S. T. Bradley

Compositional modulation and optical emission in AlGaN epitaxial films

Compositional, structural, and optical properties of molecular-beam epitaxy grown AlxGa1−xN films were characterized by transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence spectroscopy. Spontaneous modulation, phase separation, and band gap reductions were observed to vary systematically with AlN mole fraction across the full alloy series. At low AlN mole fraction (x⩽0.5), AlGaN epilayers display pronounced phase separation. With increasing AlN mole fraction, phase separation is strongly suppressed by the formation of spontaneous modulation which high spatial resolution TEM techniques unambiguously determine to be atomic-scale compositional superlattice. The formation of the spontaneous superlattice is considered responsible for the pronounced reductions in band gaps and emission energies, exceeding several hundred meV for the Al-rich AlGaN, which has been confirmed by band structure calculations.

Deep level defects and doping in high Al mole fraction AlGaN

We have used depth-dependent cathodoluminescence spectroscopy (CLS) and secondary ion mass spectrometry (SIMS) to investigate the nature of deep level defects and their effect on Si doping of high Al mole fraction (25%–100%) AlGaN. SIMS results provide correlations between AlGaN deep level emissions from CLS and elemental impurities distributed through the epitaxial bulk films. The highest Al mole fraction (xAl) samples exhibit deep level optical emissions that correlate with O and C impurities measured by SIMS. These O impurities appear to introduce donors at low and intermediate Al compositions versus deep levels in Al-rich alloys. The CLS energy onset of near band edge peak emissions track the b=1 theoretical band gap for 0⩽xAl⩽0.98 while their peak emissions deviate monotonically. Temperature-dependent CLS reveal an activation energy decrease of the near band edge emission intensity from 54 to 36 meV for xAl>∼0.80. The absence of free carriers for xAl>0.80 is consistent with Si donor compensation due ...

Surface cleaning and annealing effects on Ni∕AlGaN interface atomic composition and Schottky barrier height

Internal photoemission spectroscopy reveals changes in the Schottky barrier height of Ni on AlGaN∕GaN high electron mobility transistor structures with premetallization processing conditions and postmetallization ultrahigh-vacuum annealing. These variations in the internal photoemission Schottky barrier height are correlated with AlGaN near-band-edge emissions from low-energy electron-excited nanoluminescence spectroscopy and Ni∕AlGaN interface impurities by secondary ion mass spectrometry. We show that changes in the Schottky barrier height and the appearance of dual barriers are dominated by changes in the local Al mole fraction. Interfacial oxygen and carbon have secondary but systematic effects as well.

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.

Si doping of high-Al-mole fraction AlxGa1−xN alloys with rf plasma-induced molecular-beam-epitaxy

Very high levels of n-type doping of AlxGa1−xN alloys were recently achieved by rf plasma-induced molecular-beam epitaxy on sapphire substrates and Si as a dopant. Electron concentrations were obtained up to 1.25×1020 cm−3 when the Al mole fraction was 50%, and 8.5×1019 cm−3 electrons were measured even when the Al mole fraction was 80%. Other material properties were determined by optical absorption, photoluminescence, cathodoluminescence, x-ray diffraction, and atomic force microscopy measurements and high optical and morphological qualities were shown.

Spontaneous compositional superlattice and band-gap reduction in Si-doped AlxGa1−xN epilayers

Combined transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence spectroscopy measurements of AlxGa1−xN thin films grown by molecular-beam epitaxy reveal spontaneous modulation, phase separation, and band-gap reductions that vary systematically with AlN mole fraction across the full alloy series. At low AlN mole fraction (x⩽0.5), AlGaN epilayers display pronounced phase separation. With increasing AlN mole fraction, phase separation is strongly suppressed by the formation of spontaneous modulation, which high spatial resolution TEM techniques unambiguously determine to be an atomic-scale compositional superlattice. Superlattice-induced reductions from band gaps expected for compositionally disordered epilayers exceed several hundred meV for the Al-rich average alloy composition.

Pre-metallization processing effects on Schottky contacts to AlGaN∕GaN heterostructures

Changes in the Schottky barrier height of Ni on AlGaN∕GaN heterostructure field effect transistor structures are characterized by internal photoemission spectroscopy (IPE) as a function of pre-metallization processing conditions and postmetallization ultrahigh vacuum annealing. Low energy electron-excited nanoluminescence spectroscopy and mapping reveal AlGaN near band edge emission variations that correlate with IPE Schottky barrier height. Ni∕AlGaN interface impurities measured by secondary ion mass spectrometry are also correlated with IPE Schottky barrier height. We show that changes in the Schottky barrier height and the appearance of dual barriers are dominated by changes in the local Al mole fraction. Interfacial oxygen and carbon have secondary but systematic effects as well.

Atomic layer diffusion and electronic structure at In0.53Ga0.47As/InP interfaces

We have used secondary ion mass spectrometry and cathodoluminescence spectroscopy to determine the effects that growth and postgrowth conditions have on interdiffusion and near band edge emissions in In0.53Ga0.47As/InP heterojunctions grown by molecular beam epitaxy. This lattice-matched interface represents a model system for the study of atomic movements and electronic changes with controlled anion overlap during growth. Structures subjected to anneals ranging from 440 to 495 °C provide a quantitative measure of concentration-driven cross diffusion of group-III and group-V atoms. By measuring anneal-induced broadening at the InGaAs-on-InP interface we have determined an activation energy for As diffusion into InP of ∼2.44±0.40 eV. An interface layer with Ga–P bonds indicates Ga competes favorably versus As for bonding in the preannealed InP near-surface region. In addition, we present evidence that interface chemical effects manifest themselves electronically as variations of the InGaAs band gap energy.

Effects of deep-level defects on ohmic contact and frequency performance of AlGaN/GaN high-electron-mobility transistors

We have characterized AlGaN/GaN high-electron-mobility-transistors on sapphire and silicon carbide substrates with electrical and microcathodoluminescence spectral measurements. Quarter wafer-scale comparisons of spectral features in the GaN attributed to donor–acceptor pair (DAP) transitions and yellow luminescence (YL) from deep acceptors show that the specific contact resistance is related to the ratio of the DAP to YL defect emission intensities. This suggests that these defects interact to change the contact resistance locally on the GaN side of the AlGaN/GaN interface. We show that changes in the frequency response of these transistors can be attributed to these defects at the interface.

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.