Bogatyrenko V. Visible-to-infrared light conversion in germanium monocrystals

The work deals with the investigation of nonequilibrium thermal emission (TE) of Ge monocrystals in the spectral band of free carrier absorption induced by band-to-bad light excitation at temperatures above 300 К. Increase of free charge carrier concentration with light from the intrinsic absorption region results in increase of power of TE of semiconductor beyond its intrinsic absorption edge. This process may be considered as conversion of short-wavelength light to long-wavelength one (light down conversion). A lot of the TE power occurs at the 3-5 and 8-12 µm ranges that are of importance for thermal imaging. Monocrystalline germanium is shown to be suitable for the light down conversion. It is shown, that thermal dependence of excess TE power is non-monotone; the main reason for this is that the coefficient of absorption by equilibrium charge carriers increases with temperature due to increase of their concentration. As crystal thickness decreases and concentration of uncompensated impurities increases, the temperature range of activation of the light down conversion becomes wider. The optimal impurity concentration that is required to obtain maximal excess TE power is determined. It is shown that, due to application of single-layer antireflecting coatings, maximal excess TE power in the 3-5 and 8-12 µm ranges increases up to almost the TE power of blackbody in these ranges, and the excess TE spectrum is modified. On the basis of the investigations, a prototype of contactless converter of visible images in IR ones is made. It may be used for dynamic IR scene projection and testing of thermal imaging systems. It is shown that germanium IR emitter can function efficiently up to a temperature of, at least, 300 С; in this case its radiating power is tens of mW/cm2 and the time constant lies in the microsecond region. Key words: crystalline semiconductors, intrinsic absorption edge, absorption by free charge carriers, thermal emission, light down conversion, photonic emitter, 3-5 and 8-12 µm infrared ranges.