Interdiffusion phenomena in InGaAs/GaAs superlattice structures

We have studied structural properties of InGaAs/GaAs superlattice sample prepared by Molecular Beam Epitaxy (MBE) using high resolution X‐ray diffractometer (HRXRD). Increasing strain relaxation and defect generations are observed with the increasing Rapid Thermal Annealing (RTA) temperature up to 775 °C. The higher temperatures bring out relaxation mechanisms; interdiffusion and favored migration. The defect structure and the defects which are observed with the increasing annealing temperature were analyzed. Firstly, the in‐plane and out‐of‐plane strains after the annealing of sample were found. Secondly, the structural defect properties such as the parallel X‐ray strain, perpendicular X‐ray strain, misfit, degree of relaxation, x composition, tilt angles and dislocation that are obtained from X‐ray diffraction (XRD) analysis were carried out at every temperature. As a result, we observed that the asymmetric peaks especially in asymmetric (224) plane was affected more than symmetric and asymmetric planes with lower polar or inclination angles due to c‐direction at low temperature. These structural properties exhibit different unfavorable behaviors for every reflection direction at the increasing temperatures. The reason is the relaxation which is caused by spatially inhomogeneous strain distribution with the increasing annealing temperature. In the InGaAs superlattice samples, this process enhances preferential migration of In atoms along the growth direction. Further increase in the annealing temperature leads to the deterioration of the abrupt interfaces in the superlattice and degradation in its structural properties. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

两步法溅射中缓冲层厚度对Ge薄膜质量的影响

采用低温缓冲层技术制备Ge薄膜,利用AFM和Raman光谱研究缓冲层厚度对低温Ge缓冲层残余应变弛豫的影响.实验结果显示:随着缓冲层厚度的增加,残余应变弛豫度增大.在30 nm厚的低温Ge缓冲层上生长800nm厚的Ge外延层.Ge薄膜具有良好的结晶性,表面粗糙度RMS为2.06 nm.     

Dislocation Structure and Strain Localisation in Nickel Single Crystals Cyclically Deformed at 77 K

An experimentally based investigation is presented of the dislocation structure and of glide effects occurring in single slip oriented nickel crystals cyclically deformed at 77 K until saturation of the stress amplitude. Special attention is paid to a comparison of slip and structure phenomena observed in fatigue tests at 77 K and those found after cycling at room temperature (RT) and elevated temperatures. At strain amplitudes within the plateau region of the cyclic stress‐strain curve, where at higher temperatures in the crystal two structure types co‐exist, at 77 K nearly the entire specimen volume is occupied by one structure “phase”, a dislocation‐“condensed” wall configuration. On different scale levels the main characteristics of this extended wall structure were found to be independent of the imposed amplitude, and they turned out to fit in the temperature dependence of the structure features of the ladder‐like wall phase characterising the zones of intense slip (persistent slip bands) at RT and elevated temperatures. At 77 K the strain is localised in narrow slip bands (SBs) in the same way as at higher temperatures, although there is no indication of a “two‐phase” structure. From the experimental findings it is concluded that WINTERs “two‐phase” model remains valid, when averaging the plastic strain values over all SBs and over a sufficient number of cycles.     

Electrical and structural characteristics of metamorphic In0.38Al0.62As/In0.37Ga0.63As/In0.38Al0.62As HEMT nanoheterostructures

The influence of the metamorphic buffer design and epitaxial growth conditions on the electrical and structural characteristics of metamorphic In_0.38Al_0.62As/In_0.37Ga_0.63As/In_0.38Al_0.62As high electron mobility transistor (MHEMT) nanoheterostructures has been investigated. The samples were grown on GaAs(100) substrates by molecular beam epitaxy. The active regions of the nanoheterostructures are identical, while the metamorphic buffer In _x Al_{1 ? x} As is formed with a linear or stepwise (by Δ _x = 0.05) increase in the indium content over depth. It is found that MHEMT nanoheterostructures with a step metamorphic buffer have fewer defects and possess higher values of two-dimensional electron gas mobility at T = 77 K. The structures of the active region and metamorphic buffer have been thoroughly studied by transmission electron microscopy. It is shown that the relaxation of metamorphic buffer in the heterostructures under consideration is accompanied by the formation of structural defects of the following types: dislocations, microtwins, stacking faults, and wurtzite phase inclusions several nanometers in size.     

MOVPE growth and characterization of GaInAs(P) on (001) InP using diethyltertiarybutylarsine (DEtBAs) and tertiarybutylphosphine (TBP) as the group-V sources

We report on low pressure MOVPE results of (GaIn)As grown with the arsenic precursor DEtBAs as well as (GaIn)(AsP) grown on InP using the precursor combinations DEtBAs/TBP, DEtBAs/PH₃ which we carefully compared with samples grown in the standard system (AsH₃/PH₃). Lattice matched (GaIn)As layers have been obtained at a growth temperature of 600°C, a V/III ratio of 3 and at a growth rate of 1 μm/h at a reactor pressure of 50 mbar. Changing the As precursor from AsH₃ to DEtBAs simultaneously alters the incorporation coefficient ratio k_Ga/k_In from 1.15 to 1. At room temperature n-type mobilities of 9060 cm²/V s have been achieved for (GaIn)As grown with DEtBAs at a V/III ratio of 2. Besides the excitonic luminescence at 802 meV a lower energy peak appears in the photoluminescence (PL) spectra at 753 meV (carbon-carbon donor-acceptor pair). The excitonic luminescence peak has a FWHM of about 9 meV. The (GaIn)(AsP) layers have been grown at a growth temperature of 625°C and V/III ratios of 15–25 with DEtBAs/TBP and about 60 with DEtBAs/PH₃. Layers grown with higher V/III ratios show an enhanced As content. However, considering the P incorporation we have observed a qualitative difference between the standard hydride system and systems using alternative group-V precursors. Even in the system DEtBAs/PH₃ the phosphorus incorporation is much higher than in the AsH₃/PH₃ system indicating the influence of vapor phase exchange reactions.     

Characterization and growth of CdS nanoparticles by a cost effective chemical reduction method

A simple chemical reduction method is followed to grow CdS nanoparticles at room temperature with different duration of time. The grown samples are ultrasonicated in ethanol. The dispersed samples are characterized using electron diffraction techniques. Simultaneously optical absorption, photoluminescence and longtime photorelaxation of these samples are studied at room temperature. An increase in band gap is observed in each case as compared to bulk CdS. Also particle size increases with increase of growth time and hence there is decrease of band gap with increase of growth duration. Simultaneously long time relaxation increases with increase of growth time. An attempt is made to correlate the structural, optical, electrical and opto‐electrical properties. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect Structure of Strained Heteroepitaxial In(1—x)Al(x)P Layers Deposited by MOVPE on (001) GaAs Substrates

Weak-beam, large angle convergent beam electron diffraction and high resolution transmission electron microscope experiments have revealed, that after strain relaxation due to plastic deformation dislocation networks can be observed in In_(1—x)Al_(x)P heteroepitaxial layers grown on (001) GaAs substrates under compressive stress. The 60° slip dislocations are mostly dissociated into partials of Shockley type whereas in the particular case of layers grown under tension twins are predominantly formed by successive nucleation and slip of 90° Shockley partials on adjacent {111} glide planes lying inclined to the (001) surface. When a few 90° Shockley partials pile up during extension of twins, then planar incoherent twin boundaries with {112} coincidence planes have been formed during strain relaxation. Due to the space group symmetry ((InAl)P belongs to the space group F4-3m) there is a striking asymmetry in defect formation, i.e. defect nucleation and slip on the planes (111) and (1-1-1) slip of the [1-10] zone are preferred to nucleation and slip on the {111} planes of the [110] zone. Apparently, the occupacy of the atomic sites in the dislocation core with either group-III or group-V atoms is responsible for this behaviour. The nature of the defects implies that their spontaneous nucleation should have taken place at the growing surface. Under tensile strain the 90° Shockley partial is nucleated first and the 30° one trails. Under compressive strain this sequence is reversed. It is evident, for dissociated dislocations lying at the interface always the 30° partial, i.e. the partial with less mobility or with higher friction force, is detained near or directly in the interface. Thus, in layers grown under tension the stacking fault associated with the dissociated 60° dislocation lies inside the GaAs substrate. For layers grown under compression it is located inside the ternary layer.     

The growth and mechanical properties of paratellurite single crystals at high temperatures

α-TeO₂ single crystals have been grown by the Czochralski method on specially developed systems with automatic growth control. The mechanical properties of crystals under compression in the [100], [110], and [221] directions at a constant strain rate of about 10⁻⁴ s⁻¹ in the temperature range T = 733−993 K have been investigated. The distribution of plastic shear strain in deformed samples is has been experimentally studied. Manifestations of localized strain in covalent paratellurite crystals at T > 900 K are revealed for the first time. The active slip systems along which localized shear band are oriented are determined. The temperature dependence of the critical stress of transition to localized flow is investigated.     

Growth and characterization of Bis Thiourea Zinc Acetate (BTZA)

Single crystals of BTZA have been grown by low temperature solution growth method using slow cooling process at an optimized pH of 3.5. The grown crystals have been examined under an optical microscope to study its surface morphology. The morphological studies show that the growth takes place by spreading of growth layers. Formation of rectangular shaped etch pit on the as‐grown crystal has been explained in relation to the growth conditions. Single crystal X‐ray diffraction analysis has been carried out to confirm the monoclinic system. Transmission spectrum reveals that the crystal has a low UV cut off of 434.5 nm and has a transmittance of 100%. Dependence of micro hardness on load has been studied. Powder X‐ray Diffraction and FT‐IR have been carried out to characterize the grown crystals. BTZA forms metal – sulfur bond and has good optical transmission in the entire visible region, which is the essential requirement for a non‐linear crystal. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of a buried misfit dislocation network on the pyramid-to-dome transition size of Ge self-assembled quantum dots on Si(0 0 1)

The critical sizes of the pyramid-to-dome transition of Ge self-assembled quantum dots (SAQDs) grown on relaxed SiGe buffer layers were investigated for the relationship between the misfit strain built in dots and nucleation sites. The strain field of arrays of buried dislocations in a relaxed SiGe buffer layer provided preferential nucleation sites for quantum dots. Burgers vector analysis using plan-view transmission electron microscopy verified that the preferential nucleation sites of Ge SAQDs depended on the Burgers vector direction of corresponding dislocations. The measurement of the lateral distance between SAQDs and dislocations clarified that the location of SAQDs was at the intersection of the dislocation slip plane and the top surface. The samples are fabricated to contain low dislocation densities. The average dislocation spacing is larger than the surface migration length of Ge adatoms, resulting in two groups of SAQDs, those that are located along the dislocations, and those that are not. Atomic force microscopy observations showed a distinctively larger critical size for Ge SAQDs grown over the intersection of the dislocation slip plane and the top surface than those grown in regions between dislocations. These experimental observations indicate that the critical size of the pyramid-to-dome transition is strongly dependent on misfit strain in SAQDs with lower strain being associated with a larger critical size.     

Metamorphic growth of tensile strained GaInP on GaAs substrate

Optical and structural properties of tensile strained graded Ga_xIn_1−xP buffers grown on GaAs substrate have been studied by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements. The Ga composition in the graded buffer layers was varied from x=0.51 (lattice matched to GaAs) to x=0.66 (1% lattice mismatch to GaAs). The optimal growth temperature for the graded buffer layer was found to be about 80–100 °C lower than that for the lattice matched GaInP growth. The photoluminescence intensity and surface smoothness of the Ga_0.66In_0.34P layer grown on top of the graded buffer were strongly enhanced by temperature optimization. The relaxation of tensile GaInP was found to be highly anisotropic. A 1.5 μm thick graded buffer led to a 92% average relaxation and a room temperature photoluminescence peak wavelength of 596 nm.     

The influence of substrate temperature on the growth of sexiphenyl on mica(001)

Sexiphenyl thin films were grown by Hot Wall Epitaxy on air‐cleaved mica (001) surfaces at substrate temperatures between 293 K and 440 K. For the entire temperature range, organic thin films show nano‐needle like morphology. The nano‐needles grown at low substrate temperature (293 K) are shortest, and their growth is accompanied by a simultaneous formation of flat islands which disturbs the growth of nano‐needles. On the contrary, unusually long nano‐needles with typical lengths up to the mm range evolve during the growth at a substrate temperature close to the material's thermal desorption temperature at about 440 K. X‐ray diffraction reveals two different crystalline orientations for nano‐needles in the entire temperature range. At low substrate temperatures dominantly the (11 ) plane of the β‐phase is formed parallel to the mica (001) surface. At elevated temperatures another strong texture becomes dominant which is close to the (11 ) crystal orientation. In contrast to this, crystallites with the preferred orientation (001) parallel to the surface of the substrate are formed at low substrate temperature (293 K). This crystal orientation can be associated with flat islands observed in the early growth stage. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of low temperature dielectric processes in Poly(dicyclohexyl-itaconate)

Thermally stimulated depolarization current experiments (TSDC) have been carried out on poly(dicyclohexyl itaconate) (PDCHI) in the glassy state. The polymer exhibits three relaxation zones in the interval of temperature studied: a δ-relaxation at ∼130 K, a complex γ-process, in the temperature interval of 140 K to 200 K, and the β-process in the range of 210-270 K. The loss factor of PDCHI in the glassy zone has been reproduced from the partial depolarization data by using the elementary relaxation times and activation energies. The results show the existence of at least two relaxation processes in γ- and β-zones. A tentative explanation of the molecular origin of the observed secondary relaxations has been done by means of molecular mechanics (MM). At this respect, we perform a comparison of the relaxational data of PDCHI and those corresponding to PCHMA. This comparison allows us to confirm the relevant role played by the cyclohexyl ring in the δ- and γ-relaxations.     

Study of transport properties co – evaporated lead telluride (PbTe) thin films

Thin films of lead telluride (PbTe) of thicknesses ranging from 1000 Å to 2500 Å have been prepared by co‐evaporation (three temperature) technique, onto precleaned amorphous glass substrates at various temperatures. The deposited samples were annealed and annealed samples were used for characterization. Resistivity of these samples was measured by four‐probe technique as a function of thickness and temperature. Activation energy for charge transport have been evaluated and found in the range of 0.09 to 0.106 eV. Thermoelectric power has been measured and found to be positive indicating that the samples are p‐type semiconducting material. Mobility variation with temperature has been estimated (evaluated) and correlated with scattering mechanism in the entire range of temperature studied. The X‐ray diffraction analysis confirmed that films are polycrystalline having cubic structure cell and lattice parameters are reported.     

Epitaxial low-temperature growth of In<Subscript>0.5</Subscript>Ga<Subscript>0.5</Subscript>As films on GaAs(100) and GaAs(111)<Emphasis Type="Italic">A</Emphasis> substrates using a metamorphic buffer

A complex investigation of epitaxial In_0.5Ga_0.5As films grown on GaAs substrates with crystallographic orientations of (100) and (111)A in the standard high- and low-temperature modes has been performed. The parameters of the GaAs substrate and In_0.5Ga_0.5As film were matched using the technology of step-graded metamorphic buffer. The electrical and structural characteristics of the grown samples have been studied by the van der Pauw method, atomic force microscopy, scanning electron microscopy, and transmission/ scanning electron microscopy. The surface morphology is found to correlate with the sample growth temperature and doping with silicon. It is revealed that doping of low-temperature In_0.5Ga_0.5As layers with silicon significantly reduces both the surface roughness and highly improves the structural quality. Pores 50–100 nm in size are found in the low-temperature samples.     

Bias field induced impedance relaxation of TlGaSe2 semiconducting layered crystal at room temperature

Complex Impedance Spectroscopy has been employed for the study of absorption kinetics at TlGaSe₂ layered crystal under Bias voltage at room temperature. Two relaxations mechanisms have been observed. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The complex impedance spectra were fitted by the superposition of two Cole‐Cole types of relaxations. The fast relaxation in the higher frequency region may be due to dipolar relaxation and the slow relaxation in the low frequency region may be due to Maxwell‐Wagner type relaxation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer

The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy.     

Structure characterization of MHEMT heterostructure elements with In<Subscript>0.4</Subscript>Ga<Subscript>0.6</Subscript>As quantum well grown by molecular beam epitaxy on GaAs substrate using reciprocal space mapping

The crystallographic parameters of elements of a metamorphic high-electron-mobility transistor (MHEMT) heterostructure with In_0.4Ga_0.6As quantum well are determined using reciprocal space mapping. The heterostructure has been grown by molecular-beam epitaxy (MBE) on the vicinal surface of a GaAs substrate with a deviation angle of 2° from the (001) plane. The structure consists of a metamorphic step-graded buffer (composed of six layers, including an inverse step), a high-temperature buffer of constant composition, and active high-electron-mobility transistor (HEMT) layers. The InAs content in the metamorphic buffer layers varies from 0.1 to 0.48. Reciprocal space mapping has been performed for the 004 and 224 reflections (the latter in glancing exit geometry). Based on map processing, the lateral and vertical lattice parameters of In_xGa_1–xAs ternary solid solutions of variable composition have been determined. The degree of layer lattice relaxation and the compressive stress are found within the linear elasticity theory. The high-temperature buffer layer of constant composition (on which active MHEMT layers are directly formed) is shown to have the highest (close to 100%) degree of relaxation in comparison with all other heterostructure layers and a minimum compressive stress.     

Flux growth and low temperature dielectric relaxation in piezoelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals

Single crystals of Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT 91/9) have been grown by flux method after modifications in temperature profile, flux ratio and addition of excess ZnO/B₂O₃ which resulted in enhanced perovskite yield (more than 95%). Only a few crystals showed the presence of pyrochlore phase/variation in composition. A comparative characterization of these crystals were carried out in respect of piezoelectric charge coefficient d₃₃, dielectric constant, ac conductivity and hysteresis loop after cutting and poling the crystals along [001] direction. The total activation energy for conduction has been found to increase with Ti‐content in the sample. The effect of ZnO on growth behavior has been analyzed. A detailed analysis of PZNT (91:9) has been carried out at low temperature in respect of the various thermodynamic parameters related to the dielectric relaxation mechanism, like optical dielectric constant, static dielectric constant, free energy of activation for dipole relaxation, enthalpy of activation and relaxation time, have been calculated in the vicinity of transition temperature in the lower temperature region. The activation energy for relaxation at ‐10 and ‐49 °C have been found to be 0.09 and 0.02 eV respectively. The results were analyzed and a detailed dielectric analysis and low temperature relaxation behavior of PZNT crystals were interpreted. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Strain relaxation in InGaAs/GaAs quantum wells grown on GaAs (111)A substrates

We have grown In_0.2Ga_0.8As strained quantum wells (SQWs) on GaAs (111)A just and off-angled substrates by molecular beam epitaxy (MBE). The photoluminescence (PL) peak energy of SQWs grown on (111)A related substrates shows a large redshift as compared with the calculated values. The red-shift observed in SQWs grown on a (111)A 5° off toward [001] substrate can be explained by the presence of a built-in electric field E = 154 kV/cm due to piezoelectric effect. The larger red-shift observed in samples grown on the other substrates is partially due to strain relaxation. A strain relaxation mechanism that consists of coherently grown islands when InGaAs growth begins and the generation of misfit dislocations when these islands coalesce, gives a qualitative explanation of the observed results.