P. Kidd

Plastic relaxation of InGaAs grown on GaAs

We report measurements of the plastic relaxation of InGaAs layers grown above critical thickness on GaAs substrates. The relaxation is accurately hyperbolic, proportional to the reciprocal of the layer thickness, in agreement with a recent geometrical theory of critical thickness [D. J. Dunstan, S. Young, and R. H. Dixon, J. Appl. Phys. 70, 3038 (1991)]. At large thicknesses, work hardening is observed which leads to a residual strain dependent on the original misfit.

Design of InGaAs linear graded buffer structures

The relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, has been studied. Simple InGaAs linear‐graded layers on GaAs substrates never reach complete relaxation. The residual strain in these structures produces a dislocation‐free strained top region while the rest of the buffer is nearly completely relaxed through misfit dislocations, as observed by transmission electron microscopy (TEM). This strained top region is analyzed and its thickness compared with theoretical calculations. The effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x‐ray diffraction, TEM, and low temperature photoluminescence, and results compared with predictions of the models. The optical quality of the cap layer improves when its composition is close to the value that matches the lattice parameter of the strained surface of the grade. The design of linear graded buffers having a strain‐free cap layer with high crystalline quality is...

Strain relief in linearly graded composition buffer layers: A design scheme to grow dislocation‐free (<105 cm−2) and unstrained epilayers

The strain relaxation in linearly graded composition InGaAs layers grown on (001) GaAs substrates by molecular beam epitaxy is studied by transmission electron microscopy (TEM) and double crystal x‐ray diffraction (DCXRD). The dislocation distribution in these layers does not coincide with the predicted equilibrium dislocation distribution [J. Tersoff, Appl. Phys. Lett. 62, 693 (1993)]. The dislocation density in the dislocation‐rich layer thickness is slightly smaller than the equilibrium density. The thickness of the dislocation‐rich region is different in the [110] and [110] directions. A good correspondence exists between the TEM and DCXRD strain measurements. The dislocation distribution observed by TEM has made it possible to design a scheme to grow dislocation‐free and unstrained top layers on linearly graded composition buffer layers.

A study of surface cross-hatch and misfit dislocation structure in In0.15Ga0.85As/GaAs grown by chemical beam epitaxy

It is well known that a cross-hatch develops on the surface of low-misfit strained semiconductor layers which undergo relaxation by the introduction of arrays of a2〈101〉 misfit dislocations in the interface between the strained layer and substrate. Here we present a study of the detailed structure of these surface striations and their development with thickness in a series of InxGa1 − xAs single layers on (001) GaAs, where x is close to 0.15. Using atomic force microscopy, it is found that the striations are in fact almost triangular ridges with rounded tops separated by V-shaped grooves. They are not slip traces. These ridges are found to be asymmetric in distribution, with those parallel to [110] far higher than those parallel to [110]. The spacing and height of the ridges increases with layer thickness. The structure also becomes more disordered in the case of thicker layers, with ridges running for shorter lengths and having more complex profiles. Using transmission electron microscopy, it is possible to link the ridges to dislocations lying above, and parallel to, the interface which result from repeated operation of multiplication sources.

Plastic relaxation of metamorphic single layer and multilayer InGaAs/GaAs structures

The plastic relaxation of multilayer structures of strained InGaAs grown above critical thickness on GaAs is reported and compared with the relaxation of single layers and with theory. We show that a composite structure, taken as a whole, follows the same relaxation law as observed in single layers. However, departures of the strains of some component layers from theory show that misfit dislocations are easily pinned at an interface. Implications for the design of relaxed buffer layer growth are discussed.

Comparison of the crystalline quality of step-graded and continuously graded InGaAs buffer layers

Abstract In this paper we summarize work carried out to investigate the relaxation of epitaxial strained layers of InGaAs on GaAs, where the InGaAs composition has been increased throughout the layer in either a stepwise or linearly graded form. The results are presented from the viewpoint of exploiting the relaxed layers to provide prescribed in-plane surface lattice parameters for subsequent use as “virtual” substrates for novel devices. We compare the behaviour of step-graded and linearly graded InGaAs layers. We consider the crystalline quality of different structures and discuss the design requirements for subsequent device quality growth.

Interpretation of the diffraction profile resulting from strain relaxation in epilayers

We have studied the profiles of the Bragg peaks and diffuse scattering in reciprocal space along both the plane perpendicular (qperpendicular to ) and plane parallel (q/sub ///) directions for sample structures consisting of layers of In0.1Ga0.9As grown by molecular beam epitaxy on (001) oriented GaAs substrates. The samples have different layer thicknesses and different dislocation distributions. We have measured the dislocation distributions in the interfaces using plan view transmission electron microscopy. We find that, for thin layers with low dislocation densities, the diffraction profiles in both the plane perpendicular (qperpendicular to ) and plane parallel (q/sub ///) can be modelled by considering two components of the diffraction profile, namely, dynamical scattering from the coherently coupled regions of perfect layer between dislocations and diffuse scattering from decoupled regions around the dislocations. From the q/sub /// profile a lateral dimension can be associated with the regions that give rise to the diffuse scattering, and we show that this dimension scales with the layer thickness. For thicker layers with higher dislocation densities, the strain fields of the dislocations overlap. In this case the diffraction profiles in (qperpendicular to ) are modelled by considering the ratio of the depth of coherently scattering decoupled crystal, above the dislocation array, with the total depth of the layer, assuming that scattering from the greatly distorted crystal close to the array is lost. Along q/sub /// the diffuse scattering is discussed on the basis of a statistical distribution of finite correlation lengths and microscopic tilts.

The effect of growth temperature on plastic relaxation of In0.2Ga0.8As surface layers on GaAs

Abstract Single surface layers of In 0.2 Ga 0.8 As ranging from 10 nm to 3 μm in thickness have been grown by molecular beam epitaxy (MBE) on GaAs at two growth temperatures, 400°C and 519°C. The residual strain of each layer has been obtained from double crystal X-ray diffraction (DCXD) measurements, and the surface morphology observed by differential interference contrast microscopy. The critical thickness is found to be independent of growth temperature. The effects of the change in growth temperature on the surface morphology and the extent of relaxation with varying layer thickness are discussed.

The onset of plasticity in nanoscale contact loading

We report studies of strained‐layer semiconductor superlattice structures under nanoindentation. Coherency strain reduces the yield stress at room temperature, from 6 GPa in unstrained material to 3 GPa in the most highly strained structures. The dependence of the yield stress on the design parameters of the superlattice structures shows that the onset of plastic deformation under an inhomogeneous stress is a cooperative process that takes place simultaneously across a finite volume more than 150 nm across. In this way, we demonstrate a new yield criterion, of which the key feature is that it is to be averaged over a finite volume. This provides a natural explanation of the indentation‐hardness size effect.

Control of plasticity with coherency strain

We report measurements of the mechanical properties by nanoindentation at room temperature on coherently strained superlattices of InxGa1-xAs grown on InP. Elastic deformation and then plastic deformation are obtained. The yield pressure decreases with increasing strain modulation in the superlattice and, at the highest strains of 0.8%, the yield pressure is halved. In the plastic regime the behaviour again depends on the strain modulation. These results demonstrate the potential of tailoring coherency strain between layers in thin films to control their mechanical properties.