Reduction of threading dislocations in migration enhanced epitaxy grown GaN with N-polarity by use of AlN multiple interlayer

High quality GaN layer was obtained by insertion of high temperature grown AlN multiple intermediate layers with migration enhanced epitaxy method by the RF-plasma assisted molecular beam epitaxy on (0 0 01) sapphire substrates. The propagating behaviors of dislocations were studied, using a transmission electron microscope. The results show that the edge dislocations were filtered at the AlN/GaN interfaces. The bending propagation of threading dislocations in GaN above AlN interlayers was confirmed. Thereby, further reduction of dislocations was achieved. Dislocation density being reduced, the drastic increase of electron mobility to 668 cm²/V s was obtained at the carrier density of 9.5×10¹⁶ cm⁻³ in Si doped GaN layer.     

Investigation of GaN crystal quality on silicon substrates using GaN/AlN superlattice structures

The crystal quality of GaN thin film on silicon using GaN/AlN superlattice structures was investigated. The growth was carried out on Si(111) for GaN(0001) in a metal‐organic vapor phase epitaxy system. Various GaN/AlN superlattice intermediate layers have been designed to decrease the dislocation density. The results showed that the etch pit density could be greatly reduced by one order of magnitude. Cross‐sectional transmission electron microscopy (XTEM) study confirmed the efficiency of GaN/AlN superlattice in blocking threading dislocation propagation in GaN crystal. The design of nine period GaN/AlN (20nm/2nm) superlattice has been evidenced to be effective in reducing the dislocation density and improving the crystal quality. In addition, the dislocation bending in GaN/AlN interface and dislocation merging is investigated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Reduction of the defect density in GaN films using ultra-thin AlN buffer layers on 6H-SiC

A low dislocation density of 10^7–8 cm⁻² in GaN thin films on 6H-SiC(0001) substrates grown by metalorganic chemical vapor deposition was achieved. By considering possible origins of dislocations in the GaN/AlN/Sic structure, two major dislocation reduction routes are proposed; ultra-thin AlN buffer layers and smooth AlN surfaces in an atomic scale. Experimentally, the effects of the surface roughness and structural perfection of the AlN buffer layer on GaN film quality were extensively investigated as a function of AlN film thickness. The reduced dislocation density was realized by using ultra-thin AlN buffer layers having a thickness of 1.5 nm, which is below the critical value for misfit dislocation generation. The smoother surface morphology and enhanced structural quality of ultra-thin AlN buffer layers were found to be the main parameters in reducing the defect density in the GaN film.     

Optical and structural properties of epitaxial GaN films grown by pulsed laser deposition

Epitaxial GaN films have been grown on c-cut sapphire substrates by pulsed laser deposition (PLD) using a KrF laser. The properties of GaN films were improved by increasing the growth temperature to 800°C and the nitrogen pressure during growth to 10 mTorr. Room-temperature photoluminescence exhibits a strong band-edge emission at 3.4 eV. From transmission electron microscopy (TEM), the predominant defects in PLD-GaN observed are stacking faults parallel to the interface and screw dislocations along c-axis, the latter differing from the previously published results where most of the threading dislocation in GaN grown by other techniques are of edge type with Burgers vector of .     

TEM investigations of GaN layers grown on silicon and sintered GaN nano‐ceramic substrates

GaN nano‐ceramics were analyzed using transmission electron microscopy (TEM), showing that these ceramics are characterized by highly disoriented grains of the linear size of 100–150 nm. These GaN ceramics were used as substrates for GaN epitaxy in standard MOVPE conditions. For the comparison, MOVPE GaN layers on silicon substrates were grown using similar conditions. It is shown that MOVPE growth of GaN layers is highly anisotropic for both cases. However, the disorientation of the highly mismatched GaN layer on silicon is different from that characterizing GaN layer deposited on the ceramic substrate. In the latter case the disorientation is much higher, and three dimensional in nature, causing creation of polycrystalline structure having large number of the dislocations. In the case of the GaN layer grown on the silicon substrate the principal disorientation is due to rotation around c‐axis, causing creation of mosaic structure of edge dislocations. Additionally, it is shown that the typical grain size in AlN nucleation layer on Si is smaller, of order of 20 nm. These two factors contribute to pronounced differences in later stage of the growth of GaN layer on the ceramic. Due to high growth anisotropy an appropriately thick GaN layer can, eventually, develop flat surfaces suitable for construction of optoelectronic and electronic structures. As shown by the TEM data, this can be achieved only at the cost of creation of the relatively large density of dislocations and stacking faults. The latter defects were not observed for the GaN growth on Si substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Well parameters of two‐dimensional electron gas in Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by MOCVD

Resistivity and Hall effect measurements were carried out as a function of magnetic field (0‐1.5 T) and temperature (30‐300 K) for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two‐dimensional electron gas (2DEG) channel located at the Al_0.88In_0.12N/GaN interface with an AlN interlayer and a two‐dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al_0.88In_0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X-ray diffraction study of short-period AlN/GaN superlattices

The structure of short-period hexagonal GaN/AlN superlattices (SLs) has been investigated by X-ray diffraction. The samples have been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal reactor at a temperature of 1050°C on (0001)Al₂O₃ substrates using GaN and AlN buffer layers. The SL period changes from 2 to 6 nm, and the thickness of the structure varies in a range from 0.3 to 1 μm. The complex of X-ray diffraction techniques includes a measurement of θ-2θ rocking curves of symmetric Bragg reflection, the construction of intensity maps for asymmetric reflections, a measurement and analysis of peak broadenings in different diffraction geometries, a precise measurement of lattice parameters, and the determination of radii of curvature. The thickness and strain of separate SL layers are determined by measuring the θ-2θ rocking curves subsequent simulation. It is shown that most SL samples are completely relaxed as a whole. At the same time, relaxation is absent between sublayers, which is why strains in the AlN and GaN sublayers (on the order of 1.2 × 10^?2) have different signs. An analysis of diffraction peak half-widths allows us to determine the densities of individual sets of dislocations and observe their change from buffer layers to SLs.     

Determination of the types and densities of dislocations in GaN epitaxial layers of different thicknesses by optical and atomic force microscopy

The change in the dislocation density on the surface of GaN epitaxial layers, which were grown by hydride vapor-phase epitaxy on sapphire substrates with c and r orientations, has been investigated by optical and atomic force microscopy (AFM). It is shown that the observed decrease in the density of threading dislocations with an increase in the layer thickness is related to the annihilation of mixed dislocations. The experimental and theoretical data on the change in the density of mixed dislocations with an increase in the epitaxial-layer thickness are in good correspondence.     

HRTEM investigation of high-reflectance AlN/GaN distributed Bragg-reflectors by inserting AlN/GaN superlattice

A high-quality AlN/GaN distributed Bragg-reflectors (DBR) was successfully grown on sapphire substrate by low-pressure metal-organic chemical vapor deposition using ultra-thin AlN/GaN superlattice insertion layers (SLILs). The reflectivity of AlN/GaN DBR with ultra-thin AlN/GaN SLIL was measured and achieved blue peak reflectivity of 99.4% at 462 nm. The effect of ultra-thin AlN/GaN superlattice insertion layer was examined in detail by transmission electron microscopy, and indicated that the crack of AlN/GaN DBR can be suppress by inserting AlN/GaN SLIL. For electronic properties, the turn on voltage is about 4.1 V and CW laser action of vertical-cavity surface-emitting laser (VCSEL) was achieved at a threshold injection current of 1.4 mA at 77 K, with an emission wavelength of 462 nm.     

Molecular beam epitaxy of GaN/AlxGa1−xN superlattices for 1.52 – 4.2 μm intersubband transitions

High-quality superlattice structures of GaN/AlGaN were grown on (0 0 0 1) sapphire substrates by molecular beam epitaxy. The threading dislocation density was reduced by growing low-temperature AlN layers in between the high-temperature GaN. In addition, in situ monitoring of the growth rate was achieved using pyrometric interferometry. Cross-sectional transmission electron microscopy of the superlattice structures revealed abrupt interfaces between GaN/AlGaN and excellent layer uniformity. We observed intersubband absorption at wavelengths as short as 1.52 μm in the GaN/AlGaN material system. A range of intersubband absorption peaks was observed between 1.52 and 4.2 μm by varying the well thickness and barrier Al content. In addition, the distribution of the built-in electric field between the well and barrier layers was also found to affect the intersubband transition wavelength.     

Growth of thick,continuous GaN layers on 4-in. Si substrates by metalorganic chemical vapor deposition

We report on the growth of thick GaN epilayers on 4-in. Si(1 1 1) substrates by metalorganic chemical vapor deposition. Using intercalated AlN layers that contribute to counterbalance the tensile strain induced by the thermal mismatch between gallium nitride and the silicon substrate, up to 6.7 μm thick crack-free group III-nitride layers have been grown. Root mean-squares surface roughness of 0.5 nm, threading dislocation densities of 1.1×10⁹ cm^?2, as well as X-ray diffraction (XRD) full widths at half-maximum (FWHM) of 406 arcsec for the GaN(0 0 2) and of 1148 arcsec for the GaN(3 0 2) reflection have been measured. The donor bound exciton has a low-temperature photoluminescence line width of 12 meV. The correlation between the threading dislocation density and XRD FWHM, as well as the correlation between the wafer curvature and the GaN in-plane stress is discussed. An increase of the tensile stress is observed upon n-type doping of GaN by silicon.     

Effect of the N/Al ratio of AlN buffer on the crystal properties and stress state of GaN film grown on Si(1 1 1) substrate

The effect of the N/Al ratio of AlN buffers on the optical and crystal quality of GaN films, grown by metalorganic chemical vapor deposition on Si(1 1 1) substrates, has been investigated. By optimizing the N/Al ratio during the AlN buffer, the threading dislocation density and the tensile stress have been decreased. High-resolution X-ray diffraction exhibited a (0 0 0 2) full-width at half-maximum as low as 396 acrsec. The variations of the tensile stress existing in the GaN films were approved by the redshifts of the donor bound exiton peaks in the low-temperature photoluminescence measurement at 77 K.     

Microscopic lateral overgrowth by physical vapour transport of GaN on self‐organized diamond‐like carbon masks

Techniques to reduce dislocation densities in GaN grown on foreign substrates are an interesting alternative to bulk growth as long as efficient bulk crystals growth techniques are not available. In this paper a new approach for epitaxial lateral overgrowth (ELO) of GaN through an in‐situ grown self‐organized amorphous (diamond‐like) carbon mask is demonstrated. The ELO was done for the first time by physical vapour transport of Ga, using NH₃ as a nitrogen source. The overgrowth results in a decrease of the threading dislocation density by at least one order of magnitude compared to that of the MOCVD GaN/sapphire templates. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cross‐sectional and surface Raman mapping of thick GaN layers

Raman scattering spectroscopy was utilized for investigation of the structural properties of thick GaN layers. These layers with thickness ∼ 40 μm have been grown by HVPE technique on the sapphire substrates. The investigations have been focused on the strain distribution in GaN layer cross‐section as a function of distance from an interface sapphire/GaN and mapping of the surface and of the inner layer, near the sapphire/GaN interface. From the observed phonon shifts in the Raman spectra strain differences lower than 6.4×10^–4 corresponding to stress differences of 240 MPa were estimated across the thick GaN epitaxial layer. The measurements exhibit that strain in the layer causes changes in the Raman spectra and allow determining the relaxation process in the crystal. The obtained results confirmed, that the mode frequencies in the measured Raman spectra in both directions (parallel or perpendicular to the growth direction) for layer thicknesses over 30 µm are comparable with typical values for bulk material and match the low strain in the structure due to relaxation processes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Propagation behavior of threading dislocations during physical vapor transport growth of silicon carbide (SiC) single crystals

The dislocation formation and propagation processes in physical vapor transport (PVT) grown 4H silicon carbide (4H–SiC) single crystals have been investigated using defect selective etching and transmission electron microscopy (TEM). It was found that while the growth initiation process generally increased the density of threading dislocations in the grown crystal, for certain areas of the crystal, threading dislocations were terminated at the growth initiation. Foreign polytype inclusions also introduced a high density of dislocations at the polytype boundary. In the polytype-transformed areas of the crystal, almost no medium size hexagonal etch pits due to threading screw dislocations were observed, indicating that the foreign polytype inclusions had ceased the propagation of threading screw dislocations. Based on these results, we argued the formation and propagation of the threading dislocations in PVT grown SiC crystals, and proposed the dislocation conversion process as a plausible cause of the density reduction of threading dislocations during the PVT growth of SiC single crystals.     

Stress control by micropits density variation in strained AlGaN/GaN/SiN/AlN/Si(111) heterostructures

AlGaN/GaN heterostructures were deposited on Si utilizing in‐situ SiN masking layer as a mean to decrease stress present in the final heterostructures. Structures were grown under different V/III ratio using metalorganic vapour phase epitaxy (MOVPE). Additional approach was applied to obtain crack‐free heterostructures which was deposition of 15 nm low temperature AlN interlayer. Each of the heterostructure contained GaN layer of 2.4 μm total thickness. In‐situ SiN masking layer were obtained via introduction of SiH4 precursor into reactor under high temperature growth conditions for 100 s. In that manner, few monolayers of SixNx masking layer were deposited, which due to the partial coverage of AlN, played role of a mask leading to initial 3D growth mode enhancing longer coalescence of GaN buffer layer. To study surface morphology AFM images were observed. Three methods were used in order to obtain basal plane stress present in multilayer structures ‐ MicroRaman spectroscopy, XRD studies and optical profilometry. It was found that varying V/III precursors ratio during GaN layer growth characteristic for structures with the SiN mask approach formation of triangular micropits can be minimized. Outcomes for three different methods turned out to be coherent. It was found that certain amount of micropits on the surface can be advantageous lowering stress introduced during cooling after process to the AlGaN/GaN/SiN/AlN/Si(111) structure.     

Strain analysis of InGaN/GaN multi quantum well LED structures

Five period InGaN/GaN multi quantum well (MQW) light emitting diode (LED) structures were grown by a metalorganic chemical vapor deposition (MOCVD) system on c‐plane sapphire. The structural characteristics as a strain‐stress analysis of hexagonal epilayers MQWs were determined by using nondestructive high resolution x‐ray diffraction (HRXRD) in detail. The strain/stress analysis in AlN, GaN, and InGaN thin films with a variation of the In molar fraction in the InGaN well layers was conducted based on the precise measurement of the lattice parameters. The a‐ and c‐lattice parameters of the structures were calculated from the peak positions obtained by rocking the theta axis at the vicinity of the symmetric and asymmetric plane reflection angles, followed by the in‐plane and out‐of‐plane strains. The biaxial and hydrostatic components of the strain were extracted from the obtained a‐ and c‐direction strains values.     

AlGaN epitaxial lateral overgrowth on Ti-evaporated GaN/sapphire substrate

AlGaN growth using epitaxial lateral overgrowth (ELO) by metalorganic chemical vapor deposition on striped Ti, evaporated GaN on sapphire, has been investigated. AlGaN/AlN films growth on GaN/AlGaN superlattices (SLs) structure on the Ti masks, with various SLs growth temperature (1030, 1060 and 1090 °C) were grown. With increasing the growth temperature, AlGaN surface became flat. The AlGaN film had a cathodoluminescence peak around 345 nm. However, in secondary ion mass spectrometry (SIMS) measurement, Ti signal was detected on the top of AlGaN surface when GaN/AlGaN SLs was grown on Ti striped masks. By inserting the AlN blocking layer on SLs, Ti diffusion was stopped at the AlN layer, and the AlGaN crystalline quality was improved.