Germanium, surface recombination

Many investigations with surfaces have been carried out in this and other laboratories using the ion-bombardment method of cleaning. These include (1) structure investigations of the surface plane on clean surfaces, (2) work-function determinations, (3) adsorption measurements, (4) catalysis, (5) surface recombination velocity, (6) surface conductivity, and (7) field effect. One of the significant finds indicates that the relative positions of the atoms in the clean 100 surface planes of germanium and silicon are not the same as those of similar planes in the bulk crystals, but that these relative positions are the same when a monolayer of oxygen is adsorbed on these surfaces (9). 

E. Yablonovitch, D. L. Allara, C. C. Chang, T. Gmitter, and T. B. Bright, Unusually low surface-recombination velocity on silicon and germanium surfaces, Phys. Rev. Lett. 57, 249, 1986. 

Although not of primary concern in this review a number of experimenters have developed information on the oxidation rates of amorphous and polycrystalline films of germanium. While there is some disagreement on the role of porosity in the oxidation rate of such films, (17,18) the kinetics of the reaction appear to be strongly dependent on the morphology of the films and the ambient atmospheres to which they are initially exposed. Based on that information it would appear that implant areas should be annealed in situ or at least removed from vacuum to a controlled environment until final surface preparation is affected. This is particularly true of photovoltaic and photo-conductive devices where the uniformity of oxide, interface moisture content, uptake of carbon complexes etc. strongly affect the surface recombination currents and hence the device performance (77). 

This is further aggravtated by the surface recombination. So far, no satisfactory surface passivation technique has been found for germanium PIN photodiodes, and the surface leakage current tends to be very high and unstable, especially at high ambient temperature. 

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