Calculation of displacement fields and simulation of HRTEM images of dislocations in sphalerite type A(III)B(V) compound semiconductors

The displacement fields of different kinds of both perfect and dissociated dislocations have been calculated for an isotropic continuum, and by means of linear elasticity. Additionally, the corresponding HRTEM images have been simulated by the well-established EMS program package in order to predetermine the structural aspects of dislocations, and then to compare it with experimental HRTEM micrographs. The latter ones resulted from plastically deformed GaP single crystals and InAs/(001)GaAs single epitaxial layers. It could be established that using the simple approach of linear elasticity and isotropy results can be obtained which correspond well to the experimental images. So, the structure of various Shockley partial dislocations bounding a stacking fault can be detected unambiguously. The splitting behaviour of perfect 30° dislocations (separation into a 0° and 60° partial) and 90° dislocations (separation into two 60° partials) both with line direction along 〈112〉, 60° dislocations (separation into 30°/90° and 90°/30° configuration) and screw dislocations (separation into two 30° partials) along 〈110〉 are discussed in the more detail. Moreover, the undissociated sessile Lomer dislocation, glissile 60° dislocation and edge dislocation have been considered too.     

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.     

In-situ fabrication of three-dimensionally confined GaAs and InAs volumes via growth on non-planar patterned GaAs(001) substrates

Three-dimensionally confined GaAs/AlGaAs and InAs/GaAs structures on 100 oriented square mesas patterned onto GaAs(001) substrates are realized, in-situ, via size-reducing molecular beam epitaxy. Two stages of mesa top pinch-off involving {103} and subsequently {101} side facets are revealed. GaAs and InAs quantum boxes with lateral linear dimensions down to 40 nm and confined by AlGaAs and GaAs, respectively, are reported. For InAs, the strain relief in mesas is found to enhance the well known 2 ML thickness for three-dimensional island formation on unpatterned substrates to, remarkably, >5 ML for mesa size 75 nm. Cathodoluminescence emission from the InAs on the mesa top attests to its good optical quality.     

Growth of InGaAs/GaAs on Offcut Substrates by MOVPE: Influence on Macrosteps and Dislocations Formation

A study of the relationship between the macrosteps caused by the substrate misorientation and dislocation nucleation in MOVPE-grown InGaAs/GaAs is presented. The macrosteps could favour strain relaxation and the decrease of the critical thickness, also by generation of misfit dislocations in the 1/2〈110〉{011} glide system, as they can provide sites for stress accumulation above the average value far from the macrosteps. This adds up to the enhanced homogeneous dislocation nucleation associated with the offcut angle. The use of offcut substrates thus produces both compositional inhomogeneities and an increase of the overall dislocation density.     

Nucleation,Glide Velocity and Blocking of Misfit Dislocations in SiGe/Si

Relaxation of strained layer systems is still not well understood. It is time dependent and changes considerably for samples with different growth history. This has to be discussed in terms of nucleation, glide velocity and blocking of misfit dislocations. We have investigated these phenomena at samples with SiGe layer thicknesses ranging from 60 nm up to 120 nm grown by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) by means of X-ray topography. The samples were annealed at temperatures between 500° and 600° C. Nucleation of misfit dislocations is heterogeneous and the rate is obviously dependent on layer strain and thickness. A quantified nucleation rate was not yet accessible, mainly due to the preferential formation of dislocation bundles. The propagation velocities of misfit dislocation segments were measured during annealing by means of synchrotron radiation plane wave topography (reflection geometry). The values agree well with theory and there is no evidence that they depend on growth regime. It is shown that the interaction of propagating misfit dislocations with crossing ones may lead to blocking or cross slip in different glide systems. These results are corroborated by investigations with atomic force and transmission electron microscopy.     

Dislocations and dislocation reduction in space grown GaSb

The behaviour of dislocations in GaSb crystals grown in space both from a stoichiometric melt (floating zone method, FZ) and a Bi solution (floating solution zone, FSZ) respectively, is studied. Predominantly straight 60° dislocations with Burgers vectors of the type b = a/2 <110> in (111) glide planes are identified. In the 20 mm long FZ single crystal the linear growing out of the dislocations is observed which reduces the dislocation density in the centre of the crystal to values below 300 cm^–2. The Bi incorporation in the FSZ crystal results in a misfit between seed and grown crystal and in a network of misfit dislocations at the interface. Thermocapillary convection during growth as well as the surface tension may be the reasons for the presence of curved dislocations and the higher dislocation density within a 1 – 2 mm border region at the edges of both of the crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of defects in GaP and GaAsP graded heterojunctions by transmission electron microscopy

Misfit dislocations are observed in graded heterojunctions GaAs_1?xP_x by electron microscopy. Results are in agreement with previous work concerning the nature of the dislocations (Lomer and 60° dislocations) and their density dependence on the phosphorus gradient. The discussion concerns the formation of Lomer dislocations and the possibility of reducing the density of inclined dislocations which reach the surface of the epitaxial layer. GaP substrates, S-doped, are examined by transmission electron microscopy. Numerous defects such as Frank loops, perfect loops, helical dislocations and precipitates are observed. A GaP homoepitaxial layer realized on this substrate is free from these defects but exhibits stacking faults. A zinc diffusion does not produce additional defect but a 1000 Å thick amorphous layer is observed a at the surface.     

Molecular dynamics simulation of (100)InGaAs/GaAs strained-layer relaxation processes

Molecular dynamics simulations on In_1−xGa_xAs/GaAs(100) systems are performed showing the dynamics of threading dislocations in the overlayers and the formation of misfit dislocations at the heterojunction interface. The developed code, using a modified Tersoff potential, simulates the threading dislocation dynamics in the InGaAs overlayer, and also the formation of interface misfit dislocations. Values for critical thicknesses are predicted and the atomic structure of the dislocation cores are determined.     

Defect Structure in Te-doped GaAs Single Crystals after Plastic Deformation (II). Dislocation Slip and Cell Formation

At temperatures above the brittle-to-ductile transition (490 °C) in Te-doped GaAs three types of predominant defect configurations have been observed after uniaxial compression along a [001] direction: (i) twins and stacking faults (500 … 520 °C), (ii) slip zones of dislocations (≈550 °C) and (iii) dislocation cells (580 … 590 °C). In Part II quantitative details of the appearance of slip and cell formation are given. Leading segments of gliding half loops are mainly of 60° type. Cell walls were formed by multiple slip of perfect dislocations.     

Uniformity improvement of selectively-grown InGaAs micro-discs on Si

Dislocation-free InGaAs micro-disks, having a diameter of approximately 5 μm and a thickness of approximately 0.2 μm, have been grown on Si(111) substrate patterned with SiO₂, with excellent uniformity in shape. This shape uniformity was dependent on the initial growth of InAs on Si windows. For the growth of well-controlled lateral islands, it is mandatory to cover the openings completely with a single-domain InAs crystal prior to the subsequent growth of InGaAs, the Ga content of which is essential for lateral growth. Moreover, the shape uniformity of InAs islands strongly affected the uniformity of the InGaAs islands' shape. On the basis of these observations, the diameter of Si openings was reduced to 1 μm to make it easier to fill the openings with InAs without failure. A two-step growth of InAs was devised to achieve void-less nucleation and lateral growth at the same time. It was also found that the recess at the boundary between Si and the SiO₂ mask played a vital role to limit the lateral size of InAs islands: when an InAs island hit that boundary, the lateral growth was suppressed and a stable {?110} facet emerged, and the growth seemed to wait for all the {?110} sidewalls to emerge. This mechanism compensated variation in the progress of InAs growth due to scattered timing of InAs nucleation. Therefore, for the purpose of improving the shape uniformity of InGaAs micro-disks, transition from InAs to InGaAs growth should be attempted after all the sidewalls are covered with {?110} facets and before vertical growth starts.     

Realization of optically active strained InAs island quantum boxes on GaAs(100) via molecular beam epitaxy and the role of island induced strain fields

We report here the realization of strained InAs three-dimensional islands on GaAs(100) with optical characteristics that reveal lateral quantum confinement (i.e. quantum box behavior). The importance of the cap layer growth conditions and methodology in achieving optically active InAs islands and the existence, range, and impact of island-induced strain fields on the cap layer growth are uncovered via marker layer experiments. Strong optical emission from the InAs islands is observed in correlation with the transmission electron microscope (TEM) observation of uniform coherent islands under optimized growth conditions. Photoluminescence excitation (PLE) spectroscopy reveals the presence of the energy transitions due to the three-dimensional electronic confinement in such InAs islands. The InAs islands buried under the GaAs were found to be quite stable upon annealing to 100°C higher than the growth temperature.     

Facilitating growth of InAs–InP core–shell nanowires through the introduction of Al

InAs nanowires were grown on GaAs substrates by the Au-assisted vapour–liquid–solid (VLS) method in a gas source molecular beam epitaxy (GSMBE) system. Passivation of the InAs nanowires using InP shells proved difficult due to the tendency for the formation of axial rather than core–shell structures. To circumvent this issue, Al_xIn_1?xAs or Al_xIn_1?xP shells with nominal Al composition fraction of x=0.20, 0.36, or 0.53 were grown by direct vapour–solid deposition on the sidewalls of the InAs nanowires. Characterisation by transmission electron microscopy revealed that the addition of Al in the shell resulted in a remarkable transition from the VLS to the vapour–solid growth mode with uniform shell thickness along the nanowire length. Possible mechanisms for this transition include reduced adatom diffusion, a phase change of the Au seed particle, and surfactant effects. The InAs–AlInP core-shell nanowires exhibited misfit dislocations, while the InAs–AlInAs nanowires with lower strain appeared to be free of dislocations.     

Effect of atomic hydrogen in highly lattice-mismatched molecular beam epitaxy

Effects of atomic hydrogen on the growth of lattice-mismatched InAs/GaAs and GaAs/InP systems have been investigated. The irradiation of atomic H has resulted in a delay of the onset of formation of three-dimensional islands maintaining flat surface morphology and increase of the critical layer thickness (CLT) from 4 to 10 å in the case of the InAs/GaAs system. The effect of atomic H was shown to be dependent on the growth conditions such as the growth temperature and V/III flux ratio. The increase of CLT with atomic H irradiation may be explained by the uniform distribution of the total misfit stress in the plane of the surface as a result of enhanced two-dimensional growth by atomic H acting as a surfactant.     

Edge misfit dislocations in the GeSi/Si(001) pair: Conditions and specific features of high-quantity generation

Dislocation structure of Ge_xSi_1?x films (x=0.4?0.8) grown by molecular-beam epitaxy on Si(001) substrates was studied by means of transmission electron microscopy. It was found that the density of edge MDs formed at the early stage of plastic strain relaxation in the films could exceed the density of 60° MDs. In our previous publications, a predominant mechanism underlying the early formation of edge misfit dislocations (MD) in Ge_xSi_1?x/Si films with x>0.4 was identified; this mechanism involves the following processes. A 60° glissile MD provokes nucleation of a complementary 60° MD gliding on a mirror-like tilted plane (111). A new edge MD forms as a result of interaction of the two complementary 60° MDs, and the length of the newly formed edge MD can then be increased following the motion of the “arms” of the complementary 60° MDs. Based on this scenario of the edge MD generation process, we have calculated the critical thickness of insertion of an edge MD into GeSi layers of different compositions using the force balance model. The obtained values were found to be more than twice lower than the similar values for 60° MDs. This result suggests that a promising strategy towards obtaining dislocation arrays dominated by 90° dislocations in MBE-grown Ge_xSi_1?x/Si films can be implemented through preliminary growth on the substrate of a thin, slightly relaxed buffer layer with 60° MDs present in this layer. The dislocated buffer layer, acting as a source of threading dislocations, promotes the strain relaxation in the main growing film through nucleation of edge MDs in the film/buffer interface. It was shown that in the presence of threading dislocations penetrating from the relaxed buffer into the film nucleation of edge MDs in the stressed film can be initiated even if the film thickness remains small in comparison with the critical thickness for insertion of 60° MDs. Examples of such unusual MD generation processes are found in the literature.     

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.     

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.     

Thermally activated stress relief in garnet layers grown by liquid phase epitaxy

The lattice misfit of (Lu, Tb)₃Fe₅O₁₂ epitaxial layers grown in compression on Gd₃Ga₅O₁₂ substrates has been found to decrease as a result of annealing at temperatures between 1000 and 1300°C. The deformation rate is thermally activated and depends on the degree of compressive misfit stress at the annealing temperature, the layer thickness and the reducing nature of the anneal atmosphere. Layers which were in tension at the annealing temperature (obtained by using Y₃Fe₅O₁₂ as the substrate material) did not exhibit stress relief. The process has been found to occur nonhomogeneously by the formation of regions of almost total relief which grow and multiply with continued annealing. The shape symmetry of these regions is consistent with dislocation climb in {112} planes. The results are interpreted in terms of a mechanism involving dislocation climb loops (∽1μm diameter) which develop as a result of the formation of oxygen vacancies.     

Defect Structure in Te-doped GaAs single Crystals after Plastic Deformation (I). Twins and Stacking Faults

At temperatures above the brittle-to-ductile transition (490 °C) in Te-doped GaAs three types of predominant defect configurations have been observed after uniaxial compression along a [001] direction: (i) twins and stacking faults (500 … 520 °C), (ii) slip zones of dislocations (≈ 550 °C) and (iii) dislocation cells (580 … 590 °C). In Part I quantitative details of the appearance of twins and stacking faults are given. Most frequently found were 30° partials in twins and stacking faults.     

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.     

How to Avoid Plastic Deformation in GaAs Wafers during Molecular Beam Epitaxial Growth

Plastic deformation in two‐inch diameter GaAs wafers resulted from standard thermal treatments which accompanied epitaxial growth in molecular beam epitaxy (MBE) machines of three different makes. Synchrotron based X‐ray transmission topography was used to distinguish between thermal treatment induced dislocation bundles and misfit dislocations. Eradication of the wafer slip related dislocation bundles has been achieved by modifications to the sample holder of a user built MBE machine. These modifications are discussed, the extent of the problem is briefly outlined, and an extrapolation of the susceptibility of GaAs wafers of higher diameters to this type of plastic deformation is given.