J. E. Angelo

The coherency loss microstructure at a CdTe/GaAs(001) interface

Abstract The dislocation microstructure associated with the loss of coherency of CdTe(001), as grown on GaAs(001), is characterized using high-resolution trans-mission electron microscopy. The examination of cross-sectional specimens allowed characteristic local variations in the relative orientation of the overgrowth and the substrate to be quantified and related to associated local changes in the form and distribution of the interface dislocation array. In the interface array, the relative proportion of 60° dislocations and Lomer-Cottrell locks proved to be related to the local misorientation between the CdTe and GaAs. The extent to which this is indicative of the mode of stress relief during the loss of coherency is discussed. Interestingly, it was also found that such variations in the relative proportions of the different types of dislocation proved to be associated with microtwin formation. The significance of this with regard to the strain relief mechanism is also noted.

Reflection high energy electron diffraction measurements of molecular beam epitaxially grown GaAs and InGaAs on GaAs(111)

Abstract Reflection high energy electron diffraction (RHEED) measurements were performed during the molecular beam epitaxial (MBE) growth of GaAs and InGaAs on GaAs(111) A and (111) B surfaces. Under a fixed Ga flux the period of these intensity oscillations was observed to increase with increasing As 4 flux on the 2 × 2 reconstructed GaAs(111) B surfaces. Layer thickness measurements, using cross-sectional transmission electron micrographs of AlAs/GaAs superlattices, indicated that the real growth rate did not correspond to the measured period of the intensity oscillations. The results are explained in terms of a reduction in Ga incorporation and an enhancement of Ga surface diffusion as the arsenic coverage of the 2 × 2 reconstructed (111) B surfaces is increased. The reduced Ga incorporation, on GaAs(111) B, promotes the formation of facets, commonly observed as three-dimensional islands or hillocks, which rob a portion of the Ga flux. The MBE growth and relaxation of strained InGaAs layers on GaAs(111) B were also studied by RHEED intensity oscillations and in situ surface lattice constant measurements. It is shown that by tuning the MBE parameters, during the growth of GaAs buffers and InGaAs layers on GaAs(111) B, premature strain relaxation due to the formation of twin defects can be prevented. Unlike the growth of InGaAs on GaAs(100) no two-dimensional to three-dimensional transition was observed even at high strains.

Effects of surface reconstruction on CdTe/GaAs(001) interface structure

Abstract CdTe/GaAs(001) heterostructures were fabricated by molecular beam epitaxy on chemically etched and thermally deoxidized GaAs(001) substrates, as well as GaAs(001) (3×1) buffer layers grown in situ by molecular beam epitaxy. Different growth protocols were also explored, leading to Te-induced (6×1) or (2×1) surface reconstructions during the early growth stage. High-resolution cross-sectional transmission electron microscopy was used to examine the final interface structure resulting from the different substrate preparations, and surface reconstructions. The (2×1) surface reconstruction led to pure (001) growth, while the (6×1) reconstruction led to an interface which included small (111)-oriented inclusions. In addition, deposition on etched and deoxidized GaAs(001) wafers led to preferential CdTe growth within etch pits and resulted in a macroscopically rough interface region.

Identification of the misfit dislocations at an FeAl/AlAs/GaAs interface using moiré fringe contrast in a transmission electron microscope

Moire fringe contrast in plan‐view transmission electron microscopy (TEM) is adapted to measure the Burgers vector of misfit dislocations at the interface between FeAl and AlAs. This technique had originally been used to determine the Burgers vector of dislocations in bulk materials. The aluminide was grown by molecular beam epitaxy on AlAs which was pseudomorphic on GaAs(001). The observed misfit dislocations are determined to have [100] and [010] Burgers vectors, as measured in the FeAl, with [010] and [100] line directions, respectively. These are pure edge dislocations which cannot glide on the {110} or {112} slip systems of FeAl. This requires that the misfit dislocations either form at the edges of islands, during three dimensional (3D) growth or by climb from the free surface during two‐dimensional (2D) growth. The TEM results along with in situ reflection high‐energy electron diffraction (RHEED) results show that the growth is indeed 2D which suggests the misfit dislocations must form by dislocati...

Growth and characterization of iron aluminide films on compound semiconductors

Transmission electron microscopy (TEM), selected area electron channeling patterns (SACP), and reflection high-energy electron diffraction (RHEED) are used to investigate the growth and quality of epitaxial iron aluminide films on GaAs(100) substrates. These films are observed to grow layer-by-layer even when the constituents are codeposited. During growth the lattice relaxation is determined fromin situ RHEED measurements of the separation of two diffracted beams. After growth TEM measurements of Moiré fringes are used to determine the local residual strain and the Burgers vector of the iron aluminide film. Selected area electron channeling is used to determine the residual strain in the films without thin specimen artifacts. The results indicate that strain in the FeA1 films relaxes by the formation of misfit dislocations with anα<100> Burgers vector. The relaxation does not occur as quickly as the Matthews equilibrium model predicts. Since the FeAl and GaAs have different slip systems, the dislocations inducing the relaxation must nucleate in the epilayer. The defects present suggest growth by nucleation and subsequent climb of dislocation half-loops.

Local Interface Composition and Band Offset Tuning in ZnSe-GaAs(001) Heterostructures

ZnSe-GaAs(001) heterostructures have been grown by molecular beam epitaxy and characterized in situ by means of reflection high energy electron diffraction and x-ray photoemission spectroscopy, and ex-situ by near edge photoluminescence spectroscopy and by cross sectional transmission electron microscopy. By changing the Zn/Se flux intensity ratio (we explored the 0.1-10 range) we were able to control the Zn/Se relative concentration in the interface region, while maintaining a similar structure and high degree of long range order at the interface. Correspondingly, the valence band discontinuity is found to vary from 0.6eV (Se-rich interface) to 1.2eV (Zn-rich interface) in the interface composition range examined.

Defect structure at a CdTe(111)GaAs(001) interface

Abstract This paper addresses the defect structure, as observed by transmission electron microscopy, at a CdTe(111) GaAs(001) interface. When viewed along the [110]CdTe direction, the overlayer exhibits a high density of microtwins parallel to the CdTe(111) growth direction. Meanwhile, when viewed along [112]CdTe, an array of misfit dislocations, exhibiting edge component of Burgers vector equal to a 4 [ 1 10] , a = 6.48 A , are observed at the interface between the CdTe and GaAs lattices. These misfit dislocations are of such a spacing as to be consistent with only partial relief of the misfit strain. The residual misfit strain is found to relax in a region extending about 200 A from the interface, with the source of the strain relief being layer dislocations in the CdTe overlayer. The distance over which the misfit relaxes is consistent with the results of X-ray measurements of the misfit relaxation in thin overlayers. Furthermore, the distance over which the misfit relaxes is the same as that which the high density of microtwins are observed, suggesting that the microtwins act as nucleation sites for the observed layer dislocations.

TEM Investigations of CdTe/GaAs(001) Interfaces

In this study, cross-sectional transmission electron microscopy (XTEM) was used to investigate the defect structure which occurs at the interface between CdTe(001) and GaAs(001). The heterostructures were fabricated by molecular beam epitaxy on chemically etched and thermally deoxidized GaAs(001) substrates as well as GaAs(001) buffer layers grown in-situ by molecular beam epitaxy. This allowed for investigation of the GaAs surface preparation on the subsequent interfacial structure. The as-etched substrate led to a microscopically rough interface with the CdTe depositing in etch pits on the GaAs surface, while growth on the buffer layer led to a macroscopically flat interface. Further, growth was accomplished on different Te-induced surface reconstructions ((6×1) vs (2×1)) in an effort to understand the role of the precursor surface treatment on the subsequent interfacial structure. In this case growth on the (6×1) reconstruction led to the introduction of (111)-oriented inclusions at the interface, while the (2×1) reconstruction led to pure (001)-oriented growth. A mechanism for the formation of planar defects at the CdTe/GaAs(001) interface is described which is based on local misorientations of the CdTe and GaAs. Finally, preliminary results of ex-situ annealing experiments on the interfacial defect structure will be discussed.

Microstructural Characterization of Low Temperature GaAs(111)B MBE Growth by AFM and Tem

Atomic Force Microscopy (AFM) images are correlated with Transmission Electron Microscopy (TEM) plan-view images in a structure consisting of oriented GaAs layers grown by molecular beam epitaxy (MBE) at 500°C. We present results on the applicability of AFM, which requires short sample preparation and imaging time relative to TEM, in obtaining information on twin density and growth pits of these low temperature samples. Also, we discuss the behavior of twin boundaries by comparing plan-views and cross sectional TEM images.

Defects and strains at magnetic and semiconductor interfaces

Defects and strains at surfaces and interfaces are examined with respect to how they can be measured and how they affect resulting magnetic, electronic or mechanical properties. Emphasis is on MnZn ferrite, GaAs and PbS with deformation being examined by selected area channeling pattern (SACP), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) techniques. Damage produced during preparation of magnetic surfaces is examined by broadening of lines in SACP patterns while epitaxial strains and their relaxations by dislocations are estimated using both line shifts of SACP patterns and direct imaging by TEM. Of particular importance is that a new SACP method with accuracy approaching 0.1 percent has been developed for non-destructive examination of epitaxial strains in InGaAs/GaAs. Also, for the first time microcrack features in a low band gap semiconductor have been examined with STM.