Surface chemistry and transport effects in GaN hydride vapor phase epitaxy

A simple quasi-thermodynamic model of surface chemistry in GaN hydride vapor phase epitaxy (HVPE) is presented. The model is coupled with the detailed 3D simulations of species transport in a horizontal-tube reactor and validated by the comparison with the data on the GaN growth rate obtained by laser reflectometry. Parametric study of the growth rate as a function of temperature and species flow rates has been performed over a wide range of growth conditions. The important role of species transport in an HVPE reactor is demonstrated. In particular, a strong effect of the natural concentration convection resulting in the formation of recirculation zones and in a non-uniform vapor composition is revealed by modeling. The impact of these effects on the GaN growth rate and V/III ratio on the growth surface is discussed in detail.     

Properties of Ge-doped,high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy

We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×10¹⁹ cm⁻³. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl₄ as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×10⁶ cm⁻². The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146 cm² V⁻¹ s⁻¹ for n=3.3×10¹⁸ and 3.35×10¹⁹ cm⁻³, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0 W cm⁻¹ K⁻¹. Thermal expansion coefficients along the a- and m-axes were approximately constant at 5.0×10⁻⁶ K⁻¹ in the measured doping concentration range.     

The impact of an intermediate temperature buffer on the growth of GaN on an AlN template by hydride vapor phase epitaxy

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×10⁷ cm⁻²) and a higher edge-dislocation density (8.157×10⁹ cm⁻²) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.     

Influence of growth parameters on crack density in thick epitaxially lateral overgrown GaN layers by hydride vapor phase epitaxy

Using hydride vapor phase epitaxy the influence of growth parameters on the crack density is studied for thick epitaxially lateral overgrown (ELOG) GaN layers. Reactor pressure, growth rate, and substrate temperature are key factors to obtain crack-free thick GaN layers. The cracking mechanism is discussed and void formation on top of the SiO₂ stripes is proposed to play a key role in stress relaxation and crack suppression.     

InGaN growth with various InN mole fractions on m-plane ZnO substrate by metalorganic vapor phase epitaxy

We have demonstrated In_xGa_1−xN epitaxial growth with InN mole fractions of x=0.07 to 0.17 on an m-plane ZnO substrate by metalorganic vapor phase epitaxy for the first time. The crystalline quality of the epilayers was found to be much higher than that of epilayers grown on a GaN template on an m-plane SiC substrate.     

Hydride vapor phase epitaxy of GaN boules using high growth rates

The boule-like growth of GaN in a vertical AIXTRON HVPE reactor was studied. Extrinsic factors like properties of the starting substrate and fundamental growth parameters especially the vapor gas composition at the surface have crucial impact on the formation of inverse pyramidal defects. The partial pressure of GaCl strongly affects defect formation, in-plane strain, and crystalline quality. Optimized growth conditions resulted in growth rates of 300–500 μm/h. GaN layers with thicknesses of 2.6 and of 5.8 mm were grown at rates above 300 μm/h. The threading dislocation density reduces with an inverse proportionality to the GaN layer thickness. Thus, it is demonstrated that growth rates above 300 μm/h are promising for GaN boule growth.     

GaN/GaAs(1 0 0) superlattices grown by metalorganic vapor phase epitaxy using dimethylhydrazine precursor

Superlattices of cubic gallium nitride (GaN) and gallium arsenide (GaAs) were grown on GaAs(1 0 0) substrates using metalorganic vapor phase epitaxy (MOVPE) with dimethylhydrazine (DMHy) as nitrogen source. Structures grown at low temperatures with varying layer thicknesses were characterized using high resolution X-ray diffraction and atomic force microscopy. Several growth modes of GaAs on GaN were observed: step-edge, layer-by-layer 2D, and 3D island growth. A two-temperature growth process was found to yield good crystal quality and atomically flat surfaces. The results suggest that MOVPE-grown thin GaN layers may be applicable to novel GaAs heterostructure devices.     

Hydrogen and nitrogen ambient effects on epitaxial growth of GaN by hydride vapour phase epitaxy

This study presents the influence of the composition of the carrier gas on the growth of GaN by HVPE. Since no hydrogen is introduced in the vapour phase, the deposition is expected to be controlled by Cl desorption in the form of GaCl₃, as has been proposed for GaAs. However, our published model predicts much lower growth rates than those observed. We can account for both the observed parasitic deposition and GaN growth rate if we assume that GaCl₃ is not at its equilibrium pressure in the deposition zone and where nucleation takes place on the walls as well as on the substrate. This yields a high rate of parasitic nucleation even though the nominal supersaturation is vanishing small. Very little growth takes place on the substrate where the equilibrium pressure of GaCl₃ is reached. We describe similar experiments performed with a H₂/N₂ mixture as the carrier gas. In this case, we expect GaN deposition to be controlled by desorption of Cl as HCl, which is known as the H₂ mechanism. It is speculated that the results show the existence of a new growth mechanism.     

Transmission electron microscopy study of an AlN nucleation layer for the growth of GaN on a 7-degree off-oriented (0 0 1) Si substrate by metalorganic vapor phase epitaxy

We have investigated the morphology of the high-temperature-grown AlN nucleation layer and its role in the early stage of GaN growth, by means of transmission electron microscopy. The nitride was selectively grown on a 7-degree off-oriented (0 0 1) patterned Si substrate by metalorganic vapor phase epitaxy. AlN was deposited on the inclined unmasked (1 1 1) facet in the form of islands. The size of the islands varied along the slope, which is attributable to the diffusion of the growth species in the vapor phase. The GaN nucleation occurred at the region where rounded AlN islands formed densely. The threading dislocations were observed to generate in the GaN nucleated region.     

Accelerated surface flattening by alternating Ga flow in hydride vapor phase epitaxy

GaN films were grown on c$c$-plane sapphire substrates by using hydride vapor phase epitaxy (HVPE) with a pulsed flow of HCl over Ga metal. NH₃${\mathrm{NH}}_3$ gas supply was controlled to flow in a constant rate or in a modulated way. The surface morphology dependence of these films on the various flow modulation schemes was investigated. Depending on the duty cycle of NH₃${\mathrm{NH}}_3$ flow, the surface morphology of GaN films was sensitively modified. This sensitive response of surface morphology of GaN films to the flow modulation was attributed to diffusion efficiency variation of Ga species under different gas environment. Under proper modulation conditions, flattened top-surface morphology of nucleated domains was found to be obtained.     

Subtle interplay between polytypism and preferred growth direction alignment in GaN nanorods non-catalytically grown on Si(1 1 1) substrates by using hydride vapor phase epitaxy

GaN nanorods were grown on Si(1 1 1) substrates by using hydride vapor phase epitaxy, and the crystallographic characteristics associated with their preferred growth directions were investigated by utilizing synchrotron X-ray reciprocal space mapping in a grazing incidence geometry and scanning electron microscopy. Crystallographic analysis reveals that the nanorods containing both wurtzite and zinc blende phase tend to have narrower distribution of the preferred growth directions than those containing only wurtzite phase. This tendency is partly attributed to the subtle interplay between polytypism and the preferred growth directions of GaN nanorods.     

Method for HVPE growth of thick crack-free GaN layers

Twenty-five micrometer thick GaN was grown with hydride vapor phase epitaxy (HVPE) on metal-organic chemical vapor deposition (MOCVD) grown templates on sapphire substrates with the gallium treatment step (GTS) technique with varying buffer layer thickness. The samples are studied with atomic force microscopy (AFM), etching and scanning electron microscopy (SEM), photo-luminescence (PL), X-ray diffraction (XRD) and optical microscopy. The results show that the thickness of the buffer layer is not important for the layer quality once the growth in MOCVD starts to make the transition from 3D growth to 2D growth and HVPE continues in the same growth mode. We show that the MOCVD templates with GTS technique make excellent templates for HVPE growth, allowing growth of GaN without cracks in either sapphire or GaN.     

Liquid phase epitaxy (LPE) of GaN on c- and r-faces of AlN substrates

We present the optical properties of MBE-grown GaAs–AlGaAs core–shell nanowires (NWs) grown on anodized-aluminum-oxide (AAO) patterned-Si (1 1 1) substrate using photoluminescence and Raman scattering spectroscopy. The GaAs NWs were grown via the vapor–liquid–solid method with Au-nanoparticles as catalysts. Enhancement in emission of at least an order of magnitude was observed from the GaAs–AlGaAs core–shell NWs as compared to the bare GaAs NWs grown under similar conditions, which is an indication of improved radiative efficiency. The improvement in radiative efficiency is due to the passivating effect of the AlGaAs shell. Variation in bandgap emission energy as a function of temperature was analyzed using the semi-empirical Bose–Einstein model. Results show that the free exciton energy of the GaAs core–shell agrees well with the known emission energy of zinc blende (ZB) bulk GaAs. Further analysis on the linear slope of the temperature dependence curve of photoluminescence emission energy at low temperatures shows that there is no difference between core–shell nanowires and bulk GaAs, strongly indicating that the grown NWs are indeed predominantly ZB in structure. The Raman modes show downshift and asymmetrical broadening, which are characteristic features of NWs. The downshift is attributed to lattice defects rather than the confinement or shape effect.     

Effect of arsenic species on the kinetics of GaAs nanowires growth by molecular beam epitaxy

GaAs nanowires (NWs) are grown on GaAs (1 1 1) B substrates in a molecular beam epitaxy system, by Au-assisted vapor–liquid–solid growth. We compare the characteristics of NWs elaborated with As₂ or As₄ molecules. In a wide range of growth temperatures, As₄ leads to growth rates twice faster than As₂. The shape of the NWs also depends on the arsenic species: with As₄, regular rods can be obtained, while pencil-like shape results from growth with As₂. From the analysis of the incoming fluxes, which contributes to the NWs formation, we conclude that the diffusion length of Ga adatoms along the NW sidewalls is smaller under As₂ flux as compared to that under As₄ flux. It follows that As₂ flux is favourable to the formation of radial heterostructures, whereas As₄ flux is preferable to maintain pure axial growth.     

Influence of AlN nucleation layer growth conditions on quality of GaN layers deposited on (0 0 0 1) sapphire

The influence of AlN nucleation layer (NL) growth conditions on the quality of GaN layer deposited on (0 0 0 1) sapphire by organometallic chemical vapor phase epitaxy (OMVPE) has been investigated by X-ray diffraction, atomic force microscopy and transmission electron microscopy. Growth pressure, temperature and time were varied in this study. Results indicate that there exists an optimal thickness of the NL is required for optimal growth. Both thin and thick NLs are not conducive to the growth of high-quality GaN layers. Arguments have been developed to rationalize these observations.     

Approach for dislocation free GaN epitaxy

The characteristics of confined epitaxial growth are investigated with the goal of determining the contributing effects of mask attributes (spacing, feature size) and growth conditions (V/III ratio, pressure, temperature) on the efficiency of the approach for dislocation density reduction of GaN. In addition to standard (secondary electron and atomic force) microscopy, electron channeling contrast imaging (ECCI) is employed to identify extended defects over large (tens of microns) areas. Using this method, it is illustrated that by confining the epitaxial growth, high quality GaN can be grown with dislocation densities approaching zero.     

Lateral confined epitaxy of GaN layers on Si substrates

We developed a novel, simple procedure for achieving lateral confined epitaxy (LCE). This procedure enables the growth of uncracked GaN layers on a Si substrate, using a single, continuous metalorganic chemical vapor deposition (MOCVD) run. The epitaxial growth of GaN is confined to mesas, defined by etching into the Si substrate prior to the growth. The LCE-GaN layers exhibit improved morphological and optical properties compared to the plain GaN-on-Si layers grown in the same MOCVD system. By performing a set of LCE growth runs on mesas of varying lateral dimensions, we specified the crack-free range of GaN on Si as 14.0±0.3 μm.     

Enhanced growth rates and reduced parasitic deposition by the substitution of Cl2 for HCl in GaN HVPE

The main limitation in the application of hydride vapor phase epitaxy for the large scale production of thick free-standing GaN substrates is the so-called parasitic deposition, which limits the growth time and wafer thickness by blocking the gallium precursor inlet. By utilizing Cl₂ instead of the usual HCl gas for the production of the gallium chlorine precursor, we found a rapid increase in growth rate from ∼80 to ∼400 μm/h for an equally large flow of 25 sccm. This allowed us to grow, without any additional optimization, 1.2 mm thick high quality GaN wafers, which spontaneously lifted off from their 0.3° mis-oriented GaN on sapphire HCl-based HVPE templates. These layers exhibited clear transparencies, indicating a high purity, dislocation densities in the order of 10⁶ cm⁻², and narrow rocking curve XRD FWHMs of 54 and 166 arcsec in for the 0002 and 101−5 directions, respectively.