Effects of crystalline quality on the phase stability of cubic boron nitride thin films under medium-energy ion irradiation

To investigate the effect of radiation damage on the stability and the compressive stress of cubic boron nitride (c-BN) thin films, c-BN films with various crystalline qualities prepared by dual beam ion assisted deposition were irradiated at room temperature with 300 keV Ar+ ions over a large fluence range up to 2 x 10(16) cm(-2). Fourier transform infrared spectroscopy (FTIR) data were taken before and after each irradiation step. The results show that the c-BN films with high crystallinity are significantly more resistant against medium-energy bombardment than those of lower crystalline quality. However, even for pure c-BN films without any sp(2)-bonded BN, there is a mechanism present, which causes the transformation from pure c-BN to h-BN or to an amorphous BN phase. Additional high resolution transmission electron microscopy (HRTEM) results support the conclusion from the FTIR data. For c-BN films with thickness smaller than the projected range of the bombarding Ar ions, complete stress relaxation was found for ion fluences approaching 4 x 10(15) cm(-2). This relaxation is accompanied, however, by a significant increase of the width of c-BN FTIR TO-line. This observation points to a build-up of disorder and/or a decreasing average grain size due to the bombardment. (c) 2005 Elsevier B.V. All rights reserved.

Low-cost high-efficiency GaN LED on large-area si substrate

The use of large size Si substrates for epitaxy of nitride light emitting diode (LED) structures has attracted great interest because Si wafers are readily available in large diameter at low cost. In addition, such wafers are compatible with existing processing lines for the 6-inch and larger wafer sizes commonly used in the electronics industry. With the development of various methods to avoid wafer cracking and reduce the defect density, the performance of GaN-based LED and electronic devices has been greatly improved. In this paper, we review our methods of growing crack-free InGaN-GaN multiple quantum well (MQW) LED structures of high crystalline quality on Si(111) substrates. The performance of processed LED devices and its dependence on the threading dislocation density were studied. Full wafer-level LED processing using a conventional 6-inch III-V processing line is also presented, demonstrating the great advantage of using large-size Si substrates for mass production of GaN LED devices.

Effects of crystalline quality on the ultraviolet emission and electrical properties of the ZnO films deposited by magnetron sputtering

The ZnO films were deposited on c-plane sapphire, Si (0 0 1) and MgAl2O4 (1 1 1) substrates in pure Ar ambient at different substrate temperatures ranging from 400 to 750 degrees C by radio frequency magnetron sputtering. X-ray diffraction, photoluminescence and Hall measurements were used to evaluate the growth temperature and the substrate effects on the properties of ZnO films. The results show that the crystalline quality of the ZnO films improves with increasing the temperature up to 600 degrees C, the crystallinity of the films is degraded as the growth temperature increasing further, and the ZnO film with the best crystalline quality is obtained on sapphire at 600 degrees C. The intensity of the photoluminescence and the electrical properties strongly depend on the crystalline quality of the ZnO films. The ZnO films with the better crystallinity have the stronger ultraviolet emission, the higher mobility and the lower residual carrier concentration. The effects of crystallinity on light emission and electrical properties, and the possible origin of the n-type conductivity of the undoped ZnO films are also discussed. (C) 2009 Elsevier B. V. All rights reserved.

Effects of doping on the crystalline quality and composition distribution in InGaN/GaN structure grown by MOCVD

The effects of Si and Mg doping on the crystalline quality and In distribution in the InGaN films were studied by atomic force microscope (AFM), triple crystal X-ray diffraction (TCXRD) and Rutherford backscattering spectrometry (RBS). The undoped, Si-doped and Mg-doped InGaN films were grown by metalorganic chemical vapor deposition (MOCVD) on (0 0 0 1) sapphire substrates. The electronic concentration in the Si-doped InGaN is about 2 x 10(19) cm(-3). It is found that the crystalline quality and In distribution in InGaN is slightly affected by the Si doping. In the Mg doped-case, the hole concentration is about 4 x 10(18) cm(-3) after annealing treatment. The surface morphology and crystalline quality of the Mg-doped InGaN are deteriorated significantly compared with the undoped InGaN. The growth rate of Mg-doped InGaN is higher than the undoped InGaN. Mg doping enhances the In incorporation in the InGaN alloy. The increase in In composition in the growth direction is more severe than the undoped InGaN. (c) 2006 Elsevier B.V. All rights reserved.

High-performance EML grown on taper-masked pattern substrates by ultra-low-pressure MOCVD

A novel in-plane bandgap energy controlling technique by ultra-low pressure (22 mbar) selective area growth (SAG) has been developed. To our knowledge, this is the lowest pressure condition during SAG process ever reported. In this work, high crystalline quality InGaAsP-InP MQWs with a photoluminescence (PL) full-width at half-maximum (FWHM) of less than 35meV are selectively grown on mask-patterned planar InP substrates by ultra-low pressure (22 mbar) metal-organic chemical vapor deposition (MOCVD). In order to study the uniformity of the MQWs grown in the selective area, novel tapered masks are designed and used. Through optimizing growth conditions, a wide wavelength shift of over 80 nm with a rather small mask width variation (0-30 mu m) is obtained. The mechanism of ultra-low pressure SAG is detailed by analyzing the effect of various mask designs and quantum well widths. This powerful technique is then applied to fabricate an electroabsorption-modulated laser (EML). Superior device characteristics are achieved, such as a low threshold current of 19mA and an output power of 7mW. (c) 2005 Elsevier B.V. All rights reserved.

High-performance EML grown on taper-masked pattern substrates by ultra-low-pressure MOCVD

A novel in-plane bandgap energy controlling technique by ultra-low pressure (22 mbar) selective area growth (SAG) has been developed. To our knowledge, this is the lowest pressure condition during SAG process ever reported. In this work, high crystalline quality InGaAsP-InP MQWs with a photoluminescence (PL) full-width at half-maximum (FWHM) of less than 35meV are selectively grown on mask-patterned planar InP substrates by ultra-low pressure (22 mbar) metal-organic chemical vapor deposition (MOCVD). In order to study the uniformity of the MQWs grown in the selective area, novel tapered masks are designed and used. Through optimizing growth conditions, a wide wavelength shift of over 80 nm with a rather small mask width variation (0-30 mu m) is obtained. The mechanism of ultra-low pressure SAG is detailed by analyzing the effect of various mask designs and quantum well widths. This powerful technique is then applied to fabricate an electroabsorption-modulated laser (EML). Superior device characteristics are achieved, such as a low threshold current of 19mA and an output power of 7mW. (c) 2005 Elsevier B.V. All rights reserved.

LD side-pumped high beam quality passive Q-switched and mode-locked Nd:YAG laser based on SESAM

We report a LD side-pumped fundamental-mode (Mx(2) = 1.35 and My(2) = 1.27) passive Q-switched and mode-locked Nd:YAG laser based on a semiconductor saturable absorber mirror (SESAM). At a pump current of 12.5 A, the average output power of 5.68 W with 80 kHz repetition rate and 2 mu s pulse width of the Q-switched envelope was generated. The repetition rate of the mode-locked pulse within the Q-switched envelope of 88 MHz was achieved.

High Optical Quality Nanoporous GaN Prepared by Photoelectrochemical Etching

Nanoporous GaN films are prepared by UV assisted electrochemical etching using HF solution as an electrolyte. To assess the optical quality and morphology of these nanoporous films, micro-photoluminescence (PL), micro-Raman scattering, scanning electron microscopy (SEM), and atomic force microscopy (AFM) techniques have been employed. SEM and AFM measurements revealed an average pore size of about 85-90 nm with a transverse dimension of 70-75 nm. As compared to the as-grown GaN film, the porous layer exhibits a substantial photoluminescence intensity enhancement with a partial relaxation of compressive stress. Such a stress relaxation is further confirmed by the red shifted E₂(TO) phonon peak in the Raman spectrum of porous GaN.

Ectomycorrhizal symbiosis can enhance plant nutrition through improved access to discrete organic nutrient patches of high resource quality

It is known that roots can respond to patches of fertility; however, root proliferation is often too slow to exploit resources fully, and organic nutrient patches may be broken down and leached, immobilized or chemically fixed before they are invaded by the root system. The ability of fungal hyphae to exploit resource patches is far greater than that of roots due to their innate physiological and morphological plasticity, which allows comprehensive exploration and rapid colonization of resource patches in soils. The fungal symbionts of ectomycorrhizal plants excrete significant quantities of enzymes such as chitinases, phosphatases and proteases. These might allow the organic residue to be tapped directly for nutrients such as N and P. Pot experiments conducted with nutrient-stressed ectomycorrhizal and control willow plants showed that when high quality organic nutrient patches were added, they were colonized rapidly by the ectomycorrhizal mycelium. These established willows (0.5 m tall) were colonized by Hebeloma syrjense P. Karst. for 1 year prior to nutrient patch addition. Within days after patch addition, colour changes in the leaves of the mycorrhizal plants (reflecting improved nutrition) were apparent, and after I month the concentration of N and P in the foliage of mycorrhizal plants was significantly greater than that in non-mycorrhizal plants subject to the same nutrient addition. It seems likely that the mycorrhizal plants were able to compete effectively with the wider soil microbiota and tap directly into the high quality organic resource patch via their extra-radical mycelium. We hypothesize that ectomycorrhizal plants may reclaim some of the N and P invested in seed production by direct recycling from failed seeds in the soil. The rapid exploitation of similar discrete, transient, high-quality nutrient patches may have led to underestimations when determining the nutritional benefits of ectomycorrhizal colonization.

Post-CMOS compatible high-throughput fabrication of AIN-based piezoelectric microcantilevers

A post-complementary metal oxide semiconductor (CMOS) compatible microfabrication process of piezoelectric cantilevers has been developed. The fabrication process is suitable for standard silicon technology and provides low-cost and high-throughput manufacturing. This work reports design, fabrication and characterization of piezoelectric cantilevers based on aluminum nitride (AlN) thin films synthesized at room temperature. The proposed microcantilever system is a sandwich structure composed of chromium (Cr) electrodes and a sputtered AlN film. The key issue for cantilever fabrication is the growth at room temperature of the AlN layer by reactive sputtering, making possible the innovative compatibility of piezoelectric MEMS devices with CMOS circuits already processed. AlN and Cr have been etched by inductively coupled plasma (ICP) dry etching using a BCl3–Cl2–Ar plasma chemistry. As part of the novelty of the post-CMOS micromachining process presented here, a silicon Si (1 0 0) wafer has been used as substrate as well as the sacrificial layer used to release the microcantilevers. In order to achieve this, the Si surface underneath the structure has been wet etched using an HNA (hydrofluoric acid + nitric acid + acetic acid) based solution. X-ray diffraction (XRD) characterization indicated the high crystalline quality of the AlN film. An atomic force microscope (AFM) has been used to determine the Cr electrode surface roughness. The morphology of the fabricated devices has been studied by scanning electron microscope (SEM). The cantilevers have been piezoelectrically actuated and their out-of-plane vibration modes were detected by vibrometry.