Final Report Summary - HOLOVIEW (Single active dopant detection in semiconductor nanowires using electron holography.)
The summary should be a stand-alone description of the project and its outcomes. This text should be as concise as possible and suitable for dissemination to non specialist audiences. Please notice that this summary will be published.
Holoview was conceived as project to simultaneously improve the spatial resolution and sensitivity of electron holography to enable the visualization of the electrical potentials at atomic resolution in electrically connected (working) semiconductor devices. Electron holography is a transmission electron microscopy based technique that can be used to recover the phase of the transmitted electrons which is directly proportional to the electrostatic potential. In order to achieve this goal it was also necessary to prepare a perfect electron transparent specimen with no contamination present on the surfaces. The ultimate goal of the project was to measure the electrostatic potential arising from single ionized dopant atoms which would require a phase sensitivity of 2pi/1000 at atomic resolution using low microscope operating voltages to reduce the effects of electron beam damage.
Within the project Holoview, we have been able to optimize imaging conditions and acquire stacks of holograms such that the required sensitivity can be achieved. Electron holograms are acquired and then the beam and electron biprism drift, specimen drift and drift of microscope aberrations are corrected. By combining large stacks of holograms, a low beam intensity can be used which reduces specimen damage. Although these methods allow us the precision to be able measure the potential distribution around single ionized dopant atoms, problems with diffraction and contamination in real specimens mean that the measured changes in electron phases are too complicated to interpret. This is especially the case when considering the small signal that we expect to detect from the dopant atoms. Nonetheless, by introducing experimental methods that can be used to simultaneously improve spatial resolution and precision of the measurement we are now able to solve many materials problems that were not accessible before. For example we have been able to measure the piezo-electric fields in 3nm wide quantum wells in IIIV materials with better than 1 nm spatial resolution. Have also demonstrated measure the magnetic fields around miniaturized MRAM type devices which will have important implications for the next generation of memory devices. In addition, we are able to measure the electrostatic potential distribution around single defects in a monolayer of MoS2 and the ability to achieve these types of information is unprecedented and can have a major impact in the understanding of how defects and impurities can effect the properties of 2D materials.
Regarding electrical operation of semiconductor materials, within Holoview two different methodologies were debugged and put into place to allow the routine application of electrical biases on specimens in-situ in the TEM. Electrical biasing has been achieved by using a nanoscaled probe which is controlled by a piezoelectric motor to locally apply the voltages. In addition we have also been able to develop a new method of connecting a specimen to a silicon chip which is then electrically connected using focused ion beam metal deposition. By developing reproducible methods for operating specimens in-situ in the TEM we have been able to demonstrate for the first time a range of OXRAM memories the movement of oxygen vacancies as the device is switched from a high resistive state to a low resistive state. The ability to be able to routinely electrically connect semiconductor and PV specimens in the TEM to link their electrical properties to information about the structure, composition and fields measured at atomic resolution will be a powerful tool to improve the nanoscaled devices of the future. In the aftermath of Holoview, there are already several funded projects which work in this direction, being the observation of how nano-scaled devices work with atomic resolution.
Holoview was conceived as project to simultaneously improve the spatial resolution and sensitivity of electron holography to enable the visualization of the electrical potentials at atomic resolution in electrically connected (working) semiconductor devices. Electron holography is a transmission electron microscopy based technique that can be used to recover the phase of the transmitted electrons which is directly proportional to the electrostatic potential. In order to achieve this goal it was also necessary to prepare a perfect electron transparent specimen with no contamination present on the surfaces. The ultimate goal of the project was to measure the electrostatic potential arising from single ionized dopant atoms which would require a phase sensitivity of 2pi/1000 at atomic resolution using low microscope operating voltages to reduce the effects of electron beam damage.
Within the project Holoview, we have been able to optimize imaging conditions and acquire stacks of holograms such that the required sensitivity can be achieved. Electron holograms are acquired and then the beam and electron biprism drift, specimen drift and drift of microscope aberrations are corrected. By combining large stacks of holograms, a low beam intensity can be used which reduces specimen damage. Although these methods allow us the precision to be able measure the potential distribution around single ionized dopant atoms, problems with diffraction and contamination in real specimens mean that the measured changes in electron phases are too complicated to interpret. This is especially the case when considering the small signal that we expect to detect from the dopant atoms. Nonetheless, by introducing experimental methods that can be used to simultaneously improve spatial resolution and precision of the measurement we are now able to solve many materials problems that were not accessible before. For example we have been able to measure the piezo-electric fields in 3nm wide quantum wells in IIIV materials with better than 1 nm spatial resolution. Have also demonstrated measure the magnetic fields around miniaturized MRAM type devices which will have important implications for the next generation of memory devices. In addition, we are able to measure the electrostatic potential distribution around single defects in a monolayer of MoS2 and the ability to achieve these types of information is unprecedented and can have a major impact in the understanding of how defects and impurities can effect the properties of 2D materials.
Regarding electrical operation of semiconductor materials, within Holoview two different methodologies were debugged and put into place to allow the routine application of electrical biases on specimens in-situ in the TEM. Electrical biasing has been achieved by using a nanoscaled probe which is controlled by a piezoelectric motor to locally apply the voltages. In addition we have also been able to develop a new method of connecting a specimen to a silicon chip which is then electrically connected using focused ion beam metal deposition. By developing reproducible methods for operating specimens in-situ in the TEM we have been able to demonstrate for the first time a range of OXRAM memories the movement of oxygen vacancies as the device is switched from a high resistive state to a low resistive state. The ability to be able to routinely electrically connect semiconductor and PV specimens in the TEM to link their electrical properties to information about the structure, composition and fields measured at atomic resolution will be a powerful tool to improve the nanoscaled devices of the future. In the aftermath of Holoview, there are already several funded projects which work in this direction, being the observation of how nano-scaled devices work with atomic resolution.