimum of 55 arcsec, indicating a great improvement of the tilting feature of the grain structures in the AlN epilayer. More importantly, there is no inversion domains (IDs) found in the AlN epilayers, which clarifies that optimal sapphire nitridation is promising in the growth of high quality AlN. It is deduced that the different interfacial atomic structures caused by various pretreatment conditions influence the orientation of the AlN nucleation layer grains, which eventually determines the tilting features of the AlN epilayers.