X-ray diffraction study and quantitative measurements of high dislocation densities in superlattices and single crystal layers of heteroepitaxial systems with pronounced lattice mismatch

Plastic deformation in a single-crystal layer of the In_0.12Ga_0.88As/(111)InP solid solution is identified by the methods of X-ray diffractometry (XRD) and the double-crystal pseudorocking curves (DCPRC). X-ray topographs showed the generation of three intersecting systems of straight dislocations in the layer. In a one-layer ZnSe/GaAs structure and multilayer ZnSe/ZnSe_{1 − x} S_x/ZnSe/GaAs structures, the elastic and plastic strains were detected by the combined XRD-DCPRC method. The major components of the thermoelastic and plastic-deformation tensors were determined as ε_xx = ε_yy = 3.5 × 10⁻³ and ε_zz = 2.35 × 10⁻³. Using these data, the dislocation densities were determined as N _d ∼ 2.5 × 10⁸ cm⁻² and N _d ∼ 3 × 10¹⁰ cm⁻² for the 7 μm-thick ZnSe and 1 μm-thick InAs layers, respectively. In a superlattice of the Al_xGa_{1 − x} As/GaAs/⋯/GaAs-type with a large lattice parameter, the plastic deformation was detected. X-ray topography confirmed that the dislocation density in this superlattice equals ∼10⁵ cm⁻². __________ Translated from Kristallografiya, Vol. 45, No. 2, 2000, pp. 326–331. Original Russian Text Copyright ? 2000 by Kuznetsov.     

Formation of dislocation structure in silicon layers on non-orienting substrates

A dislocation structure of Si layers crystallized from a floating grain on quartz glass and mullite ceramics substrates has been investigated by transmission electron microscopy (TEM) including the high-voltage one. The effect of the layer orientation on the crystallographic features of dislocation distribution and brittle fracture in Si-SiO₂ system has been considered. The dislocation structure is proved to form mainly at temperatures lower than 0.8 of absolute melting temperature (T_m) of Si. Dislocation sources are located inside the crystallizable layer, and they are dislocations appearing from grain as well as the dislocation bundles near the interface. The cross slip of screws plays an essential role in dislocation multiplication. The difference of thermal expansion coefficients of the layer and substrate determines the finite dislocation density near the interface and in the bulk of the layer.     

Thick GaN layers on sapphire with various buffer layers

Thick GaN layers deposited in HVPE system on composite substrates made on sapphire substrates in Metalorganic Vapour Phase Epitaxy (MOVPE) system have been investigated. The following substrates were used: (00.1) sapphire substrates with AlN, AlN/GaN and GaN thin layers. The crystallographic structure and the quality of the epitaxial thick GaN layers were determined. Comparison of the three types of thick layers was performed. Significant differences were observed. It was found that thick GaN deposited on the simplest MOVPE‐GaN/sapphire composite substrate has comparable structure's properties as the other, more complicated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

MOVPE growth of InN buffer layers on sapphire

We report on the MOCVD growth of InN buffer layers on sapphire substrate for InN growth. The approach used assumes that an optimized InN buffer layer has to exhibit at least the same crystalline quality and sapphire surface coverage than the GaN buffer layers allowing to grow high crystalline quality GaN on sapphire. The buffer layers were characterized by AFM and GID measurements. Sapphire nitridation was investigated: it has a strong influence on in-plane crystalline quality. Two kinds of buffer layers were optimized according to the GaN buffer layer specifications: one of them only presented In droplets at its surface. It was shown that the small amount of In droplets increases the adatoms mobility of the main layer overgrown, leading to a 25% decrease of its in-plane mosaicity, compared to InN films directly grown on sapphire. To achieve a same improvement on InN buffer layer free of In droplets, the InN main layer growth temperature had to be increased from 550 °C. to 600 °C.     

Effect of the sign of misfit strain on the formation of a dislocation structure in SiGe epitaxial layers grown on Si and Ge substrates

Comparative analysis of the specific features of the formation of a dislocation structure in the single-layer epitaxial heterostructures Si_1?xGe_x/Si and Ge_1?ySi_y/Ge is performed. It is ascertained that, at a relatively low lattice mismatch between an epitaxial layer and a substrate, the sign of misfit strain at the interface significantly affects the processes of defect formation. The most probable reasons for the observed phenomena are analyzed with allowance for the specific features of the state of the ensemble of intrinsic point defects in epitaxial layers subjected to elastic strains of a different sign.     

Investigation of GaN heteroepitaxial layers by means of the kossel effect technique

GaN heteroepitaxial layers grown on spinel substrates have been investigated by the Kossel effect technique. The excitation of GaK_α X-rays inside the crystal lattice was carried out either by means of 1.25 MeV protons or 40 KeV electrons. Both methods give similar results concerning the values of the lattice parameters and the sign of polarity. Following the proton irradiation a lattice expansion normal to the surface occurs. A comparison of our results with those of other authors shows that the growth direction is the same for {111} spinel and {0001} sapphire substrates.     

Epitaxy of CdTe on sapphire substrates with titanium buffer layers

The formation of a developed electrical relief on the sapphire substrate surface is investigated. A technique is proposed for introducing Ti⁴⁺ impurity atoms into the sapphire crystal lattice by depositing titanium layers with a thickness of about 5 nm and their annealing in air (oxidizing atmosphere) to a temperature of 1400°C. It is shown that this preliminary treatment of the sapphire substrate surface results in epitaxial growth of (111) СdTe films parallel to the sapphire (0001) plane at a temperature of 350°C.     

Epitaxial growth of InN films on lattice-matched EuN buffer layers

Indium nitride (InN) films have been grown on lattice matched europium nitride (EuN) buffer layers by pulsed laser deposition (PLD) and their structural properties have been investigated. It has been revealed that the growth of EuN films on Al₂O₃ (0 0 0 1) substrates leads to the formation of polycrystalline EuN films, whereas epitaxial EuN (1 1 1) films grow on MgO (1 1 1) substrates at 860 °C. By using the EuN (1 1 1) films as buffer layers for InN growth, we have succeeded in the epitaxial growth of InN (0 0 0 1) films at 490 °C with an in-plane epitaxial relationship of [1 1 2¯ 0]_InN∥[1 1¯ 0]_EuN∥[1 1¯ 0]_MgO, which minimized the lattice mismatch between InN and EuN.     

Investigation of InAs—InSb solid solution heteroepitaxial layers and structures

The results of investigations concerning the peculiarities of obtaining InAs—InAs_1−xSb_x heterostructure and of its properties revealed that the quality of epitaxial layers as well as photoelectric and electroluminescent device structure characteristics are to a great extent determined by a disagreement magnitude of heterostructure materials. — To decrease the magnitude of disagreement at the layer-substrate boundary and to achieve the composition with x > 0.1 at some distance the method of layer-by-layer growth (ANDREWS et al.) and the obtaining a graded heterojunction with optimization of component distribution along epilayer thickness seems to be promising.     

Influence of amorphous buffer layers on the crystallinity of sputter-deposited undoped ZnO films

Undoped ZnO films were deposited by radio frequency (RF) magnetron sputtering on amorphous buffer layers such as SiO_x, SiO_xN_y, and SiN_x prepared by plasma enhanced chemical vapor deposition (PECVD) for dielectric layer in thin film transistor (TFT) application. ZnO was also deposited directly on glass and quartz substrate for comparison. It was found that continuous films were formed in the thickness up to 10 nm on all buffer layers. The crystallinity of ZnO films was improved in the order on quartz>SiO_x >SiO_xN_y>glass>SiN_x according to the investigated intensities of (0 0 2) XRD peaks. The crystallite sizes of ZnO were in the order of SiO_x～glass >SiN_x. Stable XRD parameters of ZnO thin films were obtained to the thickness from 40 to 100 nm grown on SiO_x insulator for TFT application. Investigation of the ZnO thin films by atomic force microscope (AFM) revealed that grain size and roughness obtained on SiN_x were larger than those on SiO_x and glass. Hence, both nucleation and crystallinity of sputtered ZnO thin films remarkably depended on amorphous buffer layers.     

Mechanism of condensation of heteroepitaxial A3B5 layers pulse of moderate power

The mechanism of condensation of heteroepitaxial layers in laser deposition was studied on GaAs films on NaCl. Films were deposited in a superhigh vacuum using laser pulses of moderate power. Pulsed deposition proved to be more suitable for nucleation and growth than continuous deposition. However in this case it is important to compare the number of atoms deposited per pulse and the number of preferential adsorption centres in the substrate temperature of 310 °C. The density of dislocations in the films was about 10⁹ cm⁻². The main reasons for the dislocations are the excess of low volatile component and thermal stresses. Using a laser with quantum energy higher than the energy gap of the deposited semiconductor GaAs, which provides congruent evaporation, and making an additional deposition of As, we could reduce dislocations to 10⁷ cm⁻². A mechanism is proposed to explain the reduction of temperature in single crystal growth under laser deposition: high energy ion component of laser plasma gives rise to orienting defects on the substrate surface, and these defects determine epitaxial growth.     

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm⁻¹ was found in Raman scattering spectra for all films with buffers, which is corresponding to the E₂ mode.     

Graded Ila-fluoride buffer layers for heteroepitaxy of lead chalcogenides and CdTe on Si

Graded IIa-fluoride layers consisting of nearly lattice matched CaF₂ at the Si interface, and of BaF₂ with 14% increased lattice constant at the opposite surface, form suitable substrates for subsequent growth of device quality narrow band gap lead chalcogenides for infrared applications, as well as for the growth of epitaxial CdTe. The optimised fluoride layers grown by simple MBE procedures exhibit specular surfaces, are crackfree and untwinned. Minimum ion channeling yields are nearly as low as in bulk materials. Mechanical stresses are almost completely relaxed at room temperature. The quality of the layers was rather insensitive to the exact composition grading procedure using stacked or continuously graded binary or ternary compositions within the CaF₂, SrF₂ and BaF₂ materials, but metastable (Ca,Ba)F₂ did not form on CaF₂/Si.     

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.     

Phase structure of boride layers on iron substrates with electroplated cobalt layers

Specimens made of Armco iron were electroplated with cobalt and then boronized with 2.2 vol% BCl₃ in H₂ at 1223 K. The boride layers obtained show the sequence of crystallographic phases which is (Co, Fe)B, FeB, Fe₂B, substrate material.     

X-ray measurement of deformation and dislocation density in semiconductor strained layers

Double-crystal X-ray diffraction is commonly used to measure the misfit strain and relaxation of epitaxial semiconductor layers. In this paper, a framework is developed which links the measured parameters Δd / d and Δø to the deformation tensor of a semicoherent layer. Isotropic elasticity theory and the Frank-Bilby equation are used to derive an analytical expression for this deformation. By combining X-ray measurements of different planes, it is possible to obtain the misfit strain and details of the misfit dislocation array in a strained layer grown on a substrate of arbitrary orientation. In (001) layers, it is shown that the misfit strain and relaxation can be found from just six rocking curves, although the most accurate measurements require twelve rocking curves.     

Improvement in crystallinity and optical properties of ZnO epitaxial layers by thermal annealed ZnO buffer layers with oxygen plasma

ZnO epitaxial layers with treated low-temperature (LT) ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si (1 0 0) substrates. The LT-ZnO buffer layers were treated by thermal annealing in O₂ plasma with various radio frequency (RF) power ranging from 100 to 300 W before the ZnO epilayers growth. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD), and room-temperature (RT) photoluminescence (PL) were carried out to investigate their structural and optical properties. The surface roughness measured by AFM was improved from 2.71 to 0.59 nm. The full-width at half-maximum (FWHM) of the rocking curve observed for ZnO (0 0 2) XRD and photoluminescence of the ZnO epilayers was decreased from 0.24° to 0.18° and from 232 to 133 meV, respectively. The intensity of the XRD rocking curve and the PL emission peak were increased. The XRD intensity ratio of the ZnO (0 0 2) to Si substrates and PL intensity ratio of the near-band edge emissions (NBEE) to the deep-level emissions (DLE) as a function of the RF power was increased from 0.166 to 0.467 and from 2.54 to 4.01, respectively. These results imply that the structural and optical properties of ZnO epilayers were improved by the treatment process.     

On the elements of the dislocation structure of deformed corundum

It is found by the method of transmission electron microscopy that the main elements of dislocation structure of deformed corundum are spatial triple dislocation nodes, recombination zones and dislocation network fragments. The structure of dislocation nodes is analyzed. It is shown that the nodes in corundum are the “rigid” constructions.     

Growth mechanism and structure of adsorbed layers on metals

In many cases surface compounds obtained by adsorption on single crystal faces are formed by nucleation processes. Models of structures for these surface compounds are proposed when there is a well defined coincidence mesh with the substrate, and eventually, when there is no coincidence mesh, for thick epitaxial layers. Examples of structures are given for sulphur and oxygen adsorbed on various metals, and for thick lead layers in epitaxy on gold surfaces.     

General orientational characteristics of heteroepitaxial layers of AIIBVI semiconductors on sapphire and semiconductor substrates with diamond and sphalerite structures (AIIIBV)

X-ray reflection and transmission diffractometric studies of nonisostructural heteroepitaxy showed that the spatial rotation of the crystal lattice of the grown heteroepitaxial layer with respect to the substrate lattice about the crystallographic direction common for both lattices is the most general characteristic feature of their mutual crystallographic orientation. This feature leads to the coincidence of the 〈0001〉 polar axis of the heteroepitaxial layer of the A^IIB^VI compounds with one of the 〈111〉 polar axes of the cubic substrate. The 30° and 90° rotations in the plane of heteroepitaxy and the vernier-type conjugating of the planar nets of the heteroepitaxial layer and the substrate are the particular cases of their mutual crystallographic orientations.