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.

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.

Low-energy electron-excited nanoscale-luminescence (LEEN) spectroscopy of GaN/InGaN/GaN double-heterojunction structures reveal the formation of electronic states localized near the quantum well interfaces under relatively In-rich conditions. These states are due to formation in a cubic GaN region comparable to the quantum well layer in thickness rather than the bulk native defects typically associated with growth quality. The nanoscale depth dependence of the noncontact, nondestructive LEEN technique enables detection of this competitive recombination channel within a few nanometers of the “buried” heterojunction interfaces.

In this work we investigate ZnSe/GaAs heterostructures with an additional 2 nm controlled interfacial layer (CIL) of Se- or Zn-rich composition to modify the band offset. The samples are analyzed as a function of annealing temperature by cathodoluminescence spectroscopy. The as-prepared samples show defect luminescence at ∼ 0.9 eV. With staged annealing at increasing temperatures, both the Zn-rich as well as the Se-rich interfacial layer exhibits luminescence at ∼ 1.9 eV, indicative of defect formation with an onset temperature of ∼400°C. Excitation-dependent spectroscopy provides evidence for defect formation near the interface, which extends into the ZnSe epilayer at higher temperatures. Compared to earlier work, where the threshold temperature for defect formation in bulk samples fabricated under Se-rich growth conditions occurs at temperatures as low as 325°C, the resistance to defect formation has now been improved to that of stoichiometric ZnSe. These results demonstrate that epitaxially grown CILs provide a means to alter ZnSe/GaAs band offsets without degrading the heterojunction’s resistance to defect formation at elevated temperatures.

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. The decreased emission at shallow (tens of nanometer) probe depths suggests a depletion of activation Er in the near-surface region. Enhancement of near-surface Er3+ luminescence with annealing may be due to lattice reordering as well as impurity redistribution.

We have used low-energy electron-excited nanoscale-luminescence (LEEN) spectroscopy combined with ultrahigh vacuum (UHV) surface science techniques to probe deep level defect states at GaN free surfaces, metal-GaN contacts and GaN/InGaN quantum well interfaces. Employing energies as low as 100eV and ranging up to 5keV, we have been able to establish the local nature of these states and their spatial variation normal to the interface plane on an incremental 10–20nm scale. Coupled with surface science techniques, these measurements show that a variety of discrete deep levels form deep within the GaN band gap due to (a)native defects, (b)metal-induced bonding, (c)reaction products, and (d), in the case of GaN/InGaN heterostructures, local interface phase changes. These results suggest that deep levels are a common feature at GaN interfaces and hence can play an integral role in charge transfer and the formation of local dipoles at GaN heterostructures.

We have measured the energies, relative intensities, and spatial distribution of deep level defect transitions near GaN/InGaN/GaN quantum well structures using low-energy electron-excited nanoscale-luminescence (LEEN) spectroscopy. Results obtained with electron excitation energies as low as 100 eV provide evidence on an incremental, 10–20 nm scale for defects within the GaN confinement layer, their variation with distance from the quantum well, and their electronic quality relative to the GaN substrate. The InGaN quantum well and GaN near-band-edge luminescence intensities exhibit strong variations as a function of excitation depth. Combined with a model of energy-dependent penetration, diffusion, and recombination, these variations indicate a value of 25–28 nm for the minority carrier diffusion length within the GaN confinement layer. Depth-dependent spectra also reveal the presence of cubic GaN phase formation at the InGaN/GaN substrate interface of a relatively In-rich quantum well structure. The contrast in LEEN features between structures of two different InGaN quantum well compositions demonstrates the effect of growth composition on local state formation.

An experiment to measure P23, the probability for Andreev reflection of rotons at the free surface, gives the upper limit P23≤P242, where P24 is the probability of single-particle conversion of an R- roton to an atom.