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.     

Deposition of bulk GaN from solution in gallium under high N2 pressure on silicon carbide and sapphire substrates

Results of high-pressure directional growth of GaN on foreign substrates: SiC, sapphire and GaN/sapphire MOCVD templates are presented. The role of nitrogen pressure and supersaturation in the growth process is discussed. The conditions for stable growth of the nitride are determined. The results of the crystallization process are compared with those obtained for directional growth on pressure grown GaN crystals.     

Growth of AlN films on SiC substrates by RF-MBE and RF-MEE

Epitaxial AlN films have been grown on SiC substrate by molecular beam epitaxy (MBE) and migration-enhanced epitaxy (MEE) using radio frequency (RF) plasma-excited nitrogen. In the RF-MBE growth, the growth rates have been found to be almost constant and the crystal quality improved with increasing the substrate temperature up to 850°C. Further increases of substrate temperature decreased the growth rate and degraded the crystal quality. Using the optimum substrate temperature of 850°C and optimizing the shutter open time, smooth AlN films with atomic force microscope roughness as low as 0.2 nm have been grown by RF-MEE growth.     

The effect of the AlxGa1−xN/AIN buffer layer on the properties of GaN/Si(1 1 1) film grown by NH3-MBE

We describe the growth of GaN on Si(1 1 1) substrates with Al_xGa_1−xN/AlN buffer layer by ammonia gas source molecular beam epitaxy (NH₃-GSMBE). The influence of the AlN and Al_xGa_1−xN buffer layer thickness and the Al composition on the crack density of GaN has been investigated. It is found that the optimum thickness is 120 and 250 nm for AlN and Al_xGa_1−xN layers, respectively. The optimum Al composition is between 0.3<x<0.6.     

Ca3N2 as a flux for crystallization of GaN

In this work Ca₃N₂ was investigated as a potential flux for crystallization of GaN. Melting temperature of the potential flux at high N₂ pressure evaluated by thermal analysis as 1380 °C is in good agreement with the theoretical prediction. It is shown that Ca₃N₂ present in the liquid gallium in small amount (1 at%) dramatically accelerates synthesis of GaN from its constituents. On the other hand, it does not influence significantly the rate of GaN crystallization from solution in gallium in temperature gradient for both unseeded and seeded configurations. However the habit and color of the spontaneously grown GaN crystals change drastically. For 10 mol% Ca₃N₂ content in the liquid Ga it was found that the GaN thick layer and GaN crystals (identified by micro-Raman scattering measurements) were grown on the substrate. For growth from molten Ca₃N₂ (100%) with GaN source, the most important observations were (i) GaN source material was completely dissolved in the molten Ca₃N₂ flux and (ii) after experiment, GaN crystals were found on the sapphire substrate.     

Solubility of GaN in supercritical ammonia with ammonium chloride as a mineralizer

The solubility of GaN in supercritical ammonia with ammonium chloride as a mineralizer was measured with a weight-loss method. Temperature-, pressure-, and mineralizer concentration-dependence of the solubility of GaN were investigated. The solubility increased with increase in temperature, and its pressure dependence was very low. The solubility behavior was quite different from the case of using the mineralizer KNH₂ as a basic mineralizer.     

The influence of indium on the growth of GaN from solution under high pressure

The influence of significant fraction (10–50 mole%) indium in liquid gallium on GaN crystallization from a ternary Ga–In–N solution was analyzed. Crystallization experiments of GaN on GaN-sapphire templates from Ga–In solutions, at 1350–1450 °C, with prior to the growth seed wetting at 1500 °C, and 1.0 GPa N₂ pressure, without solid GaN source showed faster growth of GaN on the seed (by a factor of 1.5–2) than using pure gallium solvent. Nevertheless the new grown crystals were morphologically unstable. The instability was reduced by decrease of the wetting temperature down to 1100 °C or by omitting the wetting procedure entirely, which indicated that GaN dissolves much faster in Ga–In melt than in pure Ga and that the unstable growth was caused most likely by complete dissolution of GaN template before the growth. It was observed that the crystals grown on bulk GaN substrates did not show morphological instability observed for GaN-sapphire templates. The influence of indium on thermodynamic and thermal properties of the investigated system is discussed.     

Structure of nanocrystalline ammonothermal GaN in dependence of temperature and type of acidic mineralizer

This study investigated ammonothermal synthesis of nanocrystalline gallium nitride (GaN) in supercritical ammonia with acidic mineralizers NH₄X (X=Cl, Br, I) at 400–500 °C. Results showed that three types of acidic mineralizers could effectively accelerate the formation of GaN. The mixed hexagonal/cubic phase fractions and lattice parameters of nanocrystalline GaN were calculated by the Rietveld refinement method. SEM showed an agglomerate of nanocrystalline GaN. A considerable amount of GaN was synthesized using NH₄Cl as the mineralizer, however, there was no yield using NH₄Br or NH₄I at 400 °C. For acidic mineralizers, both hexagonal structures (wurtzite) and cubic structures (zincblende) were obtained in ammonothermal synthesis by XRD and Raman measurement. GaN synthesized with NH₄Br and NH₄I showed mixed phases of hexagonal-GaN (h-GaN) and cubic-GaN (c-GaN) at 450–500 °C. In the case of NH₄Cl mineralizer, GaN only exhibited mixed phases of h-GaN and c-GaN at 500 °C, but pure h-GaN at 400–450 °C. Based on the results, NH₄Cl favored pure h-GaN, and NH₄Br and NH₄I favored c-GaN at 400–450 °C.     

Thermal stability of thin InGaN films on GaN

The thermal stability of ∼200-nm-thick InGaN thin films on GaN was investigated using isothermal and isochronal post-growth anneals. The In_xGa_1−xN films (x=0.08–0.18) were annealed in N₂ at 600–1000 °C for 15–60 min, and the resulting film degradation was monitored using X-ray diffraction (XRD) and photoluminescence (PL) measurements. As expected, films with higher indium concentration showed more evidence for decomposition than the samples with lower indium concentration. Also for each alloy composition, decreases in the PL intensity were observed starting at much lower temperatures compared to decreases in the XRD intensity. This difference in sensitivity of the PL and XRD techniques to the InGaN decomposition suggest that defects that quench luminescence are generated prior to the onset of structural decomposition. For the higher indium concentration films, the bulk decomposition proceeds by forming metallic indium and gallium regions as observed by XRD. For the 18% indium concentration film, measurement of the temperature-dependent InGaN decomposition yields an activation energy, E_A, of 0.87±0.07 eV, which is similar to the E_A for bulk InN decomposition. The InGaN integrated XRD signal of the 18% film displays an exponential decrease vs. time, implying InGaN decomposition proceeds via a first-order reaction mechanism.     

Photoluminescence study of homoepitaxial N-polar GaN grown on differently misoriented single crystal substrates

We report results of a photoluminescence (PL) study of homoepitaxial N-polar GaN films grown by metal-organic chemical vapour deposition on vicinal GaN single crystal substrates. Off-angles of 2° and 4° towards the direction as well as 4° in the direction were investigated. Along with a remarkable improvement of the epilayer morphology, a significant reduction of the unintentional/intrinsic donor concentration is achieved for all considered misorientations. As a consequence, PL spectra with narrow bound and free excitonic lines were observed. The misorientation of 4° towards the direction results in an N-polar epilayer of the best optical quality.     

Reducing dislocation density in GaN films using a cone-shaped patterned sapphire substrate

The threading dislocation (TD) density in GaN films grown directly on flat sapphire substrates is typically >10¹⁰/cm², which can deteriorate the properties of GaN-based LEDs significantly. This paper reports an approach to reducing the TD density in a GaN layer using a variety of patterned sapphire substrates (PSS). A cone-shaped PSS produced by metal organic chemical vapor deposition (MOVCD) was used for GaN deposition. Three types of GaN specimens were prepared at the initial nucleation stage, middle growth stage and final growth stage. The TDs generated on the cone-shaped PSS were analyzed by transmission electron microscopy (TEM) and a strain mapping simulation using HRTEM images, which evaluated the residual strain distribution. A large number of TDs were generated and the residual strain by the lattice distortions remained above the top of the cone-shaped regions. However, no TDs and residual strain were observed at the slope of the cone-shaped regions. This might be due to the formation of a GaN layer by lateral overgrowth at the slope of the cone-shaped regions, resulting in less lattice mismatch and incoherency between the GaN and sapphire. In conclusion, the TD density in the GaN layer could be reduced significantly, approximately 10⁷/cm², using the cone-shaped PSS.     

Characterization of polycrystalline GaN grown on silica glass substrates

The structural, optical, and electrical properties of GaN films grown on silica glass substrate by metalorganic chemical vapor deposition were studied. X-ray diffraction showed that the films were grown in hexagonal structure with a predominant (0 0 0 2) peak. A broad and strong band-edge emission and very weak yellow luminescence in photoluminescence (PL) spectra were observed. And the temperature dependence of the PL spectra was extensively studied. The thermal quenching activation energy was found to be very close to the donor activation energy determined from the temperature dependence of the carrier concentration. Longitudinal optical phonons were found to be responsible for the PL broadening above 100 K.     

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.     

Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer

Nonpolar (1 1–2 0) a-plane GaN films have been grown using the multi-buffer layer technique on (1–1 0 2) r-plane sapphire substrates. In order to obtain epitaxial a-plane GaN films, optimized growth condition of the multi-buffer layer was investigated using atomic force microscopy, high resolution X-ray diffraction, and transmission electron microscopy measurements. The experimental results showed that the growth conditions of nucleation layer and three-dimensional growth layer significantly affect the crystal quality of subsequently grown a-plane GaN films. At the optimized growth conditions, omega full-width at half maximum values of (11–20) X-ray rocking curve along c- and m-axes were 430 and 530 arcsec, respectively. From the results of transmission electron microscopy, it was suggested that the high crystal quality of the a-plane GaN film can be obtained from dislocation bending and annihilation by controlling of the island growth mode.     

Growth of epitaxial thin films of scandium nitride on 100-oriented silicon

A series of 100-oriented ScN films was grown under N-rich conditions on 100-oriented Si using different Sc fluxes. The ScN films grew in an epitaxial cube-on-cube orientation, with [0 0 1]_ScN//[0 0 1]_Si and [1 0 0]_ScN//[1 0 0]_Si, despite the high (11%) lattice mismatch between ScN and Si. The film grain size increases and the film ω-FWHM decreases with increasing Sc flux, but the film roughness increases. Films grown under similar conditions on 111-oriented Si resulted in mixed 111 and 100 orientations, indicating that the 100 orientation is favoured both due to texture inheritance from the substrate and due to the growth conditions used.     

Growth of height-controlled InGaN quantum dots on GaN

InGaN height-controlled quantum dots (HCQDs) were grown by alternately depositing In_0.4Ga_0.6N QD and In_0.1Ga_0.9N spacer layers on a seed In_0.4Ga_0.6N QD layer. Structural and optical studies showed that the height of the InGaN QDs was controlled by the deposition cycle of In_0.4Ga_0.6N/In_0.1Ga_0.9N layers. Photoluminescence studies showed that the In_0.4Ga_0.6N HCQDs provided deep potential wells and the piezoelectric field-induced quantum-confined Stark effect was negligibly small. These phenomena are attributed to variation in quantum confinement energy in the electronically coupled InGaN HCQDs providing deep potential wells.     

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.     

Ammonothermal crystal growth of gallium nitride – A brief discussion of critical issues

The acidic ammonothermal technique is used to develop a technology for production of free-standing gallium nitride (GaN) crystals to match the demand driven by the device technology for the wide-band-gap semiconductor group-III element nitrides. Here we report on advances toward a deeper understanding of parameters that govern mass transport and seeded crystallization of GaN under the conditions of acidic ammonothermal crystal growth with the ultimate goal to improve the process control. Comparison with the basic ammonothermal environment has been made.     

Method for modulating the wafer bow of free-standing GaN substrates via inductively coupled plasma etching

The bowing curvature of the free-standing GaN substrate significantly decreased almost linearly from 0.67 to 0.056 m⁻¹ (i.e. the bowing radius increased from 1.5 to 17.8 m) with increase in inductively coupled plasma (ICP) etching time at the N-polar face, and eventually changed the bowing direction from convex to concave. Furthermore, the influences of the bowing curvature on the measured full width at half maximum (FWHM) of high-resolution X-ray diffraction (HRXRD) in (0 0 2) reflection were also deduced, which reduced from 176.8 to 88.8 arcsec with increase in ICP etching time. Decrease in the nonhomogeneous distribution of threading dislocations and point defects as well as V_Ga–O_N complex defects on removing the GaN layer from N-polar face, which removed large amount of defects, was one of the reasons that improved the bowing of the free-standing GaN substrate. Another reason was the high aspect ratio of needle-like GaN that appeared at the N-polar face after ICP etching, which released the compressive strain of the free-standing GaN substrate. By doing so, crack-free and extremely flat free-standing GaN substrates with a bowing radius of 17.8 m could be obtained.