John E. Ayers

The measurement of threading dislocation densities in semiconductor crystals by X-ray diffraction

Abstract The measurement of threading dislocation densities in heteroepitaxial semiconductor layers is important for the development of injection lasers, microwave transistors, and also the integration of devices in disimilar semiconductors. Dislocation density measurements have been made by destructive techniques such as etching and by transmission electron microscopy (TEM). However, X-ray rocking curves provide non-destructive measurements of dislocation densities with accuracy equal to crystallographic etches or TEM. The theory of this technique has been described by Gay, Hirsch, and Kelly [P. Gay, P.B. Hirsch, and A. Kelly, Acta Met. 1 (1953) 315] and Hordon and Averbach [M.J. Hordon and B.L. Averbach, Acta Met. 9 (1961) 237], for the case of highly dislocated metal crystals. In this paper, the theory of dislocation density measurement from rocking curves is extended to the case of (001) zinc-blende semiconductors. It is shown that the measurement of several ( hkl ) rocking curve widths with a particular X-ray wavelength allows the calculation of the dislocation density by two independent techniques, thus allowing for a check of self-consistency. It is shown that for the case of epitaxial GaAs on Si(001), dislocation densities determined by these two methods are in good agreement.

Misfit dislocation density and strain relaxation in graded semiconductor heterostructures with arbitrary composition profiles

We present a computational approach for the determination of the equilibrium misfit dislocation density and strain in a semiconductor heterostructure with an arbitrary compositional profile. We demonstrate that there is good agreement between our computed results and known analytical solutions for heterostructures containing a single linearly graded layer or a single uniform composition layer. We have calculated the dislocation density and strain profiles in Si1−xGex/Si(001), InxGa1−xAs/GaAs(001), and ZnSySe1−y/GaAs(001) heterostructures, each containing a uniform composition layer (uniform layer) on a linearly graded buffer layer (graded layer). The density of misfit dislocations in the graded layer is inversely proportional to its grading coefficient and is unchanged by the presence of the uniform layer, but the dislocated thickness increases with the uniform layer thickness. If the uniform layer is sufficiently thick, misfit dislocations will exist throughout the graded layer, but additional misfit dis...

New model for the thickness and mismatch dependencies of threading dislocation densities in mismatched heteroepitaxial layers

A new model is proposed for the thickness dependence of the threading dislocation densities in mismatched heteroepitaxial semiconductor layers. This ‘‘glide model’’ has been developed based on the premise that dislocations establish mechanical equilibrium between the line tension in their misfit segments and the glide force experienced by their threading segments. The glide model correctly predicts the inverse relationship between the film thickness and the dislocation density in mismatched heteroepitaxial layers which are much thicker than the critical layer thickness. It also predicts the weak dependence of the dislocation density on the lattice mismatch. The quantitative results of the glide model are in fair agreement with published experimental results although no unknown or adjustable parameters have been incorporated.

X-ray characterization of dislocation density asymmetries in heteroepitaxial semiconductors

We demonstrate an x-ray rocking curve method which allows detection of an asymmetry in the dislocation densities in an heteroepitaxial (001) zinc blende semiconductor layer. These dislocations exist on two types of slip systems with their misfit dislocation line segments oriented along either a [1−10] direction (type A) or a [110] direction (type B). An imbalance in the densities of dislocations on these slip systems produces an observable azimuthal variation in the rocking curve width for symmetric x-ray reflections. An approximate quantitative model allows the estimation of the dislocation densities on the two types of slip systems.

Annealing induced nanostructure and photoluminescence property evolution in solution-processed Mg-alloyed ZnO nanowires

Solution-processed Mg-alloyed ZnO nanowire arrays have been achieved recently without using high temperature annealing process. By introducing thermal annealing processes in oxygen-rich ambient condition, the UV near-band-edge (NBE) emission was surprisingly mitigated until disappeared with annealing temperature increasing from 400 to 900 °C. As the annealing temperature increased, intensity of UV peak decreased while intensity of visible peak (490–520 nm) increased. The structure evolution upon thermal annealing was revealed to be responsible for these abnormal photoluminescence property variations, where unusual (Zn,Mg)1.7SiO4 epitaxially evolved on ZnMgO nanowires surface and contributed to the quenching of UV NBE emission. The structure evolution induced UV-NBE quenching and nanoscale localized alloying in semiconductor ZnMgO nanowires could bring up opportunities in catalysis, optoelectronics, spintronics, and sensors.

Complete removal of threading dislocations from mismatched layers by patterned heteroepitaxial processing

We report the effectiveness of patterned heteroepitaxial processing (PHP) in removing threading dislocations (TDs) from ZnSe epitaxial layers grown on GaAs substrates by metalorganic vapor phase epitaxy. The PHP approach used here involves postgrowth patterning of continuous epitaxial layers followed by annealing. In this study, each as-grown ZnSe/GaAs sample was first cut into pieces forming four types of samples, namely: (1) as grown, (2) postgrowth annealed, (3) postgrowth patterned, and (4) PHP prepared (patterned and annealed). The epitaxial layers with thicknesses of 2000–6000 A were patterned to create 500–6000-A-high and 3–70-μm-wide square mesas that were separated by 20 μm trenches. TD densities were determined by the etch pit density (EPD) technique and comparisons were made between the four types of samples. The first three types of samples exhibited EPDs of approximately 107 cm−2, which indicate that neither patterning alone nor annealing alone was effective at reducing TDs. In contrast, PHP ...

Design of S-Graded Buffer Layers for Metamorphic ZnSySe1−y/GaAs (001) Semiconductor Devices

We present design equations for error function (or “S-graded”) graded buffers for use in accommodating lattice mismatch of heteroepitaxial semiconductor devices. In an S-graded metamorphic buffer layer the composition and lattice mismatch profiles follow a normal cumulative distribution function. Minimum-energy calculations suggest that the S-graded profile may be beneficial for control of defect densities in lattice-mismatched devices because they have several characteristics which enhance the mobility and glide velocities of dislocations, thereby promoting long misfit segments with relatively few threading arms. First, there is a misfit-dislocation-free zone (MDFZ) adjacent to the interface, which avoids dislocation pinning defects associated with substrate defects. Second, there is another MDFZ near the surface, which reduces pinning interactions near the device layer which will be grown on top. Third, there is a large built-in strain in the top MDFZ, which enhances the glide of dislocations to sweep out threading arms. In this paper we present approximate design equations for the widths of the MDFZs, the built-in strain, and the peak misfit dislocation density for a general S-graded semiconductor with diamond or zincblende crystal structure and (001) orientation, and show that these design equations are in fair agreement with detailed numerical energy-minimization calculations for ZnSySe1−y/GaAs (001) heterostructures.

Equilibrium strain and dislocation density in exponentially graded Si1−xGex/Si (001)

We have calculated the equilibrium strain and misfit dislocation density profiles for heteroepitaxial Si1−xGex/Si (001) with convex exponential grading of composition. A graded layer of this type exhibits two regions free from misfit dislocations, one near the interface of thickness y1 and another near the free surface of thickness h−yd, where h is the layer thickness. The intermediate region contains an exponentially tapered density of misfit dislocations. We report approximate analytical models for the strain and dislocation density profile in exponentially graded Si1−xGex/Si (001) which may be used to calculate the effective stress and rate of lattice relaxation. The results of this work are readily extended to other semiconductor material systems and may be applied to the design of exponentially graded buffer layers for metamorphic device structures including transistors and light emitting diodes.

The influence of impurities on the dislocation behavior in heteroepitaxial ZnSe on GaAs

We have studied the influence of impurities on the dislocation behavior in heteroepitaxial layers of ZnSe on GaAs, grown by photoassisted organometallic vapor phase epitaxy. In undoped ZnSe layers, the dislocation densities are similar to those obtained by [S. Akram, H. Eshani, and I. B. Bhat, J. Cryst. Growth 124, 628 (1992)] whose data show that there is an inverse relationship between layer thickness and dislocation density. Incorporation of the electronically active impurity Cl increases the dislocation densities relative to undoped layers of the same thickness. Also, there is a correlation between the normalized dislocation density and the concentration of incorporated Cl. The isoelectronic impurity Cd has a similar effect on the dislocation density. Incorporation of Cd to a concentration of ∼1020/cm3 increases the dislocation density by a factor of about 3 compared to undoped layers of equal thickness. We also observed that doping of both Cd and Cl together resulted in low dislocation densities simi...

Factors influencing low-temperature photo-assisted OMVPE growth of ZnSe

The influence of temperature and ultraviolet (UV) light intensity on the optical characteristics of low-temperature, photo-assisted OMVPE growth of ZnSe on GaAs substrates has been studied. The epitaxial ZnSe was grown using the sources DMSe, DMZn and DMCd, with growth temperature varied from 360 to 400°C. We found that the UV intensity strongly influenced the optical quality of the epitaxial ZnSe layer. More intense near band-edge emission (NBE) is observed when the temperature is reduced from 400 to 360°C. The deep-level emissions (DLE) show a threshold-like dependence on UV light intensity for a given temperature. Beyond the threshold value of UV intensity the surface morphology improves and deep-level emissions are suppressed. Also, the UV threshold decreases with increasing growth temperature. The resulting ratio of near band-edge to deep-level emission intensity suggests the use of lower growth temperature and high UV intensity for blue-green light-emitting devices. The effect of cadmium doping on the optical properties of ZnSe was also investigated. Cadmium doping is found to improve the NBE/DLE photoluminescence intensity ratio, and this improvement is more pronounced at higher growth temperature.