Thick GaN Films Research Articles

Hydride vapor phase epitaxy of strain-reduced GaN film on nano-island template produced using self-assembled CsCl nanospheres

Thick GaN films were overgrown by HVPE on nano-island templates, which was fabricated by inductively coupled plasma etching using self-assembled CsCl islands as etch masks. The truncated cone-shape nano-islands exhibited significant effects on improvement of crystalline quality and reduction of crack density for subsequent GaN growth. Photoluminescence and micro-Raman measurements showed improved optical properties and partial strain relaxation in the overgrown GaN when compared to that grown on as-grown template, originated from the initial selective growth. Compared to conventional epitaxial lateral overgrown GaN, the economic and rapid method would put forward new promising pathway to fabricate high quality GaN.

Gradual variation method for thick GaN heteroepitaxy by hydride vapour phase epitaxy

Two strain-state samples of GaN, labelled the strain-relief sample and the quality-improved sample, were grown by hydride vapour phase epitaxy (HVPE), and then characterized by high-resolution X-ray diffraction, photoluminescence and optical microscopy. Two strain states of GaN in HVPE, like 3D and 2D growth modes in metal-organic chemical vapour deposition (MOCVD), provide an effective way to solve the heteroepitaxial problems of both strain relief and quality improvement. The gradual variation method (GVM), developed based on the two strain states, is characterized by growth parameters' gradual variation alternating between the strain-relief growth conditions and the quality-improved growth conditions. In GVM, the introduction of the strain-relief amplitude, which is defined by the range from the quality-improved growth conditions to the strain-relief growth conditions, makes the strain-relief control concise and effective. The 300-μm thick bright and crack-free GaN film grown on a two-inch sapphire proves the effectiveness of GVM.

Hydrogen etching of GaN and its application to produce free-standing GaN thick films

This work investigates the morphology of GaN etched in hydrogen (H 2) at different temperatures, the activation energies of the rate-limiting steps of H 2 etching, and the overgrowth on a H 2-etched GaN template. The surfaces of GaN have different profiles after being etched in H 2; they resemble a plane decorated with columns and mooring posts in a low-temperature etching condition, and with deep cavities in a high-temperature etching condition. The etched profiles show that H 2 etching has controllable etching directions: vertical and lateral. In a low-temperature condition, H 2 etching has both vertical and lateral etching directions; however, in a high-temperature condition, it has only the vertical etching direction. The activation energies of the rate-limiting steps under etching pressures of 100 and 700 Torr are estimated to be 3.22 and 3.77 eV, respectively. A thick GaN layer has been grown on a H 2-etched GaN template, and it has self-separated from the underlying sapphire substrate.

Extended defect structure of a-plane GaN produced by sidewall lateral epitaxial overgrowth

This paper demonstrates the use of Sidewall Lateral Epitaxial Overgrowth (S-LEO) on a-plane GaN thick films on r-plane sapphire by hydride vapor phase epitaxy (HVPE). Comprehensive study of the extended defect microstructure of the a-plane GaN films was carried out using cross-sectional and plan-view transmission electron microscopy (TEM). A very low density of primary threading dislocations and a heterogeneous microstructure can be found in the GaN films. In the window region and N-face wing region, the extended defects included type I 1 and type I 2 basal stacking faults (BSFs), as well as prismatic stacking faults (PSFs) on a-planes. The density of type I 1 BSFs was in the order of ∼2×10 5 cm −1, type I 2 BSFs in the order of ∼10 4 cm −1, and corresponding localized partial dislocation density less than 1.5×10 9 cm −2. PSFs on a-planes were connected to two neighboring type I 1 BSFs with an estimated density of 3×10 2 cm −1 in the plan-view images. In the Ga-face overgrowth regions, the density of BSFs was lower than 10 4 cm −1. However, inversion domains bounded by (1 1̄ 0 2), (1 1̄ 0 2̄), and (1 1̄ 0 0) planes were found in the Ga-face wing regions. The nature of inversion domain boundaries (IDB) on m-planes can be explained by the Auserman-Gehamn model using high-angle annular dark-field TEM images.

Free‐standing GaN substrates fabricated by a combination of substrate fracturing and chemical lift‐off

AbstractFree‐standing GaN thick films are successfully fabricated by employing intentional mechanical fracturing of Al2O3 substrates during HVPE growth followed by chemical lift‐off of Al2O3 substrates after growth. To do so, thermal stress induced in the Al2O3 substrates and overgrown GaN layers are controlled by adjusting the thickness of the Al2O3 substrates. The grown layers however consist of multi domains, which deteriorate crystal quality. Domains coalesce to form larger domains as the growth proceed, which results in an improvement in crystal quality. Bending of free‐standing GaN thick layers is of concave shape and its typical bending radius is 2.5 m (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of GaN in a planetary MOCVD hotwall system

AbstractWe report on a new planetary hotwall research system with independent control of the ceiling and susceptor temperatures. This allows a study of the influence of an actively heated ceiling during a MOCVD process. It was found that the temperature profile across the substrate becomes more uniform when the ceiling temperature is increased while maintaining the same susceptor surface temperature.The parasitic coating on the ceiling was found to be stable and no source of particles up to 1050 °C. The influence of the ceiling temperature on the depletion profile and the possibility to manipulate it was investigated. Good thickness homogeneity of 1.35% (1·σ) on a 4″ sapphire wafer was achieved with excellent XRD results of 206 arcsec for the (0002) reflex and 272 arcsec for the (10‐12) reflex for a 4.7 µm thick GaN film. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thick GaN Films Grown on Patterned Sapphire Substrates

GaN thick films, grown on specially patterned 2" sapphire substrates by HVPE methods have lower bowing and are less susceptible to fracture then ones, grown on unpatterned substrates under the same growth conditions. Numerical calculation shows good agreement with experiments. Such substrates could be an alternative to expensive GaN wafers sliced from GaN boules.

Effect of Growth Parameters on Stress in HVPE GaN Films

The effect of growth parameters on stress in thick GaN films grown on sapphire by HVPE method was investigated. We have found two modes of growth with different growth stress. Films grown in one mode have rough surfaces and low stress. The second mode leads to smooth surfaces but the films contain many cracks due to high growth stress. A combination of these modes allows growth of films without cracks and with smooth surfaces.

Thick GaN Films Grown on Patterned Sapphire Substrates

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Investigation of cracks in GaN films grown by combined hydride and metal organic vapor-phase epitaxial method

Cracks appeared in GaN epitaxial layers which were grown by a novel method combining metal organic vapor-phase epitaxy (MOCVD) and hydride vapor-phase epitaxy (HVPE) in one chamber. The origin of cracks in a 22-μm thick GaN film was fully investigated by high-resolution X-ray diffraction (XRD), micro-Raman spectra, and scanning electron microscopy (SEM). Many cracks under the surface were first observed by SEM after etching for 10 min. By investigating the cross section of the sample with high-resolution micro-Raman spectra, the distribution of the stress along the depth was determined. From the interface of the film/substrate to the top surface of the film, several turnings were found. A large compressive stress existed at the interface. The stress went down as the detecting area was moved up from the interface to the overlayer, and it was maintained at a large value for a long depth area. Then it went down again, and it finally increased near the top surface. The cross-section of the film was observed after cleaving and etching for 2 min. It was found that the crystal quality of the healed part was nearly the same as the uncracked region. This indicated that cracking occurred in the growth, when the tensile stress accumulated and reached the critical value. Moreover, the cracks would heal because of high lateral growth rate.

HVPE生长GaN厚膜光致发光特性研究

HVPE生长GaN厚膜光致发光特性研究

A-plane GaN hydride vapor phase epitaxy on a-plane GaN templates with and without use of TiN intermediate layers

Thick a-plane GaN films were grown by hydride vapor phase epitaxy on a-plane GaN templates prepared by metalorganic chemical vapor deposition (MOCVD) and also on a-plane MOCVD templates using in situ nitridized Ti underlayers. The growth on a-GaN showed improved crystalline quality with increasing hydride vapor phase epitaxy thickness, while MOCVD template quality had little effect. With 30 nm Ti films deposited on the templates and converted to TiN islands by nitridation during growth, the authors obtained thick (350 μm), freestanding a-GaN films detached from the template. Microcathodoluminescence spectra of the growth surface showed intense band edge luminescence at 3.47 eV at 90 K with no defect bands. Spectra taken from the surface turned to the substrate were dominated by stacking fault-related bands at 3.42, 3.3, and 3.0 eV, similar to the spectra of the a-GaN templates. X-ray measurements showed the freestanding a-GaN layers consisted of misoriented large grains of a-GaN with halfwidth for individ...

Hydride Vapor Phase Epitaxy Growth of Semipolar ( 101̄3̄ ) GaN on Patterned m-Plane Sapphire

We have investigated the hydride vapor-phase epitaxy growth of -oriented GaN thick films on patterned sapphire substrates (PSSs) . From characterization by atomic force microscopy, scanning electron microscopy, double-crystal X-ray diffraction, and photoluminescence (PL), it is determined that the crystalline and optical qualities of GaN epilayers grown on the cylindrical PSS are better than those on the flat sapphire. However, two main crystalline orientations and dominate the GaN epilayers grown on the pyramidal PSS, demonstrating poor quality. After etching in the mixed acids, these GaN films are dotted with oblique pyramids, concurrently lining along the direction, indicative of a typical N-polarity characteristic. Defect-related optical transitions of the GaN epilayers are identified and detailedly discussed in virtue of the temperature-dependent PL. In particular, an anomalous blueshift–redshift transition appears with an increase in temperature for the broad blue luminescence due to the thermal activation of the shallow level.

Thick homoepitaxial GaN with low carrier concentration for high blocking voltage

High voltage GaN Schottky diodes require a thick blocking layer with an exceptionally low carrier concentration. To this aim, a metal organic chemical vapor deposition process was developed to create a (14 μm) thick stress-free homoepitaxial GaN film. Low temperature photoluminescence measurements are consistent with low donor background and low concentration of deep compensating centers. Capacitance–voltage measurements performed at 30 °C verified a low level of about 2×10 15 cm −3 of n-type free carriers (unintentional doping), which enabled a breakdown voltage of about 500 V. A secondary ion mass spectrometry depth profile confirms the low concentration of background impurities and X-ray diffraction extracted a low dislocation density in the film. These results indicate that thick GaN films can be deposited with free carrier concentrations sufficiently low to enable high voltage rectifiers for power switching applications.

Novel approach to the fabrication of a strain- and crack-free GaN free-standing template: Self-separation assisted by the voids spontaneously formed during the transition in the preferred orientation

A c-oriented GaN film was grown on a r-plane sapphire substrate by utilizing hydride vapor phase epitaxy. This non-typical orientational relation between the film and the substrate was possible by spontaneous transition in preferred orientation, and voids were spontaneously formed in an orientation-transition layer. This c-oriented thick GaN film was then self-separated from the substrate during cool-down process to room temperature after the growth. This intriguing phenomenon of the self-separation of crack- and strain-free GaN is attributed to the spontaneous formation of voids during the preferred orientation-transition, and suggests a novel method to fabricate crack- and strain-free GaN free-standing templates.

Characterization of Thick GaN Films Directly Grown on Wet-Etching Patterned Sapphire by HVPE

Thick GaN films of high quality are directly grown on wet-etching patterned sapphire in a vertical hydride vapour phase epitaxy reactor. The optical and structural properties of GaN films are studied using scanning electronic microscopy and cathodoluminescence. Test results show that initial growth of hydride vapour phase epitaxy GaN occurs not only on the mesas but also on the two asymmetric sidewalls of the V-shaped grooves without selectivity. After the two-step coalescence near the interface, the GaN films near the surface keep on growing along the direction perpendicular to the long sidewall. Based on Raman results, GaN of the coalescence region in the grooves has the maximum residual stress and poor crystalline quality over the whole GaN film, and the coalescence process can release the stress. Therefore, stress-free thick GaN films are prepared with smooth and crack-free surfaces by this particular growth mode on wet-etching patterned sapphire substrates.

Effects of growth temperature modulated by HCl flow rate on the surface and crystal qualities of thick GaN by HVPE

We studied the influence of the growth temperature and HCl flow rate on the morphological evolution of crack-free thick GaN films by using a home-made horizontal hydride vapor phase epitaxy on sapphire substrates. Optical difference microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and cathodoluminescence (CL) were carried out to reveal the surface property of the GaN epilayer. It was found that the higher growth temperature is a key factor to obtain mirror, colorless and flat GaN surface. However, this key effect of temperature was modulated by HCl flow rate (HCl > 15 sccm). The surface RMS roughness was reduced from 206 to 2.51 nm for 10 μm × 10 μm scan area when GaN was grown at 1070 °C with HCl flow rate up to 30 sccm. These samples also reduced their (0 0 0 2) FWHM result from 1000 to 300 arcsec and showed a strong near-band-edge peak in CL spectra. Results indicated that growth temperature influence growth velocities on different crystalline planes, which will lead to the different morphologies obtained. High growth temperature can improve the lateral growth rate of vertical {1 1 − 2 0} facets and reduce the vertical growth rate of top {0 0 0 1} facet combined with higher HCl flow rate, which leads to completely coalescence of surface.

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 thanc-plane GaN. After indentation, lateral cracks emerge on the nonpolar GaN surface and preferentially propagate parallel to the 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 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.

Microstructural Analysis of Void Formation Due to a NH4Cl Layer for Self-Separation of GaN Thick Films

The role of a NH4Cl layer grown between GaN and a sapphire substrate for self-separation of GaN films was investigated in detail. Microstructural analysis of the NH4Cl layer reveals that self-separation of a GaN film was effectively assisted by the sub-micrometer-sized voids formed due to etching of GaN by evaporated NH4Cl. An areal fraction of nonetched GaN in contact with sapphire substrate was found to be linearly dependent on the thickness of NH4Cl layer and to have two different proportionality constants below and above an NH4Cl thickness at which GaN begins to be self-separated. This intriguing behavior was well explained by a simple model that takes account of void merging.

New design of nozzle structures and its effect on the surface and crystal qualities of thick GaN using a horizontal HVPE reactor

High-quality thick GaN films without cracks were achieved by using a new nozzle structure in the reactor grown by the hydride vapor phase epitaxy on sapphire substrates. Optical contrast microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray double diffraction (XRD) and cathodoluminescence (CL) were carried out to reveal the surface, crystal and optical properties of the GaN epilayer. It was found that the nozzle structure in the reactor has a large effect on the growth rate, surface flat, crystal quality, and the uniformity of the growth. Compared with the old one, the new nozzle structure (denoted as multi-layers nozzle) can improve dramatically the properties of thick GaN. Mirror, colorless and flat GaN thick film was obtained and its (0 0 0 2) FWHM results were reduced from 1000 to 300 arcsec when the new nozzle was used. AFM result revealed a step flow growth mode for GaN layer with the new nozzle. Room-temperature CL spectra on the GaN films showed a strong near-band-edge peak for the new nozzle, but there is only weak emitting peak for the old nozzle. New nozzle structure can improve the uniform of flow field near the surface of substrates compared with the old one, which leads to the improvement of properties of GaN thick film by hydride vapor phase epitaxy (HVPE).