E. Valcheva

Lattice parameters of GaN layers grown on a-plane sapphire: Effect of in-plane strain anisotropy

We have studied GaN films grown on a-plane sapphire by hydride vapor phase epitaxy and metalorganic vapor phase epitaxy. The in-plane lattice parameter was determined from sets of equivalent interplanar distances measured for six different directions in order to examine the effect of strain anisotropy. It is found that, in both types of films, the obtained six values of the in-plane lattice parameter can be grouped around two values. The strain anisotropy is estimated to have different value in the films grown by the two techniques and possible explanations are suggested.

Deformation potentials of the E1 (TO) and E2 modes of InN

The deformation potentials of the E1(TO) and E2 modes of InN are determined by combining infrared spectroscopic ellipsometry, Raman scattering, and x-ray diffraction measurements, and using a reported value of the mode Gruneisen parameter. The deformation potentials are obtained for two sets of stiffness constants. Strain-free values of the InN E1(TO) mode of 477.9 cm−1 and of the E2 mode of 491.1 cm−1 have been determined.

Optical and dielectric properties of dc magnetron sputtered AlN thin films correlated with deposition conditions

Abstract Thin films of AlN were grown on glass and aluminium foil substrates by reactive dc magnetron sputtering. The films exhibit a columnar growth with surface roughness in the order of 8.8–5.2 nm. The optical and dielectric properties of the films were studied as a function of the nitrogen concentration in the reactive gas mixture. The refractive index and extinction coefficient were found to be in the range of 1.93 to 2.3 and 10 −4 –10 −3 at λ =520 nm. The real part of dielectric constant and dielectric losses were measured to be about 7.0 and 0.006–0.085, respectively, using metal–insulator–metal (MIM) structures.

Interface structure of hydride vapor phase epitaxial GaN grown with high-temperature reactively sputtered AlN buffer

Thick hydride vapor phase epitaxy GaN layers have been grown on a-plane sapphire using high-temperature ion-assisted reactively sputtered AlN as a buffer layer. Transmission electron microscopy and atomic force microscopy were carried out to study the formation of the two interfaces sapphire/AlN and AlN/GaN, and their influence on the microstructure of both the buffer layer and the main GaN layer. It was demonstrated that the high-temperature reactively sputtered buffer layer provides a good alternative for hydride vapor phase epitaxy growth of GaN layers. In particular, the buffer promotes a specific interface ordering mechanism different from that observed on low-temperature buffers.

Misfit defect formation in thick GaN layers grown on sapphire by hydride vapor phase epitaxy

The nucleation and microstructure of large-scale columnar domains present in hydride vapor phase epitaxial (HPVE)-GaN layers grown directly on sapphire have been studied using cathodoluminescence and transmission electron microscopy. The domains are distributed in a quasicontinuous layer close to the GaN/sapphire interface. The domain boundaries are found to be associated with stacking mismatch defects. They are initiated at steps on the sapphire surface and are formed between nucleation islands growing on adjacent terraces. The formation of these domains in the initial stages of HVPE-GaN heteroepitaxial growth is proposed to play an important role in the strain relaxation mechanism.

Elimination of nonuniformities in thick GaN films using metalorganic chemical vapor deposited GaN templates

Thick hydride vapor phase epitaxial GaN layers are grown on metalorganic chemical vapor deposited GaN template layers as well as directly on sapphire, with the aim of investigating the effect of the template on the strain relaxation and spatial distribution of free carriers in the overgrown GaN films. Spatially resolved cross-sectional micro-Raman spectroscopy, cathodoluminescence, and transmission electron microscopy show improved crystalline quality for growth on metalorganic chemical vapor deposited GaN templates. The highly doped and highly defective near-substrate layer composed of columnar domains, typically present in hydride vapor phase epitaxial GaN films grown directly on sapphire, is absent in the layers grown on templates. Consequently, this results in elimination of the nonuniformities of free electron distribution, a lower residual free carrier concentration (<1017 cm−3), and improved strain relaxation.

Epitaxial growth and orientation of AlN thin films on Si(001) substrates deposited by reactive magnetron sputtering

Epitaxial domain formation and textured growth in AlN thin films deposited on Si(001) substrates by reactive magnetron sputtering was studied by transmission electron microscopy and x-ray diffraction. The films have a wurtzite type structure with a crystallographic orientation relationship to the silicon substrate of AlN(0001)‖Si(001). The AlN film is observed to nucleate randomly on the Si surface and grows three dimensionally, forming columnar domains. The in-plane orientation reveals four domains with their a axes rotated by 15° with respect to each other: AlN⟨112¯0⟩‖Si[110], AlN⟨011¯0⟩‖Si[110], AlN⟨112¯0⟩‖Si[100], and AlN⟨011¯0⟩‖Si[100] An explanation of the growth mode based on the large lattice mismatch and the topology of the substrate surface is proposed.

Effect of Si doping of metalorganic chemical vapor deposition-GaN templates on the defect arrangement in hydride vapor phase epitaxy-GaN overgrown layers

Two different types of dislocation arrangements have been observed in hydride vapor-phase epitaxial GaN films grown on sapphire substrates using both undoped and Si-doped GaN templates grown by metalorganic chemical vapor deposition: (i) predominantly straight threading dislocations parallel to the [0001] direction in the layer grown on an undoped template, and (ii) a network of interacting dislocations of edge, screw, and mixed character in the layer grown on a Si-doped template. The two types of defect distribution result in essentially different surface morphologies, respectively: (i) low-angle grain boundaries formed by pure edge dislocations around spiral grown hillocks, and (ii) smooth surface intersected by randomly distributed dislocations. The Si doping of the GaN templates was found to enhance defect interaction in the templates and to enable a reduction of the dislocation density in the overgrown thick GaN films, although it does not lead to an improvement of the overall structural properties o...

B implantation in 6H–SiC: Lattice damage recovery and implant activation upon high-temperature annealing

Implantation of B into n-type 6H–SiC has been investigated. B ion implantation was performed in n-type 6H–SiC single crystals at room temperature and 500 °C. The implanted specimens were annealed at 1700 °C in SiH4 atmosphere. Lattice damage induced by implantation and its recovery was characterized by Raman scattering. The net boron concentration and the amount of damage created were predicted by TRIM simulations. The redistribution of the implanted atoms during high-temperature annealing was obtained by simulation of the diffusion process. Capacitance–voltage characteristics of Ni/6H–SiC Schottky barrier were numerically simulated assuming different boron incorporation lattice sites. The degree of p-type activation of B ions after annealing at 1700 °C was estimated to be in the range of 8%–15%. A compensation ratio of about 48% was determined. The conductivity type was not converted from n to p type. The comparison between the experimentally obtained dopant concentrations and those predicted by simulati...

Extended defects in GaN films grown at high growth rate

A description of the present knowledge of the layer microstructure typical for thick GaN films grown at high growth rates is presented. The GaN layers are grown by hydride vapour phase epitaxy on sapphire and different buffers are used. The variety of extended defects present in such highly mismatched system are summarized, with the emphasis on their impact on the crystal quality. The defects are reviewed in two main categories according to the microstructural development during growth: large-scale nonuniformities and microstructural crystallographic defects. The first category comprises three-dimensional structural features developed mainly in the interface region, while the second are typical extended defects, i.e., dislocations with different Burgers vectors, nanopipes, inversion domain boundaries and stacking faults. The quality of the layers was improved vastly as a consequence of our understanding of the correlation of growth parameters and microstructure.