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TECHNICAL
STRATEGY |
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Semiconductor quantum nanowire development |
| Semiconductor nanowires made from group III-nitrides are showing great promise for the next generation of high-efficiency, miniature light sources. Several research laboratories worldwide have demonstrated optically pumped lasing action in group III-nitride nanowires. To accelerate and support commercial development of these materials, we are growing nanowires made of GaN, AlN, InN and related alloys with ultra-low defect densities and high luminescence efficiencies, and are pursuing fundamental studies of nanowire crystal growth using molecular beam epitaxy (MBE) and chemical beam epitaxy. Advanced structures incorporating quantum disks, axial alloy variation, and nucleation control are designed, grown, and characterized. Extensive high-resolution electron microscopy, scanning probe microscopy, and x-ray diffraction have shown that the wires are hexagonal in cross-section and strain-free, with a reproducible crystal orientation relative to the silicon substrates. Additional optical and structural characterization performed in collaboration with the Optical Materials Metrology Project and the NIST Materials Science and Engineering Laboratory also confirms that the materials are essentially free of chemical impurities and structural defects. Nanowires have also been supplied to a number of collaborators within and outside of NIST. We are developing prototype optical and electronic nanowire devices for metrology, sensing, and other applications. A particular goal is the electrically pumped group III-nitride nanowire laser Contact: Dr. Kris Bertness |
| Structural characterization and control of quantum dots and nanowires |
| Quantum dots are semiconductor structures with quantized energy levels that result in improved efficiency and tuning range for semiconductor lasers and less sensitivity to environmental changes for lasers and photodetectors. Dot formation is driven by strain during epitaxial crystal growth, but measuring the strain in structures less than 100 nm in lateral dimension presents new challenges. We have contributed to this field through studies correlating substrate preparation and growth parameters with dot density and size as measured by atomic force microscopy (AFM). We are also evaluating the shape of the dots and strain in the region of the dots with transmission electron microscopy (TEM). In addition, we are studying ways to control growth of quantum dots and nanowires and improve uniformity using strain and appropriate growth conditions. Contact: Dr. Alexana Roshko |
| Semiconductor composition standards development |
| Inaccuracy of semiconductor composition measurement has been an impediment to achieving consistency of device performance across production lines. It has also inhibited the collection of sufficiently accurate materials parameters for use in the simulation of devices, which is critical to fast product cycle times. A goal of this project is to develop certification techniques for standard reference materials having composition uncertainty specified to a level one tenth as low as that of techniques currently in use by industry. Our approach has been to combine conventional methods of determination of composition such as (photoluminescence (PL) and x-ray diffraction (XRD)) with less common methods (in situ monitoring, electron microprobe analysis (EMPA), and inductively coupled plasma optical-emission spectroscopy (ICP-OES)) to enable certification of alloy composition. This program is the basis for the production of standard reference materials (SRMs) in the AlGaAs and AlInGaN alloy systems. As part of this research, we have quantified error sources and accuracy limits of the indirect composition measurement techniques currently in use by industry, specifically PL and XRD. Contact: Dr. Kris Bertness |
| Source gas purity measurement |
| Contamination is a serious problem in phosphine, arsine, silane, ammonia, and similar gases used in the epitaxial growth of high-purity semiconductor layers. The critical concentrations of the impurities are not well known; however, it is believed that >10 nmol/mol oxygen or water in most process gases is undesirable. This project uses a cavity-ring-down spectroscopic (CRDS) system it developed with researchers in the NIST Chemical Science and Technology Laboratory to measure impurities with very low concentrations in semiconductor source gases. The advantages of this technique are that its accuracy relies primarily on accurate time measurement and detector linearity, rather than measurement of absolute light intensities, and it is insensitive to absorption outside the cavity. We have improved the detection limit of this system to 50 nanomol/mol of water in phosphine. We are using our system to measure the lineshape, absorption coefficients, and frequency of optical transitions for water, phosphine, ammonia, and arsine in the vicinity of 935 nm. At longer wavelengths the absorption lines are even stronger, enabling greater sensitivity. This absorption information is critical to facilitate the use of high-sensitivity spectroscopy techniques in these gases. The CRDS capability should ultimately lead to improvements in semiconductor source gas purity, which will allow crystal growers to choose less expensive growth conditions without sacrificing optical emission efficiency and yield in LEDs, semiconductor lasers, and photodetectors. We collaborate with gas suppliers and purity instrumentation manufacturers in this project, including on-site testing of commercial instrumentation built to detect water in hazardous gases. Contact: Dr. Kris Bertness |