A. M. Keir

Surface topography changes during the growth of GaAs by molecular beam epitaxy

Changes in surface roughness taking place during (001) GaAs molecular beam epitaxy growth have been studied in situ using laser light scattering and ex situ using atomic force microscopy (AFM). Substantial increases in light scattering are found to occur firstly during oxide thermal desorption, associated with surface pit formation, and secondly during continued layer growth, due to the buildup of atomic step arrays. Monolayer height GaAs steps are readily resolved using AFM in air.

In-situ X-ray imaging of III–V strained-layer relaxation processes

Abstract The important value of the X-ray topography (XRT) technique for the investigation of III–V strained-layer relaxation processes is described. In addition to post-growth ex-situ XRT studies, a unique combined XRT/MBE growth facility has been constructed which allows the generation, motion and interaction of misfit dislocations to be monitored in-situ during epilayer growth, for the first time. The in-situ data already obtained for (100) InGaAs strained-layer growth on both Czochralski- and vertical-gradient freeze-grown GaAs substrates indicates technologically important differences in the initial relaxation process, including, in the latter case, the observation of a previously unreported secondary relaxation phase. Initial results relating to the influence of both post-growth annealing and the subsequent cool-down process are also described.

High resolution X-ray diffraction studies of CdxHg1-xTe/CdTe epitaxial layers grown by MOVPE on GaAs substrates

Abstract We have applied high resolution X-ray diffractometry and topography techniques to investigate both the lateral uniformity and structural properties of Cd x Hg 1− x Te layers grown by MOVPE onto CdTe buffer layers on GaAs. On samples ∼1–2 cm square, maps of rocking curve width (β) have shown values varying from 60 arc sec (comparable to the best reported) to over 1000 arc sec on the same slice, indicating the superior value of mapping over single point measurements on this material. A good correlation has been observed between rocking curve widths, lattice tilts and the density of pyramid-like surface defects, the last of which are also associated with an increased twin density. However, on rotating the sample about its surface normal, the 400 surface symmetric β-value varies by up to an order of magnitude, indicating that lattice tilts play an important role in broadening the rocking curve. X-ray topography reveals large tilt boundaries in the CMT epilayer which correlate with the dislocation structure in the GaAs substrate.

Dependence of the critical thickness on Si doping of InGaAs on GaAs

The formation of misfit dislocations during the initial stages of relaxation of In0.04Ga0.96As epitaxial layers on (001) GaAs has been studied by in situ high-resolution double crystal x-ray topography during molecular beam epitaxy growth. Relaxation is initially anisotropic with the fast B(g) dislocations being nucleated before the slow A(g) set. On doping with Si up to a maximum concentration of 4×1018 atoms/cm3, an increase in critical thickness was observed for both dislocation sets. The data can be fitted to an extension of the Matthews–Blakeslee model that includes a lattice friction force varying linearly with the dopant concentration.

Composition measurement in strained AlGaN epitaxial layers using x-ray diffraction

An x-ray diffraction technique is described which, by careful choice of the x-ray reflection used, minimizes errors in composition measurements resulting from strain and uncertainties in the elastic constants of a material. The method is applied to the AlGaN system, which shows a wide range of values for Poisson’s ratio in the literature and significant variation in strain state due to the high dislocation content and large thermal expansion mismatch with the substrate. It is demonstrated that accurate composition measurements of partially relaxed AlxGa1−xN layers (x 20nm can be made from a single measurement.

In-situ direct measurement of activation energies for the generation of misfit dislocations in the InGaAs/GaAs(001) system

Abstract In-situ X-ray topography (XRT) studies of misfit dislocation generation and movement in epitaxial InGaAs strained-layer structures on (001) GaAs are described. Examination of the changes in dislocation structure during a series of successive post-growth in-vacuo sample anneals has, for the first time, yielded activation energies of 0.7 and 0.8 eV for the formation of α-and β-misfit dislocations (MDs) by the initial glide of substrate threading dislocations (TDs) in the InGaAs epilayer. The introduction of MDs by this method is supplemented by the presence of an additional MD generation process. The activation energy for this is found to be comparable to that required to initiate the glide of a TD. The XRT studies have also confirmed the existence of MD cross-slip events, where α to β cross-slip was found to have an activation energy of 1.2 eV and to be much more common than the reverse β-α cross-slip process.

Effect of Si doping on the relaxation mechanism of InGaAs on GaAs

We report measurements on the initial stages of relaxation of Si-doped In0.04Ga0.96As epitaxial layers on (001) GaAs using in situ high-resolution double-crystal x-ray topography during molecular beam epitaxial growth. For Si concentrations up to 5×1018 cm−3, the critical thickness for formation of the first B(g) misfit dislocations is modeled accurately by the Matthews–Blakeslee model, extended to include a lattice friction force varying linearly with the dopant concentration. Below a Si concentration of 2×1018 cm−3, the model can be used to predict the critical thickness for generation of the orthogonal A(g) dislocation set, with the x-ray topographs showing that the B(g) misfit multiplication occurs at damaged edges of the wafer. However, above 2×1018 cm−3 Si concentration, the critical thickness for A(g) nucleation becomes almost independent of concentration and the x-ray topographs show that cross slip becomes important in the multiplication process. For most of the Si concentrations examined, the cr...

Z-contrast imaging of AlN exclusion layers in GaN field-effect transistors

The structural characteristics of AlN exclusion layers used to enhance the electron mobility in GaN-based field-effect transistor structures are investigated using scanning transmission electron microscopy. It is shown that a peak in electron mobility is achieved for an AlN exclusion layer with a nominal thickness of 2.3 nm, although significant compositional grading at the interfaces appears to be present. For longer growth times (i.e., 30 s), a transition to three-dimensional growth occurs, roughening the GaN/AlN/AlGaN interfaces. This roughening is likely to be associated with scattering of carriers in the two-dimensional electron gas (2DEG) consistent with an observed increase in 2DEG sheet resistivity.

A combination of high-resolution X-ray diffractometry and diffraction imaging techniques applied to the study of MOVPE-grown CdxHg1−xTe/CdTe on GaAs

Abstract We have applied a range of high-resolution X-ray diffractometry and diffraction imaging techniques to study the structural properties of CdxHg1-xTe (CMT) grown epitaxially on GaAs by MOVPE. In this paper we specifically describe three such techniques and evaluate and compare the results from each. Automated double crystal diffractometry with a mapping facility provides information on the quality and uniformity of the layers. 004 rocking curve widths vary from A number of layers with varying degrees of structural quality have been examined using a combination of the above techniques. Recent results are reported illustrating the value of each technique and we demonstrate how the application of a combination of X-ray diffraction techniques can be a powerful tool for investigating the nature of structural defects in this highly mismatched heteroepitaxial system.

Observation of hardening during relaxation of InGaAs on GaAs

The relaxation of Si-doped In0.04Ga0.96As epitaxial layers on (001) GaAs has been studied by in situ high-resolution x-ray diffraction during molecular beam epitaxy growth. By use of a novel technique to measure the wafer curvature with very high sensitivity, we have identified three regions in the relaxation process as a function of epilayer thickness. For thickness below that for multiplication of the slow A(g) misfit dislocations, the behaviour is effectively elastic, there being no detectable change in curvature at the critical thickness for dislocation nucleation. Beyond the thickness at which the A(g) dislocations multiply there is total or substantial relaxation, with very little increase in elastic strain as a function of thickness in this region corresponding to stage 1 of work-hardening in cubic materials. At much greater thickness, corresponding to stage 2 of bulk work-hardening, only partial relaxation occurs as the lattice hardens to the creation of misfit dislocations. Substantial elastic strain is observed to be locked into highly mismatched epilayers many times thicker than the critical thickness.