A. P. Young

DEPTH-DEPENDENT SPECTROSCOPIC DEFECT CHARACTERIZATION OF THE INTERFACE BETWEEN PLASMA-DEPOSITED SIO2 AND SILICON

We demonstrate the use of low-energy cathodoluminescence spectroscopy (CLS) to study optical transitions at defect bonding arrangements at Si–SiO2 interfaces prepared by low-temperature plasma deposition. Variable-depth excitation achieved by different electron injection energies provides a clear distinction between luminescence derived from (i) the near-interface region of the oxide film, (ii) the Si–SiO2 interface, and (iii) the underlying crystalline Si substrate. Cathodoluminescence bands at ∼0.8 and 1 eV are assigned to interfacial Si atom dangling bonds with different numbers of back-bonded Si and O atoms. CLS also reveals higher photon energy features: two bands at ∼1.9 and 2.7 eV assigned to suboxide bonding defects in the as-grown oxide films, as well as a substrate-related feature at ∼3.4 eV. The effects of hydrogenation at 400 °C and rapid thermal annealing at 900 °C, and especially the combination of both process steps is shown to dramatically reduce the intensities of the CLS features assigne...

Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrier confinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at /spl sim/3.8 eV or /spl sim/4.0 eV, GaN band edge emission at /spl sim/3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region.

Luminescence spectroscopy of GaN in the high-temperature regime from room temperature to 900 °C

We have measured the optical luminescence spectrum of GaN from the near infrared to the near ultraviolet at elevated temperatures. Despite intense blackbody radiation above 600 °C, luminescence is observable at 3 eV or greater at temperatures as high as 900 °C, i.e., including near-band-edge emissions over the entire range of GaN growth temperatures. Elevated-temperature measurements also reveal thermally activated quenching of the midgap “yellow” defect luminescence. These results show that electron beams already used to monitor molecular-beam epitaxy growth can also provide temperature and defect information during the growth process.

Microcathodoluminescence of impurity doping at gallium nitride/sapphire interfaces

We have used low-temperature cathodoluminescence spectroscopy (CLS) to probe the spatial distribution and energies of electronic defects near GaN/Al2O3 interfaces grown by hydride vapor phase epitaxy (HVPE). Cross sectional secondary electron microscopy CLS shows systematic variations in impurity/defect emissions over a wide range of HVPE GaN/Sapphire electronic properties. These data, along with electrochemical capacitance–voltage profiling and secondary ion mass spectrometry, provide a consistent picture of near-interface doping by O diffusion from Al2O3 into GaN, over a range 100–1000 nm.

Localized states at GaN surfaces, Schottky barriers, and quantum well interfaces

Abstract We have used low energy electron-excited nanoluminescence (LEEN) spectroscopy to probe the localized electronic states at GaN free surfaces, metal–GaN contacts, and GaN/InGaN quantum well interfaces. These depth-resolved measurements reveal the presence of deep electronic states near GaN interfaces whose energies and relative densities depend sensitively on the local chemical structure and growth conditions. The physical properties of these states correlate with mobility variations in thin GaN films grown by molecular beam epitaxy, Fermi level positions at Mg and Al/GaN Schottky barriers, and the appearance of new phases localized near GaN/InGaN/GaN quantum well interfaces. The growth and processing dependence of deep GaN levels highlights new methods to understand and control the fundamental electronic structure of GaN heterointerfaces.

Depth-resolved detection and process dependence of traps at ultrathin plasma-oxidized and deposited SiO2/Si interfaces

Low-energy electron-excited nanoluminescence spectroscopy reveals depth-resolved optical emission associated with traps near the interface between ultrathin SiO2 deposited by plasma-enhanced chemical vapor deposition on plasma-oxidized crystalline Si. These near-interface states exhibit a strong dependence on local chemical bonding changes introduced by thermal/gas processing, layer-specific nitridation, or depth-dependent radiation exposure. The depth-dependent results provide a means to test chemical and structural bond models used to develop advanced dielectric-semiconductor junctions.

CATHODOLUMINESCENCE SPECTROSCOPY OF NITRIDED SIO2-SI INTERFACES

We use cathodoluminescence spectroscopy (CLS) to investigate the electronic states of ultrathin gate dielectrics with nitrided SiO2–Si interfaces, known to improve reliability in advanced complementary metal–oxide–semiconductor devices. The 5 nm thick films investigated were: (i) as-deposited (at 300 °C) structures, (ii) 400 °C hydrogen anneal, (iii) 900 °C rapid thermal anneal (RTA), and (iv) a combination of both anneals. CLS emission energies and intensities versus excitation energy were essentially unchanged for the as-deposited interface compared to non-nitrided plasma-processed interfaces. In the near-infrared, features appear at 0.8 and 1.0 eV, with the 1.0 eV peak Si substrate intensity increasing with increasing depth. From depth variation measurements at higher photon energy, a 3.4 eV peak is also shown to arise from the Si substrate, and a 2.7 eV feature is shown to come from the interface region. After hydrogenation, the CLS is essentially the same as for non-nitrided interfaces, except for an...

Low-energy cathodoluminescence spectroscopy of erbium-doped gallium nitride surfaces

We have used cathodoluminescence spectroscopy with variable incident beam energies to study the energy levels and activation of Er impurities in GaN as a function of depth below the free surface. The GaN films were doped in situ during either metalorganic molecular-beam epitaxy (MOMBE) or molecular-beam epitaxy (MBE). Besides the well-known Er3+ luminescence at 0.80 eV, we observe emissions at 1.2, 1.8, 2.2, and 2.3 eV, corresponding to higher energy Er 4f shell transitions. For unannealed MOMBE-grown GaN:Er, these higher energy emissions appear only for excitation depths of hundreds of nanometers. The MOMBE-grown GaN;Er annealed to 500 °C shows a dramatic increase in the 1.8, 2.2, and 2.3 eV peak intensities at shallow probe depths, with its yield increasing with increasing depth. These three features become pronounced at all depths after a 700 °C anneal. MBE-grown GaN:Er grown with lower C and O impurity levels than the MOMBE-grown sample exhibits strong emission at all these energies without annealing....

Low energy cathodoluminescence spectroscopy of etched 6H-SiC surfaces

We have performed low energy cathodoluminescence spectroscopy (CLS) measurements of localized states near 6H-SiC (0001) and (0001) surfaces under ultrahigh vacuum (UHV) conditions and with varying depth sensitivities. CLS reveals several electronic states deep within the SiC band gap at least two of which exhibit strong surface-dependent properties. HF etched C and Si surfaces yielded pronounced emission from near band edge transitions and also from transitions involving a set of deep levels with substantial emission intensity only a few tens of nanometers from the free surface. Lower incident beam voltages result in excitation only a few tens of nanometers from the surface and a dramatic appearance of new peaks in CLS spectra reveal a dramatic dependence of emission intensities on the excitation energy, indicating bulk-related features at 0.95 eV versus surface emissions at 1.10, 1.20, 1.35, and 1.65 eV. These measurements reveal the sensitivity of UHV-CLS techniques to localized states at SiC surfaces ...

Low energy electron-excited nano-luminescence spectroscopy of GaN surfaces and interfaces

Abstract We have used low energy electron-excited nano-luminescence (LEEN) spectroscopy to obtain electronic band gap, confined state, and deep level trap information from GaN surfaces and buried interfaces on a nanometer scale. This local spectroscopy provides information available only indirectly by other electronic techniques. Using LEEN in combination with other surface science methods, we have probed the localized electronic states at GaN free surfaces, metal–GaN contacts, GaN/InGaN quantum wells, AlGaN/GaN pseudomorphic heterostructures, and GaN/sapphire template layers. Their properties are sensitive to the interface chemical composition, bonding, and atomic structure and in turn to the specifics of the epitaxial growth. The results highlight new methods for understanding and controlling electronic properties of GaN interfaces and their future applications.