Strain relaxation in multilayer III–N structures on Si(111) substrates

X-ray diffractometry and X-ray scattering reciprocal space maps have been used to study strain relaxation in a complex buffer composed of seven intermediate layers of Al _x Ga_{1 ? x} N composition with different values of x, decreasing with an increase in the distance from the substrate. The layers have been grown by hydride metalorganic vapor phase epitaxy on silicon and sapphire substrates. Differences in the structural quality of the first four layers of a multilayer buffer grown on different substrates have been revealed. A gradual smoothing out of these differences in the next three layers with an increase in the layer serial number has been shown. The last grown intermediate Al _x Ga_{1 ? x} N layer and the GaN layer grown on it have identical thicknesses and degrees of mosaicity, regardless of the substrate type. Device structures grown on a complex buffer demonstrate emission in approximately the same wavelength range.     

Orientation relationships of aluminium nitride on sapphire

Orientation relationships of aluminium nitride on sapphire (112 6) AlN/(011 2) Al₂O₃ have been defined more exactly by X-ray diffractometry techniques. It has been found that, depending on the substrate misorientation from the surface plane (011 2), there can exist two kinds of azimuthal orientation of the epitaxial layer of AlN, namely, the projection of [0001] AlN on plane (011 2) Al₂O₃ can coincide either with [2 110] Al₂O₃ or with [21 1 0] Al₂O₃ direction. The vector field of the epitaxial layer inclination relative to the substrate misorientation has been drawn. The symmetry of this field obeys Neumann's rule. The epitaxial layer orientation has been found to depend on the sapphire substrate misorientation in direction [21 1 0].     

X-ray diagnostics of P-HEMT AlGaAs/InGaAs/GaAs structures

The structural characteristics of the P-HEMT AlGaAs/InGaAs/GaAs heterostructure have been studied by high-resolution X-ray diffractometry. The parameters of the heterostructure layers were determined by simultaneous analysis of the X-ray reflection curves for the (004) and (113) crystallographic planes. Interface diffusion has been established for the In_yGa_1?yAs quantum well and the Al_xGa_1?x As spacer layer, which are characterized by reconstructed profiles of the lattice parameter distribution and anisotropic distribution of random displacements in the layer plane and in the perpendicular direction.     

Growth and characterization of l-Tartaric acid,an NLO material

Single crystals of l-Tartaric acid (C₄H₆O₆), an organic nonlinear optical (NLO) material, have been grown by hanging seed solution and submerged seed solution techniques at room temperature. The crystal system has been confirmed from the single crystal X-ray diffraction analysis. The crystalline perfection was evaluated using high-resolution X-ray diffractometry (HRXRD). From this analysis, it was found that the quality of the crystal is quite good. The functional groups were identified using FTIR spectroscopy. UV–vis–NIR spectrum showed the absence of absorption in the wavelength region of 220–900 nm. The second harmonic generation efficiency is 89% compared to that of standard potassium dihydrogen phosphate (KDP). The laser damage threshold value is much higher than that of KDP and closer to that of β-barium borate.     

Growth of ZnSe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> solid solution crystals from vapor phase

Partially faceted ZnSe_{1 − x} S _x solid solution crystals have been grown from a vapor phase in a closed stationary horizontal system. The growth has been performed in cylindrical quartz ampoules—∼10 mm in diameter and evacuated to a residual pressure of less than ∼10⁻³ Pa-at a temperature of 1050°C and a temperature drop between the source and growth zones of 10–16°C. The crystal structure, optical absorption, and composition inhomogeneity along the ingot length have been investigated by X-ray diffractometry and optical spectroscopy in the wavelength range of 300–3000 nm. The solid-solution composition range corresponding to cubic polymorphs is determined. The concentration dependences of the lattice parameter and band gap are presented.     

Studies on the structural,thermal and optical behaviour of solution grown organic NLO material: 8‐hydroxyquinoline

Single crystal of 8‐hydroxyquinoline (8HQ) having chemical formula C₉H₇NO, an organic nonlinear optical (NLO) material has been successfully grown by slow evaporation solution growth technique at room temperature. The crystal system has been confirmed from the powder X‐ray diffraction (PXRD) analysis. The crystalline perfection was evaluated by high resolution X‐ray diffractometry (HRXRD). From this analysis we found that the quality of the crystal is quite good. However, a very low angle (tilt angle 14 arc sec) boundary was observed which might be due to entrapping of solvent molecules in the crystal during growth. Its optical behavior has been examined by UV‐Vis. analysis, which shows the absence of absorbance between the wavelengths ranging from 400 to 1200 nm. From the thermal analysis it was observed that the material exhibits single sharp weight loss starting at 113°C without any degradation. The laser damage threshold was measured at single shot mode and the SHG behavior has been tested using Nd:YAG laser as a source. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterisation of MoSexTe2−x (0 ⩽ x ⩽ 2) electrodes in terms of energetic location of valence and conduction bands

The layer type MoSe_xTe_2−x (0 ≦ x ≦ 2) has been grown by chemical vapour transport technique. The photoelectrochemical solar cells have been fabricated using the grown crystals as photoelectrodes and platinum grid as counter electrode in aqueous iodine/iodide solution. The optical band gap determination has been done from the study of the spectral response of the cells. The location of valence and conduction band edges and flat kind potentials have been evaluated using Mott-Schottky plots.     

Low-temperature growth and doping of InP:S single crystals from melt-solution in microgravity

The effect of microgravity on the growth of bulk InP:S single crystals from a melt with an initial equilibrium composition (84 at % In, 16 at % P, and ～2.2 × 10¹⁸ at cm^?3 of S) on board the Foton-11 satellite was investigated. The growth of crystals on board the satellite and on Earth (a reference crystal) was carried out by the traveling heater method. The samples of the grown crystals were investigated by metallography, double-crystal X-ray diffractometry, single-and double-crystal X-ray topography, and secondary-ion mass spectrometry. It is shown that the mass transfer in the melt in microgravity is similar to the diffusion mode. Hence, the mass transfer in the melt results in the following: the formation of a nonstationary boundary layer, depleted in phosphorus; the constitutional supercooling at the crystallization front accompanied with the development of a cellular substructure in the early growth stage; and the hypothetical phase structurization of the transition layer with the formation of In-based associates (clusters), which were found in the grown crystals in the form of spherical defects 10–20 μm in diameter. The coefficients of sulfur distribution k₀ = 0.274 and k_eff = 0.43, the sulfur diffusivity in the melt D_S = 4.2 × 10^?7 cm²/s, and the effective thickness of the transition layer δ = 0.07 cm in terrestrial gravity are determined. The data obtained are necessary to develop a mathematical model of crystallization in zero gravity.     

Compound formation in metal—semiconductor interactions

He⁺ ion backscattering and X-ray diffractometry have been used to study the formation of low temperature compounds between near-noble metals (M) and semiconductors (S). In the initial stage of compound formation where both unreacted semiconductor and metal layer are present the M₂S phase was found. At increasing time and/or temperature the formation of the second phase is controlled by the relative quantity of semiconductor and metal present in the sample. In excess of semiconductor or metal, phases richer in semiconductor or metal, respectively, have been found as end phases.     

Growth of ultrathin twin-free <Emphasis Type="Italic">b</Emphasis>-oriented YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 – <Emphasis Type="Italic">x</Emphasis></Subscript> films

Twin-free b-oriented YBa₂Cu₃O_{7 – x} films with a thickness less than 40 nm have been epitaxially grown on (100)SrLaGaO₄ crystals. Based on the temperature dependence of resistance, the onset temperature of the transition to the superconducting state is found to be 90 K; the transition width is 4 K. The film growth has been performed in two stages. A (100)PrBa₂Cu₃O_{7 – x} buffer layer was previously grown on a (100)SrLaGaO₄ substrate by rf magnetron sputtering in an Ar–O₂ gas mixture at a continuous and monotonic increase in temperature from 660 to 830°C. The main YBa₂Cu₃O_{7 – x} film was grown on the buffer layer surface by pulsed laser deposition in an oxygen medium at a fixed temperature (800°C). The above processes were implemented in different chambers, which were connected by a vacuum channel for transporting samples. Both films were grown in situ, without contacting atmosphere in all growth stages. An X-ray diffraction study has shown that the YBa₂Cu₃O_{7 – x} films are single-crystal and free of precipitates of other phases and domains of other orientations.     

Solution of the inverse problem for reconstructing the real structure of materials from the data of different X-ray methods

A software program has been developed for joint solution of the inverse problem on the basis of the X-ray diffractometry and reflectometry data. A system of basic objects supporting some functions of automatic data flow processing in the program and oriented to mathematical calculations has been created. It is shown by the example of experimental data for an In _x Ga_{1 ? x} As-In _y Al_{1 ? y} As/InP(001) sample that joint fitting of X-ray diffractometry and reflectometry curves makes it possible to reconstruct the model parameters of a multilayer with smaller rms errors. The data were processed taking into account the angular dependences of the background intensity and the aperture factors stemming from the geometry of the experiment.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

Diamond Synthesis by Thermal Plasma Chemical Vapour Deposition (TP-CVD)

This paper reviews results about diamond synthesis by high-temperature pyrolysis of methane in a H₂/Ar-plasma. By means of a modified direct-current (DC) plasma torch with an arc power of 4–8 kW diamond growth rates in the order of about 70 μm/h were obtained. Diamond films grown on unscrached single-crystalline silicon have a diameter of about 8 mm. Scanning electron microscopy (SEM), X-ray diffractometry (XRD), and Raman scattering spectroscopy have been used to analyse diamond films characterized by a transition of a crystalline diamond centre to an amorphous random region by passing a fine-crystalline region. In the film centre (100) planes with an average size of about 25 μm were made out by SEM.     

Growth and characterization of magneto‐optical YFeO3 crystals

The floating zone growth of magneto‐optical crystal YFeO₃ has been investigated. The polycrystalline feed rod was prepared by a pressure of 250MPa and sintering at about 1500°C. A crack‐ free YFeO₃ single crystal has been successfully grown. The crystal preferred to crystallize along <100> direction with about 10° deviation. The X‐ray rocking curve of the crystal has a FWHM of 24 arcsec, confirming the high crystal quality of the sample. The (100) plane was etched by hot phosphoric acid and the dislocation density was about 10⁴/cm². A thin outer layer with Y₂O₃‐rich composition was found at the periphery of as‐grown crystals, which was attributed to the Fe₂O₃ evaporation during growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of bismuth on liquid-phase epitaxy (LPE) grown GaAs layer using Ga–As–Bi melt

GaAs epitaxial layers of high structural quality have been realised from Ga–As–Bi melt using liquid-phase epitaxy (LPE). LPE grown GaAs epitaxial layer using bismuth solvent on GaAs substrate has been found to be of good structural perfection as compared to layers using gallium solvent. The temperature-dependent PL spectra of GaAs layer, grown from Ga+Bi mixed solvent has shown that the use of bismuth does not change the band energy. ECV depth profile of heavily zinc-doped epitaxial layer shows uniform doping in the GaAs layer grown using gallium solvent as compared to the layer grown using bismuth solvent.     

Structure of the interfaces of the InxGa1 − x As quantum well from X-ray diffraction data

The structure of the interfaces of a 10-nm-thick In_xGa_{1 ? x} As quantum well buried in the semiconductor GaAs matrix has been studied by the method of double-crystal X-ray diffractometry. It has been shown that, in comparison with the well-known photoluminescence method, the X-ray diffraction method has considerable advantages in characterization of multilayer systems. The detailed analysis of the rocking curves provided the reconstruction of the profiles of indium distributions in quantum wells for specimens with different indium concentrations.     

Characterization of melt grown phthalic anhydride single crystal

The bulk organic single crystal of phthalic anhydride (PA) was grown by vertical Bridgman technique. The structural perfection of the grown crystal was analysed by high resolution X‐ray diffraction (Multicrystal X‐ray diffractometry) studies. The thermal and mechanical properties have been studied. The second harmonic generation relative efficiency of Phthalic anhydride crystal is compared with KDP crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of glycine picrate—Remarkable second-harmonic generation in centrosymmetric crystal

Remarkable second-harmonic generation (SHG) efficiency has been observed in the glycine picrate (GP) though it crystallizes in centrosymmetric structure. Bulk single crystals of GP with a good size of ~20×10×3 mm³ have been successfully grown by the slow cooling method in aqueous medium. Powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) and FT-Raman studies have confirmed, respectively, the crystal structure and functional groups of the grown crystal. Crystalline perfection of single crystals has been evaluated by high-resolution X-ray diffractometry (HRXRD) using a multicrystal X-ray diffractometer and found that the grown crystals are nearly perfect. Nonlinear optical (NLO) behavior of glycine picrate crystals has been studied for the first time by Kurtz powder technique and its second-harmonic generation efficiency is found to be 2.34 times higher than that of KDP. Transparency of crystals in UV–vis–NIR region has also been studied. Dielectric measurements have been carried out using an impedance analyzer over a wide range of frequency (100 Hz–3 MHz) at room temperature. The slight decrease in dielectric constant has been observed as the frequency is increased and the dielectric loss is very low for the entire frequency range. The ac conductivity is almost constant up to 1 MHz and sudden increase has been observed above this frequency.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.     

Raman Scattering Studies on the Thin Graded Band Gap AlGaAs Hetero-Epitaxial Layer

Thin graded hetero-epitaxial AlGaAs layer has been grown from the undersaturated Liquid Phase Epitaxial (LPE) technique. The grown layers have been characterized using Laser Raman scattering studies. The peak position and intensity ratio of GaAs and AlAs like LO phonon frequencies have been measured and compared with conventional LPE grown AlGaAs epitaxial layer. The behaviour of GaAs and AlAs like LO phonons has been found to vary with the aluminum composition in the grown layer. Raman peak positions have been observed to shift to lower wavenumber in GaAs like LO phonon and higher wavenumber side of AlAs like LO phonon. Aluminum free features have been noticed in IEE grown AlGaAs (x > 0.8) hetero epitaxial layers.