Doping density

Quantum well interface roughness Carrier or doping density Electron temperature Rotational relaxation times Viscosity Relative quantity Molecular weight Polymer conformation Radiative efficiency Surface damage Excited state lifetime Impurity or defect concentration... 

This shows that the penetration depth decreases dramatically with increasing conductivity of the medium to be penetrated. This has been plotted (Fig. 2) for different specific resistivities of the medium and the frequency of 10-40 Gc/s11 at which microwave conductivity measurements are typically performed. It can be seen that with a specific resistivity of 10 Q cm, a penetration depth of only 2 mm can be expected. Figure 2 furthermore shows the doping densities at which the respective penetration depths can be expected for silicon. Whereas the lower frequency X-band of microwaves (8-12.5 Gc/s) offers some advantages for materials with very low resistance, the high-frequency microwave Ka-band (26.5 10... 

Aqueous cathodic electrodeposition has been shown to offer a low-cost route for the fabrication of large surface n-CdS/p-CdTe solar cells. In a typical procedure, CdTe films, 1-2 xm thick, are electrodeposited from common acidic tellurite bath over a thin window layer of a CdS-coated substrate under potential-controlled conditions. The as-deposited CdTe films are stoichiometric, exhibit strong preferential (111) orientation, and have n-type conductivity (doping density typically... 

Criteria 6 and 7 are important because in order to vary the kinetically relevant parameters such as the spatial distribution of charge carriers and the thermodynamic driving force, semiconductor electrodes with different majority carrier types, doping densities and band gaps must be used. 

Figure 2. Band edge positions obtained over a period of three weeks for p-and n-type WSe2 -CH3CN interfaces containing metallocene redox couples (ferrocene, FER decamethylferrocene, DFER and acetylferrocene, AFER) each at three concentrations (preceding letter refers to high.H medium,M and low, L). Two different electrodes were used to obtain the data for n-WSe2 with doping densities between 1016 -1017 cm-3.

The figure on the inner front cover of this book can be used to convert between doping density, carrier mobility and resistivity p for p- or n-type doped silicon substrates. One of the major contaminants in silicon is oxygen. Its concentration depends on the crystal growth method. It is low in FZ material and high (about 1018 cm-3) in Czochralski (CZ) material. 

Em is limited by the breakdown field strength Ebd of silicon, which is about 3x 105 V cm4. The figure on the inner front cover shows the width of the SCR as a function of doping density and applied bias, as well as the limitation by avalanche breakdown. 

The SCR capacitance for a given doping density and applied bias is given in the figure on the inner front cover. 

The morphology of alkaline-etched (100) and (110) silicon surfaces varies from rough surfaces that exhibit micron-sized pyramids or ridges [Sc5] to smooth orange peel-like surfaces, depending on the etchant composition and substrate doping density. Mirror-like surfaces can be obtained on (111) crystal planes. 

Highly p-doped layers can also be used as masking layers. If the p-type doping level of silicon substrates is high enough to cause degeneracy (NA > 1019 cm-3), a decrease in etch rate with doping density is observed in all alkaline solutions inde-... 

It is known that HF-HN03-based solutions etch highly doped substrates faster by a factor of about three compared to moderately doped ones. A higher selectivity is reported on addition of chemicals that reduce the HNOz concentration, like H202 or NaN3 [Mul], However, this report suffers from the fact that the etch rate was measured for separate wafers of a homogeneous doping density. For pp+ or nn+ structures, which are not spatially separated, only a low selectivity is observed, because of the autocatalytic behavior of the etchant. 

This is the region of the OCP. For an HF electrolyte without an oxidizing agent the electrode is inert, because no chemical reaction occurs at the front (emitter) at this potential range. The OCP depends on illumination condition, substrate doping density, illumination condition, HF concentration and DOC [Otl]. For moderately doped Si substrates in 5% aqueous HF the OCP is usually close to -0.6 V versus SCE in the dark. Under illumination a small negative (cathodic) shift to -0.64 versus SCE is observed for n-type electrodes, while the OCP for p-type substrate shifts significantly in positive (anodic) direction to -0.2 V versus SCE [Be9]. 

Fig. 3.3 The I—V curves, as recorded and compensated for ohmic losses (iRcor.), of Si electrodes in aqueous HF (1M HF, 0.5M NH4CI) are found to shift cathodically with increasing p-type doping density. In a V versus log(i) plot (inset) a p-type electrode (1 2 cm,...

As shown in Fig. 3.3, the I-V curve in this regime shows a cathodic potential shift and a slight change of slope, if the doping density is increased. Compared to p-type substrates the I-V curve of p+ is shifted cathodically by about 0.1 V and that of n+ by about 0.2 V [Gal, Zh5[. This shift can be exploited for etch stops and selective formation of PS, as discussed in Section 4.5. 

While n.. shows no dependence on doping density, current density or electrolyte concentration in the electropolishing regime, it does in the PS regime [Le23, Fr6]. enerally n., increases with current density. This is shown for the mesoporous rein Fig. 6.9 a, and microporous regime in Fig. 4.6. From the data of the latter gure the dependence of n.. on formation current density J (in mA cm4) in etha-HF can be fitted to ... 

C-V and I-V measurements of Si electrodes of different doping density in electrolytes free of fluoride show that in this case the dark current becomes dominated by thermally activated electron transfer over the Schottky barrier rather than by carrier generation in the depletion region [ChlO]. Note that the dark currents discussed above may eventually initiate the formation of breakdown type meso-pores, which causes a rapid increase of the dark current by local breakdown at the pore tips, as shown in Fig. 8.9. This effect is enhanced for higher values of anodic bias or doping density. 

Small leakage currents or a transistor-like action of the junction are sufficient to generate a small current that may cause undesired passivation. This can be circumvented by application of an additional potential to the etching layer, shown by the broken line in Fig. 4.16 a. This electrochemical etch-stop technique is favorable compared to the conventional chemical p+ etch stop in alkaline solutions, because it does not require high doping densities. This etch stop has mainly been apphed for manufacturing thin silicon membranes [Ge5, Pa7, Kll] used for example in pressure sensors [Hil]. 

An initially flat silicon electrode surface will develop a surface topography if the photocurrent varies locally. This variation can be caused by a lateral variation of the recombination rate or by a lateral variation of the illumination intensity. The photoelectrochemical etching of a silicon electrode is related to the etch-stop techniques discussed in Chapter 3. While different etching rates for different areas of the electrode may be obtained by electrical insulation or by a different doping density, the etch rate may also be altered by a difference in illumination intensity. Basically four photoelectrochemical etching modes are possible for homogeneously doped substrates ... 

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