DIRECT OBSERVATION OF MULTIPLICITY OF IMPURITY CHARGE STATES IN SEMICONDUCTORS FROM LOW-TEMPERATURE HIGH-FREQUENCY PHOTOCAPACITANCE

A simple method is proposed and demonstrated which provides a direct and reliable observation of the multiplicity of charge states of impurity centers. This is achieved by recording the high-frequency photocapacitance changes in a reverse-biased p-n junction at a low temperature during monochromatic illuminations at different wavelengths. The photon energy is first chosen so that it is above the threshold required to excite one of the trapped electrons at the impurity center, but below the threshold of exciting the next electron. After the capacitance change is completed, the photon energy is increased to the next range to excite the second trapped electron into the conduction band. The equality of the change of the squared capacitance during these two illumination periods then implies a multiplicity of three of the charge state. The procedure may be repeated for more than three charge states. An example is given for a sulfur-doped silicon p + n junction whose double donor levels have thermal activation energies of 0.30 and 0.55 eV below the conduction-band edge. © 1969 The American Institute of Physics.