Dislocations In Epitaxial Layers Research Articles

Effect of temperature on conversion of basal plane dislocations to treading edge dislocations during 4H-SiC homoepitaxiy

Abstract 4H-SiC homoepitaxial layer grown on 4° off-axis substrates is performed at various growth temperature by using the traditional chemical vapor deposition. The effect of growth temperature on propagation of basal plane dislocations from substrate to epitaxy was studied using the deep KOH etching and Synchrotron reflection X-ray topography, and the conversion mechanism was also investigated. It is found that the majority basal plane dislocations are converted to threading edge dislocations during epitaxial growth. Meanwhile, the two types of threading edge dislocations in epitaxial layer are observed after KOH etch. By the optimization of growth temperature, high quality epitaxial wafer with extremely low basal plane dislocations density (

Diffuse X-ray scattering from correlated dislocations in epitaxial layers

Diffuse X-ray scattering from correlated dislocations in epitaxial layers

Nondestructive evaluation of misfit dislocation densities in ZnSe∕GaAs heterostructures by x-ray diffuse scattering

Estimating the low densities of misfit dislocations in epitaxial layers in the early stages of relaxation is difficult, requiring a very sensitive method. The characteristic patterns of diffuse x-ray scattering are therefore utilized to quantify the linear density of misfit dislocations. This is demonstrated for ZnSe∕GaAs(001) heterostructures with ZnSe layer thickness slightly above the critical thickness. The densities that resulted from calculated patterns of x-ray scattering perpendicular to the diffraction vector are found to be in good agreement with the results of cathodoluminescence and transmission electron microscopy. Moreover, the diffuse x-ray scattering is used to quantitatively study the thermally induced increase in the linear densities of misfit dislocations. Samples are annealed at various temperatures for increasing periods successively and are analyzed between the annealing steps by high-resolution x-ray diffraction. The density of dislocations is found to saturate for longer annealing periods. The saturation value and the rate of increase of the dislocation density depend both on the annealing temperature and on the crystallographic direction.

Microstructure of III‐N semiconductors related to their applications in optoelectronics

There has been more than a decade since Shuji Nakamura from Japanese company Nichia constructed the first blue LED based on structure of (AlGaIn)N semiconductor and eight years since he made the first blue laser diode (LD). This work gives a survey on the current technological status with green/blue/violet/UV optoelectronics based on III-N semiconductors in relation with their microstructure. The following devices are presented: i) Low-power green and blue LEDs, ii) High-power LEDs targeting solid-state white lighting, iii) Low-power violet LDs for high definition DVD market, iv) High-power violet LDs, v) UV LEDs. The discussion will be focused on three main technological problems related to the microstructure of (AlGaIn)N layers in emitters based on III-N semiconductors: i) high density of dislocations in epitaxial layers of GaN on foreign substrates (sapphire, SiC, GaAs), ii), presence of strains, iii) atom segregation in ternary and quaternary compounds. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray scattering from dislocation arrays in GaSb

The structure of GaSb layers grown epitaxially on GaAs(001) substrateshas been studied by high-resolution x-ray diffraction. The samples withthicknesses between 95 and 2845 Å were grown in Oxford by metal–organicvapour phase epitaxy. The large lattice mismatch is largely relaxed by twointersecting regular arrays of 90̂ dislocations along the [110] and [11̄0 ]directions. The density of the dislocations changes with the thickness but the dislocationsremain localized at the interface and both the GaSb layers and GaAs substrates aredistorted. The spacing between the dislocations in both the [110] and [11̄0 ]directions decreases while the relaxation increases as the layer thicknessincreases. We have found the spacing of the dislocations, relaxationand residual in-plane strain along the [110] and [11̄0 ]directions and showed a strong asymmetry in dislocation spacing andrelaxation between the two directions. This is because adjacent {110}planes in the zincblende structure are not equivalent as discussed byAbrahams and Buiocchi. Due to this asymmetry the structure of theGaSb layers is orthorhombic. Scattering from the thinner, ≲450 Å, layers shows that the GaSb grows in islands first and theirsize and distribution is different along the [110] and [11̄0 ]directions. The 60̂ dislocations in this system only appear in thick, ≳ 1200Å, layers and their density is significantly higher along the [110] direction. Theresults show the power of high-resolution x-ray diffraction for studyingnon-destructively growth, relaxation and dislocation structure of thin films andfor providing detailed quantitative information about the distortions caused bymisfit dislocations in epitaxial layers.

Recent progress in group-III nitride light-emitting diodes

We review the recent progress of nitride-based light-emitting diodes (LEDs) and discuss the dislocation issue and luminous efficiency. First, candela-class blue LEDs have been developed. InGaN layer was used for nitride LEDs instead of GaN active layer. The quantum-well-structure InGaN active layer dramatically improved the external quantum efficiency. There are a number of threading dislocations in epitaxial layer of nitride-based LEDs. InGaN-LEDs, however, have quite high external quantum efficiency. With regard to this, it is thought that the fluctuation of indium mole fraction is strongly related to the high external quantum efficiency. We also discuss the method to improve the external quantum efficiency of nitride-based LEDs.

Interaction characteristics of dislocations in strained epitaxial layers

In connection with nucleation and multiplication processes of misfit dislocations in epitaxial layers, elastic interactions of some configurations of dislocation surface half-loops and angular dislocations are investigated. By using the programme code produced by Gosling and Willis [J. Mech. Phys. Solid 42, 1199 (1994)], which calculates the image stresses of a straight dislocation half-line in a half-space and enables one to construct the stress field of any configuration of dislocation loops, the interaction energy of two surface half-loops is obtained. With the change of the relative positions of the two half-loops, the sign of the interaction energy varies from minus to plus, in accordance with which the interaction force between the two half-loops varies from an attractive force to a repulsive one. The interaction forces exerted on threading segments are also estimated for two angular dislocations, and their effects on the Matthews and Blakeslee equilibrium are discussed.

Diffuse x-ray scattering from misfit dislocations in SiGe epitaxial layers with graded Ge content

A theory has been developed describing x-ray diffuse scattering from misfit dislocations in epitaxial layers. This approach has been used for explaining the origins of diffuse x-ray scattering from SiGe layers with linearly graded Ge content. The distribution of the diffusely scattered intensity in reciprocal plane measured by triple-axis x-ray diffractometry has been compared with theoretical predictions and a good agreement has been achieved. It is demonstrated that the main part of the diffusely scattered intensity originates from random strains caused by misfit dislocations at the substrate–epilayer interface or in the relaxed part of the compositionally graded layers. The contribution of the threading dislocation segments to the diffuse scattering is rather small.

Ultrasonic etching of GaAs in CrO3-HF aqueous solutions

An ultrasonic vibration aided etching technique for revealing dislocations in GaAs has been developed. This technique, which employs CrO3-HF aqueous solutions aided by ultrasonic vibration, procedures clear etch pit patterns in thinly etched layers of GaAs, without light irradiation and agitation, at room temperature. Therefore, this technique is suitable to reveal dislocations in epitaxial layers of GaAs.

Multiplication of dislocations in Si1-xGex layers on Si(001).

A mechanism describing how dislocations in epitaxial layers multiply is presented. It is shown that a single threading dislocation can give rise to an array of dislocation sources, where each source generates a separate dislocation loop perpendicular to the primary misfit dislocation. The conditions controlling the activation of these sources are discussed in detail. Most notably, it is demonstrated that a minimum layer thickness, referred to as the ``multiplication thickness,'' is required to accommodate the cross-slip processes that are necessary for source activation. An experimental value of 0.67 \ensuremath{\mu}m for the multiplication thickness ${\mathit{h}}_{\mathit{x}}$ is measured from a single ${\mathrm{Si}}_{0.87}$${\mathrm{Ge}}_{0.13}$ layer on Si(001), and a general expression for ${\mathit{h}}_{\mathit{x}}$ is developed. The dislocation patterns produced by this mechanism are also considered, as are the implications of such a mechanism in light of the established view of strain relaxation.

A Multiplication Mechanism for Misfit Dislocations

ABSTRACTA new mechanism which describes how misfit dislocations in epitaxial layers multiply is presented. This work demonstrates how a single threading dislocation can give rise to an array of dislocation sources, where each source generates a single dislocation loop perpendicular to the primary misfit dislocation. As a threading dislocation with pure screw character glides through an epilayer, certain processes occur which lead to the production of a single dislocation half-loop, and the regeneration of the original threading dislocation. The regenerated threading dislocation continues to propagate on its primary glide plane, which allows the process to repeat itself at some later time. The result of this sequential process is an array of half-loops perpendicular to the primary misfit dislocation. The shape and symmetry of the arrays also contains information regarding how the mechanism operates. The proposed mechanism is related to misfit dislocation arrays in a single Si0.87Ge0.13 layer on Si(001).

The effect of various buffer-layer structures on the material quality and dislocation density of high composition AlxGa1−xAs laser material grown by metalorganic chemical vapor deposition

The effect of various types of buffer layers on the generation and propagation of dislocations in epitaxial layers of high composition (x=0.85) AlxGa1−xAs grown by metalorganic chemical vapor deposition (MOCVD) on horizontal Bridgman (HB) and liquid-encapsulated Czochralski (LEC) substrates is examined. Bulk epilayers of high composition (x=0.85) AlxGa1−xAs and graded-barrier quantum-well laser structures with confining layers of the same composition were grown simultaneously on high-quality/low etch-pit density (EPD) HB substrates and comparatively lower quality/high EPD LEC substrates with one of four types of compositionally graded and/or superlattice buffer-layer structures. The bulk material was characterized by delineation and measurement of surface EPD and the observation of overall surface morphology. Data are also presented on the device characteristics of graded-barrier quantum-well laser diodes grown with these same buffer layers in order to determine the correlation between dislocation density and laser threshold current. The various buffer-layer structures were seen to be effective in reducing the defect density and improving the surface morphology of high composition epilayers grown on both HB and LEC substrates. The threshold-current density of the laser diodes, however, was independent of both the type of prelayer and/or substrate utilized.

Misfit Dislocations in Epitaxial Layers of SI on Gap (001) Substrates

AbstractMisfit dislocations in epitaxial layers of Si grown by MBE at 570°C on GaP(001) substrates have been studied by TEM. It is found that layers as thick as 500 Å at least reside coherently on the substrate without misfit dislocations. In 1000 Å layers of Si the misfit strain is accommodated in part by 60-degree type dislocations with their Burgers vector inclined with respect to the interface, and by stacking faults intersecting the Si layer. The dislocations are dissociated into 30- and 90-degree Shockley partial dislocations. It is shown that in the case of a biaxial strain field, which is tensile in a (001)-plane, the 90-degree partial must be nucleated first. Only then can the 30-degree partial follow on the same glide plane. This geometrical effect explains the presence of dislocations as well as stacking faults in the Si layer.

Carrier recombination at dislocations in epitaxial gallium phosphide layers

Recombination at dislocations in epitaxial layers of GaP is studied by measuring the minority carrier lifetime around single dislocations. Dislocation lines, introduced by means of plastic deformation at a temperature of about 650°C, appear to be less efficient non-radiative recombination centres than the grown-in dislocations. The recombination efficiency of dislocations is significantly increased by the presence of impurities of different metals. Annealing at a temperature of 800°C decreases the lifetime by a factor of 2, probably due to the production of P-vacancies. Lifetime measurements in annealed samples and in as-grown etched by steps LPE and VPE layers show that the recombination efficiency of dislocations follows the P-vacancy and donor doping concentration. Taking the results as a whole, together with the temperature dependence of lifetime, we suggest that the enhanced non-radiative recombination at dislocations in GaP is due to a Cottrell atmosphere of S-donor/ P-vacancy complexes around the dislocations (i.e. traps similar to those which limit the bulk lifetime) rather than due to the core structure of the dislocation.

SEM cathodoluminescence studies of dislocation recombination in GaP

Single dislocations in epitaxial layers of GaP have been investigated using high resolution scanning electron microscope (SEM) cathodoluminescence (CL) techniques. The minority carrier lifetime was measured from the CL decay time constant, and impurity concentrations were assessed from a detailed analysis of the CL spectra. The minority carrier lifetime reduction at single dislocations varied between 10 and 40% depending on doping and growth procedure, decreasing to zero at approximately one diffusion length from the dislocation. A model based on recombination at an internal cylindrical surface did not adequately fit the experimental results, and it was therefore assumed that recombination occurred at a Cottrell atmosphere of impurities or defects surrounding the dislocation. Evidence to support such behaviour was obtained from CL spectra, which showed aggregation of nitrogen and a centre, probably a vacancy-donor complex, at 1.72 eV around dislocations. The close similarity between bulk and dislocation temperature dependence of lifetime suggests that the bulk recombination centre may be aggregating around dislocations.