Ohmic electrodes

An example of the difficulties encountered when trying to fabricate an ohmic electrode, able to sustain a space-charge-limited current, is the recent work of the Neher group [179]. The authors deposited barium as an electron injection cathode on top of an electron transporting polymer based on a naphthalene diimide core whose LUMO is as low as 4 eV below vacuum level. Although the Fermi level of barium should be above the LUMO of the polymer, the electron current is. 

Fig. 6. Impedance spectra in the Z plane and in the Bode plot (log(Z) vs. log(cu)) for (a) one RC element (b) two RC elements (c) three RC elements representing the situation of a polycrystal with non-ohmic electrodes and highly resistive grain boundaries and (d) three RC elements with two similar relaxation frequencies (cor2 — 3ror3) leading to overlapping semicircles.

Figure 59 Location (xT) and width (w) of the recombination zone for a small space-charge overlap in a plate shaped EL material of thickness d, provided with two injecting ohmic electrodes (anode, cathode), dh and de denote the penetration depths of injected holes and electrons, respectively. We note that xr = dh, d /de = /ih/and dh de = d for w —> 0.

L/tE (where t is the transit time), the steady-state photocurrent density (with Ohmic electrodes) becomes... 

Thus the current is proportional to the rate of carrier generation. From the above, it is obvious that greater photocurrents can be drawn with Ohmic electrodes in the circuit than with blocking electrodes. With the latter, the current saturates if the two free carriers can reach the electrodes without recombination. Figure 8.2 shows schematically the maximum photocurrent densities that can be reached with Ohmic or blocking contacts respectively in the absence of space-charge effects. 

Fig. 8.2 (a) Schematic representation of the flow of photocurrent in a photoconductive insulator, (b) Steady-state photocurrents in insulating photoconductors with blocking and Ohmic electrodes. With blocking electrodes, the current saturates when both sign carriers reach the respective electrodes without recombination. 

The contact resistance between cBN and electrodes of conventional materials is usually lO -lO n atroom temperature (184). Although the contact resistance decreases at high temperatures, it is still quite large. In order to form an ohmic contact and reduce the contact resistance, a few materials have been examined. Trials of materials such as Cu (208), Ag (4,184), Au (210), A1 (210), Cr-Ni (177), and Mo and Pt (200) have been reported in the literature and patents. Ohmic electrodes with relatively low contact resistance have been made using Ti-Au and Al-Au on Be-doped p-type crystals (210). The contact resistance of Ti-Au was 10 -10 Q at room temperature. Annealing procedures are considered to be effective. Formation of Ti or A1 nitride probably occurred. 

For electronic applications, lowering the resistivity of cBN (especially that of the n-type crystal) is desirable. In conjunction with this, sulfur doping (4) is attractive and should be tested. Forming ohmic electrodes is the important problem to be solved. 

Option for impedance transformation very near the high-ohmic electrode with input impedance > 10 12, guarded signal and capacitance neutralization for physiological measurements. 

Guarded Signal Wires for Extremely High Ohmic Electrodes... 

In polymeric insulators, due to small concentrations and low mobility of free charges, the measured currents are very low, even at high electric fields. The dependence of steady-state current density jscLc on applied voltage y in the simplest form for the trap-free insulator (assuming that the ohmic electrodes are applied and there is no potential barrier for carrier injection) can be represented by Child s equation ... 

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