# Presentation: 916299

## Lateral Composition Modulation in InAs/GaSb Superlattices:  Nanometer Sized Quantum WiresDonna W. Stokes					DMR 0237811

| The objectives of this research is to understand the structural properties of the nanowires formed by lateral composition modulation (LCM) in InAs/GaSb  superlattices (SL) grown on GaSb and AlSb buffer layers with both InSb and GaAs interfacial (IF) layers..
Figure 1 shows a diagram of the 3D structure of the InAs/GaSb nanowire array as seen in a STM image of the sample. 100-150 nm InAs wire-like structures are observed.  
An X-ray diffraction reciprocal space map, Fig. 2, and various line scans were used to determine the composition and strain state of the layers. The strain, including the magnitude and sign, and the IF bond type are crucial factors governing the growth of the SL. Our results suggest that it is possible to grow nanowire arrays in semiconductor heterostructures with a small strain.
The strain also has a significant affect on the optical absorption of the system, Fig. 3. For the SL1, with no LCM, typical optical transitions are observed, while for SL2, with LCM, these transitions are smeared.  For SL3, with LCM and reduced strain due to growth on an AlSb buffer, the typical transitions as well as additional transitions are observed. These data suggest that that by manipulating the strain state of the SL using various buffer layers, the optical response of the system can be controlled. |  |  |
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- Fig. 2. X-ray reciprocal space map of the nanowire sample.

- Fig. 3. Absorption spectra taken in the NIR at 77K of a sample with no LCM, with LCM and with LCM and reduced strain achieved by using a different buffer layer.

**Notes:**
Research objective:
InAs/GaSb superlattices (SL) possess the unique type-II band alignment which gives the system an optical bandgap from 3 – 30 microns making it an interesting candidate for mid to far infrared detectors and lasers. One of the objectives of this research is to obtain a complete understanding of the structural properties of nanowires formed by lateral composition modulation (LCM) in InAs/GaSb superlattices. 
Introduction:
The formation of self-organized semiconductor nanoscale structures (wells, wires and dots), based on the morphological instability of strained molecular beam epitaxial (MBE) grown films, has been observed in many III-V systems and is of great importance from both a fundamental and technological point of view.  Unusual instability phenomena have been observed in some non-common anion strained III-V epitaxial single layers and heterostructures resulting in nanowire array formation. In particular, InAs/GaSb (001) heterostructures demonstrates unstable growth at a thickness of a few ML, which is not typically seen in common anion III-V systems.  We show that this instability is due to the strain state of the layers and to the type of interfacial (IF) chemical bond which exists between the layers.
Using x-ray diffraction and numerical modeling, we have analyzed InAs/GaSb structures grown on a GaSb (001) substrate with either InSb or GaAs IF bonds. Samples grown with InSb IF resulted in nanowire structures while those with GaAs IF remained planar. Kinematical x-ray scattering theory was used to determine the composition of the layers, since composition mixing does occur in this system, and to model the strain state of the layers. We found that the type of IF bond and the sign and magnitude of the strain are crucial for the spontaneous formation of the nanowires.   
Experimental techniques:
The samples studied were 140-period InAs/GaSb superlattices grown by solid-source molecular beam epitaxy (MBE) at 390C using As4 and Sb4. Each period contains 13 monolayers (MLs) of InAs, 13 MLs of GaSb and a 1 ML interfacial (IF) layer, which was intentionally grown, using migration enhanced epitaxy (MEE), to be InSb or GaAs. GaSb and AlSb buffer layers were used to manipulate the strain in the SL.  The growth rate for all materials was 0.5 ML /s. High resolution X-ray diffraction measurements were carried out on a standard four-circle diffractometer at beamline X14A of the National Synchrotron Light Source (NSLS) at the Brookhaven National laboratory with an x-ray energy of 8.0 keV. A Bruker high resolution FTIR was used for optical absorption analysis.
Results:
A reconstructed 3D structure of the nanowire sample, based on XSTM measurements, is shown in Fig. 1. The thickness of the InAs layers (dark regions) undulates periodically along the [1-bar10] direction and  the InAs regions are almost completely surrounded by the nearly uniform GaSb layers (bright regions). Along the [110] directions, the length of the layers is on the order of a few to tens of microns resulting in the InAs nanowire array. An XRD reciprocal space map, shown in Fig. 2, shows the crystalline structure in both the vertical and lateral directions. This data along with additional XRD lines scans were modeled to determine the composition and strain state of the layers.  The layer compositions for both the nanowire and planar sample were determined to be InAs0.88Sb0.12 and GaAs0.05Sb0.95 rather than the expected pure binary layers.  The InAs0.88Sb0.12 layers in the nanowire sample grown on the GaSb buffer with a smaller misfit and the AlSb buffer with an even smaller strain than that of the sample grown on the GaSb buffer were unstable, whereas the same layers in the planar sample with a larger misfit were stable. The difference between the nanowire and planar samples is that they were grown with different IF bonds, which have opposite strain with respect to the substrates. The InSb IF bonds in the nanowire samples have a large positive strain with respect to the substrate, which is an order of magnitude larger than that of the InAs0.88Sb0.12 layer.  The planar sample with the GaAs bonds experience a large negative strain with respect to the substrate, which is opposite to the sign of the strain in the InAs0.88Sb0.12  layers.  This suggests that both the magnitude and sign of the strain of the IF layer is key to the morphological instability of the InAs0.88Sb0.12 layers; therefore with proper design of the interfacial bonding, self-assembled nanostructures can be grown in material systems with small strain, which would otherwise be impossible. 
We also found that the strain also has a significant affect on the optical absorption of the system, Fig. 3. For the SL1, grown on a GaSb buffer with GaAs IF and with no LCM , typical optical transitions are observed, while for SL2, grown on a GaSb buffer with an InSb IF and with LCM, these transitions are smeared.  For SL3, grown with an InSb IF on an AlSb buffer resulting in reduced strain and with LCM, the typical transitions as well as additional transitions are observed. This data suggests that that by manipulating the strain state of the SL using various buffer layers, the optical response of the system can be  can be controlled.

## Lateral Composition Modulation in InAs/GaSb Superlattices:  Nanometer Sized Quantum WiresDonna W. Stokes					DMR 0237811

| Education and Outreach:
 During the Spring of 2006 semester, approximately 160 Bear Branch Elementary School fifth-graders visited the PI’s laboratory for an informative tour and a demonstration in nanotechnology.  See the website  http://www.nsm.uh.edu/newsroom/2006/0519_bearbranch.html for additional information. 
 During the summer 2006 Ross Sterling High School mathematics teacher, Katrina Cushenberry, worked in the PI’s laboratory on a project based on nanoscale technology and how it will affect our future. She was also involved in data collection for a project based on how to improve the math skills of students taking introductory physics courses.  Her input on this project was invaluable as it gave first hand information on how the students are prepared mathematically before entering college and how this could be adjusted to help the student be successful in a physics course.  
 The PI and members of her group also served as judges for the 2006 Houston Engineering and Science Fair in Houston, Texas.  |  |  |
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**Lateral Composition Modulation in InAs/GaSb Superlattices:  Nanometer Sized Quantum Wires**Donna W. Stokes**	****	****	****	****	**DMR 0237811

- Ms. Cushenberry reviews statistical data with Dr. Stokes.

- Bear Brach Elementary students observed a magnet levitating over a superconductor during a tour of the PI’s  laboratory.