C. Dorin

Intermixing and lateral composition modulation in GaAs/GaSb short period superlattices

Lateral composition modulation on the group V sublattice has been investigated in GaAs/GaSb short period superlattices. The effect of As species and growth temperature on the appearance of lateral composition modulation was studied. Cross-sectional transmission electron microscopy and x-ray diffraction reciprocal space maps reveal that structures grown using As tetramers are always disordered, defective, and phase separated. Also, in these structures the As-rich regions appear to be composed of stacked GaAs quantum dots embedded in a GaSb matrix. The structures grown with As dimers show improved crystalline quality. Short period superlattices grown at T<420 °C have flat interfaces and are laterally homogeneous, however, there is significant anion intermixing across the interfaces. Structures deposited at 420 °C<T<445 °C roughen during growth, and exhibit lateral composition modulation and anion intermixing. Growing at higher temperatures destroys both the superlattice structure and the lateral composition...

Lateral composition modulation in AlAs/InAs and GaAs/InAs short period superlattices structures: The role of surface segregation

The effect of In surface segregation on the microstructure of short period superlattices (SPSs) in two different material systems with nominally equivalent lattice misfit, AlAs/InAs and GaAs/InAs, has been investigated and compared. It was found that the quality of the SPSs and the appearance of lateral composition modulation are remarkably different in these two systems. For AlAs/InAs SPSs grown at temperatures of T=500 °C, uniform structures devoid of lateral composition modulation were obtained. Samples grown at T>500 °C exhibit lateral composition modulation. Uniform and homogeneous SPS structures were not obtained in the GaAs/InAs structures over the entire temperature range examined in this study (475 °C⩽T⩽510 °C). Instead, lateral composition modulation with varying degrees of regularity was always observed. It was found that In segregation and roughening determine the microstructure. A kinetic exchange model predicts that at an optimum temperature the SPS layers are more intermixed for the AlAs/In...

Lateral composition modulation in short period superlattices: The role of growth mode

The role of the growth mode on lateral composition modulation is studied in short period superlattices of AlAs/InAs and GaAs/InAs. Reflection high energy electron diffraction and scanning tunneling microscopy are used to monitor the growth mode and the quality of the interfaces. Cross-sectional transmission electron microscopy indicates that samples that grow via the layer-by-layer growth mode do not exhibit lateral composition modulation and the superlattice structure is well defined. Lateral composition modulation forms when roughening occurs during growth. However, too much roughening, i.e., three-dimensional island nucleation destroys the regularity of the composition modulation in both the lateral and vertical directions. These results are in general agreement with theoretical predictions.

Lateral composition modulation in mixed anion multilayers

Lateral composition modulation on the group V sublattice has been observed in GaAs/GaSb short period superlattices. Cross sectional transmission electron microscopy and x-ray diffraction reciprocal space maps reveal that all structures are phase-separated with Sb compositions for the strongest modulated structure of x=0.73 in the Sb-rich regions, x=0.55 in the As-rich regions, and wavelengths 15⩽Λ⩽20 nm. The composition modulation observed in these films is not due to spinodal decomposition, because an alloy grown at the same conditions results in a homogeneous layer, but may be related to vertical stacking of quantum dots that nucleate during the growth of the structure.

Composition Modulation in GaAs/GaSb Short Period Superlattices

Quantum dots and/or wells are of much interest due to novel electronic properties and the potential for improvements in photovoltaic efficiency and optoelectronic devices. Spontaneous composition modulation in mixed cation systems, where the modulation occurs on the group III sublattice, has been reported previously [1]. Composition modulation is demonstrated here for the first time in mixed anion systems, where the modulation occurs on the group V sublattice.

Imaging and manipulating heterostructure interfaces

Epitaxial heterostructures constitute a wide variety of modern microelectronics devices. In the limit of ever decreasing feature dimensions, now entering the nanoscale in some cases, the interfaces of such devices are crucial to their operation and performance. In general the properties of the interfaces will differ significantly from those of the bulk structure of either the substrate or the heteroepitaxial film. To date, direct, non-destructive characterizations of the atomic-level structure of films and interfaces have not been readily available and this has hampered the design and optimization of heteroepitaxial devices. We describe here a novel x-ray interference method which is useful for imaging such structures with sub-Ångstrom spatial resolution while also providing chemical composition information from a map of the electron density. We illustrate the method, known as Coherent Bragg Rod Analysis (COBRA), with recent results on GaSb-InAs heterostructures of interest as infrared sources and detectors. We show that, with detailed knowledge of the interfaces from COBRA, it is now feasible to correlate specific molecular beam epitaxy growth conditions with desired electronic characteristics associated with the interface bonding. The COBRA method is quite general and only requires an epitaxial relationship between the substrate and the nanostructure that is deposited on it.

The effect of as incorporation on phase separation in GaAs/GaSb short period superlattices

Spontaneous phase separation in GaAs/sub n//GaSb/sub m/ short period superlattices (SPS) deposited on [001] InP substrates grown by Molecular Beam Epitaxy (MBE) is investigated as a function of As species and growth temperature. Structures grown with As/sub 4/ and 1<n<2 ML and m=2 ML exhibit significant roughening during growth thus the SPS individual layers are intermixed. For example, cross section transmission electron micrograph (XTEM) in [002] dark field imaging condition of a GaAs/sub 2ML//GaSb/sub 2ML/ sample grown at T=400/spl deg/C shows lateral variation of contrast due to both composition and strain (figure la). High Angle Annular Dark Field image (HAADF) of this sample (figure 1b) shows image contrast that is due only to compositional variations in the film perpendicular to the growth direction. The Sb composition x in the Sb-rich and As-rich regions as determined from the X-ray Energy Dispersive Spectroscopy (XEDS) line profiles (Figure 1c) are 0.73<x/sup Sb/<0.77 and 0.33<x/sup As/<0.55. The wavelength of lateral composition modulation is 15 nm</spl Lambda/<23 nm. Surprisingly, the Ga composition varies laterally as well, suggesting that Sb interstitials or antisite defects may be present. The phase separation in these films is not due to spinodal decomposition since an alloy growth in the same conditions results in a homogeneous film. However the As composition is less then expected in both SPS and alloys films.

Compositionally Modulated Structures Studied by in Situ Scanning Tunneling Microscopy

Two different short period superlattice (SPS) structures with nominally equivalent lattice mismatch, InAs/AlAs and InAs/GaAs are examined using in situ scanning tunneling microscopy (STM). Depending upon the growth conditions, the composition of the InAs/AlAs SPS structure can be either homogeneous or modulated in the lateral direction. Distinct periodic structures are clearly visible in images of the modulated SPS while no periodicity is observed in the homogeneous SPS. For a 30 period SPS consisting of 2 monolayers of AlAs and 2 monolayers of InAs we observe structures 20 nm in size with an average spacing of ∼25 nm in orthogonal directions, which is approximately the same length scale as the composition modulation. Despite the nominally equivalent lattice mismatch, the InAs/GaAs SPS structures are quite different. Some degree of modulation is always observed in these structures. Homogeneous structures are not observed. For the modulated SPS structures, scanning tunneling spectroscopy can be used to characterize the chemical profile.

Lateral Composition Modulation in Mixed Anion Short Period Superlattices

In this work, we demonstrate for the first time lateral composition modulation in GaAs/GaSb short period superlattices (SPS). Several different structures were investigated in order to study the role of Group V overpressure on the final composition and uniformity of the structure. Cross sectional transmission electron diffraction and xray diffraction reciprocal space maps reveal that the film is indeed phase separated. The composition modulation observed in these films is a result of the SPS growth and not due to spinodal decomposition, as evidenced by the fact that an alloy grown at the same conditions results in a homogeneous layer.