Optical measurements can be generally described by the active interaction of light in the test structure—light can be absorbed, reflected, transmitted, and/or emitted. In photoluminescence measurements, an incident light source—usually higher energy than the primary absorption transition—is made incident on a photoactive film. Light of energy lower than the energy gap of the semiconductor may be used to probe midgap states and impurities. The light emitted by the sample by fluorescence or phosphorescence is then detected. (See Fig. 9.5 for a generalized example of steady-state photoluminescence experiment schematics.) Under excitation energy E > Eg, absorbed light creates a photoexcitation. The photoexcitation decays via a series of relaxations to the band edge, from where it can recombine to the ground state nonradiatively or radiatively (i. e., by emitting a photon).
Figure 9.5 Steady-state photoluminescence experiment schematic.
Steady-state photoluminescence maps the emission spectrum due to excitation and is sometimes described as the fluorescence emission spectrum. The energy shift, and shape and structure of the photoluminescence reveal the energetic structure and disorder of the probed material in the excited state. Fluorescence excitation spectra are obtained when photoluminescence is detected at a given wavelength, while the spectrum of the excitation is scanned. By using pulsed laser experiments, time-resolved photoluminescence can be recorded. The time resolution depends on the duration of the laser pulses and typically spans the widest range of any other experiment— from femtoseconds (10-15 s) to steady state. A technique called time – correlated single-photon counting is often used to record nanosecond fluorescence dynamics, while fluorescence up-conversion (a gated experiment) is the most common to obtain the femtosecond evolution of the emission. Finally, fluorescence typically occurs from neutral excited states or from bound CT states, so that emission techniques are not appropriate to investigate free charge carriers.