S. H. Goss

Electrical, spectral, and chemical properties of 1.8 MeV proton irradiated AlGaN/GaN HEMT structures as a function of proton fluence

We have characterized high-electron mobility transistors and corresponding unprocessed material as a function of 1.8 MeV proton fluence. Electrical data shows degradation of the electrical contacts at low fluences (10/sup 11/-10/sup 14/ p/sup +//cm/sup 2/) and degradation of the channel properties for higher fluences. In conjunction with the electrical data, cathodoluminescence and secondary-ion mass spectrometry results suggest mechanisms for the higher fluence degradation.

Depth-dependent investigation of defects and impurity doping in GaN/sapphire using scanning electron microscopy and cathodoluminescence spectroscopy

Cathodoluminescence (CL) imaging and temperature-dependent cathodoluminescence spectroscopy (CLS) have been used 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 imaging, CLS, and CL imaging show systematic variations in defect emissions with 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 out-diffusion from Al2O3 into GaN over hundreds of nanometers. Low-temperature CL spectra exhibit a new donor level at 3.447 meV near the interface for such samples, characteristic of O impurities spatially localized to the nanoscale interface. CLS emissions indicate the formation of amorphous Al–N–O complexes at 3.8 eV extending into the Al2O3 near the GaN/sapphire interface. CLS and CL images also reveal emissions...

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.

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.

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.

On microscopic compositional and electrostatic properties of grain boundaries in polycrystalline CuIn1−xGaxSe2

We report on the microscopic characteristics of polycrystalline CuIn1−xGaxSe2 thin films probed with Auger electron spectroscopy, cathodoluminescence spectroscopy, secondary ion mass spectrometry, and work function measurements. Confirming theory, we find a substantial reduction in Cu content from grain interior to boundary and a p-type potential barrier that acts to reduce hole recombination. Such compositional and electrostatic variations between grain boundaries and grain interiors in CuIn1−xGaxSe2 solar cell absorber layers may improve the overall photovoltaic efficiency. The high degree of intergranular inhomogeneity emphasizes the importance of detailed grain-by-grain analysis. These results show that careful specimen preparation and ultrahigh vacuum conditions, coupled with nanoscale instrumental resolution, are pivotal for such analysis.

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 Hf...

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.

Atomic diffusion and band lineups at In0.53Ga0.47As-on-InP heterointerfaces

We have used secondary ion mass spectrometry (SIMS), cathodoluminescence spectroscopy (CLS), and an analysis of secondary electron thresholds (SETs) to determine how extended anion soaks during molecular beam epitaxial (MBE) growth transitions affect band lineups at the lattice-matched In0.53Ga0.47As-on-InP interface. Growth transitions consisting of 20–150 As soaks result in SIMS-measured interfacial broadening of up to 8 nm. By monitoring SETs across an in situ cleaved InP∕In0.53Ga0.47As∕InP double heterostructures, we measure a type I conduction-band offset of 190±30meV at an abrupt InGaAs-on-InP interface. For diffused structures exposed to long As soak times, we observe an effective decrease of ΔEc by up to 210±40meV. The changes in InGaAs and InP CL intensities are consistent with both the SET-measured decrease in conduction-band offset and an increase in nonradiative recombination at the diffused InGaAs-on-InP interface.