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Photoluminescence Studies of Polytype Heterostructured InP Nanostructures

Författare

Summary, in English

The interface between two semiconductors significantly influences their optical and electronic properties. In contrast to traditional material heterostructures, polytype heterostructures between wurtzite (wz) and zincblende (zb) segments in homomaterial InP nanostructures exhibit sharp interfaces with minimal strain. The wz/zb interface of heterostructured InP has a type-II band alignment, which results in the accumulation of charge carriers on either side of the interface, electrons on one side and holes on the other side. This separation leads to the formation of spatially separated (indirect) excitons (IXs) at the interface. This thesis focuses on studying the spatial and temporal recombination characteristics of IXs in these polytypic heterostructures in InP nanostructures. Two types of nanostructures, nanowires (NWs) and platelets, are employed to investigate single type-II wz/zb interfaces. NWs have cylindrical or hexagonal cross-sections with a very small diameter and longer length, resulting in a small interface cross
section (< 1 mm2) while platelets on the other hand are characterized by much larger interface areas (> 500 mm2).

The methodology in this thesis utilizes non-invasive optical techniques, specifically steady-state (SS) and time-resolved (TR) photoluminescence (PL) to analyze the emission spectra of InP polytype heterostructures. The confirmation of a type-II band alignment is established through excitation power density (EPD)-dependent SSPL, where the IX emission redshifts with reduced EPD. In addition, the TRPL data reveals a long lived and non-exponential decay profile of the IX emission.

Paper-I delves into the recombination dynamics of IXs at the single wz/zb interface in InP NWs. Overcoming the small cross-section limitation of NWs, platelets, with a substantially larger cross-section, provide a larger sample area and, thus, provides access to investigating the transport of IXs. Paper-II and Paper-III explore the IX dynamics at the wz/zb single-interface of platelets. Spatially resolved SSPL in undoped platelets unveils EPD-dependent transport of IXs at the type-II interface. The broad spatial distribution of the IX emission suggests repulsive-force-driven transport at high EPD, while the narrow spatial distribution at low EPD indicates a diffusive transport mechanism. The temporal evolution, observed through spatially resolved TRPL, depicts an initial rapid expansion of the IXs driven by Coulomb repulsion - a result of the dipole alignment of the IXs at the interface. After this initial step, it is transitioning to a linear expansion indicative of diffusive-driven transport in undoped platelets. This behavior is characteristic for a low scattering of IXs which is indicative of a minor impact of interface fluctuations caused by the polytype interface in our system.

Beyond characterizing undoped nanostructures, our research explores the impact of a two-dimensional electron gas (2DEG) on the recombination and spatial dynamics of IXs for n-type doped nanostructures. These doped structures maintain the same geometry as the undoped counterparts, except for the n-type doping applied to the entire wz segment. Doped samples exhibit a smaller redshift with reduced EPD and shorter recombination times for IX emission compared to the undoped ones. Additionally, the spatial distribution of IX emission is independent of EPD.

Ämne

  • Nano Technology

Nyckelord

  • III-V semiconductor
  • Photoluminescence
  • time-resolved photoluminescence (TRPL)
  • polytype
  • crystal phase heterostructure
  • InP
  • wurtzite (WZ)
  • zincblende
  • Indirect Exciton

Status

Published

ISBN/ISSN/Övrigt

  • ISBN: 978-91-8039-967-8
  • ISBN: 978-91-8039-966-1

Försvarsdatum

22 mars 2024

Försvarstid

13:15

Försvarsplats

Lecture Hall Rydbergsalen, Department of Physics, Professorsgatan 1, Faculty of Engineering LTH, Lund University, Lund. The dissertation will be live streamed, but part of the premises is to be excluded from the live stream.

Opponent

  • Sebastian Lourdudoss (Prof.)