Photocurrent, photovoltage and microwave reflectance methods

Photocurrent spectroscopy involves quantitative measurement of the ratio of the electron flux in the external circuit to the incident photon flux (in some cases 

Before photocurrent excitation spectra can be normalised to allow for the wavelength dependence of the illumination intensity, it is essential to establish the relation between the photocurrent response and the incident photon flux. This can be done using calibrated neutral density fdters. The relationship is generally not linear for photoconductive systems or for systems in which processes such as surface recombination or photocurrent multiplication occur (Peter, 1990). The incident photon flux can be measured using a UV-enhanced silicon photodiode with known sensitivity. 

Expansion of the Gartner equation (eq. 12.2) for small values of 6T shows that the photoeurrent is a linear funetion of the absorption eoeffieient near the band edge. The bandgap ean therefore be obtained using the plot illustrated in Fig. 12.8. 

Direct-bandgap plot for Gao.7Alo.3As used to determine the bandgap. Adapted from Hutton and Peter (1993). 

Photocurrent spectroscopy can be used to determine the minority-carrier diffusion length of semiconductors. The Gartner equation (eq. 12.2) can be written in the linear form 

---