Holmarc’s Thermally stimulated current (TSC) spectrometer is used to study energy levels in semiconductors or insulators, especially solar cells. The sample energy levels are first filled by exposing it to a halogen lamp light source for a user specified time. The temperature of the sample at this time can be set as low as 70°C. The sample can then be heated gradually up to 150°C at a specified ramping rate which causes emission of electrons and holes in the sample.
This emission is measured at different points of temperatures, by applying a voltage and measuring the resulting current. A curve of emitted current is recorded and plotted against temperature, by the software, resulting in a TSC spectrum. By analyzing TSC spectra, information can be obtained regarding energy levels in the sample.
A semiconductor is characterized by a relatively narrow energy gap, typically a fraction of an electron volt to a few electron volts. Electrons can be excited by a photon from the valence band to the conduction band and form an electron – hole pair. The electron – hole pair stores a substantial portion of the photon energy. After formation of the electron- hole pairs, the pn-junction separates the electrons and holes to generate external electric current.
A driving force is required for emitted carriers to flow when the sample temperature is being increased. This driving force can be an electric field or a temperature gradient. In Holmarc’s TSC Spectrometer, this driving force is a voltage in the range of 0.2 to 2V. The sample heating is accomplished using a sample holder with embedded resistive heating element. The sample holder also has piping to carry liquid nitrogen to facilitate cooling up to -70 Degree Celsius. The Nitrogen cylinder can be purchased along with the spectrometer, which should be filled with liquid Nitrogen before operation.
The sample chamber can accommodate substrates of diameters up to 5 cm. The high resolution current measurement circuit has a resolution of 0.1 nA and accuracy of 1 nA. The T-Spectro software which comes with the system enables automatic scanning and data acquisition from the system. The scan parameters and results can be saved for later use. The software works with any PC with minimal specifications and which runs Windows XP, 7, 8.1 or 10.