Middle-frequency magnetron sputtering for GaN growth

A middle-frequency magnetron sputtering system was designed and constructed for GaN growth, in which a pair of back cooled pool-shaped twin magnetrons were used for Ga metal targets. GaN films were prepared using this system under various gas pressure (0.5-3.0 Pa) in a mixture of N₂ and Ar with N₂/Ar ratio of 6:1. X-ray diffraction showed that the GaN films had a strong (0 0 0 2) orientation, and the film deposited at 1.5 Pa had two more weak peaks attributed to and . The full width at half maximum (FWHM) of the (0 0 0 2) peak for the GaN film deposited at 1.5 Pa and 0.5 Pa is ∼721 and ∼986 arcsec, respectively. The deposition rate was in the range of 43.5-87.8 nm/min and was mainly influenced by the deposition pressure. The films deposited at higher pressures are columnar in structure. A root-mean-square roughness of 4.4 nm was obtained from the atomic force microscopy (AFM) surface morphology of the film deposited at 0.5 Pa.     

Chemical composition and elastic strain in AlInGaN quaternary films

High-quality AlInGaN quaternary layers were grown on c-Al₂O₃ using a thick GaN template. A full width at half maximum of 0.075° from AlInGaN(0004) rocking curve and a minimum yield of 5.6% from Rutherford backscattering/channelling spectrometry (RBS) prove the AlInGaN layer of a comparable crystalline quality with GaN layers. The chemical compositions (both of Al and In contents) of AlInGaN layers are directly obtained from RBS and elastic recoil detection analysis. The lattice parameters both in perpendicular and parallel directions are deduced from X-ray diffraction. The AlInGaN layer is found to process a compressive strain in parallel direction and a tensile strain in perpendicular direction.     

Performance enhancement of GaN-based light emitting diodes by transfer from sapphire to silicon substrate using double-transfer technique

ABSTRACT: GaN-based light emitting diodes (LEDs) fabricated on sapphire substrates were successfully transferred onto silicon substrates using a double-transfer technique. Compared with the conventional LEDs on sapphire, the transferred LEDs showed a significant improvement in the light extraction and thermal dissipation, which should be mainly attributed to the removal of sapphire and the good thermal conductivity of silicon substrate. Benefited from the optimized wafer bonding process, the transfer processes had a negligible influence on electrical characteristics of the transferred LEDs. Thus, the transferred LEDs showed a similar current-voltage characteristic with the conventional LEDs, which is of crucial importance for practical applications. It is believed that the double-transfer technique offers an alternative way to fabricate high performance GaN-based thin-film LEDs.     

Ion beam analysis of rare earth nitride thin films

We report compositional measurements on highly disordered GdN, DyN, ErN and SmN thin films, grown using ion-assisted deposition and capped with GaN AlN and Al, grown using the same technique. Ion beam analysis technique of RBS, PIXE and nuclear reaction analysis (NRA) were used to determine the composition of the capped films ex situ, and show that GaN and AlN protects the GdN, DyN and SmN films from oxidation over a timescale of at least a few days. NRA depth profiles indicate that oxygen is incorporated into the films during deposition and is located at the GaN/GdN interface. The ion beam analysis measurements showed that stoichiometric ratios can be obtained and oxygen impurities are significantly reduced by varying the film deposition parameters. The successful protection of the rare earth (RE) nitride films from oxidation allows for a reliable analysis of the RE films in the as-deposited state.     

The impact of implantation dose on the characteristics of diluted-magnetic nonpolar GaN:Cu films

Diluted-magnetic nonpolar GaN:Cu films have been fabricated by implanting Cu ions into p-type nonpolar a-plane(112?0) GaN films with a subsequent thermal annealing process. The impact of the implantation dose on the structural, morphological and magnetic characteristics of the samples have been investigated by means of high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and superconducting quantum interference device (SQUID). The XRD and AFM analyses show that the structural and morphological characteristics of samples deteriorated with the increase of implantation dose. According to the SQUID analysis, obvious room-temperature ferromagnetic properties of samples were detected. Moreover, the saturation magnetization per Cu atom decreased as the implantation dose increased.     

Study on the transdermal penetration mechanism of ibuprofen nanoemulsions

Objective: The purpose of this study was to research the mechanism of percutaneous penetration of Ibuprofen (IBU) nanoemulsion.

Method: Transdermal penetration mechanism of IBU nanoemulsion was investigated by using Fourier transform infra-red spectral analysis (FTIR), differential scanning calorimeter thermogram (DSC), and activation energy (Ea) measurement. The in vivo skin penetration test of rats was carried out using Rhodamine B nanoemulsion to simulate the process of drug penetration into the skin, and the frozen section of the skin was observed by confocal laser scanning microscopy (CLSM).

Result: FTIR spectra and DSC thermogram of rat skin treated with IBU nanoemulsion showed that infiltration occurred due to disruption of the stratum corneum (SC) protein–lipid structure and increasing of fluidity, hydration, and disruption of the lipid bilayer structure of the SC. The significant reduce in Ea (1.255?kcal/mol) for IBU permeating rat skin suggested crucial disruption of the SC lipid bilayers (P?<?0.05), which is speculated that nanoemulsion may create new pathways to promote drug penetration. CLSM revealed that Rhodamine B penetrated into the SC in a shorter period of time and it accumulated around the sebaceous glands.

Conclusion: The study of skin penetration mechanism indicated that nanoemulsion can be perfectly well used as the transdermal penetration of poorly soluble drugs.     

Electrochemical and optical characterizations of anodic porous n-InP(1 0 0) layers

In this paper, electrochemical and optical characterizations of anodic porous n-InP(1 0 0) are reported. The direct relation between the observed pore morphology and the physical properties is demonstrated using electrochemical methods such as cyclic voltammetry and impedance spectroscopy as well as optical techniques like photocurrent spectroscopy and photoluminescence measurements. An enhancement of the interfacial capacitance, proportional to the anodic charge, is revealed by voltammetry and Mott-Schottky analysis. It is related to the drastic increase of the area of the porous electrode. However, when the porous samples are sufficiently reverse-biased, the capacitance enlargement disappears because the nanosized pore walls are fully depleted and the electroactive area recovers its initial value. Photocurrent spectroscopy and photoluminescence measurements show the porous film behaves like an absorbent layer. This effect is also ascribed to the specific geometry of the space charge layer within the pore walls. A model based on the absorption coefficient and the effective optical path length is thus used to describe the phenomenon. However the model is not sufficient to depict the phenomenon and the charge recombination in the additional surface states created during the pore formation and the long transit time of electrons in the porous matrix are also significant. Additional effects such as the initial enhancement of the photocurrent response and the redshift of the absorption edge of the photocurrent spectra are observed. Inversely, no shift of the photoluminescence peak is detected. However an exponential quenching of the photoluminescence is also attributed to an absorbent behavior of the porous layer.     

Study on structure of AlGaN on AlN interlayer by synchrotron radiation XRD and RBS

The AlGaN samples have been grown on AlN interlayer (IL) by metalorganic vapor phase epitaxy (MOVPE). The effects of AlN interlayer (IL) on improvement of crystalline quality of AlGaN and Al incorporation efficiency were investigated. The samples were characterized by synchrotron radiation X-ray diffraction (XRD) and MeV He ion Rutherford backscattering spectrometry (RBS). The AlN IL played a role in suppressing edge threading dislocations (TDs) and enhancing the screw ones. It also changed the state of stress in AlGaN from tension to compression. Crack-free AlGaN films were grown successfully by inserting AlN IL.     

Fe3GaAs/GaAs(0 0 1): a stable and magnetic metal-semiconductor heterostructure

We show that in agreement with the ternary Fe-Ga-As phase diagram, the solid-state interdiffusions in epitaxial Fe/GaAs(0 0 1) heterostructures lead, at a temperature of approximately 500 °C, to the formation of thermodynamically stable Fe₃GaAs/GaAs(0 0 1) contacts quite similar to the well-known silicide/Si ones. The Fe₃GaAs films are made of grains epitaxial on GaAs with a well-defined interface. Their magnetic and electrical properties make Fe₃GaAs on GaAs an attractive metallization scheme for future magnetoelectronic devices. The results we report concern (25 or 80 nm Fe)/GaAs(0 0 1) heterostructures annealed at 480 and 500 °C for 10 min and characterized ex situ by He⁺ Rutherford backscattering and ion channeling, X-ray diffraction, transmission electron microscopy and alternating gradient field magnetometry.