Ohmic contacts

Copper Sulfide—Cadmium Sulfide. This thin-film solar cell was used in early aerospace experiments dating back to 1955. The Cu S band gap is ca 1.2 eV. Various methods of fabricating thin-film solar cells from Cu S/CdS materials exist. The most common method is based on a simple process of serially overcoating a metal substrate, eg, copper (16). The substrate first is coated with zinc which serves as an ohmic contact between the copper and a 30-p.m thick, vapor-deposited layer of polycrystaUine CdS. A layer is then formed on the CdS base by dipping the unit into hot cuprous chloride, followed by heat-treating it in air. A heterojunction then exists between the CdS and Cu S layers. 

Fig. 9. Schottky barrier band diagrams (a) a rare situation where the metal work function is less than the semiconductor electron work affinity resulting in an ohmic contact (b) normal Schottky barrier with barrier height When the depletion width Wis <10 nm, an ohmic contact forms.

Metals for Schottl Contacts. Good Schottky contacts on semiconductor surfaces should not have any interaction with the semiconductor as is common in ohmic contacts. Schottky contacts have clean, abmpt metal—semiconductor interfaces that present rectifying contacts to electron or hole conduction. Schottky contacts are usuaHy not intentionaHy annealed, although in some circumstances the contacts need to be able to withstand high temperature processing and maintain good Schottky behavior. 

Recent applications of e-beam and HF-plasma SNMS have been published in the following areas aerosol particles [3.77], X-ray mirrors [3.78, 3.79], ceramics and hard coatings [3.80-3.84], glasses [3.85], interface reactions [3.86], ion implantations [3.87], molecular beam epitaxy (MBE) layers [3.88], multilayer systems [3.89], ohmic contacts [3.90], organic additives [3.91], perovskite-type and superconducting layers [3.92], steel [3.93, 3.94], surface deposition [3.95], sub-surface diffusion [3.96], sensors [3.97-3.99], soil [3.100], and thermal barrier coatings [3.101]. 

In bilayer LEDs the field distribution within the device can be modified and the transport of the carriers can be controlled so that, in principle, higher efficiencies can be achieved. On considering the influence of the field modification, one has to bear in mind that the overall field drop over the whole device is given by the effective voltage divided by the device thickness. If therefore a hole-blocking layer (electron transporting layer) is introduced to a hole-dominated device, then the electron injection and hence the efficiency of the device can be improved due to the electric field enhancement at the interface to the electron-injection contact, but only at expense of the field drop at the interface to the hole injection contact This disadvantage can be partly overcome, if three layer- instead of two layer devices are used, so that ohmic contacts are formed at the interfaces [112]. 

Eq. (14.1) is known as the Mott-Schotlky equation. We note llial for a given n-lype semiconductor, the harrier height increases as the work function of the metal increases. It is therefore expected that high work function metals will give a rectifying junction, and low work function metals an ohmic contact (it is the reverse for a p-type semiconductor). 

Reduction in resistive and capacitive (RC) delays is continuing by the use of higher conductivity materials and improved ohmic contact. 

Gate electrodes in MOS devices for high value resistors to insure good ohmic contact to crystalline... 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

Fig. 12 a and b. Reactions at catalyst loaded particles a) catalyst forming an ohmic contact b) forming a Schottky barrier... 

In most cases, however, noble metals such as Pt, Ru and Rh or RUO2 and Rh203 have been deposited on different semiconductors such as CdS , Ti02 , SrTiOa and WO3 although these catalysts do not form an ohmic contact . In connection with the photocleavage of H2O mainly Gratzel introduced the idea of using semiconductor particles loaded even with two types of catalysts, one (e.g. Pt)... 

Electrons donated to the surface oxygen species are instantaneously replenished from the ohmic contact to maintain a steady state population of electron-filled (reduced) surface states determined by E cath-... 

Fig. 4.16. The scheme of a working plate (element) / - a plate of Si02 (AI2O3) 2 - particles of deposited metal 3 strips of pure (not activated with metal) substrate (0.03 cm wide) 4 - ZnO film serving as a sensor of hydrogen atoms 5,5 - platinum ohmic contacts.

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