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Terahertz Measurement

The part of the electromagnetic spectrum between microwaves and infrared is denoted as THz radiation. The THz frequency range (1011 Hz - 1013 Hz) is used in many applications including imaging for security or quality control of industrial products. THz radiation transmits various nonconductive materials like e.g. plastics, ceramics, semiconductors or textiles, however it is neither harmful nor damaging the material due to its low photon energies. Moreover, several materials show specific absorption lines within this spectral region. Thus THz radiation has a high application potential especially for material characterization.

Over the past years different concepts for the generation of THz radiation have been investigated. In the early 90's it was demonstrated that bare semiconductor surfaces emit THz radiation when illuminated with fs laser pulses. One radiation mechanism found was the acceleration of photo generated carriers in the so called "surface built-in field" of the semiconductors. These electric fields are induced by space charge layers formed at semiconductor surfaces.

Space charge layers also appear at the transition region in semiconducting multilayer systems like the SIS-system. Therefore, THz radiation is observable there as well.

The terahertz field strength depends on the acceleration characteristics of the photo induced carriers within these layers. Consequently, the measurement of the emitted THz-field using a THz-time domain spectrometer can be used to characterize the electric field distribution at the pn-junction.

Typically, ultrashort pulsed THz radiation is generated by the acceleration of photo injected carriers in an electric field.

This electrical field can be induced externally or internally by space charge layers at semiconductor surfaces and interfaces.

Using a time resolved detection of the THz radiation enables the spectroscopic analysis of various materials.


Within the Nano-SIS project we establish THz emission spectroscopy for the analysis of  the semiconductor interfaces of the SIS-system. In contrast to other spectroscopic techniques, THz emission spectroscopy deploys the sample (SIS-system) itself as a source of THz radiation. The temporal pulse shape reflects the acceleration of the photogenerated carriers within the electrical field of the pn-junction.

The emitted THz field is mainly determined by material properties such as doping concentration and carrier lifetime, as well as the direction of the induced photocurrent.

Aufbau

Set up

Graph1

Graph

Measured THz field strength of SIS solar cells, sample 1 without and sample 2 with photovoltaic behavior. The processing of sample 2 led to the formation of a pn-junction at the ITO Silicon interface. Therefore the direction of the induced photocurrent reverses, which results in a flipping of the THz field polarity.

Startseite UNI Jena

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