The unique features of nanowires (NW), such as the high aspect ratio and extensive surface area, are expected to play a key role in the development of very efficient semiconductor surface emitters in the terahertz (THz) spectral range. Here, we report on optically excited THz emission from catalyst-free grown arrays of intrinsically n-type InAs NWs using THz time-domain spectroscopy. Depending on the aspect ratio, the THz emission efficiency of the n-type InAs NWs is found to be up to ∼3 times stronger than that of bulk p-type InAs, known as currently the most efficient semiconductor-based THz surface emitter. Characteristic differences from bulk p-type InAs are particularly revealed from excitation wavelength-dependent measurements, showing monotonously increasing THz pulse amplitude in the NW arrays with increasing photon energy. Further polarization-dependent and two-color pump-probe experiments elucidate the physical mechanism of the THz emission: In contrast to bulk p-type InAs, where the anisotropic photoconductivity in the surface electric field is the dominant cause for THz pulse generation, the origin of the intrinsic THz emission in the NWs is based on the photo-Dember effect. The strong THz emission from high aspect ratio NW arrays further suggests an improved out-coupling of the radiation, while further enhancements in efficiency using core-shell NW geometries are discussed.
We report the terahertz (THz) emission properties of composition-tunable, intrinsically n-type InGaAs nanowire (NW) arrays using THz time-domain spectroscopy. By tuning the alloy composition of In1-xGaxAs NWs from pure InAs (x(Ga)=0) up to the intermediate composition (x(Ga)∼0.5), a substantially enhanced (>3-fold) THz emission efficiency is found, which is ascribed to a reduction in electron accumulation at the NW surface and respective electron scattering at donor-type surface defects. These findings are also confirmed by photoexcitation wavelength dependent measurements, while the THz emission characteristics are further found to be different from corresponding bulk-type planar InGaAs. In particular, NWs exhibit no distinct maxima in THz excitation spectra as caused by electron scattering to subsidiary conduction band valleys and commonly observed in the majority of bulk semiconductors. The wavelength-dependent emission spectra further reveal distinct signatures of modified intervalley scattering, revealing the underlying polytypism of intermixed wurtzite and zincblende phases in the investigated InGaAs NWs.
In this study we report the investigation of terahertz (THz) emission efficiency dynamics in p-type InAs using a double-pump-pulse (DPP) THz emission method. We also suggest a novel modification of the standard DPP method which allows us to measure the indirectly modulated THz pulse. The obtained results reveal that the first optical pulse increases the free carrier concentration and enhances the surface electric field. This field prevents perpendicular but improves parallel to the surface electric dipole formation after sample excitation with the second optical pulse. Our suggested method is shown to be a more precise and sensitive way to study electric fields and photocarrier dynamics in semiconductors after photoexcitation.
Terahertz pulse emission from the photoexcited semiconductor surface heavily depends on the orientation of the photo-induced electric dipole. Two methods to determine the orientation of the terahertz pulse emitting dipole have been demonstrated. Method I relies on the measurement of THz emission dependencies on the angle of incidence in the transmission geometry, while method II is based on the measurements of THz emission in the reflection geometry. Theoretical reasoning for both of these methods has been presented. Both of these methods have been tested with a semi-insulating GaAs substrate using an external magnetic field in order to change the tilt angle of the electric dipole. The investigation presented in this work leads to an assumption that the magnetic field induced change in the dipole tilt angle is proportional to the mobility of electrons. Thus, the suggested method may become a promising tool for testing the quality of substrates and epitaxial layers.
A non-stoichiometric (NS) GaAs layer by the means of terahertz (THz) emission spectroscopy is investigated. THz emission azimuthal dependencies and THz pulse amplitude dependence on the excitation angle were measured. Obtained results were explained by the existence of parallel to the sample surface components of THz radiating electric dipoles. The results were compared with those gained investigating GaAs nanowires. In addition, it was shown that a NS GaAs layer could be a very promising material for a compact bias-free THz emitter with good spectral characteristics.
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