Initial GaN Growth Research Articles

Growth of thin AlN nucleation layer and its impact on GaN-on-SiC heteroepitaxy

Abstract AlN nucleation layer (NL) with thickness of 35 nm was grown on semi-insulating 4H–SiC substrate by metal organic chemical vapor deposition (MOCVD). Focused on the evolutions of the surface microstructural morphology and crystalline quality, the growth mechanism of the thin NL was studied by series of growth procedures. As interval supply of ammonia (NH3) and alternative supply of Trimethylaluminum (TMA)/NH3 are applied to NL growth, complete coalescence of AlN nucleate islands with a smooth surface and low density of defects is obtained. In addition, the impact of the thin NL on the subsequent GaN buffer layer (BL) was also discussed. Overgrown on the well-coalesced NL, surface roughening and three-dimensional mode get predominant at the initial GaN growth, which facilitates high crystalline quality of GaN BL with low wafer bow.

Growth of AlGaN/InGaN/GaN Heterostructure on AlN Template/Sapphire

GaN films are grown directly on AlN templates/sapphire substrates without using low‐temperature (LT) buffer layers by metalorganic chemical vapor deposition. AlN templates are complimentarily deformed at the initial growth of GaN, adjusting the a‐lattice constant and tilting crystal orientation slightly. Compared with the film on sapphire substrates using an LT buffer layer, the GaN on the AlN template forms a smoother surface and has better crystalline quality after a shorter growth time at a lower temperature. Higher‐quality InGaN films on the GaN/AlN template are subsequently grown to optimize the thickness exhibiting the minimum Urbach energy which is evaluated by photothermal deflection spectroscopy. A high‐quality AlGaN/InGaN heterostructure is fabricated in which the Shubnikov–de Haas oscillation can be clearly observed from the electron gas of the InGaN channel at the interface.

Stress control and dislocation reduction in the initial growth of GaN on Si (111) substrates by using a thin GaN transition layer

A thin GaN transition layer technology is proposed to control stress and reduce the dislocation density in the initial GaN growth.

Ultraviolet and visible micro‐Raman and micro‐photoluminescence spectroscopy investigations of stress on a 75‐mm GaN‐on‐diamond wafer

Investigations of stress distributions and material quality across a 75‐mm wafer consisting of device‐quality GaN integrated with a diamond substrate are presented. Stress in the GaN are mapped both over the full wafer and across the layer along the growth direction. Ultraviolet (UV) and visible micro‐Raman and UV photoluminescence (PL) spectroscopy from both sides of the wafer reveal an unexpected gradient between the tensile stress at the free GaN surface (∼0.86–0.90 GPa) and the GaN/diamond interface (∼0.05–0.23 GPa). The stresses obtained exhibit good cross‐wafer uniformity. The stress gradient is understood through variations in the material along the growth direction of the layers due to the presence of threading dislocations which result in local stress relaxation. Transmission electron microscopy confirms the presence of extended defects to be greater near the interface with diamond, corresponding to the initial GaN growth regime, and diminished toward the surface where transistors would be fabricated in a full device technology. Finite element (FE) simulations describing the observed stress dependence along with TEM imaging of the GaN cross‐section support the relaxation interpretation.

Three-dimensionally-architectured GaN light emitting crystals

We demonstrate the epitaxial growth of three-dimensional (3D) GaN single crystal arrays through metal–organic vapor phase epitaxy (MOVPE) on lattice-matched ZnO templates that were achieved via hydrothermal growth in which we controlled the position, size and morphology of the layer. To prevent collapse of the crystals during MOVPE growth, graphene sheets were employed as a protection/mask layer, and initial GaN growth was performed at a relatively low temperature under hydrogen-depleted conditions. Temperature-dependent growth behaviors of GaN crystals on diverse types of ZnO templates can facilitate the control of hexagonal crystal shapes including pencil, tent and plate shapes. Furthermore, a 3D light emitting crystal array was demonstrated through subsequent growth of high-quality n-type and p-type GaN layers.

Effect of N2 microplasma treatment on initial growth of GaN by metal–organic molecular beam epitaxy

N2 atmospheric microplasma was applied to improve the yields and reproducibility of the initial growth of GaN by metal–organic molecular beam epitaxy (MOMBE). The plasma treatment was found to be effective in cleaning the surface, and excellent flat growth was achieved even in the early stage of the growth. The effect of the air exposure after plasma treatment was also studied, and the yield of the growth was found to be largely decreased by the air exposure even after the treatment. Therefore, the oxidation of the substrate is one of main causes of the poor initial growth and the installation of the microplasma equipment in the MBE loading chamber is useful for suppressing the oxidation after the treatment. Atomic force microscopy (AFM) measurement shows that the microplasma treatment is also effective for undoing the surface double steps through etching, which is helpful for a very smooth layer-by-layer growth in the early stage of growth.

Interruption-free growth of 10 μm-thick GaN film prepared on sputtered AlN/PSS template by hydride vapor phase epitaxy

GaN films (10μm-thick) of high crystalline quality were prepared on sputtered AlN/PSS template by hydride vapor phase epitaxy (HVPE). By introducing the two-step growth method into HVPE, one can reduce the steps in the procedure, realize uninterrupted growth, and improve the crystal quality of the films. The effects of initial GaN growth on the AlN/PSS template by HVPE were also investigated.In this study, 10μm-thick GaN films prepared on sputtered AlN/PSS template by HVPE showed improved crystal quality using X-ray diffraction and etching pits density. Compared with conventional undoped GaN film grown by metal organic chemical vapor deposition, the full width at half maximum of the (002) and (102) peaks of GaN decreased from 450arcsec to 290arcsec and from 376arcsec to 344arcsec, respectively. Transmission electron microscopy results showed that the gaps observed between the convex regions would eventually turn into dislocations during coalescence, because the number of dislocations increased with the number of gaps observed between the convex regions after step-1 growth.

MOVPE of III-N LED structures with short technological process

The paper presents results on optimizing the MOVPE technology of light-emitting diode (LED) structures in a Dragon-125 system to accelerate the technological cycle. Due to the high growth rate of GaN layers and optimization of the initial GaN growth phase, the total duration of the epitaxial process is reduced from 4 h 45 min to 2 h 44 min. The LED diode structures grown by this technique compare well in quality with LEDs grown by the standard method in the commercially available AIX2000HT system.

Effects of initial GaN growth mode on the material and electrical properties of AlGaN/GaN high-electron-mobility transistors

AlGaN/GaN high-electron-mobility transistors (HEMTs) with different initial GaN growth modes were prepared on AlN/sapphire substrates. Secondary ion mass spectroscopy and Hall effect measurements confirmed that a highly conductive n-type region was generated at the GaN/AlN buffer interface during the three-dimensional to two-dimensional (3D–2D) growth mode transition. This n-type region was created by oxygen impurity incorporation during the 3D–2D growth mode transition, and its thickness increased with the transition time. The existence of this n-type conduction path not only changed the material properties but also degraded the device performance of HEMTs.

MOVPE growth of GaN on Si substrate with 3C-SiC buffer layer

We investigated the effect of the thickness of a 3C-SiC buffer layer on the growth of GaN on a Si substrate. GaN samples with thicknesses of 2.0 and 4.5 µm were grown by metal organic vapor phase epitaxy. Islands were observed at the initial growth of the GaN samples, and they became larger and then coalesced with each other with increasing growth time. The crystalline quality of the GaN samples was affected by the thickness of the 3C-SiC buffer layer and was improved by increasing their thickness. This indicates that the crystalline quality trend for thick GaN growth is different from that for initial GaN growth. Moreover, the GaN sample with a 100-nm-thick SiC buffer layer had a low initial curvature, and the crystalline qualities on GaN(0004) and planes were 389 and 460 arcsec for 4.5-µm-thick GaN growth, respectively.

Interface science of virtual GaN substrates on Si(111) via Sc2O3/Y2O3 buffers: Experiment and theory

The final film quality of GaN on foreign substrates is known to crucially depend on the initial GaN interface and nucleation characteristics. To shed light on these characteristics of recently pioneered virtual, hexagonal GaN(0001) substrates on Si(111) via step graded Sc2O3(111)/Y2O3(111) buffers, a complex GaN(0001)/Sc2O3(111) interface structure model and the initial nucleation scenario is derived from a combined experimental (reflection high energy electron diffraction and X-ray photoelectron spectroscopy) and theoretical ab initio study. It is shown that the GaN/Sc2O3 interface chemistry is determined by a N-Ga-O-Sc atomic arrangement leading to N-polar GaN films. However, the atomic GaN(0001)/Sc2O3(111) interface configuration is complex and local perturbations might be at the origin of Ga-polar inversion domains in the mainly N-polar GaN films. The initial growth of GaN on Sc2O3 is characterized by an ultrathin N-Ga-O-Sc wetting layer which carries tensile strain and relaxes with increasing thickness. Further GaN deposition results in the formation of 3D islands which fully relax before island coalescence occurs. The implications of the GaN/Sc2O3 interface configuration, the 3D nucleation growth mode, and the coalescence process of misaligned islands are discussed with respect to the defect characteristics (inversion domains, cubic inclusions, threading dislocations) of the final GaN layer.

Effects of Initial GaN Growth Mode on Patterned Sapphire on the Opto-Electrical Characteristics of GaN-Based Light-Emitting Diodes

We have studied the initial growth modes of GaN on patterned sapphire substrate (PSS) with different initial TMGa flow rates. The FWHM of the (102) XRD spectrum of GaN on PSS increased from 470 to 580 arcsec when the initial TMGa flow rate was increased from 80 to 200 sccm. A low TMGa flow rate sufficiently suppresses GaN island growth on the top of the pattern and hence improves GaN crystal quality. The electrical and optical characteristics of GaN-based LEDs on PSS with low initial TMGa were also improved. More than 90% of the GaN LED chips with low initial GaN growth rate can hold the 1-kV machine-mode electrostatic discharge level.

Structural evolution of AlN buffer and crystal quality of GaN films on a‐ and c‐sapphire grown by metalorganic vapor phase epitaxy

AbstractLow‐temperature‐deposited (LT‐) AlN layers and the initial growth of GaN on a‐ and c‐sapphire have been investigated and compared. The as‐deposited LT‐AlN layers on both a‐ and c‐sapphire are composed of high‐density islands. By annealing, the islands become large. The density of islands on a‐ and c‐sapphire are 5.6 × 1010 and 7.9 × 1010 cm‐2, respectively, indicating that the evolution of the solid phase epitaxy of AlN islands induced by annealing is different on a‐ and c‐sapphire. The moiré spacings obtained by plan‐view electron microscopy reveal that the strain of the AlN islands on a‐sapphire is larger than that of islands on c‐sapphire. These results imply that development of the solid phase epitaxy is influenced by the strain. The coalescence of GaN islands grown on a‐sapphire, which has larger AlN islands, is advanced compared with that of islands on c‐sapphire. The relationship between the annealed AlN island density and the threading dislocation (TD) density in the GaN films is investigated on a‐ and c‐sapphire. The TD density of the GaN films is small when the density of annealed AlN islands is low. These results indicate that decreasing the island density of the annealed AlN layers is essential for improving the crystalline quality of GaN films. It is therefore important to control the solid phase epitaxy of LT‐AlN layers by annealing. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement of structural quality in the initial stage of GaN growth by basic ammonothermal method

Study on initial growth of GaN revealed evolutional improvement of grain structure in the initial stage of ammonothermal growth. GaN crystals grown on N-polar surface of GaN platelets for various growth durations were characterized with the X-ray diffraction and Nomarski microscopy. The full width half maximum (FWHM) of X-ray rocking curve from 002 reflection became larger than that of a seed crystal at approximately 100 μm, followed by improvement in thickness larger than approximately 200 μm. On the other hand, FWHM of X-ray rocking curve from 201 reflection did not change. The Nomarski microscope showed larger hillocks for thicker layer indicating coalescence of grains. These results imply that GaN crystal quality in the initial stage was improved through reduction of c-axis tilting associated with coalescence of grains in the grown layer.

Influence of native silicon oxides on the growth of GaN nanorods on Si(001)

AbstractInfluence of native Si oxides on the growth of GaN nanorods on Si(001) substrates have been investigated. At the initial stage of GaN growth on clean Si(001) substrates at 450 °C, (2x1) reflection high‐energy electron diffraction patterns immediately change into halo patterns, indicating growth of amorphous materials. The cross‐sectional transmission microscopy observation and secondary ion mass spectroscopy measurement reveal that there exists an amorphous Six Ny layer at the GaN/Si interface. This provides the evidence that during the initial growth of GaN on clean Si substrates with out native oxides, nitridation of Si takes place to form amorphous Six Ny as well as GaN. The structural properties and surface morphologies of these interfacial layers affect GaN nanorod growth. By contrast, native oxides act as a mask against nitridation of Si substrates. Since plasma‐activated nitrogen mainly reacts with Ga to form GaN, GaN growth always proceeds under a desired N‐rich growth condition. As a result, GaN nanorods are uniformly grown on Si(001) with native oxides. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ab initio calculation for an initial growth process of GaN on (0001) and (000$ \bar 1 $) surfaces by vapor phase epitaxy

AbstractThe adsorption processes of group‐III and group‐V sources used in metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) were investigated using ab initio calculations based on the density functional theory (DFT) to understand the initial growth process of GaN on GaN(0001) and (000$ \bar 1 $) substrates. In order to understand the influence from the decomposition of ammonia, NH2, NH and N were considered as adsorption species. We assume group‐V source preflow in the case of GaN(0001) and group‐III preflow in the case of GaN(000$ \bar 1 $). From the calculations, the initial GaN growth process in the vapor phase epitaxy is clarified. When GaN is grown on the GaN(0001) substrate, NH3 molecules adsorb to the on‐top site or NH2 molecules adsorb to the bridge site. Also, it was found that the surface structures with both NH2 and NH3 are stable under a hydrogen atmosphere. When GaN is grown on the GaN(000$ \bar 1 $) substrate, Ga atoms adsorb to the H3 site in the case of MOVPE and GaCl molecules adsorb to the H3 site with the Ga atom facing the surface in the case of HVPE. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Initial Growth Mode of GaN Film on Stepped Sapphire

The initial GaN growth mode on stepped sapphires by plasma enhanced metal organic molecular beam epitaxy (PEMOMBE) has been analyzed using in-situ, real time synchrotron x-ray diffraction and x-ray absorption. The sapphire substrate annealed at high temperature had flat terraces and regular atomic steps. The crystal quality and the vicinal angle of sapphire substrate had an effect on the width of terraces and the step arrangement. The initial growth mode of the GaN film on the regular atomic step (AS) surface was the layer-by-layer mode and changed to the 3D growth mode within 2 bilayer thickness. In the meanwhile, the growth mode of the GaN film grown on the sapphire with random roughness (RR) surface made the flat surface in the early stage and changed the 3D growth mode. As increasing the film thickness, the nucleation layer grows strain-free hexagonal GaN on stepped sapphires

Atomistic study of GaN surface grown on Si(111)

GaN is directly grown on Si(111) by radio-frequency plasma-assisted molecular-beam epitaxy, and the surface is studied using in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). By optimizing the growth condition, well-defined surface reconstructions are observed in atomically-resolved STM images after the additional Ga deposition, indicating the uniform N-polarity of the grown film. We show that N-rich condition in the initial GaN growth and slightly Ga-rich condition in the subsequent growth are critical in order to achieve monopolar uniform GaN films.

Photo-electron emission and atomic force microscopies of the hydrogen etched 6H-SiC(0 0 0 1) surface and the initial growth of GaN and AlN

Photo-emission electron microscopy (PEEM) and atomic force microscopy (AFM) have been used to characterize the surfaces of hydrogen etched 6H-SiC(0 0 0 1) wafers and the microstructure of the initial stages of growth of GaN and AlN on these surfaces via molecular beam epitaxy. The PEEM images were obtained using a free electron laser as the photon source. A stepped structure was evident in these images of the surfaces etched at 1600–1700 °C for 15 min. Comparison with the AFM images revealed that emission was occurring from the intersection of the steps and the terraces. Images of the initial stages of deposition of the GaN thin films at 700 and 800 °C revealed three-dimensional island growth. The degree of coalescence of these films was dependent upon the step structure: regions containing steps having unit cell height exhibited complete or nearly complete coalescence; regions containing steps with half unit cell height showed voids in the films parallel to the steps. PEEM of the initial stages of growth of AlN revealed immediate nucleation and rapid coalescence during deposition at 900 °C, except in areas on the substrate surface containing steps having half unit cell height. Incomplete coalescence and pits were also observed in the latter areas.

Initial growth of GaN and InN over GaAs (110) substrates

We present, a systematic theoretical study of the adsorption of Ga, In and N over GaAs (110) surfaces based on parameter-free, self-consistent total energy and force calculations using the density functional theory. We analyzed the changes in the bond-lengths and in the bond-angles before and after deposition, as well as the total energy behaviour with the adsorbate chemical potential variation. Our results show that the GaN growth over GaAs (110) is energetically more favorable than the InN growth. However, the resulting InN film can be cubic only if we apply small strains on the substrate, while the GaN one is hexagonal.