Effect of critical thickness on structural and optical properties of InxGa1-xN/GaN multiple quantum wells

InGaN/GaN multiquantum-well (MQW) structures grown by metalorganic chemical-vapor deposition on n-type GaN and capped by p-type GaN were investigated by cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. For the sample with strained-layer thicknesses greater than the critical thicknesses, a high density of pure edge type threading dislocations generated from MQW layers and extended to the cap layer was observed. These dislocations result from a relaxation of the strained layers when their thicknesses are beyond the critical thicknesses. Because of indium outdiffusion from the well layers due to the anneal effect of Mg-doped cap layer growth and defects generated from strain relaxation, the PL emission peak was almost depressed by the broad yellow band with an intensity maximum at 2.28 eV. But for the sample with strained-layer thicknesses less than the critical thicknesses, it has no such phenomenon. The measured critical thicknesses are consistent with the calculated values using the model proposed by Fischer, Kuhne, and Richter. (C) 2004 American Institute of Physics.

Optical analysis of dislocation-related physical processes in GaN-based epilayers

In this paper, recent progresses in optical analysis of dislocation-related physical properties in GaN-based epilayers are surveyed with a brief review. The influence of dislocations on both near-band edge emission and yellow luminescence (YL) is examined either in a statistical way as a function of dislocation density or focused on individual dislocation lines with a high spatial resolution. Threading dislocations may introduce non-radiative recombination centers and enhance YL, but their effects are affected by the structural and chemical environment. The minority carrier diffusion length may be dependent on either dislocation density or impurity doping as confirmed by the result of photovoltaic spectra. The in situ optical monitoring of the strain evolution process is employed during GaN heteroepitaxy using an AIN interlayer. A typical transition of strain from compression to tension is observed and its correlation with the reduction and inclination of threading dislocation lines is revealed. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Investigation of oxygen vacancy and interstitial oxygen defects in ZnO films by photoluminescence and x-ray photoelectron spectroscopy

ZnO films prepared at different temperatures and annealed at 900 degrees C in oxygen are studied by photoluminescence (PL) and x-ray photoelection spectroscopy (XPS). It is observed that in the PL of the as-grown films the green luminescence (GL) and the yellow luminescence (YL) are related, and after annealing the GL is restrained and the YL is enhanced. The O 1s XPS results also show the coexistence of oxygen vacancy (Vo) and interstitial oxygen (O-i) before annealing and the quenching of the V-o after annealing. By combining the two results it is deduced that the GL and YL are related to the V-o and O-i defects, respectively.

The influence of V/III ratio in the initial growth stage on the properties of GaN epilayer deposited on low temperature AlN buffer layer

The V/III ratio in the initial growth stage of metalorganic chemical vapor deposition has an important influence on the quality of a GaN epilayer grown on a low-temperature AIN buffer layer and c-plane sapphire substrate. A weaker yellow luminescence, a narrower half-width of the X-ray diffraction peak, and a higher electron mobility result when a lower V/III ratio is taken. The intensity of in situ optical reflectivity measurements indicates that the film surface is rougher at the beginning of GaN growth, and a longer time is needed for the islands to coalesce and for a quasi-two dimensional mode growth to start. A comparison of front- and back-illuminated photoluminescence spectra confirms that many threading dislocations are bent during the initial stage, leading to a better structural quality of the GaN layer. (C) 2007 Elsevier B.V. All rights reserved.

Spatial distribution of deep level defects in crack-free AlGaN grown on GaN with a high-temperature AlN interlayer

The deep level luminescence of crack-free Al0.25Ga0.75N layers grown on a GaN template with a high-temperature grown AlN interlayer has been studied using spatially resolved cathodoluminescence (CL) spectroscopy. The CL spectra of Al0.25Ga0.75N grown on a thin AlN interlayer present a deep level aquamarine luminescence (DLAL) band at about 2.6 eV and a deep level violet luminescence (DLVL) band at about 3.17 eV. Cross-section line scan CL measurements on a cleaved sample edge clearly reveal different distributions of DLAL-related and DLVL-related defects in AlGaN along the growth direction. The DLAL band of AlGaN is attributed to evolve from the yellow luminescence band of GaN, and therefore has an analogous origin of a radiative transition between a shallow donor and a deep acceptor. The DLVL band is correlated with defects distributed near the GaN/AlN/AlGaN interfaces. Additionally, the lateral distribution of the intensity of the DLAL band shows a domainlike feature which is accompanied by a lateral phase separation of Al composition. Such a distribution of deep level defects is probably caused by the strain field within the domains. (c) 2006 American Institute of Physics.

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au/GaN Schottky contacts

Under identical preparation conditions, Au/GaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers. (c) 2006 American Institute of Physics.

Effect of substrate temperature on the growth and photoluminescence properties of vertically aligned ZnO nanostructures

Vertically well-aligned ZnO nanoridge, nanorod, nanorod-nanowall junction, and nanotip arrays have been successfully synthesized on Si (100) substrates using a pulsed laser deposition prepared ZnO film as seed layer by thermal evaporation method. Experimental results illustrated that the growth of different morphologies of ZnO nanostructures was strongly dependent upon substrate temperature. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies showed that the ZnO nanostructures were single crystals with a wurtzite structure. Compared with those of the other nanostructures, the photoluminescence (PL) spectrum of nanorod-nanowall junctions showed the largest intensity ratio of ultraviolet (UV) to yellow-green emission and the smallest full-width at half-maximum (FWHM) of the UV peak, reflecting the high optical quality and nearly defect free of crystal structure. The vertical alignment of the nanowire array on the substrate is attributed to the epitaxial growth of the nanostructures from the ZnO buffer layer. The growth mechanism was also discussed in detail. (c) 2006 Elsevier B.V. All rights reserved.

Structure and visible luminescence of ZnO nanoparticles

ZnO nanoparticles were synthesized in ethanolic solution using a sol-gel method. The structural and optical properties were investigated by X-ray diffraction, transmission electron microscopy, UV absorption, and photoluminescence. After annealing at 200 degrees C, the particle size is increased and the peak of defect luminescence in the visible region is changed. A yellow emission was observed in the as-prepared sample and a green emission in the annealed sample. The change of the visible emission is related to oxygen defects. Annealing in the absence of oxygen would increase oxygen vacancies. (c) 2006 Elsevier Ltd. All rights reserved.

Effect of lightly Si doping on the minority carrier diffusion length in n-type GaN films

We investigate the effects of lightly Si doping on the minority carrier diffusion length in n-type GaN films by analyzing photovoltaic spectra and positron annihilation measurements. We find that the minority carrier diffusion length in undoped n-type GaN is much larger than in lightly Si-doped GaN. Positron annihilation analysis demonstrates that the concentration of Ga vacancies is much higher in lightly Si-doped GaN and suggests that the Ga vacancies instead of dislocations are responsible for the smaller minority carrier diffusion length in the investigated Si-doped GaN samples due to the effects of deep level defects. (c) 2006 American Institute of Physics.

Influence of cracks generation on the structural and optical properties of GaN/Al0.55Ga0.45N multiple quantum wells

Both cracked and crack-free GaN/Al0.55Ga0.45N multiple quantum wells (MQWs) grown on GaN template by metalorganic chemical vapor deposition have been studied by triple-axis X-ray diffraction, grazing-incidence X-ray reflectivity, atomic force microscope, photoluminescence spectroscopy and low-energy positron annihilation spectroscopy. The experimental results show that cracks generation not only deteriorates the surface morphology, but also leads to a period dispersion and roughens the interfaces of MQWs. The mean density of dislocations in MQWs, determined from the average full-width at half-maximum of to-scan of each satellite peak, has been significantly enhanced by the cracks generation. Furthermore, the measurement of annihilation-line Doppler broadening reveals a higher concentration of negatively charged vacancies in the cracked MQWs. The combination of these vacancies and the high density of edge dislocations are assumed to contribute to the highly enhanced yellow luminescence in the cracked sample. (c) 2005 Elsevier B.V. All rights reserved.

Growth temperature effect on the optical and material properties of AlxInyGa1-x-yN epilayers grown by MOCVD

AlxInyGa1-x-yN epilayers have been grown by metalorganic chemical vapor deposition (MOCVD) at different temperatures from 800 to 870degreesC. The incorporation of indium is found to increase with decreasing growth temperature, while the incorporation of Al remains nearly constant. The optical properties of the samples have been investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL) at different temperatures. The results show that the sample grown at 820 C exhibits the best optical quality for its large PL intensity and the absence of the yellow luminescence. Furthermore the temperature-dependent PL and TRPL of the sample reveals its less exciton localization effect caused by alloy fluctuations. In the scanning electron microscopy measurement, much uniform surface morphology is found for the sample grown at 820degreesC, in good agreement with the PL results, The improvement of AlxInyGa1-x-yN quality is well correlated with the incorporation of indium into AlGaN and the possible mechanism is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Effects of residual C and O impurities on photoluminescence in undoped GaN epilayers

Photoluminescence (PL) was investigated in undoped GaN from 4.8 K to room temperature. The 4.8 K spectra exhibited recombinations of free exciton, donor-acceptor pair (DAP), blue and yellow bands (Ybs). The blue band (BB) was also identified to be a DAP recombination. The YB was assigned to a recombination from deep levels. The energy-dispersive X-ray spectroscopy show that C and O are the main residual impurities in undoped GaN and that C concentration is lower in the epilayers with the stronger BB. The electronic structures of native defects, C and O impurities, and their complexes were calculated using ab initio local-density-functional (LDF) methods with linear muffin-tin-orbital and 72-atomic supercell. The theoretical analyses suggest that the electron transitions from O-N states to C-N and to V-Ga states are responsible for DAP and the BB, respectively, and the electron transitions between the inner levels of the C-N-O-N complex may be responsible for the YB in our samples. (C) 2002 Elsevier Science B.V. All rights reserved.

Residual donors and compensation in metalorganic chemical vapor deposition as-grown n-GaN

In our recent report, [Xu , Appl. Phys. Lett. 76, 152 (2000)], profile distributions of five elements in the GaN/sapphire system have been obtained using secondary ion-mass spectroscopy. The results suggested that a thin degenerate n(+) layer at the interface is the main source of the n-type conductivity for the whole film. The further studies in this article show that this n(+) conductivity is not only from the contribution of nitride-site oxygen (O-N), but also from the gallium-site silicon (Si-Ga) donors, with activation energies 2 meV (for O-N) and 42 meV (for Si-Ga), respectively. On the other hand, Al incorporated on the Ga sublattice reduces the concentration of compensating Ga-vacancy acceptors. The two-donor two-layer conduction, including Hall carrier concentration and mobility, has been modeled by separating the GaN film into a thin interface layer and a main bulk layer of the GaN film. The bulk layer conductivity is to be found mainly from a near-surface thin layer and is temperature dependent. Si-Ga and O-N should also be shallow donors and V-Ga-O or V-Ga-Al should be compensation sites in the bulk layer. The best fits for the Hall mobility and the Hall concentration in the bulk layer were obtained by taking the acceptor concentration N-A=1.8x10(17) cm(-3), the second donor concentration N-D2=1.0x10(18) cm(-3), and the compensation ratio C=N-A/N-D1=0.6, which is consistent with Rode's theory. Saturation of carriers and the low value of carrier mobility at low temperature can also be well explained. (C) 2001 American Institute of Physics.

Investigation on the origin of wurtzite domains in thick cubic GaN using reactive ion etching

We measured the depth profiling of photoluminescence (PL) in cubic GaN films. The depth-resolved PL of normal grown GaN layers showed that the near-band-edge luminescence intensities of both cubic and wurtzite domains remained constant only until an etching depth of up to 2.7 mu m, but their ratio remained unchanged at all etching depths. Moreover, when a thin In0.1Ga0.9N layer was sandwiched between two GaN layers, the content of the wurtzite domains increased, and its distribution showed a dependence on thickness. As the reactive ion etching depth increased, the PL intensity ratio of cubic GaN to wurtzite domains increased. Based on the distribution, the strain relaxation, instead of the instability of cubic GaN at high temperature, was attributed to the origin of wurtzite domains. (C) 2000 Elsevier Science S.A. All rights reserved.

Photoluminescence and optical quenching of photoconductivity studies on undoped GaN grown by molecular beam epitaxy

Deep levels in undoped GaN materials grown by modified molecular beam epitaxy (MBE) are investigated by photoluminescence (PL) and optical quenching of photoconductivity measurements. A broad band which extends from 2.1 to 3.0 eV with a maximum at about 2.7 eV is observed, and four prominent quenching bands were found located at 2.18, 2.40, 2.71, and 2.78 eV above the valence band, respectively. These levels are attributed to four holes trap levels existence in the material. The defects cannot be firmly identified at present. (C) 2000 Elsevier Science B.V, All rights reserved.