Electron-excited Nanoscale Luminescence Research Articles

Low energy electron-excited nanoscale luminescence spectroscopy studies of intrinsic defects in HfO2 and SiO2–HfO2–SiO2–Si stacks

Low energy electron-excited nanoscale (LEEN) luminescence spectroscopy and secondary ion mass spectrometry have been used to probe the defect states and chemical composition in as-deposited relatively thick (∼100nm) HfO2 films and in SiO2∕HfO2∕SiO2∕Si (5nm∕15nm∕5nm) heterojunction stacks grown by plasma enhanced chemical vapor deposition including as well changes in bonding and defects after high temperature (900°C) annealing. LEEN measurements of optical transitions in the thicker HfO2 films are assigned to defect-associated radiative transitions centered at approximately 2.7, 3.4, 4.2 and 5.5eV. These spectra exhibited significant changes in as-deposited films (300°C) and after a 900°C anneal in forming gas (N2∕H2). Qualitative differences in LEEN spectra of stacked films are correlated with (i) formation of Hf silicate during deposition of the HfO2 film onto the SiO2 substrates in the as-deposited films, and (ii) a chemical phase separation of these Hf silicates into a heterogeneous mixture SiO2 and HfO2 nanocrystallites after the 900°C anneal.

Electronic defect states at annealed metal∕4H–SiC interfaces

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to study the formation of electronic surface states at metal∕4H–SiC contacts. These junctions were formed using both low and high reactivity metals to study how the nature of interface chemical bonding affects the interface state formation. We observe evidence for the formation and removal of localized states at energies that have been associated with morphological SiC defects. Metals such as Au and Ag with no strong chemical reactivity exhibited the most pronounced changes. Conversely, chemically-reactive metals such as Ti and Ni exhibited only minor changes and only with high temperature annealing. These observations suggest that native defects rather then metal-specific chemical bonding dominate the interface electronic features.

Local electronic and chemical structure at GaN, AlGaN and SiC heterointerfaces

Defects and intermediate chemical phases at nanoscale heterointerfaces of GaN, AlGaN, and SiC can dominate their macroscopic electronic properties. We have used low energy electron-excited nanoscale luminescence spectroscopy in combination with secondary ion mass spectrometry and internal photoemission spectroscopy to correlate interface physical and electronic properties for a variety of Schottky barrier and heterointerfaces involving these semiconductors. These results demonstrate the key role of initial surface processing and subsequent chemical interaction on the heterointerface electronic states, barriers, and carrier concentrations.

Chemical composition, morphology, and deep level electronic states of GaN (0001) (1×1) surfaces prepared by indium decapping

Ordered GaN (0001) (1×1) surfaces are prepared after removal from the growth chamber via thermal desorption of a thin In cap layer at 650 °C in combination with one thermal flash of Ga metal to reduce residual O and C contamination. Auger electron spectroscopy (AES) and low energy electron-excited nanoscale luminescence (LEEN) spectroscopy results show that In can prevent contamination during atmospheric exposure if it can cover the surface uniformly. LEEN spectra of the ordered surface show that the In capping layer is desorbed from the GaN without reacting to produce InxGa1−xN or diffusing into the GaN to produce new localized states. Subsequent atomic force microscopy (AFM) measurements reveal an atomically smooth film plus Ga droplets residual to the flash annealing. These results suggest that In decapping may be useful in obtaining clean, LEED-ordered GaN surfaces after transport in air with a minimum of UHV treatment.

Role of barrier and buffer layer defect states in AlGaN/GaN HEMT structures

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures to correlate their effect on two-dimensional electron gas (2DEG) confinement. Also, we have used Auger electron spectroscopy (AES) to detect the chemical composition as a function of lateral position on a growth wafer and to correlate chemical effects with electronic properties. We investigated several high-quality AlGaN/GaN heterostructures of varying electrical properties using incident electron beam energies of 0.5–15 keV to probe electronic state transitions within each of the heterostructure layers. The LEEN depth profiles reveal differences between sucessful and failed structures and highlight the importance of acceptor deep defect levels in the near 2DEG region. Variations in the GaN and AlGaN band edge emissions, as well as the yellow defect emission across an AlGaN/GaN heterostructure growth wafer have been observed. AES and LEEN spectroscopy of the growth wafer suggest that variation in the cation concentration may play a role in the mechanism responsible for the deep aceceptor level emission in the AlGaN barrier layer.

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.

Low energy electron-excited nanoscale luminescence: a tool to detect trap activation by ionizing radiation

Ultra-thin SiO/sub 2//Si gate dielectric structures exposed to heavy X-ray irradiation exhibit optical emission characteristic of interface traps. Low energy electron-excited luminescence spectroscopy with nanometer-scale depth resolution yields a characteristic spectral energy and excitation depth dependence. Ultra-thin (5 nm) oxide films on Si substrates exposed to 10 keV, 7.6 Mrad(SiO/sub 2/) [13.7 Mrad (Si)] X-ray irradiation introduces trap densities on the order of 10/sup 11/ cm/sup -2/ ev/sup -1/, localized near the intimate SiO/sub 2/-Si interface. This density is consistent with the trapped oxide and interface charge densities expected based on observed capacitance-voltages shifts of thicker oxides, their corresponding charge densities, and the proportionally smaller charge densities expected for the thinner oxide layers in this study.

Low energy electron excited nanoscale luminescence spectroscopy of erbium doped AIN

We have investigated the optical activity of Er3+ ions in AlN via depth-resolved, (5 nm to 250 nm), low energy electron-excited nanoscale (LEEN) luminescence spectroscopy and compared it with the luminescence of an Er-free AlN film. For the Er-free film, there was no emission in the IR from the AlN at any depth, and at higher energies we measured only a broad, weak feature between 1.7–3.25 eV along with an O defect related feature at 3.8 eV, which is significantly enhanced toward the surface. We found strong emission in the AlN:Er films from the first excited → ground state transition of Er3+ at 0.80 eV along with many other excited state transitions, although the features are broad compared to those of GaN:Er. The AlN:Er luminescence saturates near a concentration of 1021 cm−3, at which point we also observe enhanced O defect related luminescence uniformly distributed throughout the film. This finding suggests a role for O in activating the Er at low Er concentrations, while inhibiting the Er activity at high O concentrations.