Pump-probe spectroscopy on cubic III-nitride structures

Whale, Joshua J. (2019) Pump-probe spectroscopy on cubic III-nitride structures. PhD thesis, University of Nottingham.

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This thesis presents a series of studies applying the picosecond acoustics technique to investigate the ultrafast phonon dynamics in cubic III-nitride epilayers grown by molecular beam epitaxy on GaAs (001) substrates. The aim of these studies was twofold, firstly for the investigation and characterisation of the photo-elastic properties of zinc-blende (cubic) nitride materials. And secondly for the study of high frequency coherent phonons in sub-film-thick cubic AlGaN structures with the general aim of modulating light in the UV spectrum at up to terahertz frequencies.

A series of c-Al(x)Ga(1-x)N layers were produced with aluminium fractions (x) ranging from 0 - 0.66. So called Brillouin oscillations were studied in three of the samples around the 50% Al fraction by optically pumping the GaAs substrate through the transparent epilayer and tracking the strain pulse as it propagates toward the surface. We obtain the values for complex refractive index and their dependence on Al content and the optical wavelength in the range 250-300nm. Spectroscopic ellipsometry measurements in the same energy range are taken for comparative analysis. Analysis of the surface reflection of phonons matching the Brillouin backscattering frequency are used to estimate the surface roughness of the samples and are in close agreement with measurements of the surface topology by atomic force microscopy.

The phononic properties of these epilayers were further investigated in a free-standing membrane geometry by etching away the substrate. In the membranes the optical absorption is weak so strain is generated approximately uniformly in the layer. This leads to a periodic modulation of the layer thickness with frequency related to the round trip time of acoustic phonons in the layer. The optical reflectivity response did not rely on the photoelastic effect but predominantly the optical cavity thickness modulation, so confined membrane modes were observed in the full series of samples. From this data the longitudinal speed of sound in material released from the interfacial stress is defined.

In the second part of this work single cubic GaN quantum well (QW) with c-Al(x)Ga(1-x)N barrier cladding layers are utilised for the generation and detection of narrow strain pulses in buried layers. The QW did not function well as a detector at the probe wavelengths applied because the photo-elastic response could not be disentangled from the barrier layers. Acoustic pulses generated in the QW are observed in the GaAs substrate after propagating some distance and are independently resolved with a time resolution ~3.2ps. Theoretical calculations of the excited carrier distribution in the well indicate the efficient generation of frequencies up to 10 THz dependent on the abrupt electronic junction between the well and barrier layers.

Terahertz coherent folded longitudinal acoustic (FLA) phonon modes in a zincblende GaN/AlN superlattice (SL) are observed and studied by two colour time-resolved pump-probe spectroscopy. We observe the characteristic triplet of first-order mini-Brillouin zone folded modes at the zone center (q = 0), with frequency 1.2 THz, and with sidebands at phonon vector q = 2klaser where the detection mechanism is strongest. The observed frequencies show good agreement with the theoretical dispersion curves in the elastic limit. Picosecond acoustics is demonstrated as a diagnostic tool for measuring the period of SLs.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Kent, Anthony
Keywords: phonon phonons gallium aluminium nitride pump probe spectroscopy acoustic superlattice quantum well
Subjects: Q Science > QC Physics > QC170 Atomic physics. Constitution and properties of matter
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 56521
Depositing User: Whale, Josh
Date Deposited: 25 Jul 2019 08:12
Last Modified: 07 May 2020 11:17
URI: http://eprints.nottingham.ac.uk/id/eprint/56521

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