R. Beanland

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.

Relaxation of InGaAs layers grown on (111)B GaAs

The relaxation behavior of InGaAs layers grown by molecular beam epitaxy on (111)B GaAs is investigated and compared with simultaneously grown (100) reference samples. Surface morphology, defect microstructure, and optical quality of the layers during the relaxation process are studied by Nomarski interference contrast, transmission electron microscopy, low‐temperature photoluminescence, and Raman spectroscopy. These techniques reveal an inhomogeneous and anisotropic relaxation in (111) samples. In (111) samples, the increase of critical thickness and the slower relaxation dependence on thickness, as compared with the (100) reference samples, is discussed

On the development of misfit dislocation distributions in strained epitaxial layer interfaces

Electronic devices such as lasers with performance characteristics that are unique may be constructed by the growth of a thin layer of semiconductor epitaxially on a substrate. Since the materials are often selected for reasons other than a perfect match of lattice constants, the mismatch between the two layers produces strain in the epitaxial layer that has to be accommodated in some way. The two possibilities are firstly, that the layer is in perfect register with the substrate leaving a strain in the layer, or secondly, the strain can be partially relieved in the layer by an array of misfit dislocations in the substrate-epilayer interface. It has been shown that the distribution of dislocation spacings in samples with a dislocation density less than approximately 2 {times} 10{sup 4} cm{sup {minus}1} has a broad distribution, with a single peak and an extended tail. A second peak forms in the distribution as the dislocation density increases to approximately 1.5 {times} 10{sup 5} cm{sup {minus}1}. This peak is found initially at 1.5 times the first modal peak, and as the dislocation density increases further, the second peak increases in magnitude and shifts to 2 times the first modal peak. This second peak occursmorexa0» when the dislocation density reaches approximately 2.5 {times} 10{sup 5} cm {sup {minus}1}. The authors have provided an explanation for why this should occur, suggesting that it is due to the formation of edge dislocations by the interaction of gliding 60{degree} dislocations.«xa0less

Optical evaluation of an AlAs/AlGaAs visible Bragg reflector grown by chemical beam epitaxy

A 21 layer AlAs/Al0.4Ga0.6As multilayer structure, designed as a Bragg reflector centered at 670 nm, has been grown by chemical beam epitaxy. The growth was monitored in real time by dynamic optical reflectivity (DOR) using a 670 nm semiconductor diode laser. The resultant DOR trace was compared to a computer simulation for the growth structure and good agreement is obtained using layer thicknesses measured by transmission electron microscopy. The wavelength dependent reflectivity of the Bragg reflector was measured using a grating spectrometer and good agreement is obtained to a computer simulation once the dispersive complex refractive index is taken into account.

X-ray measurement of deformation and dislocation density in semiconductor strained layers

Abstract Double-crystal X-ray diffraction is commonly used to measure the misfit strain and relaxation of epitaxial semiconductor layers. In this paper, a framework is developed which links the measured parameters Δ d / d and Δo to the deformation tensor of a semicoherent layer. Isotropic elasticity theory and the Frank-Bilby equation are used to derive an analytical expression for this deformation. By combining X-ray measurements of different planes, it is possible to obtain the misfit strain and details of the misfit dislocation array in a strained layer grown on a substrate of arbitrary orientation. In (001) layers, it is shown that the misfit strain and relaxation can be found from just six rocking curves, although the most accurate measurements require twelve rocking curves.

An in-situ laser-light scattering study of the development of surface topography during GaAs and InxGa1 − xAs chemical beam epitaxy

The development of surface topography during the annealing of epi-ready (001) GaAs wafers and the subsequent CBE growth of GaAs and InxGa1 − xAs layers were investigated using laser-light scattering (LLS) from a rotating substrate. The change in scattered light intensity during the pre-growth anneal is usually attributed to oxide desorption. However, we have found that a wide range of behaviour is possible depending on the growth history of the chamber. We suggest that this is due to the desorption of previously deposited species from the cryopanel as a result of radiant heating from the substrate, leading to contamination of the wafer surface. Procedures to minimise this contamination are outlined. LLS was used to study the changes in surface topography during growth, particularly the development of elliptical islands during the growth of GaAs and the onset of the “cross-hatch” pattern during the growth of InGaAs. It has recently been established that the ridges of the cross-hatch pattern are associated with dislocations which lie parallel to the surface. The development of these ridges parallel to 〈110〉 is related to the critical thickness of the layer. Interference oscillations in scattered light intensity during the growth of multilayer structures are also shown to be observable using LLS.

A novel design method for the suppression of edge dislocation formation in step-graded InGaAs/GaAs layers

Abstract A set of design principles for stacks of epitaxial layers with different lattice parameters is described. The thickness and composition of each layer is determined using a novel design method which aims to reduce the likelihood of edge dislocation formation within the epilayer by ensuring that the 60° dislocations at each interface are separated to such an extent that dislocation glide along {111} planes to form an edge dislocation necessitates the traversing of another interface. This implies that the two 60° dislocations which could glide to form an edge dislocation would have to interact with the dislocation array present at the InxGa1‒xAs/InyGa1‒yAs interfaces. Such interactions will reduce the number of edge dislocations created in the structure. Edge dislocation suppression is important since these dislocations are sessile and can block gliding threading dislocations, trapping them within the layer and reducing the extent of strain relief and thus the quality of possible devises. Stacks of ...

A model for the distribution of misfit dislocations near epitaxial layer interfaces

We present Monte Carlo simulations of the process of misfit relief in strained epitaxial layer interfaces. The emphasis is on the effect on the distribution of dislocation spacings owing to the reactions of 60° dislocations to form edge dislocations. When there are no edge dislocations the distribution is a decaying exponential function, but as the edge dislocation density increases a peak appears in the distribution. The position and magnitude of this peak depend on the proportion of edge dislocations found within the misfit dislocation array. We also take into account the effects of the experimental resolution on the appearance of the histograms. The results are consistent with detailed experimental measurements of dislocation spacing distributions and are important since the energy of an array increases significantly when the dislocation spacings are non-periodic.

MICROSTRUCTURE OF GAAS GROWN BY EXCIMER LASER-ASSISTED CHEMICAL BEAM EPITAXY

Selective-area CBE growth of GaAs on (100) GaAs substrates has been achieved using focused XeCl (308 nm) excimer laser assistance. The highest selectivity, at the growth rates used ( approximately 1 mu m h-1), occurred when the substrate temperature was <400 degrees C and the laser fluence was approximately 90 mJ cm-2. GaAs grown by laser-assisted CBE was found to contain dislocation tangles and to have an associated regular ripple structure on a scale of 300 nm.

Non-uniform strain relaxation in InxGa1−xAs layers

Abstract The deformation properties of the surface roughening associated with the strain relaxation process have been studied in a set of In x Ga 1− x As single layers (0.10≤ x ≤0.50) grown by different epitaxial techniques on GaAs and InP substrates. Raman spectroscopy performed on samples showing a rough surface reveals that, as the probing depth is reduced, a shift of the GaAs-like longitudinal optical phonon frequency, as well as an increase of the normally forbidden transverse optical mode, are produced. These effects tend to disappear in samples showing a flat relief. The results can be explained if a considerable strain relaxation together with a non-tetragonal distortion of the relaxed material occurred at the surface. Using the information on the surface geometry provided by atomic force microscopy and talystep measurements we have developed a simple elastic model which explains the observations.