Gallium phosphide on silicon(001) (GaP/Si) represents a structurally well characterized model system of a polar/non-polar inorganic semiconductor interface. This system is of particular interest since the lattice mismatch between GaP(001) and Si(001) is small and strain induced defects can be neglected. It has been proven to be a promising link between silicon based technology and III/V-based optoelectronic applications. We are studying the buried interface of GaP/Si(001) by means of polarizational anisotropy second-harmonic generation (SHG), time-resolved SHG measurements as well as coherent phonon spectroscopy.
Rotational anisotropy measurements probing the SHG response as a function of the crystal orientation offer direct access to the symmetry of the studied materials. The rotational anisotropy measurements of heteroepitaxial GaP films on Si(001) differ remarkably from the results of the individual GaP and Si bulk wafers, both in overall signal intensity and in particular in their anisotropy. Unexpected large, isotropic signals are observed which can clearly be attributed to the buried GaP/Si interface. These interface signals furthermore differ for samples fabricated under different growth conditions. In close collaboration with our colleagues K. Volz and W. Stolz of the Structure and Technology Research Laboratory in Marburg, we were able to show that the SH signals of these samples can be correlated to specific defects that form at different growth conditions in metal-organic vapor phase epitaxy. Our results reveal that SHG is a non-invasive, sensitive probe for the structural quality of the interface. As an all-optical technique, it could also be applied as an in situ monitor of the growth process complementary to established methods such as reflection anisotropy spectroscopy.
Charge Transfer & Electronic States
Our time-resolved optical-pump SHG-probe measurements reveal an unexpected strong transient response of the buried GaP/Si(001) interface upon 800-nm photoexcita-tion. The SH transients exhibit a complex temporal behaviour with a fast initial increase of the SH intensity followed by a decay within approximately 400 fs. Afterwards, a second component dominates which rises delayed within several picoseconds and decays comparatively slow. These observations are not only surprising because of the low excitation energy of 1.55 eV (which is below the direct bandgaps of both GaP and Si), but also because of the magnitude of the pump-induced effects. Our results can be understood if one postulates that interface-specific electronic states exist at the GaP/Si boundary and that the electrons excited by the pump pulse via these interface states are efficiently injected into the conduction band of Si. The resulting transient electric field then gives rise to a SH response from the Si bulk whose inversion symmetry is broken perpendicular to the interface.
contact: Dr. Gerson Mette, Prof. Dr. Ulrich Höfer
- K. Ishioka, A. Rustagi, A. Beyer, W. Stolz, K. Volz, U. Höfer, H. Petek, and C. J. Stanton
Sub-picosecond acoustic pulses at buried GaP/Si interfaces
Appl. Phys. Lett. 111, 062105 (2017).
- K. Ishioka, A. Rustagi, U. Höfer, H. Petek, and C. J. Stanton
Intrinsic coherent acoustic phonons in the indirect band gap semiconductors Si an GaP
Phys. Rev. B 95, 035205 (2017).
- K. Ishioka, K. Brixius, A. Beyer, A. Rustagi, C. J. Stanton, W. Stolz, K. Volz, U. Höfer, and H. Petek
Coherent phonon spectroscopy characterization of electronic bands at buried semiconductor heterointerfaces
Appl. Phys. Lett. 108, 051607 (2016).