Accumulation region

It is well known that photoelectrochemical measurements do not indicate photocurrents in the accumulation region of an illuminated semiconductor. The reason is that majority carriers control interfacial reactions, which 

Photoinduced microwave conductivity measurements obviously allow the measurement of minority carriers in the accumulation region (Fig. 17). In fact, both charge carriers are measured simultaneously since the PMC signal can be assumed to be proportional to the photoinduced conductivity change jicr. (This condition is fulfilled when the microwave field is not significantly attenuated within the illuminated layer.) 

This means that the minority carriers are measured, however formally, with an effectively changed mobility, which also includes the mobility of photogenerated majority carriers. 

The potential-dependent behavior of minority carriers in the accumulation region has up to now not been accessible to electrochemistry. 

Therefore, no experimental knowledge is available on interfacial reaction mechanisms under such conditions. These now become accessible via PMC measurements. As theory shows [Fig. 13(b)], the PMC signals in the accumulation region are controlled by potential-dependent surface recombination and charge-transferrates, as well as by the bulk lifetime of charge carriers. 

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In the accumulation region, the situation is much more complicated, so that a reliable analytical expression is difficult to obtain. However, it can be shown17 that the PMC signals increase toward increased accumulation in a smooth, steplike function. The ratio between the PMC maximum and the PMC minimum (at the flatband potential) can be calculated... 

Figure 13. Numerically calculated PMC potential curves from transport equations (14)—(17) without simplifications for different interfacial reaction rate constants for minority carriers (holes in n-type semiconductor) (a) PMC peak in depletion region. Bulk lifetime 10" s, combined interfacial rate constants (sr = sr + kr) inserted in drawing. Dark points, calculation from analytical formula (18). (b) PMC peak in accumulation region. Bulk lifetime 10 5s. The combined interfacial charge-transfer and recombination rate ranges from 10 (1), 100 (2), 103 (3), 3 x 103 (4), 104 (5), 3 x 104 (6) to 106 (7) cm s"1. The flatband potential is indicated.

Figure 17(a) shows the PMC peak in the accumulation region (at negative potentials) of silicon in contact with a propylene carbonate... 

Figure 17. PMC behavior in the accumulation region, (a) PMC potential curve and photocurrent-potential curve (dashed line) for silicon (dotted with Pt particles) in contact with propylene carbonate electrolyte containing ferrocene.21 (b) PMC potential curve and photocurrent-potential curve (dashed line) for a sputtered ZnO layer [resistivity 1,5 x 103 ft cm, on conducting glass (ITO)] in contact with an alkaline electrolyte (NaOH, pH = 12), measured against a saturated calomel electrode.22...

We conclude that the interfacial kinetics of excess majority carriers control the PMC signal in the accumulation region, while it is the minority carriers, as we have seen, that control the PMC signal in the depletion region. 

Surface recombination processes of charge carriers are mechanisms that cannot easily be separated from real semiconductor interfaces. Only a few semiconductor surfaces can be passivated to such an extent as to permit suppression of surface recombination (e.g., Si with optimized oxide or nitride layers). A pronounced dip is typically seen between the potential-dependent PMC curve in the accumulation region and the photocurrent potential curve (e.g., Fig. 29). This dip may be partially caused by a surface... 

Another way to determine the sensitivity factor consists in determining the difference between the PMC minimum (flatband potential) and the PMC maximum in the accumulation region (the infinite and negligible surface recombination rate). This difference can be calculated to be17... 

Equation (40) relates the lifetime of potential-dependent PMC transients to stationary PMC signals and thus interfacial rate constants [compare (18)]. In order to verify such a correlation and see whether the interfacial recombination rates can be controlled in the accumulation region via the applied electrode potentials, experiments with silicon/polymer junctions were performed.38 The selected polymer, poly(epichlorhydrine-co-ethylenoxide-co-allyl-glycylether, or technically (Hydrine-T), to which lithium perchlorate or potassium iodide were added as salt, should not chemically interact with silicon, but can provide a solid electrolyte contact able to polarize the silicon/electrode interface. 

Figure 35. Dynamic change of lifetime in an n-type silicon/polymer (poly(epichlorhydrine-co-elhylenoxide-co-allyl-glycylether plus iodide) junction during a potential sweep. The arrows show the direction of sweep (0.25 V s" ). A shoulder in the accumulation region and a peak in the depletion region of silicon are clearly seen.

Figure 44. Energy scheme showing essential phenomena for photoinduced microwave conductivity mechanisms (a) Accumulation of minority carriers near the onset of photocurrents in the depletion region, (b) Drift of minority carriers into the interior of an accumulation region, thus escaping surface recombination.

Figure 45. (a) Schematic of PMC signal behavior in accumulation region (i), flatband region (ii), and depletion region (iii) with (b) visualization of energy band situation of an n-type semiconductor. 

Fig. 4.2 For an n-type bulk semiconductor in the presence of an electrolyte illustrated is (left) no space charge layer, (center) a space charge layer in a depletion region, (right) a space charge layer in an accumulation region.

We have operated the University of Washington MKV impactor as a low-pressure impactor to provide for chemical analysis, four discretely sized fly-ash fractions in the sub-half-micrometer- diameter aerosol accumulation region. Instrumental neutron activation analysis provided the sensitivity to determine accurately the concentrations of 28 major, minor, and trace elements with sufficient precision to reveal fine structure in the elemental distributions that might be missed by techniques of lesser accuracy and precision. 

An epitaxial layer 11 of Hg0.nCd0.2Te is grown on a CdTe substrate 10. Charge accumulation regions 7 are provided in the epitaxial layer. Read-out diode regions 8 are formed by selectively growing Hgo.7Cdo.3Te. Accumulation electrodes 1, transfer electrodes 2 and readout electrodes 9 complete the structure. 

Accumulation region — An accumulation region is any part of a —r semiconductor device that has an increased... 

Accumulation region the region near a semicondnctor interface where negative charges (for an n-type semicondnctor) accnmnlate... 

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