2.1.3. Cyclic stacking

like states. As mentioned in Section 1.1.4, there is a charge neutrality level, Φ<sup>0</sup> in the band gap where the gap-state charges are balanced. The Fermi level is pinned close to the charge neutrality level because of dipole formation. To prevent the Fermi level pinning, the free electron wave function penetration has to be reduced. This can be done by introducing a thin dielectric layer. Si3N4 [17–19] has low dielectric constant and moderately high band gap to prevent the free electron wave function from penetrating into the semiconductor band gap and hence releasing the Fermi level. Al2O3 [19] has also been reported to reduce the Fermi level

There are principally three different methods of depositing metal on a substrate: plating, metal evaporation and sputtering. Metal plating is generally used to deposit thick layers. Only metal

Evaporation techniques are based on heating up a source to a temperature where the material starts vaporizing. The vaporized material is then deposited on the sample and cools down forming a thin film. Thermal evaporation can either be achieved by heating the source with a resistive element or by using an electron beam. Resistive heating takes place by passing a current through a heating element, often made out of tungsten, which heats up a crucible containing the source material. Resistive evaporation has the disadvantage of potential contamination from the crucible if the melting temperature of the crucible is close to the melting temperature of the source material, resulting in a poor film quality. Electron beam evaporation uses an electron beam generated from a cathode to heat up the source material locally. The crucibles are water cooled to minimize contamination. The electron beam is generated by a thermionic emission filament and is accelerated towards the crucible using a high accelerating voltage. The beam is then focused into a spot on the surface of the source material and the interaction between the accelerated electrons and the source material will cause the material to start heating up and vaporize. The combination of local heating and water cooled sources prevents crucible metal contamination, resulting in a high purity film deposited on the substrate. The evaporation processes take

–10�<sup>4</sup> mTorr) in order to create a mean free path of the evaporat-

evaporation and sputtering were used in the work reported on in this chapter.

ing flux, which is greater than the distance between the source and the sample.

While evaporation requires a source to be heated to produce a flux of gas, sputtering targets make use of a physical plasma process rather than heat. The plasma is formed using an inert gas (normally Argon) and is excited by either a direct current (DC) or radio frequency (RF) source. The target source is negatively biased and the plasma sputters neutral atoms of source

pinning effects.

2.1. Metallization

2.1.1. Metal evaporation

place under high vacuum (10�<sup>3</sup>

2.1.2. Sputtering

2. Experimental techniques

64 Advanced Material and Device Applications with Germanium

To explain the process of cyclic stacked we take the example of the production of an NiGe layer on a Ge substrate. Multi layers of Ni and Ge are formed by RF magnetron sputtering on an ntype germanium substrate at room temperature and the average composition of the whole multi-layers is controlled so as to have a stoichiometric equivalence to the atomic ratio of Ni

Figure 16. A schematic diagram of an RF magnetron sputtering unit.

Acknowledgements

Author details

Adrian Habanyama

References

11.202

242104

The author would like to thank the Copperbelt University for the use of the institution's facilities.

Interface Control Processes for Ni/Ge and Pd/Ge Schottky and Ohmic Contact Fabrication: Part One

http://dx.doi.org/10.5772/intechopen.78692

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[4] Martens K, Firrincieli A, Rooyackers R, Vincent B, Loo R, Locorotondo S, Rosseel E, Vandeweyer T, Hellings G, Jaeger BD, Meuris M, Favia P, Bender H, Douhard B, Delmotte J, Vandervorst W, Simoen E, Jurczak G, Wouters D, Kittl JA. Record low contact resistivity to n-type Ge for CMOS and memory applications. Technical Digest – International Electron

[5] Martens K, Rooyackers R, Firrincieli A, Vincent B, Loo R, De Jaeger B, Meuris M, Favia P, Bender H, Douhard B, Vandervorst W, Simoen E, Jurczak M, Wouters DJ, Kittl JA. Contact resistivity and fermi-level pinning in n-type Ge contacts with epitaxial Si-passivation.

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Address all correspondence to: adrian.habanyama@cbu.ac.zm Department of Physics, Copperbelt University, Kitwe, Zambia

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Figure 17. A schematic illustration of the sample configuration in cyclically stacked Ni/Ge films on an n-type Ge substrate.

and Ge atoms of 1 to 1 as in the phase, NiGe [20]. Figure 17 is a schematic diagram showing the sample configuration in a cyclically stacked Ni/Ge film.

The idea behind this stacking of layers is to suppress the reaction between Ni and the Ge substrate upon annealing. In this way it is possible to get a high-quality NiGe film with a smooth interface on the Ge substrate. It is hoped that this smooth interface would reduce the Fermi level pinning effects of the interface electron energy states.

#### 2.2. Donor implantation

Implanting atomic species like selenium (Se) into the surface of n-type germanium before metallization helps to reduce the Schottky barrier height by introducing local interfacial doping. In order to achieve a reasonable amount of implantation into the surface of the Ge substrate, the atoms to be implanted need to be energized to around 130 keV. The implantation is usually followed by heating at a high activation temperature to activate the diffusion of the dopant atoms further into the semiconductor surface, before metallization.

#### 2.3. Four-terminal sheet resistivity measurement

The results of sheet resistance measurements presented in this chapter were obtained using a four-terminal resistor structure also known as a Kelvin resistor [21] structure. The structure consists of four contact pads: two pads are connected to the doped bulk semiconductor material and two pads contact to the metal used to form the contact. Current is then passed through two terminals between the semiconductor and the metal and the corresponding voltage drop is measured using the other two terminals between the metal and the semiconductor. In this way a sheet resistivity, rsh can be extracted.
