Study of Compositional Intermixing in GaAs/AlAs Superlattices using Raman Spectroscopy MASc. Project Philip Scrutton

Outline Motivation: Quantum Well Intermixing Motivation: Quantum Well Intermixing Raman and PL Raman and PL Phonon Modes in Superlattices Phonon Modes in Superlattices MicroRaman System MicroRaman System Raman Study of GaAs/AlAs SL Structures Raman Study of GaAs/AlAs SL Structures Continuing Work Continuing Work

Quantum Well Intermixing Quantum-well intermixing (QWI) technologies have become a strong contender for the realization of photonic integrated circuits Quantum-well intermixing (QWI) technologies have become a strong contender for the realization of photonic integrated circuits QWI of GaAs/AlAs SL has lead to controlled  (2) modulation enabling quasi-phase matching (QPM) QWI of GaAs/AlAs SL has lead to controlled  (2) modulation enabling quasi-phase matching (QPM) Optimization of the QWI process is Optimization of the QWI process is needed to improve QPM efficiency We aim to develop microRaman as a non-destructive characterization method for our intact and intermixed SL structures We aim to develop microRaman as a non-destructive characterization method for our intact and intermixed SL structures A non-destructive characterisation method is needed for compositional and bandgap modulation A non-destructive characterisation method is needed for compositional and bandgap modulation

Raman and PL PL is typically used to characterize optical materials but Raman has advantages:  Resolution  Structural Information  Phonon modes correspond to different configurations by which the lattice can vibrate  Reveal composition, lattice order, crystal orientation and strain

Phonon Modes in SL In SL several unique effects occur to phonon modes: Unique modes arise due to the interaction of the standing electric potential between the QWs and the charged constituent atoms Unique modes arise due to the interaction of the standing electric potential between the QWs and the charged constituent atoms Extended SL period causes Brillouin zone-folding of the phonon bands leading to phonon confinement analogous to electron confinement Extended SL period causes Brillouin zone-folding of the phonon bands leading to phonon confinement analogous to electron confinement These modes can reveal layer thickness, period and interface quality

Phonon Modes in SL Confined Optical Phonons (COPs) If the phonon bands of the layers do not overlap, phonons cannot propagate If the phonon bands of the layers do not overlap, phonons cannot propagate Phonon harmonics result nλ/2 = d Phonon harmonics result nλ/2 = d Abrupt interfaces are required for confinement Abrupt interfaces are required for confinement Interface Optical Phonons (IF modes) Mode shifts vary with interface thickness Mode shifts vary with interface thickness Peak positions show dependence on the thickness ratio of the layers Peak positions show dependence on the thickness ratio of the layers Wang et al. 1988

MicroRaman System Obtain phonon modes with high spatial resolution Utilizes backscattering geometry and a confocal aperture MicroRaman system developed by JY Horiba MicroRaman system developed by JY Horiba Bundled software allows spectral analysis Bundled software allows spectral analysis

Raman Study of GaAs/AlAs SL Structures Periodically intermixed SL core waveguide samples have been characterized: Periodically intermixed SL core waveguide samples have been characterized: As-grown and fully-intermixed and As-grown and fully-intermixed and grating samples were examined Measurements were taken Measurements were taken along the cleaved edge of the sample to access the SL layers

Raman Study of GaAs/AlAs SL Structures We first identified the spectral features that give the clearest SNR between the SL and intermixed alloy We first identified the spectral features that give the clearest SNR between the SL and intermixed alloy PeakPosition ShiftFWHM shiftSignal Quality GaAs-TO/IF vs. GaAs-like TO6.19.5Strong peak AlAs-TO vs. AlAs-like TO0.94Strong peak AlAs-LO/IF vs. AlAs-like LO3.18Noisy peak GaAs-IFComplete disappearance on intermixed sample

Raman Study of GaAs/AlAs SL Structures We use these modes to identify the intermixed grating: We use these modes to identify the intermixed grating:

Raman Study of GaAs/AlAs SL Structures We recover the layer thickness by subtracting the spot size from the width on the Raman profile. The PL profile widths are complicated by carrier diffusion.

Continuing Work We will study samples with varying degrees of QWI We will study samples with varying degrees of QWI We wish to be able to detect not only the presence of intermixing but the severity as best as possible We wish to be able to detect not only the presence of intermixing but the severity as best as possible The mode positions, widths and relative intensities must be examined in greater detail The mode positions, widths and relative intensities must be examined in greater detail These features must be related to predictive models These features must be related to predictive models

Summary Raman spectroscopy and PL were used to study GaAs/AlAs SL gratings produced by QWI Raman spectroscopy and PL were used to study GaAs/AlAs SL gratings produced by QWI Spectral features correlated to intact and intermixed SL Spectral features correlated to intact and intermixed SL Presence of QWI easily identified through GaAs- IF mode Presence of QWI easily identified through GaAs- IF mode Layer thickness determined from Raman profile Layer thickness determined from Raman profile Work is continuing toward goal of characterization of degree of intermixing Work is continuing toward goal of characterization of degree of intermixing

References K. Zeaiter, et al., "Quasi-phase-matched second-harmonic generation in a GaAs/AlAs superlattice waveguide by ion-implantation-induced intermixing," Opt. Lett. 28(11), (2003). K. Zeaiter, et al., "Quasi-phase-matched second-harmonic generation in a GaAs/AlAs superlattice waveguide by ion-implantation-induced intermixing," Opt. Lett. 28(11), (2003). G. Abstreiter et al.,“Raman Spectroscopy—A Versatile Tool for Characterization of Thin Films and Heterostructures of GaAs and AlxGa1- xAs,’’Appl. Phys. 16, , (1978). G. Abstreiter et al.,“Raman Spectroscopy—A Versatile Tool for Characterization of Thin Films and Heterostructures of GaAs and AlxGa1- xAs,’’Appl. Phys. 16, , (1978). E.P. Pokatilov and S.I. Beril, “Electron-Phonon Interaction in Periodic Two- Layer Structures,” Phys. Status Solidi (b) 110, K75 (1982). E.P. Pokatilov and S.I. Beril, “Electron-Phonon Interaction in Periodic Two- Layer Structures,” Phys. Status Solidi (b) 110, K75 (1982). M.P. Chamberlain, M. Cardona and B.K. Ridley, “Optical Modes in GaAs/AlAs Superlattices,” Phys. Rev. B, 48 (19), (1993). M.P. Chamberlain, M. Cardona and B.K. Ridley, “Optical Modes in GaAs/AlAs Superlattices,” Phys. Rev. B, 48 (19), (1993). A.K. Sood et al., “Interface Vibrational Modes in GaAs-AlAs Superlattices,” Phys. Rev. Lett., 54, (1985). M. Zunke et al., “Angular Dispersion of Confined Optical Phonons in GaAs/AlAs Superlattices Studied by Micro-Raman Spectroscopy,’’Sol. Stat. Comm., 93, (1995). M. Zunke et al., “Angular Dispersion of Confined Optical Phonons in GaAs/AlAs Superlattices Studied by Micro-Raman Spectroscopy,’’Sol. Stat. Comm., 93, (1995).