Nichtlineare optische Spektroskopie an der Galliumphosphid-Silizium(001)-Grenzfläche

In dieser Arbeit wurde die Grenzfläche zwischen Galliumphosphid (GaP) und Silizium(001) (Si), einem polaren, respektive nicht polaren indirekten Halbleiter, mittels nichtlinearer optischer Spektroskopie untersucht. Dabei stand die optische Frequenzverdopplung an der Grenzfläche im Vordergrund. Es ko...

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Bibliographic Details
Main Author: Brixius, Kristina
Contributors: Höfer, Ulrich (Prof. Dr.) (Thesis advisor)
Format: Dissertation
Published: Philipps-Universität Marburg 2014
Online Access:PDF Full Text
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Table of Contents: In this thesis the interface between Gallium Phosphide (GaP) and Silicon (Si), a polar and a nonpolar indirect semiconductor, respectively, has been investigated by means of nonlinear optical spectroscopy. Within the experimental techniques optical second harmonic generation from the interface was most prominent. It has been demonstrated that the second harmonic signal originating from the heterostructure is being dominated by a strong interface contribution which is absent for the pure GaP and Si wafers. The rotational anisotropy depends both on the quality of the interface and the properties of the GaP layer. A correlation with transmission electron microscopy (TEM) data allows for direct demonstration of the sensitivity of this technique on twins, Ga-droplets, and anti-phase domains. Analysis of the rotational anisotropy data as a function of layer thickness results in an oscillating signal which is due to phase matching effects within the GaP bulk. On the other hand, the interface contribution decays exponentially, which is attributed to the absorption within the GaP bulk. The reason for the strong interface contribution has been discussed: generally an electric field leads to a symmetry break within bulk layers, this leads to an additional contribution to the second harmonic light, the so called electric field induced second-harmonic (EFISH). An electric field due to the band alignment within the interface appears to be a good explanation for this additional contribution. Within a conventional pump-/probe setup the optical second harmonic generation was used in order to investigate transient phenomena at the interface. Again the results for the heterostructure differ remarkably from the single crystals. The second harmonic intensity increases considerably, about several hundreds of percent, as a function of delay. The increase is followed by a complex decay of the signal. A rate equation model was used in order to quantitatively analyse the dependence of the transients on the pump intensity, on the character of the interface and on the polarization of the pump pulse. First temperature dependent measurements have been executed and contributed to the interpretation in a qualitative way. In addition it has been demonstrated impressively that the interface specific transient SH-signal could even be detected for a 65nm thick film of GaP on Si. The results could be interpreted by means of a charge transfer between the Si substrate and the GaP film. In this case the primary contribution to the SH-signal could be a transient EFISH signal. In particular the linear dependence of selected components of the signal on the pump intensity points to the build-up of an electric field. The formation of an interface state, which allows the direct excitation into the conduction band, is very probable. Still, spectroscopic information is needed in order to confirm this hypothesis. Coherent phonon spectroscopy at the GaP/Si hetero interface provides unexpected results as well. The resonant excitation with 400nm brought up a totally new sharp oscillation frequency at 11THz, which was absent for both the oxidized GaP wafer and Si/SiO_2 interface. The frequency of that mode does not show any variation within the pump intensity, but varies sinusoidally with the polarization of the pump pulse. The result suggests that the underlying generation mechanism is dominated by resonant impulsive stimulated Raman scattering. Basically, two options have been discussed for the origin of the sharp interface specific mode: firstly, a frequency conversion from a longitudinal optical mode to a transversal optical mode and secondly, a totally screened longitudinal optical oscillation. Both approaches need the geometrical form of the anti-phase-domains which have been found in TEM measurements. Choosing special growth conditions within metalorganic vapour phase epitaxy forces the domain boundaries, which are formed by homopolar bonds, to tilt on {111}-planes. On the one hand, the modified bonds can give rise to oscillating components normal to the interface, in this way a transversal oscillation would be formed. On the other hand, these homopolar bonds are also known to form in between two phosphorus atoms. On average such a wrong bond leads to an additional quarter electron. The number of wrong bonds has been quantified in an easy model based on the TEM measurements. It is possible to generate high electron densities, which can be localized especially at the interface depending on the size of the anti-phase. The presence of such a high electron density would lead to a complete screening of the polar character of the crystal. The frequency of the longitudinal oscillation would shift towards the frequency of the transversal mode. The results of this thesis confirm that optical second harmonic generation is a powerful tool in order to investigate single semiconductor hetero interfaces non invasively. The benefit of this method lies in its sensitivity to symmetry breaks, amongst others it can be used as a probe for electric fields. Moreover time resolved measurements enable the investigation of transient phenomena at the internal interface. Though coherent phonon spectroscopy has not been known for a special interface sensitivity, it could be demonstrated impressively that within the GaP/Si model system the detection of an interface specific mode is possible. It is highly recommended to combine both methods in order to gain further information about the GaP/Si interface and even other semiconductor hetero interfaces in general. The manipulation of the interface properties within in the epitaxial growth offers a multitude of further experiments in order to gain a higher level of understanding of this hetero interface. At the same time structural analysis within the TEM serves as an excellent complementary method for the nonlinear optical spectroscopies.