Theses and Dissertations
This collection includes both ASU Theses and Dissertations, submitted by graduate students, and the Barrett, Honors College theses submitted by undergraduate students.
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- Creators: Zhang, Yong-Hang
Description
Sb-based type-II superlattices (T2SLs) are potential alternative to HgCdTe for infrared detection due to their low manufacturing cost, good uniformity, high structural stability, and suppressed Auger recombination. The emerging InAs/InAsSb T2SLs have minority carrier lifetimes 1-2 orders of magnitude longer than those of the well-studied InAs/InGaSb T2SLs, and therefore have the potential to achieve photodetectors with higher performance. This work develops a novel method to measure the minority carrier lifetimes in infrared materials, and reports a comprehensive characterization of minority carrier lifetime and transport in InAs/InAsSb T2SLs at temperatures below 77 K.
A real-time baseline correction (RBC) method for minority carrier lifetime measurement is developed by upgrading a conventional boxcar-based time-resolved photoluminescence (TRPL) experimental system that suffers from low signal-to-noise ratio due to strong low frequency noise. The key is to modify the impulse response of the conventional TRPL system, and therefore the system becomes less sensitive to the dominant noise. Using this RBC method, the signal-to-noise ratio is improved by 2 orders of magnitude.
A record long minority carrier lifetime of 12.8 μs is observed in a high-quality mid-wavelength infrared InAs/InAsSb T2SLs at 15 K. It is further discovered that this long lifetime is partially due to strong carrier localization, which is revealed by temperature-dependent photoluminescence (PL) and TRPL measurements for InAs/InAsSb T2SLs with different period thicknesses. Moreover, the PL and TRPL results suggest that the atomic layer thickness variation is the main origin of carrier localization, which is further confirmed by a calculation using transfer matrix method.
To study the impact of the carrier localization on the device performance of InAs/InAsSb photodetectors, minority hole diffusion lengths are determined by the simulation of external quantum efficiency (EQE). A comparative study shows that carrier localization has negligible effect on the minority hole diffusion length in InAs/InAsSb T2SLs, and the long minority carrier lifetimes enhanced by carrier localization is not beneficial for photodetector operation.
A real-time baseline correction (RBC) method for minority carrier lifetime measurement is developed by upgrading a conventional boxcar-based time-resolved photoluminescence (TRPL) experimental system that suffers from low signal-to-noise ratio due to strong low frequency noise. The key is to modify the impulse response of the conventional TRPL system, and therefore the system becomes less sensitive to the dominant noise. Using this RBC method, the signal-to-noise ratio is improved by 2 orders of magnitude.
A record long minority carrier lifetime of 12.8 μs is observed in a high-quality mid-wavelength infrared InAs/InAsSb T2SLs at 15 K. It is further discovered that this long lifetime is partially due to strong carrier localization, which is revealed by temperature-dependent photoluminescence (PL) and TRPL measurements for InAs/InAsSb T2SLs with different period thicknesses. Moreover, the PL and TRPL results suggest that the atomic layer thickness variation is the main origin of carrier localization, which is further confirmed by a calculation using transfer matrix method.
To study the impact of the carrier localization on the device performance of InAs/InAsSb photodetectors, minority hole diffusion lengths are determined by the simulation of external quantum efficiency (EQE). A comparative study shows that carrier localization has negligible effect on the minority hole diffusion length in InAs/InAsSb T2SLs, and the long minority carrier lifetimes enhanced by carrier localization is not beneficial for photodetector operation.
ContributorsLin, Zhiyuan (Author) / Zhang, Yong-Hang (Thesis advisor) / Vasileska, Dragica (Committee member) / Johnson, Shane (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2016
Description
This work demonstrates novel nBn photodetectors including mid-wave infrared (MWIR) nBn photodetectors based on InAs/InAsSb type-II superlattices (T2SLs) with charge as the output signal, and visible nBn photodetectors based on CdTe with current output. Furthermore, visible/MWIR two-color photodetectors (2CPDs) are fabricated through monolithic integration of the CdTe nBn photodetector and an InSb photodiode.
The MWIR nBn photodetectors have a potential well for holes present in the barrier layer. At low voltages of < −0.2 V, which ensure low dark current <10-5 A/cm2 at 77 K, photogenerated holes are collected in this well with a storage lifetime of 40 s. This charge collection process is an in-device signal integration process that reduces the random noise significantly. Since the stored holes can be readout laterally as in charge-coupled devices, it is therefore possible to make charge-output nBn with much lower noise than conventional current-output nBn photodetectors.
The visible nBn photodetectors have a CdTe absorber layer and a ZnTe barrier layer with an aligned valence band edge. By using a novel ITO/undoped-CdTe top contact design, it has achieved a high specific detectivity of 3×1013 cm-Hz1/2/W at room temperature. Particularly, this CdTe nBn photodetector grown on InSb substrates enables the monolithic integration of CdTe and InSb photodetectors, and provides a platform to study in-depth device physics of nBn photodetectors at room temperature.
Furthermore, the visible/MWIR 2CPD has been developed by the monolithic integration of the CdTe nBn and an InSb photodiode through an n-CdTe/p-InSb tunnel junction. At 77 K, the photoresponse of the 2CPD can be switched between a 1-5.5 μm MWIR band and a 350-780 nm visible band by illuminating the device with an external light source or not, and applying with proper voltages. Under optimum conditions, the 2CPD has achieved a MWIR peak responsivity of 0.75 A/W with a band rejection ratio (BRR) of 52 dB, and a visible peak responsivity of 0.3 A/W with a BRR of 18 dB. This 2CPD has enabled future compact image sensors with high fill-factor and responsivity switchable between visible and MWIR colors.
The MWIR nBn photodetectors have a potential well for holes present in the barrier layer. At low voltages of < −0.2 V, which ensure low dark current <10-5 A/cm2 at 77 K, photogenerated holes are collected in this well with a storage lifetime of 40 s. This charge collection process is an in-device signal integration process that reduces the random noise significantly. Since the stored holes can be readout laterally as in charge-coupled devices, it is therefore possible to make charge-output nBn with much lower noise than conventional current-output nBn photodetectors.
The visible nBn photodetectors have a CdTe absorber layer and a ZnTe barrier layer with an aligned valence band edge. By using a novel ITO/undoped-CdTe top contact design, it has achieved a high specific detectivity of 3×1013 cm-Hz1/2/W at room temperature. Particularly, this CdTe nBn photodetector grown on InSb substrates enables the monolithic integration of CdTe and InSb photodetectors, and provides a platform to study in-depth device physics of nBn photodetectors at room temperature.
Furthermore, the visible/MWIR 2CPD has been developed by the monolithic integration of the CdTe nBn and an InSb photodiode through an n-CdTe/p-InSb tunnel junction. At 77 K, the photoresponse of the 2CPD can be switched between a 1-5.5 μm MWIR band and a 350-780 nm visible band by illuminating the device with an external light source or not, and applying with proper voltages. Under optimum conditions, the 2CPD has achieved a MWIR peak responsivity of 0.75 A/W with a band rejection ratio (BRR) of 52 dB, and a visible peak responsivity of 0.3 A/W with a BRR of 18 dB. This 2CPD has enabled future compact image sensors with high fill-factor and responsivity switchable between visible and MWIR colors.
ContributorsHe, Zhaoyu (Author) / Zhang, Yong-Hang (Thesis advisor) / Vasileska, Dragica (Committee member) / Goryll, Michael (Committee member) / Johnson, Shane (Committee member) / Arizona State University (Publisher)
Created2016
Description
InAs/InAsSb type-II superlattices (T2SLs) can be considered as potential alternatives for conventional HgCdTe photodetectors due to improved uniformity, lower manufacturing costs with larger substrates, and possibly better device performance. This dissertation presents a comprehensive study on the structural, optical and electrical properties of InAs/InAsSb T2SLs grown by Molecular Beam Epitaxy.
The effects of different growth conditions on the structural quality were thoroughly investigated. Lattice-matched condition was successfully achieved and material of exceptional quality was demonstrated.
After growth optimization had been achieved, structural defects could hardly be detected, so different characterization techniques, including etch-pit-density (EPD) measurements, cathodoluminescence (CL) imaging and X-ray topography (XRT), were explored, in attempting to gain better knowledge of the sparsely distributed defects. EPD revealed the distribution of dislocation-associated pits across the wafer. Unfortunately, the lack of contrast in images obtained by CL imaging and XRT indicated their inability to provide any quantitative information about defect density in these InAs/InAsSb T2SLs.
The nBn photodetectors based on mid-wave infrared (MWIR) and long-wave infrared (LWIR) InAs/InAsSb T2SLs were fabricated. The significant difference in Ga composition in the barrier layer coupled with different dark current behavior, suggested the possibility of different types of band alignment between the barrier layers and the absorbers. A positive charge density of 1.8 × 1017/cm3 in the barrier of MWIR nBn photodetector, as determined by electron holography, confirmed the presence of a potential well in its valence band, thus identifying type-II alignment. In contrast, the LWIR nBn photodetector was shown to have type-I alignment because no sign of positive charge was detected in its barrier.
Capacitance-voltage measurements were performed to investigate the temperature dependence of carrier densities in a metal-oxide-semiconductor (MOS) structure based on MWIR InAs/InAsSb T2SLs, and a nBn structure based on LWIR InAs/InAsSb T2SLs. No carrier freeze-out was observed in either sample, indicating very shallow donor levels. The decrease in carrier density when temperature increased was attributed to the increased density of holes that had been thermally excited from localized states near the oxide/semiconductor interface in the MOS sample. No deep-level traps were revealed in deep-level transient spectroscopy temperature scans.
The effects of different growth conditions on the structural quality were thoroughly investigated. Lattice-matched condition was successfully achieved and material of exceptional quality was demonstrated.
After growth optimization had been achieved, structural defects could hardly be detected, so different characterization techniques, including etch-pit-density (EPD) measurements, cathodoluminescence (CL) imaging and X-ray topography (XRT), were explored, in attempting to gain better knowledge of the sparsely distributed defects. EPD revealed the distribution of dislocation-associated pits across the wafer. Unfortunately, the lack of contrast in images obtained by CL imaging and XRT indicated their inability to provide any quantitative information about defect density in these InAs/InAsSb T2SLs.
The nBn photodetectors based on mid-wave infrared (MWIR) and long-wave infrared (LWIR) InAs/InAsSb T2SLs were fabricated. The significant difference in Ga composition in the barrier layer coupled with different dark current behavior, suggested the possibility of different types of band alignment between the barrier layers and the absorbers. A positive charge density of 1.8 × 1017/cm3 in the barrier of MWIR nBn photodetector, as determined by electron holography, confirmed the presence of a potential well in its valence band, thus identifying type-II alignment. In contrast, the LWIR nBn photodetector was shown to have type-I alignment because no sign of positive charge was detected in its barrier.
Capacitance-voltage measurements were performed to investigate the temperature dependence of carrier densities in a metal-oxide-semiconductor (MOS) structure based on MWIR InAs/InAsSb T2SLs, and a nBn structure based on LWIR InAs/InAsSb T2SLs. No carrier freeze-out was observed in either sample, indicating very shallow donor levels. The decrease in carrier density when temperature increased was attributed to the increased density of holes that had been thermally excited from localized states near the oxide/semiconductor interface in the MOS sample. No deep-level traps were revealed in deep-level transient spectroscopy temperature scans.
ContributorsShen, Xiaomeng (Author) / Zhang, Yong-Hang (Thesis advisor) / Smith, David J. (Thesis advisor) / Alford, Terry (Committee member) / Goryll, Michael (Committee member) / Mccartney, Martha R (Committee member) / Arizona State University (Publisher)
Created2015
Description
The molecular beam epitaxy growth of the III-V semiconductor alloy indium arsenide antimonide bismide (InAsSbBi) is investigated over a range of growth temperatures and V/III flux ratios. Bulk and quantum well structures grown on gallium antimonide (GaSb) substrates are examined. The relationships between Bi incorporation, surface morphology, growth temperature, and group-V flux are explored. A growth model is developed based on the kinetics of atomic desorption, incorporation, surface accumulation, and droplet formation. The model is applied to InAsSbBi, where the various process are fit to the Bi, Sb, and As mole fractions. The model predicts a Bi incorporation limit for lattice matched InAsSbBi grown on GaSb.The optical performance and bandgap energy of InAsSbBi is examined using photoluminescence spectroscopy. Emission is observed from low to room temperature with peaks ranging from 3.7 to 4.6 μm. The bandgap as function of temperature is determined from the first derivative maxima of the spectra fit to an Einstein single oscillator model. The photoluminescence spectra is observed to significantly broaden with Bi content as a result of lateral composition variations and the highly mismatched nature of Bi atoms, pairs, and clusters in the group-V sublattice.
A bowing model is developed for the bandgap and band offsets of the quinary alloy GaInAsSbBi and its quaternary constituents InAsSbBi and GaAsSbBi. The band anticrossing interaction due to the highly mismatched Bi atoms is incorporated into the relevant bowing terms. An algorithm is developed for the design of mid infrared GaInAsSbBi
quantum wells, with three degrees freedom to independently tune transition energy, in plane strain, and band edge offsets for desired electron and hole confinement.
The physical characteristics of the fundamental absorption edge of the relevant III-V binaries GaAs, GaSb, InAs, and InSb are examined using spectroscopic ellipsometry. A five parameter model is developed that describes the key physical characteristics of the absorption edge, including the bandgap energy, the Urbach tail, and the absorption coefficient at the bandgap.
The quantum efficiency and recombination lifetimes of bulk InAs0.911Sb0.089 grown by molecular beam epitaxy is investigated using excitation and temperature dependent steady state photoluminescence. The Shockley-Read-Hall, radiative, and Auger recombination lifetimes are determined.
A bowing model is developed for the bandgap and band offsets of the quinary alloy GaInAsSbBi and its quaternary constituents InAsSbBi and GaAsSbBi. The band anticrossing interaction due to the highly mismatched Bi atoms is incorporated into the relevant bowing terms. An algorithm is developed for the design of mid infrared GaInAsSbBi
quantum wells, with three degrees freedom to independently tune transition energy, in plane strain, and band edge offsets for desired electron and hole confinement.
The physical characteristics of the fundamental absorption edge of the relevant III-V binaries GaAs, GaSb, InAs, and InSb are examined using spectroscopic ellipsometry. A five parameter model is developed that describes the key physical characteristics of the absorption edge, including the bandgap energy, the Urbach tail, and the absorption coefficient at the bandgap.
The quantum efficiency and recombination lifetimes of bulk InAs0.911Sb0.089 grown by molecular beam epitaxy is investigated using excitation and temperature dependent steady state photoluminescence. The Shockley-Read-Hall, radiative, and Auger recombination lifetimes are determined.
ContributorsSchaefer, Stephen Thomas (Author) / Johnson, Shane R (Thesis advisor) / Zhang, Yong-Hang (Committee member) / Goryll, Michael (Committee member) / King, Richard (Committee member) / Arizona State University (Publisher)
Created2020