A detailed study of the structure of a short-period superlattice based on alternating layers of cadmium and calcium fluorides grown by molecular beam epitaxy on a silicon substrate (111) by transmission electron microscopy and X-ray diffractometry has been performed. It has been established that the superlattice is in a pseudomorphic state and a lateral inhomogeneity with a fragment size of 10–40 nm has been found. The reason for the broadening of the main and satellite peaks of the superlattice on the diffraction curve (111) has been elucidated.

The properties of ferroelectric sol–gel PZT films deposited on a silicon-on-sapphire (SOS) substrate are compared with those of PZT films formed on a Si substrate. The crystalline structure, asymmetry of the hysteresis loops, polarization dependences of the transient current, short-circuit photocurrent, and open-circuit photovoltage, as well as the substrate bending have been studied. The PZT-on-SOS films are textured in a single (111) direction and exhibit symmetric hysteresis loops with a strong remanent polarization. The PZT films on Si are textured in the main (111) and weaker (100) directions, have weaker polarization, and exhibit an asymmetry of the hysteresis loops, which is reflected in the magnitude of the transient current and photocurrent. It is shown that the sapphire substrate has a convex bending that causes a compressive stress in the film plane, which weakens the effect of the lattice mismatch between PZT and Pt. By contrast, the Si substrate has a concave curvature, which causes the film to stretch. The deformations and mechanical stresses within the films were estimated. For PZT-on-Si, an estimate of the strain gradient along the (111) axis was obtained, which makes it possible to relate the asymmetry of the hysteresis loops to the flexoelectric polarization, with the flexoelectric coefficient for sol–gel PZT films found to be 0.0154 μC/cm. The results obtained show that the sapphire substrate provides a better quality of thin PZT films.

The work is devoted to the study of p-GaN: Mg epitaxial layers grown by the ammonia MBE technique. We find that the conductivity of GaN layers doped with Mg does not change with a postgrowth heat treatment. Formation of Mg3N2 nanocrystallites on GaN surface during epitaxial growth of the GaN layer with a high magnesium doping level was detected by the RHEED technique for the first time. It was shown that the Mg3N2 nanocrystallites formation competes with the acceptor states formation process. It has been proposed that the growth temperature can be applied as an additional “tuning” mechanism which affects the Mg incorporation into the growing GaN:Mg layers.

Two types of ferroelectric PZT films deposited at the same conditions on Silicon-on-Sapphire (SOS) and Si substrates are studied. The PZT on SOS shows a single (111) texture, symmetric hysteresis loops with high polarization values. The PZT film on Si is textured in the main (111) and weaker (100) directions, has lower polarization values and exhibits strongly asymmetrical hysteresis loops. Analysis of the substrate bending, mechanical stresses and deformations shows that the asymmetry of the hysteresis loops is due to the flexoelectric polarization caused by the compression strain gradient acting along (111) axis of the film. The flexoelectric coefficient for solgel PZT is measured to be 0.0154 μC/cm.

A detailed study of the structure of a short-period superlattice based on alternating layers of cadmium and calcium fluorides, grown by molecular beam epitaxy on a Si (111) substrate, by transmission electron microscopy and X-ray diffractometry, has been carried out. It was found that the superlattice is in a pseudomorphic state, and a lateral inhomogeneity with a fragment size of 10 - 40 nm was found. The reason for the broadening of the main and satellite peaks of the SL on the (111) diffraction curve has been clarified.

The properties of ferroelectric sol-gel PZT films deposited on a silicon-on-sapphire (SOS) substrate are compared with those of PZT films formed on a Si substrate. The crystalline structure, asymmetry of the hysteresis loops, polarization dependences of the transient current, short-circuit photocurrent, and open-circuit photovoltage, as well as the substrate bending have been studied. The PZT on SOS films are textured in a single (111) direction and exhibit symmetric hysteresis loops with a strong remanent polarization. The PZT films on Si are textured in the main (111) and weaker (100) directions, have weaker polarization, and exhibit an asymmetry of the hysteresis loops, which is reflected in the magnitude of the transient current and photocurrent. It is shown that the sapphire substrate has a convex bending that causes a compressive stress in the film plane, which weakens the effect of the lattice mismatch between PZT and Pt. By contrast, the Si substrate has a concave curvature, which causes the film to stretch. The deformations and mechanical stresses within the films were estimated. For PZT on Si, an estimate of the strain gradient along the (111) axis was obtained, which makes it possible to relate the asymmetry of the hysteresis loops to the flexoelectric polarization, with the flexoelectric coefficient for sol-gel PZT films found to be 0.0154 µC/cm. The results obtained show that the sapphire substrate provides a better quality of thin PZT films.

AlN/c-Al2O3 templates were grown by plasma-assisted molecular beam epitaxy using migration enhanced epitaxy (MEE) and metal modulated epitaxy (MME) employed for consequent growing the nucleation and buffer layers (NL and BL). Structural quality and stress evolving were compared using in situ stress measurements, x-ray diffraction, transmission and atomic force microscopies. Optimization of MEE mode of NL led to a high degree of initial nuclei coalescence and the weak tensile stress (<0.35 GPa) in the template. The optimal stoichiometric conditions were found for a double-stage MME of BL. During the first stage with the aluminum to active nitrogen flux ratio F-Al/F-N*=1.1 at 780 degrees C, the most effective bending of both screw and edge threading dislocations occurs, followed by their efficient filtration. This bending in the stretched BL is explained by the coarse grain AlN morphology, which can be smoothed dramatically during the top BL growth at higher temperature of 850 degrees C and F-Al/F-N*=2.1.

The results of transmission electron microscopy study and X-ray diffraction analysis of AlN/c Al2O3 templates with GaN ultrathin insertions grown by plasma-assisted molecular beam epitaxy are presented. It is shown that AlN buffer layers with faceted surface morphology provide a much higher threading dislocations density reduction then with smooth layers. The filtering action of ultrathin GaN insertions is confirmed.

We present the results of a transmission electron microscopy and X-ray diffractometry investigation of AlN/ c -Al 2 O 3 templates with GaN ultrathin insertions grown by plasma-assisted molecular beam epitaxy. It has been shown that AlN buffer layers with faceted surface morphology provide a greater threading dislocation density reduction than smooth layers. The filtering effect of GaN ultrathin insertions has been confirmed.

AlN/c-sapphire templates grown by plasma-activated molecular beam epitaxy have been studied using multicrystal X-ray diffractometry and a multi-beam optical stress meter system. Studies of the seed and buffer layers grown at different ratios of Al and N* growth flows and substrate temperatures have shown that templates with small tensile elastic stresses (<0.5 GPa) and densities of screw and edge grown-in dislocations 4 × 10 8 and 8 × 10 9 cm −2 , respectively, can be produced.

Multiple-crystal X-ray diffraction and a multi-beam optical stress sensor were used to study AlN/c-sapphire templates grown by plasma-assisted molecular beam epitaxy. The influence of the nucleation and buffer layers growth regimes, temperature, the ratio between Al and N* growth fluxes on the stress generation and the character of the dislocation structure were analyzed. Templates with the best crystal quality with screw and edge threading dislocation densities in a range of 4∙10^8 and 8∙10^9 cm-2, respectively, were obtained at the flux ratio of Al to N* close to 1 by using two-stage temperature regimes.

We describe stress evolution during plasma-assisted molecular beam epitaxy (PAMBE) of AlN nucleation and buffer layers on c-Al2O3 substrates at varying growth temperatures of 780 and 850°C. Moreover, different mechanisms of stress generation in the growing AlN films, related to the processes of grain coalescence and impact of Me-excess during PA MBE by migration enhanced epitaxy and metal-modulated epitaxy, are considered.

The deformation mode and defect structure of Al x Ga1 – xN:Si epitaxial layers (x = 0–0.7) grown by molecular beam epitaxy and doped with Si under a constant silane flux are studied by X-ray diffractometry. The concentration of Si atoms in the layers measured by secondary ion mass spectrometry is (4.0–8.0) × 1019 cm–3. It is found that the lateral residual stresses are compressive at x < 0.4 and become tensile at x > 0.4. The stresses after the end of growth are estimated and the contribution to the deformation mode of the layers of both the coalescence of nuclei of the growing layer and misfit stresses in the layer–buffer system are discussed. It is found that the density of vertical screw and edge dislocations are maximal at x = 0.7 and equal to 1.5 × 1010 and 8.2 × 1010 cm–2, respectively.

AbstractThe deformation mode and defect structure of Al_ x Ga_1 – x N:Si epitaxial layers ( x = 0–0.7) grown by molecular beam epitaxy and doped with Si under a constant silane flux are studied by X-ray diffractometry. The concentration of Si atoms in the layers measured by secondary ion mass spectrometry is (4.0–8.0) × 10^19 cm^–3. It is found that the lateral residual stresses are compressive at x < 0.4 and become tensile at x > 0.4. The stresses after the end of growth are estimated and the contribution to the deformation mode of the layers of both the coalescence of nuclei of the growing layer and misfit stresses in the layer–buffer system are discussed. It is found that the density of vertical screw and edge dislocations are maximal at x = 0.7 and equal to 1.5 × 10^10 and 8.2 × 10^10 cm^–2, respectively.

The paper describes experimental study of stress relaxation and generation in (1-2)-μm-thick GaN and AlGaN layers grown on AlN/c-Al2O3 buffer layers by low temperature (<720 °C) metal-rich plasma-assisted molecular beam epitaxy (PA MBE). The atomically smooth undoped GaN layers demonstrate only gradual relaxation of the compressive stress, which is probably related to thermodynamically driven inclination of threading dislocations (TDs).The slower stress relaxation at the lower growth temperature is explained by kinetic limitation of this process. The switch of compressive to tensile stress in the less-strained undoped Al0.7Ga0.3N layers, attributed mostly to the same effect of TD inclination, occurs in the low-temperature PA MBE conditions at much larger thickness (∼0.6 with) as compared to MOVPE ones. Introduction of high Si doping (n∼1019cm-3) reduces noticeably the initial compressive stress in the AlGaN film due to substitution of small Si atoms in the group-III sublattice. At larger thickness, Si atoms seem to effect the TD propagation and suppress generation of tensile stress related to TD inclination, which makes possible to grow ∼1μm-thick Al0.7Ga0.3N: Si films without cracking.

AbstractThe effect of atomic aluminum deposited onto sapphire substrates with different nitridation levels on the quality of AlN layers grown by ammonia molecular-beam epitaxy is investigated. The nitridation of sapphire with the formation of ~1 monolayer of AlN is shown to ensure the growth of layers with a smoother surface and better crystal quality than in the case of the formation of a nitrided AlN layer with a thickness of ~2 monolayers. It is demonstrated that the change in the duration of exposure of nitrided substrates to the atomic aluminum flux does not significantly affect the parameters of subsequent AlN layers.

The effect of atomic aluminum deposited onto sapphire substrates with different nitridation levels on the quality of AlN layers grown by ammonia molecular-beam epitaxy is investigated. The nitridation of sapphire with the formation of ~1 monolayer of AlN is shown to ensure the growth of layers with a smoother surface and better crystal quality than in the case of the formation of a nitrided AlN layer with a thickness of ~2 monolayers. It is demonstrated that the change in the duration of exposure of nitrided substrates to the atomic aluminum flux does not significantly affect the parameters of subsequent AlN layers.

The paper describes experimental results on low temperature plasma-assisted molecular beam epitaxy of GaN/AlN heterostructures on both 6H-SiC and Si(111) substrates. We demonstrate that application of migration enhanced epitaxy and metal-modulated epitaxy for growth of AlN nucleation and buffer layers lowers the screw and edge(total) threading dislocation (TD) densities down to 1.7.10(8) and 2.10(9) cm(-2), respectively, in a 2.8-mu m-thick GaN buffer layer grown atop of AlN/6H-SiC. The screw and total TD densities of 1.2.10(9) and 7.4.10(9) cm(-2), respectively, were achieved in a 1-mu m-thick GaN/AlNheterostructure on Si(111). Stress generation and relaxation in GaN/AlN heterostructures were investigated by using multi-beam optical stress sensor (MOSS) to achieve zero substrate curvature at room temperature. It isdemonstrated that a 1-mu m-thick GaN/AlN buffer layer grown by PA MBE provides planar substrate morphology in the case of growth on Si substrates whereas 5-mu m-thick GaN buffer layers have to be used to achieve the same when growing on 6H-SiC substrates.

The structure of AlN/c-sapphire templates obtained by plasma-activated molecular beam epitaxy (PAMBE) has been studied by X-ray diffractometry techniques. The results show the advantages of using coarse-grained AlN nucleation layers prepared by high-temperature (780°C) adatom-migration-enhanced epitaxy. Using 3.5-nm-thick GaN inserts (obtained by three-dimensional growth under N-rich conditions), it is possible to obtain templates with insignificant residual macrostresses and relatively narrow widths (FWHM) of 0002 and 10\(\bar 1\)5 diffraction reflections.

Self-polarization directed from the top electrode to the bottom one is found in the ferroelectric Pt/PZT/Pt capacitor using the method of depolarizing hysteresis loops. We attribute the self-polarization to the flexoelectric effect caused by the mismatch between the lattice parameters of the bottom Pt film and PZT-film. This result is consistent with the measurements of photocurrent in the short-circuited structure that also indicate the presence of the downward polarization in the PZT-film.