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- All Subjects: mobility
- Creators: Barton, C. M.
Description
The first part of this dissertation reports the study of the vertical carrier transport and device application in InAs/InAs1-xSbx strain-balanced type-II superlattice. It is known that the low hole mobility in the InAs/InAs1-xSbx superlattice is considered as the main reason for the low internal quantum efficiency of its mid-wave and long-wave infrared photodetectors, compared with that of its HgCdTe counterparts. Optical measurements using time-resolved photoluminescence and steady-state photoluminescence spectroscopy are implemented to extract the diffusion coefficients and mobilities of holes in the superlattices at various temperatures from 12 K to 210 K. The sample structure consists of a mid-wave infrared superlattice absorber region grown atop a long-wave infrared superlattice probe region. An ambipolar diffusion model is adopted to extract the hole mobility. The results show that the hole mobility first increases from 0.2 cm2/Vs at 12 K and then levels off at ~50 cm2/Vs as the temperature exceeds ~60 K. An InAs/InAs1-xSbx type-II superlattice nBn long-wavelength barrier infrared photodetector has also been demonstrated with a measured dark current density of 9.5×10-4 A/cm2 and a maximum resistance-area product of 563 Ω-cm2 at 77 K under a bias of -0.5 V. The Arrhenius plot of the dark current density reveals a possible high-operating-temperature of 110 K.The second part of the dissertation reports a lift-off technology using a water-soluble sacrificial MgTe layer grown on InSb. This technique enables the seamless integration of materials with lattice constants near 6.5 Å, such as InSb, CdTe, PbTe, HgTe and Sn. Coherently strained MgTe with a lattice constant close to 6.5 Å acts as a sacrificial layer which reacts with water and releases the film above it. Freestanding CdTe/MgxCd1-xTe double-heterostructures resulting from the lift-off process show increased photoluminescence intensity due to enhanced extraction efficiency and photon-recycling effect. The lifted-off thin films show smooth and flat surfaces with 6.7 Å root-mean-square roughness revealed by atomic-force microscopy profiles. The increased photoluminescence intensity also confirms that the CdTe/MgxCd1-xTe double-heterostructures maintain the high optical quality after epitaxial lift-off.
ContributorsTsai, Cheng-Ying (Author) / Zhang, Yong-Hang YZ (Thesis advisor) / Vasileska, Dragica DV (Committee member) / Johnson, Shane SJ (Committee member) / Zhao, Yuji YZ (Committee member) / Arizona State University (Publisher)
Created2020
Description
This study examines almost 5000 lithic artifacts from three rockshelters in the Belize, part of the Maya Lowlands. These sites bear evidence of human occupation throughout the last 14,000 years, a period over which I assess change in four lithic traits: 1) cortex, or the weathered outer surface of a toolstone cobble; 2) platform type, or the degree of preparation of striking platforms; 3) bifaciality, or whether tools were flaked on one or both faces; and 4) retouch, or the removal of small flakes from tool edges. These traits are differentially associated with two modes of technological organization: curation and expedience. Curation involves greater effort spent creating tools with longer use-lives and is associated with low levels of cortex, more complex platforms, more bifacial flaking, and higher amounts of retouch. It is also more typical of highly mobile hunter-gatherers who move their residential base often. Expedience, which entails less effort creating lithics with shorter use-lives, is associated with higher amounts of cortex, simpler platforms, less bifacial flaking, and lower levels of retouch; it is also associated with more sedentary hunter-gatherers. My results indicate that, during the Late Pleistocene in Belize, groups favored curation and were likely highly mobile foragers inhabiting an open grassland landscape. However, not long after the Pleistocene-ending Younger Dryas climatic event (~12,600 to 11,700 cal BP), these groups began to favor expedience, indicating they’d begun to adopt a more sedentary lifestyle. This shift towards expedience and sedentism continued throughout the Early and Middle Holocene, when closed-canopy tropical forests came to dominate the landscape. I conclude that these foragers began to adopt lower levels of residential mobility much earlier than generally thought. Because the sparse, unpredictable nature of wild tropical forest flora and fauna favors high residential mobility, this means they were likely manipulating their landscapes and experimenting with cultivars several millennia before the first appearance of sedentary farming villages. This further implies that the origins of sedentism and agriculture in tropical forests must be sought in the changing lifeways of pre-agricultural, semi-sedentary forager-horticulturalists of the deep past.
ContributorsDennehy, Timothy J. (Author) / Smith, Michael E. (Thesis advisor) / Barton, C. M. (Committee member) / Marean, Curtis W. (Committee member) / Prufer, Keith M. (Committee member) / Arizona State University (Publisher)
Created2021