Surface recombination velocity

In the depletion region for a band bending U - Ujb> 100 mV, where a reasonably low surface recombination velocity is found, the PMC signal can consequently be approached by... 

If minority carrier current (1BC, dotted line, symbols in Fig. 3.2) is detected at the collector, it can be concluded that the emitter is no sink for minorities. The absolute value of IEB depends not only on the charge state of the emitter-base junction and surface recombination velocity, but as well on bulk diffusion length and on sample thickness. However, the latter two parameters are constants for a given sample. 

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. 

A current of photogenerated holes observed in regime 4 at the collector at low VeB of about 1 V for an illuminated n-type substrate indicates that no significant SCR is present at the front side the n-type electrode is in inversion. If the bias is increased this current disappears indicating an SCR or an increased surface recombination velocity at the emitter. In regime 4a all photogenerated holes are consumed by Feb- Breakdown of the junction in regime 4 does not lead to pore formation. 

For not too low doped samples (D W), however, the contribution of 1SCR is usually negligible. If the surface recombination velocity at the illuminated front is low, IBPC then only depends on sample thickness D, illumination intensity eP, and minority charge carrier diffusion length ID. 

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. 

These devices showed EL enhancements to ammonia, methylamine, di-methylamine, trimethylamine, and sulfur dioxide that increased in magnitude with concentration until saturation was reached [14]. The LEDs with larger active layers produced the greatest change in EL intensity with exposure to sulfur dioxide and the amines. Intensity changes were attributed principally to surface recombination velocity effects, as the significant forward biases employed should eliminate the depletion width. 

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... 

We explain the low surface recombination velocity by the sweeping of surface states from the region between the edges of the conduction and valence bands upon oxidation of the surface. The standard free energies of formation of crystalline and fused quartz from bulk Si are -192 and -191 Kcal/mole respectively. The standard free energy change for a Si surface is likely to be... 

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). 

Casey and Buehler have shown that the surface recombination velocity of n-InP ( 5xl017 carriers/cm3) is low, 103cm/sec.17 Suzuki and Ogawa have recently reported a sequence of surface treatments that cause substantial changes in the surface recombination velocity of InP.18 They found that in freshly vacuum cleaved (110) faces v, is much greater than at air exposed faces and that the quantum efficiency of band gap luminescence increases by an order of magnitude when the freshly cleaved face is exposed to air. This suggests that the surface recombination velocity is reduced when 02 is chemisorbed. 

The changes are explained as follows The density of surface states within the band gap on freshly cleaved InP is high. As a result, the surface recombination velocity is high and the luminescence efficiency is low. Chemisorption of oxygen splits the surface states, as large band gap, colorless InP04 is formed.19... 

Reduction in the surface recombination velocity of GaP, from 1.7 x 10s cm/sec to 5xl03 cm/sec, is observed upon exposure to a CF4 plasma in which fluorine is known to be present.20 Again, the product of chemisorption of fluorine on the surface is likely to be a large band gap material such as GaF3, which straddles the edges of the conduction and valence bands of GaP. 

The presence of arsenic at the interface implies that surface states within the band gap will be introduced (see Fig. 1). We associate the high surface recombination velocity with the presence of arsenic. The formation of elemental As on the GaAs surface explains the difference in behavior of InP and of GaAs. In InP the thermodynamically stable phase that results from oxidation of the surface is colorless InP04 which straddles the band gap. In GaAs it is Ga203 and small band gap As. 

Woodall et al.36 have analyzed the relationship between surface recombination velocity and the steady state band gap luminescence in GaAs. They calculate for 534nm excitation that a decrease in vs from 106cm/sec to 104cm/sec will triple the quantum efficiency at a 2.5Mm deep p-n junction if the hole diffusion length, Lp, is 3jim, and the electron diffusion length, L is 4/im. 

Benjamin, D. Huppert, D. Surface recombination velocity measurements of CdS single crystals immersed in electrolytes. A picosecond luminescence study, J. Phys. Chem. 1988, 92, 4676. 

Early p-i-n cells exhibited poor quantum efficiencies at short wavelengths because of recombination in the p layer (Carlson and Wronski, 1976). As discussed in Section 1, absorption losses associated with the front p layer can be reduced by alloying the p layer with carbon. Increasing the band gap of the p layer may also reduce recombination associated with the back-diffusion of local minority carriers. As shown in Fig. 11, the p layer may present a bump in the conduction band that reflects both thermal and hot electrons back into the i layer. Thus, the good short-wavelength response shown in Fig. 9 can be attributed to the reflection of minority carriers (or a small surface recombination velocity). 

Some metastable centers may be associated with doped layers or with interface states. The light-induced generation of these centers appears to increase the surface recombination velocity in some cells, causing a decrease in the spectral response at short wavelengths. This effect and the others mentioned above are reversible annealing the a-Si H cells at 200°C for several minutes restores the cells to their initial conditions. 

Fig. 64. Calculation of surface recombination velocity vs. electrode potential. Fig. 65. Reciprocal surface recombination velocity vs, excess carrier density Snw.

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). 

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