EPITAXIAL LATERAL OVERGROWTH OF GaN ON SILICON-ON-INSULATOR

From a single process, GaN layers were laterally overgrown on maskless stripe-patterned (111) silicon-on-insulator (SOI) substrates by metalorganic chemical vapor deposition. The influence of stress on the behavior of dislocations at the coalescence during growth was observed using transmission electron microscopy (TEM). Improvement of the crystallin equality of the GaN layer was demonstrated by TEM and micro-Raman spectroscopy. Furthermore, the benefits of SOI substrates for GaN growth are also discussed.

Mechanical Deformation Behavior of Nonpolar GaN Thick Films by Berkovich Nanoindentation

In this study, the deformation mechanisms of nonpolar GaN thick films grown on m-sapphire by hydride vapor phase epitaxy (HVPE) are investigated using nanoindentation with a Berkovich indenter, cathodoluminescence (CL), and Raman microscopy. Results show that nonpolar GaN is more susceptible to plastic deformation and has lower hardness than c-plane GaN. After indentation, lateral cracks emerge on the nonpolar GaN surface and preferentially propagate parallel to the < 11 (2) over bar0 > orientation due to anisotropic defect-related stresses. Moreover, the quenching of CL luminescence can be observed to extend exclusively out from the center of the indentations along the < 11 (2) over bar0 > orientation, a trend which is consistent with the evolution of cracks. The recrystallization process happens in the indented regions for the load of 500 mN. Raman area mapping indicates that the distribution of strain field coincides well with the profile of defect-expanded dark regions, while the enhanced compressive stress mainly concentrates in the facets of the indentation.

Room-temperature ferromagnetism and in-plane magnetic anisotropy characteristics of nonpolar GaN:Mn films

Diluted magnetic nonpolar GaN Mn films have been fabricated by implanting Mn ions into nonpolar aplane (1 1 (2) over bar 0) p-type GaN films and a subsequent rapid thermal annealing process. The ferromagnetism properties of the films were studied by means of superconducting quantum interference device (SQUID). Clearly in-plane magnetic anisotropy characteristics of the sample at 10 K were revealed with the direction of the applied magnetic field rotating along the in-plane [0 0 0 1]-axis. Moreover, obvious ferromagnetic properties of the sample up to 350 K were detected by means of the temperature-dependent SQUID. (C) 2009 Elsevier B.V. All rights reserved.

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 (1120) 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. (C) 2009 Elsevier B.V. All rights reserved.

Microstructural and compositional characteristics of GaN films grown on a ZnO-buffered Si(111) wafer

Polycrystalline GaN thin films have been deposited epitaxially on a ZnO-buffered (111)-oriented Si substrate by molecular beam epitaxy. The microstructural and compositional characteristics of the films were studied by analytical transmission electron microscopy (TEM). A SiO2 amorphous layer about 3.5 nm in thickness between the Si/ZnO interface has been identified by means of spatially resolved electron energy loss spectroscopy. Cross-sectional and plan-view TEM investigations reveal (GaN/ZnO/SiO2/Si) layers exhibiting definite a crystallographic relationship: [111](Si)//[111](ZnO)//[0001](GaN) along the epitaxy direction. GaN films are polycrystalline with nanoscale grains (similar to100 nm in size) grown along [0001] direction with about 20degrees between the (1 (1) over bar 00) planes of adjacent grains. A three-dimensional growth mode for the buffer layer and the film is proposed to explain the formation of the as-grown polycrystalline GaN films and the functionality of the buffer layer. (C) 2004 Elsevier Ltd. All rights reserved.

Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal-semiconductor-metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an AlN high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A IN HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process. (C) 2003 Elsevier Ltd. All rights reserved.

The influence of AlN/GaN superlattice intermediate layer on the properties of GaN grown on Si(111) substrates

AlN/GaN superlattice buffer is inserted between GaN epitaxial layer and Si substrate before epitaxial growth of GaN layer. High-quality and crack-free GaN epitaxial layers can be obtained by inserting AlN/GaN superlattice buffer layer. The influence of AlN/GaN superlattice buffer layer on the properties of GaN films are investigated in this paper. One of the important roles of the superlattice is to release tensile strain between Si substrate and epilayer. Raman spectra show a substantial decrease of in-plane tensile strain in GaN layers by using AlN/GaN superlattice buffer layer. Moreover, TEM cross-sectional images show that the densities of both screw and edge dislocations are significantly reduced. The GaN films grown on Si with the superlattice buffer also have better surface morphology and optical properties.

Study on optical properties of Er/Er+O doped GaN thin films

We extend the use of Raman spectroscopy to investigate the modes of Er-implanted and Er + O co-implanted GaN, and discuss the influence of O ions on Er3+ -related infrared photoluminescence (PL). It is found that Er3+ implantation introduces new Raman peaks in Raman spectra at frequencies 300 and 670 cm and one additional new peak at 360cm is introduced after Er + O implantation. It is proposed that the broad structure around 300 cm(-1) mode originates from disorder-activated scattering (DARS). The Raman peak at 670 cm is assigned to nitrogen vacancy related defects. The 360 cm peak is attributed to the O implantation induced defect complexes (vacancies, interstitial, or anti-sites in the host). The appearance of the 360 cm(-1) mode results in the decrease of the Er3+ -related infrared PL of GaN: Er + O.

Influence of nitridation time on the morphology of GaN nanorods synthesized by nitriding Ga2O3/ZnO films

Gallium nitride (GaN) nanorods were synthesized by nitriding Ga2O3/ZnO films which were deposited in turn on Si (111) substrates using radio frequency (RF) magnetron sputtering system. In the nitridation process, ZnO was reduced to Zn and Zn sublimated at 950 degrees C. Ga2O3 was reduced to Ga2O and Ga2O reacted with NH3 to synthesize GaN nanorods with the assistance of the sublimation of Zn. The morphology and structure of the nanorods were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The composition of GaN nanorods was studied by Fourier-transform infrared spectrophotometer (FTIR). The synthesized nanorods is hexagonal wurtzite structured. Nitridation time of the samples has an evident influence on the morphology of GaN nanorods synthesized by this method. (c) 2006 Elsevier B.V. All rights reserved.

Preparation and characterization of GaN films by radio frequency magnetron sputtering and carbonized-reaction technique

Radio frequency magnetron sputtering/post-carbonized-reaction technique was adopted to prepare good-quality GaN films on Al2O3(0 0 0 1) substrates. The sputtered Ga2O3 film doped with carbon was used as the precursor for GaN growth. X-ray diffraction (XRD) pattern reveals that the film consists of hexagonal wurtzite GaN. X-ray photoelectron spectroscopy (XPS) shows that no oxygen can be detected. Electrical and room-temperature photoluminescence measurements show that good-quality polycrystalline GaN films were successfully grown on Al2O3(0 0 0 1) substrates. (c) 2005 Elsevier B.V. All rights reserved.

Microstructure of GaN films grown on Si(111) substrates by metalorganic chemical vapor deposition

We report the transmission electron microscopy (TEM) study of the microstructure of wurtzitic GaN films grown on Si(I I I) substrates with AlN buffer layers by metalorganic chemical vapor deposition (MOCVD) method. An amorphous layer was formed at the interface between Si and AlN when thick GaN film was grown. We propose the amorphous layer was induced by the large stress at the interface when thick GaN was grown. The In0.1Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations from passing through the MQW. But no evident reduction of the edge dislocations by the MQW was observed. It was found that dislocations located at the boundaries of grains slightly in-plane misoriented have screw component. Inversion domain is also observed. (C) 2003 Elsevier B.V. All rights reserved.

Growth and structural properties of GaN films on flat and vicinal SiC(0001) substrates

Initial stage GaN growth by molecular-beam epitaxy (MBE) on SiC(0001) substrate is followed by in situ scanning tunneling microscopy. Comparison is made between growth on nominally flat and vicinal substrate surfaces and the results reveal characteristic differences between the two. Ex situ transmission electron microscopy (TEM) and X-ray diffraction (XRD) rocking curve measurements of the films show lower density of defects and better structural quality of the vicinal film. We suggest the improved structural quality of the vicinal film is related to the characteristic difference in its initial stage nucleation and coalescence proccsses than that of the flat film.

Annealing behavior of hexagonal phase content in cubic GaN thin films grown on GaAs (001) by MOCVD

The annealing behavior of the hexagonal phase content in cubic GaN (c-GaN) thin films grown on GaAs (001) by MOCVD is reported. C-GaN thin films are grown on GaAs (001) substrates by metalorganic chemical vapor deposition (MOCVD). High temperature annealing is employed to treat the as-grown c-GaN thin films. The characterization of the c-GaN films is investigated by photoluminescence (PL) and Raman scattering spectroscopy. The change conditions of the hexagonal phase content in the metastable c-GaN are reported. There is a boundary layer existing in the c-GaN/GaAs film. When being annealed at high temperature, the intensity of the TOB and LOB phonon modes from the boundary layer weakens while that of the E-2 phonon mode from the hexagonal phase increases. The content change of hexagonal phase has closer relationship with annealing temperature than with annealing time period.

Fabrication of luminescent Ge nanocrystals started from unlayered hydrogenated amorphous SiGe films or hydrogenated amorphous Si hydrogenated amorphous Ge multilayers

Nanocrystalline Ge embedded in SiOx matrix is fabricated by oxidizing hydrogenated amorphous Sice alloys or hydrogenated amorphous Si/hydrogenated amorphous Ge multilayers. The structures before and after oxidation are systematically investigated. Visible light emission was observed from both samples. The luminescence peak is located at 2.2 eV which is independent of the starting materials. Compared to the luminescence from unlayered samples, the photoluminescence spectrum from multilayered samples has a narrower band width, which can be attributed to the uniform size distribution. The light emission origin is also discussed briefly and a mechanism different from the quantum size effect is suggested.

Rapid thermal annealing processing of GaN epilayer on sapphire(0 0 0 1)

The effect of rapid thermal annealing (RTA) in a Nz ambient up to 900 degrees C has been investigated for GaN films grown on sapphire(0 0 0 1) substrates. Raman spectra, X-ray diffractometry and Hall-effect studies were performed for this purpose. The Raman spectra show the presence of the E-2 (high) mode and a shift in the wave number of this mode with respect to the annealing processing. This result suggests the presence and relaxation of residual stress due to thermal expansion misfit in the films which are confirmed by X-ray measurements and the structure quality of GaN epilayer was improved. Furthermore, the electron mobility increased at room temperature with respect to decrease of background electron concentration after RTA. (C) 1998 Elsevier Science B.V. All rights reserved.