Thick GaN Films Research Articles

Temperature dependent dielectric function and the E critical points of hexagonal GaN from 30 to 690 K

The complex dielectric function ɛ and the E0 excitonic and band-edge critical-point structures of hexagonal GaN are reported for temperatures from 30 to 690 K and energies from 0.74 to 6.42 eV, obtained by rotating-compensator spectroscopic ellipsometry on a 1.9 μm thick GaN film deposited on a c-plane (0001) sapphire substrate by molecular beam epitaxy. Direct inversion and B-splines in a multilayer-structure calculation were used to extract the optical properties of the film from the measured pseudodielectric function ⟨ɛ⟩. At low temperature sharp E0 excitonic and critical-point interband transitions are separately observed. Their temperature dependences were determined by fitting the data to the empirical Varshni relation and the phenomenological expression that contains the Bose-Einstein statistical factor.

Achieve 2-inch-diameter homogeneous GaN films on sapphire substrates by pulsed laser deposition

The 2-inch-diameter homogeneous GaN films have been epitaxially grown on sapphire substrates by pulsed laser deposition (PLD) technique with optimized laser rastering and PLD growth conditions. The as-grown GaN films are characterized by in situ reflection high-energy electron diffraction, white-light interferometry, scanning electron microscopy, atomic force microscopy (AFM), grazing incidence angle X-ray reflectivity, reciprocal space mappings, and micro-Raman spectroscopy for surface morphologies and structural properties. The as-grown 2-inch-diameter single-crystalline GaN films exhibit excellent thickness uniformity with a root-mean-square (RMS) inhomogeneity less than 3.4 % and very smooth surface with a RMS roughness less than 1.3 nm measured by AFM. There is a maximum of 1.2 nm thick interfacial layer existing between the as-grown GaN films and sapphire substrates, and the as-grown 310 nm thick GaN films are almost fully relaxed only with an in-plane compressive strain of 0.044 %. This work demonstrates a possibility for achieving high-quality large-scale GaN films with uniform thickness and atomically abrupt interface by PLD, and is of great interest for the commercial development of GaN-based optoelectronic devices.

GaN Substrate Technologies for Optical Devices

Large GaN single-crystal substrates with low dislocation density are the key materials for the commercial production of GaN-based laser diodes. We developed a new method to reduce the dislocations, named dislocation elimination by the epitaxial-growth with inverse-pyramidal pits (DEEP). A thick GaN film is epitaxially grown on a GaAs substrate with hydride vapor-phase epitaxy and then is separated from the GaAs substrate. The thick GaN layer grows with numerous large inverse-pyramidal pits. As the growth proceeds, dislocations in the GaN film are concentrated to the center of the pit and a wide area with low dislocation density is formed within the pit except the center area. To control the dislocation artificially, the position of the pits is fixed at a predetermined position by means of the selective growth of different polarity GaN film. This process was named as advanced DEEP (A-DEEP). GaN substrates with the A-DEEP method satisfied all the requirements for the violet laser diodes. We continued to develop new GaN substrates such as large-diameter c-plane substrates as well as nonpolar/semipolar GaN substrates. In particular, we overcame the green gap problem and developed the world's first true green laser diodes by selecting an optimal crystal plane.

The Impact of V/III Ratio on GaN Growth by HVPE

Thick GaN films grown with different V/III ratio on sapphire by hydride vapour phase epitaxy have been investigated. The V/III ratio is changed from 240 to 30. All the GaN films, which are n type, show only (0002) oriented peak and have the band emission with no yellow luminescence bands. When V/III ratio is 30, the full width at half maximum of (0002) X-ray rocking curve is the smallest, line-width of the band edge emission is narrow, the surface morphology shows step-flow growth and the growth rate is the highest.

A study of GaN regrowth on the micro-faceted GaN template formed by in situ HCl etching

This study demonstrates improvement of crystal quality of GaN film by growing it on a micro-faceted GaN template fabricated by in situ etching with an HCl/N2 etching gas mixture at 1050°C in hydride vapor phase epitaxy (HVPE) reactor. Cathodoluminescence images of the cross-sectional structure showed that there were six sublays in the GaN film grown with in situ etching for five times. Etch pit density (EPD) and high-resolution X-ray diffraction (HR-XRD) measurements showed that the crystal quality of GaN thick film grown on micro-faceted GaN template was better than that of the as-grown GaN. A large number of columnar structures were formed on the micro-faceted GaN template at the initial stage of regrowth process, the coalescence of which played an important role in the reduction of threading dislocation density during the GaN growth. A model has been developed to explain the mechanism of forming process of the columnar structures.

Nearly 4-Inch-Diameter Free-Standing GaN Wafer Fabricated by Hydride Vapor Phase Epitaxy with Pit-Inducing Buffer Layer

A free-standing GaN wafer was fabricated by depositing a GaN buffer that induced the formation of pits (hereafter, pit-inducing GaN buffer) on a low-temperature-grown GaN buffer on the sapphire substrate. A high-temperature-grown GaN layer was grown on the pit-inducing GaN buffer that induced the formation of pits on the high-temperature-grown GaN layer. The pit-inducing buffer suppresses crack formation in the thick GaN film thereby releasing growth stress. Thermal stress in GaN on a sapphire system is also discussed on the basis of calculations utilizing a bilayer model. We have succeeded in the fabrication of a nearly 4-in.-diameter free-standing GaN thick wafer with a pit-inducing GaN buffer by one-stop hydride vapor phase epitaxy, which will lead to a low-cost fabrication of free-standing GaN wafers.

Free‐standing GaN wafer by one‐stop HVPE with pit‐induced buffer layer

AbstractWe have succeeded in the fabrication of a 2‐inch dia free‐standing GaN thick wafer by one‐stop hydride vapor phase epitaxy using pit‐induced buffer layer. High temperature grown GaN layer on the pit‐induced layer was grown with numerous inverse‐pyramidal pits on the surface. A thick GaN film was separated from the sapphire substrate during cooling. The X‐ray rocking curves show the full width at half maximum values of 146 and 152 arcsec for (0002) and (10–12) reflections, respectively. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of thick GaN layer on ZnAl2O4 spinel layer by HVPE

Thick GaN film was grown by hydride vapor phase epitaxy (HVPE) on spinel covered ZnO layer. Single crystal ZnO was grown on c-plane sapphire by ultra high vacuum (UHV) DC sputtering. A thin Al layer was thermally deposited on it. A spinel layer was formed by a solid phase reaction of the Al and ZnO layers, and GaN thick film was grown on it. Structural and chemical properties of the spinel layer were studied by X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS), respectively. The crystal quality of GaN was observed by atomic force microscope (AFM) and transmission electron microscope (TEM). We found that the spinel layer is helpful for the reduction of zinc and oxygen diffusion into GaN during the HVPE growth.

Study on proton irradiation induced defects in GaN thick film

Proton-irradiation-induced defects threaten seriously the stable performance of GaN-based devices in harsh environments, such as outer space. It is therefore urgent to understand the behaviors of proton-irradiation-induced defects for improving the radiation tolerance of GaN-based devices. Positron annihilation spectroscopy (PAS) has been used to study proton-induced defects in GaN grown by HVPE. The result shows that VGa is the main defects and no (VGaVN) or (VGaVN)2 is formed in 5 MeV proton-irradiated GaN. Photoluminescence (PL) spectrum is carried out at 10K. After irradiation, the band edge shows a blue-shift, but the donor-acceptor pair (DAP) emission band and its LO-phonon replicas is kept at the original position. The intensity of yellow luminescence (YL) band is decreased, which means that the origin of YL band has no relation with VGa. The increased FWHM of GaN (0002) peak in proton-irradiated GaN indicates a degradation of crystal quality.

Mosaic structure in epitaxial GaN filmvarying with thickness

In this article. We report on the study of mosaic structures of different thick GaN films grown on sapphire (0001) by metalorganic chemical vapor deposition (MOCVD), using high resolution x-ray diffraction. The result from the symmetrical reflections show that the mosaic vertical and lateral correlation lengths that are calculated by two methods increase with film thickness increasing, and the vertical correlation lengths are close to the film thickness, and the same trend in the lateral correlation lengths derived from the reciprocal space maps. By the help of asymmetrical reflections and Williamson-Hall extrapolation method, the tilt and twist mosaic drop with thickness increasing at different rates. All this shows that the increase in thickness lads to the more uniform and neat grain arrangement and the higher-quality epitaxial wafers.

Stress distribution of 12 μm thick crack free continuous GaN on patterned Si(110) substrate

AbstractThe stress distribution on crack free thick continuous GaN film (12 µm) grown by Metal organic chemical vapour deposition (MOCVD) on the arrays of different sizes of the patterned silicon substrate is investigated by micro‐Raman (µRaman) spectroscopy. On the largest crack free mesa (400 μm) both µ‐photoluminescence (µPL) at low temperature and µRaman measurements are performed. The µRaman shift of the GaN E2 mode shows the U‐shape in‐plane stress distribution across the mesa. The center of the mesa is under tensile stress and it relaxes near the corner and edges. A similar trend is observed also from the µPL spectra. The size of the mesa, the trench height and the trench width of the patterned silicon are varied to study the stress of the thick epitaxial crack free GaN layer. The size and trench height of the mesa have a large influence on the GaN film stress but the trench width does not show any significant effect. The maximum stress is saturated for the large sizes of mesas. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Harmonic surface acoustic waves on gallium nitride thin films

SAW devices operating at the fundamental frequency and the 5th, 7th, 9th, and 11th harmonics have been designed, fabricated, and measured. Devices were fabricated on GaN thin films on sapphire substrates, which were grown via metal organic vapor phase epitaxy (MOVPE). Operating frequencies of 230, 962, 1338, 1720, and 2100 MHz were achieved with devices that had a fundamental wavelength, lambda0 = 20 μm. Gigahertz operation is realized with relatively large interdigital transducers that do not require complicated submicrometer fabrication techniques. SAW devices fabricated on the GaN/sapphire bilayer have an anisotropic propagation when the wavelength is longer than the GaN film thickness. It is shown that for GaN thin films, where kh(GaN) > 10 (k = 2pi/lambda and h(GaN) = GaN film thickness), effects of the substrate on the SAW propagation are eliminated. Bulk mode suppression at harmonic operation is also demonstrated.

Realization of compressively strained GaN films grown on Si(110) substrates by inserting a thin AlN/GaN superlattice interlayer

We investigate the strain properties of GaN films grown by plasma-assisted molecular beam epitaxy on Si(110) substrates. It is found that the strain of the GaN film can be converted from a tensile to a compressive state simply by inserting a thin AlN/GaN superlattice structure (SLs) within the GaN film. The GaN layers seperated by the SLs can have different strain states, which indicates that the SLs plays a key role in the strain modulation during the growth and the cooling down processes. Using this simple technique, we grow a crack-free GaN film exceeding 2-μm-thick. The realization of the compressively strained GaN film makes it possible to grow thick GaN films without crack generation on Si substrates for optic and electronic device applications.

Experimental study of light output power for vertical GaN-based light-emitting diodes with various textured surface and thickness of GaN layer

The light output power (LOP) of vertical-type GaN-based light emitting diodes (LED) with surface roughness (texture) can be changed by texture size, density, and thickness of GaN film or by the combined effects of texture formation and thickness of GaN film. We have investigated these changes experimentally and note that the enhancement of the LOP by a factor of 2.4 can be improved with optimum texturing parameters as compared to that without texturing. In addition, the LOP of GaN-based LEDs under the same texture density increase slightly as thickness of GaN film decreases. Base on these results, we have evidently demonstrated that the enhancement factors of LOP are related to the correlation between texture size (density) and thickness of GaN film.

Cross sectional CL study of the growth and annihilation of pit type defects in HVPE grown (0001) thick GaN

The growth and annihilation mechanism of pit type defects in HVPE grown thick GaN films was studied using a cross sectional cathodoluminescence (CL) technique. Two kinds of pit type defects were distinguished by their morphology: the hexagonal V-pit surrounded by {10−11} facets and the U-pit with {10−11} facets having a blunt bottom. It was found that the V-pit originated from different growth rates between (0001) plane and {10−11} facet, and was filled and annihilated by {10−12} facets’ growth, namely U-pit generation. The formation of U-pit may play an important role in annihilation of pit type defects.

Estimating the Threading Dislocation Density of Thick GaN Films

The threading dislocation density (TDD) of thick GaN films grown by hydride vapor phase epitaxy (HVPE) was estimated through counting etch pit density (EPD) and calculating full width at half maximum（FWHM）of double crystal X-Ray diffraction (DCXRD). TDD was about 108 through counting EPD, while it was about 109 through calculating the FWHMs of (0002) and (1012). The experiment results show that the two methods are both suited to estimating the TDD of the thick HVPE-GaN films with 350um thickness. But they have some difference: EPD method is fitter in evaluating the dislocation density of the surface of thick GaN film, but FWHM method can gain the total dislocation density of thick GaN film. The method calculating the FWHMs possesses more general statistical significance.

Structural and optical studies of thick freestanding GaN films deposited by Hydride vapor phase epitaxy

Thick freestanding films or bulk GaN substrates with very low background impurity levels (≤1×1015/cm3) and high crystalline quality are required for a number of electronic device applications. Low pressure chemical vapor and molecular beam epitaxy techniques can systematically deposit films with low residual impurity concentrations. However, their typical slow growth rate prevents their utilization for substrate growth. The hydride vapor phase epitaxy deposition technique can achieve hundreds of microns per hour growth rate, but these films have typically high free carrier concentration (≥3×1017/cm3). It is crucial to verify if this method can reproducibly deliver thick freestanding GaN films of high crystalline quality with exceptionally low free carrier concentration. Low temperature photoluminescence and room temperature Raman scattering experiments carried out on a number of samples indicate that they have high crystalline quality and uncommonly low donor background levels. Reduced concentration of uncompensated shallow donors verified by low temperature electron paramagnetic resonance was confirmed by detailed high sensitive SIMS analyses. In addition, it was verified by X-ray diffraction analysis that relatively low dislocation densities can be achieved.

Cathodoluminescence study of nonuniformity in hydride vapor phase epitaxy-grown thick GaN films

Cathodoluminescence study of nonuniformity in hydride vapor phase epitaxy-grown thick GaN films

The growth of high-quality and self-separation GaN thick-films by hydride vapor phase epitaxy

About 1.2 mm thick GaN bulk crystals were obtained by combining a pulsed NH 3-flow modulation (PFM) method and a self-separation method of short-shutting NH 3 flow when using hydride vapor phase epitaxy (HVPE). High crystal quality of bulk GaN was evaluated by X-ray rocking curves (XRC) and the full width at half maximum (FWHM) values were 110, 72 and 83 arcsec for (002), (102) and (100) reflection planes, respectively. The PFM method is proved to be effective in reducing cracks and keeping the surface smooth. And the method of short-shutting NH 3 flow can lead to GaN thick layer separate from sapphire substrate when cooling from the high growth temperature. Growth and separation mechanisms were investigated. Two states were found in PFM method. With PFM method modulating between high quality state and low stress state, 300 μm thick GaN layers without cracks were obtained. Study of spontaneous separation mechanism revealed that the separation attributed to formation of voids inside the GaN layer.

Estimating the Threading Dislocation Density of Thick GaN Films

The threading dislocation density (TDD) of thick GaN films grown by hydride vapor phase epitaxy (HVPE) was estimated through counting etch pit density (EPD) and calculating full width at half maximum(FWHM)of double crystal X-Ray diffraction (DCXRD). TDD was about 108 through counting EPD, while it was about 109 through calculating the FWHMs of (0002) and (10î2). The experiment results show that the two methods are both suited to estimating the TDD of the thick HVPE-GaN films with 350um thickness. But they have some difference: EPD method is fitter in evaluating the dislocation density of the surface of thick GaN film, but FWHM method can gain the total dislocation density of thick GaN film. The method calculating the FWHMs possesses more general statistical significance.