Static, transient or modulated light excitation pro and con

The time-resolved or frequency-modulated, surface photovoltage spectroscopy (SPS) is based on a time-resolved/frequency modulated measurement of the spectral dependence of the surface photovoltage (SPV). It is a powerful non-destructive and contactless characterization method. It is mainly used to study the electronic transitions and optical properties of bulk materials, thin films and heterostructures. High sensitivity and the possibility of room temperature measurements are the key advantages of the SPV method. Another advantage is that there is no need for the preparation of a front contact on the investigated sample. In general, there is no need for preparing the sample for the measurement, allowing to investigate the sample under operation/process conditions in a wide temperature range under different atmospheric conditions. The information depth and thereby the possibility to extract bulk properties is limited by the lights penetration depth and the diffusion length. In comparison to other spectroscopic methods, such as but not limited to optical transmission, deep level transient spectroscopy, photoluminescence or Raman spectroscopy, the time-resolved/frequency modulated SPS or SPV (fixed wavelength) method is fast and uncomplicated and is thus an ideal tool for production floor decisions of sample quality. We distinguish between 3 different excitation modes, but common to all of them is that the relaxation aspect of states in the samples are resolved under ideal conditions.

A static SPV measurement is sensitive to any fast or slow process that lead to the separation of photogenerated carriers in space. The sample is illuminated until a saturation of the SPV signal is observed, after which the light is switched of. Measuring 1) the static SPV signal and 2) the time-resolved relaxation time gives a lot of useful information about the state of the material.

A SPV measurement that is performed under modulated illumination in a fixed capacitor arrangement is very sensitive to small changes in the SPV signal. And, only those SPV signals, which can follow the modulation frequency are contributing to the measured signal. The response of processes with relaxation times much longer than the modulation period are simply filtered out.

The most sensitive SPV measurement that can be made is a transient measurement, where illumination pulse of different pulse width are followed by a time dependent measurement of the decay of the SPV signal – in this way charge separation distances in the nanometre range can be investigated. This is particular important for surface or tunnelling dominated processes in the material.