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.

We have investigated the roles of interfacial reaction, work function variation, and localized states of annealed Ti/Al ohmic contacts to p -type 4H-SiC. The Al was found to be absent in the near interface region. The possibility of additional p- doping by Al indiffusion in the top SiC layer was ruled out. The work function of Ti 3 SiC 2 , the direct contact layer to SiC, was determined to be intermediate between Ti and p- SiC, leading to a considerably lowered Schottky barrier height. Reaction-induced interfacial states were observed in the near-interface SiC, which may further reduce the barrier height and cause the formation of ohmic contact.

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.

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.

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.

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.

The impact of interface switching sequences on interface quality and minority carrier recombination in In0.53Ga0.47As∕InP double heterostructure (DH) grown by solid-source molecular-beam epitaxy (MBE) was studied. As2 exposure at the lower In0.53Ga0.47As∕InP interface prior to In0.53Ga0.47As growth was found to cause enhanced As diffusion into the underlying InP that correlates with steadily increased photoconductive decay (PCD) lifetimes beyond the theoretical radiative and Auger limit. Low-temperature PCD measurements reveal that a persistent photoconductivity (PPC) process is responsible for the high “apparent” lifetimes. The PPC effect increases monotonically with As2 exposure on the InP surface, implying the involvement of interfacial defects in the carrier recombination dynamics of In0.53Ga0.47As∕InP DHs grown by MBE.

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 In 0.53Ga0.47As/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 ;2.4460.40 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. © 2004 American Vacuum Society.@DOI: 10.1116/1.1651112 #

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.

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 degreesC 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 similar to2.44 +/- 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. (C) 2004 American Vacuum Society.

We have characterized high-electron mobility transistors and corresponding uprocessed material as a function of 1.8 MeV proton fluence. Electrical data shows degradation of the electrical contacts at low fluences (10(11) - 10(14) p(+) /cm(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 and laterally-resolved cathodoluminescence spectroscopy (CLS) enable electronic property measurements of III-V nitride heterojunctions and device structures on a nanometer scale and localized to individual micron-scale transistor devices. We have correlated these microscopic electronic properties to the optical and electrical features on a macroscopic scale. Our results for quantum wells, GaN/sapphire growth templates, and AlGaN/GaN high electron mobility transistor (HEMT) devices reveal clear evidence for the effects of morphological change, chemical interdiffusion and resultant defect formation on the microscopic electronic structure. Indeed, for state-of-the-art device structures, they demonstrate clear correlations between the defect features observed optically versus the transport properties measured electrically of the individual transistors. These results show that spatially-resolved spectral features can provide detailed physical explanations for optoelectronic and microelectronic properties observed macroscopically.

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 to deep levels associated with oxygen.

We have used low-energy electron-excited nanoluminescence (LEEN) spectroscopy combined with ultrahigh vacuum surface analysis techniques to obtain electronic bandgap, confined state and deep-level trap information from III nitride compound semiconductor surfaces and their buried interfaces on a nanometer scale. Localized states are evident at GaN/InGaN quantum wells, GaN ultrathin films, AlGaN/GaN pseudomorphic heterostructures, and GaN/Al2O3 interfaces that are sensitive to the chemical composition, bonding and atomic structure near interfaces, and in turn to the specifics of the epitaxial growth process. Identification of electrically active defects in these multilayer nanostructures provides information to optimize interface growth and control local electronic properties.

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 due to excitons bound to stacking faults and cubic phase GaN. The temperature dependence of the various optical transitions in the 10–300 K range provides additional information to identify the near interface defects and impurity doping.

The performance of state-of-the-art tunneling magnetoresistive (TMR) heads depends on the thickness of insulating layers less than a few nanometers thick that separate two magnetic films. We have used low energy electron nanoscale-luminescence (LEEN) spectroscopy to observe optical emission from TMR test structures with buried insulating oxides less than a few nanometers thick. TMR structures grown by metal evaporation consisted of a 0.8–3 nm Al oxide layer on a 2.4 nm CoFe alloy (84:16) on a multilayer metal-on-Si substrate, all capped with a 0.4 nm CoFe with Pt overlayer. LEEN studies with excitation energies ranging from 0.5 to 3 keV enabled emissions from the buried oxide layers versus the free surface to be distinguished. We used different compositions, thicknesses, and oxidation exposures to separate Al oxide from transition metal oxide emissions, as well as from the ambient-exposed Pt surface. A broad peak centered at 2.2–2.5 eV increased with O plasma exposure to Al films on the CoFe alloy. Emission from oxidized CoFe without Al consists of a broad emission centered at 2.5 eV. Common to all these spectra is emission at 1.8 eV, which energy-dependent LEEN demonstrates is due to the ambient-exposed Pt. Spectral changes versus oxygen exposure reveal the regime separating oxidation of the magnetic and nonmagnetic layers.

We have used an ultrahigh vacuum scanning electron microscope to carry out cross sectional secondary electron imaging, cathodoluminescence spectroscopy, and cathodoluminescence imaging on GaN grown on sapphire by hydride vapor phase epitaxy. These measurements provide evidence for deep level defects highly localized at the GaN, sapphire interface as well as defects extending into both the semiconductor film and the substrate. The different spatial distributions of these radiative defects provide information on the physical origin of these electrically active features.

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. Highly degenerate interface regions give rise to above bandgap emissions due to band filling and free electron recombination. Besides the common donor and acceptor bound exciton, CLS and CL images also reveal emissions due to excitons bound to stacking faults and cubic phase GaN.

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-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.