Robust coherent phonon mode localized at GaP/Si(001) heterointerface

TL;DR

This work investigates ultrafast electron–phonon dynamics at buried GaP/Si(001) interfaces during two-step MOVPE growth. Using interface-specific pump–probe transient reflectivity, it identifies a discrete interfacial electronic state present in thin nucleation layers and a robust $2\ \mathrm{THz}$ coherent phonon localized at the interface that survives high-temperature overgrowth but whose coupling depends on the interfacial electronic transition. The results show that overgrowth quenches the initial electronic state and induces intermixing at the interface, altering polarization dependence and boosting the LFM amplitude through a new coupling pathway. Overall, the findings demonstrate that the $2\ \mathrm{THz}$ interfacial phonon is robust, but its observable strength is controlled by the interfacial electronic structure, highlighting transient reflectivity as a powerful tool for buried-interface characterization.

Abstract

Ultrafast electron and phonon dynamics at a buried interface of GaP/Si(001) are investigated at different growth stages of the GaP layer by combining interface-specific photoexcitation and transient reflectivity detection. A discrete electronic state, which dominates the charge carrier dynamics at the interface of a thin low-temperature nucleation layer, is found to be quenched by the following high-temperature overgrowth. A coherent phonon mode localized at the heterointerface is observed for both the thin nucleation layers and the thicker overgrown layers, and their amplitude exhibits the similar resonance behavior to that of the interface electronic transition at the respective growth stages. The optical polarization-dependence of the phonon amplitude is nearly isotropic for the nucleation layer but becomes anisotropic after the overgrowth, possibly due to the formation of an intermixing layer. Our observations imply that the 2-THz phonon mode itself is robust against the high-temperature overgrowth, but its amplitude is defined by the coupling with the interface electronic transition that is more sensitively affected by the overgrowth.

Figures (5)

• Figure 1: Atomic force microscopy (AFM) images of the GaP/Si(001) samples: $d$=8nm nucleation layer A (a) and $d$=48 nm overgrown layer E (b).
• Figure 2: (a,b) Interface-contribution to TR signals of samples F (a) and E (b) pumped at different wavelength and probed at 800 nm. Incident pump fluence is $\sim0.4$ mJ/cm$^2$. Traces are offset for clarity. (c,d) Initial drop height of the interface TR signal (c) and the LFM amplitude obtained from fitting the oscillatory signals to Eq. (\ref{['dh']}) (d) obtained from samples F and E. Curves are to guide the eye. (e) Schematic band energy diagrams with possible electronic transitions at an GaP/Si interface. IS, CBM and VBM indicates an unoccupied interface state, the conduction band minimum and valence band maximum, respectively.
• Figure 3: (a) Oscillatory part of TR signals of samples A to E. Pump and probe wavelength is 815 nm, with their polarizations being parallel to the [110] direction of the Si substrate. Incident pump fluence is $\sim0.1$ mJ/cm$^2$. (b-e) Fast Fourier transform (FFT) spectra of the oscillatory TR signals measured with four different pump and probe polarization combinations. Traces are offset for clarity in (a-e). (f) Ball and stick model of an abrupt, Ga-terminated GaP/Si(001) interface Beyer2012Beyer2016Beyer2019. Red, yellow and black circles represent silicon, gallium and phosphorus atoms, respectively. (g,h) Polar plot of the LFM amplitude as a function of pump polarization angle relative to the [110] direction of the Si substrate (symbols). Probe polarization is along the [110] and [-110] directions in (g) and (h). Solid curves represent fits to Eq. (\ref{['sinu']}).
• Figure 4: GaP layer thickness-dependences of LFM amplitude (a), dephasing rate (b), frequency (c), and initial phase (d) obtained by fitting the oscillatory TR signals to Eq. (\ref{['dh']}). Pump and probe wavelength is 815 nm, with their polarizations being parallel to the [110] direction.
• Figure 5: (a,b) Schematic illustrations of a second-order Raman scattering involving large-wavevector Si-like and GaP-like phonons (steps 1 to 4 described in the main text) in the presence (a) and absence (b) of an unoccupied interface electronic state (IS). Black and orange solid curves represent the bulk electronic bands of Si and GaP; grey and orange are their projection to the (001) plane.