Fabrication of GaN epitaxial films on Al2O3/Si (001) substrates

Single crystal GaN films of hexagonal modification have been fabricated on Al2O3/Si (001) substrates via a low pressure metalorganic chemical deposition (LP-MOCVD) method. The full width at half-maximum of (0002) X-ray diffraction peak for the GaN film 1.1 mu m thick was 72 arcmin. and the mosaic structure of the film was the main cause of broadening to the X-ray diffraction peak. Al room temperature, the photoluminescence (PL) spectrum of GaN exhibited near band edge emission peaking at 365 nm.

Microstructure and strain analysis of GaN epitaxial films using in-plane grazing incidence x-ray diffraction

This paper investigates the major structural parameters, such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition, using an in-plane grazing incidence x-ray diffraction technique. The results are analysed and compared with a complementary out-of-plane x-ray diffraction technique. The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method. The method for directly determining the in-plane lattice parameters of the GaN layers is also presented. Combined with the biaxial strain model, it derives the lattice parameters corresponding to fully relaxed GaN films. The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established, reaching to a maximum level of -0.89 GPa.

Study on mechanical properties of GaN epitaxy films grown on sapphire by MOCVD

GaN epitaxy films were grown on (0001) oriented sapphire substrate by metal-organic vapor deposition(MOCVD). AFM and SEM were used to analyze the surface morphology of GaN films. Hardness and critical load of GaN films were measured by an nano-indentation tester, friction coefficient by reciprocating UMT-2MT tribometer. It is found that the surface of GaN film is smooth and the epitaxial growth mechanism is in two-dimension mode, GaN epitaxy films also belong to ultra-hardness materials, whose hardness is 22.1 MPa and elastic modulus is 299.5 GPa. Adhesion strength of epitaxial GaN to sapphire is high, and critical load reaches 1.6 N. Friction coefficient against GCr15 ball is steadily close to 0.13, while GaN films turns to be broken rapidly by using Si3N4 ceramic ball as counterpart.

Study of GaN thin films grown on intermediate-temperature buffer layers by molecular beam epitaxy

A detailed characterisation study of GaN thin films grown by rf-plasma molecular beam epitaxy on intermediate-temperature buffer layers (ITBL) was carried out with Hall, photoluminescence (PL) and deep-level transient Fourier spectroscopy (DLTFS) techniques. The unique feature of our GaN thin films is that the GaN epitaxial layers are grown on top of a double layer that consists of an ITBL, which is grown at 690 degreesC, and a conventional low-temperature buffer layer deposited at 500 degreesC. It is observed that the electron mobility increases steadily with the thickness of the ITBL, which peaks at 377 cm(2)V(-1)S(-1) for an ITBL thickness of 800 nm. The PL also demonstrated systematic improvements with the thickness of the ITBL. The DLTFS results suggest a three-order-of-magnitude reduction in the deep level at E-c-0.40 eV in the device fabricated with the GaN films grown on an ITBL thickness of 1.25 mum in comparison with the control device without an ITBL. Our analyses indicate that the utilization of an ITBL in addition to the conventional low-temperature buffer layer leads to the relaxation of residual strain within the material, resulting in an improvement in the optoelectronic properties of the films. (C) 2002 Elsevier Science BN. All rights reserved.

Biaxial stress dependence of the electrostimulated near-band-gap spectrum of GaN epitaxial film grown on (0001) sapphire substrate

The biaxial piezospectroscopic coefficient (i.e., the rate of spectral shift with stress) of the electrostimulated near-band-gap luminescence of gallium nitride (GaN) was determined as Pi=-25.8 +/- 0.2 meV/GPa. A controlled biaxial stress field was applied on a hexagonal GaN film, epitaxially grown on (0001) sapphire using a ball-on-ring biaxial bending jig, and the spectral shift of the electrostimulated near-band-gap was measured in situ in the scanning electron microscope. This calibration method can be useful to overcome the lack of a bulk crystal of relatively large size for more conventional uniaxial bending calibrations, which has so far hampered the precise determination of the piezospectroscopic coefficient of GaN. The main source of error involved with the present calibration method is represented by the selection of appropriate values for the elastic stiffness constants of both film and substrate. The ball-on-ring calibration method can be generally applied to directly determine the biaxial-stress dependence of selected cathodoluminescence bands of epilayer/substrate materials without requiring separation of the film from the substrate. (c) 2006 American Institute of Physics.

Study of GaN thin films grown on vicinal SiC (0001) substrates by molecular beam epitaxy

A detailed study of the characteristics of undoped GaN films, grown on either vicinal or nominal flat SiC (0001) substrates by molecular beam epitaxy, has been carried out using photoluminescence and Raman scattering techniques. The I I K photoluminescence spectra of the GaN film grown on the vicinal SiC (0001) substrate show a strong and sharp near-bandgap peak (full width at half maximum (FWHM) similar to 16 meV). This feature contrasts with that of the GaN film grown on the nominal flat SiC (0001) substrate where the I I K photoluminescence spectra exhibit the near-bandgap peak (FWHM similar to 25 meV) and the intensity is approximately seven times weaker than that of the vicinal film sample. The redshift of the near-bandgap peak associated with excitons bound to shallow donors is related to the stress caused by both the lattice mismatch and the thermal expansion coefficient difference between GaN and SiC substrates. The measured thermal activation energy of the shallow donor of 33.4 meV is determined by using an Arrhenius plot of the near-bandgap luminescence versus I IT from the slope of the graph at high temperature. The temperature dependence of the FWHM of the near-bandgap luminescence has also been studied. The Raman scattering measurements from the vicinal film reveal that the E-2 phonon peak is strengthened and the A(1)(LO) phonon peak is shifted towards the low-frequency side with enhanced intensity, in comparison to that from the nominal flat film, suggesting a reduction in the density of defects and a lower free carrier concentration in the vicinal GaN film.

The fabrication of GaN-based nanopillar light-emitting diodes

InGaN/GaN multiple quantum well-based light-emitting diode (LED) nanopillar arrays were fabricated using Ni self-assembled nanodots as etching mask. The Ni nanodots were fabricated with a density of 6 x 10(8)-1.5 x 10(9) cm(-2) and a dimension of 100-250 nm with varying Ni thickness and annealing duration time. Then LED nanopillar arrays with diameter of approximately 250 nm and height of 700 nm were fabricated by inductively coupled plasma etching. In comparison to the as-grown LED sample an enhancement by a factor of four of photoluminescence (PL) intensity is achieved for the nanopillars and a blueshift as well as a decrease in full width at half maximum of the PL peak are also observed. The method of additional chemical etching was used to remove the etching-induced damage. Then nano-LED devices were further completed using a planarization approach to deposit p-type electrode on the tips of nanopillars. The current-voltage curves of both nanopillars and planar LED devices are measured for comparison.

Material Growth and Device Fabrication of GaN-Based Blue-Violet Laser Diodes

Studies on first GaN-based blue-violet laser diodes(LDs) in China mainland are reported.High quality GaN materials as well as GaN-based quantum wells laser structures are grown by metal-organic chemical vapor deposition method.The X-ray double-crystal diffraction rocking curve measurements show the full-width half maximum of 180" and 185" for (0002) symmetric reflection and (10(-1)2) skew reflection,respectively.A room temperature mobility of 850cm2/(V·s) is obtained for a 3μm thick GaN film.Gain guided and ridge geometry waveguide laser diodes are fabricated with cleaved facet mirrors at room temperature under pulse current injection.The lasing wavelength is 405.9nm.A threshold current density of 5kA/cm2 and an output light power over 100mW are obtained for ridge geometry waveguide laser diodes.

Rapid fabrication of Au nanoparticle films with the aid of centrifugal force

In this work, rapid fabrication of Au nanoparticle (Au NP) films has been simply achieved by alternate adsorption of citrate-stabilized Au NPs and poly(diallyldimethylammonium chloride) with the aid of centrifugal force. In contrast to conventional electrostatic assembly, we carried out the assembly process in a centrifuge with a rotating speed of 4000 rpm, where centrifugal force can be imposed on Au NPs. Scanning electron microscopy and cyclic voltammetry were employed to characterize the assembly procedure and the thus-prepared thin solid films. Our results demonstrate that centrifugal force can promote the assembly of Au NPs and therefore enable the rapid fabrication of functional Au NP films.

The influence of substrate nucleation on HVPE-grown GaN thick films - art. no. 684105

Thick GaN films were grown on sapphire in a home-made vertical HVPE reactor. Effect of nucleation treatments on the properties of GaN films was investigated, including the nitridation of sapphire, low temperature GaN buffer and MOCVD-template. Various material characterization techniques, including AFM, SEM, XRD, CL and PL have been used to assess these GaN epitaxial films. It was found that the surface of sapphire after high temperature nitridation was flat and showed high density nucleation centers. In addition, smooth Ga-polarity surface of epitaxial layer can be obtained on the nitridation sapphire placed in air for several days due to polarity inversion. This may be caused by the atoms re-arrangement because of oxidation. The roughness of N-polarity film was caused by the huge inverted taper domains, which can penetrate up to the surface. The low temperature GaN buffer gown at 650 degrees C is favorable for subsequent epitaxial film, which had narrow FWHM of 307 arcsec. The epitaxial growth on MOCVD-template directly came into quasi-2D growth mode due to enough nucleation centers, and high quality GaN films were acquired with the values of the FWHM of 141 arcsec for (002) reflections. After etching in boiled KOH, that the total etch-pit density was only 5 x 106 cm(-2) illustrated high quality of the thick film on template. The photoluminescence spectrum of GaN film on the MOCVD-template showed the narrowest line-width of the band edge emission in comparison with other two growth modes.

Structural and Optical Performance of GaN Thick Film Grown by HVPE

Thick GaN films were grown on GaN/sapphire template in a vertical HVPE reactor. Various material characterization techniques,including AFM, SEM, XRD, RBS/Channeling, CL, PL, and XPS, were used to characterize these GaN epitaxial films. It was found that stepped/terraced structures appeared on the film surface,which were indicative of a nearly step-flow mode of growth for the HVPE GaN despite the high growth rate. A few hexagonal pits appeared on the surface, which have strong light emission. After being etched in molten KOH, the wavy steps disappeared and hexagonal pits with {1010} facets appeared on the surface. An EPD of only 8 ×10~6cm~(-2) shows that the GaN film has few dislocations. Both XRD and RBS channeling indicate the high quality of the GaN thick films. Sharp band-edge emission with a full width at half maximum（FWHM）of 67meV was observed, while the yellow and infrared emissions were also found. These emissions are likely caused by native defects and C and O impurities.

Demonstration of GaN/InGaN light emitting diodes on (100) beta-Ga2O3 substrates by metalorganic chemical vapour deposition

The growth and fabrication of GaN/InGaN multiple quantum well (MQW) light emitting diodes ( LEDs) on ( 100) beta-Ga2O3 single crystal substrates by metal-organic chemical vapour deposition (MOCVD) technique are reported. x-ray diffraction (XRD) theta-2 theta. scan spectroscopy is carried out on the GaN buffer layer grown on a ( 100) beta-Ga2O3 substrate. The spectrum presents several sharp peaks corresponding to the ( 100) beta-Ga2O3 and ( 004) GaN. High-quality ( 0002) GaN material is obtained. The emission characteristics of the GaN/InGaN MQW LED are measurement. The first green LED on beta-Ga2O3 with vertical current injection is demonstrated.

Influences of reactor pressure of GaN buffer layers on morphological evolution of GaN grown by MOCVD

The morphological evolution of GaN thin films grown on sapphire by metalorganic chemical vapor deposition was demonstrated to depend strongly on the growth pressure of GaN nucleation layer (NL). For the commonly used two-step growth process, a change in deposition pressure of NL greatly influences the growth mode and morphological evolution of the following GaN epitaxy. By means of atomic force microscopy and scanning electron microscope, it is shown that the initial density and the spacing of nucleation sites on the NL and subsequently the growth mode of FIT GaN epilayer may be directly controlled by tailoring the initial low temperature NL growth pressure. A mode is proposed to explain the TD reduction for NL grown at relatively high reactor pressure. (C) 2003 Elsevier B.V. All rights reserved.

Fabrication of novel double-hetero-epitaxial SOT structure Si/gamma-Al2O3/Si

In this paper, we report the fabrication of Si-based double-hetero-epitaxial silicon on insulator (SOI) structure Si/gamma-Al2O3/Si. Firstly, single crystalline gamma-Al2O3(100) insulator films were grown epitaxially on Si(100) using the sources of TMA (Al(CH3)(3)) and O-2 by very low-pressure chemical vapor deposition. Afterwards, Si(100) epitaxial films were grown on gamma-Al2O3 (100)/Si(100) epi-substrates using a chemical vapor deposition method similar to the silicon on sapphire epitaxial growth. The Si/gamma-Al2O3/Si SOL materials are characterized in detail by reflect high-energy electron diffraction, X-ray diffraction and Auger energy spectrum (AES) techniques. The insulator layer of gamma-Al2O3 has an excellent dielectric property. The leakage current is less than 1 x 10(-10) A/cm(2) when the electric field is below 1.3 MV/ cm. The Si film grown on gamma-Al2O3/Si epi-substrates was single crystalline. Meanwhile, the AES depth profile of the SOL structure shows that the composition of gamma-Al2O3 film is uniform, and the carbon contamination is not observed. Additionally, the gamma-Al2O3/Si epi-substrates are suitable candidates as a platform for a variety of active layers such as GaN, SiC and GeSi. It shows a bright future for microelectronic and optical electronics applications. (C) 2002 Elsevier Science B.V. All rights reserved.

STUDY ON PREPARATION OF GAN AND COSI2 EPITAXIAL-FILMS BY MASS ANALYZED LOW-ENERGY DUAL ION-BEAM EPITAXY

The Mass Analyzed Low Energy Dual Ion Beam Epitaxy (MALE-DIBE) system has been designed and constructed in our laboratory. We believe that the system, which was installed and came into full operation in 1988, is the first facility of this kind. With our system we have carried out studies, for the first time, on compound synthesis of GaN and CoSi2 epitaxial thin films. RHEED and AES results show that GaN films, which were deposited on Si and sapphire substrates, are monocrystalline and of good stoichiometry. To our knowledge, GaN film heteroepitaxially grown on Si. which is more lattice-mismatched than GaN on sapphire, has not been reported before by other authors. RBS and TEM investigations indicated a rather good crystallinity of CoSi2 with a distinct interface between CoSi2 and the Si substrate. The channelling minimum yield chi(min) from the Co profile is approximately 4%. The results showed that the DIBE system with simultaneous arrival of two beams at the target is particularly useful in the formation of novel compounds at a relatively low substrate temperature.