Open-volume defects introduced in Si(100) crystals during fluorine implantation were investigated... more Open-volume defects introduced in Si(100) crystals during fluorine implantation were investigated by variable-energy positron beam depth profiling. The behavior of the implantation-induced lattice defects upon high temperature annealing and their role in the surface-oriented diffusion of F impurities were examined. The defects become mobile and undergo recovery at temperatures below 550 °C, i.e., well before the onset of fluorine diffusion as seen by secondary ion mass spectroscopy (SIMS) profiling. This behavior suggests that after irradiation and annealing the fluorine occupies substitutional sites to which positrons are insensitive. The anomalous F diffusion seen in SIMS has been explained through a two-step diffusion mechanism, in which the diffusion kinetics is determined by dissociation of the substitutional F into an interstitial F and a vacancy, followed by a rapid diffusion of the interstitial F and the vacancy through the crystal to the surface.
Positron-annihilation spectra are used to identify interface states in a 110-nm-thick, thermally ... more Positron-annihilation spectra are used to identify interface states in a 110-nm-thick, thermally grown (dry, no HCl) SiO,/Si(100) system. A normalized shape parameter (S parameter) is used to characterize the positron-annihilation spectra. The interface-state-density variation under a low-temperature annealing (20'C-500'C) is shown to be correlated with the variation in the intensity of the interface shape parameter. Activation and passivation of interface states by atomic hydrogen are demonstrated by repeated in situ hydrogen exposure and vacuum anneal. The present study shows that characterizing the interface states with positron-annihilation techniques opens an avenue for studies involving interface states without the need for a gate electrode.
New insight into damage formation in Si(100) during self-ion irradiation is gained from processin... more New insight into damage formation in Si(100) during self-ion irradiation is gained from processing under extreme conditions. Dislocations form in the near-surface as a result of lattice relaxation in response to strain produced by precursor defects which are shown to be vacancy-type by positron analysis. A model to account for these defects and their distribution is presented. A novel technique is demonstrated which utilizes a subsequent implantation as a depth specific probe to manipulate the vacancy-type defects. Aspects of damage growth which emerge from the probe results are discussed.
Positrons implanted with varying energies (0-20 keV) have been used to study silicon epilayers gr... more Positrons implanted with varying energies (0-20 keV) have been used to study silicon epilayers grown by molecular-beam epitaxy on Si(100) substrates. Defects at the initial growth interface and throughout the overlayer have been observed and depth profiled. In addition, field-driven positron drift observed in some of the epilayers is shown to be consistent with estimated concentrations of (active) interfacial impurities. The study demonstrates that positrons can be used nondestructively to profile structural defects and electric fileds in thin films and at interfaces.
Defects in a silicon-on-insulator structure formed by high-energy (200-keV) oxygen implantation h... more Defects in a silicon-on-insulator structure formed by high-energy (200-keV) oxygen implantation has been studied utilizing a variable-energy positron beam. The positron-based probe is found to be especially sensitive to the condition of the top Si layer. Open-volume defects (cavities) are detected in the top 80-nm Si layer in the as-irradiated state. The majority of these defects are removed by high-temperature
The effects of the heat treatment of Si covered with a thermally-grown ˜50 nm overlayer of SiO2 w... more The effects of the heat treatment of Si covered with a thermally-grown ˜50 nm overlayer of SiO2 were probed by means of measurements of positron annihilation characteristics obtained with a variable-energy positron beam. From measurements at elevated temperature (˜500°C) it was observed that positrons implanted overlapping the SiO2/Si interface decay from a state with properties distinctively different from the state in Si and in SiO2. The nature of the annihilation characteristics indicates the presence of open volume defects.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1989
The applicability ofPd x Tal _ x as a diffusion barrier on Si has been investigated. For this pur... more The applicability ofPd x Tal _ x as a diffusion barrier on Si has been investigated. For this purpose Pd x Tal _ x films of 200-nm thickness (x ranges from 0 to 1) were deposited on SIC 1(0), and the reaction between overlayer and substrate was studied as a function of temperature. Interaction was found to occur at temperatures increasing with the Ta content. The as-deposited Pd x Tal _ x films with 0.2<x<0.6 were found to be amorphous. The amorphous phase had a higher reaction temperature than the crystalline one, causing a discontinuous step in the reaction temperature. Rutherford backscattering spectrometry spectra revealed that for the Pd-rich compositions, first a stoichiometric Pd 2 Si layer formed underneath a pure Ta layer. At higher temperatures TaSi 2 formed at the surface. For Ta-rich compositions Pd 2 Si formed first as well; however, the reaction temperature was so high that Pd 2 Si grains formed in a Si matrix. The defect density of the Ta layer, which remained after outdiffusion of Pd, was investigated using variable energy positron annihilation. The defect concentration is very high, as deduced from the trapped positron fraction. A model is presented that describes the composition dependence of the reaction temperature.
We have compared the shot-noise properties at T = 4.2 K of a double-barrier resonant-tunneling di... more We have compared the shot-noise properties at T = 4.2 K of a double-barrier resonant-tunneling diode and a superlattice tunnel diode, both of which exhibit negative differential-conductance (NDC) in their current-voltage characteristics. While the noise spectral density of the former device was greatly enhanced over the Poissonian value of 2eI in the NDC region, that of the latter device remained 2eI. This result implies that charge accumulation, not system instability, is responsible for shot-noise enhancement in NDC devices.
ABSTRACT The annihilation characteristics of a monoenergetic beam of positrons, after implantatio... more ABSTRACT The annihilation characteristics of a monoenergetic beam of positrons, after implantation in Si with a 350-nm overlayer of SiO2, were measured as a function of mean implantation depth. Positrons implanted overlapping the SiO2/Si interface were observed to decay from a state with properties distinctively different from the state in bulk Si and the thermally grown SiO2, i.e., a positron interface state. The momentum distribution of the annihilating positron-electron pair, as observed in the Doppler broadening of the annihilation line, is much broader for this state than for either bulk Si or SiO2, in contrast to previously observed localized positron states in solids and at surfaces which show a narrower distribution.
Positron diffusion in Si(100) and Si(111) has been studied using a variable-energy positron beam.... more Positron diffusion in Si(100) and Si(111) has been studied using a variable-energy positron beam. The positron diffusion coefficient is found to be D+-2.7+0.3 cm /sec using a Makhov-type positron implantation profile, which is demonstrated to fit the data more reliably than the more commonly applied exponential profile. The diffusion-related parameter, Eo, which results from the exponential profile, is found to be 4.2+0.2 keV, significantly longer than previously reported values. A drastic reduction in Eo is found after annealing the sample at 1300 K, showing that previously reported low values of Eo are probably associated with the thermal history of the sample. Reconstruction of the Si(111)into the 7)& 7 (low-energy electron diffraction) structure had no detectable effect on the positron diffusion behavior.
A variable-energy positron beam was used to study device-quality SiO2 (~=50-nm thick) grown therm... more A variable-energy positron beam was used to study device-quality SiO2 (~=50-nm thick) grown thermally on the Si(100) surface. The unusual observation of ortho-positronium 3gamma decay at the interface demonstrates that microvoids >1 nm in size are present, most likely as a consequence of the thermal oxidation process. Other interfacial defects were also observed, illustrating the sensitivity of positron studies for studying interfacial properties.
The environmental durabilities of several commercial bridge coatings are being investigated by ex... more The environmental durabilities of several commercial bridge coatings are being investigated by exposing samples to accelerated UV irradiation. Primary microscopic techniques include positron annihilation spectroscopy (PAS), which detects and characterizes nanometer-scale physical holes/defects, and electron spin resonance (ESR) spectroscopy, which detects broken chemical bonds. For the PAS tests, significant decreases of sub-nanometer defect parameters are observed as a function of
ABSTRACT Due to its small volume and linear energy dependence of electron density of states, grap... more ABSTRACT Due to its small volume and linear energy dependence of electron density of states, graphene has very low electronic heat capacitance compared to what is found in metals with achievable volume. This makes it a promising material for applications requiring bolometric sensing of radiation with a fast response without compromising sensitivity. Here we report fabrication of graphene-superconductor tunnel junctions and characterization of their bolometric response. When radio frequency radiation is shone onto the junction, the electrons in graphene heat up and dynamic resistance within the superconducting gap changes. The relation between absorbed power and temperature rise is used to characterize heat conductance and thermal noise equivalent power.
ABSTRACT Low electronic heat capacity and small achievable volume has made graphene a promising c... more ABSTRACT Low electronic heat capacity and small achievable volume has made graphene a promising candidate for a fast and sensitive bolometric detector. In our device scheme, in addition to low electron-phonon coupling we can further limit out-diffusion of hot electrons from graphene into the leads by placing tunnel-type superconducting contacts on graphene and preventing tunneling through the oxide barrier into the superconducting gap. We fabricate NbN contacts on graphene with a sandwich layer of titanium oxide tunnel barrier. Due to high dielectric constant of titanium oxide, our design allows device impedance matching with the antenna circuitry at THz frequencies, necessary to achieve practical device efficiency. We present our measurements of bolometric characteristics of thermal conductance, Noise equivalent Power (NEP) and responsivity for such a device.
In a study of neutron-irradiated Mo containing voids, Schultz et al. [Phys. Rev. Lett. 44, 1629 (... more In a study of neutron-irradiated Mo containing voids, Schultz et al. [Phys. Rev. Lett. 44, 1629 (1980)] presented data which showed a small fraction of positronium atoms being formed in the voids and decaying by three photons. This effect was observable at temperatures above ~=650 K. It was stated at that time that the positronium may be forming through the
The interface between graphene and the ferroelectric superlattice PbTiO3/SrTiO3 (PTO/STO) is stud... more The interface between graphene and the ferroelectric superlattice PbTiO3/SrTiO3 (PTO/STO) is studied. Tuning the transition temperature through the PTO/STO volume fraction minimizes the adorbates at the graphene/ferroelectric interface, allowing robust ferroelectric hysteresis to be demonstrated. "Intrinsic" charge traps from the ferroelectric surface defects can adversely affect the graphene channel hysteresis and can be controlled by careful sample processing, enabling systematic study of the charge trapping mechanism.
Open-volume defects introduced in Si(100) crystals during fluorine implantation were investigated... more Open-volume defects introduced in Si(100) crystals during fluorine implantation were investigated by variable-energy positron beam depth profiling. The behavior of the implantation-induced lattice defects upon high temperature annealing and their role in the surface-oriented diffusion of F impurities were examined. The defects become mobile and undergo recovery at temperatures below 550 °C, i.e., well before the onset of fluorine diffusion as seen by secondary ion mass spectroscopy (SIMS) profiling. This behavior suggests that after irradiation and annealing the fluorine occupies substitutional sites to which positrons are insensitive. The anomalous F diffusion seen in SIMS has been explained through a two-step diffusion mechanism, in which the diffusion kinetics is determined by dissociation of the substitutional F into an interstitial F and a vacancy, followed by a rapid diffusion of the interstitial F and the vacancy through the crystal to the surface.
Positron-annihilation spectra are used to identify interface states in a 110-nm-thick, thermally ... more Positron-annihilation spectra are used to identify interface states in a 110-nm-thick, thermally grown (dry, no HCl) SiO,/Si(100) system. A normalized shape parameter (S parameter) is used to characterize the positron-annihilation spectra. The interface-state-density variation under a low-temperature annealing (20'C-500'C) is shown to be correlated with the variation in the intensity of the interface shape parameter. Activation and passivation of interface states by atomic hydrogen are demonstrated by repeated in situ hydrogen exposure and vacuum anneal. The present study shows that characterizing the interface states with positron-annihilation techniques opens an avenue for studies involving interface states without the need for a gate electrode.
New insight into damage formation in Si(100) during self-ion irradiation is gained from processin... more New insight into damage formation in Si(100) during self-ion irradiation is gained from processing under extreme conditions. Dislocations form in the near-surface as a result of lattice relaxation in response to strain produced by precursor defects which are shown to be vacancy-type by positron analysis. A model to account for these defects and their distribution is presented. A novel technique is demonstrated which utilizes a subsequent implantation as a depth specific probe to manipulate the vacancy-type defects. Aspects of damage growth which emerge from the probe results are discussed.
Positrons implanted with varying energies (0-20 keV) have been used to study silicon epilayers gr... more Positrons implanted with varying energies (0-20 keV) have been used to study silicon epilayers grown by molecular-beam epitaxy on Si(100) substrates. Defects at the initial growth interface and throughout the overlayer have been observed and depth profiled. In addition, field-driven positron drift observed in some of the epilayers is shown to be consistent with estimated concentrations of (active) interfacial impurities. The study demonstrates that positrons can be used nondestructively to profile structural defects and electric fileds in thin films and at interfaces.
Defects in a silicon-on-insulator structure formed by high-energy (200-keV) oxygen implantation h... more Defects in a silicon-on-insulator structure formed by high-energy (200-keV) oxygen implantation has been studied utilizing a variable-energy positron beam. The positron-based probe is found to be especially sensitive to the condition of the top Si layer. Open-volume defects (cavities) are detected in the top 80-nm Si layer in the as-irradiated state. The majority of these defects are removed by high-temperature
The effects of the heat treatment of Si covered with a thermally-grown ˜50 nm overlayer of SiO2 w... more The effects of the heat treatment of Si covered with a thermally-grown ˜50 nm overlayer of SiO2 were probed by means of measurements of positron annihilation characteristics obtained with a variable-energy positron beam. From measurements at elevated temperature (˜500°C) it was observed that positrons implanted overlapping the SiO2/Si interface decay from a state with properties distinctively different from the state in Si and in SiO2. The nature of the annihilation characteristics indicates the presence of open volume defects.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1989
The applicability ofPd x Tal _ x as a diffusion barrier on Si has been investigated. For this pur... more The applicability ofPd x Tal _ x as a diffusion barrier on Si has been investigated. For this purpose Pd x Tal _ x films of 200-nm thickness (x ranges from 0 to 1) were deposited on SIC 1(0), and the reaction between overlayer and substrate was studied as a function of temperature. Interaction was found to occur at temperatures increasing with the Ta content. The as-deposited Pd x Tal _ x films with 0.2<x<0.6 were found to be amorphous. The amorphous phase had a higher reaction temperature than the crystalline one, causing a discontinuous step in the reaction temperature. Rutherford backscattering spectrometry spectra revealed that for the Pd-rich compositions, first a stoichiometric Pd 2 Si layer formed underneath a pure Ta layer. At higher temperatures TaSi 2 formed at the surface. For Ta-rich compositions Pd 2 Si formed first as well; however, the reaction temperature was so high that Pd 2 Si grains formed in a Si matrix. The defect density of the Ta layer, which remained after outdiffusion of Pd, was investigated using variable energy positron annihilation. The defect concentration is very high, as deduced from the trapped positron fraction. A model is presented that describes the composition dependence of the reaction temperature.
We have compared the shot-noise properties at T = 4.2 K of a double-barrier resonant-tunneling di... more We have compared the shot-noise properties at T = 4.2 K of a double-barrier resonant-tunneling diode and a superlattice tunnel diode, both of which exhibit negative differential-conductance (NDC) in their current-voltage characteristics. While the noise spectral density of the former device was greatly enhanced over the Poissonian value of 2eI in the NDC region, that of the latter device remained 2eI. This result implies that charge accumulation, not system instability, is responsible for shot-noise enhancement in NDC devices.
ABSTRACT The annihilation characteristics of a monoenergetic beam of positrons, after implantatio... more ABSTRACT The annihilation characteristics of a monoenergetic beam of positrons, after implantation in Si with a 350-nm overlayer of SiO2, were measured as a function of mean implantation depth. Positrons implanted overlapping the SiO2/Si interface were observed to decay from a state with properties distinctively different from the state in bulk Si and the thermally grown SiO2, i.e., a positron interface state. The momentum distribution of the annihilating positron-electron pair, as observed in the Doppler broadening of the annihilation line, is much broader for this state than for either bulk Si or SiO2, in contrast to previously observed localized positron states in solids and at surfaces which show a narrower distribution.
Positron diffusion in Si(100) and Si(111) has been studied using a variable-energy positron beam.... more Positron diffusion in Si(100) and Si(111) has been studied using a variable-energy positron beam. The positron diffusion coefficient is found to be D+-2.7+0.3 cm /sec using a Makhov-type positron implantation profile, which is demonstrated to fit the data more reliably than the more commonly applied exponential profile. The diffusion-related parameter, Eo, which results from the exponential profile, is found to be 4.2+0.2 keV, significantly longer than previously reported values. A drastic reduction in Eo is found after annealing the sample at 1300 K, showing that previously reported low values of Eo are probably associated with the thermal history of the sample. Reconstruction of the Si(111)into the 7)& 7 (low-energy electron diffraction) structure had no detectable effect on the positron diffusion behavior.
A variable-energy positron beam was used to study device-quality SiO2 (~=50-nm thick) grown therm... more A variable-energy positron beam was used to study device-quality SiO2 (~=50-nm thick) grown thermally on the Si(100) surface. The unusual observation of ortho-positronium 3gamma decay at the interface demonstrates that microvoids >1 nm in size are present, most likely as a consequence of the thermal oxidation process. Other interfacial defects were also observed, illustrating the sensitivity of positron studies for studying interfacial properties.
The environmental durabilities of several commercial bridge coatings are being investigated by ex... more The environmental durabilities of several commercial bridge coatings are being investigated by exposing samples to accelerated UV irradiation. Primary microscopic techniques include positron annihilation spectroscopy (PAS), which detects and characterizes nanometer-scale physical holes/defects, and electron spin resonance (ESR) spectroscopy, which detects broken chemical bonds. For the PAS tests, significant decreases of sub-nanometer defect parameters are observed as a function of
ABSTRACT Due to its small volume and linear energy dependence of electron density of states, grap... more ABSTRACT Due to its small volume and linear energy dependence of electron density of states, graphene has very low electronic heat capacitance compared to what is found in metals with achievable volume. This makes it a promising material for applications requiring bolometric sensing of radiation with a fast response without compromising sensitivity. Here we report fabrication of graphene-superconductor tunnel junctions and characterization of their bolometric response. When radio frequency radiation is shone onto the junction, the electrons in graphene heat up and dynamic resistance within the superconducting gap changes. The relation between absorbed power and temperature rise is used to characterize heat conductance and thermal noise equivalent power.
ABSTRACT Low electronic heat capacity and small achievable volume has made graphene a promising c... more ABSTRACT Low electronic heat capacity and small achievable volume has made graphene a promising candidate for a fast and sensitive bolometric detector. In our device scheme, in addition to low electron-phonon coupling we can further limit out-diffusion of hot electrons from graphene into the leads by placing tunnel-type superconducting contacts on graphene and preventing tunneling through the oxide barrier into the superconducting gap. We fabricate NbN contacts on graphene with a sandwich layer of titanium oxide tunnel barrier. Due to high dielectric constant of titanium oxide, our design allows device impedance matching with the antenna circuitry at THz frequencies, necessary to achieve practical device efficiency. We present our measurements of bolometric characteristics of thermal conductance, Noise equivalent Power (NEP) and responsivity for such a device.
In a study of neutron-irradiated Mo containing voids, Schultz et al. [Phys. Rev. Lett. 44, 1629 (... more In a study of neutron-irradiated Mo containing voids, Schultz et al. [Phys. Rev. Lett. 44, 1629 (1980)] presented data which showed a small fraction of positronium atoms being formed in the voids and decaying by three photons. This effect was observable at temperatures above ~=650 K. It was stated at that time that the positronium may be forming through the
The interface between graphene and the ferroelectric superlattice PbTiO3/SrTiO3 (PTO/STO) is stud... more The interface between graphene and the ferroelectric superlattice PbTiO3/SrTiO3 (PTO/STO) is studied. Tuning the transition temperature through the PTO/STO volume fraction minimizes the adorbates at the graphene/ferroelectric interface, allowing robust ferroelectric hysteresis to be demonstrated. "Intrinsic" charge traps from the ferroelectric surface defects can adversely affect the graphene channel hysteresis and can be controlled by careful sample processing, enabling systematic study of the charge trapping mechanism.
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Papers by Bent Nielsen