Approach for dislocation free GaN epitaxy

The characteristics of confined epitaxial growth are investigated with the goal of determining the contributing effects of mask attributes (spacing, feature size) and growth conditions (V/III ratio, pressure, temperature) on the efficiency of the approach for dislocation density reduction of GaN. In addition to standard (secondary electron and atomic force) microscopy, electron channeling contrast imaging (ECCI) is employed to identify extended defects over large (tens of microns) areas. Using this method, it is illustrated that by confining the epitaxial growth, high quality GaN can be grown with dislocation densities approaching zero.     

The reduction of the misfit dislocation in non-doped AlAs/GaAs DBRs

The non-doped AlAs/GaAs distributed Bragg reflectors (DBRs) with density of misfit dislocation (MD) close to zero had been obtained. The reduction of MD density was achieved by increasing temperature distribution homogeneity on the growing crystal in consequence of higher rotation rate of the wafer. Two structures of DBR were crystallized using molecular beam epitaxy (MBE) under the same optimal growth condition. The growth runs differ only in the rotation rate of the wafers. X-ray topograph showed no residual MDs in case of faster rotation. The DBR structure with residual MD density is still highly strained. No additional relaxation process has occurred, what was confirmed by an angular position of DBR zeroth-order peak on high-resolution X-ray diffractometry (HRXRD) rocking curve.     

Propagation of dislocations during epitaxy

The reduction of dislocation density in epitaxial layers relative to the original density in the substrate is a well-known phenomenon which seems particularly pronounced for liquid-phase processes and in compound semiconductors. Several mechanisms have previously been suggested to account for this reduction of dislocation density. We are here proposing a simple alternative in order to explain the formation of loops and the propagation parallel to the substrate surface. The initial stages of growth are assumed to consist of advancing terraces or ridges parallel to the surface. Dislocations may be dragged along with this parallel growth, and annihilation of dislocations results. We discuss the details of this model in terms of dislocation configurations and present consequences and methods for possible experimental verifications.     

Effect of thickness and heat treatment on the crystallite size and dislocation density of nanostructured zinc oxide thin films

The zinc oxide thin films were deposited by the sol–gel method on the glass microscope slide substrates. The microstructure of films was determined as a function of film thickness as well as annealing temperature using X-ray line broadening technique and applying whole powder pattern modeling (WPPM). This investigation showed that the film thickness has no significant effect on the grain size, whereas the dislocation density decreases with the film thickness. On the other hand with the rise of annealing temperature the dislocation density decreases, but the crystallite size becomes larger.     

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)     

Reducing dislocation density in GaN films using a cone-shaped patterned sapphire substrate

The threading dislocation (TD) density in GaN films grown directly on flat sapphire substrates is typically >10¹⁰/cm², which can deteriorate the properties of GaN-based LEDs significantly. This paper reports an approach to reducing the TD density in a GaN layer using a variety of patterned sapphire substrates (PSS). A cone-shaped PSS produced by metal organic chemical vapor deposition (MOVCD) was used for GaN deposition. Three types of GaN specimens were prepared at the initial nucleation stage, middle growth stage and final growth stage. The TDs generated on the cone-shaped PSS were analyzed by transmission electron microscopy (TEM) and a strain mapping simulation using HRTEM images, which evaluated the residual strain distribution. A large number of TDs were generated and the residual strain by the lattice distortions remained above the top of the cone-shaped regions. However, no TDs and residual strain were observed at the slope of the cone-shaped regions. This might be due to the formation of a GaN layer by lateral overgrowth at the slope of the cone-shaped regions, resulting in less lattice mismatch and incoherency between the GaN and sapphire. In conclusion, the TD density in the GaN layer could be reduced significantly, approximately 10⁷/cm², using the cone-shaped PSS.     

Change in dislocation distribution of KCl single crystals during thermal cyclic annealing

The change in dislocation distribution of KCl single crystals during thermal cyclic annealing was studied by etch pit method. The results obtained are summarized as follows. (1) In contrast to the case in Cu (KITAJIMA et al.; HASEGAWA et al.), the dislocation distribution in KCl crystals became non-uniform with an increase in the cyclic annealing time, i.e., in the region near the surface, many sub-boundaries were formed, but in the interior, the density of dislocations and sub-boundaries decreased. The cyclic annealing is more effective in reducing the dislocation density than the isothermal annealing. (2) The longer the period of thermal cycle, the more effective in reducing the dislocation density. (3) The larger the temperature difference between the maximum and minimum during one cycle, the more effective in reducing the dislocation density. (4) In the Sr-doped crystals, the thermal cyclic annealing was also effective in reducing the dislocation density within subgrains but sub-boundary length per unit area slightly increased by the cyclic annealing. (5) When the samples were annealed under the atmosphere of Cl₂, the dislocation density remarkably increased against our expectation, although the scattering centres had disappeared by this treatment.     

Effect of dislocation loops in macroscopically dislocation-free GaP substrates on the perfection of homo-epitactic deposits

Substrates and epitactic layers of GaP are etched under optimized conditions with the defect-revealing, preferential R-C etchant and subsequently examined using high-magnification (about 2500X) optical microscopy techniques. It is possible then to distinguish two new phenomena, viz. etch pits characteristic of dislocation loops in the substrate, and inclined dislocation dipoles in the layer. For layers grown on dislocation-free substrates we find that (i) the surface densities of both defects are equal (～5 × 10⁵cm^-2), and (ii) the average diameter of the dislocation loops in the substrate is roughly the same as the average distance between the two dislocations of the dislocation dipoles (0.5?1μm). Hence the perfection of these layers is determined by interfacial dislocation loops. Because the density of dislocation loops is only about a factor of two lower in highly (～1 × 10⁵ cm^-2) dislocated substrates, growth on these substrates results in layers which have even slightly lower dislocation density than layers grown on dislocation-free substrates. In the former case also single dislocations in the substrate propagate into the layer. Minority-carrier lifetime data indicate that minority-carrier recombination at dislocations is a restrictive factor for the luminescence quality of layers grown both on dislocation-free and highly-dislocated substrates.     

Habit,dislocation densities,and microhardness of NaClO3-NaBrO3 mixed crystals

Results of a systematic study of growth habit, dislocation densitics, and microhardness of the NaClO₃—NaBrO₃ mixed crystals are presented. A drastic change in habit of NaBrO₃ for small additions of NaClO₃ was observed. The composition dependence of dislocation density for this system is found to be highly non-linear. A non-linear composition dependence of microhardness, with positive deviations from linearity was also observed. This increase in hardness is correlated with the enhanced dislocation density in the mixed crystal.     

Size effect of dislocation density in Sb crystals grown by the bridgman method in a hard crucible

A dislocation density dependence on crystal transverse dimensions has been investigated for crystals of Sb grown in split graphite moulds at rates of 1.5; 8.5 and 15 cm/h. The dependence is shown to be the same for all growth rates which agrees with an earlier suggested theoretical equation.     

Reduction of grown-in dislocation density in Ge Czochralski-grown from the B2O3-partially-covered melt

Germanium (Ge) single crystals with an extremely low density of grown-in dislocations were grown by the Czochralski (CZ) technique with boron oxide (B₂O₃) liquid. Because attachment of particles floating on the melt surface to a growing Ge crystal leads to generation of dislocations during the growth, partial covering of the Ge melt surface with B₂O₃ liquid was attempted. Such attachment of particles was drastically suppressed or the particles were caught by the introduction of B₂O₃ liquid, and a particle-free Ge melt was realized in the central region of the melt surface. Ge single crystals were successfully grown from such melt, the grown-in dislocation density being 0–1×10³ cm⁻², which was remarkably lower than that in Ge crystals grown by a conventional CZ technique. The contaminations by B and O atoms of the grown crystal detected by SIMS analysis were very low. These Ge crystals have the potential for application to be applied as high-quality, dislocation-free substrates of GaAs solar cells for various usages including in space.     

The influences of supersaturation on LPE growth of GaN single crystals using the Na flux method

The dependency of LPE growth rate and dislocation density on supersaturation in the growth of GaN single crystals in the Na flux was investigated. When the growth rate was low during the growth of GaN at a small value of supersaturation, the dislocation density was much lower compared with that of a substrate grown by the Metal Organic Chemical Vapor Deposition method (MOCVD). In contrast, when the growth rate of GaN was high at a large value of supersaturation, the crystal was hopper including a large number of dislocations. The relationship between the growth conditions and the crystal color in GaN single crystals grown in Na flux was also investigated. When at 800 °C the nitrogen concentration in Na–Ga melt was low, the grown crystals were always tinted black. When the nitrogen concentration at 850 °C was high, transparent crystals could be grown.     

Finite element analysis of dislocation density during bulk single crystal growth (effect of doping atoms in InP single crystal)

A computer code for simulation of dislocation density in a bulk single crystal during liquid encapsulated Czochralski (LEC) or Czochralski (CZ) growth process. In this computer code, the shape of crystal–melt interface and the temperature in a crystal at an arbitrary time were determined by linear interpolation of the results that were discretely obtained by heat conduction analysis of a CZ single crystal growth system. A dislocation kinetics model called Haasen–Sumino model was used as a constitutive equation. In this model, creep strain rate is related to dislocation density, and this model extended to multiaxial stress state was incorporated into a finite element elastic creep analysis program for axisymmetric bodies. Dislocation density simulations were performed using this computer code for InP bulk single crystals with about 8″ in diameter. In the analysis, the effect of dopant atoms on the dislocation density was examined. In the case of a low doped InP single crystal, dislocations are distributed in the whole of the crystal. On the other hand, in the case of a highly doped InP single crystal, dislocations are localized at both the central and peripheral regions of the crystal.     

Edge dislocation effect on optical properties in wurtzite ZnO

The effect of edge dislocations on optical properties in wurtzite ZnO is studied using a k·p multiband Hamiltonian model. An edge dislocation is modeled as a negatively charged line due to its electron-acceptor nature with an elastic strain field due to the lattice distortion around the dislocation. The electrostatic potential strength of the negatively charged dislocation depends on the filling fraction, which describes the fraction of acceptor sites occupied by trapped electrons along the dislocation line. To understand the effect of electrostatic potential strength, the filling fraction has been varied in this work. Using the calculated energy levels and wave functions for electrons and holes from the k·p multiband Hamiltonian, the spontaneous emission spectrum has been obtained as a function of dislocation density and filling fraction. The calculated results are compared with available experimental photoluminescence data. The band edge peak intensity decreases significantly with increasing dislocation density. It is found that the electrostatic potential strength does not affect the band edge peak emission, but it generates deep level emissions. For low filling fractions, corresponding to high temperature, the most commonly observed green luminescence is found. For a high filling fraction, corresponding to low temperature, the green luminescence shifts to red luminescence, which is consistent with experimental observation.     

Crystal growth and dislocation arrangement of Czochralski alexandrite (BeAl2O4:Cr) single crystals

The [001] oriented alexandrite (BeAl₂O₄:Cr) single crystals have been grown by the Czochralski pulling technique. The X-ray topographic investigation indicates that the grown-in dislocations mainly originate from the seed and propagate along the path normal or nearly perpendicular to the growth interface. The dislocation density and arrangement is closely related to the quality of the seed crystal.     

On a delayed dislocation emission from hot hardness indentations

The change of the dislocation arrangements around microindentations after indentation test has been investigated by TEM and etching. The indentations were generated on {100} - surfaces of iron single crystals at temperatures between 800 K and 1000 K. The change of the dislocation structure was found to depend strongly on stress and dislocation density in the deformation field. The deformation field especially acts as a dislocation source operating over long periods. The results suggest that the propagation of emitted dislocations is controlled by dragging of Cottrell-atmospheres of carbon atoms. Conclusions regarding the hardness-temperature relation in pure iron are drawn.     

半导体单晶生长过程中的位错研究

阐述了现有的半导体单晶位错模型,即临界切应力模型和粘塑性模型的基本理论及应用状况.分析了熔体法单晶生长过程中影响位错产生、增殖的各种因素,以及抑制位错增殖的措施.与熔体不润湿、与晶体热膨胀系数相近的坩埚材料,低位错密度的籽晶可有效地抑制生长晶体的位错密度;固液界面的形状及晶体内的温度梯度是降低位错密度的关键控制因素,而两因素又受到炉膛温度梯度、长晶速率、气体和熔体对流等晶体生长工艺参数的影响.最后,对熔体单晶生长过程的位错研究进行了展望.     

Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire

This work assesses the relative effectiveness of four techniques to reduce the defect density in heteroepitaxial nonpolar a-plane GaN films grown on r-plane sapphire by metalorganic vapour phase epitaxy (MOVPE). The defect reduction techniques studied were: 3D–2D growth, SiN_x interlayers, ScN interlayers and epitaxial lateral overgrowth (ELOG). Plan-view transmission electron microscopy (TEM) showed that the GaN layer grown in a 2D fashion had a dislocation and basal-plane stacking fault (BSF) density of (1.9±0.2)×10¹¹ cm⁻² and (1.1±0.9)×10⁶ cm⁻¹, respectively. The dislocation and BSF densities were reduced by all methods compared to this 2D-grown layer (used as a seed layer for the interlayer and ELOG methods). The greatest reduction was achieved in the (0 0 0 1) wing of the ELOG sample, where the dislocation density was <1×10⁶ cm⁻² and BSF density was (2.0±0.7)×10⁴ cm⁻¹. Of the in-situ techniques, SiN_x interlayers were most effective: the interlayer with the highest surface coverage that was studied reduced the BSF density to (4.0±0.2)×10⁵ cm⁻¹ and the dislocation density was lowered by over two orders of magnitude to (3.5±0.2)×10⁸ cm⁻².     

Propagation behavior of threading dislocations during physical vapor transport growth of silicon carbide (SiC) single crystals

The dislocation formation and propagation processes in physical vapor transport (PVT) grown 4H silicon carbide (4H–SiC) single crystals have been investigated using defect selective etching and transmission electron microscopy (TEM). It was found that while the growth initiation process generally increased the density of threading dislocations in the grown crystal, for certain areas of the crystal, threading dislocations were terminated at the growth initiation. Foreign polytype inclusions also introduced a high density of dislocations at the polytype boundary. In the polytype-transformed areas of the crystal, almost no medium size hexagonal etch pits due to threading screw dislocations were observed, indicating that the foreign polytype inclusions had ceased the propagation of threading screw dislocations. Based on these results, we argued the formation and propagation of the threading dislocations in PVT grown SiC crystals, and proposed the dislocation conversion process as a plausible cause of the density reduction of threading dislocations during the PVT growth of SiC single crystals.     

Cathodoluminescence imaging for the determination of dislocation density in differently doped HVPE GaN

In this study we report the potential and limitations of the cathodoluminescence dark spot (DS) counting as a method for the determination of dislocation density and distribution in GaN, produced by the hydride vapour phase epitaxy (HVPE). Different GaN sample series (s.i. GaN:Fe and n-type GaN:Si) were used, in order to study the dependence of the results of the DS-counting on the dopant type and concentration. By the direct comparison of these results to classical defect selective etching, the DS-measurements were validated. It could be shown that each of the both methods have their particular restrictions, which must be considered in their application.