TSCap capacitance

Nonisothermal capacitance methods employ heating programs T(t) and are known as thermally stimulated capacitance (TSCAP) [5]. 

Fig. 8. Energy below the conduction band of levels reported in the literature for GaAs. Arrangement and notations are the same as for Figs. 4 and 5. Notations not defined there are epitaxial layer on semi-insulating substrate (EPI/SI), boat-grown (BG), vapor phase epitaxial layer on semi-insulating substrate (VPE/SI), melt-grown (M), molecular beam epitaxy (MBE), horizontal Bridgman (HB), irradiated with 1-MeV electrons or rays (1-MeV e, y), thermally stimulated capacitance (TSCAP), photoluminescence excitation (PLE), and deep level optical spectroscopy (DLOS).

The techniques of thermally stimulated capacitance (TSCAP) and thermally stimulated current (TSC) are the simplest methods for surveying the... 

Figure 35 shows the behavior for our example of a discrete level system. The TSCAP experiment takes at least two temperature scans, one at the ambient bias in the usual manner to give a capacitance baseline, then another following a cool-down in zero bias. In the latter situation both deep levels at and 2 are completely filled above at low temperatures (see Fig. 17) and therefore do not contribute positive charge to the depletion region. Thus the capacitance is due exclusively to the shallow donors alone and will differ from its usual equilibrium value Q by (iV, iVj c Ad)... 

Fig. 35. Calculated set of thermally stimulated capacitance (TSCAP) scans for the quasidiscrete-level g(E) of Fig. 16 under voltage pulse and laser pulse filling conditions as described in the text.

Other workers (Vieux-Rochaz and Chenevas-Paule, 1980 Fuhs and Milleville, 1980 Thompson et al, 1981) have also carried out TSC studies on a-Si H. The results of such studies to date are generally inconsistent with the results of the TSCAP measurements discussed above. A major concern in such measurements in these materials are sources of extraneous currents that obscure the thermal emission phenomena. These include thermal emission over the barrier itself and other kinds of leakage, high sensitivity to surface rather than bulk states, thermoelectric currents, and secondary photocurrent in the case of laser pulse studies. Capacitance measurements are largely insensitive to such effects and are thus more easily interpreted. For these reasons the results of TSC measurements must be carefully checked (for example, by verifying that the features scale properly with applied bias) before they are interpreted in terms of thermal emission from bulk states. 

This fundamental surface sensitivity for current transient measurements also applies to TSC and low frequency capacitance measurements. The latter case follows from the fact that the low frequency capacitance is proportional to the total barrier current that flows through the external circuit as a result of small voltage changes. The capacitance DLTS and TSCAP methods, on the other hand, are largely insensitive to surface effects. Thus features in current DLTS that do not appear in the corresponding capacitance DLTS spectra are likely candidates for states localized near the barrier interface. 

---