Silicon surface recombination velocity

This is the regime of cathodic currents. The silicon atoms of the electrode do not participate in the chemical reaction in this regime. An n-type electrode is under forward bias and the current is caused by majority carriers (electrons). The fact that photogenerated minority carriers (holes) are detectable at the collector indicates that the front is under flat band or accumulation. A decrease of IBC with cathodization time is observed. As Fig. 3.2 shows, the minority carrier current at the collector after switching to a cathodic potential is identical to that at VQcp in the first moment, but then it decreases within seconds to lower values, as indicated by arrows in Fig. 3.2. This can be interpreted as an increase of the surface recombination velocity with time under cathodic potential. It can be speculated that protons, which rapidly diffuse into the bulk of the electrode, are responsible for the change of the electronic properties of the surface layer [A17]. However, any other effect sufficient to produce a surface recombination velocity in excess of 100 cm s 1 would produce similar results. 

Double-sided electrolytic contacts are favorable for this method of diffusion length measurement because they are transparent and the required SCRs are easily induced by application of a reverse bias. Therefore homogeneously doped wafers need no additional preparation, such as evaporation of metal contacts or diffusion doping, to produce a p-n junction. Furthermore, a record low value of surface recombination velocity has been measured for silicon surfaces in contact with an HF electrolyte at OCP [Yal], Note that this OCP value cannot be further decreased by a forward bias at the frontside, because any potential other than OCP has been found to increase the surface recombination velocity, as shown in Fig. 3.2. Note that contaminations in the HF electrolyte, such as Cu, may significantly increase the surface recombination velocity. This effect has been used to detect trace levels (20 ppt) of Cu in HF [Re5j. 

The existence of today s silicon based microelectronics technology is evidence for the low surface recombination velocity of oxidized Si. The velocity is less than 103 cm/sec.5,6,7... 

Exposure of silicon to atomic hydrogen increases the surface recombination velocity.111213 The free energy of formation of SiH4, the most stable of the hydrides of silicon, is only — lOKcal/mole. Since four electron pairs are shared in the formation of the molecule, the free energy of formations per Si-H bond is only -2.5 Kcal or about O.leV. Because of the weak chemisorption, heating of the silicon to temperatures above 500 C is adequate to release the hydrogen. Our model explains the increase in surface recombination velocity by the weak chemisorption of hydrogen, which may increase the density of surface states within the band gap (see Fig. 2b). 

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. 

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