Electron Channeling Contrast Imaging: Rapid Characterization of Semiconductors
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Series/Report no.:2015 Edward F. Hayes Graduate Research Forum. 29th
In recent years, solar power has begun to approach cost-competitiveness with conventional power sources due to improvements in efficiency, manufacturing, and economies of scale. However, much of this advancement is reaching a saturation point; the sustainability of current and near-term costs is at risk. It is the next generation of PV technologies, with inherently higher conversion efficiencies and lower costs, that will lead the way to grid parity and greater adoption. For these emerging technologies, achievement of their ultimate potential depends greatly upon the ability to fully harness and exploit their advanced properties, which in turn depends on understanding these properties and their limiters. To this end, the detailed characterization and analysis of advanced photovoltaic materials and structures is necessary for the investigation of the fundamental structure-property relationships. Specifically, characterization via electron microscopy within these materials systems provides vital feedback into ongoing and future materials and device design, synthesis, and test efforts. This research utilizes an emerging technique, electron channeling contrast imaging (ECCI) for thin film, photovoltaic defect characterization. Detailed imaging and characterization of these defects is typically performed using transmission electron microscopy as it provides critical insight into their structure, and it allows for selective imaging through achieving various diffraction conditions. Unfortunately, this is a fundamentally low-throughput approach due to the time-intensive sample preparation, which can lead to significant bottlenecks in research and development cycles. Additionally, sample preparation is inherently destructive, thereby potentially leading to adulterated results. ECCI, on the other hand, obtains selective imaging through use of a scanning electron microscope and as such, it requires little to no sample preparation (i.e. can use as-grown samples). This feature of ECCI allows for rapid, high-throughput imaging of extended defects in crystalline materials. In this contribution, ECCI is used for the first time for imaging defects at an interface in thin films – specifically for the characterization of defects within samples of heteroepitaxial GaP grown on Si(100) substrates, in order to develop a better understanding of the dislocation dynamics in this materials system. Epitaxial GaP on Si has been of interest for nearly four decades as a route for integration of III-V materials with Si substrates for high efficiency photovoltaic applications. This makes this GaP/Si system of particular interest for characterization because further progress in the optimization of the epitaxial processes requires a deeper understanding of the dislocation dynamics, for which ECCI is an ideally suited. Here, the use of ECCI for imaging misfit dislocations at buried GaP/Si interfaces, and the use of these capabilities to determine important materials properties, such as critical thickness is discussed. Images will be shown from a range of GaP/Si samples, including studies of GaP films covering a range of thicknesses (from 30 – 250 nm), both as-grown and high-temperature annealed, for the analysis of misfit dislocation extension/glide at the strained interface. Furthermore, other applications of ECCI never before performed will be discussed such as the use of ECCI to do in-situ experiments which cannot be performed with TEM. This work provides unprecedented access to valuable information regarding the formation and evolution of defects, misfit dislocations, at this complex interface, as well as demonstrating the power of the ECCI technique for semiconductor heteroepitaxial research, in general. Ultimately, ECCI proves that it can be an equally valuable, if not superior (in many applications), technique to TEM for detailed microstructural characterization.
Engineering: 1st Place (The Ohio State University Edward F. Hayes Graduate Research Forum)
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