We have a dedicated setup to measure optoelectronic effects at electrochemcial interfaces. The typical situation is a semiconducting surface coating that is excited by light. The excitation leads to the formation of an electrical surface dipole or electrochemcial reactions, both giving rise to a transient current signal. To measure photoelectrochemical current precisely the following components are crucial:
(i) a broad excitation spectra: We employ a Xe-discharge lamp to produce a spectra from the UV to the NIR range.
(ii) a monochromator to probe single wavelength: our setup features a high-resolution Oriel monochromator in Czerny-Turner geometry.
(iii) a photoelectrochemical cell: we crafted our own cell that isolates nicely the electrical contacts from the electrochemical system.
(iv) a potentiostat: we use a Metrohm PGSTAT to control the potential in the liquid using reference and counter electrode.
(v) a sensitive current recording system: we employ a current amplifier and a lockin amplifier to measure photo-electrochemical currents down to the femto-amp range.
The setup is versatile and permits all typical electrochemical characterizations (amperometry, potentiometry, impedance spectroscopy) in the presence or absence of monochromated light or in spectroscopy mode.
The SPV method is a well-established contactless technique for the detection of surface and defect states in semiconductors, which has been used since the early 1970s by Gatos and Lagowski (1973) as an extensive source of surface and bulk information on various semiconductors and semiconductor surfaces and interfaces.
The SPV is defined as the difference between the surface potential under illumination and the surface potential in dark:
SPV= Vs(ill)-Vs(dark)
The SPV signal can be detected by illuminating the surface with above or below bandgap light, usually a tunable source is used in order to obtain a continuous spectrum of SPV versus the incident photon energy. A sketch of the experimental set up is shown below.
This method allows for the detection of electronic transitions (band-to-band, defect-band, and surface state-bands) on a huge range of semiconductors without the need of cumbersome sample preparation, junction formation, etc. The detection of such transitions can be obtained also on buried layers, deposited thin films, heterostructures, multiphase, and nanostructures, where standard transmission optical spectroscopies are not applicable. Such materials are becoming more and more interesting also for many different applicative purposes.
Several examples on the application of the method to different systems can be found in:
D Cavalcoli, M Fazio, Material Science in Semiconductor Processing, 2018 https://doi.org/10.1016/j.mssp.2018.05.027
Daniela Cavalcoli, Beatrice Fraboni, Anna Cavallini Semiconductors and Semimetals 2015 https://doi.org/10.1016/bs.semsem.2014.11.004
(iEntrance facility)
The experimental technique named photocurrent spectroscopy is based on the optoelectronic phenomenon of photoconductivity, i.e. the increase of the electrical conductivity of a material when it is exposed to electromagnetic radiation (visible light, ultraviolet light, infrared light, or gamma radiation). Essentially the physical phenomenon of photoconduction in a semiconductor is based on the absorption of a photon by an electron (internal photoelectric effect). If the photon energy is high enough, the photon absorption causes the excitation of the electron across the forbidden bandgap i.e. from the valence band to the conduction band in inorganic semiconductors, and from HOMO to LUMO in organic semiconductors. Also transition from impurity levels eventually present in the bandgap can occur and they correspond to absorption of radiation with energy lower than the bandgap.
From photocurrent spectroscopy it is possible to probe the materials energy gap, intragap defective states, vibrionic oscillations in the high conductivity region and many other effects related to charge carriers generation, transport and trapping. Photocurrent spectroscopy is thus complementary to absorption spectroscopy, and the comparison between the two spectra is fundamental for the correct interpretation of the data.
The experimental setup is equipped with a Czerny-Turner monochromator, 9055F model made by Sciencetech, with two slots for diffraction gratings that cover the spectral range UV-vis-NIR (300 nm – 1500 nm). QTH and Xe-lamp are available sources. The light output is focused on a dedicated sample positioning system equipped with micromanipulator for fast electrical connection on microelectronic elements and the possibility to perform the measurements in vacuum, under gas-controlled atmosphere (O2, N, Ar) and with temperature gradient (perltier controlled -20°C, +60°C ). The coupling of current pre-amplifier and lock-in amplifier allow the detection of very low photocurrent signal, down to 10 fA.