Kurt G. Eyink

Quantitative analysis of interfacial strain in InAs/GaSb superlattices by aberration-corrected HRTEM and HAADF-STEM.

The strain distribution across interfaces in InAs/GaSb superlattices grown on (100)-GaSb substrates is investigated by aberration corrected transmission electron microscopy. Atomic resolution images of interfaces were obtained by conventional high resolution transmission electron microscopy (HRTEM), using the negative spherical-aberration imaging mode, and by scanning transmission electron microscopy (STEM), using the high-angle annular dark-field (HAADF) imaging mode. The local atomic displacements across interfaces were determined from these images using the peak pair algorithm, from which strain maps were calculated with respect to a reference lattice extracted from the GaSb substrate region. Both techniques yield consistent results, which reveal that the InAs-on-GaSb interface is nearly strain balanced, whereas the GaSb-on-InAs interface is in tensile strain, indicating that the prevalent bond type at this interface is Ga-As. In addition, the GaSb layers in the superlattice are compressively strained indicating the incorporation of In into these layers. Further analysis of the HAADF-STEM images indicates an estimated 4% In content in the GaSb layers and that the GaSb-on-InAs interface contributes to about 27% of the overall superlattice strain. The strain measurements in the InAs layers are in good agreement with the theoretical values determined from elastic constants. Furthermore, the overall superlattice strain determined from this analysis is also in good agreement with the measurements determined by high-resolution X-ray diffraction.

Compositional analysis of mixed–cation-anion III–V semiconductor interfaces using phase retrieval high-resolution transmission electron microscopy

Employing exit‐plane wave function (EPWF) reconstruction in high‐resolution transmission electron microscopy (HRTEM), we have developed an approach to atomic scale compositional analysis of III‐V semiconductor interfaces, especially suitable for analyzing quaternary heterostructures with intermixing in both cation and anion sub‐lattices. Specifically, we use the focal‐series reconstruction technique, which retrieves the complex‐valued EPWF from a thru‐focus series of HRTEM images. A study of interfaces in Al0.4Ga0.6As–GaAs and In0.25Ga0.75Sb–InAs heterostructures using focal‐series reconstruction shows that change in chemical composition along individual atomic columns across an interface is discernible in the phase image of the reconstructed EPWF. To extract the interface composition profiles along the cation and anion sub‐lattices, quantitative analysis of the phase image is performed using factorial analysis of correspondence. This enabled independent quantification of changes in the In–Ga and As–Sb contents across ultra‐thin interfacial regions (approximately 0.6 nm wide) with true atomic resolution, in the In0.25Ga0.75Sb–InAs heterostructure. The validity of the method is demonstrated by analyzing simulated HRTEM images of an InAs–GaSb–InAs model structure with abrupt and graded interfaces. Our approach is general, permitting atomic‐level compositional analysis of heterostructures with two species per sub‐lattice, hitherto unfeasible with existing HRTEM methods.

Mechanical lithography using a single point diamond machining

A technique that produces submicron size features by machining GaAs wafers in air was studied. An apparatus was built which uses a sharp diamond tip to mechanically scribe patterns into the substrate. The debris produced from surface machining was removed by using a CO2“snow jet” sprayer, thus eliminating the use of hazardous liquid chemicals for cleaning. By changing the force between the diamond tip and sample surface, different depths of cuts were produced. This allowed the production of three dimensional patterns on the wafer in one step. With this instrument, write speeds of up to 25 μm/s on 30 μm×30 μm area patterns with a lateral resolution of 100 nm were demonstrated. In a different set of experiments, x-ray diffraction, spectroscopic ellipsometry, and cross sectional transmission electron microscopy analysis identified that subsurface damage created during the diamond turning process was removed at normal molecular beam epitaxial growth temperatures. Atomic force microscope analysis showed that t...

Mid- to long-wavelength infrared surface plasmon properties in doped zinc oxides

This work investigates properties of surface plasmons on doped metal oxides in the 2-20 μm wavelength regime. By varying the stoichiometry in pulse laser deposited Ga and Al doped ZnO, the plasmonic properties can be controlled via a fluctuating free carrier concentration. This deterministic approach may enable one to develop the most appropriate stoichometry of ZnAlO and ZnGaO in regards to specific plasmonic applications for particular IR wavelengths. Presented are theoretical and experimental investigations pertaining to ZnAlO and ZnGaO as surface plasmon host materials. Samples are fabricated via pulsed laser deposition and characterized by infrared ellipsometry and Hall-effect measurements. Complex permittivity spectra are presented, as well as plasmon properties such as the field propagation lengths and penetration depths, in the infrared range of interest. Drude considerations are utilized to determine how the optical properties may change with doping. Finite element simulations verify these plasmonic properties. These materials not only offer potential use as IR plasmon hosts for sensor applications, but also offer new integrated device possibilities due to stoichiometric control of electrical and optical properties.

Band gap formation in graphene by in-situ doping

We report the formation of band gaps in as-grown stacks of epitaxial graphene with opposite doping. Control of in-situ doping during carbon source molecular beam epitaxy growth on SiC was achieved by using different carbon sources. Doping heterostructures were grown by stacking n-type material from a C60 source on p-type material from a graphite filament source. Activation energies for the resistivity and carrier concentration indicated band gaps up to 200 meV. A photoconductivity threshold was observed in the range of the electrical activation energies. Band gap formation is attributed to electric fields induced by spatially separated ionized dopants of opposite charge.

Thin-film, wide-angle, design-tunable, selective absorber from near UV to far infrared

We experimentally demonstrate a structured thin film that selectively absorbs incident electromagnetic waves in discrete bands, which by design occur in any chosen range from near UV to far infrared. The structure consists of conducting islands separated from a conducting plane by a dielectric layer. By changing dimensions and materials, we have achieved broad absorption resonances centered at 0.36, 1.1, 14, and 53 microns wavelength. Angle-dependent specular reflectivity spectra are measured using UV-visible or Fourier spectrometers. The peak absorption ranges from 85 to 98%. The absorption resonances are explained using the model of an LCR resonant circuit created by coupling between dipolar plasma resonance in the surface structures and their image dipoles in the ground plane. The resonance wavelength is proportional to the dielectric permittivity and to the linear dimension of the surface structures. These absorbers have application to thermal detectors of electromagnetic radiation.

Combined in situ and ex situ analysis of hydrogen radical and thermal removal of native oxides from (001) GaAs

We are currently involved in the study of regrowth of InAs on nanopatterned GaAs surfaces. The nanopatterning is accomplished through the movement of the sample while in contact with a diamond tip maintained at a constant load. Native oxides present on these surfaces introduce an obstacle to the subsequent regrowth. Therefore, the removal of this oxide is a prerequisite step for the study of the subsequent regrowth on these patterned surfaces. In this study we used in situ spectroscopic ellipsometry (SE) and reflection high energy electron diffraction (RHEED) as well as ex situ atomic force microscopy to follow the hydrogen cleaning and thermal removal of the native oxides from the GaAs surface. SE and RHEED were used to follow the oxide desorption process in situ and were used to determine when the surface was clean. Post AFM analysis indicated that the thermally desorbed oxide surface contained pits which were approximately 100 A deep and covered 15% of the surface. Hydrogen radical cleaning was studied...

Strain relaxation in the growth of planar InAs

In this work, we look at the strain relaxation of InAs growth on GaAs under metal rich conditions. InAs layers, grown under these conditions, have been found to maintain a planar morphology for thicknesses well beyond a Matthews-Blakeslee critical thickness of ∼2 ML. We employed reflection high-energy electron diffraction, in situ spectroscopic ellipsometry and atomic force microscopy to follow the InAs growth under In stabilized conditions. The critical thickness was found to increase with a reduction in growth temperature. A region of growth was seen in which three-dimensional diffraction was not visible in the reflection high energy electron diffraction pattern before annealing. For thicknesses less than the critical thickness observed by atomic force microscope, the planar morphology has been found to be stable with annealing. These results indicate that strain relaxation via the formation of Lomer-type dislocations is essential to stabilizing planar morphology during growth.

In situ and ex situ spectroscopic investigation of low temperature grown gallium arsenide by molecular beam epitaxy

Low temperature growth of GaAs not only provides useful semi‐insulating layers in a variety of devices, but also is interesting from a materials and crystal growth point of view. In this work we have utilized an in situ spectroscopic ellipsometer having 44 wavelength regions in the range 4000–8000 A to monitor the low temperature growth of GaAs. Several different regions of growth have been observed, in agreement with earlier studies utilizing single wavelength ellipsometry. An initial region of homogeneous growth is followed by regions of material that exhibit varying optical properties. The complex refractive index of the epitaxial LT‐GaAs films have been extracted from the real‐time data acquired during the homogenous growth region. Ex situ characterization of these films was performed using high‐resolution x‐ray diffraction to determine the excess As concentration. Films in which the low temperature growth was stopped within the homogeneous growth regime have been characterized with a multiwavelength ...

Self-assembly of heterojunction quantum dots

The fabrication of a self-assembled heterojunction quantum dot structure composed of multiple materials is reported. This structure consists of a composite dot formed of an initial core of one material which results from normal self-assembly, followed by the epitaxy of a crown composed of a similarly strained material. Finally the entire dot structure is capped with a barrier material closely lattice matched to the substrate. In this demonstration, self-assembled InAs quantum dots were first formed on a GaAs substrate and subsequently crowned with GaSb. The entire structure was encapsulated with a GaAs cap layer. Atomic force microscopy shows that additional nucleation between the InAs layers has been minimized and cross-sectional transmission electron microscopy shows the formation of the composite structure.