Abstract
Spectral dependence of terahertz emission is a sensitive tool to analyze the structure of conduction band of semiconductors. In this work, we investigate the excitation spectra of THz pulses emitted from MOCVD-grown InN and InGaN epitaxial layers with indium content of 16%, 68%, and 80%. In InN and indium-rich InGaN layers we observe a gradual saturation of THz emission efficiency with increasing photon energy. This is in stark contrast to other III-V semiconductors where an abrupt drop of THz efficiency occurs at certain photon energy due to inter-valley electron scattering. From these results, we set a lower limit of the intervalley energy separation in the conduction band of InN as 2.4 eV. In terms of THz emission efficiency, the largest optical-to-THz energy conversion rate was obtained in 75 nm thick In0.16Ga0.84N layer, while lower THz emission efficiency was observed from InN and indium-rich InGaN layers due to the screening of built-in field by a high-density electron gas in these materials.
Highlights
Excitation of semiconductor surfaces with femtosecond laser pulses is an important way of generating ultrafast electromagnetic transients with characteristic spectra reaching into the terahertz (THz) frequency range[1]
The THz emission efficiency is diminished by free electrons that screen the built-in electrical field; the efficiency can be in part restored by doping with magnesium, though[14,16,17,18]
The unintentional doping is extremally difficult to control in InN grown by metalorganic chemical vapor deposition (MOCVD)
Summary
Excitation of semiconductor surfaces with femtosecond (fs) laser pulses is an important way of generating ultrafast electromagnetic transients with characteristic spectra reaching into the terahertz (THz) frequency range[1]. Several attempts were carried out to measure the position of the subsidiary valleys in InN by using the spectrally-dependent THz emission technique, but the highest excitation photon energy did not exceed 1.7 eV, obscuring the observation of intervalley scattering effects[5,16,22]. We investigate the dependence of THz surface emission efficiency on excitation wavelength in InN and InGaN layers, using the pump photon energy up to 3.0 eV.
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