Helen Jackson | Wright State University (original) (raw)

Papers by Helen Jackson

SPIE Proceedings, 2007

In the past, various virtual Frisch-grid designs have been proposed for cadmium zinc telluride (C... more In the past, various virtual Frisch-grid designs have been proposed for cadmium zinc telluride (CZT) and other compound semiconductor detectors. These include three-terminal, semi-spherical, CAPture, Frisch-ring, capacitive Frisch-grid and pixel devices (along with their modifications). Among them, the Frisch-grid design employing a non-contacting ring extended over the entire side surfaces of parallelepiped-shaped CZT crystals is the most promising. The defect-free parallelepiped-shaped crystals
with typical dimensions of 5x5~12m m3 are easy to produce and can be arranged into large arrays used for imaging and gamma-ray spectroscopy. In this paper, we report on further advances of the virtual Frischgrid detector design for the parallelepiped-shaped CZT crystals. Both the experimental testing and modeling results are described.
Keywords: CdZnTe, gamma ray detectors, Frisch-grid

Hard X-Ray and Gamma-Ray Detector Physics and Penetrating Radiation Systems VIII, 2006

Te inclusions existing at high concentrations in CdZnTe (CZT) material can degrade the performan... more Te inclusions existing at high concentrations in
CdZnTe (CZT) material can degrade the performance of CZT
detectors. These microscopic defects trap the free electrons generated
by incident radiation, so entailing significant fluctuations
in the total collected charge and thereby strongly affecting the
energy resolution of thick (long-drift) detectors. Such effects
were demonstrated in thin planar detectors, and, in many cases,
they proved to be the dominant cause of the low performance
of thick detectors, wherein the fluctuations in the charge losses
accumulate along the charge’s drift path. We continued studying
this effect using different tools and techniques. We employed a
dedicated beam-line recently established at BNL’s National Synchrotron
Light Source for characterizing semiconductor radiation
detectors, along with an IR transmission microscope system, the
combination of which allowed us to correlate the concentration
of defects with the devices’ performances. We present here our
new results from testing over 50 CZT samples grown by different
techniques. Our goals are to establish tolerable limits on the size
and concentrations of these detrimental Te inclusions in CZT
material, and to provide feedback to crystal growers to reduce
their numbers in the material.

IEEE Transactions on Nuclear Science, 2008

With the choice of an optimum thickness range, silicon nitride passivation (Si3N4) on AlGaN\GaN h... more With the choice of an optimum thickness range, silicon nitride passivation (Si3N4) on
AlGaN\GaN heterojunction devices can not only improve device performance but also radiation
harden the device while preserving the reliability and performance. In this study, the effects of
passivation layer thickness were investigated by using various thicknesses (0, 20, 50 and 120
nanometers) on bare epilayer AlGaN\GaN structures which were then measured before and
immediately after 1.0 MeV electron irradiation at fluences from 5 x 1015 cm-2 to 1016 cm-2. The
irradiation was applied in order to increase the electron trapping at the interface, thus providing
an enhanced interface quality. It has been shown previously that this irradiation produces point
defects and creates acceptors [1,2]. Hall measurements were used pre- and post-irradiation to
observe changes in carrier concentration and mobility as a function of fluence, energy, and total
dose. Hall carrier density data indicates the surface states are donors. Most importantly, these
measurements indicate preservation of mobility and conductivity within the optimal range of
Si3N4 thickness. This optimal range was found to be 50 to 120 nm.

Silicon nitride passivation on AlGaN\GaN heterojunction devices can improve performance by reduci... more Silicon nitride passivation on AlGaN\GaN heterojunction devices can improve performance by reducing electron traps at the surface. In this study, the effects of passivation layer thickness was investigated at various thicknesses (0, 20, 50 and 120 nm) on bare epilayer AlGaN\GaN structures and then measured before and immediately after 1.0 MeV electron irradiation at fluences of 1016 cm-2 . The irradiation was applied in order to increase the electron trapping at the interface, thus providing an enhanced measurement of the interface quality .Additionally, pre- and post-irradiation photoluminescence spectroscopy was used to reveal near-band-edge shallow electron donors, neutral donor bound excitons (D0 XA ) as well as deep center yellow/blue bands as the interface . Hall system measurements were used pre- and post-irradiation to observe changes in carrier concentration and mobility as a function of fluence, energy, and total dose. These measurements indicated preservation of mobility as a function of Si3N4(SiN) thickness

Drafts by Helen Jackson

The Effect of Electron Irradiation on the 2DEG Triangular Quantum Well shape and location in SiNx... more The Effect of Electron Irradiation on the 2DEG Triangular Quantum Well shape and location in SiNx/AlGaN/GaN High Electron Mobility Transistors
H. Jackson, Department of Physics, Wright State University, Dayton, Ohio 45431 USA
Abstract
Radiation changes the channel carrier density and increases the scattering in High Electron Mobility Transistors (HEMT’s). The channel in SiNx/AlGaN/GaN is described as a two-dimensional electron gas (2DEG) and is where the carriers exist. Structural parameters affect the 2DEG, and hence all other device metrics. An AlN inter-layer reduces the alloy scattering by acting as a barrier between the 2DEG wave function and the AlGaN. This can be shown in 2DEG wave function models. The AlN inter-layer adds to polarization, which then raises energy bands up relative to the Fermi level.
The interface roughness scattering potential amplitude V0 in the quantum well is approximately determined by assuming that local fluctuations of the interface position and of the roughness amplitude shrinks with well width. As the 2DEG waveform moves closer to the barrier (due possibly to things like increases in carrier density), there is scattering from interface roughness. Structures wherein the higher waveform sub-bands fill quicker, like those with a GaN cap, will have less interface roughness scattering and thus higher mobility because the higher sub-bands are further removed from the AlGaN interface.

A novel Photoluminescence (PL) model for the effect of 1 MeV electron irradiation on AlGaN/GaN wi... more A novel Photoluminescence (PL) model for the effect of 1 MeV electron irradiation on AlGaN/GaN with a variation in Si3N4 passivation layer thickness has been developed by examining the transitions and changes in defect populations and energy levels. The PL results of SixN/AlGaN/GaN gives information on the spatial localization of impurities such as oxygen and silicon, which are precursors to the D 0 X centers, deeper impurities like magnesium, and Vn and VGa donor information. The Si3N4 passivation layer shows a mono-atomic variation with PL intensity prior to 1MeV radiation. The post radiation Pl gives a 50nm peak and a 20 nm minimum in the deep center range, but then reverses to mono-atomic variation in the near band edge range. There is a shift in the main D 0 X center in the rear band edge due to mismatch lattice constants which results from tensile strain. The model that follows seeks to explain the observed changes in the PL due to 1 MeV electron irradiation. The changes with Si3N4 thickness pre-irradiation are explained as due to the attenuation of the PL laser beam going through the material, and corresponded linearly to the thickness of the Si3N4 pre-irradiation, but not post irradiation.

Talks by Helen Jackson

Powerpoint Summary of Radiation Effects/Detection Research

SPIE Proceedings, 2007

In the past, various virtual Frisch-grid designs have been proposed for cadmium zinc telluride (C... more In the past, various virtual Frisch-grid designs have been proposed for cadmium zinc telluride (CZT) and other compound semiconductor detectors. These include three-terminal, semi-spherical, CAPture, Frisch-ring, capacitive Frisch-grid and pixel devices (along with their modifications). Among them, the Frisch-grid design employing a non-contacting ring extended over the entire side surfaces of parallelepiped-shaped CZT crystals is the most promising. The defect-free parallelepiped-shaped crystals
with typical dimensions of 5x5~12m m3 are easy to produce and can be arranged into large arrays used for imaging and gamma-ray spectroscopy. In this paper, we report on further advances of the virtual Frischgrid detector design for the parallelepiped-shaped CZT crystals. Both the experimental testing and modeling results are described.
Keywords: CdZnTe, gamma ray detectors, Frisch-grid

Hard X-Ray and Gamma-Ray Detector Physics and Penetrating Radiation Systems VIII, 2006

Te inclusions existing at high concentrations in CdZnTe (CZT) material can degrade the performan... more Te inclusions existing at high concentrations in
CdZnTe (CZT) material can degrade the performance of CZT
detectors. These microscopic defects trap the free electrons generated
by incident radiation, so entailing significant fluctuations
in the total collected charge and thereby strongly affecting the
energy resolution of thick (long-drift) detectors. Such effects
were demonstrated in thin planar detectors, and, in many cases,
they proved to be the dominant cause of the low performance
of thick detectors, wherein the fluctuations in the charge losses
accumulate along the charge’s drift path. We continued studying
this effect using different tools and techniques. We employed a
dedicated beam-line recently established at BNL’s National Synchrotron
Light Source for characterizing semiconductor radiation
detectors, along with an IR transmission microscope system, the
combination of which allowed us to correlate the concentration
of defects with the devices’ performances. We present here our
new results from testing over 50 CZT samples grown by different
techniques. Our goals are to establish tolerable limits on the size
and concentrations of these detrimental Te inclusions in CZT
material, and to provide feedback to crystal growers to reduce
their numbers in the material.

IEEE Transactions on Nuclear Science, 2008

With the choice of an optimum thickness range, silicon nitride passivation (Si3N4) on AlGaN\GaN h... more With the choice of an optimum thickness range, silicon nitride passivation (Si3N4) on
AlGaN\GaN heterojunction devices can not only improve device performance but also radiation
harden the device while preserving the reliability and performance. In this study, the effects of
passivation layer thickness were investigated by using various thicknesses (0, 20, 50 and 120
nanometers) on bare epilayer AlGaN\GaN structures which were then measured before and
immediately after 1.0 MeV electron irradiation at fluences from 5 x 1015 cm-2 to 1016 cm-2. The
irradiation was applied in order to increase the electron trapping at the interface, thus providing
an enhanced interface quality. It has been shown previously that this irradiation produces point
defects and creates acceptors [1,2]. Hall measurements were used pre- and post-irradiation to
observe changes in carrier concentration and mobility as a function of fluence, energy, and total
dose. Hall carrier density data indicates the surface states are donors. Most importantly, these
measurements indicate preservation of mobility and conductivity within the optimal range of
Si3N4 thickness. This optimal range was found to be 50 to 120 nm.

Silicon nitride passivation on AlGaN\GaN heterojunction devices can improve performance by reduci... more Silicon nitride passivation on AlGaN\GaN heterojunction devices can improve performance by reducing electron traps at the surface. In this study, the effects of passivation layer thickness was investigated at various thicknesses (0, 20, 50 and 120 nm) on bare epilayer AlGaN\GaN structures and then measured before and immediately after 1.0 MeV electron irradiation at fluences of 1016 cm-2 . The irradiation was applied in order to increase the electron trapping at the interface, thus providing an enhanced measurement of the interface quality .Additionally, pre- and post-irradiation photoluminescence spectroscopy was used to reveal near-band-edge shallow electron donors, neutral donor bound excitons (D0 XA ) as well as deep center yellow/blue bands as the interface . Hall system measurements were used pre- and post-irradiation to observe changes in carrier concentration and mobility as a function of fluence, energy, and total dose. These measurements indicated preservation of mobility as a function of Si3N4(SiN) thickness

The Effect of Electron Irradiation on the 2DEG Triangular Quantum Well shape and location in SiNx... more The Effect of Electron Irradiation on the 2DEG Triangular Quantum Well shape and location in SiNx/AlGaN/GaN High Electron Mobility Transistors
H. Jackson, Department of Physics, Wright State University, Dayton, Ohio 45431 USA
Abstract
Radiation changes the channel carrier density and increases the scattering in High Electron Mobility Transistors (HEMT’s). The channel in SiNx/AlGaN/GaN is described as a two-dimensional electron gas (2DEG) and is where the carriers exist. Structural parameters affect the 2DEG, and hence all other device metrics. An AlN inter-layer reduces the alloy scattering by acting as a barrier between the 2DEG wave function and the AlGaN. This can be shown in 2DEG wave function models. The AlN inter-layer adds to polarization, which then raises energy bands up relative to the Fermi level.
The interface roughness scattering potential amplitude V0 in the quantum well is approximately determined by assuming that local fluctuations of the interface position and of the roughness amplitude shrinks with well width. As the 2DEG waveform moves closer to the barrier (due possibly to things like increases in carrier density), there is scattering from interface roughness. Structures wherein the higher waveform sub-bands fill quicker, like those with a GaN cap, will have less interface roughness scattering and thus higher mobility because the higher sub-bands are further removed from the AlGaN interface.

A novel Photoluminescence (PL) model for the effect of 1 MeV electron irradiation on AlGaN/GaN wi... more A novel Photoluminescence (PL) model for the effect of 1 MeV electron irradiation on AlGaN/GaN with a variation in Si3N4 passivation layer thickness has been developed by examining the transitions and changes in defect populations and energy levels. The PL results of SixN/AlGaN/GaN gives information on the spatial localization of impurities such as oxygen and silicon, which are precursors to the D 0 X centers, deeper impurities like magnesium, and Vn and VGa donor information. The Si3N4 passivation layer shows a mono-atomic variation with PL intensity prior to 1MeV radiation. The post radiation Pl gives a 50nm peak and a 20 nm minimum in the deep center range, but then reverses to mono-atomic variation in the near band edge range. There is a shift in the main D 0 X center in the rear band edge due to mismatch lattice constants which results from tensile strain. The model that follows seeks to explain the observed changes in the PL due to 1 MeV electron irradiation. The changes with Si3N4 thickness pre-irradiation are explained as due to the attenuation of the PL laser beam going through the material, and corresponded linearly to the thickness of the Si3N4 pre-irradiation, but not post irradiation.

Powerpoint Summary of Radiation Effects/Detection Research