seyoum wolde - Academia.edu (original) (raw)

Papers by seyoum wolde

Infrared Physics & Technology, Sep 1, 2016

We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectr... more We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectra of GaAs/AlGaAs heterojunction infrared photodetectors, where a highly doped GaAs emitter is sandwiched between two AlGaAs barriers. The noise and gain mechanisms associated with the carrier transport are investigated, and it is shown that a lower noise spectral density is observed for a device with a flat barrier, and thicker emitter. Despite the lower noise power spectral density of flat barrier device, comparison of the dark and photocurrent noise gain between flat and graded barrier samples confirmed that the escape probability of carriers (or detectivity) is enhanced by grading the barrier. The grading suppresses recombination owing to the higher momentum of carriers in the barrier. Optimizing the emitter thickness of the graded barrier to enhance the absorption efficiency, and increase the escape probability and lower the dark current, enhances the specific detectivity of devices.

Applied Physics Letters, Apr 28, 2014

Applied Physics Letters, Dec 9, 2013

Applied Physics Letters, Oct 13, 2014

Journal of Applied Physics, Jun 27, 2017

We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDI... more We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDIP) and Quantum Dot-in-a-well (DWELL) vary with the electric field and temperature. At lower temperatures (below ~ 100 K), the noise current of both types of detectors are dominated by generationrecombination (G-R) noise which is consistent with a mechanism of fluctuations driven by the electric field and by thermal noise. The noise gain, capture probability and carrier life time for bound-tocontinuum or quasi-bound transitions in DWELL and QDIP structures are discussed. The capture probability of DWELL is found to be more than two times higher than the corresponding QDIP. Based on the analysis, structural parameters such as the numbers of active layers, the surface density of QDs, and the carrier capture or relaxation rate, type of material and electric field are some of the optimization parameters identified to improve the gain of devices.

Infrared Physics & Technology, 2016

We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectr... more We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectra of GaAs/AlGaAs heterojunction infrared photodetectors, where a highly doped GaAs emitter is sandwiched between two AlGaAs barriers. The noise and gain mechanisms associated with the carrier transport are investigated, and it is shown that a lower noise spectral density is observed for a device with a flat barrier, and thicker emitter. Despite the lower noise power spectral density of flat barrier device, comparison of the dark and photocurrent noise gain between flat and graded barrier samples confirmed that the escape probability of carriers (or detectivity) is enhanced by grading the barrier. The grading suppresses recombination owing to the higher momentum of carriers in the barrier. Optimizing the emitter thickness of the graded barrier to enhance the absorption efficiency, and increase the escape probability and lower the dark current, enhances the specific detectivity of devices.

Infrared Physics & Technology, 2019

In this work, the activation energy obtained from the temperature dependent internal photoemissio... more In this work, the activation energy obtained from the temperature dependent internal photoemission spectroscopy (TDIPS) and thermionic dark currents using GaAs/AlGaAs photodetectors are compared. Different barrier heights within the p-type GaAs/AlGaAs heterostructures are studied. The temperature dependent spectral response shows the red-shifting of the detector threshold wavelength for increasing temperature due to the decreasing band-offset. The activation energy extracted from Arrhenius plot of the dark current-voltage-temperature (I-V-T), and measured spectral response show the carrier activation energy increases with increasing Al mole fraction and decreases with increasing doping density. For Infrared detectors with ≤ 6.5 m, the Arrhenius analysis yields the values of activation energy with less than 5 % deviation from the actual or TDIPS fitting values. However, for detectors with longer threshold wavelengths (>> λ.3 m), activation energy extracted from the Arrhenius plot leads to energy values which deviate more than ~ 10 % from the corresponding TDIPS values. The higher percentage deviation (>> 10 %) of activation energy determined by Arrhenius plot from the corresponding TDIPS values attribute to the temperature dependent Fermi distribution tailing effect and Fowler-Nordheim tunneling current.

Journal of Applied Physics, 2017

We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDI... more We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDIP) and Quantum Dot-in-a-well (DWELL) varies with the electric field and temperature. At lower temperatures (below ∼100 K), the noise current of both types of detectors is dominated by generation-recombination (G-R) noise which is consistent with a mechanism of fluctuations driven by the electric field and thermal noise. The noise gain, capture probability, and carrier life time for bound-to-continuum or quasi-bound transitions in DWELL and QDIP structures are discussed. The capture probability of DWELL is found to be more than two times higher than the corresponding QDIP. Based on the analysis, structural parameters such as the numbers of active layers, the surface density of QDs, and the carrier capture or relaxation rate, type of material, and electric field are some of the optimization parameters identified to improve the gain of devices.

Infrared Physics & Technology, 2015

h i g h l i g h t s We demonstrate high quantum efficiency (17%) InAs/GaAs p-type quantum dot inf... more h i g h l i g h t s We demonstrate high quantum efficiency (17%) InAs/GaAs p-type quantum dot infrared photodetector. The p-type hole response displays a well-preserved spectral profile, independent of the applied bias. The p-type quantum dot photodetector exhibits a strong far-infrared response up to 70 lm.

Applied Physics Letters, 2014

Terahertz (THz) response observed in a p-type InAs/In0.15Ga0.85As/GaAs quantum dots-in-a-well (DW... more Terahertz (THz) response observed in a p-type InAs/In0.15Ga0.85As/GaAs quantum dots-in-a-well (DWELL) photodetector is reported. This detector displays expected mid-infrared response (from ∼3 to ∼10 μm) at temperatures below ∼100 K, while strong THz responses up to ∼4.28 THz is observed at higher temperatures (∼100–130 K). Responsivity and specific detectivity at 9.2 THz (32.6 μm) under applied bias of −0.4 V at 130 K are ∼0.3 mA/W and ∼1.4 × 106 Jones, respectively. Our results demonstrate the potential use of p-type DWELL in developing high operating temperature THz devices.

Applied Physics Letters, 2012

An InAs/GaAs quantum dot infrared photodetector with strong, multicolor, broadband (5–20 μm) phot... more An InAs/GaAs quantum dot infrared photodetector with strong, multicolor, broadband (5–20 μm) photoresponse is reported. Using a combined quaternary In0.21Al0.21Ga0.58As and GaAs capping that relieves strain and maintains strong carrier confinement, we demonstrate a four color infrared response with peaks in the midwave- (5.7 μm), longwave- (9.0 and 14.5 μm), and far- (17 μm) infrared regions. Narrow spectral widths (7% to 9%) are noted at each of these wavelengths including responsivity value ∼95.3 mA/W at 14.5 μm. Using strain field and multi-band k⋅p theory, we map specific bound-to-bound and bound-to-quasibound transitions to the longwave and midwave responses, respectively.

Applied Physics Letters, 2013

An InAs/GaAs quantum dot infrared photodetector (QDIP) based on p-type valence-band intersublevel... more An InAs/GaAs quantum dot infrared photodetector (QDIP) based on p-type valence-band intersublevel hole transitions as opposed to conventional electron transitions is reported. Two response bands observed at 1.5–3 and 3–10 μm are due to transitions from the heavy-hole to spin-orbit split-off QD level and from the heavy-hole to heavy-hole level, respectively. Without employing optimized structures (e.g., the dark current blocking layer), the demonstrated QDIP displays promising characteristics, including a specific detectivity of 1.8×109 cm·Hz1/2/W and a quantum efficiency of 17%, which is about 5% higher than that of present n-type QDIPs. This study shows the promise of utilizing hole transitions for developing QDIPs.

Compendium of continuing education in dentistry (Jamesburg, N.J. : 1995), 2003

A s is often the case, the first sign of trouble was a front-page article in a respected newspape... more A s is often the case, the first sign of trouble was a front-page article in a respected newspaper. On November 22, 2002, The New York Times published an article entitled, "Madison Ave. Plays Growing Role in Drug Research." 1 The reporter, Melody Peterson, detailed how the three largest advertising companies in the United States "spent tens of millions of dollars to buy or invest in companies. .. that perform clinical trials of experimental drugs.. . ."

Infrared Physics & Technology, Sep 1, 2016

We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectr... more We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectra of GaAs/AlGaAs heterojunction infrared photodetectors, where a highly doped GaAs emitter is sandwiched between two AlGaAs barriers. The noise and gain mechanisms associated with the carrier transport are investigated, and it is shown that a lower noise spectral density is observed for a device with a flat barrier, and thicker emitter. Despite the lower noise power spectral density of flat barrier device, comparison of the dark and photocurrent noise gain between flat and graded barrier samples confirmed that the escape probability of carriers (or detectivity) is enhanced by grading the barrier. The grading suppresses recombination owing to the higher momentum of carriers in the barrier. Optimizing the emitter thickness of the graded barrier to enhance the absorption efficiency, and increase the escape probability and lower the dark current, enhances the specific detectivity of devices.

Applied Physics Letters, Apr 28, 2014

Applied Physics Letters, Dec 9, 2013

Applied Physics Letters, Oct 13, 2014

Journal of Applied Physics, Jun 27, 2017

We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDI... more We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDIP) and Quantum Dot-in-a-well (DWELL) vary with the electric field and temperature. At lower temperatures (below ~ 100 K), the noise current of both types of detectors are dominated by generationrecombination (G-R) noise which is consistent with a mechanism of fluctuations driven by the electric field and by thermal noise. The noise gain, capture probability and carrier life time for bound-tocontinuum or quasi-bound transitions in DWELL and QDIP structures are discussed. The capture probability of DWELL is found to be more than two times higher than the corresponding QDIP. Based on the analysis, structural parameters such as the numbers of active layers, the surface density of QDs, and the carrier capture or relaxation rate, type of material and electric field are some of the optimization parameters identified to improve the gain of devices.

Infrared Physics & Technology, 2016

We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectr... more We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectra of GaAs/AlGaAs heterojunction infrared photodetectors, where a highly doped GaAs emitter is sandwiched between two AlGaAs barriers. The noise and gain mechanisms associated with the carrier transport are investigated, and it is shown that a lower noise spectral density is observed for a device with a flat barrier, and thicker emitter. Despite the lower noise power spectral density of flat barrier device, comparison of the dark and photocurrent noise gain between flat and graded barrier samples confirmed that the escape probability of carriers (or detectivity) is enhanced by grading the barrier. The grading suppresses recombination owing to the higher momentum of carriers in the barrier. Optimizing the emitter thickness of the graded barrier to enhance the absorption efficiency, and increase the escape probability and lower the dark current, enhances the specific detectivity of devices.

Infrared Physics & Technology, 2019

In this work, the activation energy obtained from the temperature dependent internal photoemissio... more In this work, the activation energy obtained from the temperature dependent internal photoemission spectroscopy (TDIPS) and thermionic dark currents using GaAs/AlGaAs photodetectors are compared. Different barrier heights within the p-type GaAs/AlGaAs heterostructures are studied. The temperature dependent spectral response shows the red-shifting of the detector threshold wavelength for increasing temperature due to the decreasing band-offset. The activation energy extracted from Arrhenius plot of the dark current-voltage-temperature (I-V-T), and measured spectral response show the carrier activation energy increases with increasing Al mole fraction and decreases with increasing doping density. For Infrared detectors with ≤ 6.5 m, the Arrhenius analysis yields the values of activation energy with less than 5 % deviation from the actual or TDIPS fitting values. However, for detectors with longer threshold wavelengths (>> λ.3 m), activation energy extracted from the Arrhenius plot leads to energy values which deviate more than ~ 10 % from the corresponding TDIPS values. The higher percentage deviation (>> 10 %) of activation energy determined by Arrhenius plot from the corresponding TDIPS values attribute to the temperature dependent Fermi distribution tailing effect and Fowler-Nordheim tunneling current.

Journal of Applied Physics, 2017

We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDI... more We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDIP) and Quantum Dot-in-a-well (DWELL) varies with the electric field and temperature. At lower temperatures (below ∼100 K), the noise current of both types of detectors is dominated by generation-recombination (G-R) noise which is consistent with a mechanism of fluctuations driven by the electric field and thermal noise. The noise gain, capture probability, and carrier life time for bound-to-continuum or quasi-bound transitions in DWELL and QDIP structures are discussed. The capture probability of DWELL is found to be more than two times higher than the corresponding QDIP. Based on the analysis, structural parameters such as the numbers of active layers, the surface density of QDs, and the carrier capture or relaxation rate, type of material, and electric field are some of the optimization parameters identified to improve the gain of devices.

Infrared Physics & Technology, 2015

h i g h l i g h t s We demonstrate high quantum efficiency (17%) InAs/GaAs p-type quantum dot inf... more h i g h l i g h t s We demonstrate high quantum efficiency (17%) InAs/GaAs p-type quantum dot infrared photodetector. The p-type hole response displays a well-preserved spectral profile, independent of the applied bias. The p-type quantum dot photodetector exhibits a strong far-infrared response up to 70 lm.

Applied Physics Letters, 2014

Terahertz (THz) response observed in a p-type InAs/In0.15Ga0.85As/GaAs quantum dots-in-a-well (DW... more Terahertz (THz) response observed in a p-type InAs/In0.15Ga0.85As/GaAs quantum dots-in-a-well (DWELL) photodetector is reported. This detector displays expected mid-infrared response (from ∼3 to ∼10 μm) at temperatures below ∼100 K, while strong THz responses up to ∼4.28 THz is observed at higher temperatures (∼100–130 K). Responsivity and specific detectivity at 9.2 THz (32.6 μm) under applied bias of −0.4 V at 130 K are ∼0.3 mA/W and ∼1.4 × 106 Jones, respectively. Our results demonstrate the potential use of p-type DWELL in developing high operating temperature THz devices.

Applied Physics Letters, 2012

An InAs/GaAs quantum dot infrared photodetector with strong, multicolor, broadband (5–20 μm) phot... more An InAs/GaAs quantum dot infrared photodetector with strong, multicolor, broadband (5–20 μm) photoresponse is reported. Using a combined quaternary In0.21Al0.21Ga0.58As and GaAs capping that relieves strain and maintains strong carrier confinement, we demonstrate a four color infrared response with peaks in the midwave- (5.7 μm), longwave- (9.0 and 14.5 μm), and far- (17 μm) infrared regions. Narrow spectral widths (7% to 9%) are noted at each of these wavelengths including responsivity value ∼95.3 mA/W at 14.5 μm. Using strain field and multi-band k⋅p theory, we map specific bound-to-bound and bound-to-quasibound transitions to the longwave and midwave responses, respectively.

Applied Physics Letters, 2013

An InAs/GaAs quantum dot infrared photodetector (QDIP) based on p-type valence-band intersublevel... more An InAs/GaAs quantum dot infrared photodetector (QDIP) based on p-type valence-band intersublevel hole transitions as opposed to conventional electron transitions is reported. Two response bands observed at 1.5–3 and 3–10 μm are due to transitions from the heavy-hole to spin-orbit split-off QD level and from the heavy-hole to heavy-hole level, respectively. Without employing optimized structures (e.g., the dark current blocking layer), the demonstrated QDIP displays promising characteristics, including a specific detectivity of 1.8×109 cm·Hz1/2/W and a quantum efficiency of 17%, which is about 5% higher than that of present n-type QDIPs. This study shows the promise of utilizing hole transitions for developing QDIPs.

Compendium of continuing education in dentistry (Jamesburg, N.J. : 1995), 2003

A s is often the case, the first sign of trouble was a front-page article in a respected newspape... more A s is often the case, the first sign of trouble was a front-page article in a respected newspaper. On November 22, 2002, The New York Times published an article entitled, "Madison Ave. Plays Growing Role in Drug Research." 1 The reporter, Melody Peterson, detailed how the three largest advertising companies in the United States "spent tens of millions of dollars to buy or invest in companies. .. that perform clinical trials of experimental drugs.. . ."