Intermediate GaN Research Articles

Ammonia Plasma-Catalytic Synthesis Using Low Melting Point Alloys

The Haber-Bosch process has been the commercial benchmark process for ammonia synthesis for more than a century. Plasma-catalytic synthesis for ammonia production is theorized to have a great potential for being a greener alternative to the Haber-Bosch process. However, the underlying reactions for ammonia synthesis still require some detailed study especially for radiofrequency plasmas. Herein, the use of inductively coupled radiofrequency plasma for the synthesis of ammonia when employing Ga, In and their alloys as catalysts is presented. The plasma is characterized using emission spectroscopy and the surface of catalysts using Scanning Electron Microscope. A maximum energy yield of 0.31 g-NH3/kWh and energy cost of 196 MJ/mol is achieved with Ga-In (0.6:0.4 and 0.2:0.8) alloy at 50 W plasma power. Granular nodes are observed on the surface of catalysts indicating the formation of the intermediate GaN.

Surface morphology improvement of homoepitaxial GaN grown on free-standing GaN substrate by metalorganic chemical vapor deposition

Free-standing GaN is generally regarded as an ideal substrate for GaN-based devices due to its advantage of low threading dislocation density (TDD) and good thermal conductivity. However, new surface features such as hillocks and ridges appear on the GaN homoepitaxy films. In this paper, the influences of the intermediate GaN (IM-GaN) layer on the surface defects and crystal quality of GaN homoepitaxy films grown on c-plane GaN substrates by metalorganic chemical vapor deposition are investigated. It is found that hexagonal hillocks and ridges on the surface can be avoided by inserting an IM-GaN layer grown at an intermediate temperature (650850℃), prior to the growth of GaN at 1050℃. The results based on X-ray diffraction (XRD) measurements and differential interference contrast microscopy images demonstrate that the growth temperature of the IM-GaN layer has a significant influence on GaN homoepitaxy layer, which is one of the most critical parameters determining the surface morphology and crystal quality. As the IM-GaN growth temperature decreases from 1050℃ to 650℃, thed densities of hillocks and ridges on the surface reduce gradually. While, the XRD full width at half maximum (FWHM) values of (002) and (102) peaks for the homoepitaxy films are increased rapidly, indicating the adding of the TDD in the films. The atomic force microscopy (AFM) images show that the quasi-step growth mode change into layer-layer growth mode with the growth temperature decreasing from 1050℃ to 650℃ during the IM-GaN layer growing. It is speculated that the growth mode is determined by the diffusion length of adatom on the growing surface, which is proportional to the growth temperature. In the case of IM-GaN grown at low temperature, the formation of hillocks can be suppressed by reducing the adatom diffusion length. Finally, High crystal quality GaN homoepitaxy films (2 m) without hillocks is achieved by optimizing the growth parameters of IM-GaN layer, which is about 150 nm in thickness and grown at 850℃. The crystal quality of GaN homoepitaxy film is assessed by XRD rocking curve measured with double-crystal optics. The FWHMs of the (002) and (102) peaks are 125arcsec and 85arcsec respectively, indicating that rather low TDD is formed in the film. And well defined steps are observed on the image of AFM test, the root-mean square roughness value of the which is only about 0.23 nm for 5 m5 m scan area.

Composition, structure and morphology of Al1−xInxN thin films grown on Al1−yGayN templates with different GaN contents

Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1−yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of ~15% was chosen to achieve close lattice matching of Al1−xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1−yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1−xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.

UGaN growth on laser patterned sapphire substrate: quality feature

AbstractLaser patterning technique as a flexible tool to grow high quality undoped gallium nitride (uGaN) on sapphire wafers is presented. A series of stripes were formed on sapphire wafer surface using Nd:YAG laser (100 kHz, 266 nm, 30 ps pulse) with variable pulse power (40‐670 mW), laser spot velocity over the wafer (1‐46 mm/s) and distance between the stripes (4‐15 μm). Stripes width was ∼2‐5 μm. After laser patterning sapphire wafers were cleaned using aqua regia and etched using sulphuric and phosphoric acid solution. SEM images after wet chemical etching show less surface roughness and open sapphire crystalline planes compared to surfaces treated with laser. Undoped GaN was grown by metal‐organic chemical vapour deposition (MOCVD). Used III/IV ratio: 2400, reactor pressure: 150 mBar, temperature: 1065 °C. SEM and AFM results show reduction of dislocation density in grown gallium nitride layer. The growing of this layer was performed using epitaxial lateral overgrowth (ELO) method on laser patterned sapphire substrates. Our observation revealed that the optimal laser power for sapphire substrate patterning for ELO GaN growth is 70‐300 mW. Moreover, we demonstrated intermediate GaN lateral growth states. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis of InN nanowires grown on droplets formed with Au and self-catalyst on Si(111) by using metalorganic chemical vapor deposition

We demonstrated the growth of indium nitride (InN) nanowires on Si(111) substrates by metalorganic chemical vapor deposition without the use of any intermediate GaN or AlN buffer layer. The InN nanowires were grown by forming the Au + In droplets and In droplets on the Au- and In-coated Si substrate. The growth conditions such as chamber pressure, chamber temperature, reaction gas flow rate, and carrier gas flow rate were optimized to yield nanowires free from contamination. Depending on the growth parameters different growth regimes for the InN nanowires were identified. The strength of self-catalytic route has been highlighted. The morphology and microstructures of samples were characterized by x-ray diffraction and scanning electron microscopy (SEM). The transmission electron microscopy and SEM investigations showed that the InN nanowires are single crystals with diameters ranging from 40 to 400 nm, and lengths up to 3 µm. Photoluminescence spectra of the InN nanowires showed a strong broad emission peak at 0.77 eV.

Spontaneous growth of III-nitride nanowires on Si by molecular beam epitaxy

In this paper, we report on the catalyst-free growth and the optoelectronic properties of GaN and InN nanowires (NWs) grown on Si(1 1 1) substrates by nitrogen RF plasma source molecular beam epitaxy (RF-MBE) without the use of intermediate GaN or AlN buffer layer. The growth conditions were optimized in order to fabricate well-aligned, stress-free single crystalline nanowires. In both cases, the III-nitride NWs were generated from the lattice mismatch strain of the materials on Si and the high surface energy of their nitrogen stabilized surfaces. Both PL and Raman spectroscopy revealed that the NWs were fully relaxed. XRD results further assert the single crystallinity of the nanowires.

Growth of InN on Ge(1 1 1) by molecular beam epitaxy using a GaN buffer

Many novel applications are foreseen for the InN-containing materials especially when cheap, conductive substrates can be used. In this paper, we report on the growth of pure InN layers on germanium (Ge) (1 1 1) substrates. We found that high-quality InN can be grown on Ge(1 1 1) with plasma-assisted molecular beam epitaxy when using a thin GaN intermediate layer. On such intermediate GaN layers, 50 nm InN layers were grown and analyzed by RHEED, XRD, AFM, Hall and I– V measurements. Additionally, using ellipsometry we could determine that the optical bandgap of these InN layers lies around 0.85 eV. Our results indicate that Ge(1 1 1) is a promising substrate for vertical conducting devices, with In(Ga)N on top.

Specific features of electrical properties of the InxGa1 − x N alloy

Temperature dependences of the Hall coefficient and resistivity of the InxGa1 − x N alloys (0 ≤ x ≤ 1) are investigated. It is found that, at x ≤ 0.4, the temperature dependences of the Hall coefficient and resistivity have the activation-related portion. The activation energy depends linearly on the In content in the alloy. At x ≈0.5, the activation portion vanishes. The main scattering mechanism depends on the temperature, on the defect density in the film (this density is largely determined by the used intermediate GaN layers), and on the alloy composition x.

Thick GaN Growth on GaAs(111) Substratres at 1000 °C with HVPE

It was demonstrated that GaAs substrates can withstand GaN halide vapor phase epitaxy (HVPE) at 1000 °C by introducing an intermediate GaN layer grown at around 850 °C. Even if the surface of the GaN layer grown at 850 °C was rough, it became smooth when GaN was grown on it at 1000 °C by optimizing the growth conditions, though there were sometimes several hexagonal pits on the surface. Some surface flattening mechanism seems to exist for the hexagonal GaN growth at higher temperatures than 1000 °C. The θ–2θ X-ray diffraction (XRD) of the grown layer showed only the hexagonal (0002) peak.

Mechanisms of Strain Reduction in GaN and AlGaN/GaN Epitaxial Layers

ABSTRACTWe have investigated Al0.12Ga0.88N layers with and without a 1 µm GaN buffer, grown on the c-face of α – Al2O3 substrate with an intermediate AlN nucleation layer grown by LP-MOVPE. We used spatially resolved cathodoluminescence spectroscopy at a temperature of 8K to investigate the strain and the homogeneity of composition that can be determined from the energy of the luminescence peak. The larger thermal expansion coefficient of the sapphire in comparison to the nitrides leads to a biaxial compressive strain of the upper GaN layer when cooling down from growth temperature. For AlGaN layers directly grown on the nucleation layer this cannot be confirmed. The layer stays relaxed and fluctuations in the aluminium composition of 0.4% can be observed. When growing an intermediate GaN buffer, the AlGaN layer gets tensilely strained. This strain is of elastic nature and microcracks can be observed preferentially at the edges due to the smaller lattice constant of AlGaN in comparison to GaN. Even detaching of the AlGaN layers grown on the buffer can be observed. In the regions without cracks the layers are quite homogeneous. A deformation potential of (19±4)eV was estimated for Al0.12Ga0.88N.