Z. X. Qin

Effects of symmetry of GaN-based two-dimensional photonic crystal with quasicrystal lattices on enhancement of surface light extraction

We demonstrate enhancement of surface light extraction from two-dimensional photonic crystals (2D-PCs) on the electrical injected GaN-based light emitters. The effects of symmetry of PCs on light extraction were studied. 2.5 times enhancement of surface emission was obtained from the PCs with an octagonal symmetric quasicrystal lattice (8PQC) compared to that from a nonpatterned region. Additionally the surface emission from PCs with dodecagonal symmetric quasicrystal lattice (12PQC) exhibited about 1.7 and 1.4 times higher emission than regular PCs with triangular lattice and 8PQC, respectively. Consequently, the 12PQC provides a favorable consideration of 2D-PC in light extraction from light emitting diode.

Current transport mechanism of Au∕Ni∕GaN Schottky diodes at high temperatures

Current transport mechanism in Au∕Ni∕GaN Schottky diodes has been investigated using current-voltage characterization technique between 27 and 350°C. It is found that the ideality factor n of the diode decreases with increasing temperature when the temperature is lower than 230°C, and then increases with increasing temperature when the temperature is higher than 230°C. The corresponding Schottky barrier height (SBH) increases all through the temperature range. Thermionic-emission model with a Gaussian distribution of SBHs is thought to be responsible for the electrical behavior at temperatures lower than 230°C, while the generation-recombination (GR) process takes place in at temperatures above 230°C. The effective Richardson constant is determined to be 24.08Acm−2K−2, in excellent agreement with the theoretical value. The extrapolated activation energy of the GR process is determined to be 1.157eV. Based on the cathodoluminescence measurements, it is suggested that the deep level defects inducing yellow ...

Reduction in threading dislocation densities in AlN epilayer by introducing a pulsed atomic-layer epitaxial buffer layer

A method of reducing threading dislocation (TD) density in AlN epilayers grown on sapphire substrate is reported. By introducing an AlN buffer layer grown by a pulsed atomic-layer epitaxy method, TDs in epitaxial AlN films were greatly decreased. From transmission electron microscopic images, a clear subinterface was observed between the buffer layer and the subsequently continuous grown AlN epilayer. In the vicinity of the subinterface, the redirection, annihilation, and termination of TDs were observed. The increase in lateral growth rate accounted for TD redirection and annihilation in the AlN epilayer. Strain variation between the two regions resulted in the termination of TDs owing to the dislocation line energy minimization.

Growth of high quality and uniformity AlGaN/GaN heterostructures on Si substrates using a single AlGaN layer with low Al composition

By employing a single AlGaN layer with low Al composition, high quality and uniformity AlGaN/GaN heterostructures have been successfully grown on Si substrates by metal-organic chemical vapor deposition (MOCVD). The heterostructures exhibit a high electron mobility of 2150u2009cm2/Vs with an electron density of 9.3u2009×u20091012u2009cm−2. The sheet resistance is 313u2009±u20094u2009Ω/◻ with ±1.3% variation. The high uniformity is attributed to the reduced wafer bow resulting from the balance of the compressive stress induced and consumed during the growth, and the thermal tensile stress induced during the cooling down process. By a combination of theoretical calculations and in situ wafer curvature measurements, we find that the compressive stress consumed by the dislocation relaxation (~1.2u2009GPa) is comparable to the value of the thermal tensile stress (~1.4u2009GPa) and we should pay more attention to it during growth of GaN on Si substrates. Our results demonstrate a promising approach to simplifying the growth processes of GaN-on-Si to reduce the wafer bow and lower the cost while maintaining high material quality.

Capacitance characteristics of back-illuminated Al0.42Ga0.58N∕Al0.40Ga0.60N heterojunction p-i-n solar-blind UV photodiode

Capacitance-voltage and capacitance-frequency characteristics of Al0.42Ga0.58N∕Al0.40Ga0.60N heterojunction p-i-n photodiode are investigated. The slopes of the 1∕C2-V plot were nearly the same at 100Hz, 1KHz, 10KHz, and 100KHz, slightly smaller at 1MHz. A depletion region width of 63.6nm calculated under zero bias for this diode was smaller than that of the intrinsic region, which showed that the intrinsic region was depleted partly under zero bias. The difference between the capacitance measured at low and high frequencies could be due to the high resistivity of the intrinsic region.

Study of moiré fringes at the interface of GaN/α-Al2O3 (0001)

Abstract In this paper, the interface structure of GaN/α-Al2O3 (0xa00xa00xa01) is studied by high-resolution transmission electron microscopy on tilted samples around the 〈1 1 2 0〉 Al 2 O 3 axis. Sets of moire fringes are observed. There are many disparities, such as contrast, period, tilt angle, and so on, among these moire fringes located in different regions. These fringes represent magnifying images of the plane distances of (1 1 0 0) GaN and the tilts of c-axis of GaN crystallites to that of the substrate. According to these results, the growth mechanism of GaN buffer layer at the interface is discussed.

Transmission electron microscopy investigation of inversion domain boundary in Al0.65Ga0.35N grown on AlN/sapphire template

A kind of inversion domains (IDs) which originated from the tips of threading dislocations were observed in the Al0.65Ga0.35N layer by transmission electron microscopy (TEM). The IDs showed columnar structures with diameters of 10–20 nm. We find that the dislocations evolved into V-shape boundaries then form IDs. By high resolution TEM analysis, the atomic structure at the boundary was proved to be the IDB∗ structure, in which each atom remains fourfold coordinated without the formation of Ga–Ga or N–N bonds.

Spatial identification of traps in AlGaN/GaN heterostructures by the combination of lateral and vertical electrical stress measurements

We present a methodology and the corresponding experimental results to identify the exact location of the traps that induce hot electron trapping in AlGaN/GaN heterostructures grown on Si substrates. The methodology is based on a combination of lateral and vertical electrical stress measurements employing three ohmic terminals on the test sample structure with different GaN buffer designs. By monitoring the evolution of the lateral current during lateral as well as vertical stress application, we investigate the trapping/detrapping behaviors of the hot electrons and identify that the traps correlated with current degradation are in fact located in the GaN buffer layers. The trap activation energies (0.38–0.39u2009eV and 0.57–0.59u2009eV) extracted from either lateral or vertical stress measurements are in good agreement with each other, also confirming the identification. By further comparing the trapping behaviors in two samples with different growth conditions of an unintentionally doped GaN layer, we conclude ...

Ni diffusion and its influence on electrical properties of AlxGa1−xN∕GaN heterostructures

The effect of thermal annealing of Ni∕AlxGa1−xN∕GaN structures on electric properties of AlxGa1−xN∕GaN heterostructures has been studied by means of temperature-dependent Hall measurements and deep level transient spectroscopy. It is found that the mobility of the two-dimensional electron gas (2DEG) decreases from 1530to986cm2∕Vs at room temperature (RT) after annealing the Al0.25Ga0.75N∕GaN heterostructure with a 10nm thick Ni cap layer at 600°C. The density of the 2DEG is also reduced by 2.0×1012cm−2 at RT after the annealing, and decreases with increasing temperature between 100 and 460K. It is determined that an acceptorlike deep level with an activation energy of 1.23eV and apparent capture cross section of 2.8×10−13cm2 is introduced into the heterostructures. We believe that the acceptorlike deep level is induced by Ni diffusion during the annealing, and it results in the significant degradation of the transport properties of the 2DEG in the heterostructures.

Intersubband Transition in GaN/InGaN Multiple Quantum Wells

Utilizing the growth temperature controlled epitaxy, high quality GaN/In0.15Ga0.85N multiple quantum wells designed for intersubband transition (ISBT) as novel candidates in III-nitride infrared device applications have been experimentally realized for the first time. Photo-absorption originated from the ISBT has been successfully observed at infrared regime covering the 3–5u2009μm atmosphere window, where the central absorption wavelength is modulated by adjusting the quantum well width. With increasing the quantum well thickness, the ISBT center wave length blue shifts at thickness less than 2.8u2009nm and then redshifts with further increase of the well thickness. The non-monotonic trend is most likely due to the polarization induced asymmetric shape of the quantum wells.