Modulation spectroscopy

Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin films, microstructures, surfaces/interfaces, and for the evaluation of important device parameters. This spectroscopy stems from a very general principle of experimental physics, in which a periodic perturbation (applied either to the sample or probe) leads to sharp derivative-like spectral features in the optical response of the system. The derivative nature of modulation spectroscopy emphasizes even weak structures localized in the region of interband (intersubband) transitions of semiconductors (semiconductor microstructures) and suppresses undesired background effects.

In temperature modulation (thermoreflectance: TR) the sample may be mounted on a small heater attached to a heat sink and the temperature varied cyclically by passing current pulses through the heater. Alternately a secondary beam of light may be used to heat the sample. Typical operating frequency for this method cannot be greater than 10-20 Hz. The observed normalized changes are usually small so that the difference signals are closely related to a derivative of the absolute spectrum with respect to the modifying parameter (i.e. the temperature of the sample in TR).

By applying an a.c. voltage to the sample surface (electroreflectance: ER) the ER signal ΔR/R (i.e. the normalized variation of the reflectance of the sample) exhibits a third derivative behavior. This is typical of bulk semiconductor materials in the low-field regime, usually encountered performing ER measurements in standard experimental conditions. In the most popular experimental configuration (surface barrier ER) the field is developed normal to the surface in the space-charge region formed in conjunction with a nonohmic contact, the voltage being supplied by using as electrodes a thin (semitransparent) metallic film deposited on the front surface of the sample and an ohmic contact on the back surface. Contactless electromodulation can be obtained by using a capacitor-like geometry or, in a more expensive way, by using a collimated electron beam or by taking advantage from the light induced variation of the surface field (photoreflectance: PR).

Photoreflectance and the related techniques of contactless electromodulation are particularly useful because they do not require special mounting of the sample. In a typical PR experimental set-up, the modulation of the (built-in) electric field in the sample is caused by photo-excited electron-hole pairs created by the pump source (laser) which is chopped at a frequency of 30-3000 Hz. This in turn produces a modulation ΔR of the reflectance R. By using a probe light (typically a lamp source) and phase-sensitive detection techniques PR signals ΔR/R as low as 10-6 can be revealed. Moreover, because of the room temperature performance and the richness of the derivative-like spectra, the information in the line shape fits allows the critical point energy in bulk as well as in confined semiconductor systems to be determined with an uncertainty of a few meV even at room temperature. Furthermore, the influence of external perturbation such as temperature, electric field, hydrostatic pressure, etc. can be investigated by monitoring the line shape evolution as the experimental conditions are varied.