Growth Of Gallium Nitride Research Articles

PEALD-deposited crystalline GaN films on Si (100) substrates with sharp interfaces**Project supported by the Fundamental Research Funds for the Central Universities (Grant Nos. FRF-BR-16-018A, FRF-TP-17-022A1, and FRF-TP-17-069A1), the National Natural Science Foundation of China (Grant Nos. 61274134 and 51402064), USTB Start-up Program (Grant No. 06105033), China Postdoctoral Science Foundation (Grant No. 2018M631333), Beijing Natural Science Foundation (Grant Nos. 2184112 and

Polycrystalline gallium nitride (GaN) thin films were deposited on Si (100) substrates via plasma-enhanced atomic layer deposition (PEALD) under optimal deposition parameters. In this work, we focus on the research of the GaN/Si (100) interfacial properties. The x-ray reflectivity measurements show the clearly-resolved fringes for all the as-grown GaN films, which reveals a perfectly smooth interface between the GaN film and Si (100), and this feature of sharp interface is further confirmed by high resolution transmission electron microscopy (HRTEM). However, an amorphous interfacial layer (∼2 nm) can be observed from the HRTEM images, and is determined to be mixture of GaxOy and GaN by x-ray photoelectron spectroscopy. To investigate the effect of this interlayer on the GaN growth, an AlN buffer layer was employed for GaN deposition. No interlayer is observed between GaN and AlN, and GaN shows better crystallization and lower oxygen impurity during the initial growth stage than the GaN with an interlayer.

High germanium doping of GaN films by ammonia molecular beam epitaxy

Gallium Nitride (GaN) grown by ammonia molecular beam epitaxy and doped with elemental Germanium (Ge) is presented. Growth studies varying the GaN growth rate, substrate growth temperature and the elemental Ge flux reveal several incorporation dependencies. Ge incorporation increases with flux, as expected, and a doping range from ∼1017 cm−3 to 1020 cm−3 was readily achieved. A strong substrate temperature dependence on the electrical properties of films grown is observed, with an optimal growth temperature of 740 °C, lower than standard GaN growth conditions for ammonia molecular beam epitaxy. Compensation effects at higher growth temperatures are suspected, as observed with other techniques. Crystallographic defects are apparent at the highest doping concentrations from electrical and optical measurements, however thin layers of such highly doped films are of great interest for contact layers and tunnel junctions in devices.

Structural and electrical characterization of thick GaN layers on Si, GaN, and engineered substrates

A major challenge in gallium nitride (GaN) vertical power devices and other large bandgap materials is the high defect density that compromises the performance, reliability, and yield. Defects are typically nucleated at the heterointerface where there are both lattice and thermal mismatches. Here, we report the selective area growth (SAG) of thick GaN on Si and on the newly available Qromis Substrate Technology™ (QST) substrates that lead to a significant reduction of the defect densities to a level that is nearly comparable to that on native substrates by defect annihilation. We performed a parametric study of the electrical properties of the SAG GaN layers by fabricating and characterizing Schottky barrier diodes for SAG GaN layer thicknesses of 5, 10, 15, and 20 μm for GaN-on-Si, GaN-on-QST, and GaN-on-GaN diodes. While thicker layers led to a significant reduction in defect densities and improvement in the diode forward current characteristics, the GaN-on-QST diodes exhibited nearly similar characteristics to the GaN-on-GaN diodes. Further improvement in the device structure and/or SAG growth for GaN-on-Si is needed to achieve a comparable performance as the defect densities in the GaN-on-Si are comparable to that of GaN-on-QST substrates.

Effect of N2/H2 plasma on GaN substrate cleaning for homoepitaxial GaN growth by radical-enhanced metalorganic chemical vapor deposition (REMOCVD)

We developed a new method of GaN growth using Radical-Enhanced Metalorganic Chemical Vapor Deposition (REMOCVD) technology by which Gallium Nitride (GaN) grows at low temperatures without ammonia gas. In this method, we investigated the effect of N2/H2 plasma on the GaN substrate surface cleaning prior to the growth of homoepitaxial GaN. In-situ reflection high-energy electron diffraction (RHEED) and atomic force microscope (AFM) were used to investigate the surface morphology of the cleaned GaN substrates. The interface between GaN substrate and homoepitaxially grown GaN by REMOCVD was evaluated by transmission electron microscope and the crystal quality was evaluated by X-ray diffraction. The in-situ N2/H2 plasma cleaning at 600 °C shows a smooth surface morphology with streak diffraction lines observed by RHEED. Since the homoepitaxial growth of GaN was performed at 800 °C, the cleaned GaN substrate temperature was ramped up from 600 °C to 800 °C with and without plasma exposure to compare the effect of plasma. Homoepitaxially grown GaN on GaN substrates whose temperature was ramped up with plasma exposure showed good crystal quality with no threading dislocations at the interface. It was found that N2/H2 plasma plays a significant role in the GaN surface cleaning for good quality crystal growth.

Reaction Pathway of Surface-Catalyzed Ammonia Decomposition and Nitrogen Incorporation in Epitaxial Growth of Gallium Nitride

We report density functional calculations that clarify atom-scale mechanisms of vapor-phase epitaxial growth of GaN with gas sources of trimethylgallium and ammonia. We identify various stable adsorption structures of a Ga atom and NHx molecules (x = 0–3) on GaN(0001) surfaces and find that NH2 and NH units spontaneously intervene in Ga–Ga surface bonds on Ga-rich GaN(0001) surface. We then explore the reaction in which NH3 on the surface is decomposed and becomes an N-incorporated structure, −Ga–(NH)–Ga–, on the Ga-rich GaN(0001) and find that the reaction occurs with the energy barrier of 0.63 eV. Further exploration reveals that a reaction of H2 desorption occurs with the energy barrier of 2 eV, leaving the N atom incorporated in the Ga–N network. This barrier can be overcome when we consider the chemical potential of an H2 molecule in the gas phase at the growth temperature. This N incorporation on GaN is a new growth mechanism catalyzed by the growing surface.

Transformation of 2D group-III selenides to ultra-thin nitrides: enabling epitaxy on amorphous substrates

The experimental realization of two-dimensional (2D) gallium nitride (GaN) has enabled the exploration of 2D nitride materials beyond boron nitride. Here we demonstrate one possible pathway to realizing ultra-thin nitride layers through a two-step process involving the synthesis of naturally layered, group-III chalcogenides (GIIIC) and subsequent annealing in ammonia (ammonolysis) that leads to an atomic-exchange of the chalcogen and nitrogen species in the 2D-GIIICs. The effect of nitridation differs for gallium and indium selenide, where gallium selenide undergoes structural changes and eventual formation of ultra-thin GaN, while indium selenide layers are primarily etched rather than transformed by nitridation. Further investigation of the resulting GaN films indicates that ultra-thin GaN layers grown on silicon dioxide act as effective ‘seed layers’ for the growth of 3D GaN on amorphous substrates.

A thermodynamic supersaturation model for the growth of aluminum gallium nitride by metalorganic chemical vapor deposition

A thermodynamic supersaturation model for growth of AlGaN by metalorganic chemical vapor deposition was developed for experimentally accessible growth parameters. The derived non-linear relationships enabled us to estimate Ga and Al supersaturation during AlGaN growth for given growth conditions. Calculations revealed that the GaN phase was close to chemical equilibrium, while the Al supersaturation was as high as 1010 for typical growth conditions. Such a disparity in the supersaturation of reaction species plays a significant role in the stability of the growth of the resulting ternary alloy. The agreement between experiment and simulation suggests that the parasitic gas phase reactions between trimethylaluminum and NH3 were not significant at low NH3 flow rates/partial pressures, indicating that, under these conditions, the AlGaN growth was thermodynamically limited.

Growth kinetics of basic ammonothermal gallium nitride crystals

The ammonothermal method has been extensively studied for the last two decades because of its potential to produce high-quality bulk gallium nitride (GaN) boules at low cost. Currently, the ammonothermal literature lacks in situ, internal fluid temperature measurements during the crystal growth process. This study reports on internal fluid temperature measured simultaneously in the dissolution and growth zones for twelve basic ammonothermal GaN growth experiments (pressure range = 180–261 MPa; molar NH3:Na fill ratio = 20:1) on {0 0 0 1} and {1 0 1¯ 0}-orientation GaN seeds. Knowledge of the fluid temperature combined with mass-change measurements of the polycrystalline source and seed crystals has enabled the distinction of three growth regimes: a mass transport-limited regime at internal fluid density differences below 1.2 mol/L between dissolution and growth zones; a surface reaction-limited regime above the aforementioned critical fluid density difference and below growth zone fluid temperatures of ∼570 °C; and an apparent loss of solvent-limited regime above growth zone fluid temperatures of ∼570 °C. Analysis of growth in [0 0 0 1], [0 0 0 1¯], and [1 0 1¯ 0] has enabled the determination of activation energies of each respective GaN crystallographic orientation in the surface reaction-limited regime.

Effect of HfO2 nitridation on structural, optical and electrical properties of GaN films grown on HfO2/Si(100) by laser molecular beam epitaxy

Growth of high quality III-nitride films on high-k oxide/Si substrates is crucial for the fabrication of nitride based metal-oxide-semiconductor devices. Here, we report the effect of nitridation of HfO2 surface on the physical properties of gallium nitride (GaN) films grown on HfO2 (5 nm)/p-Si(100) substrates using laser molecular beam epitaxy (LMBE) technique. The grazing incidence x-ray diffraction (GIXRD) measurements showed the prominent GaN growth along (0002) direction. It is observed that the HfO2 nitridation significantly improves the surface coverage of GaN film with larger grains of size in the range 200 ∼ 250 nm. Room temperature photoluminescence (PL) spectroscopy exhibits a sharp, intense near band-edge emission line at 3.4 eV with negligible defect-related deep bands for the GaN film grown with HfO2 nitridation. Also, the current-voltage characteristics of Au/GaN Schottky contacts show a reduced leakage current for HfO2 nitridation. Thus, the surface nitridation of HfO2 is found vital to improve the quality of GaN films for the development of GaN-on-Si technology.

Theoretical Study of the Electronic Structure of Threading Edge Dislocations in GaN

Although silicon is the material mainly used for power devices, we are now approaching the performance limits of power devices based on silicon. Moreover, it is difficult to improve their performance due to the intrinsic physical properties of silicon. Gallium nitride (GaN), on the other hand, is attracting a lot of attention as an alternative material for future power devices. The bandgap of GaN is three times as wide as that of silicon, and GaN has some superior features, such as its high thermal conductively, large dielectric breakdown field, and high saturation velocity for the electrons. Therefore GaN is expected to become a key material in the next generation of power devices, enabling higher voltages and higher speed operation than current devices. Moreover, the use of GaN will allow miniaturization of the devices and higher temperature operation. During the crystal growth of GaN, however, a high density of threading dislocations appears due to the lattice mismatch with the sapphire (Al2O3), silicon carbide (SiC). Moreover, it has been reported that threading dislocations degrade the performance of GaN-based electronic devices fabricated even on GaN substrates. There have been some experimental studies in which screw or mixed dislocations have been reported as being the primary sources for the leakage current in GaN-based devices [1], whereas threading edge dislocations were found not to contribute to the leakage current [2]. Therefore, we need to fabricate GaN-based power devices to examine the electronic behavior at threading dislocations in GaN and to clarify the origin of the leakage current. As for theoretical studies, the electronic structure of threading dislocations in GaN is not fully understood, although Akiyama et al. reported the electronic structures of threading dislocations by determining the atomistic structures by empirical inter-atomic potential calculations [3]. The purpose of this theoretical study is to discover whether threading edge dislocations contribute to the leakage current or not. To do this, we used first principles calculations based on density functional theory (DFT) to examine the electronic structure at threading edge dislocations with Burghers vectors of 1/3[11-20]. In order to investigate the electronic states of threading edge dislocations in GaN, it was necessary to prepare an initial atomic structure for the dislocations. Our initial atomic structure, which has a supercell containing about 200 atoms, is shown in Fig. 1. Fig. 1(a) and Fig. 1(b) show this model in the [0001] and [1-210] directions, respectively. The model has a periodic boundary condition in the [0001] direction, parallel to the dislocation. The surface of the model shown Fig. 1(a) is terminated by fictitious hydrogen atoms [4] and the model is surrounded by a vacuum region. The shape of the model is like a square pillar. First, the model structure was optimized. It is known that several types of core configurations are proposed for 1/3[11-20] threading dislocations in GaN. In this study we focused on an 8-atom ring core configuration, a symmetric 4-atom ring core configuration, and 5/7-atom ring core configurations. The electronic structures for these relaxed atomic models were calculated. Structural optimization and electronic state calculations were performed using VASP (Vienna Ab-initio Simulation Package) [5], which is a first principles calculation code based on DFT. The electronic densities of states of several core configurations are shown in Fig. 2. Fig. 2(a), Fig. 2(b), Fig. 2(c) and Fig. 2(d) show the electronic densities of states for bulk wurtzite GaN, an 8-atoms ring core, a 5/7-atoms ring core, and a symmetric 4-atoms ring core, respectively. In Fig. 2, the Fermi level is shown by the black lines at 0 eV, and the bandgaps are shown by the shaded blue or green parts. It is known that undoped GaN generally has n-type semiconductor characteristics. Therefore, focusing on the blue parts above the Fermi level, the bandgap of every core configuration is similar to that of bulk GaN. In other words, no defect levels can be seen near the bottom of the conduction band. These results demonstrate that threading edge dislocations in gallium nitride do not contribute to leakage current in GaN-based devices. Moreover, we also discuss the atomic and electronic structures using larger scale systems. Acknowledgement This work was conducted as part of a project entrusted with “research and development for next generation of power devices that contributes to the realization of an energy saving society” (MEXT; Ministry of Education, Culture, Sports, Science and Technology). Figure 1

Basic ammonothermal growth of Gallium Nitride – State of the art, challenges, perspectives

Recent progress in ammonothermal technology of bulk GaN growth in basic environment is presented and discussed in this paper. This method enables growth of two-inch in diameter crystals of outstanding structural properties, with radius of curvature above tens of meters and low threading dislocation density of the order of 5 × 104 cm−2. Crystals with different types of conductivity, n-type with free electron concentration up to 1019 cm−3, p-type with free hole concentration of 1016 cm−3, and semi-insulating with resistivity exceeding 1011 Ω cm, can be obtained. Ammonothermal GaN of various electrical properties is described in terms of point defects present in the material. Potential applications of high-quality GaN substrates are also briefly shown.

Strain Effects in Gallium Nitride Adsorption on Defective and Doped Graphene: First-Principles Calculations

Transferable, low-stress gallium nitride grown on graphene for flexible lighting or display applications may enable next-generation optoelectronic devices. However, the growth of gallium nitride on graphene is challenging. In this study, the adsorptions of initial nucleation process of gallium nitride on graphene were investigated using first-principles calculations based on density functional theory. The adsorption energies and the role of in-plane strains were calculated for different possible configurations of the adatoms on the surfaces of vacancy defect and doped graphene. Compared with the results of the gallium adatom, adsorption of the nitrogen atom on graphene was found to exhibit greater stability. The calculations reveal that the vacancy defect core enhanced the adsorption stability of the adatom on graphene, whereas the incorporation of oxygen impurity greatly reduced the stable adsorption of the gallium and nitrogen adatoms. Furthermore, the calculations of strain showed that the lattice expansion led to increased stability for all adsorption sites and configuration surfaces, except for the nitrogen adatom adsorbed over the gallium atom in Ga-doped graphene. The study presented in this paper may have important implications in understanding gallium nitride growth on graphene.

Removal of Ca and K from Metallic Na in TFSI Ionic Liquids Using an Exchange Reaction

To produce high purity sodium used in the sodium-sulfur secondary batteries and as flux for crystal growth of gallium nitride, behavior of the exchange reaction between Ca or K in the Na and Na ions in the hydrophobic ionic liquids was investigated. Na containing Ca and K was floated on the NaTFSI or NaFSI and TEATFSI mixed at a molar ratio of 1: 4. At a reaction time of 720 min and temperature of 160°C, the purity of Na calculated from the concentrations of Ca and K in the Na increased to the 99.998% and 99.999% in NaTFSI-TEATFSI and NaFSI-TEATFSI ionic liquid, respectively. It was confirmed that Ca and K dissolved by the exchange reaction form a precipitate with little accumulation in the ionic liquid.

Removal of Ca and K from Metallic Na in NaTFSI-TEATFSI Ionic Liquids Using an Exchange Reaction

Introduction Recently, high purity metallic Na is used in both large scale secondary batteries and in the flux for crystal growth of gallium nitride (GaN), increasing the demand for high purity Na. We have studied the electrorefining process using hydrophobic ionic liquids1,2). In electrorefining, Ca and K in metallic Na dissolve into the electrolyte, and there is preferential reduction reaction of Na ions in the electrolyte. Further, we have identified an exchange reaction between Ca or K in metallic Na and Na ions in the electrolyte both before and after the electrorefining. In this study, the effect of exchange reaction is quantitatively investigated. Accumulation of Ca ions and K ions in the electrolyte may result in electrodeposition with Na. In this study, we control the simulated conditions of the accumulation of Ca and K in the ionic liquid by repetition of the exchange reaction, and evaluate the purity of the generated metallic Na. Experimental Two solid powder, NaTFSI (sodium-bis(trifluoromethanesulfonyl)imide) and TEATFSI (tetraethylammonium-bis(trifluoromethanesulfonyl)imide) were mixed at a molar ratio of 1 : 4 in an Ar filled glove box. In the exchange reaction experiment, metallic Na (99.95 %, Ca:340 ppm, K:120 ppm, Aldrich) was floated on the ionic liquid mixture, at temperatures of 120 or 160 °C, and with a maximum reaction time of 720 min. After the experiments, a small amount of metallic Na was sampled and dissolved in pure water. The concentrations of Ca and K in the metallic Na were analyzed by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy). For accumulating impurities in the ionic liquid, 150 g of NaTFSI - TEATFSI (1:4) mixed ionic liquid melted at 160 °C was used, and 10 g of metallic Na (99.95 %, Ca : 330 ppm, K : 220 ppm, Aldrich) was floated on this ionic liquid and Ca and K were introduced into the ionic liquid by maintaining the exchange reaction for 60 min. A part of the floated metallic Na and ionic liquid were sampled after the exchange reaction. The samples were dissolved in pure water or ethanol. Further, 10 g of other new metallic Na was replaced in the previously used metallic Na on the same ionic liquid. The Ca and K contents in the ionic liquid were increased by repeating the experiment of 20 times. Concentrations of Ca and K in the prepared 20 aqueous solutions were analyzed by ICP-AES. Results and discussion The relationship between the reaction time and the Ca or K concentrations at 120 °C using the NaTFSI-TEATFSI mixed ionic liquid was investigated. The Ca and K concentrations rapidly decrease up to about 120 min of exchange reaction and thereafter decrease more gradually. A similar behavior was also observed in the experiment at 160 °C, however the duration of the rapid decrease was shorter than at 120 °C. Further, the Ca and K concentrations in the metallic Na obtained at 160 °C by the exchange reaction at 720 min were lower than those at 120 °C. The minimum concentration in the purifyed Na was 4 ppm for Ca and 11 ppm for K. The reason is that the reaction rate increases at the higher temperature, as well as that the ions move more easily when increasing the electric conductivity of the ionic liquid, and the convection becomes faster as the viscosity decreases. Therefore, it is possible to remove more Ca and K by conducting the exchange reaction at 160 °C for a longer period, and metallic Na with an initial purity of 99.95 % can be purified to 99.99 %. After repeating the exchange reaction 20 times, the concentration of Ca in the ionic liquid was close to 0 ppm, and K was accumulated at a concentration lower than the expected accumulated concentration. Precipitate containing Ca and K was identified in the ionic liquid after the experiments. Since the concentrations of Ca and K in all metallic Na after the exchange reaction became lower than the initial concentration, it may be assumed that the Ca and K components dissolved into the ionic liquid from the metallic Na and may have formed this precipitate. At a reaction time of 480 min, 99.99 % of metallic Na could be purified even for the ionic liquid after 20 exchange reaction cycles. References 1) Mikito Ueda, Kazuya Honda, Toshiaki Ohtsuka, Electrochimica Acta 100, 265 (2013). 2) Mikito Ueda, Ryuichi Inaba, Hisayoshi Matsushima, Toshiaki Ohtsuka, ECS Electrochemistry Letters, 4, E1 (2015).

Gallium nitride grown by molecular beam epitaxy at low temperatures

Growth of gallium nitride at low temperatures broadens the opportunity for its integration into optoelectronic devices that contain thermally sensitive substrates or active layers. As temperature is a very critical growth parameter, changes in crystallinity, defect density, optical, and structural properties are expected as temperatures fall below those typical of molecular beam epitaxy growth. In this contribution, energetic neutral atomic-beam lithography and epitaxy, a molecular beam epitaxy method that utilizes energetic neutral atomic nitrogen as the active nitrogen species, is used to grow gallium nitride directly on nitridized sapphire at temperatures between 800 and 200°C. Photospectroscopy, photoluminescence, Raman spectroscopy, scanning electron microscopy and X-ray diffractometry are applied to determine changes in optical, morphological and structural properties induced by the unconventional low-temperature growth process. As anticipated, we observe that defect density, disorder, and light absorptance increase as growth temperature decreases. Interestingly, X-ray diffraction and photoluminescence reveal the presence of the cubic phase of gallium nitride in films grown at low temperatures under a nitrogen-rich regime, which differs from growth conditions reported by plasma-assisted molecular beam epitaxy and metalorganic molecular beam epitaxy. These discrepancies are presented in a critical review of several studies reporting the stabilization of the cubic phase over the energetically-favored hexagonal phase, with emphasis on relation to growth temperature, Ga/N flux ratio and surface kinetics during growth.

RF Dielectric Loss Due to MOCVD Aluminum Nitride on High Resistivity Silicon

The contribution of high-frequency losses from an aluminum nitride (AlN) layer on high resistivity silicon (Si) is reported. The AlN, deposited on silicon via metalorganic chemical vapor phase deposition as a nucleation layer for subsequent gallium nitride growth, is analyzed for its contribution to the dielectric losses from 6–20 GHz and differentiated from the loss due to the p-type layer formed in the silicon substrate. It is found that AlN is a stronger contributor to overall dielectric loss in comparison with the silicon substrate.

Phosphor-Free InGaN White Light Emitting Diodes Using Flip-Chip Technology.

Monolithic phosphor-free two-color gallium nitride (GaN)-based white light emitting diodes (LED) have the potential to replace current phosphor-based GaN white LEDs due to their low cost and long life cycle. Unfortunately, the growth of high indium content indium gallium nitride (InGaN)/GaN quantum dot and reported LED’s color rendering index (CRI) are still problematic. Here, we use flip-chip technology to fabricate an upside down monolithic two-color phosphor-free LED with four grown layers of high indium quantum dots on top of the three grown layers of lower indium quantum wells separated by a GaN tunneling barrier layer. The photoluminescence (PL) and electroluminescence (EL) spectra of this white LED reveal a broad spectrum ranging from 475 to 675 nm which is close to an ideal white-light source. The corresponding color temperature and color rendering index (CRI) of the fabricated white LED, operated at 350, 500, and 750 mA, are comparable to that of the conventional phosphor-based LEDs. Insights of the epitaxial structure and the transport mechanism were revealed through the TEM and temperature dependent PL and EL measurements. Our results show true potential in the Epi-ready GaN white LEDs for future solid state lighting applications.

Non-polar GaN film growth on (0 1 0) gallium oxide substrate by metal organic chemical vapor deposition

Abstract

The process and mechanism of the GaN nanoparticles formed by nitridation of β-Ga2O3 crystal

The process of conversion from [Formula: see text]-Ga2O3 single crystal to gallium nitride (GaN) in an atmosphere of NH3 by chemical vapor deposition is investigated. The surface morphology and microstructure of the GaN nanoparticles are observed by scanning electron microscope, which indicates that the growth of GaN is via the Volmer–Weber mechanism. The [Formula: see text]-Ga2O3 is firstly evaporated at high temperature to form the porous layer, followed by the surface-defect induced GaN nucleation formation. The crystalline structure and epitaxial relationship of the GaN nanoparticles are investigated by X-ray diffraction (XRD) via [Formula: see text]–[Formula: see text], showing GaN (0002) and (0004) diffraction peaks in the XRD spectra. It is concluded that the polycrystalline GaN film with hexagonal structure has a strong c-axis preferential orientation.

Misfit dislocation locking and rotation during gallium nitride growth on SiC/Si substrates

The effect of changing the misfit dislocation propagation direction during GaN layer growth on the AlN/SiC/Si(111) structure surface is detected. The effect is as follows. As the GaN layer growing on AlN/SiC/Si(111) reaches a certain thickness of ~300 nm, misfit dislocations initially along the layer growth axis stop and begin to move in the direction perpendicular to the growth axis. A theoretical model of AlN and GaN nucleation on the (111) SiC/Si face, explaining the effect of changing the misfit dislocation motion direction, is constructed. The effect of changing the nucleation mechanism from the island one for AlN on SiC/Si(111) to the layer one for the GaN layer on AlN/SiC/Si is experimentally detected and theoretically explained.