Evidence from EELS of Oxygen in the Nucleation Layer of a MBE grown III-N HEMT
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INTRODUCTION Within the last decade, the III-N semiconductors have experienced rapid development to the point that commercial optical devices are now available. Recently the importance of the piezoelectric properties of the III-N has come to light. With this realization, the knowledge and control of the polarity of III-N thin films has become paramount for device design. The piezoelectric and spontaneous polarizations in this system are large enough to induce a two-dimensional electron gas (2DEG) at Al1-xGaxN/GaN heterojunctions [1-7]. For Molecular Beam Epitaxy (MBE) material grown on sapphire substrates using a plasma nitrogen source, the nucleation layer determines the polarity of the resulting layers [8]. With this control of the polarity of the material, high quality MBE High Electron Mobility Transistor (HEMT) devices have been achieved [8]. In this paper, the composition and structure of the AlN nucleation layer in a HEMT device structure studied in the Cornell University Scanning Transmission Electron Microscope (STEM), will be presented. The Cornell University STEM produces an electron probe approximately 2.1Å in diameter and thus is distinctly qualified for near atomic resolution interface
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investigations. As this probe is scanned across a TEM type sample, the scattered electrons are collected. Electrons scattered to high angles are collected by the Annular Dark Field (ADF) detector producing an image where the contrast depends mainly on atomic number (Z) and thickness. Thus ADF imaging is commonly referred to as Zcontrast imaging. Electrons scattered to small angles are either detected by the Bright Field Detector, producing an image analogous to normal bright field TEM, or are recorded as EELS by either the Serial or Parallel detector. EELS provides information about the local bonding and electronic structure by probing unoccupied electronic states. The fast moving electrons from the STEM probe interact with the core electrons in the specimen, which are excited from their ground state to unoccupied states. Since the energy lost by the fast moving electrons in the probe is equal to the difference in energy between the core level and excited level, the measurement of the intensity of the probe electrons as a function of energy loss provides substantial details of the electronic structure and chemistry.
EXPERIMENTAL DETAILS The III-N HEMT sample with an AlN nucleation layer was grown in a Varian GenII MBE at Cornell University. The specifics of the growth conditions, electrical characterization, and device results are given elsewhere [8]. The sample was prepared by standard tripod polishing techniques to form a wedge specimen [9]. The sample was ion milled with a BAL-TEC Res 010 for final thinning. The Cornell University STEM has a maximum energy resolution of ~0.7eV over a large energy range (from 0 to 2keV). With high spatial and energy resolution, the STEM is particularly suited for sub-nanometer chemical and structural studies. With such a focused small probe, high spatial resolution EELS can
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