Photo-reflectance is an optical technique for investigating the material and electronic properties of thin films. Photo-reflectance measures the change in reflectivity of a sample in response to the application of an amplitude modulated light beam. In general, a photo-reflectometer consists of an intensity modulated "pump" light beam used to modulate the reflectivity of the sample, a second "probe" light beam used to measure the reflectance of the sample, an optical system for directing the pump and probe beams to the sample, and for directing the reflected probe light onto a photodetector, and a signal processor to record the differential reflectance. The pump light is typically modulated at a known frequency so that a lock-in amplifier may be used to suppress unwanted noise, resulting in the ability to detect reflectance changes at the ppm level.
The utility of photo-reflectance for characterization of semiconductor samples has been recognized since the late 1960s. In particular, conventional photo-reflectance is closely related to electroreflectance in that the sample's internal electric field is modulated by the photo-injection of electron-hole pairs. The electro-reflectance response is sharply peaked near semiconductor interband transitions, which accounts for its usefulness in semiconductor characterization. Photo-reflectance spectroscopy has been used to determine semiconductor bandstructures, internal electric fields, and other material properties such as crystallinity, composition, physical strain, and doping concentration.
Etymology
The name "photo-reflectance" or "photoreflectance" is shortened from the term "photo-modulated reflectance," which describes the use of an intensity modulated light beam to perturb the reflectance of a sample. The technique has also been referred to as "modulated photo-reflectance," "modulated optical reflectance," and "photo-modulated optical reflectance." It has been known at least since 1967.
Basic principles
Photo-reflectance is a particularly convenient type of modulation spectroscopy, as it may be performed at room temperature and only requires the sample have a reflecting surface. It is an established tool for non-contact determination of material and electronic properties of semiconductor films. In photo-reflectance, a pump laser beam is used to modulate the free charge density in a semiconductor sample (via photo-injection), thereby modulating one or more physical quantities (e.g. the internal electric field). The measured signal ΔR is the change in amplitude of the reflected probe light as the intensity modulated pump radiation interacts with the sample. The normalized signal is ΔR/R, i.e. the pump-induced change in reflectance (AC) divided by the baseline reflectance (DC). The conventional photo-reflectance apparatus uses a spectroscopic source for the probe beam, such that the signal may be recorded as a function of the probe light's wavelength. Generally, the signal may be written:
where ΔR/R is the normalized change in reflectance, α (≡1/R×∂R/∂ε1) and β (≡1/R×∂R/∂ε2) are the "Seraphin coefficients" which contain filmstack information, and Δε1 and Δε2 are the pump induced changes in the complex dielectric function. However, in conventional photo-reflectance analysis, it is not necessary to independently determine the refractive and absorptive components (the first and second terms in ΔR/R, respectively) of the signal. Rather, a fit to the overall signal is performed using the third derivative functional form given by Aspnes. This fit procedure yields the interband transition energies, amplitudes, and widths. However, because the signal depends on the uniformity of the perturbation, the extraction of such parameters must be treated with care.