SAS Offers for Commercial Sphere

Versatile transient charge processor (VTCP).

Code: IMC-P-00-0026-FU

Institute of Physics

Contact Person:

Description:

It deals with processing transient charges of the system under test equipped with two (three) electrodes in response to excitation pulses superimposed on a quiescent or swept bias. Apart from the sample holder selected, the setup comprises three modules:

  • stand-alone charge-to-voltage converter (CVC),
  • main application card (MAC),
  • subsidiary application card (SAC).
The input CVC module comprises a charge-sensitive amplifier (integrator) with a feedback capacitor CF that is charged by the input transient current. Output voltage of the integrator is buffered by an amplifier, so the single-ended output voltage of the CVC module is proportional to the charge Q(t) on CF times the gain of the buffer at any point in time t. An electronic switch across CF is closed before the measurement (during the pulse) to restore baseline. A potentiostat needed for electrochemical experiments has been included in the CVC module. The output voltage of the CVC is processed by the MAC module that contains a programmable time base generator, bias pulse generator and sampling (averaging) circuitry assembled in two three-channel correlators. The latter represent filters of the time constant tt of the decay Q(t), the correlator output being a maximum at well defined delays of the sampling events with respect to the trailing edge of the bias pulse. In this way the kinetic parameters of an exponential response can be assessed. Having a decay of the type Q(t) µµ tm, the widely observed case of electrochemical reactions, the kinetic coefficient m and the standard reaction potential can be deduced from the peak of the signal observed while sweeping the quiescent bias. On board of the SAS module there are four digital-to-analog (D/A) converters and one analog-to-digital (A/D) converter, all the converters possessing 12 bits resolution. Two of the D/A converters supply voltages to the bias pulse generator in the MAC module, the remaining D/A couple is used for CVC input current compensation. Finally, the A/D serves for both signal processing at the outputs of the correlators and diagnostics of the hardware. Both application cards are designed for the standard ISA bus of PC IBM compatible computers. Our original software package comprises the application software for programming the instrument and data lodging in a built-in database, along with evaluation software, all written in C++. All the programmes were developed for operation under DOS environment.

Inovation:

Before passing to details, let us mention that we deal with a versatile facility which integrates several table-top powerful instruments, reducing to a minimum space and costs:

  • spectrometer of deep defect levels (charge DLTS - commercially not available),
  • feedback charge capacitance- and current-voltage meter (C-V, I-V),
  • multimode voltammetric analyser (double-step voltcoulometry, differential pulse voltammetry, steady-state voltammetry, derivative voltammetry).
It is now well established that measurements of transient currents or charges are obscured mainly by parasitic currents (leakage in solid-state devices, nonfaradaic currents in polarography and voltammetry). To eliminate these undesirable effects, we developed a combined compensation of the parasitic steady-state currents. The combination covers both an active compensation (compensation current injection at the input) and a passive compensation by means of higher-order filtering (correlation), thereby completely eliminating parasitic drifts. To maintain high detection sensitivity, the active compensation current has been split to a coarse and a fine components, respectively. The kinetics- sensitive double-step voltcoulometry, capable of assessing the kinetic coefficient m defined above, may be regarded as a complementary tool to the state-of-the-art voltammetric (polarographic) instrumentation.

Application:

Dielectric and polarization properties of insulators Knowing precisely the geometry of the capacitor containing either solid or liquid dielectric between its electrodes one can define the time-dependent permittivity as an equivalent (complement) of the frequency domain permittivity determined by ac measurements. From the total amount of the collected charge concentrations of complexes (e. g. dipoles) contributing to dielectric polarization can be obtained.

Spectroscopy of defects in semiconductors

Nowadays there is a huge number of electronic devices which could be explored by the charge DLTS method from the point of view of defects in the band gap of many types of semiconducting materials. The commercially available DLTS spectrometers use exclusively relatively slow capacitance meters as input units. Our CVC input unit is capable of collecting charges down to a few microseconds, a time region not accessible by the capacitance meters. Moreover, the charge DLTS has a complementary spatial detection sensitivity to its capacitance counterpart, a valuable feature if reconstructing spatial density profiles of defects in the vicinity of surfaces (interfaces). Extensive charge DLTS experiments are being conducted on thin films of amorphous undoped (hydrogenated) Si for solar cells, a material known for its complex spatial and energetical distribution of electron traps.

Analytical electrochemistry of solids and liquids

The multimode feature of our setup, if viewed as a voltammetric (polarographic) analyser, makes it suitable for electroanalytical studies of electrically active species (speciation) in waters and drinks even in situations where a supporting electrolyte cannot be added for whatever reasons. In addition, our instrumentation is especially suitable for assessing real concentrations of the species while facing significant deviations of the reaction kinetics from the ideal one.

There is quite a new branch of the voltammetry, known as solid- state voltammetry. It adheres to solids containing high densities of „redox“ centers (centers exhibiting at least two different charge states), responsible for hopping of charge carriers between adjacent sites at relatively low temperatures. As an accompanying attribute, a steady-state voltammetric wave is observed, from which the density of the centres as well as the diffusion coefficient of the hopping charge carriers may be deduced. This phenomenon is well pronounced if a concentration gradient of the centres could initially be induced in an electrochemical way.

State:

The VCTP instrument characterized above is now commercially available with two types of sample holders (cells). For applications labelled A and B, respectively, in the previous paragraph there is a simple evacuated metallic cryostat available, covering the temperature range 80 K – 450 K. As an alternative we are able to supply an electrochemical cell utilizing a carbon fibre microelectrode as the working electrode. A new version of the setup in the form of a condensed single application card is in preparation as a prototype.

Cooperation:

We are ready to start both types of experiments on request for either the spectroscopy of defects in insulators and semiconductors, or speciation of metals and acids in waters or drinks. The outcome would be a corresponding protocol, other possible form could be offering the complete VCTP setup to potential customers for buying it.