W.E. Hagston

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.

Molecular dynamics simulation of (100)InGaAs/GaAs strained-layer relaxation processes

Abstract Molecular dynamics simulations on In 1− x Ga x As/GaAs(100) systems are performed showing the dynamics of threading dislocations in the overlayers and the formation of misfit dislocations at the heterojunction interface. The developed code, using a modified Tersoff potential, simulates the threading dislocation dynamics in the InGaAs overlayer, and also the formation of interface misfit dislocations. Values for critical thicknesses are predicted and the atomic structure of the dislocation cores are determined.

An MBE growth facility for real-time in situ synchrotron x-ray topography studies of strained-layer III--V epitaxial materials

This paper describes a unique combined UHV MBE growth x‐ray topography facility designed to allow the first real‐time synchrotron radiation x‐ray topography study of strained‐layer III–V growth processes. This system will enable unambiguous determination of dislocation nucleation and multiplication processes as a function of controlled variations in growth conditions, and also during post‐growth thermal processing. The planned experiments have placed very stringent demands upon the engineering design of the system, and design details regarding the growth chamber; sample manipulator, x‐ray optics, and real‐time imaging systems are described. Results obtained during a feasibility study are also presented.

In situ X-ray topography studies during the molecular beam epitaxy growth of InGaAs on (001) GaAs: effects of substrate dislocation distribution on strain relaxation

We report results from a novel facility constructed to enable in situ X-ray diffraction studies during the molecular beam epitaxy growth of Ill-V strained layer device structures on 50 mm diameter substrates. This new facility, used in conjunction with the Daresbury synchrotron source, permits X-ray topographic imaging of individual misfit dislocations formed during the molecular beam epitaxy growth process. The misfit dislocation growth and interactions can be imaged as a function of layer thickness, strain, growth and post-growth conditions. Our recent results show that the nature and distribution of dislocations threading up from the substrate are crucial in determining the initial pattern of epilayer relaxation. Under certain growth conditions and substrate dislocation distributions, large areas of the epilayer remain free of misfit dislocations at epilayer thicknesses significantly higher than the measured initial critical thickness tc1. We have observed in situ for the first time a second critical thickness tc2 (under certain conditions tc2>2tc1) at which there is a rapid increase in misfit dislocation density as a second misfit dislocation source(s) becomes active.

Interpretation of the temperature-dependent behaviour of the emission from isoelectronic tellurium centres in epitaxial ZnSe1-xTex

The emission from isoelectronic tellurium centres in metal-organic chemical vapour deposition ZnSe1-xTex has been investigated with photoluminescence and photoluminescence excitation spectroscopy. The emission is dominated by two bands, previously called the S1 and S2 bands and attributed to excitonic emission at isolated tellurium ions or tellurium ion pairs or clusters. The authors account for the temperature-dependent behaviour of these bands with a model in which the tellurium isoelectronic centres behave as spherical potential wells which trap holes via low lying resonant states in the valence band. Changes in intensity of the emission when both bands are observed together have been satisfactorily accounted for within the framework of this model and indicate that the S1 emission results from exciton recombination at tellurium pairs whilst the S2 emission results from recombination at tellurium triplets or larger clusters.

Atmospheric pressure metalorganic chemical vapour deposition growth and optical studies of ZnSe1−xTex thin film alloys

Abstract We report on the growth of ZnSe 1−x Te x layers on (100) oriented GaAs substrates by atmospheric pressure metal-organic chemical vapour deposition (MOCVD) with a new zinc adduct dimethylzinc-triethylamine. Optical studies were confined to samples with a low tellurium (Te) concentration, x

The metalorganic chemical vapour deposition and photoluminescence of tellurium-doped ZnS/CdS: Te strained layer superlattices

Abstract Tellurium (Te) exciton traps have been doped into the wells of ZnS/CdS: Te superlattices to investigate the potential for achieving enhanced blue emission intensities at room temperature. Exciton trapping and corresponding blue emission was confirmed from photoluminescence studies, however, the emission was found to quench above about 200 K, which is probably attributable to dislocation formation in highly strained layers when large ions such as Te are incorporated. Comparison of the emission linewidths with that of bulk CdS: Te show that the nature of electron-phonon coupling to the Te centres changes in the superlattice structures compared to bulk material.

Spectroscopic determination of the band offset in CdTe/Cd1-xMnxTe multiple quantum wells grown on InSb substrates

Abstract The effect of magnetic fields on the photoluminescence from CdTe / Cd 1- x Mn x Te multiple quantum wells grown by molecular beam epitaxy on InSb has been studied. We have attempted to determine the band offsets by measuring the field-induced shifts in the emission energies for structures with periods greater than 140 A. Difficulties caused by uncertainties concerning the electron-hole binding energy and the extent of magnetic polaron formation are highlighted. For x ≈ 0.25 we confirm that the structures are of type I and find a valence band offset in the range 30–55 meV (provided that magnetic polarons do not form in the barriers). Preliminary results for small period (40 to 100 A) structures are also discussed.

Exciton binding energies in CdTe/Cd1−xMnxTe multiple quantum wells

Abstract These results are compared with binding energies calculated with a variational approach which accounts for motion along the z-direction as well as in the x-y plane. The 1S and 2S states of both light hole, (e1, l1), and heavy hole (e1h1) excitons have been observed in CdTe/Cd1−xMnxTe multiple quantum wells. Clear identification of the 2S states arises from their large diamagnetic effects and their observation has allowed the light and heavy hole exciton binding energies to be investigated with changes in depth of the electron and hole confining potentials by application of a magnetic field. In a sample with x ∼ 0.75 it is found that changes of the heavy hole exciton binding energies are small, ⪅ 1 2 meV, even for large changes, ≈40 meV, of the hole confining potential. For the light hole exciton changes of the binding energy are larger, ≅ 2meV, for smaller changes, ≅ 14meV, of the hole confining potential.

Growth and structure of CdTe/Cd1−xMnxTe multiple quantum wells showing excitonic 2S states

Abstract Multiple quantum well structures of CdTe and the dilute magnetic semiconductor Cd 1−x Mn x Te (x⪅0.08) have been grown by molecular beam epitaxy on InSb substrates. High resolution double crystal X-ray diffraction analysis using rocking curve simulations based on the dynamical theory of diffraction shows that these layers are of excellent structural quality and uniformity. The diffraction data indicate the presence of a highly mismatched layer a few monolayers thick at the interface between the InSb substrate and the CdTe buffer layer, which is consistent with previous Raman spectroscopy studies identifying the phase as In2Te3. Photoluminescence and photoluminescence excitation spectroscopy measurements indicate that well width fluctuations occur on a monolayer scale. Observation of the 2S state of both light and heavy hole excitons allows the binding energy of both excitons to be deduced—17.1 meV and 19.7 meV respectively—considerably enhanced from their three-dimensional values.