Subsequent GaN Growth Research Articles

Effect of fully strained AlN nucleation layer on the AlN/SiC interface and subsequent GaN growth on 4H–SiC by MOVPE

AlN nucleation layers (NL) with different thickness were grown on 4H-SiC substrates using MOVPE. The growth evolution of the AlN layer on SiC was investigated. The effect of fully strained AlN nucleation layer (NL) on the AlN/SiC interface and on the subsequent GaN growth was also investigated. The thickness of the NL was varied between 10 nm and 100 nm in order to study the surface morphology and strain of AlN NL layer. The interface quality between AlN and SiC was characterized using HRXRD. It was observed that 100 nm NL was partially relaxed whereas 40 nm AlN NL was fully strained with improved SiC/AlN interface. GaN layer grown with fully strained NL showed improved surface morphology and lower screw dislocation density. 2DEG properties of AlGaN/GaN HEMT structures grown on fully strained and partially relaxed NL were found to be almost similar.

Polarity-inverted lateral overgrowth and selective wet-etching and regrowth (PILOSWER) of GaN

On an SiO2-patterned c-plane sapphire substrate, GaN domains were grown with their polarity controlled in accordance with the pattern. While N-polar GaN was grown on hexagonally arranged circular openings, Ga-polar GaN was laterally overgrown on mask regions due to polarity inversion occurring at the boundary of the circular openings. After etching of N-polar GaN on the circular openings by H3PO4, this template was coated with 40-nm Si by sputtering and was slightly etched by KOH. After slight etching, a thin layer of Si left on the circular openings of sapphire,but not on GaN, was oxidized during thermal annealing and served as a dielectric mask during subsequent regrowth. Thus, the subsequent growth of GaN was made only on the existing Ga-polar GaN domains, not on the circular openings of the sapphire substrate. Transmission electron microscopy analysis revealed no sign of threading dislocations in this film. This approach may help fabricating an unholed and merged GaN film physically attached to but epitaxially separated from the SiO2-patterned sapphire.

Kinetic surface roughening and wafer bow control in heteroepitaxial growth of 3C-SiC on Si(111) substrates.

A thin, chemically inert 3C-SiC layer between GaN and Si helps not only to avoid the “melt-back” effect, but also to inhibit the crack generation in the grown GaN layers. The quality of GaN layer is heavily dependent on the unique properties of the available 3C-SiC/Si templates. In this paper, the parameters influencing the roughness, crystalline quality, and wafer bow are investigated and engineered to obtain high quality, low roughness 3C-SiC/Si templates suitable for subsequent GaN growth and device processing. Kinetic surface roughening and SiC growth mechanisms, which depend on both deposition temperature and off-cut angle, are reported for heteroepitaxial growth of 3C-SiC on Si substrates. The narrower terrace width on 4° off-axis Si enhances the step-flow growth at 1200 °C, with the roughness of 3C-SiC remaining constant with increasing thickness, corresponding to a scaling exponent of zero. Crack-free 3C-SiC grown on 150-mm Si substrate with a wafer bow of less than 20 μm was achieved. Both concave and convex wafer bow can be obtained by in situ tuning of the deposited SiC layer thicknesses. The 3C-SiC grown on off-axis Si, compared to that grown on on-axis Si, has lower surface roughness, better crystallinity, and smaller bow magnitude.

Optimized Silicon Nanostructures Formed by One-Step Metal-Assisted Chemical Etching of Si(111) Wafers for GaN Deposition

Nanostructuring of silicon substrates has been used as a technique to reduce defects in epitaxial GaN growths on them. To fabricate the nanostructures, metal-assisted chemical etching (MACE) is a promising method. In particular, the one-step MACE (using silver nitrate and HF solution) is highly attractive, due to its simple and low cost fabrication method. Here we report experimental results of etched silicon, using different etching conditions, in order to obtain an optimized etching conditions. Process conditions of 5.0 M of HF and 0.02 M of AgNO3 at 25°C without stirring was found to be suitable for patterning silicon substrates for subsequent GaN growth. We have also evaluated the effect of the presence of dendrites on the etch rate of silicon and determined that the etch rate was enhanced by the coverage of silver dendrite. It was found that the etching mode changed from a short etching time regime (Regime I) to a long etching time regime (Regime II). Etch rate was increased by almost 80% when silver dendrites coverage was at 64%. Further increase in dendrites coverage did not increase the etch rate. Additionally, GaN has been successfully grown on substrates prepared by this method.

Less strained and more efficient GaN light-emitting diodes with embedded silica hollow nanospheres

Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources due to high efficiency and long lifetime. However, different material properties between GaN and sapphire cause several problems such as high defect density in GaN, serious wafer bowing, particularly in large-area wafers, and poor light extraction of GaN-based LEDs. Here, we suggest a new growth strategy for high efficiency LEDs by incorporating silica hollow nanospheres (S-HNS). In this strategy, S-HNSs were introduced as a monolayer on a sapphire substrate and the subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively for improved light extraction, and reduce wafer bowing due to partial alleviation of compressive stress in GaN. The incorporation of S-HNS into LEDs is thus quite advantageous in achieving high efficiency LEDs for solid-state lighting.

Surface Polarity Effects on the Hydride Vapor Phase Epitaxial Growth of GaN on 6H-SiC with a Chrome Nitride Buffer Layer

A CrN buffer layer was introduced to 6H-SiC substrate to aid the subsequent hydride vapor phase epitaxial growth of GaN. Crystallinities of GaN layers grown on both Si- and C-terminated surfaces of 6H-SiC wafers with and without CrN layers were compared. On Si-terminated SiC, CrN formed with a trigonal morphology, indicating a good crystallographic relationship with growth along the 111 axis. CrN formed randomly on C-terminated SiC, leading to the subsequent GaN growth occurring with no preferred orientation, suggesting deteriorated crystallinity. High-quality GaN layers were grown on the CrN buffered Si-terminated SiC.

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.

(Invited) The Resurgence of III-N Materials Development: AlInN HEMTs and GaN-on-Si

Heterostructure devices based on the AlInN material system have demonstrated unprecedented high frequency performance but are still limited by materials issues. Likewise, improved crystal growth schemes are envisioned as a key component in the realization of GaN-on-Si high voltage devices for power electronics applications. This work presents materials optimization results from MOCVD growth of quaternary barrier AlInGaN/AlN/GaN HEMTs formed on sapphire and SiC substrates and also a study of AlN nucleation and subsequent GaN growth for crack-free films on silicon (111).

GaN vertical and lateral polarity heterostructures on GaN substrates

A method to control local GaN polarity on GaN substrates without using Mg-induced inversion has been developed. This method rests on the development of thin alternative inversion layers and the use of the confined epitaxial growth technique to pattern and selectively grow the inversion layer. Used in conjunction with optimized subsequent GaN growth processes, the technique has resulted in structures demonstrating both vertical and lateral polarity inversion on GaN substrates. Scanning electron microscopy shows a smooth interface between regions of different polarities and a smooth surface, the result of equivalent growth rates for both polarities. Electron channeling contrast imaging is used to determine the dislocation density of each region. Transmission electron microscopy (TEM) images show full polarity conversion of the inverted layer. This process enables a wide range of novel variable polarity devices.

Multi-Wavelength Emitting InGaN/GaN Quantum Well Grown on V-Shaped GaN(1101) Microfacet

InGaN/GaN multiple quantum wells (MQWs) were successfully grown on the inclined GaN(1101) microfacets. Conventional photolithography and subsequent growth of GaN were employed to generate the V-shaped microfacets along (1120) direction. The well-developed microfacets observed by scanning electron microscopy and the clear transmission electron microscope interfacial images indicated that the MQW was successfully grown on the GaN microfacets. Interestingly, cathodoluminescence (CL) spectra measured on the microfacets showed a continuous change in the luminescence peak positions. The CL peaks were shifted to a longer wavelength from 420 nm to 440 nm as the probing points were changed along upward direction. This could be attributed to the nonuniform distribution of the In composition and/or the wavefunction overlapping between adjacent wells. Present works thus propose a novel route to fabricate a monolithic white light emitting diode without phosphors by growing the InGaN/GaN MQWs on (1101) facet.

Electron Microscopy Investigation of the Role of Surface Steps in the Generation of Dislocations during MOCVD Growth of GaN on 4H-SiC

Through the use of specially-prepared on-axis SiC substrates with patterned mesa tops completely free of atomic-scale surface steps, we have previously reported the growth of highquality GaN heteroepitaxial films with greatly reduced threading dislocation densities on the order of 107/cm2. In these films, we reported a defect substructure in which lateral a-type dislocations are present in the nucleation layer but do not bow into threading dislocations during the subsequent GaN growth. This study focuses further on the role of SiC substrate surface steps in the generation of misfit, a-type, and threading dislocations at the heteroepitaxial interface. By using weak-beam imaging (both to eliminate Moiré effects and to observe narrow dislocation images) from plan-view transmission electron microscopy (TEM), we identify dislocations generated on stepped and unstepped mesas and compare their geometries. We observe that misfit dislocations nucleated on an unstepped SiC mesa are confined to one set of a-type Burgers vectors of the form g=1/3 [2110] _ _ , straight and well-ordered so that they are less likely to interact with each other. On the other hand, misfit dislocation structures on a stepped SiC mesa surface are not nearly as well-ordered, having bowed structure with threading dislocations that appear to nucleate at SiC surface steps.

InGaN quantum well epilayers morphological evolution under a wide range of MOCVD growth parameter sets

AbstractThis study exemplifies the use of TappingModeTM atomic force microscopy (AFM) surface morphology imaging to investigate and optimise the metalorganic chemical vapour deposition (MOCVD) growth conditions and post‐growth stability of thin (<40 Å) InGaN layers with direct implications to the structural and optical properties of blue (460 nm) and green (520 nm) LEDs. InGaN epilayers less than 40 Å thick of ∼20% solid phase indium were produced on thick (3‐4 µm) 2″ GaN templates grown on (0001) c ‐plane sapphire substrates. The morphological evolution of the InGaN material was studied utilising a DI3100 AFM tool. Surface morphology and its correlation with photoluminescence and X‐ray diffraction results will be discussed for every set of conditions employed. More specifically, the post‐growth ambient exposure and thermal stability of the uncapped InGaN epilayers were investigated. In addition, the initial stage of subsequent GaN growth, which is an essential step towards the manufacture of LED active regions, was examined. Based on the above findings, a flexible MOCVD growth parameter space and improved LED constituent layer sequencing techniques have been established leading to more efficient and stable LED devices. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN full-vertical p-i-n rectifiers employing AlGaN:Si conducting buffer layers on n-SiC substrates

The development of a full-vertical GaN p-i-n rectifier on a 6H n-type SiC substrate by employing a conducting AlxGa1−xN:Si (x=∼0.1) buffer layer scheme is reported. In this vertical configuration, the n contact is made on the backside of the SiC substrate using a Ni∕Au metallization scheme. Epitaxial layers are grown by low-pressure metal organic chemical vapor deposition. The AlxGa1−xN:Si nucleation layer is proven to provide excellent electrical properties while also acting as a good buffer layer for subsequent GaN growth. The reverse breakdown voltage for a relatively thin 2.5μm thick i region was found to be over −330V. The devices also show a low on resistance of Ron of 7.5×10−3Ωcm2. This full-vertical configuration provides the advantage of the reduction of sidewall damage from plasma etching and lower forward resistance due to the reduction of current crowding in the bottom n-type layer.

V-shaped pits formed at the GaN/AlN interface

A high-quality GaN layer has been achieved using an AlN buffer layer directly grown on a sapphire substrate at high temperature. The microstructure was investigated by high-resolution transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX). A high density of V-shaped pits, with sizes of 20–40 nm, was observed at the interface between AlN and GaN layers. Unlike the previously reported V-shaped defects in nitride-based ternary alloys, these V-shaped pits were formed on top of the AlN layer due to H 2 etching of the surface during a high-temperature growth interruption between AlN and GaN layers. The pits were preferentially formed around the surface terminations of threading dislocations (TDs) in the AlN, similar to chemical etching-induced surface pits. The TD density of the AlN layer is generally very high, leading to a high density of V-shaped pits. Subsequent growth of GaN resulted in the bending over of the pre-existing TDs, significantly reducing the threading dislocation density in the overlaying GaN. The high crystal quality is further proved by highly improved photoluminescence intensity, compared to the case of conventional GaN.

Integration of CdSe quantum dots with GaN optoelectronic materials

Material structures on the nanoscale can enable enhancement of optoelectronic device performance. For example, in the InGaN active layers of MOCVD grown blue light emitting diodes, indium segregation plays a critical role in the interplay between blue luminescent channels and non-radiative recombination centers such as crystal defects. Unfortunately, high efficiency luminescence of InGaN does not extend into the “deep green” spectral region, around the wavelength of peak human eye response. We are investigating whether commercially available luminescent nanostructures such as CdSe quantum dots can be incorporated into III-nitride devices to extend their high-efficiency performance into the “deep green”. Surfactant stabilized CdSe particles in liquid dispersions are drop-cast onto HVPE grown GaN. Physical properties of resultant CdSe surface structures are examined. Luminescence is reported before and after subsequent growth of GaN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Progress Toward Making Gallium Nitride Seed Crystals Using Hydride Vapor-Phase Epitaxy

The essential steps required to create GaN seed crystals for bulk crystal growth from thick films are described. These steps include the growth of low-dislocation-density GaN films using a hydride vapor-phase epitaxy route, the separation of the films from the sapphire substrate on which they are grown, and the subsequent growth of GaN on both sides of the free-standing films. These steps are illustrated with examples that emphasize the growth of material with low threading dislocations.

Inversion domains and pinholes in GaN grown over Si(111)

High-resolution transmission electron microscopy is used to analyze the connection between pinholes and inversion domains at the AlN/GaN interface. From some pinholes on top of the AlN buffer layer, the subsequent growth of GaN was observed to lead to the formation of inversion domains. In addition, the pinhole area which was originally the termination of defects from the buffer layer is highly strained and its boundary facets in {112̄2} and {112̄3} atomic planes. Inside the GaN the inversion domain boundary quickly settles to the usual {101̄0} planes.

Hydride vapor phase epitaxy growth of GaN on sapphire with ZnO buffer layers

The initial stages and subsequent growth of GaN on sapphire using ZnO buffer layers is reported for the hydride vapor phase epitaxy technique. A high gas-phase supersaturation in the growth ambient was used to favor a rapid initial growth on the substrate. A subsequent growth step was employed under conditions that favor a high lateral growth rate in order to promote the coalescence of the initial islands and provide optimal material properties. The specific gas-phase mole fractions of the GaCl and NH3 at the growth front control both the vertical and lateral growth rates. The use of a two-step growth process in the GaN growth leads to a controlled morphology and improved material properties for GaN materials when grown with a ZnO buffer layer. An optimized set of growth conditions, utilizing this two-step process, was found to also improve the growth directly on sapphire without a ZnO buffer layer.

Buffer free direct growth of GaN on 6H–SiC by metalorganic vapor phase epitaxy

The growth of GaN on 6H–SiC is three dimensional (3D) and results in the formation of large islands presenting hexagonal truncated shape with {1–101} lateral facets and a top {0001} facet. In this work, we present a three steps growth process that enables us to grow high quality mirrorlike GaN layers without using AlN buffer layers. During a first step, a thin 3D GaN layer is deposited at high temperature. This layer is smoothed under ammonia flow for several minutes when the growth is interrupted. The subsequent growth of GaN is two dimensional. 600 nm thick GaN films were grown. They were analyzed by high resolution x-ray diffraction, reflectivity, and photoluminescence. All the layers are under strong tensile biaxial strain. The correlation between residual tensile strain in GaN layers and their optical properties is reported for biaxial deformations εxx ranging up to 0.37%.

Metal Organic Vapour Phase Epitaxy (MOVPE) Growth of GaN(n)/SiC(p) Heterostructures

GaN epilayers about 600 nm thick were grown directly on 6H-SiC by metal-organic vapour phase epitaxy (MOVPE) with a three-step process. A thin GaN nucleation layer grown at 1080 °C, initially 3D, is smoothed in several minutes under ammonia flux when the growth is interrupted. A flat layer is obtained through a large mass transport mechanism. The subsequent growth of GaN is 2D and 600 nm thick GaN films were grown on both on-axis and 3.5° off substrates. The structural properties of the samples were characterised by High Resolution X-Ray Diffraction (HRXRD) and Transmission Electron Microscopy (TEM). The full width at half maximum (fwhm) in ω-scan rocking curve of the (0002) and (1-015) lines of GaN are 124 and 188 arcsec, respectively. The defect structure of the GaN film away from the substrate consists mostly of threading dislocations with a density of 3 × 109 cm—2 and of (112-0) prismatic defects.