Photoelectric Spectroscopy

the determination of the chemical composition of impurities in semiconductors and the study of the energy structure of the impurities on the basis of extrinsic-photoconductivity spectra.

An impurity atom in a semiconductor may be in the ground state or an excited state. The spectrum of the states is unique for each impurity element in a given semiconductor. If a semiconductor is irradiated by monochromatic radiation and the frequency ω—that is, the photon energy ħω, where ħ is Planck’s

Figure 1. Photoelectric spectrum of Ge containing B, Al, and Ga impurities

constant—is changed continuously, every time ħω coincides with an energy gap between the ground state and an excited state, an impurity atom of the appropriate species undergoes a transition to the particular excited state, absorbing a photon in the process. The temperature of a crystal may be selected so that the energy of the crystal’s thermal vibrations is sufficient to ionize an excited atom but insufficient to ionize a ground-state atom. Two-step photothermal ionization of impurity atoms then ensues; optical excitation occurs, followed by thermal ionization. The result of such two-step ionization is the ejection of an electron or hole from an impurity atom into the conduction band and, consequently, photoconductivity.

An extrinsic-photoconductivity spectrum consists of a set of peaks, each of which corresponds to the energy of the photons responsible for the transition of an impurity atom of a given species to an excited state (see Figure 1). Over a wide range of variation in the impurity concentrations, the heights of the peaks are independent of the concentrations. As a result, infinitesimally small amounts of impurities may be detected by means of photoelectric spectroscopy. For example, in the Ge sample whose spectrum is shown in Figure 1, the total concentration of impurity atoms is 10^–11 percent of the total number of atoms. The theoretical threshold sensitivity of photoelectric spectroscopy is several orders of magnitude lower than this value.