S.J. Barnett

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.

Photo-stimulated II–VI crystal growth: A study of low temperature epitaxy

Abstract A comparison is made with low temperature epitaxial growth of II–VI compounds using photo-MOVPE and alternatives such as MBE and pyrolytic MOVPE. A major attraction for photo-MOVPE is the potential for selective area deposition which could work in the substrate range of 200 to 300°C. However, at temperatures below 200°C, the growth rates are low ( ≈0.1 μm/h) and in this paper reasons for this are sought. The growth rate of HgTe decreases with an activation energy of 24.3 kcal/mol, however, with increased UV intensity the low temperature growth rate could be enhanced. CdTe photo-MOVPE growth rates are reduced at low temperatures to avoid the onset of homogeneous CdTe formation. This process has been modelled and can explain experimental conditions of growth rate and temperature where epitaxial growth will occur. The quality of epitaxial layers has been assessed by double crystal X-ray diffraction. HgTe layers are of very high quality and rocking curve widths approximately two times the theoretical half width have been measured. CdTe and CMT layers are more variable and an unusual feature of tilt for homo-epitaxy has been seen. However, the best CdTe layers had rocking curve widths within a factor of two of theoretical and were similar in quality to HgTe photo-MOVPE.

Effect of growth temperature on the optical, electrical and crystallographic properties of epitaxial indium gallium arsenide grown by MOCVD in an atmospheric pressure reactor

Indium gallium arsenide was grown epitaxially by metalorganic chemical vapour deposition (MOCVD) on (100) indium phosphide using trimethylindium. At an optimum growth temperature of 680°C, material of very good properties can be grown. The carrier concentration is 1x1015 cm-3 n-type with a mobility 80000 cm2 s-1 V-1 at 7 K. Double crystal X-ray diffraction rocking curve widths are <30 arc seconds using the (400) reflection with Cu radiation and the non-dispersive mode. The peak width in photoluminescence spectra at 2 K is < 2 meV. At lower growth temperatures the photoluminescence deteriorates dramatically. In previous work this has been attributed to enhanced carbon incorporation. In this work X-ray diffraction and transmission electron microscopy (TEM) measurements indicate that the deterioration is structural in origin and possibly due to the onset of longwavelength spinodal decomposition. TEM studies show that the interface between alloy and substrate is essentially defect free for optimally grown near lattice matched layers. Interfaces showing 2D electron gas effects and quantum wells can readily be grown. For wells, pauses in the growth can be used to sharpen interfaces and produce quantum wells of high photoluminescence efficiency.

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.

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.

Double‐crystal x‐ray diffraction from Si1−xGex/Si superlattices: Quantification of peak broadening effects

Double‐crystal x‐ray diffraction has been successfully used to study dimensional and compositional variations in Si1−x Gex /Si superlattices. For most samples studied an adequate agreement between theoretical and experimental diffraction profiles is obtained by using a theoretical model (kinematical) which assumes uniform superlattices with abrupt interfaces. In some samples, however, the superlattice‐related diffraction peaks reveal an asymmetric broadening which is shown by comparison with dynamical diffraction simulations to be due to a grading in layer compositions and thicknesses throughout the superlattice stack. Detailed analysis of the diffraction data identified a gradual drift of 1.25% in the Ge flux during molecular beam epitaxy (MBE) growth. The MBE system parameters were modified to compensate for this effect and allow the growth of more structurally perfect strained‐layer superlattices.

Demonstration of quantum confinement in InSb‐In1−xAlxSb multiquantum wells using photoluminescence spectroscopy

We report the observation of quantum confinement, using photoluminescence, in InSb‐In1−xAlxSb (0.08≤x≤0.23) multiquantum well samples grown by molecular beam epitaxy. A series of samples were studied with different well widths and varying concentration of aluminum in the barriers. The upshifted luminescence energies behave qualitatively as expected due to changes in confinement, and are in good quantitative agreement with calculated upshifts taking into account strain in the barriers. These results demonstrate that good quality heterostructures can be obtained in this material system and show its potential for device applications.

A study of layer composition of InGaAs/InP multiquantum wells grown by metalorganic chemical vapor deposition using double‐crystal x‐ray diffraction theory and experiment

InGaAs/InP multiquantum wells grown by metalorganic chemical vapor deposition have been studied by fitting computer simulated theoretical double‐crystal x‐ray rocking curves to experimental data. It is shown that the technique can identify the carry over of As into the InP barriers and the concentration and distribution of As are determined. The highest concentration of As observed was 21%, which was confined to the first 50 A of the barrier and fell to 2% towards the following interface. Both the maximum and minimum concentration of As was found to decrease as a function of the pause time between growth of the InGaAs quantum well and the InP barrier layers. These observations were confirmed by secondary ion mass spectrometry (SIMS) and other experimental data on the same material is shown to support the results.

X-ray topography of lattice relaxation in strained layer semiconductors: post-growth studies and a new facility for in situ topography during MBE growth

The ability of X-ray topography to image individual dislocations makes it an ideal tool with which to investigate the initial stages of lattice relaxation in strained-layer semiconductors. Topographs provide a useful insight into the origin of the first misfit dislocations, which define the critical thickness, and make possible quantitative analysis of the initial strain relaxation process. Examples are given for the InxGa1-xAs/GaAs and In1-xAlxSb/InSb materials systems. In the second case the rate of increase in misfit dislocation density with layer thickness is substantially lower. This is believed to be due to the different substrate dislocation densities of GaAs (>104 cm-2) and InSb (<102 cm-2). The early success of these experiments has led to the development of an MBE facility which allows X-ray topography during the growth and post-growth processing stages of the materials. The system is described and the potential benefits over ex situ studies are discussed.

X-ray diffraction determination of a semiconductor epilayer unit cell oriented and distorted arbitrarily

The growth of GaAlAs on misoriented GaAs (100) substrates has been shown to lead to complex distortions of the epilayer unit cell which must be accounted for if the strain state, and consequently the Al fraction, is to be deduced from X-ray diffraction studies. The growth of GaAlAs on vicinal GaAs (100) surfaces is also shown to lead to unit cell distortions other than the conventionally assumed tetragonal distortion and rigid-body rotation. The interpretation of asymmetric reflection results, particularly in situations where interfacial coherence with a growth substrate cannot be assumed, is in these cases non-trivial. A strategy for determining the structure of a unit cell, oriented and distorted arbitrarily relative to its reference crystal, is given. An example of a determination of the unit cell of a GaAlAs layer grown on a vicinal GaAs (100) substrate, on which 200 AA of AlAs has been deposited, is given. The analysis is rapid and requires no assumptions regarding the interfacial structure of the sample.