Role of edge dislocation and Si impurity in linking the blue luminescence and yellow luminescence in n-type GaN films

A close relationship is found between the blue and yellow luminescence bands in n-type GaN films, which are grown without intentional acceptor doping. The intensity ratio of blue luminescence to yellow luminescence (I-BL/I-YL) decreases with the increase in edge dislocation densities as demonstrated by the (102) full width at half maximum of x-ray diffraction. In addition, the I-BL/I-YL ratio decreases with the increase in Si doping. It is suggested that the edge dislocation and Si impurity play important roles in linking the blue and yellow luminescence.

Tunable edge-emitting microlaser on photonic crystal slab

A tunable edge-emitting microlaser is realised by a chirped line-defect photonic crystal waveguide. A tunable range of 57 nm is obtained experimentally.

Generation and behavior of pure-edge threading misfit dislocations in InxGa1-xN/GaN multiple quantum wells

The relaxation of the misfit strain by the formation of misfit dislocations in InxGa1-xN/GaN multiple quantum wells grown by metal-organic chemical-vapor deposition was investigated by the cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. It is found that the misfit dislocations generated from strain relaxation are all pure-edge threading dislocations with burgers vectors of b=1/3<11 (2) over bar0>. The misfit dislocations arise from the strain relaxation due to the thickness of strained layer greater than the critical thickness. The relaxation of strained layer was mainly achieved by the formation of dislocations and localization of In, while the dislocations changed their slip planes from {0001} to {10 (1) over bar0}. With the increasing temperature, the efficiency of photoluminescence decrease sharply. It indicates that the relaxation of the misfit strain has a strong effect on optical efficiency of film. (C) 2004 American Institute of Physics.

Large-scale synthesis of single-phase, high-quality GaN nanocrystallites

Large-scale synthesis of high-quality GaN nano-crystallites has been achieved by direct reaction of a 4:1 molar Ga/Ga2O3 mixture with ammonia at 950degreesC. X-ray diffraction, transmission electron microscopy, selected-area electron diffraction and high-resolution transmission electron microscopy revealed that the produced GaN nanocrystallites were single hexagonal wurtzite structure with an average particle size around 45 nm. A sharp near band edge emission peak and a blue light emission peak were observed in photoluminescence spectroscopy. The synthesis approach is simple and easy to be commercialized.

Cleaved-edge overgrowth of aligned InAs islands on GaAs(110)

A novel approach for positioning InAs islands on GaAs(110) by cleaved-edge overgrowth is reported. The first growth sample contains a strained InxGa1-xAs/GaAs superlattice of varying indium fraction and thickness, which acts as a strain nanopattern for the cleaved edge overgrowth. The formation of aligned islands is observed by means of atomic force microscopy. The ordering of the aligned islands and the structure of a single InAs island are found to depend on the properties of the underlying InxGa1-xAs/GaAs superlattice and molecular beam epitaxy growth conditions.

Optical properties of highly ordered AlN nanowire arrays grown on sapphire substrate

Highly ordered AlN nanowire arrays were synthesized via a simple physical vapor deposition method on sapphire substrate. The nanowires have an extremely sharp tip < 10 nm, with the average length around 3 mu m. Raman spectroscopy analysis on the AlN nanowire arrays revealed that the lifetime of the phonons is shorter than that in bulk AlN. The transmission spectra of the AlN nanowires showed a blueshift similar to 0.27 eV at the absorption edge with that of the bulk AlN, which is closely related to the small size of the nanowires. (c) 2005 American Institute of Physics.

Pressure behavior of the alloy band edge and nitrogen-related centers in GaAs0.999N0.001

The photoluminescence of a GaAsN alloy with 0.1% nitrogen has been studied under pressures up to 8.5 GPa at 33, 70, and 130 K. At ambient pressure, emissions from both the GaAsN alloy conduction band edge and discrete nitrogen-related bound states are observed. Under applied pressure, these two types of emissions shift with rather different pressure coefficients: about 40 meV/GPa for the nitrogen-related features, and about 80 meV/GPa for the alloy band-edge emission. Beyond 1 GPa, these discrete nitrogen-related peaks broaden and evolve into a broad band. Three new photoluminescence bands emerge on the high-energy side of the broad band, when the pressure is above 2.5, 4.5, and 5.25 GPa, respectively, at 33 K. In view of their relative energy positions and pressure behavior, we have attributed these new emissions to the nitrogen-pair states NN3 and NN4, and the isolated nitrogen state N-x. In addition, we have attributed the high-energy component of the broad band formed above 1 GPa to resonant or near-resonant NN1 and NN2, and its main body to deeper cluster centers involving more than two nitrogen atoms. This study reveals the persistence of all the paired and isolated nitrogen-related impurity states, previously observed only in the dilute doping limit, into a rather high doping level. Additionally, we find that the responses of different N-related states to varying N-doping levels differ significantly and in a nontrivial manner.

Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273-343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using alpha = alpha(max) + (alpha(min) - alpha(max))/[1 + exp 2(E - E-g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E-g(T) = E-g(273 K) - (8.8 x 10(-4)T(2))/(483 + T) + 0.088 (Varshni empirical formula) and E-g(T) = E-g(273 K)-0.231447/[exp(362/T)-1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor similar to 0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.

Pressure dependence of the near-band-edge photoluminescence from ZnO microrods at low temperature

The temperature and pressure dependences of band-edge photo luminescence from ZnO microrods have been investigated. The energy separation between the free exciton (FX) and its first order phonon replica (FX-1LO) decreases at a rate of k(B)T with increasing temperature. The intensity ratio of the FX-1LO to the bound exciton (BX) emission is found to decrease slightly with increasing pressure. All of the exciton emission peaks show a blue shift with increasing pressure. The pressure coefficient of the FX transition, longitudinal optical (LO) phonon energy, and binding energy of BX are estimated to be 21.4, 0.5, and 0.9 meV/GPa, respectively. (c) 2006 Elsevier Ltd. All rights reserved.

Design of two-dimensional photonic crystal edge emitting laser for photonic integrated circuits

An edge emitting laser based on two-dimensional photonic crystal slabs is proposed. The device consists of a square lattice microcavity, which is composed of two structures with the same period but different radius of air-holes, and a waveguide. In the cavity, laser resonance in the inner structure benelits from not only the anomalous dispersion characteristic of the first band-edge at the M point in the first Brillouin-zone but also zero photon states in the outer structure. A line defect waveguide is introduced in the outer structure for extracting photons from the inner cavity. Three-dimensional finite-difference time-domain simulations apparently show the in-plane laser output from the waveguide. The microcavity has an effective mode volume of about 3.2(lambda/eta(slab))(3) for oscillation -mode and the quality factor of the device including line defect waveguide is estimated to be as high as 1300.

Effects of edge dislocations and intentional Si doping on the electron mobility of n-type GaN films

The effects of dislocations and Si doping on the electrical properties of n-type GaN grown by metal organic chemical vapor deposition (MOCVD) are investigated. It is found that both electron mobility and carrier concentration are strongly influenced by edge dislocations. A moderate Si doping during the GaN growth improves the electron mobility, but the best doping effect depends on the dislocation density of the sample. High quality about 4-mu m-thick MOCVD-grown GaN film with a room temperature electron mobility as high as 1005 cm(2)/V s is obtained by optimizing growth conditions. (c) 2006 American Institute of Physics.

Role of edge dislocations in enhancing the yellow luminescence of n-type GaN

We investigate the origin of yellow luminescence in n-type GaN. It is found that the relative intensity of yellow luminescence increases as the full width at half maximum of the x-ray diffraction rocking curve at the (102) plane increases. This indicates that the yellow luminescence is related to the edge dislocation density. In addition, the relative intensity of yellow luminescence is confirmed to increase with increasing Si doping for the high quality GaN we have obtained. We propose that the yellow luminescence is effectively enhanced by the transition from donor impurities such as Si to acceptors around the edge dislocations in n-type GaN. (c) 2006 American Institute of Physics.

Synthesis and temperature-dependent near-band-edge emission of chain-like Mg-doped ZnO nanoparticles

Chain-like Mg-doped ZnO nanoparticles were prepared using a wet chemical method combined with subsequent heat treatment. The blueshifted near-band-edge emission of the doped ZnO sample with respect to the undoped one was investigated by temperature-dependent photoluminescence. Based on the energy shift of the free-exciton transition, a band gap enlargement of similar to 83 meV was estimated, which seems to result in the equivalent shift of the bound-exciton transition. At 50 K, the transformation from the donor-acceptor-pair to free-to-acceptor emissions was observed for both the undoped and doped samples. The results show that Mg doping leads to the decrease of the acceptor binding energy. (c) 2006 American Institute of Physics.

Strain and photoluminescence characterization of cubic (In,Ga)N films grown on GaAs(001) substrates

Cubic InxGa1-xN films were successfully grown on GaAs(001) substrates by metalorganic chemical-vapor deposition. The values of x content ranging from 0.10 to 0.24 obtained at different growth conditions were measured by double-crystal x-ray diffraction (XRD). The perpendicular and parallel elastic strain of the In0.2Ga0.8N layer, epsilon(perpendicular to)=0.4% and epsilon(parallel to)=-0.4% for GaN and epsilon(perpendicular to)=0.37% and epsilon(parallel to)=-0.37% for InGaN, respectively, were derived using the XRD measurements. The inhomogeneous strain and the average grain size of the In0.2Ga0.8N/GaN films were also studied by XRD. Photoluminescence spectra were used to measure the optical characterization of the InxGa1-xN thin films with different In composition, and the near-band-edge emission dependence of cubic InxGa1-xN on the x value is nearly linear with In content x less than or equal to 0.24. (C) 2000 American Institute of Physics. [S0021-8979(00)03908-6].

Photovoltaic effect of cubic GaN/GaAs(100)

We have studied the photovoltaic effect in cubic GaN on GaAs at room temperature. The photovoltaic spectra of cubic GaN epitaxial film were concealed by the photovoltaic effect from the GaAs substrate unless additional illumination of a 632.8 nm He-Ne laser beam was used to remove the interference of the GaAs absorption in the measurement. On the basis of the near-band-edge photovoltaic spectra of cubic GaN, we obtained the minority carrier diffusion lengths of about 0.32 and 0.14 mu m for two undoped n-type cubic GaN samples with background concentrations of 10(14) and 10(18) cm(-3), respectively. (C) 1999 American Institute of Physics. [S0003-6951(99)00450-7].